西天山特克斯县北乌孙山大哈拉军山组火山岩地球化学特征及构造意义

Geochemical characteristics and tectonic implications of Wusunshan volcanic rocks in northern Tekes County of West Tianshan Mountains

对西天山特克斯西北乌孙山中部地区出露的火山岩进行了地球化学研究,结果表明玄武岩、安山岩均为拉斑质火山岩,LREE较富集[（La/Yb）N介于3.44～11.36之间],具有弱的Eu负异常（δEu=0.79～1.13）,安山岩与流纹岩明显富集强不相容元素（如Cs、Rb、Ba、Th、U）和LREE.玄武岩与中酸性岩样品具有轻微的差异,但所有样品具有较明显的Nb、Ta、Ti负异常,并且除了活动性较强的元素（Cs、Rb、Ba）外,其余微量元素均接近于下地壳的含量,（Th/Nb）N介于2.02～8.12之间,（Nb/La）N介于0.40～0.45之间,远远小于1,La/Ba值低（0.01～0.07）,Ba/Nb值高（90～410）; Zr/Nb值平均为16.48,最为接近原始地幔（15.71）的比值;而Ta/Nb、Hf/Ta、Th/Yb值平均为0.10、3.77、0.64,较为接近上地壳（0.10、3.50、0.48）的比值,Zr含量大于90×10^-6,Zr/Y值在4左右,显示了板内玄武岩的特征.结合区域上地质特征,认为该地区在石炭纪时碰撞结束进入碰撞后伸展阶段,局部具有裂谷化特征.样品的Nb、Ta、Ti负异常应为地壳混染引起,火山岩的形成环境为碰撞后伸展的构造环境.

Geochemical characteristics of the volcanic rocks in middle Wusunshan of West Tianshan Mountains were studied in this paper,and the results show that all the basalts and andesites are tholeiitic volcanic rocks,with enrichment of LREE [（La/Yb）N =3.44～ 11.36] and weak depletion of Eu （δEu =0.79 ～ 1.13） ; andesite and rhyolite are significantly enriched in highly incompatible elements （Cs,Rb,Ba,Th,U） and LREE;basalt and acid rock samples show slight difference,but all the samples obviously have negative anomalies of Nb,Ta and Ti.Nevertheless,the characteristics of basalts are similar to those of intraplate basalt：the （Th/Nb）N range from 2.02 to 8.12,（Nb/La）N range from 0.40 to 0.45,by far less than 1 ; La / Ba ratios are low （0.01to 0.07） and Ba/Nb ratios are high （90 to 410）; average Zr/Nb ratio is 16.48,close to the ratio of primitive mantle （15.71）; Ta/Nb,Hf/Ta,Th/Yb ratios are averagely 0.10,3.77,0.64 respectively,close to their upper crust ratios （0.10,3.50,0.48） ; Zr content is larger than 90 × 10 ^-6,Zr/Y ratios are about 4; Except for some active elements （Cs,Rb,Ba）,values of trace elements are close to their values in the lower crust.Combined with the geological characteristics of this area,the authors consider that this region might have been in the post-collision extension phase in the Carboniferous,with local rifting characteristics.The distinct depletion of Nb,Ta and Ti might have been caused by the hybridization of the lithosphere,and the environment of the Middle Wusunshan volcanic rocks should have belonged to the post-collision extension phase.