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We present preliminary results and interpretations of the field and microscopy work in metavolcanic successions from the Dannemora area, south central Sweden. The c. 1.9 Ga metavolcanic units are represented mainly by rhyolites, interbedded with carbonates, and surrounded by granitoids. At Bennbo (2km W Dannemora) a 60 m long profile with redeposited volcaniclastic deposits is exposed. The sequence shows well-preserved sedimentary structures and evidence of soft sediment deformation. BIF which is interbedded in this succession is quite rare in Bergslagen. The BIF sequence contains Ba-feldspar, with c. 8 wt% BaO (hyalophane) which is common in manganese and manganese-ferromanganese deposits. Thin section analysis of massive skarn show anisotropic and euhedral garnets, proved by microprobe analysis to be hydro-andradite. Alteration and devitrification of ignimbrites is prominent in thin sections, where glass shards are totally altered to sericite-aggregates. Spherulites are characteristic for densely welded ignimbrites.
One steeply dipping major shear zone has been recorded. Field observation indicates east side up kinematics, which is in accordance with earlier work. The shear zone controls the boundary between rhyolite, basalt, granitoid and ultra mafic rocks. A detailed study of its nature and kinematics is in progress. Metamorphic conditions have been interpreted from deformation mechanisms in quartz and feldspar phenocrysts. Quartz show dynamic recrystallisation and feldspar show brittle deformation, indicating metamorphic temperatures not higher than 500°C. Foliation in the metavolcanic rocks is usually defined by coexisting white mica and chlorite, and the presence of chlorite and epidote in intermediate dikes indicates metamorphic conditions of upper greenschist facies.
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