喜马拉雅东段库曲锂辉石伟晶岩锡石研究及指示意义

锡石是花岗伟晶岩中重要的含锡矿物。喜马拉雅淡色花岗岩带中多个花岗岩-伟晶岩系统均产出锡石。富锂伟晶岩中锡石的显微结构与化学组成及其对岩浆分异演化的指示意义尚不清楚, 亟待开展相关研究。本文调查了喜马拉雅东段库曲岩体西侧产出的多条含锂辉石伟晶岩脉, 其规模不等, 岩相分带简单, 主要岩相带为锂辉石-石英-钠长石带或锂辉石-块体长石-石英-(白云母)带, 也见钠质细晶岩。选取3处含锂辉石伟晶岩脉中的锡石开展阴极发光(CL)和原位微区分析测试(EPMA和LA-ICP-MS)研究。锡石的元素替代机制以2(Nb, Ta)⁵⁺+Fe²⁺→3Sn⁴⁺和同价置换(Ti⁴⁺, Zr⁴⁺, Hf⁴⁺, U⁴⁺)→Sn⁴⁺为主, 部分锂辉石伟晶岩中的锡石出现Fe对Sn和Nb的置换。锡石呈现4种阴极发光特征(黑色、灰色、振荡环带以及白色): 黑色区域具有明显高的Nb、Ta、Fe、Zr、U和Ga含量, 相对高的Hf含量, 以及较大的Ti含量变化范围; 灰色区域和振荡环带区域的化学组成相近, 振荡环带具有略高的Nb、Ta和Fe含量, 相对低的U含量; 白色区域具有相对低的Nb、Ta、Fe、Zr、U和Ga含量, 以及略低的Hf含量。锡石阴极发光特征与化学组成密切相关, Zr、U、Ga、Hf含量与锡石阴极发光特征具有一定联系。锡石的阴极发光结构(CL结构)主要为均一结构和不均一结构(核-边正环带、补丁以及核-边反环带), 包括黑色晶体、灰色晶体、振荡环带晶体, 黑色核+振荡环带/灰色边及补丁、灰色核+振荡环带/灰色边及补丁、振荡环带/黑色区域+白色丝带状边/棉絮状补丁、以及振荡环带/灰色/黑色核+黑色边。锡石CL结构的演化序列自内向外表现为黑色核部/晶体→灰色或振荡环带区域/晶体→(白色/黑色区域), 向亏损Nb、Ta、Fe、U、Ga, 弱亏损Zr、Hf、Y、Ti元素, 以及高Ta/(Nb+Ta)值和低Zr/Hf值的方向演化, 晚期可出现Fe的富集。锡石的CL结构和化学组成对比发现: 同一伟晶岩脉中的含锂辉石伟晶岩的分异演化程度低于锂辉石伟晶岩, 共存矿物的结晶分异可能产生富Nb和富Ta两组锡石, 不同含锂辉石伟晶岩脉在岩浆化学特征和演化过程具有明显差异。锡石CL结构与化学组成能够揭示锂辉石伟晶岩的化学特征与形成过程。

Cassiterite of the Kuqu spodumene-bearing pegmatites in the eastern Himalaya,Tibet, and its implication

Cassiterite is one of the most important tin-bearing minerals in granitic pegmatite, which can be observed in many leucogranite-pegmatite systems in the Himalaya. The microstructures and chemical compositions of cassiterite from the lithium-rich pegmatites in this region have not been revealed so far and its significance on magma fractionation and evolution requires further study. In the field work, the spodumene-bearing pegmatite dykes in the Kuqu intrusion in the eastern Himalaya were identified and investigated, which are simply zoned and mainly composed of spodumene-quartz-albite zone or spodumene-blocky feldspar-quartz-(muscovite) zone with albite aplite. The cassiterite crystals from three spodumene-bearing pegmatite dykes were studied by cathodoluminescence (CL) and in-situ EMPA and LA-ICP-MS analyses, demonstrating that the mechanisms of the cassiterite substitution are dominant of 2(Nb, Ta)5++Fe2+→3Sn4+ together with a homovalent substitution of (Ti4+, Zr4+, Hf4+, U4+)→Sn4+. In some of the cassiterite crystals in one pegmatite dyke, Sn and Nb were replaced by Fe. Black, grey, oscillatory and white domains are displayed on CL images of cassiterite, and the results show that the black domains have high Nb, Ta, Fe, Zr, U and Ga contents with a relatively high Hf content and a larger Ti content range, the grey zones display similar components to the oscillatory zones which are slightly rich in Nb, Ta and Fe and depleted in U, while the white domains are mainly poor in Nb, Ta, Fe, Zr, U, Ga and Hf. The CL characteristics of cassiterite are influenced by its compositions, i.e., Zr, U, Ga and Hf are related with CL of cassiterite in some degree. The CL structures are homogeneous crystals, including black, grey and oscillatory grains, and heterogeneous patterns which are core-rim normal zoning, patch and core-rim reverse zoning. They are black core+oscillatory/grey border/patch, grey core+oscillatory/grey border/patch, oscillatory/black core+white convolute rim/patch, oscillatory/grey/black core+black border, etc. The sequence of the cassiterite CL structure is black core/crystal → grey or oscillatory domain/crystal → (white/black domain), and geochemically, the evolution trend is a gradual decrease of Nb, Ta, Fe, U, Ga, a slight loss of Zr, Hf, Y, Ti, an increasing of Ta/(Nb+Ta) and Zr/Hf values, and finally followed by an Fe enrichment. Comparisons on CL structures and chemical compositions of cassiterite reveal that the evolution degree of the pegmatite with sporadic small spodumene crystals is lower than that of the pegmatite hosting coarse spodumene grains, competence of coexisting minerals might produce Nb-rich and Ta-rich cassiterite in one pegmatite sample, and different spodumene-bearing pegmatite dykes are diversified in their compositions and evolution of magmas. Thus, CL structure and chemical composition of cassiterite could reflect the characteristics of magma chemistry and formation process in a spodumene-bearing pegmatite.