S U M M A R Y:The time and duration of igneous activity in the separate component areas of the British Tertiary Igneous Province (BTIP) are investigated using radiometric dates that pass internal consistency tests, used in conjunction with palaeomagnetic polarities and the available stratigraphic information.Reliable results are available only for the currently subaerial parts of the Province. It is found that activity occurred within the approximate interval 63-52 Ma, with most activity at about 59 Ma. Later activity was predominantly of acid magmas though basic rocks preponderate in the province as a whole. The types and span of igneous activity in the separate areas followed no common pattern but areas of more complex geology tend to have had a longer span of activity.Magnetic polarities are predominantly reversed, with all the lavas having this polarity. This predominance is probably largely, but not entirely, due to reversed polarity intervals being longer than normal ones at this time, and with much activity occurring in a single reversed interval. The sequence of polarities found within the BTIP cannot be fully reconciled with the polarity timescales of either Harland et at. (1982) or Berggren et al. (1985).
40 Ar- 39 incremental-heating spectra analyses were made of five microgabbro dykes from the Falkland Islands, two of which yielded an acceptable plateau, and a third a close maximum estimate. All three indicate an age of dyke intrusion of about 190 Ma, supporting a single, published K-Ar date. This provides further support for the geological continuity of the Falkland Islands and the eastern Karoo Basin where dyke swarms of similar petrological affinities were intruded at about the same time. The age constrains the previously published palaeoreconstruction of the Falkland Islands, based on palaeomagnetic data, which places the Falklands immediately to the east of southern Africa, to be valid until at least the late Early Jurassic.
Synopsis There have been a number of recent papers on ‘giant dykes’, i.e. dykes exceeding 100 km in length. This paper extends the known length of the Tertiary (Palaeogene) Cleveland Dyke (CD) by about 64 km, giving a total proven length of c. 260 km. The results of a detailed investigation using ground magnetic surveys have discovered 26 additional exposures in southern Scotland, allowing magnetic, petrographic and geochemical measurements and magnetic modelling to be carried out, resulting in one of the most detailed studies of a ‘giant’ dyke. The structure of the CD complex in the Southern Uplands of SW Scotland has been influenced by pre-existing structures, in particular faults and granitic intrusions. It is chemically heterogeneous without systematic trend along its length. This heterogeneity, plus segmentation along strike, numerous offsets, in particular one of 5 km along the Water of Ken, en-échelon structure, narrow width at many localities (2.3–0.8 m) with lack of baking of adjacent country rocks, all argue strongly against lateral intrusion from the Mull intrusive complex, as has been proposed. Instead, aeromagnetic surveys and wide-angle seismic and gravity surveys make it probable that the dyke relates to a regional magma reservoir beneath the Southern Uplands.
Controversy over the use of rectilinear plots for the interpretation of isotopic rock lead studies has prompted the following analysis of discrete two‐ and three‐stage episodic models, which is partly in the nature of a review, but which also introduces some new results. An investigation is made of the restrictive conditions under which these models lead to rectilinear plots in the isochron, lead‐lead, and modified concordia diagrams; the question of the solubility of these models and the possibility of discriminating between them are discussed. It is shown that there are two special three‐stage models that lead to rectilinear plots in all diagrams and that one of these models cannot (from uranium‐lead data alone) be distinguished from the two‐stage model. The alternative special rectilinear three‐stage model, which can be distinguished both from the two‐stage and from other three‐stage models on the basis of uranium‐lead data alone, cannot be fully solved on the basis of these data alone. The necessity of using data other than the uranium‐lead data (such as the ‘age of the earth’ or the duration of the last stage of lead evolution) either to solve or to distinguish the models makes it clear that a fully independent age of the earth cannot be derived from uranium‐lead data alone. A discussion is also given of the solubility of general three‐ and four‐stage models by use of ‘age‐corrected’ isotope ratios to correct back one stage. It is shown that careless application of a particular multistage model without proper investigation of its consistency with the data can lead to false interpretations. Recent controversy over the use of multistage models is discussed; much of the controversy is shown to arise out of a confusion of the models with the particular diagram used to plot the data. In general no one diagram is to be preferred to another, with the exceptions that if µ1 is variable the modified concordia diagram must be used, whereas the discussion of age‐corrected isotope ratios necessitates use of the lead‐lead diagram. A brief discussion is given of the interpretation of oceanic lead data, for which the analytical precision at present may often be insufficient for the proper application of consistency tests; individual sets of oceanic lead therefore are not susceptible of unambiguous interpretation at this time. It is emphasized that this real lack of information cannot be overcome (as has been implied in some papers) by the use of alternative presentations of the data. The new Tera‐Wasserburg diagram is examined, as is the recently introduced idea that the initial 207Pb/206Pb ratio may serve as an important chronometer for early planetary system evolution. Both ideas are shown to contribute no information that cannot be derived from the modified concordia diagram. The application of the Tera‐Wasserburg diagram in the attempt to derive from lunar basalt U‐Pb data an independent age of the moon is shown to rest on precisely the same principles and to be subject to the same restrictions and uncertainties as earl...
This study is the first of a series in which a main concern will be to establish the Neogene absolute motion of the African Plate. A combined palaeomagnetic and K–Ar whole rock age dating study has been made of the Haruj Assuad basaltic volcanic area of central Libya. Seventy volcanic units, largely pahoehoe basaltic flows, were sampled in the north-central Haruj Assuad in an area centred on 27°45′ N, 017°30′E. Precisely defined paleomagnetic directions were obtained for 68 of the volcanic units and a weighted mean pole at 83°N, 171°E (δp = 5°, δm = 9°) obtained. This pole is just significantly different at the 95% level from the geographic pole. The difference between the paleomagnetic and geographic poles is thought to be largely the result of a degree of nonrepresentative sampling of the geomagnetic field. K–Ar radiometric age data indicate that volcanic activity in the Haruj Assuad area continued at least over the interval 6.0 to 0.4 my. The polarities of dated units are in agreement with the predictions of the time polarity scale. The ages of dated flows indicate that most of the flows presently exposed are younger than 2.2 my (i.e.) Upper Pliocene and younger. A scheme for assigning relative ages to groups of flows by degree of surface weathering is now calibrated in part with absolute ages.
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