Ductile shearing and LA‐ICP MS zircon U–Pb dating of Sa'erqiaoke ductile shear belt in the Bogda orogen: Constraints on the process of mountain building of eastern Tianshan

Abstract

The relationship between collision orogeny and post‐orogeny tectonism remains to be a weakness of the present orogenic belt research. An E—W‐trending ductile shear zone exposed along the Bogda Mountain of the Eastern Tianshan Orogenic Belt (ETOB) has spatiotemporal correlation with the Kelameili ophiolite belt and offers important evidence for research on the evolution of the ETOB. These ductile shear belts exposed in the eastern Sa'erqiaoke display that the low greenschist facies metamorphism was followed by north‐eastward thrusting and sinistral strike‐slip shearing. LA‐ICP MS zircon U–Pb ages of the mylonitic quartz schist in Sa'erqiaoke structure section range from 226 to 1458 Ma, four old zircons with ages of 832.2, 1029.2, 1240.5, and 1458.2 Ma possess round‐shaped grains and core–mantle texture, which suggest that its protolith has a detritic and metamorphic origin. The major age peak ranges from 443.3 to 466.8 Ma, with weighted age of 452.9 ± 5.4 (MSWD = 0.62, N = 13), corresponding to the early Palaeozoic Harlic island arc magmatism. Two young metamorphic zircons with ages of 247.2 and 226 Ma could represent the time of ductile shearing, which are consistent with the final termination of the ETOB. Two weighted ages of 303.8 ± 3.4 (MSWD = 0.12, N = 16) and 305.2 ± 2.4 (MSWD = 0.46, N = 32) from Nawakuduke mylonitic belts are coeval with post‐collision A‐type granite of the Kelameili suture, which suggest that the ductile shearing should be later than the closure of the Kelameili Ocean. Based on the synthesis of voluminous evidences, we propose that the Kelameili Ocean, as part of the paleo‐Tianshan Ocean, obliquely subducted beneath the Junggar plate, bringing about the Bogda subduction‐torn‐type rift during the Carboniferous period. Although the final closure of the Kelameili Ocean has ended at late Carboniferous, the subsequent continental collision and large‐scale ductile shearing lasted until late Triassic.