Abstract
The Central Asian Orogenic Belt (CAOB) is one of the largest accretionary orogens in the world. The mechanism of continental growth and tectonic evolution of the CAOB remain debated. Here we present an overview of Early Paleozoic ophiolitic mélanges, calc-alkaline intrusions, and metamorphic rocks in West Junggar with an aim to provide constraints on the time and mechanism of subduction initiation in the Junggar Ocean, a branch of the southern Paleo-Asian Ocean (PAO). The Early Paleozoic ophiolitic mélanges are composed of ultramafic-mafic rocks, cherts, pelagic limestones, basaltic breccias and tuffs. The mafic rocks from these ophiolitic mélanges are divided into MORB-like and OIB-like types. The MORB-like rocks were formed in a fore-arc setting, but the OIB-like mafic rocks were formed by the intra-plate magmatism related to mantle plume activities. The Early Paleozoic intrusions are occurred as small stocks with a dominant composition of diorite, trondhjemite, and granodiorite. These granitoids display (high-K) calc-alkaline affinities, and have high and positive εNd(t) and εHf(t) values, formed in an arc-related setting. The metamorphic complex is mainly composed of blueschist and amphibolite blocks with metamorphism ages ranging from ~500 Ma to ~460 Ma. Their protoliths are calc-alkaline andesite and tholeiitic and alkaline basalts, formed in an arc related and seamount setting, respectively. It is clearly show that the West Junggar was under an intra-oceanic subduction regime during the Early Paleozoic, and the initial subduction of the southern PAO might have occurred in the Early Cambrian. Based on our observations, and in combination with previous work, we propose the plume-induced subduction initiation model for the Early-Middle Cambrian tectonic evolutionary of the Junggar Ocean. Our new model not only shed light on subduction initiation dynamics of the southern PAO, but also contribute to tectonic evolution of the CAOB.
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