藏北红脊山地区变质玄武岩地球化学及其形成构造环境

Characteristics and Tectonic Setting of Meta-Basalt from Hongjishan in North Tibet

藏北红脊山地区变质玄武岩按产状分为呈"岩块"产出和在变质碎屑岩中呈夹层产出两个部分,其全岩Sm-Nd等时线年龄为(401±18)Ma。变质玄武岩属于亚碱性拉斑玄武岩系列,主量元素Al2O3、TiO2低,K2O略高。呈"岩块"和呈夹层产出的变质玄武岩REE配分曲线呈LREE略亏损或平坦分布,HREE略显富集;大离子亲石元素(LILE)Rb、Ba、Th富集,高场强元素(HFSE)Ta、Zr、Ti、Y、Yb、Sc、Nb相对亏损,强相容元素Cr、Ni、Co亏损,微量元素MORB标准化曲线呈"单隆起"型曲线分布型式。岩石组合及地球化学特征反映该变质玄武岩形成于弧后盆地构造环境,可能是早泥盆世古特提斯弧后盆地扩张的产物。

Meta-basalt,in Hongjishan in north Tibet,is a unique block.It is composed of amphibole schist,plagioclase amphibole schist,plagioclase actinolite schist,plagioclase amphibolite and plagioclase quartz actinolite schist and occurred in ＂massive rocks＂,or bedded in the metamorphic clastic rocks.The isochron age of whole rock for amphibole schist is（401±18）Ma.The meta-basalt belongs to the type of sub-alkaline tholeiite,slightly low in Al2O3,TiO2,and high in K2O.Massive meta-basalt is rich in LILE（Rb,Ba,Th）,relatively depletion in HFSE（Ta,Zr,Ti,Y,Yb,Sc,Nb）.It is also depletion in strong compatible elements as Cr,Ni,Co.MORB-normalized web diagram shows the single peak.According to the association of rocks and geochemical characteristics,the meta-basalts of Hongjishan in north Tibet is formed in back arc basin,the result of back-arc basin spreading of Paleo-Tethys on the north of Qiangtang block.