Globule-rich lavas in the Razjerd district, Qazvin, Iran: a unique volcanic fabric
Résumé
A hypocrystalline silica-rich (63-67 wt.% SiO2, dacitic composition) lava flow (called G-lava) in the subaerial eruptive sequence of the Alborz Mountains (Razjerd district, Qazvin Province) of northern Iran, contains abundant (40-50 vol.%) 0.1- to 5.0-cm globules set in a matrix of rather similar composition and microtexture. Numerous globules have coalesced, showing triple-point junctions with 120A degrees angles. Both phases in the G-lava (globules and matrix) contain similar microphenocrysts (plagioclase, ortho- and clinopyroxene and magnetite) in a trachytic groundmass. However, their mesostasis differ in colour, in composition, in the amount of glass and their amount of volatiles and silica: in the globules the mesostasis is darker and richer in SiO2 but is volatile poor. Other volcanic materials in the same unit are very similar in composition to the G-lava. The globular fabric was formed with two phases: one poor in volatiles (the globules), the other rich in volatiles (the matrix). The globules are slightly more silicic (66.9 against 64.6 wt.% SiO2), more potassic (3.7 againt 2.8 wt.% K2O) and more viscous (of the order of 10(3) to 10(4)) than the matrix outside the globules. It seems that the two phases (globules and matrix) with different silica and volatiles contents and thus different vesicularities, viscosities and densities, were produced in the dacitic melt due to temperature and pressure drop and magmatic degassing in the volcanic conduit involved fluid-melt exsolution processes. Some of the volatile-rich melt was probably frothy during eruption, producing volcanic bombs and scoria.