GEOCHRONOLOGY OF OLIGOCENE MAFIC DIKES WITHIN THE SOUTHEASTERN COLORADO PLATEAU: IMPLICATIONS TO REGIONAL STRESS FIELDS OF THE EARLY RIO GRANDE RIFT

Oligocene mafic dikes exposed on the southeastern Colorado Plateau are part of a large radial array known as the Magdalena radial dike swarm (MRDS). The MRDS spans 260º of arc and is broadly focused on the westward younging Socorro-Magdalena caldera cluster (SMCC) of Oligocene age (32-24 Ma) within the central Rio Grande rift. Ignimbrite sheets from the SMCC repeatedly buried high-angle normal fault scarps in the early rift.

Six new 40Ar/39Ar ages of mafic dikes and field observations imply lateral intrusion into the Colorado Plateau margin from mafic roots of the SMCC during short-lived periods of high magma pressure, possibly coeval with pre-caldera tumescence. Closely spaced NNW- to NNE-trending dikes at Riley are attributed a local coaxial relationship between magmatic stress fields and westward tectonic extension across the rift.

Three coeval dikes that span 70 degrees of arc from Cox Peak eastward to Riley are focused on the Mt. Withington caldera, source of the South Canyon Tuff, precisely dated at 27.55 ± 0.07 Ma (n=3). From west to east, the arcuate array yielded clockwise younging 40Ar/39Ar plateau ages of 27.88± 0.18, 27.80± 0.19 and 27.70 ± 0.15 Ma. The two older dikes are high-K olivine basalts and the youngest dike is a pyroxene-plagioclase basaltic andesite. Considering analytical error (0.7%), the three radiating dikes are coeval, but the WNW-striking dike is at least 0.08 Ma older than the Mt. Withington caldera.

Two long, NNW-striking, aphyric, xenocrystic basaltic andesite dikes near Pie Town and Hickman locally show gently southeast plunging flow lineations along their margins. Sparse xenocrysts of strongly resorbed sanidine and quartz, plus the presence of small resinous to black siliceous spherules, suggest the basaltic andesite magmas were mixed with rhyolitic magma just prior to their intrusion. The Pie Town dike produced a slightly disturbed plateau age of 28.92±0.27 Ma and two widely separated samples of the Hickman dike yielded plateau ages of 28.90± 0.13 and 28.94± 0.20 Ma. All three dates are analytically equivalent to the age of the La Jencia Tuff, which was erupted from the Sawmill Canyon caldera at 29.04± 0.04 Ma (n=4). The Pie Town and Hickman dikes are orthogonal to the WSW-trending axis of the SMCC; they do not necessarily indicate the orientation of early rifting, as previously published.