Spinel-bearing symplectites in Palaeoproterozoic ultramafic rocks from two different geological settings in Finland: Thermobarometric and tectonic implications

Abstract

Spinel-bearing symplectites and reaction rims after olivine and plagioclase are common in metamorphic mafic rocks. Two such occurrences in Palaeoproterozoic rocks in Finland are described and their conditions of metamorphism calculated. Both occur in olivine cumulates between cumulus olivine and intercumulus plagioclase, and were formed at ca. 1.9 Ga during the Svecofennian orogeny. They, nevertheless, represent quite different tectonic environments, as is reflected in their mineralogy. The first is from a Svecofennian tholeiitic intrusion at Saarijärvi in SE Finland. This intrusion is assigned to juvenile island arc setting with a MORB-type substrate, and the symplectite was formed under low-P/high-T granulite-facies metamorphism during the accretion of the island arc to a continental margin following emplacement and crystallization of the intrusion. The reaction rim consists of orthopyroxene against olivine and pargasite-spinel symplectite against the plagioclase side. The calculated PT-conditions are 900°C and 5-6 kbar. The other rock unit is located at the margin of the Lapland granulite belt, which constitutes a Palaeoproterozoic mobile collision zone between two Archaean blocks or microcontinents. The origin of the intrusion and its primary age is not clear, but metamorphism took place under relatively high pressures, either at the crust-mantle boundary during 1.9 Ga metamorphism of the granulite belt or slightly afterwards in a subduction zone beneath the overthrust granulite belt and Inari-Kola craton. This symplectite consists of orthopyroxene in the olivine side and pargasite-spinel and clinopyroxene-spinel symplectite on the plagioclase side. Calculated PT conditions are 670°C and 11–12 kbar.