Development of the Long Valley rhyolitic magma system: strontium and neodymium isotope evidence from glasses and individual phenocrysts

Abstract

Pre-caldera high-silica rhyolites of Glass Mountain and the voluminous, zoned rhyolitic Bishop Tuff record the evolution of a magma system from initiation at 2.1 Ma to 0.76 Ma. Pre-1.2 Ma Glass Mountain lavas formed rapidly in two differentiation events recorded by regionally controlled Rb-Sr isochrons at ∼2.1 and 1.9 Ma. Younger, post-1.2 Ma Glass Mountain lavas have Nd isotope ratios distinct from the older Glass Mountain lavas and also define two regionally controlled Rb-Sr isochrons, 1.09 ± 0.03 Ma and 1.15 ± 0.01 Ma, that have distinct initial ratios: 0.7057 ± 1 and 0.7060 ± 1, respectively. These lavas have eruption ages as young as 0.79 Ma and therefore provide evidence of magma residence times of up to 360 kyr, comparable to that recorded in the older Glass Mountain lavas. Neodymium isotope compositions of sanidine and plagioclase from the younger Glass Mountain lavas and late erupted Bishop Tuff are within error (ε Nd − 1). Sanidine and plagioclase from the younger Glass Mountain lavas yield glass-mineral Rb-Sr isotope ages close to those of the younger regional isochrons, the exception being feldspar rims which yield ages close to the time of lava eruption. This suggests that feldspar phenocrysts were stored in the magma chamber for up to 300 kyr with little mineral growth until close to the time of eruption when minerals rims were formed. In contrast, feldspars from the early Bishop Tuff form two populations with Sr-Nd isotope systematics implying derivation from magmas that formed the older and younger Glass Mountain lavas. Feldspar rims give ages close to Bishop Tuff eruption. Strontium elemental and isotope zonation suggest that the feldspar populations from the Bishop Tuff represent xenocrytic material that may have resided in the Long Valley magma chamber(s) for up to 1.3 Myr prior to eruption of the Bishop Tuff.