海南洋壳型高温榴辉岩:基本特征及待解问题

Oceanic-type high-temperature eclogites from Hainan Island,South China:General characteristics and unsolved problems

榴辉岩常产于汇聚板块的边界,是鉴别古板块缝合带的重要标志之一。最近,这种特征的变质岩石在海南岛北部被第四系严重覆盖的木栏头地区被发现,其基本特征总结如下:(1)榴辉岩孤立地出露于潮滩鼻潮间-潮下带,主体露岩区域分布的总面积约1.8km2,由片麻理构成的优势构造走向为北东至近东西;(2)榴辉岩经历了顺时针变质演化,从绿帘角闪岩相(620~680℃、0.87~1.11GPa)、榴辉岩相/榴辉岩-高压麻粒岩过渡相(820~860℃、1.70~1.82GPa)、角闪岩相(700~730℃、0.71~0.85GPa)到绿片岩相;(3)榴辉岩的主体(占分析样品总数的65%)具有正常洋中脊玄武岩(N-MORB)属性,少数具有富集洋中脊玄武岩(E-MORB)和火山弧玄武岩(VAB)属性,初始Sr-Nd同位素成分表明他们来自于亏损的软流圈地幔;(4)榴辉岩原岩形成于355Ma之前,进变质和峰期-退变质的时代分别约为340~330Ma和310~300Ma,冷却至金红石U-Pb体系封闭温度的时代为292±6Ma。所以,海南榴辉岩主要是大洋(少数岛弧)玄武岩在石炭纪经高温高压变质作用的产物。由这种特殊的洋壳型高温榴辉岩自身及其引申出的科学问题包括:(1)海南榴辉岩是单体榴辉岩还是榴辉岩集合体?如果是单体,那将是国内出露规模最大的榴辉岩体;(2)海南榴辉岩到底是榴辉岩相岩石还是榴辉岩-高压麻粒岩过渡相岩石?榴辉岩相峰期变质的压力到底有多高?遍布的深熔作用是发生在温压峰期还是在减压过程中的温度峰期?(3)原岩形成于洋盆还是弧后盆地?其与同时代的金沙江-哀牢山-马江洋和邦溪-晨星弧后盆地具有怎样的联系?(4)榴辉岩起因于大洋热俯冲/增生还是大陆俯冲/碰撞环境?以其为代表的古板块缝合带(可称木栏头或潮滩鼻缝合带)向哪里延伸?(5)东、西古特提斯构造域的早期演化有无相似之处?海南陆块或其北部或西部地体是否在石炭纪就已与华南陆块碰撞对接在一起?显然,海南榴辉岩对重构全球古特提斯构造带的早期演化具有重要的科学意义,值得进一步深入研究。

Eclogites typically occur along the convergent plate boundaries and are generally thought to be markers of the suture zones.These diagnostic metamorphic rocks have recently been founded in the Mulantou area of northeastern Hainan Island,where is covered mostly by Quaternary deposits.The general characteristics of the eclogites are summarized as follows:(1)The eclogites isolatedly occur in the intertidal and subtidal zones at Chaotanbi.The principal part of the outcrops is distributed within an area of~1.8km2.The dominant structural strikes defined by the gneissosities of the rocks are northeast-southwest to nearly east-west.(2)The eclogites underwent a clockwise metamorphic evolution from epidote amphibolite facie(620~680℃and 0.87~1.11GPa)through eclogite or transitional eclogite-high-pressure(HP)granulite facies(820~860℃and 1.70~1.82GPa)to amphibolite facies(700~730℃and 0.71~0.85GPa)and greenschis facies.(3)Most of the eclogites(65%of total analyzed samples)have normal mid-ocean ridge basalt(N-MORB)affinities,and some are of enriched mid-ocean ridge basalt(E-MORB)and volcanic arc basalt(VAB)affinities.Relatively juvenile Sr-Nd isotopic compositions of the rocks suggest a derivation from the depleted asthenosphere mantle.(4)The protoliths of the eclogites were formed before 355Ma,and prograde and peak to retrograde metamorphism took place at ca.340~330Ma and ca.310~300Ma,respectively.The rocks cooling to the closure temperature of the rutile U-Pb system are dated at 292±6Ma.In brief,eclogites from Hainan Island are the products of oceanic basalts(including a few VAB)metamorphosed at high P-T conditions during the Carboniferous.The scientific problems derived from these exceptional oceanic-type high-temperature eclogites theirselves and their extensions include the following aspects:(1)Are the eclogites a single bigger body or a group of smaller bodies?If they are the former case,these eclogites would constitue the beggist eclogite body in China.(2)Are these eclogites belong to eclogite facies rocks or transitional eclogite-HP granulite facies rocks?How high will the pressures reach during the peak metamorphism of the eclogite facies?Whether widespread anatexis occurs at the peak P-T conditions or the peak temperatures during the decompression of the rocks?(3)Were the protoliths of the eclogites formed in an oceanic basin or a back-arc basin?How is it related to the simultaneous Jinshajiang-Ailaoshan-Song Ma Ocean and Bangxi-Chenxing back-arc basin?(4)Were the eclogites resulted from hot oceanic subduction/accretion or continental subduction/collision?Where is the paleosuture(called the Mulantou or Chaotanbi suture)indicated by the extention of eclogite outcrops?(5)Are there any similarities in the earlier evolution between the weasten and eastern Paleo-Tethyan tectonic domains?Whether or not the Hainan continental block,or its northern or western terrane,collided with the South China Block during the Carboniferous?It is obvious that eclogites from Hainan Island are of great significance for reconstructing the early evolution of the global Paleo-Tethyan tectonic belt,and therefore they deserve futher investigations.