Glass and amygdules in Precambrian diabases from Orivesi, southern Finland

Abstract

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.

Mesoproterozoic geomagnetic reversal asymmetry in light of new paleomagnetic and geochronological data for the Häme dyke swarm, Finland: Implications for the Nuna supercontinent

“…Contacts with the host rock are typically sharp. Noncrystalline glass has been discovered in some of the narrowest dykes (Lindqvist and Laitakari, 1980). In some of the dykes there are amygdoloids which indicate crystallization of magma close to the surface (Laitakari, 1987).…”

The Proterozoic granitoids of Finland: Granite types, metallogeny and relation to crustal evolution

“…1700 Ma ago. The diabase dykes with glassy apophyses and amygdules demonstrate that no significant erosion or thermal activity has affected the crust since their intrusion (Lindqvist and Laitakari 1980). The waning of the principal uplift stage of the Svecokarelian crust can be inferred from the K-Ar biotite ages, (Korsman et al 1984).…”

Assumptions about the presence of natural glasses on Mars

“…are commonly found [Bou•ka et al, 1993]. Older glasses, e.g., Jurassic to Lower Cretaceous volcanic glass in Cisbaikalia [Lebedeva, 1981], Permotriassic glass of the Targay fold [Korobov and Chernyayeva, 1981], Permian pitchstone in Saxony [Pietzsch, 1962], late Devonian microtekrites [Wang, 1992;Claeys et al, 1992], and Precambrian diabases [Miikipiiii, 1979;Lindqvist and Laitakari, 1980], are very rare. These paleoglasses occur mostly as constituents of volcanic rocks (e.g., diabase) and have usually been fully devitrified, although this is not the case for the Devonian microspherules or the Finnish diabase samples.…”