Sequential development of metamorphic fabric and structural elements in polyphase deformed serpentinites in the Svecokarelides of eastern Finland

Abstract

Serpentinites are a major component in a distinctive suite of metasedimentary and metamorphosed igneous rocks, the Outokumpu association, in the early–middle Proterozoic Svecokarelides of eastern Finland. Like the adjacent mica schists and associated supracrustal rocks, they show the effects of at least six phases of deformation (D1–D6). The tectonic history began with the emplacement of the Outokumpunappe (pre-D1), but a study of relict assemblages in the serpentinites and theirenvelope rocks reveals evidence for a complex pre-nappe history involving ultramaficmagmatism with extensive high-temperature hornfels development, and low-temperature serpentinisation contributing to sea-floor exhalation of sulphide ores.The serpentinites subsequently underwent recrystallisation during several phases of regional metamorphism giving rise to mineral fabrics, including a pressure-solution initiated segregation (M1–D1), anthophyllite growth during the thermal climax (D2–M2—600–680°C, 2·5–4 kb), and steatitisation (D2c–M2c). Steatitisation is structurally controlled, occurring where serpentinite bodies impinge upon localised shear zones (wrench faults—D2c). Talc–carbonate–brucite assemblages reflect the influx of CO2 in this environment. Deformation during late metamorphic retrogression (greenschist—D3–D4) has a dichotomous expression with crenulations developed in strongly anisotropic phyllonitised serpentinite and irregular fracturesin massive rock.The implications of such polyphase reconstitution for geochemical and isotopic studies are assessed and discussed.

Tectonized actinolite-albite rocks from the Outokumpu district, Finland: Field and geochemical evidence for mafic extrusive origin

“…The low Ti0 2 is probably only indicative of the unusual parental magma composition, and inferences from diagrams based on Ti0 2 contents in basalts from modern tectonic environments are probably not therefore justified. Low Ti0 2 contents are also found to characterize, among others, basalts of the Cyprus ophiolite complex (Pearce and Gale 1977) and an island arc environment would be consistent with the back-arc basin model proposed by Park (1983) for the Proterozoic tectonic setting of the Outokumpu assemblage.…”

Metamorphic development of cordierite-bearing layered schist and mica schist in the vicinity of Savonranta, eastern Finland

“…Increasing tectonic pressure is consistent with a compressive deformational event that characterises D 2 for the Savonranta district (Halden 1982 a). It is also compatible with increasing depth-of burial, which would be a consequence of nappe emplacement (Campbell 1978;Koistinen 1981;Halden 1982 b;Park 1983 a). Increasing pressure during the M 2 event in the Savonranta district is probably a product of both mechanisms.…”

“…1 inset); there Park (1983 a) gives c. 550°C and 3-4 kb for M, conditions and 600°-680°C and 2.5-4 kb for M 2 conditions. However the Savonranta data contrast with the 600 ± 50°C and 3.5 ± 1 kb estimate derived from a post-D,, syn-D 3 (possibly syn-D 2C ) static cordierite -amphibole assemblage at Outokumpu (Treloar et al 1981).…”

“…The intrusion of the Maarianvaara granite during D 3 and D 4 (c.f. van Breemen and Bowes 1977) between Kaavi and Outokumpu may account for this; there is evidence in the same district of granitic material being associated with the end of the D 2 deformational phase (Park 1983 a).…”

Metagabbros, metasyenites and related rocks in the Outokumpu assemblage, east Finland

“…It consists of a distinctive group of serpentinized ultramafic bodies (dunite and saxonite, Haapala, 1936;Park, 1983), mafic metavolcanic amphibolites and greenschists (Park, 1984;Rehtijärvi & Saastamoinen, 1985), chromium-rich calc-silicate skarns and dolomites, quartz rock (metachert) and metalliferous black schists (carbonaceous sulphidic pelites) (Huhma & Huhma, 1970;Gaål et al 1975;Koistinen, 1981;Fig. 1).…”