VARIATION IN MINERALOGY, TEMPERATURE, AND OXYGEN FUGACITY IN A SUITE OF STRONGLY PERALKALINE LAVAS AND TUFFS, PANTELLERIA, ITALY

摘要

Eight samples of pantelleritic lava and tuff and a lithic inclusion of trachyte from Pantelleria, Italy, have been thoroughly analyzed with an electron microprobe. These samples reveal five different mineral assemblages if classified by the presence of fayalite, aenigmatite, ilmenite, and magnetite: (1) augite + fayalite + ilmenite + magnetite, (2) augite + fayalite + ilmenite, (3) hedenbergite or sodian hedenbergite + fayalite + ilmenite + aenigmatite + quartz, (4) sodian hedenbergite or aegirine-augite + ilmenite + aenigmatite + quartz + - ferrorichterite, and (5) aegirine-augite + aenigmatite + quartz. Alkali feldspar (Or_(35-37)) is present as the dominant phyric phase in each assemblage. Whole-rock silica and peralkalinity correlate strongly with the mineral assemblage: assemblage 1 is found in the sample with the lowest agpaitic index [A.I. = molar (Na + K)/A1] and silica concentration (A.I. < 1.31, SiO_2 < 64.8 wt percent) and equilibrated at 991-888 deg C at an oxygen fugacity between 0.7 and 1.1 log units below the FMQ buffer (FMQ - 0.7 to FMQ- 1.1). Assemblage 2 is associated with a higher agpaitic index and silica concentration (A.I. = 1.42, SiO_2 = 67.1 percent) and equilibrated at ~794 deg C at FMQ - 0.5. Assemblage 3 is associated with a still higher agpaitic index and silica concentration (A.I. in the range 1.55 - 1.63, 66.8 < SiO_2 < 67.8 percent) and equilibrated at 764-756 deg C at FMQ - 0.5 to FMQ - 0.2. Assemblage 4 is associated with a slightly higher agpaitic index and yet higher silica concentration (1.61 < A.I < 1.75, 67.6 < SiO_2 < 72.0 percent) and equilibrated between 740-700 deg C at oxygen fugacities at or just below the FMQ buffer. Assemblage 5 is associated with the highest agpaitic index and highest concentration of silica (A.I. = 1.97, SiO_2 = 69.7 percent) and equilibrated at <700 deg C at an oxygen fugacity just above the FMQ buffer in a "no-oxide" field. Despite the paucity of two-oxide, two-pyroxene, or two-feldspar pairs, it may be possible to accurately constrain temperature and oxygen fugacity in peralkaline rocks with QUIIF equilibria given an equilibrium assemblage of fayalite, ilmenite, and clinopyroxene.