ALBERT

Notes: In the Alboran domain, two crustal-thickening late-orogenic extension cycles are superposed. The importance of the late Alpine thinning of the Alpujarride-Sebtide crustal section on top of the Beni Bousera peridotites is discussed here in metamorphic petrological terms. The Alpine metamorphism operated first under a HP–LT gradient, and reached the eclogite facies in the Permian–Triassic phyllites, before retrogression under a high geothermal gradient. A contrasting, higher temperature metamorphism characterizes the pre-Permian section, reaching the HP-granulite facies at the bottom of the crustal section. By reference to the western European setting, the granulites relate to the Hercynian orogeny, as supported by the isotopic ages of the enclosed, armoured monazite crystals. Thus, thinning of an overthickened crust might have occurred there during the late Hercynian extension and Tethyan opening, before being reactivated during the late Oligocene.

Notes: Abstract Three reactions limiting the stability field of the di-trioctahedral chlorite cookeite in the presence of quartz, in the system Li2O−Al2O3−SiO2−H2O (LASH) have been reversed in the range 290–480°C, 0.8–14 kbar, using natural material close to the end member composition. Combining our results with known and estimated thermodynamic properties of the other minerals belonging to the LASH system, the enthalpy (-8512200 J/mol) and the entropy (504.8 J/mol*K) of cookeite are calculated by a feasible solution space approach. The knowledge of these values allowed us to draw the first P−T phase diagram involving both the hydrated Li-aluminosilicates cookeite and bikitaite, which is applicable to a large variety of natural rock systems. The low thermal extent of the stability field of cookeite+quartz (260–480°C) makes cookeite a valuable indicator of low temperature conditions within a wide range of pressure (1–14 kbar).

Notes: Abstract Fluid inclusions have been analysed in successive generations of syn-metamorphic segregations within low-grade, high-pressure, low-temperature (HP–LT) metapelites from the Western Alps. Fluid composition was then compared to mass transfer deduced from outcrop-scale retrograde mineral reactions. Two types of quartz segregations (veins) occur in the `Schistes lustrés' unit: early blueschist-facies carpholite-bearing veins (BS) and retrograde greenschist-facies chlorite-bearing veins (GS). Fluid inclusions in both types of segregations are aqueous (no trace of dissolved gases such as CO2, CH4, N2), with significant differences in density and composition (salinity). BS fluids are moderately saline fluids (average 9.1 wt% eq. NaCl) characterized by a chronological trend towards more dilute composition (from 15 down to 0 wt% eq. NaCl), whereas GS fluids have a very constant salinity of ∼3.7 wt% eq. NaCl. Both types of inclusions were continuously reset to lower densities along the retrograde path, until a temperature of ∼300 °C. Mass-balance calculations, together with fluid inclusion data, suggest that GS fluids result from the mixing between two fluid sources: one initial, early metamorphic, moderately saline HP fluid and a second nearly pure water fluid provided by the breakdown of carpholite. Estimates of the amount of water released by carpholite breakdown result in a dilution of the interstitial fluid phase (from 10 to 2.5–4 wt% eq. NaCl) consistent with the actual shift of the fluid composition. Alkali elements required for the formation of the GS chlorite + phengite assemblage after carpholite could be locally provided by HP phengite. This is taken as an indirect evidence that, during the generation of both BS and GS fluids, mixing with externally derived fluids may have been very limited. The location, amount and constant composition of the less saline GS fluids appear to be related to an interconnected porosity at the time of inclusion formation.

Notes: Abstract The Southern Vanoise is localized in the internal part of the Western Alps, in the Briançonnais zone. In Vanoise the following units can be distinguished (Fig. 1): a pre-hercynian basement (micaschists, glaucophanites, basic rocks), a permian cover (micaschists) and a mesozoic-paleocene cover (carbonate rocks). This area has been affected by the alpine metamorphic event characterized here by high and intermediate pressure facies. The rocks paragenesis are often unbalanced. The paleozoic rocks (Table 1) contain mainly: quartz, albite, paragonite, phengite, blue amphibole, chlorite, green biotite, garnet (Table 2). These minerals were analysed by an electron microprobe (Tables 3, 4 and 5). Mineral composition is highly variable: glaucophane is zoned (Table 5), white micas are more or less substituted with phengite (3.2〈Si〈3.5), whereas chlorites display large variation of Fe and Mg content [0.3〈(Fe/ Fe + Mg)〈0.7]. Little phengitic white micas present a paragonitic substitution as high as 12%. There is usually some correlation between the chlorite and the phengite composition. The more phengitic white micas (Si=3,5) are associated with the Al poor chlorites [(Al2O3/FeO + MgO)〈0.53] whereas the Al rich chlorites [(Al2O3/FeO + MgO)〉0.6] are associated with the less substituted white micas (Si=3.2) (Tables 3 and 4). The phengites with a Si content 3.2 occur in rocks where the retromorphic evolution is the most pronounced and penetrative. A metamorphic evolution is characterized by the disappearance of glaucophane which corresponds to the appearance of Al rich chlorite and to the decrease of phengitic substitution. The samples analysis are plotted in the tetraedric diagram: K2O-Al2O3-Na2O, Al2O3-FeO, MgO, on which a special mathematical treatment was applied. This method calculates the location of rocks composition in the four minerals space. This location is internal when the per cent amounts of all four relevant minerals are positive, if any of them is negative, the point is external (Tables 6–9). In Southern Vanoise micaschists, 2 subfacies are successively present (Fig. 3): Subfacies I: glaucophane-chlorite-phengite (Si4+ 3.5)-paragonite. Then subfacies II: chlorite-albite-phengite (Si4+ 3.2)-paragonite. In basic rocks is found essentially: Subfacies III: glaucophane-garnet-phengite-paragonite or IV: glaucophane-garnet-phengite-albite. Then subfacies V: green biotite-chlorite-albite-paragonite. The assemblages I and II proceed through reaction: 2 glaucophane +1 paragonite+2 H2O→4.2 albite + 1 chlorite. The assemblage V appears with reactions: 1.8 glaucophane +2 phengite→0.4 chlorite+2 green biotite + 3.6 albite +0.4 H2O or 2 glaucophane +2 phengite +0.5 garnet+ 6 H2O→2 green biotite +1 chlorite+4 albite These reactions are controlled by hydratation: the composition variation of phengite and associated chlorite during the metamorphic evolution determines the stability of some minerals (particularly the glaucophane in Na2O poor rocks). In same rocks the results of mathematical treatment is not consistent with the data (Tables 2, 6–9). This discrepancy corresponds to a desequilibrium between chlorite and phengite. These results imply a continuous metamorphic evolution between two stages (Fig. 6): a first stage (1) at 8 kb, 350 ° C; a second stage (2) at 2 to 3 kb, 400–450 ° C.

Notes: Abstract A study of hydrothermal vein mineralization in meta-argillites subjected to high-pressure, low-temperature metamorphism reveals that ferromagnesian (e.g., chlorite) and pure aluminosilicate (e.g., pyrophyllite) mineralization can be correlated with regimes of increasing and decreasing temperature, respectively. An experimental study of the transport of silica, aluminum and magnesium in hydrothermal solutions has been undertaken to simulate variations in the physical conditions during metamorphism and the accompanying mass transport in a closed system. Thermodynamic and kinetic analysis of the experimental results indicates that local equilibrium among aqueous and mineral phases controls the distribution and composition of hydrothermal vein mineralization and that vein mineralogy can be used to infer the sense of variation of pressure and temperature during metamorphism.

Notes: Abstract Magnesiocarpholite has been synthesized on its own composition between 15 and 25 kb water pressure and 415°–600° C. Best conditions are 25 kb-550° C, starting from a mixture of oxides and synthetic cordierite. Within the MgO-Al2O3-SiO2-H2O system, possible substitutions appear to be very limited in magnesiocarpholite. Cell-parameters are a=13.706(3), b= 20.075(3), c=5.107(l) Å, space group Ccca. The larger cell, as compared with the most magnesian natural carpholites, is tentatively ascribed to structural disorder. Preliminary stability data confirm the low-temperature character of this mineral which is shown to be a high-pressure equivalent of sudoite+quartz.