西藏加查堆花岗岩锆石U-Pb年龄、地球化学及地质意义

Geochronology and Geochemistry of the Jiachadui Ganodiorite in Xizang and its Geological Implications

这是一篇地球科学领域的论文。冈底斯带岩浆岩是青藏高原基础地质研究中的热门问题。现已有的研究主要集中在中新生代,本文对早古生代代表岩体加查堆似斑状黑云母二长花岗闪长岩利用LA-ICPMS锆石U-Pb测年方法,结合岩石地球化学特征及各类图解法对加查堆花岗岩物源及区域构造属性进行分析,经研究表明岩石具有富铝(SiO_(2)平均含量68.89%)、高碱(Na_(2)O+K_(2)O平均含量6.26%)、富钾(K_(2)O/Na_(2)O=1.03)、低TiO_(2)(平均为0.50%)和准铝质(铝饱和指数A/CNK=0.933~1.033,平均0.991)的高钾钙碱性岩系特点。稀土元素表现Eu处呈负异常明显,为Eu亏损型,Ce具弱负异常;微量元素显示大离子亲石元素Rb、Th、Nd、La、K富集;高场强元素Nb、Ba、U、Ta、Ce、Sm、Ti亏损特征;反映了岩浆来源壳幔混合型,并有俯冲洋壳熔融存在。锆石LA-ICP-MS U-Pb同位素年龄为345.3±1.8 Ma,其形成时代为早石炭世。投点均落入I型花岗岩区,图解显示其形成于大陆岛弧环境,岩石经历了从板块碰撞前→同碰撞造山过程,显示了岩浆演化过程较长;火山岩组合及其地球化学特征标志着岛弧产生(初始岛弧)→发展(早期岛弧)→成熟(成熟岛弧)的演变过程。

This is an article in the field of earth sciences.Magmatic rocks of the Gondwana Belt are one of the most popular problems in basic geological research on the Xizang-Qinghai Plateau.In this paper,we analyze the physical origin and regional tectonic properties of the Gachatai granite using LA-ICP-MS zircon U-Pb dating method,combined with the geochemical characteristics of the rocks and various graphical methods.The rocks are characterized by high alkali(average Na_(2)O+K_(2)O content of 6.26%),potassium(K_(2)O/Na_(2)O=1.03),low TiO_(2)(average 0.50%)and quasi-aluminous(aluminum saturation index A/CNK=0.933~1.033,average 0.991)high potassium-calcium-alkaline rock system.Rare earth elements show negative anomalies at Eu,which is the Eu-deficient type,and Ce has weak negative anomalies;trace elements show enrichment of large ion-parental elements Rb,Th,Nd,La and K;high field strength elements Nb,Ba,U,Ta,Ce,Sm and Ti are deficient;reflecting the mixed crust-mantle type of magma origin and the presence of subduction oceanic crustal melting.The zircon LA-ICP-MS U-Pb isotopic age is(345.3±1.8)Ma,and its formation age is Early Carboniferous.The analysis of the regional geological data suggests that this magmatic event is a product of the formation of the northern part of the eastern Gondwana continent after the breakup of the Rodinia supercontinent.The project points go to I-type granite,the discriminant diagrams indicate it’s formed from continental island arc,going through time before plate collision to co-collisional orogenic movement,which indicates a long period time of magmatic evolution;the rock geochemical characteristics show the environment of magmatic evolution changed from the start of island arc(primary island arc)-development(early stage island arc)-maturement(full-grown island arc).