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Derivation of Adakitic Magma by Partial Melting of Subducted Continental Crust

ZHANG Hong-fei1, WANG Jing1, XU Wang-chun1, YUAN Hong-lin2   

  1. 1. The State Key Laboratory of Geological Processes and Mineral Resources and Faculty of Earth Sciences, China University of Geosciences, Wuhan 430074, China;2. The State Key Laboratory of Continental Dynamics, Northwest University, Xi ,an 710069, China
  • Received:2007-06-20 Revised:2007-06-20 Online:2007-06-20 Published:2007-06-20

Abstract: Present geological models for adakitic magma generation assume that most adakitic magmas were produced by partial melting of subducted oceanic slabs. However, it is also possible that the adakitic magma may be generated by partial melting of subducted continental crust in continental collisional settings. Here, we present a case study of the Guanshan granite (229 Ma) from the southeastern part of the Qilian Orogenic Belt. The Guanshan granite is characterized by high K (K2O: 4.12~5.16 %; K2O/Na2O: 0.97~1.64) and Sr/Y (13.6~84.1), low Y (6.8×10-6~15.7×10-6) and HREE (eg. Yb: 0.62×10-6~1.31×10-6), strongly fractionated REE patterns with (La/Yb)N =17.5~41.6 and evolved Sr~Nd isotopic compositions (87Sr/86Sr)229Ma = 0.70587~0.70714, εNd(229 Ma) = -10.9~-5.16,tDM = 1.10~1.49 Ga). These geochemical signatures show that the Guanshan granite has an affinity with continenttype adakitic rocks, but are distinct from the adakites, produced by partial melting of subducted oceanic slabs and/or underplated basaltic lower crust in arc settings. Pb-Sr-Nd isotopic compositions of the Guanshan granite are quite distinct from those of the Precambrian basement rocks, the Early Paleozoic volcanic rocks and the Early Paleozoic granitoids in the Qilian belt of the southern margin of the North China plate, but are similar to those of the lower crust of the northern margin of the Yangtze (South China) plate, as probed by the Early Mesozoic granites of the northern margin of the Yangtze plate. It is proposed that the magma for the Guanshan granite was derived from partial melting of the subducted Yangtze continental crust during the Triassic continental collision between the North China and Yangtze plates. Our result provides a new model that adakitic magma can be generated by partial melting of subducted continental crust in continental collisional settings.