西昆仑库地蛇绿岩地质、地球化学及其成因研究

西昆仑库地蛇绿混杂岩由方辉橄榄岩和纯橄榄岩等地幔变质橄榄岩、豆荚状铬铁矿、堆晶橄榄岩、堆晶辉石岩和辉长岩、辉绿岩墙、块状和枕状玄武岩等组成。强亏损方辉橄榄岩为主的地幔岩组合,二辉石的低Al含量和铬尖晶石的高Cr^#,以及岩石的富Mg、Ni和贫Al、Ca特征一致表明地幔橄榄岩类是经较高程度部分熔融后的地幔残余,与消减带之上蛇绿岩中的同类岩石相近。岩石富Rb、Ba、U、Th、LREE,说明地幔残余岩石受到了来自消减带的洋壳重熔组分的混染。堆晶岩以辉石岩和辉长岩为主,可能属PPG系列,指示岩浆是在消减带环境和含水条件下熔融的。辉长岩为低Ti蛇绿岩型,代表洋内弧后盆地早期环境或弧前环境。辉绿岩和玄武岩为洋中脊拉斑玄武岩和岛弧拉斑玄武岩的过渡类型;玄武岩和辉绿岩相比富Ba、Th、LREE,贫Ta,指示玄武岩较辉绿岩更多地受到来自消减带洋壳重熔组分的影响。库地蛇绿岩形成时的古构造环境是消减带之上的弧间或弧后盆地。

GEOLOGY, GEOCHEMISTRY AND GENESIS OF THE KUDA OPHIOLITE, WESTERN KUNLUN, CHINA

The Kuda ophiolite in the northern part of the western Kunlun, China, comprises harzburgites, dunites, chromite pods, cumulate pyroxenites and gabbros, diabase dykes, pillow and massive lavas. The mantle peridotites are mainly Mg-riched harzburgites and characterised by low-Al pyroxenes and Cr-riched spinels, suggesting that the metamorphic peridotites are mantle residue after higher degree partial melting, and similar to the peridotites formed in supra\|subduction zone (SSZ). The mantle residual association had been affected by partial melting material of oceanic crust rocks from the related subduction zone and make it rich in LILE, Th and LREE. The cumulate rocks, mainly pyroxenites and gabbros, belong to the PPG-subtype, indicating that the magma of the rocks melted under the H 2O-bearing condition in SSZ. The low-Ti gabbros may form in an initial back-arc basin. The diabases and lavas are the transitional MORB/IAB-type basalt. The lavas are richer in Ba, Th and LREE, and lower in Ta than the diabases, suggesting that the lavas are with much clear SSZ signature (high-Th and low-Ta). The palaeoenvironment of the Kuda ophiolite is an inter-arc or back-arc basin in SSZ.