Origin and Evolution of the Sierra Nevada and Walker Lane themed issue Miocene magmatism in the Bodie Hills volcanic fi eld, California and Nevada: A long-lived eruptive center in the southern segment of the ancestral Cascades arc David A. John1,*, Edward A. du Bray2, Richard J. Blakely1, Robert J. Fleck1, Peter G. Vikre3, Stephen E. Box4, and Barry C. Moring1 1U.S. Geological Survey, 345 Middlefi eld Road, Menlo Park, California 94025, USA 2U.S. Geological Survey, MS 973, Federal Center, Denver, Colorado 80225, USA 3U.S. Geological Survey, University of Nevada, Reno, Nevada 89557, USA 4U.S. Geological Survey, 904 West Riverside Avenue, Spokane, Washington 99201, USA ABSTRACT felsic compositions through time. Following north of Lake Tahoe in northeastern Cali- a 2 Ma hiatus in volcanism, postsubduction fornia, the scarcity of mafi c rocks, relatively The Middle to Late Miocene Bodie Hills rocks of the ca. 3.6–0.1 Ma, bimodal, high-K K-rich calc-alkaline compositions, and abun- volcanic field is a >700 km2, long-lived Aurora volcanic fi eld erupted unconform- dance of composite dome fi elds in the Bodie (~9 Ma) but episodic eruptive center in the ably onto rocks of the Miocene Bodie Hills Hills may refl ect thicker crust beneath the southern segment of the ancestral Cascades volcanic fi eld. southern ancestral arc segment. Thicker arc north of Mono Lake (California, U.S.). It At the latitude of the Bodie Hills, sub- crust may have inhibited direct ascent and consists of ~20 major eruptive units, including duction of the Farallon plate is inferred to eruption of mafi c, mantle-derived magma, 4 trachyandesite stratovolcanoes emplaced have ended ca. 10 Ma, evolving to a trans- instead stalling its ascent in the lower or along the margins of the fi eld, and numerous, form plate margin. However, volcanism middle crust, thereby promoting differentia- more centrally located silicic trachy andesite in the region continued until 8 Ma without tion to silicic compositions and development to rhyolite flow dome complexes. Bodie an apparent change in rock composition or of porphyritic textures characteristic of the Hills volcanism was episodic with two peak style of eruption. Equidimensional, poly- southern ancestral arc segment. periods of eruptive activity: an early period genetic volcanoes and the absence of dike ca. 14.7–12.9 Ma that mostly formed trachy- swarms suggest a low differential horizontal INTRODUCTION andesite stratovolcanoes and a later period stress regime throughout the lifespan of the between ca. 9.2 and 8.0 Ma domi nated by Bodie Hills volcanic fi eld. However, kine- From Eocene through Miocene time, subduc- large trachyandesite-dacite dome fi elds. A matic data for veins and faults in mining tion of the Farallon plate beneath western North fi nal period of small silicic dome em place- districts suggest a change in the stress fi eld America sustained magmatism associated with ment occurred ca. 6 Ma. Aeromagnetic and from transtensional to extensional approxi- the well-characterized ancestral (Western) Cas- gravity data suggest that many of the Mio- mately coincident with the inferred cessation cades arc segment in Washington, Oregon, and cene volcanoes have shallow plutonic roots of subduction. northernmost California, which was slightly that extend to depths ≥1–2 km below the Numerous hydrothermal systems were west of the modern Cascades arc (Fig. 1; surface, and much of the Bodie Hills may be operative in the Bodie Hills during the Mio- Atwater , 1970; McBirney, 1978; Priest, 1990; underlain by low-density plutons presum- cene. Several large systems caused altera- Smith, 1993; Sherrod and Smith, 2000; du Bray ably related to Miocene volcanism. tion of volcaniclastic rocks in areas as large et al., 2006; du Bray and John, 2011). A swath Compositions of Bodie Hills volcanic rocks as 30 km2, but these altered rocks are mostly of Oligocene to Pliocene volcanic rocks in east- vary from ~50 to 78 wt% SiO2, although devoid of economic mineral concentrations. ern California and western Nevada (Fig. 1) has rocks with <55 wt% SiO2 are rare. They More structurally focused hydrothermal sys- often been interpreted as the southern extension form a high-K calc-alkaline series with pro- tems formed large epithermal Au-Ag vein of this ancestral arc magmatism (e.g., Noble, nounced negative Ti-P-Nb-Ta anomalies and deposits in the Bodie and Aurora mining 1972; Christiansen and Yeats, 1992; Dickinson, high Ba/Nb, Ba/Ta, and La/Nb typical of districts. Economically important hydrother- 1997, 2006; Putirka and Busby, 2007; Cousens subduction-related continental margin arcs. mal systems are temporally related to inter- et al., 2008, 2011; Busby et al., 2008; Busby and Most Bodie Hills rocks are porphyritic, com- mediate to silicic composition domes. Putirka, 2009; du Bray et al., 2009; Hagan et al., monly containing 15–35 vol% phenocrysts Rock types, major and trace element 2009; Vikre and Henry, 2011; Colgan et al., of plagioclase, pyroxene, and hornblende ± compositions, petrographic characteristics, 2011; C.J. Busby, 2011, written commun.). biotite. The oldest eruptive units have the and volcanic features of the Bodie Hills vol- The southern and eastern extent of the ancestral most mafi c compositions, but volcanic rocks canic fi eld are similar to those of other large Cascades arc is debated, however, and Glazner oscillated between mafi c and intermediate to Miocene volcanic fi elds in the southern seg- and Farmer (2008) questioned the existence of ment of the ancestral Cascade arc. Relative an ancestral arc south of the southern terminus *E-mail: [email protected] to other parts of the ancestral arc, especially of the modern Cascades arc at Lassen Peak in Geosphere; February 2012; v. 8; no. 1; p. 44–97; doi:10.1130/GES00674.1; 20 fi gures; 5 tables; 2 supplemental tables; 1 supplemental fi gure fi le. 44 For permission to copy, contact [email protected] © 2012 Geological Society of America Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/8/1/44/3342254/44.pdf by guest on 30 September 2021 Bodie Hills volcanic fi eld L C a Modern arc J s L c U a Major Quaternary d A i a L eruptive center N Washington D Chemical analyses L 46° E of arc rocks S F 4–7 Ma L E Figure 1. Map of the west- U 8–17 Ma D ern United States showing the C 18–25 Ma A L inferred extent of ancestral and A 26–35 Ma S C modern Cascades magmatic u S 36–45 Ma bd (FA L arcs in Washington, Oregon, A u Mesozoic plutons C California, and Nevada (modi- c t 87 86 L ion R Sr/ Sr = 0.706 fi ed from du Bray et al., 2009; i 44° Colgan et al., 2011). Star shows A 208Pb/204Pb = 38.8 L L RN location of the Bodie Hills. L E Sample locations of ancestral O Oregon Idaho HIGH CASCADES T L Cascade arc rocks from geo- N S ) Proposed slab tear chemical databases of du Bray E L (Colgan et al., 2011) et al. (2006, 2009). Heavy gray W lines show the southern edge P L L Zo of subducting Farallon plate at A 42° the time specifi ed (Atwater and TE ne L = 0.706 Stock, 1998). Red dashed line i 86 Sr 87 86 / is the initial Sr/ Sr = 0.706 87 Sr isopleth for Mesozoic plutonic L rocks in California (Kistler , 1990) and Nevada (Tosdal et al., LP 20 8 2000). Blue dashed line is the Present San California Pb/ 208 204 20 PACIFI 4 Pb/ Pb = 38.8 isopleth (Tosdal Pb = 38.8 40° et al., 2000). Outcrops of Meso- Andreas E S zoic granitic rocks are from 4 Ma T R Ludington et al. (2005). Black A box outlines area of Figure 2. C L Nevada 6 Ma C LP—Lassen Peak; SNB—Sierra F A a Nevada batholith. u S PLA l 8 Ma t D E T S 10 Ma 38° E Figure 2 12 Ma 100 km 14 Ma SNB 124° 122° 120° 118° 116° northern California (Fig. 1). Rocks interpreted Ward, 1991; Best et al., 1989; Humphreys, map and brief summary of part of the Miocene as constituents of the southern ancestral Cas- 1995; Dickinson, 2006). Virginia City volcanic center (Fig. 2; Hudson cades arc segment are part of a diverse suite Recent studies of the southern ancestral et al., 2009), few detailed studies of Miocene– of Cenozoic volcanic rocks erupted across the Cascades arc segment have focused on Neo- Pliocene, possibly ancestral arc-related, vol- Basin and Range Province and related to mul- gene volcanic rocks in the central Sierra canic fi elds in the western Basin and Range tiple tectonic processes (Cousens et al., 2008, Nevada between Sonora Pass and Lake Tahoe east of the central Sierra Nevada have been 2011; du Bray et al., 2009; Henry et al., 2009; (Putirka and Busby, 2007, 2009; Busby et al., completed. Colgan et al., 2011), including a major pulse of 2008; Cousens et al., 2008, 2011; C.J. Busby, In the central Sierra Nevada, recent studies mid-Tertiary magmatism probably related to 2011, written commun.) and in the Warner of late Cenozoic tectonics and magmatism sug- delamination and/or foundering of the shallowly Range in northeasternmost California (Colgan gest the formation ca. 12 Ma of a Sierra Nevada east-dipping Farallon slab (e.g., Armstrong and et al., 2011). With the exception of a geologic microplate between the San Andreas fault to the Geosphere, February 2012 45 Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/8/1/44/3342254/44.pdf by guest on 30 September 2021 John et al.