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Previous studies have shown that the eastern North China Craton (NCC) lost its ancient lithospheric mantle root during the Phanerozoic. The temporal sequence, spatial extent, and cause of the lithospheric thinning, however, continue to be debated. Here we report olivine compositions, whole-rock Re–Os isotopic systematics, and platinum-group element abundances of small (< 2 cm in maximum dimension) mantle peridotite xenoliths from two basalt localities from the eastern NCC, Wudi (Cenozoic) and Fuxin (Cretaceous). These locations lie far (~ 150–200 km) from the Tan–Lu fault, which has been linked to lithospheric replacement in the eastern NCC. Peridotites at both locations have fertile to moderately refractory compositions (Fo < 91.5), while highly refractory (Fo > 92) lithospheric mantle is largely absent. Osmium isotopic data suggest the Wudi peridotites experienced melt depletion primarily during the Paleoproterozoic (~ 1.8 Ga), although an Archean Os model age for one xenolith indicates incorporation of a minor component of Archean lithospheric mantle. These data suggest that a previously unrecognized Paleoproterozoic orogenic event removed and replaced the original Archean lithospheric mantle beneath the sedimentary basin at the southern edge of the Bohai Sea. By contrast, the Fuxin peridotites, entrained in Cretaceous basalts that crop out along the northern edge of the eastern NCC, document the coexistence of both ancient (≥ 2.3 Ga) and modern lithospheric mantle components. Here, the original Late Archean–Early Paleoproterozoic lithospheric mantle was, at least partially, removed and replaced prior to 100 Ma. Combined with literature data, our results show that removal of the original Archean lithosphere occurred within Proterozoic collisional orogens, and that replacement of Precambrian lithosphere during the Mesozoic may have been spatially associated with the collisional boundaries and the strike–slip Tan–Lu fault, as well as the onset of Paleo-Pacific plate subduction. [ABSTRACT FROM AUTHOR]

Abstract: The petrology, mineral compositions, whole rock major/trace element concentrations, including highly siderophile elements, and Re–Os isotopes of 99 peridotite xenoliths from the central North China Craton were determined in order to constrain the structure and evolution of the deep lithosphere. Samples from seven Early Cretaceous–Tertiary volcanic centers display distinct geochemical characteristics from north to south. Peridotites from the northern section are generally more fertile (e.g., Al2O3 =0.9–4.0%) than those from the south (e.g., Al2O3 =0.2–2.2%), and have maximum whole-rock Re-depletion Os model ages (T RD) of ∼1.8Ga suggesting their coeval formation in the latest Paleoproterozoic. By contrast, peridotites from the south have maximum T RD model ages that span the Archean–Proterozoic boundary (2.1–2.5Ga). Peridotites with model ages from both groups are found at Fansi, the southernmost locality in the northern group, which likely marks a lithospheric boundary. The Neoarchean age of the lithospheric mantle in the southern section matches that of the overlying crust and likely reflects the time of amalgamation of the North China Craton via collision between the Eastern and Western blocks. The Late Paleoproterozoic (∼1.8Ga) lithospheric mantle beneath the northern section is significantly younger than the overlying Archean crust, indicating that the original lithospheric mantle was replaced in this region, either during a major north–south continent–continent collision that occurred during assembly of the Columbia supercontinent at ∼1.8–1.9Ga, or from extrusion of ∼1.9Ga lithosphere from the Khondalite Belt beneath the northern Trans-North China Orogen, during the ∼1.85Ga continental collision between Eastern and Western blocks. Post-Cretaceous heating of the southern section is indicated by high temperatures (>1000°C) recorded in peridotites from the 4Ma Hebi suite, which are significantly higher than the temperatures recorded in peridotites from the nearby Early Cretaceous Fushan suite (<720°C), and likely reflects significant lithospheric thinning after the Early Cretaceous. Combining previous Os isotope results on mantle xenoliths from the eastern North China Craton with our new data, it appears that lithospheric thinning and replacement may have evolved from east to west with time, commencing before the Triassic on the eastern edge of the craton, occurring during the Jurassic–Cretaceous within the interior, and post-dating 125Ma on the westernmost boundary. [Copyright &y& Elsevier]

Abstract: Xenolithic peridotites having a similar range of major element compositions from two nearby localities in the Trans-North China Orogen, North China Craton, provide a rare opportunity to explore effects resulting from both primary partial melting and secondary processes on Os isotopes and highly siderophile element (HSE) abundances. HSE patterns of peridotites from Hannuoba are similar to those of orogenic peridotite massifs worldwide, but are rare for xenolithic peridotites. These patterns can be explained by relatively low degrees of melt depletion, coupled with long-term preservation of sulfides. By contrast, peridotites from Yangyuan have major element compositions similar to or slightly more depleted than Hannuoba xenoliths, but are characterized by distinct, highly fractionated HSE patterns with lower total HSE abundances and Os, Pd and Re depletions relative to Ir. Some of the latter HSE characteristics must reflect secondary processes. The low S and Se contents of Yangyuan peridotites, coupled with scarcity of observable sulfides, suggest that they experienced sulfide breakdown, possibly as a result of interaction with a S-undersaturated melt/fluid. This may have occurred under oxidizing conditions, as suggested by the somewhat higher ƒO2 recorded in the Yangyuan peridotites compared to the Hannuoba peridotites, as well as the metal-deficient composition of rare, mono-sulfide-solid solution (mss) sulfides within the Yangyuan peridotites. We speculate that under such conditions, Os, Pd, and possibly Re, more strongly partition into a sulfide liquid, or the oxidizing medium (melt or fluid), than Ir and Pt and, thus, become depleted. These effects would have been imposed on original patterns that were similar to those in the Hannuoba suite. The good correlation between 187Os/188Os and major element indices of melt depletion in the Yangyuan rocks, coupled with the poor correlation between 187Os/188Os and 187Re/188Os, suggests that the S, Os, Pd and Re removal was recent. Hence, the long-term Re–Os isotopic systematics of these rocks would not have been affected, and Re depletion model ages, based on Os isotopes, remain viable to constrain the timing of melt deletion in these peridotites. The similarity of model age distributions between Yangyuan and Hannuoba peridotites (TRD =0 to 1.7 and 0 to 1.5Ga, respectively) is consistent with this, and further indicates that these peridotites formed in the Paleoproterozoic. [Copyright &y& Elsevier]

It is now well established that lithospheric thinning of the eastern North China Craton (NCC) occurred during the Phanerozoic. The cause and extent of the thinning, however, remains highly debated. In this study, mantle xenoliths from the Paleozoic Mengyin kimberlites, along with xenoliths from the Cenozoic Penglai and Shanwang basalts in Shandong Province, are investigated via traditional petrographic and elemental, Sr–Nd–Hf–Os isotopic and platinum-group element (PGE) analyses. Late Archean Os model ages of c. 2·5 Ga for the Mengyin peridotites provide confirmation that refractory, Archean lithospheric mantle existed beneath the easternmost portion of the NCC during the Paleozoic. Some Paleoproterozoic lithospheric mantle fragments may also be present in the Mengyin xenolith suite. In contrast, the spatially associated Penglai and Shanwang peridotite xenoliths are more fertile, and have Sr–Nd isotopic compositions similar to the depleted mantle. They differ dramatically from the Archean peridotites sampled by the Mengyin kimberlites. Osmium model ages for single samples range from mid-Proterozoic to modern, similar to variations observed in Phanerozoic convecting upper mantle as sampled by modern abyssal peridotites. This suggests that the present lithospheric mantle beneath the eastern NCC formed in the Phanerozoic, despite the fact that Os model ages extend back to the Proterozoic. Some samples from the Penglai suite yield Proterozoic Lu–Hf clinopyroxene mineral isochron ages that are consistent with the Os model ages, suggesting that Hf isotopes in modern convective upper mantle can preserve evidence for ancient melt depletion, similar to Os isotopes. Our results are consistent with thinning of the eastern NCC as a result of foundering of the deep crust and lithospheric mantle, but are inconsistent with stretching or refertilization models, as remnants of Archean mantle are expected to be present in these scenarios. The match between 187Os/188Os in convective upper mantle and the eastern China lithopspheric mantle also precludes models that seek to explain the present lithosphere as being due to lateral translation of Proterozoic lithosphere. [ABSTRACT FROM PUBLISHER]

Abstract: Lithium concentrations and isotopic compositions of 39 A-type granites and mafic enclaves from 11 plutons in Northeast China and the North China craton are used to constrain their genesis and to characterize further the average composition of the continental crust. Lithium concentrations (2.8 to 80 ppm) in 29 A-type granites are slightly higher than those of average I-type granites and lower than those of average S-type granites, reflecting source differences. Lithium isotopic compositions (δ 7Li=−1.8 to +6.9) of A-type granites fall within the range of worldwide I-type and S-type granites. By contrast, 10 mafic enclaves have significantly higher Li concentrations (32 to 179 ppm) and less variation in Li isotopic compositions (δ 7Li=−3.2 to +3.1) than their corresponding granites, due to the high modal abundance of amphiboles and biotites. Overall, the weighted mean δ 7Li for A-type granites, as well as I-type and S-type granites is lighter than that of the mantle. Lithium concentrations in granites vary as a function of fractional crystallization and are mainly controlled by modal mineralogy. Lithium mostly behaves as an incompatible element during granite differentiation, although Li is compatible in hornblende and biotite during crystallization of mafic enclaves. By contrast, Li isotopic compositions of A-type granites and mafic enclaves are not controlled by fractional crystallization, comparable to that for I- and S-type granites, indicating negligible Li isotope fractionation during granite differentiation. Instead, they reflect source heterogeneity produced by mixing of isotopically heterogeneous lower crust and/or variable amounts of re-equilibration between enclaves and host granites. The Li isotopic compositions of A-, I- and S-type granites can be used to derive an independent estimate of the average Li isotopic composition of the continental crust (i.e., δ 7Li=+1.7, weighted mean of all granites), comparable to our previous estimate (δ 7Li=+1.2) that relies on large-scale sampling of upper, middle and lower continental crust. Compared to the mantle, the continental crust has lower δ 7Li, indicating the influence of weathering and recycling in controlling the Li isotopic composition of the continents. [Copyright &y& Elsevier]

Abstract: Samples from Archean high-grade metamorphic terranes in China and granulite-facies xenoliths from Australia (Chudleigh and McBride suites) and China (Hannuoba suite) have been analyzed to assess the Li concentrations and isotopic compositions of the middle and lower continental crust, respectively. Thirty composite samples from metamorphic terranes, including tonalite–trondjhemite–granodiorite (TTG) gneisses, amphibolites and felsic to mafic granulites, show a large variation in Li concentrations (5–33 ppm) but a relatively narrow range in δ 7Li values, from +1.7 to +7.5 with a mean of +4.0±1.4 (1σ). These results suggest that the middle continental crust is relatively homogenous in Li isotopic composition and indistinguishable from the upper mantle. This may be a primary feature or may reflect homogenization of Li isotopes during exhumation of the metamorphic terranes. In contrast, Li isotopic compositions of granulite xenoliths from the lower crust vary significantly, with δ 7Li ranging from −17.9 to +15.7. δ 7Li of minerals also shows a very large spread from −17.6 to +16.7 for plagioclases and −14.6 to +12.7 for pyroxenes. Large Li isotopic variations exist between plagioclase and pyroxene, with pyroxenes (13 out of 14) isotopically equal to or lighter than coexisting plagioclases. Lithium concentrations of granulite xenoliths also vary widely (0.5 to 21 ppm) and are, on average, lower than those of terranes (5±4 vs. 13±6 ppm respectively, 1σ), consistent with a higher proportion of mafic lithologies and a higher metamorphic grade for the xenoliths. Pyroxene separates from granulite xenoliths have equal or significantly greater Li than coexisting plagioclase. These large Li isotopic variations between minerals and in whole-rock granulite xenoliths mostly reflect diffusion-driven kinetic isotopic fractionation during the interactions of xenoliths with host magma. Only those xenoliths that reach inter-mineral isotopic equilibria are likely to preserve the initial Li isotopic signatures of the lower crust. Eight such equilibrated samples have δ 7Li from −14 to +14.3, with a concentration weighted average of +2.5, which is our best estimate of the average δ 7Li of the lower continental crust. The substantial isotopic heterogeneity of the lower crust may reflect the combined effects of isotopic fractionation during granulite-facies metamorphism, diffusion-driven isotopic fractionation during igneous intrusion and variable protolith compositions. Consistent with previous B elemental and O isotopic studies, the Li isotopic heterogeneity in the lower crust indicates that pervasive fluid migration and equilibration have not occurred. Using all data for granulite xenoliths, the Li concentration of the lower crust is estimated to be ∼8 ppm. Together with previous estimates of Li concentration in the upper and middle crust, the average Li concentration of the bulk continental crust is estimated to be 18 ppm, which is similar to previous estimates. The average Li isotopic composition of the continental crust is estimated to be +1.2, which is isotopically lighter than upper mantle and may reflect the loss of isotopically heavy Li from the continents during weathering and metamorphic dehydration. [Copyright &y& Elsevier]

Assessing the K isotopic composition of the upper continental crust is important for understanding the processes by which the crust is generated and modified, and constraining the K isotopic budget of the silicate Earth. High-precision K isotopic data are reported for 70 well-characterized individual and composite samples from the upper continental crust, including diorite, granodiorite, granite, loess, shale, graywacke, pelite, and tillite, to constrain its K isotopic composition. δ41K varies significantly in eight I-type and two S-type granites from Australia (−0.57 to −0.40‰), nine A-type granites (−0.53 to −0.38‰), and three granitoid composites including diorite, granodiorite, and granite (−0.50 to −0.37‰) from China, mainly reflecting source heterogeneity. The 22 post-Archean Australian shales (PAAS) (δ41K = −0.68 to −0.12‰) and the 12 sedimentary composites including graywacke, pelite, and tillite from China (δ41K = −0.57 to −0.23‰) have heterogeneous K isotopic compositions while the 12 loess samples from around the world display more limited K isotopic variation (δ41K = −0.47 to −0.35‰). δ41K values display a smaller dispersion in loess compared to shales, which have comparatively more intense weathering and higher chemical index of alteration (CIA). δ41K correlates with Fe 2 O 3 /Al 2 O 3 and Fe 2 O 3 /K 2 O in shales and Al 2 O 3 /SiO 2 , K 2 O/Al 2 O 3 , and δ7Li in loess, suggesting that K isotopes are fractionated during chemical weathering. Overall, the upper continental crust has a heterogeneous K isotopic composition, ranging from −0.68 to −0.12‰ with an average δ41K of −0.44 ± 0.05‰ (2SD, n = 88), which is indistinguishable from the mantle. [ABSTRACT FROM AUTHOR]

Abstract: Concentrations of trace elements in orthopyroxenes and clinopyroxenes, and the Sr, Nd, Hf and Pb isotopic compositions of clinopyroxenes from 11 well-characterized spinel peridotite xenoliths carried in the ~30Ma Yangyuan alkali basalts, North China Craton, reveal a complex history of chemical modification of the lithospheric mantle. The clinopyroxenes were separated from a suite of peridotites for which whole-rock Re–Os isotopic systematics have previously suggested a melt depletion event at ~1.8Ga, and were chosen to span a range of rare earth element (REE) patterns with (La/Yb) N =0.13 to 8.2. Present-day isotopic compositions are highly variable: 87Sr/86Sr=0.70229 to 0.70443, ε Nd =−0.6 to +24, 206Pb/204Pb=15.74 to 19.08, and ε Hf =+13.5 to +167. Partial melting models indicate that the Yangyuan clinopyroxenes have light REE (LREE) abundances that are too high to be pristine residues of fractional melting, reflecting subsequent enrichment of the LREE, e.g., via melt addition. Ancient melt addition accounts for the decoupling observed between Nd and Hf isotopic data for most samples, although a few samples may also have been modified by recent melt addition, shortly before the entrainment of the xenoliths into the host basalt. Strontium, Nd and Pb isotopic systems provide no useful age constraints for this suite, although some samples have Paleoproterozoic model ages. By contrast, Yangyuan clinopyroxenes yield a Lu–Hf correlation trend corresponding to an apparent age of 1.66±0.10Ga (2σ) that is within uncertainty of their whole-rock Os model age (1.8±0.2Ga). Given that the crust in this area originally formed during the Archean, these results suggest mantle lithosphere removal and replacement during the Paleoproterozoic. Our data demonstrate that, in some instances, the Lu–Hf isotope system may be used as a complement to the whole‐rock Re–Os isotope system in dating melt depletion of peridotites. [Copyright &y& Elsevier]

Abstract: Serpentinized garnet peridotites from the Xugou peridotite body of the Sulu ultrahigh-pressure (UHP) metamorphic terrane, central eastern China, are refractory (olivines have Fo91.7–93.1), indicating their origin as residual mantle. Negative correlations between whole-rock MgO and TiO2, Al2O3, total Fe2O3 and CaO (r =−0.90 to −0.95) and positive correlations between whole-rock Al2O3 and CaO and incompatible elements [Li, V, Cu, Ga, Sr, Y, Zr, heavy rare earth elements (HREEs), Hf, Pb and U] (r =0.69 to 0.98) likely reflect melt depletion trends. Four highly refractory samples were selected for Re–Os isotopic analysis. Although they show evidence of variable enrichment of incompatible elements during serpentinization/metasomatism, no correlations exist between 187Re/188Os or 187Os/188Os with either La or Re (r =0.00 to 0.17). These results indicate that any Re addition was fairly recent and did not affect the Os isotopic composition significantly. The correlation between 187Os/188Os and 187Re/188Os ratios thus, most likely reflects an ancient melt extraction event. The T RD, T MA and errorchron ages of the Xugou peridotites are all similar, suggesting that these peridotites formed around 2.0 Ga ago. This age is similar to Os model ages of mantle peridotites from the Dabie terrane, but contrasts markedly with the Archean ages of the continental lithospheric mantle (CLM) beneath the eastern block of the North China craton (NCC). If we assume that the Dabie–Sulu belt formed by the Triassic collision of the Yangtze craton with the eastern block of NCC and that the Archean aged CLM of the latter persisted until the Triassic, the Paleoproterozoic ages suggest derivation of these Dabie–Sulu mantle peridotites from the Yangtze craton. A Yangtze craton origin is consistent with the existing tectonic model of the Dabie–Sulu UHP belt. Our results support the hypothesis that the crust and underlying lithospheric mantle of the Yangtze craton were subducted to depths of >180–200 km to form the world''s largest UHP belt. [Copyright &y& Elsevier]