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).
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.
Two species types of basic rocks contrasting in chemical and mineral composition were investigated within the Korsun-Novomyrhorod anorthosite-rapakivigranite pluton (KNP) of the Proterozoic age. In comparison to more typical basic rocks of KNP, the investigated rocks are presented by the extremely Fe-rich fayalite gabbroids and the most MgO enriched biotite gabbronorite. The first of them occupy a deeper level of the Horodishche massif in the central part of KNP. According to a high-Fe association of minerals and reduced or low An content in plagioclase they may represent the crystallization of a highly evolved basic melt (after anorthosite and gabbronorites separation), which were crystallized under abyssal conditions and low oxygen fugacity. Preliminary geochemical data indicate that trace elements concentration and negligible negative Eu-anomalies (0.72-0.95) are similar to most distributed basic rocks but unlike the last it is slightly differed by decreasing La/Yb and enriched in Sc (up to 118 ppm). Thus, we suppose those rocks might be crystallized as a result of mixing highly differentiated (iron and alkali enriched) melt with the early generation of anorthitic plagioclase, with subsequent dissolution of the last. Enrichment in iron of the mafic minerals and increasing of alkalinity of plagioclase in the basic rocks is consistent with the appearance of ferrodioritic melts as a product of prolonged crystalline differentiation of the initial melt. In contrast to fayalitic gabbroids, the pyroxene-biotite gabbronorites from the border zone in according to increased Mg# of the mafic minerals and rocks are obviously the least differentiated varieties of the anorthosite-gabbronorite series. The regularities in chemical composition in such type of rocks are consistent with the liquid line of dissent for basic rocks in KNP, which implies their crystallization at an earlier stage of magma ascending. By composition, such melt can be formed at an intermediate stage from slightly differentiated melt. This is indicated by enrichment in Sr (453-881 ppm) and Ba (910-930 ppm), Eu/Eu* (0.85-1.10), increased content MgO (up to 8 wt. %), Cr and V (59-193 and 169-350 ppm respectively). At the same time these rocks are enriched in Zr and Hf (378-478 and 10.3-12.02 ppm respectively), highly enriched in Rb (169-192 ppm), with moderate Nb and Ta content (14.6-18.1 and 0.91-2.84 ppm respectively) that point out to interaction and partial assimilation by crust material. Summarizing geological data of the deep drill-holes, it is possible to reveal a general direction of the mafic minerals evolution in the basic rocks and the evidences of cryptic layering. The last are quite clearly manifested both in the large gabbro-anorthosite massifs and individual intrusive bodies. We suppose that the evolution trend of mafic mineral composition are consistent with the tholeiitic trend differentiation of the primary melt with gradual increasing of iron content (under low oxygen fugacity) during differentiation and ascending in the upper crust. According to this interpretation of the inner structure of gabbro-anorthositic massifs, on the modern erosional level of KNP are often exposed the fragments of the upper (or lateral) layered series (Fe-enriched), less often the drill-holes reach up to the heads of the lower layered series.
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