Petrogenesis of the Xuexiumaer Biotite Quartz Monzonite Porphyry in Southern Gangdese and its Implications for Paleo-Crustal Thickness of the Qinghai–Tibet Plateau

Abstract

The Qinghai–Tibet Plateau possesses the thickest continental crust on Earth, yet the timing of its formation remains debated. In this study, we conducted zircon U-Pb isotopic dating, geochemical and Sr-Nd-Pb-Hf isotopic analyses on the Xuexiumaer biotite quartz monzonite porphyry (BQMP) sampled from the Lake Dajia area in southern Gangdese. This study aims to estimate the paleo-crustal thickness beneath this region during the early India-Asia collision stage using whole-rock Sr/Y and (La/Yb)_N ratios as proxies. Results reveal that the Xuexiumaer BQMP was formed at ~51 Ma in a collisional tectonic setting following Neo-Tethyan slab breakoff, and is an I-type granitoid derived primarily from partial melting of juvenile mafic lower crust with subordinate ancient crustal input. The estimated paleo-crustal thickness in the Lake Dajia area at ~51 Ma is less than 40 km. This indicates that although the Qinghai–Tibet Plateau had already undergone significant crustal thickening and attained an exceptionally thick crust (>50 km) prior to the India–Asia collision as demonstrated by previous studies, some regions still maintained a crust only slightly thicker than the average continental crust (~35 km) at the initial collision stage. This limited crustal thickening likely resulted from underplating of subduction-related mafic magma at the mantle-crust boundary.