Study On Accessory Minerals In UHPM Rocks From Drilling Core Of CCSD And Outcrops Of North Jiangsu, China

Rutile, apatite and epidote are common accessory minerals in Ultra-high pressure metamorphosed (UHPM) rocks from Chinese Continental Scientific Drilling (CCSD) main-hole and Sulu-Dabie terrene, besides, there are some special vein minerals in various veins in CCSD drill core, these accessory minerals and vein minerals are not only the "witness" of UHPM and continental subduction-exhumation, but also the most important carrier for some significant trace elements, such as High Field Strenth Elements (HFSE), Rare Earth Elements (REE), some Large Ion Lithophile Elements (LILE, eg. Sr, Ba, Th, U, Pb) and volatile elements (eg. F, Cl, S, C, P, OH). Therefore, study on these accessory minerals and vein minerals will have significant implication to mineralogy, elemental geochemistry, and the evolution of metamorphic fluid. However, there are few related studies about these minerals up to date. This dissertation focus on rutiles, apatites epidotes and vein minerals from CCSD drill core and outcrops of Sulu-Dabie UHPM rocks, by detailed study of petrography, petro-chemistry, and mineral-chemistry, the implication of these minerals to UHPM and continental subduction-exhumation were discussed. Major conclutions from this study can be summarized as following:1. On mineralogy, rutiles, apatites, epidotes and vein minerals from UHPM rocks were firstly and systematically studied, by detailed petrography and mineral-chemistry investigation, some mineral or mineral assemblage with special composions or texture were firstly found in UHPM rocks from Sulu-Dabie, for example, corrudum, magnesite, high-Ti biotite and garnet were found as inclusions in rutiles from eclogites, Nb-Fe rutile and zoned Si-Y apatite were found in gneiss, epidote / allanite with euhedral zones like magmatic genesis were found in elogites and gneiss, a lot of minerals with high REE, HFSE, LILE and volatiles were found in veins of gneiss. These founds enrich and improve the mineralogy study in UHPM rocks.2. On elemental geochemistry, the understanding of geochemical behavious of REE, HFSE, LILE and volatiles during UHPM were deepened by detailed mineral-chemistry study of rutiles, apatites epidotes and vein minerals. It is pointed out that Cr content of rutile in eclogite is positively related to the MgO content of bulk rock, and the protolith of rutiles with high Cr content is not necessarily meta-cumulates. It is considered that diffusion and retrogression with retrograde fluid are responsible for the lower of Zr content of rutile in eclogites from Sulu-Dabie, and the Zr-in-rutile recorded temperatures of re-equilibium but not peak metamorphism. LILE and LREE show high activity in retrograde fluid, however, HREE and HFSE will also show activity only if there are abundance of volatiles dissolved in fluid.3. On the evolution and characteristics of metamorphic fluid, the number, occurrence and distribution of veins in CCSD drill core were systematically counted and analyzed to deduce that the fluid activaty at various stage have various extents and manners. By analyzing the accessory and vein minerals, it is point out that the retrograde fluid in eclogites may be hydrous solution with NaCl and silicate, and the retrograde fluid in gneiss are rich in REE, HFSE, LILE and volatiles. The redox state of metamorphic fluid may experienced oxidation→reduction→oxidation during prograde-peak→early retrogration→amphibolite facies.4. On metallogeny, based on previous study, it is point out that the most important factors that control the mineralization of eclogitic type rutile deposit are protoliths property and UHPM. Ti-rich basaltic protoliths related to the rift magmatism in neoproterozoic are the material foundermatal for rutile deposit, high to ultra-high pressure (>1.5GPa) are prerequisite for this ore type.