The evolution of a high temperature/low pressure crustal block and its thick ultramylonitic shear zone: an insight into the Famatinian Orogen, the Sierra de Quilmes, Sierras Pampeanas, NW Argentina.

Abstract

This thesis investigated the evolution of a high temperature, low pressure crustal block of the Late Cambrian to Silurian Famatinian orogeny in the Sierra de Quilmes mountain range of the Sierras Pampeanas, NW Argentina. The crustal block is bound by the El Pichao-Ovejeria shear zone, of which the eastern strand, the El Pichao shear zone, is a focus for this thesis. The El Pichao shear zone contains a 1 km-thick ultramylonite at its base and thrusts the granulite facies Tolombon complex, onto the amphibolite facies Agua del Sapo complex. The aims of this project were (1) to characterise the structural and metamorphic evolution of the Tolombon complex and its significance for understanding the western margin of Gondwana during the Famatinian orogeny, and (2) to determine the processes that formed the thick ultramylonitic layer. Mapping in the Tolombon complex revealed that metamorphic zones increased from greenschist facies in the NE to granulite facies migmatites in the SW (chapter 2). Rocks are sheared with top-to-W or -NW thrust shear sense that intensifies near major thrust shear zones, which repeat metamorphic zones (chapters 2, 4). We determined that thrusting in the Tolombon complex began during peak metamorphism (chapter 2), between 485 and 465 Ma (U-Pb monazite, chapter 4). The kinematics remained the same as rocks were thrust to shallower structural levels and shearing localised to the amphibolite facies El Pichao shear zone (chapters 2 and 4). Major- and trace- element geochemistry revealed that the mylonitic rocks of the El Pichao shear zone define a geochemical field narrower than their protolith, suggesting that they underwent mixing and homogenisation during shearing and did not experience substantial alteration (chapter 2). Peak metamorphism in the footwall of the PSZ occurred between ~435 and 420 Ma (U-Pb monazite, chapter 4) suggesting that the Tolombon complex was thrust onto the Agua del Sapo complex prior to this time, coinciding with the activation of other mylonitic shear zones in the Sierras Pampeanas. We speculate that the heat flux during peak metamorphism in the Tolombon complex may be a result of contact metamorphism, increased rates of shallow convection in the asthenosphere, or inherited high geothermal gradients from an earlier period of shearing (chapter 4). We suggest that the Sierra de Quilmes may have been part of the Famatinian back-arc at this time, indicated by a shared high geothermal gradient and position inboard of the Famatinian magmatic arc (chapter 4). Pervasive shortening in the Sierra de Quilmes from early in the Famatinian orogeny suggests that the Famatinian back-arc was dominantly convergent and any period of extension was brief, concluding before peak metamorphism. Ultramylonites 1 km thick are rare and suggest the layer may have widened over time, perhaps a result of weakening of the host rock or hardening of the ultramylonite. These processes may be facilitated by water migration, so we used FTIR to measure the water content of mylonitic rocks (chapter 3). We found that water content decreased as strain increased, the opposite of previous studies. We speculate that water and heat diffusion out of the ultramylonitic layer caused the ultramylonite to harden and the neighbouring rocks to weaken (chapter 3). This caused shearing to refocus to the neighbouring rocks and widened the shear zone. Repetition of this cycle gradually built up the 1 km-thick layer of ultramylonite (chapter 3). The work presented in this thesis provides insight into strain localisation in shear zones and the role of water and heat diffusion in generating thick layers of ultramylonite. This work also demonstrates that the Sierra de Quilmes provides a unique insight into the tectono-metamorphic evolution of Famatinian back-arc, suggesting shortening was dominant during its development.