Reconstruction of the exhumation path of the Alpe Arami garnet‐peridotite body from depths exceeding 160 km

Abstract

ABSTRACT Chemical disequilibrium exists between all phases of the Alpe Arami garnet‐peridotite body (Ticino, Switzerland) which hampers the evaluation of P‐T conditions of origin, yet disequilibrium offers the inherent possibility to derive a P‐T‐t path for this mantle slice. We tried to tackle this problem by carrying out new mineral analyses and taking diffusion rates and bulk‐rock compositional effects into consideration. Peak metamorphic conditions from mineral core compositions were estimated as 112050C/50.2 GPa. These values are significantly higher than previously published results and were determined from a combination of the O'Neill & Wood (1979) Fe/Mg garnet‐olivine exchange thermometer and the Al‐in‐orthopyroxene barometer (Brey & Köhler, 1990), and are supported by the Ca/Cr ratios in garnet, which are in accord with these conditions. Details of the exhumation path were derived from (1) rim compositions of minerals that yield a first retrograde stage of 720 50C/2 0.25 GPa (2) a spinel lherzolite assemblage in narrow shear zones (tectonic phase F0″, after Möckel, 1969) which documents a second retrograde stage at 500–600C/0.8‐l.5 GPa. The Ca content in olivine (Köhler & Brey, 1989) can be used to evaluate further P‐T conditions along the retrograde path. We measured very low values (30–40 ppm Ca) in the cores of olivine and a remarkable increase towards the rim (120 ppm). The low core values may reflect an equilibrium stage during the main Alpine metamorphism. The increasing values towards the olivine rims probably represent a late‐stage heating event. The initial cooling rates for the peridotite body are between 2700 and 5100C Ma‐1, depending on which diffusion data are used.