Strain localization mechanisms (or lack thereof) in a 10 km wide, syn- to post-magmatic mylonite zone in the Famatinian arc

RATSCHBACHER, BARBARA; CAWOOD, TARRYN; LUSK, ALEXANDER; LARROVERE, MARIANO A.; RICK, CHRISTOPHER; ALASINO, PABLO H.; PATERSON, SCOTT R.; MEMETI, VALBONE

Vienna

Congreso; EGU General Assembly 2018; 2018

European Geosciences Union

Numerous mechanisms have been proposed to localize strain in crustal rocks (e.g. grain size reduction). Incontrast, mechanisms also exist that could act against strain localization, leading to unusually wide shear zones,(e.g. reaction hardening). Several of these processes may be active in a single shear zone, as a function oftemperature, composition and time.The 10 km wide Cuesta de Randolfo Mylonite Zone (CRMZ), is one of numerous km-scale shear zones inthe central, intrusive-rock dominated part of the Ordovician Famatinian arc, NW Argentina. We use structuralmapping, microscopy, U-Pb geochronology and MELTS modeling to understand the structural evolution of theCRMZ. This shear zone provides an excellent case study to investigate the interplay between processes localizingstrain, and processes acting against strain localization, because it comprises two distinct structural, compositionaland temporal domains: (1) a western domain, dominated by younger two-mica, K-feldspar and tourmaline-richgranite, which displays distributed moderate strain (i.e. less localized); and (2) an eastern domain, comprisingolder, biotite-plagioclase-rich granite, with narrow ultramylonites along contacts with dikes, veins, pegmatites,or rafts of volcanic rock (i.e. highly localized). A narrow fault occurs at the contact between the two domains.Deformation was likely coeval across the two domains, as structures are parallel, and structures in the older easterndomain lack an overprint due to a younger deformation event.During arc-wide regional shortening, the younger 2-mica-K-feldspar granite intruded the older, solidified biotiteplagioclase-granite. At this time, the hypersolidus younger granite was the weakest phase, and shortening wastaken up preferentially by the melt. Upon cooling of the younger granite below its solidus (630 [U+1D52]C,lowered due to boron), quartz (across both domains) became the weakest phase, and underwent dislocationcreep and subgrain rotation (SGR) recrystallization. Strain localization by reaction weakening (plagioclasesericitization) only had a major effect in the plagioclase-rich older granite, which developed narrow, highlylocalized ultramylonites (quartz dislocation creep, and slip on bands of newly-formed mica), whereas the youngergranite formed a broad zone of distributed strain (quartz dislocation creep only). With further cooling, strainlocalized along the contact between the two domains, and along the existing ultramylonites in the older granite.This lower-T strain was accompanied by bulge (BLG) recrystallization in quartz, and slip along mica-rich shearbands.Several strain localization processes have been active, dependent on compositional and temporal characteristicsof the two domains. During emplacement of the younger granite, strain localized in the melt of this unit due toits relative rheological weakness. Subsequently, strain was accommodated by quartz dislocation creep in bothdomains; however, strain localization only occurred in the E domain, where the plagioclase-rich compositionallowed reaction softening. Thus, the broad, distributed strain in the W domain does not necessarily reflect anactive mechanism acting against strain localization. Rather, we suggest a lack of a strain localization mechanismwithin this domain causing broad strain distribution (insufficient grainsize reduction by SGR or BLG to lead to amechanism switch, and an unsuitable composition for reaction weakening).