Abstract
This paper is devoted to detailed study of picritic rocks (olivine melanogabbronorites) and comag� matic gabbrodolerites from sills and dikes in the central part of the Bashkirian meganticlinorium. These rocks are ascribed to the KamaBelsk magmatic province (K BP) that was formed in the eastern East European Platform (EEP) in the Mesoproterozoic time. The study of minerals (EMPA, SIMS), rocks, and their oxygen isotope compositions showed the contribution of crustal contamination, fractional crystallization and cumu� lus processes in their formation. The geochemical indicators of crustal contamination (Nb/Nb*, (Nb/La)n , δ 18 O, and others) show strong variations, which indicates uneven crustal contribution in the parental melts during rock formation (10-25%). The study of weakly contaminated (δ 18 O = 5.3‰) olivine melanogab� bronorites (MgO = 22.55 wt %) from the small Ishlya�1 subvolcanic body, which contain subordinate amount of cumulus (24%), highmagnesian olivine ( Fo91.3), and highCr spinel (cr# 0.67), as well as HREE depleted clinopyroxenes, allowed us to retrieve the composition of parental melt. The latter contained about 20 wt % MgO and was formed by 19-26% melting of mantle source (potential mantle temperature Tm of 1530-1545°C). Geochemical characteristics of KBP reflect the formation of primary melts by melting of mantle column at different depths, mixing of the melts, and significant contamination by crustal material. The dominant role in the formation of the rocks of the Ishlya area and Mashak Complex was played by derivatives of spinel peri� dotites, while the rocks of the Bakal-Satka area were derived from garnet peridotites. The melts of olivine melanogabbronorites crystallized within a pressure range from 7 to 2 kbar and tempera� ture from 1470°C to 1000-950°C at close to QFM. The comparison of model crystallization trend of parental melt with compositions of basic and ultrabasic rocks of the province showed that the observed com� positional diversity was provided by the accumulation of clinopyroxene-plagiolcase (dolerites and gabbrod� olerites) and olivive-plagioclase (picrites and picrobasalts) cumulates. Obtained estimates of conditions of magma generations with Tm significantly exceeding (at 150-165°C) that of asthenosphere indicate that the Mesoproterozoic magmatism in the eastern part of EEP was related to the ascent of deepseated plume. Pre� liminary results of comparative analysis of petrological-geochemcial characteristics of KBP and Mesoprot� erozoic basic rocks of Northern Greenland (Upton et al., 2005; Kalsbeek and Frei, 2006) revealed significant similarity of these withinplate manifestations, which in both cases bear signs of plume nature and distinct crustal contamination.
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