Interpretation of magnetic fabrics in the early proterozoic diabase dykes of Keuruu, central Finland

Abstract

Magnetic fabrics, petrophysics, petrography and chemical compositions are described for 14 diabase dykes located in the Keuruu area, central Finland. These Early Proterozoic (1880 Ma) dykes, sharply cut both low-density granodiorites and high-density diorites and gabbros. The diabase dykes, with a NW-SE strike and an almost vertical dip, have a maximum length of a few kilometers and widths of about 1 m. Strongly developed magnetic fabrics of deformation type (anisotropy degree up to 1.4) were observed in six of the dykes, and weakly developed fabrics of flow type (anisotropy degree below 1.1) in eight dykes. No systematic differences in the dyke widths, mean densities, palaeomagnetic poles, mineral compositions, degrees of alteration or chemical characteristics were observed between the two types of dykes, suggesting that the dykes had a common magma source. All the fabrics of deformation type are dominantly ferrimagnetic with tightly aligned horizontal foliations and well-grouped lineations, which in most cases are parallel to the dyke trends. It is suggested that the deformation fabrics were generated as tholeiitic magma was forced to intrude laterally along its level of neutral buoyancy (LNB), which may have been close to the present erosion level (PEL) in the low-density granodiorite environment. Ferri- and paramagnetic fabrics of flow type characterize the dykes located in the denser mafic parts of the crust. In these parts the LNB was possibly somewhat higher in the crust, so that magma flow passed the PEL and formed weakly developed flow fabrics with gently plunging lineations and vertical or variable magnetic foliations.