Kenzie J. Turner

40Ar/39Ar geochronology, isotope geochemistry (Sr, Nd, Pb), and petrology of alkaline lavas near Yampa, Colorado: Migration of alkaline volcanism and evolution of the northern Rio Grande rift

Volcanic rocks near Yampa, Colorado (USA), represent one of several small late Miocene to Quaternary alkaline volcanic fields along the northeast margin of the Colorado Plateau. Basanite, trachybasalt, and basalt collected from six sites within the Yampa volcanic field were investigated to assess correlations with late Cenozoic extension and Rio Grande rifting. In this paper we report major and trace element rock and mineral compositions and Ar, Sr, Nd, and Pb isotope data for these volcanic rocks. High-precision 40 Ar/ 39 Ar geochronology indicates westward migration of volcanism within the Yampa volcanic field between 6 and 4.5 Ma, and the Sr, Nd, and Pb isotope values are consistent with a primary source in the Proterozoic subcontinental lithospheric mantle. Relict olivine phenocrysts have Mg- and Ni-rich cores, whereas unmelted clinopyroxene cores are Na and Si enriched with finely banded Ca-, Mg-, Al-, and Ti-enriched rims, thus tracing their crystallization history from a lithospheric mantle source region to one in contact with melt prior to eruption. A regional synthesis of Neogene and younger volcanism within the Rio Grande rift corridor, from northern New Mexico to southern Wyoming, supports a systematic overall southwest migration of alkaline volcanism. We interpret this Neogene to Quaternary migration of volcanism toward the northeast margin of the Colorado Plateau to record passage of melt through subvertical zones within the lithosphere weakened by late Cenozoic extension. If the locus of Quaternary alkaline magmatism defines the current location of the Rio Grande rift, it includes the Leucite Hills, Wyoming. We suggest that alkaline volcanism in the incipient northern Rio Grande rift, north of Leadville, Colorado, represents melting of the subcontinental lithospheric mantle in response to transient infiltration of asthenospheric mantle into deep, subvertical zones of dilational crustal weakness developed during late Cenozoic extension that have been migrating toward, and subparallel to, the northeast margin of the Colorado Plateau since the middle Miocene. Quaternary volcanism within this northern Rio Grande rift corridor is evidence that the rift is continuing to evolve.

Geologic map of the Murray Quadrangle, Newton County, Arkansas

Figure 5. Structural data from the Murray quadrangle. Number of data are “n”. A, Rose diagram of strike frequency (left ) and stereoplot of poles and orientation density contours (right) for faults in quadrangle. Contour levels are multiples of standard deviations. B, Subset of striated faults with known slip sense that are compatible with normal (left) and strike-slip (right) paleostress regimes. Arcs are lower hemisphere projections of fault planes and solid and open dots are slip lines with components of normal and reverse slip, respectively. Small arrows show movement sense of hanging wall block. Red large, medium, and small filled circles represent orientation of maximum, intermediate, and least principal paleostress axes, respectively, from analysis of Angelier (1990). Large black arrows show azimuth of least horizontal compression. C, Rose diagram of strike frequency of joints recorded within the map area. 1–2 2–4 4–6 >6