The geochronological significance of argon-40/argon-39 age determinations on White Whin from the northern Pennine orefield

Abstract

Controversy regarding the age and character of mineralization in the northern Pennine orefield is reviewed in the context of recent geological and geochronological research in the region, and a number of the unresolved problems are tabulated. A new method of potassium-argon radioisotopic age determination which utilizes the conversion of the natural isotope ³⁹K in a rock or mineral to the isotope ³⁹Ar by neutron irradi­ation is described in outline. This method avoids some of the limitations of conventional potassium-argon age determination, and, among other advantages, makes the measurement of the radioages of clay minerals possible. A geochronological and petrological investigation of White Whin samples from the northern Pennine orefield is reported in full, and eighteen new age determinations, mostly upon clay mineral concentrates are presented. The conclusions that can be drawn from this work support a hypothesis of repeated hydrothermal mineralizations within the area of the northern Pennine orefield: the first within the period 284 ± 40 My with subsequent maxima of hydrothermal activity occurring at intervals. At least two of these subsequent episodes have been identified: one around a minimum age of 230 My and another, very important episode occurring around 170 My. Comparison of these results with histograms of (1) all radio­ages of British rocks and (2) radioages from British dolerites and basalts only, suggests that episodic hydrothermal alteration of pre-existing rocks may be a widespread phenomenon in Britain.