大青山-乌拉山变质杂岩带大南沟地区含榴尖晶黑云钾长片麻岩成因及其形成的P-T条件  

Petrogenesis and metamorphic P-T conditions of garnet-spinel-biotitebearing paragneiss in Danangou area,Daqingshan-Wulashan metamorphic complex belt

作　　者：蔡佳[1] 刘福来[1] 刘平华[1] 施建荣[1] 刘建辉[1] 

机构地区：[1]中国地质科学院地质研究所,北京100037

出　　处：《岩石学报》2013年第7期2313-2328,共16页Acta Petrologica Sinica

基　　金：科技部973项目(2012CB416603);国家杰出青年科学基金项目(40725007);中国地质调查局地质大调查项目(1212010811065;1212011120150)资助

摘　　要：大南沟贫硅的含榴尖晶黑云钾长片麻岩出露于华北克拉通西北部大青山-乌拉山变质杂岩带(孔兹岩带)中,主要以大小不等的透镜状和互层状产于石榴长英质粒状岩石和石榴堇青夕线黑云二长片麻岩中。本文通过岩相学、变质反应、成因矿物学、矿物化学以及相平衡模拟的综合研究,揭示含榴尖晶黑云钾长片麻岩的成因机制及其形成的P-T条件。研究表明,大南沟含榴尖晶黑云钾长片麻岩内尖晶石的Zn含量极低(XZn=Zn/(Fe2++Mg+Zn)<0.006)。含尖晶石的后成合晶微域反映峰后退变条件下局部的再平衡过程,其中石榴石分解形成的钾长石+尖晶石±斜长石后成合晶是由Grt+Sil+Melt→Kfs+Spl+Pl反应形成的。而细粒尖晶石-刚玉-磁铁矿矿物组合及相应的退变结构是在晚期降温和氧化作用下,通过反应Splss+O2→Spl+Mag+Crn分解形成的。相平衡模拟结果表明含榴尖晶黑云钾长片麻岩峰期矿物组合为Bt+Grt+Spl+Mag+Sil+Kfs+Pl+Liq,稳定的温压条件在830～870℃和8.3～8.6kb,位于夕线石的稳定域。Danangou garnet-spinel-biotite-bearing paragneiss occurs as interlayers or various lenses within garnet-bearing quartzofeldspathic rocks and garnet-cordierite-sillimanite-bearing paragneisses,Daqingshan-Wulashan metamorphic complex belt,North China Craton.The combined studies of petrography,metamorphic reaction,genetic mineralogy,and quantitative phase modeling provide insight into the origin of the garnet-spinel-biotite-bearing paragneiss.This typical silica-undersaturated metamorphic rock mainly consists of garnet,spinel,biotite,sillimanite,K-feldspar with secondary minerals of magnetite,apatite,quartz,etc.Thereinto,spinel has extremely low gahnite content with X Zn ratio 0.006 （X Zn=Zn /（Fe 2 ＋ ＋ Mg ＋ Zn）.Spinel-bearing mineral assemblages within the same microdomain were formed at the late re-equilibration stage corresponding to the decompression or cooling evolution.The symplectite of K-feldspar ＋ spinel ± plagioclase around relic garnet is attributed to the reaction Grt ＋ Sil ＋ Melt→Kfs ＋ Spl ＋ Pl during the early retrogression.Whereas the fine-grained retrogressive mineral assemblage of spinel ＋ corundum ＋ magnetite was formed by the reaction of Spl ss ＋ O 2 →Spl ＋ Mag ＋ Crn related to the late cooling process and oxidation state of spinel solid solution.The P-T relationships from phase modeling for garnet-spinel-biotite-bearing paragneiss reveal a granulite facies mineral assemblage of Bt ＋ Grt ＋ Spl ＋ Mag ＋ Sil ＋ Kfs ＋ Pl ＋ Liq which records a peak metamorphic P-T condition of around 830 ～ 870℃ and 8.3 ～ 8.6kb.

关 键 词：尖晶石 贫硅体系 副片麻岩 相平衡模拟 大青山-乌拉山变质杂岩带 大南沟 

分 类 号：P588.347[天文地球—岩石学]