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

Abstract

Whole-rock and isotope geochemistry of six ∼1.8 Ga post-kinematic intrusions, emplaced along the ∼1.9 Ga Southern Svecofennian Arc Complex (SSAC) and in the SW part of the Karelian Domain in Finland, was studied. The intrusive age [U–Pb secondary ion mass spectrometer (SIMS)] of one of these, the Petravaara Pluton, was determined as 1811 ± 6 Ma.

Basic-intermediate rocks are alkali-rich (K₂O + Na₂O > 4 wt.%) and typically shoshonitic, strongly enriched in large ion lithophile elements and light rare earth elements, but relatively depleted in high field strength elements and heavy rare earth elements. The enrichment is much higher than can be accounted for by crustal contamination and requires previously melt-depleted mantle sources, subjected to variable metasomatism by carbonate-rich fluids and sediment-derived melts. These sources are inferred to consist of phlogopite ± amphibole-bearing peridotites from depths below the spinel–garnet transition, as shown by the high Ce/Yb ratios. ⁸⁷Sr/⁸⁶Sr(1.8 Ga) ratios in the range 0.7027–0.7031 and ‘mildly depleted’ ϵ_Nd(1.8 Ga) values (+0.1 to +1.4), with T _DM values <2.1 Ga, suggest that mantle enrichment was associated with the previous Svecofennian subduction–accretion process, when enriched sub-Svecofennian mantle sections developed, dominantly characterized by ¹⁴⁷Sm/¹⁴⁴Nd ratios of 0.14–0.17.

The associated granitoids are diversified. One group is marginally peraluminous, transitional between I (volcanic-arc) and S (syn-collisional) types, and was derived from mixed igneous and sedimentary, but juvenile Svecofennian source rocks, as supported by near-chondritic ϵ_Nd(1.8 Ga) and somewhat elevated ⁸⁷Sr/⁸⁶Sr(1.8 Ga). The other group is transitional between I and A (within-plate) types in character and had dominantly igneous protoliths. The whole-rock geochemistry and isotopes suggest that the compositional variation between ∼50 and 70 wt.% SiO₂ may be explained by hybridization between strongly enriched mantle-derived magmas and anatectic granitic magmas from the juvenile Svecofennian crust. One intrusion in the east contains a significant portion of Archaean, mostly igneous protolithic material (ϵ_Nd(1.8 Ga) = –2.8 and ϵ_Hf(t) for zircons between +2.8 and −11.9, with an average of −4.9).

The ∼1.8 Ga post-kinematic intrusions were emplaced within the SSAC subsequent to the continental collision with the Volgo-Sarmatia craton from the SE, during a shift from contraction to extension, that is, in a post-collisional setting.

Keywords:

• geochemistry
• shoshonitic
• mantle metasomatism
• Nd-Sr-Hf and U–Pb isotopes
• Svecofennian
• Fennoscandian Shield

Acknowledgements

HR acknowledges the financial support from Magnus Ehrnrooth's Foundation, Agneta and Carl-Erik Olin's Foundation, Nordenskiöld Society in Finland, Oskar Öflund's Foundation, the Research Institute of Åbo Akademi University Foundation, the Principal of Åbo Akademi University (Åbo Akademi University Foundation), The Social Insurance Institution of Finland, and Carl Cedercreuz' stipendiefond (Svenska Kulturfonden). MV was funded by the Academy of Finland (project 117311). The Laboratory for Isotope Geology at the Swedish Museum of Natural History provided the facilities, expertise, and financial support through SYNTHESYS fundings (project acronyms SE TAF 1393 and 2050), which were made available by European Community – Research Infrastructure Action under the FP6 ‘Structuring the European Research Area’ Programme. Martin Whitehouse, Lev Ilyinsky, Chris Kirkland, and Kerstin Lindén from the Nordsim laboratory are thanked for all the help with the SIMS. The Nordsim facility is financed and operated under an agreement between the research councils of Denmark, Norway, and Sweden, the Geological Survey of Finland, and the Swedish Museum of Natural History. This is a Nordsim publication #259, and an SIGL (Finland Isotope Geosciences Laboratory) contribution. Hugh O'Brien at SIGL of the Geological Survey of Finland (GTK) is thanked for letting us make the Hf isotopic analyses and helping us with the analytical procedure and data handling. Hannu Huhma and Bo Johanson from GTK are shown gratitude for lending powder of the Parkkila sample for analysis and for allowing the use of equipments and instructing the CL/BSE imaging. Hannu Huhma and Tom Andersen reviewed an earlier version of the manuscript. Dmitry Konopelko and Shauket Baltybaev are acknowledged for updating the information on the geology of Russian Karelia. Jeremy Woodard is thanked for useful discussions and help in laboratory works. Veikko Grönroos, Juha Kauhanen, and Arto Peltola are acknowledged for preparing the thin sections.