投稿时间：2019-12-03
修订日期：2020-03-06
网络发布日期：2020-07-13
中文摘要:阿拉斯加型岩体是一类具有独特的岩性环带状结构的镁铁-超镁铁质侵入体，常呈链状分布于汇聚板块边缘。其形成时代跨度较大，从元古代到新生代均有分布，以中生代最为发育。大部分阿拉斯加型岩体规模较小，出露面积约12~14 km2或更小，平面上呈近似同心环状结构，垂直剖面上呈管道状。岩体中心为纯橄岩，向外依次包括异剥橄榄岩、橄榄单斜辉石岩、单斜辉石岩、角闪单斜辉石岩、角闪石岩和辉长岩。造岩矿物为橄榄石、单斜辉石、角闪石等，副矿物为铬铁矿、磁铁矿、钛铁矿等，超镁铁质岩石中少或无斜方辉石，斜长石仅出现在边缘的辉长质岩石中。磁铁矿在单斜辉石岩和角闪石岩中为常见矿物，含量最高达15%~20%。阿拉斯加型岩体的主量元素成分揭示所有岩石均为与拉斑玄武质岩浆分异有关的亚碱性堆晶岩。微量元素成分上显示平坦的稀土元素配分型式和较低的微量元素含量，且富集大离子亲石元素，亏损高场强元素。矿物化学特征上，橄榄石富镁且Fo值变化较大；单斜辉石主要为富Ca的透辉石，其成分变化具有弧堆晶趋势；角闪石主要是镁角闪石和韭角闪石；铬铁矿富集Fe-Al，贫Cr。这些特征揭示，该类岩体成因明显不同于层状岩体和阿尔卑斯型岩体。综合岩石学、矿物学和地球化学分析表明，阿拉斯加型岩体形成于与板块俯冲作用有关的岛弧或者活动大陆边缘背景下，其母岩浆为受到熔/流体交代的地幔楔部分熔融产生的含水玄武质岩浆。各岩相为未受明显地壳混染的同源母岩浆在地壳深度结晶分异的产物。阿拉斯加型岩体的岩浆体系具有含水且高氧逸度的特征，其通常为铂族元素和铬铁矿矿床的重要载体，无或少铜镍矿化。
Abstract:Alaskan-type complexes refer to a category of lithologically zoned mafic-ultramafic intrusive suites that commonly occur along convergent plate margins. They span a wide age range from Proterozoic to Cenozoic, with a peak in Mesozoic. Most outcrops are small, with a surface area of 12~14 km2 or less, a pipe-like internal structure and a roughly concentrically zoned pattern. They have a dunite core that is enveloped sequentially by wehrlite, olivine clinopyroxenite, clinopyroxenite, hornblende clinopyroxenite, hornblendite and gabbro. The principal rock-forming minerals are olivine, clinopyroxene and hornblende, with minor chromite, magnetite and ilme-nite. Orthopyroxene is absent or rare in ultramafic rocks, and plagioclase occurs only in marginal gabbroic rocks. Magnetite is common in clinopyroxenite and hornblendite with a modal abundance up to 15%~20%. The major element compositions of the Alaskan-type complexes reveal that they are subalkaline cumulative rocks related to tholeiitic differentiation. In terms of trace element composition, they display flat rare earth element patterns and have low total trace element abundance with enrichment of large ion lithophile elements and depletion of high field strength elements. The characteristics of mineral chemistry are as follows:olivine is rich in Mg with a wide range of Fo; clinopyroxene is dominantly diopside with high CaO content and arc-cumulus tendency in composition; hornblende is magnesio-hornblende and pargasite; chromite is rich in Fe and Al and poor in Cr. These features reveal that this complex is somewhat different from layered and Alpine-type complexes. Comprehensive studies of petrology, mineralogy, and geochemistry in Alaskan-type complexes suggest that they were formed at island arcs or on active continental margins associated with subduction. The hydrous basaltic parental magmas were generated from partial melting of mantle wedge metasomatized by melts/fluids in subduction zones. Each of lithofacies in the complexes was formed by fractional crystallization from a common parental magma with negligible crustal contamination. The magmatic system is characterized by hydrous and high oxygen fugacity.Alaskan-type complexes are important hosts for platinum-group elements (PGE) and chromite, whereas Ni-Cu sulfide mi-neralization is rare or absent.
keywords:geochemistry Alaskan-type complexes petrogenesis arc magmas accumulation PGE minera-lization
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基金项目:本文得到国家重点研发计划（编号：2017YFC0601306）和国家自然科学基金项目（编号：41973012、41772055）联合资助
引用文本：
崔梦萌,白洋,罗扬,苏本勋,肖燕,王静,潘旗旗,高东林.2020.阿拉斯加型岩体的基本特征、成岩过程及成矿作用[J].矿床地质,39(3):397~418CUI MengMeng,BAI Yang,LUO Yang,SU BenXun,XIAO Yan,WANG Jing,PAN QiQi,GAO DongLin.2020.Characteristics, petrogenesis and metallogenesis of Alaskan-type complexes[J].Mineral Deposits39(3):397~418
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