Comment on 'New constraints on the P-T evolution of the Alpe Arami garnet peridotite body (Central Alps, Switzerland)' by Paquin and Altherr (2001)

Abstract

The 'Alpe Arami controversy' seems destined not to to end as more and more new analytical data become available. Recently, Paquin and Altherr (2001; hereafter P and A) presented an excellent set of major- and trace-element zoning profiles in garnets and in various generations of pyroxenes from the Alpe Arami peridotite body. The results led the authors to conclude that the peridotite experienced peak metamorphic conditions of 1180°C and 5.9 GPa. These values are in fairly good agreement with the high P-T estimates of Brenker and Brey (1997), but are at variance with the more recent estimates of Nimis and Trommsdorff(2001; hereafter Nad geological of 840°C and 3.2 GPa. Although the thermobarometric estimates of Pand A are much lower than postulated by Dobrzhinetskaya et al. (1996) and Green et al. (1997), A still invoke an ultra-high-pressure Alpine (35-43 Ma) metamorphism for Alpe Arami which does not fit the prograde, high-pressure Alpine metamorphic sequence of the Adula-Cima Lunga nappe, to which other ultramafic, mafic, ophicarbonate and pelitic rocks of this unit comply (see N and T and references therein). Overall, P and A's arguments in favour of high-T conditions are well grounded. However, we will show that (1) N and T's and P and A's temperature estimates are not in conflict with each other and reflect two distinct stages of equilibration characterized by different thermal conditions; (2) the low-T stage corresponds to N and T's estimates of 840°C and 3.2 GPa and is representative of the P-T recorded by the Alpe Arami peridotite during Alpine subduction metamorphism dated at 43 Ma (Gebauer et al., 1992; Becker, 1993; Gebauer, 1996); (3) a short-lived thermal event at T ≥ 1100°C, probably related to magmatic intrusion at 35 Ma (see Gebauer, 1996), caused complete re-equilibration of garnet Fe-Mg and Ni, but left pyroxene Ca, Cr, Al core contents virtually unaltered; (4) no unequivocal barometric estimate can be given for this high-T stage on the basis of available data, although pressures not very dissimilar from those achieved at the peak of subduction metamorphism are the most likely.