Crystal Growth of Zircons in Puttetti Syenite: Nature of Fluids and Implications on CO<SUP>2</SUP> Activity

Abstract

Fluid inclusion studies in a number of doubly polished plates of zircons showing interesting features are documented. The equilibrium balanced shapes of inclusions and their pattern of distribution are suggestive of slow growth rate, largely controlIed by stacking mechanism, resulting in completed layerites. Development of tensional fractures which eventually healed by dendritic growth and typical necking-down textures of long tabular inclusions are recorded. Thermometric studies show tbat the early fluids were of high density (0.90-0.85 g/cm³)CO₂ type with probable traces of CH₄ and/or N₂, entrapped at a pressure of 4.5 Kb. The fluids evolved into pure CO₂ (O.83 - 0.68 g/cm³) and to mixed CO₂-H₂O around 650°C and 3.7 Kb due to formation of vapour phase, while a fraction of the melt was sti!l present. The composition of fluids recorded suggests repeated CO₂ activity in south India with a peak during Late Precambrian-Early Palaeozoic times, effecting the generation and emplacement of alkaline plutons.