碱性岩中富钛钙铁榴石的成因及成岩成矿意义:以河北矾山超镁铁岩-正长岩杂岩体中的石榴石为例  

Genesis of the titanian andradite in alkaline rocks and their petrological and mineralization significance:A study example of the garnet from the Fanshan ultramafic-syenitic complex in North China Craton

作　　者：牛晓露[1,2] 冯光英 刘飞[1,2] 徐向珍 杨经绥[1,2,3] NIU XiaoLu;FENG GuangYing;LIU Fei;Xu XiangZhen;YANG JingSui(Key Laboratory of Deep-Earth Dynamics of Ministry of Natural Resources,Institute of Geology,Chinese Academy of Geological Sciences,Beijing 100037,China;Southern Marine Science and Engineering Guangdong Laboratory(Guangzhou),Guangzhou 511458,China;School of Earth Sciences and Engineering,Nanjing University,Nanjing 210023,China)

机构地区：[1]自然资源部深地动力学重点实验室,中国地质科学院地质研究所,北京100037 [2]南方海洋科学与工程广东省实验室(广州),广州511458 [3]南京大学地球科学与工程学院,南京210023

出　　处：《岩石学报》2022年第11期3435-3454,共20页Acta Petrologica Sinica

基　　金：国家自然科学基金项目(41672063、41773029);南方海洋科学与工程广东省实验室(广州)人才团队引进重大专项(GML2019ZD0201);自然资源部深地动力学重点实验室自主研究课题资助(J1901-35);中国地质调查项目(DD20221630)联合资助.

摘　　要：富钛钙铁榴石是硅不饱和碱性火成岩中的特征矿物,其Ti含量和Ti进入石榴石的方式可直接反映岩浆体系的硅饱和度和氧逸度。河北矾山超镁铁岩-正长岩杂岩体是华北克拉通北缘东西向展布三叠纪碱性岩带的重要岩体之一,其岩石中发育三种类型的富钛钙铁榴石:类型Ⅰ发育于岩体外带的石榴石辉石正长岩中,与辉石、黑云母呈岩浆共生关系,端元组成为Adr_(25-65)Mmt_(15-37)Slo_(8-28)Grs_(10-14),以高TiO_(2)含量(6.08%~18.61%)、较低的SiO_(2)含量(25.46%~33.26%)为特征,为原生岩浆成因;类型Ⅱ见于各类岩石,呈细粒他形充填于其他矿物颗粒之间,端元组成为Adr_(57-69)Mmt_(5-19)Slo_(0-6)Grs_(19-27),以高Al_(2)O_(3)含量(3.95%~5.56%)为特征,为岩浆演化后期熔体富Al_(2)O_(3)时结晶而成;类型Ⅲ亦见于各类岩石,主要呈细粒他形或细碎状发育于其他矿物内,端元组成为Adr_(68-79)Mmt_(8-14)Slo_(0-5)Grs_(6-14),以低的Al_(2)O_(3)含量(1.18%~2.89%)和较高的FeO含量(21.65%~24.62%)为特征,为岩浆期后热液成因。在矾山杂岩体的富钛钙铁榴石中,Ti主要以Si→Ti^(4+)和2Fe^(3+)→Ti^(4+)+Fe^(2+)/Mg替代方式进入到晶体结构中,反映了矾山杂岩体的母岩浆体系为二氧化硅不饱和的碱性岩浆,并具有较高的氧逸度。受石榴石的主量元素组成和相应的晶体结构控制,在微量元素组成上,矾山杂岩体中的富钛钙铁榴石富集轻稀土或中稀土、亏损重稀土,大离子亲石元素(Rb、Ba、Pb和Sr)含量较低或极低,而高场强元素(如Th、U、Nb、Ta、Zr和Hf等)含量普遍较高。本文对石榴石的成因研究结果支持矾山杂岩体不同类型的岩石形成于封闭体系下同一母岩浆系统充分的结晶-分异和堆晶作用;此演化模式可以较好地解释矾山杂岩体的同心环状特征和韵律层状结构,以及磁铁矿和磷矿的成因。Titanian andradite is a diagonositic phase of silica-undersaturated alkaline igneous rocks.The Ti abundance and the nature of Ti substitution in andradite are a function of the silica saturation and oxygen fugacity of the parental magma.The Fanshan ultramafic-syenitic complex is an ultrapotassic,silica-undersaturated alkaline pluton emplaced on the northern margin of the North China Craton in Late Triassic.We have identified three types of titanian andradites in the Fanshan complex.Type I garnets,occurring only in the garnet-clinopyroxene syenites,show a magmatic,cogenetic relationship with other constituent minerals(i.e.,clinopyroxene,biotite);they have a composition of Adr_(25-65)Mmt_(15-37)Slo_(8-28)Grs_(10-14),characterized by high TiO_(2)contents(6.08%~18.61%)and low SiO_(2)(25.46%~33.26%),and they are of magmatic origin.TypeⅡgarnets are anhedral,interstitial to other mineral grains;they have a composition of Adr_(57-69)Mmt_(5-19)Slo_(0-6)Grs_(19-27),characterized by high Al_(2)O_(3)contents(3.95%~5.56%);they are formed when the melt was enriched in Al_(2)O_(3)at the late stage of magmatic evolution.TypeⅢgarnets mainly occur as fine-grained,fragmentary grains developing on other mafic minerals;they have a composition of Adr_(68-79)Mmt_(8-14)Slo_(0-5)Grs_(6-14),characterized by low Al_(2)O_(3)contents(1.18%~2.89%)and high FeO contents(21.65%~24.62%),and they are of hydrothermal origin.Titanium is accommodated in the garnets of the Fanshan complex mainly via two substitution reactions:Si→Ti^(4+)and 2Fe^(3+)→Ti^(4+)+Fe^(2+)/Mg,arguing for low silica saturation and high oxygen fugacity of the parental magma.These garnets are enriched in light rare earth element(REE)and medium REE,and depleted in heavy REE,and have very high contents of high field strength elements,but have extremely low abundances of large ion lithophile elements.Such distribution of the trace elements in garnets is related to the major element constituents of the garnets and accordingly their crystal structures.The formation of titanian

关 键 词：富钛钙铁榴石 磷铁矿 矾山超镁铁岩-正长岩杂岩体 碱性岩 华北克拉通 

分 类 号：P571[天文地球—矿物学] P578.947[天文地球—地质学] P588.15