JOURNAL OF PETROLOGY VOLUME 48 NUMBER 5 PAGES 951^999 2007 doi:10.1093/petrology/egm007 Compositional Zoning of the Bishop Tuff WES HILDRETH1* AND COLIN J. N. WILSON2 1US GEOLOGICAL SURVEY, MS-910, MENLO PARK, CA 94025, USA 2SCHOOL OF GEOGRAPHY, GEOLOGY AND ENVIRONMENTAL SCIENCE, UNIVERSITY OF AUCKLAND, PB 92019 AUCKLAND MAIL CENTRE, AUCKLAND 1142, NEW ZEALAND Downloaded from https://academic.oup.com/petrology/article/48/5/951/1472295 by guest on 29 September 2021 RECEIVED JANUARY 7, 2006; ACCEPTED FEBRUARY 13, 2007 ADVANCE ACCESS PUBLICATION MARCH 29, 2007 Compositional data for 4400 pumice clasts, organized according to and the roofward decline in liquidus temperature of the zoned melt, eruptive sequence, crystal content, and texture, provide new perspec- prevented significant crystallization against the roof, consistent with tives on eruption and pre-eruptive evolution of the4600 km3 of zoned dominance of crystal-poor magma early in the eruption and lack of rhyolitic magma ejected as the BishopTuff during formation of Long any roof-rind fragments among the Bishop ejecta, before or after onset Valley caldera. Proportions and compositions of different pumice of caldera collapse. A model of secular incremental zoning is types are given for each ignimbrite package and for the intercalated advanced wherein numerous batches of crystal-poor melt were plinian pumice-fall layers that erupted synchronously. Although released from a mush zone (many kilometers thick) that floored the withdrawal of the zoned magma was less systematic than previously accumulating rhyolitic melt-rich body. Each batch rose to its own realized, the overall sequence displays trends toward greater propor- appropriate level in the melt-buoyancy gradient, which was self- tions of less evolved pumice, more crystals (0Á5^24 wt %), and sustaining against wholesale convective re-homogenization, while higher FeTi-oxide temperatures (714^8188C). No significant hiatus the thick mush zone below buffered it against disruption by the took place during the 6 day eruption of the BishopTuff, nearly all of deeper (non-rhyolitic) recharge that augmented the mush zone which issued from an integrated, zoned, unitary reservoir. Shortly and thermally sustained the whole magma chamber. Crystal^melt before eruption, however, the zoned melt-dominant portion of the fractionation was the dominant zoning process, but it took place chamber was invaded by batches of disparate lower-silica rhyolite not principally in the shallow melt-rich body but mostly in the magma, poorer in crystals than most of the resident magma but pluton-scale mush zone before and during batchwise melt extraction. slightly hotter and richer in Ba, Sr, andTi. Interaction with resident magma at the deepest levels tapped promoted growth of Ti-rich rims KEY WORDS: BishopTuff; ignimbrite; magma zonation; mush model; on quartz, Ba-rich rims on sanidine, and entrapment of near-rim rhyolite melt inclusions relatively enriched in Ba and CO2. Varied amounts of mingling, even in higher parts of the chamber, led to the dark gray and swirly crystal-poor pumices sparsely present in all ash- INTRODUCTION flow packages. As shown by FeTi-oxide geothermometry, the zoned The Bishop Tuff, product of one of the world’s greatest rhyolitic chamber was hottest where crystal-richest, rendering any Quaternary eruptions, was released at 760 ka during an model of solidification fronts at the walls or roof unlikely.The main episode about 6 days long from the Long Valley magma compositional gradient (75^195 ppm Rb; 0Á8^2Á2ppm Ta; chamber in eastern California. Fallout remnants are pre- 71^154 ppm Zr; 0Á40^1Á73% FeOÃ) existed in the melt, prior to served from the Pacific Ocean to Nebraska, over an area crystallization of the phenocryst suite observed, which included of 42Á5 Â10 6 km2. Concurrent with the fallout, ash flows zircon as much as 100 kyr older than the eruption.The compositions spread 470 km SE down Owens Valley, 40^50 km east to of crystals, though themselves largely unzoned, generally reflect bank against the White Mountains, 40^50 km north into magma temperature and the bulk compositional gradient, implying Mono Basin and Adobe Valley, and tens of kilometers both that few crystals settled or were transported far and that the SW down the San Joaquin River canyon (Fig. 1). Long observed crystals contributed little to establishing that gradient. Valley caldera collapsed along a 12 km  22 km elliptical Upward increases in aqueous gas and dissolved water, combined ring-fault zone that became active only after half or more with the adiabatic gradient (for the 5 km depth range tapped) of the erupting magma had escaped from the chamber. *Corresponding author. Telphone: 650-329-5231. Fax: 650-329-5203. E-mail: [email protected] Published by Oxford University Press (2007). JOURNAL OF PETROLOGY VOLUME 48 NUMBER 5 MAY 2007 N Paoha I. Adobe Hills 38°N Cowtrack a Mono Lake Mtn d in LV as Adobe a B Granite ley Mtn l v MonoM Vy e o n nton Va o C Be N Downloaded from https://academic.oup.com/petrology/article/48/5/951/1472295 by guest on 29 September 2021 r a t e Aeolian Buttes r a s Benton r B r l i n d e BHS S i GM p r e i n W g g S ° n H 37 45 a i l l h RFZ R n i t o Apron of t e n RD GM rhy e debris B M Wa ML tterson Cyn o C u topo h gra id n phic margi L. ag n Crowley o Owens Go Ca t rg ny a e on C i n n o h y a s n Sierra l a k f c e a r e ive r n q R H C ° oa uin t 37 30 J C k SH n c V a o F a S R Volcanic l l Nevad Tableland e y Owe ns Round R iv e a r Valley Bishop 010 20 km Owens Califor Sierra Nevada Valley Early Rhyolite vents (760-650 ka) ° 37 15 Bishop Tuff (760 ka) nia Glass Mtn rhyolite vents (2.2-0.8 Ma) Precaldera dacite vents (3.5-2.5 Ma) Big Pine 119°W 118°45 118°30 118°15 Fig. 1. Outline map of Long Valley caldera and the Bishop Tuff, straddling the extensionally faulted transition from Basin and Range to Sierra Nevada. Dark gray pattern indicates exposed Bishop Tuff; light gray is pre-Quaternary basement; white is Quaternary volcanic and valley-fill deposits. Shown for the caldera are its topographic margin (dashed), ring-fault zone (RFZ; dotted), and limit of structurally uplifted resurgent dome (RD; dash^dot). Exposed vents for precaldera dacites, precaldera Glass Mountain (GM) rhyolites, and early postcaldera rhyolites are identified (after Bailey,1989; Metz & Bailey,1993). Separate clusters of pre- and postcaldera rhyolite vents suggest that a shift of magmatic focus took place during growth of the caldera-forming Bishop magma chamber. Drilling has shown subsurface Bishop Tuff to extend NW beneath Paoha Island in Mono Lake, SE beneath Big Pine, and to be as thick as 1500 m beneath the caldera floor. Before erosion and burial, Bishop ignimbrite probably covered all of Mono Basin, Adobe Valley, Benton Valley, Chalfant Valley, Owens Valley at least as far south as Big Pine, and the canyon of the San Joaquin River as far as 50 km downstream; the patterned areas of present-day outcrops thus represent only about half the original ignimbrite distribution (even neglecting thin upland veneers). Plinian pumice-fall deposits are preserved only within the easterly sector confined by the two long-dash lines, though primary plinian ashfall is also recognized outside this sector as far as Owens Lake (160 km SSE) and Friant Dam (110 km SW). Large asterisk indicates initial vent site (for most of Ig1 and F1^F8 plinian fallout) as inferred by Hildreth & Mahood (1986). Place-name abbreviations: BHS, Benton Hot Springs; HCF, Hilton Creek fault; LV, Lee Vining; ML, town of Mammoth Lakes; SH, Sherwin Hill. Chidago Canyon is normally a dry wash (dot^dash line). 952 HILDRETH & WILSON ZONING OF THE BISHOP TUFF The caldera was enlarged by syneruptive slumping and between 2Á2 and 0Á79 Ma (Bailey et al., 1976; Metz & secular erosive recession of the walls to form the modern Mahood, 1985, 1991; Bailey, 1989, 2004; Metz & Bailey, 17 k m  32 km depression, which has been the site of 1993). Taking into account dispersed pyroclastics (Sarna- many postcaldera eruptions, resurgent structural uplift, Wojcicki et al., 2005), the 60 or more precaldera vents hydrothermal activity, and current unrest (Bailey et al., of Glass Mountain erupted 100 Æ20 km3 (Hildreth, 2004) 1976; Bailey, 1989; Sorey et al., 1991; Hill et al., 2002; of high-silica-rhyolite magma, most of it similar to or even Hildreth, 2004). more evolved than the most differentiated pumice in the The Bishop Tuff consists predominantly of ash and Bishop Tuff (Metz & Mahood, 1991). Early postcaldera pumice clasts of biotite^plagioclase^quartz^sanidine high- phenocryst-poor rhyolites (760^650 ka) include lavas, 3 silica rhyolite (mainly 74^77Á7% SiO2), though scarcer tuffs, and intrusions that likewise add up to 100 km .In pumice types (reported here) extend the compositional large part coextensive and contemporaneous with growth Downloaded from https://academic.oup.com/petrology/article/48/5/951/1472295 by guest on 29 September 2021 range to dacite. Proximal to medial fall deposits are pli- of a 10 km wide resurgent uplift (Bailey, 1989), these early nian pumice lapilli and crystal-rich ash, but beyond about intracaldera rhyolites (74^75% SiO2) are less evolved 200 km from source the fallout is largely vitric ash that than the precaldera rhyolites, and in most respects they includes both plinian and coignimbrite contributions.