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Geological Society, London, Special Publications; 1998; v. 142; p. 259-281;
DOI: 10.1144/GSL.SP.1998.142.01.13
© 1998 Geological Society of London

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. Sims1, T. R. Ireland2,3, A. Camacho1, P. Lyons1, P. E. Pieters1, R. G. Skirrow1, P. G. Stuart-Smith1 & R. Miró4

1 Australian 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 Servício Geológico Minero Argentino, Av. Poeta Lugones, Nueva Córdoba, Argentina

New SHRIMP zircon and monazite 206Pb/238U and 208Pb/232Th ages on structurally controlled units and 40Ar-39Ar 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.





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