Transtensional Stresses Tap Magmas From the Middle Crust, and Also Low-degree Melts in Crystal-Liquid Mush Zones: Evidence from the Sierra Crest-Little Walker Volcanic Center, Central California
Abstract
Stanislaus Group lavas and ignimbrites of the central Sierra Nevada have high K2O at a given SiO2, compared to preceding Relief Peak or later Disaster Peak volcanics. Elevated K2O might result from either (a) deep-seated partial crystallization of mafic magmas, when clinopyroxene dominates the assemblage, or (b) by the transport to shallow depths of low degree or enriched partial melts from the mantle source. These hypotheses, which are not mutually exclusive, stem from field data and age dates that show that Walker Lane transtensional faulting started at ~11 Ma and was synchronous with Stanislaus Group eruptions. This structural setting might produce new fractures that can tap deeper magma storage zones. And presumably, with the decline of transtensional stresses, such deep sources might not have viable pathways to the surface. As a test, we measure clinopyroxene, olivine and plagioclase compositions from the Relief Peak, Stanislaus Group and Disaster Peak volcanics, from the Sierra Crest-Little Walker Volcanic Center (Busby et al. 2018) near Sonora Pass, California, and estimate crystallization depths and temperatures using mineral-melt equilibria. We find that pre-eruption crystallization depths fall largely between 1-atmosphere and 5 kbar, but only Stanislaus Group lavas yield crystallization pressures > 5 kbar, and one lava flow from this unit yields an average pressure of 5.4 kbar. However, K2O contents are strongly, and negatively correlated with temperature: K2O contents systematically increase as T decreases. We tentatively conclude that the elevated K2O contents of the Stanislaus Group reflect the eruption of lavas from uniquely middle-crust depths, compared to prior and later eruptive episodes, but also that much of the high-K2O character of Stanislaus Group lavas also reflects fractional crystallization within upper crust magma chambers or mush zones. This implies that transtensional stresses might facilitate the eruption of more deeply stored material, but perhaps also lower-fraction partial melts that might otherwise be trapped within upper crust crystal-liquid mush zones.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFM.V13D0185L
- Keywords:
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- 1295 Integrations of techniques;
- GEODESY AND GRAVITY;
- 7280 Volcano seismology;
- SEISMOLOGY;
- 8178 Tectonics and magmatism;
- TECTONOPHYSICS;
- 8485 Remote sensing of volcanoes;
- VOLCANOLOGY