GARNET-FORMING AND GARNET-ELIMINATING REACTIONS IN A QUARTZ DIORITE INTRUSION AT CAPO-VATICANO, CALABRIA, SOUTHERN ITALY

Late Hercynian peraluminous granitoids (quartz diorite, tonalite, and granodiorite) intrude amphibolite-granulite-facies country rocks in th e Capo Vaticano area, Calabria, southern Italy. Small (1 cm in diamete r) grains of garnet occur in kinzigitic gneisses and amphibolites in t he country rocks, but they are rare to absent in the intrusive rocks, except for one quartz diorite pluton that contains up to 15% modal gar net. Field relations, textural relations, compositional ranges, and ch emical zonation profiles of the garnet grains indicate how these grain s nucleated, grew, and eventually disappeared. Within the quartz diori te pluton, garnet occurs in metasedimentary enclaves, particularly in the leucosomes associated with their apparent partial melting, suggest ing a formation reaction of the type: Pl + Bt + Crd + Als + Pl + Qtz r eversible arrow L + Grt +/- Kfs. This reaction may or may not have beg un with pre-existing nuclei of garnet in the metasedimentary rocks. Wi th subsequent disintegration and assimilation of these enclaves into t he magma, the garnet grains occur as large (up to 4 cm diameter). disc rete, euhedral crystals, apparently in chemical equilibrium with the m elt fraction. Most garnet associated with metasedimentary material, or isolated in the quartz diorite, has similar Mn profiles (higher conce ntrations in the cores and rims than between the core and rim). Elsewh ere in the quartz diorite, garnet crystals occur as irregular grains w ith a wide corona of biotite and tschermakite, suggesting an eliminati on reaction of the type: Grt + L --> Bt + Ts. Although garnet grains i n other granitoid rocks may be entirely of xenocrystic or magmatic ori gin, their occurrence in the Capo Vaticano quartz diorite demonstrates formation principally through a melting reaction in metasedimentary e nclaves. The later elimination of this garnet occurs at a peritectic r eaction resulting from open-system assimilation of the anatectic melt and garnet in the larger body of quartz dioritic magma.