Alaric M. Hopgood's research while affiliated with University of St Andrews and other places

Pajunen, M., Hopgood, A., Huhma, H. & Koistinen, T. 2008. Integrated structural succession and age constraints on a Svecofennian key outcrop in Västerviken, southern Finland. Geological Survey of Finland, Special Paper 47, 161-184, 21 figures and 2 tables. The Västerviken outcrop in Karjalohja, southwestern Finland, represents an exam- ple of polyphase migmatitic rocks of the Svecofennian amphibolite facies supracrustal gneiss terrain, southeast of the non-migmatitic rocks in the Orijärvi area. The struc- tural sequence on the outcrop yields two major groups of ductile deformation events, Ea-Eb and Ed-Eg, with a cooling stage in between them and later wide-scale open warp- ing structures, Dh-Di, of the earlier structures. The structural succession at Västerviken is correlated to the regional structural scheme in the southern Svecofennian domain. The primary features of the felsic gneiss suggest a volcanic or volcano-clastic ori- gin at c. 1.9 Ga. The earliest structures identified are isoclinal, intrafolial folds, Fa, in layered felsic gneisses, but they were not identified in the Db-deformed gneissic tonalites. The first strong metamorphic pulse was accompanied by structures that pro- vide evidence of an intense shortening event, Eb, that caused isoclinal Fb folding of the polyphase neosome veins at c. 1.88 Ga. Discordant relations of mafic dykes to the host rock and to Eb neosome veins, prove that the crust was rigid and cool prior to mafic dyke intrusion; this dilatational event is named as Ec. The mafic dykes were meta- morphosed and altered in a potassium-metasomatic event, Ed, that was progressively developed to high-temperature, penetrative shortening accompanied by production of garnet-bearing granitic veins and dykes. The Ec-Ed events are related to an progressive extension that was followed by contractional steep folding events. Isoclinal Fe char- acterizes the outcrop and surrounding area. Large-scale asymmetric transpressional folding, Ff with a NE-SW-trending axial plane deformed the earlier structures. The Ef event was followed by angular Fg folding with N-trending axial planes suggests c. E-W contraction. An ESE-WNW-trending coarse-grained pegmatitic granite dyke, which yielded a U-Pb monazite age of 1804 ± 2 Ma, cuts the Dg structures as shown by gneiss fragments including Fg folds in the dyke. A pegmatitic granite dyke was intruded into a dilatational fracture after or during the latest stage of the Eg event, partly regulated by ductile sinistral N-S shearing near the dyke contacts. The latest structures, Dh-Di, are open warps of the early structures. Fh folds are recumbent warps that were refolded by the open Fi folds formed under N-S compression. These deformations did not mark- edly affect the pre-existing structures on outcrop scale, but they are important when in- terpreting the regional variations in metamorphic grade, geophysical anomaly patterns or continuation of lithological sequences on the present-day erosion surface.

A study has been made to test the likelihood that migmatites cropping out in the Finnish Archipelago have undergone a different, similar, or identical, tectonic history to those in small, poorly exposed inland areas of the forested region further east in the vicinity of Orijärvi. The study is based on comparison of structural associations and has used the detailed small-scale structural succession already determined from the exceptionally well-exposed Proterozoic Svecofennian migmatites on outcrops in the Jussarö-Skåldö area of the Finnish Archipelago. Preliminary results show strong similarities between the Archipelago migmatite successions and those at Orijärvi, suggesting that they shared at least part of a common tectonic history. This indicates that further detailed structural examination would be rewarding and should confirm, or otherwise, that the regions shared a comparable tectonic history.