CHEMICAL VARIATION OF CHROMITE IN THE GORGONA OLIVINE-ORTHOPYROXENITE, THRACE, GREECE

Compositional zoning and variations of disseminated chromite grains re flect the multi-stage metamorphic history of the Gorgona olivine-ortho pyroxenite. The most abundant chromite types displaying a large range in grain-size (60-750 mu m in diameter) are the olivine related ones: either enclosed in olivine or intergranular in contact with olivine. C hromite enclosed in enstatite is the less abundant type and shows the smallest grain-size (30-100 mu m), while chromite with a grain size ra nging from 50 to 250 mu m in diameter is enclosed in chlorite. The int ergranular chromite shows similar composition with the chromite enclos ed in olivine. Chromite enclosed in chlorite shows more or less the sa me composition as the smaller (approx. <400 mu m in diameter) olivine- associated grains. Compositional zoning was identified in the larger c hromite grains associated with olivine. Chromite grains associated wit h olivine (excluding the rim of the larger ones) display a weak but cl ear trend of decreasing X(Fe2+) with increasing grain-size. Subsolidus re-equilibration reactions between chromite and silicates modified th e chemistry of chromite grains associated with olivine and enclosed in chlorite. Chromite grains enclosed in enstatite most probably were pr eserved from subsolidus re-equilibration, with their chemistry being i nherited, above the solidus, at the magmatic stage.