Zircon U-Pb age and petrogenesis of the Permian highly fractionated granites in Tongpu, eastern Tibet

How to precisely date the high-silica granites with U-rich or high-U zircons and how to effectively constrain their petrogenesis remain two puzzling questions in the research of granite. This paper reports zircon U-Pb ages and Hf isotope, whole-rock major and trace elemental data of the high-silica granites from the marginal facies of the Tongpu batholith in northern Jomda-Weixi belt, eastern Tibet. Reliable zircon U-Pb age data (260 +/- 1 Ma) of the U-rich or high-U zircons from the high-silica granite were obtained by pre-ablation dating method, i. e., conducting 2 similar to 3 pulses ablation firstly with a large size beam spot (100 mu m) before normal ablation. These samples are characterized by high SiO2 (74. 92% similar to 76. 46%), high Na-2 O + K-2 O (7. 61% similar to 8. 55%), high differentiation index (92 similar to 96), and significant negative Eu anomalies (Eu/Eu * = 0. 17 similar to 0. 41) and strong depletion of Ba, Nb, Sr, P, and Ti, similar to those of highly fractionated granites. These data, along with the data in the literature, indicate that the Tongpu high-silica granites can be assigned to be highly fractionated I-type granites. The negative zircon epsilon(Hf)(t) (-9. 9 similar to 5. 8) values indicate that they are most likely derived from partial melting of ancient crustal material. Quantitative modeling shows that the Tongpu high-silica granites can be formed by fractional crystallization of plagioclase, apatite, and Ti-rich minerals from the coeval granodioritic magma.