The tectono-magmatic evolution of the West Junggar terrane (NW China) unravelled by U-Pb ages of detrital zircons in modern river sands

摘要

The West Junggar terrane (WJT) is an outstanding laboratory for studying the tectonic evolution of the Junggar-Balkhash Ocean, because it contains widespread Paleozoic magmatism in different tectonic settings. We attempt to reconstruct the tectono-magmatic evolution of WJT through U-pb analysis of detrital zircons from three modern river sand samples from the Harabura, Baibuxie, and Aletengyemule rivers in the Barleik Mountains of the central WJT. A total of 232 concordant spots show Th/U ratios of 0.14-1.69, typical of igneous origin, and they contain abundant Paleozoic (96%) and few Precambrian (4%) ages, with major age populations at 450-530, 400-430, 320-380, and 265-320 Ma. The first two groups may be derived from the early subduction- and accretion-related magmatic rocks of the WJT, whereas the third group is congruent with magmatic activities related to the final subduction and basin-filling processes within a framework of the remnant Junggar-Balkhash Ocean. By combining with the regional data, the last group of magmatic events is referred to as post-subduction magmatism. The missing Mesozoic-Cenozoic magmatism clearly indicates a pre-Permian closure for the Junggar-Balkhash Ocean, nearly coeval with the closure of other oceans in the southwestern Palaeo-Asian Ocean.