The role of crustal anatexis in porphyry copper ore formation during flat-slab subduction: Insights from the Laramide Belt, SW USA

Abstract

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 unclear the source region of the ore-forming magmas. To resolve this paradox, we investigate deep crustal processes during the genesis of the Laramide Porphyry Province of Arizona, which formed between 80–50 Ma during flat-slab subduction of the Farallon Plate beneath North America. We show that: (1) Laramide granitic rocks have isotopic signatures implying a crustal origin suggesting that they were derived from Proterozoic aged crust; and (2) Proterozoic crustal rocks were pre-enriched in copper and underwent water fluxed anatexis between 73–60 Ma at a geothermal gradient of ~28°C/km, coincident with the zenith of granitic magmatism, porphyry genesis (73–56 Ma) and flat-slab subduction (75–65 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 mafic (garnet pyroxenite and amphibolite) and felsic pre-enriched lower crust, without necessarily requiring significant juvenile mantle-wedge derived magmatism.