Two episodes of Early Cretaceous magmatism in Geji area of the Lhasa Block, Tibet

A series of sub-parallel, NS-trending dikes of rhyolitic composition intruded the Bangba granitic complex in Geji area, western central Lhasa Block. Field investigation and LA-MC-ICP-MS zircon U-Pb dating indicate that two episodes of Early Cretaceous magamatism occurred at 131~132Ma and 127Ma respectively in this area. Earlier episode magmatism is represented by the formation of the dioritic enclaves and their hosts of granodiorite and granite. The magmatic rocks of this episode are characterized by①enrichment of LILEs(K, Cs) and depletion of HFSEs(Nb, Ti, Zr); ②negative Eu anomalies(Eu/Eu^*=0.49~0.61) and negative Ce anomalies; ③negative zircon ε_Hf(t)(-3.2~-0.3) and ancient two stage model ages(1.210~1.399Ga); and(4) unradioenic to radiogenic initial ⁸⁷Sr/⁸⁶Sr(0.70424~0.71472), and negative Nd(t)(-5.70~-5.54). The rhyolitic dikes, generated by the second episode of magmatism, are characterized by①enrichment of LILEs(K, Cs) and depletion of HFSEs(Nb, Ti, Zr); ②weak negative Eu anomalies(Eu/Eu^*=0.74~0.87); and③highly variable negative zircon ε_Hf(t)(-6.1~-0.7) and more ancient two stage model ages(1.226~1.576Ga). Both the zircon saturation thermometer(777~796℃) and monazite saturation thermometer(794~812℃) yielded slightly higher magma temperature for the rhyolitic dike than the temperatures(661~762℃and 750~769℃) either for granodiotite or for the mafic enclave. Both episodes of magmatism are considered to have been the consequence of mixing of mantle-derived magma with the mag-ma derived from melting of ancient continental crust, which might have resulted from roll-back of southward subduction of the Ban-gong-Nujiang oceanic lithosphere.