U-Pb zircon plutonic emplacement and metamorphic ages from a tilted crustal section of the Famatinian arc, northewestern Argentina

STAIR, K., DUCEA, M., OTAMENDI, J., GEHRELS, G., BERGANTZ, G.

Tucson

Conferencia; Ores and Orogenesis: Circum-Pacific Tectonics, Geologic Evolution, and Ore Deposits; 2007

The Sierra de Valle Fertil (northwestern Argentina) is a basement block in the Sierras Pampeanas representing a continuous tilted crustal section that exposes a portion of a magmatic arc from paleo-depths of over 25 km to shallow upper crustal levels. This block was uplifted during the late Cenozoic involving Laramide-style tectonism above flat slab of the central Andean subduction system. Several of the Sierras Pampeanas uplifts consists of continental basement rocks of Gondwanan affinity that were intruded during the early Paleozoic by calc-alkaline magmatic rocks of a sizeable Cordilleran-style magmatic arc, the Famatinian arc. This arc was presumably generated in response to subduction ceasing when a Laurentia derived terrane accreted to the paleo-margin of South America.New zircon U-Pb geochronoly data on fifteen samples from the Sierra Valle Fertil provide further insights into the magmatic history of the Famatinian arc and its tectonic significance. Data were obtained on single crystals (~ 25 per sample) by laser ablasion multicollector ICP-MS. Fourteen of the analyzed are tonalites and granodiorites whereas one is a meta-pelitic(?) migmatite from deeper section. This data show that the emplacement of the tonalites and granodiorites took place between 490 and 468 Ma, with most ages being identical within analytical error. A hornblende-gabbro near the bottom of the Sierra Valle Fertil section (Pankhurst et al., 2000) also overlaps with the ages reported here. This age range is also identical to previously published regional geochronological results on Famatinian arc  plutonic rocks and support two major interpretrations concerning the evolution of the arc. First, the results bolster the argument that the Famatinian arc was in a high flux mode during that time, similar to flare-ups documented elsewhere in Cordilleran arcs. Second, the emplacement of mafic magmas in the section was coeval with the felsic batholith flare-up.