Phase relations and metamorphic history of a clinohumite-chlorite-serpentine-marble from the Western Tauern Area (Austria)

104

Two impure ultrahigh-pressure (UHP) marbles, a calcite marble with the peak assemblage Grt + Phe + Cpx + Rt + (Arg) and a dolomite marble with the peak assemblage Crn + Chl + Rt + Dol (±Arg), from the same lens from the polymetamorphic complex of the Brossasco-Isasca Unit (BIU) (southern Dora-Maira Massif) have been petrologically investigated and modelled by calculating P-T phase-diagram projections for H 2 O-CO 2 mixed-volatile systems. Thermobarometric data obtained from the calcite marble suggest Alpine peak conditions in the diamond stability field (4.0 GPa at 730°C), and allow reconstruction of the earlier portion of the Alpine retrograde P-T path, which is characterized by a significant decompression coupled with a moderate and continuous cooling to 650°C at 2.50 GPa. The modelled fluid compositions at peak conditions point to 0.025 £ X(CO 2 ) £ 0.10 and X(CO 2 ) £ 0.0012 in the calcite marble and dolomite marble, respectively, suggesting fluid heterogeneity at the local scale and an internally buffered fluid evolution of the studied impure marbles. The lack of micro-diamond in the BIU marbles is explained by the very-low X(CO 2 ) values, which favoured relatively high fO 2 -conditions, preventing the formation of diamond at the UHP peak metamorphic conditions.

Section: Metamorphic Evolutionsupporting

confidence: 70%

Impure marbles from the UHP Brossasco‐Isasca Unit (Dora‐Maira Massif, western Alps): evidence for Alpine equilibration in the diamond stability field and evaluation of theX(CO₂) fluid evolution

Castelli

Rolfo

Groppo³

et al. 2007

104

“…All fields contain fluid, with X CO 2 shown as blue dashed lines. The aragonite-calcite transition is shown as predicted by the model (red solid line) and as determined experimentally by Hermann et al (2016) are metastable but their topologies are likely similar to that of olivine (Franz & Ackermand, 1980;Rice, 1980). Figure 13a shows the P-T pseudosection for Almirez meta-ophicarbonates.…”

Section: Milagrosa Serpentinite and Marble Lensesmentioning

confidence: 61%

“…Although F and Ti stabilize Ti‐clinohumite and TiChn at high temperature (Engi & Lindsley, ; Evans & Trommsdorff, ; Grützner, Klemme, Rohrbach, Gervasoni, & Berndt, ; López Sánchez‐Vizcaíno et al., ), no suitable solid solution models exist to model phase relations of Ti‐clinohumite and TiChn in fluorine‐ and titanium‐bearing carbonate systems (Weiss, ). In the computed pseudosection of Figure b (F‐ and Ti‐free system) these phases are metastable but their topologies are likely similar to that of olivine (Franz & Ackermand, ; Rice, ).…”

Section: Resultsmentioning

confidence: 92%

Subduction metamorphism of serpentinite‐hosted carbonates beyond antigorite‐serpentinite dehydration (Nevado‐Filábride Complex, Spain)

Menzel

Garrido

Sánchez‐Vizcaíno

et al. 2019

At sub‐arc depths, the release of carbon from subducting slab lithologies is mostly controlled by fluid released by devolatilization reactions such as dehydration of antigorite (Atg‐) serpentinite to prograde peridotite. Here we investigate carbonate–silicate rocks hosted in Atg‐serpentinite and prograde chlorite (Chl‐) harzburgite in the Milagrosa and Almirez ultramafic massifs of the palaeo‐subducted Nevado‐Filábride Complex (NFC, Betic Cordillera, S. Spain). These massifs provide a unique opportunity to study the stability of carbonate during subduction metamorphism at P–T conditions before and after the dehydration of Atg‐serpentinite in a warm subduction setting. In the Milagrosa massif, carbonate–silicate rocks occur as lenses of Ti‐clinohumite–diopside–calcite marbles, diopside–dolomite marbles and antigorite–diopside–dolomite rocks hosted in clinopyroxene‐bearing Atg‐serpentinite. In Almirez, carbonate–silicate rocks are hosted in Chl‐harzburgite and show a high‐grade assemblage composed of olivine, Ti‐clinohumite, diopside, chlorite, dolomite, calcite, Cr‐bearing magnetite, pentlandite and rare aragonite inclusions. These NFC carbonate–silicate rocks have variable CaO and CO2 contents at nearly constant Mg/Si ratio and high Ni and Cr contents, indicating that their protoliths were variable mixtures of serpentine and Ca‐carbonate (i.e., ophicarbonates). Thermodynamic modelling shows that the carbonate–silicate rocks attained peak metamorphic conditions similar to those of their host serpentinite (Milagrosa massif; 550–600°C and 1.0–1.4 GPa) and Chl‐harzburgite (Almirez massif; 1.7–1.9 GPa and 680°C). Microstructures, mineral chemistry and phase relations indicate that the hybrid carbonate–silicate bulk rock compositions formed before prograde metamorphism, likely during seawater hydrothermal alteration, and subsequently underwent subduction metamorphism. In the CaO–MgO–SiO2 ternary, these processes resulted in a compositional variability of NFC serpentinite‐hosted carbonate–silicate rocks along the serpentine‐calcite mixing trend, similar to that observed in serpentinite‐hosted carbonate‐rocks in other palaeo‐subducted metamorphic terranes. Thermodynamic modelling using classical models of binary H2O–CO2 fluids shows that the compositional variability along this binary determines the temperature of the main devolatilization reactions, the fluid composition and the mineral assemblages of reaction products during prograde subduction metamorphism. Thermodynamic modelling considering electrolytic fluids reveals that H2O and molecular CO2 are the main fluid species and charged carbon‐bearing species occur only in minor amounts in equilibrium with carbonate–silicate rocks in warm subduction settings. Consequently, accounting for electrolytic fluids at these conditions slightly increases the solubility of carbon in the fluids compared with predictions by classical binary H2O–CO2 fluids, but does not affect the topology of phase relations in serpentinite‐hosted carbonate‐rocks. Phase relations, mineral c...

“…Franz & Ackermand (1980) attribute the development of a clinohumite + olivine + diopside +chlorite paragenesis in marbles from Schlegeistal (western Tauern Window) to this event. In migmatitic schists of the SE Tauern, contact metamorphism resulted in the localized breakdown of large Hercynian muscovite porphyroblasts to aggregates of acicular sillimanite which were later pseudomorphed by Alpine kyanite (Droop, 1981).…”

Section: Discussion-t H E Po L Y M Eta-morphic History O F the Pementioning

confidence: 94%

Pre‐Alpine eclogites in the Pennine Basement Complex of the Eastern Alps

Droop

1983

An occurrence of quartz-eclogite is described from the Inner Schieferhiille unit of the Pennine Basement Complex in the SE Tauern Window, Austria.Field relations strongly suggest a pre-Alpine age for the primary eclogitic mineral assemblage (garnet + omphacite + quartz + rutile). This implies that there was no connection between the formation of these eclogites and the late Cretaceous and Tertiary tectonic evolution of the Eastern Alps. The quartz-eclogite mineral assemblage crystallized under conditions of 620 f 100°C and at pressures in excess of 12 kbar, and suffered amphibolitic overprinting of Alpine and possibly Hercynian age. A four-stage polymetamorphic history is proposed for the Inner Schieferhiille:1. Pre-Hercynian or early Hercynian eclogitefacies metamorphism. 2. Hercynian amphibolite-facies metamorphism. 3. Contact metamorphism by late-Hercynian granitic intrusions (the Zentralgneis). 4. Meso-Alpine amphibolite-facies metamorphism.Abbreviations used in the text andfigures ab = albite, ac = acmite, alm = almandine, and = andalusite, andr = andradite, a p = apatite, aug = augite, cpx = clinopyroxene, di = diopside, ep = epidote, gross = grossular, gt = garnet, hbl = hornblende, jd = jadeite, ky = kyanite, om = omphacite, ore = opaque oxide, pyrp = pyrope, pyt = pyrite, qz = quartz, rut = rutile, sill = sillimanite, spess = spessartine, sph = sphene.