江苏东海榴辉岩相变质脉体的流体包裹体研究

A study of fluid inclusions in the eclogite-facies metamorphic veins in Donghai,Jiangsu Province,eastern China.

苏鲁超高压变质带南部的东海地区产出大量榴辉岩体,其中存在含蓝晶石—黝帘石/褐帘石—绿辉石—金红石(+磷灰石+锆石)石英脉,它们含有与榴辉岩主岩相近似的矿物组合,故应该是在与榴辉岩相近的超高压 P-T 条件下形成的。这些脉体的褐帘石/黝帘石和金红石具有很高的稀土元素(REE)和高场强元素(HFSE)含量及变化的 Sr、Ba、V 和 Cr 含量。脉体的蓝晶石,特别是褐帘石和黝帘石含有丰富的固体与流体包裹体,包括:(1)多相固体包裹体(multiphase solidinclusions,Ⅰ型);(2)多子晶流体包裹体(multidaughter mineral-bearing fluid inclusions,Ⅱ型);(3)H_2O-CO_2包裹体(Ⅲ型);(4)高盐度盐水包裹体(Ⅳ型);和(5)中-低盐度 H_2O 包裹体(Ⅴ型)。Ⅰ型和Ⅲ型包裹体主要存在于蓝晶石中,偶尔见于黝帘石中,呈孤立分布或沿晶内裂隙分布;Ⅱ型包裹体多沿褐帘石和黝帘石晶核密集或随机分布,或沿晶内裂隙分布;Ⅳ型和Ⅴ型包裹体多见于石英中,也见于褐帘石和黝帘石中,一般沿裂隙分布。能谱和激光拉曼探针分析表明Ⅰ型包裹体中的固相有钠云母、刚玉、方解石、菱铁矿、硬石膏、重晶石、磁铁矿和黄铁矿,以及未知相;Ⅱ型包裹体中的固相有白(钠)云母、硬石膏、方解石、磷灰石、天青石、磁铁矿和黄铁矿,以及未知含水硅酸盐等;Ⅲ型包裹体主要含 H_2O 和 CO_2;Ⅳ型包裹体除含 H_2O 液相外,常见固相有石盐、方解石和不透明矿物,而气相有时可含明显数量的 CO_2和 N_2。显微测温显示,多数Ⅱ型包裹体的冰点(T_(mi))在-5℃～0℃,相应的盐度为0～7.86%NaCl equiv.。鉴于Ⅱ型包裹体含有多种子矿物,根据包裹体中固体总合量并结合溶解度资料估计,Ⅱa和Ⅱb型包裹体中溶质浓度可分别达到40%～70%和25%～40%,原始流体可能属于 Na^+-Ca^(2+)(Sr^(2+))-Mg^(2+)-Fe^(2+)-CO_3^(2-)-SO_4~2-SiO_3^(2-)±PO_4^(3-)-Cl^--H_2O 体系。Ⅳ型包裹体中 CO_2固相熔化温度为-58.1～-58.0℃,激光拉曼探针分析证实 CO_2相中还存在 N_2;CO_2(-N_2)相的均一温度(T_(hCO_(2))为9.8～18℃,相应的 CO_2(-N_2)相流体密度为0.739～0.784g/cm^3。加热时石盐熔化温度(T_s)为201～428℃,液-气均一温度(T_h)从184到≥450℃,相应的含水相盐度为31.12%～≥53%NaCl。黝帘石和石英中Ⅴ型包裹体 T_e 接近-21℃,T_(mi)分别为-4.5～-19.0℃和-3.2～-18.4℃,相应的盐度分别为7.17%～≥20.68%NaCl 和5.26%～21.26%NaCl equiv.不等。高压脉体中存在丰富的流体包裹体证明这些脉体是在有自由流体相的条件下形成的。包裹体的产状和结构关系表明,原生Ⅰ型和Ⅱ型包裹体是在蓝晶石、褐帘石和黝帘石生长期间捕获的,流体可能来自于大陆板块深俯冲期间含水矿物的脱水反应。在超高压条件下,这种流体属于含水硅酸盐熔体—含水流体体系,含有大量溶质和微量元素。流体组成与岩石(矿物)类型有关,反映矿物结晶时与周围流体介质构成了局部的缓冲体系,并从周围流体介质中获取了所需要的组分,即产生榴辉岩相脉体的流体是就地来源的。由于超高压峰期变质后,苏鲁地体经历了快速折返和抬升,超高压条件下捕获的流体包裹体都经受了部分爆裂—再平衡,流体包裹体的密度大大降低;同时,流体包裹体有可能与主矿物腔壁相互作用,引起流体成分改变。

Kyanite-zoisite/allanite-omphacite-rutile （ ± apatite ± zircon） quartz veins occur in the Chizhuang eclogite in the Donghai area, in the southern part of the Sulu ultrahigh-pressure （UHP） metamorphic belt. These veins consist of mineral assemblages similar to their host eclogite and thus are considered be formed under P-T conditions similar to their host. Abundant solid/fluid inclusions are found in kyanite （Ky） and allanite/zoisite （Aln/Zo） ; they include： （ 1 ） multiphase solid inclusions （ type Ⅰ ） ; （2） muhidanghter mineral-bearing fluid inclusions （ type Ⅱ ） ; （3） H2O-CO2 inclusions （ type Ⅲ ） ; （4） high-salinity brine inclusions （ type Ⅳ ） and （5） medium-low salinity aqueous inclusions （ type Ⅴ ）. Types Ⅰ and Ⅲ inclusions are found mainly in Ky and rarely in Zo, occurring as isolated ones or along intragranular fractures whereas type Ⅱ inclusions occur densely/randomly in the cores of Aln/Zo or along intragranular fractures; types Ⅳ and Ⅴ inclusions occur commonly along fractures in quartz and also in Aln/Zo. EDS and Raman analyses indicate that the type Ⅰ inclusions have a daughter mineral association of paragonite ± corundum ± magnetite ± anhydrite ± barite ± calcite/siderite ± pyrite, plus unknown phase （ s ）, whereas type Ⅱ inclusions contain paragonite （ muscovite ）, anhydrite, calcite, apatite, celestite, magnetite, marcasite and unknown hydrosilicate （s）. Type m inclusions contain mainly H2O and CO2 without other volatile components in the CO2 phase while type Ⅳ inclusions contain halite ± calcite ± opaque in addition to a liquid （ H2 O） and a vapor phase which sometimes contains significant amount of CO2 and N2. Microthermometric measurements show that the final melting temperature of ice in most type Ⅱ inclusions ranges from - 5 to 0℃ which correspond to salinity values of 0 - 7.86% NaCL equivalent. Since type Ⅱ inclusions contain solid phases the concentrations of total solutes in type Ⅱ a and Ⅱ b inclusions are 40% -70% and 25% - 40%, respectively based on the estimation of total solids present in the inclusions and the solubility data of the solids. The original fluids from which type Ⅱ inclusions formed probably belong to the system of Na ^＋ -Ca^2＋ （ Sr^2＋ ） -Mg^2＋ -Fe^2＋ -CO3^2- - SO4^2-SiO3^- ± PO4^3--Cl^--H2O. The final melting temperature and homogenization temperature of CO2 phases in type Ⅳ inclusions range from -58. 1 to -58. 0℃ and from 9. 8 to 18℃, respectively. Raman analysis confirmed the presence of N2 in the CO2 phase. In heating, the melting temperature of halite ranges from 201 to 428℃, corresponding to the salinity of 31.12% -≥53% NaCl for the aqueous phase. The total homogenization temperature of type IV inclusions ranges from 184 to ≥450℃. The final melting temperature of ice in type V inclusions in zoisite and quartz ranges from - 4. 5 to - 19. 0℃ and from - 3.2 to - 18.4℃, corresponding to the salinity of 7.17% -≥120. 68% NaCl and 5.26% -21.26% NaCl, respectively, although these inclusions all show eutectic melting behaviour near -21℃. The presence of abundant fluid inclusions in the high-pressure vein minerals demonstrates that these veins were deposited from a free fluid phase. The occurrence and textural relations of the inclusions suggest that the primary types Ⅰ and Ⅱ inclusions were trapped during the growth of kyanite, allanite and zoisite and the fluids may originate from dehydration reactions of hydrous minerals （e. g. zoisite, talc and phengite）. Under ultra-high pressure conditions these fluids belong to supercritical hydrous silicate melt-aqueous fluids rich in solutes and trace elements. The tluid compositions are related to lithology （mineralogy）, which reflects that during the crystallization the minerals were locally buffered with the surrounding fluids and gained constituents from them, suggesting that the fluids responsible for the eclogite-facies veins were generated in-situ. Most fluid inclusions trapped in the UHP conditions have been subjected to decrepitation and reequilibration due to the fast exhumation and uplifting of the Sulu terrain after the peak UHP metamorphism, resulting in dramatic decrease of fluid density. Meanwhile, it is very likely that the fluid inclusions have been interacted with its hosts, causing changes in fluid compositions.