The Mesozoic metamorphic–magmatic events in the Medog area, the Eastern Himalayan Syntaxis: constraints from zircon U–Pb geochronology, trace elements and Hf isotope compositions in granitoids

Abstract

Based on the regional geological mapping, several granitoid intrusions had been discovered in the Eastern Himalayan Syntaxis (EHS). In order to constrain their petrogenesis and discuss their relations with the regional tectonics, we carried out U–Pb dating, trace elements and Hf isotope geochemistry studies on zircons separated from the granitoid rocks, in the area of the EHS. In this contribution, the granitoid rocks are mainly composed of diorites (X20-1-6) and granitic gneissic rocks (X2-15-1). The U–Pb zircon dating of diorites yields a crystallization age of 193.8 ± 2.0 Ma. These zircon have eHf(t) values ranging from −6.48 to −0.05, indicating an involvement of ancient crustal materials in the generation of these igneous rocks. The zircons from the Medog granitic gneissic rock commonly show zoning structures. The REE patterns and abundances of the inherited cores are different from those of the oscillatory rims. The LA–ICP-MS U–Pb zircon in situ analyses indicate that: (1) the zircon cores give multi-stage magmatic event ages ranging from 516 to 1,826 Ma, of which six ages are converged on 1,330–911 Ma, it is considered that the migmatitic gneiss has been formed in this time, and (2) while the zircon rims yield 206Pb/238U weighted mean ages of 217.4 ± 3.0 Ma (MSWD = 3.2), which was interpreted to represent the ages of the Triassic anatexis. Their eHf(t) values range from −18.98 to −8.36 and −14.22 to 8.72, respectively. The timing of the anatexis in the Medog area is coeval with the widespread metamorphism in Lhasa terrane.