中国东部新生代玄武岩中宝石级辉石及石榴石巨晶的成因及意义——以穆棱、桦甸、明溪三地为例  

Origin and Its Significance of Gem-Quality Pyroxene and Garnet Megacrysts from the Cenozoic Basalts in Eastern China:Constraints from Muling,Huadian and Mingxi Areas

作　　者：潘少逵 吴箫龙 胡琳琳 孙海祁 PAN Shaokui;WU Xiaolong;HU Linlin;SUN Haiqi(Gemmological Institute,China University of Geosciences,Wuhan 430074,China;Hubei Gems and Jewelry Engineering Technology Research Center,Wuhan 430074,China;School of Earth Sciences,China University of Geosciences,Wuhan 430074,China)

机构地区：[1]中国地质大学(武汉)珠宝学院,湖北武汉430074 [2]湖北省珠宝工程技术研究中心,湖北武汉430074 [3]中国地质大学(武汉)地球科学学院,湖北武汉430074

出　　处：《宝石和宝石学杂志（中英文）》2024年第S01期1-5,共5页Journal of Gems & Gemmology

基　　金：国家自然科学基金面上项目(41973032)。

摘　　要：中国东部广泛分布以碱性玄武岩为主的新生代火山岩。这些火山岩含有丰富的深源岩石包体和高压巨晶,为认识我国东部深部岩石圈物质组成及过程提供了良好样品。然而,与广受关注的岩石包体相比,人们对与之共生的巨晶的形成机制与形成条件等方面的认识仍显薄弱。这些巨晶不仅种类丰富(包括辉石、石榴石、角闪石、锆石、钛铁矿、刚玉等矿物),更为重要的是,其常具有粒度大、净度高等特点,可作为优质的彩色宝石资源而加以利用(图1)。因此,本文以黑龙江穆棱、吉林桦甸、福建明溪新生代碱性玄武岩所携带的辉石、石榴石巨晶样品为研究对象,通过原位主微量元素组成分析,结合前人研究成果,探讨了我国东部新生代火山岩中辉石与石榴石巨晶的成因,进而约束陆下深部岩石圈所经历的地质过程。穆棱、明溪两个地方的火山岩均含有辉石和石榴石巨晶,但桦甸地区仅见辉石巨晶。这些巨晶的形态多为不规则状,难以识别出原始晶形及晶面特征。其中,辉石巨晶的颗粒较大,粒径多介于1~4 cm。而石榴石巨晶相对较小,粒径大多小于3 cm。辉石巨晶肉眼观察均呈黑色,透射光下呈棕褐色-浅绿色,石榴石巨晶则呈粉红色-酒红色。主量元素测试结果表明,所有巨晶在化学成分上是均一的,没有出现核-边元素含量系统性的变化。其中,穆棱与桦甸地区辉石包括了单斜辉石和斜方辉石,而明溪地区仅见有单斜辉石。这三个地区的单斜辉石都具有较高的Al_(2)O_(3)含量(6.55~10.30 wt%),可归属于富Al普通辉石。单斜辉石巨晶的主要元素含量与地幔岩石中(如橄榄岩)单斜辉石主要元素含量截然不同,表明其不是被寄主岩浆上涌过程中捕获的由地幔岩石裂解产生的矿物碎块。更值得注意的是,单斜辉石巨晶的主要元素含量(如TiO_(2)和Al_(2)O_(3))随Mg#的降低而升高,而Cr_(2)O_(3)随Mg#�The Cenozoic volcanic rocks,mainly composed of alkaline basalts,are widely distributed across Eastern China.These volcanic rocks contain abundant deep-derived rock xenoliths and high-pressure megacrysts,providing good opportunities for understanding the composition and processes experienced by deep lithosphere beneath Eastern China.However,compared to the well-studied rock xenoliths,people’s understanding of the formational mechanism and conditions of the accompanied megacrysts is relatively weak.These megacrysts include a variety of minerals such as pyroxene,garnet,amphibole,zircon,ilmenite,corundum.More importantly,they are characterized by large size and high clarity,and can be used as high-quality colored gemstone resources.Therefore,this study conducted in-situ major-and trace-element analysis on pyroxene and garnet megacrysts hosted by the Cenozoic alkali basalt from Muling(Heilongjiang Province),Huadian(Jilin Province)and Mingxi(Fujian Province)are as with the aim of revealing the origin of pyroxene and garnet megacrysts from the Cenozoic volcanic rocks in Eastern China,and further constraining the geological processes undergone by the deep lithosphere.The volcanic rocks in Muling and Mingxi areas contain pyroxene and garnet megacrystals,but only pyroxene megacrystals are found in Huadian area.The shape of these megacrysts is commonly irregular,making it difficult to identify the original crystal form and related face characteristics.Among them,the pyroxene megacrysts are large in size,with diameters mostly ranging from 1-4 cm.Garnet megacrystals are relatively small,with diameters mostly less than 3 cm.The pyroxene megacrystals are black under naked eyes and brown light green under transmitted light,while the garnet megacrysts are light pink to wine red in colour.According to major-element analytical results,the megacrysts are chemically homogeneous,without systematic differences of element contents between core and rim.The pyroxene megacrysts from Muling and Huadian include clinopyroxene and orthopyrox

关 键 词：玄武岩 巨晶 成因 中国东部 

分 类 号：P588.145[天文地球—岩石学] P619.28[天文地球—地质学]