Relationships between monazite, apatite and chevkinite-group minerals in the rhyolitic Joe Lott Tuff, Utah, USA

The high-silica rhyolitic Joe Lott Tuff was erupted at 19.2 ± 0.4 Ma from the Mount Belknap caldera in Utah, USA. Despite having a volume of ~150 km3 it is compositionally homogeneous and contains ≤3 modal% phenocrysts. The tuff also shows two very unusual aspects of the REE-bearing phases, monazite and the chevkinite-group minerals (CGM): (i) the presence of perrierite-(Ce) in a high-silica rhyolite, and (ii) the replacement of CGM in the stratigraphic sequence by monazite, with a subsequent return to a CGM. Evidence from CGM distribution and variations in monazite and apatite-group composition with stratigraphic height in the deposit are used to infer that prior to eruption the magma was compositionally zoned, the zonation being partially obliterated by convective overturn in the reservoir.