Thermal Emplacement Model for the Alpine Lherzolite Massif at Balmuccia, Italy |
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Authors: | SHERVAIS JOHN W. |
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Affiliation: | Department of Geological Sciences, University of California Santa Barbara, California 93106, U.S.A. |
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Abstract: | The spinel lherzolite massif at Balmuccia, northwest Italy,forms an elongate north-south trending lens (4.5 x 0.5 x 1.1km) within the pre-Alpine granulite basement complex of theIvrea zone. The western contact is a mylonite fault zone formedduring late emplacement cataclastic flow near the Insubric line;to the east the lherzolite massif is separated from the granulitesby a magmatic sheath of layered pyroxenites, pyroxene pegmatitesand meta-gabbros. Pyroxene reaction zones on gabbro dikes indunite pods which lie east of the main lherzolite massif showthat emplacement occurred at pressures >9 kb, based on peridotiteequilibria studies. Phase chemistry calculations on pyroxenitesand granulites show ambient PT conditions to have been850 °C (CpxOpx equilibria) and 1013 kb (OpxGt;PlgGtSillQtz) during emplacement of thelherzolite massif. Temperature calculations on 12 peridotitesfrom throughout the massif suggest an earlier high-T stage (1200°C; OlPxSp) followed by partial re-equilibrationat lower T (850950 °C; CpxOpx). The areaswithin the lherzolite massif with the highest calculated OlPxSptemperatures have the lowest CpxOpx temperatures, suggestingthat the apparent CpxOpx temperatures are due to re-equilibrationduring emplacement. The spinel lherzolite probably originatedat 12 and 20 kb, based on the mineral assemblage Ol + Opx +Cpx + Sp + Hnbd. The inferred PT ranges put both themassif and the granulites on a geotherm that is high for continentalcrust and implies a high surface heat flow at the time of emplacement(2.2 µcal/cm2 sec). The Balmuccia area later became thelocus of early Mesozoic rifting between the North and SouthAlpine plates. These relationships at Balmuccia are similarto the Great Basin of the western United States, where mantlexenoliths in young basalts that show PT conditions of11001300 °C at 1720 kb, occur in an area ofhigh heat flow (2.0 µCal/cm2 sec average) and extension.This suggests an association between up-welling of mantle peridotitesbelow continents and ensialic tensional tectonics. |
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