Three stages to form a large batholith after terrane accretion – An example from the Svecofennian orogen

Full-Scale Crustal Interpretation of Kokkola–Kymi (KOKKY) Seismic Profile, Fennoscandian Shield

“…9) as already pointed out by Korja et al (1993). Nikkilä et al (2016) suggested that the high velocity lower crust is formed by both crustal differentiation process after three granitic melting stages and by mafic underplating during the latest magmatic event. CFGC is represented by overall thicker crust (63 km) and a thicker (high velocity) lower crustal layer (18 km) compared to the supracrustal Bothnian and Häme belts on its margins with overall crustal thicknesses of 54 and 43 km and lower crustal layer thicknesses of 12 km and 4 km, respectively.…”

“…It consists mainly of granitoid plutons together with minor amounts of gabbroic plutons and mafic dykes, and less prominently of volcano-sedimentary units (Nironen 2017). Nikkilä et al (2016) suggested that the batholith was formed in three stages involving mafic underplating, differentiation, partial melting, and granitoid formation. By using FIRE1 and FIRE3A transect data and analogue modelling techniques, Nikkilä et al (2015) interpreted that the orogenic crust has laterally extended by 50% and thinned by maximum of 20% via westward gravitational spreading.…”

The Vaasa Migmatitic Complex (Svecofennian Orogen, Finland): Buildup of a LP‐HT Dome During Nuna Assembly

“…Therefore, we propose that the first‐order structures highlighted by the magnetotelluric and seismic profiles represent primary lithological and tectonic features developed during stacking of supracrustal rocks in an accretionary prism (stacks of fertile, wet sediments and large amounts of black shales) and mafic volcanic slivers (e.g., Lahtinen et al, ). Nevertheless, the main structures could have been reactivated, extended, and inverted several times and could have been enhanced by extensional block rotation as proposed by Nikkilä et al ().…”

“…In the CFGC, it developed conjugate NW‐SE striking dextral ductile shear zones and roughly E‐W striking magmatic to subsolidus planar fabrics (Nikkilä et al, ; Nironen, ; Nironen et al, ). The shear zones are interpreted to have been active in the CFGC at around 1.88 Ga or later (Nikkilä et al, ; Nironen et al, ) and the E‐W striking metamorphic fabrics a few million years before (Nikkilä et al, ). In the Tampere‐Pirkanmaa belts, the D2 deformation has produced upright tight to isoclinal F2 E‐W striking folds and associated axial planar S2 schistosity (Kilpeläinen, ; Nironen, ).…”

“…Luukas et al () divided the Finnish part of the Svecofennian orogen into the Southern and Western Finland Subprovinces (Figure a); the latter being divided into the Central Finland Granitoid Complex (CFGC) and the Savo, Tampere‐Pirkanmaa, and Pohjanmaa schist belts (Nironen et al, ). The CFGC, interpreted as a typical root zone of a magmatic arc (Rämö et al, ), comprises mainly (I‐type) calc‐alkalic to alkali‐calcic granitoids (1.89–1.87 Ga; Nironen et al, , Nikkilä et al, ) with minor amounts of mafic plutonic rocks and comagmatic volcanic sequences (Kähkönen, ; Korsman et al, ; Lahtinen et al, ; Nironen et al, ). It is surrounded by schist belts (Savo, Tampere‐Pirkanmaa, and Pohjanmaa schist belts) made of supracrustal (meta)sediments (greywackes and pelites) and volcanic rocks.…”