Detrital composition of the Laojunshan conglomerate in the Qilian Mountains, Northwest China and tectonic settings of its source regions

The Laojunshan conglomerate is a suite of continental coarse clastic rocks consisting of conglomerate and graywacke. Conglomerate is composed of fragments of ultramafic rocks, intermediate-basic volcanic rocks, chert and granite derived from the underlying basement. Graywacke contains more than 70% lithic fragments, of which quartz constitutes -10 % and feldspar -15%. Intermediate-basic volcanic rocks and granite are the dominant lithic fragments. Chert fragments are dominant sedimentary fragments of greywacke. Zircon, apatite and magnetite are the most abundant heavy minerals in graywacke. In addition, chromite, garnet, tourmaline, ruffle and pyrite also occur in sandstone. This demonstrates that ultramafic, intermediate-basic and metamorphic rocks may have outcropped in the source regions of the Laojunshan conglomerate. The sandstone detrital model and geochemical compositions of siltstone and mudstone of the Laojunshan conglomerate suggest that their source regions were the continental-margin arc and oceanic island arc. These indicate that the Laojunshan conglomerate formed in active continental margins and an arc-related basin.