The Carboniferous tectonic attributes of the Bogda orogenic belt in Xinjiang: Evidence from gabbro chronology and geochemistry

Basic intrusive rock masses are extensively developed in Bogda orogenic belt, and their origin is of great significance to the correct understanding of the geodynamic background of this area. In this paper, the authors studied systematically petrology, geochemistry and zircon U-Pb geochronology of gabbro in the west of Bogda orogenic belt, in order to discuss their petrogenesis and tectonic significance and then to provide a basis for solving geological tectonic evolution of Bogda orogenic belt. Petrogeochemical analysis shows that their SiO₂ values range from 48.08% to 51.03%, and ALK values range from 3.09% to 4.39% (less than 5%), and that they are rich in CaO (6.87%-11.41%), and depleted in Al₂O₃ (12.59%-18.49%). The MgO values range from 3.78% to 7.66%, Mg^#=31.75-65.73 (47.03 on average), and A/CNK=0.60-0.80, indicating that the rocks belong to metaluminous intermediate K calc-alkaline rock series. The analysis also shows that they have low REE (36.30×10^-6-147.72×10^-6), with no obvious fractionation between light and heavy rare earth elements and between heavy rare earth elements. They are obviously depleted in Th, Ta, Nb elements, with no obvious loss of Ti and slight enrichment of U element, showing the characteristics of continental rift magmatism. According to the relevant diagram and the ratio of trace elements and rare earth elements, the gabbro magma probably originated from depleted lithospheric mantle, with the contamination of the crust; their differentiation evolution degree was low; they experienced higher degree of partial melting of the original phase spinel peridotite, and were formed in the intraplate rift environment. Moreover, gabbro LA-ICP-MS zircon U-Pb dating results show that the formation age is (305.0±1.6) Ma, suggesting the late period of the Bogda rift volcanic activity in late Carboniferous. All these data indicate that the west of Bogda rift underwent a conversion from the overall extension stage to the confined extension stage. The analysis of regional geological data shows that there existed a large rift evolution related to magmatism during Late Paleozoic (Early Carboniferous-Early Permian), and the orogenic belt was developed on the basis of rifting, with its formation related to the northward subduction of the Junggar-Turpan-Hami block driven by the Kangurtag Ocean.