The earth's tectonosphere

Abstract

The Trégor doleritic dike swarm intrudes the Cadomian North Trégor Batholith (Armorican Massif, France). Alternating field (A.F.) and thermal demagnetization performed on 158 samples from 18 dolerite dikes and 37 samples from 4 baked contact sites reveal a multi-component magnetization and a complex magnetic mineralogy which was thoroughly investigated with thermomagnetic experiments and polished thin-section examinations. Three magnetization components were recognized, in addition to a low-temperature component with a viscous origin. The most frequently observed component, named B (D = 214°, I = 10°), was isolated at 12 sites, independent upon their magnetic mineralogy, and appears to be a remagnetization (negative baked contact test). A Carboniferous age of acquisition is inferred from comparison of the B-component palaeopole (29° S, 318°E) with an Armorican APWP. At some sites, after demagnetization of the B-component, higher-temperature and coercivity components were isolated. Their directions fall into two groups, named A and C. Positive baked contact tests were obtained for both. The C-component (D = 174°, I = 50°), whose magnetic carrier is magnetite, yields a palaeopole (11° S, 2° E) which is similar to some Ordovician palaeopoles from the British Isles, south of the lapetus suture, or Eocambrian palaeopoles from the Trégor area. The A-component (D = 191°, I = −33°), whose magnetic carrier is magnetite, partly transformed to leucoxene, yields a palaeopole (58° S, 336° E) and a palaeolatitude (17.9° N) not previously observed for the Armorican Massif in Palaeozoic time. Comparison between this palaeolatitude and those for northern Spain from Ordovician to Carboniferous time suggests a late Silurian remanence age. This interpretation supports the model of apparent polar wander for Gondwana between Ordovician and Devonian time proposed by Van der Voo, and Bachtadse and Briden. 40Ar/39Ar dating of mineral separates from the dikes do not allow us to place constraints on the acquisition age of the A- and C-components.