Petrogenesis and geodynamic implications of Early Cretaceous highly fractionated leucogranites in the northern Lanping-Simao terrane, Eastern Tibetan Plateau

摘要

A felsic intrusion in the Guaguazi Cu mining district in the northern Lanping-Simao terrane, Eastern Tibetan Plateau was revealed through drill holes and is composed of highly fractionated leucogranites. In this paper, we report zircon U-Pb ages and whole-rock compositions to constrain the petrogenesis and geodynamic setting of this intrusion. Zircon grains separated from two leucogranite samples have identical U-Pb ages of 123.9 +/- 2.0 Ma and 123.6 +/- 1.9 Ma, indicating that the crystallization age of the intrusion is Early Cretaceous (ca. 124 Ma). Leucogranites from the intrusion have SiO2 contents from 74.3 to 77.6 wt% with differentiation index values of 96.3-97.6, indicative of highly fractionated nature. They are weakly peraluminous (A/ CNK = 1.01-1.10) and have light REE enrichment ((La/Yb)(N) = 5.4-8.5) and negative Eu anomalies (Eu/Eu* = 0.26-0.45). The primitive-mantle-normalized spider diagram indicates strong enrichment in large-ion lithophile elements (Rb, Th, U, K, and Pb) relative to high-field-strength elements (Nb, Ta, P, and Ti). The high initial Sr-87/Sr-86 ratios (0.7070-0.7094) and negative epsilon Nd(t) values (-8.18 to-7.20) and T-DM2 ages of 1.58-1.51 Ga of the rocks, suggest that their source was predominantly Mesoproterozoic lower continental crust with insignificant mantle contribution. High initial Pb-207/Pb-204 (15.74-15.75) and Pb-206/Pb-204 (18.51-18.63) ratios further confirm their crustal source and highly evolved nature. Considering the petrogenetic similarity of Early Cretaceous granitoids in the Guaguazi area and the Gaoligong tectonic belt, we propose that the eastern Bangong-Nujiang Tethyan Ocean was closed at least no later than the Early Cretaceous, and that the Guaguazi granitic intrusion formed in a crustal thickening setting in response to the Lhasa-Qiangtang collision.