Zircon and monazite perspectives on the origin and evolution of Gaowu leucogranites in Yadong area, southern Tibet

Abstract

The Himalayan Orogen, formed during the ongoing India-Asia continental collision, experienced a tectonic regime transition during the Middle Miocene, which resulted in a complex extension. This tectonic evolutional history is recorded in leucogranites intruding into the Himalayan crust. However, the source characteristics of leucogranites are difficult to determine because of the strong interaction between the magma and country rock. In this study, we performed U-Pb dating and trace element analysis of zircon and monazite, Hf isotope analysis of zircon, and mineral composition, elemental, and Sr-Nd isotopic geochemistry analyses of leucogranites from the Gaowu pluton in the Yadong area, southern Tibet. Three types of leucogranites (two-mica, tourmaline, and garnet) were collected from the pluton. These leucogranites show S-type and peraluminous granite affinities with SiO₂ content ranging from 71.58 to 76.91 wt% and A/CNK ratios of 1.06–1.2. Episodic leucogranitic magmatism occurred at ∼27 Ma, 23–21 Ma, 20–17 Ma, and ∼ 13 Ma, with variable ε_Hf(t) values ranging from −27.1 to −6.7. The oldest leucogranites showed relatively low Nb_N/Pb_N and Sm_N/La_N ratios, distinct from those with ages of 23–21 and 20–17 Ma. Additionally, leucogranites with ages of ∼27 Ma and 20–17 Ma (Eu/Eu* = 0.01–0.04) showed more significant negative Eu anomalies than those with ages of 23–21 Ma (Eu/Eu* = 0.03–0.09), which was attributed to compositional variations in the host magmas. Inherited zircons from the Gaowu leucogranites display scattered ages with a peak at approximately 492 Ma, and were characterized by lower Th and Pb concentrations, similar to the High Himalayan Crystalline Series (HHCS), indicating their derivation from the HHCS. Zircon trace element data identified Paleozoic gneissic granites and pegmatites as the potential source rocks of leucogranites in the Yadong area. According to zircon saturation thermometry, the peak melt temperature of two-mica leucogranites was approximately 715 °C, which was 55 °C and 115 °C higher than those of tourmaline leucogranites (660 °C) and garnet leucogranites (600 °C), respectively. After combining our results with previous findings, we propose that the Gaowu leucogranites were derived from muscovite fluid-absent melting at 27–17 Ma and muscovite fluid-present melting at ∼13 Ma. This transition in melting behavior could be linked to the activity of the South Tibetan Detachment System (STDS) and the Yadong–Gulu rift, which led to decompression and water infiltration, respectively. Concurrently, the eastern Himalayas tectonic regime transitioned from N-S extension and HHCS exhumation to E–W extension at ∼13 Ma. Thus, Middle Miocene Himalayan leucogranite rocks provide valuable insights into this tectonic regime transitions and deep geodynamic processes.