Significance of the Early Jurassic Garamilla formation in the western Nordpatagonian Massif

•We study the volcanic rocks of Garamilla Formation in western Nordpatagonian Massif.•It is composed by three units with andesitic, dacitic and rhyolitic compositions.•Radiometric dating indicates Early Jurassic time for development of the volcanic system.•Field evidences show a half graben and trapdoor structure related to eruption.•It is comparable with equivalent units in the Neuquén Basin and Nordpatagonian Massif.

By means of facial, stratigraphic, petrographic, geochemical and geochronological studies we characterize the Garamilla Formation, cropping out in the western Nordpatagonian Massif.The studies of these volcanic rocks reveal an Early Jurassic volcanic episode formed by three volcanic units that change from normal calc-alkaline to high-K calc-alkaline series. Other geochemical features reveal a progressive change from an initial subduction-related volcanism to one intraplate-related volcanism.This volcanic episode is temporally and geochemically equivalent to those volcanic units located in half-grabens in several areas of the Neuquén Basin.The volcanic units were erupted into different structural designs. A portion of its depocenter was interpreted as a transtensional half-graben, whereas the other exhibits a trapdoor structure. The lineament trends that bound the volcanic system were also recognized in western Nordpatagonian Massif, and were assigned to the Gondwanide Orogeny.

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