Tectono-magmatic evolution during the Permian to the Early Jurassic in the Cordillera del Viento (37°05´S - 37°15´S): new geological and geochemical evidences for the early stages of the Neuquén basin.

Abstract

The southern extremity of the Cordillera del Viento is mainly composed of volcanic sequences, which form the basement of the Neuquén basin. The oldest rocks are the sediments in the Huaraco Formation, Andacollo Group, of Upper Carboniferous age. After being folded by the San Rafael orogenic phase during the Lower Permian, these sediments were intruded by granodioritic and monzogranitic plutons along with dikes and domes of rhyolitic nature. They were then covered in angular unconformity by rhyolitic ignimbrites, which are co-genetic with the intrusives. The resulting volcanic-plutonic complex is suggested to be correlative with the Permian Choiyoi Group exposed in the Cordillera Frontal and the Bloque de San Rafael geological provinces. This volcanic-plutonic complex was subjected to strong erosion during the Early to Middle Triassic. The development of an undulated erosion surface (Huarpican unconformity) resulted in the exposure of the cupolas of the plutons. This surface support the Cordillera del Viento Formation (ex Groeber's Choiyoilitense), which consists of a volcanic sequence of andesitic to rhyolitic composition up to 1.300 meters thick. The next unit, the Milla Michicó Formation, was also deposited unconformably. This unit is composed of bedded flows of basalts, andesitic basalts and rhyolites with thickness up to 200 meters. The succession continued with the unconformable deposition of the La Primavera Formation, which is the oldest unit of the Cuyo Group. It is composed of marine sediments interbedded with basaltic flows at the base and rhyolitic ignimbrites in the upper levels. The thickness varies from a few meters up to 400 meters. The La Primavera Formation is covered in apparent conformity by the Los Molles Formation, which has limestones at the base and contains pelites with andesitic lahar intercalations deposited in a turbiditic regime. Regionally, the Huarpican unconformity marks the transition between the post-orogenic stage when the Permian volcano-plutonic complex was formed, and the Triassic rifting stage when extension allowed magmas to ascend rapidly from sublithospheric levels. The unconformities that followed the Huarpican diastrophic phase are considered to be of minor hierarchy, and to only represent changes in the velocity of the rift opening. Chemicaly, the volcanic rocks belong to the calc-alkalines series, displaying moderate to high Mg# numbers and the less altered rocks are potassium-poor. The very low incompatible elements enrichments indicate that the mantle melting percentages were moderate to high and that crustal contamination was minimal. Taking these characteristics into account, Triassic volcanic rocks are proposed to have a volcanic arc associated with the subduction-related Triassic accretionary complexes found in the Chilean Cordillera de la Costa at these latitudes. During the Early Jurassic, this volcanic arc migrated westward as the region passed from a rifting to a backarc extension stage, at the time in which the huge marine ingression of the Cuyo Group took place. The moderate to high amount of mantle melting that gave rise to the volcanic arc magmas can be explained by an anomalous high geothermal gradient. Cooling of the area affected by this gradient could explain the non-lineal shape of the Neuquén basin at this latitude.