Late Archean komatiites of the Ura bay-Titovka structure, Northwestern Kola region

The extended Northern Kola greenstone belt was formed within the Kola granu lite-greenstone terrane in the Late Archean. Its lower part consists of vol canogenic basalt-komatiite sequences metamorphosed to amphibolite facies. T he most representative and well preserved these rocks were found in the wes tern part of the belt, in the middle reaches of the Ura, Western Litsa, and Titovka rivers. The rocks of the komatiite association compose massive and pillow lavas, differentiated lava flows, horizons and lenses of lava brecc ias and agglomerate tuffs of variable thickness and length, as well as subv olcanic bodies. The komatiites and basalts are products of areal subaqueous eruptions. Volcanic eruptions were accompanied by occasional phreatic expl osions. The scarce differentiated flows consist of cumulate, spinifex-textu red, and brecciated zones. Temperature and pressure in the mantle source ar e estimated as 1734 degrees C and 7-8 GPa, respectively. The temperature of erupted lavas was about 1520 degrees C. Geochemically, the komatiites appr oximate the Yilgarn type but show some what lower Al2O3/TiO2 ratio. They ex perienced intense three-stage metamorphism. The first stage produced serpen tine-magnetite acid later chlorite-tremolite assemblages with partial prese rvation of primary magmatic high-Mg olivine. The second stage resulted in t he local extensive development of olivine porphyroblasts owing to chlorite breakdown at 650-700 degrees C. The third, retrograde stage caused the form ation of carbonate and idding-site-bowlingite, The intense carbonation was responsible for partial redistribution of REE and other elements.