The Geometry of a Gabbro Unit Intruded in a Granitic Body, Billdal, Sweden

Abstract

The Billdal area is a part of the Kungsbacka bimodal suite characterized by alternating felsic and mafic intrusions in a N-S trending band stretching from Kungsbacka in the south to Trollhättan in the north. The aim of this study is to better understand the large-scale geometries of one of these gabbro intrusions in a 25 km2 area around Billdal and Hällesås, south-western Sweden. To do this, a gravimetric survey is conducted in order to interpolate the gravity anomalies over a wider area. A profile of this interpolation is compared with a simulated gravity profile in ModelVision. A bedrock profile based on mapped outcrops is also created to verify the result and to attempt to calculate the shape of the intrusion. The gravity simulation shows all the signs of being a deep magma conduit and an almost horizontal sill, but the density of the theorized magma conduit needs to be higher than previously assumed. No gravitational signs of a massive gabbro structure can be found over the area presumed to house the sill, and no other magma conduits have been identified in the study area. The bedrock profile indicates the same structure as the simulated gravity profile.