摘要
班公湖-怒江缝合带广泛分布超基性岩及蛇纹石化超基性岩,已有研究表明它们与区域成矿关系密切,其蛇纹石化过程使一些元素活化并具有一定的成矿潜力。位于班-怒带中段的昂吾地区蛇纹石化超基性岩主要矿物成分有橄榄石、单斜辉石、铬铁矿、利蛇纹石、磁铁矿和绿泥石等,原岩恢复表明该超基性岩为单辉橄榄岩。镜下鉴定、背散射电子图像、能谱成分分析和电子探针分析结果显示单辉橄榄岩的蛇纹石化及蚀变过程可分为三个阶段:(Ⅰ)以形成相对富铁蛇纹石(Mg#=75~88)为主,基本无磁铁矿析出;(Ⅱ)形成相对富镁的蛇纹石(Mg#> 90),析出磁铁矿;(Ⅲ)蛇纹石进一步蚀变成绿泥石。热力学模拟及多组分矿物相平衡图表明,在蛇纹石化过程中,昂吾地区超基性岩中的辉石脱硅致使反应体系SiO2活度升高,限制了磁铁矿的生成。同时也发现,在利蛇纹石稳定存在的温度区间内(100~300℃),本研究的蛇纹石化体系温度倾向高值区,不利于磁铁矿的析出。进而探讨了原岩成分、反应体系SiO2活度及温度等因素对蛇纹石化过程中磁铁矿析出的影响。本研究有助于理解班-怒带内超基性岩的蛇纹石化过程及磁铁矿化机制。
In Bangong-Nujiang suture zone( BNSZ),ultramafic and serpentinized ultramafic rocks are widely distributed,which was considered being closely associated with the regional mineralization. In the process of serpentinization of ultramafic rocks,some elements were activated and then owned a certain metallogenic potential. Angwu serpentinized ultramafic rocks( AURs),located in the middle section of BNSZ,are mainly composed of olivine,clinopyroxene,chromite,lizardite,magnetite and chlorite. By the method of the restoration,the original rock of AURs was identified as clinopyroxene peridotite. After comprehensively analyzing the results of microscopic photographs,back-scattered electron,energy dispersive spectrometer images and electron probe microanalysis,the alteration process of AURs including serpentinization could be divided into three stages:(Ⅰ) mainly forming The Fe-rich serpentine( Mg#= 75 ~ 88) without magnetite yielding;(Ⅱ) mainly forming the Mg-rich serpentine( Mg#> 90) with magnetite yielding;(Ⅲ)further transforming from serpentine to chlorite. Based on thermodynamic simulation and multi-component mineral phase diagrams,it is proposed that the desilicication from clinopyroxene in AURs increased the activity of SiO2 in the reaction system of alternation. Thus,the increased SiO2 activity limited the precipitation of magnetite in the system. Meanwhile, the reaction temperature for the serpentinization of AURs was deduced being at relatively high level in the range of 100 ~ 300℃ under which lizardite could stably crystallized. However,the relative high temperature was also not favorable for magnetite yielding. Furthermore,the influence factors on the magnetite yielding in the serpentinization process of ultramafic rocks were discussed,such as the composition of original rocks,the activity of SiO2 and temperature in the reaction system of alternation. This study helps us to understand the serpentinization process and the mechanism of magnetite mineralization in the ultramafic rocks in BNSZ.
作者
李静超
赵涛
刘治博
林赟
邵华胜
袁国礼
宋扬
LI JingChao;ZHAO Tao;LIU ZhiBo;LIN Yun;SHAO HuaSheng;YUAN GuoLi;SONG Yang(School of Earth Sciences and Resources, China University of Geosciences, Beijing 100083, China;MNR Key Laboratory of Metallogeny and Mineral Assessment, Institute of Mineral Resources, Chinese Academy of Geological Science, Beijing 100037, China)
出处
《岩石学报》
SCIE
EI
CAS
CSCD
北大核心
2019年第7期2158-2172,共15页
Acta Petrologica Sinica
基金
中国地质调查局项目(DD20160026)
教育部中央高校基本科研业务费(2652017239)联合资助
关键词
蛇纹石化
超基性岩
磁铁矿
班-怒带
藏北
Serpentinization
Ultramafic rocks
Magnetite
Bangong-Nujiang suture zone
North Tibet
