Geochemistry and geochronology of travertine from the internal Apennine chain (Sothern Tuscany): implication for the hydrothermal setting in a post-orogenic domain

Abstract

Thermogene travertines form through precipitation of CaCO3 from supersaturated fluids usually generated and discharged in volcano-tectonic settings, often deposited in proximity of active geothermal springs or along open fissures. This dissertation includes three main chapters presenting a multiscale and multidisciplinary study on the Quaternary travertine deposits from the Albegna basin, Southern Tuscany, Italy, with the main aim of understanding their spatio-temporal distribution and define the feedback relationships and controlling factors on their deposition. The Albegna basin is an excellent area for studying feedback relationships between travertine deposition tectonics, active geothermal systems and climate due to the coexistence of active and fossil travertine deposition, few kilometers south of the Mt. Amiata geothermal field. In the northern section of the Albegna basin, travertines are deposited at different altitudes along a N-S-striking alignment about 18 km long, comprised between the Semproniano village (to the north) and the Saturnia village (to the south). This work integrates geological-structural investigations, geochronological analyses, and geochemical investigations on travertines and associated mineralizations, focusing on the hydrothermal circulation at the scale of the basin and on selected single travertine deposits. At the scale of the basin, the main novelty is that it was possible to reconstruct the 4D tectonic-hydrothermal system evolution over a distance of about 30 km and a time span of over 600 ka, through the spatio-temporal distribution of thermogene travertine deposits and other CaCO3 mineralizations as well as their morphological, geological, structural, and geochemical characters. In particular, this work show a general rejuvenation of the travertine deposition, a cooling of the parental fluid temperature and a lowering of the depositional altitude moving southward in the Albegna basin, moving away from the Mt. Amiata volcano. The Pleistocene travertine deposition of the Albegna basin was assisted by active tectonics, modulated by paleoclimate, within a region of positive geothermal anomaly to the south of the Mt. Amiata volcanic district. Wet climate conditions regulated the abundance of water, tectonic activity provided the permeability pathways, and the geothermal anomaly provided the ascension energy (heat) for convective hydrothermal circulation of percolating meteoric fluid(s) that interacted at depth with the carbonate reservoir. At the scale of the single deposit, the modes and rates for the growth of the Semproniano fissure ridge and the relationships between travertine fissure ridges and plateaus were investigated, as well as the implications in terms of the CO2 release, and the possible relationships between banded travertine formation and Quaternary paleoclimate-hydrological oscillations. Banded travertine of the Semproniano fissure ridge is the thickest continuous vein documented in the literature (50 m), and its thickness appears to be connected with its probable shallow emplacement and with the unusual longevity of the hydrothermal activity, longer than any other fissure ridge in literature (between at least 650 and 85 ka). The epithermal fluid supply feeding the Semproniano giant vein is not directly connected with the main volcanic paroxysmal activity of the Mt. Amiata volcano (which is younger than the early growth of the vein), rather with the positive geothermal anomaly associated with its pre eruptive stages through structurally-controlled fluid pathways which created and maintained active convection and supply of CO2-enriched meteoric fluids. This work provide also an estimation of the long-term release of CO2 from the Semproniano giant vein to improve our knowledge on the CO2 cycle in the past in geothermal/volcanic provinces. Finally, in two travertine quarries in the Albegna basin, syn-diagenetic non-tectonic folding caused by progressive hydrothermal veining were recognized. Those presented are the first records of syn-diagenetic non-tectonic contractional deformations in thermogene travertines. The changes of some pristine rock properties such as fabric, porosity, stratigraphic-chronological sequence, and rheology, if disregarded or unrecognized, may significantly impact paleoclimate, neotectonics, paleohydrology, and hydrocarbon reservoirs.