Hydrothermal Alteration of Xenoliths from Miocene Tuff Breccias from Valle Guffari (Sicily-Italy)

Phyllosilicates occurr as replacements of olivine, clinopyroxene and interstitial materials as well as veins or fracture-fillings in hydrothermally altered basalts. The parageneses of phyllosilicates generally change systematically with depth and with the degree of alteration, which in turn is related to permeability of basalts. Minimal alteration in the basalts is characterized by the occurrence of saponite mixed-layer chlorite/smectite at low-temperature, and of serpentine, mixed-layer chlorite/corrensite or mixed-layer talc/chlorite at high-temperature. Moreover, variations in composition of hydrothermal fluids and precursor minerals, especially in Si/(Si+A1) and Fe/(Fe+Mg) ratios, are important factors in controlling the parageneses of phyllosilicates. Detailed characterization of phyllosilicates in terms of phase assemblages (parageneses), paragenetic sequences, and their compositions and microstructures may provide insight into the nature of submarine hydrothermal systems, convection and evolution of hydrothermal fluids, alteration mechanisms, and hydrothermal sulfide deposition. In ultramafic xenoliths, phyllosilicates are major products of hydrothermal alteration. Xenolith rich outcrops were found within the Miocene tuff breccias from Valle Guffari. This locality is situated in the Hyblean Plateau (south-eastern corner of Sicily, Italy), which consists of a thick Meso- Cenozoic carbonate sequence and of Neogene-Quaternary open shelf clastics with interbedded volcanic basic rocks. On the basis of petrographic and compositional featurs, collected xenolith population may be divided into: a) ultramafic, which consist of spinel facies peridotites and pyroxenites; b): feldspar bearing suite, mostly represented by metabasite rocks and anorthosites [Atzori et al., 1999; Punturo et al., 2000; Punturo, 2010]. Moreover, Manuella [2011] reports mineralogical investigations on some Hyblean harzburgite xenoliths, which show serpentinization and related secondary minerals (i.e. sulphides, saponite). In this contest, the purpose of the present work is to describe the formation of phyllosilicates [Catalano et al., 2014] obtained from xenoliths from Miocene tuff breccias exposed to various hydrothermal conditions: temperature, 300–600 °C; pressure 0.5-2 kbar; duration of treatment 4-16 days; pH 5 - 7. Xenoliths were characterized in detail before being used as starting material in hydrothermal synthesis. The starting material and run products were characterized by X-ray powder diffraction (XRPD), scanning electron microscopy with an energy-dispersive spectrometer (SEM/EDS) and differential scanning calorimetry (DSC) and thermogravimetry (TG). References Atzori P., Mazzoleni P., Punturo R., Scribano V., (1999). Garnet-bearing pyroxenite xenoliths from Hyblean Plateau (South-Eastern Sicily, Italy). Mineralogy and Petrology, 27, 1-17. Catalano M., Belluso E., Miriello D., Barrese E., Bloise A., (2014). Synthesis of Zn-doped talc in hydrothermal atmosphere. Cryst. Reseach and Tech. 49, 5, 283-289. Manuella F.C., (2011). Vein mineral assemblage in partially serpentinized peridotite xenoliths from Hyblean Plateau (south-eastern Sicily, Italy). Per. Min. 80, 2, 247-266. Punturo R., Kern H., Scribano V., Atzori P., (2000). Petrophysical and petrological characteristics of deepseated xenoliths from Hyblean Plateau, south-eastern Sicily, Italy: suggestions for a lithospheric model. Mineral Petrogr Acta 43, 1-20. Punturo R., (2010). Contribution to the knowlegde of the Hyblean lithosphere (south-eastern Sicily, Italy): the petrophysical approach. Rend. Online Soc. Geol. It., 11, 105-106.