The evolution of the Peach Spring Tuff magmatic system as revealed by accessory mineral textures and compositions.

Abstract

The Peach Spring Tuff (PST), a large Miocene ignimbrite located in the southwestern USA, is unique in its abundance of U, Th, and REE concentrating accessory minerals (zircon, sphene, allanite, chevkinite). Textures and compositions of these accessory minerals and magnetite were studied by a variety of methods.

Samples of pumice clasts and fiamme were collected from locations in the outflow sheet and intracaldera deposits. Samples range in composition and crystal content, from relatively crystal-poor rhyolites to crystal-rich trachytes, with intracaldera fiamme on the more mafic end of this spectrum. REE trends in zircon and sphene grains reveal a simple fractionation history in rhyolites, but MREE enrichment in sphene edges in trachytes suggest final growth from a less evolved melt. Ti-in-zircon and Zr-in-sphene thermometry reveals lower temperature growth at edges of grains from rhyolites, while edges from trachytes record warmer temperatures. Trace element variations and temperatures also suggest that zircon has a more protracted history of growth than other accessory phases. Textures corroborate the geochemical results. Phenocrysts in rhyolites tend to be euhedral, while those from intracaldera trachytes display resorption features. Sphene size distributions of accessory minerals are kinked, suggestive of growth during decompression.

These results suggest that the PST was a zoned system affected by a late-stage heating event, which may have triggered eruption, followed by eruptive decompression. One outflow pumice clast of trachyte composition follows compositional trends of intracaldera trachytes and textural trends of outflow rhyolites, suggesting that these events affected different regions of the chamber to different extents.