XENOLITH CONSTRAINTS ON RHEOLOGY OF HETEROGENEOUS DEEP CRUST BENEATH THE EASTERN MOJAVE DESERT, CALIFORNIA

Lower crustal xenoliths from a Tertiary dike in the Eastern Mojave Desert, California are lithologically heterogeneous. They broadly reflect igneous and sedimentary protoliths, but two features stand out: 1) high proportion of metapelites compared to typical lower crustal xenolith suites; 2) abundant modal garnet. Previous work by Hanchar et al. (1994) interpreted garnet-rich metasedimentary xenoliths as residues of deep crustal melting. Here, we use electron backscatter diffraction (EBSD) large area mapping to investigate rheology of three major compositional categories represented in the metasedimentary suite: quartzose, quartzofeldspathic, and aluminous. We focus on modally abundant rheology-controlling minerals quartz and plagioclase. Quartz microstructures in quartzose xenoliths are consistent with dynamic recrystallization (DRX) via grain boundary migration and subgrain rotation (SGR). In quartzofeldspathic and aluminous compositions, quartz grains display sparse interconnected bands, but isolated grains are common. Using quartz pole figure data, we find crystallographic preferred orientations (CPOs) consistent with operation of {m} <c> and {m} <a> slip systems and less commonly, (c) <a> and {r} <a> slip systems. Plagioclase microstructures are characterized by grains with deformation twinning, as well as by dynamic recrystallization via SGR. Plagioclase pole figures exhibit CPO consistent with [001](010), ½[112](11-1), and [010](100) slip systems. Our data lead to several findings with respect to strain accommodation between rock compositions, and thus have implications for bulk rheology of heterogeneous crust. First, quartz in quartzose xenoliths accommodates more deformation than quartz in other xenolith types, suggesting phase topology as a first-order control on deformation amongst diverse lower crustal compositions. Second, some samples have quartz and plagioclase CPOs consistent with dislocation creep; however limited interconnectedness and interstitial nature of the phases appear incompatible with crystal-plastic deformation. Instead, we propose CPO could develop from shape-preferred orientation induced through partial melting. Our results provide insight into contributions of various rock compositions in lower crustal strain localization and melt.