ULTRAPOTASSIC MAGMAS ALONG THE FLANKS OF THE OLIGOMIOCENE 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 concluded that their observed geophysical features require that: (1) extension affects a much wider zone of the underlying lithospheric mantle than the crust; (2) early extension involves a comparatively wide zone that narrow with time. The Neogene evolution of the segment of the Rio Grande rift between the Great Plains and Colorado Plateau shows this theoretical pattern clearly. The width of the crustal extension zone narrowed from approximately 170 km in the Oligo-Miocene to approximately 50 km in the Pliocene. In contrast, both gravity and teleseismic studies indicate that the current width of the zone of thinned lithospheric mantle (beta = 2-3) beneath the rift is approximately 750 km. To assess the contributions of lithosphere- and asthenosphere-derived melts to the magmatism associated with the early phase of development of the Rio Grande rift, we have undertaken a 670-km geochemical traverse of Oligo-Miocene volcanism between latitudes 36 and 38-degrees-N. Our section is centered on the present-day axis of the rift in the Espanola Basin. It extends from the Navajo volcanic field, Arizona, to Two Buttes, SE Colorado, and intersects hypabyssal intrusions on the rift shoulders at Dulce, west of the rift, and Spanish Peaks to the east. We have sampled a diverse range of magma types that vary in composition from ultrapotassic to Hy- and Ne-normative basalts. A geochemical profile along this traverse shows a spatially symmetrical variation in element and oxide ratios, such as Na2O/K2O and Ba/Nb, and also in Sr and Nd isotope ratios. On the rift flanks and shoulders Oligo-Miocene volcanism was dominated by K-rich mafic magmatism, whereas at the rift axis tholeiitic and alkalic basalts with whole-rock compositions similar to those of ocean-island basalts (OIB) were erupted. This symmetrical geochemical variation broadly parallels the corresponding teleseismic lithosphere thickness profile and is a mirror image of the gravity profile. We interpret the OIB-type magmas at the rift axis as predominantly asthenosphere-derived melts. These suggest that mantle upwelling, and melting by decompression, were occurring during the early development of the Rio Grande rift. The symmetrical variation of incompatible elements and isotope ratios in rocks about the rift axis suggests that the sources of the K-rich mafic magmas on the stable flanks and shoulders of the rift are not directly related to the subduction of the Farallon plate: an asymmetric process. Instead, we propose that the K-rich mafic magmas on the flanks and shoulders of the Rio Grande rift are derived from the melting of a metasomatized layer in the lithospheric mantle during extension.