CRUSTAL ANATEXIS DROVE PORPHYRY COPPER ORE FORMATION DURING LARAMIDE FLAT-SLAB SUBDUCTION

The prevailing paradigm for the formation of porphyry copper deposits along convergent plate boundaries involves deep crustal differentiation of metal-bearing juvenile magmas derived from the mantle-wedge above a subduction zone. However, many major porphyry districts formed during periods of flat-slab subduction when the mantle-wedge would have been reduced or absent, leaving the source of the ore-forming magmas unclear. To resolve this paradox, we investigate deep-crustal processes during the genesis of the Late Cretaceous–Paleocene Laramide Porphyry Province in Arizona, which formed during flat-slab subduction of the Farallon Plate beneath North America. We show that: (1) Isotopic signatures of Laramide granitic rocks are consistent with a Proterozoic crustal source; and (2) Proterozoic crustal rocks were pre-enriched in copper and underwent water-fluxed anatexis between 73–60 Ma, coincident with the zenith of granitic magmatism (78–50 Ma), porphyry genesis (73–56 Ma) and flat-slab subduction (70–40 Ma). To explain the formation of the Laramide Porphyry Province, we propose that volatiles derived from the leading-edge of the Farallon flat-slab promoted anatexis of both mafic and felsic pre-enriched lower-crust, without requiring significant magmatic input from the mantle-wedge.