Partial melting of thickened lower crust in post‐collisional setting: Evidence from high silicon adakitic granites in the North Qilian orogen

Abstract

During orogenic extension, partial melting of the continental crust usually produces post‐collisional granitoids, and the characteristics of their petrogenesis can provide crucial constraints on the geodynamics of tectonic regimes and post‐collisional lithospheric processes. A case study for this issue is the high‐silicon adakitic granites from the Jingtieshan area, North Qilian orogenic belt. In this study, we present a range of evidences on petrology, zircon U‐Pb dating, zircon Lu‐Hf isotopic composition, whole‐rock geochemistry, and Sr‐Nd isotopes from the studied high‐silicon adakitic granites to better understand the geodynamic processes of the North Qilian orogenic belt. The adakitic rocks consisted of plagioclase (55–60 %), K‐feldspar (5–10 %), quartz (20–25 %), and a small amount of biotite and accessory minerals. Zircon grains from the studied sample yielded a weighted mean 206Pb/238U age of 430 ± 2 Ma (MSWD = 0.96) with positive εHf(t) values (−1.4 to +6.3, with an average of +4.2). Geochemical results suggested that the adakitic rocks were enriched in LREE without evident Eu anomalies (except for one sample; δEu = 0.32–0.41), and these samples exhibited high Sr/Y ratios with negative εNd(t) values (−4.9 to −3.1). A combined study of petrology and geochemical arguments lead us to propose that the high‐silicon adakitic granites from the Jingtieshan area within the North Qilian orogenic belt were generated by partial melting of the thickened crust in a post‐collisional tectonic setting and that the adakitic melts were derived from the partial melting of the metabasite that was emplaced into the crust during the late Proterozoic‐early Neoproterozoic.