SHRIMP U-Pb AGES OF DETRITAL ZIRCONS FROM CUSHAMEN AND ESQUEL FORMATIONS, NORTH PATAGONIAN MASSIF, ARGENTINA : GEOLOGICAL IMPLICATIONS. Francisco Hervé 1 , Miguel J. Haller 2 , Paul Duhart 3 , and C. Mark Fanning 4 . 1 Departamento de Geología, Universidad de Chile, Plaza Ercilla 803, Santiago, Chile. fherve@cec.uchile.cl 2 Universidad Nacional de la Patagonia San Juan Bosco. Bvard. Brown 3700 (9120) Puerto Madryn. CENPAT- CONICET. haller@cenpat.edu.ar 3 Servicio Nacional de Geología y Minería, Oficina Técnica de Puerto Varas, La Paz 406.Puerto Varas, Chile. sernageomin@surnet.cl 4 Research School of Earth Sciences, The Australian National University, Canberra ACT 0200, Australia. Key Words: SHRIMP ages, Cushamen Formation, Esquel Formation, Provenance, North Patagonian Massif INTRODUCTION The crystalline basement of the south western part of the North Patagonian Massif is comprised of metamorphic rocks of the Cushamen Formation (CF; Volkheimer,1964), and the predominatly igneous rocks of the Mamil Choique Formation (MCF; Ravazzoli and Sesana, 1977). Ostera et al. (2001) suggested that the main metamorphic event affecting the CF took place in the Middle to Late Devonian, whilst the protoliths are possibly of late Precambrian age. The MCF has yielded Early Devonian ages (Cerredo et al., 2000), and López de Lucchi et al (2000) indicated the existence within the MCF of Early Permian intrusive components. From field and petrographic observations of an outcrop located 2.5 km west of the Rio Chico bridge, Duhart et al. (2002) suggested that the protolith of the Cushamen Formation was probably a diamictite. As diamictites are common components of Esquel Formation (EF), a unit of well documented Late Paleozoic age, which crops out 140 km to the west, they further implied that there was a possibility that the EF was the protolith of the CF. This view appeared untenable as it is generally accepted that the amphibolite facies staurolite-garnet-sillimanite bearing schists of the CF are older than the very low grade or non metamorphic, fossil bearing Early Carboniferous to Early Permian EF, which is part of the Tepuel Group. In this paper we will refer to Esquel Formation in the sense of López Gamundi (1980) in which the diamictites occur in the middle member of the formation. After Cucchi (1980) the middle and upper members are uncomformably overlying the lower member, and are considered as the distinct Valle Chico Formation. The diamictites being per se a distinctive lithology in the area so irrespective of the formational nomenclature they are well locted in the stratigraphic column. The purpose of this paper is to present new U-Pb SHRIMP detrital zircon data for two samples, one from CF and one from EF, to evaluate their possible relationship, as well as some implications about the provenance areas and their correlation with other geological units of the Patagonian realm in which previous detrital zircon age determinations have been made. SAMPLES The analysed samples are located in the geological sketch map of Figure 1. Sample SEC017 (42°54,99’S; 71°17,10’W) is a diamictite interbedded with sandstone and slate at Cerro Cementerio, near Esquel. It is a matrix supported conglomeratic breccia containing centimetre long clasts of granite, quartzite, and porphyritic rhyolite. Sample SEC022, (42°08,59’ S; 70°32,43’ W) , from the Cushamen Formation near Rio Chico, is a quartz-biotite-garnet-staurolite-sillimanite-muscovite schist, in which there are dispersed flattened clasts, 1 to 10 cm long, composed by granite, quartzite and vein quartz. The pervasive EW / 25ºS foliation also affects the clasts There are no continuous outcrops between the two localities, so the geological relationships are obscured.