The Proterozoic Ahvenisto rapakivi granite–massif-type anorthosite complex, southeastern Finland; petrography and U–Pb chronology

Abstract

The Ahvenisto complex belongs to the classic 1615–1645 Ma Wiborg rapakivi granite terrane of southeastern Finland. The complex covers an area of 350 km 2 of which 70% comprises silicic rocks (mainly subalkaline biotite±hornblende granites) and 25% gabbroic rocks (mainly leucogabbronorite, some olivine-bearing gabbroic rocks and rare anorthosite); a minor suite of monzodioritic rocks (as well as silicic and mafic dike rocks) is also found. On outcrop, the silicic rocks are usually found to cut the monzodioritic rocks, whereas the silicic and monzodioritic rocks always cut the gabbroic rocks. In terms of petrography and major element geochemistry, the gabbroic and monzodioritic rocks of the Ahvenisto complex are much like those described from the Proterozoic massif-type anorthosite occurrences, for example, the Harp Lake complex of Labrador. The Ahvenisto complex was emplaced into the Paleoproterozoic (1900 Ma) Svecofennian metamorphic crust 1630–1645 Ma ago and is thus one of the earliest intrusions in the Wiborg terrane. In particular, our U–Pb data on zircon and baddeleyite show that a barely measurable age difference exists between a leucogabbronorite (1643±3 Ma), a gabbroic pegmatite in leucotroctolite (1637±2 Ma), a quartz monzodiorite (1642±2 Ma), a hornblende–biotite granite (1633±6 Ma) and a quartz–feldspar porphyry dike (1632±5 Ma) of the complex.