石榴子石中的“水”揭示喜马拉雅错热锂辉石伟晶岩快速形成

东西向延伸近2500km的高喜马拉雅淡色花岗岩带不仅是新生代印度-欧亚板块碰撞的产物, 同时也与多种稀有金属矿床密切相关。近期, 在高喜马拉雅中部的错热地区发现了多条锂辉石伟晶岩脉, 为研究该地区岩浆分异演化过程、探索错热地区锂辉石伟晶岩成岩成矿时限提供了新材料。本文对该地区各类型淡色花岗岩及锂辉石伟晶岩中的石榴子石进行电子探针和傅里叶变换红外光谱分析, 研究表明: 错热地区花岗岩-伟晶岩体系中石榴子石属于岩浆成因的铁铝榴石-锰铝榴石系列, 且可能存在多种OH取代机制; 石榴子石主量元素及水含量记录了岩浆演化历程, 水含量受Si、Fe、Ca、Mn影响; 伴随岩性从石榴子石白云母花岗岩向锂辉石伟晶岩分异演化, 石榴子石逐渐富锰、富水; 在伟晶岩侵位后, 还可能与围岩发生了混染并发生了流体丢失, 导致石榴子石边部贫锰、贫水。此外, 石榴子石氢扩散模拟可用于限定伟晶岩形成时限, 热模拟结果显示2.5m宽的错热锂辉石伟晶岩脉自720℃侵位温度冷却至岩体中心温度低于450℃仅需24天, 表明错热锂辉石伟晶岩冷却迅速, 具有快速的形成速率。另外, 石榴子石水含量与MnO/(MnO+FeO)值呈正相关关系, 高水含量(>0.04%) 的锰铝榴石可能是喜马拉雅伟晶岩Li-Be矿化的潜在指示标志。

Diffusion of hydrogen in garnets reveals the rapid formation of spodumene pegmatite from Cuore in the Himalayan orogen

The Higher Himalayan leucogranite which extends east-westward for nearly 2500km is not only the product of the Cenozoic India-Asia continental collision, but also closely related to a variety of rare metal deposits. Recently, a number of spodumene pegmatite veins have been found in Cuore area in the central part of the Higher Himalayas. To study the evolution process of magma differentiation and explore the time duration of diagenesis and mineralization of these spodumene pegmatites, electron probe and Fourier transform infrared spectroscopy were used to analyze garnet from various types of granite and spodumene pegmatite in this area. The results show that the garnet in the granite-pegmatite system is an almandine-spessartine solid solution and of magmatic origin. There may be a variety of OH substitution mechanisms in pegmatite garnet. The major compositions and water content of garnet can record the evolution of magma since they can be affected by Si, Fe, Ca and Mn. In the process of magmatic differentiation and evolution from garnet muscovite granite to spodumene pegmatite, garnet is gradually rich in manganese and water. After the emplacement of pegmatite, surrounding rock contamination and fluid loss may occur, resulting in poor manganese and water on the edge of the garnet. The hydrogen diffusion simulation of garnet can be used to constrain timescales of cooling processes and pegmatite formation. The thermal simulation result shows that it takes only 24 days for the 2.5m thick spodumene pegmatite dike to cool from the emplacement temperature of 720℃ to the central temperature of the dike below 450℃, which indicates that spodumene pegmatites in Cuore area have a rapid formation rate. Besides, the water content of garnet is positively correlated with MnO/(MnO+FeO). Therefore, the high-water-content (>0.04%) of spessartine may be a potential sign of Li-Be mineralization of leucogranites and pegmatites in Himalayan region.