HR: 14:30h
AN: V23C-03    [Abstracts]
TI: Reassessing Magmatic Space-Time-Composition Patterns in the Colorado Mineral Belt, USA
AU: * Bailley, T L
EM: bailley@colorado.edu
AF: University of Colorado, Department of Geological Sciences, 399 UCB, Boulder, CO 80309, United States
AU: Farmer, G L
EM: farmer@colorado.edu
AF: University of Colorado, Department of Geological Sciences, 399 UCB, Boulder, CO 80309, United States
AU: Jones, C
EM: craig.jones@colorado.edu
AF: University of Colorado, Department of Geological Sciences, 399 UCB, Boulder, CO 80309, United States
AB: The Colorado Mineral Belt (COMB) is a northeast trending zone of Late Cretaceous to Early Tertiary (~75-50Ma) magmatism that accompanied the development of basement cored uplifts of the Laramide orogeny in Colorado. The origin of COMB magmatism remains enigmatic despite decades of study, largely because the magmatism coincided temporally with subduction at the western margin of North America but occurred some 1000 km inboard of the active trench. Many workers have attributed COMB magmatism to progressive shallowing of the oceanic lithosphere of the Farallon plate subducting beneath this region and have suggested that COMB magmatism was generally related to lower crustal melting (Simmons and Hedge, 1978, Stein and Crock, 1990). Others have attributed the COMB magmatism to upwelling of passive hot spots resulting from lithospheric deformation (Mutschler et al., 1987, 1998). Radiogenic isotopic data available for basaltic COMB volcanic clasts found in the Late Cretaceous Windy Gap member of the Middle Park Formation in northern Colorado (Farmer and Larson, unpublished data) support a mantle rather than crustal origin for COMB parental magmas. Further insights into the origin of the COMB clearly require a better understanding of the factors controlling the space-time-composition patterns in the magmatism. However, data compiled in the North American Volcanic and Igneous Rock Database (NAVDAT) illustrate the difficulty in assessing age patterns within the COMB. Little high quality age information is currently available for the COMB igneous rocks, and taken as a whole, little obvious space-time patterns can be discerned (Karlstrom and Humphreys, 1998). But when only the highest quality age determinations are used (including unpublished Ar-Ar ages from graduate theses), there is support for a progressive younging in igneous activity from the central COMB (~65 Ma) to northern COMB (~55 Ma; Cunningham et al., 1994). Cessation of COMB magmatism coincides with the onset of alkaline magmatism in the Black Hills of eastern Wyoming and western South Dakota (58-45 Ma; Duke and Singer, 2002). Recent plate reconstructions (Saleeby, 2003) reveal that the COMB trend is co-parallel with Farallon-North American motion and lies inboard of known area of continent affected by low angle subduction. Based on these observations we suggest an alternative hypothesis in which both the COMB and Black Hills magmatism ultimately represent the product of decompression melting of sublithospheric mantle produced by convective instabilities in the mantle that were induced above and/or along the margins of shallowly subducting oceanic lithosphere. Testing of this model will require additional age and chemical data from COMB igneous rocks, along with numerical modeling of mantle flow induced by the shallowly subducting oceanic lithosphere.
DE: 8415 Intra-plate processes (1033, 3615)
DE: 9350 North America
SC: Volcanology, Geochemistry, and Petrology [V]
MN: 2007 Joint Assembly