STRATIGRAPHY AND GEOCHRONOLOGY OF THE MIDDLE?-LATE JURASSIC NACIENTES DEL TENO AND RIO DAMAS FORMATIONS—NEW INSIGHTS INTO VOLCANISM, SEDIMENTATION AND PALEOGEOGRAPHY

A subaerial volcanogenic pebble conglomerate in the Principal Cordillera of the Chilean Andes at 35.5°S provides the first strong evidence for the location of a late Middle?-early Late Jurassic, volumetrically important source of intermediate volcanic products. Most likely the source was located within a few kilometers west of the present-day continental divide and consisted of one or more andesitic stratovolcanoes. The pebble conglomerate shares a depositional contact with an overlying quartz rhyolite ignimbrite that yielded a U-Pb zircon age of 162 + 3.2 Ma, the first Jurassic rhyolite reported for the southern Central Andes. This age is inconsistent with the Triassic age previously assumed for local Mesozoic rhyolitic rocks, and thus requires revisions to stratigraphy used to model the development of the Andean back-arc basin and the overall magmatic and deformational evolution of the Mesozoic Central Andean Arc.