Arc Geochemistry Research
Above: Garnet-bearing gabbroic cumulate from the Granatifera Tuff in the North Andes
The formation and loss of the lower arc-crust in convergent margins has been posited as a fundamental process for explaining the bulk composition of Earth’s continents, recycling differentiated material into the mantle, and modulating first-order geodynamic processes in cordilleran orogens. Nevertheless, direct study of the processes responsible for forming and destroying lower-crust in modern arc environments is obscured by their lack of exposure and low preservation potential in the geologic record. In collaboration with Alan Rooney (Yale University) and Elias Bloch (University of Lausanne) we are studying a remarkable suite of mantle and crustal xenolith from the northern volcanic zone of the Andean arc, which provide an unparalleled record of modern lower crust formation in this archetypal active continental margin.
This research is supported by NSF-EAR grants 1926124 (to Ibañez-Mejia) and 1926421 (to Rooney). Duration: 09/2019 to 09/2022.
Highlights
recent lower-crustal foundering underneath the active north andean arc
Thermobarometry and Lu–Hf isotope systematics of crustal garnet clinopyroxenite xenoliths of the Mercaderes-Rio Mayo field offer the first evidence of recent, and likely active, crustal foundering in the Northern Volcanic Zone of the Andean arc. Most of the xenoliths we have studied so far equilibrated between 60–80 km depths, ∼7–27 km below the seismically determined Moho in this region, and that at least one crustal garnet clinopyroxenite re-equilibrated at depths exceeding 95 km. A large-magnitude positive geoid anomaly and relatively low mean surface elevations indicate that the gravitationally unstable crustal root is still largely attached to the overriding crust in this part of the Northern Volcanic Zone. Thermobarometric calculations indicate that the lowermost crust in this region is a partial melt zone, and we argue that rheological weakening in the presence of melt has led to the foundering of relatively small parcels of gravitationally unstable crustal material, which the Mercaderes xenoliths document, without catastrophic removal of the crustal root.
See Bloch et al. (2017) in the journal Earth and Planetary Science Letters for more information.
geochemistry of recent continental crust formation in the north andean volcanic zone
UR-LIG PhD student Lisa Zieman is studying the geochemistry of the Granatifera eruption, associated xenoliths and nearby stratovolcanoes, to better understand how high-pressure magmatic fractional crystallization of arc magmas (and their foundering) may be a key process for developing the geochemical and geodynamic signatures of Andean arc magmatism.
Stay tuned for upcoming papers from Lisa’s thesis.
