Late svecofennian magmatism and tectonism, Åland, Southwest Finland

Abstract

The Proterozoic, late-Svecofennian, Group III Granites of Sederholm (Ava, Seglinge, Mosshaga and Lemland) form discrete complexes distributed along a NESW line across the Åland Archipelago in southwestern Finland. They were emplaced (1840-1800 Ma) after cessation of the main Svecofennian orogeny, during the subsequent long period of continuing mantle—crust interaction and repeated crustal adjustment by shearing which culminated in the major Rapakivi Granite event. Current investigations on the island of Seglinge suggest that these complexes were rapidly emplaced at high crustal levels. Over a short time interval, a composite pipe of hybrid rocks of intermediate composition was emplaced and, with the surrounding country rocks, was subsequently disrupted and invaded by porphyritic-porphyroclastic granite to form breccia screens and ring dykes. Field relationships and textures suggest that the granite was emplaced largely as gas-borne solids. The petrography and mineralogy of the hybrid rock suite is such as could have been produced by contact anatexis—assimilation where hot, basic melts were impounded beneath older, deeper porphyritic granite sheets. The granite phase at Seglinge may be a sample of the roof of such a source complex, mobilised by volatile-discharge when the surface was breached. Compositional change in transit would be minimal. The compositional similarity of the Seglinge granite and the rapakavi granites is so close as to suggest some genetic kinship. The Group III complexes may represent local, premature, tappings from the developing rapakivi source region. The composition indicated for the granites of the source region of the hybrid suites and granites of the Group III complexes differs markedly from that of the early Svecofennian granites of the region. It is, however, closely similar to granite sheets described from SW Sweden and Greenland which have been attributed to crustal depletion associated with basic intrusion. Similar lower crust—mantle processes may have operated during the long, spasmodically eventful, period which followed the major Svecofennian orogenic activity in SW Finland.