How old is bishop tuff

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. 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. Chidago Canyon is normally a dry wash dot—dash line.

Eruptive volume is not well known owing to 1 erosion, transport, and resedimentation of distal ash, 2 erosion or burial of distal outflow sheets, 3 thick postcaldera fill that conceals the intracaldera tuff, and 4 irregular faulting of the caldera floor beneath it. McConnell et al. The two northerly ignimbrite lobes Fig. For simplicity, we restrict use of the term Bishop Tuff to the deposit, and we refer to Bishop magma, crystals, and melt, none of which became tuff until they had ceased being magma.

Taking into account dispersed pyroclastics Sarna-Wojcicki et al. This activity is not dealt with here, as it reflects events and processes subsequent to those that built and organized the magma reservoir that released the Bishop Tuff.

The Bishop Tuff was initially mapped, described, and named by Gilbert , who characterized the pyroclastic-flow origin and welding of its outflow sheets. Bateman described the basal fall deposit as an integral part of the Bishop Tuff, and Izett et al.

Sheridan , , recognized the multi-lobate distribution of the ignimbrite, studied its mineralogy, and described its fossil fumaroles. Compositional zonation of the Bishop Tuff was first recognized and reconnoitered by Hildreth , , , , and Halliday et al. Confirmation of zoning in volatiles by analysis of melt inclusions in Bishop Tuff phenocrysts has provided important insights about the pre-eruptive magma body Anderson et al. In this paper we review these contributions and present a revised and expanded database of Bishop analytical data, supplementing and superseding that of Hildreth , We extend previous studies by 1 documenting the full compositional range of major and minor types of pumice, 2 recording the complexities of magma withdrawal reflected in fluctuating proportions of varied pumice types that were concurrently emplaced, and 3 drawing revised inferences concerning pre-eruptive processes in the magma reservoir.

The eruptive sequence Fig. Most, possibly all see below of the ignimbrite packages are demonstrably syn-plinian, rather than post-plinian as previously thought by Hildreth , Within the fall sequence exposed east of vent Fig.

Vertical distances are scaled to a composite timescale, which is separated by a short time break between F8 and F9. Fall deposits are shaded gray and labeled F1—F9. Several meters of nonwelded distal Ig2Ea rest conformably on fall unit F9.

Fall units F1—F5 are scree-covered here. Ig1 flow units failed to reach this location, terminating a few kilometers north.

All units are intensely vapor-phase altered. F9 and ignimbrite above contain rhyolite lithics; ignimbrite below does not. East and SE of vent, several ignimbrite packages successively offlapped, each one shingling progressively farther away from source. The earlier Ig1Ea and b were coeval with fall units F2—F8; they lack pyroxene phenocrysts and generally lack the rhyolite lithics derived from Glass Mountain and its volcaniclastic apron characteristic of the overlying Ig2E packages, except where they flowed eastward across the Glass Mountain apron.

Later eastern packages Ig2Ea, b, c were coeval with uppermost F8 and F9, and like those fall layers they do contain pyroxene-bearing pumice and vent-derived rhyolite lithics, the latter indicating northeastward propagation of vents along the ring-fault zone during caldera subsidence. Because the plinian plume was driven strongly eastward, proximal to medial fall deposits are not present in northern and southwestern outflow sectors Fig. Nonetheless, the northerly packages Ig2N, Ig2NW are rich in pyroxene-bearing pumice, overlie a remnant of pyroxene-free ignimbrite Ig1NW near the north margin of the caldera, and have suites of pumice types that partly overlap with that of Ig2E.

In the San Joaquin canyon to the SW, lithic suites were in large part picked up locally, but pumice suites and mineral chemistry link the lower of two packages Ig1SW to the Ig1E eruptive interval and the upper Ig2SW to the Ig2 interval.

The importance of this chronostratigraphic framework is threefold. Detailed understanding of the emplacement sequence allows sampling of pumice from fresh glassy parts of every emplacement package. Banded and composite pumice are sparsely present Table 1 but were not analyzed.

There is no evidence in our observational or chemical data for any correlation between composition and size of pumices. Ash and pumice granules are created by fragmentation and comminution of all pumice types. Variant pumice clasts. The compositional problems of bulk ash-flow vitrophyres, subject to glass—crystal fractionation and contamination during eruption and outflow, are well known. Some typical Bishop Tuff outcrops counted and sampled.

There are systematic variations in proportions of the different pumice types through the tuff, as summarized in Fig. Of the variant pumice types, the glistening variety of xp pumice is sparsely present throughout Ig1 and Ig2E but is rare north of the caldera Electronic Appendix 2.

Dark gray pumice is also represented in Ig1Ea and irregularly throughout the sequence of emplacement units. Local spikes in the count fractions of low-density dark, swirly, and glistening pumice Electronic Appendix 2 are interpreted to reflect buoyant concentration during outflow. Pumice clast proportions for Bishop emplacement units of Fig. Clasts counted at indicated number of field sites for each unit; data are given in Electronic Appendix 2.

Sherwin shr and Watterson wat subunits of Ig1Eb were counted separately. Diagonal lines indicate ranges of within-unit variability. Available data do not discriminate between deposition by flows directed eastward from the initial vent site that scoured that fan Fig. This subunit forms the topmost part of Ig1Eb, above welding zone c, principally near the rim of Rock Creek gorge in the area of Sherwin Hill, though scattered outliers occur farther east. It is absent from Chidago Canyon, and, as such, its distribution is distinct from that of the Watterson subunit.

Each pumice type identified has a significant range of composition Table 2 ; Figs 7— All such ranges are continuous, not bimodal or otherwise clustered. At the less evolved ends, however, the crystal-rich arrays extend to less differentiated compositions than the crystal-poor arrays; in this respect, the xm arrays consistently end at intermediate values for all elements Table 2. Weight per cent crystals was determined by separations in heavy liquids as tabulated in Electronic Appendix 1. Inset identifies emplacement unit as in Fig.

Dark and swirly pumices from all units; see Fig. Alkali variation and mobilization for pumiceous and dense clasts in the Bishop Tuff. Clast types described in text are identified in inset. Other clast types are sparse or only locally common see Electronic Appendix 2. For dark and swirly pumice types, see text. Fiamme are welded lenticles of any collapsed pumice-clast type. Most dense clasts are crystal-poor. Clast types identified in inset, as in Fig.

The top panel is extended as in Fig. Definitions and descriptions of clast types have been given in Table 1. Vitrophyres are dense glassy juvenile ejecta, not welded tuff.

Square brackets enclose outliers beyond continuous range of data; parentheses enclose ranges of Na 2 O and K 2 O extended by strongly hydrated samples. Siems, analyst. Trace elements in ppm, by energy-dispersive XRF; D. Siems and P. Bruggman, analysts. Precision is estimated by numerous repeat analyses of internal standards. Such extensive overlap of the compositional ranges for main-suite pumices signifies that major- and trace-element compositions are to some degree independent of crystal content Electronic Appendix 1 , there being at best a crude correlation for a few elements Fig.

This, in turn, suggests 1 that the main body of high-silica rhyolitic magma that erupted to form the Bishop Tuff had been compositionally zoned before the observed spectrum of crystal contents developed, and 2 that crystal accumulation within the magma erupted was not a leading compositional control.

Data for the dense vitrophyric clasts and fiamme extensively overlap the compositional ranges of the main xp—xm—xr arrays described above Table 2 , consistent with these being dense equivalents of normal Bishop pumice. Exceptions are two or three dense fiamme that may once have been Ba-rich swirly pumice. Whole-rock welded-tuff vitrophyre has generally been avoided owing to the obvious potential for contamination and crystal—glass sorting during eruption and emplacement.

Because the four were lithic-free and from low in thick welded emplacement packages, they may have lost little glass-enriched elutriate and suffered little sorting. Their lesser hydration and limited alkali variability suggest that carefully selected samples of bulk ignimbrite vitrophyre can in such special circumstances offer some advantages over badly hydrated pumice. Swirly pumice is by far the most abundant Bishop component not belonging to the main xp—xm—xr continuum.

It differs in texture, color, and in combining low phenocryst content with generally elevated Ba and Sr abundances. Nearly all samples analyzed were single clasts or fiamme, unaffected by the crystal—glass fractionation and lithic contamination intrinsic to emplacement of bulk ignimbrite.

To minimize effects of post-emplacement vapor transport and secular alteration, we avoided taking clasts from devitrified or vapor-phase zones, or from exposures displaying case-hardening i.

The two low-LOI samples of Fig. Because hydration of volcanic glass usually results in Na loss and, also commonly, in K gain Lipman, ; Noble, , probably in part by ion exchange between groundwater and glass Truesdell, , the Na—K data for the Bishop Tuff sample suite have been scrutinized. A few additional points can be observed in Fig. A negative K—Si correlation is opposite to expectation if K gain by ion exchange were linked with greater hydration of higher-silica samples. All three suites overlap at the high-silica ends of the arrays.

Plotting LOI vs every other element determined revealed no evidence for preferential mobilization in high-LOI samples. Scrutiny of LOI and inter-element plots identified five samples that appear to have lost SiO 2 and five more that have gained Ca and Sr; these 10 samples were omitted from the data set discussed below.

Two samples of the dark gray pumice that may have lost SiO 2 and another that may have gained SiO 2 have been retained, owing to uncertainties about the primary compositional range of this highly varied suite. A general feature of major- and trace-element plots of Bishop Tuff pumice data is an absence of narrow compositional arrays.

Instead, all x — y plots show broadly scattered trends for the main suite of normal xp—xm—xr pumices and generally different trends typically even more scattered for the subordinate suites of variant swirly and dark pumices. Variation vs SiO 2 of key major and trace element contents are shown in Figs 9 and Data for the normal xp pumices cluster at the high-silica low-Ca—Fe—Ti ends of the arrays Fig. The scattered arrays of dark and swirly pumice extend to still less silicic compositions, but both overlap extensively the main xp—xm—xr array in CaO and TiO 2.

Comparable relationships for Ba, Zr, and Rb are illustrated in Fig. Despite considerable overlap, the dark and swirly pumice arrays tend to have higher Rb and lower Zr, Ba, and Ti at equivalent SiO 2 than the xr segments of the main arrays Figs 9 and Many trace-element plots exhibit fairly coherent arrays for the main suite but broadly scattered fields for the swirly and dark pumices Figs 11— At equivalent Ba contents Fig.

Clast types identified in inset, as in previous figures. Conversely, however, the evolved low-Fe ends of the main arrays extend to somewhat lower Zr and Ti and slightly higher Nb than the swirly and dark pumices. Apart from clusters of highly evolved samples predominantly but not exclusively xp pumice that terminate the main-suite arrays, the scattered fields of swirly and dark pumice nonetheless exhibit essentially the same ranges in Nb, Ti, and Zr abundances as the main suite Fig.

The observation that a dozen or more samples of dark and swirly pumice also follow this positively sloping trend Fig. Each successive emplacement package Fig. In Fig. Symbols identified in inset. For clarity, paired panels separate the main suite of xp—xm—xr white pumice from the combined suite of subordinate dark and swirly pumice.

To facilitate comparison, the field outlines of the former are reproduced in panels of the latter. Comparable partial overlaps exist for plots of Y, Nb, and other major and trace elements not shown. It is important to observe Fig. Considering next the crystal-poor suite of swirly and dark pumices Fig.

These pumices tend to have higher Rb and lower Zr in Ig1 than later in the eruptive sequence, but there are exceptions. Figure 14 also shows that the swirly and dark pumices extend to lower SiO 2 and to higher Fe, Ti, Ba, and Sr contents than the main suite. Overall, the wide compositional range of the subordinate magma that produced the swirly and dark pumices remained available throughout the eruption, having been released unsystematically as a small but fluctuating fraction Fig.

Fluctuating proportions of pumice types throughout the eruptive sequence—especially marked in Ig2E Fig. The concurrent emplacement of most kinds of pumice virtually from start to finish strongly suggests the importance of processes 1 and 3 ; and, in a later section below, we provide abundant evidence for a unitary chamber, thus rejecting process 2. Processes 4 and 5 are unavoidable at some scale during an eruption of this magnitude and duration, and the welded-tuff clasts in Ig2 Fig.

Although the main purpose of this paper is to document the extent and continuity of the Bishop pumice magmatic compositional zoning, crystals contribute to bulk magma composition and inevitably warrant discussion. Conceptually, it is useful to distinguish among crystals in a magma that are 1 xenocrysts, accidentally entrained from contrasting magmas or older rocks during intrusion, storage, eruption, or outflow; 2 phenocrysts, which grew in the magma containing them and may have had long or short residence times; and 3 antecrysts, which were inherited by the magma now containing them but had grown as phenocrysts in a discrete but kindred magmatic precursor, known or inferred to have been an earlier component of a waxing—waning multi-stage system.

Crystals introduced by recharge batches of similar but not identical magma, crystals re-entrained from floor cumulates or mushy enveloping rinds, and the crystals liberated from precursor batches that had nearly but temporarily solidified provide examples of antecryst-vs-xenocryst ambiguity that require microbeam data and thoughtful interpretation.

Even though the Bishop crystals are largely unzoned, reflect the bulk zonation, and thus appear to have crystallized after the chamber became zoned Hildreth, , , attributes of the crystals and their melt inclusions nonetheless bear significantly on interpretation of the origin of the zoning.

Where present, euhedral augite and hypersthene virtually always occur together, in roughly equal amounts. They are restricted to Ig2 and F9 , but not all Ig2 pumices contain pyroxenes.

Hildreth found them in 26 of 36 Ig2 samples, and for the broader suite including swirly and dark pumices: Electronic Appendix 1 separated for the present study, pyroxenes were identified in 24 of 69 Ig2 pumices and in one of five pumices from F9.

Both pyroxenes contain tiny inclusions of ilmenite, titanomagnetite, zircon, apatite, sulfide and, rarely, allanite and monazite.

It should be noted that only a subordinate proportion of the Bishop Tuff is pyroxene-bearing. There is great uncertainty about the fractions of pyroxene-bearing material in the intracaldera tuff and distal ash-fall, but both are likely to be modest. Crystals are generally solitary and euhedral, whereas crystal aggregates are very rare. Tiny mineral and glass inclusions are common in most crystal species.

Compositional data for each mineral species in the main pumice suite have been tabulated by Hildreth , whose principal observations adapted to our detailed stratigraphic framework were the following. Ranges of FeTi-oxide temperature for coerupted glassy material extended to higher values as the eruption progressed, viz.

There was extensive overlap in the ranges of temperature given by the pumice suites in successive emplacement units. Temperatures correlated well with mineral and bulk-pumice compositions, but with eruptive withdrawal sequence only in the gross sense of an increasing proportion of higher-temperature ejecta with time.

Within any sample, each mineral species is unzoned or nearly so, but with rising FeTi-oxide temperature, the compositions of sanidine, plagioclase, biotite, titanomagnetite, ilmenite, zircon, and apatite are progressively less evolved. Along with declining proportions of highly evolved pumice across the eruptive sequence, the progressive addition of less evolved, higher-temperature pumices was accompanied by attendant shifts in mineral compositions.

Allanite, hypersthene, augite, and pyrrhotite crystals are likewise unzoned, but, in contrast to the other mineral species, compositions change little with emplacement sequence or across the limited temperature range over which each species occurs. The quantitative mineral separations undertaken Electronic Appendix 1 illustrate a weak tendency within the normal pumice suite for crystal content to correlate crudely with whole-pumice Zr, Fe, Ba, and Sr contents and negatively with Rb Fig.

The swirly and dark pumices, on the other hand, exhibit only wide compositional scatter at low to moderate crystal contents Fig. Crystal contents of the main-suite pumice increased with eruption progress, in the general sense of a progressive advance in the proportion of crystal-rich pumice. Nonetheless, sparse crystal-rich pumices were also ejected early, and subordinate crystal-poor pumices remained part of the eruptive mixture until the very end Figs 6 and 7.

The dark pumices were first reported by Hildreth , who noted that most are crystal-poor and commingled with bands and blebs of white rhyolite pumice. No Fe—Ti oxide temperatures were obtained for any of 13 swirly pumice samples separated, owing either to lack of ilmenite or to alteration of one or both oxide minerals. With rare exceptions among granitoids, lithic fragments lack evidence of partial melting, a generalization that pertains even to obsidian clasts of precaldera high-silica rhyolite.

Mineral concentrates from pumice include sparse anhedral quartz, hornblende, cloudy feldspars, and various intergrowths of the three, which we infer to have disaggregated from granitoid xenoliths. Broken anhedral sphene and monazite xenocrysts are likewise present at sub-ppm abundances within pumice clasts Hildreth, Many mineral separates from pumice contain distinguishable xenocrysts at parts-per-mil concentrations; a few also have traces of millimeter-sized xenoliths, mostly schistose metapelite but also rare volcanic, metavolcanic, and hornfelsed silicic and calc-silicate metasedimentary grains.

Dark and swirly pumices carry the same kinds of xenoliths as the main suite. Microprobe analyses Hildreth, of plagioclase mostly unzoned or poorly zoned oligoclase identified rare crystals in Ig2 that have irregular An 32—48 cores, suggesting xenocrystic inheritance and overgrowth.

Engulfment of part of the Glass Mountain apron of high-silica-rhyolite pyroclastic debris during the Ig2 stage of the caldera-forming eruption would inevitably have contaminated the Bishop magma with entrained xenocrysts of biotite, quartz, and feldspar difficult to distinguish from the juvenile phenocrysts.

Truly accidental material is certainly present in Bishop pumice, though rather sparsely. On the contrary, in their plinian pumice sample, several isotopically heterogeneous feldspar crystals were identified.

Despite this isotopic variability for one sample , abundant microprobe data for plinian feldspars from many samples have identified only virtually homogeneous sanidine Or 62—66 and plagioclase An 13—16 , unzoned in major and trace elements Hildreth, ; Lu, ; Anderson et al.

At odds with such a conclusion, however, are two pieces of new data. First, Simon et al. We supplement and recalculate this data set here, with data from additional locations and lithologies, and extending coverage to the variant pumices. However, only 34 of our 71 new samples separated yielded homogeneous titanomagnetite—ilmenite pairs see notes to Electronic Appendix 3.

FeTi-oxide data for the Bishop Tuff. Anomalously Mn-enriched subset illustrates probable vapor-phase modification, which yields suspect T — f O 2 results not used.

Plotted are 68 samples recalculated from Hildreth and 43 from the new data of Electronic Appendix 3. Symbols are identified in inset; the wide range of transitional package Ig2E should be noted.

Unlike the data, where each ilmenite or titanomagnetite analysis reported was an average of two or more spot analyses of five or more similar homogeneous grains, the new data involve no averaging, each representing a point analysis of a single grain. Touching pairs were preferred but not always found. The T — f O 2 values plotted Fig. No T — f O 2 data were obtained for xr pumices in Ig1, but of those for which mineral separates were undertaken Electronic Appendix 1 , none were pyroxene-bearing.

None of the 13 swirly pumices separated contained homogeneous Fe—Ti oxide pairs. It can be inferred, nonetheless, that the magma they represented had little thermal contrast to the resident magma it invaded, because it remained phenocryst-poor and the pumice ejected was glassy.

Although there are significant overlaps and lack of any simple correspondence, the new data support the old in indicating generally coordinated trends toward greater crystal content, higher pre-eruptive temperatures, and an increasing proportion of pyroxene-bearing pumice as the eruption progressed. The pyroxene-bearing domain in the magma reservoir seems not to have been simply bounded by an isothermal surface, however, because several pyroxene-free pumices yield FeTi-oxide temperatures overlapping the main range for the pyroxene-bearing assemblage.

The former cited: 1 lack of Fe—Mg exchange equilibrium between Bishop biotite which ranges compositionally with temperature and coexisting orthopyroxene, which is of virtually constant composition across the temperature range; and 2 independence of the Bishop T — f O 2 trend Fig.

They suggested that ilmenite composition may have been modified, either just before, during, or after eruption, by some thermal event not recorded by the pyroxenes. The possibility that pyroxene or oxide compositions were modified by post-eruptive processes can be ruled out because 1 both pyroxenes are euhedral e. Michael, , fig. The same homogeneity cannot be claimed for Bishop biotites, which may have interacted with a pre-eruptive or syneruptive gas phase Hildreth, Cooling rates for our samples are inferred to have been rapid see Zhang et al.

In such circumstances, ilmenite, titanomagnetite, and pyroxenes remained as pristine as erupted. Constraints bearing upon such a hypothesis include the following. We conclude that, at the time of eruption, both pyroxenes and both oxides were indigenous to the zoned Bishop reservoir. Mineralogical observations on the Bishop Tuff subsequent to Hildreth's , , reports include the following. Traces of allanite were separated by Izett et al.

For some of these crystals, Fe and Ca also increase slightly in the Ba-rich rims Lu, ; Anderson et al. Sanidine from plinian and Ig1 pumices, in contrast, was shown by ion probe to be nearly homogeneous in trace and major elements.

Whether the host quartz crystals were xenocrysts, antecrysts, or Bishop-age phenocrysts transferred by mixing within the magma reservoir or by syneruptive engulfment of earlier-erupted material is unresolved. Several Triassic zircons were apparently derived from local granitoid roof rocks, showing that old zircons could survive in the low-temperature Bishop magma.

No straightforward mechanism of monotonic compositional evolution of melt composition emerges from the MI data, however, because entrapment sequences are compositionally inconsistent and often nearly random Lu et al. Infrared spectroscopic analysis of MI in quartz crystals Anderson et al.

Inverse correlations between incompatible trace elements and CO 2 in sets of MI from plinian, Ig1, and earliest Ig2 pumices permit the inference of gas-saturated crystallization, which would be accompanied by progressive preferential partitioning of CO 2 into the gas phase. Why such a mass of bubbles, if real, would not rise to the roof and escape was not addressed. Analyses of MI from pumice at two sites indicate a more complex history for the quartz host crystals in Ig2NW Wallace et al.

Wallace personal communication, suggested that most of the 41 MI from Ig2NW may have lost H 2 O diffusively, thereby lowering their estimated entrapment pressures. Ion-probe analyses of the same sets of MI document positive correlations of Ba, Sr, Zr, and Mg abundances with CO 2 contents, thus demonstrating joint rimward increases in successively trapped MI of these compatible trace elements and incompatible CO 2. Such reverse trace-element zoning contrasts with that recorded for quartz MI from earlier in the Bishop sequence, where these elements and CO 2 either decrease slightly or scatter randomly in successively rimward MI.

Moreover, importantly, most MI analyzed from their Ig2NW samples, even those in the cores of quartz grains, have higher CO 2 contents than do MI from earlier in the eruptive sequence Wallace et al.

Cathodoluminescence CL imaging identified oscillatory growth zones in Bishop quartz grains Peppard et al. For Ig1, plinian, and Ig2Ea quartz, neither compatible Ba, Zr, Mg nor incompatible U, Nb elements determined by ion probe showed any systematic compositional evolution of MI with entrapment sequence; the within-crystal concentrations variously increased, decreased, reversed, or remained constant for core-to-rim sequences of MI.

For two sites in Ig2NW, however, bright-CL rims on 20 of 21 quartz grains from 10 pumice lumps indicated late overgrowths that are rare on grains in Ig2E and apparently absent earlier in the eruptive sequence.

Ion-probe analysis showed that MI in the sharply bounded bright-CL outer zones of Ig2NW quartz are enriched in Ba and Zr and depleted in Nb relative to MI in their own interiors as well as compared with MI in quartz grains from earlier in the sequence. This reinforces the evidence from items 3 and 8 , above, that the final growth stage of these Ig2NW quartz grains took place after an abrupt change of the rhyolitic host melt to one that was less evolved, richer in Ba, Sr, Ti, Zr, and CO 2 , and presumably somewhat hotter than had been the host melt when the interior zones had grown Peppard et al.

Peppard et al. Compositions of the virtually unzoned Ig2NW crystals biotite, plagioclase, both FeTi oxides, zircon, and apatite differ markedly from those erupted earlier from shallower levels of the chamber, and only the quartz and sanidine carry the thin overgrowths that indicate late elevation of Ba, Sr, Ti, Zr, and CO 2 contents in some but by no means all; Fig. These data suggest that a thermal pulse caused resorption, truncating the outermost CL zones Peppard et al.

These shifts are recognized only in Ig2 pumice and are pronounced mainly for crystal-rich pumice of Ig2NW and Ig2N, ejection of which terminated the eruption. These processes had little or no effect on the lower-temperature rhyolite at shallower levels of the chamber that supplied the first three-quarters of the eruptive volume. Estimates of Ti diffusivity in quartz suggest that the sharply bounded Ti-enriched bright-CL rims grew no earlier than a century or so before eruption Wark et al.

All lines of evidence indicate physical continuity of the Bishop eruptive sequence. The F1—F8 sequence brackets all of Ig1E and beyond the distal limits of coeval ignimbrite contains no discernible hiatus in plinian deposition. A short break in activity, probably representing at most a few hours, occurred at the F8—F9 boundary. The shortness of the break is indicated by pristine preservation of the fine-ash-bearing top of F8 Fig.

Our newer data show that fall unit F9 was synchronous with most or all of Ig2E emplacement see above. Moreover, failure of ash flows consisting of undiluted northern material to cross the caldera floor and join the easterly outflow sector may best be attributable to blockage by synchronous outpouring of Ig2E material from the eastern and southeastern ring-vent segments.

Concurrent foundering of the caldera floor may also have been a contributing factor, but it would not change the implications of these observations. Unlike most distal Bishop remnants, which are reworked and contaminated Izett et al.

The continuous gradation implies continuous deposition from a downwind ash cloud that included co-ignimbrite ash entrained into the plinian column. The large fraction of pyroxene-bearing ash as much as the upper third of the Utah section is inferred to be equivalent to proximal—medial fall unit F9, along with ash elutriated from the coeval Ig2E and Ig2NW and Ig2N flows.

A composite column around Chalfant Valley Fig. We infer that pyroxene-bearing magma joined the erupting mixture about two-thirds of the way volumetrically through the eruption. In addition to the physical evidence for continuity of the Bishop eruption, abundant and varied chemical and mineralogical evidence supports the notion of a single integrated magma reservoir prior to the eruption.

Evidence is compelling that the Bishop Tuff represents magma that had been stored in a unitary zoned chamber that was most differentiated, lowest in temperature, richest in gas, and poorest in phenocrysts at the roof, where withdrawal began. Geol Soc Am Mem Nature — Watson EB, Harrison TM Zircon saturation revisited: temperature and composition effects in a variety of crustal magma types.

J Geol — Download references. Ghiorso was a student of Ian Carmichael and remembers well the heated discussions between Wes and Ian that accompanied the formulation of the Standard Model. That this model invokes argument and discussion some 35 years after the fact is an enormous testament to their intuition for choosing to study timeless and important petrologic problems, and to do so in novel and creative ways.

Fred Anderson introduced Gualda to the Bishop Tuff, its sources of fascination and puzzlement; the advice and uncountable hours of discussion and collaborative work are greatly appreciated.

You can also search for this author in PubMed Google Scholar. Correspondence to Guilherme A. Reprints and Permissions. Gualda, G. The Bishop Tuff giant magma body: an alternative to the Standard Model. Contrib Mineral Petrol , — Download citation. Received : 24 October Accepted : 29 May Published : 22 August Issue Date : September Anyone you share the following link with will be able to read this content:.

Sorry, a shareable link is not currently available for this article. Provided by the Springer Nature SharedIt content-sharing initiative. Skip to main content. Search SpringerLink Search. Abstract The Bishop Tuff, one of the most extensively studied high-silica rhyolite bodies in the world, is usually considered as the archetypical example of a deposit formed from a magma body characterized by thermal and compositional vertical stratification—what we call the Standard Model for the Bishop magma body.

References Anderson JL Status of thermobarometry in granitic batholiths. J Petrol — Article Google Scholar Annen C Implications of incremental emplacement of magma bodies for magma differentiation, thermal aureole dimensions and plutonism-volcanism relationships. Ghiorso Authors Guilherme A.

Gualda View author publications. View author publications. Duplicate potassium-argon age determinations on each of three samples from widely separated localities indicate that the age of the Bishop Tuff, California, is about 0. Two of the samples are from the basal ash fall that preceded the ash flow eruptions; one of these two samples was collected within 1 m of the contact of the Bishop Tuff with the underlying Sherwin Till.

The third sample is from near the present exposed surface of the Bishop Tuff. The minimum age of the Sherwin Till Kansan? The samples used for previously published age determinations of about 1 million years were probably contaminated with older material.

Paleomagnetic results from five widely separated localities indicate that the welded part of the Bishop Tuff became magnetized when the geomagnetic field was normal and that it may have cooled in several centuries or less.

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