40AR/39AR DATING OF A LATE PROTEROZOIC PALEOMAGNETIC POLE FOR THE ARMORICAN MASSIF (FRANCE)

As part of a combined geochronologic and palaeomagnetic study of the Trégor area (Armorican Massif), we have attempted to obtain a reliable age for a Late Proterozoic palaeopole from experiments carried out on the Keralain and Petit Taurel diorites. A heterogeneous population of amphiboles analysed by the 40Ar/39Ar step‐heating method gave a flat saddle‐shaped age spectrum (characteristic of excess argon) with a minimum age of 604.9 ± 1.2 Ma, which represents a maximum estimate of the age of the mineral. The isochron calculation gives an initial 40Ar/39Ar ratio sensibly higher than the atmospheric ratio. This indicates that the isochron age, 583 ± 2 Ma, is a better estimate of the age of the Keralain diorite. Laser single step fusion and step‐heating experiments performed on single grains representative of the different components of this heterogeneous population of amphiboles show that the excess argon affects it in its entirety. The minimum age of 590.8 ± 2.3 Ma of the flat saddle‐shaped age spectrum displayed by the core of a crystal of hornblende is concordant (at the 2s̀ level) with the isochron age. The isotopic closure temperature calculated from the step‐heating results is 556 ± 43 d̀C. The palaeomagnetic study has shown, using a thermal demagnetization procedure, that the samples have a multicomponent magnetization. Two characteristic components (A and B) have been recognized respectively at Keralain and the Petit Taurel. A careful rock magnetic study has established that the magnetic carrier of the primary component A, isolated at Keralain, is magnetite with a magnetic blocking temperature of 575 d̀C. The similarity of this temperature with the calculated isotopic closure temperature allows us to assign an age of c. 583 Ma to the corresponding well‐defined palaeopole ‘DKA', located at 33d̀N, 256d̀E. The demagnetization experiments have also clearly shown that component B, well isolated at the Petit Taurel, is a remagnetization probably related to magmatic events at around 550 Ma. Furthermore, its magnetic behaviour, as observed at Keralain, indicates a possible thermochemical origin. The well‐dated DKA palaeopole allows us to refine the late Proterozoic polar wander path for Armorica and seems to confirm that the Armorican Massif and southern United Kingdom were linked together at that period. Copyright © 1990, Wiley Blackwell. All rights reserved