Abstract

The Central Asian orogenic belt is the largest tectonic assembly of continental and oceanic terranes on Earth due to closure of the paleo–Asian Ocean in the Phanerozoic. Among major suture zones in the North Xinjiang region of western China, the North Tian Shan suture zone, because of collision between the Yili terrane in the south and the Junggar terrane in the north, contains the youngest ophiolitic rocks and may represent the terminal stage of development of the Central Asian orogenic belt in western China, but the timing of the suture zone remains poorly constrained. A sensitive high-resolution ion microprobe (SHRIMP) zircon U-Pb age of 316 ± 3 Ma (i.e., the beginning of the late Carboniferous) from the undeformed Sikeshu pluton, which crosscuts the suture zone, places a crucial upper-age bound for the time of collision between the Yili and Junggar terranes. This event occurred later than, or nearly concurrent with, other accretion-collision events in the North Xinjiang region, implying that final terrane amalgamation was completed in the late Carboniferous. The Sikeshu pluton shares geochemical characteristics of the widespread late Carboniferous to Permian postcollisional A-type and I-type granitoids with depleted-mantle–like Sr-Nd isotopic signatures in the North Xinjiang region. They all occurred during a protracted (ca. 320–270 Ma) episode of postcollisional magmatism that may have been induced by basaltic underplating due to either slab breakoff or delamination of thickened mantle lithosphere beneath the Central Asian orogenic belt. The same postcollisional magmatism also generated Cu-Ni-sulfide–bearing, mafic-ultramafic magmatic complexes, adakites, and porphyry-type copper-molybdenum–bearing magmatic rocks in the North Xinjiang region.

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