LATE CENOZOIC VOLCANISM NEAR COVE FORT, CENTRAL UTAH: IMPLICATIONS FROM TRACE ELEMENT GEOCHEMISTRY

Late Cenozoic (<20 Ma) volcanism along the eastern margin of the Great Basin is generally bimodal with basalt and rhyolite. Intermediate rocks of this age are rare. However, a suite of lavas less than 2 Ma near Cove Fort, Utah, range from tholeiitic basalt to variably alkalic basaltic andesite and andesite. In order to understand the origin of andesite in this extensional region, we have examined the trace element composition of the mafic and intermediate rocks. Andesites have normalized trace element patterns with negative Nb and Ti anomalies and positive Pb anomalies, but they have much lower concentrations of Rb, Th,and U and higher concentrations of Nb and consequently lower Ba/Nb ratios than the orogenic andesites of the middle Cenozoic from the eastern Great Basin (Barr, 1993). The oldest and most primitive rocks in the Cove Fort area are olivine tholeiites with trace element characteristics consistent with an origin from “asthenospheric” mantle—they are ferroan, medium-K, and have low Ba/Nb ratios (~13). Their tholeiitic character and low concentrations of incompatible elements implies larger degrees of melting than for contemporaneous silica-undersaturated basalts in the central Basin and Range. All of the intermediate rocks are magnesian, but the alkalic intermediate composition rocks include trachybasaltic andesite and trachyandesite that lie on distinct compositional trends from basaltic andesite and andesite and erupt from separate fields within the area. If the tholeiitic basalts are parental to the intermediate magmas, then open system behavior is indicated by large variations in incompatible element ratios. Multiple differentiation trends are present, but the overall evolution of the field suggests that the intermediate composition rocks of the Cove Fort area are the result of contamination of differentiating olivine tholeiite magma by continental crust, rather than by closed system differentiation of mafic magma derived from “enriched lithospheric” mantle. Embayed quartz crystals with thick reaction rims of oxides and clinopyroxene are found in basaltic andesites in the Cove Fort Field also suggest that crustal contamination played a role in the formation of intermediate composition magmas as does the correlation of silica with higher Ba/Nb ratios.