Post-metamorphic differential cooling in the Sierra de San Luis (Argentina)

STEENKEN, A., ; LÓPEZ DE LUCHI, M. G.; SIEGESMUND, S.; WEMMER,K.

Mendoza

Simposio; Gondwana 12 Geological and biological heritage of Gondwana; 2005

Academia Nacional de Ciencias

Geochronological data on the metamorphic and structural evolution of the Sierra de San Luis (Sierras Pampeanas Orientales) provide an important insight into the geodynamic history along the southwestern margin of Gondwana. This geodynamic evolution comprises the Pampean, the Famatinian, and the Achalian tectonic cycles (cf. Steenken et al. 2003 for a recent review). Within the basement of the Sierra de San Luis geochronological evidence of the Pampean cycle has been obliterated by the subsequent Famatinian cycle. Available geochronological data on the Famatinian metamorphic evolution of the three basement domains of the Sierra de San Luis, i.e. the Nogolí, Pringles and Conlara Metamorphic Complexes (cf. Sims et al., 1998) denote to a common metamorphic peak at approximately 480 Ma (Whitmeyer and Simpson, 2004; Steenken et al., 2005) as a consequence of the accretion of the Cuyania Terrane to the western outboard of Gondwana (Casquet et al., 2001; Steenken et al., 2005). The establishment of polyphase deformation patterns in an orthogonal stress field is accompanied by the development of large-scale NNE-SSW striking mylonitic shear zones along the borders of the Pringles Metamorphic Complex, i.e. the R¨ªo Guzm¨¢n shear zone and the La Arenilla mylonite belt. Highly inclined or steeply ESE dipping mylonite fabrics superimpose a previous axial plane fabric. The Arenilla mylonite belt is characterised by a prominent near within-dip lineation that is indicated by the linear arrangement of sillimanite needles and the local appearance of staurolith prisms. Meso and microscopic shear indicators denote to an east-side-up reverse displacement. Microstructures within those high-grade mylonites record temperatures of more than 500¡ãC during deformation, as indicated e.g. by dynamic K-feldspar recrystallisation and subgrain formation in quartz above the ¦Á/¦Â transition (cf. Passchier and Trouw, 1996). Resumption of oblique compression due to the Achalian collision with the Chilenia Terrane (Sims et al., 1998) is accommodated by the sinistral reactivation of those Famatinian high-temperature shear zones at greenschist facies conditions. Cooling after the Famatinian peak metamorphism within the Sierra de San Luis is captured by K/Ar hornblende, muscovite and biotite ages with some additional substantiation by Rb/Sr mica dating. Careful evaluation of an ample collection of K/Ar muscovite ages obtained from different grain sizes of gneissic country rocks as well as coarse-grained pegmatites indicates that the entire basement complex of the Sierra de San Luis was cooled below the muscovite closure temperature interval at ~440 Ma. Moreover, fine-grained muscovites from the Pringles Metamorphic Complex provide evidence for a regional reheating of this basement domain that led to a reset of K/Ar muscovite ages at 400 Ma emphasizing the Achalian cycle as separated from the foregoing Famatinian history. Decelerated cooling within the two eastern basement domains is recorded by K/Ar biotite ages in the range between ~380 Ma and ~340 Ma. If differential cooling is interpreted in terms of differential exhumation accommodated by the R¨ªo Guzm¨¢n shear zone a change from west-side-up to east-side-up displacement is indicated.