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

Abstract

Volcanic rocks near Yampa, Colorado (USA), represent one of several small late Miocene to Quaternary alkaline volcanic fi elds along the northeast margin of the Colorado Plateau. Basanite, trachybasalt, and basalt collected from six sites within the Yampa volcanic fi eld 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 fi eld 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 fi nely 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 defi nes 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 infi ltration 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.

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“…Notably, all three of these ages attest to a signifi cant episode of volcanism at ca. 7-5 Ma in the present-day Yampa River valley, consistent with recent age determinations on relict volcanic necks in the region (Cosca et al, 2014). Relationships between deposits at Lone Spring Butte and the underlying Browns Park Formation make determination of the timing and amount of fl uvial erosion diffi cult in this locality.…”

“…We note that these values are similar to the magnitude and wavelength observed along the eastern slope of the Rockies (e.g., Leonard, 2002;McMillan et al, 2002;Nereson et al, 2013), suggesting that both fl anks of the range may be responding to changes in mantle buoyancy beneath central Colorado. We also note that extensional deformation (e.g., Buffl er, 2003) and the presence of late Cenozoic alkalic volcanism in the Yampa region (Cosca et al, 2014; this study) are both consistent with the addition of buoyancy associated with continued modifi cation of the mantle lithosphere beneath the range (e.g., Hansen et al, 2013). We suggest that long-wavelength tilting along the fl anks of the range during the past 6-10 Ma has a tectonic origin associated with differences in the buoyancy of the mantle between the northern Rocky Mountains and adjacent regions.…”

“…As noted above, the presence of ca. 7 Ma clasts within the Crowner deposits that were transported at the surface implies that some topographic relief was present during the eruption of 5-7 Ma volcanics in the Yampa River valley (e.g., Cosca et al, 2014). Unfortunately, we are unable to be place quantitative estimates on the amount of relief.…”

Perspectives on Continental Rifting Processes From Spatiotemporal Patterns of Faulting and Magmatism in the Rio Grande Rift, USA

“…Rift initiation has also historically been identified by the onset of voluminous volcanism or eruption of specific magmatic compositions, namely, mafic or bimodal volcanism (e.g., Bailey, ; Cosca et al, ; Johnson & Thompson, ; Kellogg, ; Ricketts et al, ; Tweto, ), although recent studies from the East African rift imply that volcanism cannot necessarily be used as a proxy for rift activity (e.g., Corti et al, ). Therefore, we assemble published data on Cenozoic volcanic rocks in Colorado and New Mexico to summarize and evaluate spatial, temporal, and compositional patterns in magmatism and compare the spatiotemporal patterns of rift‐related volcanism to the fault initiation and exhumation patterns determined from the thermochronometry data.…”

Lithospheric Signature of Late Cenozoic Extension in Electrical Resistivity Structure of the Rio Grande Rift, New Mexico, USA

“…This north‐to‐south increase in extension, which is apparent in the footprint of mapped synrift sedimentary units (Figure ), formed the basis for the hypothesis that the rift opened progressively from south to north. However, recent geochronologic studies on rift flank uplifts (Landman & Flowers, ; Ricketts et al, ) and petrologic studies of Miocene‐Quaternary volcanic rocks in northern Colorado (e.g., Cosca et al, ) demonstrate that this was not the case and that the rift opened simultaneously along its entire axis in the late Cenozoic. Modern geodetic observations reveal that the rift is actively extending at a rate of ~1 nanostrain/year (Berglund et al, ), with deformation over short timescales (~5 years) distributed regionally rather than focused on rift‐bounding faults.…”