Early Paleoproterozoic rift volcanism in the eastern Fennoscandian Shield related to the breakup of the Kenorland supercontinent

Abstract

The widespread early Paleoproterozoic (ca. 2.5–2.2Ga) volcanic and associated sedimentary rocks represent an episode of initial rifting of the late Archean craton in the eastern Fennoscandian Shield. These continental margin rocks cover cratonized Archean basement of the Fennoscandian Shield, in eastern-northern Finland and northwest Russia, and represent a typical Sumian-Sariolian volcanic-sedimentary succession. Physical volcanology and major element geochemistry of the subaerially erupted calc-alkaline basaltic to andesitic lavas of the Matinvaara Formation in north-eastern Finland, provide globally important characteristics for this craton-wide event. The lavas are generally massive or amygdaloidal, but flow-top breccias and variolitic lavas are also found. Laterally, different lava flows of this formation can be followed up to several hundred meters. Diagnostic pseudoforms after primary minerals and volcanic glass in the lavas are used in identifying crystallizing phases. Additionally, the primary structures of lava flows give indication of accumulation and settling processes. Specific internal mineral textures also relate to cooling history of these lavas. Flow inflation is suggested by accumulation of multiple vesicle-rich horizons. Thick massive flows may be analogues of tube pahoehoe flows produced by subaerial flow inflation. The vesicular units are, however, directly evidence for devolatilization of lava, whereas varioles, in the Matinvaara Formation, represent a form of plagioclase spherulite. Varioles consist of radiating crystals of plagioclase and pyroxene, presently identified as albite-actinolite pseudoforms. Under near equilibrium crystal growth conditions, kinetics favored euhedral, compositionally homogenous crystals. Continued undercooling facilitated the formation of compositionally zoned crystals and under greater undercooling anisotropic, skeletal and dentritic crystals of plagioclase and pyroxene rapidly developed. Tectono-magmatic evolution of the Matinvaara Formation suggests subalkaline magmatism in a intracratonic setting, which was related to the breakup of the Kenorland supercontinent ca. 2.5–2.1Ga. Geochemistry indicates that the volcanism was affected by felsic continental crustal contamination with plume-driven rifting. This volcanism likely occurred in distinct pulses with total duration of tens of millions of years.