Silica precipitates in omphacite from eclogite at Alpe Arami, Switzerland: evidence of deep subduction

Observations of oriented SiO2 precipitates in omphacite from eclogite with tholeiitic basalt protolith bordering the Alpe Arami garnet peridotite massif, Ticino, Switzerland, and petrological studies of the eclogitic mineral assemblages, suggest that this rock was subjected to higher-pressure metamorphism than previously realized. We employed various calibrations of the Fe2+ - Mg exchange thermometer and calculations of equilibria with thermodynamic data, considering the calcium-Tschermak's component (CaAl2 SiO6 ), of garnet-pyroxene pairs. From these calculations, it is concluded that the eclogitic lenses have recorded at least four stages of mineral growth corresponding to the following: Stage I (prograde) c. 2.4 GPa; 700 degreesC; Stage IIa (maximum recorded grade) c. 7.0 GPa; 1100 degreesC; Stage IIb (retrograde) c. 3.7 GPa; 900 degreesC; Stage III (retrograde) c. 2.1 GPa; 750 degreesC. Because of the preservation of Stage I, a relatively rapid subduction and exhumation of Alpe Arami eclogite is suggested. The exhumation path of the eclogitic rock is in good agreement with most exhumation paths inferred for the Alpe Arami garnet lherzolite proposed previously by several authors based upon a variety of different observations, although the eclogite and peridotite exhumation paths may diverge at depths greater than 120 km.