FINGERPRINTING CRUSTAL FOUNDERING IN THE NORTHERN ANDEAN VOLCANIC ZONE USING BASALT GEOCHEMISTRY

Foundering of dense crustal material into the mantle is thought to be an important mechanism of lithospheric recycling in subduction zones. The Quaternary Granatifera Tuff in the northern Andes contains an unparalleled load of lower-crust and mantle xenoliths, which include crustal garnet pyroxenites found to have equilibrated at supra-solidus temperatures in the Pliocene or younger and at depths close to, or greater than, the local seismic Moho. Although previous observations suggest that this locality is a potential site of recent crustal foundering, limited data exists to support this interpretation and very little remains known about the rare eruption that transported these xenoliths to the surface.

Here, primitive basalt geochemistry is used to investigate the nature of this eruption by tracing the potential source(s) of magma generation. Major and trace element concentrations were obtained by XRF and LA-ICP-MS for 13 juvenile lava bombs within pyroclastic flows of the Granatifera Tuff, all of which host garnet-free and garnet-bearing peridotite, pyroxenite, and hornblendite xenoliths. Based on their SiO₂ contents, all bombs are classified as basalts to basaltic andesites, and the most primitive samples (n = 5) are characterized by MgO wt. % > 7.7, Mg# > 60, and have high concentrations of Cr (181-285 ppm) and Ni (136-234 ppm), indicating they represent primary or near-primary magmas. Zn/Fe_T (*10⁴) ratios (12.9 – 13.7) fall well above the average for melts derived from normal peridotite (8.5) and even extreme peridotite (11), thus requiring a significant component of pyroxenite-derived melts in their source. We argue that basalts of the Granatifera Tuff record the contribution of pyroxenite-derived melts to arc magmatism in this section of the northern Andean volcanic zone and presumably formed as lower-crust garnet pyroxenites surpassed their solidus near the base of the arc-crust and/or during gravitational descent into the mantle. These observations are in agreement with geochemical and thermobarometric data from abundant garnet clinopyroxenite xenoliths found in this eruption, and also bear close geochemical similarities with lavas previously interpreted as recording lower crustal foundering under the Puna Plateau in the Central Andes.