冈底斯中段羊易盆地年波组埃达克岩的发现及其对印度-欧亚板块碰撞的启示  

The discovery and limplications for the India-Eurasia Plate collision of the Nianbo Formation adakitic rocks from Yangyi basin in Middle Gangdise Belt

作　　者：胡林[1] 唐华[1] 徐刚[1] 刘大明[1] 肖进[1] HU Lin;TANG Hua;XU Gang;LIU Daming;XIAO Jin(Northwest Sichuan Geological Party,Bureau of Geology and Mineral Resources Exploration and Development of Sichuan Province,Mianyang 621000,Sichuan,China)

机构地区：[1]四川省地质矿产勘查开发局川西北地质队,四川绵阳621000

出　　处：《地质通报》2020年第10期1507-1517,共11页Geological Bulletin of China

基　　金：中国地质调查局项目《西藏厅宫地区15万区域地质调查》(编号:DD20160015)。

摘　　要：冈底斯中段羊易盆地发现始新世年波组埃达克质火山岩,岩石组合为英安岩、流纹岩、火山熔岩等。为确定其形成时代、成因及其地质构造意义,对其进行了岩相学、LA-ICP-MS锆石U-Pb测年及地球化学研究。结果表明,用于定年的锆石为岩浆成因,其206 Pb/238 U年龄加权平均值为55±0.8 Ma,喷出时代为始新世。地球化学特征显示,该套火山岩属弱过铝质钙碱性岩石系列,具富Si(SiO 2=67.69%~71.93%)、高Al(Al 2O 3=13.13%~16.16%)、低Mg(MgO=0.52%~0.76%)、高Sr(Sr=345×10-6~875×10-6)、低Y(Y=4.40×10-6~11.30×10-6)及Yb(Yb=0.81×10-6~1.49×10-6)特征,稀土元素总量在116.86×10-6~352×10-6之间,轻、重稀土元素分馏较明显,(La/Yb)N=23.31~43.66,Eu异常不明显(δEu=0.8~1.15)。在原始地幔标准化蛛网图中,富集大离子亲石元素Rb、U、Th,亏损高场强元素Nb、P、Ti,为C型埃达克质火山岩地球化学特征。综合区域资料,羊易盆地年波组埃达克岩是加厚下地壳部分熔融形成的,说明在55±0.8 Ma左右拉萨地体的南缘部分地区已增厚到50 km左右。The Eocene Nianbo Formation Adakitic volcanic rocks were discovered for the first time from the Yangyi basin in the Middle Gangdise Belt.This set of adakitic volcanic rocks consists of dacite,rhyolite,and volcanic lava.In order to determine their formation age,origin and geological tectonic significance,the authors conducted LA-ICP-MS zircon U-Pb geochronologic,petrographic and geochemical studies.The results show that the zircons used for dating are of magmatic origin,and the zircon 206 Pb/238 U dating yielded 55±0.8 Ma,which suggests that the volcano erupted in Eocene.Geochemical characteristics show that the volcanic rocks belong to a series of weakly peraluminous calc-alkaline rocks,and have rich Si(SiO 2 of 67.69%~71.93%),high Al(Al 2O 3 of 13.13%~16.16%),low Mg(MgO of 0.52%~0.76%),higher Sr(Sr of 345×10-6~875×10-6),and lower Y(Y of 4.40×10-6~11.30×10-6)and Yb(Yb of 0.81×10-6~1.49×10-6).The total rare earth element values vary in the range of 116.86×10-6~352×10-6,exhibiting obvious fractionation between light and heavy REE((La/Yb)N=23.31~43.66),with no obviousδEu anomaly(0.8~1.15).Enrichment of LILE(Rb,U and Th)and depletion of HFSE(Nb,P and Ti)are displayed in the primitive mantle normalized multi-element variation diagram,indicating geochemical characteristics of C-type adakitic volcanic rocks.Based on regional information,the authors hold that adakitic rocks of Nianbo Formation in Yangyi basin were formed by partial melting of thickened lower crust,indicating that some areas of the southern margin of Lhasa were thickened to about 50 km around 55±0.8 Ma.

关 键 词：年波组 C型埃达克岩 冈底斯中段 锆石U-PB年龄 岩石学 

分 类 号：P534.613[天文地球—古生物学与地层学] P597.3[天文地球—地质学]