THE THERMAL STRUCTURE OF AN ACTIVE CONTINENTAL ARC: INSIGHTS FROM GARNET PYROXENITE AND PERIDOTITE THERMOBAROMETRY IN THE NORTH ANDES

The thermal structure of continental arcs controls fundamental arc processes such as magma generation and differentiation, seismicity, deformation, and crustal recycling. Foundering of the lower continental crust is a key arc process that depends on the thermal structure and composition of the lower crust and sub-arc mantle. These conditions govern density and viscosity, influencing the likelihood and timescales over which the lower crust will become density-unstable relative to the underlying mantle. Here, we investigate the thermal structure of the Andean Northern Volcanic Zone (NVZ), an active continental arc, using geochronologic and petrologic evidence from the Mercaderes xenolith field, Colombia. Ash bed zircon grains from the Granatifera Tuff, which exhumed the xenoliths, were dated to no more than ca. 240 ka using zircon U-Pb, indicating the xenolith cargo was entrained from the (near)-modern lower crust and sub-arc mantle. To constrain the thermal structure of the lower crust and sub-arc mantle, we used thermobarometry between rock-forming phases to estimate pressure and temperature for 55 lower crust-derived garnet-bearing gabbro, pyroxenite, and hornblendite fragments and 25 mantle peridotite and pyroxenite fragments. The lower crust-derived fragments equilibrated at 1.2-2.2 GPa and 920-1280°C, defining a steep conductive geothermal gradient. Many mantle-derived fragments equilibrated in a hot convective thermal regime (~1270°C, 2.9-3.2 GPa), whereas another subset of these samples record slightly lower temperatures at higher pressures (1140-1230°C, 3.5-4.1 GPa), reflecting cooling of the mantle adjacent to the subducting oceanic slab. Our results indicate the presence of a 9-17 km thick garnet-rich crustal root which is ~100 kg/m³ denser than the underlying mantle, in agreement with recent estimates from seismic wave inversion. Rayleigh-Taylor instability growth calculations for the NVZ suggests that crustal foundering occurs on shorter timescales than estimated for exhumed arc terranes (e.g., Kohistan and Talkeetna) due to the hotter Moho temperature, defining a hotter end-member of lower crustal recycling in a modern continental arc, which contrasts with the ‘cooler’ styles preserved in the exhumed geologic record.