Filling the (Daly) gap: petrographic and petrological modeling evidence for intermediate composition magmas at Yellowstone.
Abstract
The Yellowstone-Snake River Plain continental hotspot constitutes a classic example of bimodal (mafic-rhyolitic) volcanism. At its modern termination, Yellowstone, two groups of post-caldera rhyolites, the Upper Basin (UBM) and Central Plateau (CPM) members span overlapping bulk rock and glass compositions that suggest that fractional crystallization (FC) is a major driver of magma evolution. However, the bimodal nature of this system has so far hampered many geochemical modeling efforts. This study presents new results from rhyolite-MELTS models that establish a genetic link between the UBM and CPM crystal cargoes, glomerocrysts of plagioclase (An30-55)-clinopyroxene-orthopyroxene-oxides, of bulk composition ~ 55% SiO2 found in a few UBM lavas, and mafic enclaves erupted in coeval rhyolites to the North of the caldera. The rhyolite-MELTS models reveal that the UBM or CPM primitive end-member glass compositions cannot explain the crystal cargoes of Or45-55 sanidine, quartz, minor clinopyroxene, Fe-Ti oxides ± An20-30 plagioclase, instead indicating origins from more primitive melts. The basaltic andesite glass found in mafic enclaves of the 301-ka extra caldera Gardner River rhyolite lava is predicted by rhyolite-MELTS to evolve towards a residual trachy-andesitic melt (60-62% SiO2) after 20-30% FC. This melt, if allowed to fractionate at ~ 7-8 kbar with 2 wt% H2O can successfully explain the glomerocrysts. In turn, the residual melt present in that system as it becomes a mush (~40-50% vol. FC), a 69-71% SiO2 rhyolite can explain the crystal cargo in the UBM and CPM lavas, if segregated from the mush and fractionating at ~ 4-6 kbar with 2 wt% H2O. Although seldom erupted, such intermediate composition magmas might be vital to generate the rhyolites. The model proposed here involves a column of vertically stacked reservoirs (crystal mushes?), all undergoing FC and producing melt that ascends in a shallower storage zone and differentiates further. Importantly this model does not preclude assimilation of older rhyolite in the upper (rhyolitic) magma evolution zones, for which some lavas exhibit ample isotopic evidence. Concurrent assimilation of older rhyolite may in fact explain why the bulk of these lavas by volume comprises high-SiO2 rhyolite glass.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFM.V51H0138G
- Keywords:
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- 1115 Radioisotope geochronology;
- GEOCHRONOLOGY;
- 3625 Petrography;
- microstructures;
- and textures;
- MINERALOGY AND PETROLOGY;
- 3642 Intrusive structures and rocks;
- MINERALOGY AND PETROLOGY;
- 8032 Rheology: general;
- STRUCTURAL GEOLOGY