Reference 1558

Abstract

Late Miocene lava flows in southern Jackson Hole are distinctlyintermediate, calcalkaline in composition in contrast to the bimodalbasalt-rhyolite assemblages characteristic of most Neogene volcanicrocks in the Yellowstone Plateau-Snake River Plain region. Despitelimited stratigraphic continuity between units, geochemical andpetrographic analysis reveals strongly unifying and distinctivefeatures. Silica variation diagrams are remarkably coherent,suggesting a co-genetic, if not co-magmatic, origin. As a whole, theserocks are significantly lower in K and Fe and higher in Ca andespecially Mg than rocks of similar silica content from the SnakeRiver Plain and Yellowstone. The basaltic andesites in particular arecharacterized by abundant olivine phenocrysts Fo 80-90 and extremelyhigh Ni 200 ppm and Cr 550 ppm values. Within the suite, basalticandesites and andesites 54-58% SiO2, with olivine and augite differfrom dacites 60-62% SiO2, aphyric or hornblende-rich which differfrom rhyodacites 66-69% SiO2, with hypersthene and rounded andesineand rhyolites and obsidian 73-75% SiO2. Both geochemical andpetrographic features suggest involvement of varying degrees ofcrustal contamination and/or magma mixing, especially in the moresilicic rocks. Low super 143 Nd/ super 144 Nd ratios in this suiteclearly distinguish it from the Yellowstone-Snake River Plainvolcanics and suggest the absence of asthenospheric input, precludingmelt generation due to convective heat transfer. A possible meltingmechanism is suggested by the high Ba/Nb values of these rocks, afeature considered diagnostic of involvement of subducted slab-derivedhydrous flux in melt generation. It is proposed that a fragment ofsubducted slab detached as the Cascade subduction zone was disruptedby the inception of the Yellowstone mantle plume was buoyed andtransported eastward, where its descent and dehydration beneathwestern Wyoming produced calc-alkaline magmas by solidus depression inthe lithospheric mantle and lower crust.

21 samples from this reference