WHAT’S THE DIFFERENCE? A PETROGRAPHIC AND GEOCHEMICAL COMPARISON OF THE BLOODGOOD CANYON TUFF AND THE TUFF OF TRIANGLE C RANCH

The Bloodgood Canyon Tuff and the tuff of Triangle C Ranch are contemporaneous rhyolite ignimbrites in the western Mogollon-Datil Volcanic Field (MDVF) of southern New Mexico, erupting during the peak of volcanism in the region. The MDVF erupted 12,000 km³ of rhyolite ignimbrite between 36 and 24 Ma. The ignimbrite flare-up is attributed to the relaxation of compressional stresses related to the Laramide orogeny ending between 35 and 55 Ma which initiated the extension in the Rio Grande rift and Basin and Range. In the Mogollon-Datil volcanic field, there are more than 30 recognized ignimbrites and 15 calderas are dispersed over an area of 40,000 km². Here we present a comparison between the Bloodgood Canyon and the tuff of Triangle C Ranch using whole rock major and trace element geochemistry and thin section petrography to determine the relationship between the tuffs and to establish if the Triangle C Ranch tuff is a unique eruptive unit.

The composition of the volcanic rocks are bimodal basalt and rhyolite lava flows and ash flow tuffs. Both the tuff of Triangle C Ranch and the Bloodgood Canyon tuff have been dated within error at 28.05±0.04 Ma and 28.15 ±0.14 Ma respectively, but the Bloodgood Canyon Tuff and the tuff of Triangle C Ranch have different magnetic polarities. The Bloodgood Canyon Tuff is a single cooling unit crystal-rich, high silica ash-flow rhyolite with low FeO, Sr and CaO contents. The phenocryst phases include alkali feldspar, quartz, biotite and hornblende in variable percentages throughout the stratigraphic section. Pumice contents are variable, and range in size from ~1 cm to 0.5 m. The Bloodgood Canyon Tuff is the most extensive eruptive unit in the MDVF with an estimated extent of 15,000 km² and an estimated eruptive volume of ~1300 km³.

Petrographically similar, the tuff of Triangle C Ranch overlies the Bloodgood Canyon Tuff and is only distinguished where a thin volcaniclastic sandstone is present. The tuff of Triangle C Ranch is primarily a non-welded tuff, rich in silica and fine grained, with the exception of pumice and lithic fragments ranging in size from 0.1 to 0.5 cm. In some locally exposed outcrops the two units are separated by pumiceous sandstone ranging in thickness from 1 to 2 meters, but in other exposed outcrops, the sandstone is not present and the two units are nearly indistinguishable in the field.