Laguna Tordillo Formation: Eocene dacitic volcanism in the Deseado Massif, Santa Cruz province

Abstract

A new formational entity for the Deseado Massif geological province was determined. They are melanocratic volcanic rocks presented as two outcrops in the proximity of the Tordillo lagoon, in the eastern Deseado Massif. They are black to reddish lava flows, with porphiritic texture composed by labradorite (An_55-57) phenocrystals in a hialocrystalline, hialophitic texture matrix. They have also volcanic rock xenoliths with glomeroporphiritic texture composed by labradorite (An_56-59) and enstatite (En₅₆-Fs₄₀-Wo₄), with a pigeonitic rim (En₄₄-Fs₄₇-Wo₉), phenocrystals. Majority and trace element analysis define them as subalkalic, with calc-alkaline affinities, dacite lavas. The stratigraphic relationships evidence an post-upper Jurassic age for these rocks; this was confirmed by a K/Ar whole rock (vitreous fraction) isotope determination, resulting in a 40.8 ± 2.4 Ma age (Eocene). Considering the Paleogene paleogeographic and geotectonic environment for Patagonia, it is proposed that these rocks were generated as a consequence of a basic magma underplating, possibly linked with the Cerro del Doce Basalt or correlated Paleogene units. Nevertheless, it is not discarded a link between the Laguna Tordillo Formation rocks and the Sarmiento Formation or Group pyroclastic rocks.