Precambrian palaeomagnetism in Australia I: Basic dykes and volcanics from the Yilgarn Block

The palaeomagnetism of 54 dykes sampled from the many swarms that intrude the Yilgarn Block of Western Australia shows that their directions of magnetization fall into five distinct groups.Preliminary Rb-Sr age determinations made on samples used in the study however, indicate that for one, and possibly two of those groups, the dykes belong to two widely different ages suggesting that the palaeomagnetic field has been sampled at two widely separated points in time but when its direction was essentially the same. The seven groups of directions thus obtained imply that the dykes belong to at least six, possibly seven periods of intrusion. Relative-age information relating to four of those periods, determined from cross-cutting and baked contact studies, agrees with that obtained radiometrically.The poles and ages of the groups are as follows: Group YA dykes (c. 2500 or c. 1700 m.y.) 21.7S 133.7E (A₉₅ = 17.9°) Group YB dykes (750—700 m.y.) 19.9S 282.0E (A₉₅ = 28.1°) Group YC dykes (1500 m.y., Group YB) 79.7N 2.7E (A₉₅ = 13.0°) Group YD dykes (1700 m.y.) 23.5S 46.1E (A₉₅ = 9.9°) Group YE dykes (c. 2500 m.y.) 28.3S 0.4E (A₉₅ = 31.0°) Group YF dykes (c. 1700 m.y.) 24.7N 101.8E (A₉₅ = 14.0°) Ravensthorpe dykes (2500 ± 100 m.y.) 38.3S 136.2E (A₉₅ = 25.5°)Results from eight samples of the 1390 ± 140 m.y. Morawa Lavas of the Billeranga Hills yield a pole at 42.8N 22.4E (A₉₅ = 14.7°).