REE geochemistry of the recent playa sediments from the Thar Desert,India:An implication to playa sediment provenance

摘要

The playas (saline lakes) situated in the Thar Desert,north-west India,provide prominent examples of alkaline brine and varying assemblages of detrital and evaporite mineralogy.The eastern margin of the desert is relatively semi-arid,whereas the central to western region is arid to hyper-arid in nature.Rare earth elements (REEs) systematics in the sediments of nine different playas of the Thar Desert were studied to understand the provenance of the sediments and the intensity of chemical weathering in the region.Based on the REE patterns,fractionation of light REE (LREE) (La/Sm)_N and heavy REE (HREE) (Gd/Yb)_N,and Eu anomaly (Eu/Eu*),the upper continental crust normalised playa sediments are divided into two different groups.The eastern margin playa sediments show homogeneous REE contents,relatively positive Eu anomaly and depleted HREE values,whereas the western arid core playa sediments have highly variable REE contents,relatively negative Eu anomaly and similarly fractioned LREE and HREE patterns.The dissimilarity in the degree of HREE fractionations both in the eastern and western playa sediments is attributed to the differential distribution of minerals,depending upon their resistance to chemical weathering.It is believed that the relatively higher abundance of REE bearing heavy minerals and the presence of higher amounts of evaporites influence the large variation of REE distribution and enriched HREE in the western playa sediments.Apart from the relatively higher abundance of heavy minerals,the presence of rock fragments of variable petrographic character and roundness mirror the lower rock-water interaction in the arid western region.The presence of well-rounded metamorphic rock fragments and minerals,sourced from the eastern margin Aravalli mountains,indicates that the playas of the entire desert get the detrital and dissolved material mainly from the Aravalli mountains.Additionally,the western playas receive sediments from their surrounding Proterozoic and Mesozoic formations.This interpretation is supported by the presence of angular rock fragments of basalt,rhyolite and limestone in the western playas.