he Bajo Pobre Formation Jurassic in the eastern Deseado Massif, Patagonia: Relationship with the Bahía Laura Group

Abstract

New Bajo Pobre Formation outcrops found in the eastern sector of the Deseado Massif, Santa Cruz province, Patagonia, are described. This unit, together with the Bahía Laura Group, is the most important magmato-pyroclastic event of the Deseado Massif. They are melanocratic, porphyric to aphanitic lava flows, with plagioclase, pyroxene and occasionally biotite phenocrysts in an intersertal matrix with plagioclase, pyroxene, opaque and occasionally with olivine microlites. Plagioclases are An53 to An85 (labradorite - bytownite) and pyroxenes are augite (Wo39 En48 Fs13), and pigeonite microlites (Wo11 En54 Fs35) in the rocks with olivine (Fo51-45 Fa49-55) microlites. They are intermediate to basic rocks, basaltic andesite to andesite in composition with calc-alkaline affinities. Underlying and overlying stratigraphic relationships of the Bajo Pobre Formation lava flows with the Bahía Laura Group acid volcanics are described. In the basaltic andesites overlying the Bahía Laura Group it was determined a 151.3 ± 3.4 Ma whole-rock K-Ar age and an assumed plagioclase Ar-Ar age of 177 ± 4 Ma for the underlying andesitic rocks. These studies confirm the extension of the Bajo Pobre Formation outcrops up to the Laura Group it was determined a 151.3 ± 3.4 Ma whole-rock K-Ar age and an assumed plagioclase Ar-Ar age of 177 ± 4 Ma for the underlying andesitic rocks. These studies confirm the extension of the Bajo Pobre Formation outcrops up to the coastal region of the Deseado Massif. The combination of stratigraphic observations and geochronology is in agreement with the coeval and possible interbedding relationship between the Bajo Pobre Formation lavas and the Bahía Laura Group acid volcanic rocks, supporting the bimodality of the Jurassic magmatism for the Deseado Massif.