The basement complex of the Fuegian Andes: microstructures and mineral assemblages.

CERREDO, M. E.; TASSONE, A.; MENICHETTI, M.

San Luís, 17-21 de octubre

Otro; XIII Reunión de Tectónica.; 2006

ComTect-RAGA

The schists outcropping at the south-western corner of Tierra del Fuego (Bahia Lapataia, Bahía Ensenada an along the Lago Roca shoreline) have been traditionally considered as the easternmost exposures of the Early Mesozoic Basement Complex of the Fuegian Andes. This unit forms a WNW-ESE trending-belt, which extends along the northern Beagle Channel from the Cordillera Darwin (Chile) where high-grade assemblages are found. Rocks assigned to this unit in the Argentinean Tierra del Fuego display very low-grade to low-grade metamorphic associations and form restricted outcrops limited by a WNW-ESE thrust, which brings them in contact with the Upper Jurassic Lemaire Fm. and the Lower Cretaceous Yahgan Fm. The exposures are dominated by fine-grained phyllites displaying polyphase deformation and widespread quartz (± calcite, ± chlorite) veins. Phyllites are banded with quartz-feldspar-rich millimetre to cm- layers (Q domains) alternating with phyllosilicate-rich bands (M domains); typical facies variations are related to the Q/M ratio and the abundance of veins. Where Q domains dominate over M domains phyllites are characterized by ductile folds whereas more brittle, box fold morphologies dominate in M>Q phyllites The Basement Complex rocks are distinguished from the fine-grained facies of the cover units (i.e.: Yahgan and Lemaire Formations) by the abundance of quartz (±carbonate±chlorite) veins and by the presence of three cleavages, whereas within the cover units only two cleavages may be recognized. The Basement Complex phyllites display a polyphase evolution beginning with the development of an early foliation, S1, and associated compositional layering (likely parallel to former bedding) with Q and M layers. In the latter, oriented chlorite characterizes the S1 slaty cleavage whereas in Q domains S1 is a spaced discontinuous pressure solution cleavage lined by opaque dust. The oldest veins are parallel to both S1 and compositional layering and are typically coarser grained than the host. A second deformation (D2) folds S1 and produces S2 which in Q domains is defined by reddish oxides and in M layers is a discrete crenulation cleavage lined by opaque dust. Extensional fractures parallel to F2 axial planes with epidote infillings are found within Q domains. Within more strained domains in M-rich varieties S2 (lined by high aspect ratio muscovite and chlorite (17:1) ± opaque dust) becomes the main foliation. S1 is rarely preserved either as relic microfolds within S2 microlithons or as folded quartz hinges with axial planes parallel to S2. The third cleavage S3 occurs either as a non-penetrative crenulation of S2, or as penetrative spaced crenulation cleavage defined by thin strings of opaque dust ±white mica of high aspect ratio (7:1).