DEVONIAN MAGMATISM IN SIERRA DE SAN LUIS, ARGENTINA: STRUCTURAL EVIDENCES FOR A SYNKINEMATIC EMPLACEMENT

The zoned and elliptical Devonian Renca and La Totora batholiths intruded the Early Paleozoic basement of the Sierra de San Luis. Both granitoids are concentrically foliated and show an outer porphyric granodiorite-monzogranite unit and an inner equigranular monzogranites. Although combined microstructural, petrofabric and anisotropy of magnetic susceptibility studies indicate that interactions between magmatism and emplacement related fabrics played the main role in the strain patterns, a far field control by the regional deformation is present. The batholiths exhibit at a regional scale, internal fabrics that are decoupled from host rock fabric except for the the narrow contact aureoles. Concentric dipping-inwards foliations and deflection of country rock foliation are the result of magma dynamics. In the Renca batholith, magnetic data indicate a flattening deformation with a weak lineation that varies from vertical to subhorizontal in the external units and is radial and subhorizontal in the central unit. Localized subhorizontal to shallower dipping inwards foliations in the outer unit result from radial compression preferentially accomodated in the west and east contacts. Radial compression is the result of the emplacement of the inner granitoid. Proximity to the roof zone is inferred from the concentric subhorizontal foliations and subhorizontally plunging lineations in inner unit. In La Totora batholith the increase of the intensity of the foliation towards the margins is related with the emplacement of the central granite. Magnetic data indicate a flattening deformation with a weak lineation that is mostly shallowly dipping with a preferred NNE direction. Previous interpretations of the Sierra de San Luis considered that the Devonian plutons were post-tectonic or -kynematic in relation with the Ordovician deformation. The WNW to NW elongation of the batholiths is controlled by a regional transtensional direction. The Devonian stress field controls the movement along sinistral transcurrent NNE ductile shear zones. The close association of this magmatism with mantelic derived rocks support the hypothesis of regional processes helping in creating conduits for the ascent of deep-seated magmas. In this connection we propose that the Devonian magmatism must be considered as syn-kynematic.