四川红原地区三叠纪花岗岩的地球化学特征及地质意义

Geochemical characteristics of Triassic granites in Hongyuan area,Sichuan Province,and their geological significance

红原地区三叠纪花岗岩位于松潘-甘孜造山带的北部,侵位于三叠纪西康群砂板岩系中,岩石类型主要为中细粒花岗闪长岩和二长花岗岩。岩石地球化学分析结果表明该区岩体属于准铝质-过铝质富钾钙碱性系列,大部分为I型花岗岩(羊拱海和达盖寨岩体部分表现为A型),稀土元素总量变化较大,轻稀土元素相对富集,稀土元素球粒陨石标准化配分型式为右倾型,Eu异常不明显。轻重稀土元素分馏明显,轻稀土元素分镏大于重稀土元素。岩石地球化学特征指示该区花岗岩体可能为同源的地壳物质的部分熔融作用所形成,源岩可能为变杂砂岩和变中性火山岩,热源可能来自于地幔岩浆的底侵作用。

Located in northern Songpan-Garzê orogenic belt and intruding into Triassic Xikang Group mainly comprising sandstone and slate,the Hongyuan granites are composed of medium-fine grained granodiorite and monzogranite.Geochemical analyses show that they have high SiO2 （62.36%～73.7%） and Al2O3 （13.84%～16.69%） content,low TiO2 （0.18%～0.72%）,Na2O （2.64%～4.43%） and K2O （2.24%～5.07%） content,and similar Na and K content.These granites are of metaluminous-peraluminous high-K calc-alkaline series and can be mostly classified as I type granite （some Yanggonghai and Dagaizhai granites can be classified as A type granite）.Their total REE amounts vary considerably （84.39×10-6～241.07×10-6）.Light REE elements are of relative enrichment.They are enriched with large ion lithophile elements such as Ba,Rb and Th and depleted in high field strength elements like Nb and Ta.In chondrite-normalized REE patterns,all samples are LREE-enriched with insignificant Eu anomalies （δEu=0.54～1.28）.Light and heavy REE are significantly fractionated.The fractional extent of light REE [（La/Sm）N=6.13～11.16] is higher than that of heavy REE [（Gd/Yb）N=2.13～9.52].LA-ICP-MS U-Pb zircon ages （211.0±0.11 Ma） of the granites suggest that their intrusion occurred in late Indosinian.The melt composition is different in different source positions.It is thought that these granites most likely resulted from partial melting of the homologous crust,and that there was a substantial proportion of biotite in the source region because they were metaluminous-peraluminous high-K calc-alkaline granites with high SiO2 and Rb content and relatively high K2O/Na2O ratio.They were probably derived from metamorphic greywacke and intermediate volcanic rocks.Based on Hf-Rb/10-Ta×3 triangular diagram and Rb-（Y＋Nb） discrimination diagram,the authors have reached the conclusion that the granites might have been formed in an collision environment after the main intracontinental collision and required an underplating or intraplating scheme of mantle-derived magma.