Intraoceanic back-arc magma diversity: Insights from a relic of the Proto-Tethys oceanic lithosphere in the western Qilian Orogen, NW China

Abstract

The Qilian orogeny formed by subduction of the proto-Tethyan ocean along the northern margin of Gondwana. In this paper we report geochemical, zircon U-Pb-Hf, and mapping data of the Duoruonuoer volcanic rocks from the western Qilian Orogen. Based on distinct geochemical compositions, four melt types can be distinguished: boninite-like basalt, OIB-like basalt, MORB, and high-K to shoshonitic volcanic rocks (HKSV). Field relationships show a compositional change from HKSV, MORB, OIB to boninite-like basalt, from north to south. The boninite-like basalt is low to medium-K calc-alkaline, with low TiO2 (0.13–0.18 < 0.5 wt%) and high CaO/Al2O3 ratio (0.72–0.80), similar to high-Ca back-arc boninite. The geochemical characteristics of Duoruonuoer OIB-like basalt, MORB, and HKSV are similar to those of the Izu-Bonin (IB) back-arc magmatic assemblages. Boninite-like basalt is enriched in LILEs, and has low Ce/Pb, and high Sr/Nd, Ba/Nb ratios, indicating an addition of a shallow subduction-derived fluid phase. Inferred pressures of magma segregation are 0.9–1.4 GPa with temperatures of 1200–1370 °C consistent with melting of a highly depleted, low pressure hydrous mantle source. The MORB-like rock formed by mixing of depleted mantle and a subduction component, and is identical to back-arc basin floor basalt (BABB). The HKSV, dated at 475 Ma, shares the characteristics with lava from the IB back-arc seamount chains (rear-arc). Their low Ce/Pb and high Th/Yb and Th/Nb ratios are compatible with contributions from melts of deep subducting sediment and oceanic crust. High pressure (1.8–2.0 GPa) may reflect sourcing from a mantle melt at the base of thickened arc crust. The OIB-like basalts have high TiO2, Zr/Yb, Nb/Yb and Ce/Pb ratios, suggesting an enriched component from partial melting of the asthenosphere. The high-Fe8 and Ti8 composition of the Duoruonuoer OIB-like basalt is produced by fractional melting, similar to a spreading ridge segment developed a deeper source. The variety of Duoruonuoer back-arc magmatism highlights the importance of changes to the melt source region linked to replenishment of the mantle wedge by material extracted from the subducting slab and influx of more fertile mantle material. It is also controlled by evolution of the mantle wedge over time in the building of the back-arc basin. These findings provide evidence for c.475 Ma back-arc basin developed on a thickened arc lithosphere at the western Qilian Orogen.