Abstract
The spinel lherzolite massif at Balmuccia, northwest Italy, forms an elongate north-south trending lens (4.5 × 0.5 × 1.1 km) within the pre-Alpine granulite basement complex of the Ivrea zone. The western contact is a mylonite fault zone formed during late emplacement cataclastic flow near the Insubric line; to the east the lherzolite massif is separated from the granulites by a magmatic sheath of layered pyroxenites, pyroxene pegmatites and meta-gabbros. Pyroxene reaction zones on gabbro dikes in dunite pods which lie east of the main lherzolite massif show that emplacement occurred at pressures >9 kb, based on peridotite equilibria studies. Phase chemistry calculations on pyroxenites and 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 the lherzolite massif. Temperature calculations on 12 peridotites from throughout the massif suggest an earlier high-T stage (∼1200 °C; Ol−Px−Sp) followed by partial re-equilibration at lower T (850–950 °C; Cpx−Opx). The areas within the lherzolite massif with the highest calculated Ol−Px−Sp temperatures have the lowest Cpx−Opx temperatures, suggesting that the apparent Cpx−Opx temperatures are due to re-equilibration during emplacement. The spinel lherzolite probably originated at ≥12 and ≤20 kb, based on the mineral assemblage Ol + Opx + Cpx + Sp + Hnbd. The inferred P–T ranges put both the massif and the granulites on a geotherm that is high for continental crust and implies a high surface heat flow at the time of emplacement (≥2.2 μcal/cm2 sec). The Balmuccia area later became the locus of early Mesozoic rifting between the North and South Alpine plates. These relationships at Balmuccia are similar to the Great Basin of the western United States, where mantle xenoliths in young basalts that show P–T conditions of 1100–1300 °C at 17–20 kb, occur in an area of high heat flow (≥2.0 μCal/cm2 sec average) and extension. This suggests an association between up-welling of mantle peridotites below continents and ensialic tensional tectonics.

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