Stratigraphy of the Rocas Verdes basin: Petrography and new U-Pb ages from the Argentine Fuegian Andes

Abstract

New mapping, detailed petrographic analyses and isotopic ages of the Jurassic-Lower Cretaceous rocks of the Argentine Fuegian Andes allow us to organize the stratigraphic framework of the Rocas Verdes basin. Accordingly, we confirm a vast lithological heterogeneity associated with the Jurassic rifting that initiated this back-arc basin, including acid and basic volcanic and subvolcanic rocks amongst several pyroclastic and sedimentary deposits, and propose the status of Complex for the stratigraphic classification of these rocks (the Lemaire Complex). U-Pb crystallization ages of rhyolitic lavas and equivalents of the Lemaire Complex reported in this and previous work suggest continuous magmatic activity related to the breakup of Gondwana during the MiddleLate Jurassic, rather than episodic as proposed in the literature. Additionally, detrital maximum depositional ages of ~158 and 143 Ma in volcaniclastic levels near its top extend the upper limit of the Lemaire Complex to the Berriasian (Lower Cretaceous), slightly younger than previously reported in southern Tierra del Fuego. The overlying Early Cretaceous deposits of the back-arc basin exposed in the Fuegian Andes (Yahgan and Beauvoir formations) include conglomerates, turbidite sandstones, mudstones and several tuffaceous facies that attest the coeval rimming of the basin by a magmatic arc, with maximum depositional ages of ~106 and 102 Ma (Albian). The latter is the first report of an isotopic age from the Beauvoir Formation, confirming its lateral correlation with the Yahgan Formation.