Robert J. Fleck

Interpretation of discordant 40Ar/39Ar age-spectra of mesozoic tholeiites from antarctica

Conventional K-Ar ages of tholeiitic basalts of the Ferrar Group in the central Transantarctic Mountains indicate significant loss of radiogenic 40Ar from this unit over much of its outcrop area. Argon loss varies inversely with amount of devitrified matrix in the basalts, which have not been thermally or tectonically disturbed since extrusion. 40Ar/19Ar age-spectra of these tholeiites are generally discordant and indicate significant inhomogeneity in the distribution of radiogenic 40Ar with respect to 39Ar, but are distinctly different from release patterns of thermally disturbed samples. Amounts of argon redistribution vary directly with amounts of devitrification and are reflected in progressive modification of the age spectra. A model of redistribution of radiogenic 40Ar by devitrification of originally glassy matrix is suggested that is consistent with disturbance of the conventional K-Ar systematics as well as the 40Ar/39Ar age-spectra. Samples with substantial redistribution but minor loss of radiogenic argon yield age spectra whose apparent ages decrease from low-temperature to high-temperature steps, similar to those reported for some lunar basalts, breccias, and soils. Modification of all the age spectra is attributed to redistribution of radiogenic 40Ar during progressive devitrification, although 39Ar-recoil effects suggested by Turner and Cadogan (1974) may be a factor in some cases. Where devitrification involves most potassium sites within the basalt, 40Ar/39Ar age-plateaux may be formed that have no geologic significance.

Episodic caldera volcanism in the Miocene southwestern Nevada volcanic field: Revised stratigraphic framework, 40Ar/39Ar geochronology, and implications for magmatism and extension

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 40 Ar/ 39 Ar 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 ∼0.3% (1σ), 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 km 3 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 intra-caldera deformation. Significant stratal tilting and paleomagnetically determined dextral shear affected the southwestern margin of the SWNVF between the Paint-brush and Timber Mountain caldera episodes. Larger magnitude detachment faulting in the Bullfrog Hills, southwest of the central SWNVF caldera cluster, followed the climactic 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 SWNVF area were broadly concurrent, but in detail they were episodic in time and not coincident in space

Fallout tuffs of Trapper Creek, Idaho—A record of Miocene explosive volcanism in the Snake River Plain volcanic province

A 900-m-thick section of tuffaceous sedimentary rock, vitric fallout tuff, and ash-flow tuff is well exposed along Trapper Creek in south-central Idaho. This section provides nearly continuous exposure through the fill of the Goose Creek basin, a major north-trending Miocene extensional basin located along the southern margin of the Snake River Plain volcanic province (SRPVP). Some 51 separate units of vitric fallout tuff are recognized in the Trapper Creek section. Petrographic and chemical characteristics of these vitric tuffs indicate that most are from SRPVP sources. New 40Ar/39Ar laser-fusion dating, along with prior isotopic age determinations, show that the Trapper Creek tuffs span the period ca. 13.9 – 8.6 Ma. Chemical correlation indicates that fallout tuffs in the central part of the Trapper Creek section (12.5 – 10.0 Ma) are from sources in the Bruneau-Jarbidge volcanic field of the SRPVP centered ≈100 km west of Trapper Creek. Underlying fallout tuffs may have had sources in the Owyhee-Humboldt field of the SRPVP centered ≈200 km west of Trapper Creek, while overlying fallout tuffs, interlayered with several ash-flow tuffs, had a relatively proximal source, possibly in the proposed Twin Falls volcanic field centered ≈60 km north of Trapper Creek. The Trapper Creek tuffs provide insight into the characteristics of explosive silicic volcanism within the SRPVP during middle – late Miocene time. From ca. 13.9 to ca. 9.5 Ma, major eruptions (those depositing ≥1.5 m of fallout tuff) were frequent (about one event per 200 k.y.); their products display a trend toward the eruption of progressively less evolved, higher temperature silicic magma after 12.5 Ma. This trend to higher temperature eruptions, termed the Cougar Point “flare-up,” culminated in the eruption of high-temperature (≈1000°C), plagioclase-rich magma during the period 10.5 – 9.5 Ma. In contrast to these eruptions, later (<7.0 Ma) major silicic eruptions within the SRPVP were characterized by the lower temperature (≈850°C) of the erupted magma and by the longer intervals (about one event per ≈500 – 600 k.y.) between eruptions. Variations in the character of SRPVP explosive silicic eruptions may reflect changes in the structure, composition, or state of stress in the crust beneath the eastward propagating SRPVP, or, perhaps, changes in the Yellowstone hot-spot plume that may drive the SRPVP volcanism.

Strontium and oxygen isotopic variations in Mesozoic and Tertiary plutons of central Idaho

Regional variations in initial 87Sr/86Sr ratios (ri) of Mesozoic plutons in central Idaho locate the edge of Precambrian continental crust at the boundary between the late Paleozoic-Mesozoic accreted terranes and Precambrian sialic crust in western Idaho. The ri values increase abruptly but continuously from less than 0.704 in the accreted terranes to greater than 0.708 across a narrow, 5 to 15 km zone, characterized by elongate, lens-shaped, highly deformed plutons and schistose metasedimentary and metavolcanic units. The chemical and petrologic character of the plutons changes concomitantly from ocean-arc-type, diorite-tonalite-trondhjemite units to a weakly peraluminous, calcic to calcalkalic tonalite-granodiorite-granite suite (the Idaho batholith). Plutons in both suites yield Late Cretaceous ages, but Permian through Early Cretaceous bodies are confined to the accreted terranes and early Tertiary intrusions are restricted to areas underlain by Precambrian crust. The two major terranes were juxtaposed between 75 and 130 m.y. ago, probably between 80 and 95 m.y.Oxygen and strontium isotopic ratios and Rb and Sr concentrations of the plutonic rocks document a significant upper-crustal contribution to the magmas that intrude Precambrian crust. Magmas intruding the arc terranes were derived from the upper mantle/subducted oceanic lithosphere and may have been modified by anatexis of earlier island-arc volcanic and sedimentary units.Plutons near the edge of Precambrian sialic crust represent simple mixtures of the Precambrian wall-rocks with melts derived from the upper mantle or subducted oceanic lithosphere with ri of 0.7035. Rb/Sr varies linearly with ri, producing “pseudoisochrons” with apparent “ages” close to the age of the wall rocks. Measured δ18O values of the wall rocks are less than those required for the assimilated end-member by Sr-O covariation in the plutons, however, indicating that wall-rock δ18O was reduced significantly by exchange with circulating fluids. Metasedimentary rocks of the Belt Supergroup are similarly affected near the batholith, documenting a systematic depletion in 18O as much as 50 km from the margin of the batholith.Plutons of the Bitterroot lobe of the Idaho batholith are remote from the accreted terranes and represent mixtures of Precambrian wall-rocks with melts dominated by continental lower crust (ri>0.708) rather than mantle. “Pseudoisochrons” resulting from these data are actually mixing lines that yield apparent “ages” less than the true age of the wall rocks and meaningless “ri”. Assimilation/ fractional-crystallization models permit only insignificant amounts of crystal fractionation during anatexis and mixing for the majority of plutons of the region.

Implications of the northwestwardly younger age of the volcanic rocks of west-central California

Erosional remnants of volcanic fields in west-central California form a linear northwest-trending belt growing younger in age to the northwest. Major fields within the belt are represented by the Neenach Volcanics, Pinnacles Volcanic Formation, Quien Sabe Volcanics, volcanic rocks in the Berkeley Hills, Tolay Volcanics, Sonoma Volcanics, and Clear Lake Volcanics. Dispersion in the age-distance relation is reduced by restoration of inferred offsets on transecting right-lateral fault systems. The offsets include 115 km on the San Gregorio–Hosgri fault, 314 km on the San Andreas fault, 43 km on the Hayward-Rodgers Creek fault, and 28 km on the Carneros-Franklin-Sunol-Calaveras fault. On the basis of the age and restored position of the volcanic rocks, we judge that the locus of initial active volcanism migrated northwestward ∼3.75 cm/yr from 25 to 12 Ma, and ∼1.35 cm/yr from 12 Ma to the present. The volcanic rocks apparently formed south of the northwardly retreating edge of the subducted part of the Juan de Fuca plate, corroborating one corollary of a published model of an expanding hole in the subducted Farallon-Juan de Fuca-Cocos plate. The present position of the locus of melting at Clear Lake, California, requires substantial overthrusting of the Juan de Fuca plate by the Pacific plate, as was postulated on the basis of foreshortening of magnetic anomalies in the Gorda basin. The change in rate of northwestward migration ∼12 Ma reflects a change in spreading direction of the Juan de Fuca plate vis-a-vis the Pacific plate, previously recognized from changes in orientation of oceanic magnetic anomalies. From the migration rates, it can be inferred that the relative movement between the Pacific plate and the westernmost fringe of the North American plate averaged ∼3.5 cm/yr from 27 m.y. ago to the present.

Geochronology of the Arabian Shield, western Saudi Arabia: K-Ar results

An orogenic event, correlated with the Pan-African event in eastern Africa, affected the Arabian Peninsula between 510 and 610 m.y. ago and is well-recorded geochronologically. The event probably included two thermal pulses or maxima, the first occurring between 560 and 610 m.y. ago and the second between 510 and 540 m.y. ago. The earlier pulse, the more severe one, included the majority of the igneous activity and metamorphism. During the last part of the 510- to 610-m.y. period, left-lateral strike-slip faulting occurred along a set of northwest-trending en echelon fracture zones, whose composite displacement may be as large as 240 km. At least one and probably more orogenic events affected the Arabian Peninsula before the Pan-African event, but only minimum ages can be assigned to these, because thermal effects of the 510- to 610-m.y. event have reset K-Ar ages. Major diorite-granite batholiths, however, formed before 760 m.y. ago.

Geology of the Spring Mountains, Nevada

The northwest-trending Spring Mountains, Nevada, contain a 45-mi-wide (75-km) cross section of the eastern part of the North American Cordilleran orogenic belt and geosyncline. This cross section is probably the most southerly exposed section which exhibits structure and stratigraphy “typical” of the eastern part of the Cordillera. Stradgraphically, the transition from Paleozoic craton to miogeosyncline is present from east to west across the Spring Mountains. The sedimentary succession through the middle Permian thickens from 8,800 ft (2,660 m) east of the Spring Mountains to approximately 30,000 ft (9,000 m) in the west. Thickening of individual formations accounts for 6,800 ft (2,070 m) of added section, addition of formations at unconformities accounts for 4,600 ft (1,400 m) of added section, and addition of a thick terrigenous late Precambrian sequence accounts for 9,800 ft (3,000 m) of added section. Three major thrust plates are exposed in the Spring Mountains, each structurally higher plate containing a thicker sequence of Paleozoic rocks. The easternmost thrust is the Keystone thrust, except where the earlier Red Spring thrust plate is present below the Keystone as isolated remnants. The Keystone thrust appears to be a decollement thrust, but complications at depth suggest that additional thrust slices may be present below the thrust or several thousand feet of late Precambrian terrigenous rocks may be present above the thrust. The structurally higher Lee Canyon thrust plate probably contains at least 4,000 ft (1,200 m) of these terrigenous rocks at its base, and the Wheeler Pass thrust plate contains at least 11,000 ft (3,300 m) of these rocks. Pregeosynclinal basement could be involved in some of the higher thrust plates, particularly the Wheeler Pass plate, but depths of exposure are inadequate to determine its role. Thrust faulting has produced a shortening of from 22 to 45 mi (36.6 to 75 km) in the geosynclinal rocks based on geometric constructions of cross sections at depth. This range probably represents a minimum figure. Some folding and thrusting occurred during the early Late Cretaceous, but data within the Spring Mountains only establish a much wider time bracket, post–Early Jurassic to pre–late Cenozoic for the easternmost thrust faults and post–Early Permian to pre–late Cenozoic for the westernmost thrusts.

Age of structural differentiation between the Colorado Plateaus and Basin and Range provinces in southwestern Utah

Distribution of Tertiary regional ash-flow tuffs indicates that the Colorado Plateaus and Basin and Range provinces in southwestern Utah became separate structural terrains some time after 29 m.y. ago; the structural differentiation probably was underway by 26 m.y. ago and had produced significant topographic contrasts between the two provinces by 24 m.y. ago. Initial differentiation may be older than the main phase of basin-range faulting in southwestern Utah, which appears to have begun about 21 to 20 m.y. ago.

An evaluation of the zircon method of isotopic dating in the Southern Arabian Craton

A zircon study has been made on eleven samples of igneous rocks from the Saudi Arabian Craton. Ages of sized and magnetic fractions of zircon concentrates show variable degrees of discordance which seem to result from a very young disturbance that produces linear arrays in the Concordia plot. Model age calculations based on a statistically and geologically reasonable lower intercept produce very consistent internal relationships. The Pan African Orogeny, considered to be responsible for loss of radiogenic argon and strontium from minerals of many rocks, does not appear to have affected the zircon data, even though uplift had exposed the rocks of the Arabian Shield at that time.Tonalite, granodiorite, and crosscutting leucoadamellite bodies in the southern part of the An Nimas Bathylith yield ages in the time range 820−760 Ma. A narrow time range of 660 to 665 million years was indicated for ages of widely separated and compositionally different intrusive bodies all to the east of the An Nimas Bathylith. This work suggests that the younger end of the age spectrum established from regional K-Ar and Rb-Sr measurements may be underestimated, and that magmatic activity could be more episodic than previously assumed.

Age and character of basaltic rocks of the Yucca Mountain region, southern Nevada

Volcanism in the Yucca Mountain region of southern Nevada in the last 5 m.y. is restricted to moderate-to-small volumes of subalkaline basaltic magmas, produced during at least 6 intervals, and spanning an age range from 4.6 Ma to about 125 ka. Where paleomagnetic evidence is available, the period of volcanism at individual eruptive centers apparently was geologically short-lived, even where multiple eruptions involved different magma types. K-Ar studies are consistent with most other geochronologic information, such as the minimum ages of exposure-dating techniques, and show no evidence of renewed volcanism after a significant quiescence at any of the centers in the Yucca Mountain region. A volcanic recurrence interval of 860 ± 350 kyr is computed from a large K-Ar data set and an evaluation of their uncertainties. Monte Carlo error propagations demonstrate the validity of uncertainties obtained for weighted-mean ages when modified using the goodness of fit parameter, MSWD. Elevated 87Sr/86Sr initial ratios (Sri) in the basalts, nearly constant at 0.707, combined with low SiO2 and Rb/Sr ratios indicate a subcontinental, lithospheric mantle source, previously enriched in radiogenic Sr and depleted in Rb. Beginning with eruptions of the most voluminous eruptive center, the newly dated Pliocene Thirsty Mountain volcano, basaltic magmas have decreased in eruptive volume, plagioclase-phenocryst content, various trace element ratios, and TiO2, while increasing in light rare earth elements, U, Th, P2O5, and light REE/heavy REE ratios. These time-correlated changes are consistent with either increasing depths of melting or a decreasing thermal gradient in the Yucca Mountain region during the last 5 m.y.