J. A. Miller

Potassium-argon age determinations on some British Tertiary igneous rocks

The timing and duration of volcanism within each igneous province of the North Atlantic region during the Tertiary is required to evaluate possible relationships between this magmatism, plate tectonics and continental drift. Published radioisotopic dates from the British Tertiary volcanic province are listed and briefly reviewed in the context of the data available from other North Atlantic areas. New conventional K–Ar and 40Ar/39Ar age spectrum ages of rocks from Mull, Staffa, Arran, Antrim, the Mountains of Mourne, Carlingford, the Cleveland dyke and the North Welsh dyke swarm are reported. Volcanism began in the British Province around or just before 65–66 m.y. in the latest Cretaceous or earliest Palaeocene. The earliest episode of basalt lava eruption in Northern Ireland may not be represented in the Hebrides. The major plutonic centres are between 60 and 58 m.y. while minor activity continued at various localities until at least 50 m.y. in the Eocene.

The Fort Ternan hominoid site, Kenya: Geology, age, taphonomy and paleoecology

Fort Ternan is of unusual interest, both because it is the oldest, directlydated site containing remains of Ramapithecus and because the well-known Early Miocene sites in the area afford the opportunity to trace hominoid evolution in one area over time. A phonolitic lava flow destroyed the lowland forest that had blanketed western Kenya since the Oligocene at about 15 Ma (date obtained by the 39Ar/40Ar step-heat method). The site itself is an inverted basin developed by erosion of a horst upthrust near the foot of the volcano, Tinderet. At 14 Ma, volcanic activity associated with the developing Western Rift Valley produced tuffs that washed into the basin and weathered into paleosols, in which the fossils are preserved. Mica flakes from the upper paleosol gave a date of 13·9 Ma, so the time span sampled is relatively brief. Evidence of the relative abundance and diversity of species and skeletal elements, the state of preservation of the fossils and their spatial distribution suggests that: (1) most of the fauma was locally derived; (2) only a small percentage of the species were derived from the forests on Tinderet; (3) Ramapithecus shared a common taphonomic history with the open-country animals and probably lived with them or in the ecotone; (4) the appearance of Ramapithecus and other new species may have been related to the ecological shift towards more open country.

Glauconite dating of Palaeocene-Eocene rocks from East Kent and the time-scale of Palaeogene volcanism in the North Atlantic region

Glauconite dates obtained by conventional K-Ar and 40Ar/39Ar age spectrum techniques from Palaeocene horizons in Kent are examined against a Palaeogene K-Ar time-scale derived from the Tertiary igneous rocks of the North Atlantic region. Isotope ratio plots (correlation diagrams) are used to analyse and test the validity of the results. The boundary ages suggested by the dating of North Atlantic igneous rocks are: base Danian –66Ma, base Thanetian ~60Ma, base Lower Sparnacian ~56Ma, base Upper Sparnacian ~54 Ma, base Ypresian ~53 Ma and base Lutetian ~49 Ma (when calculated using Geological Society of London 1964 standard constants). A variety of natural deficiencies may impair the dating of authigenic glauconite grains, some specific to glauconite in addition to those generally applicable in K-Ar dating. These shortcomings prevent direct use of the majority of unconfirmed glauconite dates in the definition of the standard K-Ar time-scale. Nevertheless, accurate and precise glauconite dates can be obtained from ideal glauconite dating samples. It is suggested that ideal glauconite dating samples can be recognised and confirmed as such from the character of the 40Ar/39Ar step heating correlation diagrams obtained from them and that careful experimental procedures are essential if argon loss error in the laboratory is to be avoided.

A new approach to radio-isotopic dating in orogenic belts

Summary The theory of conventional K-Ar dating and the opposed ‘overprinting’ and ‘slow-cooling’ interpretations of radio metric age sequences in orogenic belts are examined briefly. A new K-Ar age-spectrum method is described which enables intermediate ages obtained from partially overprinted rocks and minerals to be unmixed. Under ideal conditions one or both of the component ages can be determined. Applications of the K-Ar age-spectrum method to a number of dating problems in the British Caledonides are reported as examples of the importance of the new method in orogenic studies. In particular, there is a discussion of the geochronology of the Moine and Dalradian Series in Scotland and the Tanygriseau microgranite in North Wales. Finally, there is a discussion of the possible implications of the new data for orogenic theory.

Radiogenic argon and major-element loss from biotite during natural weathering: A geochemical approach to the interpretation of potassium—argon ages of detrital biotite

Abstract Major-element geochemical data from fresh biotites are used to distinguish an “invariant” parameter, Σ, which is the scaled sum of elemental concentrations, 0.7 (Fe 2+ + Mg + Ti + Al + Si) + 0.3K, having the value 35.3 ± 0.5 wt.%. Analyses of biotites from a sub-tropical modern weathering section of the Beni-Toufout Granite (northern Algeria) (K-Ar age 17.5 ± 0.2 Ma) are used to demonstrate that, during weathering, Σ is reduced in a fashion which is linearly correlated with radiogenic Ar loss. By this means, a quantification of the effect of weathering on the K-Ar age system is established, and used to “correct” conventional K-Ar ages of detrital biotites from Middle Triassic sandstones of the Cheshire Basin, U.K. Following “correction”, the biotite ages are in closer accord with K-Ar ages of coexisting detrital muscovite than they were previously. The “corrected” ages support the view that the palaeo-drainage system transporting the micas originated in the Armorican uplands lying to the south, where both the Cadomian and Variscan orogenic belts are represented. No evidence is found to indicate significant “Caledonian” input to the “Keuper” of the Cheshire Basin.

Age of a new carbonatite locality in northern Kenya

THE ages of carbonatites which occur within the Suregei–Asille Volcanics of northern Kenya cannot be obtained at present by directradioisotope dating, but can be derived from the 40Ar/39Ar K–Ar isochron ages of intercalated felsic ignimbrites. The Suregei–Asille Volcanics crop out over an area of some 1,000 km2 to the north-east of Lake Rudolf. They rest unconformably on the Precambrian basement and comprise several volcanic formations. The rock types include flood basalts; Strombolian cinder cones; aa- and pahaoehoe-flows varying in composition from ankaramitic basalt to trachyte; felsic air-fall and ash-flow deposits; lahars; carbonatite volcanics; and a variety of reworked volcanic ashes, volcaniclastic and fluviatile sediments.

Geochronological problems and radioisotopic dating in the Gregory Rift Valley

Geochronology is the science of geological time and has a wider connotation than radioisotopic dating. The two major tools of geochronology: stratigraphical palaeontology and radioisotopic dating are equally fallible and are best used in conjunction. Basic large-scale geological mapping and the erection of local rock-stratigraphies are necessary pre-requisites for the detailed geochronological analysis of an area. In the Gregory Rift Valley these requirements are beginning to be met at such places as Olduvai, Baringo, East Lake Turkana and Omo. Conventional K-Ar age determination of volcanic rocks and minerals has been used widely in the dating of East African Cenozoic successions. It is important that the theoretical and practical limitations of this otherwise very successful technique are fully appreciated. K-Ar and 40Ar39Ar isochron dating are developments of the K-Ar technique that can be used to resolve some of the difficulties inherent in the conventional method. Isochron dating of rocks from Olduvai, the plateau phonolites of Kenya, East Lake Turkana and the Omo illustrates this point. Arguments are presented for rejecting discrepant apparent ages of less than 2.0 m.y. from the KBS Tuff, East Lake Turkana. It is suggested that fission track dating of volcanic zircons will resolve this controversy and provide independent time-scales for many East African and other volcanic successions in future.

Tectonic and Radiometric Age Comparisons

The present-day structure of the North Atlantic Ocean is dominated by the mid-Atlantic ridge and the three major crustal plates which meet at (and probably originate at) this feature. These major crustal plates are the Eastern North Atlantic or Eurasian, the Western Atlantic or American, and the African plates. The oldest dated ocean-floor rocks from the area are of Upper Jurassic (Oxfordian) age and are thought to indicate that the opening of the North Atlantic began about 180 m.y. ago (Early Jurassic, Le Pichon and Fox, 1971). The mid-Atlantic ridge is the site of present-day volcanism: eruptions have occurred within the last five years in the Azores, Iceland, and Jan Mayen. Modern dating research has generally confirmed the predictions made by Wilson (1963) that the ocean floor and oldest volcanic rocks on the Atlantic oceanic islands would be found to increase in age away from the ridge (Funnel and Smith, 1968; JOIDES results; Moorbath et al., 1968; Aumento et al., 1968; Fleisher et al., 1968; and many other authors).