EVIDENCE FOR PROPAGATION OF THE NORTHERN RIO GRANDE RIFT: 40AR/39AR AGES AND GEOCHEMISTRY OF ALKALINE LAVAS NEAR YAMPA, CO

A geochemical and isotopic study was conducted on a suite of volcanic necks near Yampa, in northwestern Colorado, to determine their ages, compositions, and potential relationship to Rio Grande rifting. The volcanic necks, remnants of small volume eruptions that protrude through Miocene sediments of the Browns Park Formation, are associated with normal faults bounding low-relief grabens and half-grabens. The lavas are mafic (SiO₂ = 41.5 to 47.5 wt.%; MgO = 5 to 10 wt. %) and alkalic (Na₂O + K₂O = 4 to 7 wt.%), corresponding to basanite, alkali basalt, and tephrite rock types. ⁴⁰Ar/³⁹Ar ages of the alkalic lavas range from ~4.5 to 6 Ma. Although crustal xenoliths are common, low ²⁰⁷Pb/²⁰⁴Pb of the Yampa lavas (15.45 to 15.5) are inconsistent with significant upper crustal assimilation, suggesting rapid ascent from their source. The Yampa lavas are enriched in light rare earth elements (La_N/Lu_N >23) with generally negative εNd₍₀₎ values (0.6 to -7.7). These values are consistent with partial melting and eruption from a much older, previously enriched sub-continental mantle source. The ~1.77 Ga arc magmatic rocks recognized in neighboring areas of Colorado and Wyoming represent a likely period for such lithospheric enrichment. We conclude that the Yampa lavas represent small degrees of partial melting of Precambrian sub-continental mantle during Neogene crustal thinning. Crustal thinning was probably caused by rising asthenospheric mantle along the eastern margin of the Colorado Plateau in response to post-12 Ma regional extension within the Piman subtaphrogen. Other occurrences of alkalic magmatism in northwestern Colorado, some as young as 4 ka, have similar geochemical signatures characteristic of rift environments. Collectively, these rocks are interpreted to represent northerly propagation of the Rio Grande Rift system. This propagating rift tip may continue around the northern margin of the Colorado Plateau, and may be responsible for the occurrence of 0.9 to 3 Ma alkalic lavas of the Leucite Hills in southwest Wyoming.