Fluid history from crystal cavities in rapakivi, Pyterlahti, southeastern Finland

Abstract

The crystal cavities lined with quartz, feldspars, biotite, fluorite and a number of accessory phases occurring in the Haikonvuori pyterlite quarry in the Wiborg rapakivi massif were studied with mineralogical, petrographical and chemical methods. Primary fluid inclusions of quartz showed filling temperatures of 292°C to 342°C. The cavities as structures are considered to be miarolitic vugs of chamber pegmatites, but their filling is interpreted as largely secondary and deposited during at least three of the fluid stages present. The nature of the filling material was not reflected in the chemical composition of the pyterlites, but the presence of the cavities, irrespective of their filling, could be inferred from the La and Th depletion and from various petrographic characteristics indicating intensified fluid action. Thus, crystal cavities seem to provide ideal fluid traps for studies of the local fluid history of the crust.

Keywords

• quartz
• fluid inclusions
• topaz
• molybdenite
• miarolitic
• rapakivi
• chemical composition
• Finland
• Virolahti
• Pyterlahti
• Haikonvuori