SMALLER TRANSTENSIONAL BASINS ERUPT COOLER LAVAS FROM SHALLOWER DEPTHS: EVIDENCE FROM THE SIERRA CREST-LITTLE WALKER & EBBETTS PASS VOLCANIC CENTERS, CENTRAL SIERRA NEVADA, CALIFORNIA

Volcanic rocks from the Ebbetts Pass (ca. 6-4.6 Ma) and the Sierra Crest-Little Walker (ca. 12-5 Ma) volcanic centers provide a test on how structural setting may influence magma storage and transport. Both of these volcanic centers offer well-exposed lavas erupted in pull-apart basins, within, and adjacent to, the ancestral Cascades arc. Previous work on the Sierra Crest-Little Walker volcanic center, shows that highly potassic lavas there erupted during periods of higher degrees of Walker Lane transtensional faulting, and these high-K lavas were stored at greater depths prior to eruption. The smaller, and younger Ebbetts Pass volcanic center shows no evidence for high-K lavas, which may be due to the smaller size of this pull-apart basin, and lower transtensional stresses during eruption. Here, we present new mineral composition data for the Ebbetts Pass volcanic center to compare whether the smaller size and lower transtensional stresses had any effect on the depths and temperatures at which its magmas were stored, compared to the larger Sierra Crest-Little Walker volcanic center. We measure amphibole, clinopyroxene, orthopyroxene, olivine, and plagioclase compositions from both lava flows and intrusions at Ebbetts Pass. We find that pre-eruption crystallization pressures are between 0 and <2.5 kbar, which are on average lower than the 1 to <5 kbar (> 5 kbar for high-K lavas), for lava flows from the Sierra Crest-Little Walker volcanic center. For both volcanic centers, temperature does not significantly vary from one eruptive unit to another, but the temperature of crystallization for the majority of the minerals at Ebbetts Pass are cooler than that of the Sierra Crest-Little Walker volcanic center. We tentatively conclude that the lower pressures and temperatures associated with Ebbetts Pass volcanic center are caused by the smaller size of the pull-apart basin, and the associated lower transtensional strains. We posit that the lower transtensional stresses and strains may have limited the amount of mafic recharge that could have reached the shallow crustal reservoirs that act as staging zones for volcanic eruptions. The Ebbetts Pass system thus erupts few to any magmas that equilibrated below the upper crust, whereas the larger Sierra Crest-Little Walker system can readily tap magmas stored in the middle-crust.