Magmatism and strike-slip tectonics in the Southernmost Andes: Ushuaia Pluton. Tierra del Fuego

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

San Salvador de Jujuy

Congreso; XVII Congreso Geológico Argentino; 2008

CGA-RAGA

The Ushuaia Pluton (UP) is located along the strike of the Beagle Channel Fault System (BCFS) in the southernmost tip of Andean Cordillera where there is an abrupt change of about 90° in its strike from a roughly north-south direction in continental Patagonia into an east-west orogen in the island of Tierra del Fuego. It has been proposed that this sharp orogene curvature is due to major strike-slip offsets (Cunningham, 1993). The BCFS is one of these WNW-ESE trending strike-slip faults. The UP is an epizonal intrusive body of around 14 km2 hosted in the Early Cretaceous Yahgan Formation (YF) on the northern margin of Beagle Channel. An extended metamorphic contact aureole (> 1km) is recognized within the turbidites of YF reaching up to the K-feldspar zone where local pockets of partial melts were produced. The contact assemblages overprint the foliation of the host defined by very low grade associations. UP is compositional varied and includes several facies: a basic to ultrabasic facies, a heterogenous facies where magma mixing processes are evident and marginal roof facies of volcanic/hypabyssal character. Rocks are metaluminous and span from the medium-, through the high- K up to the shoshonite series. Mg# and Ni, Cr and Co contents of basic and ultrabasic rocks indicate some Fe-Mg silicate fractionation at the source area. Some trace element contents and interelemental ratios clearly indicate the subduction-related nature of UP. The pluton display a suite of down-T microstructures from the submagmatic stage to the britlle low greenschist facies condicitions (Cerredo et al., 2007). UP is affected by E-W, vertical, left lateral strike slip faults which are crosscut by a set of high angle to vertical normal faults. Kynematic analysis over the whole fault population indicates a N-S extensional stress field (Menichetti et. al., 2007). A suite of brittle, low greenschist facies conditions, microstructures are associated with this extensional regime (i.e. pyrite strain fringes, extensional veins, kink bands in amphibole and plagioclase). A preliminary ASM survey has been carried out on UP aiming to recover high-T submagmatic fabric which can provide new insights into the history of pluton assembly. Presently available data suggest a probable layered nature which agrees with the laccolith geometry proposed on magnetic modeling grounds (Peroni et al., 2008).