SOUTHERN ROCKY MOUNTAIN COOKBOOK FOR THE MAKING OF LARGE IGNIMBRITE ERUPTIONS, 37-23 MA

Mid-Tertiary growth of the Southern Rocky Mountain Volcanic Field (SRMVF) has been intensively documented during the past 50 years as detailed mapping and new petrologic, geochronologic, and geophysical methods and data have permitted increased resolution of stratigraphic sequence and eruptive processes. Notable have been the identification and precise dating of 30 large-volume (100-5,000 km³) ignimbrites and associated calderas, thus permitting recognition of commonalities and uniqueness in ignimbrite-forming processes. The SRMVF also provides broad perspectives on tectonic setting, magma-generation environment, crustal contributions, petrologic-fractionation controls, and magma-accumulation processes that give rise to continental ignimbrite eruptions. Deemed particularly significant are: (1) location on Proterozoic lithosphere along the eastern Cordilleran margin of the American plate, straddling Laramide Rocky Mountain uplifts and the more stable Colorado Plateau; (2) eruption of ignimbrites intermittently during a 14-m.y span, in association with even larger volumes of intermediate-composition lavas erupted from central volcanoes, that record incremental growth of the subvolcanic batholith; (3) geochronologic evidence for rapid regeneration of diverse magma compositions, contrasting with m.y.-periods of magmatic quiescence; (4) petrologic and isotopic evidence for open-system processes driven by mafic magma from mantle depths, major input from the lower continental crust, and shallow crystal-liquid fractionation in the upper crust to generate the ignimbrites; (5) gravity and seismic evidence for shallow presence of a low-density batholith of substantial vertical extent beneath central parts of the SRMVF, within continental crust that is not exceptionally thick despite high regional topography and low Bouguer gravity; (6) close similarities to younger continental-margin ignimbrite regions such as the Altiplano of the central Andes. Incompletely resolved is the geometry and physical state of magma chambers (including nonerupted residua and erupted portions) at times of SRMVF ignimbrite eruptions in relation to the present-day geophysically imaged batholith. Based on these constraints, a general recipe for Cordilleran ignimbrite generation will be presented.