Garnets in volcanic breccias of the Phlegraean Fields (southern Italy): mineralogical, geochemical and genetic features

Summary

¶Post-magmatic garnets occur in volcanic breccias at the base of the Neapolitan Yellow Tuff (NYT) formation in the north-western area of the Phlegraean Fields. We report the results of a comprehensive study of these grandites. Garnet is found on the surfaces of tuffaceous blocks or inside their micropores, and is associated with sodalite, sanidine, marialite and amorphous silica. Garnet samples were examined by scanning electron microscopy (SEM), electron probe microanalysis (EPMA), powder and single-crystal X-ray diffraction (XRD) and infrared spectroscopy (IR). SEM observations on morphology showed typical dodecahedral and icositetrahedral habits. EPM analysis showed that they are close to grossular or andradite end members, with only moderate solid solution between them. X-ray study of single crystals showed cubic cell dimensions a_o of 11.86 Å (grossular) and 12.04 Å (andradite). IR spectroscopy confirmed the presence of hydroxyls in coexisting garnet and sanidine, 0.06 wt% H₂O (garnet) and 0.05–0.07 wt% H₂O (sanidine), respectively. Well-crystallized sanidine of an earlier generation showed significantly higher water contents, in the range 0.13–0.23 wt% H₂O. Type of occurrence and mineralogical features suggest a post-magmatic (pneumatolitic) genesis for these garnets. This is consistent with the physico-chemical processes linked to the eruptive dynamics of the breccias. Experimental studies of garnet synthesis at 550 °C and 2 kbar provide further support for this concept.