Petrographic study of high-grade gneisses, Halland area

Sammanfattning: This is a petrographic study of high-grade gneisses in the Halland area, Sweden. During bedrock mapping by the Swedish Geological Survey, three principal petrographically distinct gneiss domains have been recognized: the Skene, Varberg, and Halmstad domain. The purpose is to compare the petrographic textures and minerals that occur within this area, to determine possible metamorphic differences between the domains. Within this study, the petrographic descriptions will also be compared with technical data made on the gneisses, to evaluate if the gneisses could be used as aggregates, in for example road materials. The study was conducted in the south-western part of Sweden, in the lower Eastern Segment. A total of 19 thin samples from 10 different localities wad studied under an optical microscope, of which four localities are active quarries. The Skene domain orthogneisses typically have blue-green hornblende, dark brown/green brown biotite, no antiperthitic plagioclase, no perthitic orthoclase, microcline with tartan twinning, titanite and opaque minerals but no garnet are present. The textures of the Skene samples are inequigranular to seriate with both polygonal and interlobate grain boundaries. The Varberg domain orthogneisses typically have brown-green hornblende, red-brown biotite, antiperthitic plagioclase, perthitic orthoclase, no microcline with tartan twinning, garnet and opaque minerals are present. The texture of the Varberg samples are seriate with both polygonal and interlobate grain boundaries. The Halmstad domain orthogneisses typically have brown-green hornblende, red-brown biotite, antiperthitic plagioclase, no perthitic orthoclase and no microcline with tartan twinning, opaque minerals and occasionally garnet and epidote are present. The textures of the Halmstad samples are seriate to inequigranular with both polygonal and interlobate grain boundaries. From the study, it is concluded that even though the protolith classifications plot fairly similar, the metamorphic overprint seems to differ between the domains. From hydrous conditions in the amphibolite facies in the Skene gneisses, anhydrous conditions in the amphibolite to granulite facies in the Varberg gneisses, to anhydrous conditions in the amphibolite to granulite facies in the Halmstad gneisses. The Skene and Halmstad gneisses have a more pervasive recrystallization than the Varberg gneisses. From the petrographic study, rocks suitable for road aggregate production correlate with the presence of antiperthitic plagioclase and the absence of tartan twinning and titanite. Furthermore, higher sericitization has been correlated to higher technical values for road aggregates, which is inconsistent with previous work. This could be due to the Halland area being subjected to several metamorphic events, where prograde and retrograde reactions have affected the plagioclase differently, than in areas from previous work. It is concluded that the Varberg and the Halmstad gneisses seems to have the best properties for production of aggregates for roads while the Skene gneisses unsuitable for this purpose. This could be due to the fact that the Skene domain have a higher retrogression in comparison to the other domains.