Geochemistry, Geochronology, and Tectonic Significance of Yuejin Sumu Bimodal-like Intrusive Rocks in Xilinhot, Inner Mongolia

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Abstract
The Early Permian Yuejin Sumu intrusive rocks are located approximately 40 km northwest of Xilinhot City, Inner Mongolia. The magmatic rocks primarily composed of gabbro, diorite, and syenogranite. Through the study of zircon LA-ICP-MS U–Pb geochronology and rock geochemistry, age dates of the three lithologies show that the three formed simultaneously in the late Early Permian (gabbro formation age is 276.7 ± 1.3 Ma; diorite is 276.9 ± 0.8 Ma; syenogranite is 275.0± 1.4 Ma). The geochemical characteristics of the three lithologies are primarily calc-alkaline, and some samples, such as gabbro and diorite, show a transition to the alkaline series. The oxides in the gabbro and diorite are linearly related to SiO2. With magma MgO content, Al2O3 content increases first and then decreases, while CaO content decreases sharply, suggesting that the magma for the gabbro and diorite is a mantle-derived magma that undergoes separation and crystallization. The crystalline minerals include magnesia-rich pyroxene in the early stage and the plagioclase and pyroxene in the late stage. The syenogranite has high aluminum content (12.8–14.6%), and the ratio of FeOT/Mg# is high. Rare earth elements are characterized by a mid-range of the light and heavy rare earth elements. Eu shows significant negative anomalies, and the whole rare earth elements (REEs) are characterized by V-shaped distribution. The syenogranite are enriched in K, Rb, Th, U, Zr, and depleted in Eu, Ba, Sr, Ti, P, Nb, Ta, indicating that the magma may derived from crust in a typical post-orogenic environment. Petrology and geochemical characteristics of the Early Permian intrusive rocks indicate that in the Early Permian (~274–278 Ma) in the Xilinhot area was back-arc extensional environment at the active continental margin. Mantle-derived magma caused partial melting of the crustal material. The mantle-derived magma and the melt of the crustal material formed the Early Permian bimodal-like Yuejin Sumu intrusive rocks.

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