HIGH-POTASSIUM GLASS SEGREGATIONS IN LATE CENOZOIC ALKALIC LAVAS OF THE EASTERN SAYAN

Abstract

In this article, we give the results of our geologic study of the inhomogeneity in a Pliocene lava flow in the Monkres River basin in the Iya-Uda interfluve in the Eastern Sayan. The flow basalt here is black or dark gray and aphyric. Its groundmass consists of olivine, plagioclase, clinopyroxene, an ore mineral, nepheline and glass. The normative nepheline content of the rock ranges from 8.7 to 13.8 percent. In degree of silica undersaturation, this basalt is comparable to nepheline hawaiite from the Bartoy cinder cones. K2O/Na2O ratios of 0.57 to 0.68 are lower on the whole, than in late Cenozoic leucite-bearing basaltic rocks of the Udokan and the Hangay Ranges. The inhomogeneity within one nepheline hawaiite sheet was studied in two sections sampled 500 m apart. The lava bed is 15 to 25 m thick. The chemical composition of the rocks and their trace-element content are given.