Magmatic evidence for Late Carboniferous-Early Permian slab breakoff and extension of the southern Mongolia collage system in Central Asia

Oceanic subduction and its last underthrusted part can both triggers arc-like magmatism. As the existence of multi-subduction zones in the Central Asian Orogenic Belt, controversy still surrounds on when and especially how the subduction of the (Paleo-Asian Ocean) PAO terminated. Here, we present geochronological, geochemical, and LuHf isotopic data for a suite of basalt-andesites, dacite-rhyolites and later trachyandesite-mugearitic dykes from the Khan Bogd area in the Gobi Tianshan Zone (GTZ) of the southern Mongolia. UPb dating of zircons indicate the basalt-andesites and dacite-rhyolites were formed at ~334–338 Ma, and the dykes at ~300 Ma. These Early Carboniferous volcanic rocks display high U/Th, Ba/Th, low La/Sm and variable Zr/Nb ratios, implying the involvement of subduction fluids or sediment melt. They display arc geochemical features such as calc-alkaline and metaluminous nature and positive Ba and U and negative Nb, Ta and Ti anomalies. Moreover, their continental geochemical signals (e.g. positive Pb, K anomalies) and old captured zircons (~1.9–2.5 Ga, basalt sample MG19) implying a continental arc setting. Comparatively, the ~300 Ma dykes are characterized by high alkaline contents, which are common for coeval (~320–290 Ma) and widespread post-subductional granites there. Given a mainly crust-derived magma source for those granites, these dykes likely reflect a mantle disturbance due to: (1) their relative low SiO2 (51.71–55.85 wt%) and high Mg# (40.3–67.3) values, and (2) positive zircon ƐHf(t) (most >12). Considering a slab rollback model during the Carboniferous and Triassic, the mantle disturbance was possibly induced by the oceanic slab breakoff. Combined with previous work, this ~320–290 Ma slab breakoff-induced extension marks the closure of a wide secondary ocean (North Tianshan-Hegenshan ocean) north of the main ocean basin of the PAO.