西藏狮泉河地区高钾-钾玄质火山岩的岩石学、地球化学及锆石U-Pb年龄  

Petrology, geochemistry and zircon U-Pb age of high potassic shoshonitic volcanic rocks in Shiquanhe area,Tibet

作　　者：尼玛次仁[1,2] 王国灿[2] 顿都 次仁央宗 焦文龙 普尺 叶强 达娃 

机构地区：[1]西藏地勘局区域地质调查大队,西藏堆龙德庆851400 [2]中国地质大学地球科学学院,湖北武汉430074

出　　处：《地质通报》2015年第9期1656-1667,共12页Geological Bulletin of China

基　　金：中国地质调查局项目(编号:1212011121232;1212011221057)

摘　　要：报道的高钾-钾玄质火山岩位于狮泉河镇南东方向约20km处,向东延伸。高钾-钾玄质火山岩Si O2变化于60.35%-68.68%之间,属中酸性岩范畴;具有高的K2O＋Na2O含量（8.8%-10.66%）,K2O/Na2O值在1.92-2.49之间,Mg O含量较低,介于0.88%-3.47%之间,Al2O3含量为14.02%-14.91%,属于高钾-钾玄质系列。岩石强烈富集大离子亲石元素（LILE）Rb、Ba、Th、U和轻稀土元素（LREE）,高场强元素（HFSE）Nb、Ta、Ti具有明显负异常,Cr、Ni、Co相容元素含量低于或接近地壳的平均含量,结合Th/Yb-Ta/Yb、（Th×100）/Zr-（Nb×100）/Zr判别图及La-La/Yb图解,暗示岩浆源区可能为下地壳。在左左乡南东约2km处和狮泉河水泥厂北东约1km处各采集1个高钾-钾玄质火山岩样品,对其中的锆石进行LA-ICP-MS U-Pb同位素测定,得到的206Pb/238U年龄加权平均值分别为22.04±0.42Ma和22.29±0.31Ma,此年龄被解释为狮泉河一带高钾-钾玄质火山岩的喷发时代,即中新世阿启塔期。由此表明,该火山岩是印度板片向北俯冲时在狮泉河一带俯冲板片断离,岩浆发生部分熔融的产物。In this paper, a series of high potassic shoshonitic volcanic rocks are identified for the first time from Late Oligocene R.igongla Formation in southwest Shiquanhe Town, Tibet. The volcanic rocks are composed of quartz-trachyte and biotite trachyte. The zircon U-Pb ages of two samples from different locations are respectively 22.04±0.42Ma and 22.09±0.13Ma. These rocks have SiO2 values from 60.35% to 68.68%, suggesting intermediate-acid rocks. They have high K2O＋Na2O values from 8.8% to 10.66%, and K2O/Na2O values from 1.92 to 2.49; the MgO values are comparably low, ranging from 0.88% to 3.47%, and Al2O3 values vary from 14.02% to 14.91%. The major element data show that these rocks belong to high potassic shoshonitic series. The trace element data suggest that magma source might have been derived from the lower crust. The newly-found high potassic shoshonitic volcanic rocks in Shiquanhe area may be considered as the consequence of magma partial melting caused by breakup of the subducted slab when the Indian plate subducted northward in Aquitanian period of Miocene.

关 键 词：狮泉河地区 高钾-钾玄质火山岩 LA-ICP-MS锆石U-PB年龄 中新世 

分 类 号：P588.14[天文地球—岩石学] P597.3[天文地球—地质学]