西藏雅鲁藏布江缝合带萨嘎碱性玄武岩地球化学和年代学研究  

Geochemical and chronological studies of the alkaline basalt in Saga along the Yarlung Zangbo suture zone, Tibet

作　　者：赵慧[1,2] 杨经绥[1,2] 刘飞[2] 熊发挥[2] 张岚[1] 连东洋[2,3] 

机构地区：[1]中国地质大学(北京)地球科学与资源学院,北京100083 [2]大陆构造与动力学国家重点实验室地幔研究中心,中国地质科学院地质研究所,北京100037 [3]中国地质大学(武汉)地球科学学院,湖北武汉430074

出　　处：《中国地质》2015年第5期1242-1256,共15页Geology in China

基　　金：国家自然科学基金重点项目(40930313);国家行业专项(SinoProbe-05-02);中国地质调查局工作项目(1212011121263、1212011121272);创新研究群体科学基金项目(40921001)联合资助

摘　　要：萨嘎蛇绿岩位于雅鲁藏布江缝合带（YZSZ）的中段,主要由地幔橄榄岩、辉长岩、辉绿岩和玄武岩组成。在萨嘎蛇绿岩南侧分布一条熔岩带,与萨嘎蛇绿岩呈断层接触。该熔岩带呈北西西向展布,宽约1~3 km,长约150km。熔岩呈块状构造,气孔和杏仁状构造发育,斑状结构,基质为间粒间隐结构。斑晶主要为斜长石（钠长石化,含量15%~20%）,及少量单斜辉石（约5%）,单斜辉石（Wo=44~48,En=40~45,Fs=10~12）为含钛透辉石（Ti O2=1.46%）。根据硅碱图（TAS）分类,该套熔岩属于碱性玄武岩类,整体表现为高钛、高铝和高钾,富集Rb、Ba、Th等大离子亲石元素和Zr、Hf等高场强元素,轻重稀土强烈分馏,（Ce/Yb）N=7.30~14.18,平均值为11。萨嘎玄武岩的地球化学特征与大陆裂谷火山岩一致,可能形成于板内裂谷环境。本研究表明,萨嘎碱性玄武岩的地幔源区具有石榴子石残余,早期经历了橄榄石、单斜辉石等镁铁矿物的分离结晶。锆石U-Pb定年结果表明萨嘎碱性玄武岩中锆石年龄较分散,产出的最年轻的一组锆石平均年龄为（54.2±1.4）Ma,它们的εHf（t）介于-11-＋1.7之间,具有中元古代的Hf同位素二阶段模式年龄,为壳源的捕获锆石,表明该套碱性玄武岩形成时代不早于始新世早期。Located in the central part of Yarlung Zangbo Suture Zone（YZSZ）, the Saga ophiolite is mainly composed of peridotites,gabbro, diabase and lava. This paper reports a suite of alkaline basalts to the south of the Saga ophiolite, which extends in NWW direction and crops out 1~3km in width. This suite of basaltic lavas has vesicular/ amygdaloid structure and porphyric texture with phenocrysts of plagioclase（15 modal %- 20 modal.% in content） and clinopyroxene（~5 modal % in content）. The plagioclase is albite and the clinopyroxene is titaniferous diopside（Ti O2=1.46 wt.%）. Based on the composition and TAS diagram, the authors hold that the lava belongs to alkaline basalt. The lava shows the characteristics of low Si O2, high Ti O2,K2 O and Al2O3 content as well as enrichment of large ion lithophile elements（ LILE）（Rb, Ba, Th） and high field strength elements（ HFSE）（ Zr, Hf, Nb, Ta）. The rare earth elements（REE） content of the basalt is higher（ ∑ REE=266.93 × 10- 6）, with a strong fractionation between LREE and HREE（（ Ce/Yb）N= 7.30-14.18）. The chemical characteristics are similar to features of continental rift volcanic rocks. This study shows that garnets exist as residual minerals in the source. In addition, at the early stage of magmatism, there existed crystallization fractionation of olivine, clinopyroxene and other mafic minerals. U-Pb dating of zircons by LA-ICP-MS yielded a wide range of ages. The youngest group has average age of（54.2±1.4）Ma,with εHf（t）being-11-＋1.7, and has the Mesoproterozoic two-stage model age, indicating that these zircons were captured from the crust and the lava had not formed until early Eocene.

关 键 词：雅鲁藏布江缝合带 萨嘎碱性玄武岩 锆石U-PB定年 地球化学 

分 类 号：P588.1[天文地球—岩石学] P597.3[天文地球—地质学]