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Marvin A. Lanphere

Publications and source records attributed to Marvin A. Lanphere.

At least 55 records · Page 3Linked to original sources
USGS1995

40Ar/39Ar and 18O/16O studies of the Chegem ash-flow caldera and the Eldjurta Granite: Cooling of two late Pliocene igneous bodies in the Greater Caucasus Mountains, Russia

Volcanic and intrusive rocks of the Chegem caldera and the nearby Eldjurta (Eldzhurtinskiy) Granite record a late Pliocene episode of silicic magmatism in the north-central Caucasus Mountains. Surface exposures, created by the recent rapid uplift and erosion of the Caucasus Mountains, span a 2 km vertical section of Chegem caldera fill and 1150 m of the Eldjurta Granite; cored mineral-exploration drillholes in the Eldjurta Granite extend the sampling to a depth of 4 km. The unique sampling range available in these two young igneous bodies affords an excellent opportunity to study their denudation and cooling histories, which we examine by means of 40 Ar/ 39 Ar and 18 O/ 16 O measurements on an extensive sample suite. Total-fusion biotite and sanidine ages from eight Chegem Tuff samples, both intracaldera and outflow, are analytically indistinguishable with a weighted mean of 2.82 ± 0.02 Ma. A cross-cutting granodiorite porphyry intrusion has a sanidine total fusion age of 2.84 ± 0.03 Ma, and whole-rock incremental heating of a post-caldera andesite flow, which caps the caldera fill, yields an age of 2.82 ± 0.02 Ma. Thus, caldera formation and post-caldera resurgence and volcanism all occurred within a very short time (< 50,000 yr). Biotite total-fusion ages of ten Eldjurta Granite samples, including seven from ∼ 500 m intervals in the 4 km deep drillhole, show a systematic linear decrease in age with depth from 1.90 Ma near the roof contact of the granite to 1.56 Ma at a depth of 3700 m. Assuming these ages were set at the same temperature, this age/depth gradient implies an isotherm migration rate of 13 mm/yr between 1.90 and 1.56 Ma. This migration rate is due to a combination of rapid denudation and downward relaxation of isotherms, with cooling rates between 200 and 500°C/Ma during this period. Oxygen isotopic compositions of quartz, K-feldspar, plagioclase and biotite from the drillhole samples below the 800 m depth are fairly uniform and record primary igneous δ 18 O values with little evidence for subsolidus hydrothermal activity. However, in surface outcrop samples and in the shallowest drillhole sample, mineral δ 18 O values have been lowered by up to 3‰ by interaction with an external (meteoric-hydrothermal?) fluid. The primary mineral δ 18 O values of the Eldjurta Granite are distinctly higher than the corresponding phenocryst δ 18 O values in the Chegem volcanic rocks, indicating that the two bodies evolved as separxate magma batches.

Caucasus Mountains, Chegem Caldera↗
USGS1995

Volcanic and structural evolution of Taupo Volcanic Zone, New Zealand: A review

The Taupo Volcanic Zone (TVZ) in the central North Island is the main focus of young volcanism in New Zealand. Andesitic activity started at c. 2 Ma, joined by voluminous rhyolitic (plus minor basaltic and dacitic) activity from c. 1.6 Ma. The TVZ is c. 300 km long (200 km on land) and up to 60 km wide, as defined by vent positions and caldera structural boundaries. The total volume of TVZ volcanic deposits is uncertain because a sub-volcanic basement has not been identified, but present data suggest bulk volumes of 15–20,000 km 3 , and that faulted metasediments form most of the immediate subvolcanic basement. Rhyolite (≥15,000 km 3 bulk volume, typically 70–77% SiO 2 ) is the dominant magma erupted in the TVZ (mostly as calderaforming ignimbrite eruptions), andesite is an order of magnitude less abundant, and basalt and dacite are minor in volume (< 100 km 3 each). The history of the TVZ is here divided into ‘old TVZ’ from 2.0 Ma to 0.34 Ma, and ‘young TVZ’ from 0.34 Ma onwards, separated by the Whakamaru eruptions, which obscured much of the evidence for older activity within the zone. The TVZ shows a pronounced segmentation into northeastern and southwestern andesite-dominated extremities with composite cones and no calderas, and a central 125-km-long rhyolite-dominated segment. Eight rhyolitic caldera centres have so far been identified in the central segment, of which two (Mangakino and Kapenga) are composite features, and more centres will probably be delineated as further data accumulate. These centres account for 34 inferred caldera-forming ignimbrite eruptions, in the c. 1.6-Ma lifetime of the central TVZ. The modern central TVZ is the most frequently active and productive silicic volcanic system on Earth, erupting rhyolite at c. 0.28 m 3 s −1 , and available information suggests this has been so for at least the past 0.34 Ma. The rhyolites show no major compositional changes with time, though the extent of magma chamber zonation may have changed with the incoming of rifting and crustal extension in the past c. 0.9 Ma. Within the central TVZ, non-rhyolitic compositions have been erupted apparently irregularly in time and space; in particular there is no evidence for a geographic separation of basalts from andesites. Between 0.9 and 0.34 Ma, a major episode of uplift affected areas around the TVZ, while at the same time the main focus of activity may have migrated eastwards within the TVZ accompanying rifting along the axis of the zone. The modern TVZ is rifting at rates between 7 and 18 mm a −1 and restoration of the thin (15km) ‘crust’ ( V p ≤ 6.1 km s −1 ) beneath the central TVZ to its pre-rifting thickness (25 km) implies that rifting at such rates may have begun only at c. 0.9 Ma. The TVZ is a rifted arc, but its longitudinally segmented nature, high thermal flux and voluminous rhyolitic volcanism make it unique on Earth.

Taupo Volcanic Field↗
USGS1995

Chronology and dynamics of a large silicic magmatic system: Central Taupo Volcanic Zone, New Zealand

The central Taupo Volcanic Zone in New Zealand is a region of intense Quaternary silicic volcanism accompanying rapid extension of continental crust. At least 34 caldera-forming ignimbrite eruptions have produced a complex sequence of relatively short-lived, nested, and/or overlapping volcanic centers over 1.6 m.y. Silicic volcanism at Taupo is similar to the Yellowstone system in size, longevity, thermal flux, and magma output rate. However, Taupo contrasts with Yellowstone in the exceptionally high frequency, but small size, of caldera-forming eruptions. This contrast reflects the thin, rifted nature of the crust, which precludes the development of long-term magmatic cycles at Taupo.

Bay of Plenty, Taupo Volcanic Zone↗
USGS1992

Map, tables, and summary of fossil and isotopic age data, Mount Hayes Quadrangle, eastern Alaska range, Alaska

This report describes, summarizes, and interprets all known bedrock fossil and isotopic age studies for the Mount Hayes quadrangle, eastern Alaska Range, Alaska. The accompanying map shows the location of all known bedrock fossil and isotopic sample localities in the quadrangle on a generalized geologic base map. These fossil and isotopic age data are obtained from new studies, unpublished data of the U.S. Geological Survey, contributed unpublished data, and published data. This report is one result of a five-year mineral resource assessment of the quadrangle that was done during the summers of 1978 through 1982, with additional topical studiesin 1985 and 1986. This report is one part of a folio on the geological, geochemical, geophysical, and mineral-resource assessment studies of the quadrangle prepared as part of the Alaskan Mineral Resource Assessment Program (AMRAP) of the U.S. Geological Survey.

Alaska↗
USGS1991

K‐Ar ages from the Western Dome Belt and associated rhyolitic lavas in the Maroa‐Taupo area, Taupo Volcanic Zone, New Zealand

The Western Dome Belt is a 32 km long belt of rhyolitic lava domes west of Maroa caldera. New K‐Ar age data suggest that the belt was a significant locus of extrusive volcanism between the voluminous pyroclastic Whakamaru eruption(s), and the onset of caldera formation at Maroa, during which time it marked the western boundary of rhyolitic volcanism in Taupo Volcanic Zone. A smaller, younger cluster of lava flows and domes to the south at Ben Lomond station forms part of a transitional region between the youthful Maroa and Taupo calderas.

Maroa, Taupo↗
USGS1991

The Briggs Creek Amphibolite, Klamath Mountains, Oregon: Its origin and dispersal

The Briggs Creek Amphibolite (BCA) in southwestern Oregon is an allochthonous slice of metamorphosed oceanic crust emplaced during the Nevadan orogenic event. Even though the evidence is not clear, the authors prefer to have the protolith for the BCA formed in a back‐arc basin contemporaneously as part of the Josephine Ophiolite during late Callovian to early Oxfordian. Although major rearrangement of some of the chemical elements took place during metamorphism, it is possible, by use of major and rare earth elements, to suggest that the BCA formed in an oceanic spreading centre rather than an island arc setting. Meta‐morphic mineral assemblages and composition of the hornblendes indicate that metamorphism attained amphibolite facies grade. Widespread occurrence of allochthonous blocks of BCA in Western Oregon indicate that its dispersion following metamorphism can be related to the accretionary history of the continental margin. Westward thrusting of these blocks of BCA over the Chetco Arc during the Nevadan orogenic event provided blocks that slid into the outboard active Early Cretaceous trench. These blocks were then incorporated into the downgoing subduction complex and overprinted by high P/T metamorphic assemblages. Tectonic blocks within the Early Cretaceous Otter Point trench melange (Franciscan Complex equivalent) contain amphibolites correlated with the BCA and overprinted by high P/T mineral assemblages.

Oregon↗
USGS1989

Direct evidence for the origin of low-18O silicic magmas: quenched samples of a magma chamber's partially-fused granitoid walls, Crater Lake, Oregon

Partially fused granitoid blocks were ejected in the climactic eruption of Mount Mazama, which was accompanied by collapse of Crater Lake caldera. Quartz, plagioclase, and glass in the granitoids have much lower δ 18 O values (−3.4 to +4.9‰) than any fresh lavas of Mount Mazama and the surrounding region (+5.8 to +7.0‰). Oxygen isotope fractionation between phases in granitoids is consistent with equilibrium at T ⩾ 900°C following subsolidus exchange with hydrothermal fluids of meteoric origin. Assimilation of ∼ 10–20% of material similar to these granitoids can account for the O and Sr isotopic compositions of lavas and juvenile pyroclasts derived from the climactic magma chamber, many of which have δ 18 O values ∼ 0.5‰ or more lower than comparable lavas of Mount Mazama. The O isotope data provide the only clear evidence for such assimilation because the mineralogy and chemical and radiogenic isotopic compositions of the granitoids (dominantly granodiorite) are similar to those of erupted juvenile magmas. The granitoid blocks from Crater Lake serve as direct evidence for the origin of 18 O depletion in large, shallow silicic magma bodies.

Oregon↗
USGS1987

In search of the Abrams post office, Trinity County

An understanding of earth history depends in part on stratigraphy, a division of geology in which the distinctive features of natural units or formations of layered rocks are studied and described and names are assigned to them. The procedures for describing and naming rock units in a uniform way are incorporated in documents known as stratigraphic codes. The North American Stratigraphic Code (1983) is currently used by most geologists in the United States when formation names are selected. Rock unit names consist of a geographic name, generally taken from a natural feature near the locality where the unit was first described, followed by a descriptive feature, usually the dominant rock type in the unit. Although the procedure for naming a rock unit seems straightforward, stratigraphic nomenclature can lead to confusion when the principles outlined in the stratigraphic code are ignored or incorrectly applied. This paper traces the naming of the Abrams Mica Schist, one of the major units of the northern California Klamath Mountains. It describes how uncertainty about the location of the geographic feature after which the unit was named has led to conflicting terminology. The search revealed some interesting history of the early days of mining in the Coffee Creek region of the Trinity Alps in Trinity County.

California Geology↗
USGS1985

Age of biostratigraphic horizons within the Ordovician and Silurian systems

Three samples that have a bearing on the age of horizons within the Ordovician and Silurian systems, two previously dated by the conventional K-Ar method and one by the 40 Ar/ 39 Ar total-fusion method, have been reanalysed using the 40 Ar/ 39 Ar age-spectrum method. Conventional K-Ar and total-fusion 40 Ar/ 39 Ar ages can always be questioned because of the relative ease with which the K-Ar system can be disturbed, either thermally or chemically (i.e. Dalrymple & Lanphere 1969; Clauer et al. 1982). The 40 Ar/ 39 Ar age-spectrum method has the potential for identifying disturbed K-Ar systems (i.e. Berger 1975; Harrison & McDougall 1980). The authors feel that the age-spectrum data from these samples are significant because the previous results for these samples have been questioned in recently proposed Palaeozoic time-scales because of a possible disturbance of the K-Ar isotopic system (i.e. Gale et al. 1979, 1980; Gale 1982).

Geological Society Memoir↗
USGS1983

87Sr/86Sr ratios for basalt from Loihi Seamount, Hawaii

87 Sr/ 86 Sr ratios of 15 samples of basalt dredged from Loihi Seamount range from 0.70334 to 0.70368. The basalt types range from tholeiite to basanite in composition and can be divided into six groups on the basis of abundances of K 2 O, Na 2 O, Rb and Sr and 87 Sr/ 86 Sr ratio. The isotopic data require that the various basalt types be derived from source regions differing in Sr isotopic composition. The Loihi basalts may be produced by mixing of isotopically distinct sources, but the tholeiites and alkalic basalts from Loihi do not show a well-developed inverse trend between Rb/Sr and 87 Sr/ 86 Sr that is characteristic of the later stages of Hawaiian volcanoes such as Haleakala and Koolau.

Hawaii↗
USGS1983

40Ar 39Ar ages and tectonic setting of ophiolite from the Neyriz area, southeast Zagros Range, Iran

An ophiolite, considered to be an allochthonous fragment of Tethyan oceanic crust and mantle, crops out near Neyriz in the Zagros Range, Iran. 40 Ar 39 Ar ages ranging from 76.8 ± 23.8 Ma to 105 ± 23.3 Ma were measured on hornblende from five samples of plagiogranite and diabase from the ophiolite. The most precise ages are 85.9 ± 3.8 Ma for a diabase and 83.6 ± 8.4 Ma for a plagiogranite. The weighted mean age of hornblende from the five samples is 87.5 ± 7.2 Ma which indicates that the igneous part of the Neyriz ophiolite formed during the early part of the Late Cretaceous. Pargasite from amphibolite below peridotite of the Neyriz ophiolite has a 40 Ar 39 Ar age of 94.9 ± 7.6 Ma. The pargasite age agrees within analytical uncertainty with the ages measured on diabase and plagiogranite. Comparable ages have been measured on igneous rocks from the Samail ophiolite of Oman and on amphibolite below peridotite of the Samail ophiolite.

Neyriz area, southeast Zagros Range↗
USGS1977

Intrusive rocks of the Yakutat-St. Elias area, south-central Alaska

Twenty-three plutons, exposed over a total area of nearly 1200 km 2 , have been studied in the Alaska part of the St. Elias Mountains between long 138° and 141°W. Results of potassium-argon age determinations combined with field relations, petrography, and major- and trace-element chemistry suggest six major intrusive events: (1) late Paleozoic gabbro to quartz diorite intruded Paleozoic metamorphic rocks that are probably equivalent to the Kaskawulsh Group in adjacent areas of Canada, (2) Triassic quartz diorite formed one small pluton in undated metamorphic rocks near Mt. St. Elias, (3) Jurassic tonalite and granite intruded upper Paleozoic(?) and lower Mesozoic(?) metamorphic rocks, (4) Late Cretaceous or Tertiary altered tonalite formed three widely separated plutons in metasedimentary rocks of Jurassic(?) and Cretaceous age in the Yakutat Group, (5) Eocene granodiorite and granite, and (6) late Cenozoic tonalite and granodiorite intruded both the Yakutat Group and upper Paleozoic(?) and lower Mesozoic(?) metamorphic rocks. The Paleozoic, Jurassic, and Cretaceous or Tertiary plutonic suites are restricted to particular geologic terranes, and the Jurassic and Eocene suites correlate with regional plutonic belts present elsewhere in southern Alaska. The distribution of the Tertiary plutons does not require large-scale horizontal displacements along the Fairweather and other major high-angle faults. The available data indicate that the mineral resource potential of the Yakutat-St. Elias area is low for those deposits that are generally related to magmatic processes.

Alaska↗
USGS1976

Age and tectonic significance of volcanic rocks on St. Matthew Island, Bering Sea, Alaska

Reconnaissance investigations of the heretofore little known volcanic assemblage on St Matthew Island provide significant information on the tectonic history of the Bering Sea shelf. St. Matthew Island is made up of approximately 500 m of subaerial calc-alkaline volcanic rocks ranging in composition from high-alumina basalt to rhyolite. Four K-Ar analyses of samples from this volcanic sequence give Late Cretaceous ages of 65-77 m.y., and intercalated carbonaceous tuff layers yield Cretaceous pollen assemblages. Along the northeast coast of St. Matthew Island, the volcanic rocks are intruded by granodiorite that gives an early Tertiary K-Ar age of 61 m.y. Correlations with on-land geology in northeast Siberia and marine geophysical data from the western Bering Sea strongly suggest that St. Matthew Island represents a southeastward extension of the Okhotsk-Chukotsk belt, a Cretaceous and early Tertiary volcanic arc that borders the Pacific margin of Siberia for nearly 3,000 km. The apparent continuation of this volcanic arc along the margin of the Bering shelf at least as far east as St. Matthew Island supports suggestions by Burk and by Scholl and others that in late Mesozoic time the Pacific plate margin coincided with the present Bering shelf margin and did not shift to the Aleutian trench until the end of Cretaceous or the beginning of Tertiary time.

Alaska↗