HR: 15:00h
AN: V23C-05    [Abstracts]
TI: Mantle Source Volumes and the Origin of Mid-Tertiary Ignimbrite Flare-up in the Southern Rocky Mountains, Western U.S.
AU: * Farmer, G L
EM: farmer@colorado.edu
AF: Dept. of Geological Sciences and CIRES, University of Colorado, Boulder, CO 80309, United States
AU: Bailley, T
EM: Treasure.Bailley@Colorado.EDU
AF: Dept. of Geological Sciences and CIRES, University of Colorado, Boulder, CO 80309, United States
AB: Voluminous intermediate to silicic composition volcanic rocks were generated throughout the southern Rocky Mountains, western U.S., during the mid-Tertiary -ignimbrite flare-up", principally at the San Juan (SJVF) and Mogollon-Datil (MDVF) volcanic fields. At both volcanic centers, existing radiogenic isotope data have been interpreted as evidence that 50% or more of the volcanic rock (by mass) was derived from mantle-derived, mafic parental magmas. However, unlike other portions of western North America affected by major mid-Tertiary magmatism, existing xenolith and seismic data suggest that thick mantle lithosphere (>100 km) was present beneath the southern Rockies in the mid-Tertiary and remains present there today. As a result, basaltic magmas parental to the mid-Tertiary volcanism in this region were unlikely to have been generated by decompression melting of upwelling, normal potential temperature, sublithospheric mantle. The main alternative possibility is that the basaltic magmas were generated by conductive melting of -lithospheric" mantle that had been hydrated and refrigerated by oceanic lithosphere that subducted at a low angle beneath the interior portions of the continent during the Late Cretaceous/Early Tertiary Laramide Orogeny. Melting of this hydrated -lithospheric" mantle was then triggered in the mid-Tertiary by exposure to upwelling sublithospheric mantle during slab roll- back. To test this possibility, we generated first-order estimates of the volumes of basaltic magma, and of the mantle source volume needed to produce those magmas, required to fuel the ignimbrite flare-up in the southern Rockies. Conservative estimates of the volume of mantle that must have partially melted to supply the MDVF and SJVF volcanism are ~2Mkm³ and ~7Mkm³, respectively. If derived by conductive heating of the base of pre-existing mantle lithosphere during the ~20 m.y. duration of volcanic activity at these centers, as a consequence of upwelling of -normal" potential temperature mantle (1290°C), then these estimated mantle source volumes require that the lower ~20 km of the mantle lithosphere beneath the entire southern Rocky Mountains have partially melted during the mid-Tertiary. Such widespread melting of lithospheric mantle implies that this mantle was uniformly fertile and primed for melting and so must have experienced widespread hydration and refrigeration during early Tertiary low angle subduction. However, the mafic magmas must also have been focused laterally within the lithosphere for distances of up to ~300 km into each major volcanic center. Restricting mantle melting to a smaller footprint removes the problem of long lateral magma transport distances to each major volcanic center, but requires that upwelling sublithospheric mantle have contributed a significant proportion of the parental basaltic magmatism, which, in the absence of significant lithospheric thinning, would require that the upwelling mantle have had high (-excess") potential temperatures.
DE: 1031 Subduction zone processes (3060, 3613, 8170, 8413)
DE: 1037 Magma genesis and partial melting (3619)
DE: 1065 Major and trace element geochemistry
DE: 8178 Tectonics and magmatism
SC: Volcanology, Geochemistry, and Petrology [V]
MN: 2007 Joint Assembly