Abstract:  The Pliocene mixed rocks contain large amounts of salt minerals in the Nanyishan structure of the western Qaidam Basin, and their provenance analyses can be valuable in finding the origin of the salt minerals. Based on field observation and microscopic identification, we analyzed the geochemical characteristics of mixed rocks from the Pliocene Shizigou Formation in the Nanyishan structure. Combined with rare earth element (REE) characteristics of granites from the surrounding orogenic belts, we discussed the provenance of terrigenous clastics in mixed rocks. The results showed that the detrital, carbonate and clay minerals in mixed rocks were homogeneously mixed, and the elemental contents of the rocks were closely related to the mineral composition. Except for Ca, Na, and Sr, the mixed rocks had generally lower mineral contents than those of the upper continental crust. Considering that CaO and Na2O are affected by the formation of carbonate and halite, Ca and Na are not suitable for identifying tectonic settings. In contrast, trace elements and REE such as Sc, Th, Co, Zr and Hf, retain mostly the geochemical information of source rocks and therefore can be used to evaluate the tectonic settings of the source area for provenance analysis. The REE patterns showed good consistency, indicating the terrigenous clastics of mixed rocks have the same provenance characteristics. Together with LaThSc, ThScZr/10, La/ThHf diagrams and Al2O3/TiO2 ratio, our data suggest that the terrigenous clastics components of mixed rocks were derived from the felsic igneous rocks of continental island arc. In comparison with REE patterns in granites from the surrounding orogenic belts, we consider the Late Silurian and Late Permian granites to be the main source rocks for terrigenous clastics and salt minerals in the mixed rock. In addition, we hypothesize that the south part of the study area, Qimantag, is the main source area, and South Altyn Tagh in the west also takes part in the sedimentary source recharge.

Key words: mixed rocks, geochemistry, tectonic setting, provenance, Nanyishan structure, western Qaidam Basin