Strongly potassic mafic magmas from lithospheric mantle sources during continental extension and heating: evidence from Miocene minettes of northwest Colorado, U.S.A.

Minette occurs as sparse dykes and sills in the Upper-Miocene Elkhead Mountains igneous province, NW Colorado. At the time of magma emplacement, the region was undergoing crustal extension and probably also heating of the sub-continental lithospheric mantle by the approaching Yellowstone plume. The predominant magmatism of the province comprises lava field remnants and hypabyssal plutons. Elemental variation within the minette suite is explicable in terms of fractional crystallisation which involved phlogopite separation from even the most magnesian (MgO = 10.68%) samples. Wide ranges of incompatible-element abundances and ratios occur in the minettes with MgO > 6.0%. Some of these ratios (e.g. Ti/Zr andLa/Nb) correlate well with 143Nd/144Nd in this suite. The minettes have a combination of relatively low values of both 87Sr/86Sr (0.70387–0.70413) and 143Nd/144Nd (0.51201–0.51227), with 206Pb/204Pb (17.28–17.47), 207Pb/204Pb (15.45–15.54) and 208Pb/204Pb (36.55–37.02). Taken together, these isotopic characteristics fall far outside the range of all oceanic igneous rocks and therefore rule out an exclusively asthenospheric source for the magmas. Genetic models involving crustal contamination of either basaltic (s.l.) or lamproitic liquids do not appear to explain satisfactorily the geochemical features of the minettes. Alternative models invoke either separate subcontinental lithospheric mantle sources for each magma batch or mixing between upwelling basaltic liquids and varying amounts of ultrapotassic lithospheric melts. Both models fit the geochemical data reasonably well but the latter is, in addition, consistent with a recent analysis by D. McKenzie [8] of the physical constraints on strongly potassic magma genesis during continental lithospheric extension and/or heating above a mantle plume. A brief survey of the tectonic settings of minettes and ultrapotassic rocks worldwide shows that a strong case can be made for their association in space and time with heating and/or thinning of sub-continental lithospheric mantle.