Revisiting lithospheric dripping: Modern crustal xenoliths from the Colombian arc suggest heterogeneous viscosity and cyclic instability development

Abstract

Lithospheric dripping is often invoked to remove dense material from arcs. However, the thickness, timing, and mechanical properties that lead to detachment of lower arc crust remains poorly constrained. The Mercaderes xenolith suite erupted <280 kyr in the Northern Volcanic Zone of the Colombian Andes and includes mantle and crustal xenoliths providing a near-complete lithospheric section through a negatively buoyant root geophysically observable today. A subset of lower crustal xenoliths span amphibole to clinopyroxene dominated assemblages with igneous to metamorphic textures. In-situ U-Pb dating provides lower intercept 238 U/ 206 Pb ages of 1.7 ± 1.1 Ma for euhedral titanite in an amphibole cumulate, 2.5 ± 2 Ma for titanite in an amphibolite, and apatite dates with means between ∼3 to 7 Ma but with large uncertainties given their much lower U contents compared to the titanite - all recording recent arc lithologies and processes. The garnet clinopyroxenites and amphibole-rich cumulates are strong with weak crystallographic preferred orientations (CPOs) and low intragranular deformation. The crystal-plastically deformed metamorphic amphibolites are the weakest sampled lithology with strong CPOs and intragranular deformation. Rayleigh-Taylor Instability modeling based on recent constraints predict exceptionally fast (∼10 4–5 years) dripping of gravitationally unstable crust once this layer reaches 1–3 km thickness, yet thermobarometry and receiver functions document an 8.5–14 km thick negatively buoyant root that must have existed for at least a few million years. We document the importance of compositional heterogeneity and petrologic evolution and consider a framework of cyclical small drips active simultaneously as the “overthickened” root is built.