PLAGIOCLASE ZONATION IN A BASALT TO RHYODACITE ERUPTIVE SUITE, SEGUAMISLAND, ALASKA - OBSERVATIONS BY NOMARSKI CONTRAST INTERFERENCE

Plagioclase zoning in lavas from the Seguam volcanic center, Aleutian arc, has been studied using the Nomarski Differential Interference Con trast (NDIC) imaging technique on etched polished sections. Plagioclas e in basalts records a complex history including periods of oscillator y euhedral growth interrupted repeatedly by patchy cellular growth, an d less commonly, by dissolution events, whereas andesites, dacites, an d rhyodacites crystallized oscillatory euhedral zoned plagioclase with only rare evidence of subtle dissolution events. The complexity of pl agioclase in basalts and the simplicity of plagioclase in the evolved lavas at Seguam contrast with NDIC observations from previously studie d orogenic lavas. These first-order observations suggest that the inte nsive parameters T, P, and X of equilibrium crystallization for the ba saltic magmas may have been disturbed by thermal effects of magma mixi ng or convection, or by decompression during magma ascent. Andesitic t o rhyodacitic magmas apparently were less strongly affected by thermal , baric, or compositional shifts. The contrasts in plagioclase zoning between Seguam and other orogenic volcanic centers reflect the differe nce between systems dominated by closed-system fractionation (Seguam) and more typical open systems where magma mixing may be common.