2005 Salt Lake City Annual Meeting (October 16–19, 2005)

Paper No. 5
Presentation Time: 2:35 PM


WALKER, J. Douglas1, FARMER, G. Lang2, GLAZNER, Allen F.3, CARLSON, Richard W.4 and BOWERS, Todd D.1, (1)Department of Geology, Univ of Kansas, Lawrence, KS 66045, (2)Univ Colorado - Boulder, PO Box 399, Boulder, CO 80309-0399, (3)Dept. of Geological Sciences, University of North Carolina, CB# 3315, Chapel Hill, NC 27599, (4)Department of Terrestrial Magnetism, Carnegie Institution of Washington, 5241 Broad Branch Road, NW, Washington, DC 20015, jdwalker@ku.edu

Magmatism often is assumed to relate closely to tectonic configuration and activity.  Since the advent of plate tectonics,  the loci and characteristics of igneous rocks in western North America have been used to better understand the geologic history of the region.   Many  previous inferences  were based on incomplete geochemical and age datasets.  Construction of the NAVDAT database (navdat.geongrid.org)  allows  many of the previously posited models to be rigorously tested using a large compilation of rock and age data.  We present two examples of such tests.  

Age, geochemical, and isotopic data compiled for Colorado allow reassessment of space-time-composition patterns for Laramide (~70-55 Ma) magmatism.  When interrogated in toto, the age and chemical data available for Laramide igneous rocks of the Colorado Mineral Belt (COMB) are too sparse and imprecise to allow space-time patterns in these rocks to be clearly resolved, despite assertions to the contrary by earlier workers.  The compiled isotopic data, however, demonstrate that Laramide magmatism was initiated in the upper mantle and was not solely a product of crustal anatexis.  In addition, the COMB cuts known Precambrian crustal shear zones and so the spatial distribution of the Laramide magmatism was unlikely to have been controlled only by the orientation of these crustal flaws.  Reexamination suggests an alternative possibility that COMB delineates the southern margin of a "flat slab" segment of the subducting Farallon Plate and this margin localized convective instabilities in asthenospheric mantle ultimately responsible for the Laramide magmatism.  

Late Cenozoic magmatism (~25-0 Ma) should reflect processes related to the intersection of the North American with the Pacific plate and the formation of two migrating triple junctions.  NAVDAT data clearly show that the onset of magmatism swept northward through Arizona, California, and Nevada coincident with the arrival of the Mendocino triple junction. Curiously, magmatism in this region was sparse during subduction and only blossomed after the triple junction passed by a given latitude. In this case, the more complete database confirms interpretations made by previous workers on smaller datasets.