EMPLACEMENT OF THE RENCA BATHOLITH, SIERRAS DE SAN LUIS, ARGENTINA: AN ANISOTROPIC BALLOONING PLUTON

Magnetic and rock fabrics are used in order to constrain the emplacement of the zoned elliptical Late Devonian Renca Batholith (Sierra de San Luis, Argentina) which is mainly made up by ferrimagnetic porphyric granodiorites (Unit 1) and paramagnetic equigranular monzogranites (Unit 2). Magnetic foliation can be directly related to the mesoscopic foliation in Unit 1, the steep to vertical foliation results from a combination of both magmatic and locally high temperature solid state flows. K »O is measured both in kynematic indicators and in AMS data with a distribution of magnetic foliation planes generally sub-vertical. The fabric is parallel to the external and internal boundaries of the ring-like Unit 1, the X-Y plane of the enclaves, synplutonic dykes attitude. These internal features are concordant with the ductile aureole that developed by meso- and micro-folding of the regional NNE S2/S3 of the Ordovician metamorphic country rocks. AMS allows to infer proximity to the roof zone from the concentric subhorizontal foliations and gently plunging lineations in Unit 2. K close to O, oblate magnetic ellipsoids, weak lineation and concentric foliation, indicate a ballooning type process, related with magma pressure. However, the ellipsoidal shape of the body would point to a preferred direction of extension that would be in agreement with some far-field influence of regional structures active during the batholith construction. The regional strain field at 400 Ma is related with NNE dextral shear-zones that are recognized towards the west of the area. Those zones could both promote or channalize the ascent of the first melts and control the final expansion as the result of the emplacement of the central monzogranites