科研之友
微信
新浪微博
Facebook
分享链接
摘要
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) multidaughter mineral-bearing fluid inclusions (type H) (3) H(2)O-CO(2) 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 H 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 III inclusions contain mainly H(2)O and CO(2) without other volatile components in the CO(2) phase while type IV inclusions contain halite calcite opaque in addition to a liquid (H(2)O) and a vapor phase which sometimes contains significant amount of CO(2) and N(2). Microthermometric measurements show that the final melting temperature of ice in most type II inclusions ranges from -5 to 0 degrees C which correspond to salinity values of 0 similar to 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% similar to 70% and 25% similar to 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+) -CO(3)(2-)- SO(4)(2)-SiO(3)(2-) +/- PO(4)(3-) -Cl(-) -H(2)O. The final melting temperature and homogenization temperature of CO(2) phases in type IV inclusions range from -58.1 to -58.0 degrees C and from 9.8 to 18 degrees C, respectively. Raman analysis confirmed the presence of N(2) in the CO(2) phase. In heating, the melting temperature of halite ranges from 201 to 428 degrees C, corresponding to the salinity of 31.12% similar to >= 53% NaCl for the aqueous phase. The total homogenization temperature of type IV inclusions ranges from 184 to >= 450 degrees C. The final melting temperature of ice in type V inclusions in zoisite and quartz ranges from -4.5 to -19.0 degrees C and from -3.2 to -18.4 degrees C, corresponding to the salinity of 7.17% similar to >= 20.68% NaCl and 5.26% similar to 21.26% NaCl, respectively, although these inclusions all show eutectic melting behaviour near -21 degrees C. 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 inlusions 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 fluid 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 re-equilibration 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.
- 出版日期2008-9
- 单位山东省地质科学研究院; 中国地质科学院
