Evolution of Microstructural Heterogeneity in the Deep Arc Lithosphere During Delamination

Abstract

The microstructural properties of deep arc cumulates (arclogites) are poorly understood, but are essential in gaining a comprehensive picture of the rheology of continental lithosphere. Here, we analyze 16 arclogite xenoliths, comprising a low MgO and a high MgO suite, from Arizona, USA using electron backscatter diffraction to map microstructures, clinopyroxene shape preferred orientations (SPO), and clinopyroxene crystallographic preferred orientations (CPO). The lower pressure (∼1 GPa) low MgO arclogites show a variety of different clinopyroxene fabrics (S, L, and LS‐type), whereas the high pressure (>2 GPa) high MgO arclogites show predominantly LS‐type fabrics. Furthermore, clinopyroxenes in low MgO arclogites all show a pronounced correspondence between the long axis of their grain shape ellipsoids with the [001] crystal direction, indicating an SPO control on the CPO. In contrast, high MgO arclogite clinopyroxenes lack such a correspondence. We propose that both arclogite types originated as igneous cumulates, consistent with previous studies, but that the high MgO suite experienced substantial recrystallization which diminished the original igneous SPO‐induced CPO. Using strain rates appropriate for arc settings, we calculate a strength profile for the lithosphere and argue that the deepest arclogite textures are consistent with lithospheric foundering through ductile deformation under high shear strain (10−14–10−12 s−1). Our study shows that there is a high degree of shear strain localization in deep arc roots while shallower portions are relatively undeformed.