Pyroclastic fall deposits of the paired Rotoiti and Earthquake Flat eruptions from the Taupo Volcanic Zone (New Zealand) combine to form a widespread isochronous horizon over much of northern New Zealand and the southwest Pacific. This horizon is important for correlating climatic and environmental changes during the Last Glacial period, but has been the subject of numerous disparate age estimates between 35.1±2.8 and 71±6ka (all errors are 1s.d.), obtained by a variety of techniques. A potassium–argon (K–Ar) age of 64±4ka was previously determined on bracketing lavas at Mayor Island volcano, offshore from the Taupo Volcanic Zone. We present a new, more-precise 40Ar/39Ar age determination on a lava flow on Mayor Island, that shortly post-dates the Rotoiti/Earthquake Flat fall deposits, of 58.5±1.1ka. This value, coupled with existing ages from underlying lavas, yield a new estimate for the age of the combined eruptions of 61.0±1.4ka, which is consistent with U–Th disequilibrium model-age data for zircons from the Rotoiti deposits. Direct 40Ar/39Ar age determinations of plagioclase and biotite from the Rotoiti and Earthquake Flat eruption products yield variable values between 49.6±2.8 and 125.3±10.0ka, with the scatter attributed to low radiogenic Ar yields, and/or alteration, and/or inheritance of xenocrystic material with inherited Ar. Rotoiti/Earthquake Flat fall deposits occur in New Zealand in association with palynological indicators of mild climate, attributed to Marine Isotope Stage (MIS) 3 and thus used to suggest an age that is post-59ka. The natures of the criteria used to define the MIS 4/3 boundary in the Northern and Southern hemispheres, however, imply that the new 61ka age for the Rotoiti/Earthquake Flat eruption deposits will provide the inverse, namely, a more accurate isochronous marker for correlating diverse changes across the MIS 4/3 boundary in the southwest Pacific.

We present new 40Ar/39Ar ages for hornblende, muscovite, and biotite from metamorphic and plutonic rocks from the Yukon–Tanana Upland, Alaska. Integration of our data with published 40Ar/39Ar, kinematic, and metamorphic pressure (P) and temperature (T) data confirms and refines the complex interaction of metamorphism and tectonism proposed for the region. The oldest metamorphic episode(s) postdates Middle Permian magmatism and predates the intrusion of Late Triassic (215–212 Ma) granitoids into the Fortymile River assemblage (Taylor Mountain assemblage of previous papers). In the eastern Eagle quadrangle, rapid and widespread Early Jurassic cooling is indicated by ~188–186 Ma 40Ar/39Ar plateau ages for hornblende from plutons that intrude the Fortymile River assemblage, and for metamorphic minerals from the Fortymile River assemblage and the structurally underlying Nasina assemblage. We interpret these Early Jurassic ages to represent cooling resulting from northwest-directed contraction that emplaced the Fortymile River assemblage onto the Nasina assemblage to the north as well as the Lake George assemblage to the south. This cooling was the final stage of a continuum of subduction-related contraction that produced crustal thickening, intermediate- to high-P metamorphism within both the Fortymile River assemblage and the structurally underlying Lake George assemblage, and Late Triassic and Early Jurassic plutonism in the Fortymile River and Nasina assemblages. Although a few metamorphic samples from the Lake George assemblage yield Jurassic 40Ar/39Ar cooling ages, most yield Early Cretaceous 40Ar/39Ar ages: hornblende ~135–115 Ma, and muscovite and biotite ~110–108 Ma. We interpret the Early Cretaceous metamorphic cooling, in most areas, to have resulted from regional extension and exhumation of the lower plate, previously tectonically thickened during Early Jurassic and older convergence.

The age and paleomagnetism were determined on basalt from 27 lava flows represented in about 1,900 feet of core from corehole ANL-OBS-AQ-014 in the area of the Argonne National Laboratory-West facilities of the Idaho National Engineering and Environmental Laboratory (INEEL).Paleomagnetic study was also made on an additional core from a shallow corehole located a mile east of that facility.Paleomagnetic measurements were made on 462 samples from the two coreholes, which are compared to each other, and to surface outcrop paleomagnetic data.40Ar/39Ar measurements were made on 5 basalt samples over the length of core ANL-OBS-AQ-014, and these samples range in age from 565 ka to 1.75 Ma.The pattern of accumulation suggested by these ages is irregular.Very rapid rates (>3,000'/m.y.) quickly piled up hundreds of feet of lava in different time-separated events, interspersed with eruptive hiatuses, some of which may have lasted 650 k.y.

Mauna Kea lava flows cored in the Hilo hole range in age from <200 ka to about 400 ka based on 40Ar/39Ar incremental heating and K‐Ar analyses of 16 groundmass samples and one coexisting plagioclase. The lavas, all subaerially deposited, include a lower section consisting only of tholeiitic basalts and an upper section of interbedded alkalic, transitional tholeiitic, and tholeiitic basalts. The lower section has yielded predominantly complex, discordant 40Ar/39Ar age spectra that result from mobility of 40Ar and perhaps K, the presence of excess 40Ar, and redistribution of 39Ar by recoil. Comparison of K‐Ar ages with 40Ar/39Ar integrated ages indicates that some of these samples have also lost 39Ar. Nevertheless, two plateau ages of 391 ± 40 and 400 ± 26 ka from deep in the hole, combined with data from the upper section, show that the tholeiitic section accumulated at an average rate of about 7 to 8 m/kyr and has a mean recurrence interval of 0.5 kyr/flow unit. Samples from the upper section yield relatively precise 40Ar/39Ar plateau and isotope correlation ages of 326 ± 23, 241 ± 5, 232 ± 4, and 199 ± 9 ka for depths of −415.7 m to −299.2 m. Within their uncertainty, these ages define a linear relationship with depth, with an average accumulation rate of 0.9 m/kyr and an average recurrence interval of 4.8 kyr/flow unit. The top of the Mauna Kea sequence at −280 m must be older than the plateau age of 132 ± 32 ka, obtained for the basal Mauna Loa flow in the corehole. The upward decrease in lava accumulation rate is a consequence of the decreasing magma supply available to Mauna Kea as it rode the Pacific plate away from its magma source, the Hawaiian mantle plume. The age‐depth relation in the core hole may be used to test and refine models that relate the growth of Mauna Kea to the thermal and compositional structure of the mantle plume.

Mauna Kea lava flows cored in the Hilo hole range in age from <200 ka to about 400 ka based on 4øAr/39Ar incremental heating and K-Ar analyses of 16 groundmass samples and one coexisting plagioclase. The lavas, all subaerially deposited, include a lower section consisting only of tholeiitic basalts and an upper section of interbedded alkalic, transitional tholeiitic, and tholeiitic basalts. The lower section has yielded predominantly complex, discordant 4øAr/39Ar age spectra that result from mobility of 4øAr and perhaps K, the presence of excess 4øAr, and redistribution of 39Ar by recoil. Comparison of K-Ar ages with 4ømr/39Ar integrated ages indicates that some of these samples have also lost 39mr. Nevertheless, two plateau ages of 391 _+ 40 and 400 _+ 26 ka from deep in the hole, combined with data from the upper section, show that the tholeiitic section accumulated at an average rate of about 7 to 8 m/kyr and has an mean recurrence interval of 0.5 kyr/flow unit. Samples from the upper section yield relatively precise 4øAr/39Ar plateau and isotope correlation ages of 326 _+ 23, 241 _+ 5, 232 + 4, and 199 + 9 ka for depths of -415.7 m to -299.2 m. Within their uncertainty, these ages define a linear relationship with depth, with an average accumulation rate of 0.9 m/kyr and an average recurrence interval of 4.8 kyr/flow unit. The top of the Mauna Kea sequence at -280 m must be older than the plateau age of 132 +_ 32 ka, obtained for the basal Mauna Loa flow in the corehole. The upward decrease in lava accumulation rate is a consequence of the decreasing magma supply available to Mauna Kea as it rode the Pacific plate away from its magma source, the Hawaiian mantle plume. The age-depth relation in the core hole may be used to test and refine models that relate the growth of Mauna Kea to the thermal and compositional structure of the mantle plume.

The Ruby terrane is an elongate fragment of continental crustal rocks that is structurally overlain by thrust slices of oceanic crust. Our results from the Kokrines Hills, in the south-central part of the Ruby terrane, demonstrate that the low-angle schistose fabric formed under high-P/low-T conditions, at peak conditions of 10.8-13.2 kbar and 425-550-degrees-C, consistent with the rare occurrence of glaucophane. White mica Ar-40/Ar-39 cooling ages from these blueschists indicate that the metamorphism occurred prior to 144 +/- 1 Ma. The blueschist facies assemblages are partially replaced by greenschist facies assemblages in the eastern Kokrines Hills. In contrast, in the central and western Kokrines Hills, upper amphibolite to lower granulite facies metamorphism associated with extensive late Early Cretaceous plutonism has completely overprinted any evidence of an earlier high-P/T metamorphic history. Deformation accompanying the plutonism produced recumbent isoclinal folds in the plutonic rocks and pelitic gneisses of the wallrock; decompression reactions in the pelitic gneisses suggest that the deformation occurred during exhumation. Thermochronological data bracket the time of intrusion and cooling below 500-degrees-C between 118 +/- 3 and 109 +/- 1 Ma.Our data from the schists of the Ruby terrane support the general assumption of many authors that the Ruby terrane was subducted beneath an oceanic island arc. This tectonic history is similar to that described for other large continental crustal blocks in northern and central Alaska, in the Brooks Range, Seward Peninsula and Yukon-Tanana Upland. The current orientation of the Ruby terrane at an oblique angle to these other crustal blocks and to the Cordilleran trend is due to post-collisional tectonic processes that have greatly modified the original continental margin.

The middle Miocene southwestern Nevada volcanic field (SWNVF) is a classic example of a silicic multicaldera volcanic field in the Great Basin. More than six major calderas formed between >15 and 7.5 Ma. The central SWNVF caldera cluster consists of the overlapping Silent Canyon caldera complex, the Claim Canyon caldera, and the Timber Mountain caldera complex, active from 14 to 11.5 Ma and centered on topographic Timber Mountain. Locations of calderas older than the Claim Canyon caldera source of the Tiva Canyon Tuff are uncertain except where verified by drilling. Younger peralkaline calderas (Black Mountain and Stonewall Mountain) formed northwest of the central SWNVF caldera cluster. We summarize major revisions of the SWNVF stratigraphy that provide for correlation of lava flows and small-volume tuffs with the widespread outflow sheets of the SWNVF. New laser fusion Ar-40/Ar-39 isotopic ages are used to refine and revise the timing of eruptive activity in the SWNVF. The use of high-sensitivity mass spectrometry allowed analysis of submilligram-sized samples with analytical uncertainties of approximately 0.3% (1 sigma), permitting resolution of age differences as small as 0.07 Ma. These results confirm the revised stratigraphic succession and document a pattern of episodic volcanism in the SWNVF. Major caldera episodes (Belted Range, Crater Flat, Paintbrush, Timber Mountain, and Thirsty Canyon Groups) erupted widespread ash-flow sheets within 100-300 k.y. time spans, and pre- and post-caldera lavas erupted within 100-300 k.y. of the associated ash flows. Peak volcanism in the SWNVF occurred during eruption of the Paintbrush and Timber Mountain Groups, when over 4500 km3 of metaluminous magma was erupted in two episodes within 1.35 m.y., separated by a 750 k.y. magmatic gap. Peralkaline and metaluminous magmatism in the SWNVF overlapped in time and space. The peralkaline Tub Spring and Grouse Canyon Tuffs erupted early, and the peralkaline Thirsty Canyon Group tuffs and Stonewall Flat Tuff erupted late in the history of the SWNVF, flanking the central, volumetrically dominant peak of metaluminous volcanism. Magma chemistry transitional between peralkaline and metaluminous magmas is indicated by petrographic and chemical data, particularly in the overlapping Grouse Canyon and Area 20 calderas of the Silent Canyon caldera complex. Volcanism in the SWNVF coincided with the Miocene peak of extensional deformation in adjoining parts of the Great Basin. Although regional extension was concurrent with volcanism, it was at a minimum in the central area of the SWNVF, where synvolcanic faulting was dominated by intracaldera deformation. Significant stratal tilting and paleomagnetically determined dextral shear affected the southwestern margin of the SWNVF between the Paintbrush and Timber Mountain caldera episodes. Larger magnitude detachment faulting in the Bullfrog Hills, southwest of the central SWNVF caldera cluster, followed the climatic Timber Mountain caldera episode. Postvolcanic normal faulting was substantial to the north, east, and south of the central SWNVF caldera cluster, but the central area of peak volcanic activity remained relatively unextended in postvolcanic time. Volcanism and extension in the SWNVF area were broadly concurrent, but in detail they were episodic in time and not coincident in space.

The Saih Hatat tectonic window in NE Oman exposes basement and shelf units that structurally underlie the Semail ophiolite. These units were metamorphosed under high-pressure conditions, as evidenced by the occurrence of lawsonite schists, carpholite-bearing metasediments, blueschists, and eclogites. Conventional K-Ar and $$^{40}Ar/^{39}Ar$$ dating of whole rocks and white mica separates from the structurally highest Region I of Saih Hatat and the lowest-grade blueschists of the northern part of the structurally lowest Saih Hatat Region III yield ages of 72-80 Ma. White micas from the high-grade blueschists and eclogites of Region III, which formed at T >340°C, yielded variably discordant age spectra with weighted mean "plateau" ages of 106-111 Ma. The age data, combined with structural and petrological criteria, suggest that many units exposed in Saih Hatat experienced two high-pressure, low temperature (high P/T) metamorphic events. The first, in the Early Cretaceous, was possibly a result of partial subduction of the continental margin beneath a microcontinental fragment of Gondwanaland. The second high P/T event was a result of the Late Cretaceous emplacement of the Semail ophiolite onto the Oman continental margin and is characterized by lower temperatures.

The Saih Hatat tectonic window in NE Oman exposes basement and shelf units that structurally underlie the Semail ophiolite. These units were metamorphosed under high-pressure conditions, as evidenced by the occurrence of lawsonite schists, carpholite-bearing metasediments, blueschists, and eclogites. Conventional K-Ar and Ar-40/Ar-39 dating of whole rocks and white mica separates from the structurally highest Region I of Saih Hatat and the lowest-grade blueschists of the northern part of the structurally lowest Saih Hatat Region III yield ages of 72-80 Ma. White micas from the high-grade blueschists and eclogites of Region 111, which formed at T > 340-degrees-C, yielded variably discordant age spectra with weighted mean "plateau" ages of 106-111 Ma. The age data, combined with structural and petrological criteria, suggest that many units exposed in Saih Hatat experienced two high-pressure, low temperature (high P/T) metamorphic events. The first, in the Early Cretaceous, was possibly a result of partial subduction of the continental margin beneath a microcontinental fragment of Gondwanaland. The second high P/T event was a result of the Late Cretaceous emplacement of the Semail ophiolite onto the oman continental margin and is characterized by lower temperatures.

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Summary 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 40Ar/39 Ar total-fusion method, have been reanalysed using the 40Ar/39Ar age-spectrum method. Conventional K-Ar and total-fusion 40Ar/39Ar 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 40Ar/39Ar 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).

A system is described that utilizes five separate Faraday-cup collector assemblies, aligned along the focal plane of a mass spectrometer, to collect simultaneous argon ion beams at masses 36–40. Each collector has its own electrometer amplifier and analog-to-digital measuring channel, the outputs of which are processed by a minicomputer that also controls the mass spectrometer. The mass spectrometer utilizes a 90° sector magnetic analyzer with a radius of 23 cm, in which some degree of z-direction focussing is provided for all the ion beams by the fringe field of the magnet.

The middle Tertiary ignimbrites of the Sierra Madre Occidental of western Mexico form the largest continuous rhyolitic province in the world. Minor amounts of intermediate lavas occur interlayered with the ignimbrites, and Sr isotope data on the complete range in rock compositions must be considered when evaluating hypotheses for the origin of the rhyolites. Initial 87Sr/86Sr ratios of andesites, dacites and rhyolites from the Batopilas region of western Chihuahua lie in the range 0.7042–0.7050, showing no systematic variation with rock composition. The data reported here support a crystal fractionation hypothesis for the origin for the voluminous rhyolitic ignimbrites.

SummaryK—Ar age measurements on Esquibel Island, southeastern Alaska yield a minimum age of about 433±3 Ma for the Zone of Monograptus cyphus of the Lower Silurian. An age of approximately 435—437 Ma is estimated for the Ordovician—Silurian boundary.

K/Ar age of 35.8 m.y. for hydrothermal biotite that crystallized contemporaneously with bornite, chalcopyrite, and molybdenite, indication that a short time interval exists between emplacement of host rocks and fracture-controlled mineralization

Ar^(40)-K^(40) and Sr^(87)-R^(87) age measurements in the eastern Mojave Desert indicate two separate early Precambrian events (see table). The older event is approximately 1650 m.y. old and is evidenced by pegmatites and associated metamorphic rocks in the Mountain Pass district. Ages were measured on coarse muscovite and potassium feldspar, MP-1 and MP-2, from a pegmatite which cuts across biotite-bearing gneisses, MP-7 and MP-9. These data confirm the widespread areal extent of this ancient metamorphic terrane. Ages of biotite, MP-21 and MP-22, from the shonkinite, which intrudes the metamorphic rocks, at Mountain Pass and the Rb-Sr age of potassium feldspar, MM-3f, from granite in the Marble Mountains suggest a period of igneous intrusion in the 1350 to 1410 m.y. interval. Metamorphic rocks in the central Panamint Range have been mapped and are shown to be stratigraphically early Precambrian. K-Ar ages of approximately 80 m.y. have been measured on biotite, muscovite, and hornblende. The minerals show no memory of a Precambrian age. The early Precambrian rocks show no evidence of a younger period of metamorphism. However, a younger metamorphism can be recognized in the overlying Precambrian(?) Noonday dolomite and Johnnie formation.