Rb–Sr ages from phengite inclusions in garnets from high pressure rocks of the Swiss Western Alps
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Phengite
Greenschist
Geochronology
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Abstract The Seve Nappe Complex ( SNC ) of the Scandinavian Caledonides records a well‐documented history of high pressure ( HP ) and ultra‐high pressure ( UHP ) metamorphism. Eclogites of the SNC occur in two areas in Sweden, namely Jämtland and Norrbotten. The Jämtland eclogites and associated rocks are well‐studied and provide evidence for late Ordovician UHP metamorphism, whereas the Norrbotten eclogites, formed during the late Cambrian (Furongian)/Early Ordovician, have not been studied in such detail, especially in terms of the P–T conditions of their formation. Within the studied eclogite, clinopyroxene contains a high‐Na core and two rims: inner, medium‐Na and outer, low‐Na. Garnet consists of a high‐Ca euhedral core, low‐Ca inner rim and medium‐Ca outer rim. A similar pattern occurs within phengite, where high‐Si cores are enveloped by medium and low‐Si rims. The compositions of the mineral cores, inner rims and outer rims reflect three stages in the metamorphic evolution of the eclogite. Applied Quartz‐in‐Garnet geobarometry, coupled with Zr‐in‐rutile geothermometry reveal that garnet nucleation (E0 stage) took place at 1.5–1.6 GP a and 620–660°C. The eclogite peak‐pressure assemblage developed during the E1 stage, it consists of garnet+omphacite+phengite+rutile+coesite? and yields P–T conditions of 2.8–3.1 GP a and 660–780°C as constrained by conventional geothermobarometry and thermodynamic modelling in the NCKFMM n ASHT system. Later, lower‐pressure stages E2 and E3 record conditions of 2.2–2.8 GP a, 680–780°C and 2.1 GP a, 735°C, respectively. The prograde metamorphic evolution of the eclogite is inferred from inclusions of epidote, amphibole and clinopyroxene within garnet. The presence of amphibole–quartz–plagioclase symplectites, secondary epidote/zoisite and titanite replacing rutile record the later retrograde changes taking place at <1.5 GP a (referred as E4 stage). The obtained P–T conditions indicate that the Norrbotten eclogites underwent a metamorphic evolution characterized by a clockwise P–T path with peak metamorphism reaching up to coesite stability field within a relatively cold subduction regime (7.8°C/km). The obtained results provide the first evidence for UHP metamorphism in the SNC above the Arctic Circle and document cold subduction regime and multistage exhumation of the deeply subducted Baltican margin at early stage of the Caledonian Orogeny.
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Coesite
Omphacite
Phengite
Geothermobarometry
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Coesite
Omphacite
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Amphibole
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Titanite
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P-T conditions of the Alpine regional metamorphism of carbonate successions which form the cover sequence of the southern Veporicum crystalline basement have been determined. The P-T estimates were calculated on the basis of microprobe chemical analyses of equilibrium mineral assemblages together with analyses of bulk rock chemical composition. The calibrated geothermic reactions Cal-Dol and Kfs-Phl were used. Geothermometric and geobarometric data were obtained using the SOLVCALC, TWQ and PERPLEX software programs in combination with phengite geobarometry. The metacarbonates were formed from a sedimentary-carbonate protolith in low-pressure and low-temperature conditions of greenschist facies in the kyanite stability field: TCal = 354-476°C, TAb-Or = 329-453°C, P = 0.3-0.5 GPa, during Cretaceous orogeny
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Kyanite-phengite eclogites can yield robust P-T equilibration conditions that do not depend on the ferrous/ferric ratios of the constituent minerals. Even rocks lacking kyanite or phengite—for which the ferrous/ferric ratios are calculated stoichiometrically—can yield equilibration conditions that differ by < 0.4 GPa. The range of robust P-T conditions for kyanite-phengite UHP eclogites is more restricted than that reported for eclogites generally, with most in the range 600-800°C and < 4 GPa; none fall in the "forbidden zone" defined by geothermal gradients of ≤ 5 K/km. Most UHP eclogites equilibrated at temperatures below the H2O-saturated solidus for crustal rocks. This is probably because hotter UHP terranes are sufficiently low in viscosity that they mix with their surroundings and lose their identity or re-equilibrate continuously during cooling until the solidus is reached. Lawsonite and chloritoid are not known from UHP eclogites.
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Coesite provides direct evidence for ultrahigh pressure metamorphism. Although coesite has been found as inclusions in zircon in paragneiss of the north Qaidam Mountains, it has never been identified in eclogite. In this contribution, based on petrographic observations and in situ Raman microprobe spectroscopy, coesite was identified as inclusions in garnet of eclogite from the Aercituoshan, Dulan UHP metamorphic unit, north Qaidam Mountains. Coesite is partly replaced by quartz, showing a pali-sade texture. This is the first report on coesite in eclogite from the north Qaidam Mountains, and is also supported by garnet-omphacite-phengite geothermobarometry (2.7―3.25 GPa, 670―730℃). Coesite and its pseudomorphs have not been found in eclogites and associated rocks of other units of the north Qaidam Mountains. Further studies are required to confirm if all metamorphic units in the north Qaidam Mountains underwent the ultrahigh-pressure metamorphism.
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Coesite
Omphacite
Phengite
Glaucophane
Amphibole
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Blueschist
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Abstract The Western Gneiss Region (WGR) of Norway consists largely of Proterozoic orthogneisses, but also contains paragneisses, peridotites, anorthosites, gabbros and coarse-grained intermediate-acid ‘rapakivi granites’. All of these lithologies enclose eclogites. Structural and isotopic data suggest that many of these rocks were juxtaposed by early Caledonian thrusting prior to eclogite formation at ca. 425 Ma. Low- P protoliths can be demonstrated for many eclogites. Prograde metamorphism to eclogite facies is demonstrated by inclusion suites within garnet grains and zoning of eclogite minerals. The regional distribution of K D (gnt/cpx) and X cpx jd shows a decrease in T max , and in the corresponding P , away from the present coastline. The lowest values (500 °C, 10 kbar) are found in the Sunnfjord area and the highest (∼ 800 °C) along the coast of Sunnmøre and Nordmøre. Maximum pressures were reached at temperatures 100–200° < T max . This P-T-t path is consistent with the preservation of jadeite-rich cpx (and possible coesite) in the coastal regions. Earlier overestimates of P max , based on partitioning of Al between opx and gnt, resulted from combination of early low- T (low-Al) opx and T values derived from cpx/gnt equilibrated at T max . Despite pervasive later amphibolitization, high- P assemblages (phengite + kyanite, omphacite + quartz) are locally preserved within gneisses near the coast. The high- P metamorphism can be explained by westward subduction of the Baltic continental plate beneath the Greenland plate, during the Caledonian orogeny. At least some of the Mg-Cr garnet peridotites of the WGR were derived from low- P protoliths (spinel ± chlorite peridotites, enclosing high-Al pyroxenites). While Sm-Nd mineral ages of most eclogites cluster around 425 Ma, garnet peridotites and their enclosed garnet pyroxenites give Proterozoic Sm-Nd mineral ages (1700–1000 Ma). The tectonic position of the Mg-Cr garnet peridotites, relative to the Caledonian high-pressure metamorphism, remains to be resolved.
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