The Study Of Garnet And Zircon In HP To UHP Eclogites From The Hong’an Orogen:Implications For Subduction-zone Procsses

Continental collision orogeny involves a series of tectonic processes from oceanic subduction to continental collision. Many studies have been focused on arc volcanics and ultrahigh pressure (UHP) metamorphic rocks, respectively, the typical products of the oceanic-and continental subduction. However, less attention has been paid to the transition from oceanic subduction to continental collosion. The Qinling-Tongbai-Hong’an-Dabie-Sulu orogen is one of the largest UHP metamorphic belts in continental subduction zones in the world, making it a nature laboratory to investigate the tectonic evolution of continental orogen. The Qinling-Tongbai orogens in the west have been proved to be derived from subduction of Paletethyan oceanic crust and subsequently arc-continental collision in the Paleozoic, whereas the Dabie-Sulu orogens in the east are due to subduction of the South China Block beneath the North China Block in the Mesozoic. The Hong’an orogen lie between the two types of collisional orogens, providing temporal and spatial constraints on the transition from oceanic subduction to continental collision for this composite orogenic belt.This dissertation focuses on high-pressure (HP) and UHP metamorphic rocks from the Hong’an orogen. Through a series of mineralogical and geochemical studyes, three aspects of achievement are acquired as follows.1. Garnet zoning in HP eclogites from the Huwan shear zoneThere are two occurrences of eclogites (massive and foliated) on the same outcrop at Xiongdian in the Huwan shear zone in the northwestern margin of the Hong’an orogen. Whole-rock major and trace elements, element profiles for some rock-forming minerals including garnet, omphacite and epidote as well as mineral inclusions therein were analyzed for the two occurrences of eclogites. The results show that most garnet grains in both eclogites have evident core-rim zonings on the element and inclusion distributions. However, the zoning patterns in the two occurrences of eclogites are different from each other.In the massive eclogite, garnets exhibit increased MgO but decreased CaO contents from core to rim. In addition, garnet cores contain abundant mineral inclusions such as quartz, chlorite, amphibole, rutile and phengite, whereas the rims only contain a few mineral inclusions such as quartz, rutile and titanite. In addition, garnet rims exhibit depleted middle rare earth element (MREE) and steeper MREE/HREE patterns relative to the cores, with chondrite-normalized (Yb/Tb)N values increasing from0.95-2.34in the cores to3.32-18.6in the rims, respectively.In the foliated eclogite, garnet grains dispaly increased MgO but decreased CaO and REE contents from core to rim, which is in inverse with the zonings in the massive eclogite. The inclusions in garnet cores are abundance and mainly composed of quartz and carbonates, which are less in garnet rims. In addition, garnet rims show much higher HREE and equivalent MREE contents, resulting in the chondrite-normalized (Yb/Tb)N values of4.83-82.4for garnet rims that are obviously higher than those1.88-7.05for garnet cores.Together with pseudosection calculations using the whole-rock compositions of the two eclogites, it is suggested that hydrous minerals such as chlorite, epidote and amphibole were probably the major reactants for the growth of garnet cores during prograde subduction, whereas epidote breakdown and/or the dissolution of previously formed garnet were involved in the growth of garnet rims. It is concluded that the differences in the protolith compositions of eclogites potentially dictate the differences in garnet zoning patterns and mineral assemblages of matrix and inclusions.2. Garnet zoning in eclogite from the Xinxian UHP zoneGarnet major-trace elements and zircon U-Pb ages were measured on thin sections of eclogites from the Xinxian UHP metamorphic zone. Major elements show homogeneous distribution across garnet grains, except weak or no zonation of Ca. However, their REE distributions display two groups of patterns. Group I garnets are characterized by steep patterns from LREE to HREE with (Yb/Tb)N values of3.33-33.5, whereas Group Ⅱ garnets show enrichment of MREE relative to LREE and HREE, resulting in flat MREE-HREE patterns with (Yb/Tb)N values of0.42-1.74. These two groups of garnets also have different assemblages of mineral inclusions. Group I garnets always contain abundant inclusions of amphibole+rutile+epidote+quartz+zircon, whereas Group Ⅱgarnets occasionally contain inclusions of omphacite±rutile±apatite±zircon. According to the chemical compositions of the inclusion and matrix minerals, it is suggested that the decomposition of epidote contributed for to the enrichment of MREE in the Group Ⅱ garnets. The LA-ICPMS U-Pb dating and trace element analysis of zircon provide constraints on the growth time of the two groups of garnets. Three types of zircons were identified based on their mineragraphy, REE distributions and U-Pb ages. Type I zircons are characterized by relatively high Th and U contents, high Th/U ratios, pre-Triassic206pb/238U ages, high REE contents, significantly negative Eu anomalies and steep MREE-HREE patterns. These characteristics are consistent with those of magmatic zircon that was modified by solid-state recrystallization. Type II zircons share the similar REE patterns to Type I zircons, but have medium Th, U and REE contents, and concordant Triassic U-Pb ages, suggesting dissolution recrystallization of protolith zircons. Type Ⅲ zircons are unzoned or display cloudy zoning, with low Th, U, and REE contents, low Th/U ratios (<0.1), concordant Triassic U-Pb ages, weak or no Eu anomalies, and flat MREE-HREE patterns. This indicates that these zircons were newly grown from aqueous fluids in the stability field of garnet at eclogite-facies, and thus could be used to constrain the time of garnet growth. Furthermore, the Triassic U-Pb ages obtained from Type Ⅲ zircons can be categorized into two subgroups at～240Ma and～220Ma, respectively. The ages of-220Ma are also obtained from two zircon grains enclosed by Group Ⅱ garnets. Thus the growth time of Group Ⅱ garnets can be constrained at～220Ma, corresponding to the retrograde UHP to HP metamorphic event during the initial exhumation. On the other hand, the ages of～240Ma can be used to contrain the growth time of Group I garnets, corresponding to the prograde HP to UHP eclogite-facies metamorphic event during the late subduction.3. The tectonic evolutions of the Hong’an orogenA combined study of whole-rock major-trace elements and Sr-Nd-Hf isotopes as well as zircon U-Pb ages, trace elements and Lu-Hf isotopes was carried out for the three zones of HP and UHP eclogite-facies metamorphic rocks in the Hong’an orogen. The results provide insights into the tectonic evolution from oceanic subduction to continental collision during the closure of Paleotethyan ocean between the South and North China Blocks. Based on the whole-rock geochemistry, eclogites in the orogen are categorized into continental-type and oceanic-type, respectively. The continental-type eclogites widely occur in the whole orogen, exhibiting general enrichments of LILE and LREE but depletion of HFSE and HREE. Zircon U-Pb dating yields protolith ages of about750to1200Ma, demonstrating a tectonic affinity to the South China Block. They have both positive and negative εNd(t) and εHf(t) values for whole-rock and zircons, suggesting that their protolith were originated from both juvenile and ancient crustal rocks. The oceanic-type eclogites only occur in the northwestern edge of the orogen, exhibiting mid-ocean ridge basalts (MORB)-like flat REE patterns and arc-like trace element patterns. Zircon U-Pb dating on the relict zircon cores of magmatic origin yields protolith ages of-420Ma. They have variable εNd(t) and positive εHf(t) values and slightly high (87Sr/86Sr)i ratios. These geochemical features suggest that their protoliths are equivalent to backarc basin basalts (BABB) in a continental margin.The eclogite-facies metamorphic age and grade are also different for these two types of eclogites. The oceanic-type eclogites were only metamorphosed under HP conditions in the Carboniferous at～315Ma with little overprint by subsequent Triassic metamorphism at～230Ma, whereas the continental-type eclogites were mostly metamorphosed under HP to UHP conditions in the Triassic at～215to240Ma. Thus, the two types of eclogites record the tectonic transition from oceanic subduction to continental collision during the closure of Paleotethyan ocean between the South and North China Blocks. It is possible that the backarc rift basins were developed in the northern margin of the SCB during the Early Paleozoic, and then became a new subduction zone in which both backarc basin basalts and overlying terrigenous sediments were carried to a depth in the Carboniferous for the HP eclogite-facies metamorphism. Afterwards the continental crust would start to subduct northwards, eventually leading to the HP to UHP eclogite-facies metamorphism in the Triassic. Therefore, the Hong’an orogen is a composite one that is tectonically different not only from the Qinling-Tongbai orogens to the west but also from the Dabie-Sulu orogens to the east. The occurrence of both oceanic-type and continental-type eclogites in the Hong’an orogen suggests that the subduction of continental crust would be gravitationally pulled by the subduction of oceanic crust at the same subduction zone and the HP eclogite-facies metamorphic melanges would be exhumed together along the same subduction channel.