Petrological and Geochemical Peculiarities of Alkaline Rocks in the North-Western Region of the Ukrainian Shield

The data of chemical composition and content of trace elements in alkaline-ultrabasic rocks of the northwestern region of the Ukrainian Shield are summarized. The occurrences of alkaline rocks of this composition were discovered as results of drilling in four areas (Horodnytska, Hlumchanska, Bolyarkivska, and Hubkivska). They are presented by small hypabyssal intrusions, stem- and dike-like bodies. The rocks are represented by the jacupirangite-melteigite series, which belong to high-Mg (Mg# 0.7–0.8) varieties. Less common are alkaline and subalkaline gabbroids found in thin apophyses and veins near alkaline-ultrabasic bodies. It is assumed that these gabbroids were formed as a result of crust material contamination of the primary alkaline-ultrabasic melts. The alkaline-ultrabasic rocks of all occurrences have high Cr (233–2737 ppm) and Ni (95–1022 ppm), but are unusually depleted in incompatible elements (Nb, Ta, Zr, Hf, REE, Y), with moderate Ba and Sr. Their geochemical features sharply differ from similar rocks, carbonatite and alkaline complexes known as in the Ukrainian Shield and other regions of the world. Despite the primitive composition of alkaline-ultrabasic rocks and anomalous REE depletion, their chondrite-normalized patterns have negative slopes and are similar to differentiated rocks with a prevailing of LREE (La/Yb = 6–16). The petrogenesis of such primitive and trace-element depleted alkaline-ultrabasic rocks is enigmatic. We suppose that these unusual geochemical features are the result of primary melt(s) generation from mantle source and their evolution by a similar way. For the alkaline rocks of the Horodnytska and Hlumchanska intrusions, and possibly the Hubkivska occurrence, the primary melts arose through partial melting of garnet (with amphibole?) peridotites. At the same time, the primary melt for the Bolyarkivska intrusion was generated by partial melting of spinel peridotite, probably in a shallower environment. The anomalous depletion at HFSE and REE can be considered as the result of specific conditions of partial melting of depleted mantle source at low temperature and pressure, when refractory minerals concentrating these elements (rutile, amphibole) remained in the restite, or due to early crystallization and fractionation of concentrating minerals (perovskite, apatite).