Petrogenesis of post-collisional magmatism at the Carboniferous–Permian boundary in central Inner Mongolia, NE China: insights into when the Hegenshan Ocean closed?

The tectonic evolution of the Hegenshan Ocean is an important but poorly understood issue in the geology of the southeastern Central Asian Orogenic Belt (CAOB). Here we present an integrated study of petrology, geochronology, geochemistry, and Sr–Nd–Hf isotopes on a volcanic suite from the Baoligaomiao Formation near Dongwuqi, central Inner Mongolia, NE China. LA-ICP-MS zircon U–Pb dating indicates that the volcanic rocks formed at the Carboniferous–Permian boundary (ca. 301–297 Ma). Geochemistry data reveal that the rocks belong to alkaline to sub-alkaline series, and are characterized by enrichments in light rare earth elements (La_N/Yb_N = 13.01–39.30) and large ion lithophile elements and depletions in heavy rare earth elements and high field strength elements (e.g., Nb, Ta, and Ti). They have relatively low initial ⁸⁷Sr/⁸⁶Sr ratios (0.702570–0.708084) and highly positive whole-rock Ԑ_Nd(t) (2.27–2.72) and zircon Ԑ_Hf(t) values (0.66–11.92). These geochemical characteristics and Sr–Nd–Hf isotopic compositions reveal that the rocks originated from partial melts of a juvenile, arc-type middle-lower crust, with fractional crystallization as a dominant factor controlling the magma differentiation. The rocks were generated in a post-collisional regime, and occurred as a magmatic response to the final closure of the Hegenshan Ocean at the Late Carboniferous (ca. 305 Ma). In addition, based on a review of the pre-existing literature data, a model of continental arc–back-arc basin system at the southern margin of South Mongolian microcontinent is proposed to explain the tectonic evolution of the Hegenshan Ocean.