New Mexico Geological Society Annual Spring Meeting — Abstracts

The Eocene Palm Park and Rubio Peak Formations record a major calc-alkaline magmatic episode throughout southern New Mexico. This volcanism deposited a blanket of volcaniclastic debris, mainly lahars, sandstones, and conglomerates that are intercalated with lava flows and tuffs. Three volcanic centers of this age are defined in south-central New Mexico by the presence of near-vent volcanic facies or hypabyssal intrusions: Rubio Peak lavas in the southern Cookes Range and in the Goodsight Mountains, and the Cleofas intrusion in the Dona Ana Mountains. However, the location of volcanic centers that produced the volcaniclastic rocks is equivocal, because of the virtual impossibility of determining paleocurrent directions in lahars. This study compares the major and trace element compositions of volcanic clasts in lahars from three sections in the Palm Park and Rubio Peak Formations (southern Cookes Range, Faulkner Canyon, and the East Selden Hills) with the in situ lavas and intrusions as a method of correlating volcanic aprons with the related volcanic centers.

All of the samples are predictably similar in composition, having been generated in the same tectonic environment. The largest differences in composition are related to the extent of weathering and alteration: totai volatile content ranges from 1% to 8%, with large variations in K₂O, Na₂O, Sr, and Ba. Although it is difficult to assess the geochemical impact of weathering, this study uses the high field strength elements, such as Y, Zr, Sc, and TiO₂, because they are more reliable petrogenetic indicators in weathered samples.

Based on the concentrations of these elements, the three volcaniclastic sections appear to have originated from three separate vents. The Cookes Range clasts exhibit only very weak elemental correlations, even for samples with low total volatile content, suggesting that the magmas evolved through open system processes. In contrast, the East Selden Hills and Faulkner Canyon samples show correlations between Sc, Zr, and Y. The East Selden Hills samples have higher Sc and Y that the Faulkner Canyon section, suggesting that, they evolved from distinct parental magmas with different concentrations of these elements.

Lavas from the Cookes Range and Goodsight Mountains have higher Y and Zr than any of the suites of clasts, precluding them as volcanic centers for the lahars. However, the Cleofas intrusion is similar in composition to clasts from the East Selden Hills and Faulkner Canyon, and is probably the volcanic center for one or both of these lahar suites. Unfortunately, data from the Cleofas intrusion do not exhibit the detail necessary to discriminate between the East Selden Hills and Faulkner Canyon suites.