西藏冈底斯岩基羊八井地区古新世I型含石榴子石花岗岩的成因  

Petrogenesis of I-type garnet-bearing granite in the Yangbajing area within the Gangdese Batholith,Tibet

作　　者：李广旭[1] 曾令森[2] 高利娥[1] 严立龙 赵令浩[3] LI GuangXu;ZENG LingSen;GAO LiE;YAN LiLong;ZHAO LingHao(Research Center of Earth Science System,Institute of Geology,Chinese Academy of Geological Sciences,Beijing 100037,China;Beijing SHRIMP Center,Institute of Geology,Chinese Academy of Geological Sciences,Beijing 100037,China;National Research Center for Geoanalysis,Chinese Academy of Geological Sciences,Beijing 100037,China)

机构地区：[1]地球系统科学研究中心,中国地质科学院地质研究所,北京100037 [2]北京离子探针中心,中国地质科学院地质研究所,北京100037 [3]中国地质科学院国家地质实验测试中心,北京100037

出　　处：《岩石学报》2023年第12期3619-3640,共22页Acta Petrologica Sinica

基　　金：国家自然科学基金项目(92055202,42172261);第二次青藏高原科学考察项目(2019QZKK0702);国家重点研发计划(2021YFC2901901);中国地质调查局地质调查项目(DD20221817,DD20221630)联合资助.

摘　　要：高硅花岗岩(SiO_(2)>74%)是许多大型花岗岩岩基的重要组成部分,对理解弧岩浆作用过程中的地壳深熔作用和大陆地壳的演化过程具有重要意义。冈底斯岩基羊八井地区发育一套高硅I型含石榴子石花岗岩,结晶时代为古新世早期(锆石U-Pb年龄为64.9±0.3Ma和64.1±0.3Ma)。该套花岗岩具有以下特征:(1)属于高钾钙碱性系列,具有高SiO_(2)和K 2O,低MgO、MnO、CaO和P 2O 5,海鸥状REE分配模式,强烈的Eu负异常,富集K、Rb、Pb、Th和U,亏损Nb、Ti和P的特征,是高分异I型花岗岩;(2)发育岩浆型石榴子石,端元组分为铁铝榴石和锰铝榴石,富集HREE,亏损LREE,富集关键微量元素Sc、Y和Zn,归因于斜长石强烈的分离结晶作用;(3)熔体结构的改变和锆石的分离结晶导致了较低的Zr和Hf含量以及Zr/Hf比值;(4)锆石Hf同位素和全岩Nd同位素组成显示明显的富集特征(εHf(t)=-17.1~-11.7;εNd(t)=-9.0~-7.6)。结合区域研究成果,这套花岗岩的原始熔体来源于古老岩浆岩基底物质的部分熔融,之后经历了强烈的分离结晶作用,最终形成了I型含石榴子石花岗岩。High silica granites(HSGs)are common to large granitic batholiths worldwide.It is important for understanding crustal anatexis during arc magmatism and the evolution of continental crust.Within the Gangdese batholith,field observations and zircon U-Pb dating reveal a suite of HSGs occurred in the Yangbajing area,which formed at 64.9±0.3Ma~64.1±0.3Ma.Here,the petrographic,mineralogical,whole-rock geochemical and isotopic composition were carried out on the Yangbajing granite,and the results show that:(1)it belongs to the high-K calc-alkaline series,with high content of SiO_(2) and K 2O,but low MgO,MnO,CaO and P 2O 5,and a gull-like REE pattern,and strong negative Eu anomalies,and enrichment in K,Rb,Pb,Th and U,and depletion of Nb,Ti and P;(2)magmatic garnets with almandine and spessartite significant enrichment in HREE,Sc,Y,and Zn,attributed to strong fractional crystallization of plagioclase;(3)fractional crystallization of plagioclase with high An components has also resulted in the decrease of Al/NK ratio in the evolved magma,which increased the solubility of zircon;and(4)zircon Hf and whole rock Nd isotopic compositions show enrichment characteristics(εHf(t)=-17.1~-11.7;εNd(t)=-9.0~-7.6),suggesting that the melting of ancient crust magmatic materials and then underwent strong fractional crystallization,leading to the formation of I-type garnet-bearing granite.

关 键 词：冈底斯岩基 羊八井 I型含石榴子石花岗岩 分离结晶 熔体结构 

分 类 号：P588.121[天文地球—岩石学] P597.3[天文地球—地质学]