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

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