The nature and geochemistiy of Miocene magmatism coeval to major extension is studied in detail along the Colorado River between 34-36°N. A tectonomagmatic model is proposed and developed into a general model for the Basin and Range.
Mafic magmas are divided into three groups on the basis of geochemistry: group 1 (most recent) magmas have [Nb/La] >1, and Sr and Pb isotope ratios indistinguishable from OIB, these basalts are thought to derive by <10% melting of an asthenospheric source; group 2 & 3 magmas have [Nb/La] <1 and elevated Sr and Pb isotope ratios suggesting a subcontinental lithospheric mantle (SCLM) source. Group 3 magma geochemistry is rather variable, but consistent with an increasing degree of partial melting (1-14%) of a trace element enriched garnet Iherzolite source with time. Group 2 magmas have HREE, Y and Sc enriched relative to groups 1 & 3, although relatively depleted LILE and LREE. It is argued that the group 2 & 3 sources are related by depth and that melting in the SCLM progressed to shallower, less enriched, levels with time.
Intermediate to high silica magmas display a positive 87Sr/86Sr vs. Si02 correlation and evolution to moderately alkaline compositions. Fractionation and AFC processes are insufficient to replicate the observed geochemical variations, instead these magmas are modelled by binary mixing between mafic and silicic endmember melts. The distribution and volume of these ‘hybrid’ magmas suggests that a ductile melting zone developed in the middle crust, which dominated the Miocene crustal extension.
Geophysical evidence, combined with plate tectonic reconstructions and the changes in magma geochemistry with time, indicate that plate tectonics was the driving force behind extension and magmatism. The influence of a mantle plume is thought to be unnecessary, rather SCLM melting was initiated by heat input from the asthenosphere as it re-equilibrated to fill the ‘void’ left by the northward migration of the Farallon Plate beneath the W. USA
