Petrogenesis and tectonic setting of the K-feldspar granites in Mangshi area, western Yunnan： Evidence from petro-geochemistry and zircon U-Pb isotope chronology

Abstract：Located in Tengchong block of western Yunnan, the Paleocene K-feldspar granites in northern Mangshi are mainly composed of K-feldspar granites. Most of the Mangshi K-feldspar granites resulted from the disintegration of original Gaoligong Group, as evidenced by 1∶50000 regional geological mapping. To precisely limit the age of the K-feldspar magmatic mineralization in Mangshi area, the authors analyzed the granites by using LA-ICP-MS zircon U-Pb technique and geochemical methods. The zircon LA-ICP-MS dating indicates that the K-feldspar granites was emplaced at (60.8±0.8) Ma. The granite mass was formed in the Paleocene period. Geochemical data show that the Lianghe granites are K basaltic, high-K calc-alkaline and peraluminous to strongly peraluminous series, characterized by high content of SiO2 （67.97%～80.62%）, high content of K2O+Na2O (5.08%～8.79%）, medium to high K（K2O/Na2O=1.11～17.80）, and lower content of P2O5 (0.01%～0.15%) with peraluminous ratio (A/CNK) being 1.03～2.31. ACF plot indicates that the granites are S type granites. Mangshi granites are also enriched in Rb, Th, U, Ta, La, Pb, Nd, Sm and Gd，but strongly depleted in Ba, Nb, Ce, Sr, Zr, Eu and Hf. The REE content is between 56.42×10-6 and 322.91×10-6, 183.15×10-6 on average, with (La/Yb)N ratios being 1.58～41.60 and δEu being 0.18～0.71. REE diagram shows right inclination and obvious negative Eu anomalies. The intrusion is remarkably characterized by enriched LREE and depleted HREE. The chondrite-normalized REE patterns show negative Eu anomalies. Magma source was probably derived from the emplacement of partially melted granitic magma dominated by greywacke and claystone, which is typical of crustal genesis. The K-feldspar granite was derived from partial melting of the Mid-Proterozoic metamorphosed basement (Gaoligong Group). According to geochemistry and the trace element discrimination diagrams for tectonic interpretation, the Paleocene K-feldspar granites are considered to be of the island arc and syn-collision type formed in a transitional tectonic setting from the compressional system to the island arc system. The Paleocene K-feldspar granite was formed in the syn-collision and island arc tectonic setting. The Paleocene K-feldspar granites in northern Mangshi area indicate that the Yaluzangbu-Mizhila Ocean had been closed and the subduction and collidion between the Indian plate and the Gangdise-Tengchong massif had occurred before 60.80Ma. Tengchong block was formed in magmatic arc tectonic setting at that time. The Gangdise-Tengchong block tectono-magmatic orogenesis was intimately related to the closure and subduction and collidion orogenesis between the Indian plate and the Gangdise-Tengchong massif. The determination of the crystallization and origin of Mangshi K-feldspar granites provides new evidence for the Paleocene tectonic evolution of the Tengchong massif．