Chemiostratigrafia E petrografia della parte superiore della formazione dolomitica triassica di Filettino (monti Simbruini, Lazio)

The main petrographic and geochemical features of the Triassic Dolomitic Formation of Filettino (Norian-Rhaetian), Simbruini Mountains, eastern Latium, have been determined. The carbon and oxygen isotopic composition of selected samples has also been determined. Petrographically, the rocks are generally dolomicrites, showing no significant structural difference between massive and stratified samples. A few samples display large euhedral dolomite crystals scattered throughout the microdolomitic mass, indicating that dolomitization took place occasionally in two stages. Calcite is rare and occurs only in a sample as scattered large crystals. Other rare minerals, identified by XRD, are quartz, K-feldspar, illite and muscovite. The scarce fossils are represented mainly by ostracods and foraminifera; these latter occur mostly in the samples from the middle part of the sequence. Geochemically, the rocks show very low (x̄ = 0.46 mass %) insoluble residue, except the uppermost stratigraphic sample (2.18%). After CaO and MgO the most abundant oxides are SiO2 (x̄ = 0.29%), Al2O3 (x̄ = 0.19%) and Na2O (x̄ = 0.02%), all the others being often below detection limit. Among trace elements Cl is the most abundant (x̄ = 373 ppm), stoichiometrically linked to Na. Sr is low (x̄ = 43 ppm) because of release during dolomitization. Rb, S, Zn, Cu and Cr are scarce (x̄ = 19, 15, 13, 7 and 5 ppm, respectively), whereas Ba, Y and Zr are generally below detection lilimit, in agreement with the very scarce insoluble residue of the rocks. The dolomite crystals analyzed by microprobe show variable composition even on the sample scale, suggesting accordingly variable Ca/Mg ratios of the dolomitizing solutions through time and space. However, crystals with Ca excess are generally dominant, indicating that dolomitization was performed by non evaporitic solutions. Stoichiometric crystals, that formed from hyperaline solutions, are more abundant in the lower and upper stratigraphic levels. The isotopic composition of the dolostones is uniform: the δ13C (x̄ = 39‰ PDB) and δ18O (x̄ = 33,1‰ SMOW) values are typical of marine dolomites. As a whole, the petrographic and geochemical features of the Triassic Filettino dolostones indicate that the rocks formed under rather steady conditions in a coastal environment with lagoons near a peneplaned continent composed of acidic rocks and subordinate basic rocks, as suggested by the high Cr/Zr ratios of dolostones, or during a period of low erosional rate, likely because of arid climate. The detrital load became significant only at the top of the sequence towards the Liassic. The sediments were dolomitized just after deposition layer by layer. Finally, the elemental and isotopic compositions of the Filettino dolostones are roughly similar to those of the Antrodoco dolostones. that represent the lower part of the Triassic Dolomitic Formation: however, the latter rocks show wider variability of the detrital input, likely reflecting major tectonic activity.