Transformation of Spinel Lherzolite to Garnet Lherzolite in Ultramafic Lenses of the Austridic Crystalline Complex, Northern Italy |
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Authors: | OBATA, MASAAKI MORTEN, LAURO |
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Affiliation: | 1Department of Geology, Faculty of Science, Kumamoto University Kumamoto, 860 Japan 2lstituto di Mineralogia e Petrografia, Universita di Bologna Bologna, Italy |
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Abstract: | Numerous lenticular bodies of ultramafic rocks occur withinthe upper amphibolite- to granulitefacies metamorphic terraneof the Austrides between the Non and Ultimo valleys (Nonsbergregion), northern Italy. The ultramafic rocks are divided intotwo textural types: (a) coarse-type; and (b) finetype. The coarse-typerocks have the protogranular texture and are predominantly spinellherzolite. Some coarse-type spinel lherzolites have partlytransformed to garnet lherzolite. The fine-types are consideredto be metamorphic derivatives of the former, and the observedmineral assemblages are: (1) olivine + orthopyroxene + clinopyroxene+ garnet + amphibole ? spinel, (2) olivine + orthopyroxene +garnet + amphibole + spinel; (3) olivine + orthopyroxene + amphibole+ spinel; and (4) olivine+ orthopyroxene + amphibole + chlorite.Based on the microprobe analyses of constituent minerals fromten representative peridotite samples, physical conditions ofthe metamorphism, particularly that of the spinel to garnetlherzolite transformation, are estimated. Applications of pyroxenegeothermometry yield temperature estimates of 11001300?Cfor the formation of the primary spinel lherzolite, and 700800?Cfor that of the fine-type peridotites. A pressure range of 1628kb is obtained for the garnet lherzolite crystallization dependingon the choice of geobarometers. Two alternative P-T paths, i.e.(1) isobaric cooling or (2) pressure-increase and temperaturedecrease are considered and their geodynamic implications discussed. |
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