基墨里造山与松潘-甘孜锂矿链成因的探讨

The Cimmeride orogeny and the genesis of the Songpan-Ganze“lithium deposit chain”

晚三叠世—早侏罗世,基墨里大陆和泛华夏陆块与欧亚大陆的碰撞,形成了4000 km长的基墨里造山带。位于三大陆之间、现今青藏高原北部的松潘-甘孜地体的造山属性的确定,对于探索松潘-甘孜锂矿链的成因有重要意义。松潘-甘孜地体西部和北部的早侏罗世陆相火山岩、底砾岩和煤系地层(~201 Ma)不整合在晚三叠世褶皱地层及花岗岩体之上,为晚三叠世—早侏罗世的基墨里造山时限提供了直接证据。松潘-甘孜地体中的甲基卡、可尔因、扎乌龙和白龙山-大红柳滩等稀有金属矿集区具有共同特征:赋存在由核部高分异S型花岗岩和幔部中晚三叠世浊积岩组成的片麻岩穹隆构造中、经历了巴罗-巴肯变质作用、含锂伟晶岩脉侵位在花岗岩体上部的伸展空间。通过对松潘-甘孜地体区域地质调查和对矿集区的变质、变形、岩浆和成矿作用及同位素年代学研究,提出该地体经历晚三叠世—早中侏罗世的基墨里造山过程:(1) 230~220 Ma:地壳缩短和加厚阶段,以盖层大规模强烈褶皱、逆冲带,盖层与基底之间向南的滑脱变形为特征,伴随深熔和巴罗式变质作用;(2) 220~190 Ma:地壳减压折返阶段,大量花岗岩侵位在中晚三叠世浊积岩中、形成片麻岩穹隆并伴随巴肯式变质作用。由于花岗质岩浆的高度分异及岩浆不混溶作用,导致侵位在片麻岩穹隆顶部伸展空间的伟晶岩稀有金属富集成矿。此外,伟晶岩型锂矿科学钻探(JSD)揭示了甲基卡矿集区多层次穹状花岗岩席控制含锂伟晶岩脉的成矿机制,推测大规模中下地壳基底深熔驱动岩浆上升,岩浆体沿上地壳中的构造面推叠形成岩席。

During the Late Triassic-Early Jurassic,the Cimmerian,Cathaysian and Eurasian continents in the Tethys domain on the eastern side of the Pangea super-continent collided and formed the 4000 km-long Cimmeride orogenic belt.The Songpan-Ganze terrane in the northern part of Tibet Plateau,was located among these three continents.The orogenic properties of the Songpan-Ganze terrane are of great significance to understand the genesis of the Songpan-Ganze“lithium deposit chain”.The unconformable Early Jurassic continental volcanic rocks,the basal conglomerates and the coal-bearing strata(~201 Ma)in the western and northern Songpan-Ganze terrane settled on the Late Triassic folded sediments and granites,providing the direct evidence for the timing of the Cimmeride orogeny.The rare metal ore deposits in Songpan-Ganze terrane,such as Jiajika,Keeryin,Zhawulong and Bailongshan share the same characteristics:The ore deposits are hosted in gneiss dome that is composed of the highly differentiated Stype granites in the core and the Upper Triassic turbidites in the mantle,which have experienced Barrovian-Buchan metamorphism,and lithium-rich pegmatites are emplaced in the extensive spaces above the granites.Based on the regional geological survey of the Songpan-Ganze terrane and the study of metamorphism,deformation,magmatism and metallogenic processes and isotopic chronology of the ore deposits,it is proposed that this terrane has undergone the Late Triassic to Early Middle Jurassic Cimmeride orogenic process:①230~220 Ma:the crust shortening and thickening stage is characterized by the intensive large-scale folding and thrusts,as well as top-to-the-south decollement between the sedimentary covers and the Neoproterozoic basement.This stage is also characterized by anatexis and Barrovian metamorphism;②220~190 Ma:during the crustal decompression and exhumation stage,a large number of granites emplaced in the Late Triassic turbidites,forming gneiss domes accompanied with Buchan metamorphism.Due to high-degree differentiation and liquid immiscibility of the granitic magma,the rare metal-rich pegmatites had emplaced in the extensive spaces on top of gneiss domes.In addition,The Jiajika Scientific Drilling reveal the metallogenic mechanism of the lithium-bearing pegmatite veins controlled by the domal granitic sheets.It is speculated that the magma rising was driven by large-scale melting in the middle and lower crust.The granitic sheets were formed by thrusts along structural planes in the upper crust.