塔里木大火成岩省熔结凝灰岩中富磷-稀土矿物集合体的识别及其意义  

The discovery of Fe-P-REE enriched mineral assemblages in the ignimbrite from Tarim large igneous province and its implication

作　　者：安秀慧 程志国[1] 张招崇[1] 刘秉翔 孔维亮 AN Xiu-hui;CHENG Zhi-guo;ZHANG Zhao-chong;LIU Bing-xiang;KONG Wei-liang(State Key Laboratory of Geological Processes anil Mineral Resources,China University of Geosciences,Beijing 100083,China)

机构地区：[1]中国地质大学地质过程与矿产资源国家重点实验室,北京100083

出　　处：《岩石矿物学杂志》2021年第4期717-728,共12页Acta Petrologica et Mineralogica

基　　金：国家自然科学基金资助项目(41772057,41702064);中央高校基本科研业务费优秀导师资助项目(2652018118)。

摘　　要：高分异的高硅流纹岩具有稀土元素(REE)成矿的潜在价值。塔里木大火成岩省以发育高比例的长英质岩石为特征,但其是否具有稀土成矿的潜力还缺少有力证据。本次研究以来自塔里木大火成岩省塔北隆起S42井的高硅熔结凝灰岩为研究对象,从中识别出多种富Fe-P-REE的矿物集合体:(1)熔结凝灰岩的岩屑具有由磁铁矿、独居石和碱性长石等矿物组成的核部,并被石英组成的边部包裹;(2)熔结凝灰岩胶结物中的磁铁矿+磷灰石+金红石+独居石矿物组合;(3)晶屑石英包裹独居石和黄铁矿等。进一步的石英SIMS原位氧同位素分析表明,岩屑石英的δ^(18)O_(V-SMOW)值的变化范围为11.15‰-14.60‰,而晶屑石英的氧同位素组成偏轻(7.23‰-7.84‰),与全岩氧同位素组成基本一致(5.00‰-7.21‰)。石英钛饱和温度计估算晶屑石英的形成温度为756-815℃,岩屑石英为566-645℃。另外,岩屑中与独居石共生的磁铁矿的微量元素特征与热液成因的磁铁矿一致。以上证据表明,岩屑中的Fe-P-REE矿物集合体主要来源于热液作用。而熔结凝灰岩的胶结物中与晶屑石英包裹体中出现富Fe-P-REE矿物集合体表明岩浆在演化的晚期REE已足够富集并结晶出稀土矿物。因此,熔结凝灰岩中的富Fe-P-REE的矿物集合体主要与岩浆的分离结晶作用和热液作用有关,它们的识别对认识塔里木大火成岩省长英质岩石的稀土成矿潜力具有重要意义。The highly fractionated high-silica rhyolite is a potential host of the rare earth elements(REE)deposits.The Tarim large igneous province(T-LIP)is characterized by a high proportion of felsic rocks;nevertheless,the potential capacity of the REE mineralization remains unclear.In this study,ignimbrite from well S42 in Northern Tarim Uplift was studied,and several Fe-P-REE enriched mineral assemblages were recognized:(1)the lithoclast consisting of quartz rim and magnetite+monazite+alkali feldspar core;(2)the magnetite+monazite+rutile+apatite in the ground mass;(3)the monazite+pyrite in the quartz phenocryst.Further analysis of quartz SIMS oxygen isotopes in situ showed that the δ^(18)O_(V-SMOW) values of lithic quartz range from 11.15‰ to 14.60‰,while those of the quartz phenocrysts are from 7.23‰ to 7.84‰,consistent with the bulk-rock oxygenic composition(5.00‰-7.21‰).A titanium thermometer of quartz shows that the formation temperature of quartz phenocryst was 756〜815℃,and that of lithic.quartz was 566-645℃.In addition,trace elemental compositions of magnetite associated with quartz in the lithoclast also show consistency with those of the hydrothermal origin.The above evidence indicates that the Fe-P-REE mineral assemblage in the lithoclast was derived from the late hydrothermal fluids.On the other hand,the occurrence of Fe-P-REE mineral aggregates in the groundmass of the ignimbrite and quartz phenocryst indicates that REE were sufficiently concentrated in magma and then REE minerals were crystallized in the late stage of evolution.Therefore,the Fe-P-REE enriched mineral aggregates in the ignimbrite were mainly related to the magmatic fractional crystallization and the late hydrothermal fluids.Their recognition provides an important implication for the REE mineralization of felsic rocks in the T-LIP.

关 键 词：塔里木大火成岩省 熔结凝灰岩 稀土元素 液态不混溶 氧同位素 

分 类 号：P618.7[天文地球—矿床学]