Emplacement and magnetic fabrics of rapakivi granite intrusions within Wiborg and ?land rapakivi granite batholiths in Finland

摘要

Anisotropy of magnetic susceptibility (AMS) studies were carried out in two areas in Finland: the Ruotsinpyht?? intrusionwithin the Wiborg rapakivi granite batholith and the Saltvik intrusionswithin the ?land rapakivi granite batholith. The main aim of this study was to understand the emplacement history of these mid-Proterozoic rapakivi granites. Aeromagnetic images reveal structures of ca. 5-10 km intrusions that build up the large rapakivi granite batholiths of ?land andWiborg. Magnetic susceptibility data fromthe database of the Geological Survey of Finland, includingmore than 1700 samples fromtheWiborg rapakivi batholith and almost 900 samples fromthe ?land rapakivi batholith,were compared with measurements fromthe present study. Themean susceptibility is ca. 1500 μSI for the Wiborg batholith and ca. 10,000 μSI for the ?land batholith. Samples taken for this study demonstrate that the mean value for the Ruotsinpyht?? intrusion is ca. 1200 μSI and for the Saltvik intrusions ca. 24,000 μSI. Thermomagneticmeasurements reveal that the magnetic susceptibility is mainly derived either from paramagnetic minerals or from magnetite. The absence of solid-state deformation features such as breccia or contact deformation indicates a cauldron-type subsidence emplacement. The AMS measurements from Ruotsinpyht?? confirm these proposals, with concentric gently dipping magnetic foliations that support a ring complex structure above a piston-type subsidence system. The Saltvik area consists of a number of smaller elliptical intrusions of different rapakivi granites forming a multiple intrusive complex. The magnetic fabric shows a generalwestward dipping of the pyterlite and eastward dipping of the contiguous even-grained rapakivi granite, which indicates a central inflow of magma batches towards the east and west resulting from a laccolitic emplacement of magma batches, while the main mechanism for space creation was derived from subsidence.