Clinopyroxene geobarometry of pyroxenitic xenoliths from Hyblean Plateau (SE Sicily, Italy)

An unconventional geobarometric approach based on clinopyroxene unit-cell and site volume determination (Nimis, 1995) was used to constrain the pressures of magmatic crystallization for pyroxenitic xenoliths in alkaline volcanic rocks from the Hyblean Plateau (SE Sicily, Italy). Coarse-grained (O > 1 cm) Al-augite +/- orthopyroxene +/- spinel-bearing pyroxenites crystallized from basaltic melts within the mantle at greater than or equal to 37 km depth. The presence of pyroxenitic bodies or veins at mantle depths may account for the existence of an LREE-enriched mantle source beneath eastern Sicily, Medium-grained, recrystallized pyroxenites were segregated from their parent magmas at shallower depths. in some cases possibly as monomineralic clinopyroxenites. The pressure conditions for their crystallization are poorly constrained because of the difficulty in determining the extent of orthopyroxene exsolution. The most reliable estimate (8 kbar), as obtained fr om an orthopyroxene-poor sample, corresponds to a depth of 28 km. This value matches the inferred depth of the Moho discontinuity and support the hypothesis of extensive magmatic intrusions having occurred near the Hyblean crust-mantle boundary. Combination of clinopyroxene pressure estimates with temperature data based on two-pyroxene geothermometry suggests the existence of a high geothermal gradient, comparable with those defined for environments dominated by advective heat-transfer mechanisms. The proposed P-T profile is consistent with petrological constraints (absence of garnet, presence of spinel) and with the paleogeodynamic setting of the region. Pressure estimates retrieved by means of clinopyroxene geobarometry are only slightly biased by exsolution of orthopyroxene during subsolidus processes ten. 1-2 kbar for 10 vol% of exsolved orthopyroxene). It is therefore suggested that application of this method may be extended to reequilibrated rocks, provided the composition of the primary (i.e. magmatic) clinopyroxene can be modeled from the compositions and relative proportions of the unmixed phases. Semi-quantitative estimates (i.e. minimum pressures) can still be obtained when the amount of exsolved phases is unknown, using the clinopyroxene occurring in the rock as a limiting case.