Late Ordovician to early Silurian calc-alkaline magmatism in the Xiemisitai Mountains, northern West Junggar: a response to the subduction of the Junggar-Balkhash Ocean

摘要

The Early Palaeozoic tectonic evolution of northern West Junggar is essential for understanding the tectonic framework and accretionary processes in the southwestern segment of the Central Asian Orogenic Belt. However, its evolutionary history is controversial due to insufficient pre-late Silurian magmatic data. In order to clarify this issue, we conducted geochronological, geochemical, and Sr-Nd-Hf isotope analyses on samples from the Dengdeer and Maodun complexes in the Xiemisitai Mountains of Boshchekul-Chingiz (BC) arc, northern West Junggar. LA-ICP-MS zircon U-Pb dating yielded ages ranging from 445 Ma to 429 Ma, revealing the Late Ordovician to early Silurian magmatic events in northern West Junggar. The dioritic and metaluminous Dengdeer complex has high Mg-# (49.5-52.9), moderate La/Yb (11.0-12.3), and low initial Sr-87/Sr-86 (0.70324-0.70410) and epsilon Nd(t) (+3.7 to +4.5). This complex exhibits variable zircon epsilon Hf(t) (+11.31 to +17.62), young Hf model ages (470-440 Ma), and appears to have been produced by differentiation of melts from metasomatized mantle. The intermediate to felsic, metaluminous to weakly peraluminous Maodun complex has abundant enclaves and inherited zircons. This complex seems to have fractionated from mantle- and crustal-derived magma mixing with some crustal contamination, based on its high Mg-# (36.6-49.5), low Sr/Y (8.7-29.6), Nb/Ta (8.3-13.9), initial Sr-87/Sr-86 (0.70377) and epsilon Nd(t) (+4.0 to +4.2), and variable zircon epsilon Hf(t) (+11.59 to +15.21). The Dengdeer and Maodun samples are high-K calc-alkaline, rich in hydrous minerals, Rb, Th, and LREEs, depleted in Nb, Ta, and Ti, and show decoupling Hf-Nd isotopic signatures. These characteristics indicate that their source had been modified by melts fromslab sediments. After combining our data with previous observations, we conclude that a trench-ward migration of an arc-back-arc system occurred during the Late Ordovician to early Silurian in northern West Junggar, while the Dengdeer and Maodun complexes were related to subduction-accretionary processes of the Junggar-Balkhash oceanic slab beneath the BC arc.