Absolute (U–Pb) and relative age determinations of intrusive rocks in the Ragunda rapakivi complex, central Sweden

Abstract

The Ragunda complex (ca 550 km 2) is one of the two major rapakivi occurrences in central Sweden and forms, together with seven smaller complexes, the youngest group (1.53–1.47 Ga) of rapakivi complexes in the Fennoscandian Shield. In contrast to most of the classic Finnish rapakivi plutons the Ragunda complex consists of large amounts of mafic rocks and a substantial volume of syenite and quartz syenite. At the present erosion level, the Ragunda complex includes ca 30% gabbro, 5% syenite and quartz syenite and 65% amphibole–biotite and biotite granites. Minor amounts of hybrid rocks and a large number of acid and basic dykes are found as well. The complex consists of three massifs with distinctive aeromagnetic anomalies. The Western Massif is a centred complex in which magmatic activity began in the west and moved towards the east, forming a linear sequence of approximately circular gabbro, syenite and granite intrusions. The last intrusion, a amphibole–biotite granite, gave a U–Pb zircon age of 1514±5 Ma. The Central Massif is a large centred complex without any obvious linear intrusion sequence, and consists of several consecutively emplaced plutons including a late granite with a U–Pb zircon age of 1505±12 Ma. The Northeastern Massif is a small centred complex consisting of gabbro closely followed by granite. Magma mingling structures are observed throughout the complex, most commonly in contact zones between gabbro and granite. The generation of the Ragunda complex and the small complexes in central Sweden cannot be explained by a lithospheric plate moving over a stationary hot spot. Deep fractures formed in response to an extensional regime probably controlled the emplacement of the complexes. This is supported by the large number of subparallel mafic and acid dykes associated with them.