| In the 1980s and 1990s, the discovery of ultrahigh-pressure (UHP) index minerals such as coesite and diamond in the supracrustal rocks of metamorphic origin led to recognition of continental deep subduction. Since then the study of continental deep subduction and UHP metamorphism has been one of the frontiers and hotspots in solid earth science. The Dabie-Sulu orogenic belt, which is located in east-central China, is one of the largest UHP metamorphic zones in the word. It is an excellent natural laboratory for studying the petrological and geochemical changes during subduction and exhumation of continental crust. In this PhD thesis, I have focused on UHP metamorphic rocks from the Taohang and Wehai areas, respectively, that are located in the central and northeastern part of the Sulu orogen. An integrated study of petrology, zirconology and geochemistry was carried out for different types of UHP rocks including migmatites. The results not only provide new evidence for the anatexis and fluid activity during continental collision, but also reveal the details of anatectic processes, anatectic conditions and anatectic mechanisms as well as minerals reactions, accessory growth and element mobility. These results further provide new constraints on exhumation mechanism of the deeply subducted continental crust and on the subduction channel model.A combined petrological and zirconological study was carried for UHP metagranite from the Taohang area. The results reveal differential behaviors of dehydration and anatexis between two samples from the same UHP slice. The dehydration and anatexis of UHP metamorphic rocks during continental collision are two key processes that have great bearing on the physicochemical properties of deeply subducted continental crust at mantle depths. Determining the time and P-T conditions at which these events took place is a key to understanding of subduction-zone tectonism. Zircon mantle domains in gneiss 10SL16 record eclogite-facies dehydration metamorphism at 236±5 Ma during subduction, exhibiting low REE contents, steep MREE-HREE patterns without negative Eu anomalies, low Th, Nb and Ta contents, low temperatures of 651-750℃, and inclusions of quartz, apatite and jadeite. A second mantle domain records high-T anatexis at 223±3 Ma during exhumation, showing high REE contents, steeper MREE-HREE patterns with marked negative Eu anomalies, high Hf, Nb, Ta, Th and U contents, high temperatures of 698-879℃, and multiphase solid (MS) inclusions of albite+muscovite+quartz. In contrast, in gneiss 10SL38, one zircon mantle domain records limited hydration anatexis at 237±3 Ma during subduction, exhibiting high REE contents, steep MREE-HREE patterns with marked negative Eu anomalies, high Hf, Nb, Ta, Th and U contents, medium temperatures of 601-717℃, and multiphase solid inclusions of albite+muscovite+hydrohalite. A second mantle domain in this sample records low-T dehydration metamorphism throughout the whole continental collision in the Triassic, showing low REE contents, steep MREE-HREE patterns with weakly negative Eu anomalies, low Th, Nb and Ta contents, low temperatures of 524-669℃, and anhydrite+gas inclusions. Garnet, phengite and allanite/epidote in these two samples also exhibit different variations in texture and major-trace element compositions, in accordance with the zircon records. The distinct P-T-t paths for these two samples suggest separate processes of dehydration and anatexis which are ascribed to the different geothermal gradients at different positions inside the same crustal slice during continental subduction-zone metamorphism. Therefore, the subducting continental crust underwent variable extents of dehydration and anatexis in response to the difference in subduction-zone P-T conditions, and finally decided by their different positions in subducted block.An elaborate petrological study, especlally mineral-pair thermobarometric calculations and microstructureal comparison with the results from experimental studies, was carried out for a series of eclogite samples with different degree of migmatization from the Taohang area. The results identified two anatectic events of eclogite during continental collision. The first episode of anatexis took place at the early stage of exhumation with the pressure high than 2.0 GPa by phengite fluxed melting at low degree, and produced the felsic melt, which further formed MS inclusions mainly consisting of K-feldspar and quartz in garnet and zoisite. Some phengite and zoisite grains included in garnet also partial melted in this episode, and produced MS inclusions and newly grown garnet around the inclusions. The second episode of anatexis took place at 1.6-1.8 GPa and 800-850℃ in the rocks rich in phengite and zoisite mainly by phengite and zoisite dehydration melting during hot exhumation. The melting degree is higher (15-30%) and the melts were separated to form the leucosome. The leucosome is mainly composed of quartz, plagioclase and phengite, with minor amounts of epidote, amphibolite and garnet. In the samples, plagioclase has the homogeneous composition, indicating that the melts are homogeneous and reach equilibrium. In one sample, the kyanite was reacted with the melt to form the corona structure aroud the residual kyanite. From the inner to outside, the corona consists of margarite, phengite and plagioclase, which rich in Ca, K and Na, respectively. In situ analysis of trace elements and Sr isotopes in epidote group minerals in eclogite indicate their sources and reactions. However, new growth zircon rims from the migmatized eclogite may growth from eutectic melts, it’s low trace element contents characters similar as the metamorphic zircon. This indicates the eutectic melts has the low trace elements solubility, and similar as the metamorphic fluid. In general, the water content of eclogite, especially the amount of hydrous minerals, dictates the degree of partial melting.A combined study of zircon U-Pb ages, oxygen isotopes and trace elements, and mineral compositions and inclusion analyses was carried out for a suite of migmatites (diatexite, metatexite and leucosome) from the Weihai area. The results revealed two episodes of anatexis in deeply subducted continental crust during continental collision, and confirmed the prediction of anatexis at the top of subducted curst block at subarc depth by numerical model. The nanogranite is observed to occur not only in zircon, garnet and monazite from the diatexite but also in zircon cores from the leucosome, indicating those minerals growth from anatectic melts. The anatectic zircon domains exhibit U-Pb ages of 230-227 Ma and flat REE patterns with weak or no negative Eu anomalies, suggesting that this episode of anatexis took place at the transition from amphibolite-to eclogite-facies metamorphism during subduction. Zircon in the diatexite and zircon cores in the leucosome both exhibit high δ18O values of 8.3 to 17.3‰. The diatexite shows high A/CNK value and the occurrence of peritectic garnet. Thus, the diatexite was produced by anatexis of metasedimentary rocks. The Ti-in-zircon thermometry and garnet growth zoning indicate consistent melting temperatures of 700-800℃ and subsequent melt crystallization at 650-700℃. On the other hand, zircons in metatexite and zircon rims in leucosome record the second episode of anatexis at 218-214 Ma during exhumation. They exhibit oscillatory zoning, steep HREE patterns with negative Eu anomalies, low temperatures of 550-700℃, and significant variations in Th, U, Nb and Ta contents. These characteristics indicate their crystallization from evolved hydrous melts, consistent with the pegmatitic structure of leucosome. Zircon in the metatexite exhibits low δ18O values of-1.5 to 3.5‰ and Neoproterozoic U-Pb ages for relict magmatic cores, indicating that the anatexis of low δ18O UHP metagranite produces the metatexite. Therefore, the metasedimentary rocks would be predominated atop the deeply subducted continental crust, and they were heated by the overlying mantle wedge to undergo the first episode of anatexis during the subduction. The exhumation of the upper crustal slice in the continental subduction channel would cause the subsequent anatexis of UHP metamorphic rocks and the mixing of different types of rocks.An integrated study of SIMS U-Pb and O isotope analyses with LA-ICPMS U-Pb isotope and trace element analyses was carried for zircons from pegmatite vein and host UHP gneiss in the Taohang area. The results reveal the anatectic event and genetic transition between aqueous fluid and hydrous melt in these UHP metamorphic rocks. The pegmatite veins yield zircon U-Pb ages of 147-153 Ma for new growths, and 700-800 Ma for relict cores. One meters-width pegmatite vein exhibits two episodes of zircon growth at 153 ±3 Ma for inner domain and at 147 ± 2 Ma for outer domain. The two types of domains have a series of differences in CL image, inclusion type, REE content and pattern, trace element contents and ratios, and Ti-in-zircon temperature. The inner domain grew from the hydrous melt at 730-840℃, whereas the outer domain grew from the aqueous fluid at 520-650℃. Nevertheless, they have similarly low δ18O values of 1.0-2.3‰, suggesting their growth from O isotope homogeneous media despite the transition from melt to fluid. The host gneiss of this pegmatite vein exhibits three generations of zircon growth, with 180-205 Ma at 700-770℃ during amphibolite-facies metamorphism,157±3 Ma at 610-670℃ for fluid-assisted growth, and 147±2 Ma at 780-860℃ for melt-assisted growth. The last two types of growth record the development of crustal anatexis from hydration melting to dehydration melting. Another decimeter-width pegmatite vein only exhibits one episode of zircon growth from melt at 149±2 Ma and 660-860℃. Its host gneiss exhibits a residual melt texture but no significant growth of zircon, indicating low degree of dehydration melting and quickly cooling of the small volume melt, which do not evoluted to the aqueous fluid stage. In summary, the zircon domains in the host gneisses record their growth during a transition from aqueous fluid to hydrous melt along a temperature-increasing path, whereas the zircon domains in the pegmatite veins record their growth during a transition from hydrous melt to aqueous fluid along a temperature-decreasing path. Despite the difference in the direction of fluid/melt evolution, the all zircon domains in these gneisses and pegmatite veins record the same event of crustal anatexis in the UHP gneisses. The present study also provides insights into a genetic definition of anatectic melt and thus anatectic zircon. |
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