THE VARICOLOURED SLATES OF THE GRÀSSERA UNIT (CENTRAL-EASTERN ELBA, TUSCANY): PETROGRAPHICAL-MINERALOGICAL DATA AND COMPARISONS WITH OTHER TUSCAN AND LIGURIAN-PIEDMONTESE UNITS

Abstract

The varicoloured slates of the Cavo Formation in the Gràssera Unit represent a succession of debated stratigraphic and paleogeographic pertinence (Tuscan Domain, Ligurian or Ligurian-Piedmontese Units) within the complex tectonic puzzle of the Elba Island. With this in mind, the authors carried out petrographical, illite crystallinity and mineralogical analyses on the slates of the Gràssera Unit and on the pelites of some other Tuscan, Ligurian and Ligurian- Piedmontese units cropping out in the Eastern Elba Island (the Tuscan Verrucano Group, Varicoloured Sericitc Schists and Posidonia Marlstones Fm. and the Ligurian Palombini Shales) and in Southern Tuscany (the “Scaglia Toscana” Fm. and the “Schistes Lustrés” of the Argentario Promontory). The Cavo Fm. slates show a mineralogical association of illite+chlorite+quartz (±chlorite-vermiculite) and IC values in the anchimetamorphic field (°D2q = 0.28¸0.39). The local hydrothermal circulation and heating processes due to the Neogene magmatic intrusions locally increase the IC and vary the phyllosilicates association. The comparison of these data with those obtained from the other units ruled out the correlation of the Cavo Fm. with the pelitic rocks either of both the Tuscan Nappe (Posidonia Marlstones and Tuscan Scaglia Fms.) and the Ophiolitic unit (Palombini Shales Fm.). The correlation with the metapelites of the Monticiano- Roccastrada Unit (e.g. “Verrucano” Group) is not clear, whereas the attribution of the Cavo Fm. slates to a shallow-buried Ligurian-Piedmontese Unit succession seems more probable because of some lithological and mineralogical analogies with the “Schistes Lustrés” of the Argentario Promontory.