Talc-Phengite: a Widespread Assemblage in High-Grade Pelitic Blueschists of the Western Alps

Talc-phengite, an assemblage hitherto believed to be rare, isfound in regional distribution in the Gran Paradiso area, whereit occurs in the characteristic mineral association chloritoid-talc-phengite(Si_3·4–_3·5). Talc contains up to 15 moleper cent minnesotaite, and chloritoid up to 45 mole per centof the magnesium end member. The talc-phengite stability resultsbasically from the disappearance of chlorite + quartz in rockswith low and moderate MgO/FeO ratios through the divariant reactionsfirst recognized here: Fe-Mg-Chlorite+quartz talc + garnet + H₂O and Fe-Mg-chlorite + quartz talc + Chloritoid + H₂O These reactions imply the disappearance of the join biotite-chloritein the presence of quartz and thus open a talc-phengite stabilityfield (±garnet or chloritoid or Mg-chlorite) which extends,with increasing P and T, toward Mg-richer compositions. Whetheror not it reaches the magnesian subsystem in the Gran Paradisoarea cannot be ascertained. However, the sporadic occurrenceof the high-pressure assemblage talc-kyanite-chloritoid 50 to70 km further northeast in the vicinity of the Monte Rosa massifwithin the same lithological unit (Zermatt-Saas Fee zone s.l.)indicates the instability of any chlorite in quartz-bearingrocks, and implies that talc-phengite must also be stable forpurely magnesian compositions in that area. This progressivestabilization of talc-phengite with increasing metamorphic gradesupports Abraham & Schreyer's (1976) hypothesis of a high-pressurefield for this assemblage, and rules out Chernosky's construction(1978) implying a low-pressure field. The following paragenetic sequence is proposed for pelitic compositionswith intermediate Mg/Fe ratios and excess quartz subjected tohigh-pressure metamorphism with maximum temperatures near 400–500°C: chlorite-illite chlorite-phengite chloritoid-talc-phengite.The absence of biotite is a compositional effect due to thehigh degree of phengite substitution in the white mica. ^*Present address: Institut fr Mineralogic, Ruhr-Universitt, Postfach 10 21 48, D-4630 Bochum 1, Federal Republic of Germany.     

Eclogitization of the Monviso ophiolite (W. Alps) and implications on subduction dynamics

To constrain deep (40–100 km) subduction dynamics, extensive P–T data are provided on the eclogitic Monviso ophiolite derived from the subducted Liguro‐Piemontese oceanic lithosphere (which was exhumed, together with associated continental units, before the Alpine collision). The Monviso ophiolite has so far been interpreted either as a fossilized subduction channel, with tectonic blocks detached from the slab at different depths and gathered in a weak serpentinized matrix, or as a more or less continuous portion of oceanic lithosphere. To evaluate potential heterogeneities within and between the various subunits, extensive sampling was undertaken on metasedimentary rocks and Fe–Ti metagabbros. The results indicate that the Monviso ophiolite comprises two main coherent tectonic subunits (the Monviso and Lago Superiore Units) detached during subduction at different depths and later juxtaposed at epidote–blueschist facies during exhumation along the subduction interface. Raman spectroscopy of carbonaceous material suggests (i) a difference in peak temperature of ～50 °C between these two subunits and (ii) a good temperature homogeneity within each subunit. Pseudosections and average P–T estimates using thermocalc in the Lago Superiore Unit suggest for the first time homogeneous HP to UHP conditions (～550 °C, 26–27 kbar). Parageneses, peak conditions and tectonic setting are very similar to those of the Zermatt‐Saas ophiolite, 200 km northwards, thus suggesting a common detachment mechanism for the whole Western Alpine belt.     

Tectono‐metamorphic history recorded in garnet porphyroblasts: insights from thermodynamic modelling and electron backscatter diffraction analysis of inclusion trails

In a Barrovian metamorphic sequence, garnetiferous mica schists document a heterogeneously developed superposition of sub‐orthogonal fabrics and multiple garnet growth episodes. In the variably deformed domains, four types of garnet porphyroblasts have been defined based on inclusion trail patterns. Modelled garnet zoning in the MnNCKFMASHTO system indicates a prograde evolution from 4–4.5 kbar and 490–510 °C to 5–6 kbar and 520–550 °C in the earliest subhorizontal fabric progressing towards 6.5–7.5 kbar and 560–590 °C in the subsequent subvertical foliation. This fabric is heterogeneously deformed into a shallow‐dipping retrograde foliation associated with garnet resorption. In situ electron backscatter diffraction measurements of ilmenite inclusions in individual garnet grains yield precise data on included planar and linear elements. Consistent orientations of internal foliations, lineations and foliation intersection axis sets indicate a superposition of three sub‐orthogonal foliation systems. Weak variations of internal records with increasing intensity of deformation suggest that a moderate buckling stage occurred, but apparent lack of porphyroblast rotation is interpreted as a result of dominant passive flow. Coupling the orientation of internal fabric sets with P–T estimates is used to complement the tectono‐metamorphic evolution of the thickened crust. We demonstrate that garnet porphyroblasts preserve features which reflect large‐scale tectonic processes in orogens.     

How continuous and precise is the record of P–T paths? Insights from combined thermobarometry and thermodynamic modelling into subduction dynamics (Schistes Lustrés,W. Alps)

Pressure–temperature (P–T) paths as complete as possible and with a precision on the km‐scale or less are needed to further improve the knowledge of deformation, re‐equilibration processes and element/fluid transfer, in particular along subduction zones. This contribution attempts to (i) critically evaluate the precision and continuity with which metamorphic P–T histories are retrieved today and (ii) discuss implications for regional‐scale accretionary processes in subduction zones, through application to the Schistes Lustrés complex (Haute Maurienne, W. Alps). P–T estimates are compared and combined using several independent approaches: (i) from minerals assumed to be in textural equilibrium; (ii) from electron microprobe compositional maps; and (iii) from pseudosection modelling predictions. Multi‐equilibrium calculations were performed with tweequ and thermocalc, and pseudosections were built with Perple_X and Theriak/Domino. These P–T estimates were also compared with maximum temperatures (T_max) deduced from the Raman spectroscopy of carbonaceous matter. The different methods used here yield the peak of pressure for the lower structural unit of the Schistes Lustrés at 480 °C and 23 kbar and document the retrograde path for both the Median and Lower Units. The results show that P–T conditions are recorded almost continuously and can be determined with a precision of ±1 kbar and ±30 °C at best. This study underlines the complementarity of the various thermobarometric methods and demonstrates that precision could be increased by improving solid solution models for chlorite. Observed tectonic patterns, major lithological boundaries, pressure–temperature and T_max data suggest that underplating processes and early structural development played a key role in the Schistes Lustrés accretionary complex.     

The juxtaposition of eclogite and mid‐crustal rocks in the Orlica–Śnieżnik Dome,Bohemian Massif

Eclogite, felsic orthogneiss and garnet–staurolite metapelite occur in a 5 km long profile in the area of Mi?dzygórze in the Orlica–?nie?nik dome (Bohemian Massif). Petrographic observations and mineral equilibria modelling, in the context of detailed structural work, are used to document the close juxtaposition of high‐pressure and medium‐pressure rocks. The structural succession in all lithologies shows an early shallow‐dipping fabric, S1, that is folded by upright folds and overprinted by a heterogeneously developed subvertical foliation, S2. Late recumbent folds associated with a weak shallow‐dipping axial‐plane cleavage, S3, occur locally. The S1 fabric in the eclogite is defined by alternation of garnet‐rich (grs = 22–29 mol.%) and omphacite‐rich (jd = 33–36 mol.%) layers with oriented muscovite (Si = 3.26–3.31 p.f.u.) and accessory kyanite, zoisite, rutile and quartz, indicating conditions of ～19–22 kbar and ～700–750 °C. The assemblage in the retrograde S2 fabric is formed by amphibole, plagioclase, biotite and relict rutile surrounded by ilmenite and sphene that is compatible with decompression and cooling from ～9 kbar and ～730 °C to 5–6 kbar and 600–650 °C. The S3 fabric contains in addition domains with albite, chlorite, K‐feldspar and magnetite indicating cooling to greenschist facies conditions. The metapelites are composed of garnet, staurolite, muscovite, biotite, quartz, ilmenite and chlorite. Chemical zoning of garnet cores that contain straight ilmenite and staurolite inclusion trails oriented perpendicular to the external S2 fabric indicates prograde growth, from ～5 kbar and ～520 °C to ～7 kbar and ～610 °C, during the formation of the S1 fabric. Inclusion trails parallel with the S2 fabric at garnet and staurolite rims are interpreted to be a continuation of the prograde path to ～7.5 and ～630 °C in the S2 fabric. Matrix chlorite parallel to the S2 foliation indicates that the subvertical fabric was still active below 550 °C. The axial planar S2 fabrics developed during upright folding are associated with retrogression of the eclogite under amphibolite facies conditions, and with prograde evolution in the metapelites, associated with their juxtaposition. The shared part of the eclogite and metapelite P–T paths during the development of the subvertical fabric reflects their exhumation together.     

The assemblage diaspore+quartz in metamorphic rocks: a petrological, experimental and thermodynamic study

The upper pressure limit of pyrophyllite is given by the equilibria (i) pyrophyllite=diaspore+quartz and (ii) pyrophyllite=diaspore+coesite. High- P experimental investigations carried out to locate equilibrium (i) yield brackets between 497 °C/24.8  kbar and 535 °C/25.1  kbar, and between 500 °C/23  kbar and 540 °C/23  kbar. Equilibrium (ii) was bracketed at 550 °C between 26.0 and 28.3  kbar. In the experimental P–T  range, equilibria (i) and (ii) are metastable with respect to kyanite. A stable P–T  grid is calculated using thermodynamic data derived under consideration of the present experimental results. According to these data, the lower pressure limit of the assemblage diaspore+quartz according to equilibrium (i) range from about 12  kbar/300 °C to 20  kbar/430 °C (in the presence of pure water). The upper stability of diaspore+quartz is limited by the reaction diaspore+quartz=kyanite+H₂O at about 450 °C (nearly independent of pressure) and, to higher pressure, by the quartz=coesite transition. Equilibrium (ii) is metastable over the whole P–T  range.
Natural occurrences600.S of the diaspore–quartz assemblage in metamorphic rocks in Sulawesi, New Caledonia, Amorgos and the Vanoise are characterized by minerals indicative of high- P such as ferro-magnesiocarpholite, glaucophane, sodic pyroxene and lawsonite. The metamorphic P–T  conditions of these rocks are estimated to be in the range 300–400 °C, >8  kbar. These data are compatible with the derived P–T  stability field of the diaspore+quartz assemblage. We conclude that, in metamorphic rocks, diaspore+quartz is, as ferrocarpholite, an indicator for unusual low- T  /very high- P settings.     

Microstructural and metamorphic evolution of a high‐pressure granitic orthogneiss during continental subduction (Orlica–Śnieżnik dome,Bohemian Massif)

A microstructural and metamorphic study of a naturally deformed medium‐ to high‐pressure granitic orthogneiss (Orlica–?nie?nik dome, Bohemian Massif) provides evidence of behaviour of the felsic crust during progressive burial along a subduction‐type apparent thermal gradient (～10 °C km^?1). The granitic orthogneisses develops three distinct microstructural types, as follows: type I – augen orthogneiss, type II – banded orthogneiss and type III – mylonitic orthogneiss, each representing an evolutionary stage of a progressively deformed granite. Type I orthogneiss is composed of partially recrystallized K‐feldspar porphyroclasts surrounded by wide fronts of myrmekite, fully recrystallized quartz aggregates and interconnected monomineralic layers of recrystallized plagioclase. Compositional layering in the type II orthogneiss is defined by plagioclase‐ and K‐feldspar‐rich layers, both of which show an increasing proportion of interstitial minerals, as well as the deformation of recrystallized myrmekite fronts. Type III orthogneiss shows relicts of quartz and K‐feldspar ribbons preserved in a fine‐grained polymineralic matrix. All three types have the same assemblage (quartz + plagioclase + K‐feldspar + muscovite + biotite + garnet + sphene ± ilmenite), but show systematic variations in the composition of muscovite and garnet from types I to III. This is consistent with the equilibration of the three types at different positions along a prograde P?T path ranging from <15 kbar and <700 °C (type I orthogneiss) to 19–20 kbar and >700 °C (types II and III orthogneisses). The deformation types thus do not represent evolutionary stages of a highly partitioned deformation at constant P?T conditions, but reflect progressive formation during the burial of the continental crust. The microstructures of the type I and type II orthogneisses result from the dislocation creep of quartz and K‐feldspar whereas a grain boundary sliding‐dominated diffusion creep regime is the characteristic of the type III orthogneiss. Strain weakening related to the transition from type I to type II microstructures was enhanced by the recrystallization of wide myrmekite fronts, and plagioclase and quartz, and further weakening and strain localization in type III orthogneiss occurred via grain boundary sliding‐enhanced diffusion creep. The potential role of incipient melting in strain localization is discussed.