Discrimination and origin of the sanukitoid series: Geochemical constraints from the Neoarchean western Karelian Province (Finland)

Abstract

Neoarchean late to post-tectonic sanukitoid intrusions are found in the western part of the Karelian Province in Finland. Variable-sized, even-grained to K-feldspar porphyritic intrusions form a series of diorites, tonalites, and granodiorites, that are calc-alkaline to alkali-calcic, magnesian, and mostly peraluminous. The major and trace element geochemistry of the intrusions show typical sanukitoid affinities: a mantle signature (high content of Mg, Ni, Cr and high Mg#) and enrichment in LILE (especially K, Ba, and Sr). The intrusions form a distinctive series that can be distinguished from the TTG series of the Karelian Province by their high Ba–Sr signature, low Na 2O/K 2O ratio, and uniform HREE pattern. The sanukitoid series can be compositionally constrained as follows: SiO 2 = 55–70 wt.%, Na 2O/K 2O = 0.5–3, MgO = 1.5–9 wt.%, Mg# = 45–65, K 2O = 1.5–5.0 wt.%, Ba + Sr > 1400 ppm, and (Gd/Er) N = 2–6. The probable source was enriched subcontinental lithosphere mantle with high LILE concentrations in phlogopite and/or sanidine. The accumulation of K, Ba, and Sr in the mantle source may have occurred as a consequence of two separate metasomatic events: 1) by subduction-related fluids/melts and 2) by metasomatism related to asthenospheric mantle upwelling caused by slab breakoff. The slab breakoff at the end-stage of subduction is also a viable trigger for the sanukitoid magmatism.