Peralkaline rocks in the Late Cretaceous Del Salto Pluton,

Abstract

The Late Cretaceous Del Salto Pluton is one of the four Late Jurassic to Late Miocene intrusives that are recognized ca. 60 km inboard from the eastern limit of the Patagonian Batholith, close to the Chilean-Argentinian border. The pluton is a fluorite-bearing biotite syenogranite, except for part of its northeastern sector, where an alkali feldspar granite contains a typical peralkaline assemblage composed of relict augite and aegirine, together with arfvedsonite, ferrorichterite, astrophyllite, pyrochlore and zektzerite. Most of these minerals had not been recognized previously, making this the first example of an alkaline intrusion with a local peralkaline facies in this southern part of the continent. These mineral assemblages are unusual for a subduction-related margin in which subalkalic magmatism is characteristic. Judging from its trace element composition and rare earth element patterns, the pluton probably reflects the end product of extreme fractionation in a within-plate to post-collisional environment. The lack of clear-cut contacts between the rock that contains the peralkaline assemblage and the biotite syenogranite argues in favor of podiform bodies in which F-rich solutions have induced alkalic metasomatism.