Interplay between fluid circulation and Alpine metamorphism in the Monte Rosa whiteschist from white mica and quartz in situ oxygen isotope analysis by SIMS
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Abstract In situ oxygen isotope compositions of white mica and quartz have been used to characterize the interplay of metamorphism and fluid events between a metasomatic whiteschist and its granite protolith in the Monte Rosa nappe, Western Alps. New natural muscovite and phengite reference materials were calibrated for in situ Secondary Ion Mass Spectrometry (SIMS) oxygen isotope measurement. White mica and quartz oxygen isotope compositions were measured in situ in one whiteschist and two metagranites. Based on microtextural observation, phengite composition of white mica, and phase petrology modeling, it is possible to identify two events of fluid infiltration and one event of fluid expulsion, all of which were responsible for forming this unique whiteschist occurrence and for tracing its metamorphic evolution from late Permian intrusion to Alpine subduction and finally to the present day, exhumed whiteschist. Metagranite samples contain three generations of white mica: igneous, high-P metamorphic, and late Alpine, retrograde compositions. In the whiteschist samples, we distinguish two distinct Alpine white mica generations: (1) prograde to peak generation and (2) retrograde generation. The δ18OVSMOW values of white mica and quartz from a whiteschist of 5.3 to 7.3‰ and 9.1 to 10.6‰ are significantly lower than in the metagranites, with 9.1 to 10.8‰ and 13.2 to 14.6‰, respectively. This indicates a complete recrystallization of the whiteschist protolith during intense fluid-rock interaction. Subsequent Alpine metamorphism transformed the protolith into the whiteschist. The isotopic composition of the whiteschist, fine-grained, retrograde white mica (5.3 to 6‰) is lower than that of the high-pressure phengite (6.2 and 7.5‰). The low δ18O values could be explained by infiltration of external fluids with δ18O values of 2 to 6‰. Such fluids would carry the isotopic signature of the serpentinites of the Piemonte-Liguria Ocean by either equilibration of fluids with or dehydration of serpentinites. Another, more simple explanation would be the infiltration of very small quantities of fluids leading to the breakdown of chloritoid. Local inheritance of the oxygen composition would then hide the origin of the fluids. Isotope exchange temperatures calculated from high-P phengite-quartz pairs in whiteschist give an average temperature of 440 ± 50 °C. These are lower than the best T-estimates from phase petrology of 570 °C, at 2.2 GPa. Igneous muscovite-quartz pairs in the metagranite yield 400 ± 40 °C. Only one high-P phengite-quartz pair was analyzed, resulting in 350 ± 40 °C. Greenschist facies, low silica phengites give an average temperature of 310 ± 10 °C. Propagation of analytical uncertainty suggests large errors of 60 to 120 °C, due to the relatively small T-dependence of the quartz-white mica fractionation factor for oxygen isotopes.Keywords:
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Ultrahigh-pressure (UHP) metamorphic rocks from the Qinglongshan region of the Sulu orogen are comprehensively studied for their whole-rock geochemistry, mineral O isotopes and zirconology. The metamorphic minerals, which experienced eclogite- to amphibolite-facies metamorphism, exhibit low to negative δ^18^O values, suggesting that the ^18^O-depletion of UHP rocks was acquired from their igneous protolith due to high-T meteoric-hydrothermal alteration during the Neoproterozoic. The O isotope heterogeneity in the protolith was not homogenized during the Triassic UHP metamorphism, indicating very limited fluid flow during orogenesis. However, the fluid flow is locally significant during exhumation of the UHP rocks, resulting in the formation of quartz veins, symplectites and coronas. Geochemical transport due to fluid action is evident in whole-rock geochemistry and mineralogical composition. The UHP rocks exhibit unreasonably low ^87^Sr/^86^Sr ratios at t~1~ = 750 Ma but much radiogenic Sr isotopes at t~2~ = 230 Ma, suggesting the mobility of water-soluble elements due to hydrothermal alteration during protolith emplacement and metamorphic dehydration during continental collision. Fluid-rock interaction during exhumation would also have mobilized Al, Si, Ca and LREE, resulting in the formation of high-pressure veins in the UHP eclogites. The protolith zircon of magmatic origin underwent different types of metamorphic recrystallization in response to fluid-mineral interaction, leading to differential redistribution of trace elements and O-Hf isotopes. Newly grown zircons of metamorphic origin exhibit negative δ^18^O values, indicating precipitation from negative δ^18^O fluids that were likely generated by metamorphic dehydration of the hydrothermally altered negative δ^18^O rock-forming minerals during the Triassic. The metamorphic zircons exhibit relatively homogeneous Hf isotope compositions, suggesting that fluid Hf isotopes originated from the same Hf isotope composition of the protolith. Relict zircon domains of magmatic origin exhibit both positive ε~Hf~(t) and negative ε~Hf~(t) values, indicating that the protolith of UHP rocks formed by reworking of both juvenile and ancient crustal rocks.
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Extremely 18O-depleted zircons from granitic gneisses, with δ 18O values as low as -7.8‰, were found in the Zaobuzhen area in the Weihai region, at the northeastern end of the Sulu orogen. SHRIMP zircon U-Pb ages and the oxygen isotope compositions were determined for the low δ 18O zircons. CL images reveal that the low δ 18O zircons are primarily of igneous origin, but some grains underwent metamorphic recrystallization. The igneous zircons from a granitic gneiss sample yield a concordant U-Pb age of (760±49)Ma and an upper intercepted age of (751±27)Ma, indicating a protolith of Middle Neoproterozoic age. Metamorphic zircons from the same sample yield a concordant U-Pb age of (232±4)Ma and a lower intercepted age of (241±33)Ma, pointing to Triassic UHP metamorphism. Most of the igneous zircons have unusually variable δ 18O values of -7.76‰ to 5.40‰, indicating that the gneiss protolith was intruded as low δ 18O magma that was generated by the partial melting of altered rocks, which suffered intensive water-rock interaction with a low δ 18O fluid at high temperatures during the Neoproterozoic. The preservation of extreme 18O-depletion in the zircons suggests that there is no remarkable oxygen isotope exchange between the metagranite and the mantle during the processes of Triassic subduction and exhumation. The protolith nature, metamorphic timing and oxygen isotope compositions of the granitic gneisses in the Weihai region are similar to those of granitic gneisses in the Qinglongshan area in the southwestern part of the Sulu orogenic belt, indicating that the gneisses along the Sulu orogenic belt share the same nature of protolith origin, water-rock interaction and UHP metamorphism. The present study provides tight constraints not only on the origin of extremely 18O-depleted zircons, but also on the protolith nature of granitic gneisses in the Sulu orogenic belt.
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Abstract The presence of ternary feldspar in high‐grade meta‐igneous rocks, and the recognition of the thermometric significance of this mineral, has led recent researchers to postulate peak metamorphic temperatures in excess of 1000 °C. However, it needs to be established that such ternary feldspar is not in fact a survivor of the original high‐temperature crystallization of the igneous protolith. After exsolution, the host and lamellae in the ternary feldspar grains may be stable throughout subsequent history as long as recrystallization does not occur. Such a history may involve rehydration and metamorphism, including H 2 O‐saturated conditions, with the compositions and proportions of the host and lamellae being modified to reflect the P – T conditions experienced. In the case of the high‐grade meta‐igneous rocks from the Moldanubian of the Bohemian Massif, some samples that contain ternary feldspar preserve a substantial measure of their igneous heritage. Orthopyroxene‐bearing granulites not only include types that are barely affected by the metamorphism, but also others that have undergone hydration of the igneous protolith prior to the development of a metamorphic overprint. A key to establishing the igneous origin of the ternary feldspar grains is their preservation in garnet that is either itself igneous, or of a relatively low‐temperature metamorphic origin. Applying the logic to the other ternary feldspar‐bearing meta‐igneous rocks deprives the Moldanubian of its ultrahigh temperature (UHT) metamorphic status.
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