Search

The Yukon-Koyukuk Basin is a large depression that covers ~118,000 km2 in western interior Alaska and is divided into two subbasins by a volcanic arc assemblage. Interpretations of the depositional setting of the northern Kobuk Koyukuk subbasin vary from a syncollisional forearc basin to a postorogenic successor basin formed by lithospheric extension. New results from sandstones and conglomerates collected from the Kobuk Koyukuk subbasin provide evidence for the timing of basin development, insight into the provenance of coarse siliciclastic sediments, and understanding of the nature of Cretaceous paleogeography and paleodrainage of Arctic Alaska. Early sedimentary rocks of the Kobuk Koyukuk subbasin contain abundant mafic to ultramafic volcanic and plutonic lithic fragments and mafic heavy minerals (e.g., spinel, clinopyroxene, and amphibole). They also contain abundant Middle Triassic to early Late Jurassic zircons (240-160 Ma; peak maximum ca. 200 Ma) that yield highly juvenile Hf isotopic compositions. Geochemistry of chromium spinels (Cr# = 0.17-0.86) suggests crystallization in an immature arc setting that likely developed over mid-ocean-ridge basalt-type crust. These early sediments originated from the mafic and ultramafic rocks of the Angayucham terrane, which was once much more extensive. These results suggest that the Angayucham terrane consists of an obducted Middle Triassic to early Late Jurassic oceanic arc complex that was coeval with oceanic- to continental-margin mafic arc magmatism in the Canadian Cordillera. Our generalized stratigraphy, along with U-Pb ages and Lu-Hf isotope analyses of zircons from sedimentary rocks of the Kobuk Koyukuk subbasin, reflects the tectonic and/or erosional unroofing of the adjacent southern Brooks Range and Ruby terrane. U-Pb ages of detrital zircons collected from the stratigraphically lowest mafic- to ultramaficrich strata yield maximum depositional ages (107 Ma) that reflect initial erosion of the structurally highest Angayucham terrane and initiation of basin formation and deposition in the late Early Cretaceous. Continued uplift and erosion exposed structurally deeper metamorphic rocks, as revealed by incorporation of low-grade phyllites and eventually higher-grade metamorphic schistose lithic detritus and intermediate-composition (e.g., biotite) to metamorphic (e.g., chloritoid and xenotime) heavy mineral suites into the basin sediments. Differences in detrital zircon signatures between similar-age strata in the Colville foreland basin to the north of the Brooks Range and the Kobuk Koyukuk subbasin indicate that the sediments within the two basins were derived from two different sources, and the Brooks Range orogen acted as a drainage divide during late Early Cretaceous deposition. [ABSTRACT FROM AUTHOR]

The U-Pb system is a prime geochronometer, mainly due to its occurrence as a pair of isotopically distinct but chemically identical decay systems with 235U decaying to 207Pb and 238U to 206Pb, respectively. In addition, U has suitable half-lives and is hosted in some very convenient minerals, such as the widespread and robust mineral zircon. These twin decay systems, running at different speeds, allow an immediate verification of the validity of their ages, which must be concordant to be considered valid, although under favorable circumstances, discordant data can be extrapolated to the correct age. In detail, the degree of discordance can vary greatly, and discordance can have many different causes. The progress in developments of the U-Pb method is both a history of technical discoveries and advances, as well as a history of a long struggle toward concordance, toward an understanding of the causes of discordance, and toward ways to eliminate it. Despite the enormous progress achieved in this field, the problems of U-Pb discordance have not yet been completely resolved and will be one of the main hurdles to overcome in the future. This paper reviews some of the main stages in the evolution of the method, focusing especially on the style, causes, and implications of U-Pb discordance in modern geochronology. [ABSTRACT FROM AUTHOR]

Zircons from the Black Forest Bed, Petrifled Forest Member, Chinle Formation, in Petrified Forest National Park, yield ages that range from Late Triassic to Late Archean. Grains were analyzed by multigrain TIMS (thermal-ionization mass spectrometry), single-crystal TIMS, and SHRIMP (sensitive, high-resolution ion-microprobe). Multiple-grain analysis yielded a discordia trajectory with a lower intercept of 207 ± 2 Ma, which because of the nature of multiple-grain sampling of a detrital bed, is not considered conclusive. Analysis of 29 detrital-zircon grains by TIMS yielded UPb ages of 2706 ± 6 Ma to 206 ± 6 Ma. Eleven of these ages lie between 211 and 216 ± 6.8 Ma. Our statistical analysis of these grains indicates that the mean of the ages, 213 ± 1.7 Ma, reflects more analytical error than geologic variability in sources of the grains. Grains with ages of ca. 1400 Ma were derived from the widespread plutons of that age exposed throughout the southwestern Cordillera and central United States. Twelve grains analyzed by SHRIMP provide [sup 206]Pb*/[sup 238]U ages from 214 ± 2 Ma to 200 ± 4 Ma. We use these data to infer that cores of inherited material were present in many zircons and that single-crystal analysis provides an accurate estimation of the age of the bed. We further propose that, even if some degree of reworking has occurred, the very strong concentration of ages at ca. 213 Ma provides a maximum age for the Black Forest Bed of 213 ± 1.7 Ma. The actual age of the bed may be closer to 209 Ma. Dating continental successions is very difficult when distinct ash beds are not clearly identified, as is the case in the Chinle Formation. Detrital zircons in the Black Forest Bed, however, are dominated by an acicular morphology with preserved delicate terminations. The shape of these crystals and their inferred environment of deposition in slow-water settings suggest that the crystals were not far removed from their site of deposition in space and likely not far in time. Plinian ash clouds derived from explosive eruptions along the early Mesozoic Cordilleran margin provided the crystals to the Chinle basin, where local conditions insured their preservation. In the case of the Black Forest Bed, the products of one major eruption may dominate the volcanic contribution to the unit. Volcanic detritus in the Chinle Formation was derived from multiple, distinct sources. Coarse pebble- to cobble-size material may have originated in eastern California and/or western Arizona, where Triassic plutons are exposed. Fine-grained detritus, in contrast, was carried in ash clouds that derived from caldera eruptions in east-central California or western Nevada. [ABSTRACT FROM AUTHOR]

Presents an example of how the sensitive high-resolution ion microprobe (SHRIMP) zircon dating method can provide a terrane-specific geochronological fingerprint for a rock and thus help to reveal major tectonic boundaries within orogens. Geologic background of the European continent; SHRIMP dating of the inherited zircons from the moravo-silesian.

Examines crystal residence times using in situ uranium-lead (U-Pb) dating of zircons from the Bishop Tuff in east-central California. Maximum mean crystal residence time for zircon; Analysis of zircons from fallout and intercalated ash-flow deposits and unpolished rims; Storage of Bishop Tuff rhyolite prior to eruption.

Studies the felsic granulites from the Bohemian masdif of Central Europe. Role of zircon in granulite evolution; Geochronometry of zircon; Behavior of crustal rocks under extreme pressure-temperature conditions.

The article focuses on the results analyzed by geological researchers Paquette and Le Pennec on zircon uranium-lead (U-Pb) dating data from the ignimbrites in Central Anatolia, Turkey. It also informs that the researchers detected zircons which reveal the geochronological results for Paleoproterozoic and Archean era for U-Pb ages ranging from 2.3 to 3.8 gigaampere. It mentions that research postulated the existence of an Archean basement terrane in lower parts of Central Anatolia.