Petrogenesis and tectonic setting of the Early Permian gabbro–granite complex in the southeastern Central Asian Orogenic Belt, central Inner Mongolia, China

Abstract

Gabbro–granite complexes formed in active continental margins provide critical constraints on magmatic processes, crust–mantle interactions, and tectonic evolution. The petrogenesis and tectonic setting of the Early Permian igneous rocks in the southeastern Central Asian Orogenic Belt have been a subject of dispute for a long time. This contribution presents an integrated study on the field occurrence, petrology, petrography, zircon U–Pb ages, in situ zircon Hf isotopes, whole-rock geochemistry, and Sr–Nd isotopes of the Early Permian Yiheshaorong gabbro–granite complex, Xi Ujimqin Banner, central Inner Mongolia. Lithologically, the Yiheshaorong complex consists predominantly of gabbro, pyroxene diorite, and alkali feldspar granite. Results of LA–ICP–MS zircon U–Pb dating suggest that crystallization ages of the gabbro, pyroxene diorite, and alkali feldspar granite are 281–280, 278, and 281–280 Ma, respectively. Silica (SiO2) content varies from 41.26 to 79.64 wt% and exhibits a gap between 56.84 and 75.45 wt%, displaying a good bimodal geochemical distribution. Geochemically, the gabbros and pyroxene diorites are characterized by enrichment of large ion lithophile elements (LILEs; e.g., Ba and K) and depletion of high field strength elements (HFSEs; e.g., Nb, Ti, and P). These rocks show similar homogeneous zircon Hf isotopic (positive εHf (t) values from +7.8 to +10.1) and whole-rock Sr–Nd isotopic (low initial 87Sr/86Sr ratios from 0.7044 to 0.7051, positive εNd (t) values from +3.7 to +4.7) compositions. The gabbros and pyroxene diorites originated from partial melting of depleted asthenospheric mantle metasomatized by subduction slab fluids; and they experienced various degrees of crystallization differentiation during ascension. The alkali feldspar granites are ferroan, alkali-calcic–calc-alkalic series rocks marked by high SiO2, K2O + Na2O, and FeOT/MgO; and low Al2O3, CaO, and MgO contents. Their trace elements are marked by enrichment of LILEs (such as K and Rb); depletion of HFSEs (such as Nb and Ti), Ba, Sr, and P; and strongly negative Eu anomalies (Eu/Eu* = 0.17–0.57), with coincident distribution patterns forming a seagull-type distribution form, showing an A-type granite affinity. The alkali feldspar granites are characterized by positive εHf (t) values (+8.6 to +10.3, young TDM2 ages, 645–758 Ma) and εNd (t) values (+3.5 to +3.8 as well as young TDM2 ages, 667–687 Ma). All these geochemical characteristics indicate that the alkali feldspar granites originated from the partial melting of juvenile crust under high temperature and low pressure condition. The Yiheshaorong complex was most likely formed in a back-arc spreading basin. Such an extensional regime can likely be attributed to the upwelling of asthenospheric mantle caused by the rollback and break-off of a subducted Paleo-Asian Ocean slab in the Early Permian, which induced large-scale extension and magmatism in the study area.