Bernard Moine

Geochemistry and stratigraphic correlations ― application to the investigation of geothermal and mineral resources of Tuscany, Italy (Contribution to the knowledge of the ore deposits of Tuscany. II)

Abstract Major-element geochemistry (364 analyses) is used to obtain quantitative information on the initial mineralogical composition and the depositional environment of Paleozoic, more or less metamorphic, rocks from Tuscany and Elba. Furthermore, this method can be used to improve stratigraphic correlations by comparing the chemical compositions of the metamorphic rocks and their possible non-metamorphic counterparts of known stratigraphic position. The stratigraphic units were divided into two sets. The first includes the units of known stratigraphic or tectonic position, taken as reference groups: Permian Red Porphyries, Carboniferous rocks, Buti Group l.s., Porphyritic Schists, Porphyroids, Lower Phyllites and metabasites from the Apuan Alps and Elba. The second set comprises the units of uncertain position: metapelites and metapsammites from outcrops, mines and boreholes from the Boccheggiano—Niccioleta area, Calamita Schists and metabasites associated with these two units, Micaschists, Gneisses and associated amphibolites. The main results are: 1. (1) The Porphyroids and Porphyritic Schists differ strongly from the Permian Red Porphyries, thus confirming that they belong to two different volcanic episodes. 2. (2) All the reference units consist of shales, sandstones s.s. or graywackes, differing by their degree of maturity (increasing from the Lower Phyllites to the Buti Group and to the Carboniferous formations). 3. (3) The Carboniferous rocks, the Buti Group and the Lower Phyllites have distinct chemical compositions. 4. (4) The Boccheggiano Formation l.s. and the Calamita Schists are similar and include rocks chemically equivalent to the Lower Phyllites, the Buti Group and the Carboniferous formations. 5. (5) The Micaschists and Gneisses derive from shales and graywackes respectively, and chemically recall the Lower Phyllites. 6. (6) The metabasites from the Apuan Alps and from Niccioleta are “within-plate basalts”, whereas the amphibolites interlayered within the Micaschists and Gneisses seem “ocean-floor basalts”.