Magmatism associated with lithospheric thinning, mantle upwelling, and extensional tectonics: Evidence from Carboniferous-Permian dyke swarms and granitoids from Inner Mongolia, Central Asian Orogenic Belt

Abstract

The east-Ujimqin complex located north of the Erenhot-Hegenshan ophiolite belt in North China is composed of extensive Carboniferous and Permian magmatic rock suites, which preserve the records of an extensional tectonic regime following collision. These rocks provide important clues to the closure of the Hegenshan ocean and intraplate processes. Here we investigate the diorite dyke swarms in association with granitic rocks from the Xing'an-Mongolian Orogenic Belt (XMOB) through field investigations, petrology, geochemistry, zircon U-Pb and Lu-Hf studies, to gain insights into the timing, magma source and tectonic setting during Late Carboniferous and Early Permian. The diorite rocks display sub-alkaline, calc-alkaline, and high-K calc-alkaline features. Their extensional setting is suggested by plots in the fields of within plate basalt and active continental margin + alkaline oceanic arc. Zircon U-Pb data on magmatic zircons indicate that the granitoids crystallized at ca. 301 Ma, whereas the diorite dykes were emplaced at 286 Ma. Xenocryst zircon grains with 320 to 300 Ma ages in the dyke samples suggest incorporation from the surrounding granites. Zircon Lu-Hf isotopic data show consistently positive εHf(t) values (5.3 to 10.8) and plot between CHUR (Chondritic Uniform Reservoir) and depleted mantle lines suggesting that depleted mantle sources contributed significantly to the magma source. The computed model ages suggest that the magma source involved Neoproterozoic juvenile components.We propose a model where the magmatism occurred within an extensional tectonic setting caused by asthenosphere upwelling and lithospheric thinning. Our study also constrains the timing of Early Permian extension which suggests that the Hegenshan Ocean, as part of the Central Asian Orogenic Belt closed prior to 286 Ma.