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TitleThe =6-km Cuesta de Randolfo mylonite zone in Ordovician Famatinian peraluminous granites, NW Argentina: strain localization as a function of protolith composition
AuthorRatschbacher, BORCID logo; Cawood, T KORCID logo; Larrovere, M A; Alasino, P; Lusk, A D; Memeti, V
SourceJournal of South American Earth Sciences vol. 112, pt. 1, 103561, 2021 p. 1-17,
Alt SeriesNatural Resources Canada, Contribution Series 20230114
Mediapaper; digital; on-line
File formatpdf; html
Lat/Long WENS -69.0761 -59.8203 -19.8594 -35.2894
SubjectsScience and Technology; general geology; Cuesta de Randolfo mylonite zone; Ordovician
Illustrationslocation maps; cross-sections; photographs; tables; charts; diagrams
Released2021 09 23
The Cuesta de Randolfo mylonite zone (CRMZ) in the southern Puna of Argentina comprises >6-km of moderately to steeply dipping proto-to ultramylonites developed in peraluminous intrusive rocks of the Famatinian arc. From youngest to oldest, these include a tourmaline-bearing granite, K-feldspar-biotite granite, and biotite-plagioclase granodiorite. Based on whole rock chemistry, the former is identified as a distinct, more evolved and particularly silica-rich intrusive phase. LA-ICP-MS U-Pb zircon ages indicate that the intrusive units crystallized during two periods of Famatinian arc magmatism (~484 and ~471 Ma). Distinctive solid-state deformation microstructures and temperatures characterize four structural zones, revealing strain localization governed by protolith mineralogy during cooling. Early deformation by quartz subgrain-rotation recrystallization and minor K-feldspar bulging at ~500-450 °C resulted in moderate strain distributed across the width of the youngest, quartz-rich intrusive unit (the tourmaline-bearing granite), forming the Western Distributed Zone. With cooling, strain localized into reverse-sense shear zones active at 450-400 °C, forming the Western and Eastern Shear Zones along the contacts between the youngest and the older intrusive units. Lastly, narrow ultramylonites developed along the contacts between all intrusive units and along country-rock rafts, in particular in the Eastern Domain. Deformation at ~400-280 °C was driven by plagioclase-reaction weakening in the feldspar-rich older intrusive units. Progressive CRMZ deformation therefore transitioned from broadly- and homogenously-distributed at moderate temperatures-enabled by the abundance and weakness of quartz of the youngest units-to localized at low temperatures, driven by reaction softening of plagioclase in the older units. We conclude that the composition of intrusive units (silicic and peraluminous) has a first order control on strain localization. The CRMZ is part of a larger N-S striking network of wide ductile shear zones deforming peraluminous Famatinian-aged igneous rocks. Their similar composition likely caused the development of wide shear zones in the eastern part of the Famatinian orogen.
Summary(Plain Language Summary, not published)
Magmatic rocks in the southern Puna region of Argentina are cut by an unusually broad (= 6 km) zone of deformation. This study aims to explain what special conditions caused this deformation zone to be so wide. We document how deformation affected different parts of this zone at different times, and propose that these changes were controlled by the composition of the affected rocks. The deformation zone affects three different rock units: an older unit rich in the mineral plagioclase, a second unit with more K-feldspar, and a younger unit with abundant quartz. Different minerals respond to deformation in different ways depending on the temperature, so we can estimate the temperature of deformation by observing the structure of the rocks under a microscope. By combining these observations with field mapping, we show that the earliest deformation occurred at ~500-450°C, and affected a broad zone (km-scale) dominated by the quartz-rich rock. At these temperatures, quartz was the weakest mineral, which is why the quartz-rich rock accommodated most of the deformation. At this time, the deformation zone was wide, because the quartz-rich unit is wide. Later, as the region cooled, deformation became concentrated into narrower zones, and the last stages of deformation occurred at ~400-280°C, affecting very narrow zones (cm-scale) within the plagioclase-rich unit. At these lower temperatures, plagioclase reacts with water to form a very weak mineral called sericite, which is why later deformation was concentrated in narrow zones along which water had affected the rock.

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