Deformation and Migmatization in the Stensjostrands Naturreservat, Halland Province SW Sweden

The Stensjostrand Naturreservat is in the N-S trending Eastern Segment of the Proterozoic Sveconorwegian Orogen: a high grade tectometamorphic province roughly equivalent to the North American Grenville. Geologic mapping in the Naturreservat reveals that the metamorphic rocks in the research area underwent at least two episodes of shortening and leucosome generation. The first identified generation of folds (F1) that affects the main gneissic banding in the area (S1) and that is particularly well developed in sillimanite-bearing gneisses, is characterized by angular hinges, NE-SW trending axis and associated centrimetric-scale parasitic folds. Locally, a fanning foliation is associated to the parasitic folds. Subsequent compression led to the development of a second fold set (F2) without foliation development. During this younger shortening event, mostly upright to moderately plunging cylindrical open folds with NE-SW to E-W trending axis reoriented F1 structures.

Leucosomes in sillimanite-bearing gneisses are granitic in composition and their mineral assemblages suggest that they formed by muscovite and biotite consuming dehydration melting of the surrounding gneisses. The oldest generation of leucosomes (G1) is folded by F1 and forms centimetric-scale parasitic folds whose sense of shear reverses across the F1 limbs suggesting that G1 is syn-deformational to F1. A younger generation of coarse grained granitic leucosomes (G2) crosscuts F1 parasitic folds, postdating the first shortening event. Leucosomes in mafic gneisses are tonalitic (T1, T2) in composition and formed dehydration melting reactions in the mafic gneisss that locally produced large orthopyroxene crystals. T1 leucosomes are parallel to the S1 gneissic banding while T2 leucosomes occur in shears that cut this banding. The relationship between leucosomes and F1 structures should make it possible to use zircons in the leucosomes to date both migmatization and the F1 deformational event.