Single-grain Ages Research Articles

A titanite fission track profile across the southeastern Archæan Kaapvaal Craton and the Mesoproterozoic Natal Metamorphic Province, South Africa: evidence for differential cryptic Meso- to Neoproterozoic tectonism

Titanite fission track data are presented from samples taken at regular intervals along a ∼ 250 km long traverse from the ca 1.1 Ga Natal Metamorphic Province into the Archæan Kaapvaal Craton, southeastern South Africa. The titanite fission track traverse was intended to distinguish any differences in low-temperature (280°C) cooling trajectories between the two provinces and to identify, so far unrecorded, exhumation events. Titanite fission track age data in four groupings were obtained, namely at 1100 ± 200, 670 ± 100, 520 ± 50 and 170 ± 15 Ma. In addition to these main groupings, the older group contains a few single-grain ages of Palæoproterozoic age in the northernmost craton sample, from near Piet Retief. Samples from the craton, and the northernmost sample from the Natal Metamorphic Province, show a concentration of Grenville age ( ca 1100 Ma) single-grain ages. They reveal a cooling event that indicates that the craton margin was affected during the Mesoproterozoic to a larger extent then previously thought. Possibly, the dates reflect the timing of differential uplift of the rigid craton in response to loading by the over-thrusting of the Tugela nappes some 200 km to the south during the Mesoproterozoic collision event of the Natal Metamorphic Province. Alternatively, the data may suggest that the Tugela nappes extended far further north than previously thought, and the dates represent uplift from their unroofing. The age group of 670 ± 100 Ma is interpreted to represent mixed ages, with some titanite grains having predominantly Grenville-age age components and others having mainly Pan-African age components. Generally, the intensity of Pan-African overprint increases to the south and east. The southern craton samples and all samples from the Natal Metamorphic Province show a concentration of Pan-African single-grain ages of ca 520 ± 50 Ma. These dates are considered to represent fairly rapid uplift of the western foreland during the Pan-African collision of East and West Gondwana, which took place far to the east. Finally, samples taken from the northeastern part of the traverse give Jurassic dates, taken to represent thermal resetting during the Karoo volcanic event as a result of the break-up of Gondwana. The titanite fission track data presented thus aim to demonstrate the power of medium- to low-closure temperature geochronometer to reveal and distinguish younger, largely unseen but nevertheless important, tectonic events.

Apatite fission-track evidence of widespread Eocene heating and exhumation in the Yukon-Tanana Upland, interior Alaska

We present an apatite fission-track (AFT) study of five plutonic rocks and seven metamorphic rocks across 310 km of the Yukon–Tanana Upland in east-central Alaska. Samples yielding ~40 Ma AFT ages and mean confined track lengths > 14 µm with low standard deviations cooled rapidly from >120°C to <50°C during a 3–5 Ma period, beginning at about 40 Ma. Data from samples yielding AFT ages >40 Ma suggest partial annealing and, therefore, lower maximum temperatures (~90–105°C). A few samples with single-grain ages of ~20 Ma apparently remained above ~50°C after initial cooling. Although the present geothermal gradient in the western Yukon–Tanana Upland is ~32°C/km, it could have been as high as 45°C/km during a widespread Eocene intraplate magmatic episode. Prior to rapid exhumation, samples with ~40 Ma AFT ages were >3.8–2.7 km deep and samples with >50 Ma AFT ages were >3.3–2.0 km deep. We calculate a 440–320 m/Ma minimum rate for exhumation of all samples during rapid cooling. Our AFT data, and data from rocks north of Fairbanks and from the Eielson deep test hole, indicate up to 3 km of post-40 Ma vertical displacement along known and inferred northeast-trending high-angle faults. The predominance of 40–50 Ma AFT ages throughout the Yukon–Tanana Upland indicates that, prior to the post-40 Ma relative uplift along some northeast-trending faults, rapid regional cooling and exhumation closely followed the Eocene extensional magmatism. We propose that Eocene magmatism and exhumation were somehow related to plate movements that produced regional-scale oroclinal rotation, northward translation of outboard terranes, major dextral strike-slip faulting, and subduction of an oceanic spreading ridge along the southern margin of Alaska.

New ages for the climactic eruptions at Yellowstone: Single-crystal 40Ar/39Ar dating identifies contamination

Ash beds associated with the three climactic Yellowstone ignimbrites form important Quaternary chronostratigraphic markers over much of the continental United States. Previous K-Ar ages determined on crystal concentrates from these ashes varied by as much as 60–100 k.y. Laser-fusion 40Ar/39Ar dating of single sanidine grains from these units reveals a small number of grains with anomalously old ages. Eliminating these from the weighted averages results in highly precise refined ages of 2.003 ± 0.014, 1.293 ± 0.012, and 0.602 ± 0.004 Ma (2σ errors) for the Huckleberry Ridge Tuff, Mesa Falls Tuff, and member B of the Lava Creek Tuff, respectively. Individual single-grain ages that are slightly too old could result from incomplete degassing of xenocrysts in the magma. Electron-microprobe analyses of sanidine splits reveal no obvious xenocrystic compositions, suggesting another possibility—that phenocrysts from the crystallized rind of the magma chamber were re-entrained into the magma prior to eruption. Contamination and natural variation in phenocryst age may create larger uncertainty in bulk-crystal dating of young silicic volcanic rocks than incomplete extraction of Ar from sanidine.

Natural long-term annealing of the zircon fission-track system in Vienna Basin deep borehole samples: constraints upon the partial annealing zone and closure temperature

Fission-track (FT) analysis of zircon from Cretaceous and Tertiary sandstone samples in Vienna Basin boreholes has provided constraints for the thermal stability of the zircon FT system over a geological timescale. Confined track lengths and ages were measured for samples from depths of ∼ 1.5-7.5 km, the deepest with a present environmental temperature of ∼ 200°C. Mean track lengths range from 10.3 to 10.8 μm, indistinguishable within error from the unannealed reference length of 10.7 ± 0.1 μm (± 1 standard error), thus showing no sign of systematic downhole reduction. Central zircon ages are consistently older than stratigraphic ages of parent rocks, with no single-grain ages significantly younger than their respective stratigraphic ages. Such FT age and length evidence strongly suggests that the tracks have not been significantly annealed since sediment deposition. Because the present geothermal regime as well as sample burial depths have been near-constant for at least the past 5 m.y. the lower temperature limit of the zircon FT partial annealing zone is > 200°C for a heating duration of the order of 5–10 m.y. Such evidence from long-term natural annealing is compatible with a zircon FT partial annealing zone cf ∼ 200–350°C derived by the extrapolation of laboratory annealing results based on o̧nfined track length measurements, but cannot alone discriminate between different annealing models. For the zircon FT closure temperature, a perhaps over-simplified concept, these results are consistent with the oft-quoted temperature of ∼ 250°C for cooling rates of ∼ 10–100°C/m.y.

The importance of apatite composition and single-grain ages when interpreting fission track data from plutonic rocks: a case study from the Coast Ranges, British Columbia

Fission track results determined from granitic rocks exposed within the Coast Ranges of British Columbia indicate that significant variations in apatite chemistry may occur in small plutonic rock samples. To test ages determined earlier by Parrish using the population method of fission track dating, new apparent apatite ages were completed using the external detector method so that single-grain ages could be generated. In most cases, the new apatite ages were similar to the earlier ones (within ±2σ). However, in many cases, it was found that individual samples contained significant variations in single-grain apatite ages. Microprobe analyses showed the spread in ages were directly related to variations in apatite chemistry, such that chlorine-rich apatites retained older ages than fluorine-rich grains. Examination of the single-grain age data suggested that the spread in single-grain ages provided information which constrained the thermal history of the sample and that distributions of single-grain ages reflected the temperatures at which they resided during cooling. These results suggest that it is important to generate more than a few single-grain fission track ages when dating apatite from granitic rocks, since variations in the chlorine concentration in apatite grains may exist in kg-sized granitic rock samples. Therefore, it is no longer a valid assumption that each apatite grain separated from a small granite sample will have a similar chlorine concentration, and in future apatite fission track studies of granitic terrains it is important to recognize that the apatite composition might be heterogeneous when interpreting results.

Formula omitted] and [(Mg,Fe)Si-Al 2] compositional control on argon behaviour in high-pressure white micas: A [formula omitted] continuous laser-probe study from the Dora-Maira nappe of the internal western Alps, Italy

High-pressure white micas from a restricted area in the Dora-Maira nappe of the western Alps (Italy) were analyzed by the conventional 40Ar 39Ar bulk-sample step-heating method and by the step-heating and the spot-fusion 40Ar 39Ar continuous laser-probe techniques on single grains. High-Si, Ferich phengites from fine-grained gneisses display homogeneous 35–40 Ma plateau and spot-fusion ages, consistent with the upper Eocene 40Ar 39Ar phengite ages documented over the whole internal western Alps. In contrast, phengite separates and single-grains of high-Si and Mg-rich to intermediate composition selected from metapelitic and Mg-rich mica schist samples yield older discordant apparent ages, even when sampled in the same outcrop as the fine-grained gneisses. These local age variations are interpreted to reflect primarily an increasing Ar retentivity with increasing Mg content in phengites. In metapelite-derived white micas, the isotopic discordance is marked by 1. (1) a characteristic bulk-sample upward-convex spectrum shape, 2. (2) single-grain disturbed age spectra, 3. (3) a systematic radially outward decrease in spot ages across individual mica cleavage planes and 4. (4) strong grain-to-grain spot-age variations in rock sections. 1. (1) In Mg-rich mica schists, the isotopic discordance is observed as a reproducible upward-convex bulk-sample age spectrum with high (140 Ma) intermediate apparent ages, 2. (2) upward-convex single-grain age spectra and 3. (3) irregular intra- and intergrain age gradients. On geochemical grounds, it is argued that the isotopic discordance occurring in the metapelites is mainly controlled by the compositional changes introduced by the [(Mg,Fe)Si-Al 2] celadonitic re-equilibration of the high-pressure phengites during greenschist overprinting. In Mg-rich phengites, this chemical re-equilibration may induce Ar loss below their presumed higher blocking temperature. A model of Ar behaviour in high-pressure white micas is proposed which explains the age variations recorded throughout the different lithologies in terms of a simple interaction between the Mg Fe compositional control on Ar retentivity and the [(Mg,Fe)Si-Al 2] modifications occurring in phengites during progressive overprinting.