New Mexico Geological Society Annual Spring Meeting — Abstracts

We have updated previous analyses of the timing, geochemistry and distribution of magmatism within the Rio Grande rift with datasets that have become available in the last decade, and compare the results with new tomographic images and tectonism. It is well understood that two major pulses of magmatism occurred in and around the Rio Grande rift: an early stage following Laramide subduction of the Farallon plate, and a later stage related to extension of the Basin and Range. These pulses can be distinguished using major element, trace element and isotopic compositions of igneous rocks.

Our new analysis supports earlier work showing that the well-documented shift from intermediate-rhyolitic compositions in the Oligocene to predominantly basalt, with minor intermediate and rhyolitic magmas, in the middle Miocene, records the transition from a lithospheric melt source to a predominantly asthenosphere-derived source. This is supported by a shift in K2O/NaO2 ratios that indicate a transition from relatively evolved magmas in the Oligocene to more juvenile magmas from Pliocene to Holocene.
The concentration of magmatism along the Jemez lineament in New Mexico during the middle Miocene to Pleistocene is apparent. The Jemez Lineament is underlain by low seismic wave velocity upper mantle and we suggest that it results from shear-driven upwelling and decompression melting in small-scale convection cells that formed along the Colorado Plateau keel as the keel became more pronounced in the early Miocene. The Jemez Lineament extents into the Great Plains (Ocate and Clayton volcanic fields), and also this can be explained with the shear-driven small scale convection model.

We find that, contrary to previous interpretations based on fewer data or on inclusion of basaltic andesite with basalt, a truly bimodal distribution of dominantly basaltic and rhyolite volcanic rocks is observed only in Pleistocene volcanic rocks of the Jemez field. Rhyolites were erupted as ignimbrites at 1.6 and 1.2 Ma and domes from ~1.6 Ma to ~40 ka from the Valles caldera.
The lack of spatial correlation between volcanic centers and the rift itself throughout much of the rift history is intriguing. Volcanism occurs within and westward of the rift, and is largely absent east of the rift. The Rio Grande rift beneath westernmost Texas-southcentral New Mexico and Mexico has been devoid of any magmatic activity since the Oligocene. This region is underlain by fast seismic wave velocities in the upper mantle, and may reflect colder or compositionally different remnants resulting from Farallon subduction that have inhibited partial melting.