GEOCHEMICAL SIGNATURE OF PROGRESSIVE EXTENSION WITHIN AN ACCOMMODATION ZONE, EASTERN BLACK MOUNTAINS, AZ

The Grasshopper Junction (GJ) anticline is part of a regionally extensive accommodation zone located in the eastern Black Mountains in northwestern Arizona. It includes Middle Tertiary volcanic rocks that were erupted prior to, and during, extension and crustal thinning. Mafic to rhyolitic volcanic rocks record volcanism accompanying both pre- and syn-extensional periods between ~18.5 and 13.8 Ma. Lower, primarily andesitic lavas are overlain by a thick sequence of rocks ranging in composition from basalt through andesite to rhyolite, but dominated by rhyolite. The uppermost unit comprises basaltic (and minor andesitic) lavas, and is separated from the underlying rhyolite sequence by a transitional lens of andesitic/rhyolitic tuffs and basaltic lavas.

The GJ volcanic rocks are predominately calc-alkaline and range from 48 to 77 wt % SiO₂. Abundances of TiO₂ and P₂O₅ can be used to distinguish between two groups of rocks with <55 wt % silica, which we refer to as types I and II. Type I rocks have higher TiO₂, P₂O₅, alkalis and incompatible elements abundances, and lower abundances of CaO, MgO, Al₂O₃, Cr and Sc compared to type II rocks. There is no clear relation between these two mafic types and stratigraphic sequence, with both occurring throughout the entire eruptive series. Among all GJ rocks, there is a general increase in Sr and O isotope ratios and Rb and Ta contents and decrease in e_Nd, Zr, and Y with increasing silica.

Mafic rocks of both types I and II have geochemical signatures common to subduction-related magmas (depletions in Nb and Ta, enrichments in LILEs). The rhyolites have geochemical characteristics consistent with an origin via crustal anatexis, involving intermediate source rocks. The data support an origin for the intermediate magmas via mixing/mingling of mafic magmas with the rhyolites. This petrogenetic model implies that the endmember rhyolitic and mafic magmas coexisted at some level in the crust at the same time. Furthermore, the role of mixing/mingling appears to diminish with time; mixed/mingled magmas dominate the early eruptive products, whereas endmember rhyolite and mafic compositions dominate the final eruptive products. This trend tracks the structural evolution of the accommodation zone wherein the transition to bimodal volcanism is concomitant with the beginning of extension.