The Late Permian highly fractionated I-type granites from Sishijia pluton in southestern Inner Mongolia, North China: A post-collisional magmatism record and its implication for the closure of Paleo-Asian Ocean

Abstract

Whole-rock major and trace elements, zircon UPb geochronology and Hf isotopic data have been implemented for the felsic rocks from the Sishijia pluton in southeastern Inner Mongolia, North China, in order to determine their petrogenesis and futher constrain the termination of Paleoasian Oceanic subduction in southeastern Central Asian Orogenic Belt. Petrographic observation indicates that Sishijia granitoids mainly consist of syenogranites and porphyritic granites, with the major mineral compositions of alkaline feldspar, quartz, plagioclase and biotite, without any alkaline melanocratic or peraluminous minerals. Miarolitic structures are widely developed in these granitoids, indicating for the characteristics of shallow emplacement depth and rapid condensation. Zircon LA-ICP-MS UPb dating yields weighted mean ages of 256.2 ± 1.4 and 250.9 ± 1.2 Ma for syenogranites, and 251.6 ± 1.4 Ma for porphyritic granites, representing a Late Permian magmatism record. These granitoids exhibit weakly peraluminous nature (A/CNK = 1.03–1.08) with high SiO2 (76.50–81.92 wt%) and K2O (3.64–5.56 wt%) concentrations and differentiation index values (D.I. = 93.8–97.2). Chondrite-normalized REE patterns of these samples display enrichment of LREE with high ratios of (La/Yb)N (12.29–30.34) and significant negative Eu anomalies (δEu = 0.15–0.53). In mantle-normalized multielement variation diagrams, they are all characterized by typical depletions of Ba, Nb, Ta, Sr, P, and Ti and enrichments of Rb, Th, U, and K. Petrological and geochemical features of the pluton indicate a highly fractionated I-type affinity. Zircon Hf isotope analyses show variably negative εHf(t) values ranging from −13.7 to −4.0 and corresponding two-stage model ages varying of 2148–1540 Ma, indicating that they were derived from an Paleo- to Mesoproterozoic crustal source with minor juvenile materials. In combination with previous geological evidences, it is reasonable to propose that the Sishijia granitoids emplaced in a post-collisional extension regime during the transformation from pre-collision to post-orogenic, which symbolize a Late Permian final closure of Paleo-Asian Ocean in southeastern segment of the Central Asian Orogenic Belt.