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ta-Nb МІНЕРАЛІЗАцІя в ЛуЖНИХ пОРОДАХ СХІДНОГО пРИАЗОв'я (уКРАїНА) наведено результати мікрозондового дослідження мінералів Nb і та у лужних магматичних і метасоматичних породах східного Приазов'я. в октябрському масиві досліджено мінерали надгрупи пірохлору в біотит-егіринових дайкових мікрофойяїтах, агпаїтових фонолітах і маріуполітах. у лужних метасоматитах (фенітах) сіл дмитрівка і хлібодарівка, а також балки калмицька (притока р. кальміус) мінерали групи пірохлору є також головними мінералами Nb і та, підпорядковане значення мають колумбіт і фергусоніт-(Y). у мінералах групи пірохлору переважає власне пірохлор із низьким або помірним вмістом Ta, Ree, Y, Zr, U, Th, Pb, але порівняно часто трапляються різновиди з високим вмістом цих елементів (Ta 2 o 5 до 13-24 %), Ree 2 o 3 (до 13-30), Y 2 o 3 (до 7-15), Zro 2 (до 12), Uo 2 (до 14), Tho 2 (до 10), Pbo (до 15-33 %). Переважання мінералів групи пірохлору над іншими ніобатами пояснюється високою лужністю вмісних порід. Значна частина Nb ізоморфно входить у титанові та титанвмісні силікати та оксиди (%): ільменіт (до 6), рутил (до 10,7), псевдорутил (до 13), баотит (до 7-18), цирконоліт-(Y) (до 8), куплетськіт (до 5-8).

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It was made an attempt to generalize geochemical data of Nb and Ta concentration in the most common igneous rocks of the Ukrainian Shield (USh). In the majority of widely distributed rocks of the USh (normal and subalkaline granitoids) the Nb and Ta concentration are similar to upper crust but lower the accepted Clark value for acidic rocks. In the more differentiated rapakivi granites concentrations of these elements reach or exceed the Clark’s concentrations (up to 35 ppm). Only highly differentiated alkaline rocks and alkaline feldspar granites (Perga, Kamiani mogyly, Ruska Poliana massifs) have high Nb concentrations (up to 800, 120 and 370 ppm, respectively). Medium rocks of the normal range are the least geochemically studied and typically are characterized low Nb and Ta concentrations. Almost all gabbroids as well as their metamorphosed analogues in the greenstone structures, are characterized by very low Nb (and Ta) concentration (two or more orders of magnitude) compared with Clark’s values for basic rocks (20 ppm Nb and 0.48 ppm Ta) according to A.P. Vinogradov. Against this background, increased of Nb and Ta concentration is observed in the main rocks of anorthosite-rapakivi-granite plutons. Howover even in these rocks concentration of these elements rarely reach or does not achieve the values which are typical for subalkaline and alkaline basalts of continental rifts. The regional heterogeneity in Nb and Ta distribution is observed in alkaline rocks of different composition: in the Azov Sea region these rocks are characterized by high concentrations, while in the western part of the USh the content of these elements is extremely low. The increased concentration of these elements is also revealed in kimberlites from the Azov Sea region and the Kyrovohrad megablock of the USh. The authors give some considerations and assumptions about the dependence on geochemical features of the Precambrian igneous rocks of the USh from the geodynamic conditions of their formation.

We present new geochemical data on alkali and nepheline syenites from various complexes of dif ferent age within the Ukrainian Shield. The results reveal a correlation between the content of trace elements in the syenites, their assignment to a particular rock complex, the chemistry of primary melts, and the degree of their differentiation. The data also suggest regional geochemical heterogeneity in the ultramafic-alkaline complexes of the Ukrainian Shield. The alkali and nepheline syenites in the ultramafic-alkaline massifs from the eastern and western parts of the region exhibit similar REE contents and Eu/Eu* ratios but are markedly different in Nb, Ta, Zr, and Hf content and are of the miaskitic type. These rocks have lower REE, Nb, and Zr and higher Sr and Ba compared with early foidolites. The rocks of the gabbro-syenite complexes define a distinct Fe enrichment fractionation trend from early syenitic intrusions to more differentiated varieties; they are also characterized by lower Sr, Ba, and Eu/Eu* and significantly lower contents of some major elements, e.g., Ti, Mg, and P. The agpaitic index and concentrations of Zr, Nb, Y, and REE increase in the same direc tion. A similar geochemical feature is observed in the alkali syenites genetically associated with anorthositerapakivi granite plutons, which show incompatible element enrichment and strong depletion in Sr and Ba. The distinctive evolutionary trends of alkali and nepheline syenites from different rock complexes of the Ukrainian Shield can be explained by different mechanisms of their formation. The main petrogenetic mech anism controlling the distribution of trace elements in the rocks of ultramafic-alkaline complexes was the separation of parent melts of melanephelinite and melilitite types into immiscible phonolite and carbonatite liquids. The gabbro-syenite complexes and alkali syenites from anorthosite-rapakivi granite plutons evolved via crystallization differentiation, which involved extensive feldspar fractionation.