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Sub-alkaline gabbro-diabases in the southwestern Ukrainian Shield



Sub-alkaline gabbro-diabases in the southwestern Ukrainian Shield



Mineralogicheskiy Zhurnal 29(1): 44-57



Minero-petrographic, petro- and geochemical characteristics of gabbro-diabases of Proterozoic (Early-Middle-Riphean) age of the Upper Bug dike field have been presented. A distinguishing feature of these rocks is a broad development of corona structures (reaction rims) around olivine grains at the boundary with plagioclase. The rim composition mainly includes hornblende of tremolite type, amphiboles of cummingtonite-grunerite and antophillitic series occur rarely, and orthopyroxene--very rarely. Gabbro-diabases belong to K-Na series of subalkaline basic rocks; they passes low magnesium content and high titanium content, they are poor in Cr, Ni, Co and incompatible elements (Nb, Ta, Th, REE). Rare-earth elements are moderately fractionated (La (sub N) /Yb (sub N) = 6.3-6.6) and europium anomaly (Eu/Eu (super *) = 1.07-1.21) is displayed weakly in their spectra. It is supposed that dikes of subalkaline gabbro-diabases have penetrated from the intermediate magmatic chamber distributed near the boundary the crust-upper mantle. The initial magma of the basic composition suffered considerable differentiation even before arriving to the intermediate chamber. The residual melt, from which the subalkaline gabbro-diabases were crystallized, was rather high-temperature (T = 950-1050 degrees C) and lightly contaminated by crystal material ( (super 87) Sr/ (super 86) Sr (sub 2000) = 0.703576; epsilon Sr (sub 2000) = 18.4). The source of generation of the initial melt could be a mantle reservoir of PM type.

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