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冈底斯岩浆带中段岩浆混合作用:来自花岗杂岩的证据 被引量:87

Magma mixing in middle part of Gangdise magma belt:Evidences from granitoid complex.
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摘要 巨型冈底斯岩浆岩带横旦于青藏高原南部,主要由花岗质岩石组成,其中存在大量的镁铁质微粒包体,以辉长岩为主的基性岩相对集中在岩浆岩带南部。辉长岩类与花岗质岩石呈渐变过渡关系。辉长岩中出现有石英,暗色矿物沿着长石颗粒边部分布,花岗闪长岩中出现有基性倍长石(An>80),辉长岩中有中长石(An=45)出现,镁铁质微粒包体(MME)包体中倍长石-钙长石(An=72~90)和中长石(An=37)共存,这些矿物组合的不协调现象是岩浆演化过程中混合作用的表现形式;MME的化学成分相对高钾、铝和铁镁,MME、花岗岩类寄主岩及辉长岩类岩石化学成分呈直线变异趋势(相对MgO), MME的SiO_2和K_2O、Na_2O成分的过渡性变化也都反映出混合作用特征;基性辉长岩类、酸性花岗质岩石和MME包体的稀土配分模式基本一致,其微量元素特征也具有明显的一致性,反映了岩浆混合作用的特征;辉长岩类的^(143)Nd/^(144)Nd比值较高,^(87) Sr/^(86)Sr比值较低,而偏酸性的花岗质岩石中^(143)Nd/^(144)Nd比值较低,^(87)Sr/^(86)Sr比值较高,构成直线性分布趋势,并位于地幔演化线的延伸方向上,显示区域岩浆混合作用特征;辉长岩、花岗岩类和MME的同位素年龄值十分接近,也表明三者属于岩浆混合作用产物。研究证明,冈底斯岩浆岩带中不同岩性均由不同比例的基性端元和酸性端元成分混合而成,其中基性端元成分所占的比例变化在16%~90%不等,仅从侵入岩浆推算,幔源基性岩浆对冈底斯地壳垂向增厚的贡献率超过5%。可以推断,随着俯冲-碰撞过程中产生的基性岩浆底侵作用,冈底斯岩浆岩带发生壳幔岩浆混合作用,其时代为50~45Ma,属于印度-欧亚大陆碰撞开始15Ma后的主碰撞期内,岩浆混合作用是碰撞过程中壳幔物质与成分交换的主要形式之一,是研究主碰撞带北部青藏高原的陆壳增生与改造、地壳结构及成分变化重要途径之一。 Gangdise giant magma belt lays down in southern Tibet, consisting mainly of granitoids with abundant mafic macrogranular enclaves. Gabbro as predominant marie rock type concentrated along the southern magma belt and had transitional contact with the granitoid. Researches have found that some quartz crystals and andesine ( An = 45 ) occurred in the gabbro and marie bytownite ( An 〉 80 ) in granodiofite, bytownite-anorthite ( An = 72 - 90 ) associated with andesine ( An = 37 ) and dark minerals distributed along plagioclase grains. All the inconsistent mineral assemblages proved to be characteristics of magma mixing during magma evolution. The relative high content of K, Al and Fe, Mg, a linear variation trend of lithological chemical composition among MME, granitoid and gabbros, and transitional variation between SiO2 and K2O, Na2O in MME are all likely respondent to magma mixing. The similar REE pattern and trace element spider distribution for MME, granitoid and gabbro gave a image of magma mixing. The high ^143Nd/^144Nd ratio and low ^87Sr/^86 Sr in gabbro and low ^143Nd/^144 Nd ratio and high ^87Sr/^86 Sr ratio fell in linear in mantle evolution range, representing magma mixing happening. The SHRIMP zircon U-Pb ages were quite similar for the REE, granitoid and gabbro, indicating that they were the products of magma mixing in same geothermal event. Much evidence indicates that the various lithologies in Gangdise magma belt were mixed by different proportional marie end-member and acid end-member. As a result, the marie end-member occupied 16% - 90% between acid to marie end-member. It was estimated base on the intrusions that marie magma from mantle sources contributed over 5% to the Gangdise crust. It deduced that, with the underplating of mantle-derived magma during India-Eurasin continental collision, magma mixing from both mantle source and crust source took place during 50 - 45Ma. This magma mixing event post-dated the initiation of India-Eurasia continental collision by 15 million years. It is believed that magma mixing was one of the major mechanisms for mass and energy exchange between mantle-crust during the Indian-Eurasia collision, and also for key approaches of study on crust accretion, reconstruction, crust texture and composition in north to the collision zone.
出处 《岩石学报》 SCIE EI CAS CSCD 北大核心 2006年第4期835-844,共10页 Acta Petrologica Sinica
基金 国家重点基础研究发展规划项目(编号:2002CB412600) 国家自然科学基金(40473020 40103003 40503005 40572048) 国土资源部青藏专项计划(200101020401) 中国地调局综合研究项目联合资助.
关键词 花岗岩类 辉长岩类 岩浆混合作用 岩浆岩带 冈底斯 西藏 Granitoid, Gabbro, Magma mixing, Magma belt, Gangdise, Tibet
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