MINERALOGY AND GEOCHEMISTRY OF ISLAND ARC PICRITES, AS ILLUSTRATED BY AN EXAMPLE FROM THE SOUTHERN PART OF THE OLYUTORSKAYA ZONE OF THE KORYAK HIGHLANDS

Abstract

The determination of the composition of primary melts is one of the most important steps in the analysis of the evolution of basaltic magmas of different series. The high magnesian lavas occur in central and top parts of the section of lower subformation of Achayvayam Formation, specifically in a unit of psammitic tuff. The ore minerals are represented by spinel of two generations. The earliest generation consists of grains of Cr-picotite enclosed in olivine. The later generation consists of titanomagnetite. The volcanic rocks exhibit a fairly persistent composition. They consist of low-titanium picrite and picrobasalt. In terms of their chemical composition, they belong to the shoshonite series. The data then show that the picrite of Achayvayam Formation was generated from melts that are almost primary, and may itself be the matter from which the rocks of the shoshonite series (which exhibits close spatial relationship to picrite were formed.