Origin of LateTriassic high-Mg adakitic granitoid rocks from the Dongjiangkou area, Qinling orogen, central China Implications for subduction of continental crust

The origin of high-Mg adakitic granitolds in collisional orogens can provide important information about the nature of the lower crust and upper mantle during the orogenic process Late-Triassic high-Mg adakitic granite and its mafic enclaves from the Dongjiangkou area the Qinling orogenic belt central China were derived by partial melting of subducted continental crust and underwent interaction with the overlying mantle wedge peridotite Adakitic affinity of the different facies of the Dongjiangkou granite body are high Sr Ba high La/Yb and Sr/Y low Y Yb Yb/Lu and Dy/Yb and no significant Eu anomalies suggesting amphibole + garnet and plagioclase-free restite in their source region Evolved Sr-Nd-Pb isotopic compositions [(Sr-87/Sr-86)(t) = 0 7050 to 0 7055 epsilon(Nd)(t) = -6 6 to -3 3 (Pb-206/Pb-204)(t)= 17 599 to 17 799 (Pb-207/Pb-204)(t) = 15 507 to 15 526 (Pb-208/Pb-204)(t) = 37 775 to 37 795] and high K2O Rb together with a large variation in zircon Hf isotopic composition (epsilon(Hf)(t) = 9 8 to + 5 0) suggest that the granite was derived from reworking of the ancient lower continental crust CaO P2O5 K2O/Na2O Cr Ni Nb/Ta Rb/Sr and increase and SiO2 Sr/Y and Eu/Eu* decrease with increasing MgO consistent with interaction of primitive adakitic melt and overlying mantle peridotite Zircons separated from the host granites have U-Pb concordia ages of 214 +/- 2 Ma to 222 +/- 2 Ma compatible with exhumation ages of Trassic UHP metamorphic rocks in the Dabie orogenic belt Mafic microgranular enclaves and mafic dykes associated with the granite have identical zircon U-Pb ages of 220 Ma and are characterized by lower SiO2 high TiO2 Mg# and similar evolved Sr-Nd-Pb isotopic composition Zircons from mafic microgranular enclaves (MMEs) and mafic dykes also show a large variation in Hf isotopic composition with epsilon(Hf)(t) between 11 3 and + 11 3 It is Inferred that they were formed by partial melting of enriched mantle lithosphere and contaminated by the host adakitic granite magma In combination with the regional geology high Mg# adakitic granitold rocks in the Dongiangkou area are considered to have resulted from interaction between subducted Yangtze continental crust and the overlying mantle wedge Triassic continental collision caused detachment of the Yangtze continental lithosphere subducted beneath the North China Craton at ca 220 Ma causing asthenosphere upwelling and exhumation of the continental crust Triassic clockwise rotation of the Yangtze Craton caused extension in the Dabie area which led to rapid exhumation of the subducted continental lithosphere while compression in the Qinling area and high-P partial melting (amphibole +/- garnet stability field) of the subducted continental crust produced adakitic granitic magma that reacted with peridotite to form Mg-rich hybrid magma Crown Copyright (C) 2010 Published by Elsevier B V All rights reserved