Sr-Nd-Pb isotopic compositions of Early Cretaceous granitoids from the Dabie orogen: Constraints on the recycled lower continental crust

摘要

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 Sr-Nd-Pb isotopes. Combined with literature data, LMA have low ε_(Nd)(t) (-27.8 to -14.7) and ~(206)Pb/~(204)Pb(i). (15.69-17.16) and low to moderately high ~(87)Sr/~(86)Sr(i) (0.7066 to 0.7087) ratios. Normal granitoids yield iso-tope ratios similar to adakitic rocks, except a few with ~(87)Sr/~(86)Sr (i) up to 0.7105. Dabie LMA define a linear trend parallel to the North Hemisphere Reference Line (NHRL) in a ~(208)Pb/~(204)Pb(i)- ~(206)Pb/~(204)Pb(i) diagram. For a given ~(206)Pb/~(204)Pb(i), the ~(208)Pb/~(204)Pb(i) or A8/4 (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 (A8/4<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 Sr-Nd-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 Sr-Nd-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 0- 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 U-Pb age, element and C-O isotope 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-O isotope evidence for crust-mantle interaction during continental deep subduction. Earth and Planetary Science Letters 308, 229-244]. Combining the Sr-Nd-Pb-Hf and O-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. .