A counter-clockwise P–T path for the Voltri Massif eclogites (Ligurian Alps, Italy)

Integrated petrological and structural investigations of eclogites from the eclogite zone of the Voltri Massif (Ligurian Alps) have been used to reconstruct a complete Alpine P–T deformation path from burial by subduction to subsequent exhumation. The early metamorphic evolution of the eclogites has been unravelled by correlating garnet zonation trends with the chemical variations in inclusions found in the different garnet domains. Garnet in massive eclogites displays typical growth zoning, whereas garnet in foliated eclogites shows rim‐ward resorption, likely related to re‐equilibration during retrogressive evolution. Garnet inclusions are distinctly different from core to rim, consisting primarily of Ca‐, Na/Ca‐amphibole, epidote, paragonite and talc in garnet cores and of clinopyroxene ± talc in the outer garnet domains. Quantitative thermobarometry on the inclusion assemblages in the garnet cores defines an initial greenschist‐to‐amphibolite facies metamorphic stage (M1 stage) at c. 450–500 °C and 5–8 kbar. Coexistence of omphacite + talc + katophorite inclusion assemblage in the outer garnet domains indicate c. 550 °C and 20 kbar, conditions which were considered as minimum P–T estimates for the M2 eclogitic stage. The early phase of retrograde reactions is polyphase and equilibrated under epidote–blueschist facies (M3 stage), characterized by the development of composite reaction textures (garnet necklaces and fluid‐assisted Na‐amphibole‐bearing symplectites) produced at the expense of the primary M2 garnet‐clinopyroxene assemblage. The blueschist retrogression is contemporaneous with the development of a penetrative deformation (D3) that resulted in a non‐coaxial fabric, with dominant top‐to‐the‐N sense of shear during rock exhumation. All of that is overprinted by a texturally late amphibolite/greenschist facies assemblages (M4 & M5 stages), which are not associated with a penetrative structural fabric. The combined P–T deformation data are consistent with an overall counter‐clockwise path, from the greenschist/amphibolite, through the eclogite, the blueschist to the greenschist facies. These new results provide insights into the dynamic evolution of the Tertiary oceanic subduction processes leading to the building up of the Alpine orogen and the mechanisms involved in the exhumation of its high‐pressure roots.     

A Low-Variance Mineral Assemblage with Talc and Phengite in an Eclogite from the Saxonian Erzgebirge, Central Europe, and its P-T Evolution

A light-coloured, fine-grained eclogite sample from near thevillage of Hammerunterwiesenthal in the Erzgebirge (NW BohemianMassif) preserves the low-variance mineral assemblage of garnet,omphacite, phengite, talc, amphibole, clinozoisite, quartz,rutile, and accessory phases. Porphyroblasts of amphibole, clinozoisite,and phengite formed during a late stage (III) of metamorphism.Paragonite joined the assemblage late in this stage (IIIb).The chemical zonation of the minerals was carefully studied.Various geothermobarometric methods were applied, especiallyinvolving phengite and talc. The constrained P–T pathfor the eclogite starts at about 480°C and 25 kbar (stageIb), followed by a significant temperature rise (stage II) atslightly increasing pressure. At the peak P–T conditionsof 720°C and 27 kbar, blastesis of amphibole, clinozoisite,and phengite was caused by infiltrating hydrous fluids. Theresulting density reduction may have allowed buoyant upliftof the eclogite. Subsequently, significant cooling occurredat high pressures. Stage IIIb is characterized by P–Tconditions around 520°C and 18 kbar at reduced water activities.This unusual late P–T evolution might explain the freshnessof the eclogite, including the preservation of chemical zonationon the micrometre scale. KEY WORDS: eclogite; Saxonian Erzgebirge; P–T evolution; talc; phengite     

The upper thermal stability of chlorite + quartz: an experimental study in the system MgO-Al2O3-SiO2-H2O

Experiments up to water pressures of 21 kbar have been undertaken to bracket the reactions chlorite + quartz = talc + kyanite + H₂O, chlorite + quartz = talc + cordierite + H₂O, and talc + kyanite + quartz = cordierite ± H₂O by reversed runs in the system MgO-Al₂O₃-SiO₂-H₂O (MASH). These reaction curves intersect at an invariant point (IP1) at PH₂O = 6.4 ± 0.2 kbar and a temperature of 624 ± 4°C. The curve of the chlorite + quartz breakdown to talc + kyanite + H₂O at water pressures above 6.4 kbar shows a negative dP/dT, with the slope decreasing with rising pressure, whereas the slope of the breakdown curve to talc + cordierite + H₂O at water pressures is clearly positive. The composition of the chlorite solid solution reacting with quartz has been estimated to be approximately Mg_4.85Al_1.15[Al_1.15Si_2.85O₁₀](OH)₈ over the entire pressure range investigated. The composition of the talc solid solution forming by the breakdown of chlorite + quartz appears to be Mg_2.94Al_0.06[Al_0.06Si_3.94O₁₀](OH)₂ at PH₂O = 2kbar. With increasing pressure, the Al content of talc decreases, reaching a value of about 0.06 atoms per formula unit at P,H₂O = 21 kbar. As a consequence of the new experimental data, the existing phase topologies of the MASH-system and K₂O-MASH-system have been revised. For example, the invariant point IP1 and the univariant reaction curve kyanite + talc + H₂O = chlorite + cordierite are stable. For this reason, the development of medium- to high-temperature metamorphic rocks compositionally approximating the MASH-system must be reconsidered. The whiteschists from Sar e Sang, Afghanistan, are treated as an example. The application of the present experimental data to metamorphic rocks of more normal composition requires the examination of the influence of further components. This leads to the conclusion that the introduction of Fe²⁺ into magnesian chlorite extends its stability field in the presence of quartz by 10°-15°C in comparison with pure Mg-chlorite.     

Mineral Chemistry and Pressure-Temperature Evolution of Two Contrasting High-pressure-Low-temperature Belts in the Chonos Archipelago, Southern Chile

The Chonos Metamorphic Complex forms part of a belt of low-grademetamorphic rocks in the Chilean Coastal Cordillera that areinterpreted as Palaeozoic–Mesozoic accretionary complexes.It comprises metapsammopelitic schists, metabasites and meta-ironstonesoccurring in two contrasting units. Special attention duringmicroprobe study of key samples was given to the chemical zonationof minerals. Subsequently, conventional geothermobarometry andthat using thermodynamic calculations were applied. The Easternbelt comprises rocks that are metamorphosed to pumpellyite–actinolitefacies conditions and show a low degree of deformation withwell-preserved sedimentary and igneous structures. Maximum P–Tconditions were around 5·5 kbar and 250–280°C.The rocks of the Western belt are characterized by a transitionbetween greenschist and albite–epidote–amphibolitefacies metamorphism and show a penetrative tectonic transpositionfoliation S₂ formed close to the pressure maximum. Maximum P–Tconditions vary around 8–10 kbar and 380–500°Coverstepping the stilpnomelane + phengite stability. High pressuresin this belt are confirmed by regionally distributed phengiteswith high Si contents up to 3·5 Si per formula unit.Regional distribution of maximum temperatures is reflected bythe composition of actinolitic hornblendes within the metabasites.In a garnet-bearing metabasite of the Western belt, oscillatorygrowth zoning of garnet was observed. The composition of correspondingmineral inclusions suggests that a prograde P–T path duringgarnet growth evolved from 7·5 kbar and 375°C toabout 9·4 kbar and 500°C. Late garnet grew synkinematicallywith penetrative deformation. The retrograde P–T pathin the rocks of the Western belt is constrained by the compositionof mainly late strain-free minerals and involves slight coolingduring decompression. Both belts are part of a subduction system.The apparent P–T gap between the belts is due to theirjuxtaposition during exhumation. The Eastern belt constitutesthe transition towards the backstop system of the accretionaryprism that is represented by the Western belt, whereas the absenceof very low grade rocks west of the Western belt is attributedto tectonic erosion, which was possibly caused by subductionof a ridge. KEY WORDS: Chonos Metamorphic Complex; accretionary complex; high-pressure–low-temperature metamorphism; oscillatory garnet zonation; phengite; P–T paths     

Decrepitated UHP fluid inclusions: about diverse phase assemblages and extreme decompression rates (Erzgebirge, Germany)

Minute polyphase inclusions in garnet of quartzofeldspathic rocks (saidenbachite) from the Saxonian Erzgebirge, Germany, contain microdiamond or graphite, phlogopite, quartz, paragonite, phengite and other minerals in minor amounts. These inclusions are interpreted to represent an original dense COH + silicate fluid, trapped in crystallizing garnet at depths of >150 km. Inspection of the inclusion population in a single garnet by SEM reveals two characteristic features: (i) The shape of most inclusions indicates healed radial cracks in the host garnet, and, thus, the buildup of a significant differential pressure ΔP, i.e. a contrast in pressure between the inclusion (P_i) and the host mineral (P_e). The mineral assemblages sealing the cracks and showing an equilibrated microstructure indicate temperatures of ～750 ± 50 °C and pressures below 2.5 GPa. (ii) The diverse types of inclusions appear to be randomly distributed in the garnet host. Thus, the variable phase assemblages do not reflect a compositional evolution of the fluid trapped in the garnet. Combining observations (i) and (ii), we propose that the diversity of the phase assemblage in the inclusions is the result of decrepitation at different times, and thus, of distinct histories of P_i, as ΔP at decrepitation is primarily controlled by inclusion size and shape. Applying a flow law for dislocation creep of garnet, a low strength of garnet at 750 ± 50 °C for low geological strain rates is predicted. Thus, differential pressure should have been kept low (i.e. P_i≈ P_e) by continuous stretching of the inclusion for typical exhumation rates of metamorphic rocks. To attain the differential pressure (P_i >> P_e) required for catastrophic brittle failure of the garnet host, the decompression rate must have been extremely high. As a robust lower bound, a minimum exhumation rate on the order of 100 m year^?1 is suggested, which corresponds to ascent rates of magma.