A GEOCHEMICAL STUDY OF LOESS AND DESERT SAND IN NORTHERN CHINA - IMPLICATIONS FOR CONTINENTAL-CRUST WEATHERING AND COMPOSITION

Chemical compositional variations of desert sands and loess, loess-lik e deposits, river deposits and saline lacustrine deposits collected mo stly from the southern margin of the Taklimakan Desert, China, provide strong constraints on the chemical weathering processes of continenta l crust, and also on the provenance compositions of these Quaternary d eposits. Constant but lower concentrations of Al relative to the avera ge upper continental crust (UCC) in these Quaternary deposits could be attributed to chemical weathering of Al-rich minerals such as feldspa r and mica, and removal of clay minerals by wind and water. Large vari ations of mobile Na, Ca, Mg and Sr compared to relatively immobile Al, K and Ba resulted from the chemical weathering of plagioclase and car bonate minerals, salinization and carbonation of the arid lands. Most trace elements are not significantly fractionated. Their variations in absolute concentration are caused by the dilution of quartz, evaporat es and carbonate minerals. Variations in Sn concentrations probably de pend on relative contents of some heavy minerals. High Sr/Ba, low Th/U , K/Cs ratios of the saline lake deposits suggest differences in aqueo us chemistry of these elements. The occurrence of the M-type tetrad ef fect (a REE pattern with four convex curves) of REE compositions, foun d mainly in the river deposits, is considered the result of water-part icle interaction during chemical weathering of crustal materials. Y/Ho ratios of these samples are significantly higher than those of chondr ite and the UCC. This suggests different chemical behavior of these tw o elements, and possibly the enrichment of carbonate materials. The lo w LREE/HREE feature of the saline lake deposits can be interpreted as due to the different aqueous chemical behavior of the REE's. However, it can also be reasonably considered as the result of inheritance from their provenance, since LREE and LREE/HREE variations may be related to the geological setting of the source. According to some elements th at are not significantly fractionated, the average provenance composit ion of these Quaternary deposits are probably characterized by enrichm ents of Ni, Cr and V over Fe, higher La/Th, lower La/Sc and Th/Sc, and lower Ta/W and Nb/W ratios, compared with the UCC composition.