Emplacement mechanisms of late-orogenic granites: structural and geochemical evidence from southern Finland

Abstract

The country rock in southern Finland formed mainly during the Svecofennian orogeny ca. 1.9 Ga ago. The middle and lower crust was partially melted 1.83 Ga ago due to crustal thickening and subsequent extension. During this event, S-type migmatites and granites were formed along a 100·500 km zone. This Late Svecofennian Granite-Migmatite zone (LSGM zone) is a large crustal segment characterised by roughly E-W trending sub-horizontal migmatites and granites. Combined ductile E-W shear movements and NNW-SSE compressional movements defined a transpressional tectonic regime during the emplacement. Partial melts that moved through the crust pooled as granite sheets or froze as migmatites. Major transpressive shear zones border the LSGM zone, which forms a tectonic and metamorphic zone that crosscuts the earlier Svecofennian granitoids. Based on field observations and geochemical data from two sets of outcrops, we show that the great volumes of late-orogenic granites and migmatites in southern Finland were transported and emplaced as small chemically variable batches, possibly extracted from different protoliths. These melt batches were transported along repeatedly activated channels and collected at some horizontal level in the crust. In the Nagu area, the melt batches were trapped under a rooflayer of amphibolite and the whole complex was synchronously folded into open folds with steep axial surfaces and E-W trending fold axes. The sheets of microcline granite are, in places, strongly sheared; the microcline phenocrysts are imbricated and subsequent deformation of the microcline phenocrysts indicates syntectonic movements of the layers as well as a syn-tectonic mechanism for the late-magmatic fractionation. Depending on the degree of crystallisation of the individual melt batches during shearing at different intensities, the granites have slightly different appearances. Some sheared zones show a cumulate-like trace element geochemistry, indicating that melt fractions were expelled from the system, producing layers of deformation enhanced fractionated granites and cumulate layers. Our interpretation is that the Nagu area shows shear-assisted fractionation mechanisms in granitic melts, and that similar processes are responsible for the fractionation trends seen in the sub-horizontal sheeted granites in Ha¨meenlinna at higher levels in the crust.

Microstructures associated with the Sottunga-Jurmo shear zone and their implications for the 1.83-1.79 Ga tectonic development of SW Finland

“…The relationship between the LGSM and the SJSZ (and SFSZ) remains somewhat uncertain, although an obvious spatial connection can be observed as both the SJSZ and the SFSZ closely follow the southern margin of the LSGM. Stålfors & Ehlers (2005) suggest that the emplacement and fractionation of the~1 .83 Ga LSGM granitic melts in Nagu (c. 40 km to NE from Kökar) was a shear-assisted processes, and propose a model where subvertical crustal-scale shear zones functioned as transport channels for the magmas. Such processes, however, seem to be absent within and to the south of the SJSZ; on the other hand, the northern margin of the SJSZ is not well exposed.…”

“…The LSGM rocks are extensively migmatised and intruded by subhorizontal sheets of anatectic granites and tonalites at 1.85-1.82 Ga (e.g. Johannes et al, 2003;Kurhila et al, 2005;Stålfors & Ehlers, 2005). In contrast to the LSGM, the SJSZ and the area to the SW consist mainly of amphibolite facies orthogneisses dominated by steep foliations and fold axial surfaces (Fig.…”

From ductile to brittle deformation: structural development and strain distribution along a crustal-scale shear zone in SW Finland

“…The LSGM rocks consist of structurally overturned supracrustal rocks that are extensively metamorphosed, migmatised and intruded by sub-horizontal sheets of anatectic granites and tonalites at 1.85-1.82 Ga (e.g. Ehlers et al 1993;Väisänen and Hölttä 1999;Johannes et al 2003;Kurhila et al 2005;Stålfors and Ehlers 2005). In contrast to the LSGM, the SJSZ and the area SW of it shows mainly steep foliations and fold axial surfaces.…”

“…The relationship between the LGSM and the SJSZ (and SFSZ) remains somewhat uncertain, although an obvious spatial connection can be observed as both the SJSZ and the SFSZ closely follow the southern margin of the LSGM. Stålfors and Ehlers (2005) suggest that the emplacement and fractionation of the *1.83 Ga LSGM granitic melts in Nagu (c. 40 km NE from Kökar) was a shear-assisted processes and propose a model where sub-vertical crustalscale shear zones functioned as transport channels for the magmas.…”

1.8 Ga magmatism in southern Finland: strongly enriched mantle and juvenile crustal sources in a post-collisional setting

“…In addition to the data reported here, plotted data from these studied intrusions are from Hackman (1931), Nykänen (1968), Pitkänen (1985), Eklund et al (1998), Väisänen et al (2000), and Nironen (2005 and references therein). The reference data of the late-orogenic granitoids in southern Finland and 1.8 Ga post-collisional rocks from southern Finland and Russian Karelia are collected from Rutanen et al (1997 and references therein), Eklund et al (1998), Nironen (2005, Andersson et al (2006a), Stålfors and Ehlers (2006), and the Rock Geochemical Database of Finland (see in the reference list). Some of these analyses were also received from the late-Matti Vaasjoki (Geological Survey of Finland) in 2003.…”