INSTITUTO DE GEOCRONOLOGIA Y GEOLOGIA ISOTOPICA
Unidad Ejecutora - UE
congresos y reuniones científicas
Famatinian granulite facies metamorphism in the central basement domain of the Sierra de San Luis, Eastern Sierras Pampeanas, Argentina.
STEENKEN, A., LÓPEZ DE LUCHI, M.G., SIEGESMUND, S., WEMMER, K
Conferencia; Granulite Conference; 2006
Universidad de Brasilia
In the Pringles Metamorphic Complex1 (PMC) the central basement domain of the Sierra de San Luis (Eastern Sierras Pampeanas), 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 melts1,2,3,4,5. The corresponding PT path displays an anti-clockwise loop, characterised by a prograde evolution with an inferred coeval heating and limited pressure increase and by a retrograde stage with a near isobaric cooling initial phase2. A relatively high geothermal gradient (around 40ºC/km3) is registered in the complex5. The simulation code SHEMAT6,7 was used to characterise the heat source that could be causal for the granulite facies metamorphism and widespread migmatisation of the PMC. Numerical modelling used a finite-difference approximation of mutually coupled equations describing the conduction of heat in a solid medium. The description of the thermal aureole in the vicinity of the mafic and ultramafic intrusions within the PMC indicates the requirement of a large near surface seated heat source in order to explain the distribution of the observed granulite facies metamorphism. Simulation runs indicate that the injection of mafic melts was not a single event but rather a long-lasting episode of continuous magma supply and storage. These results were combined with geochronological data. U-Pb SHRIMP zircon ages1 and mineral-whole rock Sm/Nd5 isochrons indicate a protracted (506-478 Ma) period of mafic magma input in the crust which agrees with the modelling prediction. The ca 500-468 Ma emplacement of the Famatinian granitoid magmatism, the 490-460 Ma granulite facies metamorphism and the emplacement of mafic rocks in the PMC are essentially contemporaneous. The backarc signature of the mafic units, the protracted period of mafic input to the lower crust as reflected both by the model predictions and the crystallization ages of the mafic rocks together with the coeval peak metamorphic conditions and crustal derived granitoid magmatism suggest that the granulite facies metamorphism results from an anomalous heat input in a crust that after undergoing extension changed to compressive regime with the consequent closure of the back-arc basin. Segregation of crustal melts led to the contemporary sheet-like peraluminous tonalitic to granite intrusions. Magma migration 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.