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Roberta L Rudnick - One of the best experts on this subject based on the ideXlab platform.
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barium isotopic composition of the upper Continental Crust
Geochimica et Cosmochimica Acta, 2018Co-Authors: Xiaoyun Nan, Roberta L Rudnick, Richard M Gaschnig, Qun Zhang, Zhangdong Jin, Fang HuangAbstract:Abstract The upper Continental Crust (UCC) is an important reservoir of Ba within the Earth. We report high precision (≤±0.05‰, 2SD) Ba isotopic data for 71 samples (including granites, granodiorites, loess, glacial diamictites, and river sediments) to constrain the Ba isotopic composition of the UCC. I-type granites from the Fogang batholith, Southeastern (SE) China, exhibit variable δ137/134Ba (−0.16‰ to 0.01‰), which may be due to isotopic fractionation during the latest stages of magmatic differentiation. The δ137/134Ba of S-type granites from Darongshan-Shiwandashan, SE China (−0.03‰ to 0.11‰) correlate with ɛNd (t), likely reflecting mixing of heterogeneous Crustal source materials. Five A-type granites with high SiO2 contents (∼76 wt%) from Nankunshan, SE China have remarkably low δ137/134Ba (−0.47‰ to −0.33‰), which possibly arose from magmatic differentiation or assimilation of Crustal materials with light Ba isotopic compositions. Loess from northwestern China has homogeneous δ137/134Ba (−0.02‰ to 0.03‰) that shows no correlation with bulk compositions, sample locations, or degree of chemical weathering, suggesting that loess is representative of the average Ba isotopic composition of the UCC. Three river sediments from northern China have δ137/134Ba similar to that of loess. The δ137/134Ba of glacial diamictites vary with CIA values: those with high CIA (≥60) have heterogeneous δ137/134Ba (−0.19‰ to 0.35‰), while those with low CIA ( √ n , n = 71), which is similar to the average Ba isotopic composition of the upper mantle, but significantly lower than δ137/134Ba of seawater.
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insights into chemical weathering of the upper Continental Crust from the geochemistry of ancient glacial diamictites
Geochimica et Cosmochimica Acta, 2016Co-Authors: Richard M Gaschnig, Roberta L RudnickAbstract:Abstract Glacial diamictites, with ages ranging from ∼2900 to 0.01 Ma, record the changing composition of the upper Continental Crust through time ( Gaschnig et al., 2014 ). Li concentrations and isotopic compositions, combined with Pb isotopic compositions, chemical index of alteration (CIA) values and relative Sr concentrations are used here to assess the degree of chemical weathering recorded in these deposits and the origin of this signature. The δ7Li values of most of the diamictites (ranging from −3.9 to +3.5) are lower than those of mantle-derived basalts (+3.7 ± 2, 2σ), and the low δ7Li values are generally accompanied by high CIA and low Sr/Sr∗ values (or Sr depletion factor, Sr/Sr∗ = Sr/(Ce∗Nd)0.5), reflecting a weathering signature that may have derived from pre-depositional, syn-depositional, and/or post-depositional weathering processes. Profiles through three glacial diamictites with relatively high CIA (a fresh road cut of the Neoproterozoic Nantuo Formation (CIA = 62–69), and drill cores through the Paleoproterozoic Timeball Hill (CIA = 66–75) and Duitschland Formations (CIA = 84–91)) do not show evidence of significant post-depositional weathering. High Th/U, reflecting loss of uranium during oxidative weathering, is seen in all Paleozoic and Neoproterozoic diamictites and a few Paleoproterozoic deposits. Pb isotopic systematics suggest that this signature was largely inherited from preexisting Crust, although a subset of samples (the Neoproterozoic Konnarock, Paleozoic Dwyka, and several of the Paleoproterozoic Duitschland samples) appears to have experienced post-depositional U loss. Modern glaciomarine sediments record little weathering (CIA = 47, Sr/Sr∗ = 0.7, δ7Li = +1.8), consistent with the cold temperatures accompanying glacial periods, and suggesting that limited syn-depositional weathering has occurred. Thus, the chemical weathering signature observed in ancient glacial diamictites appears to be largely inherited from the upper Continental Crust (UCC) over which the glaciers traversed. The strength of this weathering signature, based on the CIA, is greatest in the Mesoarchean and some of the Paleoproterozoic diamictites and is weaker in the Neoproterozoic and Phanerozoic glacial diamictites. Combining these data with data for Archean shales and other types of post-Paleoproterozoic sedimentary rocks (i.e., shales, mudstones, etc.), it appears that post-Paleoproterozoic upper Continental Crust experienced less intense chemical weathering, on average, than Archean and Paleoproterozoic upper Continental Crust.
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composition of the Continental Crust
Treatise on Geochemistry, 2014Co-Authors: Roberta L Rudnick, Shan GaoAbstract:This chapter reviews the present-day composition of the Continental Crust, the methods employed to derive these estimates, and the implications of the Continental Crust composition for the formation of the continents, Earth differentiation, and its geochemical inventories. We review the composition of the upper, middle, and lower Continental Crust. We then examine the bulk Crust composition and the implications of this composition for Crust generation and modification processes. Finally, we compare the Earth's Crust with those of the other terrestrial planets in our solar system and speculate about what unique processes on Earth have given rise to this unusual Crustal distribution.
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heterogeneous magnesium isotopic composition of the lower Continental Crust a xenolith perspective
Geochemistry Geophysics Geosystems, 2013Co-Authors: Fangzhen Teng, Wei Yang, Roberta L RudnickAbstract:[1] We report 26 high-precision whole-rock Mg isotopic analyses for two suites of well-characterized granulite xenoliths from Chudleigh and McBride, North Queensland, Australia, in order to constrain the behavior of Mg isotopes during deep Crustal processes and the Mg isotopic composition of the lower Continental Crust. Previous studies suggest that the Chudleigh granulites are a suite of cogenetic cumulates crystallized from mafic magmas that intruded into and assimilated the preexisting lower Crust via combined assimilation and fractional crystallization (AFC). The δ26Mg values of the xenoliths range from −0.31 to −0.21‰ and correlate with radiogenic isotopes, reflecting mixing of mantle-derived mafic magma (δ26Mg = −0.31‰) with preexisting isotopically heavy Crustal materials (δ26Mg = ∼ +0.5‰) through the AFC process. The McBride granulites range compositionally from mafic to felsic, and originated as cumulates, solidified mafic/felsic melts, and restites that formed during basaltic underplating and reworking of preexisting lower Crust. Their δ26Mg values vary widely from −0.72 to +0.19‰. The large Mg isotopic variation in the McBride xenoliths reflects both distinct source compositions and metamorphic enrichment of garnet, which is isotopically light. Based on these results, the lower Continental Crust has a heterogeneous Mg isotopic composition, with a weighted average δ26Mg of −0.18‰. The bulk Continental Crust, based on available data, has an average Mg isotopic composition of −0.19‰, and is slightly heavier than the mantle. The highly heterogeneous Mg isotopic distribution in the Crust indicates that chemical weathering not only modifies the upper Crust compositions but also significantly influences lower Crust compositions through emplacement of upper Crustal materials into the deep Crust.
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recycling lower Continental Crust in the north china craton
Nature, 2004Co-Authors: Roberta L Rudnick, Shan Gao, Yongsheng Liu, Hongling Yuan, Xiaoming Liu, Wenliang Xu, Wenli Ling, John C Ayers, Xuanche WangAbstract:Foundering of mafic lower Continental Crust into underlying convecting mantle has been proposed as one means to explain the unusually evolved chemical composition of Earth's Continental Crust1,2, yet direct evidence of this process has been scarce. Here we report that Late Jurassic high-magnesium andesites, dacites and adakites (siliceous lavas with high strontium and low heavy-rare-earth element and yttrium contents) from the North China craton have chemical and petrographic features consistent with their origin as partial melts of eclogite that subsequently interacted with mantle peridotite. Similar features observed in adakites and some Archaean sodium-rich granitoids of the tonalite-trondhjemite-granodiorite series have been interpreted to result from interaction of slab melts with the mantle wedge. Unlike their arc-related counterparts, however, the Chinese magmas carry inherited Archaean zircons and have neodymium and strontium isotopic compositions overlapping those of eclogite xenoliths derived from the lower Crust of the North China craton. Such features cannot be produced by Crustal assimilation of slab melts, given the high Mg#, nickel and chromium contents of the lavas. We infer that the Chinese lavas derive from ancient mafic lower Crust that foundered into the convecting mantle and subsequently melted and interacted with peridotite. We suggest that lower Crustal foundering occurred within the North China craton during the Late Jurassic, and thus provides constraints on the timing of lithosphere removal beneath the North China craton.
Shan Gao - One of the best experts on this subject based on the ideXlab platform.
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magnesium isotopic composition of the deep Continental Crust
American Mineralogist, 2016Co-Authors: Wei Yang, Fangzhen Teng, Shengao Liu, Yongsheng Liu, Hongfu Zhang, Shan GaoAbstract:To constrain the behavior of Mg isotopes during deep Crustal processes and the Mg isotopic composition of the middle and lower Continental Crust, 30 composite samples from high-grade metamorphic terranes and 18 granulite xenoliths were investigated. The composites derive from eight different high-grade metamorphic terranes in the two largest Archean cratons of China, including 13 TTG gneisses, 5 amphibolites, 4 felsic, 4 intermediate, and 4 mafic granulites. They have variable bulk compositions with SiO 2 ranging from 45.7 to 72.5%, representative of the middle Crust beneath eastern China. The δ 26 Mg values of these samples vary from −0.40 to +0.12‰, reflecting heterogeneity of their protoliths, which could involve upper Crustal sediments. The granulite xenoliths from the Cenozoic Hannuoba basalts also have a diversity of compositions with MgO ranging from 2.95 to 20.2%. These xenoliths equilibrated under high temperatures of 800–950 °C, corresponding to depths of the lower Continental Crust (>30 km). They yield a large δ 26 Mg variation of −0.76 to −0.24‰. The light Mg isotopic compositions likely result from interactions with isotopically light metamorphic fluids, probably carbonate fluids. Together with previously reported data, the average δ 26 Mg values of the middle and lower Continental Crusts are estimated to be −0.21 ±0.07‰ and −0.26 ±0.06‰, respectively. The bulk Continental Crust is estimated to have an average δ 26 Mg of −0.24 ±0.07‰, which is similar to the average of the mantle. The large Mg isotopic variation in the Continental Crust reflects the combination of several processes, such as Continental weathering, involvement of supraCrustal materials in the deep Crust, and fluid metasomatism.
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composition of the Continental Crust
Treatise on Geochemistry, 2014Co-Authors: Roberta L Rudnick, Shan GaoAbstract:This chapter reviews the present-day composition of the Continental Crust, the methods employed to derive these estimates, and the implications of the Continental Crust composition for the formation of the continents, Earth differentiation, and its geochemical inventories. We review the composition of the upper, middle, and lower Continental Crust. We then examine the bulk Crust composition and the implications of this composition for Crust generation and modification processes. Finally, we compare the Earth's Crust with those of the other terrestrial planets in our solar system and speculate about what unique processes on Earth have given rise to this unusual Crustal distribution.
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recycling lower Continental Crust in the north china craton
Nature, 2004Co-Authors: Roberta L Rudnick, Shan Gao, Yongsheng Liu, Hongling Yuan, Xiaoming Liu, Wenliang Xu, Wenli Ling, John C Ayers, Xuanche WangAbstract:Foundering of mafic lower Continental Crust into underlying convecting mantle has been proposed as one means to explain the unusually evolved chemical composition of Earth's Continental Crust1,2, yet direct evidence of this process has been scarce. Here we report that Late Jurassic high-magnesium andesites, dacites and adakites (siliceous lavas with high strontium and low heavy-rare-earth element and yttrium contents) from the North China craton have chemical and petrographic features consistent with their origin as partial melts of eclogite that subsequently interacted with mantle peridotite. Similar features observed in adakites and some Archaean sodium-rich granitoids of the tonalite-trondhjemite-granodiorite series have been interpreted to result from interaction of slab melts with the mantle wedge. Unlike their arc-related counterparts, however, the Chinese magmas carry inherited Archaean zircons and have neodymium and strontium isotopic compositions overlapping those of eclogite xenoliths derived from the lower Crust of the North China craton. Such features cannot be produced by Crustal assimilation of slab melts, given the high Mg#, nickel and chromium contents of the lavas. We infer that the Chinese lavas derive from ancient mafic lower Crust that foundered into the convecting mantle and subsequently melted and interacted with peridotite. We suggest that lower Crustal foundering occurred within the North China craton during the Late Jurassic, and thus provides constraints on the timing of lithosphere removal beneath the North China craton.
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lithium isotopic composition and concentration of the upper Continental Crust
Geochimica et Cosmochimica Acta, 2004Co-Authors: Fangzhen Teng, Roberta L Rudnick, Shan Gao, William F Mcdonough, Claude Dalpe, Paul B Tomascak, B W ChappellAbstract:The Li isotopic composition of the upper Continental Crust is estimated from the analyses of well-characterized shales, loess, granites and upper Crustal composites (51 samples in total) from North America, China, Europe, Australia and New Zealand. Correlations between Li, δ7Li, and chemical weathering (as measured by the Chemical Index of Alteration (CIA)), and δ7Li and the clay content of shales (as measured by Al2O3/SiO2), reflect uptake of heavy Li from the hydrosphere by clays. S-type granites from the Lachlan fold belt (–1.1 to –1.4‰) have δ7Li indistinguishable from their associated sedimentary rocks (–0.7 to 1.2‰), and show no variation in δ7Li throughout the differentiation sequence, suggesting that isotopic fractionation during Crustal anatexis and subsequent differentiation is less than analytical uncertainty (±1‰, 2σ). The isotopically light compositions for both I- and S-type granites from the Lachlan fold belt (–2.5 to + 2.7 ‰) and loess from around the world (–3.1 to + 4.5‰) reflect the influence of weathering in their source regions. Collectively, these lithologies possess a limited range of Li isotopic compositions (δ7Li of −5‰ to + 5‰), with an average (δ7Li of 0 ± 2‰ at 1σ) that is representative of the average upper Continental Crust. Thus, the Li isotopic composition of the upper Continental Crust is lighter than the average upper mantle (δ7Li of + 4 ± 2‰), reflecting the influence of weathering on the upper Crustal composition. The concentration of Li in the upper Continental Crust is estimated to be 35 ± 11 ppm (2σ), based on the average loess composition and correlations between insoluble elements (Ti, Nb, Ta, Ga and Al2O3, Th and HREE) and Li in shales. This value is somewhat higher than previous estimates (∼20 ppm), but is probably indistinguishable when uncertainties in the latter are accounted for.
Taras Gerya - One of the best experts on this subject based on the ideXlab platform.
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Continental Crust formation on early earth controlled by intrusive magmatism
Nature, 2017Co-Authors: Antoine Rozel, Gregor J Golabek, Charitra Jain, Paul J Tackley, Taras GeryaAbstract:The global geodynamic regime of early Earth, which operated before the onset of plate tectonics, remains contentious. As geological and geochemical data suggest hotter Archean mantle temperature and more intense juvenile magmatism than in the present-day Earth, two Crust-mantle interaction modes differing in melt eruption efficiency have been proposed: the Io-like heat-pipe tectonics regime dominated by volcanism and the "Plutonic squishy lid" tectonics regime governed by intrusive magmatism, which is thought to apply to the dynamics of Venus. Both tectonics regimes are capable of producing primordial tonalite-trondhjemite-granodiorite (TTG) Continental Crust but lithospheric geotherms and Crust production rates as well as proportions of various TTG compositions differ greatly, which implies that the heat-pipe and Plutonic squishy lid hypotheses can be tested using natural data. Here we investigate the creation of primordial TTG-like Continental Crust using self-consistent numerical models of global thermochemical convection associated with magmatic processes. We show that the volcanism-dominated heat-pipe tectonics model results in cold Crustal geotherms and is not able to produce Earth-like primordial Continental Crust. In contrast, the Plutonic squishy lid tectonics regime dominated by intrusive magmatism results in hotter Crustal geotherms and is capable of reproducing the observed proportions of various TTG rocks. Using a systematic parameter study, we show that the typical modern eruption efficiency of less than 40 per cent leads to the production of the expected amounts of the three main primordial Crustal compositions previously reported from field data (low-, medium- and high-pressure TTG). Our study thus suggests that the pre-plate-tectonics Archean Earth operated globally in the Plutonic squishy lid regime rather than in an Io-like heat-pipe regime.
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generation of new Continental Crust by sublithospheric silicic magma relamination in arcs a test of taylor s andesite model
Gondwana Research, 2013Co-Authors: Antonio Castro, Taras Gerya, Katharina VogtAbstract:The paradox of the Earth's Continental Crust is that although this reservoir is generally regarded as having differentiated from the mantle, it has an andesitic bulk composition that contrasts with the intrinsic basaltic composition of mantle-derived melts. Classical models for new Crust generation from the mantle in two-stage processes fail to account for two fundamental facts: the absence of ultramafic residues in the lower Crust and the hot temperature of batholith magma generation. Other models based on the arrival of already-fractionated silicic magmas to the Crust have not received the necessary attention. Addition of new Crust by relamination from below of subducted materials has been formulated as a process complementary to delamination of mafic residues. Here we show important support to relamination from below the lithosphere as an important mechanism for new Crust generation in magmatic arcs of active Continental margins and mature intraoceanic arcs. The new support is based on three independent lines: (1) thermo-mechanical modeling of subduction zones, (2) experimental phase relations and melt compositions of subducted materials and (3) geochemical relations between mafic granulites (lower Crust) and batholiths (upper Crust). The mineral assemblage and bulk geochemistry of lower Crust rocks are compared with solid residues left after granite melt segregation. The implication is that an andesite magma precursor is responsible for the generation of new Continental Crust at active Continental margins and mature oceanic arcs. According to our numerical and laboratory experiments, melting and eventual reaction with the mantle of subducted oceanic Crust and sediments produce the andesite magmas. These ascend in the form of mantle wedge diapirs and are finally attached (relaminated) to the Continental Crust, where they crystallize partially and produce the separation of the solid fraction to form mafic granulites (lower Crust) and granitic (sl) liquids to form the batholiths (upper Crust).
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inherent gravitational instability of hot Continental Crust implications for doming and diapirism in granulite facies terrains
Geological Society of America Special Papers, 2004Co-Authors: Taras Gerya, Walter V Maresch, L L Perchuk, Arne P WillnerAbstract:Modeling of in situ rock properties based on a Gibbs free energy minimization approach shows that regional metamorphism of granulite facies may critically enhance the decrease of Crustal density with depth. This leads to a gravitational instability of hot Continental Crust, resulting in regional doming and diapirism. Two types of Crustal models have been studied: (1) lithologically homogeneous Crust and (2) heterogeneous, multilayered Crust. Gravitational instability of relatively homogeneous Continental Crust sections is related to a vertical density contrast developed during prograde changes in mineral assemblages and the thermal expansion of minerals with increasing temperature. Gravitational instability of lithologically heterogeneous Crust is related to an initial density contrast of dissimilar intercalated layers enhanced by high-temperature phase transformations. In addition, the thermal regime of heterogeneous Crust strongly depends on the pattern of vertical interlayering: A strong positive correlation between temperature and the estimated degree of lithological gravitational instability is indicated. An interrelated combination of two-dimensional, numerical thermomechanical experiments and modeling of in situ physical properties of rocks is used to study the processes of gravitational redistribution within a doubly stacked, heterogeneously layered Continental Crust. It is shown that exponential lowering of viscosity with increasing temperature, in conjunction with prograde changes in metamorphic mineral assemblages during thermal relaxation after collisional thickening of the Crust, provide positive feedback mechanisms leading to regional doming and diapirism that contribute to the exhumation of high-grade metamorphic rocks.
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inherent gravitational instability of thickened Continental Crust with regionally developed low to medium pressure granulite facies metamorphism
Earth and Planetary Science Letters, 2001Co-Authors: Walter V Maresch, Arne P Willner, Dirk D Van Reenen, Taras Gerya, Andre C SmitAbstract:Abstract Petrological arguments show that regionally developed low- to medium-pressure, high-temperature granulite facies metamorphism may critically enhance the lowering of Crustal density with depth. This leads to gravitational instability of homogeneously thickened Continental Crust, mainly due to changes in mineral assemblages and the thermal expansion of minerals in conjunction with the exponential lowering of the effective viscosity of rocks with increasing temperature. It is argued that Crustal processes of gravitational redistribution (Crustal diapirism) contributing to the exhumation of granulite facies rocks may be activated in this way.
Fang Huang - One of the best experts on this subject based on the ideXlab platform.
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barium isotopic composition of the upper Continental Crust
Geochimica et Cosmochimica Acta, 2018Co-Authors: Xiaoyun Nan, Roberta L Rudnick, Richard M Gaschnig, Qun Zhang, Zhangdong Jin, Fang HuangAbstract:Abstract The upper Continental Crust (UCC) is an important reservoir of Ba within the Earth. We report high precision (≤±0.05‰, 2SD) Ba isotopic data for 71 samples (including granites, granodiorites, loess, glacial diamictites, and river sediments) to constrain the Ba isotopic composition of the UCC. I-type granites from the Fogang batholith, Southeastern (SE) China, exhibit variable δ137/134Ba (−0.16‰ to 0.01‰), which may be due to isotopic fractionation during the latest stages of magmatic differentiation. The δ137/134Ba of S-type granites from Darongshan-Shiwandashan, SE China (−0.03‰ to 0.11‰) correlate with ɛNd (t), likely reflecting mixing of heterogeneous Crustal source materials. Five A-type granites with high SiO2 contents (∼76 wt%) from Nankunshan, SE China have remarkably low δ137/134Ba (−0.47‰ to −0.33‰), which possibly arose from magmatic differentiation or assimilation of Crustal materials with light Ba isotopic compositions. Loess from northwestern China has homogeneous δ137/134Ba (−0.02‰ to 0.03‰) that shows no correlation with bulk compositions, sample locations, or degree of chemical weathering, suggesting that loess is representative of the average Ba isotopic composition of the UCC. Three river sediments from northern China have δ137/134Ba similar to that of loess. The δ137/134Ba of glacial diamictites vary with CIA values: those with high CIA (≥60) have heterogeneous δ137/134Ba (−0.19‰ to 0.35‰), while those with low CIA ( √ n , n = 71), which is similar to the average Ba isotopic composition of the upper mantle, but significantly lower than δ137/134Ba of seawater.
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high mg adakitic rocks in the dabie orogen central china implications for foundering mechanism of lower Continental Crust
Chemical Geology, 2008Co-Authors: Fang Huang, Feng Dong, Fukun ChenAbstract:Abstract The late Mesozoic high-Mg adakitic rocks from Eastern and Central China provide important insight into the foundering mechanism of the over-thickened lower Continental Crust. The Chituling high-Mg adakites (131 ± 3 Ma, SHRIMP zircon U–Pb age) from the eastern margin of the Southern Dabie ultrahigh pressure metamorphic zone, adjacent to the Tan–Lu fault, have high Al 2 O 3 and Sr contents, high Sr/Y, La/Yb, and Mg# (44–63), but low Y and Yb contents. The samples also have moderately enriched 87 Sr/ 86 Sr i (0.70691–0.70775), very low e Nd ( T ) (− 20.7 to − 24.9), unradiogenic Pb isotopes, enrichment of large ion lithophile elements, and depletion of high field strength elements. These geochemical features indicate that they did not result from melting of young oceanic Crust, assimilation and fractional crystallization, or magma mixing. Instead, they were derived from partial melting of delaminated lower Continental Crust, with subsequent reaction with surrounding mantle peridotites during ascent to Crustal depths. The reactions between the adakitic melt and peridotites also generated the enriched mantle source of the post-collisional basaltic rocks in the Dabie orogen. Distribution of the late Mesozoic high-Mg adakites in eastern and central China generally forms a high-Mg adakite belt along the southern Tan–Lu fault. Therefore, we propose that the large strike-slip motion of the Tan–Lu fault in eastern and central China due to the western subduction of the Pacific plate in the early Cretaceous might trigger the foundering of some fragments of the over-thickened lithosphere near the Tan–Lu fault, which caused mantle upwelling and partial melting of the thickened lower Crust in the Dabie orogen and eastern boundary of the North China Craton. This further weakened the gravitationally instable lithosphere, consequently resulting in delamination and foundering of the mountain root underneath the Dabie orogen, which could be an important foundering mechanism of lower Continental Crust.
Fangzhen Teng - One of the best experts on this subject based on the ideXlab platform.
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magnesium isotopic composition of the deep Continental Crust
American Mineralogist, 2016Co-Authors: Wei Yang, Fangzhen Teng, Shengao Liu, Yongsheng Liu, Hongfu Zhang, Shan GaoAbstract:To constrain the behavior of Mg isotopes during deep Crustal processes and the Mg isotopic composition of the middle and lower Continental Crust, 30 composite samples from high-grade metamorphic terranes and 18 granulite xenoliths were investigated. The composites derive from eight different high-grade metamorphic terranes in the two largest Archean cratons of China, including 13 TTG gneisses, 5 amphibolites, 4 felsic, 4 intermediate, and 4 mafic granulites. They have variable bulk compositions with SiO 2 ranging from 45.7 to 72.5%, representative of the middle Crust beneath eastern China. The δ 26 Mg values of these samples vary from −0.40 to +0.12‰, reflecting heterogeneity of their protoliths, which could involve upper Crustal sediments. The granulite xenoliths from the Cenozoic Hannuoba basalts also have a diversity of compositions with MgO ranging from 2.95 to 20.2%. These xenoliths equilibrated under high temperatures of 800–950 °C, corresponding to depths of the lower Continental Crust (>30 km). They yield a large δ 26 Mg variation of −0.76 to −0.24‰. The light Mg isotopic compositions likely result from interactions with isotopically light metamorphic fluids, probably carbonate fluids. Together with previously reported data, the average δ 26 Mg values of the middle and lower Continental Crusts are estimated to be −0.21 ±0.07‰ and −0.26 ±0.06‰, respectively. The bulk Continental Crust is estimated to have an average δ 26 Mg of −0.24 ±0.07‰, which is similar to the average of the mantle. The large Mg isotopic variation in the Continental Crust reflects the combination of several processes, such as Continental weathering, involvement of supraCrustal materials in the deep Crust, and fluid metasomatism.
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heterogeneous magnesium isotopic composition of the lower Continental Crust a xenolith perspective
Geochemistry Geophysics Geosystems, 2013Co-Authors: Fangzhen Teng, Wei Yang, Roberta L RudnickAbstract:[1] We report 26 high-precision whole-rock Mg isotopic analyses for two suites of well-characterized granulite xenoliths from Chudleigh and McBride, North Queensland, Australia, in order to constrain the behavior of Mg isotopes during deep Crustal processes and the Mg isotopic composition of the lower Continental Crust. Previous studies suggest that the Chudleigh granulites are a suite of cogenetic cumulates crystallized from mafic magmas that intruded into and assimilated the preexisting lower Crust via combined assimilation and fractional crystallization (AFC). The δ26Mg values of the xenoliths range from −0.31 to −0.21‰ and correlate with radiogenic isotopes, reflecting mixing of mantle-derived mafic magma (δ26Mg = −0.31‰) with preexisting isotopically heavy Crustal materials (δ26Mg = ∼ +0.5‰) through the AFC process. The McBride granulites range compositionally from mafic to felsic, and originated as cumulates, solidified mafic/felsic melts, and restites that formed during basaltic underplating and reworking of preexisting lower Crust. Their δ26Mg values vary widely from −0.72 to +0.19‰. The large Mg isotopic variation in the McBride xenoliths reflects both distinct source compositions and metamorphic enrichment of garnet, which is isotopically light. Based on these results, the lower Continental Crust has a heterogeneous Mg isotopic composition, with a weighted average δ26Mg of −0.18‰. The bulk Continental Crust, based on available data, has an average Mg isotopic composition of −0.19‰, and is slightly heavier than the mantle. The highly heterogeneous Mg isotopic distribution in the Crust indicates that chemical weathering not only modifies the upper Crust compositions but also significantly influences lower Crust compositions through emplacement of upper Crustal materials into the deep Crust.
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lithium isotopic composition and concentration of the upper Continental Crust
Geochimica et Cosmochimica Acta, 2004Co-Authors: Fangzhen Teng, Roberta L Rudnick, Shan Gao, William F Mcdonough, Claude Dalpe, Paul B Tomascak, B W ChappellAbstract:The Li isotopic composition of the upper Continental Crust is estimated from the analyses of well-characterized shales, loess, granites and upper Crustal composites (51 samples in total) from North America, China, Europe, Australia and New Zealand. Correlations between Li, δ7Li, and chemical weathering (as measured by the Chemical Index of Alteration (CIA)), and δ7Li and the clay content of shales (as measured by Al2O3/SiO2), reflect uptake of heavy Li from the hydrosphere by clays. S-type granites from the Lachlan fold belt (–1.1 to –1.4‰) have δ7Li indistinguishable from their associated sedimentary rocks (–0.7 to 1.2‰), and show no variation in δ7Li throughout the differentiation sequence, suggesting that isotopic fractionation during Crustal anatexis and subsequent differentiation is less than analytical uncertainty (±1‰, 2σ). The isotopically light compositions for both I- and S-type granites from the Lachlan fold belt (–2.5 to + 2.7 ‰) and loess from around the world (–3.1 to + 4.5‰) reflect the influence of weathering in their source regions. Collectively, these lithologies possess a limited range of Li isotopic compositions (δ7Li of −5‰ to + 5‰), with an average (δ7Li of 0 ± 2‰ at 1σ) that is representative of the average upper Continental Crust. Thus, the Li isotopic composition of the upper Continental Crust is lighter than the average upper mantle (δ7Li of + 4 ± 2‰), reflecting the influence of weathering on the upper Crustal composition. The concentration of Li in the upper Continental Crust is estimated to be 35 ± 11 ppm (2σ), based on the average loess composition and correlations between insoluble elements (Ti, Nb, Ta, Ga and Al2O3, Th and HREE) and Li in shales. This value is somewhat higher than previous estimates (∼20 ppm), but is probably indistinguishable when uncertainties in the latter are accounted for.