TWO-STAGE GROWTH MODEL OF THE JUNGGAR BASIN BASEMENT: CONSTRAINS FROM GEOCHRONOLOGY AND GEOCHEMISTRY OF THE BOREHOLE CARBONIFEROUS–PERMIAN VOLCANIC ROCKS

Junggar Basin is located in the central part of the Central Asian Orogenic Belt (CAOB). Its basement nature is a highly controversial scientific topic, involving the basic style and processes of crustal growth.

In this paper, based on the borehole data from over 300 wells drilled into the Carboniferous System, together with the high-resolution gravity and magnetic data (in a 1:50,000 scale), we made a detailed analysis of the basement structure, formation timing and process and later evolution on basis of core geochemical and isotopic analysis. Firstly, we defined the Mahu Precambrian micro-continental block in the juvenile crust of Junggar Basin according to the Hf isotopic analysis of the Carboniferous volcanic rocks. Secondly, the results of the tectonic setting and basin analysis suggest that the Junggar area incorporates three approximately E-W trending island arc belts (from north to south: Yemaquan–Wulungu–Chingiz, Jiangjunmiao–Luliang–Darbut and Zhongguai–Mosuowan–Baijiahai–Qitai respectively) and intervened three approximately E-W trending retro-arc or inter-arc basin belts from north to south, such as Santanghu–Suosuoquan–Emin, Wucaiwan–Dongdaohaizi–Mahu (Mahu block sunk as a bathyal basin during this phase) and Fukang–western well Pen1 accordingly. Thirdly, the closure of these retro-arc or inter-arc basins gradually toward the south led to the occurrence of collision and amalgamation of the above-mentioned island arcs during the Carboniferous, constituting the basic framework of the Junggar “block”. Fourthly, the emplacement of large-scale mantle-derived magmas occurred in the latest Carboniferous or Early Permian. For instance, the well Mahu 5 penetrate the latest Carboniferous basalts with a thickness of over 20m, and these mantle-derived magmas concreted the above-mentioned island arc-collaged body. Therefore, the Junggar basin basement mainly comprises pre-Carboniferous collaged basement, and its formation is characterized by two-stage growth model, involving the Carboniferous lateral growth of island arcs and the latest Carboniferous to Early Permian vertical crustal growth related to emplacement and underplating of the mantle-derived magmas.