Geochemical Gradients in the Topopah Spring Member of the Paintbrush Tuff: Evidence for Eruption Across a Magmatic Interface

摘要

The Topopah Spring Member of the Paintbrush Tuff in southern Nevada is a classic example of a compositionally zoned ash-flow sheet that is inferred to have resulted from eruption of a compositionally zoned magma body. Geochemical and petrographic analyses of whole-rock tuff samples indicate that the base of the ash-flow sheet and the dominant volume of erupted material are composed of crystal-poor high-silica rhyolite, with a gradational transition into overlying crystal-rich quartz latite at the top of the sequence. These compositional variations are consistent with a model of progressive eruption of a stratified magma body in which relatively cooler, crystal-poor high-silica rhyolitic magma overlay hotter, crystal-rich quartz latitic magma. Major and trace element chemical analyses of whole glassy pumices and analyses of coexisting ilmenite and magnetite phenocrysts from within the pumices provide closer approximations to the chemical and thermal gradients within the inferred magma body. The magmatic gradients inferred from these data indicate that the transition from high-silica rhyolitic to quartz latitic magma within the chamber was abrupt rather than gradational, with a distinct liquid-liquid interface separating the two contrasting magmas. Throughout the ash-flow sheet, individual pumice lumps with distinct and variable textural characteristics are present within outcrop, hand-sample, and thin-section scale. Within the lower portion of the ash-flow sheet, the individual pumices are all high-silica rhyolites with relatively small variations in trace-element composition and estimated quench temperatures, and thus are chemically similar to their associated whole-rock tuff composites. In contrast, the chemical variability among pumices within the uppermost quartz latite is as great as that of the entire ash-flow sheet. (ERA citation 12:032050)