EXTENSION-RELATED VOLCANISM ALONG THE MARGIN OF THE NORTHERN RIO GRANDE RIFT IN SOUTHERN COLORADO AND NORTHERN NEW MEXICO, USA

In southern Colorado and northern New Mexico, late Oligocene to Miocene volcanism associated with early Rio Grande rift extension has been characterized as bimodal and previously grouped as a single formational unit, the Hinsdale Formation. New field, geochemical, and geochronological data suggest a more complex and diverse magmatic evolution for these lavas than alluded to by their grouping into a single formation, particularly for those largely underlain by ignimbrites of the San Juan volcanic locus.

New ⁴⁰Ar/³⁹Ar geochronology and geochemistry coupled with previous data suggest Hinsdale lavas fall into two groups: (1) subalkaline tholeiitic basalt to trachyandesite lavas (ca. 28 to 25 Ma) located principally beyond the margins of the San Juan volcanic locus; (2) trachybasalt to trachydacite with volumetrically minor rhyolite (ca. 23 to 19 Ma) erupted unconformably on top of the San Juan volcanic locus deposits. Collectively, these lavas span a broad range of compositions from 48-77 wt.% silica, with a significant intermediate component comprising Group 2. Eruption ages generally get younger to the northwest, from the San Luis Hills associated with the early rift axis, to the southeastern and central San Juan volcanic locus, where lavas are underlain by nested caldera complexes. Extension-related volcanic activity waned in this area after 19 Ma.

Group 2 eruptive centers are underlain by heterogeneous, thicker crust that persisted following the Oligocene ignimbrite flare-up. Associated lavas indicate influence of the precursor transcrustal magmatic system on differentiation and many of the lavas that erupted proximal to the Oligocene calderas exhibit extensive evidence of magma mixing. They are porphyritic, include abundant entrained crystals in disequilibrium with groundmass, and in many cases contain mafic enclaves and xenocrysts. The more evolved nature of Group 2 lavas, as well as evidence of magma mixing, verify the role of crustal contamination in the evolution of these lavas. Group 1 lavas near the rift axis are beyond the margins of the San Juan volcanic locus and are consistently more mafic with less compositional diversity suggesting less modification during transport through the crust prior to eruption.