Revised Thermobarometry of Alpe Arami and other Garnet Peridotites from the Central Alps

摘要

The four, currently best constrained, independent thermobarometers for garnet peridotites, namely Taylor's (Neues Jahrbuch für Mineralogie, Abhandlungen 172, 381-408, 1998) pyroxene solvus and Krogh's (Contributions to Mineralogy and Petrology 99, 44-48, 1988) clinopyroxene-garnet Fe-Mg exchange thermometers, and Taylor's (1998) Al-in-orthopyroxene and Nimis & Taylor's (Contributions to Mineralogy and Petrology, 139, 541-544, 2000) Cr-in-clinopyroxene barometers, have been applied to garnet lherzolites from the Central Alps. Analyses from the literature, as well as new in-house analyses, all pertaining to core compositions of first-generation, garnet lherzolite minerals, have been selected for thermobarometric calculations. The P-T data obtained for the three known garnet lherzolite occurrences in the Central Alps are tightly constrained, consistent with one another, and summarized as follows: Alpe Arami, 3·2 GPa and 840°C; Monte Duria, 3·0 GPa and 830°C; Cima di Gagnone, 3·0 GPa and 740°C. These values are consistent with experimental data on pargasite stability and composition in peridotitic systems. Our P-T estimates, along with microstructural and field geological observations, indicate that the garnet lherzolite parageneses form part of the prograde, Alpine, high-pressure metamorphic sequence of the Adula-Cima Lunga unit. Thermobarometry shows that the garnet lherzolites reached a maximum depth of subduction of ∼100 km, leaving little room for an extraordinary high-pressure, Alpine metamorphism at Alpe Arami. The very high pressure estimates obtained by some other workers are ascribed to inaccuracies in thermometric evaluations and to the strong temperature dependence of the Al-in-Opx barometer. Uncertainties in the determination of garnet-olivine equilibrium compositions, combined with the small sensitivity of garnet-olivine chemical exchanges to temperature variations, and unreliability of garnet-orthopyroxene thermometry at low-temperature conditions may account for the observed inconsistencies. A discrepancy between thermobarometric data for the garnet lherzolites and the associated eclogites is non-existent in our example, and in other localities it may also be a thermobarometric artefact