花岗质熔体结构的改变与稀有金属W-Sn-Nb-Ta的富集:以夏如早古生代花岗岩为例  

Changes in the melt structure and enrichment of rare metals W-Sn-Nb-Ta in granitic magma: An example from the Xiaru Early Paleozoic granites

作　　者：高利娥[1] 曾令森[1] 严立龙 赵令浩[2] 李广旭[1] 邸英龙 王亚莹[1] 王海涛 GAO LiE;ZENG LingSen;YAN LiLong;ZHAO LingHao;LI GuangXu;DI YingLong;WANG YaYing;WANG HaiTao(Institute of Geology,Chinese Academy of Geological Sciences,Beijing 100037,China;National Research Center for Geoanalysis,Chinese Academy of Geological Sciences,Beijing 100037,China;School of Geographic and Environmental Sciences,Tianjin Normal University,Tianjin 300387,China)

机构地区：[1]中国地质科学院地质研究所,北京100037 [2]中国地质科学院国家地质实验测试中心,北京100037 [3]天津师范大学地理与环境科学学院,天津300387

出　　处：《岩石学报》2022年第11期3281-3301,共21页Acta Petrologica Sinica

基　　金：国家重点研发计划(2021YFC2901901);国家自然科学基金项目(41873023、92055202、42002071);第二次青藏高原科学考察项目(2019QZKK0702);中国地质调查局地质调查项目(DD20221817、DD20221630)联合资助.

摘　　要：藏南喜马拉雅造山带新生代高硅淡色花岗岩富集稀有金属元素(Nb、Ta、W、Sn、Be、Li等),成矿潜力大,是未来矿产勘探的重要靶区。除了新生代花岗岩,早古生代花岗岩分布广泛,具有和新生代花岗岩相似的地球化学特征,是否也具有成矿潜力,是有待深入研究的一个重要课题。夏如穹窿主体由早古生代花岗片麻岩以及侵入其中的新生代淡色花岗岩和伟晶岩组成,这些花岗岩具有与新生代高硅淡色花岗岩相似的地球化学特征,在Sn-W和Nb-Ta系统关系上,可以分成两组:一组富集W和Sn(W=5×10^(-6)~42×10^(-6),Sn=12×10^(-6)~35×10^(-6)),另一组富集Nb和Ta(Nb=23×10^(-6)~108×10^(-6),个别高达217×10^(-6),Ta=8×10^(-6)~38×10^(-6),个别高达143×10^(-6))。与富集W-Sn的花岗片麻岩相比,富集Nb-Ta的花岗片麻岩具有:(1)较高的Na_(2)O,为富Na花岗岩,(2)较低的K_(2)O、FeO^(T)、TiO_(2)、P_(2)O_(5)、Sr、Zr;(3)略微富集MREE,亏损LREE和HREE,显著的负Eu异常;(4)较高的Nb、Ta,但较低的W、Sn。元素的系统关系表明,这两类花岗片麻岩都是较原始岩浆经历了不同程度斜长石、锆石、云母等矿物分离结晶作用的产物,富集Nb-Ta的花岗片麻岩分异程度最高。夏如早古生代两类花岗岩的Nb/Ta比值都小于5,但富集W-Sn花岗岩中Zr/Hf>20,富集Nb-Ta花岗岩中Zr/Hf<20。随着花岗质岩浆的分异逐步增强,当Zr/Hf=20时,熔体结构发生实质性变化,花岗质熔体从富钾质变成富钠质,从富集W-Sn变成富集Nb-Ta。本研究表明,在喜马拉雅造山带,不仅新生代花岗岩,而且古生代花岗岩都富集稀有金属元素,熔体结构的改变是控制花岗岩富集稀有金属的主要因素。A number of Cenozoic high slica leucogranites in the Himalayan orogenic belt are enriched in rare metals(e.g.,Nb,Ta,W,Sn,Be,Li)and have high potential to produce economic rare metal deposits.In addition to the Cenozoic granites,the Early Paleozoic granites are widely distributed and have similar geochemical characteristics to the Cenozoic granites.Whether they also have mineralization potential is an important question to be further explored.The Xiaru Gneiss Dome hosts dominantly Early Paleozoic granitic gneisses intruded by Cenozoic leucogranites and pegmatites.These granitic gneisses show similar geochemical characteristics to those in the Cenozoic leucogranites and can be subdivided into two groups in terms of their enrichment in Sn-W and Nb-Ta.Group-1 granitic gneisses are enriched in W and Sn(W=5×10^(-6)~42×10^(-6),Sn=12×10^(-6)~35×10^(-6)),whereas Group-2 enriched in Nb and Ta(Nb=23×10^(-6)~108×10^(-6),up to 217×10^(-6),Ta=8×10^(-6)~38×10^(-6),up to 143×10^(-6)).Compared with Group-1 granitic gneisses,Group-2 ones are characterized by(1)higher Na_(2)O but lower K_(2)O,FeO^(T),TiO_(2),P_(2)O_(5),Sr,Zr;(2)slightly enrichment in MREE,but depletion in LREE and HREE with pronounced Eu negative anomalies;and(3)higher Nb,Ta,but lower W,Sn.The systematic relationship of elements suggests that(1)both groups of granite gneisses are the products from more primary magmas by different degrees of fractional crystallization of plagioclase,zircon,mica,and(2)Group-2 granites represent the most evolved melts.Both groups of granitic gneisses in the Xiaru Dome have Nb/Ta lower than 5,but Zr/Hf ratios in Group-1 are greater than 20 in contrast to less than 20 in Group-2.Therefore,we suggest that with fractional crystallization of granitic magma,when Zr/Hf ratios drop to 20,the melt structure changes substantially in conjunction with changes in the melt’s composition from potassium-rich to sodium-rich.Such changes further induce changes in the dissolution behaviors and subsequent enrichment of Sn,W,Nb,and Ta in the m

关 键 词：稀有金属元素 W-Sn-Nb-Ta 熔体结构 古生代花岗片麻岩 喜马拉雅造山带 

分 类 号：P588.121[天文地球—岩石学] P597.3[天文地球—地质学] P618.6