Field and geochemical studies of the melilite-rearing Arydzhangsky Suite, and an overall perspective on the Siberian alkaline-ultramafic flood-volcanic rocks

This paper presents the first comprehensive geologic; petrographic, geochem ical, and Sr- and Nd-isotopic study of the 350 m thick Arydzhangsky lava su ite, the last suite in the entire north Siberian, flood-volcanic sequence t o require study by modem methods. Within this sequence, only the Arydzhangs ky Suite includes melilite-bearing lavas; it is composed of mela-nephelinit es to limburgites (both melilite-bearing and melilite-free), with rare meli litites and picrites. The lavas contain from 0 to 60 vol% melilite, with Mg O contents ranging from 5.7 to 29.5 wt%. Nonetheless, these compositionally diverse lavas are all quite similar in incompatible-element geochemistry. They are distinct from all other Siberian alkaline-ultramafic lavas and, am ong these lavas, show the most resemblance to the Yakutian kimberlites. With this contribution, all of the north Siberian, alkaline-ultramafic lava s will have received equal geochemical and isotopic characterization. Five rock groups have been identified among them on the basis of distinctive rar e-earth-element (REE) patterns: melilitite-related, mela-nephelinite-relate d, meymechite-related, trachybasalt-related, and ankaramite-related. The RE E ratios and patterns that distinguish the groups have not evolved by fract ionation, because they display no relation to MgO content. Judging from the isotopic data, crustal contamination bad little influence on magma evoluti on. All rock groups, despite their geochemical dissimilarities, show close geochemical linkages among themselves, and significant geochemical similari ty to kimberlites of the Yakutian province and ocean-island basalts (OIB). Thus, all of these continental and oceanic magmas may have originated in th e same part of the mantle. Geochemical distinctions among the five rock gro ups could have been caused by various degrees of partial melting and differ ing amounts of dissolved volatiles.