MANTLE-DRIVEN RHYOLITE VOLCANISM IN THE PLEISTOCENE COSO VOLCANIC FIELD (CA): THE RADIOGENIC ISOTOPE RECORD

Pleistocene magmatism at the Coso Volcanic Field (CA) is associated with vigorous geothermal production and is strongly bimodal. Eruptions of high-SiO₂ (≥76 wt.%) rhyolite occurred within the area of highest heat flow from ca. 600 ka to ca. 80 ka, and basalt volcanism along the western, southern, and eastern edges of the rhyolite field overlaps temporally with rhyolite magmatism. Although basalt does not erupt within the rhyolite field, rhyolites commonly contain quenched basalt-andesite inclusions that range from 53 to 59 wt.% SiO₂.

The least radiogenic Sr, and most radiogenic Nd and Hf isotopic data for the Pleistocene basalts have Pacific OIB-like isotopic (Sr_i = 0.703, ε_Nd = +7.5,ε_Hf = +11) and trace element signatures, but range to more radiogenic Sr and less radiogenic Nd and Hf, indicating contamination by Mesozoic basement locally. Sr, Nd, and Hf isotopic data for the Pleistocene rhyolites (Sr_i = 0.705-0.708;ε_Nd = +1.6 to +3.6, ε_Hf = +4.5 to +7.2) indicate that they are derived from hybrid crustal and mantle sources with significant local mantle input (50-75%, depending on plausible isotopic mixing end members). Sr isotopes for the rhyolites are sensitive to the age correction and/or late stage assimilation of higher Sr basement rocks, as well as minor hydration (rhyolites have high Rb/Sr and very low Sr, 1-9 ppm). Basaltic-andesitic magmatic inclusions have Sr, Nd, and Hf isotopic compositions that overlap their rhyolite hosts.

The radiogenic isotope data require either: (1) a two-stage process for forming the rhyolites, where mixing of mantle basalts and crustal components form intermediate hybrid magmas that undergo extreme fractionation to form high-SiO₂ rhyolite, or (2) derivation of the rhyolites by low-degree partial melting of a mafic-intermediate hybrid source. Isotopic similarity of the inclusion-forming magmas to host rhyolites is permissive of the first scenario. However, earlier erupted Pliocene mafic-intermediate volcanic rocks at Coso have radiogenic isotopic compositions similar to the rhyolites. Intrusive equivalents of these Pliocene volcanic rocks could also have been partially melted to source the rhyolites, in which case the overlap in isotopic composition of magmatic inclusions and the rhyolites is coincidental.