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Post-collisional granitoids from the Dabie orogen: New evidence for partial melting of a thickened continental crust |
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| Authors: | Yongsheng He Shuguang Li Jochen Hoefs Sheng-Ao Liu |
| Institution: | a CAS Key Laboratory of Crust-Mantle Materials and Environments, School of Earth and Space Sciences, University of Science and Technology of China, Hefei, 230026 Anhui, China b Department of Geosciences, University of Göttingen, Goldschmidtstr. 1, Göttingen 37077, Germany c Institute of Geochemistry and Petrology, ETH Zurich, 8092 Zurich, Switzerland |
| Abstract: | The geological implications of granitoid magmas with high Sr/Y and La/Yb are debated because these signatures can be produced by multiple processes. This study presents comprehensive major and trace element compositions and zircon SHRIMP U-Pb age data of 81 early Cretaceous granitoids and 4 mafic enclaves from the Dabie orogen to investigate partial melting of the thickened lower continental crust (LCC). On the basis of Sr/Y ratios, granitoids can be grouped into two magma series: (i) high Sr/Y granitoids (HSG) and (ii) normal granitoids with low Sr/Y. Relative to normal granitoids, HSG display the following distinct chemical features: (1) at given SiO2 and CaO contents, the HSG have significantly higher Sr than normal granitoids, defining two different trends in Sr versus SiO2, CaO diagrams; (2) highly depleted heavy rare earth element (REE) relative to middle and light REE with (Dy/Yb)N and (La/Yb)N up to 3.2 and 151, respectively; (3) variable and higher Nb/Ta; and (4) positive correlations among Sr/Y, (Dy/Yb)N, (La/Yb)N, and Nb/Ta.High Sr/Y, (La/Yb)N, (Dy/Yb)N, and Sr/CaO of HSG do not correlate with major elements (e.g., SiO2). Large variations in these ratios at a given SiO2 content indicate that these features do not reflect magma mixing or fractionation. HSG have higher Sr at a given CaO content and larger variation of (Dy/Yb)N than old crustal rocks (including exposed basement, global mafic LCC xenoliths, high Sr/Y TTG suites, and adakites in modern arcs). This precludes inheritance of the HSG chemical features from these source rocks. Instead, the chemical features of the HSG are best explained by partial melting of the thickened LCC with garnet-dominant, plagioclase-poor, and rutile-present residual lithologies. The coupled chemical features of the HSG are not observed in post-collisional granitoids younger than ca.130 Ma, indicating removal of the eclogitic source and/or residuum of HSG underneath the orogen. These characteristic chemical relationships in the Dabie HSG may be applied to distinguish partial melts of thickened LCC from high Sr/Y intermediate-felsic magmatic rocks which do not show clear indications for melting depth. |
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