Geology of the Pikku-Vaiskonselkä differentiated sill in the Kevitsa area, northern Finland

• Main Author: Suvanto, K. (Kalle)
• Format: Dissertation
• Language: English
• Published: University of Oulu 2014
• Subjects: Geology
• Online Access: http://urn.fi/URN:NBN:fi:oulu-201406041682; http://nbn-resolving.de/urn:nbn:fi:oulu-201406041682

The Pikku-Vaiskonselkä sill, situated in the Kevitsa area, northern Finland, was studied using core logging, field observations, petrography and whole-rock and mineral analysis. The aim of the Master’s project was to describe the intrusion and compare it with other layered intrusions in central Finnish Lapland, including the Kevitsa, Satovaara, Rantavaara intrusions, the Lower Suite of the Koitelainen intrusions, and layered sills the ca. 2.22 Ga gabbro-wehrlite association. The sample material from the Pikku-Vaiskonselkä sill consisted of drillcore samples (drillcores KV300 and KV305) and outcrop samples. Twenty polished thin sections were used in petrographical studies and one polished thin section was used to determine the composition of olivine, clinopyroxene and chromite with electron microprobe micro-analyzer. Whole-rock compositions were measured with the XRF and ICP methods. The Pikku-Vaiskonselkä sill is differentiated, consisting of chromite-bearing wehrlite, clinopyroxenite and gabbro. The studied rocks are strongly altered and the primary magmatic minerals of the rocks have been mostly replaced by secondary minerals, with preserved olivine and clinopyroxene being only observed in few outcrops. No significant sulfide mineralization seems to be present in the intrusion. The parental magma composition of the intrusion was not very primitive as shown by the forsterite content (~84.5 mol.%) of olivine. The shape of chondrite-normalized REE patterns suggests that the magma was LREE-enriched. Ratios of incompatible elements in olivine cumulates were used to estimate the character of the parental magma and to compare it with that of other mafic-ultramafic intrusions. The Pikku-Vaiskonselkä parental magma had Al2O3/TiO2 ratio of ~20 that is higher than in the other intrusions. Also the Zr/TiO2 ratio in the Pikku-Vaiskonselkä parental magma was higher. The Koitelainen Lower Suite olivine cumulates are compositionally closest to the Pikku-Vaiskonselkä sill rocks, but have higher CaO/Al2O3 ratios. Al2O3/TiO2 ratios in clinopyroxene suggest compositional differences between the Pikku-Vaiskonselkä, Kevitsa and the GWA intrusions. The differences in the mineral and whole-rock compositions between the Pikku-Vaiskonselkä sill and the other intrusions indicate compositionally different parental magmas.