When batholiths exploded: The Mogollon-Datil volcanic field, southwestern New Mexico

stretched-out accordion. Rio Grande rift the north-trending lesser branch, system of big cracks and late volcanism the A BSTRACT — Beginning in 1950, studies of the Mogollon-Datil volcanic field contributed to documenting a mid-Tertiary ignimbrite flareup (~40-15 Ma on a continental scale, 36-24 Ma in New Mexico). During this event, parts of western Mexico and the southwestern USA were periodically flooded by incandescent magma foam erupted from heaving calderas up to tens of kilometers in diameter, atop exploding granitic batholiths. The discovery of similar events on other continents and geologic periods was strong evidence for the then-disputed magmatic origin of granite, a problem fundamental to understanding the origin and nature of continents. During a proposed extensional orogeny , volcanism of the North American ignimbrite province began in the continental back arc of an ocean-to-continent convergent western plate margin. Regional chemical variations reflect location, thickness, and state of stress of the extending cratonic lithosphere. The unique 1200-km maximum distance of the province from the plate boundary resulted in part from ductile extension, in response to underplating of the continent by hot young oceanic lithosphere. Extension accelerated after the collision of the North American and Pacific plates and the transition from plate convergence to a lengthening transform plate boundary. Rising under extensional stress, mantle diapirs induced crustal melting. Siliceous magmas evolved as melting rose from lower to upper crustal levels. Contemporaneous high-Fe intermediate-to-mafic magmas evolved in the opposite direction, toward less depleted “primitive” magmas from progressively deeper mantle levels. Subse-quent brittle extension of a cooling plate resulted in the present Basin-and-Range configuration and basaltic magmatism.