超高压变质流体的组成与演化:中国大陆科学钻探工程主孔岩心的流体包裹体研究

Composition and evolution of ultrahigh-pressure metamorphic fluids:a fluid inclusion study of the drill cores from the main hole of Chinese Continental Scientific Drilling Program

中国大陆科学钻探工程主孔位于大别-苏鲁超高压变质带东段的江苏东海县,孔深为5100m,其上部2050m钻遇的岩石主要为榴辉岩,其次是正、副片麻岩、石榴橄榄(辉石)岩以及少量片岩和石英岩。它们经历了超高压变质作用和随后的角闪岩相退变质作用。通过对上述各种岩石的详细流体包体观察和RAMAN光谱分析,发现了五种不同成分的流体包裹体:(1)中-低盐度水溶液包裹体(Ⅰ型),呈原生的孤立和小群存在于榴辉岩和片麻岩锆石的岩浆核和超高压变质边缘,或存在于绿辉石、黝帘石和被绿辉石包裹的方解石和石英中,偶尔呈出溶包裹体产于磷灰石中,而主要沿绿辉石、石榴石、蓝晶石、黝帘石和石英等矿物的穿颗粒裂隙分布;也呈孤立和小群产于切穿榴辉岩的方解石脉和片麻岩重结晶石榴石和绿帘石中;(2)CO2(±CH4)-H2O包裹体(Ⅱ型),存在于锆石的岩浆核和变质边缘,或沿石英裂隙分布;(3)含石盐±SiO2±CaCO3的复杂盐水包裹体(Ⅲ型),呈原生流体包裹体产在榴辉岩的绿辉石中,与石英出溶棒一起平行于绿辉石的C轴分布,或产在石榴辉石岩透辉石的晶内裂隙中;(4)富CO2包裹体(Ⅳ型),在榴辉岩的石英中随机分布;(5)单气相包裹体(Ⅴ型),沿各种矿物穿颗粒裂隙分布。流体包裹体产状及其与捕获时代关系表明,Ⅰ型和Ⅱ型包裹体可以出现在超高压变质岩原岩、峰期变质和退变质各阶段。Ⅲ型包裹体出现在超高压变质岩的早期减压退变质阶段。而Ⅳ型和Ⅴ型包裹体主要形成于角闪岩相及更晚的退变质阶段。本研究的主要认识是:(1)低盐度H2O和CO2流体在进变质、超高压变质和退变质作用各阶段均有存在,这表明在整个超高压变质演化过程中流体具有继承性。(2)超高压变质岩原岩和角闪岩相退变质岩中存在较丰富的流体包裹体,但在超高压峰期捕获的流体包裹体却很少见,这表明丰富的原岩流体或在超高压进变质过程中被排出岩石体系,或进入含水超高压矿物和结合进名义无水矿物。(3)复杂成分原生流体包裹体的发现证明在超高压变质峰期后的早期减压退变质阶段存在一种高盐度似熔体流体,名义上的无水矿物在超高压条件下可以保存相当量的流体,并在退变质过程中分离出来,产生流体-岩石相互作用。(4)角闪岩阶段的流体包裹体具有各种不同的化学组成,且在局部富集,推测可能有部分外部加入的流体。(5)流体包裹体类型、丰度和成分在不同岩石类型中和不同钻孔深度都存在明显差异,表明超高压变质作用过程中没有大规模的透入性流体活动。(6)根据超高压变质峰期包裹体等容线得到的压力值大大低于根据矿物温压计获得的近峰期变质压力,这表明包裹体的密度在捕获后发生了改变。这些改变是由于流体渗漏、部分爆裂和流体-岩石相互作用所引起。

The main hole of the Chinese continental scientific drilling project is located in Donghai county in the east segment of the Dabic-Sulu ultrahigh-pressure (UHP) metamorphic belt. With a depth of 5 100 m, the lithology of the upper part 2050 m of the hole consists mainly of eclogite, ortho- and paragneisses, garnet peridotite (pyroxenite) and some schist and quartzite. These rocks have experienced UHP metamorphism and later amphibolite-facies retrogression. Five types of fluid inclusions with different compositions are found in the rocks by detailed petrographic observation and Raman analysis of fluid inclusions. They are: (1) aqueous inclusions of medium-low salinity (type I) which Occur either as primary inclusions in the magmatic cores and UHP-metamorphic rims of zircon grains of eclogite and gneisses, in omphacite, zoesite and in calcite- and quartz blebs encompassed by omphacite, or as exsolution inclusions in apatite, or mainly as secondary inclusions along transgranular fractures in omphacite, garnet, kyanite, zoesite and quartz of eclogite; aqueous inclusions also occur in isolation or Cluster in calcite veins crosscutting eclogite, in recrystallized garnet and epiclote in gneiss; (2) CO(2)(CH(4))-H(2)O inclusions (type II) which are present in both magmatic cores and metainorplue rims of zircon and are distributed along quartz fractures; ( 3) complex saline inclusions containing halite +/- SiO(2) CaCO(3) which either occur as primary inclusions in oniphacite of eclogite, coexisting with quartz rods and paralleling to the c-axis of oniphacite, or distributed along intragranular fractures in diopside of garnet pyroxenite; (4) CO(2) rich inclusions (type IV) which are distributed randomly in quartz of eclogite; and (5) monophase gas inclusions (type V) which occur randomly or along fractures of various minerals. The occurence of fluid inclusions and their relative chronology suggest that the inclusions of types I and II occurred in the protoliths, in the peak- and retrograde stages of UHP metamorphic rocks but the type III was present primarily in the early decompression-retrogression stage of UHP rocks, whereas types BI and V were trapped mainly in the amphibolite-facies or later stages. This study gains the following understanding: (1) The low-salinity H(2)O and CO(2) fluids are present in all the prograde-, peak- and retrograde nietamorphic stages, suggesting that the fluids were inherited during the entire UHP metamorphic evolution. (2) While fluid inclusions are abubdant in the protoliths of UHP rocks and in the amphibolite-facies rocks those trapped in the peak stage are rare, implying that abundant fluids contained in the protoliths were removed from the rock system or entered into UHP hydrous minerals or n nominally anhydrous minerals. (3) The identification of primary inclusions with complex composition demonstrates that a nielt-like fluid was present in the early decoin press ion -retrograde stage after the peak UHII metamorphism and norninally anhydrous minerals might hold a significant amount of fluids under UHP conditions and released in retrograde process, resulting in fluid-rock interactions. (4) Fluid inclusions trapped in the amphibolite-facies stage have various compositions and they are concentrated locally and may partly derived from external fluids. (5) Great differences in type, abundance and composition of fluid inclusions exist in different lithologies and depths of the drill hole, suggesting that there was no large-scale pervasive fluid flow during UHP metamorphism. ( 6) The pressures obtained from the isochores of the fluid inclusions trapped during the peak of UHP metamorphism are mush less than the peak metamorphic pressures as derived from the mineral thermobarometers, indicating density changes of the fluid inclusions due to fluid leakage, partial decrepitation and fluid-rock interactions, etc.