2013

Geochemical and petrological features of alkaline rocks of Ukraine

Kryvdik S., Dubyna O.

N.P. Semenenko Institute geochemistry, mineralogy and ore formation NAS of Ukraine, kryvdik@ukr.net

In Ukraine, the alkaline rocks are spread within the Ukrainian Shield (USh) and surrounding depressions. In USh alkaline rocks have mainly Proterozoic age (1.7-2.1 Ga) but in in the surrounding depression and the Azov area, in the joining zone of USh and folded Donbas, the Devonian alkaline rocks are developed.

Proterozoic alkaline rocks are belonging to two complexes - alkaline-ultrabasic (carbonatitic) and gabbro-syenitic. There are massifs of alkaline rocks of different erosion - from abyssal (20 km) to the hypabyssal and rarely of volcanic analogues of these species in this region.

Most of alkaline rocks have typical geochemical features - high content of Zr, Nb, REE, Y, Sr. However, in the western part of USh alkaline rocks of carbonatitic complex are unusual depleted in these elements.

The two different evolution trends of alkaline complexes are revealed. In the gabbro-syenitic complexes final differentiates are enriched in incompatible trace elements (Zr, Nb, REE, Y) and depleted in Sr, Ba and P. Jointly to this the alkali femic minerals are enriched in iron. This is due intensive crystallization differentiation during formation of gabbro-syenitic complex. At the same time, the differentiation of these elements in carbonatitic complexes is more complicated. Usually in the late alkaline and nepheline syenites concentration of REE and Y, and often Zr are lower than the earlier alkaline melanocratic rocks. In carbonatites P, REE and Nb are concentrated. The trend of enrichment in iron of femic minerals in the final differentiates doesn't usually took place, i.e. it is not expressed the crystal fractionation.

Within of the USh very clearly displayed the dependence of the mineral composition in alkaline rocks of carbonatitic complexes on the erosion depth. For hypabyssal complexes magnesian diopside, aegirine-diopside and is common forsterite, while in the deeply eroded massifs olivine and pyroxene are highly enriched in iron. There is some enrichment in REE and Nb of alkaline rocks and carbonatites with increasing of erosion depth.

Another interesting fact of changing in the isotopic composition of Sr in the differentiated series of gabbro-syenite complexes is revealed. In the apatite of the earliest rocks ⁸⁷Sr/⁸⁶Sr ratio has a "deep-derived" values ​​(0,702-0,704), while in some residual syenites strongly depleted in Sr and enriched in Rb ⁸⁷Sr/⁸⁶Sr ratio in apatites varies from 0,715 to 0,858. Chondrite-normalized patterns of REE in these rocks have a deep negative Eu-anomalies (Eu/Eu* up to 0.06).