Compositionally-Diverse Miocene—Recent Rift-Related Magmatism in Northwest Colorado: Partial Melting, and Mixing of Mafic Magmas from 3 Different Asthenospheric and Lithospheric Mantle Sources

Propagation of the Rio Grande rift into northwest Colorado at 26–8 m.y. BP was accompanied by eruption of mafic magmas with varied elemental and isotopic compositions. The volcanic rocks were erupted during two Miocene episodes (24–20 and 13·4–7·5 m.y. BP), and in the Quaternary (2 m.y. BP–4000 BP). Three mafic magma end-member compositions are identified which were not related to each other by fractional crystallization or assimilation of crust. Most of the volcanic rocks represent hybrids of two or three of these end-members, although textural evidence for such mixing is normally absent. Group 1 end-members are basanites, derived by partial melting of an OIB-source asthenospheric mantle. Group 3 end-members are mainly alkali basalts, derived by partial melting of asthenosphere modified by subduction of oceanic lithosphere. Group 1 source mantle replaced Group 3 source mantle as the asthenosphere underlying the region 11·6–10 m.y. BP, but Group 3 magmas continued to be erupted into the Quaternary. This replacement of asthenosphere post-dated cessation of subduction of oceanic lithosphere at the west margin of the North America plate by about 10 m.y. Group 2 end-members are minettes, with 143 Nd/ 144 Nd and 87 Sr/ 86 Sr ratios comparable to those of the Cretaceous-Recent Montana-Wyoming potassic province. These minettes are thought to represent partial melts of sub-Colorado lithospheric mantle. Group 2 magmas were available as end-members for mixing during the entire period 24 m.y. BP to the present, but biotite-phyric minettes were erupted only during the period c. 11·6–7·5 m.y. BP, during the main rifting event in northwest Colorado.