LARGE MELT VOLUMES AS POSSIBLE INDICATOR FOR WATER PRESENT CRUSTAL MELTING IN THE NORTH AMERICAN CORDILLERA

Crustal melting during orogenesis is the primary process that differentiates the continents into a more mafic lower crust and a more felsic upper crust. This process is primarily thought to occur in continent-collisional orogens in the absence of a free water phase. However, the North American Cordillera, an ocean-continent subduction system, contains a ~3,000 km long belt of anatectic rocks in the orogenic interior that formed in the Sevier-Laramide Orogeny during the Cretaceous to Paleogene. The North American Cordilleran anatectic belt rivals the largest collisional anatectic provinces on Earth. The three main hypotheses for crustal melting in the North American Cordillera – radiogenic heating, added heat from the mantle, and decompression – all propose that melting occurred via water-absent dehydration melting. These hypotheses are difficult to reconcile with low crystallization temperatures recorded in anatectic melts and the large volume of melt produced. Here we show that it is not possible to produce observed melt volumes by water-absent dehydration melting using existing constraints on crustal source compositions and pressure-temperature conditions. We propose that added water was necessary to produce the large melt volumes present and that this water was derived from the Farallon slab, which was subducting beneath the Cordillera at a low-angle concurrent with crustal anatexis. Cordilleran orogenic systems may thus play a more important role in the differentiation of continental crust.