Integrated analyses constraining the provenance of sandstones, mudstones, and conglomerates, a case study: the Laojunshan conglomerate, Qilian orogen, northwest China
The Qilian orogenic belt in the northern Tibetan plateau connects the Altaids to the north with the Tethyan orogenic system to the south and occupies a key tectonic position in the evolution and assembly of Asia. The belt con- tains a wide range of subduction-accretion-related petrotectonic units. The Early-Middle Devonian Laojunshan con- glomerate, deposited unconformably upon Cambrian-Silurian strata along the northern margin of the North Qilian terrane, contains a record of the late Paleozoic tectonism of the Qilian orogen. Its provenance and tectonic setting are critical in understanding not only the tectonic evolution of Tibetan plateau, but Paleozoic global reconstructions as well. The composition of clastic conglomerates and heavy mineral assemblages of sandstones suggests that coeval mafic, felsic, metamorphic, and sedimentary rocks were the main sources. The geochemistry of volcanic clasts and paleocurrent and paleogeographic data suggest derivation from subduction-accretion complexes in the North Qilian terrane. The geochemistry of siltstones and mudstones indicates that the Laojunshan conglomerate was derived from an arc and accumulated in an active continental margin. Geochemical data of granitoid clasts suggest that they were de- rived from Ordovician-Silurian subduction-related magmatic rocks. Mafic and ultramafic clasts, chromite, and magne- tite decrease upwards in the stratigraphy whereas metamorphic, sedimentary and granitoid clasts, and garnet increase. These data imply that mafic rocks were the predominant source during initial deposition. Regional studies suggest that the North China plate subducted southwards and produced subduction-related arc magmatism along the southern margin of the North Qilian terrane during the Early-Middle Devonian. Therefore, we interpret the Laojunshan conglomerate as a fore-arc basin fill.