米仓山坪河新元古代二长花岗岩成因及其地质意义  

Petrogenesis and Implications for the Neoproterzoic Monzogranite in Pinghe,Micang Mountain

作　　者：甘保平 赖绍聪[1] 秦江锋[1] 

机构地区：[1]西北大学大陆动力学国家重点实验室/地质学系,西安710069

出　　处：《地质论评》2016年第4期929-944,共16页Geological Review

基　　金：国家自然科学基金(批准号:41372067);国家自然科学基金重大计划(批准号:41190072);国家自然科学基金委创新群体(批准号:41421002);教育部创新团队(批准号:IRT 1281)联合资助~~

摘　　要：扬子板块西北缘新元古代岩浆作用成因的研究对于研究Rodinia超大陆在该区的构造演化具有重要意义。本文对扬子板块西北缘米仓山地区二长花岗岩进行了LA-ICP-MS锆石U-Pb定年和同位素、全岩主量和微量元素分析。结果表明二长花岗岩锆石U-Pb年龄为742.1±5.9 Ma,属于新元古代花岗岩。岩石具有高Si O_2(76.84%~80.08%),高碱(Na_2O+K_2O=7.64%~8.99%),相对富钾(K_2O/Na_2O=0.91~1.36),低P_2O_5含量等特征,铝饱和指数(A/CNK)介于0.77~0.89之间。岩石具有稀土元素含量较高、相对富集轻稀土元素的特征,具有明显负Eu异常,Eu*=0.05~0.13,Rb、U、Th、K、Pb等元素相对富集,Ba、Nb、Sr、P、Zr、Ti和Eu等元素明显亏损。岩石n(^(87)Sr)/n(^(86)Sr)=0.747067~0.795283,n(^(143)Nd)/n(^(144)Nd)=0.512472~0.512661,ε_(Nd)(t)=+3.6^+5.2,二阶段模式年龄T_(2DM)值介于0.96~1.07Ga之间。综合地球化学、同位素特征及米仓山区域构造资料,我们认为坪河二长花岗岩为高钾钙碱性高分异I型花岗岩,该岩浆起源于新生玄武质下地壳岩石的局部熔融,在上升阶段可能经历了地壳的混染作用,形成于伸展的构造环境中,是新元古代晚期Rodinia超大陆裂解作用的岩浆响应。Neoproterozoic magmatic rocks are widespread in northwestern margin of the Yangtze block,and further works for petrogenesis of these rcoks have significant implications to understand the tectonic evolution of Rodinia supercontinent in this region.Methods： In this paper,we present zircon U Pb age, whole rock major and trace elements,and Sr—Nd isotopic compositions of the monzogranite from the northwestern margin of the Yangtze Block, Micang Mountain.Results： Monzogranite zircon U Pb result show that the crystallization age of 742.1 ± 5.9Ma,which belongs to Neproterozoic granites. The Pinghe monzogranites are characterized by relatively high SiO2 （76.84% ~ 80.08%）contents,enriched in alkali（Na2O ＋ K2O=7.64% ~ 8.99%） and potassium （K2O/Na2O=0.91~1.36）,and low P2O5 contents. These rocks belong to metaluminous granite with A/CNK=0.77 ~ 0.89. They also show high REE compositions,enriched in LREE, Rb,U,Th,K,Pb and depleted in Ba,Nb,Sr,P,Zr,Ti,and a strong negative Eu anomaly （δEu=0.05 ~ 0.13）. Isotopic results of these rock reveal evoluted Sr and Nd n（87Sr）/n（86Sr） = 0.747067 ~ 0.795283,n（143Nd）/n（144Nd） = 0.512472 ~ 0.512661,εNd（t）=＋3.6 ~ ＋5.2）,with two stage model age of 0.96~1.07Ga.Conclusions： In combination with geochemistry, isotopic signatures and contemporaneous geological events of Micang Mountain, we argued that the Pinghe monzogranites are high K calc alkaline and high fractionated I type granites, which derived from partial melting of lower crustal juvenile basaltic rocks,and subsequently undergone crustal contamination during the upwelling process. It was generated in an extensional tectonic setting, regarded as the precursor of the Neoproterozoic break up of Rodinia supercontinent.

关 键 词：米仓山 花岗岩 地球化学 锆石U-PB定年 RODINIA超大陆 

分 类 号：P588.121[天文地球—岩石学]