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THE ORIGINS AND IMPLICATIONS OF FELDSPAR MEGACRYSTS IN SILICIC LAVAS AND DOMES OF THE INYO CHAIN, LONG VALLEY CALDERA, EASTERN CALIFORNIA
This study examines the similarities and differences between the crystal cargoes of the 700, 40 ka, and 150 ka lavas in the Inyo Chain, California to elucidate the spatial and temporal continuity of small volume silicic magma plumbing systems.
The three youngest (~700 years old) silicic lava domes of the Inyo Chain, Long Valley caldera, eastern California, are Obsidian dome, Glass Creek dome, and Deadman dome. All three domes simultaneously erupted crystal-poor (~70 to 74 w.t.% SiO2, ≤ 5 % crystals) and crystal-rich (~71 w.t.% SiO2, ≥ 20% crystals) magmas with variable Ba concentrations (≤ 250 ppm to ≥ 1000 ppm). The crystal-rich lava has sanidine and albite megacrysts (≥ 5mm and ≤ 2cm, respectively).
The 700-year-old crystal-rich lava is texturally similar to an ~40 ka dacitic lava (~100 meters from Glass Creek dome) and an ~150 ka rhyolite lava (~1 km south of Deadman dome) that all erupted along the same lineament. The ~150 ka Deer Mountain lava (~72 w.t.% SiO2) contains ~35 to 40 % crystals and displays variable Ba concentrations (<250 ppm to >1000 ppm). The ~40 ka ‘northwest dacites’ (~66 w.t.% SiO2) contain ~20 to 25 % crystals. A second population of smaller (≤ 2 mm) sanidine and albite phenocrysts are identified in all crystal-rich and crystal-poor lavas.
Preliminary feldspar-liquid geothermometry results show unrealistic eruption temperatures (> 1100 °C) when considering the sanidine megacrysts and the groundmass glass for all lavas investigated (KD > 0.5). Two-feldspar geothermometry using touching pairs of the smaller (≤ 2 mm) sanidine and albite populations reveals realistic eruption temperatures between 720 °C and 830 °C for the crystal-rich lavas of the Inyo Chain and the Deer Mountain rhyolite.
We suggest the large population of sanidines are antecrysts due to their high Ba content, sieve textures, and unrealistic calculated temperatures. The sodic rim and similar eruption temperatures when considering the large zoned albite populations suggest these crystal-rich lavas possibly all resided or sampled the same magmatic reservoir prior to eruption. Future work aims to compare the mineralogy and trace element chemistry of the Inyo Chain with that of the Bishop Tuff to gain further insight into the spatial continuity of these plumbing systems.