PETROLOGY, MINERALOGY AND GEOCHEMISTRY OF A MAFIC DIKE FROM MONTE CASTELLO, ELBA ISLAND, ITALY

Abstract

A mafic dike is intruded into two SW-NE trending normal faults, and crosscuts the Mt. Alpe Cherts and the Nisportino Fm. in the Monte Castello area, Elba Island, Tuscan Archipelago. The dike is dark grey to brownish in colour, it has a porphyritic texture with phenocrysts of plagioclase + clinopyroxene + olivine, and seldom xenocrysts of large k-feldspar. The original mineralogy is strongly altered and replaced by secondary minerals. Clinopyroxene and plagioclase in some cases are still preserved, whereas olivine is entirely replaced by smectite aggregates. Euhedral Mg-chromite inclusions occur in the olivine ghosts. In the freshest samples the groundmass is made up of clinopyroxene, k-feldspar, plagioclase, magnetite and apatite. Large k-feldspar xenocrysts probably scavenged from a monzogranite are also present. No olivine analyses are available due to strong weathering. Mg-chromites hosted by olivine ghosts have a mild residual character with a Cr# between 0.40 and 0.65. Plagioclases range in composition from An75 to An65, although those enclosed in the k-feldspar xenocrysts have a compositional range from An39 to An35. Clinopyroxenes are mainly sub-calcic diopside (augite) with an Mg# ranging from 0.88 to 0.83. Step-heating 40Ar/39Ar dating was performed on the k-feldspar-rich groundmass of the less altered sample. The result establishes a date of 5.83±0.14 Ma (~70% of the released 39Ar), although the slightly radiogenic initial Ar isotopic composition and the scatter observed during the release of Ar at low temperature make the result only indicative of the age. Secondary minerals are ubiquitous even in the freshest samples, the whole rock compositions indicate that the parental magma has a shoshonitic composition, with a clear alkaline-potassic affinity. Mineral chemistry of k-feldspar and clinopyroxene crystals indicate that the magma belongs to the Italian Potassic Suites, with a strong resemblance to the potassic rocks cropping out at the Capraia Island and in the Southern Tuscany. The presence of olivine ghosts with euhedral Mg-chromite inclusions suggest that the magma has a strong primitive composition, with an alkaline character. Initial Sr-isotope values are in the range between 0.70878-0.70900, which indicate that the parental magma of the dike has not been contaminated with the monzogranite but the radiogenic Sr has been diluted by small scale magma-wall rock interaction. The presence of xenocrysts from a monzogranite, and lack of reaction paragenesis clearly indicates that the mafic magma intruded the monzogranite successively to its cooling. The 5.8 Ma age of this dike constitutes a very important datum: in fact it indicates that block tectonics should began in the Island before this date, and could be contemporaneous or also older of the gravitational detachments triggered by the Monte Capanne pluton uplift.