Intrusion mechanisms and magnetic fabrics of the Vehmaa rapakivi granite batholith in SW Finland

摘要

The c. 1580 Ma Vehmaa rapakivi granite batholith, southwestern Finland, consists of four intrusive pulses forming a semi-concentric pattern. The central part of the batholith consists of two batches of almost identical medium-grained porphyritic rapakivi granites forming the inner and outer zone of the central part. The two zones can easily be distinguished on aeromagnetic images and by their rtetrophysical properties. A magnetic survey, including anisotropy of magnetic susceptibility (AMS), was carried out to follow up the magnetic properties and internal fabric of the two central units. The bulk susceptibility (MS) and thermomagnetic measurements verify that the main magnetic mineral in the inner zone is magnetite (average MS value is –14000 OD while the outer zone is paramagnetic (average MS value –250 IASI). Composition and geochemical signature of the two magma batches is compatible with fractional crystallization as a dominant process of magmatic evolution of the central part of the batholith. The magnetic fabric shows magnetic foliations that follow the concentric shape of the inner and outer zone and generally dip gently to moderately outwards. The magnetic lineations plunge gently in the direction of the slightly elongated medium-grained porphyritic rapakivi granites. The shape of the AMS ellipsoid within the inner zone is more oblate in the centre, while towards the contact to the outer zone the ellipsoid becomes more prolate. Field observations combined with the AMS data are interpreted to indicate that the Vehmaa batholith was emplaced as a succession of gently dipping sheets above a piston-shaped collapsing structure