Physics
Scientific paper
Dec 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008agufm.v13e2157f&link_type=abstract
American Geophysical Union, Fall Meeting 2008, abstract #V13E-2157
Physics
1033 Intra-Plate Processes (3615, 8415), 1065 Major And Trace Element Geochemistry, 3619 Magma Genesis And Partial Melting (1037), 8148 Planetary Volcanism (5480, 8450), 8415 Intra-Plate Processes (1033, 3615)
Scientific paper
Space-time-composition patterns in Late Cretaceous and Cenozoic igneous activity in southwestern North America are critical to models of the evolution of the deep continental lithosphere in this region. In the 1970's, for example, consideration of igneous rocks found some 1,000km inland from the continental margin in the southern Rocky Mountains led directly to suggestions that significant variations in the dip angle of subducting oceanic lithosphere must have occurred beneath this region during the Cenozoic. But no effort has been made to refine these models for at least the past 20 years, despite the large amount of high quality age and compositional data from Cenozoic igneous rocks that has been generated over that time. These data are now readily accessible through the on-line igneous rock database, NAVDAT. For example, NAVDAT includes published age and compositional data from over 3,000 samples of mid-Tertiary volcanic rocks from the Rocky Mountains and vicinity. Interrogation of these data in toto clearly illustrates that significant spatial variations exist in the major and trace element compositions of volcanic rocks produced at the major Rocky Mountain mid-Tertiary (20-50 Ma) volcanic fields. Volcanic rocks throughout the region span the entire compositional spectrum from basalt to high silica rhyolite, but volcanic rocks of the Trans-Pecos volcanic field in west Texas are more sodic than their "calc-alkaline" counterparts further north in the Mogollon-Datil, San Juan, Absaroka and Challis volcanic fields and show little evidence for their derivation from slab metasomatized mantle source regions. In contrast, the -calc-alkaline" centers typically have significantly lower Nb and Ti contents, and higher LIL/HFSE ratios that are consistent with their derivation from mantle rocks affected by fluids derived from dehydrating oceanic lithosphere. These observations generally support the concept that fluids derived from shallow subducting oceanic lithosphere were generated as far inboard as the present-day Colorado Rocky Mountains. In fact, volcanic rock LIL and Pb contents (at any given wt% SiO2) are highest in the San Juan volcanic field in Colorado, suggesting that lateral variations may have existed in the magnitude or duration of slab metasomatism beneath the interior portion of North America during Tertiary low-angle subduction. Maximum exposure to such fluids apparently occurred in Colorado and diminished both to the north and south, with no metasomatism occurring as far to the east as the Trans Pecos volcanic field. These variations are likely related to the disposition of the subducting Farallon plate through time beneath the interior portions of southwestern North America.
Farmer Lang G.
Fornash K.
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