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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 Al2O3 and Sr contents, high Sr/Y, La/Yb, and Mg# (44–63), but low Y and Yb contents. The samples also have moderately enriched 87Sr/86Sr i (0.70691–0.70775), very low ε 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. [Copyright &y& Elsevier]

Distinct thermal states of mid-ocean ridges and adjacent transform faults on subducting plates tend to produce distinct magmatic responses on overlying plates. The origins and spatial distribution of the widespread late Mesozoic adakitic rocks in mid-eastern China may provide critical insights into understanding magmatic responses under various thermal states of the underlying Pacific plates. Among them, the intrusive rocks from the Xu-Huai (Xuzhou-Huaibei) region are spatially important but their petrogenesis remains controversial. These intrusions were proposed to be formed by foundering of an 'eclogitic' lower continental crust represented by the mafic xenoliths hosted by these intrusions. By eliminating abundant tiny xenoliths that potentially affect chemical and isotopic compositions of the host rocks, our new data reveal a positive correlation between Sr and Nd isotopic ratios defining two end-members on the EM-I and EM-II arrays respectively, which cannot be explained by a delaminated lower crustal source alone. The radiogenic Pb isotopic ratios of the Xu-Huai intrusions are as high as those of the oceanic crust and marine sediments. Meanwhile, these rocks are sodic with high Na 2 O/K 2 O, and have relatively low (La/Yb) N but high (Dy/Yb) N , Sr/La and Sr/Y, resembling those of oceanic adakites. Given the contemporaneous subducting oceanic ridges under mid-eastern China, the Xu-Huai intrusions were most likely derived from partial melting of a hot subducted oceanic crust that was close to the mid-ocean ridge. Consequently, a sandwich-like distribution pattern of Mesozoic adakitic rocks in mid-eastern China can be recognized, and subduction of a ridge-transform fault-ridge system is proposed to explain this observation. [ABSTRACT FROM AUTHOR]

Black shales represent one of the main reservoirs of metals released to hydrosphere via chemical weathering and play an important role in geochemical cycling of metals in the ocean. The stable isotope systematics of transitional metals (e.g., Cu and Zn) may be used as a proxy for evaluating their geochemical cycling. To investigate the behaviors of Cu and Zn isotopes during metal enrichment of black shales and the migration during weathering, in this study we reported Cu and Zn concentration and isotope data for unweathered and weathered metalliferous shales and siliceous interbeds from the Maokou Formation in central China. The unweathered shales and cherts have moderately enriched Cu and Zn concentrations with silicate-like δ 65 Cu (+ 0.14 ± 0.09‰, 1σ) but heavy δ 66 Zn (0.51 ± 0.11‰, 1σ). The elevated δ 66 Zn values reflect an important contribution from seawater via sulfide precipitation and/or organic matter (OM) adsorption. The Zn isotopic compositions of these metalliferous shales are different from those of the ‘normal’ shales, highlighting the potential of Zn isotopes as a tracer for metal enrichment in natural systems. The weathered shales and cherts have an extreme δ 65 Cu range from − 6.42‰ to + 19.73‰ and a modest δ 66 Zn range of + 0.25‰ to + 0.78‰. The strongly weathered samples have lower Cu and Zn concentrations and lighter isotopic compositions compared to the weakly weathered samples. The leaching of Cu- and Zn-rich sulfides in shallow depths and their downward transport and refixation by Fe-sulfide account for the Cu and Zn isotope fractionation, with the huge Cu isotope variation generated by multistage redox leaching. In general, δ 66 Zn values of the weathered shales shift towards light values compared to the unweathered protoliths, suggesting that shale weathering releases Zn which is isotopically heavier than igneous rocks and the global riverine average (+ 0.33‰). Our results therefore indicate that Cu isotopes can be extremely fractionated during weathering of Cu-rich shales and both heavy Cu and Zn isotopes are preferentially released into fluids during shale weathering. These results should be considered when evaluating geochemical cycling of Cu and Zn in the modern or past oceans. [ABSTRACT FROM AUTHOR]