Imnaha Basalt Research Articles

Province-Wide Tapping of a Shallow, Variably Depleted, and Metasomatized Mantle to Generate Earliest Flood Basalt Magmas of the Columbia River Basalt, Northwestern USA

The Miocene Columbia River Basalt Group (CRBG) of the Pacific Northwest of the United States is the world’s youngest and smallest large igneous province. Its earliest formations are the Imnaha, Steens, and now the Picture Gorge Basalt (PGB), and they were sourced from three different dike swarms exposed from SE Washington to Nevada to northcentral Oregon. PGB is often viewed to be distinct from the other formations, as its magmas are sourced from a shallow, relatively depleted, and later subduction-induced metasomatized mantle, along with its young stratigraphic position. It has long been known that the lowermost American Bar flows (AB1&2) of the Imnaha Basalt are chemically similar to those of the PGB, yet the Imnaha Basalt is generally thought to carry the strongest plume source component. These opposing aspects motivated us to revisit the compositional relationships between AB1&2 and PGB. Our findings suggest that tapping a shallow, variably depleted, and metasomatized mantle reservoir to produce earliest CRBG lavas occurred across the province, now pinpointed to ~17 Ma. Moreover, compositional provinciality exists indicating regional differences in degree of depletion and subduction overprint that is preserved by regionally distributed lavas, which in turn implies relatively local lava emplacement at this stage.

Columbia River Rhyolites: Age-Distribution Patterns and Their Implications for Arrival, Location, and Dispersion of Continental Flood Basalt Magmas in the Crust

Columbia River province magmatism is now known to include abundant and widespread rhyolite centers even though the view that the earliest rhyolites erupted from the McDermitt Caldera and other nearby volcanic fields along the Oregon–Nevada state border has persisted. Our study covers little-studied or unknown rhyolite occurrences in eastern Oregon that show a much wider distribution of older centers. With our new data on distribution of rhyolite centers and ages along with literature data, we consider rhyolites spanning from 17.5 to 14.5 Ma of eastern Oregon, northern Nevada, and western Idaho to be a direct response to flood basalts of the Columbia River Basalt Group (CRBG) and collectively categorize them as Columbia River Rhyolites. The age distribution patterns of Columbia River Rhyolites have implications for the arrival, location, and dispersion of flood basalt magmas in the crust. We consider the period from 17.5 to 16.4 Ma to be the waxing phase of rhyolite activity and the period from 15.3 to 14.5 Ma to be the waning phase. The largest number of centers was active between 16.3–15.4 Ma. The existence of crustal CRBG magma reservoirs beneath rhyolites seems inevitable, and hence, rhyolites suggest the following. The locations of centers of the waxing phase imply the arrival of CRBG magmas across the distribution area of rhyolites and are thought to correspond to the thermal pulses of arriving Picture Gorge Basalt and Picture-Gorge-Basalt-like magmas of the Imnaha Basalt in the north and to those of Steens Basalt magmas in the south. The earlier main rhyolite activity phase corresponds with Grande Ronde Basalt and evolved Picture Gorge Basalt and Steens Basalt. The later main phase rhyolite activity slightly postdated these basalts but is contemporaneous with icelanditic magmas that evolved from flood basalts. Similarly, centers of the waning phase span the area distribution of earlier phases and are similarly contemporaneous with icelanditic magmas and with other local basalts. These data have a number of implications for long-held notions about flood basalt migration through time and the age-progressive Snake River Plain Yellowstone rhyolite trend. There is no age progression in rhyolite activity from south-to-north, and this places doubt on the postulated south-to-north progression in basalt activity, at least for main-phase CRBG lavas. Furthermore, we suggest that age-progressive rhyolite activity of the Snake River Plain–Yellowstone trend starts at ~12 Ma with activity at the Bruneau Jarbidge center, and early centers along the Oregon–Nevada border, such as McDermitt, belong to the early to main phase rhyolites identified here.

Constraints on the post-orogenic tectonic history along the Salmon River suture zone from low-temperature thermochronology, western Idaho and eastern Oregon

ABSTRACTThe abrupt boundary between accreted terranes and cratonic North America is well exposed along the Salmon River suture zone in western Idaho and eastern Oregon. To constrain the post-suturing deformation of this boundary, we assess the cooling history using zircon and apatite (U–Th)/He thermochronology. Pre-Miocene granitic rocks, along a regional transect, were sampled from accreted terranes of the Blue Mountains Province to cratonic North America (Idaho batholith). Each sample was taken from a known structural position relative to a paleotopographic surface represented by the basal unit of the Miocene Columbia River basalts. An isopach map constructed for the Imnaha Basalt, the basal member of the Columbia River Basalt Group (CRBG), confirms the presence of a Miocene paleocanyon parallel to the northern part of Hells Canyon. The (U–Th)/He zircon dates indicate mostly Cretaceous cooling below 200°C, with the ages getting generally younger from west to east. The (U–Th)/He apatite dates indicate Late Cretaceous–Paleogene cooling, which post-dates tectonism associated with the western Idaho shear zone (WISZ). However, (U–Th)/He apatite dates younger than the Imnaha Basalt, with one date of 3.4 ± 0.6 Ma, occur at the bottom of Hells Canyon. These young (U–Th)/He apatite dates occur along the trend of the Miocene paleocanyon, and cannot be attributed to local exhumation related to faults. We propose that burial of Mesozoic basement rocks by the Columbia River basalts occurred regionally. However, the only samples currently exposed at the Earth’s surface that were thermally reset by this burial were at the bottom of the Miocene paleocanyon. If so, exhumation of these samples must have occurred by river incision in the last 4 million years. Thus, the low-temperature thermochronology data record a combination of Late Cretaceous–Paleogene cooling after deformation along the WISZ that structurally overprinted the suture zone and Neogene cooling associated with rapid river incision.

The three-stage petrochemical evolution of the Steens Basalt (southeast Oregon, USA) compared to large igneous provinces and layered mafic intrusions

The Steens Basalt, southeast Oregon, USA, initiated at 17 Ma as the earliest pulse of the Columbia River Flood Basalt of the northwestern USA. New and existing stratigraphically controlled data reveal temporal changes in lava flow character, and whole-rock and mineral compositions, which we use to evaluate how the balance of magma differentiation processes change in time. Temporal petrochemical variations in the Steens Basalt are analogous to the transition from Imnaha Basalt to Grande Ronde Basalt units of the Columbia River Flood Basalt and have parallels to the temporal evolution of the Deccan and Siberian traps, in India and Russia, respectively, as well as to the stratigraphic sequences of the Bushveld, of South Africa, and Stillwater, in southern Montana, USA, layered mafic intrusions. The excellent stratigraphic control from the Steens Basalt provides a detailed record for comparison across this variety of large mafic systems, providing ability to focus on commonalities among differentiation processes in time.

New 40Ar/ 39Ar dating of the Grande Ronde lavas, Columbia River Basalts, USA: Implications for duration of flood basalt eruption episodes

Grande Ronde Basalt (GRB) lavas represent the most voluminous eruptive pulse of the Columbia River–Snake River–Yellowstone hotspot volcanism. With an estimated eruptive volume of 150,000 km 3, GRB lavas form at least 66% of the total volume of the Columbia River Basalt Group. New 40Ar/ 39Ar dates for GRB lavas reveal they were emplaced within a maximum period of 0.42 ± 0.18 My. A well-documented stratigraphy indicates at least 110 GRB flow fields (or individual eruptions), and on this basis suggests an average inter-eruption hiatus of less than 4000 years. Isotopic age-dating cannot resolve time gaps between GRB eruptions, and it is difficult to otherwise form a picture of the durations of eruptions because of non-uniform weathering in the top of flow fields and a general paucity of sediments between GR lavas. Where sediment has formed on top of GRB lavas, it varies in thickness from zero to 20–30 cm of silty to fine-sandy material, with occasional diatomaceous sediment. Individual GRB eruptions varied considerably in volume but many were greater than 1000 km 3 in size. Most probably eruptive events were not equally spaced in time; some eruptions may have followed short periods of volcanic repose (perhaps 10 2 to 10 3 of years), whilst others could have been considerably longer (many 1000 s to > 10 4 years). Recent improvements in age-dating for other continental flood basalt (CFB) lava sequences have yielded estimates of total eruptive durations of less than 1 My for high-volume pulses of lava production. The GRB appears to be a similar example, where the main pulse occupied a brief period. Even allowing for moderate to long-duration pahoehoe flow field production, the amount of time the system spends in active lava-producing mode is small — less than c. 2.6% (based on eruption durations of approximately 10,000 years, compared to the duration of the entire eruptive pulse of c. 420,000 years). A review of available 40Ar/ 39Ar data for the major voluminous phases of the Columbia River Basalt Group suggests that activity of the Steens Basalt–Imnaha Basalt–GRB may have, at times, been simultaneous, with obvious implications for climatic effects. Resolving intervals between successive eruptions during CFB province construction, and durations of main eruptive pulses, remains vital to determining the environmental impact of these huge eruptions.

A plume-triggered delamination origin for the Columbia River Basalt Group

The Columbia River Basalt Group reveals a complete and detailed stratigraphic succession to assess the interplay of lithospheric and asthenospheric processes. This record of chemical change through time is used to evaluate genetic models for Columbia River Basalt volcanism. We recognize four primary constraints on source melting: (1) a plume component appears to be the dominant source of Imnaha Basalt; (2) Grande Ronde Basalt is best interpreted as being derived from a mafic pyroxenite or eclogite source; (3) the sequence of source melting must correspond with the stratigraphic record; and (4) working models must explain a step-function chemical change at the Imnaha–Grande Ronde stratigraphic boundary. We can envision only three potential models to satisfy these primary constraints: (1) melting of a mantle plume entrained with eclogite, (2) plume interaction with the Juan de Fuca plate, and (3) delamination triggered by plume emplacement. The first two of these are inconsistent with the time-stratigraphic sequence of melting and cannot satisfy all four primary constraints. In contrast, a model of plume-triggered delamination accurately predicts a progressive sequence of melting that satisfies each of the primary constraints. Such a model is consistent with recent numerical experiments demonstrating that delamination is the expected result of plume emplacement beneath thin Mesozoic lithosphere lying adjacent to a thick cratonic boundary. We test this model by comparing the observed history of uplift and tectonism in eastern Oregon and adjacent Washington to that predicted by the numerical models to reveal consistent stress regimes and strikingly similar topographic and structural profiles.

Pb isotopic heterogeneity in basaltic phenocrysts

The Pb isotopic compositions of phenocrystic phases in young basaltic lavas have been investigated using the Getty-DePaolo method (Getty S. J. and DePaolo D. J. [1995] Quaternary geochronology by the U-Th-Pb method. Geochim. Cosmochim. Acta 59, 3267–3272), which allows for the resolution of small isotopic differences. Phenocryst, matrix, and whole rock analyses were made on samples from the 17 Myr-old Imnaha basalts of the Columbia River Group, a zero-age MORB from the Mid-Atlantic Ridge, and a ca. 260 kyr-old tholeiite from Mount Etna. Plagioclase feldspar phenocrysts have low-(U, Th)/Pb, and in each sample the plagioclase has significantly lower 206Pb/ 207Pb and 208Pb/ 207Pb values than whole rock, matrix, and magnetite-rich separates. The Pb isotopic contrast between plagioclase and matrix/whole rock is found in three samples with varying grain sizes (0.5–2 cm for the Imnaha basalt and MORB and <1 mm for the Etna sample) from different tectonic settings, suggesting that these results are not unique. The isotopic contrasts are only slightly smaller in magnitude than the variations exhibited by whole rock samples from the region. The Imnaha basalts also have Sr isotopic heterogeneity evident only in plagioclase phenocrysts, but the MORB and Etna lavas do not. The isotopic heterogeneities reflect magma mixing, and indicate that isotopically diverse magmas were mixed together just prior to eruption. The results reinforce indications from melt inclusion studies that magma source region isotopic heterogeneities have large amplitudes at short length scales, and that the isotopic variations imparted to the magmas are not entirely homogenized during segregation and transport processes.

Ages of the Steens and Columbia River flood basalts and their relationship to extension related calc-alkalic volcanism in eastern Oregon: Discussion

[Hooper et al. (2002)][1] present stratigraphic, chemical, and field relationships for Tertiary volcanic rocks of eastern Oregon and conclude that (1) the Imnaha Basalt are ∼15.5 m.y. old and (2) the Steens Basalt (ca. 16.6 Ma) represents the earliest eruptions of the Columbia River Basalt flood-

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Research Article| January 01, 2004 Reply Peter Hooper Peter Hooper 1Department of Geology, Washington State University, Pullman, Washington 99164, USA Search for other works by this author on: GSW Google Scholar Author and Article Information Peter Hooper 1Department of Geology, Washington State University, Pullman, Washington 99164, USA Publisher: Geological Society of America Accepted: 07 Jul 2003 First Online: 02 Mar 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Geological Society of America GSA Bulletin (2004) 116 (1-2): 249–250. https://doi.org/10.1130/B25310R.1 Article history Accepted: 07 Jul 2003 First Online: 02 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Peter Hooper; Reply. GSA Bulletin 2004;; 116 (1-2): 249–250. doi: https://doi.org/10.1130/B25310R.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Baksi questions the ages of the various units presented in Hooper et al. (2002) and reiterates his belief that Imnaha basalt flows on the main Columbia Plateau are older than the Steens Basalt. In particular he questions two new 40Ar/39Ar ages of Imnaha basalt flows from R.A. Duncan's laboratory at Oregon State University that are based on multiple step plateaus comprising >50% of gas released, concordant with isochron ages. In support of this position he quotes his own 40Ar/39Ar ages (Baksi and Farmer, 1990). In part this is a disagreement between specialized... You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Helium and neon isotopes in the Imnaha Basalt, Columbia River Basalt Group: Evidence for a Yellowstone plume source

Helium, neon, and argon isotopic compositions were measured in two flows of the Columbia River flood basalt. The Imnaha Basalt has a 3He/4He ratio of 11.4 times atmospheric and 20Ne/22Ne and 21Ne/22Ne ratios characteristic of a plume component. The measured 3He/4He is a lower limit, due to possible preferential 3He loss and/or addition of radiogenic 4He. A Wanapum Basalt flow, erupted approximately 2 Ma later in the waning stages of volcanism, has more MORB-like noble gases. The He, Nd and Sr isotopic compositions of these lavas suggest that the Columbia River basalts were derived from the Yellowstone plume head which contained both ‘high-helium’ plume material and entrained depleted mantle. As the eruptions progressed the plume component in the melting region was gradually diluted or replaced.

Lead isotopes and the sources of the Columbia River Basalt Group

A detailed study of 208Pb/204Pb, 207Pb/204Pb, and 206Pb/204Pb ratios suggests that the number of Pb isotopic reservoirs required for the Columbia River Basalt Group (CRBG) (as currently defined) must be increased from the presently accepted four to at least six. The identities of the six reservoirs are two of mid‐ocean ridge basalt (MORB) slightly contaminated with sediment (Rl and R2, Rl being “Cascades” Pb); one a probable plume component (R3); two of unspecified mantle material but probably of Rl type, contaminated with local crustal Pb (R4 and R5); and the sixth a complex enriched reservoir of 2150 Ma age (R6). Rl is an end‐member for all CRBG, except the Saddle Mountains Basalts. R2 is a second end‐member for the Picture Gorge Basalt and appears to be unique to it. Both Rl and R2 are related to the Juan de Fuca subduction zone and the back arc spreading associated with it. R3 is the principal source for most of the Wanapum Basalt, as well as for most of the chemically evolved portions of the Grande Ronde Basalt. Volumetric and tectonic considerations require that one source of the CRBG be a plume or, at least, a nonlocal crustal or uppermost mantle source, and R3 is the ideal candidate for that role. R4 is the second source for the Imnaha Basalt, and R5 is the source for the 206Pb rich varieties of the Grande Ronde Basalt. R4 and R5 are identified with contamination by local Phanerozoic crust because of their similarity to local crustal Pb and because the rocks which contain them also have Cu contents correlated with their Pb isotopic compositions. R6 is the parent for Pb in all the Saddle Mountain Basalts. This last source appears to have been homogeneous 2150 m.y. ago but has since split into a number of separate, discrete pockets, each with its own characteristic Pb isotopic signature. Some of these appear in individual Saddle Mountain Basalt flows, while other sources have mixed with R4 to produce individual Saddle Mountain Basalt flows of the Ice Harbor set. R6 is similar in all its isotopic properties to the Precambrian basement that is exposed to the immediate north of the Columbia River Basalt Group. The overall model for Columbia River Basalt Group genesis is termed “plume reinforced back arc magmatism”.

Crystal Fractionation and Recharge (RFC) in the American Bar Flows of the Imnaha Basalt, Columbia River Basalt Group

Variable isotope and incompatible element ratios require multiple sources for the Columbia River Basalt Group (CRBG) as a whole, but smaller, stratigraphically coherent, groups of flows within the CRBG show much smaller variations in these ratios and appear to be derived from a single source. One such group of flows, the American Bar (AB) subgroup of the Imnaha Basalt, is examined in detail. Sixty-nine samples from the six major AB flows have been reanalyzed for 23 major and trace elements in order to further constrain the processes responsible for the variations between their compositions. It is shown that the compositional differences between the AB flows include the compatible behavior of Sr, which requires the participation of plagioclase, and can be accommodated in a model which fractionates a crystal assemblage of plagioclase, clinopyroxene, and orthopyroxene (16:10:4). Apparent differences in the degree of fractionation between major elements (˜30%) and trace elements (˜50%) are mainly explicable in terms of recharge accompanying the crystal fractionation (RFC), but some ambiguity in the enrichment factors for Y, P, and Zr (but not for Nb) remains unexplained and permits the possibility of some crustal assimilation. The fractionation assemblage (plagioclase+augite±orthopyroxene) differs from the observed phenocryst suite (plagioclase+olivine+augite) and, in the absence of critical experimental data, is assumed to represent a higher pressure assemblage stable near the crust/mantle boundary. The physical model which has been developed to explain the AB eruption entails a large magma reservoir in the lower crust immediately above the crust/mantle boundary in which fractionation and recharge accompanied periodic eruption, probably without significant crustal assimilation either within the magma reservoir or during the rapid expulsion of magma to the surface. Chemical, isotopic, and mineralogic differences between coherent flow subgroups (Rock Creek, American Bar, and the lower flows of Grande Ronde Basalt, for example) are abrupt. They include changes in isotope and incompatible element ratios as well as changes in SiO2 and K concentrations which require magma derivation from a different combination of sources.

A Miocene subcontinental plume in the Pacific Northwest: geochemical evidence

Most of the recent discussions on the number and kinds of components which can be distinguished in the Columbia River Basalt Group (CRBG) magmas have used arguments developed from isotopic evidence. In this paper, we consider relative contents of excluded trace elements, interpreted by means of spidergrams and abundance ratios, and compare data from the CRBG and related lavas with those of selected oceanic basalts. Although many recent models for CRBG petrogeneses do not include a plume component, spidergrams for the American Bar flows of the Imnaha Basalt of the CRBG have broad humps at Ba, Th, and Nb (Ta). This feature suggests that an enriched mantle plume component is present in the American Bar magmas, an inference which is weakly confirmed by 87Sr/ 86Sr data. This plume may have been identical to that presently beneath Yellowstone. The inferred hot spot track lies almost directly east-west, and requires a vector of motion of the Pacific Northwest part of the North American plate significantly different from that inferred by Minster and Jordan (1978) [31]. Our proposed vector for the North American plate agrees, however, with that suggested by the Anahim Belt, a volcanic lineament in southern British Columbia that has been proposed as a hot spot track. This proposed vector requires alterations in current models for Pacific and North American plate movement during the Cenozoic.

Imnaha Basalt, Columbia River Basalt Group

Imnaha Basalt, Columbia River Basalt Group

Geochemistry and origin of basalt of the Columbia River Group, Oregon and Washington

Research Article| May 01, 1976 Geochemistry and origin of basalt of the Columbia River Group, Oregon and Washington IAN McDOUGALL IAN McDOUGALL 1U.S. Geological Survey, Menlo Park, California 94025 Search for other works by this author on: GSW Google Scholar GSA Bulletin (1976) 87 (5): 777–792. https://doi.org/10.1130/0016-7606(1976)87<777:GAOOBO>2.0.CO;2 Article history first online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation IAN McDOUGALL; Geochemistry and origin of basalt of the Columbia River Group, Oregon and Washington. GSA Bulletin 1976;; 87 (5): 777–792. doi: https://doi.org/10.1130/0016-7606(1976)87<777:GAOOBO>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract Strontium isotope and major- and trace-element data from the voluminous middle Miocene tholeiitic basalts of the Columbia River Group differentiate the lavas stratigraphically and reflect the origin of the basalt. The Picture Gorge Basalt has relatively low Sr87/Sr86 ratios (0.7035 to 0.7039) and a major- and trace-element composition consistent with an origin from the upper mantle. The Picture Gorge Basalt is clearly distinguished from the Imnaha basalt in northeast Oregon and from the widespread, mainly younger Yakima Basalt. The Yakima Basalt, including lava flows in the Vantage, Grande Ronde, and Imnaha regions, forms a relatively coherent geochemical group with moderate to high initial Sr87/Sr86 ratios (0.7045 to 0.7080), which increase with decreasing age of the lava and show an inverse relation with Sr abundance. Possible explanations include progressive contamination of parental mantle—derived magmas by crustal material or derivation from an inhomogeneous upper mantle.It is suggested that the Yakima Basalt formed during a major readjustment episode related to plate motions in an extensional tectonic regime. This caused diapiric upwelling of upper mantle peridotite with formation of great volumes of olivine tholeiite magma, which underwent extensive olivine fractionation. The Columbia Plateau may be a continental analogue of the marginal seas of the western Pacific Ocean. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Petrology and chemistry of a Columbia River basalt section, Rocky Canyon, west-central Idaho

Field characteristics, petrography, and major-element analyses are given for 17 of the 19 flows of the Columbia River Group present in a canyon in the southeast corner of the Columbia Plateau. The 17 flows are divided into 4 flows of Imnaha Basalt at the base, 12 flows of lower Yakima Basalt above, and 1 flow of middle Yakima Basalt at the top. The 4 Imnaha Basalt members are feldsparphyric with lower SiO 2 (48.0 to 52.0 percent) and lower K 2 O (0.60 to 1.14 percent) than the lower Yakima Basalt. They may be subdivided into the upper two flows of Rock Creek chemical type that have less SiO 2 (48.8 to 50.5 percent) than the other two members. The Rock Creek flow has uniquely high values of Al 2 O 3 and MgO associated with high modal olivine. The lower two members may be distinguished from each other by their TiO 2 contents. The lower Yakima Basalt flows are fine grained and nonporphyritic with an intergranular to intersertal texture, SiO 2 varies from 52.0 to 56.6 percent and K 2 O varies from 1.14 to 2.00 percent. The earlier nine members show an increase in SiO 2 and K 2 O and a decrease in MgO upward; the top three members show a reversal of this trend. The less siliceous flows contain small subhedral olivine crystals or their pseudomorphs, and the more siliceous contain a pigeonitic in addition to an augitic clinopyroxene. Each member has a unique set of properties that is consistent vertically through the thickness of a flow and laterally between two sections 4 km apart and separated by a major fault. The top flow exposed, with phenocrysts of plagioclase and olivine, falls within the Pomona chemical type previously recorded only from upper Yakima Basalt. Flow-by-flow correlation can be made of the lower Yakima Basalt with the Whitebird section 20 km to the south. Thinning and cutting out of flows from north to south suggests that many flows are local in origin. Similar types of Imnaha Basalt occur at Rocky Canyon and Whitebird, but flow-by-flow correlation is less convincing.