A NEW MODEL FOR LARGE-VOLUME LOW δ18O SILICIC MAGMATISM? INSIGHTS FROM ZIRCONS OF THE COUGAR POINT TUFF

The series of multiple supereruptions represented by the Cougar Point Tuff (CPT) of the Bruneau-Jarbidge eruptive center (BJEC) documents the first ~ 2 m.y. of a ~ 5 m.y. interval (~13–8 Ma) of magmatism in the largest low δ¹⁸O silicic volcanic province known on Earth. Results of in situ analysis by ion microprobe of oxygen isotope ratios in zircon from 10 units of the CPT (≥ 7,000 km³) indicate characteristic δ¹⁸O values < 2‰; available data from Heise and Yellowstone show similarly ¹⁸O depleted magmas represent ~45 % and <10 % respectively of total eruptive volumes at these younger volcanic centers of the Yellowstone hotspot track. Combined results of δ¹⁸O, U-Th-Pb isotopes and trace elements in zircons distinguish the BJEC from other low δ¹⁸O magmatic systems that implicate shallow crustal processes of hydrothermal alteration and cycles of caldera collapse. An initial onset of large volume normal δ¹⁸O magmatism is absent at the BJEC; all 10 members of the CPT are strongly depleted in ¹⁸O (ave. δ¹⁸O_zircon=1.0 ‰) with the third representing the singular occurrence of a super-eruption of magma ≥ 2‰ lower than other large-volume low δ¹⁸O magmas known worldwide (δ¹⁸O_WR ≤ 0.9 ‰ vs. 3.4 ‰). Systematic variations over the sequence of CPT eruptions in the spectrum of intra-unit δ¹⁸O_Zrc and intra-crystal zoning patterns (direction and magnitude of Δ¹⁸O_core-rim), zircon U-Pb ages, and magma temperatures (900-1000°C) and compositions are difficult to reconcile with the model for “cannibalistic” low δ¹⁸O magma genesis that has been convincingly argued for Heise and Yellowstone. An alternative model is developed using results of Leeman et al. (2008) for possible degrees of ¹⁸O depletion as a function of depth in an extending mid-upper crust that was hydrothermally altered by infiltrating meteoric waters prior to the onset of silicic magmatism. The model proposes that BJEC silicic magmas were generated in response to the propagation of a melting front over a ~ 4 m.y. interval through a crustal volume in which a vertically asymmetric O-isotopic gradient had previously developed. Within the context of the model, CPT zircon data are consistent with events of incremental melting and mixing in roof zones of silicic magma reservoirs during surfaceward advance of the system, and concomitant incremental growth of an underlying vast mafic sill complex.