元古代蛇绿岩及铬铁矿  

Proterozoic ophiolite and chromite

作　　者：鲍佩声[1] BAO PeiSheng(MNR Key Laboratory of Deep-Earth Dynamics,Institute of Geology,Chinese Academy of Geological Sciences,Beijing 100037,China)

机构地区：[1]自然资源部深地动力学重点实验室中国地质科学院地质研究所

出　　处：《岩石学报》2019年第10期2971-2988,共18页Acta Petrologica Sinica

基　　金：国家自然科学基金项目(41872240、91755103);第二次青藏高原综合科学考察(2019QZKK0703);中国地质科学院地质研究所基本科研业务费项目(J1705、YYWF201704);中国地质调查局工作项目(DD20190370、DD20190060)联合资助

摘　　要：本文总结了国外典型元古代蛇绿岩的岩石组合、野外产状、地球化学资料以及成矿特征,并与显生宙蛇绿岩进行了对比,继而探讨元古代板块构造演化和铬铁矿成因。资料表明,早元古代和中-新元古代均有蛇绿岩的存在,但前者较少,仅见于Canadian地盾的Cape Smith Belt中的Prutuniq蛇绿岩(2. 05~2. 0Ga)和芬兰Fennoscandian地盾的Outokumpu和Jormua蛇绿岩(时代为1. 97~1. 96Ga),而中-新元古代的蛇绿岩则见于世界许多地区,如埃及东部沙漠区(~750Ma)和非洲东北部地区(ca.900~800Ma)等。与显生宙蛇绿岩相比,这些老蛇绿岩具如下特征:(1)它们均为被肢解的蛇绿岩,大多与"弧火山岩"和(或)混杂岩伴生,经历不同程度的变形和变质(具绿片岩相-角闪岩相组合);(2)岩石组合大多较齐全,壳层组合发育,以镁铁-超镁铁岩(堆晶岩)、辉长岩、镁铁质席状岩床(墙)杂岩、火山岩为代表;层状镁铁-超镁铁岩的韵律层以及矿物的隐晶变化等均提示了岩浆多期次活动及开放岩浆房的特征;(3)元古代蛇绿岩中既有高铝型铬铁矿,也有高铬型铬铁矿,且主要寄主于纯橄岩(或蛇纹岩)中;高铝型和高铬型直接受控于熔体的熔融程度及含水流体的参与,反映了铬铁矿形成于俯冲带演化的不同阶段;铬铁矿规模均较小,且均以低TiO 2为特征,均为岩浆分异作用的产物,明显区别于显生宙熔融残余成因的豆荚状铬铁矿;(4)元古代蛇绿岩常伴有硫化物Cu-Co-Zn-Au矿,且铬铁矿含Zn较高(Zn=0. 11%~0. 18%)(如芬兰Outokumpu蛇绿岩);橄榄岩及铬铁矿中常含较高的MnO (高达1. 79%,如埃及东部的Wizer蛇绿岩);(5)元古代蛇绿岩具多种成因:主要为俯冲带成因(如埃及蛇绿岩、北东非蛇绿岩、芬兰Outokumpu蛇绿岩),少量为洋中脊成因(加拿大Purtuniq蛇绿岩)及裂谷成因(芬兰Jormua蛇绿岩)等。Based on a large number of publications,this paper summarized the outcrops and characteristics of the Proterozoic ophiolites outside China,as well as the similarities and differences between the Proterozoic and Phanerozoic ophiolites. Also,it introduced the development and genesis of chromite deposits in the Proterozoic ophiolites in the world,mainly to clarify whether the chromite deposits were formed by magma differentiation or by partial melting of mantle peridotite. The existing data show that ophiolites occurred either in Early Proterozoic or in Middle-Late Proterozoic. The Early Proterozoic ophiolites are rare,and are represented by the Prutuniq ophiolite( 2. 05 ~ 2. 0 Ga) in the Cape Smith belt of the Canadian Shield and the Outokumpu and Jormua ophiolites( 1. 97 ~1. 96 Ga) in the Fennoscandian shield( Finland). The Middle-Late Proterozoic ophiolites crop out in many places,such as the desert of the east of Egypt( ~ 750 Ma) and the northeast of Africa( ca. 900 ~ 800 Ma). Several typical characters of the Proterozoic ophiolites are as following:( 1) The ophiolites are all dismembered. They are generally associated with arc-type volcanic rocks and/or mélanges,and have undergone different degrees of deformation and metamorphism( greenschist to amphibolite facies);( 2) They mostly have relatively complete rock assemblage of oceanic crust,including mafic-ultramafic cumulate,gabbro,sheeted dike complex,and volcanic rock. The rhythmic layering of the mafic-ultramafic rocks and compositional variations in minerals indicate multistage magmatic activities in an open magma chamber;( 3) Either high-Al or high-Cr chromite deposits may occur in the Proterozoic ophiolites,mainly in the dunite unit. The compositions of the chromites are controlled by the degree of partial melting and the activity of hydrous fluids in different evolution stages of supra-subduction zone( SSZ). The chromites usually have low Ti O2 contents. The chromite deposits formed by magma differentiation and are generally small in scale,different from

关 键 词：元古代 蛇绿岩 铬铁矿 构造环境 

分 类 号：P542[天文地球—构造地质学] P578.46[天文地球—地质学]