Strain (de)-localization in the ~10km wide Cuesta de Randolfo mylonite zone, Famatinian Arc

CAWOOD, T.; PATERSON, S.; RATSCHBACHER, B.; LUSK, A.; LARROVERE M.A.; ALASINO P.H.; RICK, C.; MEMETI, V.

Congreso; GSA Annual Meeting 2017; 2017

The ~470 Ma Famatinian Arc in Argentina hosts numerous wide shear zones in its central, S-type pluton dominated part, including the ~10 km Cuesta de Randolfo Mylonite Zone (RMZ). An understanding of the mechanisms of strain localization active (or not) in the RMZ may help explain the occurrence of so many wide shear zones in this arc, and contribute to our understanding of strain localization processes in general. We combine field mapping with microscopy to examine the distribution of strain between compositionally- and texturally different map units within the RMZ. We compare how the distance over which protomylonite gives way to ultramylonite in outcrop, and the proportion and grain size of recrystallized quartz in thin section, change along transects through units of different composition and/or age. The RMZ is a composite shear zone, comprising two distinct structural domains. Across both domains, much of the shear strain occurs in bodies of relatively younger alkali-feldspar, two-mica and tourmaline-rich granite. This unit displays distributed strain, accommodated by extensive Subgrain Rotation (SGR) recrystallization of quartz. In contrast, bodies of older, more biotite-rich but tourmaline-absent granodiorite appear significantly less deformed, with weak to no solid-state foliation, except where cut by narrow (cm- to m-scale) mylonite to ultramylonite zones. The younger two-mica granites were emplaced during regional shortening, as evidenced by steep magmatic fabrics that parallel regional solid-state foliations, and indicate a continuous magmatic to solid-state deformation transition. They were therefore less competent than the older, colder granodiorite, and could accommodate greater strain. The granites thus developed only a moderately intense foliation, distributed over a wide zone of weak rock. Furthermore, we propose that the solidus temperature of the tourmaline-bearing granites was lowered by the presence of abundant boron in the melt. Strain localization mechanisms such as strain softening by water addition or reaction, or a transition to grain size sensitive creep, either did not occur or did not have a significant impact at these near-solidus conditions. The granodiorites, however, were significantly below their solidus, and thus incapable of distributing strain over wide zones.