Early Jurassic high-temperature granites along eastern Bangong-Nujiang suture in Tibet and its geodynamic implications

Abstract

The varying geochemical compositions of granites are generally considered as the proxy record of different geodynamic processes. The granites collected in the Kaqiong microblock along the eastern Bangong-Nujiang suture in Tibet show A-type granite affinity, but do not strictly follow the definition of A-type granite in some aspects. Zircon U Pb dating shows that the Kaqiong granite pluton was emplaced in the Early Jurassic (186–176 Ma), bracketing in the timing of the lithospheric and asthenospheric mantle-derived magmatism (197–175 Ma) in the Kaqiong microblock. These granites are metaluminous to peraluminous and high-K calc-alkaline to shoshonitic rocks. They have high contents of alkalis (K 2 O + Na 2 O), high field strength elements (e.g., Zr, Nb), and rare earth elements (except Eu), with high magmatic temperatures; these features are typical of A-type granite. However, these granites show slightly lower 10,000 × Ga/Al ratios of 2.3–3.0 and lower FeO tot /(FeO tot + MgO) values of 0.58–0.68 than the typical A-type granite. Their whole-rock Sr Nd isotopic compositions ( I Sr = 0.72996–0.73481, ε Nd (t) = −12.4 to −10.8, T DM C = ~1.9 Ga) and zircon Hf isotopic compositions (ε Hf (t) = −18.9 to −6.1, T DM2 = ~2.0 Ga) indicate that they mainly originated from pre-existing crustal materials. We suggest that the Kaqiong granites were produced by protracted partial melting of the basement rocks at relatively high temperature and high oxygen fugacity conditions. The Early Jurassic high-temperature granites were formed due to a significant mantle heat addition into the crust in an extensional continental arc, caused by the rollback of the subducted Bangong-Nujiang Tethyan oceanic slab. The study is an example of how high-temperature granites could shed light on the geodynamic processes. • Geochronology, geochemistry, and Sr-Nd-Hf isotope compositions of the Kaqiong granites • The Kaqiong granites are the high-temperature granites. • The Kaqiong granites formed in an extensional continental arc • The type of granites provides insight into the geodynamic processes of a subducting slab.