Geochemical Characteristics of Cenozoic Potassic Volcanic Rocks in the Eastern Hoh Xil

The major trace element and rare earth element of potassic volcanic rocks in the studied area have been analyzed in this paper.The results show that these rocks have high SiO₂ contents (59.35%~63.85%), alkali content (K₂O+Na₂O=7.89%~8.47%), K₂O/Na₂O ratio (K₂O/Na₂O>1), MgO content (1.76%~2.46%) and Mg^# values.In the TAS diagram, these rocks are located in the range of shoshonite series.In the primitive mantle normalized distribution pattern, the incompatible elements of these volcanic rocks present as rightist type, the heavy ion lithophile elements are enriched, and the high field strength elements are relatively depleted.In addition, the pattern exhibits three peak values of Th, La and Pb and three losses groove of Ta, Nb and Ti, indicating that the shoshonite has the characteristics of the intraplate alkali basalt and it has the geochemical characteristics of the arc magma mixed with more crustal material in the magma source area.The total content of rare earth is very high, ΣREE, LREE and HREE vary from 442.44×10^-6 to 681.53×10^-6, from 425.94×10^-6 to 657.26×10^-6 and from 12.53×10^-6 to 24.27×10^-6, respectively, and the LREE/ΣREE ratios range from 0.96 to 0.97, with strong enrichment of light rare earth element.The potassic volcanic rocks exhibit weakly negative Eu (δEu=0.69~0.77), and the La/Sm ratios show an increasing trend with the raising of La content, which indicate that these rocks serve as the products of partial melting, with weak fractional crystallization.The geochemical characteristics of Shoshonitic rocks indicates that, they were formed by the partial melting of mantle materials in the thickened crust continental setting, and these mantle materials were contaminated by oceanic sediments and crust materials, with enrichments of light rare earth elements and large ion lithophile elements.