LATE MIOCENE BASALTS OF THE ROBLEDO MOUNTAINS, NEW MEXICO, IN CONTEXT OF THE HISTORY OF MAFIC MAGMATISM IN THE SOUTHERN RIO GRANDE RIFT

Rift magmatism is a hallmark of continental extension, and the initiation of rift magmatism is used to determine the onset of extension. The Rio Grande rift (RGR) of central New Mexico is thought to have started ~ 36 Ma based on: (1) ages of the oldest ignimbrites and calderas and thickness variations of ignimbrites in half-grabens in the Goodsight-Cedar Hills (GCH) of southern NM; (2) timing of inversion of Laramide faults in central NM; and (3) transition from subduction-related intermediate volcanism to silicic ignimbrites. Rapid cooling of fault blocks in the southern RGR started at around 25 Ma in the San Andres Mtns., but these data do not constrain the onset of extension.

Bimodal magmatism is a key indicator of crustal extension. In the southern RGR, silicic ignimbrites are rarely associated with mafic-intermediate volcanism with two known exceptions. We dated a hb-bearing dacite at the base of the Schoolhouse Mtn. Caldera (SMC) in the Burro Mtns. of SW NM that yielded a U-Pb zircon date of 35 ± 1 Ma. Is this the end of the subduction episode or the beginning of a bimodal suite? Younger andesites in the SMC are 35–34 Ma and voluminous rhyolite ignimbrites preceded and followed the intermediate rocks. The Bell Top Formation in the GCH has an undated basalt that is overlain by an ash-flow tuff dated at 35.9 Ma, but it is unclear whether it is a sill or a lava flow. In the southern RGR, the earliest basaltic magmatism is the Uvas basaltic andesite at 28 Ma. Could extension have finally proceeded sufficiently to allow mantle-derived magmatism at this time? Intermediate volcanism in southern RGR occurred at 19–18 Ma (Seager et al., 1984; McLemore et al., 2012). A magmatic gap occurred from 18–10 Ma. Evidence for rift magmatism at ~10 Ma comes from basalts and 10 Ma rims on zircons from granulite xenoliths in 2.7 Ma basalts at Elephant Butte.

We present six ⁴⁰Ar/³⁹Ar dates on basalt plugs, dikes, and sills from in the Robledo Mtns. in southern NM that range from 7.6–5.5 Ma. These intrusive rocks contain mantle and crustal xenoliths and plagioclase megacrysts indicating crustal mafic intrusions of unknown age. Megacrysts have initial ⁸⁷Sr/⁸⁶Sr of 0.7030 and are likely sourced from an asthenospheric mafic magma. Host basalts have initial ⁸⁷Sr/⁸⁶Sr of 0.7034 also indicating an asthenospheric source with minor crustal contamination.