Low δ18O magmas in the carboniferous intra-oceanic arc, central Tibet: Implications for felsic magma generation and oceanic arc accretion

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Low-δ18O magmas (δ18Ozircon < 4.7‰) are rare but significant compared to the more abundant high-δ18O magmas in diverse tectonic settings. Here, we report mantle-like δ18O lavas and low-δ18O granites from the Carboniferous Riwanchaka intra-oceanic arc of the Qiangtang Terrane, central Tibet. SIMS (secondary ion mass spectroscopy) zircon UPb dating indicates that they were formed simultaneously at ~350 Ma, slightly younger than the Early Carboniferous ophiolites (~355 Ma). The forearc ophiolites and volcanic rock assemblages constitute an intra-oceanic subduction system, evidenced by their similar SrNd isotopes. SIMS zircon O isotope analyses indicate that the felsic rocks have δ18Ozircon values similar to or lower than that of the mantle. The dacite and trondhjemite samples have homogeneous δ18Ozircon values of 5.1 ± 0.4‰ (2SD) and 4.6 ± 0.5‰ (2SD), respectively. The tonalite samples exhibit a large range of δ18Ozircon values from 2.7 to 5.0‰, most of which are lower than the mantle-like value of 5.3 ± 0.6‰ (2SD). We suggest that the dacites and trondhjemites were produced by partial melting of unaltered lower crust, whereas the low-δ18O tonalites originated by melting of hydrothermally altered lower crust, and both were generated in the early stage of this intra-oceanic arc. This process resulted in a weak middle arc crust of intermediate composition. By allowing decoupling of upper and lower arc crust, this plays an important role in the accretion of intra-oceanic arcs to continents and ultimately in the crustal growth.