Crustal thickening of the western Tibetan Plateau during the Early Cretaceous: Evidence from adakitic granodiorites, leucogranites, and monzogranites in the Songxi area, western Qiangtang Terrane

Abstract

Felsic igneous rocks in the Songxi area are a key part of the Qiangtang Terrane and India–Asia collision zone, and can constrain the evolution of the Tibetan Plateau. This paper presents new geochronological, geochemical, and isotopic data for Early Cretaceous granodiorites (119 Ma), leucogranites (111 Ma), and monzogranites (107–102 Ma) from the Songxi area in the western Qiangtang Terrane. The granodiorites are characterized by enriched Sr–Nd–Pb–Hf isotopic compositions (87Sr/86Sri = 0.70721–0.70925, εNd(t) = −7.6 to −5.6, 206Pb/204Pbi = 18.907–18.964, 207Pb/204Pbi = 15.782–15.825, and εHf(t) = −24.0 to −2.0) and are adakitic with low Mg# values (39–44) and high K2O contents (2.6–3.9 wt%). The leucogranites have low Sr concentrations (80–100 ppm), high Rb/Sr (2.83–3.53) and low CaO/Na2O (0.19–0.22) ratios, high initial 87Sr/86Sr ratios (0.71653–0.71721), and low εNd(t) (−12.4 to −11.8) and εHf(t) (−18.8 to −6.8) values. The monzogranites are I-type granitoids and have enriched Sr–Nd–Pb–Hf isotopic compositions (87Sr/86Sri = 0.70956–0.70966, εNd(t) = −8.5 to −8.3, 206Pb/204Pbi = 18.740–18.749, 207Pb/204Pbi = 15.759–15.778, and εHf(t) = −16.9 to −1.4). Our data suggest that these three types of granitoids were likely generated by partial melting of a thickened lower crust, metapelite-dominated source by fluid-absent muscovite melting, and normal lower crust, respectively. Integration of our new and existing data allows us to conclude that the Songxi granitoids formed in a continental margin arc setting. This further suggests that some sediment-derived leucogranites form in a continental arc setting, and are unrelated to continental collision. Tectonic compression and crustal thickening of the western Tibetan Plateau during the Early Cretaceous occurred when the Bangong–Nujiang oceanic lithosphere was subducted northward at a low angle beneath the western Qiangtang Terrane before ca. 119 Ma.