Glass and amygdules in Precambrian diabases from Orivesi, southern Finland

Glassy apophyses of a Precambrian diabase dyke appear at Orivesi, southern Finland. In chemical composition the glass is a subalkaline tholeiite. The glass content of the apophyses is 90 °/o, the only crystals found being two generations of plagioclase and some sporadic olivine. The intrusion temperature is 1150°—1160°C and the mineral assemblage indicates low pressure. Although the water content of the glass is high, only weak signs of devitrification are present. The preservation of the glass proves that no temperature rise has taken place in the area during the last 1600 Ma. Diabase in another dyke of the same set shows amygdaloidal structure. The amygdules that appear at the upper contact are spheroidal and consist of calcite and euhedral quartz. Since the formation of amygdules is a phenomenon of shallow depths, the present erosion level is close to the 1600-Ma-old Precambrian one. K. Lindqvist and I. Laitakari, Geological Survey of Finland, SF02150 Espoo 15, Finland.