Rautalammin Kiviniemen granaattipitoinen fayaliittiferrogabro

Kiviniemi intrusion is located in Rautalampi, Central Finland. In addition to a garnet-bearing fayalite ferrogabbro with elevated scandium concentrations, the intrusion consists of a wider area of heterogeneous ferrogabbro, leucogabbro and diorite. The intrusion is located in the northeastern part of the Central Finland Granitoid Complex, right next to the primitive arc-type rocks of the Savo Belt. The mafic intrusion is surrounded by a porphyritic granite. This study is based on the results of field work (field observation reports and samples) and analyses made by Geological Survey of Finland, and also on the reports of nine drill holes (combined length 1251,50 m) drilled during the years 2008-2010. Whole-rock geochemical analyses were made with the XRF-method for the main oxides (n=74) and ICP-MS for the trace elements (n=28). Petrographic studies were based on 51 thin sections, from which 12 were chosen for electron microprobe analysis, in order to examine the compositions of solid solution series minerals and map the distribution of some trace elements. Concordant age results based on the U-Pb system are reported for the garnet-bearing fayalite ferrogabbro (TIMS, 1857±2 Ma) and the country rock granite (LA-ICP-MS, 1859±9 Ma). Neodymium isotope compositions were also determined at the Geological Survey of Finland. The initial neodymium epsilon values are +0,1 for ferrogabbro and -2,4 for granite. The ferrogabbro of the main deposit is coarse-grained, enriched in iron (FeOtot 11-40%), scandium (130-281 ppm) and zirconium (235-5210 ppm). Locally, the concentrations of titanium and phosphorus can also be high (max TiO2 3,6% and P2O5 1,8%). The main rock-forming minerals are fayalite, almandine, ferroaugite/ferrohedenbergite, hastingsite/actinolite and plagioclase. Potassium feldspar, quartz, ilmenite, fluorapatite and zircon are found as accessory minerals. The high pressure iron-manganese silicate mineral pyroxferroite was revealed by microprobe analysis. Main deposit is surrounded by small to medium-grained ferrogabro/diorite, which has a slightly heterogeneous mineral composition and texture. Small shreds of leucogabbro are located in the proximity of main deposit. The study of the geochemical features suggests that the distribution of trace elements and REE is uniform throughout the Kiviniemi intrusion, including the country rock granite. The high concentration of iron refers to a highly evolved tholeiitic magma, which originated from the mafic melts of upper mantle and emplaced into the lower crust. High concentrations of some rare earth elements (Hf, Y, Sc) and locally elevated percentages of alkali metals may indicate crustal contamination during magma ascent. The age of the Kiviniemi intrusion and its country rock represent the magmatism of the post-kinematic stage of the Svecofennian orogeny (1880-1860 Ma). The presence of garnet and pyroxferroite in the ferrogabbro may reflect high crystallization pressures (7-12 kbar). To examine the enrichment of scandium and zirconium, further isotope measurements (Hf, Sr) are needed. Modeling the contamination and magma mixing as well as investigating the effects of possible fluid activity are proposed.