INTERNAL STRUCTURE AND CHEMICAL COMPOSITION OF HYALOIGNIMBRITE PHASES IN THE OLA FORMATION, OKHOTSK-CHUKOTKA VOLCANIC BELT

Abstract

The chemical composition of haloignimbrite phases has not yet been adequately studied, so existing views on their origin are somewhat hypothetical. To investigate the nature of such phases, the authors used the Superprobe-733 electron-probe microanalyzer, made by the JEOL Company (Japan). The work was done on the classic hyaloignimbrite from the Ola formation in the Kheta volcanic field, OCVB. The volcanic rock section studied in the Kheta River Canyon consists of three parts. The ignimbrite studied was collected from the top of a hyaloignimbrite bed, directly overlain by basalt. The image in back-scattered electrons, as viewed under the scanning microscope of the electron-probe microanalyzer, reveals the internal hyaloignimbrite structure remarkably completely. The rock studied is equivalent in bulk chemical composition to rhyodacite high in water. Additional study results are discussed.