ZIRCON AND APATITE GEOCHEMISTRY OF RHYOLITES FROM THE CENTRAL SNAKE RIVER PLAIN: GENETIC IMPLICATIONS

Whole-rock and zircon and apatite chemistry of three eruptive deposits from the Twin Falls caldera in Idaho, USA, associated with the Yellowstone mantle plume, provide a window into melt generation and evolution of hot, dry, A-type rhyolites. Three rhyolitic units were sampled via drill-core: the Shoshone Rhyolite (6.06 Ma, 120 m thick), the Kimberly Member (7.70 Ma, 169 m thick), and the Castleford Crossing Member (7.96 Ma, >1400 m thick).

Average Ti-in-zircon crystallization temperatures are highest in the Castleford Crossing member (844 °C), followed by the Shoshone Rhyolite (791 °C), and Kimberly member (786 °C). Oxygen fugacities derived from the zircons confirm that they crystallized at low fO₂ near the QFM buffer.

REE patterns for zircon in all 3 units are similar, but REE concentrations in Shoshone and Kimberly zircons are higher (~1.5x) than in the Castleford Crossing zircon. But U and Th concentrations are lowest in Shoshone zircons; Hf concentrations overlap. Zircons in all three units have Cl-dark cores enriched in REEs, U, and Th; similar cores are found in other YSRP rhyolites and are interpreted to indicate partial dissolution followed by rim overgrowth from an incompatible-element poor melt; alternatively, the dark cores crystallized from highly fractionated melts that were incorporated in more primitive, less REE-rich melts—the interpretation we favor.

Zircons in the Kimberly hole scatter between Ocean Island and continental arc element ratios (high Th and U vs low Nb, and intermediate Yb/Nb), while younger zircons from the Yellowstone Plateau are more similar to mantle plume zircons.

Apatites have high F/Cl and F/OH ratios, probably because of low H₂O and high-F in the A-type parent magmas. REE³⁺ substitution for Ca²⁺ is significant and accommodated by replacement of P⁵⁺ by Si⁴⁺ (as much as 6% SiO₂). Apatite from the Kimberly rhyolite has higher REEs than either the Shoshone or Castleford Crossing rhyolites.

Based on these zircon and apatite compositions and zoning patterns (as well as whole rock compositions) the Kimberly rhyolite, which is only about 0.26 Ma younger, did not differentiate from the more mafic Castleford Crossing magma. Instead, complex zircon and apatite chemical trends indicate that all three rhyolitic units were generated independently.