西昆仑库地蛇绿岩铬铁矿中铬尖晶石化学特征及其地质意义

Chemical Characteristics of Cr-spinel of Chromites in Kudi Ophiolite of Western Kunlun and Its Geological Implications

库地蛇绿岩属于西昆仑早古生代蛇绿岩带,主要由变质橄榄岩、堆晶橄榄岩、基性火山岩和石英岩等组成。库地蛇绿岩体具有较好的岩相分带,其中纯橄岩相与辉橄岩相中产出豆荚状铬铁矿。与其他岩相对比,铬铁矿具有最高的Cr2O3含量、最大的Mg#值和最小的Fe#值,表明铬铁矿形成于富Mg的环境,且经历了更高程度的部分熔融。利用铬铁矿(铬尖晶石)矿物的化学成分,得出铬尖晶石的结晶温度为1366～1404℃,平均1379℃;压力为2.98～3.03GPa,平均3.00GPa;地幔部分熔融程度F为17.33%～18.84%,平均18.28%。结合已有的研究成果,推测库地蛇绿岩的基底橄榄岩单元源区为石榴石二辉橄榄岩,形成于亏损的软流圈地幔,对应的大地构造位置为消减带之上岛弧环境。

The Kudi ophiolite belongs to the early Paleozoic ophiolite belt in Western Kunlun Mountains and is composed of metamorphic peridotites, cumulate peridotites, basic volcanic rocks and quartzites. The Kudi ophiolite can be divided into four lithofacies, and chromite with the podiform texture occurs in the dunite and pyroxene peridotite facies. In the light of their geochemical composition, the chromites contain the highest Cr2O3 content, maximum Mg# value and minimum Fe# value in the all samples. It can be in- ferred that the chromites formed a Mg-rich environment and experienced a higher grade of partial melt- ing. On the basis of olivine-spinel geothermometry , the crystallized temperature of the Cr-spinel is esti- mated to be from 1 366 ℃ to 1 404℃, 1 379 ℃ on average. Similarly , according to the Cr-spinel geoba- rometer, the mantle pressure is estimated to be from 2. 98 GPa to 3.03 GPa, 3.00 GPa on average. The degree of partial melting of the upper mantle is estimated to be from 17.33% to 18.84%, 18.28% on aver- age. These parameters provide some clues of the physicochemical conditions： these peridotites were derived from garnet lherzolites located in the asthenosphere of the upper mantle, and the incompatible element-de- pleted mantle might have resulted from previous partial melting. The tectono-discrimination diagrams im- ply that the Kudi ophiolite was formed in a island-arc island-arc setting overlying subduction zones.