Zircon U–Pb dating, geochemical, and Sr–Nd–Pb–Hf isotopic constraints on the age and origin of intermediate to felsic igneous rocks at South Altyn, Xinjiang, China

Abstract

As a part of a giant trending fault system in the Asian continent and one where a strong zone of left strike-slip fault is present, the Altyn Orogenic belt (AOB) has become an important focus for research. Magmatic rocks are widely distributed across the AOB. However, many investigations have focused primarily on Paleozoic igneous rocks; discussion of Mesozoic related igneous activity is often ignored. Here we present the result of studies of representative diorite and granite rocks outcropping in the AOB, within the Xinjiang Uygur Autonomous Region, South Altyn, China. We present new zircon LA-ICP-MS U–Pb age, geochemical, and Sr–Nd–Pb–Hf isotopic data for these sample suites, identifying them as typical igneous rocks formed between 238 ± 1.5 and 238.8 ± 1.1 Ma. The rocks that we studied fall into the alkaline series, also enriched in light rare earth elements (LREE), some large ion lithophile elements (LILE; e.g., Rb, Ba, Sr, and K), Pb, Th and U, and depleted in heavy rare earth elements (HREE), Nb, Ta, Hf, and Ti. The granite and diorite have high initial 87Sr/86Sr ratios (0.7062–0.7114), negative eNd (t) values (− 8.8 to − 11.3), eHf (t) values (− 8.7 to − 18.7), and relatively constant Pb isotopic ratios ((206Pb/204Pb)i = 6.74–17.884, (207Pb/204Pb)i = 15.51–15.58, and (208Pb/204Pb)i = 35.36–38.04), respectively. This suggests that the magmas parental to these rocks were generated from the partial melting of the ancient crust. The parental magmas to these rocks experienced a degree of fractionation of plagioclase, K-feldspar, and hornblende, possibly during rapid magma ascent. Based on these studies, we propose a reasonable model for the origin of the investigated rocks from the Xinjiang Uygur Autonomous Region of South Altyn, which involves crustal thickening, lithospheric extension, and asthenosphere upwelling, that induced crustal melting.