Do intrusive bodies weaken or strengthen the arc crust ? or both? An example from the Famatinian arc, Argentina

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

Pasadena

Congreso; GSA 116th Annual Meeting; 2020

GSA

Voluminous magmatism appears to have conflicting effects on the strength of the arc crust into which it is emplaced: the intrusion oflarge volumes of hot, weak material can weaken the crust and result in broad distributed deformation; on the other hand, the additionof large, competent bodies of plutonic rock appears to strengthen the crust overall, and cause strain to localize into narrow shearzones. We present a case in which both of these processes appear to have occurred, but at different times during protracted cooling ofintrusive units. The Cuesta de Randolfo Mylonite Zone (RMZ) is one of numerous km-scale shear zones in the central, S-type granitedominatedbelt of the Ordovician Famatinian Arc, Argentina. The RMZ comprises several distinct structural domains characterized bydistinctive compositional and deformation features: The ca. 4km wide Western Domain comprises the youngest intrusive unit of twomica, tourmaline-bearing granite with a NW-trending, moderate- to strong, pervasive ductile foliation and steeply-dipping lineations.We propose that the relatively low solidus temperatures of this strongly peraluminous and boron-rich unit caused it to remain above itssolidus for an extended amount of time. This, together with the high quartz content, facilitated a continuous transition from distributedmelt-present deformation into high-T, quartz-dominated subsolidus deformation, during which strain remained broadly distributed. TheEastern Domain, in contrast, is ca. 5km wide and comprises the oldest intrusive phase of plagioclase and biotite-rich granodiorite witha very weak to absent fabric, but which is cut by highly localized, cm- to m-scale ultramylonitic shear zones. These narrow shearzones trend NW, parallel the overall trend of the RMZ, typically occur along the edges of quartz veins or granitic dykes of the two-micagranite, and are characterized by intense sericitization of magmatic feldspar. Therefore, emplacement of the two mica-granite in theWestern Domain created a broad weak zone, which took up strain while the granite cooled from hyper- to subsolidus temperatures.After further cooling, the two mica-granite acted as a strong, competent unit, and strain was preferentially localized into narrowultramylonites in the adjacent granodiorite, which underwent reaction weakening.