Fabio Carmelo Manuella

ORIGIN OF SAPONITE-RICH CLAYS IN A FOSSIL SERPENTINITE-HOSTED HYDROTHERMAL SYSTEM IN THE CRUSTAL BASEMENT OF THE HYBLEAN PLATEAU (SICILY, ITALY)

A diapiric intrusion of clays in the Carlentini Formation (Tortonian) was discovered in a quarry at S. Demetrio High (Hyblean Plateau, Sicily, Italy). Seven clay samples were analyzed by different analytical methods, including X-ray powder diffraction (XRD) and Fourier-transform infrared (FTIR) spectroscopy, to determine the composition and mechanism of formation (sedimentary vs. hydrothermal) of these clays. Ferric saponite, carbonates (calcite and traces of ankerite), quartz, pyrite, and zeolites (phillipsite and harmotome) were detected using XRD and FTIR. This mineral assemblage, dominated by Fe-rich saponite, and the abundance of light rare-earth elements (LREE), Eu, fluid-mobile elements (FME > 10 × primordial mantle: Li, Be, B, As, Sb, Pb, U, Ba, Sr, Cs), and other incompatible elements (Zr = 169 ppm, Nb = 46 ppm, Th = 11 ppm, on average) imply that S. Demetrio clays precipitated from a mixture of hot Si-rich hydrothermal fluids (350–400°C) and cold seawater. The evidence is in accord with the affinity of clays for hydrothermally modified mafic and ultramafic rocks, forming the Hyblean lower crust, based on multi-element comparisons, and on the occurrence of trace amounts of chrysotile 2Mc1 and sepiolite. The association of long-chain aliphatic-aromatic hydrocarbons (intensity ratios I2927/I2957 > 0.5) with hydrothermal clays, the lack of fossils, and the similarity of the IR absorption bands with those of organic compounds detected previously in some metasomatized Hyblean gabbroic xenoliths suggest a possible abiogenic origin of hydrocarbons via a Fischer-Tropsch-type reaction. The S. Demetrio clay diapir was emplaced at shallow crustal levels in the Late Miocene as a consequence of the interaction, at a greater depth, of an uprising basalt magma and the products of an early, serpentinite-hosted hydrothermal system.

Micro-Raman spectroscopy and SIMS characterization of oxykinoshitalite in an olivine nephelinite from the Hyblean Plateau (Sicily, Italy)

A Ba-Ti rich oxymica occurs in an olivine nephelinite from S. Demetrio High in the northern margin of the Hyblean Plateau (Sicily, Italy). The rock sample exhibits a porphyritic texture formed by olivine, clinopyroxene, nepheline, titanian magnetite, apatite, and rare subhedral tabular mica crystals, which have a perfect cleavage on {001}, and a strong pleochroism with X pale brown and YZ brown. The studied oxykinoshitalite, characterized by micro-Raman spectroscopy, electron microprobe WDS (wavelength- dispersive system), and secondary-ion mass spectrometry (SIMS), was compared with the type material from the Fernando de Noronha island (Pernambuco, Brazil). Structural formula of the Hyblean oxykinoshitalite, calculated on the basis of 7 (Si, Al, Fe, Mg, Ti), is (Ba0.51K0.41Na0.04Ca0.01)0.97(Mg1.98Fe0.55Ti0.48)3.01(Si2.42Al1.56)3.98 O10(O1.17F0.62(OH)0.21)2.00. The lack of chemical zoning and the enrichment in Zr and Nb in the groundmass crystals of the Hyblean oxykinoshitalite suggest formation during the final crystallization stage of a basaltoid magma with ocean island basalt (OIB) affinity. Most likely, the magma originated by partial melting of metasomatized ultramafic rocks in the Hyblean crustal basement.

Reply to “Comment on Manuella et al. ‘The Hyblean xenolith suite (Sicily): an unexpected legacy of the Ionian–Tethys realm’ by Beccaluva et al. (2015)”

One of the most important pieces of background information left in our pen (Manuella et al. 2015) regards the circumstance that, during the last 25 years, international marine geology expeditions brought crucial advances in understanding the composition and tectonic evolution of present and fossil oceanic lithosphere (e.g., Pearce 2002; Dick et al. 2003; Boschi et al. 2006; Snow and Edmond 2007; Ildefonse et al. 2007; Miranda and Dilek 2010; Silantyev et al. 2011). In particular, we would draw attention to some fault-bounded abyssal highs, called oceanic core complexes (OCCs), located in the crest zone of (ultra) slow-spreading mid-ocean ridges. OCCs mostly consist of serpentinized mantle peridotites and gabbroic rocks exhumed to the ocean floor along systems of detachment faults, related to serpentinite diapirism. Most elevated blocks even reach the ocean surface to form non-volcanic ocean islands, as well as the St. Peter and St. Paul Rocks located near the axial zone of MAR in the equatorial region (e.g., Campos et al. 2010; Sharkov 2012). More in general, magmatic layers of the normal oceanic crust are very thin or even absent at OCCs sites, seismic profiles being compatible with a serpentinite layer overlying almost unaltered mantle ultramafics (e.g., Blackman et al. 2004a, b). In this respect, the concept of a “crust” had to be called into question, and hence, the Moho can be regarded as a serpentinization front (e.g., Minshull et al. 1998). Oxide-rich gabbros with sheared texture are considered obliged components of the gabbroic suite of present and fossil OCCs (e.g., Sharkov 2012). Veins of plagiogranites are also relatively common in these oceanic structures, intruding both gabbros and peridotite bodies. Oxide gabbros and plagiogranites from OCCs typically bear zircon as accessory phase (e.g., Aranovich et al. 2013). OCC basalts, Introduction