New U-Pb ages from the Wiborg rapakivi area: constraints on the temporal evolution of the rapakivi granite-anorthosite-diabase dyke association of southeastern Finland

Abstract

Abstract New U-Pb data on zircons, monazites, and baddeleyite suggest that the Wiborg rapakivi batholith and associated mafic rocks in southeastern Finland were emplaced mainly between 1650 and 1625 Ma. The earliest anorogenic magmatism related to the intrusion of the rapakivi granites was the emplacement of some diabase dykes about 1665 Ma ago, while the youngest porphyries intruded the rapakivi granites at 1615 Ma. The process involved three peaks of diabase activity at 1665, 1645, and 1635 Ma and two major granite events at 1640±5 and 1630±5 Ma, the former of which comprises also the intrusion of minor gabbroic-anorthosite bodies. On the whole, the result was the emplacement of at least 105 km3 of rock material over a period of 50 Ma. Within southern Finland, the rapakivi magmatism continued until 1540 Ma by emplacement of the West Finnish intrusions, which combined are as extensive as the Wiborg area. Globally, the Proterozoic rapakivi event probably represents the largest pulse of intracratonic magmatism which occurred during geological history and may be a consequence of rapid growth of continental masses in the Early Proterozoic.