Late Cretaceous magmatism in central Tierra del Fuego: Kranck Pluton, Fuegian Andes, Argentina.

CERREDO, M. E.; TASSONE A.; PERONI J. I.; MENICHETTI, M.

San Miguel de Tucumán

Simposio; 1º Simposio Sobre Petrología Ignea y Metalogénesis Asociada; 2011

Universidad de Tucumán

The Kranck pluton (KP) is a small (<10 km2), subcircular, epizonal intrusive body exposed at the northern shore of Lago Fagnano, in Sierra de Beauvoir, Tierra del Fuego. The intrusive body is hosted in the pelite/mudstone sequence of the Lower Cretaceous Beauvoir Formation. Although KP outcrops are restricted and largely covered by forest, aeromagnetic surveys have revealed an outstanding anomaly with a subcircular pattern and average diameter of around 6 km. Modeling of the aeromagnetic anomaly related to KP yielded a thin sheet body (Peroni et al., 2008). The KP is located within the Fuegian thin-skinned fold-thrust belt, the intrusive body is flanked by two major structures: a low angle NNE verging thrust (belonging to the main stack of the Fuegian Cordillera thrust front at the boundary with the Magellan foreland) in the north, and the eastern tip of the M. Hope-Catamarca-fault segment belonging to the Magallanes-Fagnano Fault System, at the south (Menichetti et al., 2008). A newly acquired amphibole K/Ar mineral age (95.1+2.9 Ma) allows to constrain the KP emplacement time to the Late Cretaceous. Field and magnetic fabric data indicate concordant foliations within the pluton and its country rocks (Cerredo et al., 2010). No petrographic evidence of pluton thermal impact was recognized in the host mudstones; thermal modeling of the KP predicted maximum temperatures below 300 ºC in a narrow rim around the intrusion. The KP displays a large compositional span, from monzogabbro to syenite, of metaluminous, shoshonitic nature, as is typical in other Fuegian plutons (Cerredo et al., 2005; Peroni et al., 2009). The KP encompasses minor cumular ultrabasic facies, gabbros, monzodiorite and monzonite facies and late stage syenite veins and dykes. Monzodiorite facies are generally heterogeneous, bearing mafic microgranular enclaves (MMEs) several cm to dm in size, either with typical crenated outlines or as diffuse ghosts parallel to magmatic banding. Some MMEs show clear textural features of undercooling suggesting a scenario of sudden mixing of magmatic liquids of contrasting temperatures. Petrographic evidence of this quenching is the presence of high aspect ratio apatite, sphene and large (up to 1 cm) amphibole crystals with dendritic outlines. These textural features provide evidence for the fast cooling of the MMEs in contact with the host monzonitic/dioritic magma and points to several injection episodes in the assembly of the KP.