New Mexico Geological Society Annual Spring Meeting — Abstracts

Forty years ago, mid-Tertiary volcanic rocks were regarded as mere overburden on Laramide ore deposits. Beginning in 1950, the NMBM&MR sponsored systematic studies which built a stratigraphic framework, not yet complete. The Director, Eugene Callaghan, recognized the dominance of ignimbrites; a surprise, because granitic magmas were then unpopular. The 1960's brought federal money, workers from the USGS and many universities, oligocene K-Ar dates, resurgent cauldrons, connections between siliceous plutonism and volcanism, and plate tectonics. The 1970's and 1980's introduced petrogenetic models based on mineralogical, geochemical and isotopic studies, concepts of ductile extension, and ⁴⁰Ar/³⁹Ar dates. We now recognize dozens of ignimbrites cauldrons and andesite stratovolcanoes, results of partial melting of, respectively, lower crust and upper mantle, in an extending lithosphere. Many mid-Tertiary ore deposits are structurally controlled by cauldron ring fractures (e.g., Mogollon), as are some modern geothermal systems (e.g., Lightning Dock). Some ore deposits are also genetically controlled by cauldrons (e.g., Questa). Locations of volcanic centers are critical for evaluating hydrocarbon potential of certain sedimentary basins (e.g., Pedregosa basin).

Oligocene volcanism in New Mexico was a small part of an "ignimbrite flareup" that buried -1 million km2 of western Mexico and southwestern USA under hundred to thousands of meters of ash and lava. It occurred during an "extensional orogeny" that may have doubled the width of the Basin and Range province. It is a major geological phenomenon of a type now recognized in many continents and periods (e.g., Permian of Eurasia, Quaternary of Sumatra and New Zealand).