'New constraints on the P-T evolution of the Alpe Arami garnet peridotite body (Central Alps, Switzerland)': Reply to comment by Nimis and Trommsdorff (2001)

Abstract

Nimis and Trommsdorff (2001a) (hereafter N and T) offer an alternative and somewhat challenging interpretation of the textural and mineral compositional features displayed by the Alpe Arami (AA) peridotite body. Although N and T dispute some of the arguments used by Paquin and Altherr (2001) (hereafter P and A), they now agree that the AA periodotite has indeed experienced high temperatures of 1100-1200°C as indicated by the partitioning of elements with high diffusivities (Fe, Mg, Ni, Co) between the cores of grt, ol, opx and cpx grains (Brenker and Brey, 1997; P and A). We are therefore left with the key problem of why the application of two-pyroxene thermometers based on elements with low diffusivities (Ca, Sc, Cr, V, Ti) results in low and strongly scattering temperature values (i.e. 690-925°C; P and A; Nimis and Trommsdorff, 2001b). P and A conclude that the discrepancy between the high and low temperature values is due to a lack of equilibration during rapid subduction and exhumation and, therefore, assume peak metamorphic conditions of ~ 1180°C and 5·9 GPa. N and T postulate a two-stage evolution whereby the AA peridotite was first subducted and equilibrated with respect to all elements at low T and P (~800°C and 3 GPa) and subsequently subjected to a short-lived thermal event at T ≥ 1100 °C and P ≤ 3 GPa. The arguments that led N and T to promote their hypothesis will be considered under various aspects: (1) effective bulk compositions and system equilibration; (2) selection of thermobarometers; (3) geological and geochronological constraints. We will demonstrate that the textural and mineral compositional features of the AA garnet peridotite are not in favour of the hypothesis put forward by N and T.