川西甲基卡308号伟晶岩脉年代学和地球化学特征及其地质意义
Geochronology, Geochemistry and Their Geological Significances of No.308 Pegmatite Vein in the Jiajika Deposit, Western Sichuan, China

甲基卡稀有金属矿床是我国目前规模最大的伟晶岩型稀有金属矿床，308号伟晶岩脉为其中出露面积最大的伟晶岩脉，由于勘查及研究程度较低，其形成时代及成矿机制尚不明确.通过LA-MC-ICP-MS锡石U-Pb测年，首次获得产于308号伟晶岩脉中间带含锂辉石伟晶岩的年龄为210.9±4.6 Ma，表明其形成于印支晚期，为印支旋回强烈造山运动之后相对稳定阶段的产物.元素地球化学特征表明，308号伟晶岩脉中边缘带无矿细晶岩与矿床内二云母花岗岩具有相似的过铝质S型花岗岩特征，二者具有同源性，并认为其成矿机制为：花岗质岩浆在中浅成、偏还原的环境上升侵位，由细晶岩至伟晶岩演化过程中，相对分异程度升高，熔体相和富挥发分的流体相之间发生强烈碱交代作用，并在一定的结构分带中发生大规模稀有金属矿化.
No. 308 pegmatite vein has the maximal exposed area in the Jiajika rare metal deposit, which is the largest pegmatite type deposit in China. Because of the low degree of exploration and research, its metallogenic epoch and metallogenic mechanism remain unclear. Detailed geochronology and lithogeochemistry of the granitoids and pegmatites were carried out to explore the petrogenesis and tectonic setting cassiterite LA-MC-ICP-MS dating yields concordant ages with (210.9±4.6) Ma of spodumenebearing pegmatite in the middle zone, which reflects that No.308 pegmatite vein in the Jiajika pegmatite deposit formed at the late stage of Indosinian and was the result of the relative stable period after the strong Indosinian Orogeny. Geochemical data show that REEs and other trace elements patterns of the barren aplite in marginal zone of No. 308 pegmatite vein are similar to the two-mica granite in the Jiajika deposit, and both of them are characterized by the peraluminous S-type granite and homology. Dfferentiation of the magmatic water gradually increased from granitic aplites to pegmatites. The granitic magma ascended and intruded under a medium-shallow deep and partial reducing environment. During the evolution of magma, there is a strong alkali metasomatism between the melt phase and the fluid phase rich in volatile, and resulting in that a large-scale rare metal mineralization occurred in the certain structural zoning of No.308 pegmatite vein in the Jiajika deposit