西昆仑北段木吉地区三叠纪岩体与稀有金属伟晶岩的关系探讨

Genetic discussion between the Triassic plutons and rare metal pegmatite in the Muji area from north section of the Western Kunlun,China

近年发现的西昆仑-松潘-甘孜稀有金属伟晶岩成矿带延绵数千千米,向我国最西边延伸至西昆仑北段木吉地区,该地区稀有金属伟晶岩成矿与三叠纪高分异花岗岩密切相关。本文重点研究了该地区四个三叠纪岩体,包括石英闪长岩、二长花岗岩、花岗闪长岩和白云母花岗岩,对其开展了系统的年代学、地球化学和Sr-Nd-Hf同位素工作,以探讨岩体成因及其与稀有金属伟晶岩成矿之间的关系。年代学研究表明它们的侵位时代在214~220Ma之间。其中白云母花岗岩具有明显不同于其它岩体的地球化学和同位素特征,富集Rb、K、U、Th、P、Nb、Ta,亏损Sr、Ba、Zr、Hf、Ti及重稀土,有明显的轻重稀土分馏和Eu负异常,为强分异花岗岩;具有更高的I_(Sr)(t)值(0.71734~0.72044)、更负的ε_(Nd)(t)值(-8.30~-7.78)和ε_(Hf)(t)值(-4.96~-0.85)。由于木吉地区白云母花岗岩尤其富集P等挥发分元素和Li、Be等碱金属元素这些重要特征,因此本文倾向于认为白云母花岗岩可能遭受了后期伟晶岩流体大规模出溶及迁出的强烈影响,即富挥发分伟晶岩流体的大量出溶、聚集和迁出改变了残留熔体的物理化学特征,如Ca-Na-K的选择性迁出打破了同位素体系平衡,致使白云母花岗岩同位素体系不能反映其源区特征。同时挥发分出溶、聚集和迁出在萃取熔体和围岩中Li、Be等碱金属元素的同时,也加速了含矿伟晶岩流体的开放迁移,从而沿断裂系统长距离迁移至围岩中卸载成矿。因此,挥发分聚集可能是稀有金属超常富集的主要控制因素之一,P异常可能是一有效的评判标准。

The recently discovered Western Kunlun-Songpan-Ganze rare metal belt stretches thousands of kilometers and extends to the westernmost part of China to the Muji area in Western Kunlun.It is closely related to the Triassic highly differentiated granites.In this paper,we focus on four types of Triassic granites,including quartz diorite,monzogranite,granodiorite and muscovite granite,and study their chronology,geochemistry and Sr-Nd-Hf isotopes,in order to discuss their genesis and their relationship with the mineralization of rare metal pegmatite.The geochronology studies show that their emplacement ages are similar between 214Ma and 220Ma.Therein,the muscovite granites are different from other plutons in geochemical and isotopic characteristics.The muscovite granites are enriched in Rb,K,U,Th,P,Nb,Ta,depleted in Sr,Ba,Zr,Hf,Ti and HREE,with obviously fractionation of LREE and HREE and negative Eu anomaly,belonging to strongly differentiated granite and higher I_(Sr)(t)values(0.71734~0.72044),more negative ε_(Nd)(t)(-8.30~-7.78)and ε_(Hf)(t)values(-4.96~-0.85).Traditionally,the muscovite granite is considered derived from partial melting of the original crustal material,but a few scholars have proposed a new understanding of the high degree of crystallization differentiation and large-scale mixing of crustal material during allogenetic emplacement.This paper tends to consider that these obvious features of the muscovite granite are attributed to the mixing of surrounding rock and the large-scale dissolution and migration of pegmatite fluid.Combined with the enrichment of volatile elements(P)and alkali metal elements(Li,Be),this paper considers that the large amount of the dissolved,accumulation and migration of volatile from the muscovite granite melt might change the physical and chemical characteristics of the residual melt,such as selected migration of K-Na-Ca breaking the isotopic balance,resulting in the distortion to mirror the origin of protolith.Volatile dissolution,accumulation and migration would not only extract Li and Be from the melt,but also facilitate the migration of rare metal pegmatite fluid,thus,long-distance migrating along the faults to the surrounding rock for rare metal mineralization.Therefore,volatile accumulation may be one of the dominating controlling factors of rare metal super-enrichment,and P anomaly may be an effective evaluation criterion.