西藏冈底斯带始新世曲水岩基的岩浆混合作用:来自斜长石阴极发光特征和成分变化的证据

斜长石作为主要造岩矿物,是研究岩石成因、示踪岩浆演化和岩浆混合过程的有效工具.对冈底斯带曲水岩基始新世花岗闪长岩、二长花岗岩、闪长岩脉和暗色包体中的斜长石进行了阴极发光图像结构特征、电子探针主量元素和LA-ICP-MS微量元素成分的分析,揭示了斜长石复杂环带的成因和相关的岩浆过程.该区斜长石的阴极发光图像呈现出多种颜色且与其An值相对应,随着An值降低依次为绿色、蓝色和暗灰色或暗红色等,并发育补丁状环带、筛状环带、韵律环带等.花岗闪长岩、二长花岗岩中斜长石的An值具有相似的变化范围(20~55),而闪长岩脉和暗色包体中An值的变化范围较大(25~85),表明曲水岩基经历了复杂的开放过程.微量元素结果表明:花岗闪长岩与闪长岩脉和暗色微粒包体具有相同的Sr含量范围(600×10^-6~1 100×10^-6);而二长花岗岩的Sr含量(1 000×10^-6~2 400×10^-6)整体高于前者.以上研究表明,花岗闪长岩中阴极发光呈现绿色的核部或幔部是偏中性岩浆注入寄主岩岩浆混合的结果;具有高Sr含量的二长花岗岩认为是高Sr含量的岩浆结晶形成的;闪长岩脉和暗色微粒包体中的筛状结构斜长石为寄主岩捕掳晶.

Magma Mixing of the Eocene Quxu Batholith from the Gangdese Magmatic Belt,South Tibet: Evidence from Cathodoluminescence Characteristics and Composition Changes of Plagioclase

As the main rock-forming mineral, plagioclase is an effective tool for studying petrogenesis, magma evolution and magma mixing. The cathodoluminescence image, electron probe micro-analysis and LA-ICP-MS composition analysis were carried out for the plagioclase from the granodiorite, monzogranite, diorite dykes and mafic microgranular enclaves (MMEs) in the Quxu batholith in the Gangdese magmatic belt, which can reveal the formation mechanisms and relative magma evolution process of plagioclase complex zoning. The cathodoluminescence images of the plagioclase from the Quxu batholith shows that their color displays a corresponding relationship with the An value. With the decreasing with the An values, the colors are green, blue and dark gray or dark red in turn. The plagioclases have obvious three types of zoning:patchy zonation, sieve texture and oscillatory zoned. The An values of plagioclase from the Quxu granodiorite and monzonite have similar ranges (20-55), while the An values in diorite dikes and MMEs vary widely (25-85), all indicating that the Quxu batholith has undergone a complex opening process. Insitu trace elements analyses show that the granodiorite has similar Sr content (600×10-6-1 100×10-6) with the diorite dykes and MMEs; the Sr content of monzogranite (1 000×10-6-2 400×10-6) is higher than the granodiorite, diorite dykes and MMEs. The above studies show that:the green luminescence of cores and mantle in granodiorite is the result of the mixing of the intermediate magma and felsic magma; the high Sr content of monzogranite is considered to be derived from a Sr-enriched melt. The above studies show that the complex zonings of the plagioclase are the result of the injection of mafic magma into felsic magma. The core of sieve texture plagioclase in diorite vein and MMEs could be xenocrystals, which are captured from the host rocks.