On the Origin of Late Cretaceous and Younger Continental Interior Magmatism, Western U.S.

HR: 1340h
AN: V13E-0602 [Abstracts]
TI: On the Origin of Late Cretaceous and Younger Continental Interior Magmatism, Western U.S.
AU: * Bailley, T L
EM: bailley@colorado.edu
AF: Department of Geological Sciences, University of Colorado, Boulder, CO 80309 United States
AU: Farmer, G L
V13E-0602 AF: Department of Geological Sciences, University of Colorado, Boulder, CO 80309 United States
AB: It has long been recognized that the continental interior of the western U.S. experienced discrete pulses of magmatic activity during Late Cretaceous and Early Tertiary (Laramide; 70-55 Ma) and the mid-Tertiary (35-25 Ma). However, the relative importance of plate boundary vs. sublithospheric processes in triggering magmatism, and the controls on the space-time-composition patterns in magmatism remain unclear. Laramide magmatism was small volume and largely restricted to three linear ``bands'' in Montana, Colorado, and southern Arizona. The Laramide magmatism in Colorado (the Colorado Mineral Belt) is aligned with the southern margin of a ``flat slab'' segment of subducted oceanic lithosphere known to have been present beneath Colorado. Mantle melting may have been induced by convective instabilities along the slab margin and account for the localization in this region. In contrast to the Laramide igneous activity, mid-Tertiary magmatism was widespread from Colorado to northern Mexico and corresponds to the continental interior portion of the ``ignimbrite flare-up'' that produced large volumes of silicic magmas throughout the western United States. Rollback of shallowly subducted oceanic lithosphere beneath North America which induces melting in slab-metasomatized upper mantle(?) has been proposed by many workers as the trigger mechanism for melting, but it is not obvious whether this hydrated mantle is upwelling asthenosphere or what had been, prior to Tertiary hydration, mantle lithosphere. We suggest that the spacing of major continental interior volcanic centers may bear on this issue. The large volumes of magmas generated in the western U.S. during the mid-Tertiary event (>10,000 km³ in the San Juans alone) require either ``dynamic'' melting in upwelling asthenosphere, or ``static'' melting of recently hydrated lithosphere over a large area. The regular, but wide, spacing (~300km apart) of major mid-Tertiary volcanic centers in the western U.S., the San Juans, Mogollon-Datil and Trans Pecos fields may be a function of geometry of the source volume. The wide spacing may be a result of each center tapping magmas generated over a wide area in ``static'', lithospheric mantle. Fluid dynamic calculations further help constrain the plausible width of the mantle source footprint responsible for generating the magmas.
DE: 1033 Intra-plate processes (3615, 8415)
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005