Asthenosphere-derived magmatism in the Rio Grande rift, western USA: implications for continental break-up

Abstract Magmas that are generated at continental rift zones provide an insight into the processes operating during the early stages of continental break-up. Our detailed study of mafic volcanism along the axis of the Rio Grande rift shows that, throughout both phases (30–17 and < 13 Ma) of its evolution, magmas with compositions interpreted as melts from the asthenospheric mantle have reached the surface. This recognition of early phase (26 Ma) magmas with incompatible trace element concentrations and radiogenic isotope ratios resembling those normally associated with ocean-island basalts and small seamounts (OIB) is significant because: (1) magmas dominated by the composition of asthenosphere-derived melts are not usually thought to be characteristic of early-phase continental rifting; (2) Tertiary mafic magmatism of an age greater than late Miocene in Colorado and New Mexico was hitherto regarded as subduction-related. Previous studies have shown that the final erupted composition of asthenosphere-derived melts is determined by the potential temperature of the convecting mantle, the amount and rate of lithosphere extension, fractional crystallization and crustal contamination. However, in the Rio Grande rift and elsewhere, such as the Basin and Range province, Eifel, NW Sardinia and the Cameroon Line, the final composition of these melts is also significantly influenced by earlier magmatic episodes. During the initial stages of asthenosphere melt generation the magma batches that first penetrate may heat a previously undisturbed segment of lithosphere and mix with strongly potassic, low temperature melt fractions. When these segments have been subsequently temporarily purged of such fusible potassic fractions the asthenosphere-derived melts can rise unimpeded through the sub-continental lithosphere.