Dati petrografici su una mela-nefelinite ad haüyna degli Iblei (Sicilia sud-orientale)

During the lower Quaternary fissural eruptions, mostly submarine, occured in the present northern margin of the Hyblean Plateau, along a NWSE wrench fault system. The related magmatic products consist of basalts, basanites and melanocratic nephelinites. The latter rarely bear sodalite group phenocrysts. The studied rock derives from an abandoned quarry nearby Poggio Santalanea, in the northern margin of the Plateau, a few kilometres away the Ioniar coast (Fig.1). The exposed nephelinite shows columnar jointing: it belongs to a large volcanic body (Fig.1) widely covered by younger carbonatic sandstones which do not allow us to recognize whether it is a shallow-depth subvolcanic intrusion or the inner part of a thick lava flow. The hand-specimen appears massive, quite fresh and dark-grey coloured. Under the microscope it exhibits porphiritic texture (P.1=48), with phenocrysts and microphenocrysts of clinopyroxenes (zoned salite to Ti-augite), microphenocrysts of haüyne zoned to nosean and minor olivine (FO_86-82). The microcrystalline groundmass consists of Ti-augite, Fe-Ti oxides, nepheline and apatite. The haüyna microphenocrysts (200-500 μm in size) are generally euhedral and exhibit opacitic outer rims. The latter consist of a mass of interwoven needles of apatite. In the inner parts of the crystal, scattered apatite needles form a regular pattern according to crystallographic directions. The haüyna grains often contain variable amounts of trapped groundmass and a lot of fluid inclusions, decrepitated in most cases. Whole-rock major and trace elements are similar to those of the other Hyblean nephelinites, except for the higher CaO (15.74 wt%) and Sr (2952 ppm) contents. Moreover, it is noteworthy that the Zr/Hf (55.34), is the highest detected in any Hyblean volcanic rocks. The occurrence of euhedral haüyne microphenocrysts implies a significant concentration of volatiles (particularly S and CI) in the melt, even during the latest crystallization stage. This fact may depend on the sealing effect of a glassy crust or ductile shale country rocks laying over the original magmatic body. In addition, haüyne crystalization requires an effective silica undersaturated nature of the magma, with a relatively low SiO₂/CaO. These chemical features depend on the mantle source nature and the (low) partial melting degree. Nevertheless, the assimilation of re-mobilisized evaporitic sediments, likely occurring underneath the studied area, might be also accounted for the extra supply of Ca, Sr, S and Cl to the nephelinitic magma, as well as for its high Zr/Hf.