Garnet-spinel-pyroxenite xenoliths from hyblean plateau (South-eastern Sicily, Italy)

Abstract

Rare garnet-spinel pyroxenite xenoliths occur in some basaltic tuff-breccia levels of Miocene age from the Valle Guffari (Hyblean Plateau, Sicily), together with a number of spinel-bearing mantle xenoliths. The garnet-bearing pyroxenites may be divided into two groups (“a” and “b”) on textural and mineralogical bases. Garnet-bearing spinel websterites with a fully recrystallized texture represent the first group (a). Here the garnet (Py54.5 A1m32 Gr13.5), with a diffuse kelyphitic alteration, forms a reaction corona between coarse spinel grains and the in contact pyroxenes. The transition from the spinel-pyroxenite to the garnet-pyroxenite field may depend on isobaric cooling from higher (magmatic?) temperatures. Garnet-pyroxene geothermometry indicates that the last equilibration most probably occurred at P = 1.0 GPa (ca.), T = 750 °C (ca). The second lithotype (b) is an orthopyroxene-bearing garnet-spinel clinopyroxenite, exhibiting a complex texture. It consists of zones of coarse clinopyroxene grains enclosing euhedral spinel passing to zones where tiny rounded crystals of the same pyroxene and spinel are enclosed in relatively large patches of extensively kelyphitisized garnet (Py64.8 Alm25.6 Gr9.6). Garnet also occurs as inclusion-free grains up to 4 mm in diameter. P-T calculations give significantly higher values than for the former case (a). The origin of the b-type garnet may also depend on subsolidus reaction of spinel and pyroxenes after an isobaric cooling from still higher temperatures, but a primary magrnatic origin might also be possible, especially for the “granular” garnets. P-T estimates for both the pyroxenite types closely match a steady geotherm for 100 mW/m2 surface heat flow. Such a relatively intense heat flow may suggest the occurrence of huge masses of hot magma intruding the Hyblean lithospheric mantle and lower crust at different levels.