Discovery of Early Cambrian grandiorite north of Xar Moron River, Inner Mongolia: petrogenesis and tectonic significance

In this paper, the geochronology, petrology and geochemistry of the Dong’gou gneissic granodiorite, which was previously classified as part of the early Precambrian ‘Shuangjing Paleocontinent’ north of the Xar Moron River in southeast Inner Mongolia, are studied. Zircon U-Pb dating shows that the granodiorite was formed in the Early Cambrian (526.4±2.2 Ma). Combined with other geochronological age data published in the region, it is suggested that ‘Shuangjing Paleocontinent’ was not an early Precambrian micro-continent in the Central Asian Orogenic Belt (CAOB). The Dong’gou granodiorite is mainly composed of plagioclase, quartz and amphibole. The contents of SiO₂ of the rocks range from 64.20 to 67.53 wt%, and the contents of Na₂O and K₂O are 5.21 to 5.75 wt% and 0.37 to 0.56 wt%, respectively, indicating the characteristics of subalkaline series magmatic rocks, which are rich in sodium and poor in potassium. The aluminum saturation index (A/CNK) of the rocks is between 0.77 and 0.85, further suggesting that the Dong’gou granodiorite belonging to meta-aluminous I-type granite. In terms of trace elements, the rocks are depleted in high field strength elements (HFSE) such as Nb, Ta, P and Ti, indicating the geochemical characteristics of the magmatic rocks formed in subduction setting. The whole rock ε_Nd(t) values and lower crust two-stage model ages of the Dong’gou granodiorite are -13.76 to -15.84 and 2361 Ma to 2523 Ma, respectively, suggesting that there was Archaean-Paleoproterozoic crustal material in the magmatic source. Based on the analysis of regional data, the Dong’gou granodiorite might belong to the Early Paleozoic Bainaimiao magmatic arc between the North China Craton and the Xar Moron River suture zone. The discovery of the pluton suggests that the subduction of the Paleo-Asian Ocean in southeast CAOB might have started in Early Cambrian.