The dolerite breccia of Tärnö, Late Proterozoic of southern Sweden

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Abstract
A Late Proterozoic tholeiitic dolerite dyke in southeastern Sweden abounds in xenoliths. The dyke belongs to a N to NW-trending swarm intruding Early Proterozoic granitoids and gneisses. The xenoliths consist predominantly of irregularly shaped bodies of finegrained feldspathic sandstone lacking counterparts among the wallrocks of the dyke. The dyke has normal chilled contacts with the wallrocks. Apart from the xenoliths, the central part of the dyke is a dolerite breccia composed of fragments of fine-grained to aphanitic dolerite in a clastic matrix, similar in texture and composition to the sandstone of the xenoliths. Many fragments of aphanitic dolerite are large, may show flow structures and/or carry small recrystallized sandstone inclusions. Devitrification textures of the aphanitic dolerite, and its fragmental structure, suggest that the rock formed mainly as a peperite when mafic magma at shallow levels intruded wet sediments or sandstones. The dolerite breccia probably formed largely during emplacement in the dyke by reversal of magma ascent. The intruded sedimentary sequence had features in common with the Jotnian Almesåkra Group of southeastern Sweden. The much larger onetime extent, often discussed, of this or other similar continental sandstones of Fennoscandia receives support by this study.

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