摘要
西藏阿里地区的南公珠错蛇绿岩产在公珠错的南侧,空间上属于雅鲁藏布江缝合带西段之南亚带蛇绿岩。该蛇绿岩主要由地幔橄榄岩和辉长岩等基性岩类组成。地幔橄榄岩中约80%为方辉橄榄岩,20%为二辉橄榄岩,纯橄岩较少。南公珠错地幔橄榄岩矿物化学特征表现为橄榄石具有较低的Fo(89.3~91.4)值、辉石具有较高的Al_2O_3含量(1.89%~6.06%)、尖晶石具有较低的Cr~#(12.7~28.3)值。与原始地幔相比南公珠错地幔橄榄岩的全岩地球化学特征具有较高的MgO含量和较低的Al_2O_3、CaO和TiO_2等易熔元素含量;方辉橄榄岩和二辉橄榄岩的稀土元素总含量分别介于0.66×10-6~1.10×10-6和0.90×10^(-6)~3.78×10^(-6)之间,明显低于原始地幔值,其稀土元素配分模式为轻稀土元素轻微富集型;在原始地幔标准化微量元素蜘蛛图中,南公珠错地幔橄榄岩显示出强烈的U正异常、Nd轻微正异常和强不相容元素Zr的负异常;方辉橄榄岩和二辉橄榄岩的铂族元素总量分别介于15.26×10^(-9)~25.23×10^(-9)和18.74×10^(-9)~26.86×10^(-9)之间,二者含量的变化较小,南公珠错地幔橄榄岩PGEs球粒陨石标准化图解显示其为接近于原始地幔的"平坦型"。南公珠错地幔橄榄岩的矿物化学和全岩地球化学特征与深海橄榄岩相似,指示它们可能形成于大洋扩张脊环境。定量模拟估算表明,南公珠错地幔橄榄岩可能来源于地幔中的尖晶石相二辉橄榄岩源区,系经历了至多16%部分熔融的残余。LREE的微富集和较高的Pd/Ir、Rh/Ir比值指示它们还经历了岩石-熔体反应作用。初步结论认为南公珠错地幔橄榄岩形成于大洋脊环境,为尖晶石相二辉橄榄岩地幔源区较低程度部分熔融的残余,但经历了后期岩石-熔体反应作用。
The South Gongzhucuo ophiolite is located to the south of Gongzhu Lake, Ali area, Tibet, belonging to the southern sub- belt of the western Yarlung-Zangbo ophiolitic belt. The South Gongzhucuo ophiolite is mainly composed of mantle peridotites and gabbro dikes. About 80 percent peridotites are harzburgites and 20 percent are lherzolites, few of them are dunites. Mineral chemistry of the South Gongzhucuo peridotites are characterized by low Fo contents （ 89. 3 - 91.4） of olivine ; high Al2O3 contents （ 1.89% - 6. 06% ） in pyroxenes ; and low Cr values （ 12. 7 - 28.3 ） of Cr-spineL Compared to the primitive mantle, the South Gongzhucuo peridotites have relatively higher MgO contents, and lower CaO, Al2O3, and TiO2 contents. The total rare-earth element （REE） contents of the South Gongzhucuo harzburgites and lherzolites are0.66×10-6～1.10×10-6 and 0.90×10-6～3.78×10-6, respectively, which are significantly lower than those of the primitive mantle. The chondrite-normalized REE patterns of the South Gongzhucuo mantle peridotite display slight enrichments in LREE. In the primitive mantle-normalized spider diagrams, the South Gongzhucuo peridotites exhibit intensively positive U anomalies, slightly positive Nd anomalies, and intensively negative Zr anomalies. The PGE contents of South Gongzhucuo harzburgites and lherzolites are 15.26×10-9～25.23×10-9 and 18.74×10-9～26.86×10-9, respectively. The chondrite-normalized PGE patterns of the South Gongzhucuo peridotites are consistent with that of the primitive mantle. Mineral and whole-rock geochemical characteristics of the South Gongzhucao peridotites show an affinity to abyssal peridotites, indicating that it may have formed in the mid-ocean ridge setting. Through quantitative modeling, we conclude that the South Gongzhucuo peridoties formed after at most 16% degree of partial melting of the spinel-phase lberzolite mantle source. The LREE enrichments and relatively high Pd/ Ir and R1CIr ratios suggest that they have experienced melt-rock reaction. Thus, we conclude that the South Gongzhucuo mantle peridotites have suffered low degrees of partial melting in the spinel-lherzolite stability field beneath a mid-ocean ridge environment, and were modified by melt-rock reaction in the same tectonic setting.
出处
《岩石学报》
SCIE
EI
CAS
CSCD
北大核心
2016年第12期3649-3672,共24页
Acta Petrologica Sinica
基金
中国地质调查局地质调查项目(12120115027201
121201102000150005
201511022)
国际科技合作专项(2014DFR21270)
国家重点实验室自主课题(Z1301-a20)联合资助
