A. Boven

Timing of the Jiufotang Formation (Jehol Group) in Liaoning, northeastern China, and its implications

The timing of the Jiufotang Formation remains speculative despite recent progress in the study of the Jehol Biota. In this paper we contribute to this topic with Ar-40/Ar-39 dating on K-feldspar (sanidine and orthoclase) from tuffs interbedded within the fossil-bearing shales of the Jiufotang Formation, from the upper part of the Jehol Group in Chaoyang, Liaoning, northeastern China. Ar-40/(39) Ar step heating analyses of K-feldspar and the SHRIMP U-Pb zircon data indicate that tuffs at the Shangheshou section erupted at 120.3 +/- 0.7 million years ago. This result confirms an Aptian age for the Jiufotang Formation that was mainly based on biostratigraphic evidence. It also places stringent controls on the age of the fossils from the formation, providing a minimum age (120 Ma) for the four-winged dinosaur, Microraptor, and the seed-eating bird, Jeholornis.

40Ar/39Ar dating of ignimbrite from Inner Mongolia, northeastern China, indicates a post‐Middle Jurassic age for the overlying Daohugou Bed

The Ar-40/Ar-39 step heating analyses of K-feldspar derived from the ignimbrite in Inner Mongolia, China indicates that they were formed at 159.8 +/- 0.8 Ma, which provides a maximum age for the overlying fossil-bearing lacustrine deposits (Daohugou Bed). This result favors a post-Middle Jurassic ( Late Jurassic or younger), rather than the Middle Jurassic age for the Daohugou Bed. Such a result is generally consistent with vertebrate biostratigraphic evidence, providing a maximum known age for the first appearance of several major animal groups such as Cryptobranchidae of Urodela ( salamanders) and Maniraptora ( birds and their closest dinosaurian relatives).

The 40Ar/39Ar dating of the early Jehol Biota from Fengning, Hebei Province, northern China

The bird fossil-bearing deposits at the Jiecaigou section, correlative to the Dabeigou Formation, in Fengning, Hebei Province, northern China, is well known for yielding a fossil assemblage representing the earliest evolutionary stage of the Jehol Biota. The precise age of the fossil-bearing deposits, however, is unknown. The Ar-40/Ar-39 age spectrum obtained on bulk K-feldspars from the tuff layer about 2 m below the bird fossil-bearing layer gave a plateau age of 129.0 +/- 1.3 Ma (2 sigma, full external error) and an isochron age of 132.3 +/- 4.5 Ma (2 sigma, full external error). Seventeen total-fusion Ar-40/Ar-39 ages on K-feldspars from the interbedded tuff about 6 m below the fossil-bearing layer resulted in a weighted mean of 130.7 +/- 1.4 Ma (2 sigma, full external error). These dates suggest an age of similar to 131 Ma for the early Jehol Biota and combined with previous dating indicate that this biota lasted at least from 131 Ma to 120 Ma (Late Hauterivian to Aptian). These dates also represent the earliest absolute age for known enantiornithine birds in the world.

40Ar/39Ar dating of Lujiatun Bed (Jehol Group) in Liaoning, northeastern China

The Lujiatun bed of the Yixian Formation is famous for its extraordinary preservation of three-dimensional fossils and its implication for the most dramatic catastrophic mass mortality event in the Jehol Biota. The precise age of the fossil bearing deposits, however, remains to be established. Ar-40/Ar-39 step heating analyses on bulk K-feldspars from the fossil bearing tuff gave a weighted mean age of 123.2 +/- 1.0 Ma (2 sigma). This date suggests the Lujiatun bed was most likely deposited at about the same time as the Jianshangou bed of the lower Yixian Formation, representing a stage when the dinosaurs displayed the most significant radiation in the Early Cretaceous.

Quaternary perpotassic magmatism in Uganda (Toro-Ankole Volcanic Province): age assessment and significance for magmatic evolution along the East African Rift

Abstract The Toro-Ankole volcanics in the northern sector of the Western Branch of the East African Rift are generally assumed to be of Quaternary age. New field observations and comparative KAr and ArAr analyses on whole rock samples and phenocrysts of perpotassic lavas reveal the presence of excess Ar and led to the conclusion that apparently no volcanics have been erupted prior to 50 ka. The present geochronological data, compared with previous ones for the Virunga and South Kivu volcanic fields, suggest that a Late Quaternary pulse of volcanic activity occured in the three areas, with the emission of perpotassic, mildly potassic or sodic alkaline magmas. The genetic implications of such a synchronous activity with wide compositional variation are briefly discussed with respect to basement age and assumed lithosphere characteristics.

The Malagarazi supergroup of southeast Burundi and its correlative Bukoba supergroup of northwest Tanzania: neo- and mesoproterozoic chronostratigraphic constraints from Ar-Ar ages on mafic intrusive rocks

Abstract Neo- and Mesoproterozoic subtabular sedimentary units with associated mafic volcanic and shallow depth intrusive rocks are exposed in southeastern Burundi and northwestern Tanzania along the border of the Tanzania Craton. This paper presents the first attempts to date these mafic intrusive rocks by the Ar-Ar technique. The 40Ar-39Ar step-wise heating results obtained on mineral separates of five (micro)gabbros provide temporal constraints for the two episodes of magmatism previously considered as Neo- and Mesoproterozoic on the basis of lithostratigraphic arguments. The Ar-Ar results reveal the presence of excess Ar in samples formerly attributed to the Neoproterozoic magmatic episode (intrusive mafic rocks within the Musindozi and Kavumwe Groups in Burundi and their correlatives, the ‘Kabuye-Gagwe amygdaloidal lavas’ exposed in Burundi and Tanzania) and yield a better lower age estimate of 795 ± 7 Ma compared to the formerly reported K-Ar ages. Two distinct Mesoproterozoic magmatic episodes characterised by the emplacement of shallow depth intrusive mafic rocks have been recognised. One yields a range of comparable cooling ages of 1340 ± 9 Ma in the Kavumwe Group (Burundi) and of 1379 ± 10 Ma and 1355 ± 10 Ma in the Bukoba Sandstone Group (Tanzania). The second is recorded in the Nkoma Group (Burundi) with the emplacement of a dyke at 1282 ± 5 Ma. The Mesoproterozoic ages correspond to minimum ages for the deposition of the sedimentary country rocks belonging to these groups and elucidate former uncertainties regarding their chronostratigraphic position. These data confirm a previously proposed Kibaran foreland setting for the Nkoma, Kavumwe and Bukoba Sandstone Groups and establish that the extent of the Neoproterozoic Malagarazi (Burundi) and Bukoba (Tanzania) Supergroups is restricted to a more confined area than previously admitted.

Hornblende 40Ar/39Ar geochronology across terrane boundaries in the Sveconorwegian Province of S. Norway

Abstract Hornblende incremental heating 40 Ar/ 39 Ar data were obtained from augen gneiss and amphibolite of the Sveconorwegian Province of S. Norway. In the Rogaland-Vest Agder and Telemark terranes, four pyroxene-rich samples, located close (≤ 10 km) to the anorthosite-charnockite Rogaland Igneous Complex, define an age group at 916 + 12/ − 14 Ma and six samples distributed in the two terranes yield another group at 871 + 8/ − 10 Ma. The first age group is close to the reported zircon UPb intrusion age of the igneous complex (931 ± 2 Ma) and the regional titanite UPb age (918 ± 2 Ma), whereas the second group overlaps reported regional mineral RbSr ages (895-853 Ma) as well as biotite KAr ages (878-853 Ma). In the first group, the comparatively dry parageneses of low- P thermal metamorphism (M2) associated with the intrusion of the igneous complex are well developed, and hornblende 40 Ar/ 39 Ar ages probably record a drop in temperature shortly after this phase. In other hornblende + biotite-rich samples, with presumably a higher fluid content, the hornblende ages are probably a response to hornblende-fluid interaction during a late Sveconorwegian metamorphic or hydrothermal event. A ca 220 m.y. diachronism in hornblende 40 Ar/ 39 Ar ages is documented between S. Telemark ( ca 870 Ma) and Bamble ( ca 1090 Ma). Differential uplift between these terranes was mostly accommodated by shearing along the Kristiansand-Porsgrunn shear zone. The final stage of extension along this zone occurred after intrusion of the Herefoss post-kinematic granite at 926 ± 8 Ma. On the contrary, the southern part of the Rogaland-Vest Agder and Telemark terranes share a common cooling evolution as mineral ages are similar on both sides of the Mandal-Ustaoset Line the tectonic zone between them. The succession within 20 m.y. of a voluminous pulse of post-tectonic magmatism at 0.93 Ga, a phase of high- T -low- P metamorphism at 0.93-0.92 Ga, and fast cooling at a regional scale ca 0.92 Ga, suggests that the southern parts of Rogaland-Vest Agder and Telemark were affected by an event of post-thickening extension collapse at that time. This event is not recorded in Bamble.

40Ar/39Ar study of plagioclases from the Rogaland anorthosite complex (SW Norway); an attempt to understand argon ages in plutonic plagioclase

Abstract The combined use of stepwise-heating and UV-laser ablation 40 Ar/ 39 Ar techniques with microprobe analyses on plutonic plagioclase allows the investigation of complex argon release and retention. The complexity arises from the multiphase nature of the samples comprising plagioclase, K-feldspar (in antiperthite), secondary “sericite” and various inclusions, in which some of the components also exhibit variable ages. Fragments of plagioclase megacrysts and plagioclase separates from whole rock samples were selected from a well-constrained series of anorthosite–leuconorite intrusions of the Rogaland Igneous Complex (SW Norway) intruded at ca. 930 Ma. All samples exhibit saddle-like staircase-shaped argon-release spectra. Ages rise from around 400–600 to 750–900 Ma, though three samples exhibit excess argon released at high temperatures. The most remarkable aspect of this data is, however, the apparently linear correlation of 40 Ar*/ 39 Ar with Ca/K. The stepwise-heating data reveal late stage cooling ages, considerably younger than the intrusive age and low temperature reheating of the Rogaland Igneous Complex during Caledonian overthrusting. Arrhenius plots for the stepwise heated samples show a linear trend of diffusion data at low temperatures followed by a systematic break around 900–1060°C. Single spot ages obtained using a UV-laser show extreme age variations with heterogeneous excess argon contained in inclusions but no correlation of 40 Ar*/ 39 Ar with Ca/K. This lack of correlation indicates that the two-component mixing model (with two end-members of distinct ages), suggested by the stepwise-heating data, is an artefact resulting from a more complex mechanism of argon release. The most apparent interpretation of this data is that both plagioclase and another potassium-rich minor component, probably K-feldspar antiperthite yield variable but correlated ages. Because the excess argon is found exclusively in samples from marginal leuconorite bodies and never in the core of the megacrysts, it is probably derived from the outgassing of the K-rich country rock.

Applicability of the K/1bAr method to whole-rock samples of acid lava and pumice: Case of the Upper Pleistocene domes and pyroclasts on Kos Island, Aegean Sea, Greece

Abstract Kos pumice which may represent an important marker for the Upper Pleistocene stratigraphy in the Aegean area has been dated at 0.24 ± 0.02 Ma by conventional K/1bAr dating on feldspar concentrates and whole-rock samples. This figure is at variance with the interpretation of the tephrochronological record according to which this tephra deposit would be 0.145 Ma old. This raises the question of a possible excess Ar presenting itself in lithic fragments, phenocrysts and glassy matrix of the pumice. The fact that the same apparent age (within error limits) is obtained in the three cases makes this presence a priori improbable. A large part of the common Ar present in pumices is associated with the vacuoles and liberated upon grinding. Ar purification may be made difficult by the presence of residues of the original volcanic gases or other volatiles, of uncertain origin.

Ultra-violet laser probe measurement of40Ar/39Ar age profile in phlogopite

Ultra-Violet Laser Ablation Microprobe (UVLAMP) extraction technique enables the direct investigation of Ar-Ar age profile in crystals, and yields more information on rates and durations of geological process than conventional single time snapshots. Phlogopite flakes from lamprophyre at Pishan dyke in western Kunlun were dated by using an UV laser (λ = 213 nm) microprobe with spot analyses. The results show good agreement with those from the conventional40Ar/39Ar step heating experiments. This indicates that the Ar isotopes are distributed homogenously in the phlogopite and the UVLAMP can be a powerful tool in the study of thermal history.