REASSESSING SPACE-TIME-COMPOSITIONS PATTERNS IN LATE CRETACEOUS AND YOUNGER MAGMATISM, WESTERN NORTH AMERICA

Spatial variations in the locus of Late Cretaceous and younger magmatism in western North America have long been attributed to variations in the disposition of oceanic lithosphere subducted beneath the continent. However, these patterns have not been reassessed in light of the high quality age and composition data that have been obtained from North American igneous rocks over the past twenty years. These data are now largely accessible through the online North American Volcanic and Intrusive Igneous Rock database (NAVDAT), allowing igneous patterns in the western U.S. to be readily investigated in detail. For example, Late Cretaceous to Early Tertiary magmatism in southern California and Arizona is generally thought to have migrated eastward with time, in concert with a shallowing in the dip angle of Farallon oceanic lithosphere subducting beneath this region. However, geochronologic data now available demonstrate that magmatism occurred almost synchronously in southern California, southern Arizona, southern New Mexico and central Colorado, commencing at ~75 Ma. As a result, while magmatism of this age may still be related to slab flattening, it is unlikely to represent the product of a gradual eastward migration of otherwise “normal” continental arc magmatism. Volcanic rock trace element data acquired over the past 20 years also demonstrate that significant variations exist in the intensity of the “slab component” incorporated in mid-Tertiary igneous rocks of the ignimbrite flare-up, at least among Rocky Mountain volcanic fields. The alkalic Trans Pecos volcanic field show little slab influence (e.g. low Sr/Ta ratios for most mafic compositions volcanic rocks) while calc-alkaline centers (Challis, Absaroka, San Juan and Mogollon-Data VF) show evidence of their derivation from parental mafic magmas derived from slab-fluid modified mantle. These data suggest that it may be possible to map the geographic extent of lithospheric and/or sublithospheric mantle modified by fluids derived from the “flat” Farallon slab on the basis of mid-Tertiary volcanic rock compositions.