Superposed parageneses in the gehlenite-bearing, high-temperature skarns from Oraviţa (Banat, Romania).
Résumé
A shallow-level dioritic pluton of Upper Cretaceous age caused extensive contact metamorphism of a Callovian - Aptian calcareous sequence at Oraviţa - Ciclova (Banat, Romania), at about 35 km SSW of Reşita, the major city in the area. The resulted high-temperature skarn can be better observed on Crişenilor Brook, where it crops out at about 200 m upstream from its confluence with the Oraviţa Valley. The skarn from Crişenilor Brook is clearly zoned, the zones being individualized by the massive presence of one distinct mineral species. This zoning is, from the outer to the inner part of the metasomatic area: calcite (marble) / vesuvianite (+ wollastonite 2M) / gehlenite / plagioclase + diopside + calcic garnet / diorite. The inner endoskarn is largely dominated by the abundant development of calcic garnet (Grs 33.78 – 82.07 mol.%, Adr 13.45 – 64.36 mol.% with up to 9.47 mol.% morimotoite in the solid solution), titanite and diopside, in a mass conserving albite-substituted plagioclase (An 14 – 42 mol.%) and orthoclase (Or 95-97 Ab 5-3). The outer endoskarn is practically monomineralic, consisting in a gehlenite groundmass (åkermanite 34.10 - 51.18 mol.%, Na-melilite a up to 3.60 mol.% ) with minor monticellite (with 8.53 to 11.66 mol.% kirschsteinite and from 0.91 to 2.52 mol.% glaucochroite in solid solution), grossular-andradite (Grs 34.05 – 86.27 mol.%, Adr 10.23 – 64.41 mol.% with up to 3.80 mol.% morimotoite in the solid solution), wollastonite (the 2M polytype), hydroxylellestadite, vesuvianite, pyrrhotite, scarce magnetite and fukalite and secondary minerals such as hibschite, thomsonite, tobermorite, calcite and allophane. The exoskarn is also monomineral, consisting in a mass of vesuvianite, with minor wollastonite, clintonite, aluminian diopside, hydroxylellestadite, scarce andradite, and secondary clinochlore, talc and scolecite. Based on the reciprocal relationships of the main mineral species we identified four different mineral assemblages that correspond to different stages of the metasomatic evolution. The peak metamorphic assemblages (stage I) include wollastonite, gehlenite, diopside, grossular-andradite, perovskite, pyrrhotite. A second, subsequent paragenesis (stage II) is partly retrograde and includes magnetite, monticellite, titanian garnet, vesuvianite, aluminian diopside, hydroxylellestadite, clintonite and occasional fukalite. A late hydrothermal stage (stage III) produced retrogression of the first two stages and a paragenesis including katoite, clinochlore, talc, scolecite, thomsonite. A weathering paragenesis, corresponding to the late stage of evolution, is also present, and prompted to the formation of aragonite (probably on portlandite), calcite, tobermorite and allophane. The compositional data combined with experimental established and calculated equilibria accounts for a peak temperature of ~750°C that perfectly agrees with the temperature estimated for the dioritic magma.