Structure and geophysics of the Gåsborn granite, central Sweden: an example of fracture-fed asymmetric pluton emplacement

Abstract

The emplacement mechanisms of the Palaeoproterozoic Gfisborn granite, a satellite of the Transscandinavian Igneous Belt (TIB), are !nvestigated using an integrated structural and geophysical approach. The pluton is discordant to c. 1.89 Ga folded supracrustal rocks that were deformed and metamorphosed at c. 1.85-1.80 Ga during the Svecokarelian Orogeny. Emplacement occurred at a depth of c. 10 km, within a regime of late Svecokarelian dextral transpression. Deformation of the pluton during cooling resulted in the formation of a variably developed foliation in the granite and deflection of less competent wall-rock units around its western and eastern contacts. Later E-W Sveconorwegian shortening resulted in the formation of shear zones that affect one of the pluton margins and may contribute to a component of the observed wall-rock distortion. The granite is situated above strong NNW-trending linear magnetic and negative gravity anomalies, which are interpreted to correspond to an important early Svecokarelian shear zone. The geophysical data indicate that the pluton is markedly asymmetric and modelling of the residual gravity field suggests that it consists of a deep root zone in the west and a thin sill-like body, which makes up most of the east and south parts of the body. Emplacement of the sill-like part occurred by lateral flow of magma from the root zone accommodated by downwarping of the underlying units. Intrusion of the thicker, discordant west part may have been accommodated by a combination of roof lifting and floor depression, aided by displacement on an active shear zone.

Mafic plutonic rocks in a continental‐arc setting: geochemistry of 1.87–1.78 Ga rocks from south‐central Sweden and models of their palaeotectonic setting

“…Cruden et al 1999;Koistinen et al 2001;Andersson et al 2004a; Figure 1). The eastern contact to the Svecofennian margin is partly tectonic and irregular, and in part intrusive.…”

Granitic magmatism by melting of juvenile continental crust: new constraints on the source of Palaeoproterozoic granitoids in Fennoscandia from Hf isotopes in zircon

“…The Gåsborn pluton (sample UB98:70) is a small (6 km 3 6 km) granitic intrusion in Svecofennian supracrustal rocks (Cruden et al 1999). The intrusion consists of coarse-grained, high-silica, metaluminous to peraluminous biotite granite, with å Nd of +1 to +2 (Andersson et al 2004a).…”

“…Isotopic decoupling of mafic-intermediate and felsic magmas can be achieved by a density filtering mechanism analogous to that envisaged for the Phanerozoic magmatism of the Oslo Rift in southern Norway by Neumann (1980). When the ascent of mafic magma through the continental crust is arrested at the level of neutral buoyancy, the heat released from crystallizing magma in the middle to lower crust will lead to anatectic melting in fertile crust above the magma chambers, and to the further rise of less dense, felsic anatectic melts (Clemens 1990), along conduits provided by Svecofennian shear zones (see Cruden et al 1999). As long as the separation and ascent of anatectic melts was rapid, little exchange of material is likely to have taken place at depth.…”

Magmatic history and geophysical signature of a post-collisional intrusive center emplaced near a crustal-scale shear zone: the Plechý granite pluton (Moldanubian batholith, Bohemian Massif)

“…Gravimetric surveys have been routinely used to provide some indirect clues to the three-dimensional subsurface shape of granitoid intrusions (e.g., Améglio and Vigneresse 1999;Aranguren 1997;Aranguren et al 1996Aranguren et al , 2003Benn et al 1999;Cruden et al 1999;Vigneresse 1990). Here we use gravimetry to complement our field observations and structural data and approximately constrain the gross shape and the minimum vertical dimension of the Plechý pluton.…”