Petrology of Modern Continental Crust Formation and Recycling in the Northern Andean Arc

Abstract

Lower continental crust formation and recycling at continental margins is thought to influence topography, magmatism, and the bulk composition of the continental crust; however, the roots of continental arcs are typically poorly preserved in the geologic record and inaccessible for study. In this dissertation, I explore continental crust formation and recycling using petrologic evidence from the Mercaderes (ultra)mafic xenoliths, a suite of well-preserved lower crust- and mantle-derived intrusive fragments from the active Northern Volcanic Zone (NVZ), Colombia. The dissertation consists of four manuscripts focused primarily on the lower crustal garnet clinopyroxenite xenoliths (arclogites) and, secondarily, on primitive magmatic bombs erupted with the xenoliths in the Granatifera Tuff (GT) as well as evolved felsic extrusive rocks from the Doña Juana polygenetic volcanic complex, in close proximity to the GT. In Appendix A, I investigate the thermal, density, and seismic structure of the recent arc root and sub-arc mantle using thermobarometry, and calculate timescales over which lower crustal recycling can occur via a Rayleigh-Taylor instability. Zircon U-Pb geochronology of the GT and an overlying eruption is also presented and constrains the eruption to no earlier than 238 (±19) ka. In Appendix B, I use radiogenic (Re-Os, Sm-Nd) and stable isotopes (δ18O) to distinguish between crustal-affinity (arclogite) and mantle-affinity (metasomatic) pyroxenite xenoliths. The isotopic data unequivocally distinguishes one pyroxenite fragment as the first direct evidence for recycled crustal material. In Appendix C, I explore how the zirconium (Zr) stable isotopes system, a novel heavy stable isotope system, behaves during calc-alkaline arc magma differentiation by measuring the Mercaderes xenoliths, mafic bombs, and Doña Juana felsic volcanics. Zirconium stable isotopes are highly variable among rock-forming phases in the lower arc crust, which is likely induced by sub-solidus diffusion-driven isotopic fractionation. Finally, Appendix D is focused on the stratigraphy of the GT eruption and geochemistry of mafic magmatic bombs that erupted with xenoliths, which resemble near-primary arc magmas. I present trace element linkages between the arclogite root and the primitive magmas bombs using fractional crystallization and partial melting models. Altogether, this dissertation presents new thermobarometry, geochronology, major and trace element geochemistry, and stable and radiogenic isotope data of the NVZ lower arc crust, which advances our understanding of continental crust formation and recycling in a modern continental arc.