INSTITUTO DE GEOCRONOLOGIA Y GEOLOGIA ISOTOPICA
Unidad Ejecutora - UE
congresos y reuniones científicas
TECTONIC HISTORY OF THE BASEMENT DOMAINS OF THE SIERRA DE SAN LUIS, ARGENTINA
ANDRÉ STEENKEN1, MÓNICA G. LÓPEZ DE LUCHI, SIEGFRIED SIEGESMUND, KLAUS WEMMER
Otro; Reunión de Téctonica y Geología Estructural; 2006
Universidad Nacional de San Luis
The geodynamic scenario of the Sierra de San Luis (Sierras Pampeanas, Central Argentina) is analysed based on the combination of structural observations and precise geochronological data. Provenance and timing of deposition of the sedimentary precursors and the metamorphic and igneous history of the different basement domains of the Sierra de San Luis are addressed through SHRIMP U/Pb dating of zircon and monazites. Additional substantiation of the data set is given by results from PbSL experiments applied to two garnet and one staurolith separates. The approach and accretion of crustal fragments integrated to the southwestern margin of Gondwana: the Pampean, Precordillera/Cuyania and Chilenia terranes controlled the tectonic evolution of the Sierra de San Luis. The sedimentary precursors of the Conlara Metamorphic Complex were deposited previous to ~590 Ma, whereas the Pringles metasediments appear to be sourced from the Pampean Orogen in the Early Cambrian. The incorporation of folded xenoliths within the ca. 496 Ma old El Peñón batholith suggests that the host Conlara Metamorphic Complex already suffered a Pampean compressive event. The Famatinian metamorphic peak at ~480 Ma is present within all basement domains. Metamorphic ages older than the Early Ordovician have not been detected in the Nogolí Metamorphic Complex. A U/Pb monazite age from a migmatite from the northern part of the complex is suggestive for an entirely post-Pampean structural evolution. Ductile folding without development of a new metamorphic plane but with complete resetting of the U/Pb system of monazite, as it would be required to maintain the interpretation of the Pampean origin of fabrics, turns out to be highly unlikely. Geochronological constraints for the high-grade metasediments of the Pringles Metamorphic Complex and U/Pb dating of zircons from dacitic dikes in the Mica-schist Group and the San Luis Formation suggest of one single Early Cambrian period of sedimentation. The onset of orthogonal compression in the high-grade metamorphic rocks is contemporaneous with the deposition of younger sediments on top of the sequence. This scenario is related to the opening and subsequent closure of a back-arc basin in proximity to the Pampean Orogen. In the Pringles Metamorphic Complex 490-460 Ma high temperature low/medium pressure (5.7-6.4 kbar/740-790ºC) granulite facies rocks are spatially associated with a NNE trending belt of maficultramafic intrusions. The metamorphism was generally related to the D2 controlled accommodation of these back-arc mafic and ultramafic melts. The corresponding PT path displays a prograde evolution with an inferred coeval heating and limited pressure increase and a retrograde stage with a near isobaric initial cooling phase. The initiation of granulite facies metamorphism due to the contact heat of mafic and ultramafic intrusions is confined to the Middle Cambrian (~506 Ma) and is closely followed by the emplacement of crustal derived granitoids, e.g. the northern stock of the Paso del Rey granite at 491 Ma, that are parallel to the S1 foliation of the country rock. Segregation of crustal melts led to the contemporary sheet-like peraluminous tonalitic to granite intrusions. Magma migration to higher levels of the crust was favoured by the compressive event that would be related to a convergent active continental margin i.e. the approaching Cuyania terrane to the SW margin of Gondwana The inferred syn-D2 emplacement of those granitoids indicates that the D1 to D2 (i.e. D1 of the San Luis Formation) structural evolution pre-dates the generally assumed Middle-Ordovician accretion of the Precordillera/Cuyania Terrane. Probably the initial compression resulted rather from changes in the geometry of the subduction zone than from the docking of the Cuyania Terrane itself which is considered to having occurred shortly before 460 Ma. Refolding (D3) of the tight to isoclinal D2 folds is poorly constrained but older than the cooling of the basement below the closure temperature for the K/Ar Bt system at ~380 Ma. Therefore it could be related with the approach of the Chilenia Terrane.