U-Pb Zircon Ages of the Knight Peak Outflow Sheet and Lava Sequence, Mogollon-Datil Volcanic Field, New Mexico, USA: Implications for Magmatism and Extension in the Southeastern Basin and Range Province.

The volcanic rocks of the Knight Peak region of the southern Mogollon-Datil volcanic field (MDVF) consist of a stratigraphic sequence of ignimbrite outflow sheets and lava flows. These rocks were deposited unconformably on a ca. 1.46 Ga granite. The total thickness is more than 1100 m. 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 58.6 ± 0.6 Ma (all new ages are weighted mean 238U/206Pb dates with errors reported at the 2σ level). This dike is part of a swarm likely related to ore-bearing intrusions, such as the Tyrone pluton, that were emplaced during the Laramide orogeny. The dike swarm orientations are consistent with northeast-southwest Laramide shortening. The oldest volcanic rock in the Knight Peak sequence that we dated is the 36.2 ± 0.4 Ma JPB Mountain trachyte. This lava flow is among the oldest eruptions in the western MDVF and has no currently known correlative units. The next unit consists of a series of thick rhyolite ignimbrites, collectively mapped as the C-Bar Canyon tuff and tuff breccia and originally mapped as a tuff breccia, with a total thickness of 400 m in the study area. This unit forms the cliffs of the summit of Knight Peak. The tuff is crystal-poor and did not yield sanidine. The U-Pb ages of samples from the tuff breccia and the rhyolite tuff were 35.2 ± 0.4 and 35.2 ± 0.6 Ma, respectively. Given the overall thickness and presence of coarse lithic fragments in the tuff, this unit was likely sourced from near Knight Peak, but the overall stratigraphic continuity makes it unlikely to be intracaldera fill. The next-youngest tuff was originally mapped as the Kneeling Nun Tuff (Hedlund, 1980), but this unit, here named the KN78 rhyolite, has an 40Ar/39Ar date of 34.0 ± 0.2 Ma (McIntosh et al., 1991), making it correlative with a series of coeval MDVF tuffs collectively known as the Box Canyon Tuff. The youngest unit is the 32.6 ± 0.4 Ma Malpais Hills basaltic trachyandesite lava flow. Most of the volcanic units contained 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 minor Paleogene zircons derived from Laramide igneous rocks. The abundance of xenocrystic zircon implies significant contamination of magmas by crustal rocks. The sequence at Knight Peak represents magmatism from a typical cycle in the Mogollon-Datil and Boot Heel volcanic fields, beginning with a trachyte followed by massive crystal-poor rhyolite followed by a crystal-rich eruption (KN78) and finishing with a basaltic trachyandesite flow. These new data allow for the Knight Peak sequence to be placed in context with magmatism in adjacent volcanic fields. The entire volcanic section, plus the base of the overlying Miocene to 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, consistent with previously published thermochronology, which demonstrates Oligocene–Miocene cooling of exhumed basement and confirms the postulated Miocene age for the lower Gila Conglomerate that fills half grabens in the hanging wall of Basin and Range normal faults. [ABSTRACT FROM AUTHOR]