Volcanism and the Tectonic Development of the Rio Grande Rift and Environs, New Mexico - Colorado, from Analysis of Petrochemical Data

Abstract

A computerized data bank of over 1200 published major element analyses has been created, listing volcanic rocks from the Rio Grande rift and its flanks in New Mexico, Colorado and Texas. In this preliminary study, the data for each of the major volcanic fields, summarized on histograms and binary and triangular plots, are compared to establish a relation between petrochemistry and tectonic position with respect to the rift, and to determine temporal changes in volcanicity. No consistent systematic chemical variations across the rift were found, except locally. The only observed chemical change along the rift axis is the southward increase in TiO2-index of rift basalts. Three volcanic districts (Mt. Taylor-Mesa Chivato, Sierra Blanca and Trans-Pecos Texas) differ significantly from the others, in higher total alkali (especially Na2O), total iron oxide, and TiO2 and lower SiO2. These differences do not appear to be related to the Rio Grande rift. The strongly alkalic composition of the Mt. Taylor-Mesa Chivato field (Pliocene-Pleistocene) may arise from its location on the Jemez lineament, which may penetrate to greater depths in the mantle. The considerably older (Oligocene) Sierra Blanca and Trans-Pecos Texas districts, on the other hand, form the eastern­ most of a series of parallel magmatic belts in northern Mexico and Texas, which may have been associated with a broad, eastward-dipping subduction zone during the middle Tertiary. The absence of a distinct petrochemical "signature" for the Rio Grande rift lavas implies that the physico­ chemical environment of melting has remained the same throughout the region. Although the Rio Grande rift is a well-defined geophysical entity, it belongs to a broader zone of crustal extension in the Basin and Range province. The rift has been localized in part by structural "inheritance", and in part by its position as the eastern boundary between the Colorado Plateau and Southern Rockies. Transverse structures, such as the Jemez lineament, have also played an important role in localizing contemporaneous volcanism of highly variable and differentiated compositions, often strongly alkalic.