U-Pb, Th-Pb and Ar-Ar geochronology from the southern Sierras Pampeanas, Argentina: implications for the Palaeozoic tectonic evolution of the western Gondwana margin J. P. SIMS 1, T. R. IRELAND 2'3, A. CAMACHO 1, P. LYONS 1, P. E. PIETERS 1, R. G. SKIRROW 1, P. G. STUART-SMITH 1 & R. MIRO 4 1Australian Geological Survey Organisation, PO Box 378, Canberra ACT 2601, Australia 2 Research School of Earth Sciences, Australian National University, Canberra ACT, Australia 3 Dept of Geological & Environmental Sciences, Stanford University, CA 94305, USA 4 Servicio Geol6gico Minero Argentino, Av. Poeta Lugones, Nueva C6rdoba, Argentina Abstract: New SHRIMP zircon and monazite 2~ and 2~ ages on structurally controlled units and 4~ step-heating ages from shear fabrics, define three distinct regional tectonic events in the southern Sierras Pampeanas. The first, the Pampean orogeny, involved closure of a late Neoproterozoic basin on the western margin of Gondwana. New rims on detrital zircons and concurrent monazite growth suggest that the metamorphic peak was attained by c. 530 Ma. The second event, the Famatinian orogeny, marks the initiation of eastward-dipping subduction on the western Gondwana margin, and may represent a continuation of the earlier Pampean event. Metasedimentary rocks from the Sierras de San Luis have zircons with a predominantly Early Cambrian detrital age, indicating a Pampean source. The metamorphic peak in these rocks was contemporaneous with the emplacement of felsic, mafic and ultramafic rocks at c. 480 Ma in a collisional setting. Monazite ages and limited new zircon growth in the metasedimentary rocks suggest that the Famatinian orogeny had ceased by about 450 Ma. This correlates well with a 450- 460 Ma Ar-Ar age for late shearing in the southern sierras of La Rioja province. The third tectonic event, the Achalian orogeny, involved W-directed compression and emplacement of multiple, voluminous, granite intrusions. Deformation during this event was partitioned between discrete shear-zones and regions of open to tight folding. The shear zones alternate between W-directed thrusts and NNW-trending, sinistral shear-zones. Ar-Ar data from the low-grade shear fabrics indicate that transpressional deformation continued through most of the Devonian. The western part of the palaeocontinent of Gondwana comprised numerous cratonic blocks that amalgamated during various events from the Proterozoic through to the earliest Palaeo- zoic (e.g., Unrug 1992). Critical to our under- standing of the make-up of the supercontinent and times of accretion and dispersal, are con- straints on the ages of formation and deforma- tion of the various cratonic blocks. In this respect, the southern Sierras Pampeanas ter- rane in NW Argentina is somewhat enigmatic. Placed between the Grenville-aged (c. 1000- 1200 Ma) basement of the Precordillera in the west (e.g., Kay et al. 1996) and the Archaean to Palaeoproterozoic Rio de La Plata craton in the east (e.g., Cordani & Brito Neves 1982), there are a number of conflicting views about the age of: (1) sedimentation; (2) initial deformation and metamorphism in the basement rocks; and, (3) the duration and timing of subsequent events. In most plate reconstructions, the high-grade basement rocks of the Sierras Pampeanas are considered to be Neoproterozoic in age with significant deformation dating from the Brasi- liano orogeny (640-570Ma; e.g., Bond et al. 1984; Ramos 1988; Brito Neves & Cordani 1991; Unrug 1992). Ramos (1988) suggested that this deformation in the Sierras Pampeanas involved westward-directed subduction of a marginal basin at the western edge of the Rio de La Plata craton, beneath the Pampean terrane. Alternatively, Acefiolaza & Miller (1982) used stratigraphical evidence to suggest that the metasedimentary rocks of the Sierras Pampeanas were deposited in a marginal basin, the Puncov- iscana Trough of Je2ek et al. (1985), adjacent to the Rio de La Plata craton during the latest Proterozoic to earliest Cambrian. Acefiolaza & Miller (1982) further suggested that the terrane then underwent orogenesis during a Mid-Late Cambrian event that they termed the Pampean Orogeny (after Acefiolaza & Toselli 1976), followed by a period of sedimentation during the Late Cambrian. SIMS, J. P., IRELAND, T. R., CAMACHO, A., LYONS, P., PIETERS, P. E., SKIRROW, R. G., STUART-SMITH, P. G. & MIRO, R. 1998. U-Pb, Th-Pb and Ar-Ar geochronology from the southern Sierras Pampeanas, Argentina: implications for the Palaeozoic tectonic evolution of the western Gondwana margin. In: PANKHURST,R. J. & RAPELA,C. W. (eds) The Proto-Andean Margin of Gondwana. Geological Society, London, Special Publications, 142, 259-281.