Ultrapotassic Magmas along the Flanks of the Oligo-Miocene Rio Grande Rift, USA: Monitors of the Zone of Lithospheric Mantle Extension and Thinning Beneath a Continental Rift

Recent theoretical studies of rift tectonics have concludedthat their observed geophysical features, require that (1) extensionaffects a much wider zone of the underlying lithospheric mantlethan the crust; (2) early extension involves a comparativelywide zone that narrows with time. The Neogene evolution of thesegment of the Rio Grande rift between the Great Plains andColorado Plateau shows this theoretical pattern clearly. Thewidth of the crustal extension zone narrowed from {small tilde}170km in the Oligo-Miocene to {small tilde}50 km in the Pliocene.In contrast, both gravity and teleseismic studies indicate thatthe current width of the zone of thinned lithospheric mantle(ß = 2–3) beneath the rift is {small tilde}750km. To assess the contributions of lithosphere- and asthenosphere-derivedmelts to the magmatismassociated with the early phase of developmentof the Rio Grande rift, we have undertaken a 670-km geochemicaltraverse of Oligo-Miocene volcanism between latitudes 36 and38N. Our section is centered on the present-day axis of therift in the Espanola Basin. It extends from the Navajo volcanicfield, Arizona, to Two Buttes, SE Colorado, and intersects hypabyssalintrusions on the rift shoulders at Dulce, west of the rift,and Spanish Peaks to the east. We have sampled a diverse rangeof magma types that vary in composition from ultrapotassic toHy- and Ne-normative basalts. A geochemical profile along thistraverse shows a spatially symmetrical variation in elementand oxide ratios, such as Na₂O/K₂O and Ba/Nb, and also in Srand Nd isotope ratios. On the rift flanks and shoulders Oligo-Miocenevolcanism was dominated by K-rich mafic magmatism, whereas atthe rift axis tholeiitic and alkalic basalts with whole-rockcompositions similar to those of ocean-island basalts (OIB)were erupted. This symmetrical geochemical variation broadlyparallels the corresponding teleseismic lithosphere thicknessprofile and is a mirror image of the gravity profile. We interpret the OIB-type magmas at the rift axis as predominantlyasthenosphere-derived melts. These suggest that mantle upwelling,and melting by decompression, were occurring during the earlydevelopment of the Rio Grande rift The symmetrical variationof incompatible elements and isotope ratios in rocks about therift axis suggests that the sources of the K-rich mafic magmason the stable flanks and shoulders of the rift are not directlyrelated to the subduction of the Farallon plate: an asymmetricprocess. Instead, we propose that the K-rich mafic magmas onthe flanks and shoulders of the Rio Grande rift are derivedfrom the melting of a metasomatized layer in the lithosphericmantle during extension. *Present address: British Antarctic Survey, High Cross, Madingley Road, Cambridge CB3 0ET, UK