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Magma mixing in middle part of Gangdise magma belt:Evidences from granitoid complex.

DONG GuoChen~(1,2) MO XuanXue~(1,2) ZHAO ZhiDan~(1,2) ZHU DiCheng~3 WANG LiangLiang~1 CHEN Tao~1 LI Bing~1 1.State Key laboratory of Geological Processes and Mineral Resources,China University of Geosciences,Beijing 100083,China 2.School of Earth Science and Resources,China University of Geosciences,Beijing 100083,China 3.Chengdu Institute of Geology and Mineral Resources,Chengdu 610082,China  
Gangdise giant magma belt lays down in southern Tibet,consisting mainly of granitoids with abundant mafic macrogranular enclaves.Gabbro as predominant mafic 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 mafic hytownite(An>80)in granodiorite,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 SiO_2 and K_2O,Na_2O 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 ~(143)Nd/~(144)Nd ratio and low ~(87)Sr/~(86)Sr in gabbro and low ~(143)Nd/~(144)Nd ratio and high ~(87)Sr/~(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 mafic end-member and acid end-member.As a result,the mafic end-member occupied 16%~90% between acid to mafic end-member.It was estimated base on the intrusions that mafic 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.
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