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

Abstract

ABSTRACTSerpentinites 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.