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ABSTRACT

Sand samples from piston cores taken in the Peru-Chile Trench and adjacent areas were examined petrographically in an attempt to characterize sands from the oceanward side of a continent-margin volcanic arc. Framework composition averaged Q18F44L38, close to the average reported for this type of setting, but showed wide variation in the ratio of feldspar to lithic fragments; quartz was uniformly low, less than 30%. Samples rich in lithic grains were from sites that lie off areas of recent volcanism on the adjacent continent. Samples from off southern Central America had compositions more like those found in island-arc settings, averaging Q3F21L76, whereas those from the Straits of Magellan increased in quartz and decreased in lithic fragments toward the Atlantic. Plagioclase was greatly predominant over K-feldspar in all samples, and averaged An28. A considerable portion of the plagioclase was close to end-member albite, suggesting recycling from pre-existing sedimentary rocks. Climate had no obvious effect on the framework proportions or the feldspar composition, but the volume of sediment in the trench increases with increasing rainfall on the continent.