CONICET | Buscador de Institutos y Recursos Humanos

This contribution reports a reassessment of the units of the Central Patagonian Batholith (southern North-Patagonian Massif) based on detailed field mapping, petrography, thermobarometry and three new 40Ar/39Ar biotite cooling ages. Mineral data allowed us to constrain emplacement depth and crystallization temperature for the different units of this batholith. The Central Patagonian Batholith consists of the Gastre and the Lipetrén superunits, where the Gastre Superunit is the older unit which displays evidence of felsic and mafic magma interaction, and the Lipetrén Superunit is made up by fine-grained granite which intrude all the other units postdating magma hybridization processes. Five units can be clearly distinguished in the I-type granitoids of the Gastre Superunit: 1) equigranular hornblende-biotite granodiorites, 2) porphyritic biotite-hornblende monzogranites, 3) equigranular biotitic monzogranites, 4) hornblende quartz-diorites and 5) equigranular biotitic granodiorites (Horqueta Granodiorite, Rapela et al. 1991, 1992). Emplacement depth of the Gastre Superunit is between 1.8 to 3 kbar (6 to 11 kilometers depth), and the maximum recorded temperatures are bracketed between 760 and 800ºC. 40Ar/39Ar cooling age for biotites of the porphyritic biotite-hornblende monzogranites of the Gastre Superunit is 213±5 Ma. The Horqueta Granodiorite, a unit which had been previously set apart of the Central Patagonian Batholith because of its uncertain age and of its different modal characteristics, is now included into the Gastre Superunit as the ?equigranular biotitic granodiorites?, because of its new 40Ar/39Ar cooling age calculated in biotites (214 ± 2 Ma) and also because of the fact that this granodiorite bears magma mingling features. The Lipetren Superunit is is composed of fine-grained biotitic monzo- and syenogranites, and yielded a 40Ar/39Ar biotite age of 206 ± 4 Ma. Fine-grained and granophyric textures in the Lipetrén Superunit suggest rapid cooling, and therefore the cooling age is very near to the real crystallization age of this unit. These cooling ages suggest a time span of ≥7 Ma elapsing between the intrusions of the Gastre and the Lipetrén superunits. Fine-grained acid granites attributed to the Lipetrén Superunit crop out northwest of the study area in the Sierra de Lipetrén (Fig. 1), where no outcrops attributable to the Gastre Superunit were recognized yet. The different ages and the different spatial distribution suggests that the Gastre and Lipetren Superunit are disconnected in time and possibly also in space. This apparent disconnection is also supported by the fact that the Lipetrén Superunit does not bear textural evidence of interaction between mafic and felsic magmas. Geochemical modeling also would show that no straightforward petrogenetic process could be invoked to derive the Lipetrén Superunit from the Gastre Superunit. The age of ~185 Ma of the overlying volcanites of Lonco Trapial Formation points out that this Late Triassic batholith was exposed sometime between latest Triassic and the earliest Jurassic times, roughly contemporaneously with a major accretionary episode in the margin of southwest Gondwana.