Li-Be-Nb-Ta mineralogy of the Kuqu leucogranite and pegmatite in the Eastern Himalaya, Tibet, and its implication

Abstract

稀有金属矿物记录了花岗伟晶岩成岩成矿的重要信息。喜马拉雅是全球著名的淡色花岗岩带，库曲岩体位于喜马拉雅东段的特提斯喜马拉雅岩系中。本文调查了库曲岩体的二云母花岗岩、白云母花岗岩、电气石花岗岩和花岗伟晶岩，其中，花岗伟晶岩涉及花岗岩的伟晶岩相和独立伟晶岩脉。库曲岩体产出的稀有金属矿物包括锂辉石、锂绿泥石、绿柱石、铌铁矿-钽铁矿、钇铀钽烧绿石和细晶石，它们主要赋存于似文象伟晶岩、石英-钠长石-白云母伟晶岩、块体长石-钠质细晶岩、块体长石-电气石钠质细晶岩、锂辉石-块体长石-细晶岩、白云母花岗岩的伟晶岩相以及电气石花岗岩内。显微镜观察、电子探针和LA-ICP-MS测试结果显示锂辉石具有四种产状，包括粗粒锂辉石自形-半自形晶、细粒锂辉石-石英镶嵌晶、中细粒锂辉石-钾长石-钠长石-云母镶嵌晶以及发育锂绿泥石的粗粒锂辉石，揭示了其形成时复杂的熔流体动荡结晶环境。绿柱石背散射电子图像（BSE）下呈均一结构和不均一结构（蚀变边、不规则分带和补丁分带），元素替代机制包括通道-八面体替代、通道-四面体替代以及通道中碱金属阳离子间的置换。铌铁矿族矿物包括原生、蚀变边和不规则分带结构，部分被钇铀钽烧绿石和细晶石交代。与原生铌铁矿相比，蚀变边和不规则分带铌铁矿族矿物总体上富钽贫锰，显示了结晶分异、过冷却引起的过饱和以及流体作用。根据稀有金属矿物揭示的成因信息，独立伟晶岩脉（似文象伟晶岩）、白云母花岗岩的伟晶岩相和电气石花岗岩在岩浆分异程度、经历的演化过程、以及流体活动方面存在差异，很可能是不同期次岩浆活动的产物。库曲岩体绿柱石的Rb和Zn含量、以及铌铁矿族矿物的Sc₂O₃、SiO₂和PbO含量，与已有指示标志存在相关性，作为潜在指示标志仍需开展更多的研究工作。综合含锂辉石伟晶岩的产出、岩浆分异演化程度、多期花岗质岩浆活动、复杂的流体作用以及所属锂丰度高值区等因素，库曲岩体是喜马拉雅东段找锂的有利地段。;The rare-element minerals record important information of rock-forming process and metallogenesis. Himalaya is a famous leucogranite belt. The Kuqu intrusion is located in the Tethys Himalayan sedimentary sequence in the Eastern Himalaya. This study investigated two-mica granite, muscovite granite, tourmaline granite and granitic pegmatites in the Kuqu intrusion. The granitic pegmatites refer to either the pegmatite rocks that are included in and related to granite or the pegmatite zones that make up a single pegmatite dyke. The rare-element minerals identified in the Kuqu intrusion include spodumene, cookeite, beryl, columbite-tantalite, Y-hatchettolite and microlite. They are distributed in quartz-feldspar-muscovite pegmatite, quartz-albite-muscovite pegmatite, blocky feldspar-aplite, block feldspar-tourmaline aplite, spodumene-blocky feldspar-aplite, pegmatite rock in muscovite granite, and tourmaline granite. The EMPA and LA-ICP-MS analytical results indicate that spodumenes show four occurrences which consists of coarse-grained subhedral to euhedral spodumene, fine-grained symplectitic reaction border composed of spodumene and quartz, middle to fine-grained mosaic structure of spodumene, K-feldspar, albite and muscovite, and coarse-grained spodumene filled by cookeite veins. These occurrences of spodumene reveal a complex and fluid-rich unstable crystallization environment. There are homogeneous and heterogeneous beryl crystals in the intrusion. The internal zonation patterns for heterogeneous beryls are alteration border, patches and complex irregular zoning in BSE images. The substitution mechanisms of beryl in the Kuqu intrusion are channel-octahedral, channel-tetrahedral and alkali cations in channel. The columbite-group mineral crystals are composed of primary part and alteration border or display irregular zoning pattern, some of which are altered by Y-hatchettolite and microlite. Compared with primary parts, the alteration border and irregular zoned crystals have higher Ta/(Nb+Ta) values and lower Mn/(Fe+Mn) values, indicating fractional crystallization, supersaturation caused by strong undercooling, and fluid activities. Based on the studies of rare-element minerals, the single pegmatite dyke (quartz-feldspar-muscovite pegmatite), pegmatite rock in muscovite granite and tourmaline granite display differences on the degree of fractionation of magma, evolution processes and fluid activities and they might be magma products of different stages. The Rb and Zn contents of beryl, and Sc₂O₃, SiO₂ and PbO contents of columbite-group minerals show correlation with the existed indicators for evolution degree of magma, but more studies are needed to carry out for them to be possible indicators. Considering the spodumene-bearing pegmatite indentified in the intrusion, the high degree of magma evolution, multistage of magmatism, complex fluid activities and being in a high lithium abundance value area, the Kuqu intrusion is an important potential area for future lithium prospecting in the Eastern Himalaya.