TEMPERATURE CONSTRAINTS ON DEFORMATION OF A CAMBRIAN GNEISS IN AN ORDOVICIAN BRITTLE-DUCTILE FAULT ZONE, CENTRAL ARGENTINA

On the westernmost edge of the Sierras de Cordoba, Eastern Sierras Pampeanas, central Argentina, a reverse fault zone thrusts sillimanite-bearing gneisses and schists over chloritic phyllites. The Los Tuneles fault zone provides a unique opportunity to observe deformation and microstructures that formed at depths comparable to the brittle-ductile transition. Protolith sillimanite-bearing gneiss and felsic rocks preserve Regime 3 recrystallization and checkerboard extinction patterns in quartz, and core and mantle structures in feldspars, giving a deformation temperature estimate of 550-650^o C. In the cores of boudinaged amphibolites, chevron folds of a layered meta-sedimentary rock have been overgrown by post-folding, euhedral new hornblende grains. At the margins of these boudins, the chevron folds were deformed into a planar fabric during amphibolite grade deformation. Timing of this high temperature event is based on U-Pb ages of metamorphic monazites from paragneisses of the central Sierras de Cordoba and is constrained at 515-520 Ma.

The fault zone contains a 2km wide zone of west-directed shear bands. In this zone, quartz grains preserve undulatory extinction, deformation bands and Regime 1 grain boundary migration. Muscovites grains are kinked, but feldspars have remained rigid. These microstructures lead to a deformation temperature estimate of 250-350 ^oC. Assuming a "normal" geothermal gradient, this constrains depth to ca. 8-12km during deformation. Numerous pseudotachylyte veins, consistent with brittle-ductile deformation, are present in the shear-banded gneisses above a brittle fault contact with the phyllite. The chloritic phyllites below the fault exhibit pressure solution structures that are kinked. Subgrain formation and a good grain shape preferred orientation in quartz grains in the phyllites also indicate temperatures of 300-350^o C. The microstructures observed are consistent with movement of the hanging wall through the brittle-ductile transition on the Los Tuneles Zone. Movement is presumed to be correlative with late Ordovician shearing that occurred elsewhere in the region as a result of terrane amalgamation on the margin of Gondwana.