PETROLOGY OF A 2.41 GA REMARKABLY FRESH KOMATIITIC BASALT LAVA LAKE IN LION HILLS, CENTRAL VETRENY BELT, BALTIC SHIELD

Abstract

In the central Vetreny Belt, southeastern Baltic Shield, an areally extensive 110 m deep lava lake is exposed consisting of remarkably fresh differentiated komatiitic basalt. During eruption, the liquid had a temperature of 1380–1400 °C and contained ∼15% MgO. The lava ponded in a large topographic depression soon after eruption. The differentiation of the lava lake was controlled by settling of transported olivine and chromite phenocrysts and caused the origin of prominent internal layering. The last portions of the trapped liquid crystallized at temperatures of 1250– 1070 °C. A Sm-Nd isochron of 2410±34 Ma for whole rock samples, olivine, augite and pigeonite separates from the lava lake provides a reliable estimate for the time of formation of the uppermost sequences in the Vetreny Belt. This age is in good agreement with the Sm-Nd and Pb-Pb isochron ages of 2449±35 and 2424±178 Ma for the volcanic rocks from the same stratigraphic level in the northwestern Vetreny Belt. Modeling of Nd-isotopes and major and trace elements shows that the komatiitic basalts at Lion Hills may have had a komatiite parent depleted in highly incompatible elements. It can be shown that this initial liquid was contaminated by 7–9% of Archaean upper crustal material from the adjacent Vodla and Belomorian Blocks en route to the surface thus acquiring the observed geochemical and isotope signatures including relative enrichment in Zr, Ba, and LREE, negative Nb- and Ti-anomalies and ɛNd(T) of −1.