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Abstract: In order to characterize the recycled lower continental crust (LCC) in the Dabie orogen, 17 Early Cretaceous low-Mg adakitic (LMA) and 9 normal (non-adakitic) granitoids have been investigated for Srph name="sbnd" />Pb isotopes. Combined with literature data, LMA have low ƐNd(t) (−27.8 to −14.7) and 206Pb/204Pb(i) (15.69–17.16) and low to moderately high 87Sr/86Sr(i) (0.7066 to 0.7087) ratios. Normal granitoids yield isotope ratios similar to adakitic rocks, except a few with 87Sr/86Sr (i) up to 0.7105. Dabie LMA define a linear trend parallel to the North Hemisphere Reference Line (NHRL) in a 208Pb/204Pb(i)– 206Pb/204Pb(i) diagram. For a given 206Pb/204Pb(i), the 208Pb/204Pb(i) or ∆8/4 (152–217) of Dabie LMA are close to the majority of UHP gneisses and the Neoproterozoic mafic rocks from the northern margin of the South China Block (SCB), but significantly higher than adakitic rocks from the North China Block (∆8/4<150). Considering the commonly present Neoproterozoic inherited zircons, we suggest that the LMA in the Dabie orogen are derived from a thickened LCC that could be dominantly composed of ancient SCB lower crust. The Srph name="sbnd" />Pb isotopic composition of LMA are similar to those of Post-Collisional Mafic Igneous rocks (PCMI) from the Dabie orogen, but different to exhumed UHP rocks. The Srph name="sbnd" />Pb isotopic system of the mantle source of the PCMI could be enriched in components dominantly from delaminated LCC of the Dabie orogen instead of deeply subducted continental crust, which is in contrast to O- and C-isotope data by [Zhao, Z.F., Zheng, Y.F., Wei, C.S., Wu, Y.B., Chen, F.K., and Jahn, B.M., 2005. Zircon Ulement and C geochemistry of post-collisional mafic–ultramafic rocks from the Dabie orogen in east-central China. Lithos 83(1–2), 1–28; Dai, L.Q., Zhao, Z.F., Zheng, Y.F., Li, Q.L., Yang, Y.H., and Dai, M.N., 2011. Zircon Hf evidence for crust–mantle interaction during continental deep subduction. Earth and Planetary Science Letters 308, 229–244]. Combining the Srph name="sbnd" />Pbce:glyph name="sbnd" />C isotopic data, two types of commonly present magma components have been identified in the PCMI, reflecting crust–mantle interaction during the Triassic subduction and the Early Cretaceous orogen collapse, respectively. [Copyright &y& Elsevier]

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]

Abstract: Post-collisional mafic–ultramafic intrusive rocks from the North Dabie zone show evidence for derivation from a mantle source that was overprinted by partial melts derived from subducted crust of the South China Block. All the samples are characterized by enrichment of large ion lithophile elements (LILE, e.g., Ba, Th) and depletion of high field strength elements (HFSE, e.g., Nb, Ti). The Zhujiapu intrusion developed in the North Dabie zone has moderately enriched 87Sr/86Sr i (0.70605–0.70827), negative ε Nd(T) values (−2.3 to −18.8), and unradiogenic Pb isotopes. Such typical “continental” geochemical features do not result from crustal contamination or magma mixing, but reflect the properties of the mantle source. Because the trace element and Sr–Nd isotopic compositions of the post-collisional mafic–ultramafic intrusive rocks from the North Dabie zone are similar to those of the Mesozoic mantle-derived rocks and lower crust granulites and xenoliths from the North China Block, it is unclear whether the “continental” features are due to the contribution of lithospheric mantle and lower crust from the North China Block or deeply subducted materials from the South China Block. However, for a given 206Pb/204Pb i or Δ7/4, the 208Pb/204Pb i or Δ8/4 (159–180) of the post-collisional mafic–ultramafic intrusive rocks from the North Dabie zone is close to the Mesozoic granitoids, eclogites, and gray gneisses from the Dabie–Sulu orogen, but significantly higher than the Mesozoic mantle-derived rocks and lower crust granulites and xenoliths from the North China Block (Δ8/4<160). This indicates that the subducted slabs of the South China Block experienced a higher time-integrated Th/U but similar U/Pb as the deep lithosphere of the North China Block. We therefore propose that the mantle source of the post-collisional mafic–ultramafic intrusive rocks from the Dabie–Sulu orogen result from the depleted upper mantle metasomatised by melts from recycled deeply subducted crust. Due to the continent–continent collision between the South China Block and North China Block in the late Triassic, the lithosphere of the Dabie orogen was over-thickened resulting in eclogitic metamorphism in the mafic lower crust, producing a gravitational instability. The subducted eclogitic lower crust foundered and was recycled into the upper mantle beneath the orogen, resulting in mountain root removal, less mafic composition of unusually evolved continental crust, and upper mantle heterogeneity. [Copyright &y& Elsevier]

Iron (Fe) isotope fractionation could occur during crustal melting, which may contribute to the isotopic variations of granites. Our current knowledge on Fe isotope fractionation during crustal melting remains rare. Thirteen leucosomes, 9 melanosomes and 1 amphibolitic schollen in 11 migmatites from the Dabie orogen, Central China were measured to investigate Fe isotope fractionation during crustal anatexis. The melanosomes and amphibolitic schollen yield δ 56 Fe values from 0.018 ± 0.031‰ to 0.152 ± 0.027‰ with an average of 0.106‰. The leucosomes have δ 56 Fe values from 0.107 ± 0.035‰ to 0.512 ± 0.028‰, variably higher than their coexisting melanosomes by 0.00 ~ 0.36‰. The δ 56 Fe of leucosomes and melanosomes, as well as apparent isotope fractionation (Δ 56 Fe L-M ) between them, do not correlate with the abundance of crystalline carbonate, loss on ignition, and Th/U, indicating the insignificant effect of fluid components. High δ 56 Fe values of three leucosomes (≥ 0.34‰), giving large Δ 56 Fe L-M from 0.22 to 0.36‰, were likely produced by feldspar accumulation, evidenced by their high Eu*, low FeOt/Al 2 O 3 and high plagioclase abundance up to 50 vol%. Capture of peritectic amphiboles in another leucosome may explain its low Δ 56 Fe L-M ~ 0‰. After screening out these samples, 7 leucosome – melanosome pairs yield identical Δ 56 Fe L-M averaging 0.093 ± 0.056‰ (2SD, N = 7) within analytical uncertainties. No correlation between Δ 56 Fe L-M and Mg#, FeOt/Al 2 O 3 or TiO 2 /FeOt rules out the possibility that the fractionation observed here is produced by fractional crystallization and/or sub-solidus isotope re-equilibrium between leucosomes and melanosomes. Therefore, we suggest these consistent Δ 56 Fe L-M should record equilibrium Fe isotope fractionation during crustal anatexis producing these migmatites. The leucosomes and melanosomes have comparable Fe 3 + /ΣFe ~ 0.40, and Δ 56 Fe L-M approximates 0.07‰ at Fe 3 + / ΣFe leucosome Fe 3 + / ΣFe menalosome = 1. Fe isotope fractionation during crustal anatexis revealed here thus is not dominantly controlled by distribution of Fe 3 + and Fe 2 + , but by the difference in coordination number of iron between granitic melts and residual mafic minerals. The observation here argues that Fe isotope fractionation during crustal partial melting should also contribute to the previously revealed δ 56 Fe variation in high silica granitic rocks. [ABSTRACT FROM AUTHOR]