Chronological study of the pre-Permian basement rocks of southern Patagonia

Small outcrops of the basement of the Deseado Massif, consisting of weathered, altered granitoids and their metasedimentary host rocks, were dated by the U-Pb zircon method using thermal ionization mass spectrometry and ion microprobe (SHRIMP). The provenance ages of detrital zircon in the metasediments are typical of material available in the adjacent regions of the Gondwana margin, with prominent components at 1000-1100 and 580±6 Ma, the latter probably approximating the age of sedimentation (latest Neoproterozoic). SHRIMP data for pre-Jurassic granodiorite from a borehole in the Magallanes basin in Tierra del Fuego give a Cambrian age of 521±4 Ma, which confirms published conventional U-Pb dating from the same borehole. SHRIMP ages of 476±4 and 472±5 Ma for granitic cobbles in a Permian conglomerate (La Golondrina Formation) are evidence of nearby Ordovician (Famatinian) intrusive activity. In situ granitoids indicate late Silurian to early Carboniferous ages (422±2, 395±3, and 346±4 Ma). All these events are recognised in the evolution of adjacent South America and the Antarctic Peninsula suggesting that these parts of the Gondwana margin were contiguous throughout their Cambrian to Jurassic history.