New Mexico Geology

The Knight Peak region of southwestern New Mexico between Silver City and Lordsburg contains dramatic exposures of ignimbrite outflow sheets and lava flows that are part of the southern Mogollon-Datil volcanic field. This package of rocks is over 1000 m thick and was deposited unconformably on a ca. 1.46 Ga granite. We used U-Pb zircon geochronology to date four of the previously undated volcanic units and a rhyolite dike that is part of a swarm that intruded the Proterozoic granite. The rhyolite dike had an age of 59 Ma and was likely related to ore-bearing Laramide intrusions such as the Tyrone pluton. The dike swarm orientations are consistent with northeast-southwest Laramide shortening. The volcanic rocks in the Knight Peak sequence had ages from 36.2 ± 0.4 to 32.6 ± 0.6 Ma. These include, from oldest to youngest, the JPB Mountain trachyte, the C-Bar Canyon tuff and tuff breccia (which forms the cliffs of the summit of Knight Peak), and the Malpais Hills basaltic trachyandesite lava flow. Most of the volcanic units had zircons reflecting inheritance from ca. 1.6 Ga Mazatzal province rocks, ca. 1.45 Ga A-type granites, and ca. 1.2 Ga Grenville igneous rocks, as well as a few Paleogene zircons derived from Laramide volcanic rocks. The abundance of enocrystic zircon implies significant contamination of magmas by crustal rocks. The entire volcanic section, plus the lower part of the overlying Miocene–Pliocene (?) Gila Conglomerate, is tilted 30–45° to the northeast on the southwest-dipping Knight Peak normal fault. This suggests that the Basin and Range topography in this area formed in the Miocene.