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 size) with chilled margins. Elemental data and Sr and Nd isotopic ratios suggest that the trachydacite evolved from the basalt type forming most of the pillows, by a combination of fractional crystallization and crustal assimilation. Nevertheless, post-evolution magma mixing, during and immediately before extrusion of the flow, has obscured the details of this process. A proportion of the pillow-like basic masses in the composite flow are shoshonites that did not take part in the magma mixing. They are relatively rich in incompatible minor and trace elements; e.g. Nb = 41–46 ppm in the shoshonites and 20–25 ppm in all other Walton Peak basic lavas. The Sr and Nd isotopic ratios of the Nb-rich and Nb-poor compositions overlap. Open-system processes in a long-lived pre-existing magma chamber are considered unlikely to be the main cause of the diverse basic magmas because the Walton Peak lavas directly overlie Precambrian basement. Likewise, crustal contamination models using either upper or lower crustal rock types of this region do not give satisfactory mechanisms to relate the Nb-rich and Nb-poor mafic liquids. A genetic model invoking variable degrees of partial melting of lithospheric mantle containing hydrous minerals does not explain why the Nb-rich and Nb-poor compositions are bimodal and not a range. A variety of sparse ultrapotassic magmas (minettes to lamproites) accompanied the basalt-dominated Neogene volcanism of NW Colorado and surrounding area. Their elemental and isotopic compositions support the view that the ultrapotassic liquids originated by fusion of hydrous and halogen-rich metasomatized zones within the subcontinental lithospheric mantle. Addition of about 15% of lamproitic melt to the low-Nb Walton Peak basic magmas reproduces the main geochemical and isotopic characteristics of the high-Nb magmas. Such a process seems to have been widespread throughout NW Colorado Neogene igneous activity. Although wholly lithospheric mantle sources for all the mafic magmas cannot be ruled out, there is evidence that both subduction-related calcalkaline and ocean-island basanitic melts of asthenospheric origin formed the dominant components of the NW Colorado volcanics. Addition of as little as 10% of lamproitic melt, during their uprise through the subcontinental lithospheric mantle, was sufficient to imprint on them ‘continental’ incompatible element and radiogenic isotope characteristics.