MICAS AS INDICATORS OF GEOLOGICAL PROCESSES: THE CASE OF MT. VULTURE (ITALY)

The Mt. Vulture volcanic activity lasted for the Middle Pleistocene with several unrests after long period of inactivity. The explosive-effusive activity of Mt. Vulture produced SiO₂-undersaturated volcanics ranging from carbonatites and melilitites, to foidite, phono-foidite, tephro-phonolite and phonolites. In a recent revision of the stratigraphic setting by using unconformity boundary units (Giannandrea et al., 2006), Mt. Vulture products have been classified into a number of synthems and subsynthems. The latter permitted a reliable reconstruction of its evolutionary history. In the present contribution the relationships between micas crystal chemistry and the evolution of their host rocks are highlighted. The micas here considered belong to different stages of Mt. Vulture activity and underwent multi-analytical investigation which encompassed: chemical analytical techniques (EPMA , SIMS, CHN), spectroscopic techniques (Mössbauer investigation) and structural techniques (SCXRD).

Mt. Vulture micas are trioctahedral true micas in the phlogopite-annite join, with minor content of brittle micas (kinoshitalite-ferrokinoshitalite join) . The annite component (Fe/Fe + Mg) increases from the less evolved products to the more evolved ones (range ~ 0.10 – 0.40). A notable feature of Mt. Vulture micas is the amount of ferric iron, as determined by Mössbauer spectroscopy: octahedral Fe³⁺ ranges from ~ 47% in the youngest products up to ~ 90% of the total iron in more evolved ones.

The analysis of chemical and structural parameter strongly indicate that the main substitution mechanisms in the studied micas are: Ti-oxy ( ^[VI]M²⁺ + 2(OH)- « ^[VI]Ti⁴⁺ + 2O^2- + H₂), and M³⁺-oxy (^[VI]M²⁺ + (OH)- « ^[VI]M³⁺ + O^2- + ½ H₂), with M³⁺ = Fe³⁺, Al³⁺). A trend towards the increase of the oxy-component, i.e. a decrease in the hydrogen content in micas for more evolved products is apparent, and is supported by a combination of CHN and SIMS measurements. However, in the oldest products, occurrence of more than one micas population complicates the matter.

Overall , although relationship between micas and host melt is complex, it seems that micas crystal chemistry closely reflect physico-chemical melt changes.

Giannandrea, P., La Volpe, L., Principe, C., Schiattarella, M. (2006): Bull. Soc. Geol. It., 125, 67–92.