Alkaline hybrid mafic magmas of the Yampa area,NW Colorado,and their relationship to the Yellowstone mantle plume and lithospheric mantle domains

The Yampa volcanic field (late Miocene) consists of about 70 outcrops of monogenetic cinder cones, lavas, dykes, volcanic necks and hydrovolcanic pyroclastic deposits and is situated in the most northerly part of the Rio Grande rift. Contemporaneous extension in this part of the rift was small, but there is geological and geophysical evidence that, by the late Miocene, the area was underlain by hot asthenosphere convected by the Yellowstone mantle plume. The Yampa rocks are mafic and chemically diverse, including basanites, alkali basalts, potassic trachybasalts, hawaiites and shoshonites. About half the rocks bear the xenocryst suite feldspar, pyroxene, Fe–Ti oxide, amphibole, biotite. There is a tendency for xenocryst-free rocks to be the most mafic, interpreted to indicate that the xenocrysts are cognate, and represent cumulate material from fractional crystallization of the magmas in deep crustal magma chambers. The elemental and isotopic (Nd and Sr) variations can be modelled by mixing variable proportions of partial melts of local lithospheric mantle with an OIB end-member formed by partial melting of asthenosphere. The OIB end-member appears to have the elemental and isotopic composition of typical Northern Hemisphere OIB, in particular the plume-derived basanites of Loihi seamount, Hawaii. The OIB end-member at Yampa is interpreted to have been derived from mantle convected in the Yellowstone mantle plume.