THE MECHANICAL BEHAVIOR OF NATURALLY DEFORMED QUARTZ IN THE FOLDED CONTACT AUREOLE OF THE EUREKA VALLEY – JOSHUA FLAT – BEER CREEK (EJB) COMPOSITE PLUTON

Deformed quartzite samples were collected from a 10 km transect across the contact aureole of the EJB composite pluton to study quartz behavior and fabric development during its 10-million-year emplacement. The outer transect exhibits sedimentary textures typical of the Cambrian Harkless formation, characterized by broad E-W striking folds. In the mid-aureole (5-2 km from the pluton), there is a noticeable change: bedding rotates approximately 90° to form a steep N-S foliation, with no sedimentary textures. In the inner aureole (2 km - contact), another rotation occurs: bedding aligns parallel to the contact with a steep NW-SE foliation and vertical lineation. Here, the quartzite shows intense strain under amphibolite conditions and evidence of partial melting. Microstructural analyses reveal both brittle and plastic deformation textures that correspond to changes in quartzite orientation. Cathodal luminescence of quartz grains reveals a history of fracturing and subsequent recrystallization primarily through grain boundary migration (GBM). Grains with high luminescence define the fabric in the outer and mid-aureole but show fractures oriented perpendicular to the stretching lineation. In contrast, grains with low luminescence tend to recrystallize via GBM even at lower temperatures, exhibiting no fractures. By the mid-aureole, luminescence is homogenized across grains in a framework of triple junctions and irregular grain boundaries. A moderate crystallographic preferred orientation (CPO) emerges, with a subvertical crystallographic vorticity axis (CVA), indicating dynamic recrystallization at this stage. In the inner aureole, the strongest CPO (prism <c> slip) and a CVA parallel to the stretching lineation indicate higher temperatures and strains. Samples from the innermost 500 m exhibit evidence of multiple generations of strain-induced GBM and fracturing. As temperature increases, dynamic recrystallization becomes pervasive, resulting in fully homogenized, plastically deformed quartz but as strain rates increase, lineation sub-parallel fracturing also occurs. This detailed analysis suggests that fracturing plays a crucial role in fabric formation, working alongside GBM to accommodate strain. Moreover, minor element composition in quartz grains appears to influence the dominant deformation mechanisms observed across the aureole.