Rhyolitic Tuff Flows in Southern Peru

Abstract

The numerous and widespread sillar deposits in the western Andes of southern Peru are rhyolitic tuff flows which are not welded. Two types may be easily distinguished by color in the field. One, a salmon-colored sillar, is composed largely of minute glass shards and pumice fragments and contains small amounts of oligoclase and biotite and, rarely, quartz. The other, a white sillar, is relatively free of hematite colorant, is hard, and consists largely of secondary axiolitic and spherulitic growths of potash feldspar and (rarely) tridymite with relatively little glass. The white sillar contains quartz, oligoclase, and biotite fragments. Both types of sillar contain foreign material, mostly andesite, probably from the vent walls. Five chemical analyses of sillars from the Arequipa region show remarkable uniformity, regardless of color or degree of recrystalliza-tion, and closely approach the composition of average rhyolite. Sr, Li, Rb, and Cs are present in amounts typical of rhyolitic and granitic rocks. In the upland valleys of Apurimac and Cusco departments single rhyolitic tuff flows have been traced for 20 miles. These sillar deposits are in remnants of mature valleys which are probably of early Pleistocene age. Youthful streams have cut canyons as much as 2,300 feet below the valleys which guided the course of the tuff flows. Similar relationships hold in the Arequipa region, where many deposits of both salmon and white sillar have accumulated in a broad basin between the Yura and Chili rivers. The sillar deposits all appear to be related to dominantly andesitic volcanic centers. Physiographic evidence suggests that eruption of these rhyolitic tuff flows is connected with relatively sudden epeirogenic movement. Andesitic volcanism both preceded and followed the principal early Pleistocene rhyolitic eruptions. The volume of rhyolitic sillar is small compared to that of andesite flows and pyroclastics.