Voluminous low δ18O magmas in the late Miocene Heise volcanic field, Idaho: Implications for the fate of Yellowstone hotspot calderas (original) (raw)
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We present new isotopic and trace element data for four eruptive centers in Oregon: Wildcat Mountain (40 Ma), Crooked River (32-28 Ma), Tower Mountain (32 Ma), and Mohawk River (32 Ma). The first three calderas are located too far east to be sourced through renewed subduction of the Farallon slab following accretion of the Yellowstone-produced Siletzia terrane at ∼50 Ma. Basalts of the three eastern eruptive centers yield high Nb/Yb and Th/Yb ratios, indicating an enriched sublithospheric mantle source, while Mohawk River yields trace element and isotopic (18 δ O and εHf) values that correlate with its location above a subduction zone. The voluminous rhyolitic tuffs and lavas of Crooked River (41 × 27 km) have 18 18 δ O zircon values that include seven low δ O zircon units (1.8-4.5), one high 18 δ O zircon unit (7.4-8.8 y), and two units with heterogeneous zircons (2.0-9.0), similar to ounger Yellowstone-Snake River Plain rhyolites. In order to produce these low 18 δ O values, a large heat source, widespread hydrothermal circulation, and repeated remelting are all required. In contrast, Wildcat Mountain and Tower Mountain rocks yield high 18 δ O zircon values (6.4-7.9) and normal to low εHf i values (5.2-12.6), indicating crustal melting of high-18 δ O supracrustal rocks. We propose that these calderas were produced by the first appearance of the Yellowstone plume east of the Cascadia subduction zone, which is supported by plate reconstructions that put the Yellowstone plume under Crooked River at 32-28 Ma. Given the eastern location of these calderas along the suture of the accreted Siletzia terrane and North America, we suggest that the Yellowstone hotspot is directly responsible for magmatism at Crooked River, and for plume-assisted delamination of portions of the edge of the Blue Mountains that produced the Tower Mountain magmas, while the older Wildcat Mountain magmas are related to suture zone instabilities that were created following accretion of the Siletzia terrane.
Caldera Life-Cycles of the Yellowstone Hotspot Track: Death and Rebirth of the Heise Caldera
Journal of Petrology, 2018
As one of the most geochemically unique drill cores recovered within the Yellowstone-Snake River Plain (YSRP) province, the Sugar City geothermal test well was drilled into intra-caldera rhyolite lavas and tuffs erupted during the middle to late Pliocene and the resurgent basaltic volcanism erupted during the Pleistocene. This sequence parallels the two main stages proposed for YSRP hotspot calderas: i.e. the eruption of several large-volume, ash-flow tuff sheets followed by caldera collapse, then cessation of major rhyolitic activity and gradual subsidence accompanied by filling and eventual burial of the caldera by basalt lava flows. We employ stratigraphic relationships, paleomagnetism, and major, trace element, and Sr-Nd isotope geochemistry to develop models for the origin of the basaltic and rhyolitic magmas within a geographical and temporal context. The basalts are characterized by distinct groupings based on depth and geochemistry and reflect the dominant compositions observed on the surface, e.g. Snake River olivine tholeiite (SROT) and evolved type (e.g. Craters of the Moon). We also observe contaminated basalts that interacted with rhyolite/granite. The basaltic magma formed by shallow partial melting in the plume channel carved into the lithosphere. The older rhyolites preserve the classical characteristics of A-type granites and display major element and trace element concentrations typical for Eastern SRP caldera centres and minimal stratigraphic variation. Multiple lines of evidence document extensive magmatic differentiation and coupled basalt-rhyolite interactions. We find that the most plausible origin for the rhyolites is via partial melting of a hybrid source, comprising Archean crustal components and younger juvenile mafic intrusions. Assimilation of hydrothermally altered material is also required for some eruptive units. The rhyolites did not evolve from residual magma left over from the climactic Kilgore eruption (4Á0 Ma), but instead represent discrete magma generation events in the course of a few hundred thousand years between 4Á0 to 3Á8 Ma. Beginning at approximately 3.3 Ma, basalts were able to erupt through the solidified composite pluton that formed below the caldera. The transition from rhyolite to basalt is tied to the declining flux of basaltic magma as North America moved away from the Yellowstone hotspot core.
Snake River Plain - Yellowstone silicic volcanism: implications for magma genesis and magma fluxes
Geological Society, London, Special Publications, 2008
The origin of large-volume, high-temperature silicic volcanism associated with onset of the Snake River Plain -Yellowstone (SRPY) hotspot track is addressed based on evolution of the well-characterized Miocene Bruneau-Jarbidge (BJ) eruptive centre. Although O -Sr-Pb isotopic and bulk compositions of BJ rhyolites exhibit strong crustal affinity, including strong 18 O-depletion, Nd isotopic data preclude wholesale melting of ancient basement rocks and implicate involvement of a juvenile component -possibly derived from contemporaneous basaltic magmas. Several lines of evidence, including limits on 18 O-depletion of the rhyolite source rocks due to influx of meteoric/hydrothermal fluids, constrain rhyolite generation to depths shallower than mid-upper crust (,20 km depth). For crustal melting driven by basaltic intrusions, sustenance of temperatures exceeding 900 8C at such depths over the life of the BJ eruptive centre requires incremental intrusion of approximately 16 km of basalt into the crust. This minimum basaltic flux (c. 4 mm year 21 ) is about one-tenth that at Kilauea. Nevertheless, emplacement of such volumes of magma in the crust creates a serious room problem, requiring that the crust must undergo significant extensional deformation -seemingly exceeding present estimates of extensional strain for the SRPY province.
Contributions to Mineralogy and Petrology, 2013
The nature of compositional heterogeneity within large silicic magma bodies has important implications for how silicic reservoirs are assembled and evolve through time. We examine compositional heterogeneity in the youngest (~170 to 70 ka) post-caldera volcanism at Yellowstone caldera, the Central Plateau Member (CPM) rhyolites, as a case study. We compare 238 U-230 Th age, trace-element, and Hf isotopic data from zircons, and major-element, Ba, and Pb isotopic data from sanidines hosted in two CPM rhyolites Hayden Valley and Solfatara Plateau flows) and one extracaldera rhyolite (Gibbon River flow), all of which erupted near the caldera margin ca. 100 ka. The Hayden Valley flow hosts two zircon populations and one sanidine population that are consistent with residence in the CPM reservoir. The Gibbon River flow hosts one zircon population that is compositionally distinct from Hayden Valley flow zircons. The Solfatara Plateau flow contains multiple sanidine populations and all three zircon populations found in the Hayden Valley and Gibbon River flows, demonstrating that the Solfatara Plateau flow formed by mixing extracaldera magma with the margin of the CPM reservoir. This process highlights the dynamic nature of magmatic interactions at the margins of large silicic reservoirs. More
Geological Society of America Bulletin, 2003
We present an oxygen isotope and petrologic study of four voluminous, zoned ash-flow sheets of the Southwestern Nevada Volcanic Field (SWNVF): Topopah Spring (TS, Ͼ1200 km 3 , 12.8 Ma), Tiva Canyon (TC, 1000 km 3 , 12.7 Ma), Rainier Mesa (RM, 1200 km 3 , 11.6 Ma), and Ammonia Tanks (AT, 900 km 3 , 11.45 Ma). The ␦ 18 O values of quartz, sanidine, sphene, magnetite, and zircons in rhyolites and latites of each tuff were measured and used to estimate ␦ 18 O(melt) at 700-900 ؇C. Temperatures were determined by ⌬ 18 O(quartzmagnetite) and Fe-Ti thermometers. Each tuff is characterized by a distinct range of ␦ 18 O(melt): 8.0-9.0‰ (TS), 7.1-7.8‰ (TC), 7.4-8.6‰ (RM), and 5.4-6.0‰ (AT), with higher ␦ 18 O values for rhyolites in each unit. The distinct ␦ 18 O of rhyolitic versus latitic portions of each tuff suggests that they can not be related by in situ fractionation and assimilation in a single zoned magma chamber. It is more likely that latite and rhyolite represent two magmas that were juxtaposed prior to eruption. Low-␦ 18 O AT and normal-␦ 18 O TC tuffs were erupted from the same nested caldera complex only 100-150 k.y. after eruption of voluminous high-␦ 18 O TS and RM magmas, respectively. These short time intervals, distinct ␦ 18 O, 87 Sr/ 86 Sr i , and Nd of each tuff, the same loci of their eruption, and energyconstrained assimilation modeling suggest that TS, TC, RM, and AT represent independent magma batches that were rapidly generated, fractionated, and erupted from shallow, sheet-like magma chambers. Such geometry is a result of extensional tectonics
Bulletin of Volcanology, 2008
A new category of large-scale volcanism, here termed Snake River (SR)-type volcanism, is defined with reference to a distinctive volcanic facies association displayed by Miocene rocks in the central Snake River Plain area of southern Idaho and northern Nevada, USA. The facies association contrasts with those typical of silicic volcanism elsewhere and records unusual, voluminous and particularly environmentally devastating styles of eruption that remain poorly understood. It includes: (1) large-volume, lithic-poor rhyolitic ignimbrites with scarce pumice lapilli; (2) extensive, parallel-laminated, medium to coarse-grained ashfall deposits with large cuspate shards, crystals and a paucity of pumice lapilli; many are fused to black vitrophyre; (3) unusually extensive, large-volume rhyolite lavas; (4) unusually intense welding, rheomorphism, and widespread development of lava-like facies in the ignimbrites; (5) extensive, fines-rich ash deposits with abundant ash aggregates (pellets and accretionary lapilli); (6) the ashfall layers and ignimbrites contain abundant clasts of dense obsidian and vitrophyre; (7) a bimodal association between the rhyolitic rocks and numerous, coalescing low-profile basalt lava shields; and (8) widespread evidence of emplacement in lacustrine-alluvial environments, as revealed by intercalated lake sediments, ignimbrite peperites, rhyolitic and basaltic hyaloclastites, basalt pillow-lava deltas, rhyolitic and basaltic phreatomagmatic tuffs, alluvial sands and palaeosols. Many rhyolitic eruptions were high mass-flux, large volume and explosive (VEI 6–8), and involved H2O-poor, low-δ18O, metaluminous rhyolite magmas with unusually low viscosities, partly due to high magmatic temperatures (900–1,050°C). SR-type volcanism contrasts with silicic volcanism at many other volcanic fields, where the fall deposits are typically Plinian with pumice lapilli, the ignimbrites are low to medium grade (non-welded to eutaxitic) with abundant pumice lapilli or fiamme, and the rhyolite extrusions are small volume silicic domes and coulées. SR-type volcanism seems to have occurred at numerous times in Earth history, because elements of the facies association occur within some other volcanic fields, including Trans-Pecos Texas, Etendeka-Paraná, Lebombo, the English Lake District, the Proterozoic Keewanawan volcanics of Minnesota and the Yardea Dacite of Australia.
The origin of large-volume Yellowstone ignimbrites and smallervolume intra-caldera lavas requires shallow remelting of enormous volumes of variably 18 O-depleted volcanic and sub-volcanic rocks altered by hydrothermal activity. Zircons provide probes of these processes as they preserve older ages and inherited d 18 O values. This study presents a high-resolution, oxygen isotope examination of volcanism at Yellowstone using ion microprobe analysis with an average precision of AE 0Á2ø and a 10 mm spot size. We report 357 analyses of cores and rims of zircons, and isotope profiles of 142 single zircons in 11 units that represent major Yellowstone ignimbrites, and post-caldera lavas. Many zircons from these samples were previously dated in the same spots by sensitive high-resolution ion microprobe (SHRIMP), and all zircons were analyzed for oxygen isotope ratios in bulk as a function of grain size by laser fluorination. We additionally report oxygen isotope analyses of quartz crystals in three units.The results of this work provide the following new observations. (1) Most zircons from post-caldera low-d 18 O lavas are zoned, with higher d 18 O values and highly variable U^Pb ages in the cores that suggest inheritance from pre-caldera rocks exposed on the surface. (2) Many of the higher-d 18 O zircon cores in these lavas have U^Pb zircon crystallization ages that postdate caldera formation, but pre-date the eruption age by 10^20 kyr, and represent inheritance of unexposed post-caldera sub-volcanic units that have d 18 O similar to the Lava Creek Tuff. (3) Young and voluminous 0Á25^0Á1 Ma intra-caldera lavas, which represent the latest volcanic activity atYellowstone, contain zircons with both high-d 18 O and lowd 18 O cores surrounded by an intermediate-d 18 O rim. This implies inheritance of a variety of rocks from high-d 18 O pre-caldera and low-d 18 O post-caldera units, followed by residence in a common intermediate-d 18 O melt prior to eruption. (4) Major ignimbrites of Huckleberry Ridge, and to a lesser extent the Lava Creek and Mesa Falls Tuffs, contain zoned zircons with lower-d 18 O zircon cores, suggesting that melting and zircon inheritance from the lowd 18 O hydrothermally altered carapace was an important process during formation of these large magma bodies prior to caldera collapse. The d 18 O zoning in the majority of zircon core^rim interfaces is step-like rather than smoothly inflected, suggesting that processes of solution^reprecipitation were more important than intracrystalline oxygen diffusion. Concave-downward zircon crystal size distributions support dissolution of the smaller crystals and growth of rims on larger crystals.This study suggests that silicic magmatism at Yellowstone proceeded via rapid, shallow-level remelting of earlier erupted and hydrothermally altered Yellowstone source rocks and that pulses of basaltic magma provided the heat for melting. Each postcaldera Yellowstone lava represents an independent homogenized magma batch that was generated rapidly by remelting of source rocks of various ages and d 18 O values. The commonly held model of a single, large-volume, super-solidus, mushy-state magma chamber that is periodically reactivated and produces rhyolitic offspring is not supported by our data. Rather, the source rocks for theYellowstone volcanism were cooled below the solidus, hydrothermally altered by heated meteoric waters that caused low d 18 O values, and then remelted in distinct pockets by intrusion of basic magmas. Each packet of new melt inherited zircons that retained older age and d 18 O values.This interpretation may have significance for interpreting seismic data for crustal low-velocity zones in which magma mush and solidified areas experiencing hydrothermal circulation occur side by side. New basalt intrusions into this solidifying batholith are required to form the youngest volcanic rocks that erupted as independent rhyolitic magmas. We also suggest that the Lava Creek Tuff
2001
The Yellowstone Plateau volcanic field, the site of some of the largest known silicic volcanic eruptions, is the present location of NE-migrating hotspot volcanic activity. Most volcanic rocks in the Yellowstone caldera (0.6 Ma), which formed in response to the climactic eruption of 1000 km 3 of Lava Creek Tuff (LCT), have unusually low oxygen isotope ratios. Ion microprobe analysis of both U^Pb age and N 18 O in zircons from these low-N 18 O lavas reveals evidence of complex inheritance and remelting. A majority of analyzed zircons from low-N 18 O lavas erupted inside the Yellowstone caldera have cores that range in age from 2.4 to 0.7 Ma, significantly older than their eruption ages (0.5^0.4 Ma). These ages and the high-N 18 O cores indicate that these lavas are largely derived from nearly total remelting of normal-N 18 O Huckleberry Ridge Tuff (HRT) and other pre-LCT volcanic rocks. A post-HRT low-N 18 O lava shows similar inheritance of HRT-age zircons. The recycling of volcanic rocks by shallow remelting can change the water content and eruptive potential of magma. This newly proposed mechanism of intracaldera volcanism is best studied by combining in situ analysis of oxygen and U^Pb isotope ratios of individual crystals. ß