Petrology of foiditic and meymechitic volcanism in the Maimecha-Kotui province (Polar Siberia)

Comparative analysis of ultramafic meymechites of the Maimecha Suite and alkaline volcanics of the Ary-Dzhang Suite (foidites (nephelinites, analcimites, limburgites, etc.) and melilitites) has shown their consanguinity, which indicates their relationship with the same magmatic system periodically producing large amounts of alkaline ultramafic melts. We have studied the petrogeochemical and mineralogical compositions of rocks and melt inclusions in the hosted olivines. The rocks of the Maimecha and Ary-Dzhang Suite differ considerably in MgO content, which is well explained by the accumulation of olivine. The inclusions in olivines from the meymechites and the rocks of the Ary-Dzhang Suite correspond in composition to foidites. The trace and rare-earth element patterns are similar both in the foidites and meymechites and in the melt inclusions: They show negative anomalies of Rb and K and positive anomalies of Nb and Ta. The ratios of indicator elements (Nb/Ta, Ba/La, Ta/La, etc.) in the rocks of the Maimecha and Ary-Dzhang Suite are constant and almost independent of their Mg# values. The La/Yb ratio in the foidites is significantly higher than that in the meymechites and in the melt inclusions from their olivines, which indicates that the rocks of the Ary-Dzhang Suite resulted from the fractionation of highly magnesian alkaline picritoid melt.