Jurassic cooling ages in Paleozoic to early Mesozoic granitoids of northeastern Patagonia: Ar-40/Ar-39, K-40-Ar-40 mica and U-Pb zircon evidence

摘要

U-Pb SHRIMP zircon crystallization ages and Ar-Ar and K-Ar mica cooling ages for basement rocks of the Yaminue and Nahuel Niyeu areas in northeastern Patagonia are presented. Granitoids that cover the time span from Ordovician to Early Triassic constitute the main outcrops of the western sector of the Yaminue block. The southern Yaminue Metaigneous Complex comprises highly deformed Ordovician and Permian granitoids crosscut by undeformed leucogranite dikes (U-Pb SHRIMP zircon age of 254 +/- 2 Ma). Mica separates from highly deformed granitoids from the southern sector yielded an Ar-Ar muscovite age of 182 +/- 3 Ma and a K-Ar biotite age of 186 +/- 2 Ma. Moderately to highly deformed Permian to Early Triassic granitoids made up the northern Yaminue Complex. The Late Permian to Early Triassic (U-Pb SHRIMP zircon age of 252 +/- 6 Ma) Cabeza de Vaca Granite of the Yaminue block yielded Jurassic mica K-Ar cooling ages (198 +/- 2, 191 +/- 1, and 190 +/- 2 Ma). At the boundary between the Yaminue and Nahuel Niyeu blocks, K-Ar muscovite ages of 188 +/- 3 and 193 +/- 5 Ma were calculated for the Flores Granite, whereas the Early Permian Navarrete granodiorite, located in the Nahuel Niyeu block, yielded a K-Ar biotite age of 274 +/- 4 Ma. The Jurassic thermal history is not regionally uniform. In the supracrustal exposures of the Nahuel Niyeu block, the Early Permian granitoids of its western sector as well as other Permian plutons and Ordovician leucogranites located further east show no evidence of cooling age reset since mica ages suggest cooling in the wake of crystallization of these intrusive rocks. In contrast, deeper crustal levels are inferred for Permian-Early Triassic granitoids in the Yaminue block since cooling ages for these rocks are of Jurassic age (198-182 Ma). Jurassic resetting is contemporaneous with the massive Lower Jurassic Flores Granite, and the Marifil and Chon Aike volcanic provinces. This intraplate deformational pulse that affected northeastern Patagonia during the Early Jurassic (Sinemurian-Pliensbachian) was responsible for the partial (re)exhumation of the mid-crustal Paleozoic basement along reactivated discrete NE-SW to ENE-WSW lineaments and the resetting of isotopic systems. These new thermochronological data indicate that Early Permian magmatic rocks of the Nahuel Niyeu block were below 300 degrees C for ca. 20 Ma prior to the onset of the main magmatic episode of the Late Permian to Triassic igneous and metaigneous rocks of the Yaminue block.