摘要
青海玉树县南洞卡组粗面安山岩的SiO2含量为55．92％～57．85％，高Na，O、MgO和A12O3，低K20、Ti02和P205，铝饱和指数A／CNK=0．98～1．08，具有准铝一弱过铝质的特点。岩石K：O含量低，Na2O／K2O=12．60～15．97，显示出钠质粗面安山岩特征。SiO2-K2O图解和里特曼指数（2．89～3．25），说明岩石属于钙碱性系列。稀土配分模式为轻稀土弱富集右倾型，铕弱负异常（δEu=0．90～0．96）。微量元素蛛网图显示，岩石富集Rb、Th和u等元素，亏损高场强元素Nb、Ta和大离子亲石元素Ba，样品Ce／Pb、Nb／U并未随着MgO含量的升高而升高，样品的La／Nb、Zr／Nb、Ba／Nb比值均表现出良好的稳定性，表明岩浆在上升过程中遭受地壳物质混染的可能性不大，可能是其源区特征的反映。研究表明岩浆经历了橄榄石和单斜辉石的结晶分异，且受到俯冲板片熔体对源区的改造作用。结合234Ma的形成年龄，说明该粗面安山岩是金沙江洋盆向西俯冲消减和斜向碰撞的地质记录，其形成时代和地球化学特征也为江达一维西火山弧的形成演化及成矿作用研究提供依据。
The trachyandesite from Dongka Formation in Yushu county is characterized by high Na2 O, MgO, A12 03 , and low K2 O, TiO2, P2 05, with alumina saturation index A/CNK of 0.98 1.08. These features suggest that the trachyandesite are characteristics of Metaluminous-weak peraluminous, simultaneously, this rock belongs to sodic andesite attribute to low K20, and NaEO/KEO = 12.60 - 15.97. The SiO2-K20 diagram and Rittmann index（2.89 ～ 3.25 ） show the trachyandesite belong to the calc-alkaline series. The chondrite-normalized REE patterns are rightward inclined, with Eu weak negative anomaly （ δEu = 0.90 ～ 0.96 ）. The primitive mantle normalized trace element spidergrams are characterized by enriched Rb, Th and U, depleted Nb,Ta, Ba. The ratios of CE/Pb, Nb/U do not rise, as the MgO content increase, La/Nb,Zr/Nb,Ba/Nb show a good stability,which indicates that the ascent of magma process did not suffer form crustal contamination,may be a reflection of its source region. Studies have shown that the magma experienced fractional significant crystallization of olivine and clinopyroxene during the magmatic process,and the magma source region might be converted by the subducting slab melt. It can be deduced from formation age and geochemical characteristics that the trachyandesite is a geological record of Jinsha ocean basin westward subduction and oblique collision, also provide the basis for Jomda-Weixi volcanic arc evolution and mineralization.
出处
《地质科学》
CAS
CSCD
北大核心
2013年第4期1069-1082,共14页
Chinese Journal of Geology(Scientia Geologica Sinica)
基金
国家青年基金项目(编号:41002063,41102073)
青海阿尔金1:5万打柴沟等6幅区调项目(编号:1212011121193)资助
