东天山晚古生代埃达克岩成因及铜金成矿意义  

The late Paleozoic adakites in eastern Tianshan area and their metallogenetic significance.

作　　者：熊小林[1] 蔡志勇[1] 牛贺才[1] 陈义兵[1] 王强[1] 赵振华[1] 吴金花[1] 

机构地区：[1]中国科学院边缘海地质重点实验室

出　　处：《岩石学报》2005年第3期967-976,共10页Acta Petrologica Sinica

基　　金：国家自然科学基金(40172029;40373035)广州地球化学研究所创新工程项目(GIGCX-04-03)国家重点基础研究计划项目(2001CB409803)资助;并获中国科学院广州地球化学研究所元素和同位素实验室部分资助

摘　　要：东天山晚古生代岛弧带位于古亚洲洋成矿域的腹地,该区发育许多埃达克质的英云闪长岩、斜长花岗岩和花岗闪长岩小侵入体,本文选择尾亚北、三岔口、312国道东、312国道西、土屋-延东和巴仑台等小岩体进行了地球化学研究。这些岩石具有高Sr、Na2O和Al2O3,低Y和HREE等特征,与埃达克岩地球化学特征一致。绝大部分样品的εNd(T)为正值(+1．80 -+8．47),且(87Sr／86Sr)i很低(0．7026-0．7051),与现代MORB以及新生代俯冲洋壳熔融形成的埃达克岩的Nd和Sr同位素组成接近。东天山大部分埃达克质岩石具有高Mg#(>40)特点,且它们形成于晚古生代岛弧构造背景,暗示这些埃达克岩可能由俯冲洋壳熔融产生,并经历了与地幔楔橄榄岩的相互作用;三岔口英云闪长斑岩具有低Mg#(<40),且形成于碰撞后构造背景,可能是增厚下地壳熔融的产物。土屋-延东与埃达克岩有关的超大型斑岩铜矿的发现和突破使得东天山成为寻找斑岩铜金矿的重要靶区,本文新发现的埃达克岩无疑为在该区进一步寻找相关的矿床提供了有用的线索。特别是大部分埃达克岩由俯冲洋壳熔融产生,暗示其源区含有高丰度的Cu、Au和挥发份H2O、Cl等,且其岩浆可能在角闪岩相向榴辉岩相转变时产生,此时角闪石大规模分解有利于产生含丰富挥发份和成矿元素的埃达克质岩浆,暗示良好的成矿条件和成矿潜力。Eastern Tianshan late Paleozoic volcanic zone is located at the center of the Paleo-Asian Oceanic Belt. A number of small tonalitic, trondhjemitic and granodioritic plutons occur in this volcanic zone, among which 6 typical plutons were selected to study their geochemical features and petrogenesis. These plutonic rocks are characterized by high Na2O, Al2O3 , and Sr contents, low Y and HREE contents and high Sr/Y and La/Yb ratios, similar to the modern adakite. These rocks also possess positive εNd （T） （ ＋ 1.80-＋ 8.47） and very low initial ^87Sr/^86Sr （0. 7026 - 0. 7051 ）. These Nd and Sr isotopic compositions are also similar or close to those of most subduction-related adakites which have like-MORB isotopic compositions. The above geochemical and isotopic features, in conjunction with late Paleozoic subduction setting, suggest that these adakitic rocks may be produced by partial melting of the subducted Zhungaer oceanic crust during late Paleozoic. The MORB （ basaltic oceanic crust） are a very important source for adakitic rocks and contain high abundance of Cu, Au and volatile components H2O and Cl etc. The large scale of decomposition of amphibole within the subducted oceanic crust during the generation of adakite magmas will release large amount of H2 O, Cl and Cu, Au into the magmas. The rapid decrease of pressure during ascent of the magma will cause release of Cu, Au and volatiles to form ore-forming fluid. During the entire process from magmatic production through magmatic transportation and interaction with mantle wedge to the final stage of fluid release from the magma, high oxygen fugacity （fo2） is necessary, which will restrain the stability of sulphides to make Cu, Au enriched in the magma. Eastern Tianshan area has become an important exploration target for industrial Cu deposits due to the discovery of the superlarge Tuwu-Yandong porphyry Cu deposit whose parent magmas are adakitic rocks. The newly reported adakitic rocks here are surely helpful for the guidance of further

关 键 词：埃达克岩 地球化学和成因 Cu、Au成矿作用 东天山 

分 类 号：P588.122[天文地球—岩石学]