40Ar/39Ar Geochronlogy of the Jones Camp Dike, central New Mexico: an eastward projection of the Magdalena Radial Dike Swarm from under the Oligocene Socorro-Magdalena Caldera Cluster
Richard M. Chamberlin, William C. McIntosh, and Lisa Peters
A new 40Ar/39Ar age determination of 28.88 ± 0.22 Ma for the ESE-trending, 16-km long Jones Camp dike confirms it is part of the large diameter Magdalena radial dike swarm (MRDS), which is broadly focused on the large volume (5,000 km3) Socorro-Magdalena caldera cluster (SMCC; 32.3-24.5 Ma) of the early Rio Grande rift. Precise 40Ar/39Ar ages and near horizontal flow lineations indicate that long basaltic andesite dikes of the MRDS were driven outwards and upwards from an inferred lower crustal basaltic sill complex that presumably fueled the SMCC. Long dikes at Pie Town, Jones Camp and Elephant Butte, were driven outwards—to the north, east, and south— presumably during periods of high magma pressure prior to eruption of the large volume (1250 km3) La Jencia Tuff from the Sawmill Canyon caldera. A rhyolite lava dome immediately under the La Jencia Tuff provides a precise age of pre-caldera tumescence at 28.91 ± 0.11 Ma, which is equivalent to the age of Jones Camp dike.
Locally exposed primary flow folds on the chilled margin of Jones Camp dike indicate the initial pulse of aphyric basaltic andesite magma was driven eastward at a shallow upward angle of about 25-30º, presumably along a deep penetrating Paleozoic shear zone of the Capitan lineament. From near Socorro, the east propagating subsurface trajectory of Jones Camp dike was about 50 km long and required about 5-10 km3 of basaltic magma to fill a 10 m-wide dike (typical width of La Jencia-age dikes). We hypothesize that the inferred long trajectory of Jones Camp dike intersected a somewhat older and mostly crystalline hornblende-diorite pluton at depth in the Proterozoic basement under Chupadera Mesa. Minor volumes of the older hornblende diorite contaminated the initial pulse of basaltic andesite magma, now apparent in the form of small hornblende-diorite xenoliths in the margins of the dike and adjacent mafic sills. Shortly thereafter (ca. 101-103 years) a second and more forceful pulse of hybrid andesite magma mush carried abundant small to large xenoliths of coarse-grained hornblende diorite upward and re-inflated the still hot mafic core of the initial dike injection. A near horizontal paleomagnetic direction in a north dipping sill on the north flank of Jones Camp dike (determined in a previous thesis study) becomes coincident with the reverse paleomagnetic direction of the coeval La Jencia Tuff when 40º of northward tilt are removed from the sill. The mafic sill must have cooled quickly after the initial basaltic andesite dike injection that presumably fed the sill. The second pulse of hybrid andesitic magma injection then apparently domed the adjacent strata and pushed the sill up and away from the inflating core of the dike. The unusual width of Jones Camp dike (180 m) is attributed to its emplacement within incompetent evaporite beds of the Yeso Formation, which probably arrested the initial rise of the laterally propagating dike and later allowed it to mushroom outwards at the present level of exposure, about 1 km below the Oligocene land surface.
The ENE-trending pyroxene-hornblende diorite dike at Bingham yields a less precise 40Ar/39Ar age of 28.4 ± 0.6 Ma. Field observations and significant differences in concentrations of immobile trace elements suggest the Bingham dike is probably not the offset lateral equivalent of the Jones Camp dike.
- Chamberlin, Richard M.; McIntosh, William C.; Peters, Lisa, 2009, 40Ar/39Ar Geochronlogy of the Jones Camp Dike, central New Mexico: an eastward projection of the Magdalena Radial Dike Swarm from under the Oligocene Socorro-Magdalena Caldera Cluster, in: Geology of the Chupadera Mesa, Lueth, Virgil W.; Lucas, Spencer G.; Chamberlin, Richard M., New Mexico Geological Society, Guidebook, 60th Field Conference, pp. 337-346.