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贺根山豆荚状铬铁矿成因探讨:来自硅酸盐矿物包体的制约
投稿时间:2020-01-11  修订日期:2020-04-29  点此下载全文
引用本文:
DOI:10.19762/j.cnki.dizhixuebao.2020283
摘要点击次数: 102
全文下载次数: 117
作者单位地址
陆国隆 北京大学地球与空间科学学院 北京市海淀区颐和园路5号北京大学逸夫二楼3502
祝庆敏 北京大学地球与空间科学学院 
朱永峰 北京大学地球与空间科学学院 北京市海淀区颐和园路5号北京大学逸夫二楼3509
基金项目:国家重点研发计划(编号:2017YFC0601302)
中文摘要:贺根山豆荚状铬铁矿位于内蒙古中部的锡林郭勒盟,是典型的高Al铬铁矿,其Cr#和Mg#值分别为47.8~54.9和64.1~73.7。贺根山豆荚状铬铁矿中发现的矿物包体以硅酸盐矿物为主,主要包括橄榄石、斜方辉石、单斜辉石、韭闪石和钠长石。根据硅酸盐矿物包体的形状、矿物相组合及分布特征可将其划分为三种类型。第一类矿物包体呈孤立的单矿物相,主要包括单相橄榄石和单相单斜辉石,第二类矿物包体由平衡共生的单斜辉石和斜方辉石构成,上述两类矿物包体均具有被熔蚀的边缘并零星地分布在尖晶石中,属于捕虏晶成因。第三类矿物包体具有多边形外形,包含复杂的矿物相并密集分布于尖晶石核部,是尖晶石圈闭的母熔体演化后结晶的产物,属于熔融包体。利用尖晶石颗粒内部保存完好的单相单斜辉石包体以及平衡共生的二辉石包体估算了贺根山豆荚状铬铁矿形成的温度(1148~1254 ℃)与压力(4.9~12.9 kbar),结果表明贺根山豆荚状铬铁矿形成于较浅的上地幔环境(16~43 km)。熔融包体中含有大量的钠长石和韭闪石,指示贺根山豆荚状铬铁矿的母熔体具有富集H2O、Na和Si的特征。与贺根山豆荚状铬铁矿结晶时平衡的母熔体的Al2O3含量(15.4%~16.3%),TiO2含量(0.3%~0.9%)和FeO/MgO比例(0.6~1.1)与低Ti拉斑玄武质熔体类似,而利用尖晶石和橄榄石包体计算获得形成豆荚状铬铁矿的原始熔体具有较高的Mg含量(MgO = 19.8%),原始熔体与母熔体在成分上的差异表明原始熔体存在从高Mg向低Mg的演化,该过程可能是原始熔体与方辉橄榄岩相互反应并随后熔体混合的结果。结合前人在贺根山豆荚状铬铁矿中发现的金刚石,认为贺根山豆荚状铬铁矿的矿床成因分为深部预富集和浅部成矿两个阶段,其中浅部成矿涉及两个过程:(1)熔体与方辉橄榄岩反应;(2)演化的熔体与原始熔体混合。
中文关键词:贺根山  豆荚状铬铁矿  矿物包体  母熔体  矿床成因
 
Silicate Mineral Inclusions in Spinel from Hegenshan Podiform Chromitites: Implication for Chromitite Genesis
Author NameAffiliationAddress
LU Guolong Key Laboratory of Orogenic Belts and Crustal Evolution,School of Earth and Space Science,Peking University,Beijing 北京市海淀区颐和园路5号北京大学逸夫二楼3502
ZHU Qingmin Key Laboratory of Orogenic Belts and Crustal Evolution,School of Earth and Space Science,Peking University,Beijing 
ZHU Yongfeng Key Laboratory of Orogenic Belts and Crustal Evolution,School of Earth and Space Science,Peking University,Beijing 北京市海淀区颐和园路5号北京大学逸夫二楼3509
Abstract:Located in Xilingol League of central Inner Mongolia, Hegenshan high-Al podiform chromitites has Cr# values of 47.8~54.9 and Mg# values of 64.1~73.7. The spinel-hosted inclusions from the chromitites principally consist of silicates which comprise olivine, orthopyroxene, clinopyroxene, pargasite and albite. Based on the shapes, mineral assemblages and distribution, the silicate mineral inclusions can be divided into three types. The type 1 mineral inclusions are monomineralic inclusions which are consisting of olivine and clinopyroxene. The type 2 mineral inclusions are compound inclusions which are characterized by the coexistence of orthopyroxene and clinopyroxene. Both the type 1 and type 2 mineral inclusions which are xenocrysts included by the host spinel have rounded edges and are scattered in spinel grains. The type 3 mineral inclusions, concentrated at the nucleus of host spinel, are polygonal and generally include many different mineral types. The type 3 mineral inclusions are the result of the trapped melt by the host spinel, indicating they are melt inclusions. Temperatures (1148~1254℃) and pressures (4.9~12.9 kbar) based on monomineralic clinopyroxene inclusions and the composite inclusions consisting of clinopyroxene and othorpyroxene suggest a shallow forming environment (16 km~43 km) for the chromitites, which could be the upper mantle. The compositions of melt inclusions which include albite and pargasite show that the parental melt is rich in H2O, Na, and Si. The calculated Al2O3, TiO2 and (FeO/MgO) of the parental melt corresponding to the Hegenshan podiform chromitites are 15.4%~16.3%, 0.3%~0.9% and 0.6~1.1, respectively, which resemble low-Ti tholeiitic melt. Based on spinel and olivine inclusions, we calculated the composition of primitive melt for the Hegenshan podiform chromitites, which has a high MgO content of 19.8%. The differences in composition between parental melt and primitive melt suggest an evolving process for primitive melt from high Mg to low Mg. This evolutionis probably caused by melt-rock reaction and melt mixing. Due to the discovery of diamonds in Hegenshan podiform chromitites, we believe that the genesis of Hegenshan podiform chromitites experienced the deep mantel and finally formed in shallow mantle. The formation in shallow mantle involves two processes: (1) the reaction between the melt and harzburgites; (2) mixing of evolved melt and primitive melt.
keywords:Hegenshan  podiform chromitites  mineral inclusions  parental melt  genesis
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