Mineralogic Study of Eclogitic Rocks from Alpe Arami, Lepontine Alps, Southern Switzerland

Abstract

Certain ultramafic-mafic lenses exposed in Ticino along the contact zone between the underlying Simano and overlying Adula nappes display relatively high-pressure phase assemblages. At Alpe Arami, metabasaltic layers associated with pyropic garnet-bearing Iherzolite consist mainly of an early eclogitic assemblage characterized by Alm39Py37Gross23Spess01+ Di49Hd08Jd43+rutile±kyanite. Iron-magnesium fractionation between garnet+omphacite pairs yields a KD, (Fe2+/Mg)garnet/(Fe2+/Mg)cllnopyroxence, of about 6. This earlier assemblage has been replaced by a later, somewhat pargasitic hornblende+oligoclase+clinozoisite phase compatibility. Associated primary garnet peridotites contain Ca-rich clinopyroxene and Al2O3-poor orthopyroxene. Both rock types have been affected by a still later period of incipient chloritization. Available phase equilibrium and element partitioning data are compatible with an inferred P-T condition of origin for the Alpe Arami mafic-ultramafic complex of 965–1000 °C, 30–50 kilobars, indicating deep upper mantle generation. Amphibolites could have been produced during depressurization accompanying ascent of the mass through the upper mantle, but inasmuch as plagioclase accompanies the hornblende, the assemblage probably crystallized after emplacement of the complex in the Lepontine terrane prior to the termination of the Late Alpine regional metamorphism. Incipient production of high-rank greenschist phases certainly reflects a crustal event.