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Original Article |
1 1Departamento de Geología, Universidad de Chile, Casilla 13518, Correo 21, Santiago, Chile (e-mail: caldera{at}esfera.cl)
2 2Department of Geological and Environmental Sciences, Stanford University, Stanford, CA 94305, USA
3 3Research School of Earth Sciences, The Australian National University, Canberra, ACT 0200, Australia
4 4Centro de Pesquisas Geocronológicas, Universidade de São Paulo, São Paulo, CEP 05508-900, Brazil
Magmatic and detrital zircon ages from the Rocas Verdes basin, a tectonically juxtaposed remnant of sea floor in the Magallanes fold and thrust belt (southern Patagonia, South America), indicates that a rifting phase of the Rocas Verdes basin occurred between 152 and 142 Ma, and was accompanied by bimodal magmatism. A dacite dyke cross-cutting pillow-basalt successions and a plagiogranite dyke in mixed mafic–felsic terranes of the basal Sarmiento Ophiolite Complex contain 150 Ma zircon crystals, indicating that mafic submarine volcanism had started prior to or during the Late Jurassic, 10–15 Ma earlier than previously thought. The silicic pyroclastic rocks of the Tobífera Formation, with two samples dated at 148 and 142 Ma, were heralded by synrift sedimentation along fault-bounded grabens within Palaeozoic metasediments. No evidence for an active volcanic arc during the early formation of the Rocas Verdes basin was detected in detrital zircon grains of the lower sedimentary member of the Tobífera Formation. A minimum of 25 Ma of continuous sedimentation in the Rocas Verdes basin is suggested by detrital zircon grains in the upper member of the Zapata Formation. The Rocas Verdes basin was rimmed on the western side by an incipient and subaerial magmatic arc only in its later evolution.
