Geochemistry of alkali and nepheline syenites of the Ukrainian Shield: ICP-MS data

Abstract

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.

The Telychian (early Silurian) oxygenation event in northern Europe: A geochemical and magnetic perspective

“…However, on average the Zr/Nb ratio appears to distinguish west and east Sarmatia igneous sources (average 0.1*Zr/Nb of 1.3 and 2.2 respectively; Table S2). The west Sarmatia data sources with low (<0.5) 0.1*Zr/Nb are largely from 1.7 -2.1 Ga alkali and nepheline syenites (Dubyna et al 2014). Svecofennian averages are similar to east Sarmatia sources.…”

“…S10,S11a,b). The largest Ga/Sc values are from 1.7 -2.1 Ga syenites, anorthosites and rapakivi granites (Dubyna et al 2014;Shnyukov et al 2018) and 2.6 Ga high-silica rhyolites and granites (Savko et al 2019).…”

Götzenite in the Nepheline Syenites of the Pokrovo-Kyriyivo Massif (Azov Area, Ukraine)

“…1), наводили в аналізах гетценіту попередні дослідники (Valter, Eremenko, 1963;Eliseev et al, 1965). Серед усіх відомих масивів лужних порід Східного Приазов'я нефелінові сієніти Пок ро во-Киріївського є найзбагаченішими на Sr (Dubyna, Kryvdik, Sharygin, 2014), тому високий вміст Sr в гетценіті, як і значне різноманіття інших акцесорних Sr-мінералів, відображає геохімічну специфіку цих порід, але з'ясу вання причин його накопичення у гетценіті потребує подальших досліджень.…”