Research Items (164)
Cratons consist of Archean and Proterozoic rocks on their surface. Although they are underlain by thick litho-sphere their top surfaces are usually near sea level. Some recent models argue that craton formation results from crustal thickening caused by shortening and subsequent removal of the upper crust by erosion. This process would expose a high-grade metamorphic crust. However, the data we compiled from the world's cratons show that the greenschist-grade metamorphics and even supracrustal sedimentary rocks are preserved and with few exceptions surfaces of cratons that formed during the cratonization do not expose high-grade rocks everywhere, most not even in the largest part of their surfaces. Some of them are dominated by low-grade to non-metamorphic rocks and they are not even distributed in only one part of the craton but occur in widely dispersed areas from north to south and east to west. In North America, the Superior Province, in India, the Singhbhum and Raj Mahal cratonic pieces, in North China, in the Baltic, Tanzanian, Congo, North African and Arabian, Guyana, São Francisco, Pilbara and in Yilgarn cratons, not only greenschist-facies metamorphic rocks but also undeformed sedimentary rocks are displayed at their surface. Wholesale underthrusting of the Indian plate beneath the Tibet plateau creates a persistent buoyancy which will cause erosion of the crust and when it stabilized it will reveal only high-grade rocks. In this respect, none of these cratons show a uniform Tibet-like environment for their formation. Instead, the distribution of the rock assemblages and the structural relationship between them indicate that they were formed by the amalgamation of magmatic arcs and associated subduction-accretion complexes that were eventually involved in a final continental collision. This leads us to the conclusion that craton formation does not require total removal of upper crust. Instead, the granulitization of the roots of arcs may have been responsible for weighing them down. The best candidate of a craton forming event is seen in the Altaids, a Palaeozoic superorogenic system which has not been deformed since the early Cretaceous.
Plate tectonics describes the horizontal motion of rigid lithospheric plates away from midoceanic ridges and parallel to transforms, towards deep-sea trenches, where the oceanic lithosphere is subducted into the mantle. This process is the surface expression of modern-day heat loss from Earth. One of the biggest questions in Geosciences today is “when did plate tectonics begin on Earth” with a wide range of theories based on an equally diverse set of constraints from geology, geochemistry, numerical modeling, or pure speculation. In this contribution, we turn the coin over and ask “when was the last appearance in the geological record for which there is proof that plate tectonics did not operate on the planet as it does today”. We apply the laws of uniformitarianism to the rock record to ask how far back in time is the geologic record consistent with presently-operating kinematics of plate motion, before which some other mechanisms of planetary heat loss may have been in operation. Some have suggested that evidence shows that there was no plate tectonics before 800 Ma ago, others sometime before 1.8–2.7 Ga, or before 2.7 Ga. Still others recognize evidence for plate tectonics as early as 3.0 Ga, 3.3–3.5 Ga, the age of the oldest rocks, or in the Hadean before 4.3 Ga. A key undiscussed question is: why is there such a diversity of opinion about the age at which plate tectonics can be shown to not have operated, and what criteria are the different research groups using to define plate tectonics, and to recognize evidence of plate tectonics in very old rocks? Here, we present and evaluate data from the rock record, constrained by relevant geochemical-isotopic data, and conclude that the evidence shows indubitably that plate tectonics has been operating at least since the formation of the oldest rocks, albeit with some differences in processes, compositions, and products in earlier times of higher heat generation and mantle temperature, weaker oceanic lithosphere, hotter subduction zones caused by more slab-melt generation, and under different biological and atmospheric conditions.
Collision-related magmatism in accretionary-to-collisional orogens records a tectonic transition from early subduction-accretionary processes to collisional orogenesis, and also plays a significant role in continental growth. Here, we present an integrated study of field observations, geochemistry, whole-rock Rb-Sr and Sm-Nd isotopes, and zircon U-Pb ages and Lu-Hf isotopes for the Laohushan mafic to felsic magmatic rocks related to initial collision between the Alxa terrane and the Central Qilian block along the North Qilian orogenic belt, northeastern Tibet. The Laohushan magmatic rocks are dominated by quartz diorites (ca. 426 Ma), with minor tonalites enclosing dioritic enclaves (ca. 430 Ma) and hornblendite xenoliths (ca. 448 Ma), and some coeval dolerite dikes (ca. 427 Ma) intruded into the accretionary complex. The quartz diorites are characterized by light rare earth element (LREE)- and large ion lithophile element (LILE)-enrichment but have high field strength element (HFSE)-depleted trace element patterns and negative initial εNd (−1.6 to −2.9) and positive zircon initial εHf (+3.0 to +6.2) values. The dioritic enclaves are also characterized by LREE-enriched and HFSE-depleted patterns and have mostly negative initial εNd (−9.2 to +0.03) but positive zircon initial εHf (+3.0 to +5.9) values. These geochemical and isotopic features, together with isotopic mixing calculations, suggest that the quartz diorites were likely derived from partial melting of the lower crust dominated by accreted mafic oceanic rocks with minor sediments, whereas the dioritic enclaves originated from underplated mantle-derived magmas mixed with crust-derived melts. The hornblendite xenoliths have high MgO, Cr, and Ni contents, positive Th, U, and Pb anomalies, and negative Nb, Ta, and Ti anomalies. They have negative initial εNd (−2.8), near chondritic zircon initial εHf (−0.4 to +1.4) values and an Archean Nd model age (TDM = 2.74 Ga), suggesting that the hornblendites were likely produced by partial melting of subcontinental lithospheric mantle peridotite that was metasomatized by subduction-related melts beneath the Archean−Proterozoic Alxa terrane. We propose that partial melting of the lower crust of the early Paleozoic North Qilian orogenic belt was in response to slab breakoff and asthenospheric upwelling during the initial stage of collisional orogenesis. This study demonstrates that heterogeneous magma sources, involving accretionary materials (i.e., accreted oceanic crust and sediments) and various mantle-derived components, were mixed to form the collision-related magmatic rocks. It also highlights the significance of collision-related magmatism in continental growth and stabilization of newly-assembled crust in accretionary-to-collisional orogens.
The ca. 2.54−2.51 Ga Dengfeng greenstone belt in the southern section of the Central orogenic belt of the North China craton consists of structurally juxtaposed slices of meta-ultramafic, metamafic, and felsic igneous rocks, metasedimentary rocks, including minor banded iron formation. The complex was metamorphosed to greenschist to amphibolite facies at ca. 2.5 Ga and intruded by ca. 2.50−2.42 Ga mafic and felsic plutons/dikes. Detailed field mapping and structural analyses show that the different lithostructural units, including a metamafic-dominant unit and a metasedimentary-dominant unit, are in tectonic contact, with complex thrust imbrication and multiple brittle and ductile deformation. The metasedimentary-dominant unit consists of coherent schist-metabasalt sequences, metaturbidites, and chaotic mélanges that are characterized by typical duplex structures and block-in-matrix fabrics, closely resembling the lithostratigraphy and structural patterns of Phanerozoic accretionary complexes. Together with distinctive and diagnostic geochemical signatures of metabasalts, sanukitoid-like metadiorite, and syntectonic adakitic sills/dikes, we interpret the Dengfeng greenstone belt as Neoarchean forearc and accretionary complexes consisting of dismembered forearc crustal sheets in the west and accreted oceanic plate stratigraphy in the east that were structurally imbricated at a convergent plate margin. The kinematic indicators and the spatial configurations of different tectonic units suggest a near-southwest-dipping intra-oceanic subduction zone beneath the arc in the Central orogenic belt, which later evolved into an arc-continent collision with the Eastern block. The accreted arc and accretionary prism are unconformably overlain by a clastic sedimentary wedge, the lower part of which has a maximum depositional age of ca. 2.45 Ga and is interpreted as a foreland basin sequence related to this collision. Documentation of the Neoarchean Dengfeng forearc and accretionary complexes demonstrates that ca. 2.5 Ga intra-oceanic subduction, oceanic plate stratigraphy accretion, and arc-continent collisional events occurred in the southern section of the Central orogenic belt of the North China craton. The accretion of the 2.54−2.51 Ga arc to the continental margin of the Eastern block marks an early episode of mountain building in the Central orogenic belt, which played an important role in the lateral growth of the North China craton.
- Nov 2018
- The Geological Society of America Annual Meeting 2018
In some recent models, craton formation was ascribed to excessive crustal thickening and subsequent removal of the upper crust by a diverse set of mechanisms. We have compiled data from the world’s cratons to show that in by far the most the greenschist-grade metamorphics and even supracrustal sedimentary rocks are preserved and with few exceptions surfaces of cratons that formed during the cratonization do not expose high-grade rocks everywhere, most not even in the largest part of their surfaces. Low grade metamorphic and non-metamorphic rocks dominate the large parts of the strongest craton in the world, namely the Laurentian. They are not even distributed in only one part of the craton but occur in widely dispersed areas from north to south and east to west. In the Singhbhum and Raj Mahal cratonic pieces in India, in North China, in the Baltic, Tanzanian, Congo, North African and Arabian, Guyana, São Francisco, Pilbara and Yilgarn cratons greenschist-facies to non-metamorphic rocks were present at the surface when these cratons were stabilised. None of these cratons show an uniform Tibet-like environment for their formation. Instead, it seems that they formed by the amalgamation of magmatic arcs and associated subduction-accretion complexes that were subsequently involved in a final continental collision, but that their surfaces were never very much higher than the prevailing sea-level as they do not seem to be deeply eroded. We think the eclogitisation of mafic roots of arcs may have been responsible for weighing them down. A more recent example of a craton forming event is seen in the Palaeozoic Altaid superorogenic system.
- Nov 2018
The 2722 ± 2 Ma Archaean megacrystic anorthosite-bearing Haines Gabbroic Complex in the Wawa subprovince of the western Superior Province formed contemporaneously with the spatially associated Greenwater Assemblage of the Shebandowan greenstone belt, Ontario, Canada. Both field and petrographic observations indicate that the Haines Gabbroic Complex and the spatially and temporally associated ultramafic to intermediate volcanic and intrusive rocks of the Greenwater and Burchell assemblages experienced extensive metamorphic recrystallization and alteration as a consequence of greenschist-facies metamorphism. These metamorphic processes resulted in pervasive replacement of igneous minerals by metamorphic counterparts. Metamorphic alteration also mobilised LILE and transition metals and slightly reset the U-Th-Pb and Sm-Nd isotope systems. Consequently, the Haines Gabbroic Complex and Greenwater Assemblage have respective Sm-Nd errorchron ages of 2765 +110/‐280 Ma and 3108 +120/‐380 Ma. Major and trace element compositions suggest that the least altered anorthosites, leucogabbros and gabbros of the Haines Gabbroic Complex are not cogenetic with the Greenwater and Burchell assemblages and have distinct parental magmas and depleted mantle sources. The calc-alkaline andesites of the Burchell Assemblage, the tholeiitic basalts and gabbros of the Greenwater and Burchell assemblages, the transitional gabbros of the Greenwater and Burchell assemblages and the picrites, serpentinites and peridotites of the Greenwater Assemblage are all petrogenetically distinct and have different parental magmas and depleted mantle sources. The pyroxenites are genetically distinct from the other Greenwater Assemblage lithologies, the Burchell Assemblage and the Haines Gabbroic Complex. The Haines Gabbroic Complex crystallised from hydrous, Ca- and Al-rich tholeiitic parental magmas that fractionated from more primitive, hydrous boninitic parental magmas. The initial εNd values of the Haines Gabbroic Complex (εNd = +3.0 to +4.1) and the Greenwater (εNd = +1.8 to +4.1) and Burchell (εNd = +2.0 to +3.7) assemblages are indicative of variably depleted mantle sources. The major and trace element characteristics of the Haines Gabbroic Complex and the Greenwater and Burchell assemblages and geological relationships indicate that they formed in a Japan-style mature intra-oceanic continental arc with a large volume of TTGs. Intra-oceanic arc rifting was most likely triggered by slab rollback, followed by mantle plume emplacement. On the basis of rock association and geochemical data, we suggest that the Haines Gabbroic Complex and the Greenwater and Burchell assemblages represent a dismembered subduction-related Archaean ophiolite that marks a Neoarchaean suture zone between the Wawa and Wabigoon subprovinces.
- Oct 2018
The Darbut and Karamay ophiolitic mélanges in the West Junggar region of the Chinese Altaids, Central Asia, represent the relict fragments of the Late Paleozoic Junggar ocean basin. The mélanges have typical block-in-matrix structures and contain tectonic blocks of serpentinized perdiotite, gabbro, pillow basalt, chert, and sandstone that are dispersed in a strongly-sheared serpentinized matrix. In this contribution, we present new whole-rock major and element, and Sr and Nd isotope data for mafic rocks and cherts in the Darbut and Karamay ophiolitic mélanges. Compositionally, the mafic rocks are divided into two groups: Group 1 consists of gabbros and tholeiitic basalts displaying both N-MORB- and arc-like geochemical affinities; and Group 2 is characterized by alkaline basalts with OIB-like geochemical characteristics. Geochemical data suggest that Group 1 rocks are likely to have formed in a back-arc ocean basin and were derived from a depleted mantle source that had been metasomatized by slab-derived fluids. The origin of Group 2 rocks is attributed to ocean island basalts derived from a hotspot. The cherts in the mélanges appear to have formed in a restricted oceanic basin, showing a transitional depositional setting between a deep marine pelagic environment and a shallow continental margin environment. Accordingly, we propose a revised geodynamic model for the Late Paleozoic tectonic evolution of West Junggar. We suggest that a back-arc ocean basin opened in response to a northwest-dipping intra-oceanic subduction system during the Late Silurian to Devonian and contemporaneous hotspot-derived OIBs erupted in this basin. It evolved to a relict oceanic basin resulted from the northwestward subduction of the back-arc basin oceanic lithosphere in the Early Carboniferous and was gradually filled up with volcaniclastic sedimentary rocks by the latest Carboniferous. Following the closure of the back-arc ocean basin, West Junggar was transformed into an intra-continental setting in the Early Permian.
The geological evolution of the Early Paleozoic Wuyi-Yunkai orogen in South China is a scientific question of a long-standing debate. We document the presence of a NE-NEE-striking Early Paleozoic subduction-accretion shear zone, a possible mélange belt, in the northwestern Yunkai Domain. The northwestern Yunkai shear zone consists predominantly of tectonically juxtaposed fragments of Early Paleozoic flysch, arc volcanic rocks, and a forearc ophiolite. The Yunkai shear zone displays typical mélange structures in several locations; however, these structures are not continuous throughout the shear zone. The shear zone provides evidence for greenschist to amphibolite facies metamorphism and intense deformation resulting from Early Paleozoic to Early Mesozoic tectonic events. The flysch fragments are characterized by northwestward younging, northwestward thrusting, and northwestward migration of deformation and metamorphism. The arc volcanic rocks consist of basaltic-andesite, andesite, and dacite with a mainly sanukitic composition and also include Nb-enriched basalts. They are characterized by enrichment of LREEs, LILEs, Pb and depletion of HFSEs, suggesting a continental forearc or a continental arc tectonic setting. The ophiolitic fragments consist of MORB-like basalt and dolerite/gabbro. They have slightly depleted to flat LREE patterns and are characterized by enrichment of LILEs and Pb and depletion of HFSEs, indicating a continental forearc setting. Zircon U-Pb analyses yield 460–443 Ma and 455–437 Ma ages for the sanukitic volcanic rocks and ophiolitic fragments, respectively, suggesting that they formed in the Late Ordovician to Early Silurian. Both the sanukitic volcanic rocks and ophiolitic fragments possess negative to positive zircon εHf (t) values (–11.0 to +2.3), indicating that they may have been generated by partial melting of an old subarc mantle wedge source metasomatized by slab-derived fluids and/or melts. Recognition of the Early Paleozoic subduction-related magmatism and subduction-accretion structures in the northwestern Yunkai Domain has important implications for the tectonic history of the Wuyi-Yunkai orogen, South China. Collectively, combined with previous studies, we propose that the Huanan oceanic lithosphere began to subduct southeastward beneath the Yunkai terrane (arc) as early as 460 Ma, and the subduction continued between 460 and 440 Ma.
- Sep 2018
The Darbut and Karamay ophiolitic mélanges in the west Karamay area at the southeastern margin of West Junggar, southern Altaids, represent the relict fragments of the Paleozoic Junggar ocean basin. The mélanges contain structurally complex tectonic blocks of ultramafic rocks, gabbro, basalt, and chert that are dispersed within an extensively sheared serpentinite matrix. This study presents new field observations, structural analyses, geochronological and geophysical data for the mélanges and the overlying Carboniferous strata. Three stages of deformation (D1 –D3 ) have been identified. D1 is only locally preserved in the mélanges and is characterized by thrust-imbricated faults, recording the subduction and accretion of the paleo-oceanic crust in the Early Carboniferous. D2 mainly consists of map-scale upright folds (F1 ) in the Carboniferous strata and NE-SW-striking sub-vertical right-lateral shear zones in the mélanges. The D3 is characterized mainly by NE-SW-striking transcurrent left-lateral strike-slip faults and “S-shaped” drag folds (F2 ). The extensive tectonic reworking during D2 and D3 deformations eradicated the most of D1 structures. Our structural observations, combined with geophysical data, suggest that the basement of the west Karamay area consists of trapped oceanic crust or dismembered ophiolitic fragments and that the Darbut and Karamay ophiolitic mélanges were extruded through right-lateral shear zones. Our results and the published data further suggest that the emplacement of the mélanges and the formation of F1 folds represent regional-scale amalgamation processes between different tectonic blocks and/or plates in the Late Carboniferous. These processes resulted in the closure of the West Junggar remnant ocean basin and other sub-oceanic basins in the Paleo-Asian Ocean, leading to the formation of the Kazakhstan orocline. Furthermore, the D3 deformation was associated with anticlockwise rotation of the West Junggar region in a post-collisional setting in the Permian.
The Early Palaeozoic geology of the South China Craton (SCC) is characterized by an Early Palaeozoic intracontinental orogen with folded pre-Devonian strata and migmatites, MP/MT metamorphic rocks and Silurian post-orogenic peraluminous magmatic rocks in both the Yangtze and the Cathaysia blocks. In this contribution, we present new zircon U–Pb ages and Hf isotope data for detrital zircons from the Neoproterozoic to Silurian sedimentary sequences in the southeastern Yangtze Block. Samples from Neoproterozoic rocks generally display a major peak at 900–560 Ma, whereas samples from Lower Palaeozoic rocks are characterized by several broader peaks within the age ranges 600–410 Ma, 1100–780 Ma, 1.6–1.2 Ga and 2.8–2.5 Ga. Provenance analysis indicates that the 900–630 Ma detritus in Cryogenian to Ediacaran samples was derived from the Late Neoproterozoic igneous rocks in South China that acted as an internal source. The occurrence of 620–560 Ma detritus indicates the SE Yangtze was associated with Late Neoproterozoic arc volcanism along the north margin of East Gondwana. The change of provenance resulted in the deposition of 550–520 Ma and 1.1–0.9 Ga detrital zircons in the Cambrian–Ordovician sedimentary rocks. The ε Hf( t ) values of these detrital zircons are similar to those of zircons from NW Australia–Antarctica and South India. This change of provenance in the Cambrian can be attributed to the intracontinental subduction between South China and South Qiangtang, and the convergence of India and Australia when East Gondwana finally amalgamated.
Detrital zircons are often used to constrain the maximum sedimentary age of strata and sedimentary provenance. This study aimed at reconstructing the Cryogenian palaeogeography of the Yangtze Domain based on U–Pb ages and Lu–Hf isotopic signatures of detrital zircons from sandstones in the southeastern part of the Yangtze Domain. U–Pb ages of the youngest detrital zircon grains from the Niuguping, Gucheng and Datangpo formations yielded average ages of 712±24 Ma, 679.2±6.2 Ma and 665.1±7.4 Ma, respectively, which are close to the depositional ages of their respective formations. An integrated study of detrital zircon Lu–Hf isotopes and U–Pb ages from three samples revealed six main peak ages in the samples from the Anhua section at c . 680 Ma, c . 780 Ma, c . 820 Ma, c . 940 Ma, c . 2000 Ma and c . 2500 Ma. The characteristics of the U–Pb ages and Hf isotopes indicate a link between the north and southeast margins of the Yangtze Domain as early as c . 680 Ma, and the provenance of the coeval sedimentary sequences in the SE Yangtze Domain was the South Qinling Block on the northern margin of the Yangtze Domain. The provenance analysis on the c . 680 Ma detritus composing upper Neoproterozoic strata in the Yangtze Domain revealed that the detritus was transported southward from South Qinling to the southeast margin of the Yangtze Domain through the Exi Strait, but was hindered by the Jiangnan Orogenic Belt.
Detailed field observations indicate that Neoarchean S-type granites were emplaced along and/or proximal to some terrane (tectonic) boundary zones in the western Superior Province, southeastern Manitoba. These S-type granites are characterized by the presence of at least one diagnostic indicator minerals, such as sillimanite, cordierite, muscovite, garnet, and tourmaline. They are medium- to high-K calc-alkaline, moderately to strongly peraluminous (ANKC>1.1), and contain >1% CIPW normative corundum. Compared to more voluminous, older I-type granitoids in tonalite-trondhjemite-granodiorite suites in the region, the S-type granites occur as relatively small intrusions and have high (SiO2>72 wt.%) contents with a small silica range (SiO2 = 72.2-81.2 wt.%), but a large range of Zr/Hf (17.1-43.8) and Nb/Ta (0.3-16.0) ratios. These geochemical characteristics suggest that the S-type granites were derived from partial melting of heterogeneous sedimentary rocks deposited as synorogenic flysch that underwent burial and crustal thickening during terrane collision. Although the S-type granites display geochemical variations in individual and between different plutons, their low Sr (<400 ppm) and Yb (<2 ppm) contents, and low Sr/Y (<40) and La/Yb (<20) ratios are consistent with a partial melting process that left a granulite-facies residue consisting of plagioclase, pyroxene, and ±garnet. The S-type granites display low zircon saturation temperatures (mostly <800°C) and low emplacement pressures (<300 MPa), similar to strongly peraluminous leucogranites formed in the Himalayas. Therefore, we propose that the Neoarchean S-type granites in the western Superior Province, whose source rocks were deposited between colliding tectonic blocks between 2720 and 2680 Ma, may serve as a geological marker of some Archean terrane boundary zones.
- Apr 2018
Mantle xenoliths hosted by the Quaternary Tasse alkaline basalts in the Canadian Cordillera, southeastern British Columbia, are mostly spinel lherzolite originating from subcontinental lithospheric mantle. The xenoliths contain abundant feldspar veins, melt pockets and spongy clinopyroxene, recording extensive alkaline metasomatism and partial melting. Feldspar occurs as veins and interstitial crystal in melt pockets. Melt pockets occur mainly at triple junctions, along grain boundaries, and consist mainly of olivine, cpx, opx and spinel surrounded by interstitial feldspar. The Nd, Sr and Pb isotopic compositions of the xenoliths indicate that their sources are characterized by variable mixtures of depleted MORB mantle and EM1 and EM2 mantle components. Large variations in εNd values (−8.2 to +9.6) and Nd depleted mantle model ages (TDM = 66 to 3380 Ma) are consistent with multiple sources and melt extraction events, and long-term (>3300 Ma) isolation of some source regions from the convecting mantle. Samples with Archean and Paleoproterozoic Nd model ages are interpreted as either have been derived from relict Laurentian mantle pieces beneath the Cordillera or have been eroded from the root of the Laurentian craton to the east and transported to the base of the Cordilleran lithosphere by edge-driven convection currents. The oxygen isotope compositions of the xenoliths (average δ¹⁸O = +5.1 ± 0.5‰) are similar to those of depleted mantle. The average δ¹⁸O values of olivine (+5.0 ± 0.2‰), opx (+5.9 ± 0.6‰), cpx (+6.0 ± 0.6‰) and spinel (+4.5 ± 0.2‰) are similar to mantle values. Large fractionations for olivine-opx, olivine-cpx and opx-cpx pairs, however, reflect disequilibrium stemming from metasomatism and partial melting. Whole-rock trace element, Nd, Sr, Pb and O isotope compositions of the xenoliths and host alkaline basalts indicate different mantle sources for these two suites of rocks. The xenoliths were derived from shallow lithospheric sources, whereas the alkaline basalts originated from a deeper asthenospheric mantle source.
The ~2720 Ma Greenwater Assemblage of the Shebandowan greenstone belt contains anorthosites, leucogabbros and gabbros of the Haines Complex, basalts, picrites, serpentinites, gabbros, olivine websterites and harzburgites and hosts Ni-Cu-PGE-Co-Fe-Ti mineralization. All of these lithologies were metamorphosed under greenschist facies conditions and underwent metamorphic recrystallization and hydrothermal alteration. Despite all of this, these rocks exhibit relict igneous cumulate textures and still have some relict primary igneous pyroxene, olivine and plagioclase. Major and trace element variation diagrams suggest that the rocks of the Haines Complex crystallized from a Ca- and Al-rich tholeiitic parental magma whereas those of the Greenwater Assemblage formed from tholeiitic, transitional and calc-alkaline parental magmas. N-MORB normalized trace element diagrams have negative Nb, Ti and Zr anomalies and flat REE patterns indicating that the Haines Complex and the Greenwater Assemblage formed in a suprasubduction zone arc rift-backarc setting. The basalts, gabbros and olivine websterites of the Greenwater Assemblage have distinctively more fractionated N-MORB normalized trace element patterns, suggesting that these lithologies are genetically distinct. The rocks of the Haines Complex and Greenwater Assemblage record a transition from an arc rift to a backarc environment in a suprasubduction zone setting.
The 2722 Ma Haines Complex occurs in the Neoarchean (2680-2722 Ma) Shebandowan greenstone belt near the boundary between the Wawa subprovince and the metasedimentary Quetico subprovince to the north. The associated Ni-Cu-PGE mineralised Shebandowan greenstone belt consists of komatiitic to trachytic volcanics, dunitic to syenitic intrusions and metasedimentary rocks. Petrographic studies were carried out on the Haines Complex and the adjacent mafic to ultramafic volcanic and intrusive rocks of the Shebandowan greenstone belt. The Haines Complex includes megacrystic anorthosite and leucogabbro as well as gabbro and pyroxenite, all of which have undergone greenschist facies metamorphism. The adjacent basalts, picrites, serpentinites, gabbros and dunites of the Shebandowan greenstone belt also show evidence for having undergone greenschist facies metamorphism. The primary mineralogy (pyroxene, anorthite and olivine) of the rocks of the Haines Complex and Shebandowan greenstone belt has been mostly replaced by actinolite, tremolite, serpentine, chlorite, epidote, zoisite and albite. The rocks of both the Haines Complex and the adjacent Shebandowan greenstone belt display variably negative Nb, Ti and Zr anomalies, pronounced positive Pb anomalies and relatively flat REE patterns. The anorthosites and leucogabbros of the Haines Complex all display pronounced positive Eu anomalies, a characteristic indicative of significant crystallisation of plagioclase. Some of the gabbros of the Haines Complex also exhibit this characteristic. The pyroxenites of the Haines Complex and the basalts and gabbros of the Shebandowan greenstone belt exhibit more fractionated trace element patterns on N-MORB-normalised multi-element diagrams relative to the other studied lithologies. Whole-rock major and trace element data from this study suggest that the Haines Complex and Shebandowan greenstone belt formed in a suprasubduction zone environment. The anorthosites, leucogabbros and gabbros of the Haines Complex appear to be genetically related to the tholeiitic basalts, picrites, gabbros, dunites and serpentinites of the adjacent Shebandowan greenstone belt. The pyroxenites of the Haines Complex appear to be more genetically related to the transitional gabbros and calc-alkaline basalts of the Shebandowan greenstone belt, which may explain the more fractionated nature of these rocks relative to the other studied lithologies. This suggests that the Haines Complex crystallised from both tholeiitic and transitional parental magmas and the Shebandowan greenstone belt crystallised from tholeiitic, transitional and calc-alkaline parental magmas. These preliminary conclusions are yet to be corroborated by Sm-Nd and U-Th-Pb isotopic data and trace element modelling, and the compositions and sources of the parental magma are yet to be determined.
- Feb 2018
There are voluminous ultrahigh pressure-related orthogneisses and minor metamorphic supracrustal rocks in the northeastern Sulu UHP terrane (NSL), East China. The tectonic affinities of the supracrustal rocks are crucial for unravelling the deep continental subduction processes and locating the tectonic suture between the South China (SCB) and North China (NCB) blocks. In this contribution, we report new zircon U–Pb ages and Hf isotope data for the supracrustal rocks and metagabbros in the Zeku region of the NSL. In the Zeku region, the supracrustal rocks are spatially associated with granitic gneisses, metagabbros, and eclogites. Detrital zircon U–Pb analyses yield ages between 3.39 and 0.65 Ga that cluster as three major age populations including (1) 2.15–1.68 Ga with two subpeaks at ~1.83 Ga and~1.97 Ga, (2) 2.45–2.15 Ga with a peak at ~2.37 Ga, and (3) 0.79–0.65 Ga. In addition, there is a small age population between 3.39 and 2.61 Ga. The youngest age population of 0.79–0.65 Ga indicates that the Zeku supracrustal rocks must have been deposited after 650 Ma rather than during the Palaeoproterozoic as previously thought. The 210–190 Ma metamorphic ages suggest that the Zeku rocks were affected by Triassic collision–subduction and exhumation. Most of the Archaean-Palaeoproterozoic zircons have negative εHf(t) values and two-stage Hf model ages concentrating at 2.4–3.4 Ga (peak at ~2.9 Ga), indicating that source rocks of these zircons were mainly derived from recycling of ancient crustal material. These ages, together with the Hf isotopic compositions and rock assemblages, indicate that the Zeku supracrustal rocks were mainly derived from the Precambrian basement rocks of the northern Yangzte Block and have a tectonic affinity to the SCB, rather than the NCB. Our results, together with previously published data, suggest that there are two types of supracrustal rocks with different zircon U–Pb ages and tectonic affinities in the NSL. On the basis of new data, we suggest that the surface boundary between the SCB and NCB in the Jiaodong Peninsula is a complicated tectonic mélange zone rather than a single fault.
The Dengfeng segment of the Archean Central Orogenic Belt in the North China Craton (NCC) includes circa 2.66–2.5 Ga metavolcanics, metasedimentary and plutonic rocks. All units are intruded by ∼2.5–2.4 Ga mafic dykes and a suite of coeval circa 2.5–2.45 Ga potassic granites and leucogranitic veins. Mafic dykes are characterized by mixed alkaline and calc-alkaline affinities, enriched LREE and negative Nb and Zr anomalies, indicating a lithospheric mantle source region. Potassic granitic dykes exhibit highly fractionated LREE patterns, and negative HFSE anomalies and positive zircon εHf(t) values, which are similar to those of TTG gneisses in the area. Leucogranitic veins cutting across the diorites are characterized by high contents of SiO2 and K2O, LREE-enriched patterns, positive Eu and Sr anomalies, negative Nb and Ti anomalies. We suggest that the granitic dykes and leucogranitic veins were derived from partial melting of thickened older crusts. Combined with other reported 2.5 Ga mafic dykes and potassic granites from the NCC, we propose that they demonstrate an ancient example of arc-polarity reversal and provide a record of the formation of a new plate boundary in the Archean. The underplating of the mafic dyke source beneath the newly accreted Eastern Block may have generated the widespread 2.5–2.47 HT-LP metamorphism with anticlockwise P-T paths in the Eastern Block.
Anorthosite-bearing layered intrusions are unique to the Archaean rock record and are abundant in the Archaean craton of southern West Greenland and the Superior Province of Canada. These layered intrusions consist mainly of ultramafic rocks, gabbros, leucogabbros and anorthosites, and typically contain high-Ca (>An70) megacrystic (2–30 cm in diameter) plagioclase in anorthosite and leucogabbro units. They are spatially and temporally associated with basalt-dominated greenstone belts and are intruded by syn-to post-tectonic granitoid rocks. The layered intrusions, greenstone belts and granitoids all share the geochemical characteristics of Phanerozoic subduction zone magmas, suggesting that they formed mainly in a suprasubduction zone setting. Archaean anorthosite-bearing layered intrusions and spatially associated greenstone belts are interpreted to be fragments of oceanic crust, representing dismembered subduction-related ophiolites. We suggest that large degrees of partial melting (25–35%) in the hotter (1500–1600 °C) Archaean upper mantle beneath rifting arcs and backarc basins produced shallow, kilometre-scale hydrous magma chambers. Field observations suggest that megacrystic anorthosites were generated at the top of the magma chambers, or in sills, dykes and pods in the oceanic crust. The absence of high-Ca megacrystic anorthosites in post-Archaean layered intrusions and oceanic crust reflects the decline of mantle temperatures resulting from secular cooling of the Earth.
- Aug 2017
In a recent article, Li et al. (2016) present questionable petrological, geochemical and zircon U-Pb and Lu-Hf data on a suite of rocks so-called granitoids, amphibolites, diorites, metavolcanics and metasediments (quartz-mica schist) in the Zanhuang Complex in the Central Orogenic Belt (called Trans-North China Orogen in Zhao et al. (2005)) of the North China Craton, and propose a sweeping model that the Zanhuang Complex is a Paleoproterozoic arc that collided with the Neoarchean continent of the Eastern Block of the North China Craton. However, based on many years of detailed mapping, structural, and geochronological work, we object to most of the basic interpretations of field relationships, descriptions of rock types, quality and interpretations of geochronological data, and major conclusions in Li et al. (2016). First, the paper is devoid of any systematic field descriptions, structural context of where the samples were taken from, or any rigorous description of the samples, leaving the reader unsure whether the protoliths of the rocks are what the authors suggest. Second, the zircon age data is not of publishable quality, is poorly presented and interpreted, rendering it meaningless. Third, the paper entirely ignores the circa 2.5 Ga collisional event which has been well documented in the Zanhuang Complex in recently published work (Deng et al., 2013, 2014; Kusky et al., 2016; Wang et al., 2013, 2015, 2016, 2017a,b; Xiao et al., 2014) and which refutes their proposed tectonic model. Fourth, the paper is internally inconsistent between the conclusion and the description of the tectonic model, and does not present any field evidence for the so-called Paleoproterozoic arc-continent collision between the Zanhuang arc and Eastern Block continent. Fifth, the paper has abundant spelling and grammatical mistakes.
Chromian-spinel (chromite) is a common mineral in ophiolitic rocks and the study of chromite from the mantle sections of ophiolites can help shed light on their petrogenetic origin and tectonic setting for formation. The Proterozoic Miaowan Ophiolite Complex (MOC) in the Yangtze Craton contains disseminated chromite grains in mantle harzburgites and podiform chromitites associated with serpentinised dunites. Most chromite grains display compositional zoning due to alteration but the fresh cores preserve primary igneous compositions. Podiform chromitites in the MOC dunites and harzburgites are compositionally similar to typical ophiolitic chromitites elsewhere. The chromite grains contain numerous inclusions of clinopyroxene and amphibole with minor amounts of olivine, chlorite, base-metal sulphides (BMS) and platinum-group minerals (PGM). The abundant hydrous mineral inclusions within the chromite grains suggest a hydrous mantle source. Core compositions of chromite grains indicate that the parental melts of the chromitites were similar to typical boninitic melts in a forearc settings. However, chromite grains in the harzburgites show mixed MORB and arc signatures. Thus, the mineralogy and geochemistry of the MOC peridotites suggest that the chromitites in the MOC formed in a forearc setting during reaction between boninitic melts and MORB-type harzburgite in a supra-subduction zone (SSZ) mantle wedge.
- Jun 2017
The Neoarchean (ca. 2728 Ma) anorthosite-bearing Doré Lake Complex in the northeastern Abitibi subprovince, Quebec, was emplaced into an association of intra-oceanic tholeiitic basalts and gabbros known as the Obatogamau Formation. The Obatogamau Formation constitutes the lower part of the Roy Group, which is composed of two cycles of tholeiitic-to-calc-alkaline volcanic and volcaniclastic rocks, siliciclastic and chemical sedimentary rocks, and layered mafic-to-ultramafic sills. In this study, we report major and trace element results, and Nd, Sr, Pb and O isotope data for anorthosites, leucogabbros, gabbros and mafic dykes from the Doré Lake Complex and spatially associated basalts and gabbros of the Obatogamau Formation to assess their petrogenetic origin and geodynamic setting. Field and petrographic observations indicate that the Doré Lake Complex and associated volcanic rocks underwent extensive metamorphic alteration under greenschist facies conditions, resulting in widespread epidotization (20–40%) and chloritization (10–40%) of many rock types. Plagioclase recrystallized mainly to anorthite and albite endmembers, erasing intermediate compositions. Metamorphic alteration also led to the mobilization of many elements (e.g., LILE and transition metals) and to significant disturbance of the Rb–Sr and U–Pb isotope systems, resulting in 1935 ± 150 and 3326 ± 270 Ma errorchron ages, respectively. The Sm–Nd isotope system was less disturbed, yielding an errorchron age of 2624 ± 160 Ma. On many binary major and trace element diagrams, the least altered anorthosites and leucogabbros, and the gabbros and mafic dykes of the Doré Lake Complex plot in separate fields, signifying the presence of two distinct magma types in the complex. The gabbros and mafic dykes in the Doré Lake Complex share the geochemical characteristics of tholeiitic basalts and gabbros in the Obatogamau Formation, suggesting a possible genetic link between the two rock associations. Initial εNd (+2.6 to +5.0) and δ¹⁸O (+6.1 to +7.9‰) values for the Doré Lake Complex and gabbros of the Obatogamau Formation (εNd = +2.8 to +4.0; δ¹⁸O = +7.3 to 8.0‰) are consistent with depleted mantle sources. All rock types in the Doré Lake Complex and the Roy Group share the trace element characteristics of modern arc magmas, suggesting a suprasubduction zone setting for these two lithological associations. On the basis of regional geology and geochemical data, we suggest that the Doré Lake Complex and the Obatogamau Formation represent a dismembered fragment of a suture zone, like many Phanerozoic ophiolites, resulting from closure of a back-arc basin between 2703 and 2690 Ma.
- May 2017
The Yangtze Block is separated into the eastern Yangtze Block (EYB) and western Yangtze Block (WYB) by the Chongqing–Huaying line (CHL). An integrated study of detrital zircon Lu-Hf isotopes and U-Pb ages from two sandstones in the Huashan Group together with previous studies reveals six major tectonothermal events in the EYB occurring at about 2.87 Ga, 2.67 Ga, 2.5 Ga, 2.0 Ga, 1.84 Ga and 0.8 Ga. Combined with paleogeographic map, this study reveals distinct detrital zircon U-Pb ages and Hf isotopic characteristics in the western EYB and eastern EYB. The former has most zircon U-Pb ages grouped at about 2.0 Ga, 2.67 Ga and 2.87 Ga with sporadic 2.5 Ga ages. The ∼2.5 Ga zircons show negative to slightly positive εHf(t) values. However, the latter has significant ages grouped at ∼2.0 Ga and ∼2.5 Ga with negligible 2.67 Ga and older zircon ages. The ∼2.5 Ga zircons show negative to high positive εHf(t) values. The Early Neoarchean (2.7–2.6 Ga) was a significant period for production of mature continental crust in the western EYB and the majority of crustal materials were extracted from the depleted mantle until then. Zircon Hf isotopic compositions indicate that the Late Neoarchean to Early Paleoproterozoic event (∼2.5 Ga) involved both reworking of old crust and juvenile crust growth. With previous studies of sedimentary facies, lithochemical compositions, zircon morphology and zircon Hf isotopes, it is proposed that there are unexposed ∼2.5 Ga basement rocks in the EYB. During the 2.2–1.8 Ga, subduction processes, collisional and post-collisional events were recorded in the north and west of the EYB, which can be correlated to the assembly and breakup of the supercontinent Columbia. Two changes in εHf(t) value at ∼840 Ma and ∼790 Ma have been found from detrital zircons in the northern margin of the EYB, which probably represent arc–continent collision and continent–continent collision events, respectively. This long-term Neoproterozoic subduction process occurring at 1100–790 Ma in the EYB appeals for a peripheral position for the Yangtze Block in the supercontinent Rodinia.
We report for the first time the presence of a suite of Paleoproterozoic (2.12 Ga) metamorphosed high-Mg basalts and andesites in the Huangling dome, northern Yangtze craton, China, which provide new insights into crustal growth processes in the craton during the Paleoproterozoic era. The high-Mg basalts and andesites are mineralogically and texturally amphibolites and quartz-bearing amphibolites, respectively, and occur as deformed layers within the metasedimentary rocks of the Shuiyuesi Group in the northern Huangling dome. We present new field and petrographic observations, zircon U-Pb ages, in situ zircon Hf isotope and whole-rock major and trace element and Nd isotope data, to assess the petrogenetic origin and geodynamic setting of these high-Mg rocks. The igneous zircons from the andesites yield a weighted age of ca. 2.12 Ga that is interpreted to be the formation age of the magmatic protolith. The basalts are characterized by moderate SiO2 (49 wt.%), low TiO2 (0.63–0.65 wt.%), high MgO (13.7–14.6 wt.%) and high Mg-numbers (58–59). The andesites have 53–60 wt.% SiO2, 0.45–0.50 wt.% TiO2 and 6.6–9.5 wt.% MgO contents, yielding high Mg-numbers (63–76). Both the basalts and andesites are enriched in LILE and LREE, but depleted in Nb, Ta, and HREE. Zircons in the andesites have εHf (t) values between −0.2 and +3.3 and corresponding Hf isotopic model age (TDM1) of 2.4 Ga, reflecting various degrees of crustal input. They have negative εNd (t) values, ranging from −4.4 to −2.7. Geochemical characteristics of the basalts and andesites indicate that they are equivalent to high-Mg basalts and andesites, respectively, occurring in Phanerozoic suprasubduction zones. The Nd and Hf isotope compositions of the high-Mg rocks suggest that they were derived from partial melting of subarc lithospheric mantle above a subducting oceanic slab. The formation age and general geochemical characteristics of the high-Mg rocks reveal the presence of a Paleoproterozoic (2.1–2.2 Ga) Andean-type continental margin developed on metasomatized lithospheric mantle in the northern Huangling dome. The geochemical and geochronological data presented in this study provide important insights into Paleoproterozoic evolution of the Yangtze craton.
We document for the first time the presence of a Paleoproterozoic ophiolitic mélange in the Archean–Paleoproterozoic Kongling Complex in the northern Huangling Dome, Yangtze craton, South China. Detailed field mapping, petrographic, geochronological and geochemical studies reveal that the mélange consists of a suite of amphibolite facies mafic and ultramafic tectonic blocks including serpentinized harzburgite, olivine pyroxenite, pyroxenite, diabase, gabbro and basalt dispersed in a strongly sheared metasedimentary matrix. The mélange displays polyphase deformation and northwest-verging nappe structures, and underwent amphibolite to granulite facies metamorphism in the Paleoproterozoic (2.0–1.95 Ga). The metasedimentary rocks are composed mainly of garnet-biotite-plagioclase gneiss, mica schist, mica-graphite schist, marble, quartzite, and banded iron formation (BIF). The mafic–ultramafic rock association shares the lithological and geochemical characteristics of Phanerozoic suprasubduction zone ophiolites. LA-ICP-MS U–Pb dating of zircons from the diabase yield ages between 2142 and 2148 Ma for the magmatic cores and between 2042–2048 Ma for the metamorphic rims. Zircon cores have εHf (t) values ranging from +5.4 to +10.3 (average = +7.2) with corresponding TDM1 ages of 2.24 Ga, indicating that these rocks are remnants of juvenile crust derived from the depleted mantle at ca. 2.2 Ga. All rock types in the mélange underwent Paleoproterozoic (2.0–1.95 Ga) amphibolite–granulite facies metamorphism and were intruded by syn-metamorphic granite (∼2.0 Ga), post-collisional (ca.1.85 Ga) granite and mafic dykes. Intrusion age of one quartz monzonitic dyke crosscutting the serpentinized harzburgite is 1999 Ma, constraining the minimum emplacement age of the mafic–ultramafic complex. The mafic–ultramafic rocks within the metasedimentary matrix in the northern Huangling Dome are fragments of a ca. 2.15 Ga suprasubduction zone ophiolite that was incorporated into the suture zone during the accretionary-collisional process. The mélange recognized in the northern Yangtze craton provides important evidence for the Paleoproterozoic subduction and accretion processes possibly associated with the amalgamation of the proposed Columbia supercontinent.
The Miaowan Complex in the Northern Yangtze Craton consists mainly of layered fine-grained metabasites, pillow lavas, sheeted dikes, gabbros, sepentinized harzburguite, and sepentinized dunite, with rare metasedimentary rocks in the metabasite section, and was initially regarded as an ophiolite complex. In this study, we divide the Miaowan Complex into two suites including the Miaowan Ophiolite Suite and a Late Magmatic Suite, based on differences in the degree of deformation, age and geochemical characteristics. The Miaowan Ophiolite Suite (MOS) mainly consists of ductily deformed serpentinized harzburgite, sepentinized dunite, gabbro, sheeted dikes, basalt, plagiogranite, and layered metasedimentary rocks. All these units were then intruded by the Late Magmatic Suite (LMS) consisting of pegmatitic-isotropic gabbro and massive diabase. Magmatic zircons from a deformed gabbro in the MOS yield an age of ca. 1115 Ma, consistent with the whole-rock Sm-Nd errochron age (1135±54 Ma) of harzburgite, gabbro and basalt in the MOS. Hence, the formation age of the MOS is interpreted to be ca. 1115 Ma. Harzburgites in the MOS are characterized by smooth LREE-depleted and flat MREE-HREE patterns; whereas dunites in the MOS display U-shape REE patterns. Deformed gabbro and basalt in the MOS display flat to slightly LREE enriched patterns, and low Th/Yb ratios and a lack of Nb anomalies, showing N-MORB affinities. The initial εNd (t) values of rock units in the MOS range from +6.6 to +7.8, indicating that the MOS was derived from a strongly depleted mantle source. Accordingly, the harzburgite, gabbro and basalt in the MOS are interpreted to have formed in an oceanic spreading center and the dunites in the MOS were formed by reaction between the harzburgites and the subduction-related boninitic melts when the MOS was trapped with the harzburgites as a part of the mantle wedge above a subduction zone in response to the initiation of an intra-oceanic subduction system. Igneous zircons from late pegmatitic-isotropic gabbros in the LMS yield weighted mean ²⁰⁷Pb/²⁰⁶Pb ages of 973±15 Ma, 999±17 Ma and 1002±19 Ma, respectively, suggesting that the LMS was intruded between ca. 1000 Ma and ca. 970 Ma, consistent with their whole-rock Sm-Nd errochron age (1007±62 Ma). Pegmatitic-isotropic gabbro and diabase in the LMS are characterized by enriched-LREE patterns with high Th/Yb ratios and negative Nb and Zr anomalies, consistent with a subduction-related setting. The initial εNd (t) values of rock units in the LMS range from +6.0 to +7.2. It is suggested that the LMS formed in a depleted fore-arc setting. Collectively, the Miaowan Complex consists of two magmatic suites including an older MORB-type ophiolitic suite and a younger arc-related magmatic suite, recording the evolution of geodynamic settings in a Proterozoic ocean.
- Dec 2016
Bulk-rock Lu-Hf and Sm-Nd isotope compositions, as well as major and trace element data are presented for metavolcanic rocks from the Mesoarchaean (ca. 3075 Ma) Ivisaartoq Supracrustal Belt in the Nuuk region of southern West Greenland. The Hf t calculated at 3075 Ma range from +0.8 to +3.1 and the corresponding Nd t values range from +0.7 to +3.6, which forms an array that is displaced off the mantle array for these two isotopic systems. Primitive mantle normalized trace element plots of the metabasalts display negative Nb-and Ti-anomalies in combination with the elevated Th abundances, which is consistent with a subduction zone affinity as proposed by previous studies of this metavolcanic belt. No significant correlations are observed between the isotope compositions and proxies of shallow crustal contamination in the Ivisaartoq rocks, despite clear evidence for inherited Eoarchaean zircon [Polat et al. (2009a) Chemical Geology 268, 248-271], which would have dominated the bulk-rock Hf-isotope budget. Furthermore, the measured samples are less radiogenic than the estimate for the depleted mantle composition at 3075 Ma. The lack of isotope and trace element correlation suggests incomplete equilibration between the crustal contaminant and the parental Ivisaartoq melts. We prefer a petrogenetic model with some combination of slab-derived metasomatism of the mantle source region for the Ivisaartoq magmas, which homogenized their trace element contents, in combination with the incorporation of granitoid residue with unradiogenic Hf-isotope composition at higher degrees of partial melting and finally the eruption of mechanically entrained Eoarchaean crust without significant chemical equilibration. The geo-chemical arc-affinity and lower than depleted mantle (DM) isotope compositions of these metavolcanic rocks support the notion that crustal recycling and plate tectonics has been operating on Earth since at least the Mesoarchaean Eon.
Archean cratons have map patterns and rock associations that are diagnostic of the Wilson Cycle. The North China Craton (NCC) consists of several distinctly different tectonic units, but the delineation and understanding of the significance of individual sutures and the rocks between them has been controversial. We present an actualistic tectonic division and evolution of the North China Craton based on Wilson Cycle and comparative tectonic analysis that uses a multi-disciplinary approach in order to define sutures, their ages, and the nature of the rocks between them, to determine their mode of formation and means of accretion or exhumation, and propose appropriate modern analogues. The eastern unit of the craton consists of several different small blocks assembled between 2.6 and 2.7 Ga ago, that resemble fragments of accreted arcs from an assembled archipelago similar to those in the extant SW Pacific. A thick Atlantic-type passive margin developed on the western side of the newly assembled Eastern Block by 2.6-2.5 Ga. A > 1,300 km- long arc and accretionary prism collided with the margin of the Eastern Block at 2.5 Ga, obducting ophiolites and ophiolitic mélanges onto the block, and depositing a thick clastic wedge in a foreland basin farther into the Eastern Block. This was followed by an arc-polarity reversal, which led to a short-lived injection of mantle wedge-derived melts to the base of the crust that led to the intrusion of mafic dikes and arc-type granitoid (TTG) plutons with associated metamorphism. By 2.43 Ga, the remaining open ocean west of the accreted arc closed with the collision of an oceanic plateau now preserved as the Western Block with the collision-modified margin of the Eastern Block, causing further deformation inthe Central Orogenic Belt. 2.4-2.35 Ga rifting of the newly amalgamated continental block formed a rift along its center, and new oceans within the other two rift arms, which removed a still-unknown continental fragment from its northern margin. By 2.3 Ga an arc collided with a new Atlantic-type margin developed over the rift sequence along the northern margin of the craton, and thus was converted to an Andean margin through arc-polarity reversal.
The Archean North China craton is composed of the Western block, Eastern block, and the intervening Central orogenic belt. A 4-10-km-wide and 85-km-long tectonic melange belt informally called the Zanhuang tectonic melange is documented in the Zanhuang Massif of the Central orogenic belt, separating the Eastern block from an Archean arc terrane in the Central orogenic belt. The - melange belt contains a structurally complex tectonic mixture of metapelites, metapsammites, marbles, and quartzites mixed with exotic tectonic blocks of volcanic, mafic, and ultramafic rocks, metabasalts that locally include relict pillow structures, and tonalite-trondhjemite- granodiorite (TTG) gneisses. The Zanhuang tectonic melange marks the suture of an arc-continent collisional zone between the Western Zanhuang Massif in the Central orogenic belt and the Eastern block of the North China craton, and it is one of the best-preserved Archean tectonic melanges in the world. Here, using zircon U-Pb dating of various types of blocks from the Zanhuang melange, we show that the formation and associated deformation of the Zanhuang - melange occurred in the Neoarchean (ca. 2.5 Ga). High-precision (1: 20-1: 200) lithostructural mapping of three key outcrops reveals details of the internal fabrics and kinematics of the melange and regional structural relationships along the arc-continent collisional zone. A synthesis of studies on the tectonic evolution of the North China craton, coupled with our new fabric and kinematic analysis of the Zanhuang melange, further constrains the initial amalgamation timing and geometry of the arc-continent collision between the Fuping arc terrane in the Central orogenic belt and the Eastern block with a northwest- dipping subduction polarity. The asymmetric structures and mixture of different blocks and matrices with folding and thrusting events in the Zanhuang melange record kinematic information that is consistent with the tectonic setting of an accretionary wedge that was thrust over the passive margin of the Eastern block by 2.5 Ga. Lithostructural mapping shows that the classic melange and fold-and-thrust structures along the Neoarchean arc-continent collisional zone are broadly similar to Phanerozoic collisional belts.
The Mesoarchean Tartoq greenstone belt, southern West Greenland, consists of tectonically imbricated slices of metamorphosed basalt, gabbro, peridotite, and sedimentary rocks and is intruded by felsic rocks (now mylonites) with wellpreserved duplex structures, representing a relict accretionary prism. The Tartoq greenstone belt is a remnant of a suprasubduction zone ophiolite that originated as back-arc basin oceanic crust. Following the initiation of intra-oceanic subduction, the back-arc oceanic crust accreted to the overriding plate, forming an accretionary prism. The felsic mylonites are compositionally akin to Archean tonalite–trondhjemite–granodiorite suites. Field observations, along with geochemical and zircon U–Pb age data, indicate that the protoliths of the felsic mylonites were derived from partial melting of back-arc basalts in the accretionary prism and emplaced along thrust faults between 3012 ±4 and 2993 ± 6 Ma. It is proposed that the partial melting of the basalts likely occurred in response to ridge subduction. The Upper Cretaceous turbiditic greywackes of the Chugach – Prince William accretionary complex in southern Alaska are intruded by Paleogene felsic dykes. These felsic dykes appear to have been derived from partial melting of subducted and (or) accreted oceanic crust during slab window magmatism. Archean granitoid–greenstone terrains share many geological characteristics of Phanerozoic subduction–accretion complexes such as the Alaskan and Altaid subduction–accretion complexes, consistent with the operation of uniformitarian geological processes in the Archean. The Archean Earth might have been dominated by numerous smaller plates and greater ridge length than today that would have resulted in more frequent ridge-accretionary prism interactions and larger volumes of tonalite–trondhjemite– granodiorite generation in subduction–accretion complexes.
The Great Oxidation Event signals the first large-scale oxygenation of the atmosphere roughly 2.4 Gyr ago. Geochemical signals diagnostic of oxidative weathering, however, extend as far back as 3.3–2.9 Gyr ago. 3.8–3.7 Gyr old rocks from Isua, Greenland stand as a deep time outpost, recording information on Earth’s earliest surface chemistry and the low oxygen primordial biosphere. Here we find fractionated Cr isotopes, relative to the igneous silicate Earth reservoir, in metamorphosed banded iron formations (BIFs) from Isua that indicate oxidative Cr cycling 3.8–3.7 Gyr ago. Elevated U/Th ratios in these BIFs relative to the contemporary crust, also signal oxidative mobilization of U. We suggest that reactive oxygen species were present in the Eoarchean surface environment, under a very low oxygen atmosphere, inducing oxidative elemental cycling during the deposition of the Isua BIFs and possibly supporting early aerobic biology.
- Jan 2016
There are voluminous Neoproterozoic arc-related volcano-sedimentary sequences and small intrusions on the northern margin of the Yangtze Block, South China. The understanding the origin of the Sanligang granitoid intrusion and the spatially associated mafic dikes in the region is crucial for unravelling the tectonic evolution and continental crust growth processes in the Yangtze Block. Zircon U-Pb dating suggests that the mafic dikes (ca. 871 Ma) and granitoids (ca. 860 Ma) are contemporaneous. The mafic dikes have low SiO2 (45.37–46.55 wt.%), K2O (0.32–0.82 wt.%) and Na2O (2.01–2.85 wt.%), and are characterized by enrichment in large ion lithophile elements (LILEs) and depletion in high-field strength elements (HFSEs), suggesting that their mantle source was modified by subducted materials. The Sanligang granitoids have intermediate to high SiO2 (60.35–71.38 wt.%), intermediate K2O (1.38–3.67 wt.%) and Na2O (3.97–5.33 wt.%), and high MgO (1.03–3.16 wt.%). They show LREE-enriched REE patterns (La/YbN = 7.2–12.3) with no or minor negative Eu anomalies. Their primitive mantle-normalized trace element patterns are characterized by enrichment of LILEs and depletion of HFSEs. Both the mafic dikes and granitoids share similar zircon ɛHf(t) values (+10.5 to +12.9, +7.9 to +11.7, respectively), whole-rock initial 87Sr/86Sr ratios (0.7051 to 0.7057, 0.7033 to 0.7041, respectively) and ɛNd(t) values (+4.0 to +7.1, +3.4 to +4.9, respectively), suggesting that the granitoids were generated by partial melting of juvenile basaltic crust. High Mg# values (49–58) in the granitoids may have resulted from assimilation of residual mafic minerals in their source region. Based on its arc-related geochemical affinity and contemporaneous arc-related magmatism, the Sanligang pluton is proposed to be generated in a Neoproterozoic arc setting during crustal growth and reworking. The early Neoproterozoic assemblage from the Sangligang-Sanyang fault belt provides an important record of oceanic slab subduction in the northern margin of the Yangtze Block.
The Dengfeng granite-greenstone belt (DGGB), located in the southern segment of the Central Orogenic Belt (COB) of the North China Craton (NCC), consists of a volcano-sedimentary assemblage, intruded by tonalite, trondhjemite, granodiorite (TTG suite), diorite, granites and late mafic dikes. The volcano-sedimentary assemblage in the DGGB mainly consists of tectonically imbricated basaltic amphibolites, meta-gabbroic rocks with minor ultramafic rocks, and metagreywacke, marble and quartzite, consistent with characterisitics of typical Phanerozoic subduction-accretion complexes. The basaltic amphibolites yield a metamorphic zircon 207Pb/206Pb age of 2507 ± 26 Ma, interpreted to represent the peak age of amphibolite facies metamorphism that took place during subduction/accretion of the basaltic protolith. The basaltic amphibolites are characterized by a tholeiitic affinity, and flat LREE patterns with minor negative Nb and Zr anomalies. Based on mixed MORB- and arc-affinities, the basaltic amphibolites in the DGGB are interpreted to have formed in a fore-arc tectonic setting. One late potassic granite dike cutting across the fabrics of the volcano-sedimentary assemblage yields an intrusion age of 2492 ± 35 Ma, constraining the minimum deformation age for tectonic assembly of the package. The TTG gneisses and diorites intruded the western margin and the center of the subduction-accretion complex, respectively. One TTG sample and one diorite sample yield igneous zircon 207Pb/206Pb ages of 2514 ± 26 Ma and 2518 ± 36 Ma, respectively, constraining their intrusion ages. The TTG gneisses display high ratios of (La/Yb)cn and Sr/Y, and depletion in HFSE with negative Nb, Ta and Ti anomalies, consistent with those of typical Archean TTGs. The TTG gneisses are therefore considered to be generated from partial melting of a shallowly subducting oceanic slab and/or accreted arc amphibolites. The diorites have high concentrations of MgO (2.89-6.05 wt.%), Ni (148-178 ppm) and Cr (85.7-120.6 ppm), and highly fractionated REE patterns and are depleted in HFSEs with negative Nb, Ta, Zr, Hf and Ti anomalies. We suggest that the high-Mg diorites in the DGGB may have been derived from a hydrated mantle wedge which was previously metasomatized by subduction-derived melts and/or fluids. Collectively, a Neoarchean subduction-accretion-collision event is therefore proposed to have generated the DGGB. The volcano-sedimentary assemblage in the DGGB represents a fore-arc subduction-accretion complex, which is interpreted to be related to the suture zone of ca. 2.5 Ga arc-continent collision between a TTG-dominated arc terrane in the COB and the Eastern Block of the NCC. We further propose a long N-S striking Neoarchean suture zone occurring in the eastern margin of the COB mainly consists of the ca. 2.5 Ga subduction-accretion complex in the DGGB to the south, the ca. 2.5 Ga mélange belt in the Zanhuang Complex in the central, and the ca. 2.5 Ga ophiolitic mélange belt in the Zunhua-Dongwanzi structural belt to the north, which separate an arc terrane in the COB from the Eastern Block of the NCC.
The Quaternary Tasse basalts are exposed near the north shore of Quesnel Lake in southeastern British Columbia. They host a variety of mantle xenoliths consisting predominantly of spinel lherzolite with minor dunite and pyroxenite. Mineralogically, the xenoliths are composed of olivine, orthopyroxene, clinopyroxene and spinel characterized by forsterite (Fo87–93), enstatite (En90–92), diopside (En45–50–Wo40–45–Fs5), and Cr-spinel (6 − 11 wt.% Cr), respectively. All of the mantle xenoliths are coarse-grained and show granoblastic textures. Clinopyroxene and spinel display textural evidence for chemical reactions with percolating melts.
- Sep 2015
The Bad Vermilion Lake Anorthosite Complex (henceforth, the BVLA Complex) in western Ontario is one of the well-exposed, anorthosite-bearing, Archean layered intrusions in the Superior Province, Canada. This study presents new whole-rock major and trace element data for the various units of the Complex, oxygen isotope data for the anorthosite, and major and trace element data for the spatially associated granitic rocks intruding the BVLA Complex to constrain their petrogenetic and geodynamic origin. Zircons from granitic rocks have yielded a 207Pb/206Pb age of 2716 ± 18 Ma, constraining the minimum intrusion age of the Complex.
- Jun 2015
The Paleoproterozoic (ca. 1900 Ma) Lynn Lake greenstone belt of northern Manitoba has been previously characterized as comprising a series of tectonically juxtaposed intra-oceanic-derived metavolcanic rocks. The results of more recent local and regional studies, however, support a significant contribution of continental crust during formation of the metasedimentary, metavolcanic, and intrusive igneous rocks that comprise the majority of the Lynn Lake greenstone belt. The tectonic model previously proposed for the Lynn Lake greenstone belt, however, did not consider the geodynamics of the Lynn Lake greenstone belt in the context of all available data. In this study, we report the results of outcrop mapping and petrographic analysis, as well as major, minor, and trace element geochemical analyses for 54 samples from the Northern terrane, and integrate and compare the results with data from previously published studies. These data are used to recharacterize the metavolcanic rocks and to develop a new geodynamic model for the formation of the Lynn Lake greenstone belt.
The Jiangnan orogenic belt (JOB) has been interpreted as a suture zone between the Yangtze craton and Cathaysian terranes in South China. The Neoproterozoic mafic–ultramafic rocks are extensively exposed in the western JOB, providing an ideal opportunity to study the Neoproterozoic assembly and tectonic evolution of South China. We present integrated field and geochemical studies including LA-ICP-MS zircon U–Pb dating, and whole-rock major and trace element and Sm–Nd isotope analyses of the Neoproterozoic mafic–ultramafic rocks exposed in the northern Guangxi Province, South China. Geochronological results show that the magmatic events took place in two distinct periods: the early Neoproterozoic (861–834 Ma) and the late Neoproterozoic (770–750 Ma). Early Neoproterozoic ultramafic rocks of the Sibao Group have positive εNd(t) values (+ 2.7 to + 6.6) whereas mafic rocks exhibit negative εNd(t) values (− 5.8 to − 0.9). The basaltic rocks show TiO2 contents of 0.62–0.69 wt.% and Mg-number of 59–65, and also display an enrichment of light rare earth elements (LREEs) and pronounced negative Nb, Ta and Ti anomalies on chondrite- and primitive mantle-normalized diagrams, consistent with subduction-related geochemical signatures. Late Neoproterozoic rocks of the Danzhou Group show εNd(t) values (− 1.23 to + 3.19) for both ultramafic and mafic rocks. The basaltic rocks have TiO2 contents of 1.01–1.33 wt.% and Mg-number of 57–60, and have a mixture of MORB- and arc-like geochemical affinities, inferred to have formed in an extensional arc environment. Geochemical signatures suggest that all rock types in this study were derived from subarc mantle wedge sources and underwent various degrees of crustal contamination. Thus, we suggest that subduction may have continued to ca. 750 Ma in the western JOB, implying that the amalgamation event between the Yangtze craton and Cathaysian terranes was later than 750 Ma.
- Apr 2015
The Archean craton of West Greenland consists of many fault-bounded Eoarchean to Neoarchean tectonic terranes (crustal blocks). These tectonic terranes are composed mainly of tonalite-trondhjemite-granodiorite (TTG) gneisses, granitic gneisses, metavolcanic-dominated supracrustal belts, layered anorthositic complexes, and late- to post-tectonic granites. Rock assemblages and geochemical signatures in these terranes suggest that they represent fragments of dismembered oceanic island arcs, consisting mainly of TTG plutons, tholeiitic to calc-alkaline basalts, boninites, picrites, and cumulate layers of ultramafic rocks, gabbros, leucogabbros and anorthosites, with minor sedimentary rocks. The structural characteristics of the terrane boundaries are consistent with the assembly of these island arcs through modern style of horizontal tectonics, suggesting that the Archean craton of West Greenland grew at convergent plate margins. Several supracrustal belts that occur at or near the terrane boundaries are interpreted as relict accretionary prisms. The terranes display fold and thrust structures and contain numerous 10 cm to 20 m wide bifurcating, ductile shear zones that are characterized by a variety of structures including transposed and redistributed isoclinal folds. Geometrically these structures are similar to those occurring on regional scales, suggesting that the Archean craton of West Greenland can be interpreted as a continental scale accretionary complex, such as the Paleozoic Altaids. Melting of metavolcanic rocks during tectonic thickening in the arcs played an important role in the generation of TTGs. Non-uniformitarian models proposed for the origin of Archean terranes have no analogs in the geologic record and are inconsistent with structural, lithological, petrological and geochemical data collected from Archean terranes over the last four decades. The style of deformation and generation of felsic rocks on outcrop scales in the Archean craton of West Greenland and the Mesozoic Sulu orogenic belt of eastern China are similar, consistent with the formation of Archean continental crust by subduction zone processes.
- Mar 2015
Subduction polarity reversal events following arc–continent, arc–arc or continent–continent collisions have been well-documented from Cenozoic, Mesozoic, and Paleozoic orogens, but not from the Archean. We here document a Neoarchean subduction reversal event after an arc–continent collision between the Eastern Block of the North China Craton (NCC) and the Fuping arc using field, geochemical and geochronological data. We focus our work on the Wangjiazhuang granite in the Zanhuang massif located along the eastern margin of the Central Orogenic Belt (COB) of the NCC, and a regional tectonic comparison with other granitic rocks with similar ages, geochemical and petrogenetic characteristics. The ca. 2.5 Ga A-type Wangjiazhuang granite intrudes the Neoarchean Zanhuang mélange belt and contains mafic and felsic inclusions. It has positive ε Nd (t) values (+0.12 to +1.13) and T DM2 ages between 2784 Ma and 2869 Ma. This work shows clearly, from field structural relationships, geo-chemistry and geochronology, that the Wangjiazhuang granite formed after an arc–continent collision between the Eastern Block which is defined as a continental block and the Fuping arc, after a subduction polarity reversal event placed a new slab beneath the collisionally modified margin of the Eastern Block and converted it to an Andean-type margin. The subduction polarity reversal event at ca. 2.5 Ga resulted in melting of the enriched mantle. Meanwhile, the rising magma induced partial melting of the old and thickened TTG crust leading to the intrusion of ca. 2.5 Ga Wangjiazhuang granite into the Neoarchean Zanhuang mélange. There are other gra-nitic rocks with similar ages and geochemical and petrogenetic features in the Central Orogenic Belt and Eastern Block of the North China Craton, suggesting that they formed in a similar tectonic setting as the circa 2.5 Ga gran-ites across the Eastern Block. The Neoarchean subduction polarity reversal event and prior arc–continent collision provide strong evidence that plate tectonics was operating by the end of the Neoarchean.
We report partial melting of an ultrahigh pressure eclogite in the Mesozoic Sulu orogen, China. Eclogitic migmatite shows successive stages of initial intragranular and grain boundary melt droplets, which grow into a three-dimensional interconnected intergranular network, then segregate and accumulate in pressure shadow areas and then merge to form melt channels and dikes that transport magma to higher in the lithosphere. Here we show, using zircon U-Pb dating and petrological analyses, that partial melting occurred at 228-219 Myr ago, shortly after peak metamorphism at 230 Myr ago. The melts and residues are complimentarily enriched and depleted in light rare earth element (LREE) compared with the original rock. Partial melting of deeply subducted eclogite is an important process in determining the rheological structure and mechanical behaviour of subducted lithosphere and its rapid exhumation, controlling the flow of deep lithospheric material, and for generation of melts from the upper mantle, potentially contributing to arc magmatism and growth of continental crust.
The Fiskenæsset Complex, SW Greenland, is characterized by layered anorthosite, leucogabbro, gabbro, and ultramafic rock association. Ultramafic rocks consist mainly of hornblendite, hornblende peridotite, hornblende pyroxenite, and dunite. Despite upper amphibolite to granulite facies metamorphism, poly-phase deformation and multiple granitoid intrusions, primary igneous layers and mineral assemblages have been well preserved. Petrographic studies, including SEM-BSE imaging, reveal the presence of igneous hornblende occurring as an interstitial mineral to olivine, clinopyroxene, orthopyroxene, plagioclase, and chromite, as well as inclusions in these minerals, consistent with a hydrous mantle source. Large negative Nb-anomalies in whole-rock samples and hornblende grains suggest that the magmas of the Fiskenæsset Complex originated from a hydrous sub-arc mantle peridotite. Water was recycled to the source of the Fiskenæsset rocks through subduction of hydrated oceanic crust. Phanerozoic hornblende-bearing mafic and ultramafic rocks are typically associated with supra-subduction zone ophiolites and magmatic arcs. Recycling of water to the upper mantle via subduction of oceanic crust not only resulted in the generation of hornblende-rich rocks, but also played an important role in the formation of TTG-dominated Archean continental crust.
- Sep 2014
Mafic granulitic and amphibolitic boudins dispersed in Archean felsic gneisses are widely distributed in the Central Orogenic Belt (COB) and the Eastern Block of the North China Craton (NCC) and are considered to constitute deformed mafic dike swarms. Previous studies have demonstrated that the mafic dikes in the Zanhuang Complex of the NCC intruded the fabrics of an Archean mélange belt and were boudinaged during younger deformation. Igneous zircons from an undefomed mafic dike yield a 207Pb/206Pb age of 2535 ± 30 Ma, which is interpreted as the crystallization age. In addition, pegmatites cutting across the mafic dikes in the field also yield an igneous zircon 207Pb/206Pb age of 2504 ± 16 Ma, providing strong evidence that the mafic dikes in the NCC intruded during the Neoarchean. Metamorphic zircons from one deformed mafic dike sample yield a metamorphic 207Pb/206Pb age of 2090 ± 83 Ma, and another four samples from deformed mafic dikes have a consistent metamorphic zircon age of ca. 1850 Ma, indicating the mafic dikes underwent at least two generations of Paleoproterozoic metamorphism of ca. 2.1 Ga and ca. 1.85 Ga. Previously reported trace element systematics of the mafic dikes are consistent with an arc-related lithospheric mantle source region, rather than an ocean island basalt (OIB)-like source region. The new whole rock Nd isotopic composition (ɛNd(t) = + 0.71 to + 3.70) is relatively more evolved compared to that of the depleted mantle at 2.5 Ga, indicating an enriched lithospheric mantle source. Accordingly, the mafic dikes are proposed to have formed in a subduction-related environment and their enriched mantle source was metasomatized by the melts and fluids derived from the subducted slab. Based on previous studies of the NCC and new geochronogical and isotopic data in this contribution, a new comprehensive tectonic model is proposed for the evolution of the NCC between 2.7 Ga and 1.85 Ga: (1) from 2.7 to 2.5 Ga, an oceanic arc terrane belt developed in what is now the COB of the NCC while a passive margin sequence was deposited on the western edge of the Eastern Block on the margin of the intervening ocean; (2) the oceanic arc terrane collided with the passive margin sequence at 2.5 Ga above a west-dipping subduction zone; (3) the subduction polarity was reversed at ca. 2.5 Ga and a new east-dipping subduction zone gave rise to the intrusion of the 2.5 Ga mafic dikes of the NCC; (4) the final collision between the Western Block and the Eastern Block occurred at ca. 2.1 Ga; (5) the whole NCC then collided with the Columbia Supercontinent along the north margin of the craton at 1.9-1.8 Ga.
Cretaceous granites are widespread in the North Dabie orogen, Central China, but their emplacement sequence and mechanism are poorly known. The Tiantangzhai Complex in the North Dabie Complex is the largest Cretaceous granitic suite consisting of six individual intrusions. In this study, zircon U-Pb ages are used to constrain the crystallization and protolith ages of these intrusions. The Shigujian granite is a syn-tectonic intrusion with an age of 141 Ma. This granite was emplaced under a compressional regime. Oscillatory rims of zircons have yielded two peaks at 137 +/- 1 Ma and 125 +/- 1 Ma. The 137 +/- 1 Ma peak represents the beginning of orogenic extension and tectonic collapse, whereas the 125 +/- 1 Ma peak represents widespread granitic magmatism. Zircon cores have yielded concordant ages between 812 and 804 Ma, which indicate a crystallization age for the protolith. The Tiantangzhai granites show relatively high Sr contents and high La/Yb and Sr/Y ratios. The Shigujian granite has positive Eu anomalies resulting from partial melting of a plagioclaserich source in an over-thickened crust. Correspondingly, in situ Lu-Hf analyses from zircons yield high negative epsilon(Hf)(t) values from -24.8 to -26.6, with two-stage Hf model ages from 2748 +/- 34 to 2864 +/- 40 Ma, suggesting that the magmas were dominantly derived from partial melting of middle to lower crustal rocks. The Dabie orogen underwent pervasive NW-SE extension at the beginning of the early Cretaceous associated with subduction of the Palaeo-Pacific plate beneath eastern China.
Despite multiple phases of deformation and metamorphism, the Archean (ca. 2.97 Ga) Fiskenaesset Complex, southwestern Greenland, contains well-preserved igneous olivine, pyroxene, amphibole (predominantly hornblende) and plagioclase. This study presents new major and trace element data for hornblende, plagioclase and clinopyroxene from the Majorqap qava stratigraphic section of the complex. Petrographic observations and coherent trace element patterns of hornblende, clinopyroxene and plagioclase grains in samples within and between stratigraphic units indicate that trace elements, including LILE, REE and HFSE, in these minerals were generally immobile during amphibolite to granulite facies metamorphism. Their mineral chemistry, therefore, should reflect the trace element composition of the melt from which they grew. Two distinct petrogenetic suites of rocks are recognized using the trace element composition of hornblende. The earlier Suite A is characterized by HREE-depleted hornblende whereas the later Suite B contains HREE-enriched hornblende. Within Suite A, hornblendes can be grouped into Type-A1, Type-A2 and Type-A3 hornblendes. Type-A1 hornblendes, characterized by low REE, Nb and Ta contents and the absence of Eu anomalies, most likely have derived from a primitive liquid. Type-A2 hornblendes, which have high concentrations of incompatible elements including Nb, Zr and REE and negative Eu anomalies, appear to have precipitated from a residual liquid following the extensive crystallization of plagioclase. Type-A3 hornblendes are slightly enriched in MREE compare to LREE and HREE, displaying negative Nb, Zr and Ba anomalies. Suite A typifies a hydrous crystallization sequence of olivine -> clinopyroxene/orthopyroxene -> hornblende -> plagioclase -> hornblende (intercumulus). This crystallization sequence played an important role in the formation of anorthosite layers in the Fiskenaesset Complex. Within Suite B, hornblendes are grouped into four types (TypeB1, Type-B2, Type-B3 and Type-B4 hornblendes). Irrespective of Eu anomalies, Type-B1, Type-B2 and Type-B3 homblendes display parallel patterns with coherent HREE enrichment over LREE. In contrast, Type-B4 hornblendes are enriched in LREE. Suite B illustrates a crystallization of sequence of olivine/plagioclase/hornblende -> hornblende. The presence and chemistry of the igneous hornblendes suggest the Fiskenaesset Complex formed in an Archean convergent-plate margin setting.
- Jun 2014
The Archean (ca . 2.97 Ga) Fiskenæsset layered intrusion, southwestern Greenland, consists of an association of anorthosite, leucogabbro, gabbro, hornblendite, pyroxenite, peridotite and dunite. The intrusion is characterized by well-preserved igneous layering, cumulate texture and primary igneous minerals including olivine, pyroxene, plagioclase, hornblende and chromite. We use new whole-rock (n=36n=36) and mineral (n=32n=32) oxygen isotopic data for all major lithologic units from the best preserved stratigraphic section of the Fiskenæsset Complex at Majorqap qâva to revisit geodynamic and petrogenetic hypotheses proposed for the origin of Archean terranes. The Fiskenæsset Complex has modern mantle-like whole-rock O-isotope compositions (δO18=5.8±0.5‰). Average δO18 values increase from peridotite (δO18=5.0‰), through hornblendite (δO18=5.7‰), gabbro (δO18=5.8‰), pyroxene hornblendite (δO18=6.0‰) and leucogabbro (δO18=6.3‰), to anorthosite (δO18=6.3‰). These whole-rock isotopic compositions reflect the approximate modal abundances of olivine (average δO18=4.9‰), hornblende (average δO18=5.7‰), clinopyroxene (average δO18=6.4‰) and plagioclase (average δO18=6.4‰) in each rock type, as a consequence of mineral fractionation in the magma chamber(s). Field relationships and the absence of crustal contamination suggest that the Fiskenæsset Complex formed in an oceanic setting. Subduction zone-like whole-rock trace element signatures and mantle-like δO18 and initial εNdεNd values are consistent with formation of these rocks in a juvenile oceanic island arc setting. Field and geochemical data from the Fiskenæsset region and adjacent terranes suggest that the origin of Archean crust in southwestern Greenland is consistent with Phanerozoic-like plate tectonic processes rather than density-driven sinking, delamination and diapiric processes requiring formation of greenstone belts and anorthosite complexes on pre-existing continental crust. Mantle-like δO18 values in the ca. 2.97 Ga Fiskenæsset Complex are inconsistent with substantial recycling of continental crust, indicating that the Fiskenæsset rocks originated distal from continental sources.
- Mar 2014
John Tuzo Wilson (1908–1993) was one of the greatest Canadian scientists of the 20th century. His contributions to Earth Sciences, leading the formulation of the theory of plate tectonics, have revolutionized our understanding of how the planet Earth works and evolved over the past 4 billion years. This 50th anniversary special issue of the Canadian Journal of Earth Sciences is dedicated in honour of John Tuzo Wilson, who inspired tens of thousands of students all around the world to study the Earth. This special issue contains 12 papers dealing with various aspects of the “Wilson Cycle” in the geologic record, plate tectonics, mantle plumes, and how John Tuzo Wilson accepted “continental drift” and formulated the theory of plate tectonics. The contributions have mostly been made by geoscientists who directly or indirectly associated with John Tuzo Wilson and have contributed significantly to the plate tectonics paradigm.
- Nov 2013
Laterally extensive belts of mélange characterize Phanerozoic convergent plate margins, but are rare in Archean terranes. We document a late Archean mélange in the Zanhuang Massif of the North China Craton (NCC). The Zanhuang mélange separates a passive margin to foreland basin sequence developed on the western edge of the Eastern Block of the NCC from an arc terrane consisting of trondhjemitic, tonalitic and granodioritic (TTG) gneisses in the Central Orogenic Belt (COB) of the NCC. The mélange belt contains a structurally complex tectonic mixture of metapelites, metapsammites, marbles and quartzites mixed with exotic tectonic blocks of ultramafic and metagabbroic rocks, metabasalts that locally include relict pillow structures, and TTG gneisses. All units in the mélange have been intruded by mafic dikes that were subsequently deformed, and are now preserved as garnet-amphibolite boudins. We interpret the mélange to mark the suture zone between the Eastern Block and the arc terrane in the COB. Field relationships and geochemistry suggest that the exotic ultramafic-metagabbroic-metabasaltic blocks are possible slivers of an intra-oceanic arc or fore-arc ophiolite incorporated into the mélange during the arc-continent collision process. A circa 2.5 Ga granitic pluton intrudes the mélange and undeformed circa 2.5 Ga pegmatites cut the mélange. Tectonic models for the evolution of the COB are varied, but include models that favor collision at 2.5 Ga, 2.1 Ga, and 1.8 Ga. This work shows clearly, from field structural relationships and geochronology, that the first collision must have occurred prior to 2.5 Ga, consistent with late Archean suturing of the western margin of the Eastern Block with an arc terrane (Fuping terrane) during an arc-continent collision. The presence of an Archean mélange with exotic blocks in a suture zone between an Archean arc and continental margin is clear evidence for the operation of plate tectonics at circa 2.5 Ga.
Ocean plate stratigraphy (OPS) is a term used to describe the sequence of sedimentary and volcanic rocks deposited on oceanic crust substratum from the time it forms at a spreading center, to the time it is incorporated into an accretionary prism at a convergent margin. In this study, we review the major geological characteristics of relict Cenozoic to early Archean oceanic crust and OPS persevered in Alaska, Japan, California (Franciscan Complex), Central Asia, British Isles, Canada (Slave Province), Australia (Pilbara craton), and Greenland (Isua and Ivisaartoq belts). An assessment of OPS in accretionary orogens spanning the duration of Earth's rock record shows remarkable similarities between OPS of all ages in terms of structural style, major rock components, sequence of accretion, and trace element geochemical signatures. Volcanic rocks preserved in the orogenic belts are characterized predominantly by oceanic island arc basalts, island arc picrites, mid-ocean ridge basalts, back-arc basalts, oceanic plateau basalts, ocean island basalts, and boninites, with extremely rare komatiites. This demonstrates that sea-floor spreading, lateral movement of oceanic plates with accompanying sedimentation over the oceanic substratum, and accretion at convergent margins have been major Earth processes since at least 3.8 Ga ago. There have been some secular changes in the rock types in OPS, such as changes in carbonates and radiolarian cherts whose sources were in the biota in existence in Phanerozoic times but absent in the Precambrian, but overall, there have been few changes in the style of OPS accretion with time. Komatiites and banded iron formations occur predominantly in Archean orogenic belts, reflecting higher mantle temperatures and less oxic seawater composition, respectively, before 2.5 Ga. This is clear documentation that plate tectonics, including the lateral movement of oceanic lithosphere, has been a major heat loss mechanism on Earth since the early Precambrian.
- Aug 2013
An Archean mélange belt has been recognized in the Zanhuang Complex, Central Orogenic Belt (COB), North China Craton (NCC). All units of the mélange belt are intruded by a ca. 2.5 Ga granite pluton and cross-cut by undeformed 2.5 Ga pegmatites, which constrains the Archean formation age of the mélange. Some exotic mafic-ultramafic blocks are tectonically dispersed in the metasedimentary matrix to the mélange and mainly consist of metabasaltic rocks, deformed pillow lavas with epidosite cores, gabbros, and ultramafic rocks. Both metabasalts and epidosites are characterized by enrichment in light rare earth elements (LREE) and pronounced negative Nb and Zr anomalies on chondrite- and primitive mantle- normalized diagrams. These geochemical characteristics are consistent with supra-subduction zone geochemical signatures. Based on the pillow structures and seafloor hydrothermal alteration, combined with the geochemical characteristics, an intra-oceanic arc-forearc setting is proposed for the formation of these exotic blocks. Furthermore, a west-dipping intra-oceanic subduction zone is proposed to have formed in an ocean between the Eastern Block and an arc terrane prior to 2.5 Ga. The collision between the arc and the Eastern Block of the NCC happened at ca. 2.5 Ga, resulting in the formation of the Archean mélange belt in the Zanhuang Complex. All of the units of the mélange belt were intruded by late mafic dikes, which are present as boudinaged or lenticular shapes in the mélange and surrounding gneisses and are intruded by younger ca. 2.5 Ga pegmatites. Geochemical characteristics of the late mafic dikes are consistent with an arc-related mantle source region, rather than an OIB-like source. Accordingly, pending determination of their precise age, an arc-polarity reversal event is proposed to have occurred following the arc-continent collision, resulting in east-dipping subduction beneath the newly accreted arc and Eastern Block, forming the swarm of arc-affinity mafic dikes. The new subduction zone further resulted in ponded magma below the accreted arc/Block, which induced the partial melting of thickened crust and the intrusion of ca. 2.5 Ga granitic and pegmatitic melts into the mélange and the mafic dikes. The mafic dikes were then deformed and metamorphosed when the remaining open part of the ocean between the arc-collision modified margin of the Eastern Block collided with the Western Block of the NCC.
- Apr 2013
The Xigaze ophiolite crops out along the central Yarlung-Zangbo Suture Zone (YZSZ, Tibet), which also includes the Gangdese arc and the Xigaze forearc basin. Here we report new geochronological and geochemical data from this ophiolite. The Xigaze ophiolite is dominated by mantle peridotites with low CaO and Al2O3 contents, and U-shaped REE patterns. The petrological and geochemical characteristics of these peridotites indicate that they represent residues after moderate- to high-degrees of partial melting (15-24%) and that they were metasomatized by LREE-enriched boninitic melts in a mantle wedge beneath a forearc tectonic setting. The mafic rocks of the Xigaze ophiolite are particularly thin and can be divided into two groups based on their whole-rock compositions: (1) MORB-like, and (2) boninitic rocks. Both groups display negative Nb and Ta anomalies on MORB-normalized diagrams, consistent with the metasomatism of their mantle source by slab-derived fluids. LA-ICPMS zircon U-Pb data from five representative samples indicate formation ages of 124-127 Ma. The zircons are also characterized by positive ɛHf(t) values varying from +7.5 to +17.3. These observations, combined with the geological and geochronological characteristics of the central-western Yarlung-Zangbo ophiolites (YZO), the Gangdese arc and the Xigaze forearc basin, suggest that the central-western YZO might have formed in the forearc setting where rapid crustal accretion was caused by slab rollback during subduction initiation at 130-120 Ma. Subsequently, the rollback of the subducting slab slowed down and stabilized, and the zone of melting migrated to below the Gangdese arc, producing voluminous late Cretaceous granitoids with depleted mantle-type Hf isotopic characteristics. Our new model provides a good example for the generation of the forearc-type ophiolites.
Mesoarchean to Neoarchean orthogneisses (2.95–2.79 Ga) in the Fiskenæsset region, southern West Greenland, are composed of an older suite of metamorphosed tonalites, trondhjemites, and granodiorites (TTGs), and a younger suite of high-K granites. The TTGs are characterized by high Al2O3 (14.2–18.6 wt.%), Na2O (3.4–5.13 wt.%), and Sr (205–777 ppm), and low Y (0.7–17.4 ppm) contents. On chondrite- and N-MORB-normalized trace element diagrams, the TTGs have the following geochemical characteristics: (1) highly fractionated REE patterns (La/Ybcn = 14–664; La/Smcn = 4.3–11.0; Gd/Ybcn = 1.5–19.7); (2) strong positive anomalies of Sr (Sr/Sr* = 1.0–15.9) and Pb (Pb/Pb* = 1.4–34.9); and (3) large negative anomalies of Nb (Nb/Nb* = 0.01–0.34) and Ti (Ti/Ti* = 0.1–0.6). The geochemical characteristics of the TTGs and trace element modeling suggest that they were generated by partial melting of hydrous basalts (amphibolites) at the base of a thickened magmatic arc, leaving a rutile-bearing eclogite residue. Field observations suggest that spatially and temporarily associated tholeiitic basalts (now amphibolites) in the Fiskenæsset region might have been the sources of TTG melts. The high-K granites have steep REE patterns (La/Ybcn = 3.8–506; La/Smcn = 2.7–18.9; Gd/Ybcn = 0.92–12.1) and display variably negative Eu anomalies (Eu/Eu* = 0.37–0.96) and moderate Sr (84–539 ppm) contents. Four outlier granite samples have variably positive Eu (Eu/Eu* = 1.0–12) anomalies. Given that the granodiorites have higher K2O/Na2O than the tonalites and trondhjemites, it is suggested that the granites were derived from partial melting of the granodiorites. It is speculated that the dense eclogitic residues, left after TTG melt extraction, were foundered into the sub-arc mantle, leading to basaltic underplating beneath the lower rust. Melting of the granodiorites in response to the basaltic underplating resulted in the production of high-K granitic melts. Formation of the Fiskenæsset TTGs, the foundering of the eclogitic residues into the mantle, and the emplacement of the high-K granites led to the growth of Archean continental crust in the Fiskenæsset region.
The primary driving force behind present-day structural, magmatic, sedimentary and metamorphic processes is plate tectonics, resulting from the fl ow of matter and energy between the lithosphere and mantle along divergent, convergent, and transform plate boundaries. Operation of plate tectonics and eruption of hot spot lavas from mantle plumes, stemming from the core-mantle boundary, appears to be coupled in that subducting oceanic plates pile up at the core-mantle boundary and then rise as buoyant plumes to feed hot spot volcanoes (see Hofmann, 1997; Burke, 2011). How far back in Earth history were these geological processes driven by plate tectonics? Were Archean geological processes dominated by density-driven, vertical crustal overturns and diapirs, without modern analogs? How did Archean continents grow? How did Archean oceanic crust form? Did Archean oceanic crust recycle into the mantle at subduction zones? These questions remain controversial. Excellent exposures and a prolonged geological record spanning 3.85‐2.5 Ga in the Archean craton of southwestern Greenland provide a unique opportunity to test hypotheses proposed for the early evolution of Earth. This craton consists mainly of Eoarchean to Neoarchean (ca. 3.8‐
Geological investigation of the rocks in the Elu Link has provided new information on the geodynamic origin of the Neoarchean (ca. 2716–2663 Ma) Hope Bay and Elu granite–greenstone belts. Stratigraphic and geochemical features of these rocks and those of the nearby Flake Lake area in the Hope Bay belt suggest that the two greenstone belts are contiguous, having similar mafic-dominated bimodal rocks comprising abundant basalts to andesites and less common dacites and rhyolites hosting gabbroic and trondhjemite–tonalite–granodiorite (TTG) intrusions. The corresponding parental magmas, whose evolution likely occurred via fractional crystallization and juvenile crustal contamination, formed from both deep and shallow mantle sources. The basalts, andesites, gabbros, and felsic volcanic rocks are variably tholeiitic to calc-alkaline. Chondrite- and primitive mantle-normalized profiles demonstrate (1) flat to slightly fractionated heavy rare-earth element (HREE) patterns with a weak negative Eu anomaly and (2) light rare-earth element (LREE) enriched and strongly fractionated HREE patterns with variable negative to positive Eu anomalies. In contrast, TTG rocks are calc-alkaline, with strong LREE enrichment, HREE depletion, and variable positive Eu anomalies. Altogether, the rocks exhibit Nb and Ti troughs, and variable Nb/Ta, La/Ta, and Zr/Hf ratios indicative of crustal contamination. Chalcophile elements and related ore deposits in the area are inferred to be formed from hydrothermal fluids mobilized during emplacement and after crystallization of their host rocks. An extensional, high-heat-flow back-arc tectonic environment is proposed to explain the stratigraphic and geochemical characteristics and the presence of large gold resources in these greenstone belts.
- Jan 2013
This study reports detailed stable Cr, Sm–Nd and Rb–Sr isotope data for a ca. 1.9 Ga old subaerial weathering profile at Schreiber Beach, Ontario, Canada, from which detailed major and trace element signatures and δ¹⁸O values were previously reported. The weathering profile developed on Neoarchean (∼2.7 Ga) pillow basalts and is unconformably overlain by the Paleoproterozoic (∼1.88 Ga) Gunflint Chert and basal conglomerates. This stratigraphy suggests that the basalts were uplifted and subaerially weathered prior to deposition of the Gunflint Formation.
- Oct 2012
This study reports petrographic, major and trace element, and oxygen isotopic data for a subaerial weathering profile at Schreiber Beach, Ontario, Canada. The weathering profile developed on Neoarchean (ca. 2.7 Ga) pillow basalts and is unconformably overlain by the Paleoproterozoic (ca. 1.88 Ga) Gunflint Chert and basal conglomerates. This stratigraphy suggests that the basalts were uplifted and subaerially weathered prior to deposition of the Gunflint Formation. Rocks at Schreiber Beach have been classified as pillow cores, pillow rims, hyaloclastites, weathered red basalts, and weathered brown to green basalts. There are gradual textural, mineralogical, and geochemical transitions from unweathered basalts to intensely weathered hematite-bearing basalts with stratigraphic height.
The epidosites are interpreted to form in upflow zones at the base of ore-forming oceanic hydrothermal systems that vent as black smokers on the sea floor. This study presents new field, major and trace element, and oxygen isotope data for the recently discovered epidosites in the ca. 1.0 Ga Miaowan (庙湾) ophiolite located near the northern margin of the Yangtze craton. The epidosites occur mainly in the cores of strongly deformed, lensoidal amphibolites. Field observations, major and trace elements and oxygen isotopes suggest that the epidosites were formed by metasomatism of ocean floor basalts, diabase dykes, and gabbros during seafloor hydrothermal alteration.
- Sep 2012
The Fiskenæsset Complex, SW Greenland, contains the world's best preserved Archean (∼2970 Ma) layered anorthosite, leucogabbro, gabbro, and ultramafic association. The complex was emplaced into Archean oceanic crust distal from continental lithosphere and later intruded by tonalites, trondhjemites and granodiorites (TTG) constituting Archean continental crust. The complex and bordering TTG intrusions were variably affected by granulite facies metamorphism and retrogressed under amphibolite facies conditions.
- Jul 2012
The Mesoarchean Fiskenæsset Complex was emplaced as multiple sills and dykes of magma and crystal mush into oceanic crust, forming an association of ca. 550-meter-thick layered anorthosites, leucogabbros, gabbros, hornblendites, pyroxenites, dunites, and peridotites. The complex was intruded by tonalites, trondhjemites and granodiorites (TTG), and underwent multiple phases of deformation. The Fiskenæsset Complex and bordering TTG intrusions were variably affected by granulite facies metamorphism and retrogressed under amphibolite facies conditions.This study presents new field, petrographic, and major and trace element geochemical data for the six major stratigraphic units of the Fiskenæsset Complex at Majorqap qâva, SW Greenland. In addition, new high-precision trace element data are reported for hornblendes from three stratigraphic units. The sampled major lithological units from bottom to top are: Lower Gabbro, Ultramafic, Lower Leucogabbro, Middle Gabbro, Upper Leucogabbro, and Anorthosite. Both whole-rock and hornblende trace element data are consistent with crystallization of the complex from two different parental magmas, originating from different mantle sources. Subsequent differentiation of these parental magmas in the oceanic arc crust resulted in two evolved geochemical rock suites (Suite A and Suite B), which are both characterized by depletions of HFSE (Nb, Ta, Zr and Hf) relative to Th, U, LREE and LILE (Rb, Ba), and enrichments of Sr and Pb relative to MREE; these data are consistent with a magmatic arc geodynamic setting.
The Central Eastern Desert (CED) is characterized by the widespread distribution of Neoproterozoic intra-oceanic island arc ophiolitic assemblages. The ophiolitic units have both back-arc and forearc geochemical signatures. The forearc ophiolitic units lie to the west of the back-arc related ones, indicating formation of an intra-oceanic island arc system above an east-dipping subducted slab (present coordinates). Following final accretion of the Neoproterozoic island arc into the western Saharan Metacraton, cordilleran margin magmatism started above a new W-dipping subduction zone due to a plate polarity reversal. We identify two belts in the CED representing ancient arc–forearc and arc–back-arc assemblages. The western arc–forearc belt is delineated by major serpentinite bodies running ∼NNW–SSE, marking a suture zone. Ophiolitic units in the back-arc belt to the east show an increase in the subduction geochemical signature from north to south, culminating in the occurrence of bimodal volcanic rocks farther south. This progression in subduction magmatism resulted from diachronous opening of a back-arc basin from north to south, with a bimodal volcanic arc evolving farther to the south. The intra-oceanic island arc units in the CED include coeval Algoma-type banded iron formations (BIFs) and volcanogenic massive sulphide (VMS) deposits. Formation of the BIFs was related to opening of an ocean basin to the north, whereas development of the VMS was related to rifting of the island arc in the south. Gold occurs as vein-type mineral deposits, concentrated along the NNW–SSE arc–forearc belt. The formation of these vein-type gold ore bodies was controlled by the circulation of hydrothermal fluids through serpentinites that resulted in Au mobilization, as constrained by the close spatial association of auriferous quartz veins with serpentinites along the western arc–forearc belt.
- Apr 2012
The processes and the geodynamic settings that generated Earth's oldest parts of continental crust are still a matter of debate. A pertinent issue is the genetic relationship between the tonalite-trondhjemite-granodiorite (TTG) suite and the mafic fragments that are found as inclusions within this felsic crust. Here we propose a coherent model for the geodynamic evolution of the oldest (3.65 to 3.85 Ga) continental crust in southern West Greenland. Within the Isua Supracrustal Belt, the best preserved and largest fragment of mafic Eoarchean crust worldwide, tholeiitic and boninite-like amphibolites dominate the sequence, both yielding trace element patterns consistent with a subduction-related origin. The tholeiites yield correlated trace element variations in Nb/Th, La/Yb, Gd/Yb, Zr/Nb, in agreement with a subduction zone setting where a depleted mantle source is overprinted by melt-like slab components (Hoffmann et al., 2011a). Boninite-like rocks in Isua are derived from ultradepleted sources with epsHf(3720) of up to ca. +12.9 (Hoffmann et al., 2010). Petrological phase equilibrium modeling combined with trace element modeling suggests a relationship between the typical Isua arc tholeiites and the TTGs (Nagel et al., 2012). Notably, Hf-Nd isotope signatures between the two lithologies overlap (epsHf(t) = -0.7 to +2.5; epsNd(t) = -0.8 to +4.4), both showing the characteristic decoupling of initial Hf-Nd isotope compositions. Systematically elevated 142Nd anomalies of tholeiites and TTGs are also in agreement with a related origin of both rock types (e.g., Caro et al., 2006). Trace element modeling shows that the Isua TTGs likely formed by melting of thickened mafic arc crust with tholeiite compositions (Hoffmann et al., 2011b) and that the decoupled Hf-Nd signature is likely an inherited feature from melting of the tholeiites. This is also underlined by new Hf and O in zircon data from TTGs in the area (Næraa et al., submitted) that indicate melting of a thickened mafic crust to form the TTGs. The cause for the decoupled behavior of the Hf and Nd isotope compositions is most likely a subduction-related mantle source overprint, because Nd behaves more mobile in subduction components compared to the less mobile Hf (Hoffmann et al., 2011a). Other scenarios explaining the Hf-Nd decoupling may include cumulate segregation processes in an early magma ocean or an early metamorphic overprint during intrusion of the TTGs might also (Hoffmann et al., 2011a; Rizo et al., 2011). These scenarios are possible, however, they are only very difficult to reconcile with all observed trace element and isotope features of Isua rocks. Overall, we therefore propose that the most likely geodynamic setting to form the TTGs in the Isua region is an arc-arc collisional model, where the arc tholeiites melt at 10-20% of partial melting to form the TTGs. Caro, G., Bourdon, B. et al. (2006) Geochim. Cosmochim. Acta 70, 164-191. Hoffmann, J.E., Münker, C. et al. (2010) Geochim. Cosmochim. Acta 74, 7236-7260. Hoffmann, J.E., Münker, C. et al. (2011a) Geochim. Cosmochim. Acta 75, 6610-6628. Hoffmann, J.E., Münker, C. et al. (2011b) Geochim. Cosmochim. Acta 75, 4157-4178. Nagel, T., Hoffmann, J.E., Münker, C. (2012) Geology DOI: doi:10.1130/G32729.1. Næraa, T., Scherstén, A., et al., (submitted). Rizo, H., Boyet, M. et al. (2011) Earth Planet. Sci. Lett. 312, 267-279.
Understanding the origin of the continental crust is one of the key objectives of earth sciences because as a land species we owe our existence to continents. In addition, change in the volume of the continental crust and distribution of continents on Earth's surface have profound effects on major
- Dec 2011
We report the presence of a Late Archean-Paleoproterzoic mélange belt in the Zanhuang massif, southern Taihang Mountains, of the North China craton (NCC). The mélange belt separates a circa 2.7-2.5 Ga passive margin sequence developed on the western edge of the Eastern Block of the NCC, from an arc terrane consisting of TTG gneisses in the Central Belt of the NCC. The mélange belt is 4-10 km wide, and contains a structurally complex tectonic mixture of metapelites, metapsammites, marble, metalimestone, and tectonic blocks of ultramafites and metagabbroic rocks, metabasites that locally include relict pillow lavas, and TTG gneisses. All units in the mélange have been intruded by mafic dikes that were subsequently deformed, and now preserved as garnet-amphibolite boudens. We interpret the mélange belt to mark the suture zone between the Eastern Block and the arc terrane in the Central Belt. The field relationships suggest that the ultramafites-metagabbro-metabasaltic blocks are possible slivers of oceanic crust incorporated into the suture zone during the collisional process. The mélange belt is cut by a circa 2.2 Ga pluton, providing a minimum age on the collision. Tectonic models for the evolution of the Central Belt of the NCC are varied, but include models that favor collision at 2.5 Ga, 2.1 Ga, and 1.8 Ga. This work shows clearly, from field and structural relationships alone, that the collision must have occurred prior to 2.2 Ga, favoring Late Archean suturing of the western margin of the Eastern Block with an arc terrane during an arc/continent collision.
- Sep 2011
This study reviews the geochemical characteristics of Eoarchean to Mesoarchean ultramafic to mafic volcanic rocks (now amphibolites) in SW Greenland and compares them with those of Cenozoic oceanic island arc basalts, to evaluate Archean subduction zone petrogenetic processes. Emphasis is placed on the Th–REE–HFSE (Zr, Ti, and Nb) systematics of the ca. 3800 and ca. 3700 Ma arc suites in the Isua greenstone belt, the ca. 3075 Ma Ivisaartoq–Ujarassuit greenstone belt, and amphibolites associated with the ca. 2970 Ma Fiskenæsset layered anorthosite complex.
- Aug 2011
Radiogenic isotope compositions of Hf and Nd are typically coupled in Phanerozoic and Proterozoic mafic rocks due to a similar behaviour of Lu–Hf and Sm–Nd during mantle melting. Eoarchean rocks, for instance those from southern West Green-land, exhibit an apparent decoupling of Hf and Nd isotope compositions. This apparent decoupling may either indicate meta-morphic disturbance or, alternatively, mirror early differentiation processes in the silicate Earth. To evaluate the issue, we performed combined measurements of Hf–Nd isotope compositions together with major and trace element concentrations for well preserved >3720 to >3800 Ma old tholeiitic metabasalts and gabbros from the $3700 Ma and $3800 Ma old terranes of the Isua Supracrustal Belt, southern West Greenland. In contrast to younger mafic rocks, calculated initial eHf–eNd values of the Isua tholeiites show similar spreads and are both near chondritic to strongly depleted (À0.7 to +6.3 and À0.8 to +4.4, respec-tively), also in contrast to previously reported more depleted signatures in nearby boninite-like metabasalts of the Garbenschiefer unit. An evaluation of alteration effects based on preserved major and trace element arrays reveals pristine magmatic trends and therefore the measured isotope compositions indeed in most cases characterize contrasting Eoarchean mantle sources. In accord with this view, compositions of the Isua metabasalts yield Eoarchean regression ages in Sm–Nd and Lu–Hf isochron spaces, overlapping with emplacement ages inferred from crosscutting relationships with tonalites. Lutetium–Hf systematics of the Isua metabasalts studied here, yield clear isochron relationships. For both terranes, there is some scatter in Sm–Nd space, indicating early disturbance of the Sm–Nd system close in time to the extrusion ages, possibly by seafloor alteration. Trace element com-positions of the metabasalts indicate an arc setting and a strong source overprint by melt-like subduction components. It is likely, that the source overprint may have caused partial decoupling of the eHf–eNd values, due to selective addition of Nd as observed in modern subduction settings. In this case, the most radiogenic initial eNd and eHf isotope values characterize the most depleted mantle sources, and less radiogenic values would reflect increased contributions of isotopically more enriched subduction com-ponents. However, the most depleted samples still exhibit decoupled Hf–Nd compositions, making a case for the presence of even older mantle heterogeneities. A proposed superchondritic composition of the silicate Earth (SCHEM), however, cannot account for the most depleted sample compositions. Conversely, a depleted upper mantle formed by crystallization of perovskite-rich cumulates in the early Hadean may well explain these observed compositions. A literature survey reveals an overlap in initial
- Apr 2011
The Fiskenæsset Complex, SW Greenland, is one of the best preserved layered Archean intrusions in the world, consisting of an association of ca. 550-meter-thick anorthosite, leucogabbro, gabbro, and ultramafic rocks (dunite, peridotite, pyroxenite, and hornblendite). Despite poly-phase deformation and amphibolite to granulite facies metamorphism, primary cumulate textures and igneous layering are well-preserved in the complex.This study reports new major and trace element data for three variably thick (1 to 5m) differentiated (dunite, through peridotite, pyroxenite, gabbro leucogabbro, to anorthosite) sequences (Sequences 1, 2 and 3) in the Sinarssuk area of the Fiskenæsset region. On several variation diagrams, samples from these sequences plot along a well-defined liquid line of descent, consistent with in situ fractional crystallization. The average chemical compositions of these sequences are used to constrain their approximate parental magma compositions. Petrographic observations and geochemical data suggest that Sequences 2 and 3 solidified from evolved magmas that underwent olivine fractionation prior to their intrusion. In contrast, Sequence 1 appears to have been derived from a near-primary parental magma (SiO2=43wt.%, MgO=20wt.%, Al2O3=16wt.%, CaO=9.3wt.%, Ni=840ppm, Mg-number=80). The trace element patterns of this parental magma are comparable to those of Phanerozoic boninites, consistent with a supra-subduction zone geodynamic setting.If the relative thickness of ultramafic layers, the sum of dunite, peridotite and pyroxenite layers, in differentiated sequences is taken as an analog for the original complex emplaced into Archean oceanic crust, the Fiskenæsset Complex might have had a minimum thickness of 1000m, with a 500m thick ultramafic unit at the bottom. The thickness of the ultramafic unit in the preserved complex is less than 50m, suggesting that more than 90% of the original ultramafic unit was either delaminated and recycled back into the mantle as a residual cumulate, or was destroyed during thrusting and TTG intrusion.Both the Fiskenæsset Complex and associated tholeiitic basalts display similar Th–Nb–LREE patterns and plot along the same differentiation trend on Zr versus incompatible trace element diagrams, suggesting a possible petrogenetic link between the two suites of rocks. However, basalts do not display the same differentiation trend as the complex on several major and trace element diagrams. In addition, basalts and parental magma to the complex do not plot along the same fractionation line on Al2O3/TiO2 versus incompatible trace element diagrams, implying that the Fiskenæsset layered intrusive rocks were not derived from tholeiitic basalts through fractional crystallization, as previously thought. Accordingly, we infer that the Fiskenæsset Complex and spatially associated basalts were derived from different mantle sources. Parental magmas to the Fiskenæsset Complex originated from a mantle source that was metasomatized by highly aluminous slab-derived melts.Layered anorthosite complexes are mostly restricted to the Archean and typically associated with tonalite–trondhjemite–granodiorite (TTG) gneisses. The petrogenesis of both suites appears to have been controlled mainly by slab melting, endorsing independent evidence for a secular change from slab-dominated melting to wedge-dominated melting in arc magmatism at the end of the Archean, reflecting higher geothermal gradients in Archean arcs than post-Archean counterparts.
- Jan 2011
The Storø greenstone belt, southern West Greenland, consists of thrust-imbricated slices of Mesoarchean (>3060Ma) and Neoarchean (ca. 2800Ma) mafic to ultramafic volcanic rocks, volcaniclastic sediments, and gabbro–anorthosite associations. The belt underwent polyphase metamorphism at upper amphibolite facies conditions between 2650 and 2600Ma. The contacts between the Mesoarchean and Neoarchean volcanic rocks, and surrounding Eoarchean to Neoarchean tonalite–trondhjemite–granodiorite (TTG) gneisses are tectonic and typically bounded by high-grade mylonites. Regardless of age, the volcanic rocks are dominated by mafic amphibolites with a tholeiitic basalt composition, near-flat to slightly enriched light rare earth element (LREE) patterns (La/Smcn=0.91–1.48), relatively flat to slightly depleted heavy-REE (HREE) (Gd/Ybcn=1.0–1.28), and pronounced negative Nb–Ta anomalies (Nb/Nb*=0.34–0.73) on chondrite- and primitive mantle-normalized diagrams. These geochemical characteristics are consistent with subduction zone geochemical signatures and partial melting of a shallow (
Growing evidence from the accessible geological record reveals that crust–mantle differentiation on Earth started as early as 4.4 Ga. In order to assess the extent of early Archean mantle depletion, we obtained 176Lu–176Hf, 147Sm–143Nd, and high field strength element (HFSE) concentration data for the least altered, well characterized boninite-like metabasalts and associated metasedimentary rocks from the Isua supracrustal belt (southern West Greenland). The metasediments exhibit initial εHf(3720) values from −0.7 to +1.5 and initial εNd(3720) values from +1.6 to +2.1. Initial εHf(3720) values of the least altered boninite-like metabasalts span a range from +3.5 to +12.9 and initial εNd(3720) values from −0.3 to +3.2. These initial Hf-isotope ratios display coherent trends with SiO2, Al2O3/TiO2 and other relatively immobile elements, indicating contamination via assimilation of enriched components, most likely sediments derived from the earliest crust in the region. This model is also consistent with previously reported initial γOs(3720) values for some of the samples. In addition to the positive εHf(3720) values, the least disturbed samples exhibit positive εNd(3720) values and a co-variation of εHf(3720) and εΝd(3720) values. Based on these observations, it is argued, that the most depleted samples with initial εHf(3720) values of up to +12.9 and high 176Lu/177Hf of ∼0.05 to ∼0.09 tap a highly depleted mantle source with a long term depletion history in the garnet stability field. High precision high field strength element (HFSE) data obtained for the Isua samples confirm the contamination trend. Even the most primitive samples display negative Nb–Ta anomalies and elevated Nb/Ta, indicating a subduction zone setting and overprint of the depleted mantle sources by felsic melts generated by partial melting of eclogite. Collectively, the data for boninite-like metabasalts support the presence of strongly depleted mantle reservoirs as previously inferred from Hf isotope data for Hadean zircons and combined 142Nd–143Nd isotope data for early Archean rocks.
In contrast to the strongly coupled behaviour of the Lu-Hf and Sm-Nd systems in Phanerozoic and Proterozoic rocks, early Archean rocks, e.g. those from SW-Greenland, exhibit an apparent decoupling of both systems [e.g., 1]. This apparent decoupling may either indicate metamorphic disturbance or, alternatively, mirror early differentiation processes such as magma ocean crystallisation involving cumulate segregation . To evaluate the issue, we performed combined measurements of Hf-Nd isotope compositions together with major and trace element measurements for well preserved >3.72 to >3.80 Ga old tholeiitic metabasalts and gabbros from the inner western part and the southwestern unit of the Isua Supracrustal Belt, SW-Greenland. A careful evaluation of alteration based on major and trace elements reveals pristine magmatic trends, and therefore near chondritic to moderately depleted initial εHf values of -0.2-+2.7 and initial εNd values of 0.0-+3.0 indeed most likely characterize the early Archean mantle sources. These values confirm a decoupling of the Hf-Nd systematics, but are in marked contrast to more depleted signatures in nearby boninite-like metabasalts of the Garbenschiefer unit . In Sm-Nd and Lu-Hf isochron space, the Isua metabasalts yield early Archean ages, in agreement with emplacement ages inferred from crosscutting relationships with tonalites. Trace element compositions of the metabasalts indicate a source overprint by subduction components. It is likely, that the source overprint may have caused partial decoupling of εHf-εNd values, due to selective addition of Nd as in modern subduction settings. In this case, the most radiogenic initial εNd and εHf isotope values characterize the most depleted mantle sources, and less radiogenic values might reflect addition of more enriched subduction components. A comparison of our data with Hf-Nd isotope data from the literature for early Archean rocks from Greenland reveals an overlap in initial Hf-Nd values of Greenland TTGs with compositions of the tholeiitic metabasalts. We interpret this overlap as indicating a genetic relationship between these rock types, where the TTGs possibly inherited their Hf-Nd compositions from mafic precursors similar in composition to the tholeiites preserved in Isua. Altogether, we therefore conclude that the decoupled Hf-Nd isotope signatures in the Isua tholeiites might at least partially be a pristine feature, reflecting mantle source overprint by subduction components and possibly the decoupling of εHf-εNd during global Hadean differentiation events.
- Oct 2010
The Archean Fiskenæsset Complex, SW Greenland, consists of an association of ca. 550-meter-thick layered anorthosite, leucogabbro, gabbro, and ultramafic rocks (peridotite, pyroxenite, dunite, hornblendite). The complex was intruded by tonalite, trondhjemite, and granodiorite (TTG) sheets (now orthogneisses) during thrusting that was followed by several phases of isoclinal folding. The trace element systematics of the Fiskenæsset Complex and associated volcanic rocks are consistent with a supra-subduction zone geodynamic setting.
New major and high-precision ICP-MS trace element data on the Neoarchean (ca. 2970 Ma) layered Fiskenæsset Complex and associated volcanic rocks, southern West Greenland, provide new constraints on the petrogenesis and geodynamic setting of the complex. The complex appears to have been emplaced as multiple sills of magma and crystal mush into oceanic crust (tholeiitic basalts and gabbros), forming an association of ca. 550 m thick anorthosite, leucogabbro, gabbro, and peridotite layers. The Fiskenæsset Complex and the associated volcanic rocks were intruded by Neoarchean tonalite, trondhjemite, and granodiorite (TTG) sheets during thrusting that was followed by several phases of isoclinal folding. Despite the intense deformation and amphibolite to granulite facies metamorphism, primary cumulate textures and igneous layering are locally well preserved throughout the complex. The presence of calcic-plagioclase (An75-95) and igneous amphibole in anorthosites, gabbros and leucogabbros, and hornblendite veins in peridotites suggests a hydrous magma source(s).
The geodynamic origin of the Neoproterozoic ophiolites of the Arabian-Nubian Shield exposed in the Eastern Desert of Egypt remains controversial. In this study, we present new geochemical and field data from the Fawakhir ophiolite and from some mélange blocks along the Qift-Qusier Road in order to constraint the tectonic evolution of this part of the Central Eastern Desert. The Fawakhir ophiolite contains most lithological units of a Penrose-type ophiolite sequence, and includes ultramafic rocks in the west overlain by isotropic gabbro, sheeted dikes and pillow basalt in the east. These ophiolitic units are enriched in LREE (light rare earth elements) and LILE (large ion lithophile elements) but depleted in high field strength elements (La/Smcn = 0.40–1.22, Th/Nbpm = 1.7–10.9, La/Nbpm = 1.4–6.6). Their magmas appear to have been derived from a depleted (N-MORB-like) mantle source, and their geochemical characteristics are comparable to those of the Izu-Bonin-Mariana forearc oceanic crust formed during the initiation of an intra-oceanic subduction zone. Pillow lava blocks in the eastern mélange have geochemical signatures similar to those of oceanic crust generated in back-arc basins. The decrease in the magnitude of mantle depletion and the change of the geochemical signature along the Qift-Qusier Road from a forearc in the west to a back-arc in the east suggest the formation of the Fawakhir intra-oceanic arc system over an east-dipping subduction zone. With continued subduction and arc migration, this intra-oceanic arc system finally collided with the passive margin of the West Gondwana (the Saharan craton), resulting in the accretion of the Fawakhir arc–forearc units. Following its tectonic accretion onto the West Gondwana continental margin, the Fawakhir ophiolite was intruded by calc-alkaline dikes, whose magmas were derived partly from partial melting of the sub-continental lithospheric mantle. These dikes have geochemical characteristics similar to those of modern active continental margin (Andean-type) rocks (La/Smcn = 2.13–2.48, Gd/Ybcn = 2.04–4.25, Th/Nbpm = 3.2–5.8, La/Nbpm = 2.5–4.9), suggesting that the West Gondwana passive margin (Atlantic-type) was converted to an Andean-type margin subsequent to the arc-continent collision. The inferred conversion of the Atlantic-type margin to an Andean-type margin resulted from the collision-induced reversal of the subduction direction.
The Neoarchean Fiskenaesset Complex, SW Greenland, is characterized by about 550 meter thick layered anorthosites, leucogabbros, gabbros, and ultramafic rocks. The complex is spatially associated with basaltic amphibolites displaying a tholeiitic composition. The major and trace element compositions of tholeiitic basalts suggest that they are petrogenetically related to the Fiskenæsset Complex by fractional crystallization. The complex appears to have been emplaced as multiple sills of magma and crystal mush into oceanic crust (tholeiitic basalts and gabbros). Anorthosites, gabbros, leucogabbros and ultramafic rocks (45 samples) yield a Sm-Nd isochron age of 2972 ± 28 Ma, constraining the maximum age of the complex. All rock types in the complex are characterized by large positive initial epsilon-Nd values (+2.9 to +5.4), consistent with a long term depleted mantle source. The Fiskenaesset rocks yield a Pb-Pb isochron age of 2945 ± 36 Ma. The Pb isochron intercepts the average depleted mantle growth curve at 3036 Ma, suggesting a Mesoarchean depleted mantle source. The trace element systematics of the least-altered anorthosites, gabbros, leucogabbros, ultramafic rocks, and tholeiitic basalts is collectively consistent with a supra-subduction zone geodynamic setting. On the log-transformed tectonic discrimination diagram, including La/Th, Sm/Th, Yb/Th, and Nb/Th ratios, tholeiitic basalts display a trend projecting from mid-ocean ridge basalt (MORB) to island arc basalt (IAB) field. This trend is interpreted as reflecting a transition from the depleted Neoarchean upper mantle to a subarc mantle wedge following the initiation of intra-oceanic subduction and arc migration. Collectively, on the basis of field relationships and geochemical characteristics, we interpret the Fiskenæsset Complex as a fragment of a Neoarchean oceanic island arc.
- Nov 2009
The Mesoarchean Ivisaartoq greenstone belt consists of tholeiitic pillow basalts, picrites, boninites, gabbroic to dioritic dykes and sills, actinolite schists and serpentinites. In addition, the belt includes volumetrically minor siliceous volcaniclastic sedimentary rocks, cherts, biotite schists, and quartzitic gneisses. Siliceous volcaniclastic rocks yield an average U–Pb zircon age of 3075 ± 15 Ma, and pillow basalts and gabbros yield a Sm–Nd errorchron age of 3080 ± 160 Ma, constraining the maximum age of the belt. The Ivisaartoq rocks underwent high-temperature seafloor hydrothermal alteration, resulting in extensive epidote-diopside calc-silicate metasomatism. Th–REE–Nb systematics of the least metasomatized volcanic rocks are consistent with a supra-subduction zone geodynamic setting, displaying a trend projecting from mid-ocean ridge basalt (MORB) field to island arc basalt (IAB) field. This trend is interpreted as reflecting a transition from the depleted upper mantle to a subarc mantle wedge following the initiation of intra-oceanic subduction and arc migration.