桂东北茅安塘伟晶岩中石榴子石的特征及对岩浆演化的指示意义

为了解桂东北伟晶岩岩浆的形成环境及演化过程，对桂东北茅安塘Nb-Ta-Be-Rb稀有金属矿床周围伟晶岩中的石榴子石进行了镜下观察、电子探针（EPMA）和LA-ICP-MS原位微区主微量元素研究，探讨石榴子石的成因及其对成岩及成矿作用的指示.结果表明，桂东北茅安塘地区伟晶岩中的石榴子石为岩浆成因石榴子石，属于铁铝榴石-锰铝榴石（平均Alm_49.28-Sps_47.09）固溶体系列，可分为早期形成的Ⅰ型石榴子石（GrtⅠ）和晚期形成的Ⅱ型子石（GrtⅡ）.两期石榴子石均以富集重稀土（HREE）、高场强元素（HFSE），亏损轻稀土（LREE）和缺乏大离子亲石元素（LILE）为特征，∑REE配分模式呈明显左倾趋势，显著的Eu负异常.石榴子石生长过程中的界面反应速率小于物质迁移速率，水岩作用较弱，∑REE主要以表面吸附或吸收的形式进入石榴子石中，是导致其重稀土（HREE）元素富集，轻稀土元素亏损的主要原因.随着岩浆分异演化程度的不断提高，∑REE逐渐进入并赋存于石榴子石中，促进岩浆从早期的低分馏（未分馏）的岩浆熔体逐渐向晚期的高分馏的岩浆熔体演化.石榴子石中HREE含量随岩浆演化程度逐渐增加表明，晚期分异演化的岩浆-热液中逐渐富集稀土及稀有金属元素.这些晚期富含成矿元素的热液流体交代原生矿物，导致外侧带及核部花岗伟晶岩中发育大量交代成因的稀土和稀有金属矿物. 

Characteristics of Garnets in Pegmatites of Mao'antang,Northeast Guangxi,and Their Implications for Magmatic Evolution

In order to understand the formation environment and evolution process of pegmatite magma in Northeast Guangxi, in this paper, the garnet in the pegmatite around the Nb-Ta-Be-Rb rare metal deposit in Mao'antang, Northeast Guangxi, was observed under microscope and studied by EPMA and LA-ICP-MS. The results show that the garnets in pegmatites in Mao'antang area form Northeast Guangxi are magmatic genesis garnets, which belong to the solid solution series of almandine manganese almandine (average Alm_49.28-Sps_47.09). It can be divided into early iron aluminum garnet (GrtⅠ) and late manganese aluminum garnet (GrtⅡ). Both phases of the garnet with enrichment of heavy rare earth elements (HREE) and high field strength elements (HFSE), loss of light rare earth elements (LREE) and lack of large ion lithophile elements (LILE) as the characteristics of ∑REE distribution pattern show a trend of obvious left-leaning and Eu negative anomaly. The interface reaction rate during the growth of garnet is less than the material migration rate, and the water-rock interaction is weak, ∑REE mainly in the form of surface adsorption and absorption into the garnet stone is responsible for its heavy rare earth elements (HREE), the main cause of light rare earth losses. With the increasing degree of magma differentiation evolution, ∑REE gradually enters and accumulates in the garnet, promoting the evolution of magma from the early low-fractionation (unfractionated) magma melt to the late high-fractionation magma melt. The HREE content in garnet gradually increases with the degree of magma evolution, indicating that late differential evolution of magmatic-hydrothermal fluid is gradually enriched with rare earth and rare metal elements. In the late hydrothermal fluid metasomatism primary minerals, which are rich in metallogenic elements, lead to the development of a large number of metasomatism rare earth and rare metal minerals in the lateral zone and the nuclear granite pegmatite.