Journal of South American Earth Sciences, Vol. 5, No. 3/4, pp. 321-325, 1992 Printedin GreatBritain 0895-9811/92$5.00+ .00 Pergamon PressLtd & EarthSciences & Resources Institute A revised age for the granites of the central Somuncura Batholith, North Patagonian Massif R. J. PANKHURST*1, C. W. RAPELA2, R. CAMINOSs, E. LLAMBIAS 2, and C. PARICA4 1British Antarctic Survey, c/o NERC Isotope Geosciences Laboratory, Kingsley Dunham Centre, Keyworth, Nottingham NG12 5GG, UK; 2Seeretaria de Mineria, Santa F~ 1548, Piso 12,1060 Buenos Aires, Argentina; SCentro de Investigaeiones Geolbgieas, 644 Calle No. 1, 1900 La Plata, Argentina; 4INGEIS, Pabellon 2, Ciudad Universitaria, 1428 Buenos Aires, Argentina (Received April 1992; Accepted May 1992) Abstract--The age of the granites of the La Esperanza region of the Somuncura Batholith, North Patagonian Massif has been revised, based in part on Rb-Sr whole-rock data obtained by reanalyzing samples from a previous study. The new ages are 258 + 15 Ma for the Prieto Granodiorite and 259 + 16 Ma for the Donosa Granite, both from the older La Esperanza plutenic complex, and 239+4 Ma for the Calvo Granite, from the younger volcano-plutonic Dos Lomas complex. The initial STSr/SSSrratios are all in the range 0.7070-0.7076. The ages probably correspond stratigraphically to Late Permian and Early Triassic for the two complexes, respectively, consistent with traditional geologic interpretation. Together with recently published Triassic ages from the Batholith of Central Patagonia, it is clear that the acidic volcano- plutonic associations of northern Patagonia are very latest Paleozoic and Mesozoic in age. They are not obviously related to terrane collision but are part of a Permo-Triassic acid magmatie province that extends throughout the central Andes and that preceded, or was associated with, the early rifting of Gondwana. Resumen---Se presenta una revisiSn significativa de la edad de los granitos de la regiSn de La Esperanza en el batolito de Somuncura, Macizo NorpatagSnico. Las nuevas edades, basadas en parte en dates Rb-Sr (roca total) obtenidos re-analizando muestras colectadas en un estudio previo, son 258±15 Ma para la Granodiorita Prieto y 259_+16 Ma para el Granite Donosa, ambos del complejo plutbnieo Esperanza, el m~s antiguo del area. E1 Granito Calvo, perteneciente al complejo plutonico-volcanieo Dos Lomas que superayace al anterior, dis una edad de 239±4 Ma. Las relaeiones iniciales 87Sr/SeSr est~n todas en el tango 0.7070-0.7076. Es probable que las edades de los dos complejos corresponden estratigr~ficamente a P~rmieo tardio y Tri~sico temprano respectivamente, que es compatible con la interpretacibn geolSgica tradicional. Junto con las edades tri~sicas recientemente publicadas para el Batolito de la Patagonia Central, parece claro que las asociaciones plutSnico-volc~nicas ~icidasde los batelites norpatagSnieos son de edad paleozoica tardisima y mesozoica. Estas rocas no estan obviamente relaeionadas a una colisiSn de terrenos, siendo en cambio parte de una provincia magm~tica permo-tri~sica que se extende pot todos los Andes centrales y que precediS, o se asociS, al rifting temprano de Gondwana. INTRODUCTION THE GRANITESof the North Patagonian Massif constitute two batholithic series: the Somuncura Batholith, which lies largely in the north of the massif, and the Batholith of Central Patagonia, which lies along its southwestern border. These granites were initially thought to be late Paleozoic to Early Triassic (Groeber, 1929; Stipanicic, 1967), an idea supported by rather scattered K-Ar and Rb-Sr geochronologic data (see Caminos et al., 1988 for references). Ramos (1984) interpreted this predom- inantly calc-alkaline magmatism as related to late Palaeozoic subduction of ocean floor that he hypo- thesized to the northeast, between Patagonia and the Brazilian craton. In his model, this ocean closed during a Permo-Triassic terrane collision that would also have been responsible for deformation of the Sierra de la Ventana fold-belt. This hypothesis ap- peared to receive circumstantial support from the Rb-Sr geochronologic study of Caminos et al. (1988), *Address all correspondence and reprint requests to Dr. R. J. Pankhurst: telephone [44] (602) 363263; telefax [44] (602) 363302; telex 378173 BGSKEY G. © 1992 Crown Copyright. who presented Carboniferous Rb-Sr whole-rock isochrons for the Somuncura Batholith: two of 332 _ 6 and 317_+ 4 Ma for the area around La Esperanza in the central part of the batholith, and two more of 332__.15 and 320___2 Ma from the Nahuel-Niyeu region farther east. However, Cingolani et al. (1991) have reported three analyses of granite samples from the Mencue-Chasic6 area west of La Esperanza which gave a poorly constrained but younger age of 218___9 Ma, and Rapela et al. (1992) have shown that the main magmatic phases of the Batholith of Central Patagonia were at 220_ 3 Ma (Late Triassic) and 208__.1 Ma (Triassic-Jurassic boundary). These results were largely unexpected and it was consi- dered advisable to run analyses of powder samples from La Esperanza through the same analytical procedures. It is the results of this re-analysis and their interpretation that concern us here. ANALYTICAL METHODS Rb-Sr whole-rock geochronology has been a rou- tine method at the NERC Isotope Geosciences 321