Diverse origins of late Paleozoic calc-alkaline magmatic rocks from the Bogda tectonic belt: Implications for the geodynamic evolution of the eastern Tianshan, NW China

Abstract

The changing origins of the late Paleozoic calc-alkaline magmatic rocks in the Bogda tectonic belt (BTB) during the Late Carboniferous–Early Permian bring new insights into the subduction-collision processes in the eastern Tianshan region. Based on the geochronological, geochemical and Sr–Nd–Hf isotopic data, a systematic study was conducted on the Late Carboniferous andesite and the Early Permian diabase dyke and MME-bearing granite porphyry from the Dalonggou area in the northern BTB. The ~310 Ma andesites are featured by low initial 87Sr/86Sr ratios, positive εNd(t) (+4.4 to +5.3) and εHf(t) (+10.2 to +13.6) values, enrichment in light rare earth elements and large ion lithophile elements, and depletion in Nb and Ta. They are interpreted as the products of partial melting of a metasomatized depleted mantle wedge in an extensional arc environment. The ~290 Ma diabase dykes exhibit a nearly flat REE pattern, low Fe/Mn ratios (53–59), and high TiO2 and Zr contents and mantle potential temperatures (1471–1480 °C). They were derived from a sub-slab asthenospheric mantle with the involvement of the lithospheric mantle. The MMEs were produced by fractional crystallization of the diabase magmas, and the host granite porphyries were formed by magma mixing between melts derived from the newly-formed lower crust and mantle. These results indicate that the BTB represented a backarc/intra-arc rift during the Late Carboniferous that was transitioned to a post-collisional rift in the Early Permian, witnessing a dynamic switch from rollback to detachment of the subducted North Tianshan oceanic slab. Combined with the variation of magma sources with time, we propose that the collision between the Dananhu-Harlik arc and the Central Tianshan arc and the closure of the North Tianshan Ocean occurred at ca. 310–302 Ma. Such a scenario agreed well with coeval tectonics in the Karamaili area, shedding light on the late Paleozoic spatio-temporal tectonic evolution in the eastern Tianshan.