Spatial and Temporal Framework, Evolution of Magma Sources, and Tectonic Settings of Paleozoic Magmatic Rocks in West Tianshan, China

The Central Asian Orogenic Belt (CAOB) is Earth's largest and most representative accretionary belt and records the most extensive growth and reworking of the continental crust. Accompanying the closure of the giant Paleo-Asian Ocean and the amalgamation of tectonic blocks in the CAOB regime, with different origins and evolutionary histories, voluminous magmatic rocks were formed. This study focuses on Paleozoic magmatic rocks exposed in West Tianshan and systemically summarizes the spatiotemporal frameworks, genetic types, evolution of their magma sources in space and time, and tectonic settings of these rocks. Paleozoic magmatic rocks in West Tianshan were mainly formed at three stages, i.e., Early Cambrian to Middle Devonian (～479 to ～388 Ma), Late Devonian to Early Carboniferous (～375 to ～372 Ma), and Late Carboniferous to Middle Permian (～322 to ～263 Ma). Magmatic rocks formed at the first and second stages are mainly of calc-alkaline I-type granite and intermediate and mafic rocks with "arc-like" geochemical fingerprints, with a few rocks bearing "adakite-like" features; a few A-type granites are also found. By contrast, Late Carboniferous to Middle Permian magmatic rocks show a diversity in rock types, including calc-alkaline I-type, adakite-like, and A-type felsic rocks, with a few locally exposed S-type granites; OIB-like mafic rocks formed in this period, such as gabbros and basalts, occur locally. In combination with other geological evidence, this study proposes that Early Cambrian to early Middle Devonian and Late Devonian to Early Carboniferous magmatic activities took place in convergent continental margin settings, which were associated with the subduction of branches of the Paleo-Asian Ocean. Besides, both magmatic migration and secular changes in geochemical proxies indicate the transition from advancing low-angle to retreating high-angle subduction. The final closure of oceanic basins plausibly occurred in the Late Carboniferous. Following the closure of the South Tianshan Ocean, a "hard" collision with the arriving Tarim Craton occurred; by contrast, in the north, the northern margin of the Yili-Central Tianshan Block amalgamated with an immature/nascent island arc. In terms of continental evolution, based on Hf isotopic datasets, this study identifies alternating occurrences of growth and reworking. During subduction stages, retreating subduction (slab rollback) played a predominant role, resulting in large-scale replacement of ancient, pre-accretionary materials by new-formed, syn-accretionary materials. Therefore, Yili and Central Tianshan blocks, can be viewed as ancient microcontinents that were significantly rejuvenated during accretionary processes. In the post-collisional stage, large-scale underplating of mantle-derived magmas represents another phase of continental growth. During the Paleozoic, West Tianshan and adjacent regions were characterized dominantly by continental growth.