INVESTIGATIONS ON FEATURES AND GENESIS OF HIGHER HIMALAYAN CRYSTALLINES

doi:10.3969/j.issn.0253-4967.2013.02.006

Abstract

Abstract:

The widely distributed migmatites in Higher Himalayan Crystallines(HHC)are an important clue to investigate the anatexis,crustal rheology,origins of Higher Himalayan leucogranites(HHL)and evolution of orogenic belts. We studied in detail the petrographic,petrologic and geochemical features of three basic constituent units of the migmatites,i.e.leucosome,mesosome and melanosome,collected from Nyingchi and Nyalam regions in Tibet. In order to study the chemical features of each migmatite unit,the major,trace elements and rare-earth elements of leucosomes,mesosomes and melanosomes separated from migmatites have been analyzed. Our studies indicate that leucosome is the product of crystallization of migrated melt,and mesosome is regarded as source rocks of migmatites or as a reactant of unsegregated melt and restite at source region. The melanosome represents the aggregation of melanocratic minerals crystallized from melt or the products of retro-metamorphic reaction between segregated melt and mesosome. Leucosomes and melanosomes are usually characterized by igneous textures,coarse grain size and isotropic fabric,which are important identifications to distinguish them from mesosomes. Positive Eu anomaly of leucosomes may be attributed to partially melting of plagioclase significantly and /or early crystallization of plagioclases from primary melt in nearby source region. In contrast,leaving feldspars behind source region and crystallization differentiation of plagioclase during melt aggregation contributed to negative Eu anomaly displayed by leucograintes. It seems that the existence of minerals formed in dehydration melting cannot be regarded as judgment standard about partial melting because the diagnostic minerals of dehydration melting may disappear during retrograde reactions.

Key words: migmatites, anatexis, melanosome, leucosome, mesosome, High Himalayan Crystallines

摘要：

混合岩是深熔作用的存在标志,对研究中下地壳深熔作用机制、地壳流变以及造山带演化和花岗岩的成因具有重要意义.文中对西藏林芝地区和聂拉木地区的混合岩进行了详细的岩相学、岩石学和地球化学特征的研究.岩相学特征显示,研究区的混合岩可划分为浅色体、中色体和暗色体3个基本组成部分.对三者切割分离,分别进行了主量元素和稀土元素的分析.结果表明:浅色体由迁移的熔体结晶形成;中色体可以是未发生熔融的原岩,也可以经由未发生迁移的熔体与熔融残留体反应形成;暗色体是由迁移汇集后的熔体与中色体反应形成.由于聚集的熔体可以为暗色矿物的结晶提供良好的结晶空间和物质来源,因此暗色体多数以窄条带产出于浅色体边缘.浅色体和暗色体通常具有岩浆岩的结构,矿物粒径粗大且分布不具有定向性,这是其区别于中色体的重要特征.浅色体显示出明显Eu正异常,暗示长石大量参与了部分熔融过程,并且初始熔体在近源区的冷凝过程中长石优先结晶.浅色花岗岩的Eu负异常可能与熔体在源区的长石结晶有关.退变质反应有可能使部分熔融反应形成的矿物完全消失,因此不宜将反应矿物存在与否作为发生过脱水熔融的判别标准.

关键词: 混合岩, 深熔作用, 暗色体, 浅色体, 中色体, 高喜马拉雅结晶岩系

CLC Number:

P313

YANG Yong-xin, YANG Xiao-song. INVESTIGATIONS ON FEATURES AND GENESIS OF HIGHER HIMALAYAN CRYSTALLINES[J]. SEISMOLOGY AND GEOLOGY, 2013, 35(2): 261-277.

杨永鑫, 杨晓松. 高喜马拉雅混合岩特征及其成因[J]. 地震地质, 2013, 35(2): 261-277.

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