LOWER CRUSTAL RHEOLOGY OF THE NORTHERN VOLCANIC ZONE ARC ROOT FROM NATURALLY DEFORMED XENOLITHS

The creation, refinement, and destruction of lithospheric material in the deep arc roots of continental subduction zones is as enigmatic to study as it is critical to understanding cordilleran systems. Flow in the lower crust may accommodate lithospheric removal from arc roots. However, the viscosity of the lower crust is poorly constrained and impossible to study in situ in modern arcs while the gravitationally unstable arc roots that may undergo loss of lithospheric material are poorly preserved in the exhumed paleosubduction rock record.

Young xenoliths (erupted ~280 ka) from the Granatifera Tuff sampled at the Mercaderes locality in the Andean Northern Volcanic Zone present a direct, near-modern window into a thick arc root thought to be undergoing recent to active foundering. A subset of Mercaderes xenoliths record lower crustal signatures including moderately to highly deformed garnet-bearing gabbros, hornblendites, and garnet clinopyroxenites. Petrologic and microstructural study of the lower crustal xenoliths enable reconstruction of the active deformation mechanisms, rheology, and strain rates in the lower crust of a modern arc and may constrain the lower crustal viscosities potentially accommodating lithospheric drip from a thick arc root.

EBSD analysis of 8 lower crustal Mercaderes xenoliths equilibrated at 1.2–2.2 GPa and 920–1280 °C [1] reveal both magmatic and subsolidus deformation features. Preliminary results show microstructures recording high-temperature grain boundary migration (GBM) in metamorphic quartz. Crystallographic preferred orientation (CPO), misorientation to the mean (M2M), and kernel average misorientation (KAM) analysis of amphibole and clinopyroxene show partially recovered intragrain strain and subgrain formation. CPO and KAM analysis of amphibole suggest activation of the [001](100) slip system producing a strong [001] CPO parallel to the foliation. Amphibole is abundant in many of the xenoliths and records dislocation accommodated deformation but remains deformationally and rheologically poorly constrained.

These xenoliths present an unparalleled opportunity to study the rheology of the lower crust in a modern arc system and constrain lower crustal viscosity which is mechanically significant yet poorly understood in arcs.

[1] Zieman et al., 2023. Geology, DOI: 10.1130/G50973.1