青海省布青山早古生代末期埃达克岩的发现及其构造意义  

Discovery of the end-Early Paleozoic adakite in the Buqingshan area,Qinghai province,and its tectonic implications.

作　　者：边千韬[1] Pospelov I.I. 李惠民[3] 常承法[1] 李继亮[1] 

机构地区：[1]中国科学院地质与地球物理研究所 [2]Geological Institute,Russian Academy of Sciences,Moscow 109017,Russia [3]中国地质调查局天津地质矿产研究所

出　　处：《岩石学报》2007年第5期925-934,共10页Acta Petrologica Sinica

基　　金：国家自然科学重点基金项目(40334044);国家重点基础研究发展规划项目(2002CB412602);国家自然科学基金(批准号:49872077);中国科学院卢嘉锡基金资助

摘　　要：在青藏高原北部阿尼玛卿蛇绿岩带布青山段首次发现了埃达克岩,它们多呈不规则岩脉状侵入于布青山地区北部早古生代蛇绿混杂岩中。其岩性为花岗-英云闪长岩,主矿物有斜长石、石英、黑云母、角闪石、少量钾长石和辉石;副矿物有榍石、磷灰石、锆石和磁铁矿等。属钙碱系列,具典型(Ⅰ类、O型、HSA)埃达克岩的地球化学特征,表现为:高SiO_2(63. 65%～71.59%),高Al_2O_3(14.81%～16.11%),高Na_2O(4.68%～5.33%)和Na_2O/K_2O(1.48～3.08,平均2.39),高Sr (444.08×10^(-6)～560.41×10^(-6))和Sr/Y(50.8～105.3),HREE强烈亏损,低Yb(0.38×10^(-6)～0.74×10^(-6)),高La/Yb(44.93～90.97),弱负Eu异常(δEu=0.74～0.90),富集大离子亲石元素(LILE),亏损高场强元素(HFSE),Ba、Pb正异常而Nb、P负异常。获得布青山埃达克岩的锆石TIMS U-Pb年龄为402±24Ma。布青山埃达克岩熔体来自俯冲的昆祁秦洋壳(含海洋沉积物)在石榴石角闪岩相条件下部分熔融,主要残留相为石榴石(±角闪石)。布青山埃达克岩的发现指示早古生代末期洋壳(昆祁秦多岛洋的一部分)俯冲作用的存在,俯冲方向朝北(现在方位),此前,俯冲带发生长时间的向洋迁移,早古生代末期俯冲带已后退到阿尼玛卿蛇绿混杂岩带以南;暗示典型埃达克岩也可以形成于洋壳俯冲晚期;提示阿尼玛卿蛇绿岩带可能有与典型埃达克岩有关的Au、Ag、Cu、Mo等热液和斑岩矿床。The adakite was recently revealed in the Buqingshan ophiolite complex, a western sector of the A＇nyemaqen ophiolite belt in the northern part of the Qinghai-Tibet Plateau. The adakite consists of granodiorite-tonalite intruded in the Early Paleozoic ophiolite complex. The mineralogical phases pserent in the adakite are mainly plagioclase, quartz, hornblende, biotite, rare pyroxene and potassium feldspar; and accessory phases are apatite, zircon, sphene and magnetite etc. The adakite is calc-alkaline, and characterized by high SiO2 （63.65% -71.59% ）, high Al2O3 （14.81% -16.11% ）, high Na2O（4.68% -5.33% ） and Na2O/K2O （1.48 -3.08 ,av. 2.39） ratios, high Sr（444.08 × 10^-6 - 560.41 × 10^-6） and Sr/Y （50.8 - 105.3 ） , rich in large ion lithophile elements （LILE）, depleted in high strength field elements （HFSE） , weakly negative Eu anomalies, strong depletion of heavy rare earth elements （e.g., Yb=0.38×10^-6 - 0.74×10^-6） and Y （4.93 ×10^-6 - 11.03 ×10^-6）, high l.,a/Yb（44.93 -90.97）, positive Ba and Pb but negative Nb and P anomalies. These geochemical characteristics suggest that the Buqingshan adakite belongs to typical adakite （O-type, HSA）. A TIMS zircon U-Pb age of 402±24Ma for the adakite have been obtained. The Buqingshan adakite was most probably derived by the partial melting of the subducting Kunlun-Qilian-Qinling oceanic crust （ basaltic oceanic crust plus its overlying sediment） at garnet amphibolite-facies condition. The discovery suggests that the part of the Kunlun-Qilian-Qinling oceanic crust of the study area was subducted northward, and the subduction zone was situated to the south of the A＇nyemaqen ophiolite belt in the end-Early Paleozoic period. It is also of implication that porphyry-type and hydrothermal-type Au, Ag, Cu, Mo mineralization related with typical adakite probably existed in the A＇nyemaqen ophiolite belt.

关 键 词：埃达克岩 地球化学 早古生代末期 青藏高原 阿尼玛卿蛇绿岩带 

分 类 号：P588.122[天文地球—岩石学]