New (super 40) Ar/ (super 39) Ar ages reveal contemporaneous mafic and silicic eruptions during the past 160,000 years at Mammoth Mountain and Long Valley Caldera, California 
Reference Number:
Publication Year:
GeoREF Number:
2010-029009 (View Original Data File)
Mahood, Gail A; Ring, Joshua H; Manganelli, Simone; McWilliams, Michael O
absolute age, acidic composition, Ar/Ar, basaltic composition, calderas, California, Caso Diablo Till, Cenozoic, clastic sediments, dacites, dates, eruptions, glaciation, igneous rocks, lava, Long Valley Caldera, mafic composition, magma chambers, magmas, Mammoth Mountain, Mono County California, periodicity, Pleistocene, Quaternary, sediments, till, trachytes, United States, upper Quaternary, volcanic features, volcanic rocks, volcanism, volcanoe
Geological Society of America Bulletin122.3-4 (March 2010): 396-407
We undertook a (super 40) Ar/ (super 39) Ar study of young mafic and silicic lavas at Mammoth Mountain and the Long Valley caldera (east-central California) to better understand the frequency of these eruptions and the magmatic plumbing system that drives them. Our results show that most of Mammoth Mountain, a lava-dome complex straddling the southwestern topographic rim of the caldera, consists of trachydacite lavas erupted at ca. 68 ka. These ages and new 29- and 41 ka ages for trachydacite lavas in the northwest quadrant of the caldera indicate that these silicic lavas are considerably younger than previously thought. Mafic lavas vented widely in the western third of the caldera in the past 190,000 years, suggesting that this area has not been underlain by large bodies of silicic magma during this interval, as such magma would have prevented the rise of the denser basaltic magma. We identify four eruptive sequences over the past 190,000 years: the western moat sequence ( approximately 190-160 ka), the Mammoth sequence ( approximately 120-58 ka), the northwest caldera sequence ( approximately 41-29 ka), and the Inyo chain sequence ( approximately 9 ka-present). In each eruptive sequence mafic and silicic lavas erupted contemporaneously from spatially associated vents. This suggests that intrusion of alkali basalt into the shallow crust led to the silicic eruptions. If the seismic unrest and deformation of the past three decades is a result of basalt injected beneath Mammoth Mountain and perhaps the western third of the caldera, then there is the possibility of spatially associated small-volume silicic eruptions, which would typically be considerably more explosive. In the past 40,000 years, eruptions have occurred along a N-S linear trend less than 10 km wide, limiting the zone subject to volcanic hazards. Our data bear on Pleistocene glaciation in the region. Ages of 162+ or -2 ka and 99+ or -1 ka for bracketing mafic lava flows better constrain the age of the Casa Diablo till. Our results provide equivocal support for a suggested anticorrelation between volcanism and glaciation for the past 800,000 years in eastern California (Glazner et al., 1999).
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Sample Summary:
Sample ID: Major Data: Trace Data: Isotope Data:
M-17 No No No
M-14 No No No
M-15 No No No
M-7 No No No
M-34 No No No
M-30 No No No
M-31 No No No
M-35 No No No
M-33 No No No
M-141 No No No
M-137 No No No
M-139 No No No
M-39 No No No
M-37 No No No
M-101 No No No
M-32 No No No
M-126 No No No
M-38 No No No
M-124A No No No
M-124B No No No
M-20 No No No
M-11 No No No
M-14 No No No
M-15 No No No
M-9 No No No
M-3#1 No No No
M-3#2 No No No
M-22 No No No
M-6 No No No
M-27 No No No
M-16 No No No
M-17 No No No
M-18 No No No
M-19 No No No
M-5 No No No
M-8 No No No
M-26 No No No