Thermal Emplacement Model for the Alpine Lherzolite Massif at Balmuccia, Italy |
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| Authors: | SHERVAIS JOHN W |
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| Institution: | 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 P–T conditions to have been 850 °C (Cpx–Opx equilibria) and 10–13 kb (Opx–Gt;Plg–Gt–Sill–Qtz) during emplacement of thelherzolite massif. Temperature calculations on 12 peridotitesfrom throughout the massif suggest an earlier high-T stage (1200°C; Ol–Px–Sp) followed by partial re-equilibrationat lower T (850–950 °C; Cpx–Opx). The areaswithin the lherzolite massif with the highest calculated Ol–Px–Sptemperatures have the lowest Cpx–Opx temperatures, suggestingthat the apparent Cpx–Opx 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 P–T 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 P–T conditions of1100–1300 °C at 17–20 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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