CCSD及青龙山HP-UHP变质岩中绿帘石地球化学及其对板块折返过程的示踪  

Geochemistry of epidotes in HP-UHP metamorphic rocks from Chinese Continental Scientific Drilling (CCSD) Project and Qinglongshan area: An indicator of partial melting during continental slab exhumation.

作　　者：梁金龙[1] 孙晓明[1] 徐莉[1] 翟伟[1] 汤倩[1] 梁业恒[1] 

机构地区：[1]中山大学地球科学系,广州510275

出　　处：《岩石学报》2006年第7期1845-1854,共10页Acta Petrologica Sinica

基　　金：本文得到国家自然科学基金重大项目(No.40399142);中国科学院同位素年代学和地球化学实验室合作基金(GIGIso-0504);国家重大基础研究973项目(No.2003C13716501)联合资助。

摘　　要：绿帘石是一种能在UHP变质峰值期稳定存在的含水矿物。由于其稳定的温-压条件十分宽广,其成分的变化可指示变质过程及条件。对中国大陆科学钻探(CCSD)岩芯及青龙山榴辉岩和正、副片麻岩中绿帘石较系统的岩相学观察和主、微量元素研究表明:绿帘石有多种成因,在俯冲进变质和折返退变质过程中都有绿帘石形成,每个阶段又可以划分出多个形成世代。绿帘石中主量元素的变化集中体现在XFe(XFe=Fe3+／(Al3++Cr3++Fe3++Mn3+))的变异,XFe值是变质条件(温度、压力、氧逸度)的函数。XFe在绿帘石斑晶的核部到边缘由大到小是进变质作用的标志,退变质过程形成的绿帘石则相反。在退变质过程中绿帘石斑晶边缘的∑REE及Sr、Ba、Pb等大离子半径元素比核部有降低的趋势;副片麻岩中的绿帘石核部大都包含褐帘石残余,褐帘石退变为绿帘石时∑REE急剧减少,球粒陨石配分模式也由LREE富集型逐渐过渡为LREE和HREE无明显分异的平坦型。这些微量元素的特征不可能起源于变质流体／矿物间的水／岩反应,而极有可能是板块折返过程中发生部分熔融的结果。绿帘石还是REE、Sr等大离子半径元素循环至地壳深部和地幔的合适的载体矿物。Epidote is one of the water-bearing minerals which can keep stable during peak ultra-high pressure （UHP） metamorphism. Its compositional variation can be used as indicators of metamorphic conditions and process, because of its wide range of stable conditions of temperature and pressure. In this study, major and trace elements of epidotes in eclogite, paragneiss and orthogneiss from CCSD and Qinglongshan in Sulu UHP terrance are determined by electronic microprobe and LA-ICPMS, and the results indicate that the epidotes possess multiple genesis. They were formed during both prograde and retrograde stages and can be divided into many generations. The variation of major dements can be denoted by XFe （ XFe = Fe^3＋/（ Al3^＋ ＋ Cr^3＋ ＋ Fe^3＋ ＋ Mn3^＋ ） ） which is the function of metamorphic conditions （ P, T, fo2 ） . The XFe change from high in the core to low in the margin of porphyroblastic epidotes which formed during prograde stage, but the epidotes formed during retrograde stage just on the contrary. The concentration of LILE （large ion lithophile elements） such as ∑REE, Sr, Ba, Rb, Th and Pb change from high in the core to low in the margin during the retrograde stage. Chondrite-normalized REE patterns of allanites, which were retrograded into epidotes in margin in paragneiss, vary from LREE-enriched type in the core to quite flat one in the margin. The depletion of LILE in the epidotes＇s margin may not result from metamorphic water/rock interaction during metamorphism, but probably attribute to partial melting during retrograde stage. High REE and Sr contents and wide stable conditions make epidote a suitable carrier of trace elements such as REE and Sr to the deep crust or mantle during subduction.

关 键 词：绿帘石 主量和微量元素 HP-UHP变质岩 部分熔融 CCSD 

分 类 号：P578.956[天文地球—矿物学] P595[天文地球—地质学]