INTERNAL STRUCTURE OF THE CENTRAL PATAGONIAN BATHOLITH AND ITS RELATIONSHIP WITH THE GASTRE FAULT SYSTEM

CLAUDIA BEATRIZ ZAFFARANA; RUBÉN SOMOZA

San Miguel de Tucumán

Simposio; Primer Simposio de Petrología Ígnea y Metalogénesis Asociada; 2011

The Central Patagonian Batholith is a set of Late Triassic granitoids composed of two superunits, the Gastre and the Lipetrén Superunits (Rapela et al. 1991, 1992). Their emplacement was proposed to be controlled by the activity of the NW-SE transcurrent Gastre Fault System (Coira et al. 1975). A detailed mapping of the internal structure of the granitoids of the region was performed combining petrographic, microstructural, anisotropy of the magnetic susceptibility (AMS) and rock magnetic studies in order to test this hypothesis. We divided the granitoids of the region in two groups: an older set of intrusives (the Gastre Superunit), which has mafic microgranular enclaves and titanite as an accessory mineral, and a younger set of granitoids (the Lipetrén Superunit) which postdates the mafic magmatism of the region. We chose to keep the names formerly given by Rapela et al. (1992), although our classification scheme is different. The mapping of the internal structure of the magmatic rocks showed the existence of a narrow structural corridor of magmatic fabrics of subvertical NW-SE orientation shared by all granitoid facies. Magnetic lineations inside this corridor are of shallow, subvertical and intermediate inclination. The areas outside the lineament have discordant magnetic fabric. The mafic magma in the Gastre region is represented by enclaves, dikes and small stocks; the different morphologies arise from magma mingling at different crystallization stages of the felsic magma host. Magma mingling also explains some disequilibrium textures such as oikocrysts, xenocrysts and crystal corrosion. Monogenic enclave swarms are described in the area for the first time. AMS studies of mafic microgranular enclaves and their host rock showed that enclaves constitute passive markers of magmatic deformation. Tectonic deformation is observed in small outcrops scattered among tectonically undeformed granitoids; their foliation planes and stretching lineations are highly heterogeneous. In conclusion, the pattern of obtained magnetic fabrics and the recognition of the different granitoids facies shows that the Gastre Fault System was a structure which controlled the diachronic emplacement of some tabular igneous bodies of the Central Patagonian Batholith, and which also accommodated some solid state post-emplacement deformation.