Perttu Mikkola

The Archaean Karelia and Belomorian Provinces, Fennoscandian Shield

The Archaean bedrock of the Karelia and Belomorian Provinces is mostly composed of granitoids and volcanic rocks of greenstone belts whose ages vary from c. 3.50 to 2.66 Ga. Neoarchaean rocks are dominant, since Paleoarchaean and Mesoarchaean granitoids (> 2.9 Ga) are only locally present. The granitoid rocks can be classified, based on their major and trace element compositions and age, into four main groups: TTG (tonalite-trondhjemite-granodiorite), sanukitoid, QQ (quartz diorite-quartz monzodiorite) and GGM (granodiorite-granite-monzogranite) groups. Most ages obtained from TTGs are between 2.83–2.72 Ga, and they seem to define two age groups separated by a c. 20 m.y. time gap. TTGs are 2.83–2.78 Ga in the older group and 2.76–2.72 Ga in the younger group. Sanukitoids have been dated at 2.74–2.72 Ga, QQs at c. 2.70 Ga and GGMs at 2.73–2.66 Ga. Based on REE, the TTGs fall into two major groups: low-HREE (heavy rare earth elements) and high-HREE TTGs, which originated at various crustal depths. Sanukitoids likely formed from partial melting of subcontinental metasomatized mantle, whereas the GGM group from partial melting of pre-existing TTG crust.

Alkaline-rich quartz syenite intrusions of the Western Karelia subprovince

Abstract We have studied a group of granitoids from the Western Karelia subprovince of the Fennoscandian Shield. This group is referred to as quartz syenites, but shows compositional variation from syenites to quartz monzonites, with a small number of monzonites and granites. Compositionally studied rocks are alkali and alkali-calcic, and magnesian, mostly metaluminous. Characteristically, they have a high content of alkaline (Na, K), large ion lithophile elements (LILE) (Ba, Sr), high-field strength elements (HFSE) (TiO2, Zr, Ce), as well as a low content of Mg, Ni and Cr, by which they can be distinguished from sanukitoid and quartz diorite suites of the Karelia Province. These quartz syenites were emplaced between 2.74 and 2.66 Ga, representing late-phase intrusions overlapping in age with the sanukitoids, the quartz diorites and the leucogranitoids. Initial whole-rock ϵNd values of quartz syenites vary from 1.8 to −1.8, and do not indicate a significant contribution of considerably older crust. Oxygen-isotope data for zircon indicate a varying mantle source (δ18O 5.35–7.15‰), with a contribution from source(s) with elevated δ18O values. Our data provide constraints on compositionally diverse Neoarchaean magmatism in the Archaean Karelia Province. The late Archaean evolution of the Western Karelia subprovince resembles that of the Neoarchean domains worldwide with respect to granitoid composition and temporal distribution. Supplementary material: Tables detailing geochemical analyses, analytical data for the five age samples and oxygen-isotope analyses from this study are available at https://doi.org/10.6084/m9.figshare.c.3459771

The tectonomagmatic significance of Neoarchaean variably alkali-enriched gabbro and diorite intrusions of the western Karelia Province

Abstract This study describes a group of Neoarchaean alkali enriched gabbros and diorites from the western Karelia Province of the Fennoscandian Shield. We provide new field observations, petrography, whole-rock chemical data and additional whole-rock Sm–Nd and O-isotope data from these Neoarchean rocks. Compositionally, the rocks can be classified as shoshonitic with elevated rare earth element, K2O, Ba and Sr contents together with variable MgO, Ni and Cr contents. The MgO, Ni and Cr depletion observed especially in one of the intrusions could have been caused by fractionation of olivine and clinopyroxene from the system. Zircon O-isotope values from one of the intrusions (δ18O=7.34±0.10‰) indicate the involvement of material that had undergone low-temperature fractionation of oxygen in the genesis. Samarium–Nd data imply contribution from older material in the petrogenesis of these rocks. The above-mentioned characteristics can be explained with a magma source in the mantle that was heterogeneous owing to the variable degrees of metasomatism. The alkali-enriched gabbros and diorites provide additional evidence for magmatism derived from heterogeneously enriched mantle during the Neoarchaean in the Karelia Province and associated with the cratonization of the area. Supplementary material: Table detailing oxygen-isotope analyses from this study is available at https://doi.org/10.6084/m9.figshare.c.3466575