How long did the Paleo-Asian Ocean take from initial subduction to slab rollback?: Revelation of early Paleozoic magmatism in the eastern Boshchekul-Chingiz arc
Authors: Zhang Yi , Chen Jiafu , Liu Yuqi , Ma Xu , Han Baofu , Zhang Yifan 
Published: September 2024

Abstract
To understand the accretionary processes of the Central Asian Orogenic Belt, we must first identify the period from initial subduction to slab rollback and the mature mechanism of island arcs. The Boshchekul-Chingiz arc, one of the most important Mariana-type island arcs in the southwestern Central Asian Orogenic Belt, is marked by systematic Paleozoic magmatism that occurred in response to specific evolutionary stages of the Paleo-Asian Ocean, but controversy about its tectonic evolution still exists. Thus, we conducted a detailed field investigation and geochronological, geochemical, and isotopic analyses on magmatic rocks across the Xiemisitai Mountains, the eastern extension of the Boshchekul-Chingiz arc in northern West Junggar, Northwest China. Magmatic activity in the Xiemisitai Mountains occurred mainly from the Middle Cambrian to the Early Devonian. Middle Cambrian quartz diorite (ca. 502 Ma) in the southern Xiemisitai Mountains is marked by initial arc magmas characterized by flat rare earth element patterns, enrichment in large ion lithophile elements, and depletion in high field strength elements. Its depleted mantle–like Sr-Nd-Hf isotopes suggest that it was directly extracted from depleted mantle. Late Ordovician–Early Devonian mafic–intermediate volcanic rocks (ca. 446–415 Ma) are most often exposed. These are mature arc magmas with high La/Yb, Th/Zr, and Th/Yb ratios. Their high La/Sm ratios, low Ce/Pb ratios, and depleted mantle–like Sr-Nd-Hf isotopes indicate that they were likely derived from depleted lithospheric mantle intensively metasomatized by slab-derived fluids. Widespread middle Silurian–Early Devonian felsic volcanic rocks (ca. 426–417 Ma) are geochemically equivalent to those of A2-type and I-type granitoids. The juvenile isotopic compositions and crust-like Zr/Hf ratios imply that they originated from juvenile crust remelting with the involvement of the asthenosphere. We suggest that the eastern Boshchekul-Chingiz arc was born by northward subduction (in today’s direction) of the Junggar Ocean (a branch of the Paleo-Asian Ocean) since 502 Ma. Then it transformed from a nascent arc to a mature island arc with the advancing migration of magmatism during the Late Ordovician–middle Silurian. Rollback of the Junggar oceanic slab occurred during the middle Silurian–Early Devonian and led to coeval retreating migration of magmatism and transformation to an extensional setting. The newly established tectonic framework of the Boshchekul-Chingiz arc from this study, combined with the coeval accretionary tectonic model for other parts of the Central Asian Orogenic Belt, convincingly indicates that a large-scale intra-oceanic subduction system existed in the Central Asian Orogenic Belt since the Cambrian, and a tectonic transition from advancing to retreating orogenesis exclusively occurred in the Silurian–Early Devonian due to slab rollback. Thus, it would take 70–100 m.y. from initial subduction to incipient slab rollback for the Paleo-Asian Ocean, which is important for understanding the early stage of orogenic evolution for the Central Asian Orogenic Belt. This study provides overall evidence of the potential period from initial subduction to slab rollback of the Paleo-Asian Ocean, and may be of great importance for inferring the paleo-subduction and evolution of orogens elsewhere.