| Since the first discovery of coesite and diamond in metamorphic rocks of continental affinity, ultra-high pressure (UHP) metamorphism and deep continental subduction have been one of the research frontiers in solid Earth science. Marbles are reported to be among the variety of UHP rocks but draw much less attention than associated eclogites and metapelites. It has been widely recognized that UHP marbles bear significant information on the metamorphic evolution of UHP terranes and interactions between subducted continents and overlying mantles. Besides, relevant studies may place important constraints on the formation conditions of diamond in UHP rocks. In this thesis, we conducted a detailed study on the Ganjialing UHP impure marbles and associated metasediments by a combination of field observation, optical and scanning electron microscope, electron microprobe, Laser Raman spectroscopy, cathodoluminescence and LA-ICP-MS. The main achievements in this study are summarized as follows:(1) Impure marbles and associated metasediments widely occur at the Ganjialing area. Two marble types, i.e. clinopyroxene and amphibole marbles, are identified and both of them enclose many eclogite lenses and boudins. Based on the mica types and their occurrence or absence, the eclogites can be divided into three primary types, i.e. phengite eclogite, paragonite eclogite and eclogite containing no micas. Coesite inclusions are common in the first two types, while they are nearly absent in the third type. In addition to eclogite, some other rocks are also enclosed within the marbles, such as Ti-clinohumite-diopside garnetite, dolomite-quartz garnetite, calc-gneiss and various high-pressure veins. In the dolomite-quartz garnetites, well-preserved coesite inclusions widely occur in both garnet and dolomite. More importantly, several interstitial coesite grains have been found. These discoveries underscore the role of traditional "pressure vessel" models and highlight the significance of fluid unavailability in preserving coesite.(2) Two garnet-bearing omphacite marbles from Ganjialing have a peak mineral assemblage of dolomite, aragonite, garnet, omphacite, phengite, coesite, rutile and allanite and relict coesite and aragonite inclusions are preserved in dolomite and allanite, respectively. Estimated with the compositions of garnet, clinopyroxene and phengite, a peak P-T condition of 4.05-4.45 GPa and 740-820 ℃ is obtained by conditional geothermobarometry. Neither petrological nor independent peak P-T estimations support the UHP breakdown of dolomite in the marbles. Analysis on the phase relations in the CaO-MgO-SiO2-H2O-CO2 system shows that bulk rock compositions have a large control on the stable UHP carbonate associations in carbonate-bearing rocks. Based on the petrographical data and the constructed P-T path, the software package of THERMOCALC has been used to constrain the fluid evolution in the marbles. The modeled fluid had a low X(CO2) at the peak conditions, while the CO2 concentration in the equilibrium fluids firstly increased during early isothermal exhumation and then decreased at later retrogression. The CO2-depleted feature of the peak fluid indicates a weak metamorphic decarbonation of the impure marbles under UHP conditions. The X(CO2) increased at the early stage but it was mainly internally-buffered. Given this, it can be acknowledged that metamorphic decarbonation was still weak during isothermal exhumation. Investigations on the retrograde reactions indicate that CO2 would be sequestrated as carbonates while the clinopyroxene marbles transformed into amphibole marbles. Clinopyroxene marbles are a common rock in many HP and UHP terranes so that the CO2-sequestrated process can influence the global carbon cycling in an important way.(3) Calc-gneisses interlayered with the impure marbles underwent low degree melting during exhumation. The relict peak mineral assemblage of garnet, phengite, coesite and rutile in the calc-gneisses records a peak temperature of 692-757 ℃ at 4.0-4.8 GPa, which is consistent with that of the impure marbles. Partial melting is indicated by several lines of evidence:the melting textures of phengite, the feldspar film and vein, the euhedral K-feldspar, the plagioclase+biotite intergrowth after garnet and the epidote poikiloblast. Polyphase solid inclusions in garnet are characterized with wedge-like offshoots and serrate outlines and those in epidote display negative crystal shapes, which can be best interpreted by entrapment of former melts. Given that a nearly isothermal decompression P-T path could hardly induce dehydration melting of phengite, a wet melting reaction of Phn+Q+H2O ± Na-Cpx= Bt+Pl±melts ± Grt is proposed and it likely took place at ca.650-800 ℃ and ca.1.0-2.0 GPa. Partial melting not only promoted the retrograde transformation of coesite but also altered the composition of refractory peak minerals. Chemical exchanges between garnet and melts formed new garnet domains that have elevated almandine, spessartine, MREE, HREE and Y but reduced grossular, pyrope, P, Sc, Ti, V and Zr contents. Zr-in-rutile thermometer reveals a low temperature of 620-643 ℃ at 5 GPa, implying a later reset for Zr in rutile.(4) Zircons in one tremolite marble underwent at least three episodes of metamorphic growth, which produced domain-1, domain-2 and domain-3, respectively. Domain-1 records a 206Pb/238U age of 241±3 Ma and a crystallization temperature of 515-721 ℃ and a younger 206Pb/238U age of 229±2 Ma and a higher crystallization temperature of 606-781 ℃ are obtained for domain-2. It is evident that these two domains resulted from prograde growth. Domain-2 contains inclusions of dolomite, aragonite, calcite, garnet, diopside, phengite, quartz, rutile and apatite, suggesting that it grew at eclogite-facies stage. Mineral inclusions are nearly absent in domain-1 but it displays a similar rare earth element (REE) pattern to domain-2. Consequently, domain-1 should have also been formed at eclogite-facies stage. Domain-3 contains rare calcite and plagioclase inclusions, indicating an amphibolite-facies growth. Zircons in one phengite-bearing eclogite can be divided into three main domains:domain-1, domain-2 and domain-3. All these three domains contain an assemblage of coesite, garnet, omphacite, phengite, rutile and apatite and show a flattened HREE pattern with no negative Eu-anomaly, suggesting that they were formed at UHP eclogite-facies conditions. Domain-2 and domain-3 have a low Th/U ratio and represent newly-growing zircons. Domain-2 records a 206Pb/238U age of 237±3 Ma and a crystallization temperature of 612-686 ℃ and a younger 206Pb/238U age of 224±3 Ma and a higher crystallization temperature of 681-789 ℃ are obtained for domain-3. Consequently, these two domains also resulted from prograde growth. Domain-1 has a high Th/U ratio, implying it may represent highly-recrystallized detrital cores. In spite of this, the 206Pb/238U age and crystallization temperature of domain-3 are identical to those of domain-2, and are 237±5 Ma and 634-721 ℃, respectively. Different to those in the eclogite, the eclogite-facies zircons in the marble display a higher and wider LuN/DyN ratio. This can be explained by that the tremolite marble contains low contents of garnet at eclogite-facies stage and that garnet distributed unevenly. This emphasizes the importance of garnet in differentiating the MREE and HREE in metamorphic zircons. |
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