矿物成分对东秦岭灰池子复式岩体与花岗伟晶岩成因联系的约束  

Genetic link of the Huichizi composite pluton and granitic pegmatites in the East Qinling tectonic belt:Constraints from mineral compositions

作　　者：郭腾龙 王梦玺[1] 凤永刚 梁婷[1] 周义[1] 孙小筠 GUO TengLong;WANG MengXi;FENG YongGang;LIANG Ting;ZHOU Yi;SUN XiaoYun(School of Earth Science and Resources,Chang an University,Xi an 710054,China;Key Laboratory of Western China s Mineral Resources and Geological Engineering,Ministry of Education,Xi an 710054,China)

机构地区：[1]长安大学地球科学与资源学院,西安710054 [2]教育部西部矿产资源与地质工程重点实验室,西安710054

出　　处：《岩石学报》2023年第7期2117-2137,共21页Acta Petrologica Sinica

基　　金：国家重点研发计划(2019YFC0605202);陕西省自然科学基础研究计划项目(2018JM4009、2021JM-160);中央高校基本科研业务费专项资金(300102271208)联合资助.

摘　　要：灰池子复式岩体是东秦岭构造带中规模最大的I型花岗岩,围绕该岩体发育大量赋存稀有金属和铀矿化的花岗伟晶岩脉,二者具有密切的时空联系。然而,灰池子岩体岩浆演化过程尚不清楚,与花岗伟晶岩的成因联系也还存在争论。本研究对灰池子岩体和其东南方向出露的黑云母正长花岗岩中黑云母和长石进行了原位成分分析,结合全岩成分讨论了岩体岩浆演化过程及其与花岗伟晶岩之间的关系。灰池子岩体黑云母二长花岗岩、黑云母花岗闪长岩和英云闪长岩形成压力相似且黑云母离子替代方式相同,说明它们具有演化关系,但氧逸度和SiO_(2)含量依次升高,温度及FeO^(T)和MgO含量依次降低,因此黑云母二长花岗岩演化程度最低。此外,从英云闪长岩到黑云母花岗闪长岩中黑云母与碱性长石Rb、Cs含量逐渐升高而Ba含量逐渐降低,说明后者分异程度高于前者。灰池子岩体为低-中分异花岗岩,稀有金属含量较低且在岩浆演化过程中变化不大,可能不具备成为东秦岭稀有金属矿化伟晶岩母岩的条件。另一方面,黑云母正长花岗岩中黑云母离子替代方式不同于灰池子岩体,其形成温度和压力高于灰池子岩体但SiO_(2)含量低于灰池子岩体,说明二者不存在演化关系。赋存铀矿化的黑云母花岗伟晶岩与黑云母正长花岗岩黑云母离子替代方式相同,而且黑云母花岗伟晶岩具有较高氧逸度和较低形成温度,可能是黑云母正长花岗岩分异晚期的产物。A large number of Early Paleozoic granite plutons occur in the East Qinling tectonic belt,including the largest I-type Huichizi composite pluton in regional area.Moreover,numerous granitic pegmatite veins with rare-metal-and uranium-mineralization occur around the Huichizi pluton,showing closely spatial and temporal relationships.However,the magma evolution of the Huichizi pluton is still unclear,and the genetic relationship with granitic pegmatites is still under debated.In this study,in situ analyses of biotite and feldspar of the Huichizi pluton were carried out with a combination of whole-rock geochemistry,in order to examine the evolution of the Huichizi pluton and its relationship with granitic pegmatites.Similar ionic substitution mechanism for biotite and identical emplacement depth(8.0~9.3km)for biotite monzogranite,biotite granodiorite and tonalite of the Huichizi pluton indicate that they were formed from the same primary magma,and the gradual increase of oxygen fugacity and SiO_(2)content and decreases of temperature and contents of FeO^(T) and MgO suggest that biotite monzogranite is the least evolved one.In addition,biotite and alkali feldspar crystals have a gradual increase of Rb and Cs concentrations and a gradual decrease of Ba concentration from tonalite to biotite granodiorite,indicating that samples of the biotite granodiorite are more evolved than those of tonalite.The Huichizi pluton is a low-to medium-differentiated one,with low and restricted rare-metal concentrations,ruling out the possibility for the rare-metal-mineralized pegmatites being a fractionated product of the Huichizi pluton.On the other hand,the ion substitution for biotite of the biotite syenogranite is different from that in the Huichizi pluton,with a higher formation temperature and pressure but lower SiO_(2)content than those of the Huichizi pluton,indicating that they may have no genetic relationship.Biotite of the U-mineralized biotite granitic pegmatites has the same ion substitution mechanism as those of the biotite s

关 键 词：矿物成分 岩浆演化 花岗伟晶岩 灰池子岩体 东秦岭构造带 

分 类 号：P575.1[天文地球—矿物学] P588.121[天文地球—地质学]