The Puelche Volcanic Field: extensive Pleistocene rhyolite lava flows in the Andes of central Chile

A remote volcanic field in the rugged headwaters of the Rio Puelche and Rio Invernada (35.8 degrees S) constitutes the largest cluster of Quaternary r hyolite lava flows yet identified in the Andean Southern Volcanic Zone. The Puelche Volcanic Field belongs to an intra-arc belt of silicic magmatic ce nters that extends, at least, 140 km north-south and lies well east of the volcanic front but nonetheless considerably west of the intraplate extensio nal fields of basaltic and alkaline centers of pampean Argentina. The autho rs' mapping has distinguished one shallow intrusive mass of early Pleistoce ne biotite rhyodacite (70.5% SiO2), 11 eruptive units of mid-Pleistocene hi gh-K biotite-rhyolite lava (71.3-75.6% SiO2), and 4 eruptive units of basal tic andesite (53.95-4.9% SiO2), the conduits of which cut some of the rhyol ites. Basal contacts of the rhyolite lava flows (and subjacent pyroclastic precursors) are generally scree covered, but glacial erosion has exposed in ternal flow structures and lithologic zonation superbly. Thicknesses of ind ividual rhyolite lava flows range from 75 m to 400 m. Feeders for several u nits are well exposed. Cliff-draping unconformities and intracanyon relatio nships among the 11 rhyolite units show that the eruptive sequence spanned at least one glacial episode that accentuated the local relief. Lack of ice -contact features suggests, however, that all or most eruptions took place during non-glacial intervals probably between 400 ka and 100 ka. Post-erupt ive glacial erosion reduced the rhyolites to several non-contiguous remnant s that altogether cover 83 km(2) and represent a surviving volume of about 21 km(3). Consideration of slopes, lava thicknesses, and paleotopography su ggest that the original area and volume were each about three times greater . Phenocryst content of the rhyolites ranges from 1 to 12%, with plagioclas e>>biotite>FeTi oxides in all units and amphibole conspicuous in the least silicic. The chemically varied basaltic andesites range from phenocryst-poo r to phenocryst-rich, exhibiting large differences in proportions of clinop yroxene, olivine, plagioclase, and xenocrystic quartz. Compositional bimoda lity of the volcanic field is striking, there being no Quaternary eruptive units having SiO2 contents between 55 and 70%. Major and trace element comp ositions of the mafic and silicic rocks are nonetheless typical of continen tal-margin are suites, not of intracontinental suites. The lack of intermed iate eruptive units and the differences between the mafic and rhyolitic lav as in Sr-isotope composition suggest that the rhyolites fractionated from a hybrid parent rather than continuously from basaltic magma. The rhyolites may contain larger contributions of upper-crustal partial melts than do sil icic products of the volcanic-front centers 30 km to the west.