MAGMA MIXING IN THE HINSDALE MAGMATIC SYSTEM, NORTHERN RIO GRANDE RIFT IN SOUTHERN COLORADO AND NORTHERN NEW MEXICO, USA

Late Oligocene to Miocene volcanic rocks mapped as the Hinsdale Formation in southern Colorado and northern New Mexico consist of mafic, intermediate, and rare felsic lavas. The Hinsdale was largely deposited unconformably on Oligocene volcanic deposits of the San Juan volcanic locus (SJVL), but also extend southeastward to the San Luis Hills. Traditionally characterized as bimodal volcanism associated with early Rio Grande rift-related extension, new geochemical and petrologic data suggest that much of the deposits are the product of magma mixing and/or mingling. Although lavas on the margins of the Hinsdale Formation are dominated by subalkaline tholeiitic basalt and basaltic andesite, rocks of trachybasalt to trachydacite composition are the most common composition overlying ignimbrites of the San Juan volcanic locus. These intermediate compositions are characterized by disequilibrium assemblages and include cumulate mush-like enclaves, entrained crystals, and glomerocrysts. Within a single thin-section, plagioclase crystals with different zoning patterns suggest extensive mixing and crystal entrainment from the transcrustal mush. Parental magmas were likely similar in composition to that of the mafic lavas found on the margins of the SVJL, with additional fractional crystallization, assimilation and magma mixing processes resulting in the intermediate to felsic magmas. This reflects the thickness of the crust through which these lavas transited.

New ⁴⁰Ar/³⁹Ar dates, ranging from ca. 28 to 15 Ma, expand our understanding of the temporal evolution of these post-SVJL lavas. Eruption ages generally young to the northwest, from the San Luis Hills centered about 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. As traditionally mapped, the “Hinsdale Formation” spans over 15 million years. Our new data suggest these lavas should be considered temporally and spatially distinct magmatic systems that may reflect overlapping magmatic processes occurring during the arc-to-rift tectonic transition.