Protracted Dyking of High-Viscosity Magma into Near-Solidus Rock at Casiri, Peru

Dykes are a key mechanism for material transport within volcanic arcs and typically propagate over days to months. We document an unusually slow dyking episode in the arc of southern Peru that lasted for 1.5 years. The dyke opened up to 0.5 m from 2-8 km depth and was coupled with inflation of an underlying arc-parallel magma reservoir. To reconcile the slow opening of such a thin dyke with physical models of magma flow and heat transfer, magma with viscosity $\sim$10$^{6}$ Pa s was likely injected into wall rock within $\lesssim$100 $^\circ$C of the magma's solidus. Pre-heating of the shallow crust by previous plutonic intrusions may have enabled the viscous magma to have propagated as a dyke to within 2 km of the surface. Our findings suggest that significant pre-heating will be preferential for the transport of evolved, silicic magmas in dykes to depths where magmatic-hydrothermal mineral deposits form.