Contrasted metamorphic evolutions in thrusted cover units of the Briançonnais zone (French Alps): A model for the conservation of HP-LT metamorphic mineral assemblages
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Keywords:
Lawsonite
Greenschist
Blueschist
Pyrophyllite
Mappable units (dominantly quartz-mica-lawsonite schist and phyllite, dominantly metabasalt, and dominantly graywacke and argillite), tentatively assigned to the Franciscan Formation of late Mesozoic age, have been metamorphosed in the glaucophane schist facies. Comparison of chemical analyses of rocks with glaucophane schist facies assemblages to their unmetamorphosed equivalents indicates essentially isochemical metamorphism. Distribution of lawsonite and epidote appears to be controlled by the Fe'''/Al ratio. Close association of greenschist facies mineral assemblages with glaucophane schist facies assemblages suggests the facies are transitional, and regional occurrence of lawsonite-aragonite-albite in contrast to different mineral assemblages of the glaucophane schist facies in other areas suggests that the facies may eventually be subdivided.
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Actinolite
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Abstract The Lavrion area is part of the Attic-Cycladic massif. Blue amphibole analyses revealed that they are glaucophane or ferroglaucophane. Ca-amphiboles are characterized as actinolite or actinolitic hornblende. The Ps component of epidotes from the glaucophane-bearing rocks varies from 25.42–30.89%, whereas the Ps component of epidotes from the greenschist assemblages ranges from 23.81–26.88%. Chlorites show narrow compositional variations of ferromagnesian ratios ( X mg =0.48–0.53). Albites are almost pure Ab 100 . The Kfeldspar present has a low Ab content. The evolution of the prasinites studied is characterized by the progressive transformation of eclogite facies (?) rocks through epidote blueschists into greenschists. A P-T path for the prasinites is presented indicating epidote blueschist facies at P-T conditions of around 7.0–7.5 kbar and 300–340°C. Pressures of 4.0–4.5 kbar and temperatures around 340–360°C are estimated for the subsequent overprint in the greenschist facies. The path EBS to GS conditions followed under nearly isothermal uplift.
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Blueschist
Lawsonite
Actinolite
Greenschist
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Abstract The formation of late‐stage veins can yield valuable information about the movement and composition of fluids during uplift and exhumation of high‐pressure terranes. Albite veins are especially suited to this purpose because they are ubiquitously associated with the greenschist facies overprint in high‐pressure rocks. Albite veins in retrogressed metabasic rocks from high‐pressure ophiolitic units of Alpine Corsica (France) are nearly monomineralic, and have distinct alteration haloes composed of actinolite + epidote + chlorite + albite. Estimated P – T conditions of albite vein formation are 478 ± 31 °C and 0.37 ± 0.14 GPa. The P – T estimates and petrographic constraints indicate that the albite veins formed after the regional greenschist facies retrogression, in response to continued decompression and exhumation of the terrane. Stable isotope geochemistry of the albite veins, their associated alteration haloes and unaltered hostrocks indicates that the vein‐forming fluid was derived from the ophiolite units and probably from the metabasalts within each ophiolite slice. That the vein‐forming fluid was locally derived means that a viable source of fluid to form the veins was retained in the rocks during high‐pressure metamorphism, indicating that the rocks did not completely dehydrate. This conclusion is supported by the observation of abundant lawsonite at the highest metamorphic grades. Fluids were liberated during retrogression via decompression dehydration reactions such as those that break down hydrous high‐pressure minerals like lawsonite. Albite precipitation into veins is sensitive to the solubility and speciation of Al, which is more pressure sensitive than other factors which might influence albite vein formation such as silica saturation or Na:K fluid ratios. Hydraulic fracturing in response to fluid generation during decompression was probably the main mechanism of vein formation. The associated pressure decrease with fracturing and fluid decompression may also have been sufficient to change the solubility of Al and drive albite precipitation in fracture systems.
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The Cycladic Massif is a typical example of an Alpine orogenic belt, showing indications of a collision circle which records episodes of convergence, subduction and exhumation. The study area of this thesis is the southernmost part of Evia Island (Karystos), which occupies the NW part of the periphery of the Cycladic Massif, and is characterized by the occurrence of well-preserved blueschists. The study area consists of two main tectonic units: a) the Almyropotamos Unit (AU), which represents the Basal unit of the Cycladic Massif and b) the Cycladic Blueschists Unit (CBU), which can be internally divided into two major nappes, known as the Ochi and the Styra nappes which correspond to the metaophiolite bearing sequence and the marble-schist sequence of Cyclades, respectively. The CBU records two metamorphic episodes: 1) the HP epidote-blueschist M1 episode(P > 11 kbar and T ≈ 450°C) and 2) the re-equilibration, Barrow type, pumpellyite–actinolite to greenschist facies M2 episode(P = 4–7 kbar and T = 300–350°C). The purpose of this thesis is to contribute to the decoding of the complex tectonometamorphic evolution of the region, from a mainly petrological aspect. This included microscopic observation of approximately 150 thin sections, and later use of other methods (XRD, ICP, SEM/EDS, geobarothermometry) in selected samples. The results were combined in order to obtain information about the composition of the protoliths and the paleotectonic environment in which they formed, and also to determine P-T conditions and compositional changes in mineral phases during metamorphism.
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Greenschist
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CYCLADES
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Blueschist
Greenschist
Phengite
Lawsonite
Glaucophane
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Amphibole
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Greenschist
Blueschist
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Lawsonite
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The glaucophane-bearing Raspberry Schist occurs as discontinuous, fault-bounded slivers along the northwest side of the Kodiak Islands in southern Alaska. The lithologies, predominantly metabasites and quartzites, experienced minor pre-metamorphic stratal disruption, extensive syntectonic deformation, and pervasive post-metamorphic disruption by faults. A major change in metamorphic assemblages and degree of recrystallization occurs across a fault zone, dividing the Raspberry Schist into units Js1 to the southeast and Js2 to the northwest. Js1 is characterized by relict igneous textures and phases and contains assemblages compatible with prehnite-pumpellyite, pumpellyite-actinolite, lawsonite-albite-chlorite, and lower greenschist transitional to blueschist facies. Rocks composing Js2 have been completely, dynamically recrystallized and contain assemblages described as transitional blueschist facies or high-temperature blueschist facies.
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