危地马拉硬玉岩与硬玉化绿辉石岩的岩石学特征及其地质意义  

Petrology of the Jadeitite and Jadeitized Omphacitite from Guatemala and Its Geological Implications

作　　者：张智宇[1] 施光海[1] 欧阳秋眉 柳维[3] 龚庆贺[3] 

机构地区：[1]中国地质大学地质过程与矿产资源国家重点实验室,北京100083 [2]香港珠宝学院 [3]中国地质大学珠宝学院,北京100083

出　　处：《地质学报》2012年第1期198-208,共11页Acta Geologica Sinica

基　　金：国家自然科学基金项目(编号40672046);教育部新世纪优秀人才支持计划(编号NCET-07-0771)资助的成果

摘　　要：位于北美-加勒比板块俯冲带内的危地马拉硬玉岩区产有硬玉岩、榴辉岩、钠长岩等岩石类型,但绿辉石岩还没有详细报导过。研究样品为该区的硬玉岩和绿辉石岩,前者主要由Jd端元含量较高的硬玉组成,具有粒、柱状镶嵌结构。硬玉晶体具有显著的成分振荡环带,其核部及其背散射电子图像的浅色区Jd端元含量为94.81%～95.48%;暗色区Jd含量大于97.92%。后者绿辉石岩具有交代结构,主要由绿辉石和硬玉组成。其中绿辉石的CaO(9.01%～10.80%)和MgO(6.09%～7.94%)含量较高,FeO(2.84%～4.89%)含量较低;硬玉的CaO、MgO和FeO含量变化范围较大,分别为0.59%～4.30%,0.26%～3.05%,0.76%～2.87%。硬玉岩中流体包裹体的存在,以及绿辉石岩中显著的硬玉交代绿辉石现象说明其成因与缅甸硬玉一样,是硬玉质流体通过结晶-交代作用形成。硬玉岩中硬玉振荡的环带结构反映形成时温度-压力-组分体系存在振荡变化,平直连续的环带边界反映结晶时P-T条件位于硬玉稳定的低温高压区域。该区绿辉石岩的硬玉化作用清楚,能够观察到三期硬玉岩化作用现象,说明绿辉石岩可能是在硬玉岩的形成过程中,硬玉质流体与原岩辉石岩作用的结果,这进一步阐明了俯冲带内硬玉岩区硬玉质流体的广泛透入性与结晶交代作用的多样性。In the Guatemala jadeitite area, there are several rock types such as jadeitites, eclogites and albitites etc, among which omphacitite has not been reported in detail before. Jadeitite and omphacitite studied in this paper are from Guatemala jadeitite area, which is located in the Fault Zone between North America and Caribbean plates. Jadeitite mainly consists of jadeites with high content of Jd end-member, and has granular or coarser - grained mosaic texture. Most jadeites show rhythmic zoning~ in cores and other light areas （BSE images）, Jd contents are slightly lower （from 94. 81% to 95. 48%） relative to dark areas, where Jd contents are more than 97. 92%. Omphacitite has metasomatic texture, mainly consists of omphaeites and jadeites omphacites have high CaO （9.01%- 10.80%） and MgO （6. 09%- 7.94%）, but low FeO （2.84%- 4.89%）, jadeites have CaO （0.59%-4.30%）, MgO （0.26%- 3.05%） and FeO （0.76%- 2.87%）. Presence of fluid inclusions in jadeites and textures of omphacite replaced by jadeite indicate the jadeitite was formed by crystallization from a jadeitic fluid, which subsequently leads to the replacement of adjacent pyroxenes, showing similarity to those from the Myanmar jadeitite. The zonal texture in jadeites reflects that the system of P-T-C varied rhythmically, and their straight and continuous boundaries indicate a HP/LT condition stable for jadeite crystallization. These petrographic features indicate that at least three stages of replacements existed, and the omphacitite wase possibly a production of replacement between jadeitic material and pyroxenite during formation of the jadeitite, and further indicate extensive infiltration of jadeitic fluid and diverse features of replacement within the subduction zone.

关 键 词：硬玉 硬玉岩 绿辉石岩 流体 交代 危地马拉 

分 类 号：P588.125[天文地球—岩石学]