Nature, affinities and significance of metavolcanic rocks in the Outokumpu assemblage, eastern Finland
Bulletin of the Geological Society of Finland
1983: Nature, affinities and significance of metavolcanic rocks in the Outokumpu assemblage, eastern Finland. Bull. Geol. Soc. Finland 56, Part 1-2, 25-52. The rock association of serpentinite-chrome-silicate skarn-dolomitequartz rock-black schist in the Outokumpu region, in which polymetallic sulphide ore deposits occur, has been compared to an ophiolite. However, despite evidence that the orebodies have a sea-floor exhalative origin, volcanogenic rocks have only been described recently.
... bed recently. Evidence is now presented that, in the Losomäki district, the volcanogenic rocks include pillow lavas, vesicular lava, agglomerate and tuff with autoclastic breccia. These original features indicative of a volcanogenic derivation are restricted to low strain enclaves in amphibole-chlorite (± garnet) schists, amphibole-epidote schists, zoisite-amphibole (+ chlorite) rocks and hornblende-plagioclase-quartz rocks, previously mapped as »skarn». These banded amphibolites and greenschists show evidence of repeated deformation and recrystallisation under P-T conditions pertaining successively to amphibolite and greenschist facies conditions, and grade into rocks retaining distinctive volcanogenic features. At Losomäki they constitute about 30 % of the outcrop of the serpentinite-skarn-dolomite-quartz rock -black schist assemblage, referred to as the Outokumpu assemblage. Comparable rocks occur in the same assemblage elsewhere in the Outokumpu region. Even though original volcanogenic features survive, complete metamorphic reconstitution means that the original compositions have to be deduced. Possible pseudomorphs after skeletal olivine (?spinifex) occur in some pillow cores, suggesting komatiitic affinities. Whole-rock chemistry indicates extensive alteration before the first metamorphic reconstitution; leaching of alkalis, iron, nickel, cobalt, chromium and base metals is implied, presumably in a marine environment. Some trace element ratios appear to remain well constrained, particularly Ti:Zr, Zr:Y and Ti:Y. On discriminant function diagrams affinities are suggested with primitive tholeiites of island arcs or back-arc basins; other features suggest similarities to the komatiite-tholeiite association of Archaean greenstone belts. The environment of eruption, as indicated by the relict morphology and field relations of the pillow lavas and related volcanoclastic deposits, appears to have been under shallow water on the seafloor. The geochemical parameters suggest that the tectonic setting was in an island-arc or back-arc basin and not on the deep ocean floor.