Article
To read the full-text of this research, you can request a copy directly from the authors.

Abstract

Analysis of the global distribution of U/Pb ages of both subduction-related granitoids and of detrital zircons suggests that a widespread reduction in magmatic activity on Earth beginning about 2.45 Ga and lasting for 200–250 My. There are no arc-type greenstones or tonalite–trondhjemite–granodiorite (TTG) suites and only one large igneous province (LIP) reported in this time window. There is little Nd or Hf isotopic evidence to support significant additions to the continental crust at convergent plate margins between 2.45 and 2.2 Ga. Also during this time, there are major unconformities on most cratons and a gap in deposition of banded iron formation (BIF), both consistent with a major drop in sea level. Oxygenation of the atmosphere at 2.4 Ga followed by widespread glaciation at 2.4–2.3 Ga also may be related to the initiation of the global magmatic lull. We suggest that an episodic mantle thermal regime, during which a large part of the plate circuit effectively stagnates, may explain the 250-My magmatic age gap on Earth and a remarkable feature of the Paleoproterozoic record.

No full-text available

Request Full-text Paper PDF

To read the full-text of this research,
you can request a copy directly from the authors.

... Ga Minas orogeny in the Southern São Francisco Craton (SFC), like several examples in every continent (Partin et al., 2014;Teixeira et al., 2015;Moreira et al., 2018;Barbosa et al., 2019). However, this period of the Earth's evolution is also known for a significant reduction of magmatic and tectonic activity (Condie et al., 2009;Spencer et al., 2018). ...
... Owing to the large volume of the juvenile-like Siderian-Rhyacian magmatism that formed the continental crust of South America, a geodynamic transition marked by continued accretionary growth through the Paleoproterozoic becomes apparent. At a global scale, this issue has implications at to the current debate whether continuous slab subduction or stagnant-lid plate dynamics accompanied the secular cooling of the mantle during the Paleoproterozoic and the eventual construction of Columbia/ Nuna supercontinent (e.g., Condie, 2009;Partin et al., 2014;Spencer et al., 2018). As widely known, Precambrian supercontinent cycles mark episodic records of the maximum crustal growth that may also be accompanied by LIP events (Condie et al., 2015;Ernst, 2014;Voice et al., 2011). ...
... However, for the preceding Paleoproterozoic periods, there is a debate on whether the production of continental magmatism has been continuous or episodic. For instance, Condie et al. (2009) considered that distinct minima in the global database of juvenile magmatism and detrital zircon abundance could be referred to as widespread lithospheric stagnation in an episodic mantle overturn. ...
Article
Full-text available
We document new U-Pb detrital zircon LA-MC-ICP-MS data for seven metavolcanic-sedimentary successions and metasedimentary sequences and reassess additional dates of five siliciclastic samples toward their tectonic significance in the context of the Mineiro belt, Southern São Francisco Craton. This belt represents a crustal segment of the 2.47–2.00 Ga Minas Orogen, classically known by its Siderian and Rhyacian juvenile rocks with important implications in the Earth's geodynamics. The new and compiled detrital provenance constraints unravel the long-lived magmatic and sedimentary history of the studied basins, lasting ca. 230–220 Myr. The maximum depositional dates around 2.1 Ga reflect the renewed sediment budget with the subsequent metamorphic episode ca. 2.0 Ga. Most of the unmixed relative probability diagrams are consistent with sourcing from the Siderian and Rhyacian arcs of the Mineiro belt, determining a detrital provenance change in time and space for the precursor basins. Alternative potential sources could be the youngest rocks of the Mantiqueira and Juiz de Fora terranes that constitute the other segments of the Minas Orogen, given the age match. The overall detrital fingerprints determine the study basins resumed mainly in Rhyacian fore-arc and/or back-arc settings, i.e., akin to a subduction-related system that evolved to a collisional (foreland) environment. Few samples show fingerprints of primary extensional settings, determined by major Archean detrital populations sourced from areas outside the Mineiro belt beside the Paleoproterozoic detritus. The working model considers the collage between the Mineiro belt and the ancient foreland around 2.10 Ga and eventual interaction with other crustal segments of the Minas Orogen, generating the ca. 2.0 Ga metamorphism over the metasedimentary samples. The more complete isotopic repository in detrital and igneous zircon grains for the studied supracrustal successions and the associated rocks allows new insights into the Rhyacian-Orosirian dynamics of the Minas orogeny. In a broader perspective, the juvenile nature of the Mineiro belt reinforces the paradigm of uninterrupted continental growth during the Paleoproterozoic Earth.
... The global distribution of U Pb ages for subduction-related granitoids and detrital zircons indicates a widespread reduction in magmatic activity on Earth at 2.45-2.20 Ga, which is considered to represent a global magmatic and tectonic shutdown or slowdown (Condie, O'Neill, and Aster, 2009;Partin et al., 2014;Spencer, Orogenic Belt of the Western Churchill Craton (e.g, Cui et al., 2019;Gong et al., 2014;Gong, He, Wang, Chen, and Kusky, 2019;He et al., 2018;Seixas, David, and Stevenson, 2012;Teixeira et al., 2015;Yu, Fu, Wang, Li, and Guo, 2017;Zhao et al., 2008). The TTG suites are thought to have formed in a subduction-related arc setting, whereas the other granitoids were associated with syn-or post-collisional extension. ...
... There was a remarkable reduction in magmatic activity on Earth at 2.45-2.20 Ga, which was probably due to a global plate tectonic shutdown or slowdown (Condie, O'Neill, and Aster, 2009;Partin et al., 2014;Spencer, Murphy, Kirkland, Liu, and Mitchell, 2018). Prior to the tectono-magmatic shutdown, scattered continental blocks had been amalgamated into a supercontinent at 2.7-2.5 Ga (i.e., the Kenorland supercontinent; Aspler and Chiarenzelli, 1998), which might have been responsible for the global cessation or slowdown of plate tectonics (Condie, O'Neill, and Aster, 2009). ...
... There was a remarkable reduction in magmatic activity on Earth at 2.45-2.20 Ga, which was probably due to a global plate tectonic shutdown or slowdown (Condie, O'Neill, and Aster, 2009;Partin et al., 2014;Spencer, Murphy, Kirkland, Liu, and Mitchell, 2018). Prior to the tectono-magmatic shutdown, scattered continental blocks had been amalgamated into a supercontinent at 2.7-2.5 Ga (i.e., the Kenorland supercontinent; Aspler and Chiarenzelli, 1998), which might have been responsible for the global cessation or slowdown of plate tectonics (Condie, O'Neill, and Aster, 2009). During the tectono-magmatic shutdown, mafic-ultramafic magmatism occurred in response to a mantle plume triggering the break-up of Kenorland (Eriksson and Condie, 2014;Partin et al., 2014, and references therein), suggesting that a global extensional setting prevailed. ...
Article
The Taihua Complex in the Xiaoqinling area of the southern Trans-North China Orogen (TNCO) records extensive magmatism during the early Paleoproterozoic. This contrasts with the tectono-magmatic quiescence recorded globally at this time. Based on zircon UPb geochronological and whole-rock geochemical data, the 2.36–2.30 Ga granitic gneisses in the Taihua Complex can be subdivided into three groups. Group-1 gneisses (2.36–2.33 Ga), mainly in the Dumutai area, are high-K2O granitoid rocks, which record the earliest magmatism. These rocks have fractionated rare earth elements (REE) patterns, marked negative EuSr anomalies, and variable whole-rock εNd(t) (−1.67 to +2.41) and εHf(t) values (−0.89 to +3.41), and were likely derived by partial melting of a mixture of pre-existing tonalite–trondhjemite–granodiorite (TTG) and K-rich rocks with residual plagioclase in the source at high-temperature and low-pressure conditions. Group-2 gneisses (2.33–2.31 Ga), mainly in the Jialu and Xitongyu areas, are granitoid rocks that formed after the Group-1 gneisses. These rocks have less fractionated REE patterns, low La/Yb and Sr/Y ratios, and slightly positive to negative whole-rock εNd(t) (−3.91 to +2.00) and εHf(t) values (−7.99 to +0.87), which resulted from high-degree partial melting of ancient crust in an extensional setting. Group-3 gneisses (~2.30 Ga) are TTG rocks from the Bayuan area, which represent the youngest magmatism. These rocks have high La/Yb and Sr/Y ratios, high MgO, Cr, and Ni contents, and negative whole-rock εNd(t) (−1.71 to −1.67) and εHf(t) values (−0.57 to −0.10), which indicate derivation by partial melting of delaminated lower crust and interaction with mantle materials. Samples from all groups have variable zircon εHf(t7/6) (−6.38 to +7.70) and δ¹⁸O values (3.97‰ − 6.79‰), indicative of a heterogeneous magmatic source. Most zircons have slightly positive to negative εHf(t7/6) values, indicating significant reworking of ancient crustal materials and limited crustal growth during this period. The age, geochemistry, and petrogenesis of the studied granitoid rocks record orogenic collapse and a transition from post-collisional extension to crustal delamination in the early Paleoproterozoic. This followed late Archean subduction–collision–accretion processes in the southern NCC.
... Worldwide, the Siderian rock record is as scarce as the Eo-to Paleoarchean one, which has led to the proposition for a lull in the magmatic activity during this Period (Condie et al., 2009). Although subsequent work (Pehrsson et al., 2014) has demonstrated that magmatic rocks continued to be produced in the early Paleoproterozoic, they are much less abundant than those formed at later times. ...
... Generation of crustal rocks in this age interval is more common in the geological record than in the previous 250 Myr (Condie et al., 2009;Belousova et al., 2010;Condie and Aster, 2010;Dhuime et al., 2017), but these rocks are mainly restricted to West Africa and South America (Boher et al., 1992;Sato and Siga, 2002;Delor et al., 2003;De Waele et al., 2008;Macambira et al., 2009;Baratoux et al., 2011;Sakyia et al., 2014;Block et al., 2016). Models for the assembly of northern SFC call for collision of the Gavião and Jequié blocks following westward subduction of the Jequié Paleoplate ( Barbosa and Sabaté, 2004), with production of syncollisional granites and development of a fold-thrust belt resulting from deformation of a foreland basin (Contendas-Mirante supracrustal sequence) at 2.10-2.05 ...
... Ga time interval since global compilations of igneous and detrital zircon U-Pb ages show a large peak at 1.95-1.85 Ga, interpreted to reflect a major episode of juvenile crust formation (Condie, 1998;Condie et al., 2009;Condie and Aster, 2010) followed by the final assembly of the Columbia supercontinent by 1.8-1.7 Ga (Rogers and Santosh, 2002;Hou et al., 2008b;Meert, 2012;Furlanetto et al., 2013;Pehrsson et al., 2015). ...
Article
70–80% of the continental crust was produced during the 4.0–2.0 Ga time span, but the preserved area of Archean/early Paleoproterozoic cratons is smaller than 40%. Part of this deficit can be accounted for by the presence of reworked old crust in the basement of mid-Paleoproterozoic to Phanerozoic orogenic belts. Here, I compare the crustal evolution of the Brasiliano-Pan-African Borborema Province (BP) with that of the São Francisco Craton (SFC) in eastern Brazil and highlight numerous geological aspects, several of which are uncommon in other cratons/orogenic belts, indicating their shared evolution for most of the Precambrian. These include: 1. Presence of the oldest rocks (Eo- to Paleoarchean) from the South American Platform. 2. Occurrence of Siderian (2.5–2.3 Ga) rocks. 3. Generation of juvenile crust and reworking of pre-existing rocks during the Transamazonian event (2.2–2.0 Ga). 4. Intermittent rifting and intraplate magmatic events between 1.78 and 1.50 Ga. 5. Intrusion of mafic dykes and A-type granites at 1.0–0.85 Ga. 6. Intrusion of mafic rocks, syenites and granitoids with intraplate signature between ca. 0.71 and 0.64 Ga. 7. The lack of evidence for igneous and tectonic activity between ca. 1.95 and 1.78 Ga, during most of the Mesoproterozoic, and between 0.85 and 0.73 Ga. The temporal coincidence of Rhyacian orogenic events in the SFC and BP favors the hypothesis that they were part of a continent formed by the accretion of Archean/early-Paleoproterozoic blocks and of juvenile arc crust during the Transamazonian Orogeny. In addition, the recording of several intraplate tectonomagmatic events from the late-Paleoproterozoic to the Neoproterozoic indicates that they remained united until at least the mid-Neoproterozoic. In this context, BP can be interpreted as a fragment of the SFC re-accreted and reworked during the Brasiliano-Pan-African Orogeny (ca. 640–550 Ma). Recent studies demonstrate that most of the basement of the Brasília and Araçuaí belts, which occur to the west and east, respectively, of the SFC, also resulted from its reworking. Therefore, an area c. two times larger than the current outline of the SFC can be inferred, indicating an intense process of decratonization during the Brasiliano-Pan-African Orogeny. The intermittent late Paleoproterozoic to early Neoproterozoic extension-related magmatism in this Greater São Francisco paleocontinent contrasts with the worldwide occurrence of orogenic episodes accompanying the amalgamation of the Columbia supercontinent, its fragmentation, and the build-up of Rodinia. These differences suggest that Greater São Francisco was not part of these supercontinental assemblages.
... Os picos de crescimento seriam precedidos por produção mais lenta, sendo identificada entre 2,45 e 2,2 Ga, no Sideriano, uma importante lacuna no registro do magmatismo global (Condie et al., 2009a), conhecida por "crustal/magmatic age gap", "magmatic shutdown" ou "magmatic lull" (Condie et al., 2009a;Partin et al., 2014;Spencer et al., 2018). Spencer et al. (2018) propõem que o período desta desaceleração tectono-magmática ocorreu provavelmente entre ~ 2,3-2,2 Ga. ...
... Os picos de crescimento seriam precedidos por produção mais lenta, sendo identificada entre 2,45 e 2,2 Ga, no Sideriano, uma importante lacuna no registro do magmatismo global (Condie et al., 2009a), conhecida por "crustal/magmatic age gap", "magmatic shutdown" ou "magmatic lull" (Condie et al., 2009a;Partin et al., 2014;Spencer et al., 2018). Spencer et al. (2018) propõem que o período desta desaceleração tectono-magmática ocorreu provavelmente entre ~ 2,3-2,2 Ga. ...
... Spencer et al. (2018) propõem que o período desta desaceleração tectono-magmática ocorreu provavelmente entre ~ 2,3-2,2 Ga. Para Condie et al. (2009aCondie et al. ( , 2009b este hiato temporal resulta de uma desaceleração ou desligamento da tectônica de placas. Porém, os registros de crosta continental juvenil naquele intervalo de tempo aumentaram rapidamente nos últimos anos e questiona-se se essa quietude magmática realmente existiu ou se as rochas deste período não foram preservadas (Hawkesworth et al., 2009Partin et al., 2014). ...
Article
Full-text available
The Lagoa Dourada Suite (LDS), located in the Mineiro Belt, southern margin of the São Francisco Craton, dated at ca. 2.35 Ga, is composed of biotite-hornblende tonalites crosscut by a network of dykes classified into 5 different types (Types 1 to 5). These types were individualized based on their cutting relations and spatial orientation, through the outcrop mapping of 4 workstations, besides the modal composition and texture. With microstructural features mostly igneous, the foliation observed in these dykes was generated during their emplacement, associated with the magmatic flow itself. The observed cutting relations show that Type 1 (ca. 2347 Ma) is the oldest one, whose obtained age overlaps, within the analytical error, the host rock age. It had been emplaced through a system of syn-intrusive fractures resulting from the volumetric contraction of the pluton during its crystallization. Thehost rock rupture responsible for the emplacement of the other types is probably due to the fracturing caused by the magmatic pressure, with the youngest intrusion (Type 5, ca. 2332 Ma) occurring approximately 20 Ma after its crystallization. Therefore, there is a singular record of the production of crust at ca. 2350-2330 Ma for LDS and intrusive dykes, a time that would correspond to a global paucity period in the continental crust formation (2.45 e 2.2 Ga).
... The origin and global extent of the Tectono-Magmatic Lull (TML) near 2300 Ma continue to be topics of interest and controversy. Condie et al. (2009) and Spencer et al. (2018) have suggested that it represents a global feature during which orogenic activity and global magma production decreased or even shutdown for ~ 100 Myr. In contrast, Partin et al. (2014), Pehrsson et al. (2014) and Yang and Santosh (2015) suggest that juvenile crustal production and plate tectonics both continued during the Proterozoic uninterrupted. ...
... It is the purpose of this paper to go beyond Condie et al. (2009) by reappraising and presenting more evidence for the TML and to precisely define the timing using detrital zircon ages. We show that the paucity of detrital zircon ages in the TML is not due to preservational or sampling biases. ...
... In this study, we further explore the question of whether or not the TML can be explained by a shutdown or slowdown of plate tectonics and subduction-related magmatism. To address this question, we consider two tectono-magmatic models: 1) Earth reverts to a stagnant or squishy lid regime during the TML, and 2) Plate tectonics continues during the TML, but continental crust production rate decreases or/and the crust produced is later recycled back into the mantle (Condie et al., 2009). It may be possible to eliminate one or both of these models with three types of tests as summarized below. ...
Article
There is a 87–86% drop in frequency in both detrital and igneous zircon U/Pb ages during the Tectono-Magmatic Lull (TML, 2365–2235 Ma) and a significant decrease in frequency of granitoids, but no recognized age gap. The TML is transferred in detrital zircon ages through all younger depositional time windows, indicating it is a globally robust feature. This seems to require a decrease in production rate of felsic magma during the TML, and possibly also recycling of crust of this age into the mantle. However, εHf zircon and εNd whole-rock felsic igneous data show that juvenile continental crust continued to be produced and that the ratio of juvenile/reworked crust remained about the same or increased relative to the ratio before and after the lull. During the TML there is a large decrease in the frequency of LIPs, with only four that fall in the 2365–2235 Ma time frame, and there is a global LIP age gap at 2340–2260 Ma. Komatiite frequency in greenstones decreases rapidly at 2600–2000 Ma and enrichment in incompatible elements in greenstone basalts at 2700–2400 Ma reflects increasing enriched components in mantle sources. Although 13 orogens, all with convergent margin characteristics, are recognized between 2400 and 2200 Ma, only four are known with major deformation in the TML. Paleomagnetic data indicate that the relative positions of the Superior, Kaapvaal and India cratons changed significantly between 2435 and 2175 Ma, implying the existence of plate tectonics during this time interval. Also, paleomagnetic positions from several cratons confirm that average minimal plate speeds are relatively low during the TML (<8 cm/yr) and that there are no fast plates (>10 cm/yr) between 2.35 and 2.25 Ga. We propose a new testable model for the TML related to a mantle overturn event at 2.7 Ga that initiated widespread subduction. Sinking slabs covered the core-mantle boundary decreasing the rate of mantle convection so that oceanic lithosphere was consumed faster than it was produced. This led to a slow-down in plate speeds some 400 Myr later during the TML, with consequent decreases in magma production, mantle plume generation and orogenic activity.
... The global tectono-magmatic lull during the early Paleoproterozoic has been previously constrained within the timeframe of 2.45-2.2 Ga (Condie et al., 2009), and refined between 2.26 and 2.21 Ga by a more recent global compilation (Spencer et al., 2018), in spite of opposing proposals (Diwu et al., 2014;Partin et al., 2014). The new discovery of early Paleoproterozoic igneous rocks and associated sedimentary records around the world expand the opportunities to test the hypotheses discussed above and in particular constrain the geodynamics of the early Paleoproterozoic. ...
... Geodynamic models suggest that Earth was dominated by an episodic subduction regime during the transition between a dominant stagnant lid regime towards the plate tectonic regime (O'Neill et al., 2007). The transition from a stagnant lid to plate tectonic regime was temporally associated with a period of slowdown global magmatism and perhaps in plate tectonics between 2.45 and 2.2 Ga (Condie et al., 2009). A more recent global compilation including magmatic rocks, LIPs, orogenies records has refined the interval of this lull down to ~50 Ma, from 2.26 Ga to 2.21 Ga (Spencer et al., 2018). ...
Article
It is debated whether global-scale tectono-magmatic quiescence prevails on the Earth during the early Paleoproterozoic. The early Paleoproterozoic meta-mafic and intermediate rocks in the southwestern Yangtze Block of South China could provide an opportunity to constrain not only the tectonic evolution of the Yangtze Block itself but also augment our understanding of the global tectono-magmatic lull during the early Paleoproterozoic. Zircon UPb geochronological study indicates that the meta-mafic rocks were emplaced at ca. 2.32 Ga, whereas the intermediate rocks were emplaced at ca. 2.35 Ga. These two lithologies are characterized by subduction-related whole-rock geochemical (such as depleted Nb, Ta and enriched Rb, K, Pb) and unradiogenic NdHf isotopic signatures (εHf(t) < 0, εNd(t) < 0), implying a derivation from a geochemically enriched lithospheric mantle modified by ancient subduction. Thermodynamic modeling further suggests that the intermediate rocks are products of the AFC process between mantle-derived basaltic magma and TTG-like basement rocks at shallow crustal levels. We interpret this as occurring within a post-orogenic extensional setting. Integrated with the published work, we further posit that the Yangtze Block experienced a tectonic regime transition from compressive orogenesis to post-orogenic extension during the early Paleoproterozoic, and the magmatic activities in several crustal blocks are still active until ~2.3 Ga and therefore places important constraints on the beginning of the tectono-magmatic lull.
... Our Ti isotope data alone cannot discriminate between these two processes, but as both require the downward transport of hydrated material to melting depths, the mechanisms that produced ≤3.75-Ga TTGs in Acasta may have occurred in horizontal tectonic regime akin to subduction. The shift from dry magmatism, likely in a plume setting, to wet magmatism may be a short-lived regional occurrence, evidence of a pre-subduction regime (43)(44)(45), evidence for intermittent plate tectonics (an intermediate stage between plate tectonics and stagnant lid convection) (43,46,47), or a true signature of modern-style convergent margin magmatism. ...
... Twenty bulk powder samples were prepared for Ti isotopic analysis following the methods described in (51), which involves alkali-flux fusion to ensure sample homogenization and complete dissolution of Ti before ion exchange chromatography. Following flux fusion, samples were digested and double-spiked with 47 Ti- 49 Ti solution in a fixed proportion to Ti in the sample. We use the double-spike technique to correct the data for mass bias introduced in the instrument (63). ...
Article
Full-text available
Plate subduction greatly influences the physical and chemical characteristics of Earth's surface and deep interior, yet the timing of its initiation is debated because of the paucity of exposed rocks from Earth's early history. We show that the titanium isotopic composition of orthogneisses from the Acasta Gneiss Complex spanning the Hadean to Eoarchean transition falls on two distinct magmatic differentiation trends. Hadean tonalitic gneisses show titanium isotopic compositions comparable to modern evolved tholeiitic magmas, formed by differentiation of dry parental magmas in plume settings. Younger Eoarchean granitoid gneisses have titanium isotopic compositions comparable to modern calc-alkaline magmas produced in convergent arcs. Our data therefore document a shift from tholeiitic-to calc-alkaline-style magmatism between 4.02 and 3.75 billion years (Ga) in the Slave craton.
... Ga period links two mantle events at 2.55 and 2.15 Ga and includes the Tectono-Magmatic Lull (TML) at 2.4-2.2 Ga (Fig. 8) (Condie et al, 2009;Spencer et al., 2018). During this time, mantle sources for arc-type basalts show variable contributions of enriched components. ...
... Although not included in this study, continental within-plate basalts (LIPS, flood basalts, rift basalts) show increases in LREE and LILE during this time (Greenough and McDivitt, 2018). During the TML there is a global minimum in zircon ages, and in the frequency of greenstones, granitoids, orogens, and LIPs (Condie et al, 2009;Condie, et al., 2022 in review). Although average plate speed appears to have decreased during the TML, subduction likely continued around most cratons, and at least seven convergent margins on widespread cratons are recognized . ...
Article
We use a filtered database that tracks the composition of hydrated mantle with time using arc-type basalts, including an unknown amount of plume-type mantle in the Archean. A factor of 2–10 geochemical variability, both between and within sample sites, characterizes arc-type basalts and especially so >3.5 Ga and < 2.0 Ga. Based on which geographic sites are available for sampling, this variability is probably due to an anomalously high proportion of enriched components in some arc-type mantle sources. The proportion of an enriched component in magma sources is related to the ratio of continental to oceanic arc sites in the database, and thus is at least in part controlled by tectonic setting sample biases. We suggest that online geochemical databases of mafic igneous rocks should not be used to track secular changes in mantle composition or temperature unless they have been carefully filtered for tectonic setting, alteration/metamorphism, reliable ages, rock type, and major element composition. If the relatively few geographic sites are representative, large variations in incompatible element distributions in arc-type basalts suggest poorly mixed mantle before 3.5 Ga. However, the uniformity of incompatible element distributions from 3.5 to 2.6 Ga supports the existence of well-mixed mantle sources during this time. Long-term changes in major and compatible element contents of arc-type basalt mantle sources from 3.5 to 1.0 Ga are consistent with decreasing mantle temperature and degree of melting during this time. Although enriched mantle sources for arc-type basalts appear in the Neoarchean, they do not become widespread until after 2.0 Ga. From 2.0 to 1.0 Ga, enriched mantle contributions are relatively high but variable, probably in response to some combination of increased enriched mantle in the magma sources, sampling biases from relatively few sites with an anomalously high proportion of enriched mantle, and recycling of oceanic and continental lithosphere into the mantle in response to global propagation of subduction. Superimposed on long-term geochemical trends are five geographically widespread geochemical anomalies (deviations of greater than ±2σ from a smoothed Gaussian curve) in incompatible element distributions (2.55, 2.15, 1.75, 0.65 and 0.25 Ga) and an additional anomaly at 3.6–3.3 Ga in some compatible elements. All five incompatible element anomalies occur near age peaks in the U/Pb zircon age record (at 2.5, 2.15, 1.85, 0.60, and 0.25 Ga in both detrital and igneous zircons) and the 2.55, 2.15 and 1.75 Ga anomalies are very close to peaks in Re/Os model ages for mantle ultramafic rocks. The simplest explanation for the incompatible element anomalies is episodic enrichment of mantle sources of arc-type basalts, perhaps related to widespread mantle events (overturn or/and mantle plume) that brought enriched components into subduction-related sources of arc-type basalts.
... Global tectonics may have stepped into a new stage from Archean to Paleoproterozoic era, as supported by the transitions of crustal composition to felsic and stable (Tang et al., 2016) and granite composition to non-TTG features (Moyen and Martin, 2012). According to available zircon U-Pb age data, there is a relative magmatic quiescence during this period, whatever the significant changes on Earth's surface, in-cluding the Great Oxidation Event, the termination of the striking mass-independent sulfur isotopic fractionation, the deposition of large amounts of banded iron formation, and the significant compositional change of seawater (Condie et al., 2009). During early Paleoproterozoic, the global volcanism shutdown reduced endogeneous CO 2 input into atmospherehydrosphere, while exogeneous CO 2 was largely consumed due to significant continental weathering, both of which contributed to the 2.4-2.3 ...
... During the global magmatic shutdown at 2.45-2.22 Ga (Condie et al., 2009), tectono-magmatic activities were still active in some areas, featured by active convergent plate tectonics and juvenile crust production, resulting in several accretionary orogens as found in western Canada, China, India, Africa, Australia and Greenland (Partin et al., 2014). Typical early Paleoproterozoic orogenic belts could be represented by the 2.54-2.28 ...
Article
The regime of plate tectonics on early Earth is one of the fundamental problems in Earth sciences. Precambrian era takes the majority (ca. 88%) of Earth’s history and thus plays a key role in understanding the onset of plate tectonics and the mechanism, distribution and process of Precambrian subduction zones. This paper presents a review on the progresses of subduction and subduction zones in different stages of Precambrian era, and sorts out some key issues and fields that merits further attention. We suggest that there was progressive onset and evolution of subduction and plate tectonics from Archean to Proterozoic eras. We emphasize the importance of comprehensive studies on subduction mechanism, metamorphic type, plate tectonics regime, the compositional evolution of continental crust, and petrogenesis of diverse granitoids formed in the Archean. It is proposed that innovative analytical techniques, big data, experimental petrology and numerical geodynamic modeling will facilitate future studies of Precambrian subduction zones.
... In addition, the continental crust changed from being dominantly below sea level to subaerial, as evidenced by the ubiquitous occurrence of pillow basalts in Archean greenstone belts in contrast to thick sequences of clastic sediments in the early Proterozoic (Windley, 1977;Thurston, 1990;Condie, 1994;Arndt, 1999;Kump & Barley, 2007;Flament et al., 2008;Campbell & Davies, 2017). Concurrently the late Archean mantle began to cool significantly (Korenaga, 2008;Condie & O'Neill, 2010;Herzberg et al., 2010) and the earliest Paleoprotoerozoic was heralded with a decrease in the volume of the preserved continental crust, a time referred to as the Rhyacian tectono-magmatic lull (Condie et al., 2009;Spencer et al., 2017). Superimposed on and potentially related to these massive changes in continental growth, tectonics, and mantle dynamics, the Earth's atmosphere experienced an increase in oxygen concentrations by about five orders of magnitude at c. 2Á3-2Á4 Ga [i.e. the Great Oxidation Event (GOE); Holland, 1984Holland, , 2002Holland, , 2006Canfield et al., 2000;Kasting, 2001;Kump et al., 2001;Bekker et al., 2004;Catling & Claire, 2005;Guo et al., 2009;Lyons et al., 2014;Gumsley et al., 2017]. ...
Article
Strongly peraluminous granites (SPGs) form through the partial melting of metasedimentary rocks and therefore represent archives of the influence of assimilation of sedimentary rocks on the petrology and chemistry of igneous rocks. With the aim of understanding how variations in sedimentary rock characteristics across the Archean–Proterozoic transition might have influenced the igneous rock record, we compiled and compared whole-rock chemistry, mineral chemistry, and isotope data from Archean and Paleo- to Mesoproterozoic SPGs. This time period was chosen as the Archean–Proterozoic transition broadly coincides with the stabilization of continents, the rise of subaerial weathering, and the Great Oxidation Event (GOE), all of which left an imprint on the sedimentary rock record. Our compilation of SPGs is founded on a detailed literature review of the regional geology, geochronology, and inferred origins of the SPGs, which suggest derivation from metasedimentary source material. Although Archean and Proterozoic SPGs are similar in terms of mineralogy or major-element composition owing to their compositions as near-minimum melts in the peraluminous haplogranite system, we discuss several features of their mineral and whole-rock chemistry. First, we review a previous analysis of Archean and Proterozoic SPGs biotite and whole-rock compositions indicating that Archean SPGs, on average, are more reduced than Proterozoic SPGs. This observation suggests that Proterozoic SPGs were derived from metasedimentary sources that on average had more oxidized bulk redox states relative to their Archean counterparts, which could reflect an increase in atmospheric O2 levels and more oxidized sedimentary source rocks after the GOE. Second, based on an analysis of Al2O3/TiO2 whole-rock ratios and zircon saturation temperatures, we conclude that Archean and Proterozoic SPGs formed through partial melting of metasedimentary rocks over a similar range of melting temperatures, with both ‘high-’ and ‘low-’temperature SPGs being observed across all ages. This observation suggests that the thermo-tectonic processes resulting in the heating and melting of metasedimentary rocks (e.g. crustal thickening or underplating of mafic magmas) occurred during generation of both the Archean and Proterozoic SPGs. Third, bulk-rock CaO/Na2O, Rb/Sr, and Rb/Ba ratios indicate that Archean and Proterozoic SPGs were derived from partial melting of both clay-rich (i.e. pelites) and clay-poor (i.e. greywackes) source regions that are locality specific, but not defined by age. This observation, although based on a relatively limited dataset, indicates that the source regions of Archean and Proterozoic SPGs were similar in terms of sediment maturity (i.e. clay component). Last, existing oxygen isotope data for quartz, zircon, and whole-rocks from Proterozoic SPGs show higher values than those of Archean SPGs, suggesting that bulk sedimentary 18O/16O ratios increased across the Archean–Proterozoic boundary. The existing geochemical datasets for Archean and Proterozoic SPGs, however, are limited in size and further work on these rocks is required. Future work must include detailed field studies, petrology, geochronology, and constraints on sedimentary source ages to fully interpret the chemistry of this uniquely useful suite of granites.
... One potentially distinctive feature of the middle Paleoproterozoic is a disputed tectono-magmatic lull between c. 2.3 to 2.2 Ga (Spencer et al., 2018), during which evidence of continental magmatism and orogenesis is scarce, but not entirely absent (Partin et al., 2014;Moreira et al., 2018). Juvenile magmatism reinitiated after c. 2.2 Ga (Condie et al., 2009;Spencer et al., 2018). ...
... As compiled by Condie et al. (2009a), Condie et al. (2009b), the magmatic events at 1.80-1.30 Ga occurred extensively within old cratonic blocks in Australia, Laurentia, Europe (Baltica), Africa and South America (Fig. 7f-l). ...
Article
We present a systematic study of micaschists and felsic gneisses from the Huangyuan Group of the Central Qilian block in NW China, with aims to unravel the connection with the Rodinia supercontinent. The micaschists have detrital zircon ages of 2895-928 Ma that peaking at 1.80-1.40 Ga. They show strongly increasing zircon εHf(t) values of -8.1 to +12.1 from 1.6 Ga to 1.4 Ga. Detrital zircon ages from the felsic gneisses are dominantly 960-913 Ma with εHf(t) values of -0.1 to -10.7. The micaschists have a wide range of whole-rock major element compositions, and the felsic gneisses have higher SiO2 contents, combined with lower other major element contents than those of the micaschists. All samples have trace element compositions consistent with upper continental crustal origin. The protoliths of the micaschists are dominantly shales and minor wackes with maximum depositional ages from ca. 1317 to 928 Ma. The protoliths of the felsic gneisses are mostly wackes with a maximum depositional age of ca. 927 Ma. The source materials for these metasedimentary rocks originated from intermediate to felsic igneous rocks. The variable maximum depositional ages of the metasedimentary rocks in the Huangyuan Group indicate that their protoliths constituted a sedimentary series with a long history of deposition starting at ca. 1317 Ma in an oceanic island arc-related basin that developed through a transitional continental arc-related basin into an active continental marginal basin at ca. 927 Ma. It is inferred that the 1795-1321 Ma detritus was sourced from juvenile arc crust at the margin of the Indian or the Western Australian craton. The source rocks for 1317-913 Ma detritus were arc magmatic rocks formed during assembly of Rodinia. A sequence of initial intra-oceanic subduction (ca. 1317-967 Ma) and continuous oceanic crust-continent subduction with formation of a mature continental arc (ca. 967-896 Ma) at the margin of Rodinia during the formation of the Central Qilian block is suggested.
... The zircon provenance of the São Roque Domain supports a linked Fig. 11. . (a) Trace-elemental fractionation patterns normalized to the lower crust (Weaver and Tarney, 1984) and (b) primitive mantle (McDonough and Sun, 1995 (Condie et al., 2009;Eriksson and Condie, 2014) leading the authors to propose the southwestward extension of the São Francisco plate. Nevertheless, Partin et al. (2014) disclosed that active-subduction processes during that period left few juvenile signatures due to rapid crust recycling. ...
... The Paleoproterozoic Birimian crust of the West African Craton (WAC) has been interpreted as juvenile crust formed between 2.3 Ga and 2.0 Ga (Abouchami et al., 1990;Leube et al., 1990;Taylor et al., 1992), during the period considered as global magmatic quiescence (Condie et al., 2009). The Eburnean (Birimian) represents part of large accretionary-collisional orogeny, which extends to other parts of the world and it is connected mainly to welding of West Africa and South America blocks (Zhao et al., 2002;Grenholm et al., 2019). ...
... Gravity, magnetic, and seismic constraints suggest that at least two of these Archean microplates underlie or extend into the central part of Hudson Bay; the Hudson Bay protocontinent and the Sugluk block, respectively (Figs. 2 and 3; Corrigan et al., 2009;Eaton and Darbyshire, 2010). The Sask craton, Partridge Breast block, Meta Incognita microcontinent, and Mistinibi-Raude block represent temporally distinct lithotectonic domains that contain continental crust formed between 2.5 and 2.3 Ga, considered a period of relative quiescence from a global crustal growth perspective (e.g., Condie et al., 2009). The Sask Craton and Partridge Breast blocks lie within the Trans-Hudson orogen southwest of the present study area and are not discussed further. ...
Article
There are lingering questions about how far back in geologic time plate tectonic processes began. In the Paleoproterozoic of eastern Laurentia, accretion of intra-oceanic juvenile terranes along the leading edge of the Superior craton apex (Ungava indenter) during the interval 1.87–1.83 Ga was followed by collision with the Churchill plate at ca. 1.83–1.79 Ga. Orthogonal shortening along the indenter led to early obduction of the juvenile terranes including the ca. 2.0 Ga Watts Group ophiolite, followed by out-of-sequence thrusting at ca. 1.83 Ga of granulite-facies crystalline basement of the Sugluk block (Churchill plate) along the Sugluk suture. Exhumation and erosion of the Sugluk block led to deposition of a foreland/delta fan sequence in the Hudson Bay re-entrant (Omarolluk and Loaf formations of the Belcher Group), with detritus sourced exclusively from the Sugluk block. Continued collision led to critical wedge development and orocline formation in the Hudson Bay re-entrant, forming a strongly arcuate fold-thrust belt. On the other (eastern) side of the indenter, material flow during crustal shortening was accommodated by lateral extrusion of microplates towards a then open ocean basin, in a manner similar to present-day extrusion of Indochina as a response to India – South China craton convergence. In the Churchill plate hinterland W-NW of the indenter, propagating strike-slip faults resulted in the far-field extrusion and oblique exhumation of Archean crustal slices of the Rae crustal block. The 1.83–1.79 Ga Superior-Churchill collision accommodated a minimum of 500 km of continent–continent convergence, with resulting style and mechanisms of orogenic growth and material flow similar to those observed in the Alpine-Himalayan orogenic system.
... There is a growing consensus on global cessation in magmatism during the Proterozoic as a result of episodic tectonic inactivity as reflected from the tectono-magmatic lulls during 2.45-2.2 Ga (Condie et al., 2009), 2.3-2.2 Ga (Spencer et al., 2018), and 1.8-1.2 ...
Article
Kimberlites are volatile-rich deep mantle-derived rocks that often contain diamonds. Numerous Grenvillian (ca. 1.1 Gyr) diamondiferous kimberlites, ultramafic lamprophyres, and lamproites are exposed in the Eastern Dharwar Craton and the Bastar Craton, India, and are aligned almost parallel to the Eastern Ghats (granulite) Mobile Belt (EGMB). The trigger for these kimberlite and related magmatic events still remains an open question. We review the available geochronological and radiogenic isotopic data for the ~1.1 Gyr kimberlites, lamproites, and ultramafic lamprophyres from the Eastern Dharwar Craton and the Bastar Craton of the Indian shield. We show that kimberlites and associated magmas were emplaced for a longer duration (ca. 130 Myr) than previously thought in the Indian shield during the Mesoproterozoic and sampled distinct mantle source regions. The kimberlites and ultramafic lamprophyre are characterized by slightly depleted to chondritic Nd isotopic ratios revealing their origin at deeper sub-lithospheric regions, whereas the lamproites essentially show an enriched Nd isotopic signature suggesting their derivation from enriched sub-continental lithospheric mantle. We argue that the absence of linear age progression, prolonged magmatic activity compared to the time span of coeval large igneous provinces (the Umkondo, the Keweenawan, and the Warakurna) and a cooler ambient mantle as revealed from the entrained xenoliths, constitute important limitations for a plume model earlier proposed for the emplacement of these kimberlites and related magmas. These observations together with a geographical and temporal (Grenvillian) link to the EGMB points towards edge-driven convection as a trigger for kimberlite magmatism- similar to the model proposed for the Mid-Cretaceous kimberlite corridor in North America. However, this model can't explain the coeval formation of sub-continental lithospheric mantle-derived lamproites. As the timing of kimberlite and related magmatism coincides with that of the Grenvillian orogeny and succeeded a magmatic lull of ~360 Myr in the Dharwar Craton during the Mesoproterozoic, we instead, propose that small scale partial melting of heterogeneous mantle caused by plate reorganization during Columbia to Rodinia supercontinent extroversion served as a trigger for this ca. 1.1 Gyr magmatism in the southeastern Indian shield.
... The North China Craton (NCC), as the largest and oldest craton in China, records and preserves the long evolutionary history of the early Precambrian (Zhao et al., 1998(Zhao et al., , 2005(Zhao et al., , 2012Kusky et al., 2007;Zhai andSantosh, 2011, 2013;Nutman et al., 2011;Trap et al., 2012;Wan et al., 2013;Zhai, 2014). In recent years, some researchers have proposed an early Paleoproterozoic quiescence in magmatic activity, especially at ~2.3 Ga (Gradstein et al., 2004;Barley et al., 2005;Condie et al., 2009aCondie et al., , 2009bEriksson et al., 2012). In contrast, we have identified a set of ~ 2.3 Ga metavolcanic rocks in the Lengkou area of the northern Zhongtiao Mountains on the southern margin of NCC. ...
Article
Paleoproterozoic (~2.3) Ga metavolcanic rocks occur in the Lengkou area, northern Zhongtiao Mountains of the North China Craton (NCC). The area is dominated by metabasic volcanic rocks, intercalated with metamorphosed intermediate to acid volcanic rocks. The Lengkou metavolcanic rocks have a magmatic zircon U-Pb age of 2317 ± 13 Ma, but also contain some 2508 Ma inherited zircons. The Lengkou metabasic and intermediate volcanic rocks are calc-alkaline with high SiO2 and K2O contents. They display fractionation of the HREE relative to the LREE, relative enrichments in large ion lithophile elements and depletion in high field strength elements resulting in negative Nb-Ta-Ti and Zr-Hf anomalies. Therefore, they are inferred to have formed in a volcanic arc setting. The meta-acid volcanic rocks of the Lengkou series have geochemical features similar to the surrounding Sushui Complex (Neoarchaean tonalitic gneiss), resulting from partial melting of ancient crust. The Lengkou metabasic volcanic rocks have a low ¹⁴³Nd/¹⁴⁴Nd ratio and εNd(t) value of 0.8–1.5, with TDM of 2.57–2.63 Ga, indicating the mantle-derived mafic rocks were variably contaminated by older continental material. The integrated data from the Lengkou metavolcanic rocks indicate a convergent subduction-related regime in the early Paleoproterozoic. The isotopic signatures and the xenocrystic zircons demonstrate that subduction occurred at a margin of continental crust, rather than within intra-ocean, exploring an Andean-like setting in the NCC at that time.
... In addition, a rapid growth of subaerial landmasses likely occurred at the same time, influencing global climate by increasing Earth's albedo, subaerial weathering, and continental margins (Bindeman et al., 2018). After a minimal tectonic activity driven by a magmatic shutdown between 2.4 to 2.2 Ga (Condie et al., 2009) ...
Thesis
Full-text available
La Série sédimentaire du Francevillien âgée de 2,1 milliards d’années du Gabon a fait l’objet de plusieurs investigations notamment à des fins économiques en lien avec les gisements uranifère et manganésifère. Ceci a également promu de nombreuses recherches pour reconstruire les paléoenvironnements et la paléobiodiversité du bassin de Franceville. Les sédiments abritent les plus anciens macrofossiles de tailles et de formes variées, ainsi que les traces laissées par des organismes mobiles, mettant en valeur l’enregistrement grandissant des formes primitives complexes et organisées au Paléoprotérozoïque. Cependant, le style de vie microbien, qui a émergé plus d’un milliard et demi d’années avant la sédimentation du Francevillien, a été peu décrit. Une étude multi-approches et pluridisciplinaire a révélé une grande diversité de structures liées aux voiles (MRS). Ces communautés microbiennes étaient principalement construites par des Cyanobactéries oxyphototrophiques qui ont prospéré dans des environnements peu profonds et dans la zone photique. Etant donné que ces bactéries peuvent avoir produit de grandes quantités d’oxygène très localement, ceci explique à priori la présence rependue de formes de vie avancées à proximité des MRS. Les structures fragiles bactériennes ont ensuite été analysées d’un point de vue minéralogique et géochimique. Les analyses montrent un assemblage minéralogique argileux riche en potassium (K) localisé dans les MRS mais inexistant dans les sédiments encaissants sous-jacents constitués de grès et d’argiles riches en matière organique (black shales). Cela suggère un piégeage des cations K+ par les MRS. Ce K, qui provient de l’eau de mer, a été ensuite relargué dans l’espace interstitiel pendant la dégradation de la matière organique, permettant ainsi la néoformation argileuse riche en K. Ceci confirme l’enrichissement potassique induit par des microbes. En ce qui concerne la teneur en éléments traces (TE) dans les MRS, aucun enrichissement en lien avec les microorganismes a été observé. La concentration de certains TE dans les MRS est plus élevée que celle du sédiment encaissant, mais des facteurs physiques environnementaux et non biologiques pourraient avoir causé ces enrichissements. Les données du redox local de l’eau de mer pour le sédiment encaissant montrent que le milieu de dépôt se traduit par des fluctuations des conditions redox (oxiques/suboxiques). Les signaux isotopiques du carbone organique et de l’azote de la roche totale sont similaires dans les structures bactériennes et le sédiment encaissant. La composition des isotopes du carbone suggère l’occurrence d'un recyclage secondaire d’un matériel carboné dérivé de la photosynthèse. Par ailleurs, les isotopes de l’azote indiquent une limitation azotée où les fixateurs de l’azote n’ont pas efficacement compensé la perte de ce dernier. La fixation de l’azote dans la colonne d’eau aurait été passagère et potentiellement contrôlée par la structure redox de l’océan, tandis que cette voie métabolique associée aux MRS est vraisemblablement commune au royaume des voiles benthiques à travers l’histoire de la Terre. L’ensemble de ces résultats soulignent la manifestation fréquente du mode de vie bactérien dans la série du Francevillien et révèlent si les microbes ont laissé des biosignatures spécifiques. La conservation exceptionnelle des MRS en association avec les macrofossiles du Gabon représente un écosystème marin unique à la suite de la première montée significative de la teneur en oxygène dans l’atmosphère terrestre.
... However, the data is inconclusive for the~300 Myr interval between the youngest S-MIF and the oldest S-MDF sample. The preserved rock record between 2.4 and 2.2 Ga is sparse (Condie et al., 2009;Spencer et al., 2018) making it challenging to fill this temporal gap. The lifespan of sedimentary basins ranges from <1 Myr for trench basins to >100 Myr for intracratonic basins (Woodcock, 2004). ...
Article
The Archean-Proterozoic transition marks a time of fundamental geologic, biologic, and atmospheric changes to the Earth system, including oxygenation of the atmosphere (termed the Great Oxygenation Event; GOE), and the emergence of continents above sea level. The impacts of the GOE on Earth’s surface environment are imprinted on the geologic record, including the disappearance of mass-independent fractionation of sulfur isotopes (S-MIF). Temporally overlapping geologic and geochemical observations (e.g. a change in oxygen isotope ratio of sediments and an increase in subaerial volcanism) imply the widespread subaerial emergence of continents was coeval with atmospheric oxygenation. Here we present triple sulfur isotope ratios in pyrite and oxygen isotope ratios in garnet and zircon in a global suite of Archean and Proterozoic granitoids derived from the partial melting of sedimentary protoliths. These crustal melts record an increase in average garnet and zircon δ¹⁸O from 7.2‰ before 2.3 Ga to 10.0‰ post-2.3 Ga. Pre-2.3 Ga granitoids show small S-MIF signatures with Δ³³S ranging from -0.29‰ to 0.13‰, whereas post-2.3 Ga granitoids record S-MDF (i.e. Δ³³S = 0‰). The combination of sulfur and oxygen isotope signatures in the same sample with zircon U-Pb geochronology provides new insights on a potential causal link between the emergence of continents and Paleoproterozoic atmospheric oxygenation.
... One potentially distinctive feature of the middle Paleoproterozoic is a disputed tectono-magmatic lull between c. 2.3 and 2.2 Ga (Spencer et al. 2018), during which evidence for juvenile magmatism and orogenesis is scarce, but not entirely absent (Partin et al. 2014;Moreira et al. 2018). Juvenile magmatism reinitiated after c. 2.2 Ga (Condie et al. 2009;Spencer et al. 2018) as well as episodic rifting, which eventually succeeded in the break-up of the Superia supercraton. The chronometric Rhyacian-Orosirian boundary (2050 Ma) possibly correlates also with an abrupt increase in magnitude of a mass-independent Oisotope anomaly of photochemical origin that is carried in sedimentary sulfate minerals (gypsum/anhydrite and barite). ...
Article
Full-text available
The geological time scale before 720 Ma uses rounded absolute ages rather than specific events recorded in rocks to subdivide time. This has led increasingly to mismatches between subdivisions and the features for which they were named. Here we review the formal processes that led to the current time scale, outline rock-based concepts that could be used to subdivide pre-Cryogenian time and propose revisions. An appraisal of the Precambrian rock record confirms that purely chronostratigraphic subdivision would require only modest deviation from current chronometric boundaries, removal of which could be expedited by establishing event-based concepts and provisional, approximate ages for eon-, era- and period-level subdivisions. Our review leads to the following conclusions: 1) the current informal four-fold Archean subdivision should be simplified to a tripartite scheme, pending more detailed analysis, and 2) an improved rock-based Proterozoic Eon might comprise a Paleoproterozoic Era with three periods ( early Paleoproterozoic or Skourian, Rhyacian, Orosirian), Mesoproterozoic Era with four periods (Statherian, Calymmian, Ectasian, Stenian) and a Neoproterozoic Era with four periods (pre-Tonian or Kleisian , Tonian, Cryogenian and Ediacaran). These proposals stem from a wide community and could be used to guide future development of the pre-Cryogenian timescale by international bodies.
... In addition, a rapid growth of subaerial landmasses likely occurred at the same time, influencing global climate by increasing Earth's albedo, subaerial weathering, and continental margins (Bindeman et al., 2018). After a minimal tectonic activity driven by a magmatic shutdown between 2.4 to 2.2 Ga (Condie et al., 2009) ...
Thesis
Full-text available
The 2.1-billion-year old (Ga) Francevillian Series in Gabon has been intensively studied because of economic interests in their uranium and manganese ore deposits. This also promoted numerous scientific investigations to reconstruct the palaeoenvironments and palaeobiodiversity of the Francevillian basin. The sediments host the oldest reported macrofossils of various sizes and shapes, and traces left by motile organisms, highlighting the growing record of early complex forms in the Palaeoproterozoic. However, the microbial lifestyle, which emerged more than a billion and a half years before the Francevillian deposition, has been poorly described. Through a combination of analytical technics, a large diversity of mat-related structures (MRS) has been observed. These microbial communities were mainly built by oxyphototrophic Cyanobacteria that thrived in shallow water environments within the photic zone. Considering that these bacteria may have locally produced higher amount of oxygen than in the oxygen-stressed water column, this likely explained the widespread presence of advanced forms of life in the vicinity of MRS. The delicate bacterial structures were then analyzed for their mineralogical and geochemical compositions. A distinct potassium (K)-rich clay assemblage characterizes the MRS, but not the underlying host sandstone and black shale sediments. It suggests that the MRS trapped K+ from the seawater and released it into the pore waters during degradation of organic matter, resulting in K-rich clay neoformation. This confirms the microbially induced K enrichment in the geological rock record. However, the trace element (TE) content in the MRS does not reveal particular microbially mediated enrichments. The concentration of some TE in the MRS is higher relative to that of the host sediments, but physical environmental factors may overwhelm any potential biological signal. The local seawater redox data for the host sediments show that the depositional setting reflects fluctuations in redox conditions (oxic/suboxic). The organic carbon and bulk nitrogen isotopes between the bacterial structures and host sediments are mostly similar. The carbon isotope composition suggests the occurrence of secondary recycling of photosynthetically derived carbonaceous material, while the nitrogen systematic points to a nitrogen limitation by which the N2 fixers did not sufficiently replenish the nitrogen loss. The nitrogen fixation in the water column would have been transient and likely controlled by the ocean redox structure, whereas this metabolic pathway in the MRS is likely common to the realm of benthic mats over Earth’s history. Combined, these results underline the common occurrence of bacterial lifestyle in the Francevillian Series and reveal whether the microorganisms left typical biosignatures. The exceptional conservation of the MRS in association with the Gabonese macrofossils represents a unique marine ecosystem in the aftermath of the first significant rise of oxygen content in Earth’s atmosphere.
... 2.4-2.3 Ga tectonomagmatic lull (TML); Condie et al., 2009;Spencer et al., 2018), we maintain that the remainder of the Proterozoic through to the Phanerozoic was characterized by substantial global orogenesis that not only led to the initiation of the supercontinent cycle (Wan et al., 2020;Wang et al., 2021), but also ushered in a fundamental shift in the tectonothermal evolution of Earth (Holder et al., 2019). Accordingly, we investigate whether the decrease in crustal thickness from 2.0 to 0.8 Ga is more directly linked to the rheology of hotter orogens in the mid-Proterozoic rather than orogenic quiescence. ...
Article
Full-text available
Since the Archean, secular change in orogenic style is demonstrated through evolution of metamorphic conditions and geochemical proxies. Linked to orogenic style is the amount of crustal thickening and elevation, whereas orogenic vigor is related to the supercontinent cycle. An array of Proterozoic orogens spanned the assembly of supercontinents Columbia and Rodinia, but the vigor of orogenesis is debated, with proposals for both Mesoproterozoic quiescence and climax. We show mid-Proterozoic orogenesis occurred globally and was broadly continuous; furthermore, orogens exhibit elevated metamorphic thermobaric ratios with large volumes of high-temperature felsic magmatic rocks. These features reflect higher mantle heat flux leading to increased mid-crustal flow and lower elevation. In this context, proposals that geochemical proxies for crustal thickness record orogenic quiescence are inconsistent with the geological record. Alternatively, secular change in crustal thickness is attributed to orogenic style, namely the prevalence of hot, thin, and low orogens in the mid-Proterozoic.
... Global zircon U-Pb ages and geologic records suggested a widespread shutdown or slowdown of magmatism at 2.45-2.2 Ga in the early Paleoproterozoic (Condie et al., 2009;Spencer et al., 2018). However, newly reported 2.45-2.2 ...
Article
Tectonic switch from rift zones to subduction zones is common along convergent plate boundaries. While this process is susceptible to retrieving from Phanerozoic rock records, the difficulty has been encountered for Precambrian rock records because of the relative lack of characteristic geological signatures. Nevertheless, such a difficulty can be overcome by finding of specific geochemical signatures in ancient rock records. This paper reports our finding of low δ¹⁸O zircons from the Trans-North China Orogen (TNCO) in the North China Craton (NCC), where the tectonic switch would occur during the early to middle Paleoproterozoic in association with the amalgamation of supercontinent Columbia. A combined study of zircon U-Pb ages and Hf-O isotopes as well as whole-rock major-trace elements and Nd isotopes were carried out for magmatic rocks from the Taiyue complex in the southern part of the TNCO. Zircon U-Pb dating yields two episodes of magmatism at ca. 2.34–2.31 Ga and ca. 2.2–2.1 Ga. These magmatic rocks are dominated by the ca. 2.18–2.16 Ga granites, 2.18–2.17 Ga diorites and 2.17–2.11 Ga mafic–ultramafic cumulates that intruded the 2.34–2.30 Ga granites and diorites. The 2.2–2.1 Ga diorites, mafic-ultramafic cumulates, and the regional mafic dykes/intrusions exhibit continuously varying major element compositions, arc-like trace element patterns, and consistent zircon Hf and whole-rock Nd isotope compositions, indicating their derivation from the same suite of continental arc magmas. Such primitive arc magmas would evolve through fractionation and accumulation of pyroxenes and plagioclase into dioritic magmas. The two episodes of granites are similar in major and trace element compositions, generally belonging to alkali-calcic or calc-alkalic A-type granitoids. Although both groups show a small difference in their zircon Hf isotope compositions, they exhibit a big difference in their zircon O isotope compositions. The 2.31 Ga granites show variably low zircon δ¹⁸O values of 3.4–5.5‰, mostly lower than normal mantle zircon values. The 2.18 Ga granites also exhibit variable zircon δ¹⁸O values from 3.6 to 6.0‰, but mostly mantle-like values. It is inferred that the 2.31 Ga granites would acquire their low δ¹⁸O signatures from partial melting of the high-T seawater-hydrothermally altered Archean crust in an early Paleoproterozoic continental rift. This rift would extend for at least 300 km along the TNCO. The 2.18 Ga granites are closely associated with the 2.2–2.1 Ga diorites and mafic–ultramafic cumulates. They would be most likely to form through differentiation of the 2.2–2.1 Ga continental arc magma, with their low ¹⁸O signatures being contaminated by the low δ¹⁸O 2.3 Ga granites. The low ¹⁸O signatures in the 2.3 Ga and 2.18 Ga granites indicate that the southern part of the TNCO would have probably evolved from a lithospheric rift to an active continental margin during ca. 2.3–2.1 Ga.
... which is similar to the concordia age of 2456 ± 22 Ma calculated for the youngest detrital zircon of magmatic source found in the Gouap BIFs . These ages, combined with slightly older concordia ages observed in this study (see Supplementary Tables S6 and S7) Condie et al., 2009;Partin et al., 2014;Pehrsson et al., 2013). The Gouap gneiss sample yielded twenty-five detrital zircon grains with typical oscillatory magmatic zoning, predominantly high Th/U ratios (> 0.2), and concordia age populations of ca. ...
Article
Full-text available
The Congo craton hosts banded iron formations (BIFs) in southern Cameroon. The Gouap iron deposit is in the central part of the Nyong Complex, next to the Ntem Complex in the northwestern part of the East Gabonian Block of the Congo craton. The Gouap BIFs are interbedded with metasiliciclastic rocks consisting of gneisses, schists, and quartzites. We present the first integrated study combining whole-rock and Sm-Nd isotope geochemistry with SIMS and LA-ICP-MS U-Pb-Hf-O isotope analyses of detrital zircons from metasiliciclastic rocks from the Gouap iron deposit to assess the provenance and tectonic setting during deposition of BIFs as well as the crustal evolution at the northwestern margin of the East Gabonian Block. Geochemical data for the metasiliciclastic rocks indicate that their protolith comprised a sequence of greywacke. Along with regional geology, our U-Pb-Hf-O detrital zircon and Sm-Nd whole-rock isotope data suggest that these rocks were predominantly derived from Neoarchean charnockite and TTG suites of the Ntem Complex with a variable and episodic contribution from ca. 2.2-2.0 Ga charnockites of the Nyong and Ogooué complexes. Metasiliciclastic rocks and BIFs were deposited between ca. 2.1 and 2.0 Ga in a foreland basin developed along the northwestern continental margin of the East Gabonian Block. So far unrecognized ca. 2.50-2.45 and 2.45-2.22 Ga magmatic rocks were exposed in their provenance. The Gouap succession experienced amphibolite-facies metamorphism when the Congo and São Francisco cratons collided during the Eburnean-Transamazonian orogeny to form the core of the Nuna/Columbia supercontinent. With the subduction dipping to the west, under the West Gabonian Block, slices of Paleoproterozoic oceanic crust and sediments scraped off the margin of the East Gabonian Block formed an accretionary prism. BIFs of the Nyong Complex likely represent a deep-water temporal equivalent of shallow-marine Mn-deposits of the Francevillian basin, Gabon, and together they belong to the same redox-fractionated deep-water upwelling system that was charged with iron and manganese via hydrothermal alteration of oceanic crust.
... The existing geological records suggest that the continental crust experienced thickening due to increasing lithospheric strength at the end of the Archean, heralded by the formation of the supercratons at that time . The rapid increase of zircon δ 18 O at ∼2.36 Ga may be explained with either enhanced recycling of crustal materials or involving a high-δ 18 O sedimentary reservoir (Bindeman, 2020;Payne et al., 2015;Spencer et al., 2019), while the "tectono-magmatic lull" during the early Paleoproterozoic precludes the first possibility (Condie et al., 2009;Spencer et al., 2018). In this case, the formation of a high-δ 18 O sedimentary reservoir preceding ∼2.36 Ga due to significant continental emergence above sea level which has been involved in the subsequent magmatic processes likely reconcile the rapid increase of zircon δ 18 O at Table S8). ...
Article
Full-text available
Subaerial continental crust plays a fundamental role in modulating the composition of the ocean, atmosphere, and biosphere, but the timing and rate of continental emergence above sea level remain unclear. Here we use the zircon oxygen isotopic compositions of early Paleoproterozoic metasediment-derived granitoids from the southwestern Yangtze Craton to constrain the rapidity of continental emergence. Statistical analyses of compiled igneous and detrital zircon oxygen isotopic database show a rapid increase in zircon δ18O at ~2.36 Ga. We suggest that this isotopic shift is best explained by a significant increase of continental freeboard between ~2.43 Ga and ~2.36 Ga due to the increasing strength of the continental lithosphere since the late Archean, concomitantly yielding a high-δ18O sedimentary reservoir. Subsequent melting of these high-δ18O sediments in a variety of tectonic regimes results in high zircon δ18O.
... across Gondwana have prominent age populations at 1.3-1 Ga and 0.78-0.5 Ga derived from the formation and erosion of the Rodinia and Gondwana supercontinents (Condie et al., 2009). This source significantly contributes to the zircon budget of sediments in the Adelaide Rift (Lloyd et al., 2020;Preiss, 2000) and ...
Article
Full-text available
The provenance of the upper Cambrian to Upper Ordovician sedimentary rocks of Tasmania and Waratah Bay in southern Victoria provides information about the complex and dynamic tectonic environment present during their deposition. This paper uses U–Pb detrital zircon data to constrain stratigraphic comparisons and tectonic reconstructions of these rock sequences. Multivariate statistics are used to investigate the similarity between the U–Pb ages and quantify the disparity among different samples from various locations. In western and central Tasmania, the Tyennan region supplied most detrital zircons during the late Cambrian and Early Ordovician. The overlying Middle Ordovician Pioneer Sandstone records a switch in provenance with zircons derived from the Mount Read Volcanics (MRV) mixed with zircons similar to those from continental-derived Paleozoic sedimentary rocks deposited throughout east Gondwana. The Middle to Upper Ordovician Gordon Group in western and central Tasmania lacks detrital zircons younger than 1.2 Ga, which indicates a return to a local provenance from Precambrian rocks. In southern Tasmania, the switch to zircons derived from the MRV and east Gondwana-like sources occurred earlier within the Cambrian Deadmans Bay Formation, which is dominated by the east Gondwana Paleozoic zircon age signature. In the East Tasmania Terrane, Ordovician sedimentary rocks from Lefroy have detrital zircon populations dominated by Neoproterozoic and earliest Paleozoic sources similar to the Ordovician sedimentary rocks in the Lachlan Orogen. In southern Victoria, the Bear Gully Chert from Waratah Bay exhibit both Tyennan and distal Gondwana detrital sources. The switching of detrital zircon sources in the west Tasmanian sedimentary sequences implies the docking of Tasmania with mainland Australia during the Cambrian Tyennan Orogeny. The arrival of the distal zircons into these basins occurred at different times in the different areas, reflecting a complex local topography and paleogeography.
... The Siderian Period (2.5 to 2.3 Ga) of the Paleoproterozoic era has been the target of intense debate in the world's geological literature because there is no consensus about its geotectonic evolution. Lines of research advocate the existence of an apparent tectonic calm-stagnation (shutdown by Condie et al. 2009 or tectono-magmatic Lull by Spencer et al. 2018), while other lines of research argue that this period, in fact, is not so calm in comparison to the processes that generate juvenile crusts (Pehrsson et al. 2014). The use of new tools into regional geological mapping, e.g., aerogeophysical data from gamma spectrometry combined with the popularization and refinement of geochronological (U-Pb) and isopotonic (Sm-Nd) methods, has promoted, in recent years, significant advances in the cartographic individualization of geological units other than those already known in the Borborema Province. ...
Article
Full-text available
In the Rio Piranhas-Seridó Domain (PSD), Borborema Province, geological mapping and aerogammaspectrometric data, combined with studies of Sm-Nd isotopes in whole rock and U-Pb in tonalitic orthogneiss and metamorphic clinopyroxene-hornblendite zircons enabled the identification and individualization of geological units with crystallization ages of 2456 ± 4 Ma and 2381 ± 16 Ma, respectively (U-Pb in zircon). These Siderian rocks are part of the basement of the central and northern portions of the PSD and are called the Arabia Complex. The tonalitic orthogneiss has an Sm-Nd (TDM) model age of 2.56 Ga, with an εNd value of +1.20, which allows inferring a juvenile source for the magma that gave rise to the protoliths of the rocks from the Arabia Complex. The identification and improvement of the geological cartography of Siderian units in the central portion of the PSD has contributed to a better understanding of the events of juvenile crust generation for the Borborema Province in general and inserts the province in the worldwide debate on geotectonic evolution during the Siderian Period.
... For example, plate tectonics is far more efficient at long-term mixing of geochemical reservoirs, and its inception may have promoted the destabilization of a deep mantle reservoir (3,23). A lull in global volcanic activity around 2.45 to 2.2 Ga (30,31), accompanied by a decrease in volcanic degassing, could have resulted in the loss of a significant oxygen sink, permitting the rise of biogenic O 2 during the GOE. The huge shift from primarily submarine volcanism before 2.5 Ga to predominantly subaerial volcanism, which is generally more oxidizing, in the Proterozoic (32) may have further reduced the volcanic gas sink for atmospheric O 2 , allowing it to rise. ...
Article
The rise of Earth's atmospheric O2 levels at ~2.4 Ga was driven by a shift between increasing sources and declining sinks of oxygen. Here, we compile recent evidence that the mantle shows a significant increase in oxidation state leading to the Great Oxidation Event (GOE), linked to sluggish upward mixing of a deep primordial oxidized layer. We simulate this scenario by implementing a new rheological model for this oxidized, bridgmanite-enriched viscous material and demonstrate slow mantle mixing in simulations of early Earth's mantle. The eventual homogenization of this layer may take ~2 Ga, in line with the timing of the observed mantle redox shift, and would result in the increase in upper mantle oxidation of >1 log(fO2) unit. Such a shift would alter the redox state of volcanic degassing products to more oxidized species, removing a major sink of atmospheric O2 and allowing oxygen levels to rise at ~2.4 Ga.
Chapter
The Ediacaran volcanosedimentary successions are well exposed and preserved in north and central Eastern Desert in addition to southern Sinai of Egypt. They are composed of intercalations of Dokhan Volcanics and terrestrial Hammamat sedimentary rocks. The volcanosedimentary basins have been geochronologically classified into (1) old volcanic arc volcanosedimentary basins (>650 Ma) and (2) young post-amalgamation volcanosedimentary basins overlaying amalgamated arc terranes (
Article
Using field, petrography and geochemistry (elemental and Nd isotopic compositions), an integrated study has been carried out on the Neoarchean lithologies comprising mafic magmatic enclaves (MMEs) and undeformed granitoids of the Bundelkhand Craton (Central India) to understand their genesis and crustal evolution. Based on the degree of mingling/mixing with the host granitoids, the MMEs and are classified into three types as E-1, E-2 and E-3 types. The E-1 type MMEs represent the early fragments formed by fractional crystallization of mafic/ultramafic magma. The E-2 type MMEs formed due to restricted mingling and rapid crystallization of a hot mafic magma that interacted with felsic granitoid magma. The E-3 type MMEs show compositional similarity with its felsic host and are proposed to have formed by magma mixing process. The MMEs underwent varying degrees of isotopic equilibration with the host granitoids. The E-1 and E-2 type MMEs resulted from depleted mantle-wedge and the E-3 type MMEs formed from a metasomatized mantle-wedge during subduction. Underplating of basaltic magma probably played a key role in the crustal formation and also provided heat for large scale melting of the crustal rocks giving rise to granitic magma. Such tectono-magmatic events also produced mafic magmas that variably interacted with the granitic magma and played a significant role in the generation of huge Bundelkhand granitoid massif during the Neoarchean.
Chapter
Estimates of when plate tectonics began range from the last 20% of Earth history to within the first 5%. While there is no observation that precludes plate tectonics from operating at 4.3 Ga, evidence that it was is indirect. Although subduction initiation is a robust feature of the modern plate tectonic system and we can calculate with some accuracy when oceanic lithosphere attains negative buoyancy, we don’t yet understand how strong the lithosphere weakens sufficiently for subduction to initiate. Most approaches used to estimate when Earth first entered the mobile lid regime—preservation of modern plate tectonic features, detrital zircon age spectra, trace element and radiogenic isotope geochemistry, atmosphere-crust-mantle exchange, and model-based estimates—can be interpreted in multiple ways and are all underlain by assumptions that cannot be independently tested. All share the flaw that absence of evidence is not evidence of absence. Of special concern is that the Precambrian geologic record is likely biased to rock compositions most likely to resist deformation and thus exposure to erosion at newly rifted continental margins where loss to subduction erosion could occur. Thus any look-back comparison is flawed to some degree by a preservation bias. A more recently recognized limitation is the failure to consider how a hotter, early Earth would differ petrologically from, say, Phanerozoic behavior (e.g., lower incompatible trace element concentrations in mantle magmas, higher geothermal gradients). Historically, computational limitations in early geophysical modelling methods led to skepticism regarding the possibility of plate tectonics on early Earth. Influenced by this view, the geologic community was reluctant to take a dynamic view of the preserved crustal record, instead inferring the apparent absence of a Hadean rock record as evidence that there never was one. The unknown extent to which ancient continental crust was recycled into the mantle and thoroughly mixed, the abovementioned selection biases in the rock record, and the assumption of uniformitarian conditions throughout Earth history limit virtually all continental growth estimates to providing only lower age bounds and thus minimum estimate on the initiation age of subduction.
Article
Proterozoic sedimentary covers in the Helan-Qianli Mountain area, western margin of the North China Craton (NCC), include the Huangqikou, Wangquankou and Zhengmuguan formations from bottom to top. The Huangqikou Formation, mainly distributed in the Middle Helan Mountain (MHM), North Helan Mountain (NHM) and Qianli Mountain (QM), is composed of conglomerate, quartz sandstone and silty-argillaceous shale that are locally metamorphosed into quartzite and slate. Detrital zircon U-Pb dating and stratigraphic correlation bracket that the Huangqikou Formation deposited during 1.75-1.60 Ga. Petrologic and geochemical features suggest that the provenance was sourced from felsic rocks and pre-existed sedimentary rocks. U-Pb ages of detrital zircons from the Huangqikou Formation in the MHM area yield age ranges at 2.5-2.35 Ga, 2.2-2.0 Ga and 1.95-1.8 Ga, while those in the NHM-QM areas yield age ranges at 2.5-2.4 Ga, 2.0-1.95 Ga and 1.85-1.78 Ga. The remarkable geochronological and isotopic similarity suggests that the early Paleoproterozoic granitic gneiss in the Ordos Block and Zhaochigou complex in the Khondalite Belt are the dominant source rocks for the Huangqikou Formation in the MHM. However, the provenances of the Huangqikou Formation in the NHM-QM are dominated by the granitoids in Guyang area of the Yinshan Block, the S-type granites and paragneisses in the Khondalite Belt and the TTG gneisses, Bayanwulashan complex and Boluositanmiao complex in the Alxa Block, as well as a small amount basement rocks in Alxa Block. The characteristics of geochemistry, provenance and regional geological data indicate that the Huangqikou Formation deposited in a rift basin. Besides, detritus sourced from the Alxa Block in the clastic sedimentary rocks of the Huangqikou Formation imply that the Alxa Block was already a part of the NCC before ca.1.75 Ga. The U-Pb age and Lu-Hf isotopic characteristics of detrital zircons from the Statherian show that rapid generation of juvenile crust in the western Block was at 2.9-2.7 Ga, yet the ca.2.5 Ga magmatic activity strongly reworked pre-existed crust with negligible juvenile crust generation. The magmatic activities quite possibly developed in the Western Block during the global magmatic shutdown (2.45-2.2 Ga).
Article
The paper presents a synthesis of geochronological, geochemical, and isotopic data on Paleoproterozoic granitoids in the Siberian craton and, in some cases, on volcanics related to these granitoids. Different evolution stages of the craton, including its assembly, are recorded in several major events in the history of Paleoproterozoic granitic magmatism. They are 2.52-2.40 Ga and 2.15–2.04 Ga granitoids of different tectonic settings; 2.06–2.00 Ga subduction-related granitoids; 2.00–1.87 Ga collisional granitoids; 1.88–1.84 postcollisional granitoids, and 1.76–1.71 Ga within-plate (anorogenic) varieties. Granitoids with ages of 2.5–2.4 Ga and 2.15–2.04 Ga are distributed locally in separate blocks and terranes which later entered the craton structure. These rocks are of different types and represent different tectonic settings of the respective blocks and terranes, i.e., the Siberian craton did not exist yet as a single unit at that time. The 2.06–2.00 Ga subduction-related granitoids and volcanics are found in the southern and southeastern craton parts. Magmatism of that time interval probably was associated with subduction beneath the Archean Olekma-Aldan and Anabar continental microplates and with the formation of their active margins. Granitoids of the 2.00–1.87 Ga age interval represent a collisional stage of the craton evolution. Collisional granitoids that intruded between 2.00 and 1.95 Ga record the first large-scale stage of craton assembly with collisions of terranes that had built the core of the Anabar, Aldan, and Olenek superterranes. The intrusions of 1.95–1.90 Ga granitoids mark the consolidation of the southeastern craton part. Collisional granitoids with the 1.90–1.87 Ga ages, which are especially abundant in the south and southwest of the craton but almost lack from the north, led to general assembly of the craton. The granitoid and volcanic magmatic activity of 1.88–1.84 Ga in the craton south produced the South Siberian postcollisional magmatic belt that had completed the assembly of the craton which, in its turn, may have been part of the Paleoproterozoic supercontinent of Columbia. Granitic intrusions at 1.76–1.71 Ga are limited to the southwestern and southeastern craton parts and correspond to a setting of continental extension which never led to the breakup of the craton.
Article
The basement of the northern Brasília Belt, in Central Brazil, is dominated by large volumes of TTG magmatism generated during the Early Paleoproterozoic, and represents a continental block formed prior to the welding of the São Francisco paleocontinent during the Rhyacian, holding important information concerning the mechanisms and products of Paleoproterozoic crustal growth and amalgamation. New U-Pb zircon geochronological data was obtained from metagranitoids and mafic-ultramafic intrusions from the Almas-Conceição do Tocantins Domain (ACTD). This was coupled with geochemical data from the metagranitoids to better constrain the processes of continental crust evolution in this region during the Siderian period. Two main plutonic episodes were recognized in the mapped area: (i) an early suite (2.47 Ga) dominated by tonalites and trondhjemites with subordinate biotite granites (Ribeirão das Areias Complex - RAC); and (ii) a later suite (2.30 Ga) including tonalites and trondhjemites (Ribeirão Itaboca Suite - RIS). Most of the RAC trondhjemite-tonalite-granodiorite (TTG) rocks are marked by fractionated REE patterns with high (La/Yb)N = 15 – 107 and samples have medium to high Sr/Y ratio (28 - 257). These TTG include both high- and low-Al types and high- and medium-pressure groups. The RIS TTG suite displays higher HREE contents with high (La/Yb)N = 65 – 89 and high Sr/Y ratio (59 - 95) and contains the high-Al type and the medium pressure group. In spite of the wide age gap between them, both RAC and RIS are characterized by TTG rocks formed in a subduction-like tectonic setting. The biotite granites are related to a magmatic stage after the TTG formation. Mafic rocks of the Gameleira Suite have an abundance of zircon, interpreted as crustal contamination, with dominant ²⁰⁷Pb/²⁰⁶Pb age populations of 2.48 and 2.30 Ga, and are interpreted to be associated with an extensional magmatic event following the Siderian period. Regional geotectonic correlations within the São Francisco Craton based on data compiled from the literature indicate a significant volume of Siderian crust generation that has been overlooked in continental amalgamation models and that TTG magmatism is a more common geochemical signature in Paleoproterozoic rocks than has been previously suggested.
Article
Geological data imply plate tectonics was not active on the Archaean Earth pre-2.5 Ga. Key arguments include: the absence from the Archaean rock record of ophiolites and high-pressure metamorphic rocks; the rarity of Archaean andesites and lahars; the absence of arc-like, source-metasomatic, trace element signatures in Archaean calc-alkaline magmatic suites; and fundamental differences in overall structural style and constituent lithologies. Conversely, unstable stagnant-lid models better account for many features of Archaean geology. Thermo-mechanical models imply many unstable stagnant-lid planets would experience mobile-lid phases linked to periodic mantle overturns of 30-100 million years duration, separated by stable-lid phases lasting 100-300 My. Mantle overturn upwelling zones would be characterized by high magma fluxes that may have generated continental nuclei, and would have reworked and resurfaced tracts of pre-existing oceanic and continental lithosphere. Overturns would also have generated large-scale lateral mantle flow patterns that would have pushed against the sub-continental lithospheric mantle keels underlying continents, and so induced continental drift and orogenesis, allowing mobile-lid behaviour despite the absence of plate-boundary forces such as slab pull. Major resurfacing of Earth’s surface during overturns would have heated the hydrosphere and atmosphere, with many negative impacts on biota. The high magmatic fluxes associated with mantle overturn events are likely to have induced periodic mass extinctions that may have retarded biological evolution on Earth. Evolutionary progress towards more complex metazoan organisms may only have been possible after the more efficient plate tectonic cooling system helped create a stabler, more temperate planet; although causal relationships remain uncertain. Did the start of a plate tectonic, mobile-lid cooling system on Earth gradually end (or moderate) mantle overturn behaviour? Or was the world-girdling plate tectonic system only allowed to begin because ‘other factors’ gradually acted to suppress mantle overturns during the Proterozoic? These ‘other factors’ include: secular decay of radioactive isotopes, scavenging of radioactive elements into continents, and shrinkage of the fertile lower mantle reservoir that fed early overturns. When overturn behaviour ended on Earth is also uncertain. The 2.1-1.9 Ga magmatic and tectonic pulse on Earth is plausibly interpreted as an Archaean-style mantle overturn, but more research is needed to determine whether the Large Igneous Provinces (LIPs) of the Meso- and Neo-Proterozoic are as well. It is predicted that Earth-sized exoplanets with surface water and chondritic mantle compositions will spend at least the first 2 Ga of their evolution as unstable stagnant-lid planets, with periodic overturns preventing evolution of complex metazoan organisms. Many such planets could remain trapped in this cooling mode, with only rare cases transitioning into the more efficient plate tectonic cooling mode. If correct, this greatly decreases the probability that complex metazoans are present elsewhere in the universe.
Article
The Precambrian metasedimentary rocks that are extensively distributed in the Beishan Orogenic Belt (BOB) can provide crucial geological information to constrain its tectonic affinity and geological evolution. Field investigation, zircon U–Pb dating and Lu–Hf isotopic analyses were conducted in combination for investigating them. Detrital zircon U–Pb dating shows that a quartzite sample of the Beishan Complex yields the youngest age at ca. 1710 Ma, with a major age peak at 1828–1778 Ma. Five quartzite samples from the Gudongjing Group and Pingtoushan Formation exhibit similar detrital zircon age distributions mainly from the late Paleoproterozoic to Mesoproterozoic, with the youngest ages at ca. 953 Ma, ca. 1000 Ma, ca. 978 Ma, ca. 960 Ma and ca. 985 Ma. Considering the 1.4 Ga granitoids and age peak of the early Neoproterozoic metasedimentary rocks, the supracrustal rocks within the Beishan Complex were probably deposited during the late Paleoproterozoic to early Mesoproterozoic (ca. 1.7–1.4 Ga). Moreover, owing to the presence of ca. 933–871 Ma granitoids, the deposition ages of the Gudongjing Group and Pingtoushan Formation can be constrained during the early Neoproterozoic (ca. 953–933 Ma). The metasedimentary rocks, the same as the Meso-Neoproterozoic granitic magmatism, support the existence of a Precambrian microcontinent in the BOB. The early Neoproterozoic metasedimentary rocks have continuous detrital zircon ages during the Mesoproterozoic, mainly with positive εHf(t) values. Integrated with the magmatic records of ca. 0.9 Ga and ca. 1.4 Ga, we believe that the Precambrian basement of the BOB could be a fragment drifted from the Fennoscandia of the Baltica Craton. Field investigation revealed that carbonate rocks, mature sandstones and siltstones primarily comprise the protoliths of the early Neoproterozoic metasedimentary rocks, implying the sedimentation in a passive continental margin. However, an almost contemporaneous granitic magmatism was formed in a continental arc setting associated with the assembly of Rodinia. Therefore, it can be inferred that the BOB could be located in the periphery of the Rodinia supercontinent, where the sedimentation of the passive continental margin occurred at its outboard position.
Article
Geologic observations and numerical models imply that Archean continents were mostly submarine. In contrast, approximately one‐third of modern Earth’s surface area consists of subaerial continental crust. To temporally constrain changes in the subaerial exposure of continents, we evaluate the eruptive environment (submarine vs subaerial) of 3.4‐2.0 Ga continental large igneous provinces (LIPs). Our results indicate that up until 2.4 Ga LIPs predominantly erupted onto submerged continents. This period of low freeboard was punctuated by local subaerial eruptions at 2.8‐2.7 Ga and 2.5 Ga. From 2.4 Ga‐2.2 Ga, extensive subaerial continental volcanism is recorded in six different cratons, supporting widespread subaerial continents at this time. An increase in exposed continental crust significantly impacts atmospheric and oceanic geochemical cycles, and the supply of nutrients for marine bioproductivity. Thus, the 2.4‐2.2 Ga high freeboard conditions may have triggered the earliest global glaciation event and the first significant rise of atmospheric oxygen.
Article
The Huayangchuan ore belt is located in the western segment of Xiaoqinling Orogen in the southern margin of the North China Craton (NCC), and hosts voluminous magmatism and significant U−REE−Mo−Cu−Fe polymetallic mineralization. However, geochronological framework of the various mineralization phases in this region is poorly understood. Here, we present new Re−Os isochron ages on magnetite from the Caotan Fe deposit (2 675 ± 410 Ma, MSWD = 0.55), and on pyrite from the Jialu REE deposit (2 127 ± 280 Ma, MSWD = 1.9) and Yuejiawa Cu deposit (418 ± 23 Ma, MSWD = 11.5), and Re−Os weighted average model age on pyrite from the Taoyuan Mo−U deposit (235 ± 14 Ma, MSWD = 0.17). These ages, combined with regional geology and mineralization ages from other deposits, suggest that mineralization in the Huayangchuan ore belt lasted from the Neoarchean to the Late Mesozoic. The mineralization corresponds to regional tectono-magmatic events, including the Neoar-chean alkali magmatism (REE mineralization), Paleoproterozoic plagioclase-amphibolite emplacement (Fe mineralization), Paleoproterozoic pegmatite magmatism (U mineralization), Paleozoic Shangdan oceanic slab subduction-related arc magmatism (Cu mineralization), Early Mesozoic Paleo-Tethys Ocean subduction-related arc magmatism (Mo−U mineralization), and Late Mesozoic Paleo-Pacific oceanic plate subduction direction change-related Mo(-Pb) mineralization. We proposed that the Huayang-chuan ore belt has undergone prolonged metallogenic evolution, and the magmatism and associated mineralization were controlled by regional geodynamic events.
Article
The time of 2.45–2.2 Ga in the early Paleoproterozoic era is called as ‘Quiet Interval’ because of the decreased magmatic activity worldwide. However, tectonic evolution of old continents during this time interval is still in dispute due to the rare geological records, especially for magmatism. In the southern North China Craton (NCC), the Jingwan mafic intrusions intruded into the Songshan Group and its zircon U-Pb dating yielded a weighted mean ²⁰⁷Pb/²⁰⁶Pb age of 2301 ± 16 Ma, and thus they are the only ca. 2.3 Ga mafic magmatic records founded in this region. These diabases belong to low-K tholeiitic series with relatively low K2O (0.1–0.7 wt%) and total alkalis (Na2O + K2O) (2.4–4.7 wt%). They have low SiO2 (44.8–54.1 wt%), slightly variable Fe2O3 T (10.1–14.4 wt%), Al2O3 (14.8–17.0 wt%), CaO (5.5–10.6 wt%), and medium to high MgO (6.1–10.8 wt%) and Mg# values (58–65). The rocks display LREE enrichment ((La/Yb)N = 3.02–3.58) without obvious Eu anomaly (Eu/Eu* = 0.87–1.18). They are characterized by obvious depletion in Nb, Zr and Ti, and enrichment of Sr, Ba and Pb, indicative of geochemical features of island arc magmas. The rocks have experienced crystallization from clinopyroxene to orthopyroxene, then plagioclase, suggesting the hydrous fractionation order. They also exhibit variable εNd(t) values ranging from − 0.2 to + 0.8 and zircon εHf(t) values from − 3.0 to + 6.6. These features suggest that the mafic intrusions are formed by partial melting of a garnet-spinel lherzolite and derived from an enriched mantle source metasomatized by slab-derived fluids at a shallow depth. Combined with the previous reported magmatic records, the ca. 2.3 Ga magmatic activity may be a result of the oceanic slab subduction beneath the Eastern Block of the NCC, and the subduction probably continue throughout the early Paleoproterozoic from ca. 2.5 Ga to ca. 2.3 Ga, at least 200 Ma. Therefore, the prevalence of early Paleoproterozoic magmatic activities in the southern NCC indicate that the tectonic activity did not shutdown globally during the interval of the ‘tectono-magmatic lull’.
Article
As one of the most ancient cratons in Asia, the North China Craton (NCC) provides an opportunity to examine the evolution of the early earth. However, it is poorly known about the tectonic setting of the Eastern Block of the NCC at 2.47-2.17 Ga. In this study, zircon U-Pb dating was conducted to constrain the provenance, depositional age, and depositional environment of the Langzishan Formation in the Liaodong peninsula, to investigate the tectonic setting of the Eastern Block during the earliest Paleoproterozoic. Provenance analysis suggests that the basal sandstones (protolith of quartzite) in the Langzishan Formation were sourced from 3.8-2.5 Ga continental crust, while the overlying sandstone and pelite units were sourced exclusively from 2.5 Ga continental crust. The distribution of detrital zircon ages shows that the basal sandstones of the Langzishan Formation were deposited at 2.47-2.2 Ga, during which the tectonic setting changed from an early collisional setting (2.47-2.35 Ga) to a stable continental margin (2.35-2.2 Ga), whereas pelites in the upper layer of the Langzihsan Formation were deposited at 2.2-2.17 Ga, and also experienced a transition in tectonic setting from a passive continental margin to an active continental margin. The Langzishan Formation is believed to be the lowest sedimentary unit in the Eastern Block. The transition of tectonic setting of this formation shows that the Eastern Block probably has undergone the same transition at 2.47-2.17 Ga, from a collisional setting to a passive continental margin, and then to an active continental margin, possibly related to both onset of the 2.3-2.2 Ga Paleoproterozoic tectono-magmatic lull (TML) and its cessation.
Chapter
Knowledge of the Precambrian history of the Earth between the birth of the Solar System (c. 4.567 Ga) and the end of the Mesoproterozoic (1.0 Ga) has improved dramatically as a result of the acquisition of a range of isotopic and geochemical data and new fossil discoveries. None the less, subdivision of the Precambrian in a comparable manner to that achieved for the Phanerozoic continues to be extremely challenging. Global Standard Stratigraphic Ages (GSSAs) have long been used to subdivide the Precambrian into eons, eras, and periods. It accepted that there are shortcomings in this approach and a move toward a revised time scale based as closely as possible around geobiological events would be desirable. However, development of such a “naturalistic” time scale is still a “work in progress” as shown by the state of flux in the understanding of particular biogeochemical events and the extent to which they are globally synchronous given the fragmentary state of the preserved rock record. Until these issues are resolved (if at all possible) the GSSA approach continues to serve geoscientists well in broadly delimiting major parts of the Earth’s history that are characterized by particular tectonic and biogeochemical regimes.
Book
Several different Neoarchean–Paleoproterozoic supercontinents or supercratons have been proposed, including Kenorland, Protopangea, Vaalbara, Superia, Supervaalbara, Sclavia, and Nunavutia. We used high-quality paleomagnetic data and an updated magmatic record to test these various cratonic reconstructions. Based on these analyses, we suggest that a Vaalbara configuration might be possible through at least part of the Neoarchean–Paleoproterozoic transition, contradicting recent suggestions. We also propose a modified Superia reconstruction with a looser fit of the Karelia–Kola and Superior cratons than the original Superia configuration. Disagreement between the paleomagnetic poles, different drift velocities, and the latitudinal positions of Superia and Kaapvaal at 2.7–2.2 Ga and Superia and Nunavutia at 2.4–2.2 Ga indicate that these supercratons were separate, negating a single Neoarchean–Paleoproterozoic supercontinent and the proposed Supervaalbara configuration, and thus also arguing against the existence of a full-fledged Kenorland landmass during that interval of time. This also argues against stagnant-lid tectonics during the Archean–Paleoproterozoic transition. In addition, drift velocities at 2.4–2.2 Ga that are in the range of current plate motions contradict the proposed tectono-magmatic shutdown or a tectono-magmatic lull in the Paleoproterozoic.
Chapter
We review models of the Precambrian supercontinent Nuna (c.1.80�1.20 Ga) and present a new model of the Nuna life-cycle. We explore the option where Amazonia 2West Africa and Congo 2Sa˜o Francisco cratons were not a part of Nuna but form a cluster of cratons named Atlantica. Nuna was finally assembled at c.1.65 Ga, encompassing Laurentia, Baltica, Siberia, proto-Australia, North China, and India. The break up of Nuna is here suggested to have started at c.1.3 Ga with Australian cratons moving away from Laurentia. Varying drift rates and length between coeval poles from separate cratons support the operation of modern style plate tectonics dur- ing the Nuna supercycle. Moreover, if there was no link between Nuna and the Atlantica cratons, the size of Nuna might not meet the criteria for a supercontinent, but should be regarded as a huge landmass.
Article
Reconstructing Earth history during the Hadean defies the traditional rock-based approach in geology. Given the extremely limited locality of Hadean zircons, some indirect approach needs to be employed to gain a global perspective on the Hadean Earth. In this review, two promising approaches are considered jointly. One is to better constrain the evolution of continental crust, which helps to define the global tectonic environment because generating a massive amount of felsic continental crust is difficult without plate tectonics. The other is to better understand the solidification of a putative magma ocean and its consequences, as the end of magma ocean solidification marks the beginning of subsolidus mantle convection. On the basis of recent developments in these two subjects, along with geodynamical consideration, a new perspective for early Earth evolution is presented, which starts with rapid plate tectonics made possible by a chemically heterogeneous mantle and gradually shifts to a more modern-style plate tectonics with a homogeneous mantle. The theoretical and observational stance of this new hypothesis is discussed in conjunction with a critical review of existing proposals for early Earth dynamics, such as stagnant lid convection, sagduction, episodic and intermittent subduction, and heat pipe. One unique feature of the new hypothesis is its potential to explain the evolution of nearly all components in the Earth system, including the atmosphere, the oceans, the crust, the mantle, and the core, in a geodynamically sensible manner.
Article
The Archean‐Proterozoic transition heralded a number of fundamental changes on Earth, including the oxygenation of the atmosphere, a marked emergence of continents above sea‐level, and an increase in δ18O of felsic magmas. The potential drivers for the latter are changes in the composition of supracrustal material or increased crustal reworking. Although the onset of subduction‐induced continental collision and associated enhanced crustal recycling could produce high‐δ18O felsic magmas, temporally constrained zircon δ18O reveals an increase in δ18O at ~2.35 Ga that predates the oldest widely recognized supercontinent. In this work, we use the O and Hf isotope ratios of magmatic zircon crystals in Archean and Proterozoic sediment‐derived granitoids of the North China Craton to track the incorporation of supracrustal material into magmas. The results are consistent with a Paleoproterozoic increase of continental freeboard producing sedimentary reservoirs with comparatively elevated δ18O that subsequently partially melted to generate the granitoids.
Article
Full-text available
The mid-Proterozoic or "boring billion" exhibited extremely stable environmental conditions, with little change in atmospheric oxygen levels, and mildly oxygenated shallow oceans. A limited number of passive margins with extremely long lifespans are observed from this time, suggesting that subdued tectonic activity—a plate slowdown—was the underlying reason for the environmental stability. However, the Proterozoic also has a unique magmatic and metamorphic record; massif-type anorthosites and anorogenic Rapakivi granites are largely confined to this period and the temperature/pressure (thermobaric ratio) of granulite facies metamorphism peaked at over 1500 °C/GPa during the Mesoproterozoic. Here, we develop a method of calculating plate velocities from the passive margin record, benchmarked against Phanerozoic tectonic velocities. We then extend this approach to geological observations from the Proterozoic, and provide the first quantitative constraints on Proterozoic plate velocities that substantiate the postulated slowdown. Using mantle evolution models, we calculate the consequences of this slowdown for mantle temperatures, magmatic regimes and metamorphic conditions in the crust. We show that higher mantle temperatures in the Proterozoic would have resulted in a larger proportion of intrusive magmatism, with mantle-derived melts emplaced at the Moho or into the lower crust, enabling the production of anorthosites and Rapakivi granites, and giving rise to extreme thermobaric ratios of crustal metamorphism when plate velocities were slowest.
Chapter
We review paleomagnetic data and paleoclimatological indicators of Baltica and its subcratons. Between Neoarchean and middle Mesoproterozoic Karelia and Kola, and later the united Baltica were located mostly at the latitudes between 35�N and 35�S. Location of Baltica oscillated between high latitudes and the equator at late Mesoproterozic�Neoproterozoic. Drift velocities of the separate cratons between Neoarchean and middle Proterozoic are lower than the velocities of the united Baltica at late Paleoproterozoic�middle Mesoproterozoic. At Late Mesoproterozoic�Neoproterozoic Baltica shows high velocity peaks, which correlate temporarily with the ones obtained for Laurentia and can be ascribed to true polar wander. Increase in drift rates correlate temporarily with orogenies related to the formation of the supercontinents Nuna and Rodinia and in smaller scale to the crustal growth of Baltica. Based on the results, we review possible nearest neighbors for the Kola and Karelia in the Superia supercraton and for Baltica in the Nuna and Rodinia supercontinents.
Chapter
This chapter discusses evolution of the crust and mantle through 4.6 Gyr of Earth's history. It begins with a discussion of the Hadean era focusing on new and exciting research on the ages of detrital zircons and the origin and composition of the earliest crust. It includes a brief summary of Earth's oldest rocks. Models of crustal origin and growth are presented with a detailed discussion of the growth rate of continental crust including problems of recycling into the mantle and juvenile crust production. The crustal age gap at 2.4–2.2 Ga is discussed, as are secular changes in the composition, rock associations, and tectonic settings of the crust, the controversial topic of when plate tectonics began. The chapter ends with discussion of secular changes in the mantle, including the lithosphere, and models for Earth's thermal history.
Article
The article is about the series evolution events that happened in Quanji massif and Tarim Craton in North West China through the ancient time, in this paper we focus on the Post-collision Basaltic Magmatism in Quanji Massif during the Early Paleoproterozoic
Article
Full-text available
We have compiled a record of the geochronology of mantle plume activity between 3.8 and 1.6 Ga. Over this time period, the ages of komatiites, and those of global plumes, correlate strongly, at the 99% confidence level, with the ages of banded iron formations (BIFs). The ages of continental plumes correlate more weakly, at an overall 85% confidence level. Using the geochronological records of these events, we can define four periods characterized by mantle superplume activity. Three of these periods are also times of enhanced BIF deposition. The fourth mantle plume period may similarly be coeval with increased BIF accumulation, but the BIF chronostratigraphic resolution is not accurate enough to test this rigorously. Mantle superplume volcanism may promote BIF deposition by increasing the Fe flux to the global oceans through continental weathering and/or through submarine hydrothermal processes. It may also be enhanced by increasing the number of paleotectonic environments appropriate for BIF deposition (particularly plume-induced ocean plateaus, seamounts, and intracratonic rifts) and by promoting global anoxic, Fe-rich hydrothermal plumes in the shallow to intermediate marine water column.
Chapter
Full-text available
Modern-style plate tectonics can be tracked into the geologic past with petrotectonic assemblages and other plate-tectonic indicators. These indicators suggest that modern plate tectonics were operational, at least in some places on the planet, by 3.0 Ga, or even earlier, and that they became widespread by 2.7 Ga. The scarcity of complete ophiolites before 1 Ga may be explained by thicker oceanic crust and preservation of only the upper, basaltic unit. The apparent absence of blueschists and ultrahigh-pressure meta-morphic rocks before ca. 1 Ga may reflect steeper subduction geotherms and slower rates of uplift at convergent margins. It is unlikely that plate tectonics began on Earth as a single global "event" at a distinct time, but rather it is probable that it began locally and progressively became more widespread from the early to the late Archean.
Article
Full-text available
A defense is conducted of the proposition that continental crust is recycled into the mantle and that the earth is in a near-steady state, with essentially constant volumes of ocean and crust through geological time. A contrasting view, that the continental crust has grown with time, has been repeatedly expressed by Moorbath (1977, 1978). It is pointed out that his arguments against recycling are not persuasive, and that the evidence which he presents is easily accommodated in a no-continental-growth model. Attention is given to evidence of continent and ocean volume, early planetary differentiation, continental accretion and destruction, evidence for the subduction of sediment, a quantitative simulation of the no-growth hypothesis, the Pb isotope evolution, the apparent single-stage evolution of Pb and Sr isotopes in ancient rocks, the inert gases He-3 and Ar-36, stable isotopes in sediments and ocean water, and steady-state chemical sedimentology.
Article
Full-text available
The isotope record of organic matter and calcium carbonate is often used to infer the burial history of organic carbon through time. As organic carbon burial is widely held to control long-term oxygen production, the isotope record also relates to the production rates of oxygen on Earth. Current interpretations of the record suggest a long-term consistency in the proportion of total carbon buried as organic carbon (f ratio), with some important periods of much higher burial proportions. The isotope record is analyzed here with a new carbon isotope mass balance model, which considers submarine hydrothermal weathering of ocean crust as a significant removal pathway of inorganic carbon. With this model the f ratio is considerably reduced if isotopically depleted inorganic carbon is precipitated during hydrothermal weathering and if hydrothermal weathering dominates inorganic carbon removal from the surface environment. In contrast to previous calculations, our analysis of the carbon isotope record shows that organic carbon burial in the Archean accounted for only between 0% and 10% of the total carbon burial. These low burial proportions would have contributed to a slow accumulation of atmospheric oxygen in the Archean.
Article
Full-text available
We have compiled a record of the geochronology of mantle plume activity between 3.8 and 1.6 Ga. Over this time period, the ages of komatiites, and those of global plumes, correlate strongly, at the 99% confidence level, with the ages of banded iron formations (BIFs). The ages of continental plumes correlate more weakly, at an overall 85% confidence level. Using the geochronological records of these events, we can define four periods characterized by mantle superplume activity. Three of these periods are also times of enhanced BIF deposition. The fourth mantle plume period may similarly be coeval with increased BIF accumulation, but the BIF chronostratigraphic resolution is not accurate enough to test this rigorously. Mantle superplume volcanism may promote BIF deposition by increasing the Fe flux to the global oceans through continental weathering and/or through submarine hydrothermal processes. It may also be enhanced by increasing the number of paleotectonic environments appropriate for BIF deposition (particularly plume-induced ocean plateaus, seamounts, and intracratonic rifts) and by promoting global anoxic, Fe-rich hydrothermal plumes in the shallow to intermediate marine water column.
Article
Full-text available
We present a compilation of strontium, carbon, and oxygen isotope compositions of roughly 10,000 marine carbonate rocks of Archean - Ordovician age (3800 Ma – 450 Ma). The Precambrian Marine Carbonate Isotope Database (PMCID) has been compiled from 152 published and 3 unpublished articles and books of the past 40 years. Also included are 30 categories of relevant “metadata” that allow detailed comparisons and quality assessments of the isotope data to be made. The PMCID will be updated periodically as new data and better age constraints come to light. Here we outline the structure of the first published version of the database and its inherent merits and limitations.
Article
Full-text available
Large ion lithophile and high field strength element distributions in juvenile upper continental crust are controlled chiefly by the abundance of tonalite-trondhjemite-granodiorite (TTG) in the Archean shifting to a combination of TTG, calc-alkaline granitoid, and graywacke control thereafter. Geochemical differences between TTG and high-silica adakites do not require production of most TTG magmas in descending slabs. Changes in the ratio of TTG to calc-alkaline granitoids after 2.5 Ga indicate that Archean subduction zones must have differed from younger subduction zones in two very important ways: (1) a deep mafic crust served as a TTG magma source (either as thickened crust or in descending: slabs), and (2) they did not give rise to significant volumes of calc-alkaline magma. Thickened mafic crust in the Late Archean may have resulted from plate jams in subduction zones caused by thicker oceanic crust and oceanic plateaus produced during Late Archean mantle thermal events.
Article
Full-text available
The existence, spatial distribution, and style of volcanism on terrestrial planets is an expression of their internal dynamics and evolution. On Earth a physical link has been proposed between hot spots, regions with particularly persistent, localized, and high rates of volcanism, and underlying deep mantle plumes. Such mantle plumes are thought to be constructed of large spherical heads and narrow trailing conduits. This plume model has provided a way to interpret observable phenomena including the volcanological, petrological, and geochemical evolution of ocean island volcanoes, the relative motion of plates, continental breakup, global heat flow, and the Earth's magnetic field within the broader framework of the thermal history of our planet. Despite the plume model's utility the underlying dynamics giving rise to hot spots as long-lived stable features have remained elusive. Accordingly, in this review we combine results from new and published observational, analog, theoretical, and numerical studies to address two key questions: (1) Why might mantle plumes in the Earth have a head-tail structure? (2) How can mantle plumes and hot spots persist for large geological times? We show first that the characteristic head-tail structure of mantle plumes, which is a consequence of hot upwellings having a low viscosity, is likely a result of strong cooling of the mantle by large-scale stirring driven by plate tectonics. Second, we show that the head-tail structure of such plumes is a necessary but insufficient condition for their longevity. Third, we synthesize seismological, geodynamic, geomagnetic, and geochemical constraints on the structure and composition of the lowermost mantle to argue that the source regions for most deep mantle plumes contain dense, low-viscosity material within D'' composed of partial melt, outer core material, or a mixture of both (i.e., a ``dense layer''). Fourth, using results from laboratory experiments on thermochemical convection and new theoretical scaling analyses, we argue that the longevity of mantle plumes in the Earth is a consequence of the interactions between plate tectonics, core cooling, and dense, low-viscosity material within D''. Conditions leading to Earth-like mantle plumes are highly specific and may thus be unique to our own planet. Furthermore, long-lived hot spots should not a priori be anticipated on other terrestrial planets and moons. Our analysis leads to self-consistent predictions for the longevity of mantle plumes, topography on the dense layer, and composition of ocean island basalts that are consistent with observations.
Article
Full-text available
Atmospheric oxygen concentrations in the Earth's atmosphere rose from negligible levels in the Archaean Era to about 21% in the present day. This increase is thought to have occurred in six steps, 2.65, 2.45, 1.8, 0.6, 0.3 and 0.04 billion years ago, with a possible seventh event identified at 1.2 billion years ago. Here we show that the timing of these steps correlates with the amalgamation of Earth's land masses into supercontinents. We suggest that the continent–continent collisions required to form supercontinents produced supermountains. In our scenario, these supermountains eroded quickly and released large amounts of nutrients such as iron and phosphorus into the oceans, leading to an explosion of algae and cyanobacteria, and thus a marked increase in photosynthesis, and the photosynthetic production of O2. Enhanced sedimentation during these periods promoted the burial of a high fraction of organic carbon and pyrite, thus preventing their reaction with free oxygen, and leading to sustained increases in atmospheric oxygen.
Article
Full-text available
Many hitherto puzzling features of the isotope and trace-element geochemistry of the Earth's mantle and crust can be explained if Earth history is punctuated by episodes of enhanced exchange between the lower and upper mantle. Such episodes would replenish the upper mantle with trace elements, and also cause rapid growth of continental crust. This picture is consistent with recent geophysical models in which two-layer convection alternates with episodes of penetrative or whole-mantle convection.
Article
Full-text available
Paleomagnetic, geochemical, and tectonostratigraphic data establish that plate tectonics has been active since at least 3.1 Ga. Reliable paleomagnetic data demonstrate differential horizontal movements of continents in Paleoproterozoic and Archean times. Furthermore, the dispersal and assembly of supercontinents in the Proterozoic requires lateral motion of lithosphere at divergent and convergent plate boundaries. Wellpreserved ophiolites associated with island-arc assemblages and modern-style accretion tectonics occur in the Paleoproterozoic Trans-Hudson orogen of the Canadian Shield, the Svecofennian orogen of the Baltic Shield and in the Mazatzal-Yavapai orogens of southwestern Laurentia. These rocks have trace element signatures almost identical to those found in rocks of modern intra-oceanic arcs and include ore deposits typical of modern subduction settings. The discovery of Archean eclogites in the eastern Baltic Shield; the presence of late Archean subduction-related Kuroko-type volcanogenic massive sulfide deposits in the Abitibi greenstone belt of the Canadian Shield; the discovery of mid-Archean island arc volcanics, including the oldest known boninites and adakites; and isotopic data from the world’s oldest zircons all argue for modern-style subduction processes possibly back to the Hadean. Seismic images of preserved Paleoproterozoic and Archean suture zones further support this view. These data require a tectonic regime of lithospheric plates similar to the Phanerozoic Earth.
Article
Full-text available
Paleomagnetism is the only quantitative method available to test for lateral motions by tectonic plates across the surface of ancient Earth. Here, we present several analyses of such motions using strict quality criteria from the global paleomagnetic database of pre–800 Ma rocks. Extensive surface motion of cratons can be documented confi dently to older than ca. 2775 Ma, but considering only the most reliable Archean data, we cannot discern differential motion from true polar wander (which can also generate surface motions relative to the geomagnetic reference frame). In order to fi nd evidence for differential motions between pairs of Precambrian cratons, we compared distances between paleomagnetic poles through precisely isochronous intervals for pairs of cra-tons. The existing database yields several such comparisons with ages ranging from ca. 1110 to ca. 2775 Ma. Only one pair of these ages, 1110–1880 Ma, brackets signifi cantly different apparent polar wander path lengths between the same two cratons and thus demonstrates differential surface motions. If slightly less reliable paleomagnetic results are considered, however, the number of comparisons increases dramatically, and an example is illustrated for which a single additional pole could constrain differential cratonic motion into the earliest Paleoproterozoic and late Neoarchean (in the interval 2445–2680 Ma). In a separate analysis based in part upon moderately reliable paleo-magnetic poles, if a specifi c reconstruction is chosen for Laurentia and Baltica between ca. 1265 and 1750 Ma, then those cratons' rotated apparent polar wander paths show convergence and divergence patterns that accord with regional tectonics and appear to be remarkably similar to predictions from a plate-tectonic conceptual model. Care-fully targeted and executed future paleomagnetic studies of the increasingly well-dated Precambrian rock record can imminently extend these tests to ca. 2700 Ma, and with substantially more effort, to perhaps as old as ca. 3500 Ma.
Article
Full-text available
The 2.7–2.0 Ga volcano-sedimentary records of the African, Indian and Australian cratons indicate two broadly defined periods of extensive drowning of the emergent continental areas, concomitant with lowered freeboard. Carbonate-banded iron formation (BIF) platforms characterised the first such event, at ca 2.6–2.4 Ga (Africa and Australia) to 2.7 Ga (India). These earlier globally enhanced sea levels are ascribed to increased mid-ocean ridge activity, possibly related to breakup of a postulated Late Archaean ‘southern’ supercontinent. Alternatively, a transition from global-scale catastrophic mantle overturn events to the onset of plate tectonics may have occurred in the Late Archaean (Nelson, 1998. Earth Planet. Sci. Lett. 158, 109–119). Both explanations of increased mid-ocean ridge activity are compatible with significant Early to Middle Archaean crustal growth (Armstrong, 1981. Phil. Trans. R Soc. London A 301, 443–472), with the emergent high freeboard cratons being subjected to aggressive weathering and erosion. Enhanced continental crustal growth near the Archaean–Proterozoic boundary (McLennan and Taylor, 1982. J. Geol. 90, 347–361), related to the development of significant island arc complexes, would have resulted in common lowered freeboard–enhanced sea level conditions at the passive margins of the ‘southern’ cratons. The diachronous nature of these earlier transgressions in the various cratons may reflect the effect of local tectonic movements and/or the thermal state of the cratons. From ca 2.4–2.2 Ga, cratons that make up the present-day continents of India, Africa and Australia had relatively high continental freeboard and lowered sea levels. Glacigenic deposits are preserved on the Kaapvaal (Africa), Singhbhum (India) and Pilbara (Australia) cratons. The second broadly defined drowning event, at ca <2.2 and >2.15 Ga, was probably due to post-glacial climatic amelioration. Freeboard was reduced by the combination of eustatic rise and the reestablishment of aggressive weathering as warmer palaeoclimates returned. In India, carbonates were more prominent than the siliciclastic sediments (including prominent black shales) seen in Africa and Australia.
Article
Full-text available
Calculation of sedimentation rates of Neoarchaean and Palaeoproterozoic siliciclastic and chemical sediments covering the Kaapvaal craton imply sedimentation rates comparable to their modern facies equivalents. Zircons from tuff beds in carbonate facies of the Campbellrand Subgroup in the Ghaap Plateau region of the Griqualand West basin, Transvaal Supergroup, South Africa were dated using the Perth Consortium Sensitive High Resolution Ion Microprobe II (SHRIMP II). Dates of 2588±6 Ma and 2549±7 Ma for the middle and the upper part of the Nauga Formation indicate that the decompacted sedimentation rate for the peritidal flat to subtidal below-wave-base Stratifera and clastic carbonate facies, southwest of the Ghaap Plateau at Prieska, was of up to 10 m/Ma, when not corrected for times of erosion and non-deposition. Dates of 2516±4 Ma for the upper Gamohaan Formation and 2555±19 for the upper Monteville Formation, indicate that some 2000 m of carbonate and subordinate shale sedimentation occurred during 16 Ma to 62 Ma on the Ghaap Plateau. For these predominantly peritidal stromatolitic carbonates, decompacted sedimentation rates were of 40 m/Ma to over 150 m/Ma (Bubnoff units). The mixed siliciclastic and carbonate shelf facies of the Schmidtsdrif Subgroup and Monteville Formation accumulated with decompacted sedimentation rates of around 20 B. For the Kuruman Banded Iron Formation a decompacted sedimentation rate of up to 60 B can be calculated. Thus, for the entire examined deep shelf to tidal facies range, Archaean and Phanerozoic chemical and clastic sedimentation rates are comparable. Four major transgressive phases over the Kaapvaal craton, followed by shallowing-upward sedimentation, can be recognized in the Prieska and Ghaap Plateau sub-basins, in Griqualand West, and partly also in the Transvaal basin, and are attributed to second-order cycles of crustal evolution. First-order cycles of duration longer than 50 Ma can also be identified. The calculated sedimentation rates reflect the rate of subsidence of a rift-related basin and can be ascribed to tectonic and thermal subsidence. Comparison of the calculated sedimentation rates to published data from other Archaean and Proterozoic basins allows discussion of general Precambrian basin development. Siliciclastic and carbonate sedimentation rates of Archaean and Palaeoproterozoic basins equivalent to those of younger systems suggest that similar mechanical, chemical and biological processes were active in the Precambrian as found for the Phanerozoic. Particularly for stromatolitic carbonates, matching modern and Neoarchaean sedimentation rates are interpreted as a strong hint of a similar evolutionary stage of stromatolite-building microbiota. The new data also allow for improved regional correlations across the Griqualand West basin and with the Malmani Subgroup carbonates in the Transvaal basin. The Nauga Formation carbonates in the southwest of the Griqualand West basin are significantly older than the Gamohaan Formation in the Ghaap Plateau region of this basin, but are in part, correlatives of the Oaktree Formation in the Transvaal and of parts of the Monteville Formation on the Ghaap Plateau.
Article
Full-text available
THE Earth's most active zone of mantle seismicity arises from the subduction of the Pacific plate at the Tonga trench1. It is not known why this slab generates so many more earthquakes than other subducting slabs worldwide. Above the subduction zone the active Tofua (Tonga) volcanic arc is separated by the V-shaped Lau basin from a remnant arc, the Lau ridge, located at the eastern edge of the Australian plate2. The irregular and discontinuous magnetic lineations within the basin have proven difficult to interpret3,4, and so the regional kinematic framework has been obscure. We report geodetic measurements of crustal motion within the Tonga-Lau system, which reveal the fastest crustal motions yet observed. The Lau basin is opening at a rate which increases northwards to a maximum of ~160 mm yr−1 No straining is observed within the northern Tonga ridge, suggesting that it comprises part of a rigid microplate. Convergence rates across the Tonga trench increase northwards to a maximum of ~240 mm yr−1. The extraordinary seismic activity of the subducting slab is probably related to this unusually rapid subduction.
Article
At some time in the past, average oceanic lithosphere would have only just become negatively buoyant as it reached a subduction zone. It is estimated that this condition occurred when the mantle was only about 50°C hotter than at present and as recently as 0.9-1.4 b.y. ago. Transformation of basaltic crust to eclogite might have enhanced the vigor of the plates, but possibly only modestly and intermittently. In earlier times, plate tectonics could have operated, but more slowly, so that it could not by itself have accomplished the necessary rate of heat removal from the earth. A different tectonic model must also have operated, perhaps subcrustal delamination or dripping. -from Author
Chapter
We identify 304 mafic magmatic events ranging in age from ca. 3500 Ma to the present and assess their potential for linkage to the arrival of mantle-plume heads. For each, we catalogue components (flood basalts or their erosional remnants, giant mafic dike swarms, mafic sill provinces, and mafic-ultramafic layered intrusions), age constraints, and references to published literature. On the basis of criteria involving large amounts of magma emplaced in a short time, and/or giant radiating dike swarms, 34 events have been confidently linked to the arrival of a mantle-plume head. By using other criteria including geochemistry, an additional 194 events are considered probably related to plume arrival. The remaining 76 events, the majority of which are rift related, require further study to assess the plume-head link. Our analysis of the events that are confidently or probably linked to plume arrival yields several preliminary conclusions. Plume-head events occur throughout the geologic record since at least 3500 Ma and probably since 3800 Ma, with no plume-free intervals greater than ∼200 m.y. Plume arrival does not follow any obvious periodicity. Plumehead locations are known for only a handful of events. However, in such cases, associated sills and flow packages can be emplaced (presumably via lateral flow in dikes) as far as 2500 km away from the plume center. There are numerous precise age correlations between mafic units on different continents. However, further work is required to determine which coeval events can be reconstructed into a single plume event and which represent separate events occurring at the same time.
Article
Numerical modeling of mantle convection in a spherical shell with an endothermic phase change at 670 km depth reveals an inherently three-dimensional flow pattern, containing cylindrical plumes and linear sheets which behave differently in their ability to penetrate the phase change. The dynamics are dominated by accumulation of downwelling cold material above 670 km depth, resulting in frequent avalanches of upper-mantle material into the lower mantle. This process generates long-wavelength lateral heterogeneity, helping to resolve the contradiction between seismic tomographic observations and expectations from mantle convection simulations.
Article
The western margin of the Churchill craton records a complex history of Paleoproterozoic tectonism. The most prominent of these tectonic events is recorded within the 2.0 - 1.9- Ga Taltson-Thelon magmatic zone. The widespread magmatism and high-grade metamorphism of this zone, especially in Alberta and Saskatchewan, obscures an older tectonic belt, the ca. 2.4 - 2.3-Ga Arrowsmith Orogeny. In the Uranium City region of northwestern Saskatchewan, a suite of Paleoproterozoic granites represent a magmatic product of this orogen. U-Pb zircon crystallization ages were identified for the Macintosh Bay monzogranite (Ma), the Gunnar monzogranite ( Ma), the 2330 +/- 5 2321 +/- 3 Geebee Lake tonalite ( Ma), the Yahyah Lake granite ( Ma), and the Hayter Bay monzogranite 2320 +/- 44 2287 +/- 14 ( Ma). The petrological and geochemical characteristics, emplacement ages, and Nd isotopic geochemistry 2297 +/- 10 of these plutons are most consistent with a syn- to postcollisional orogenic setting, possibly coeval with a period of orogenic extension and exhumation.
Article
Numerical models of mantle convection that incorporate the major mantle phase of the transition zone reveal an inherently three-dimensional flow pattern, with cylindrical features and linear features that behave differently in their ability to penetrate the 670-km discontinuity. The dynamics are dominated by accumulation of cold linear downwellings into rounded pools above the endothermic phase change at 670-km depth, resulting in frequent 'avalanches' of upper mantle material into the lower mantle. The effect of the exothermic phase transition at 400 km depth is to reduce the overall degree of layering by pushing material through the 670-km phase change, resulting in smaller and more frequent avalanches, and a wider range of morphologies. Large quantites of avalanched cold material accumulate above the coremantle boundary (CMB), resulting in a region of strongly depressed mean temperature at the base of the mantle. The 670-km phase change has a strong effect on the temperature field, with three distinct regions being visible. The effect of the velocity field is very different. Flow penetration across the 670-km phase change is strongly wavelength-dependent, with easy penetration at long wavelengths but strong inhibition at short wavelengths. Thus, when comparing numerical models with long-wavelength seismic tomography, diagnostics based on the density field, such as the radial correlation function, are much more sensitive to the effects of phase transitions than those based on the velocity field.
Article
At present, young oceanic lithosphere is positively buoyant, and it does not become negatively buoyant until it is older than about 20 m.y. If, in the past, the mantle was hotter, the oceanic crust would have been thicker and the lithosphere's age of neutral buoyancy greater. On the other hand, the hotter mantle would have had a lower viscosity and convected faster, so the average age of oceanic plates at subduction would have been less than the present 100 m.y. At some time in the past, average oceanic lithosphere would have only just become negatively buoyant as it reached a subduction zone. It is estimated here that this condition occurred when the mantle was only about 50 °C hotter than at present and as recently as 0.9-1.4 b.y. ago. Transformation of basaltic crust to eclogite might have enhanced the vigor of the plates, but possibly only modestly and intermittently. In earlier times, plate tectonics could have operated, but more slowly, so that it could not by itself have accomplished the necessary rate of heat removal from the earth. A different tectonic mode must also have operated, perhaps subcrustal delamination or dripping. Plate tectonics would have gradually taken over from the earlier mode. Plumes are a omplementary mode of mantle convection driven by a lower thermal boundary layer, and so they could have operated in conjunction with both regimes of the upper boundary layer.
Article
The Segwagwa Group of southeastern Botswana, a correlate of the Pretoria Group of the Transvaal Supergroup of South Africa, consists of a major sequence of siliciclastic sedimentary rocks, minor carbonates and basaltic to andesitic lavas and tuffs straddling the Western and Central Domains of the Kaapvaal Craton. The Segwagwa Group unconformably overlies the Taupone Dolomite Group, a correlative of the South African Chuniespoort/Ghaap Groups of the Transvaal Supergroup. SHRIMP U–Pb analyses of 123 detrital zircons from the top, middle and bottom of the Segwagwa Group sedimentary rocks include 96 concordant to near-concordant zircons defining three main age groups: > 3.0–2.9 Ga (n = 12), 2.8–2.5 Ga (n = 27) and 2.45–2.20 Ga (n = 57). The ≥ 2.90 Ga zircons were sourced from granitoids emplaced before and around 2915 ± 12 Ma and are related to the amalgamation of the Western, Northern and Central Domains of the Kaapvaal Craton. Concordant zircons with a mean age of 2781 ± 8 Ma originate from the Gaborone Igneous Complex. The detrital zircons in the range 2.7–2.5 Ga were likely sourced from the Kalahari continental fragment made up of the Kaapvaal Craton, Limpopo Belt and the Zimbabwe Craton, specifically from the Limpopo Belt and/or the Zimbabwe Craton where igneous rocks in this age range are widespread. The igneous sources for the Palaeoproterozoic (ca. 2.45–2.20 Ga) zircons are difficult to identify since igneous rocks in that age are not widely known or documented by reliable dates in the Kalahari Craton.
Article
U-Pb ages for 1655 individual detrital zircon grains in 18 samples of eolian and associated marine and fluvial sandstones of the Glen Canyon and San Rafael Groups from the Colorado Plateau and contiguous areas shed light on patterns of Jurassic sediment dispersal within Laurentia. Most detrital zircon grains in Jurassic eolianites were derived ultimately from basement provinces older than 285 Ma in eastern and central Laurentia, rather than from rock assemblages of the nearby Cordilleran margin. The most prominent peaks of constituent age populations at 420 Ma, 615 Ma, 1055 Ma, and 1160 Ma reflect derivation from Paleozoic, Neoproterozoic, and Grenvillian sources within the Appalachian orogen or its sedimentary cover. Sediment was transported to a position upwind to the north of the Colorado Plateau by a transcontinental paleoriver system with headwaters in the central to southern Appalachian region, but subordinate non-Appalachian detritus was contributed by both northern and southern tributaries during sediment transit across the continent. Subordinate detrital zircons younger than 285 Ma in selected Middle to Upper Jurassic eolianites were derived from the Permian-Triassic East Mexico and the Mesozoic Cordilleran magmatic arcs. Lower Jurassic fluvial sandstones typically contain a mixture of detrital zircons redistributed from eolian sand and derived from the East Mexico arc, which lay up-current to the southeast. Zircons in marine Curtis sandstone were largely reworked from underlying Entrada eolianite, with minor contributions from the Jurassic backarc igneous assemblage of the Great Basin. Once mature quartzose detritus was dispersed widely across southwest Laurentia by a transcontinental paleoriver system and paleowinds, which deposited extensive Jurassic ergs, durable zircon grains were recycled by multiple intraregional depositional systems. Lower Jurassic fluvial sand is locally composed, however, of detritus derived from the nearby Cordilleran magmatic arc assemblage and its Precambrian basement.
Article
Eleven high precision (±2–5 million years) SHRIMP zircon U–Pb ages have been obtained from felsic rocks within a single stratigraphic section of late Archaean volcanic and sedimentary rocks in the east Pilbara of Western Australia. The stratigraphic succession (Nullagine and Mount Jope Supersequences in sequence-stratigraphic terminology, Fortescue Group in lithostratigraphic terminology) is interpreted to be the rock record of three major geotectonic cycles that formed in an extensional, rift-related environment between about 2772 and 2715Ma. The geochronology is constrained by a detailed stratigraphic framework based on unconformities and supported by a preliminary magnetostratigraphy. Field mapping, geochemical and petrographic studies have shown that previously unrecognised thin felsic tuff bands are interbedded in subaerial flood basalt piles and mafic tuffs. While flood basalts and proximal felsic volcanic rocks comprise by volume most of the volcanogenic components of the succession, felsic volcanism is now known to have been active periodically through each geotectonic cycle. The succession covers a time period of about 57 million years. The lower ∼1400m of a thick (∼1700m) clastic sedimentary succession from the oldest geotectonic cycle was deposited at a rate of about 100m per million years over a mean time period of 14 million years. In contrast, a younger ∼150m thick cogenetic tuff-basalt unit accumulated in less than 3 million years, and others probably accumulated at similar rates, comparable to those of Phanerozoic flood basalts. Unconformities in the succession are shown to be of variable duration and one unconformity marking the boundary between the first and second geotectonic cycles may represent a time-gap of more than 10 million years. The unconformity-based stratigraphic framework, the new geochronology and palaeomagnetic studies [J. Geophys. Res. 108 (2003) B12, 2551, EMP 2-1 to 2-21] have been combined to determine a possible late Archaean continental drift rate for one part of the succession, implying a period of motion as fast as or up to five times faster than any known from the Phanerozoic.
Article
Eustatic sea level changes reflect variation in ocean water or ocean basin volume, or changes to the hypsometric curve, which is in itself geographically and chronologically variable. Freeboard, the elevation of a continent above mean sea level, is closely related to changes in both sea level and this curve. Relative sea level change occurs due to tectonism, sediment supply, compaction, and eustatic movements. Hydroisostatic compensation modifies first-order sea level variation by about one-third.
Article
Episodic growth of continental crust and supercontinents at 2.7, 1.9, and 1.2 Ga may be caused by superevents in the mantle as descending slabs pile up at the 660-km seismic discontinuity and then catastrophically sink into the lower mantle. Superevents, in turn, may comprise three or four events, each of 50–80 My duration, and each of which may reflect slab avalanches at different locations and times along the 660-km discontinuity. Superplume events in the late Paleozoic and Mid-Cretaceous may have been caused by minor slab avalanches as the 660-km discontinuity became more permeable to the passage of slabs with time. The total duration of a superevent cycle decreases with time reflecting the cooling of the mantle.
Chapter
Modern-style plate tectonics are mostly driven by the excess density of oceanic lithosphere sinking deeply in subduction zones and can be sustained as long as melt is produced at mid-ocean ridges. Among the silicate planets, the mechanism of plate tectonics is unique to Earth, indicating that special circumstances are required. Given that the potential temperature of Earth's mantle has decreased by several hundred degrees Celsius since Archean time, the density of oceanic lithosphere must have systematically increased, which has profound implications for the viability of plate tectonics through time. Two things must be done to advance our understanding of Earth's tectonic history: (1) uncritical uniformitarianism should be avoided; and (2) the geologic record must be thoughtfully and objectively interrogated. Theoretical considerations should motivate the exploration, but geologic evidence will provide the answers. The debate needs to address the criteria for identifying tectonic style in ancient rocks, whether this evidence is likely to be preserved, and what the record indicates. The most important criteria are the temporal distribution of ophiolites, blueschists, ultrahigh-pressure terranes, eclogites, paired metamorphic belts, passive margins, subduction-related batholiths, arc igneous rocks, isotopic evidence of recycling, and paleomagnetic constraints. This list of criteria should evolve; objective redefinitions and reviews of, especially, the eclogite paired metamorphic belt and subduction-related batholith records are needed. Also, the likely effects of major tectonic changes on other Earth systems should be considered, such as true polar wander, climate change, and biosphere changes. The modern episode of plate tectonics began in Neoproterozoic time, <1.0 Ga ago, with earlier alternating episodes of proto-plate tectonics (1.8-2.0 and 2.5-2.7 Ga); unstable stagnant-lid tectonics dominated the rest of Proterozoic time and an unknown part of Archean time.
Article
The Hf and 18O values in detrital zircons from the Slave craton,Canada, indicate three episodes of crust formation between ca.4.5 and 2.8 Ga, namely at ca. 4.4-4.5 Ga, ca. 3.8 Ga,and ca. 3.4 Ga. Most of the juvenile crust appears to have beenmafic in composition, and there is no clear evidence for initialgranitic protocrust in the Hadean of the Slave craton. The rangeof initial Hf values in zircons increases from 3.9 to 2.8 Ga,indicating that both extraction of new material from mantleand reworking of the older crust are important for the secularevolution of the continental crust. A preliminary review ofavailable Hf data in zircons suggests that the three episodesof crust generation may have been of global importance. Themafic crust formed in the Archean and the Hadean was then reworkedfor at least 0.5-1.5 b.y., as indicated by data fromthe Slave craton, Gondwana, and the Limpopo Belt of Africa.