| Recently, with the deepening of the ultrahigh pressure (UHP) metamorphism, a series of various HP-UHP rocks were recognized in the South Altyn Tagh, western China, and were determined to be the productions of the deep continental subduction. According to the field geological observation, the HP-UHP rock associations of the South Altyn Tagh and the North Qaidam were ascertained. Previous studies reported the UHP rocks were exposed in the Jianggalesayi and Yinggelisayi area, whereas the HP granulite was only discovered in the Danshuiquan area. In present study, HP felsic granulite and Grt pyroxenite are also recognized in the Danshuiquan area, and HP pelitic granulite is found in Munabulake area. Thus, HP-UHP rocks are intermittent exposed in South Altyn Tagh subduction-collision complex, and constitute a HP-UHP metamorphic belt. Combined with the petrological and experimental data, we obtained the peak metamorphic ages and the metamorphic evolution of the HP-UHP rocks, which could provide key constraints on discussing the evolution and the dynamic mechanics of the South Altyn continental subduction and exhumation; Moreover, a combined study of the petrology, geochemistry and geochronology of the felsic veins from the South Altyn Tagh and the North Qaidam Shenglikou area is performed. The study results provide constraints on the ages of fluid flow, as well as on the origin of the fluid for the vein, which may play a key in deciphering the fluid processes in subduction zones.In this paper, the main achievements are as follows:1. The carbonated inclusions and the long column polycrystal quartz aggregations after Sht pseudomorph in Grt and Omp from Jianggalesayi UHP eclogite, and the residual K-cymrite in Grt from Yinggelisayi Grt-bearing granitic gneiss provide direct evidences for the South Altyn HP-UHP belt undergoing UHP metamorphism.(1) Abundant inclusions of carbonations and long column polycrystal quartz aggregations are recognized in garnet and omphacite in the eclogite from the Jianggalesayi area by detailed petrography observations. These polycrystal quartz aggregations should be the stishovite pseudomorph based on the crystal structure analysis and the shape comparison between coesite and stishovite from synthesis experiment and nature. According to the micro-textures of the inclusions and the polycrystalline quartz aggregations, the peak mineral assemblage of the eclogite is Grt+Omp+Mgs+Arg±Sht, which indicates the peak pressure of the UHP eclogite is8-10GPa at least.(2) Various types of polycrystalline inclusions, quartz (Qz), K-feldspar (Kfs), Kfs+Qz and perthite (Pe), are identified in garnet from the granitic gneiss in the Yinggelisayi area. Raman spectroscopy shows some Kfs inclusions displaying weak peaks of cymrite and K-cymrite at～390-393cm-1except the characteristic Kfs peak (513cm-1,475cm-1,282cm-1). These characteristics indicate the Kfs inclusions are retrograded from pre-existing high-pressure potassic phases-K-cymrite, which provides direct evidence that the Grt-bearing granitic gneiss has been subjected to UHP metamorphism; The polycrystalline Qz, Kfs and Kfs+Qz inclusions might be the pseudomorphs after coesite and K-cymrite.2. In this study, we ascertain the metamorphic evolution of the HP-UHP rocks from the South Altyn Tagh and the Grt pyroxenite from Shenglikou area in the North Qaidam based on detail petrographical observation, mineral analyses and P-T estimation. Combined the chronology data and the mineral inclusions in zircons, the protolith of the HP-UHP rocks are mainly in the Neo-proterozoic period, and the peak metamorphic ages are of～500Ma, and～450Ma for the granulite-facies metamorphic stage of the UHP eclogite and politic gneiss from Jianggalesayi area; obtained the age of peak eclogite-facies metamorphism of440Ma, and the age of granulite-facies retrograded metamorphism of420Ma. And consequently, a integrated P-T-t path for the HP-UHP rocks.(1) According to petrography observation and zircon U-Pb dating, the protolith age of the UHP eclogite and pelitic gneiss are confirmed to be at Neo-proterozoic, UHPM age at500Ma, HP granulite-facies retrograded age at450Ma and amphibolite-facies retrograded age is later than450Ma, which constitute a clockwise P-T-t path.(2) Petrography, mineral chemistry, pressure-temperature (P-T) estimates and zircon U-Pb dating suggest the Neoproterozoic protolith (857Ma) of the rock was subducted to a mantle depth of～100km where experienced UHP metamorphismat～500Ma, subsequently exhumed to～60km depth where overprinted HP/UHT granulite facies metamorphism, and finally exhumed to a mid-crustal depth (-25km) where experienced amphibolite facies metamorphism. These data suggest a clockwise P-T (-t) path for the granitic gneiss with at least two stages of exhumation after subduction into mantle depths.(3) LA-ICP-MS in situ dating yields the metamorphic age of486±5Ma and the detrital protolith ages varying from719Ma to1780Ma for the HP pelitic granulite from the Qingshuiquan area. The peak metamorphic mineral assemblage of the rock is Grt+Sill+Kfs+Rt+Qz, and the metamorphic condition is7>850℃and P>11Kbar according to the THERMOCALC3.23. Four stages metamorphic evolution are described by means of P-T pseudosections for selected metapelite specimens, showing a clockwise retrograde metamorphic P-T path with rapid exhumation and fast cooling. LA-ICP-MS in situ dating also yields the metamorphic age of505±5Ma and the protolith ages of866±5Ma for the HP felsic granulite. The peak metamorphic mineral assemblage of the rock is Grt+Sill+Kfs (Per)+Rt+Qz, and the metamorphic condition is7>850℃and P>11Kbar according to the THERMOCALC3.23. Three stages metamorphic evolution are described by means of P-T pseudosections for the rock, showing a clockwise retrograde metamorphic P-T path. We also obtained a integrated clockwise P-T path for the Grt pyroxenite, showing that the rock early experienced a prograde metamorphism with increased pressure and temperature, and subjected to peak (eclogite-facies) metamorphism, subsequently experienced HP granulite-facies metamorphism with increasing temperature and decreasing pressure, and at last amphibolite-facies metamorphism with decreasing temperature and pressure. According to the U-Pb dating results, the granulites and Grt pyroxenite are the productions of the South Altyn continental deep subduction.(4) LA-ICP MS in situ trace element analyses and U-Pb dating yielded the toplimit protolith age of579Ma and the peak metamorphic age of486±5Ma for the HP pelitic granulite from the Munabulake area. The peak mineral assemblage of the rock is Grt+Ky+Kfs+Qz+Ilm, and the peak P-T condition is estimated to be T>850℃'P>11kbar according to the THERMOCALC3.33. Three stages metamorphic evolution are described by means of P-T pseudosections for the selected rock, showing a clockwise retrograde metamorphic P-T path with early decompression and late isobaric cooling. The granulite has similar metamorphic evolution and peak metamorphic age with the HP granulite from Danshuiquan area, and has identical metamorphic age with the ultra high pressure (UHP) rock from other areas in the South Altyn, indicating the granulite in this study was a production of the South Altyn continental deep subduction. Moreover, we also obtained the protolith age of <579Ma for the granulite,845Ma for the quartz shist, and<647Ma for the Mica quartz schist, belong to the late Neoproterozoic period, which is identical to those of the HP-UHP rocks from the South Altyn. Therefore, the three rocks should be a part of the South Altyn Tagh HP-UHP metamorphic belt rather than a lithostratigraphic unite belonging to "Changcheng System".(5) The petrography and mineral chemistry imply that the retrograded eclogite from the Shenglikou area in the North Qaidam may have experienced multistage metamorphism. The residual omphacite inclusions identified in the garnet imply that the retrograded eclogite was once eclogite with the assemblage Grt+Omp+Rt+Qz. The Cpx+Pl1symplectite or sieved intergrowth and the Amp+Pl2symplectite outside the Cpx+Pl1intergrowth, indicate that the rock was subsequently subjected to two steps of retrograded metamorphism after a peak eclogite-facies stage, an HP granulite-facies with Grt+Cpx+Pl1+Qz+Ilm, and an amphibolite-facies with the assemblage of Grt+Amp+Pl2+Qz±Ilm. The internal microstructure, mineral inclusions and geochronology data of the zircons from the rock show that the protolith age of the rock is909±6Ma, and the peak and the granulite-facies retrograded metamorphism took place at440and420Ma, respectively, which are identical to the peak (420-461Ma) and retrograded ages (400-425Ma) for the other HP-UHP rocks found in North Qaidam, suggesting that the Neo-proterozoic protolith of the rock experienced continental subduction and subsequently subjected to two stages of exhumations.3. By means of the studies of the felsic vein from the Jianggalesayi area in the South Altyn Tagh and the Shenglikou area in the North Qaidam, the formation age of the veins are determined, and the characteristic and the source of the fluid for veining are well confirmed.(1) The zircon U-Pb dating obtained four groups of ages for the Grt biotite gneiss from Jianggalesayi area:>586Ma、503±5Ma、459±4Ma'415±3Ma. The586Ma is interpreted as the top limit age of the protolith,503±5Ma and459±4Ma are identical to the peak UHP and granulite-facies retrograded metamorphic ages, respectively,415±3Ma should represent the age of the anatexis of the rock. The zircon cores of the felsic vein in the gneiss have consistant ages of604-1702Ma with the cores of the host gneiss, and the ages of the rims are also consistant with the rim III of the host gneiss. Petrographical and geochronological characteristics indicate that the felsic vein might be formed by the partial melting of the host gneiss at～415Ma.(2) Felsic veins were found in the garnet (Grt) pyroxenite from the Shenglikou area, located in North Qaidam. The rock near the vein is strongly amphibolized into the Grt amphibolite. Geochronological studies imply that the formation age of the felsic vein in the retrograded eclogite is yielded at422±2Ma (2a), which is identical to the granulite-facies retrograded age (420Ma) of the host retrograded eclogite. The fact that the Hf isotope compositions between the granulite-facies (type I) zircon rims from the host retrograded eclogite and the zircon from the felsic vein are identical suggests that the fluid for veining is either locally sourced or internally buffered. The felsic veins have high contents of SiO2, Al2O3, Na2O, CaO and Sr, indicating that there are significant amounts of Na, Si, Ca, Al and Sr in the vein-forming fluid. Therefore, the dehydration of Omp is interpreted to be the dominated mechanism for releasing the fluid leaving the Cpx intergrowing with Pl1as the residual phase in the retrograded eclogite. In addition, the strong amphibolization of the Grt amphibolite near the felsic vein, as well as the compositional variation of the entire rock and amphiboles between the retrograded eclogite and Grt amphibolite, the coarse-grained titanite occurring in the Grt amphibolite, the Kfs micro-veins in the Grt amphibolite and felsic vein, and the presence of biotite and muscovite in the vein, all indicate that a low flux of external pelite-derived fluid with high K, LREE, LILE and silica contents was added and transported along the vein, where it interacted with the host retrograded eclogite. It is possible that this external fluid migration occurred before and continued until after the amphibolite-facies stage. Therefore, it is shown that episodic fluid flow occurred during the exhumation of the retrograded eclogite, and that the primary internal fluid for the felsic veining flowed at the transformation from peak eclogite stage to granulite stage, then low flux external fluid was added before the amphibolite stage, corresponding to the final stage of the fluid flow.According to the above and previous research results of the HP-UHP rocks in the study area, this paper made the following new understandings:(1) Although the UHP rocks and HP granulite rocks from the South Altyn Tagh and the North Qaidam each have identical peak metamorphic ages, the rock associations, the preservation and the peak metamorphic conditions of the HP-UHP rocks are different. And some UHP rocks have different exhumated history after peak stage. Therefore, the contemporary UHP rocks and the HP granulite are coexisted in the same orogenic belt, which might be accounted for by the crustal detachment of subducting continental lithosphere at different depths with different thermal conditions in subduction channels. And the diverse retrograded path might be caused by the difference of the exhumation path.(2) Combining previous zircon U-Pb chronology, the protolith of the HP-UHP rocks in the South Altyn Tagh were mainly formed in Neo-proterozoic period. Consequently, we infer that the South Altyn HP-UHP metamorphic belt should be formed by the subduction of the Neo-proterozoic continental substance from the massif that have genetic relationship with the Yangtze Block at-500Ma which was dragged by the northward subduction of the South Altyn ocean, and then experienced two stages of exhumations at450Ma and<450Ma, respectively.(3) The peak metamorphic age of480-504Ma for the HP-UHP rocks in the South Altyn Tagh are40-60Myr older than the HP-UHP metamorphic ages of420-461Ma for UHP eclogites in North Qaidam. The retrograde metamorphic age of455±2Ma for the Altyn eclogite is30-55Myr older than～400-425Ma for the North Qaidam eclogites. These age differences suggest that the South Altyn and North Qaidam eclogites do not belong to the same HP-UHP metamorphic zone. |
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