INSIGHTS INTO MIDDLE-LATE JURASSIC ANDEAN BACKARC VOLCANISM--GEOLOGY AND GEOCHRONOLOGY OF THE NACIENTES DEL TENO FORMATION (35°10'S)

A sequence of subaerial volcaniclastic sandstone and conglomerate and an overlying rhyolite ignimbrite located in the Principal Cordillera of Chile at 35.10°S provide the first evidence in the south-Central Andes for the location of a late Middle?-early Late Jurassic arc.

Beds of clast-supported conglomerate with sub-angular to sub-rounded andesitic clasts, intercalated oxidized mudstone layers with desiccation cracks, and other sedimentological features suggest an alluvial fan and fan delta setting with alternating marine and continental deposition. Paleocurrent indicators are from the W ± 90° and produce a fanning array in map view. Most likely the sediment source for the pebble conglomerate was close to the present-day continental divide and consisted of one or more andesitic stratovolcanoes.

The pebble conglomerate is unmistakably overlain depositionally by a quartz rhyolite ignimbrite that yielded a U-Pb zircon age of 160.7 + 3.2 Ma, the first Jurassic rhyolite reported for the south-Central Andes. This age conflicts with the previously assumed much younger age for the conglomerate and much older (Triassic) age for local Mesozoic rhyolites. Thus, our new mapping and U-Pb zircon dating has profound implications for stratigraphic and structural models of the Andean back-arc basin and the overall magmatic and deformational evolution of the Mesozoic Central Andean Arc. Future provenance studies will shed additional light on the Mesozoic Andean convergent margin.