Detrital Heavy Mineral Constraints on the Triassic Tectonic Evolution of the West Qinling Terrane, NW China: Implications for Understanding Subduction of the Paleotethyan Ocean

We have examined the Triassic sediments in the west Qinling terrane, northeastern Tibet. These sediments consist mainly of flysch and shallow-sea and fluvial deposits with abundant lithic and heavy mineral detritus, sandwiched between and overlying Late Paleozoic and Early-Middle Triassic ophiolitic mélanges. Volcanic and metamorphic detritus dominates the lithic component of Lower Triassic sandstones accompanied by high Cr-spinel, pyroxene, and magnetite contents, indicating a mixed ophiolite and metamorphic source. Detrital mineral geochemistry further suggests that ophiolitic, high-grade metamorphic, basic, and intermediate-acidic igneous rocks must have been exposed and deeply eroded in their source area. Abundances of zircon, rutile, garnet, tourmaline, and epidote are greater in the Middle Triassic samples, and granitic and volcanic sources are the major contributors of detrital clasts. Considering these new observations on sedimentary petrography and detrital heavy mineral geochemistry, along with published data on paleocurrents, detrital zircon U-Pb ages, sedimentary facies, and regional magmatism, we suggest that these Triassic sediments represent the sedimentary fill of a forearc basin that overlies a late Paleozoic ophiolitic complex. A south-facing Andean-type convergent continental margin system developed along the southern margin of the North China block during the Triassic, in response to northward subduction of the Paleotethyan Ocean.