SPATIAL AND TEMPORAL VARIATIONS OF EOCENE-OLIGOCENE CALDERA VOLCANISM IN SOUTHERN NEW MEXICO
There are some notable spatio-temporal trends regarding rhyolite geochemistry. Older tuffs are predominantly hotter and crystal-poor. Younger tuffs (<35.7 Ma) are dominantly cooler and more crystal-rich. Sanidine from younger tuffs have higher Or# (Or = K/(K+Ca+Na)), Pb, and Rb, suggesting they crystallized from more evolved melts. MELTS modeling shows that the high Or# sanidines fractionated after the melt cooled and crystallized significantly to near the eutectic minimum, with modeled eruption temperatures of ~750-735°C. Additionally, many of the younger tuffs show evidence for magma underplating and feldspar cumulate melting prior to eruption, as revealed by very high Ba rims/zones in sanidine.
Tuffs older than 36 Ma are restricted to the eastern MDVF whereas the youngest tuff was erupted in the westernmost BHVF, near the AZ-NM border. This is consistent with past research (e.g., Ricketts et al., 2016) that has suggested a westward migration of volcanism in NM. However, between 36.0 – 33.0 Ma, volcanism occurred over a large region between the AZ-NM border and Las Cruces, NM (~240 km). Volcanism younger than 35.0 Ma is more restricted to the west, but still covers an extensive area (100 km). These trends suggest that slab removal and mantle upwelling likely initiated beneath the eastern MDVF, producing the oldest tuffs, but then extended across much of southern NM. Further, the abundance of volcanism across central-western New Mexico could be related to the onset of Basin and Range extension in this region at ~35 Ma (Ricketts et al., 2021), facilitating rhyolite ascent and eruption.