Mineralogic Study of Eclogitic Rocks from Alpe Arami, Lepontine Alps, Southern Switzerland |
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| Authors: | ERNST W G |
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| Institution: | Department of Earth and Space Sciences, and Institute of Geophysics and Planetary Physics, University of California Los Angeles, California 90024 |
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| Abstract: | Certain ultramafic-mafic lenses exposed in Ticino along thecontact zone between the underlying Simano and overlying Adulanappes display relatively high-pressure phase assemblages. AtAlpe Arami, metabasaltic layers associated with pyropic garnet-bearingIherzolite consist mainly of an early eclogitic assemblage characterizedby Alm39Py37Gross23Spess01+ Di49Hd08Jd43+rutile±kyanite.Iron-magnesium fractionation between garnet+omphacite pairsyields a KD, (Fe2+/Mg)garnet/(Fe2+/Mg)cllnopyroxence, of about6. This earlier assemblage has been replaced by a later, somewhatpargasitic hornblende+oligoclase+clinozoisite phase compatibility.Associated primary garnet peridotites contain Ca-rich clinopyroxeneand Al2O3-poor orthopyroxene. Both rock types have been affectedby a still later period of incipient chloritization. Available phase equilibrium and element partitioning data arecompatible with an inferred P-T condition of origin for theAlpe Arami mafic-ultramafic complex of 965–1000 °C,30–50 kilobars, indicating deep upper mantle generation.Amphibolites could have been produced during depressurizationaccompanying ascent of the mass through the upper mantle, butinasmuch as plagioclase accompanies the hornblende, the assemblageprobably crystallized after emplacement of the complex in theLepontine terrane prior to the termination of the Late Alpineregional metamorphism. Incipient production of high-rank greenschistphases certainly reflects a crustal event. |
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