Petrogenesis of Aoyougou high-silica adakite in the North Qilian orogen,NW China:Evidence for decompression melting of oceanic slab  

作　　者：YuXiao Chen XiaoHong Xia ShuGuang Song 

机构地区：[1]Peking Univ, Sch Earth & Space Sci, MOE Key Lab Orogen Belts & Crustal Evolut, Beijing 100871, Peoples R China

出　　处：《Chinese Science Bulletin》2012年第18期2289-2301,共13页

基　　金：supported by the National Basic Research Program of China(2009CB825007);the National Natural Science Foundation of China(40825007and40821002);the Chinese Geological Survey Projects(1212011121258)

摘　　要：A trondhjemitic body occurs in the Aoyougou area,the western part of the North Qilian orogen.It is geochemically characterized by high SiO 2,Na 2 O (high Na/K),elevated Sr/Y and (La/Yb) N,positive Sr anomaly,relatively enriched large ion lithophile elements (LILEs) and light rare earth elements (LREEs),and depleted Nb,Ta,Ti,resembling the high-silica adakite.Zircon U-Pb SHRIMP dating yields a weighted mean age of 438±3 Ma.This age is significantly younger than eclogitization ages of 460-490 Ma in the North Qilian orogen,suggesting that formation of the adakite postdates the subduction of oceanic crust in association with closure of the ancient Qilian Ocean.Whole-rock Sr and Nd isotopic analyses give initial ratios of I Sr =0.7044 0.7047 and Nd (t)=3.0 4.1,indicating that they are derived from partial melting of the juvenile oceanic crust.In view of the tectonic evolution of the North Qilian orogen,the high-silica adakite was probably derived from decompression melting of the exhumed eclogite at the depth of ～60 km.A trondhjemitic body occurs in the Aoyougou area, the western part of the North Qilian orogen. It is geochemically characterized by high SiO2, Na20 （high Na/K）, elevated Sr/Y and （La/Yb）N, positive Sr anomaly, relatively enriched large ion iithophile ele- ments （LILEs） and light rare earth elements （LREEs）, and depleted Nb, Ta, Ti, resembling the high-silica adakite. Zircon U-Pb SHRIMP dating yields a weighted mean age of 438＋_3 Ma. This age is significantly younger than eclogitization ages of 460--490 Ma in the North Qilian orogen, suggesting that formation of the adakite postdates the subduction of oceanic crust in association with closure of the ancient Qilian Ocean. Whole-rock Sr and Nd isotopic analyses give initial ratios of lsr=0.7044-0.7047 and eNd（t）=3.0-4.1, indicating that they are derived from partial melting of the juvenile oceanic crust. In view of the tectonic evolution of the North Qilian orogen, the high-silica adakite was probably derived from decompression melting of the exhumed eclogite at the depth of -60 km.

关 键 词：北祁连造山带 大洋地壳 部分熔融 埃达克岩 稀土元素地球化学特征 高硅 中国西北 减压 

分 类 号：P581[天文地球—岩石学]