祁连山西段古元古代北大河岩群中斜长角闪岩的地球化学特征及构造背景  

Geochemistry and tectonic setting of amphibolites in the Paleoproterozoic Beidahe Group-complex in the western segment of the Qilian Mountains

作　　者：付国民[1] 苏建平[2] 胡能高[1] 王云斌[1] 

机构地区：[1]长安大学地球科学与国土资源学院,陕西西安710054 [2]甘肃省地质调查院,甘肃兰州730050

出　　处：《中国地质》2005年第4期541-547,共7页Geology in China

基　　金：国家自然科学基金重大项目(90102003);祁连西段1:25万区域遥感调查项目(DK0002019)资助

摘　　要：以大陆碎块形式残存于北祁连造山带内部的古元古代北大河岩群早期发育有大量的斜长角闪岩,其原岩为玄武岩,Alk—∑FeO—MgO图解、REE特征、里特曼组合指数等均显示斜长角闪岩属钙碱性系列范围,个别显示弱碱性,斜长角闪岩具有较高的Al2O3、ΣFeO和低的Mg#,这些特点反映其成分演化程度较高,表明火山岩的形成与古老地壳的活化有关,有较强分异的稀土元素分配模式,在Ti—Zr—Y图、Ni—FeO/MgO图解和Ta/Yb—Th/Yb图解上斜长角闪岩均显示火山弧或活动大陆边缘的特点,在玄武岩的地球化学形式图、稀土元素整体特征与钙碱性岛弧玄武岩基本相同;87Sr/86Sr=0.71617～0.72033,143Nd/144Nd=0.512414～0.512495,εNd=-2.79～-4.37。显示出壳源性质或者是受到大陆下地壳的混染作用而形成。因此推测斜长角闪岩的原岩为不同程度部分熔融的岩浆弧作用的产物,说明在古元古代华北克拉通和柴达木克拉通之间存在古洋盆的俯冲消减。The Paleoproterozoic Beidahe Group-complex remains as continental shivers in the interior of the North Qilian orogen. In its early stage of development, there occurred abundant amphibolite. The protolith of the amphibolite is basalts. The Alk-∑FeO-MgO plot, REE features and R.ittmann index show that the protolith belongs to the calc-alkaline suite and exceptionally shows the subalkaline character. The rock is characterized by higher Al2O3 and XFeO and lower Mg^#, indicating that it has a higher degree of compositional evolution and that the formation of volcanic rocks is related to activation of older crust. Highly fractionated REE patterns and amphibolite on the Ti-Zr-Y plot, Ni-FeO/MgO plot and Ta/Yb-Th/Yb plot all indicate the characteristics of volcanic arcs and active continental margins. On the geochemical diagram of basalt, the overall REE features are in the main analogous to those of calc-alkaline island-arc basalt, with SrST/Sr^S6=0.71617 - 0.72033 ,Nd14a/Nd144= 0.512414 - 0.512495 and end= - 2.79 to - 4.37,εsr=245.78 - 305.3, showing that the magma was derived from the crust or subjected to contamination with the lower crust below the continent. So it may be inferred that the protolith of the amphibolite is the product of different degrees of partial melting of the mantle in a magma arc. This suggests that subduction of a paleo-ocean basin occurred between the Paleoproterozoic North China craton and Qaidam craton.

关 键 词：斜长角闪岩 地球化学 构造背景 北大河岩群 古元古代 祁连山西段 

分 类 号：P588.125[天文地球—岩石学] P597.3[天文地球—地质学]