Timing and rate of granulite facies metamorphism and cooling from multimineral chronology on migmatitic gneisses, Sierras de La Huerta and Valle Fértil, NW Argentina

GALLIEN, FLORIAN; MOGESSIE, ABERRA; BJERG, ERNESTO; DELPINO, SERGIO; CASTRO DE MACHUCA, BRÍGIDA; THÖNI, MARTIN; KLÖTZLI, URS

LITHOS

Elsevier

Lugar: Amsterdam; Año: 2010 vol. 114 p. 229 - 252

0024-4937

Migmatitic paragneisses of the Valle Fértil-La Huerta Ranges at the Western margin of the Sierras Pampeanas are composed of garnet?cordierite?plagioclase?biotite?quartz-bearing units that experienced peak metamorphic conditions of ca. 800 °C at 6-7 kbar. Based on petrological studies, pseudosection modeling and petrographic observations, an anticlockwise P-T path with a small pressure increment is proposed. Rare earth element LA-ICP-MS patterns acquired from rutile bearing garnets suggest a single stage of garnet growth at high-T at pressures above the ilmenite?rutile transition. U-Pb dating of zircon rims from the migmatites indicates two distinct metamorphic U-Pb ages of 525±9 Ma and 478±9 Ma. The older age is suggested to record an amphibolite facies event of the Pampean orogeny. The younger metamorphic age is contemporary with igneous zircons from metatonalites and pegmatites that yield 478±4 Ma. We suggest that the prograde high-T metamorphic Famatinian event is associated with the emplacement of large magmatic bodies in which large-scale magmatic activity gave rise to an increased geothermal gradient of about 35 °C/km. Sm-Nd garnet ages of 447±3 Ma indicate a time span of around 30 Ma for which temperatures above the garnet closure temperature prevailed. Using U-Pb, Sm-Nd and Rb-Sr isotope systems, a cooling rate of 3 to 6 °C/Myr is inferred.