Early Miocene continental extension-related basaltic magmatism at Walton Peak, northwest Colorado: further evidence on continental basalt genesis

The Walton Peak lavas erupted directly onto c. 1.8 Ga basement and were interbedded with subaerial sediments. Four new K-Ar dates for the lavas average 22.8 ± 0.3 Ma, associating them unambiguously with the earliest large-scale extension-related magmatism in NW Colorado. The lavas are basalts, trachybasalts and shoshonites. There is also one 100 m thick composite flow of trachydacite containing pillow-like basic masses (up to tens of metres in …