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Downloaded from gsabulletin.gsapubs.org on December 8, 2010 Geological Society of America Bulletin Eruption chronology and petrologic reconstruction of the ca. 8500 yr B.P. eruption of Red Cones, southern Inyo chain, California Brandon Browne, Marcus Bursik, Justin Deming, Michael Louros, Antonio Martos and Scott Stine Geological Society of America Bulletin 2010;122;1401-1422 doi: 10.1130/B30070.1 Email alerting services click www.gsapubs.org/cgi/alerts to receive free e-mail alerts when new articles cite this article Subscribe click www.gsapubs.org/subscriptions/ to subscribe to Geological Society of America Bulletin Permission request click http://www.geosociety.org/pubs/copyrt.htm#gsa to contact GSA Copyright not claimed on content prepared wholly by U.S. government employees within scope of their employment. 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Notes © 2010 Geological Society of America Downloaded from gsabulletin.gsapubs.org on December 8, 2010 Eruption chronology and petrologic reconstruction of the ca. 8500 yr B.P. eruption of Red Cones, southern Inyo chain, California Brandon Browne1,†, Marcus Bursik2, Justin Deming2, Michael Louros1, Antonio Martos1, and Scott Stine3 1Department of Geological Sciences, California State University, Fullerton, California 92834, USA 2Department of Geology, State University of New York, Buffalo, New York 14260, USA 3Department of Geography and Environmental Studies, California State University, Hayward, California 94542, USA ABSTRACT INTRODUCTION occur in this region. Addressing this basic issue is fundamental in evaluating the hazards associ- Red Cones are a pair of basaltic cinder The 60-km-long Mono-Inyo volcanic chain ated with potential basaltic eruptions in the vi- cones located 5 km SSW of Mammoth of eastern California, stretching from Mono cinity of Mammoth Mountain, particularly with Mountain at the southern end of the Mono- Lake in the north to a pair of basaltic scoria respect to how such an eruption would threaten Inyo volcanic chain, in eastern California. cones located 5 km south of Mammoth Moun- the residents, infrastructure, and economy of Charcoal recovered at two separate loca- tain in the south known as Red Cones (Fig. 1), the nearby community of Mammoth Lakes, a tions beneath the Red Cones scoria-fall de- has produced ~20 explosive volcanic eruptions rapidly growing resort town located only 5 km posits indicates that the eruption most likely over the past 5000 years, the most recent of from Mammoth Mountain (e.g., Blong, 1984; occurred shortly after 8490 ± 90 14C yr B.P. which occurred from Inyo Craters and Panum Bernknopf et al., 1990; Kaye et al., 2008a, and no later than 9325 ± 83 14C yr B.P., which Crater 500–600 years ago (Sieh and Bursik, 2008b; Ort et al., 2008). implicates Red Cones as the most recent 1986; Miller, 1989; Wood, 1983) and Paoha One way to perform this evaluation is to ex- eruption of basalt in the Mono-Inyo volcanic Island 250 years ago (Stine, 1990). Whereas amine the products of a young basaltic eruption chain. Results from geologic fi eld mapping Quaternary eruptions from the northern Mono- that occurred near Mammoth as a proxy for con- combined with geochemical and petrologic Inyo chain predominantly produced rhyolite and straining hazards associated with a future event. analysis suggest that the ca. 8500 yr B.P. rhyodacite, basalt has preferentially erupted in The Red Cones system is a logical choice for eruption produced 10.1 × 106 m3 of magma, the south, particularly within 10 km of Mam- this study because it is located only 5 km south- possibly beginning from south Red Cone moth Mountain where 35 basaltic vents and west of the Mammoth Mountain edifi ce, 4 km and later from north Red Cone via Hawai- lava fl ows have been recognized by Bailey west of the ongoing emission of magmatic CO2 ian, Strombolian, and violent Strombolian (1989, 2004) (Fig. 1). The preferential eruption from Horseshoe Lake (e.g., Farrar et al., 1995; eruptions over a minimum of 28 days. All of basalt in the vicinity immediately surround- Hilton, 1996; Rahn et al., 1996; Gerlach et al., deposits contain plagioclase, olivine, clino- ing Mammoth Mountain is signifi cant in light 1998; McGee et al., 2000; Lewicki et al., 2008), pyroxene, chrome-spinel, and titanomagne- of recent fi ndings from geophysical studies that and 10–25 km directly above the inferred site of tite. Material erupted from each cone can be suggest the presence of a dike and sill network newly intruded basaltic magma during the 1989 classifi ed as high-aluminum basalts that ex- of basaltic magma beneath Mammoth Moun- seismic swarm (Hill and Prejean, 2005). In ad- hibit calc-alkaline differentiation trends and tain’s southern and southwest fl anks (Ryall and dition, its unglaciated morphology implies that belong to the medium-K series. Red Cone Ryall, 1983; Hill, 1996; Hill and Prejean, 2005). it is younger than any other basalt in the region basalt samples are generally similar in terms Specifi cally, Hill and Prejean (2005) attribute a surrounding Mammoth Mountain; however, its of many major and trace element concentra- seismic swarm in 1989 to the intrusion of new age has not been well constrained until now. tions, but south Red Cone samples typically basaltic magma accompanied by magmatic fl uid In this study, we determine the Red Cones contain more SiO2, Sr, Zr, Rb, and Ba, and migration along a dike at depths of 10–25 km eruption age and combine geological fi eld re- less MgO, FeO, CaO, Ni, and Cr than north in a region that extends 10 km WSW from lations, petrology, petrography, geochemistry, Red Cone samples. Clinopyroxene-liquid beneath Mammoth Mountain to neighboring and thermobarometry of clinopyroxene crystals thermo barometry calculations indicate that Devils Postpile National Monument. Although from Red Cones erupted material as a proxy for the majority of Red Cones clinopyroxene the basalt emplaced at shallow depths beneath understanding the nature and hazard of potential crystal cores crystallized at temperatures of Mammoth Mountain in 1989 did not reach the future eruptions of basalt in the Horseshoe Lake 1160–1210 °C and pressures equivalent to surface, ongoing seismic activity (e.g., Hill, region near Mammoth Mountain and the town 10–25 km depth, which supports the possi- 2006) and the frequency of basaltic eruptions of Mammoth Lakes. Our results show that the bility of a basaltic dike and sill plexus located near Mammoth Mountain over the past 2 million Red Cones eruption (1) occurred shortly after 10–25 km beneath the west and southwest years validates the need for a scientifi c investi- 8490 ± 90 14C yr B.P., (2) lasted a minimum of fl anks of Mammoth Mountain. gation that addresses what should be expected 28 days, (3) varied in eruption style from Ha- in terms of eruption style, magma volume, and waiian to Strombolian to violent Strombolian, †E-mail: [email protected] duration, if a future basaltic eruption were to and (4) was fed by a basaltic dike that tapped a GSA Bulletin; September/October 2010; v. 122; no. 9/10; p. 1401–1422; doi: 10.1130/B30070.1; 13 fi gures; 8 tables. For permission to copy, contact [email protected] 1401 © 2010 Geological Society of America Downloaded from gsabulletin.gsapubs.org on December 8, 2010 Browne et al. SiO ) domes and lava fl ows located on Long 119°W 118.45°W 2 Valley caldera’s southwestern topographic rim (Figs. 1 and 2A) (Bailey, 1989, 2004; Ring, 2000; Hildreth, 2004). Although geochronological studies of Mammoth by Mankinen et al. (1986) and Ring (2000) indicate eruption ages ranging from 111,000 to 57,000 ± 5000 yr, whereas the bulk of Mammoth was produced between 67,000 and 57,000 ± 5000 yr ago, Mammoth Mountain Glass Mountain continues to display many forms of volcanic unrest in the form of ground deformation, long- period seismic activity, elevated 3He/4He ratios 97.45°N in fumarole gases, and diffuse emission of CO2 at the surface, particularly at Horseshoe Lake on Mammoth Mountain’s southern fl ank (Sher- Caldera burne, 1980; Ryall and Ryall, 1983; Welhan Mammoth margin et al., 1988; Hill et al., 1990; Sorey et al., 1993; Mountain Pitt and Hill, 1994; Farrar et al., 1995; Hill, 1996; Devils Hilton, 1996; Rahn et al., 1996; Gerlach et al., Postpile Topographic 1998; McGee et al., 2000; Hill et al., 2002, 2003; ma 97.35°N rgin Hill and Prejean, 2005; Lewicki et al., 2008). Ba- salt has repeatedly intruded the magma system Red Cones beneath Mammoth Mountain over its lifetime as evidenced by the presence of mafi c enclaves and Rhyolite (Mono-Inyo Chain, 20–0.6 ka) xenocrysts originating from disaggregated en- Rhyolite (Older, 760–100 ka) 0 5 10 15 claves (Ring, 2000; Bailey, 2004) and reversely Basalt (160–8.5 ka) km zoned magnetite-ilmenite pairs within Mam- moth’s trachydacite lavas, the latter of which Figure 1. Location map of Red Cones volcanoes with respect to possess diffusion profi les suggestive of heating Mammoth Mountain, Devils Postpile National Monument, Long events via intrusion of basalt 30–40 days prior to Valley Caldera, Glass Mountain, the Mono-Inyo volcano chain, eruption (Wolfe et al., 2007).