Field and Geochemical Studies of the Melilite-Bearing Arydzhangsky Suite, and an Overall Perspective on the Siberian Alkaline-Ultramafic Flood-Volcanic Rocks

Abstract

This paper presents the first comprehensive geologic, petrographic, geochemical, and Sr- and Nd-isotopic study of the 350 m thick Arydzhangsky lava suite, the last suite in the entire north Siberian, flood-volcanic sequence to require study by modern methods. Within this sequence, only the Arydzhangsky Suite includes melilite-bearing lavas; it is composed of melanephelinites to limburgites (both melilite-bearing and melilite-free), with rare melilitites and picrites. The lavas contain from 0 to 60 vol% melilite, with MgO 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, among these lavas, show the most resemblance to the Yakutian kimberlites. With this contribution, all of the north Siberian, alkaline-ultramafic lavas will have received equal geochemical and isotopic characterization. Five rock groups have been identified among them on the basis of distinctive rare-earth-element (REE) patterns: melilitite-related, melanephelinite-related, meymechite-related, trachybasalt-related, and ankaramite-related. The REE ratios and patterns that distinguish the groups have not evolved by fractionation, because they display no relation to MgO content. Judging from the isotopic data, crustal contamination had little influence on magma evolution. All rock groups, despite their geochemical dissimilarities, show close geochemical linkages among themselves, and significant geochemical similarity to kimberlites of the Yakutian province and ocean-island basalts (OIB). Thus, all of these continental and oceanic magmas may have originated in the same part of the mantle. Geochemical distinctions among the five rock groups could have been caused by various degrees of partial melting and differing amounts of dissolved volatiles.