Petrogenesis of Mesozoic granitoids in the Dabie UHP complex, Central China: trace element and Nd–Sr isotope evidence

Abstract

Post-collisional magmatism in the Dabie Orogen is characterized by the near contemporaneous intrusion of voluminous granitoids and minor mafic magmas in the late Mesozoic (135–110 Ma). The granitoids are represented by three major batholiths in Northern Dabie: Zhubuyuan, Baimajian and Tiantangzhai. The largest Zhubuyuan Batholith comprises hornblende-bearing intermediate rocks (Penghe suite) and hornblende-free granitic rocks (Huangbai suite). The Huangbai granites show limited variation in chemical composition, but are highly variable in textures, ranging from medium to coarse grained. They have rather homogeneous Sr and Nd isotopic compositions, with ( 87Sr/ 86Sr) i=0.708–0.709 and ε Nd( T)=−19 to −24. The Penghe intermediate suite covers a wide spectrum of rock types, including diorite, monzodiorite, quartz monzonite to monzogranitic rocks, and all of them contain hornblende and mafic enclaves. The existence of compositional and textural disequilibrium (e.g., calcic and resorbed cores of plagioclase) and the nature of chemical variation in these rock types indicate that magma mixing between an evolved mafic and a granitic magma was involved in their genesis. Fractional crystallization (FC) could have coupled with the process of magma mixing but with minor contamination by the upper continental crust during magma ascent. Isotopically, the intermediate rocks show a variation in ε Nd( T) values from −13.7 to −18, while their ( 87Sr/ 86Sr) i ratios vary from 0.7074 to 0.7088. The genesis and evolution of the Dabie post-collisional magmatic rocks may be summarized as follows. Intrusion of mantle-derived mafic/ultramafic magmas into thickened continental crust induced dehydration melting of the lower crust rocks with intermediate compositions and subsequent production of large amounts of granitic melts (Huangbai suite). Extensive interaction between the crustal and mantle-derived melts through mixing produced a hybrid parental magma which subsequently underwent a FC process en route to higher crustal levels producing a wide variety of rock types as shown by the Penghe intermediate rock suite. Injection of successive pulses of basic magma into upward mobile magma chambers could have resulted in the formation of the mafic enclaves as well as mafic dykes.