可可西里东部地区新生代钾质火山岩地球化学特征

摘要

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.%对研究区火山岩进行主量元素、微量元素、稀土元素分析,主量元素具有高SiO2(59.35%~63.85%)、富碱(K2O+Na2O=7.89%~8.47%)、富K(K2O/Na2O>1)、高MgO(1.76%~2.46%)、高Mg#值(39.96~49.82)等特征,在TAS图中落在钾玄岩系列.火山岩不相容元素原始地幔标准化配分型式图总体呈谷峰相间的右倾型式,富集大离子亲石元素,相对亏损高场强元素,具有Th、La、Pb三个峰值和Ta、Nb、Ti三个亏损槽,指示钾玄岩具有板内碱性玄武岩和岩浆源区可能有较多的地壳物质加入的岛弧岩浆的特点.稀土总量非常高,ΣREE为 442.44×10-6~681.53×10-6,LREE为425.94×10-6~657.26×10-6, HREE为12.53×10-6~24.27×10-6,LREE/ΣREE值为0.96~0.97,轻稀土元素强烈富集.无明显的负Eu异常(δEu=0.69~0.77).随La含量的增加, La/Sm值呈增加趋势,指示源区岩石是部分熔融的产物,结晶分异作用较弱.钾玄岩地球化学特征表明,其是在大陆地壳加厚背景下被具有大洋沉积物和地壳物质所混染的富含大离子亲石元素和轻稀土元素强烈富集的地幔源区的部分熔融产物.