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In-situ U–Pb dating of zircon coronas, Sr–Nd–Hf isotopes and petrological constraints on the constructional processes of the Daxigou anorthosite complex, NW China
Abstract
Massif-type anorthosites, mainly Proterozoic in age, have long been recognized as a signature rock type of crust-mantle interactions in the Proterozoic Eon. The Daxigou Anorthosite Complex (DAC) is one such massif situated in the Kuluketage block, a tectonically important domain between the Tarim Craton and the Central Asian Orogenic Belt. In this contribution, a combination of geochronological and geochemical analyses of fine-grained DAC diabase are used to constrain the characteristics of the parental magma and the initiation time of DAC magmatism. We then pinpoint the DAC’s closure time by in-situ petrographic thin section U–Pb dating of zircon coronas around Fe-Ti oxides identified in a gabbroic anorthosite. Within data uncertainty, we find that the closing age of 1813 ± 9 Ma, is indistinguishable from the 1804 ± 7 Ma crystallization age of its last episode of parental magma. MELTS simulations of the parental diabase magma suggest a pressure of 8 kbar at a fayalite-magnetite-quartz oxygen fugacity leads to broadly observed mineral compositions and mineral phases. The bulk rock major and trace elements, integrated with the Sr–Nd and Hf isotopes in zircon, indicate that the DAC was formed by partially melting of a metasomatized sub-continental lithospheric mantle, which was likely induced by a post-collisional slab break-off with minor crustal assimilation. Finally, we propose that DAC magmatism can be ascribed to a series of Paleoproterozoic tectono-thermal events along the northern and southern margins of the Tarim Craton, which could have provided important implications for the Tarim Craton and the North China Craton associated with the assembly of Columbia supercontinent.
... Morisset (2008) reached similar conclusions and proposed that most massive anorthosite and mafic rocks from the Grenville Province were generated from a less depleted mantle reservoir. Similarly, several authors have proposed a mantle source previously enriched by subduction processes in complexes such as Adirondack, Damiao, Daxigou, and Fournier Hegner et al., 2010;Teng and Santosh, 2015;Yuan et al., 2022). ...
... This Hf information could indicate the partial melting of both basement rocks or a mixture between the Mesoarchean rocks and mantle-derived melts. Yuan et al. (2022) supported the latter model based on Sr-Nd isotope data. In some cases, Sr isotopic information has played an important role in characterizing the source of MCG rocks. ...
... The linear pattern of the different anorthositic pulses in the Grenville Province could also be explained by processes of post-collisional slab detachment (e.g., Valentino et al., 2019;Groulier et al., 2020). The last tectonic scenario has been applied to the Daxigou, Damiao, and Jugsaipatna complexes (Dharma Rao et al., 2014;Teng and Santosh, 2015;Yuan et al., 2022). More recently, Bybee et al. (2014) proposed that the anorthosite exposed in the Mealy Mountains, Nain, and Rogaland Complexes were formed in long-lasting Andean-type tectonic environments. ...
The occurrence of massif-type anorthosite intrusions is a widespread Proterozoic phenomenon. They are usually associated with gabbroic, charnockitic, and granitic rocks, comprising the so-called anorthosite-mangerite-charnockite-granite (AMCG) suite. Although these rocks have been extensively studied worldwide, several aspects concerning their formation remain unsettled. Among them, the magma source and the tectonic setting are the most important. To evaluate these issues, we first compiled geochemical and isotopic data of Proterozoic anorthosite massifs and AMCG suites worldwide and stored it in a database named datAMCG. This plethora of data allows us to make some important interpretations. We argue that the wide-ranging multi-isotopic composition of this group of rocks reflects varying proportions of juvenile mantle-derived melts and crustal components. We interpret that the precursor magmas of most massive anorthosite bodies and associated mafic rocks have a mantle-dominated origin. However, we highlight that a crustal component is indispensable to generate these lithologies. Adding variable amounts of this material during succeeding multi-stage assimilation-fractional crystallization (AFC) processes gives these intrusions their typical mantle-crustal hybrid isotopic traits. In contrast, a crustal-dominant origin with a complementary mantle component is interpreted for most MCG rocks. In summary, the isotopic information in datAMCG indicates that both sources are necessary to generate AMCG rocks. Therefore, we suggest that hybridized magmas with different mantle-crust proportions originate these rocks. This interpretation might offer a more nuanced and accurate depiction of this phenomenon in future work instead of choosing a single-sourced model as in the past decades. Finally, tectonomagmatic diagrams suggest that the rocks under study were likely generated in a tectonic environment that transitioned between collision and post-collisional extension, sometimes involving subduction-modified mantle sources. This interpretation is supported by geological and geochronological information from most complexes, thus challenging the Andean-type margins as an ideal tectonic setting.
The concept that lithosphere detachment or break-off has long been conceived as a viable mechanism to explain prominent geological phenomena in Earth’s crust and the surface. One of the strengths of slab delamination mechanism is that it can account for the extensive magmatism in active orogenic belts due to the upwelling of the asthenosphere after the slab break-off. However, in the last 20 years, geodynamic simulations show that the inflow of the asthenosphere upon slab break-off is insufficient to cause significant melting of the overriding lithosphere adjacent to the slab window. The primary reasons include the occurrence of slab break-off at a location that is too deep to effectively heat the overriding lithospheric mantle. Another factor is the presence of a thin film of crustal material that is retained during the slab break- off, inhibiting a significant thermal perturbation within the lithosphere. In this work, we couple petrological–thermomechanical simulations with magmatic melting processes to examine the lithospheric melting and surface lithological expression associated with slab break-off. Our work shows that in the early Earth when the mantle temperature is relatively higher, shallow slab break-off can give rise to significant lithospheric melting during the development of slab break-off. Moreover, because the slab becomes weaker in the earlier hotter mantle, it may break-off prior to the stage of continental collision, thus the magmatism it induced may not give a direct constraint on the time of continental collision. Our study has implications for the interpretation of geological and tomography studies in orogenic belts. It also provides insights into reconciling conflicts between geodynamic and geological studies regarding slab break off-induced melting and magmatism.
The Qieganbulake deposit associated with a mafic–ultramafic–carbonatite complex in the Kuluketage block is not only the world’s second-largest vermiculite deposit, but also a medium-size carbonatite-related phosphate deposit. Field observations, radiometric dating results and Sr–Nd–Hf isotopes reveal that the parental magmas of the carbonatite and mafic–ultramafic rocks are cogenetic and formed synchronously at c . 810 Ma. Geochemical characteristics and Sr–Nd–Hf–S isotopes (( ⁸⁷ Sr/ ⁸⁶ Sr) i = 0.70581–0.70710; ϵ Nd (t) = −0.20 to −11.80; ϵ Hf (t) = −7.5 to −10.3; δ ³⁴ S = +0.7 ‰ to +3.0 ‰ (some sulfides with high δ ³⁴ S values (+3.2 to +6.6) were formed by late hydrothermal sulfur)), in combination with mineral compositions and previous research, strongly indicate that the Qieganbulake mafic–ultramafic–carbonatite complex formed via extensive crystal fractionation/cumulation and liquid immiscibility of a carbonated tholeiitic magma, possibly derived from partial melting of an enriched subcontinental lithospheric mantle previously modified by slab-released fluids and sediment input in a continental rift setting. The coupled enriched Sr–Nd isotopic signatures, in combination with previous research, suggest that the enriched subcontinental lithospheric mantle could have been metasomatized by asthenospheric mantle melts to different degrees. The Qieganbulake carbonatite-related phosphate ores were the products of normal fractional crystallization/cumulation of P–Fe ³⁺ complex enriched carbonatite magma in high oxygen fugacity conditions, which was generated by liquid immiscibility of CO 2 –Fe–Ti–P-rich residual magma undergoing high differentiation.
The Kawuliuke Fe-P-Ti oxide-rich intrusive complex is one of the largest layered mafic–ultramafic complexes associated with Fe-P-(Ti) oxide deposits in the Kuluketage Block, northeastern Tarim Craton, NW China. Here, based on new field observations, whole-rock trace element, radiometric dating analyses as well as Sr-Nd-Hf isotopes we propose a non-cogenetic origin of the parental magmas of the mafic–ultramafic rocks and syenite in the Kawuliuke intrusive complex (KIC), both of which formed synchronously at circa 808 Ma.
Geochemical characteristics and Sr-Nd-Hf-S isotopes ((⁸⁷Sr/⁸⁶Sr)i = 0.70437–0.70560; εNd(t) = -2.60 to −7.15; εHf(t) = -1.3 to −10.0; δ³⁴S = -2.98 ‰ to + 2.36 ‰), in combination with minerals composition and previous research, strongly indicate that the mafic–ultramafic rocks of the KIC formed via extensive crystal fractionation/cumulation of a tholeiitic magma, possibly derived from partial melting of an enriched subcontinental lithospheric mantle previously modified by slab-released fluids in a continental rift setting. The Kawuliuke syenite, meanwhile, is interpreted as generated by the emplacement of syenitic melts from the differentiation of newly coeval underplating basaltic magmas at depth based on lines of evidence from petrography, geochemical signatures and Sr-Nd-Hf isotopes. Our newly presented enriched Sr-Nd-Hf isotopic signatures, together with previous research, suggest that the enriched subcontinental lithospheric mantle could be metasomatized by asthenosphere mantle melts in different degrees. We further show the Kawuliuke clinopyroxenite-related apatite-rich oxide ores were the products of normal fractional crystallization/cumulation of H2O-Fe-P-Ti enriched residual magma in high oxygen fugacity condition, which experienced high differentiation. The tempo-spatial relationships of the KIC and other regional coeval mafic rocks jointly suggest that the KIC was most likely formed in response to the proposed mid-Neoproterozoic mantle plume in Tarim Craton, which likely induced partial malting of, and likely mixed with, the metasomatized subcontinental lithospheric mantle, to form the KIC with clinopyroxenite-related Fe-P-Ti oxide mineralization.
The predominant types of high-grade iron deposits in China include skarn, sedimentary metamorphic (banded iron-formation, BIF-type), continental/submarine volcanic-hosted and magmatic Fe-Ti-V oxide deposits. Based on a comprehensive review of current studies on these deposits, this paper suggests that the oxygen concentration in atmosphere played an important role for the formation of BIFs, whereas the tectonic setting and deep magmatic differentiation processes are more important for the other types. Notably, both high temperature and high pressure experiments and melt inclusion studies indicate that during the differentiation, high temperature magmas could develop iron-rich magma via liquid immiscibility but not pure oxide melt (“iron ore magma”). Fe-P melt could be generated directly by liquid immiscibility under hydrous and oxidized condition. The formation of high-grade iron deposits is mostly associated with the processes related to multiple stages of superimposition, e.g., desiliconization and iron enrichment, removal of impurity, and remobilization and re-precipitation of iron. According to the temporal evolution, the high-grade iron deposit could be divided into multi-episode superimposition type (temporally discontinuous mineralization) and multi-stage superimposition type (temporally continuous mineralization). The former is represented by the sedimentary metamorphic iron deposit, and the latter includes those related to magmatic-hydrothermal fluids (e.g., skarn, volcanic-hosted and magmatic types).
Fe–Ti–P-rich mafic to intermediate rocks (monzodiorites and oxide–apatite–gabbronorites, OAGNs) are found as small intrusions in most AMCG (anorthosite–magnerite–charnokite–granite) suites. The origin of the monzodioritic rocks is still debated, but in many studies, they are presumed to represent residual liquid compositions after fractionation of anorthositic cumulates. In the 1.64 Ga Ahvenisto complex, SE Finland, monzodioritic rocks occur as minor dike-like lenses closely associated with anorthositic rocks. We report new field, petrographic, and geochemical (XRF, ICP-MS, EMPA) data complemented with crystallization modeling (rhyolite-MELTS, MAGFRAC) for the monzodioritic rocks, apatite–oxide–gabbronorite, and olivine-bearing anorthositic rocks of the Ahvenisto complex. The presented evidence suggest that the monzodioritic rocks closely represent melt compositions while the apatite–oxide–gabbronorite and olivine-bearing anorthositic rocks are cumulates. The monzodioritic rocks seem to form a liquid line of descent (LLD) from primitive olivine monzodiorites to more evolved monzodiorites. Petrological modeling suggests that the interpreted LLD closely corresponds to a residual melt trend left after fractional crystallization (FC) and formation of the cumulate anorthositic rocks and minor apatite–oxide–gabbronorite in shallow magma chambers. Consequent equilibrium crystallization (EC) of separate monzodioritic residual magma batches can produce the observed mineral assemblages and the low Mg numbers measured from olivine (Fo25–45) and pyroxenes (En48–63, Mg#cpx 60–69). The monzodioritic rocks and apatite–oxide–gabbronorites show similar petrological and geochemical characteristics to corresponding rock types in other AMCG suites, and the model described in this study could be applicable to them as well.
The occurrence of high-aluminum orthopyroxene megacrysts (HAOMs) in several massif-type Proterozoic anorthosite complexes has been used as evidence of their polybaric crystallization. Here, we report such petrographic and geochemical (XRF and EMPA) evidence from HAOMs discovered in the 1.64 Ga Ahvenisto rapakivi granite—massif-type anorthosite complex in southeastern Finland. Two different types of HAOMs were recognized: type 1 HAOMs are individual, euhedral-to-subhedral crystals, and up to 15 cm in diameter, and type 2 HAOMs occur in pegmatitic pockets closely associated with megacrystic (up to 30 cm long) plagioclase. The type 1 megacrysts in particular are surrounded by complex corona structures composed of plagioclase, low-Al orthopyroxene, iddingsite (after olivine), and sulfides. Orthopyroxene crystallization pressure estimates based on an Al-in-Opx geobarometer reveal a three-stage compositional evolution in both textural HAOM types. The Al content decreases significantly from the core regions of the HAOM (4.4–7.6 wt% Al2O3), through the rims (1.3–3.6 wt%), into the host rock (0.5–1.5 wt%). Enstatite compositions overlap, but are generally higher in the cores (En~60–70) and rims (En~50–70) of the HAOMs than in the host rock (En~45–60) orthopyroxenes. The highest recorded Al abundances in the HAOM cores correspond to crystallization pressures of up to ~ 1.1 GPa (~ 34 km depth), whereas the HAOM rims have crystallized at lower pressures (max. ~ 0.5 GPa, 20 km depth). The highest pressure estimates for the host rock orthopyroxene were ~ 0.2 GPa (< 7 km depth). These observations confirm the polybaric magmatic evolution of the Ahvenisto anorthosites and suggest that the entire 1.65–1.55 Ga Fennoscandian rapakivi suite was emplaced at a relatively shallow level (< 7 km depth) in the upper crust. Global comparison to similar rock types reveals remarkable similarities in the petrogenetic processes controlling HAOM composition and evolution of anorthosite parental magmas.
During the past 50 years, many geological and ore-deposit investigations have led to the discovery of the Fe–P–(Ti)-oxide deposits associated with mafic–ultramafic–carbonatite complexes in the Kuluketage block, northeastern Tarim Craton. In this paper, we discuss the genetic and ore-forming ages, tectonic setting, and the genesis of these deposits (Kawuliuke, Qieganbulake and Duosike). LA-ICP-MS zircon U–Pb dating yielded a weighted mean ²⁰⁶Pb/²³⁸U ages of 811 ± 5 Ma, 811 ± 4 Ma, and 840 ± 5 Ma for Kawuliuke ore-bearing pyroxenite, Qieganbulake gabbro and Duosike ore-bearing pyroxenite, respectively. The CL images of the Kawuliuke apatite grains show core–rim structure, suggesting multi-phase crystallisation, whereas the apatite grains from Qieganbulake and Dusike deposits do not show any core–rim texture, suggesting a single-stage crystallisation. LA-ICP-MS apatite ²⁰⁷Pb-corrected U–Pb dating provided weighted mean ²⁰⁶Pb/²³⁸U ages of 814 ± 21 Ma and 771 ± 8 Ma for the Kawuliuke ores, and 810 ± 7 Ma and 841 ± 7 Ma for Qieganbulake and Duosike ores, respectively. The core–rim texture in apatite by CL imaging as well as two different ore-forming ages in the core and rim of the apatite indicate two metallogenic events for the Kawuliuke deposit. The first metallogenic period was magmatic in origin, and the second period was hydrothermal in origin. The initial ore-forming age of the Kawuliuke Fe–P–Ti mineralisation was ca 814 Ma and the second one was ca 771 Ma. On the other hand, the ore-forming ages of the Qieganbulake and Duosike deposits were ca 810 Ma and ca 841 Ma, respectively. Qieganbulake and Duosike deposits were of magmatic origin. Combined with previous geochronological data and the research on the tectonic background, we infer that the Kawuliuke, Qieganbulake and Duosike Fe–P–(Ti)-oxide deposits were formed in a subduction-related tectonic setting and were the product of subduction-related magmatism.
This paper reviews the basic principles of radiometric geochronology as implemented in a new software package called IsoplotR, which was designed to be free, flexible and future-proof. IsoplotR is free because it is written in non-proprietary languages (R, Javascript and HTML) and is released under the GPL license. The program is flexible because its graphical user interface (GUI) is separated from the command line functionality, and because its code is completely open for inspection and modification. To increase future-proofness, the software is built on free and platform-independent foundations that adhere to international standards, have existed for several decades, and continue to grow in popularity. IsoplotR currently includes functions for U-Pb, Pb-Pb, ⁴⁰Ar/³⁹Ar, Rb-Sr, Sm-Nd, Lu-Hf, Re-Os, U-Th-He, fission track and U-series disequilibrium dating. It implements isochron regression in two and three dimensions, visualises multi-aliquot datasets as cumulative age distributions, kernel density estimates and radial plots, and calculates weighted mean ages using a modified Chauvenet outlier detection criterion that accounts for the analytical uncertainties in heteroscedastic datasets. Overdispersion of geochronological data with respect to these analytical uncertainties can be attributed to either a proportional underestimation of the analytical uncertainties, or to an additive geological scatter term. IsoplotR keeps track of error correlations of the isotopic ratio measurements within aliquots of the same samples. It uses a statistical framework that will allow it to handle error correlations between aliquots in the future. Other ongoing developments include the implementation of alternative user interfaces and the integration of IsoplotR with other data reduction software.
Cumulate rocks of the Upper Main Zone and Upper Zone (UUMZ) of the Bushveld Complex South Africa, contain the world’s major resources of Fe-Ti-V±P, hosted in Ti-magnetite and apatite, and are commonly considered as having crystallized from the last major injection of magma into the magma chamber. In this study, we present the petrography, modal proportions, whole-rock major element chemistry (260 samples), electron microprobe data (∼ca.10,000 individual analyses for plagioclase, olivine, and pyroxene), and compiled analyses of Cr in magnetite (239 samples) for the UUMZ sampled over 2.1 km of the Bierkraal drill cores in the western limb of the Complex. The UUMZ section exhibits a broad normal fractionation trend upwards, but a series of reversals to more primitive anorthite contents in plagioclase, Mg# in pyroxenes and olivine, Cr in whole-rocks and Cr in magnetite separates are observed, accompanied by the appearance or disappearance of various minerals. Anorthosite or leucogabbro layers are closely linked to these reversals; the reversals in An % of plagioclase are used as boundaries to divide the UUMZ into 18 cycles. These cycles are interpreted as indications of magma chamber replenishment by plagioclase-laden magmas (up to 20 vol % plagioclase) and are also marked by spikes in Cr content. In addition, abundant Fe-Ti oxide-bearing plagioclase-rich rocks are identified in the lower half of the UUMZ. These have crystallized from a hybrid melt produced by the mixing of a new plagioclase-bearing magma batch and the resident magma. Further crystallization of this hybrid liquid may lead to the formation of magnetite layers in the lower part of the UUMZ. The Bushveld UUMZ therefore grew by multiple emplacements of crystal-laden magmas coming from deep-seated chambers. Slow cooling in a shallow chamber explains the systematic bottom-up compositional evolution in the cumulate pile within individual cycles. The residual melt reached silicate liquid immiscibility soon after the saturation of apatite. Thereafter, segregation of conjugate Fe-rich and Si-rich melts and crystallization of the paired melts produces cumulates with a smooth upward decrease in Fe-Ti oxides, while plagioclase mode increases in each apatite-bearing cycle. A comparison of systematic geochemical analyses and a detailed lithological stratigraphy between the different Bushveld limbs demonstrates the possible connectivity between the western and eastern Upper Zone but indicates notable differences with the Bellevue section of the northern limb.
Ascertaining the enigmatic characteristics of the crystalline basement of the Qaidam basin, the deepest intracontinental basin yet located in the highest orogenic plateau with the thickest continental crust, has broad implications for the Cenozoic basin evolution, in particular, and intracontinental tectonics of Asia, in general. We carried out a comprehensive investigation on the ages and geochemical-isotopic characteristics of the Qaidam and Suganhu basement rocks obtained from drill-cores. Petrological observations and U-Pb zircon dating (LA-ICP-MS) of granitoid samples from drill cores indicate that Early Neoproterozoic, Early Paleozoic and Late Paleozoic-Mesozoic intrusions are largely represented, suggesting that the Qaidam and Suganhu blocks were involved into three tectono-magmatic episodes. Similar whole-rock geochemistry and Sr–Nd isotopic characteristics between the Early Neoproterozoic granitoids beneath the Qaidam basin and the contemporaneous basement rocks in the surrounding orogenic belts confirm the existence of pervasive Early Neoproterozoic metamorphism and magmatism along the present-day northern Tibetan Plateau. By comparing the trace and rare earth elements patterns as well as the Sr-Nd compositions of the Ordovician arc-related granitoids and Silurian-Devonian post-collisional granitoids beneath the Qaidam and Suganhu basins with the corresponding granitoids from the surrounding orogenic belts, we assert that the Proto-Tethys subduction events that were recorded within the Eastern Kunlun, Altyn Tagh and the Qilian Shan orogenic belts, were different. They generated distinctive subduction-related arc magmas, whereas the closure of the oceanic basins resulted in the amalgamation of the orogenic belts inducing subsequent extensive post-collisional magmatism within the Qaidam block. The widespread Permian and Triassic granitoids beneath the northwestern/northern Qaidam basin mainly involved a lower crust or a crust-mantle source including Mesoproterozoic crust. They demonstrate that the influence of the Paleo-Tethys evolution penetrated the Qaidam block northwards, reaching to the northern edges of the block. We conclude that the Qaidam block is not a craton as typical as the rigid Tarim block, but pertains to a tectono-magmatic rejuvenated craton, which is not mechanically-stronger than its surrounding regions. However, trapped between the lithospheric left lateral strike-slip Altyn Tagh fault system to the northwest and the Qilian Shan transpressional system to the northeast, the Qaidam block forms a relatively stable domain that escaped from significant Cenozoic deformation.
Models for the nelsonite formation are currently highly contentious, with liquid immiscibility and fractional crystallization as frequently proposed formation mechanisms. The nelsonites in the Damiao massif anorthosite complex in the North China Craton are revisited here together with experimental evidence for the existence of silica-free CaO-FeO-Fe2O3-TiO2-P2O5 immiscible nelsonitic liquids. Our results of differential scanning calorimetry (DSC) and internally heated pressure vessel (IHPV) demonstrate that nelsonite with the composition of one-third apatite and two-thirds Fe-Ti oxides by weight 1) completely melts well above 1450 °C at dry condition, which is in good agreement with numerous experimental studies of the CaO-P2O5-FexO system for metallurgical purposes; 2) does not melt at the temperature up to 1200 °C with presence of considerable amount of volatiles, e.g., fluorine and water. Therefore, the composition of the nelsonite cannot be molten at temperatures relevant for crystallization of the Damiao magma. A review of experimental studies of liquid immiscibility and analyses of natural immiscible glasses show that all the liquids on the Fe- and P-rich side of the miscibility gap have at least 20 wt.% of aluminosilicate components.
We use the Hf isotope composition of zircon from the Bushveld Complex to better understand the source of its parent magmas. The data set, which consists of 141 individual LA-ICP-MS analyses from 11 samples encompassing the entire cumulate stratigraphy, shows that the parent magmas had a Hf isotope composition unlike that of the depleted mantle at 2.06 G a. Specifically, sample average epsilon(Hf(present)) values range from -55.3 to -52.5 (epsilon(Hf(2.06 Ga)) = -9.0 to -6.8) and are surprisingly homogeneous. This homogeneity is difficult to reconcile with direct assimilation of crustal material by Bushveld parent magmas because it would require that each batch of magma had assimilated just the right amount of material to all acquire the same Hf isotopic composition. Also, calculations suggest that simple mixing of regional crust into a primitive, mantle-derived liquid cannot account for both the presumed Hf and major elemental concentrations and the Hf-176/Hf-177 ratio of the Bushveld magmas. Rather, the Hf data are consistent with generation of these magmas by partial melting in a sub-continental mantle lithospheric source with an unradiogenic Hf isotopic composition equal to that of the Bushveld parent magmas. Several possibilities for the development of such a source are explored using the new Hf isotope data.
Recent zircon-based geochronological investigations in the Adirondack Mountains, in the state of New York, demonstrate that 1155 +/- 6 Ma massif anorthosite was emplaced during the post-contractional phase (1160-1140 Ma) of the ca. 1200-1140 Ma Shawinigan orogeny. Emplacement of many other anorthosite - mangerite - charnockite - granite (AMCG) suites also correlates with the waning stage of orogeny and includes the ca. 1155 Ma Morin and Lac-Saint-Jean complexes, the ca. 1650 Ma Mealy Mountain complex, and the ca. 1450 Ma complexes of eastern Labrador. The correlation also applies to the ca. 930 Ma Rogaland anorthosite complex of Norway and the ca. 1060-1020 Ma late-to post-Ottawan anorthosites of central Quebec and the Appalachians of Virginia and southeastern Pennsylvania. These correlations suggest models involving delamination of overthickened orogenic lithosphere by foundering or convective removal, followed by asthenospheric ascent, and ponding of gabbroic melt at the crust-mantle interface. Orogen rebound following delamination results in stable, dynamically balanced settings in which gabbroic magma evolves slowly at high pressure to produce high-aluminum pyroxene and coarse, intermediate plagioclase characteristic of massif anorthosite. Related melting of the lower crust produces mangeritic and charnockitic magma. Ultimately, both anorthositic and granitic magmas ascend, and lower-pressure fractionation yields the classic AMCG suites. Transtensional reactivation of lithospheric-scale shear zones and old sutures also correlates with important AMCG magmatism and provides conduits for gabbroic magma that ponds at the crust-mantle interface or in the deep crust to produce AMCG suites. Flat-slab subduction, back-arc extension, slab breakoff, and hotspots represent alternative settings that can account for gabbroic underplating and fractionation resulting in AMCG suites, if consistent with geochronological constraints.
A comprehensive database of zircon composition in West Australian magmatic rocks reveals negative correlations between both U and Th zircon/whole rock ratio and the zircon saturation temperature, with the observed change with temperature less for U(zircon/whole rock) than for Th(zircon/whole rock). This observation implies a systematic increase in the zircon/rock ratio with falling crystallization temperature, a result which replicates findings from experimental partition coefficient studies. Under equilibrium conditions there is a trend to lower zircon Th/U with increasing melt temperature which can be attributed to lattice strain. However, within a fractionating magma, Ti-in-zircon temperatures yield the opposite relationship of lower zircon Th/U in cooler melts. This is due to zircon growth under non-equilibrium conditions with greater incompatibility of Th relative to U, and the removal and segregation of mineral precipitates. These observations can be used as a tool to determine whether zircon growth was in a liquid of similar composition to the observed whole rock. We present an equation that estimates the degree of fractionation between the whole rock composition and the zircon parental liquid. This parameter demonstrates the dissimilarity between the liquid from which the zircon grew and the whole rock composition and aids in distinguishing mesostasis growth in fractionated melt versus cumulate growth in less fractionated magma. We use this ratio to investigate zircon growth in igneous rocks of the Musgrave Province. For a suite of c. 1200 Ma magmas that become progressively more fractionated, based on whole rock La/Sm, the fractionation index demonstrates increasing compositional differences between the whole rock and the zircon growth liquid. In the most extreme case independent petrographic evidence indicates mesostasis growth of zircon, whereas in the least fractionated melt zircon growth is established to be close to equilibrium with a zircon saturation temperature of c. 900°c likely being accurate. In contrast, zircon crystals from a rhyolite of the c. 1070 Ma Giles Supervolcano have distinctive compositional discordances indicative of antecrystic components. The fractionation factor in this rock implies some zircon growth under higher temperature conditions than the whole rock zircon saturation temperature.
The Damiao type iron deposit is hosted in a typical Proterozoic anorthosite complex in the northern North China Craton. The types of ores in Damiao mainly comprise massive Fe ores, massive Fe–P ores, and disseminated Fe and Fe–P ores. The disseminated Fe and Fe–P ores formed by fractional crystallization are generally hosted in oxide-apatite gabbronorite and account for 70% of the proven reserve of the Damiao type iron ore. The massive Fe and Fe–P ores account for 30% of the proven reserve of the Damiao type deposit iron ore and generally occur as irregular dykes or veins filling vertical fractures of the previously consolidated anorthosite, showing typical features of hydrothermal mineralization. The contact between the massive orebodies and wall rocks is sharp and straight. The anorthosite comprises white and dark varieties, with the former resulted by the alteration of the latter that occurs as relicts. Petrographic observation and electron microprobe analyses show abundant Fe–Ti oxide inclusions in plagioclase which impart the dark color to the rock. The similar spider diagram patterns between fresh and altered plagioclase and between dark- and white-colored anorthosite imply a genetic relationship between the dark and white types. During the alteration of anorthosite, CaO and MgO were slightly decreased, the SiO2, Al2O3 and Na2O were significantly increased, and the TFe2O3 and TiO2 were significantly decreased. The TFe2O3 and TiO2 in the dark-colored anorthosite have a range of 4.86–12.18 wt.% and 0.37–1.65 wt.%, respectively. However, The TFe2O3 and TiO2 in the white-colored anorthosite have a range of 1.67–3.1 wt.% and 0.14–0.31 wt.%, respectively. These features suggest that the alteration of the anorthosite led the Fe element by leaching from the dark-colored anorthosite at highly oxidized condition, and then precipitated within the fractures of the anorthosite, thus forming the massive Fe and Fe–P orebodies. Because the estimated amount of transported Fe is much more abundant than the proven ore reserve, we infer that there should be huge potential for prospecting Damiao type iron ores.
A boring billion for mountains
Earth's crust has changed over time as supercontinents formed and broke apart. Tied into this cycle are the building and erosion of high mountains, which are tied to collisions between tectonic plates. Tang et al. use europium anomalies in zircons to estimate the mean thickness of crust over Earth's history. This proxy shows that mountain building has not always been as active as it is today or as it was very early in Earth's history. Mountain building, and the subsequent erosion, was less intense for about a billion years, roughly correlated with a so-called “boring billion” period of biological evolution.
Science , this issue p. 728
We present a new machine learning method for calculating biotite (sensu lato) structural formula from electron microprobe analysis (EMPA) data, which is based on principal components regression (PCR) of a dataset consisting of 155 fully analyzed biotite references that have chemistry and crystal structure refinement. The dataset is randomly grouped into a training set (75% in amount) and a test set (25% in amount). The training set is used to implement the structural formula and the test set is used to evaluate the performance of the model. The resulting linear regression coefficient matrix is then applied to calculate mole proportions of cations and anions of biotite samples using their compositional data from EMPA. Through this method, the distribution of the different cations and anions in the different sites can be calculated, including the tetrahedral Fe3+, octahedral Fe2+, octahedral Fe3+, OH and WO2- at the O(4) site. The O(4) site is assumed to be occupied by anions with a relation of 2 = F + Cl + OH + WO2-. Octahedral and interlayer vacancies could also be estimated in this model. The prediction quality for major elements is perfect with R2 >0.95. The absolute errors in the estimated octahedral Fe2+, octahedral Al and OH at O(4) site are determined to be ±0.2 apfu (atom per formula unit based on 11O + 2(F, Cl, OH, O)), while those in total Fe3+ and WO2- at O(4) site are approximately ±0.3 apfu. A funnel-shaped relationship between absolute error in Fe3+/ΣFe ratio and FeOT wt% is observed, with the majority falling in the range of ±20%. Compared to previous normalization schemes, our model shows significant improvements in estimating Fe3+/ΣFe and WO2- at O(4) site. Our model is capable for calculating mineral formulae of common igneous and hydrothermal biotites, but not suitable for those that have been modified in a post-formation oxidation or reduction process. A supplementary Excel spreadsheet is provided that can be easily used for performing calculation from EMPA data.
Proterozoic massif-type anorthosites are voluminous, plagioclase-dominated plutonic suites with characteristic intermediate compositions (An50±10) that represent mantle-derived magmas ponded at Moho depths and crystallized polybarically until emplacement at mid-crustal levels. We present new ID-TIMS and LA-ICPMS U–Pb ages, as well as detailed field observations from the Kunene Anorthosite Complex (KAC) of SW Angola. Our main objective in this work is to contextualize a variety of important petrographic and geochronological features of the KAC that have implications for Proterozoic anorthosite petrogenesis and the geological evolution of the SW Congo Craton. High-precision U–Pb zircon and baddeleyite ages for anorthositic rocks from across the complex (n = 11) range between 1376 and 1438 Ma—a period of ~60 Myr—showing that such systems may be remarkably long-lived. Dating of magmatic zircons from a comagmatic pegmatoidal enclave in the KAC, which also contains high-Al orthopyroxene megacrysts (HAOM), gives an age of ~1500 Ma, indicating as much as 120 Myr between some of the earliest and latest crystallization products of the system. These enclaves provide a snapshot of the liquid line of descent of the anorthosites and are compelling evidence for the comagmatic nature of the HAOM. Field evidence for block structure, magma mingling, and extreme textural heterogeneity in the KAC supports emplacement as multiple influxes of magma, of various volumes, carrying variable proportions of crystals and autoliths from below. We infer that the KAC represents a long-lived mushy magmatic system that was likely emplaced in distinct pulses. These observations refine our understanding of Proterozoic anorthosite petrogenesis. They also provide evidence for an array of magmatic processes operating at various levels of crystallinity that may create textural and chemical heterogeneity in evolving magma products at lower- to mid-crustal levels.
The Th/U ratios of zircon crystals are routinely used to help understand their growth mechanism. Despite the wide application of Th/U ratios in understanding the geological significance of zircon U–Pb ages, the main controls on the Th/U ratio in metamorphic zircon are poorly understood. Here, phase equilibria modelling coupled with solubility expressions for accessory minerals are used to investigate the controls on the Th/U ratios of suprasolidus metamorphic zircon in an average amphibolite‐facies metapelite composition. We also present a new database of metamorphic Th/U ratios in zircon from Western Australia. Several factors affecting the Th/U ratio are investigated, including the bulk rock concentrations of Th and U, the amount of monazite and apatite in the system, and open versus closed system behaviour. Our modelling predicts that the main controls on the Th/U ratio of suprasolidus metamorphic zircon are the concentrations of Th and U in the system and the breakdown and growth of monazite in equilibrium with zircon. Furthermore, the relative timing of zircon and monazite growth during cooling and melt crystallization has an important role in the Th/U ratio of zircon. Early grown zircon near the peak of metamorphism is expected to have elevated Th/U ratios whereas zircon that grew near the solidus is predicted to have relatively low Th/U ratios, which reflects the coeval growth of monazite during cooling and melt crystallization. Our modelling approach aims to provide an improved understanding of the main controls of Th/U in metamorphic zircon in migmatites and hence better apply this geochemical ratio as a tool to assist in interpretation of the genesis of metamorphic zircon.
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Types of rock, such as komatiite, which formed entirely or dominantly during restricted time periods in solar system history, are important indicators of how planetary bodies evolved in deep time. In this paper we characterize three different types of temporally-restricted anorthosites, and discuss their significance to the broad-scale evolution of planetary processes. Primordial anorthosites, which constitute the bulk of the lunar crust, were sampled during Apollo and Luna missions, and have subsequently been identified in our meteorite collections. They are characterized by very calcic plagioclase (An93-98), and all have magmatic crystallization ages >4.3Ga, suggesting that the earliest planetary crust-forming processes on the Moon, and possibly elsewhere, involved substantial, if not total melting. The "magma ocean" hypothesis, which has endured for nearly 50years, argues that the lunar anorthositic rocks represent global flotation cumulates of plagioclase, which crystallized after extensive precipitation and sinking of olivine and pyroxene. Another possibility, revitalized by critical filtering of age data, involves "serial magmatism", whereby the lunar crust was constructed by younger, smaller, episodic magmatic events. On Earth, two distinct types of temporally-restricted anorthosites offer opportunities to understand the evolution of terrestrial geodynamics, tectonics and magmatic processes. Archean megacrystic anorthosites formed only between 3.73 and 2.49Ga as small bodies (<500km²) associated with the mafic magmatic assemblages of many greenstone belts. They are typified by accumulations of equidimensional, almost spherical megacrysts (1-30cm across) of calcic plagioclase (An61-94, avg. An80) in a mafic groundmass of broadly basaltic composition, and probably formed in shallow magma chambers. The unusually calcic plagioclase compositions may reflect crystallization from hydrous and/or Ca-rich basaltic parents that were fractionated derivatives of more primitive magmas, possibly even komatiites. Isotopic and geochemical data for many Archean anorthosite complexes indicate little to no evidence for contamination by silicic continental crust, suggesting magma formation in oceanic environments, most likely by subduction in oceanic arcs. Their temporal restriction might be explicable in terms of a hotter or wetter Archean mantle, especially if a genetic connection to komatiites can be demonstrated. Alternatively, higher Archean heat flow may have influenced oceanic crustal thickness, subduction rate, sites of magma generation and/or melt composition, producing favorable conditions for the generation of calcic megacrystic anorthosites. Proterozoic massif-type anorthosites are distinctly different from other anorthosite types, and occur as small plutons (1-10km²), to large (up to 18,000km²) composite batholiths, that are temporally restricted to a ~2000m.y. period between ~0.5 and ~2.6Ga. The rocks are dominated by lath-shaped, intermediate plagioclase (An30-70, avg. An53), with lesser pyroxenes and/or olivine. There is extensive evidence from petrology, geochemistry and isotope data for variable contamination with continental crust, and the most plausible physical model involves deep-crustal ponding of basaltic magmas, crystallization and sinking of mafic silicates, and eventual flotation of buoyant plagioclase. Massifs were constructed by the coalescence of plagioclase-rich mushes, which ascended to mid-crustal emplacement sites. The basaltic parent melts or magmas for massif-type anorthosites were mantle-derived; alternative models involving melting of crustal sources, mafic or otherwise, cannot be supported. Collective features most readily support an Andean-type continental arc setting, and careful geochronology is beginning to suggest that magmatism in individual massifs may have occurred over long time-scales of up to 100m.y. The onset of massif-type anorthosite formation at ~2.6Ga probably reflects global secular cooling of the Earth, resulting in increased lithospheric strength and crustal thickness, which promoted Moho-depth ponding and slow crystallization of basaltic magmas in continental arc environments. The apparent disappearance of anorthosite massifs at ~0.5Ga may also relate to global secular cooling that changed the thermal structure of subduction zones, and promoted the onset of high and ultra-high pressure metamorphism.
The Bushveld Complex in South Africa is the largest layered intrusion on Earth. Its upper part is known for huge resources of iron, titanium, vanadium and phosphorus. Associated with the layered character of the rocks, these resources are enriched at certain levels of the intrusion, which makes it important to understand the formation processes of those layers. In this paper we give an introduction and overview of recent debates and challenges.
The Tarim Craton is an important tectonic unit and a suitable target to investigate and understand the Proterozoic tectonic framework of the Central Asian Orogenic Belt and supercontinent Rodinia. Precambrian ultramafic–mafic-carbonatite rocks are widely distributed in the Quruqtagh domain of NE-Tarim. In the Quruqtagh, Cu-Ni, Fe-P and apatite-vermiculite deposits hosted in the ultramafic-mafic-carbonatite rocks occur in the Xingdi, Qieganbulake, Daxigou, and Kawuliuke. These deposits associated with the ultramafic-mafic-carbonatite rocks in Quruqtagh formed in a period between the Paleoproterozoic and Neoproterozoic. The Paleoproterozoic Fe-P deposit is represented by the Daxigou deposit that yielded a SIMS U-Pb zircon age of 2452 ± 10 Ma. The other Cu-Ni, Fe-P and apatite-vermiculite deposits, hosted in the ultramafic–mafic rocks, formed in the Neoproterozoic, represented by the Xingdi, Kawuliuke, and Qieganbulake deposits that formed between 812 Ma and 707 Ma. The Paleoproterozoic mineralization event was coincident with the global collisional events that led to the assembly of the Paleo-Mesoproterozoic Columbia (Nuna) supercontinent. And the emplacement of these Neoproterozoic ultramafic-mafic-carbonatite rocks was related to a mantle plume event that led to the breakup of the Tarim Craton from the Rodinia supercontinent.
Marking the northern boundary of the Tibetan plateau, the Altyn Tagh fault plays a crucial role in accommodatingthe Cenozoic crustal deformation affecting the plateau. However, its initiation time and amount of offset are stillcontroversial despite being key information for the understanding of Tibet evolution. In this study, we present1122 single LA-ICP-MS detrital zircon U–Pb ages obtained from 11 Mesozoic to Cenozoic sandstone samples, collectedalong two sections in the northwestern Qaidam basin (Eboliang and Huatugou). These data are combinedwith new3D seismic reflection profiles to demonstrate that: (1) fromthe Paleocene to early Eocene, the Eboliangsection was approximately located near the present position of Anxi, 360 ± 40 km southwest from its currentlocation along the Altyn Tagh fault, and sediments were mainly derived from the Altyn Tagh Range. At thesame period, the Huatugou section was approximately located near the present position of Tula, ca. 360 kmsouthwest from its current location along the Altyn Tagh fault, and the Eastern Kunlun Range represented a significantsediment source. (2) Left-lateral strike-slip movement along the Altyn Tagh fault initiated during theearly-middle Eocene, resulting in northeastward displacement of the two sections. (3) By early Miocene, the intensivedeformation within the Altyn Tagh Range and northwestern Qaidam basin strongly modified the drainagesystem, preventing the materials derived fromthe Altyn Tagh Range to reach the Eboliang and the Huatugousections. The post-Oligocene clastic material in the western Qaidam basin is generally derived fromlocal sourcesand recycling of the deformed Paleocene to Oligocene strata. From these data, we suggest enhanced tectonic activitywithin the Altyn Tagh Range and northwestern Qaidam basin since Miocene time, and propose an earlymiddleEocene initiation of left-lateral strike-slip faulting leading to a 360 ± 40 km offset along the Altyn Taghfault
Slab breakoffis one of the primary processes in the evolution of many collisional orogens. In the Tibet-Himalaya orogen, the timing of breakoffof the Neo-Tethyan slab remains controversial because of a scarcity of solid evidence. This study reports the discovery of Eocene gabbros, dated at 45.0 ± 1.4 Ma (in situ U-Pb age of titanite) using secondary ion mass spectrometry, from the eastern segment of Tethyan Himalaya in southern Tibet. These rocks show geochemical characteristics similar to those of HIMU (high μ)-type oceanic island basalt and have depleted Sr-Nd isotopes [87Sr/86Sr(t) = 0.70312-0.70317; εNd(t) = +4.9 to +5.0]. It is suggested that the gabbros stand as the first direct evidence for partial melting of the asthenosphere followed by rapid magma ascent with negligible crustal contamination. This event, combined with results from relevant studies along the Indus-Yarlung suture zone, is best explained by a sudden and full-scale detachment of subducted Neo-Tethyan slab at great depth. The breakoffmodel may account for coeval tectonomagmatic activities (development of small-scale, short-lived magmatism and subsequent termination of the Gangdese arc magmatism) in southern Tibet and for the abrupt slowdown (ca. 45 Ma) of Indo-Asia convergence.
The Daxigou huge iron-phosphorite deposit, with low-grade, is located at basic-ultra basic complexes in the southern Kuruktag faulted uplift of the north margin of Tarim platform. Based on the geological feature, geochemical compositions, trace elements, rare earth elements (REES) of ore-bearing anorthosite mass as well as characters of the Daxigou deposit, the iron-phosphorite mineralization has been discussed. It is suggested that ore-bearing lithofacies belong to the alkaline series, major and trace elements in complex are not inconsistent with each other. Such incompatible elements as Ba, Sr, Zr and P are obviously enriched, but the compatible elements Cr, Co and Ni are depleted. Contents of REE are relatively higher, and LREE highly enriched. There is no obvious Eu abnormality. The magma of complex rocks was originated from deep crust. In the late period of magmatism, large amounts of magnetite and apatite crystallized and formed ore body in some local section. By analysing the present exploitation situation of low grade endogenetic iron-phosphorite deposit in North China, it can be concluded that the Daxigou iron-phosphorite deposit will have a good comprehensive utilization perspective.
Ilmenite in the Gabbro Picrite Zone of the Basistoppen sill, East Greenland, contains exsolution lamellae of baddeleyite and Fe-Cr-spinel. The relative proportions of baddeleyite and Fe-Cr-spinel in isolated ilmenite grains indicate that the ilmenite was reduced during subsolidus re-equilibration, although the sill in general was oxidized during cooling. Calculated pre-exsolution composition for the ilmenite contain up to 2. 4% Cr//2O//3 and up to 7100 ppm Zr. The calculated Zr-content is higher than that of any previously reported lunar or terrestrial ilmenite. The reduction-exsolution of Fe-Cr-spinel is considered the result of the incorporation of Cr in the ilmenite during crystallization at concentrations that exceeded the solubility limit of Cr in ilmenite at the temperature of re-equilibrium.
Zircon U-Pb chronology of high-grade metamorphic rooks from the northern margin of the Tarim Craton, was reported in this paper. The metamorphic rocks located on the Tiemenguan of northern Korla, was composed of gneiss, schist, amphibolite and calc-silicate rocks. They have mineral assemblages of plagioclase + K-feldspar + quartz + biotite ± muscovita ± hornblende ± garnet, indicating amphibolite-facies conditions. The zircon U-Pb dating demonstrates that these high-grade metamorphic rocks have been subjected to three-stages of tectono-thermal events, i.e. the Early Paleoproterozoic magmatic event of ∼ 2370Ma, the Late Paleoproterozoic metamorphic event of 1.890 ∼1850Ma and the Early Neoproterozoic metamorphic event of 980 ∼91.0Ma. These results supply significant constrains on Preeambrian tectonic evolution of Tarim Craton.
Major anorthositic massifs intruded stable cratonic crust in the North Atlantic region about 1.4 to 1.7 Ga ago. Similar (meta-) anorthosite massifs (0.9-1.6 Ga) in Grenville and Sveconorwegian terranes exhibit varying degrees of deformational and metamorphic over print and consequently have ambiguous tectonic settings. Close correspondence of rock and mineral associations in both types points to common origins. Mineral assemblages, chemical, and isotopic data for pristine and meta-anorthosite massifs strongly imply varying degress of interaction between mantle-generated magmas and deep continental crust occurred prior to final emplacement. -from Author
Kuluketage block is one of the best areas for Precambrian geology studies in Xinjiang, however, the regional metallogeny of the research area is still too poon. In this paper, all the published literatures are summarized. Then, the geology of main ore deposits and the classified metallogenic series in this block are systematically described. The following seven tectonothermal periods are distinguished: Archean crust nucleus growing stage (3.3-3.0 Ga), Neoarchean-Paleoproterozoic crust growth and transformation stage (2.6-2.3 Ga), middle-late Paleoproterozoic crust transformation stage (2.1-1.8 Ga), Late Mesoproterozoic to Early Neoproterozoic orogeny stage (1.1-0.86 Ga), Middle Neoproterozoic orogenic extensional stage (830-800 Ma), middle-late Neoproterozoic intracontinental breakup stage (770-630 Ma) and Early Paleozoic land building stage. Mineralization mainly occurred at Paleoproterozoic, Neoproterozoic and Early Paleozoic. According to the ore-forming tectonic settings, ore-bearing rock formations and ore deposit genesis, six typical metallogenic series are identified in our study, including Fe-P-Cu-Au metallogenic series formed at Paleoproterozoic crust growth and transformation stage, Cu-Au metallogenic series formed at Early Neoproterozoic orogeny stage, Cu-Mo-Au-Fe-P-REE metallogenic series formed at Neoproterozoic post-collision stage, Cu-Ni metallogenic series formed at Neoproterozic rifting stage, Ag-V-Mo-Au-U-P metallogenic series formed at Early Paleozoic sedimentary basin and Cu-Au metallogenic series formed at Early Paleozoic arc subduction stage.
The Kuluketage block is the best area for Precambrian geology in north western China, because it contains the most complete Precambrian lithology units. Thus, the study of this ancient basement can improve the understanding of the Precambrian evolution of the Tarim Craton. In this study, we report LA-ICPMS zircon U-Pb ages and Hf isotopes of detrital zircons from a magnetite quartzite from the Shayiti Formation of the Xingditage Group. The 65 zircon ages and Hf isotopes obtained are used not only to constrain the maximum depositional ages of the Shayiti Formation but also to obtain the information about the evolution of regional tectonic-magmatic activities in the Paleoproterozoic of the Kuluketage block. According to the youngest concord Pb-207/Pb-206 zircon age of 1851 +/- 36 Ma in magnetite quartzites and the 1.47 Ga of the diabase sills which intrude into the Shayiti Formation, the most probable depositional age of the Shayiti Formation is between 1.47 Ga and 1.85 Ga. The detrital zircon dates are mainly clustered at 1806 Ma to 1889 Ma, 1898 Ma to 1981 Ma, and 1988 Ma to 2054 Ma, with the most prominent age peak appearing at around 1900 Ma and the subordinate peak age at around 1960 Ma. The magmatic features of Cathodoluminescence (CL) images indicate that two large magmatic tectonic-magmatic activities occurred in this district. The metamorphic rims of magmatic zircons and some baddeleyites also show regional metamorphism in the Paleoproterozoic, which may be related to the amalgamation of the Columbia supercontinent. We obtained two sets of concordant U-Pb ages older than 2.5 Ga, and several sets of two-stage Hf model ages older than 3.0 Ga. Combined with previous data in the literature, we suggest that Meso- to Neo-Archean basement rocks existed in the Kuluketage block, but were strongly reformed by tectonics, magmatism, and metamorphism in the Paleoproterozoic.
Grains consisting of finely exsolved members of the hematite-ilmenite solid-solution series, such as are present in some slowly cooled middle Proterozoic igneous and metamorphic rocks, impart unusually strong and stable remanent magnetization. TEM analysis shows multiple generations of ilmenite and hematite exsolution lamellae, with lamellar widths ranging from millimeters to nanometers. Rock-magnetic experiments suggest remanence is thermally locked to the antiferromagnetism of the hematite component of the intergrowths, yet is stronger than can be explained by canted antiferromagnetic (CAF) hematite or coexisting paramagnetic (PM) Fe-Ti ordered (R3̄) ilmenite alone. In alternating field demagnetization to 100 mT, many samples lose little remanence, indicating that the NRM is stable over billions of years. This feature has implications for understanding magnetism of deep rocks on Earth, or on planets like Mars that no longer have a magnetic field. Atomic-scale simulations of an R3̄ ilmenite lamella in a CAF hematite host, based on empirical cation-cation and spin-spin pair interaction parameters, show that contacts of the lamella are occupied by "contact layers" with a hybrid composition of Fe ions intermediate between Fe2+-rich layers in ilmenite and Fe3+-rich layers in hematite. Structural configurations dictate that each lamella has two contact layers magnetically in phase with each other, and out of phase with the magnetic moment of an odd non-self-canceling Fe3+-rich layer in the hematite host. The two contact layers and the odd hematite layer form a magnetic substructure with opposite but unequal magnetic moments: a lamellar "ferrimagnetism" made possible by the exsolution. Because it is confined to magnetic interaction involving the moments of just three ionic layers associated with each individual exsolution lamella, lamellar magnetism is unique and quite distinct from conventional ferrimagnetism. Simulation cells indicate that the magnetic moments of contact layers are locked to the magnetic moments of adjacent AF hematite layers and are parallel to the basal plane (001). Thus, lamellar magnetism is created at the temperature of chemical exsolution, and is a chemical remanent, rather than thermal remanent, magnetization. However, in thermal demagnetization experiments, too short for lamellar resorption, demagnetization temperatures are those of the CAF hematite, considerably higher than temperatures of original lamellae formation. Internal crystal structure cannot dictate that the contact layers of different lamellae will form magnetically in phase with each other to give the highest net magnetic moment, but magnetic moments of lamellae can be made to form in phase by the external force of the magnetizing field at the time of exsolution. A thesis of this paper is that an external magnetic field can dictate the magnetic moments and hence the chemical location of ilmenite lamellae in a hematite host, and that once in place, neither the location nor the magnetic moment will be easily disturbed. In an ilmenite host, the external magnetic field cannot control the chemical location of a hematite lamella, which is dictated by the enclosing ilmenite, but once lamellae have formed, the field can dictate their magnetic moments. These moments, however, are not locked chemically to the host, resulting in lower coercivity. The effectiveness of the external force in single crystals is dictated by their orientation with respect to the magnetizing field. In grains with (001) oriented parallel to the field, it would be effective in producing in-phase magnetic moments and very strong remanence. In grains with (001 normal to the field, the field would be less effective in producing in-phase magnetic moments, hence producing weak remanence. The most intense lamellar magnetism per formula unit occurs with in-phase magnetization, high lamellar yields, and the largest number of lamellae per unit volume (i.e., smallest lamellar size). Compared to the magnetic moment per formula unit (Mpfu) and magnetic moment per unit volume (Mv) of end-member magnetite (Mpfu = 4 μB, Mv = 480 kA/m) and hematite (Mpfu = 0.0115 μB, Mv = 2.1 kA/m), results for some atomic models reasonably tied to natural conditions are Mpfu = 0.46-1.36 μB and Mv = 84-250 kA/m.
Detrital zircon geochronology is rapidly developing into an essential tool in Earth science research because of the widespread occurrence of zircon in sedimentary systems; the wide range of information that can be extracted from zircon crystals; the ability to determine ages with reasonable precision, accuracy, and efficiency; and the wide range of new ideas about how to use detrital zircon geochronologic information. The U-Pb system is particularly powerful because three chronometers are available (238U→ 206Pb, 235U→207Pb, and 232Th→208Pb), but challenges arise because of complexities from inheritance and Pb loss. Ages can be used to constrain the age of deposition of the host sediment, reconstruct provenance, characterize a sedimentary unit, and characterize many different aspects of source regions. Detrital zircon geochronology has an exciting future given the growth history recorded in individual crystals; the variety of detrital minerals that can provide complementary information; and the large number of geochemical, isotopic, and chronologic systems that can be applied to these minerals.
Primitive nephelinites and basanites from the Tertiary Hocheifel area of Germany (part of the Central European Volcanic Province;
CEVP) have high Mg-number (>0·64), high Cr and Ni contents and strong light rare earth element enrichment but systematic depletion
in Rb, K and Ba relative to trace elements of similar compatibility in anhydrous mantle. Alkali basalts and more differentiated
magmatic rocks have lower Mg-number and lower abundances of Ni and Cr, and have undergone fractionation of mainly olivine,
clinopyroxene, Fe–Ti oxide, amphibole and plagioclase. Some nephelinites and basanites approach the Sr–Nd–Pb isotope compositions
inferred for the EAR (European Asthenospheric Reservoir) component. The Nd–Sr–Pb isotope composition of the differentiated
rocks indicates that assimilation of lower crustal material has modified the composition of the primary mantle-derived magmas.
Rare earth element melting models can explain the petrogenesis of the most primitive mafic magmatic rocks in terms of mixing
of melt fractions from an amphibole-bearing garnet peridotite source with melt fractions from an amphibole-bearing spinel
peridotite source, both sources containing residual amphibole. It is inferred that amphibole was precipitated in the asthenospheric
mantle beneath the Hocheifel, close to the garnet peridotite–spinel peridotite boundary, by metasomatic fluids or melts from
a rising mantle diapir or plume. Melt generation with amphibole present suggests relatively low mantle potential temperatures
(<1200°C); thus the mantle plume is not thermally anomalous. A comparison of recently published Ar/Ar ages for Hocheifel basanites
with the geochemical and isotopic composition of samples from this study collected at the same sample sites indicates that
eruption of earlier lavas with an EM signature was followed by the eruption of later lavas derived from a source with EAR
or HIMU characteristics, suggesting a contribution from the advancing plume. Thus, the Hocheifel area represents an analogue
for magmatism during continental rift initiation, during which interaction of a mantle plume with the overlying lithosphere
may have led to the generation of partial melts from both the lower lithosphere and the asthenosphere.
We report the petrology, whole-rock geochemistry, zircon LA-ICP-MS U-Pb chronology and zircon Hf isotopic data of Daxigou granitoids (western part of the Kuluketage Block, NW China) to evaluate their likely petrogenesis and tectonic setting. Zircons from syenogranite can be divided into two groups: 1) those that display oscillatory zoning and high Th/U ratios (average = 1.38), implying their magmatic origin and 2) those that exhibit weak zoning and extremely high U and Pb contents but low Th/U ratios (average = 0.35), resembling zircons that experienced hydrothermal alteration. The zircon LA-ICP-MS U-Pb dating of the two groups of zircons yielded weighted mean ages of 1830 ± 12 Ma (MSWD = 0.78) and 1798 ± 21 Ma (MSWD = 1.6) respectively. The Daxigou granitoids belong mostly to normal-K and sodium-rich metaluminous calc-alkaline type, systematically enriched in LREE and large ion lithophile elements (LILE, e.g., K, Ba and Rb), but significantly depleted in high field strength elements (HFSE, e.g., Ti, P, Nb, Ta and U). Their εHf(t) values and two-stage Hf model ages range from -7.16 to -5.03 and 2.69 to 2.76 Ga, respectively. Taken together, it is suggested that Daxigou granitoids are of I-type affinity and that they were derived by partial melting of a Neoarchaean TTG (e.g., Tuoge Complex) rocks in a continental-arc environment. These new data, combined with previous regional geological studies, demonstrate that a series of Palaeoproterozoic (c. 2.0-1.8 Ga) tectono-magmatic events occurred in Kuluketage Block during the assembly of Columbia.
Identifying the relative contribution of various crustal and mantle materials in the source of granitoids is crucial for the study of granite petrogenesis and crustal growth. Extensive and diverse late Paleoproterozoic metamorphosed granitoids are exposed in the western Kuruktag block, northern Tarim craton, marking an important tectonothermal event. Here, we report sensitive high-reso lution ion microprobe (SHRIMP) and laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) zircon U-Pb ages, in situ zircon Hf-O isotopic data, and whole-rock geochemical data for a variety of granitoids, including monzogranite, quartz diorite/quartz monzonite, garnet-bearing granodiorite, tonalite, and trondhjemite. Geochronological data show that all these granitoids were emplaced in a relatively short period at ca. 1.93-1.94 Ga and were immediately metamorphosed at ca. 1.91-1.92 Ga. In situ zircon Hf-O isotopic data suggest that both mantle-derived juvenile materials and supracrustal rocks (i.e., the Xingditag Group) were involved in magma genesis. The sodic trondhjemite and tonalite were generated by partial melting of newly underplated mafic lower crust originating from both depleted and relatively enriched mantle, with some assimilation of sedimentary materials. In contrast, the other (mostly high-K) granitoids (monzo granite, quartz diorite/quartz monzonite, and garnetbearing grano diorite) were produced by partial melting of metasedimentary rocks due to intrusion of, and mixing with, variable amounts of mantle-derived mafic magmas, suggesting that these granitoids may also have recorded substantial crustal growth. These petrogenetic interpretations imply that most granitoids in the study area were a result of synchronous crustal growth and reworking, which must be carefully considered in models of continental growth. The adakitic trondhjemite and tonalite have high Sr/Y, La/Yb, Gd/Yb, Nb/Ta, and Zr/Sm ratios, suggesting partial melting with a rutile eclogite residue and implying significant crustal thickening (>50 km). This tectonothermal event most likely occurred in an Andean-type continental arc and was followed by continental collision at ca. 1.85 Ga along the newly recognized late Paleoproterozoic North Tarim orogen. Such an accretion-tocollisional orogen implies an interior position of this area within the Columbia (or Nuna) supercontinent. Based on geological correlations, a Tarim-North China connection is suggested for Columbia reconstruction.
Two layers of basaltic flows intercalated with Late Neoproterozoic (Sinian) sandstones have been identified in the Sugetbrak region in the northwest Tarim block, Northwest China. The basaltic rocks are composed of augite and plagioclase phenocrysts set in a groundmass of plagioclase laths with interstitial subophitic clinopyroxene grains and minor anhedral opaque minerals (magnetite and ilmenite). LA-MC-ICP-MS U–Pb dating of zircons from the lower basaltic flows yields a crystallization age of 783.7 ± 2.3 Ma. Both layers of the basaltic rocks are characterized by low SiO2 and high total FeO (>12 wt.%), TiO2 (>3 wt.%) and P2O5 (>0.5 wt.%) contents and extremely high Na2O/K2O ratios, and display a Fenner trend of differentiation that could be ascribed to significant fractional crystallization of clinopyroxene and plagioclase. The mineralogical and geochemical characteristics suggest the affinities of a transitional series between alkaline basalt and tholeiite. However, the lower basaltic flows have higher Nb/Y ratios than the upper ones, indicating that they are more alkaline. Positive age-corrected Nd isotope ratios [ɛNd(t) = +0.24–1.07] and positive ɛHf(t) values (+1.1–4.5) of the basaltic rocks suggest absence of any significant crustal contamination. High ɛNd(t) lavas are isotopically similar to those of several modern oceanic hotspots, and have ocean island-like patterns of incompatible elements. The estimated potential mantle temperature is ∼100–150 °C higher than normal asthenospheric mantle, consistent with a plume-head origin. Moderate ratios of light rare earth elements (REE) to heavy REE indicate that the source magma was probably generated by partial melting of garnet–spinel transition facies of peridotite, but the upper basaltic rocks were derived from a relatively shallower mantle source, reflecting progressive lithosphere thinning possibly through plume–lithosphere interaction. We correlate the Sugetbrak basalts to the second Neoproterozoic mantle plume event (780–745 Ma) related to the breakup of the Rodinia supercontinent.
Damiao anorthosite suite derived through differentiation of high alumina basalt.•Fractional crystallization in a long-lived magma chamber.•Polybaric crystallization at depth followed by diapiric uplift.•Magma generation related to Paleoproterozoic slab-break off and mantle upwelling.
Neoproterozoic igneous rocks are widely distributed in the Kuluketage block along the northern margin of the Tarim Craton. However, the published literature mainly focuses on the ca. 800 Ma adakitic granitoids in the area, with the granites that intrude the 735–760 Ma mafic–ultramafic rocks poorly studied. Here we report the ages, petrography and geochemistry of two granites in the Xingdi mafic–ultramafic rocks, in order to construct a new view of the non-adakitic younger granites. LA-ICP-MS zircon U–Pb dating provided weighted mean 206Pb/238U ages of 743.0 ± 2.5 Ma for the No.I granite (G1) and 739.0 ± 3.5 Ma for the No.II granite (G2). A clear core-rim texture of similar age and a high zircon saturation temperature of ca. 849 ± 14 °C were observed for the No.I granite; in contrast, G2 has no apparent core-rim texture but rather inherited older zircons and a lower zircon saturation temperature of ca. 763 ± 17 °C. Geochemical analysis revealed that G1 is an alkaline A-type granite and G2 is a high-K calc-alkaline I-type granite. Both granites share similar geochemical characteristics of arc-related magmatic rocks and enriched Sr–Nd–Hf isotopes, likely due to their enriched sources or mixing with enriched magma. Whereas G1 and its host mafic rocks form typical bimodal intrusions of the same age and similar Sr–Nd–Hf isotope compositions, G2 is younger than its host mafic rocks and its Sr–Nd–Hf isotope composition indicates a lower crust origin. Although they exhibit arc-related geochemical features, the two granites likely formed in a rift setting, as inferred from thier petrology, Sr–Nd–Hf isotopes and regional tectonic evolution.
Kuruqtagh block is the best area for Precambrian geology in Xinjiang Autonomous Region, NW China, since it exposed complete Precambrian lithology units. The study of this ancient base will deepen the understanding of the Precambrian evolution of the Tarim Basin. In this paper, we studied the petrology, geochemistry, zircon LA-ICPMS U-Pb chronology and zircon Hf isotope of Daxigou anorthosite (DA) which is located at the northern margin of Tarim craton and discussed the rock formation, tectonic and geological significance. Zircons from the intrusions display oscillatory zoning and high Th/U ratios (0.39–1.35), implying their magmatic origin. Zircon LA-ICP-MS U-Pb dating results indicate that they formed during the Paleoproterozoic age with the weighted 206Pb/238U average age of 1818±9 Ma, which is significantly different from former’s Neoproterozoic age, and is coincidentally identical with its associated syenite granite age within the error range. Studies on petrogeochemistry suggest that DA belongs to medium-sodium peraluminous alkaline type, rich in Pb, La, Th and LILE, and poor in HFSE (Gd, Nd, and Ta). The chondrite-normalized REE pattern is slightly to the right form. The average ΣREE is 317.2×10−6; HREE show moderate fractionation [average LREE/HREE is 14.71, average (La/Yb)N is 24.77; average (La/Sm)N is 3.85, and average (Gd/Yb)N is 3.46]; and the δEu and δCe are not obvious. Their initial Hf isotope ratios and Hf two-stage model ages range from −6.6 to −4.43 and 2.63 to 2.74 Ga, respectively. Taken together, it is suggested that Daxigou anorthosite is a typical volcanic anorthosite and its primary magma could be contaminated by the partial melt Neoarchaean crust and mainly formed in the arc environment, which recoded the tectonic-magma activities response of the Tarim refers to the amalgamation of the supercontinent Columbia.
We have characterized the distribution of 25 trace elements in magnetite (Mg, W, and Pb), using laser ablation ICP-MS and electron microprobe, from a variety of magmatic and hydrothermal ore-forming environ-ments and compared them with data from the literature. We propose a new multielement diagram, normalized to bulk continental crust, designed to emphasize the partitioning be-havior of trace elements between magnetite, the melt/fluid, and co-crystallizing phases. The normalized pattern of mag-netite reflects the composition of the melt/fluid, which in both magmatic and hydrothermal systems varies with temperature. Thus, it is possible to distinguish magnetite formed at different degrees of crystal fractionation in both silicate and sulfide melts. The crystallization of ilmenite or sulfide before magne-tite is recorded as a marked depletion in Ti or Cu, respectively. The chemical signature of hydrothermal magnetite is distinct being depleted in elements that are relatively immobile during alteration and commonly enriched in elements that are highly incompatible into magnetite (e.g., Si and Ca). Magnetite formed from low-temperature fluids has the lowest overall abundance of trace elements due to their lower solubility. Chemical zonation of magnetite is rare but occurs in some hydrothermal deposits where laser mapping reveals oscillato-ry zoning, which records the changing conditions and composition of the fluid during magnetite growth. This new way of plotting all 25 trace elements on 1 diagram, normalized to bulk continental crust and elements in order of compatibil-ity into magnetite, provides a tool to help understand the processes that control partitioning of a full suit of trace ele-ments in magnetite and aid discrimination of magnetite formed in different environments. It has applications in both petrogenetic and provenance studies, such as in the explora-tion of ore deposits and in sedimentology.
Spinel lherzolite xenoliths within alkali basalts exposed at Rayfield River and Big Timothy Mountain, south-central British Columbia, represent samples of the underlying lithospheric mantle. Electron microprobe analysis shows that most xenoliths comprise compositionally homogeneous grains of olivine, orthopyroxene, clinopyroxene, and spinel. We applied the following mineral-pair geothermometers to these rocks: orthopyroxene–clinopyroxene, spinel–orthopyroxene, and spinel–olivine. Temperatures calculated using the Brey and Köhler calibration of two-pyroxene thermometry were constrained in pressure by being required to lie on a model geotherm we develop for this region of B.C. The model geotherm is constrained to produce a temperature at the Moho (33 km) of 825 ± 25 °C to match the lowest temperature peridotite xenoliths recovered in this study. Although the overall effect of pressure on the temperature calculations is negligible (∼2 °C for 0.1 GPa), the simultaneous solution of the model geotherm and the pressure-dependent Brey–Köhler two-pyroxene thermometry removes the need for adopting an arbitrary pressure. We take these temperatures to represent peak mantle lithosphere temperatures. Fourteen Rayfield River xenoliths return two-pyroxene temperatures between 841 and 962 °C corresponding to depths of 34–42 km. Orthopyroxene–spinel and olivine–spinel results are 889 ± 60 and 825 ± 88 °C, respectively. Five Big Timothy xenoliths have two-pyroxene temperatures spanning 840–1058 °C and corresponding to depths of 34–48 km. Mean orthopyroxene–spinel and olivine–spinel temperatures are 844 ± 63 and 896 ± 232 °C, respectively. We argue that the differences in ranges of temperature do not represent closure temperatures imposed during cooling either in the mantle or during transport by the magma. Rather, these differences reflect differences in the original calibrations of the geothermometers or different degrees of equilibration in exchange reactions in dry rocks. Isochemical phase diagrams (pseudosections) constrain the pressure–temperature (P–T) field in which spinel is stable. These diagrams suggest that the spinel-bearing peridotites equilibrated at pressures ranging from ∼9.6 to 14 kbar (10 kbar = 1 GPa).
The Tarim Craton is one of three large cratons in China. Presently,
there is only scant information concerning its crustal evolutionary
history because most of the existing geochronological studies have
lacked a combined isotopic analysis, especially an in situ Lu-Hf
isotope analysis of zircon. In this study, Precambrian basement rocks
from the Kuluketage and Dunhuang Blocks in the northeastern portion of
the Tarim Craton have been analyzed for combined in situ laser ablation
ICP-(MC)-MS zircon U-Pb and Lu-Hf isotopic analyses, as well
as whole rock elements, to constrain their protoliths, forming ages and
magma sources. Two magmatic events from the Kuluketage Block at
˜2.4 Ga and ˜1.85 Ga are revealed, and three stages of
magmatic events are detected in the Dunhuang Block, i.e., ˜2.0 Ga,
˜1.85 Ga and ˜1.75 Ga. The ˜1.85 Ga magmatic rocks
from both areas were derived from an isotopically similar crustal source
under the same tectonic settings, suggesting that the Kuluketage and
Dunhuang Blocks are part of the uniform Precambrian basement of the
Tarim Craton. Zircon Hf model ages of the ˜2.4 Ga magmatism
indicate that the crust of the Tarim Craton may have been formed as
early as the Paleoarchean period. The ˜2.0 Ga mafic rock from the
Dunhuang Block was formed in an active continental margin setting,
representing an important crustal growth event of the Tarim Craton in
the mid-Paleoproterozoic that coincides with the global episode of crust
formation during the assembly of the Columbia supercontinent. The
˜1.85 Ga event in the Kuluketage and Dunhuang Blocks primarily
involved the reworking of the old crust and most likely related to the
collisional event associated with the assembly of the Columbia
supercontinent, while the ˜1.75 Ga magmatism in the Dunhuang Block
resulted from a mixture of the reworked Archean crust with juvenile
magmas and was most likely related to a post-collisional episode.
Proterozoic anorthosites from the 1630–1650 Ma Mealy Mountains Intrusive Suite (Grenville Province, Canada), the 1289–1363 Ma Nain Plutonic Suite (Nain–Churchill Provinces, Canada) and the 920–949 Ma Rogaland Anorthosite Province (Sveconorwegian Province, Norway), all entrain comagmatic, cumulate, high-alumina orthopyroxene megacrysts (HAOMs). The orthopyroxene megacrysts range in size from 0.2 to 1 m and all contain exsolution lamellae of plagioclase that indicate the incorporation of an excess Ca–Al component inherited from the host magma at pressures in excess of 10 kbar at or near Moho depths (>30–40 km). Suites of HAOMs from each intrusion display a large range in 147Sm/144Nd (0.10 to 0.34) making them amenable for precise age dating with the Sm–Nd system. Sm–Nd isochrons for HAOMs give ages of 1765±12 Ma1765±12 Ma (Mealy Mountains), 1041±17 Ma1041±17 Ma (Rogaland) and 1444±100 Ma1444±100 Ma (Nain), all of them older by about 80 to 120 m.y. than the respective 1630–1650, 920–949 and 1289–1363 Ma crystallization ages of their host anorthosites. Internal mineral Sm–Nd isochrons between plagioclase exsolution lamellae and the orthopyroxene host for HAOMs from the Rogaland and Nain complexes yield ages of 968±43968±43 and 1347±6 Ma1347±6 Ma, respectively – identical within error to the ages of the anorthosites themselves. This age concordance establishes that decompression exsolution in the HAOM was coincident with magmatic emplacement of the anorthosites, ∼100 m.y. after HAOMs crystallization at the Moho. Correspondence of Pb isotope ages (206Pb/204Pb vs. 207Pb/204Pb) with Sm–Nd ages and other strong lines of evidence indicate that the older megacryst ages represent true crystallization ages and not the effects of time-integrated mixing processes in the magmas. Nd isotopic evolution curves, AFC/mixing calculations and the age relations between the HOAMs and their anorthosite hosts show that the HAOMs are much less contaminated with crustal components and are an older part of the same magmatic system from which the anorthosites are derived. Modeling of these anorthositic magmas with MELTS indicates that their ultramafic cumulates would have sunk in the magma and been sequestered at the Moho, where they may have sunk deeper into the mantle resulting in large-scale compositional differentiation. The HAOMs thus represent a rare example of part of a cumulate assemblage that was carried to the upper crust during anorthosite emplacement and, together with the anorthosites, illustrate the dramatic influence that magma ponding and differentiation at the Moho has on residual magmas traveling towards the surface. The new geochronologic and isotopic data indicate that the magmas were derived by melting of the mantle, forming magmatic systems that could have been long-lived (e.g. 80–100 m.y.). A geologic setting that would fit these temporal constraints is a long-lived Andean-type margin.
The methods used for precise calibrations of Sm/Nd ratios and the average isotopic abun-dances obtained for normal Sm and Nd are given. A mixed Sm–Nd normal solution with a precisely known 147Sm/144Nd ratio close to the nominal average chondritic value is described and the calibration discussed. Aliquots of this standard solution are available on request and may be useful for precise interlaboratory calibration of Sm and Nd.
Formation of ubiquitous migmatites in high-grade metamorphic terranes may involve both closed- and open-system processes, e.g., in situ anatexis, infiltration of external melts and fluids. Coupled CL-imaging, in situ U–Th–Pb and Lu–Hf isotopic analyses of zircons from various migmatites and their components, i.e., leucosomes and melanosomes, enabled us to determine the time and mechanism(s) of three episodes of migmatization in the Korla Complex, northern Tarim Craton. The first episode of migmatization took place at ca. 1.85 Ga and probably resulted from in situ partial melting due to a regional high-grade metamorphic event. The second migmatization event was marked by the presence of abundant ca. 830 Ma leucogranitic veins, dykes and small plutons. Zircon Hf isotopic data indicate that these leucogranites were probably derived from anatexis of a relatively juvenile crustal source, rather than their immediate country rocks, implying large-scale melt migration and infiltration. These two episodes of migmatization might have resulted from two regional orogenic events at ca. 1.85 Ga and 830 Ma, respectively. In contrast, the third episode of migmatization at ca. 660 Ma was probably a local remelting event induced by intrusion of small quartz syenite plutons into previously migmatized rocks. In addition, zircon domains with extremely low Th (0.003–10 ppm) and U (3–30 ppm) contents were found in samples related to the ca. 830 Ma migmatization event. These domains generally occur as CL-bright rims that penetrate the primary zoned zircons without changing their morphology, and are ascribed to strong hydrothermal alteration via the interface-coupled dissolution–reprecipitation mechanism. A compilation of published zircon Hf isotopic data reveals three episodes of crustal growth during ca. 3.1–3.4 Ga, 2.5–2.8 Ga and 1.6–1.9 Ga in the northern Tarim Craton. However, these ‘peaks’ of crustal growth may be biased by selective sampling and magma mixing. Zircon Hf isotopic data from a ca. 2.29 Ga orthogneiss and the least recrystallized detrital cores from the migmatized sedimentary rocks suggest that continental crust older than ca. 3.3 Ga might have existed in the northern Tarim Craton.
Widespread Paleoproterozoic supracrustal rocks in the northern Tarim Craton contain important information about its geological evolution and correlation with adjacent blocks. We present new in situ LA-(MC-)ICP-MS zircon U–Pb and Lu–Hf isotopic data for six mica schist samples from the Korla Complex. Field and petrological studies indicate a pelitic to semi-pelitic protolith and a high pressure upper amphibolite-facies peak metamorphic condition (T = 690 ± 50 ̊C and P = 11 ± 2 kbar) for these samples. CL-images reveal that zircons in these samples are dominantly metamorphic origin and only a few detrital zircons occur as relics in sample T1, the ages of which suggest a maximum deposition age of ca. 2.0 Ga and a sedimentary provenance from the Tarim Craton itself. All metamorphic zircons consistently record a metamorphic age of ca. 1.85 Ga, despite of various degrees of discordance probably due to later Pb-loss. Both recrystallization and new zircon growth are recognized for the genesis of these metamorphic zircons. The metamorphic zircon domains in sample T1 show a relatively large range of initial 176Hf/177Hf ratios similar to the detrital cores, whereas those in the other samples show similar initial 176Hf/177Hf ratios (ca. 0.28140 ± 0.00010, 2σ) regardless of their internal structures and degrees of discordance. The former is interpreted as a result of complete U–Pb resetting through fluid-mediated recrystallization, whereas the later probably implies a large-scale Hf isotopic homogenization during new zircon growth. Petrological and zircon isotopic evidence supports that the new zircon growth and Hf isotopic homogenization probably resulted from the mixing of Hf–Zr derived from dissolution of tiny detrital zircons and decomposition of garnet to chlorite in hydrothermal fluids during retrograde metamorphism. Accordingly, the ages of these new zircon growths may postdate the peak metamorphism, which was probably related to a late Paleoproterozoic collisional orogenic event in the northern Tarim Craton. A compilation of available geological and geochronological data enables us to identify two Late Paleoproterozoic orogenic belts: the ca. 1.9–1.8 Ga North Tarim Orogen and the ca. 2.0–1.9 Ga South Tarim Orogen. It is suggested that the Tarim Craton, including the Dunhuang and Quanji Blocks, was correlative with the Alxa–Yinshan Block of the North China Craton, and they probably formed a coherent massif during the Neoarchean–early Paleoproterozoic, which collided with the Ordos Block and its western extension along the ca. 1.95 Ga Khondalite Belt–South Tarim Orogen to form a larger landmass in the Columbia Supercontinent.