Prefolding and overprint magnetic signatures in Precambrian (∼2.9–2.7 Ga) Igneous rocks from the Pilbara Craton and Hamersley Basin, NW Australia

Abstract

A total of 446 oriented samples have been collected from an Archaean intrusion and the Fortescue Group volcanics and intrusions in the Pilbara and Hamersley regions of northwestern Australia. The paleomagnetism and rock magnetism of these samples have been investigated using susceptibility and hysteresis properties and both alternating field (af) and thermal demagnetization. The most consistent paleomagnetic results were achieved by first applying af demagnetization and then stepwise thermal demagnetization. Evidence of the effects of lightning are widespread which accounts for the efficacy of this particular demagnetization strategy. Application of the fold test has resulted in placing significant constraints on the acquisition ages of some of the magnetizations and their corresponding pole positions. A prefolding pole position from the “Millindinna Complex,” is latitude = 11.9°S, longitude = 161.3°E (dp = 6.8°, dm = 8.4°) and is correlated with a radiometric age of 2.86±0.02 Ga. Folding in the “Millindinna Complex” occurred prior to the deposition of the Fortescue Group. New geochronological data indicate that the extrusion of the flood basalts commenced about 2.8 Ga. This provides a minimum age for the magnetization of the “Millindinna Complex.” Deformation responsible for the formation of the mobile belt separating the Pilbara and Yilgarn blocks during the Ophthalmian Orogeny has affected the Fortescue Group and produced a moderate degree of folding in the southern part of the Hamersley Basin. In the region of “Millindinna Complex” outcrop, the Fortescue Group is generally flat lying. Prefolding pole positions from the Fortescue Group are for the Mount Roe Basalt, latitude = 52.4°S, longitude = 178.0°E (dp = 6.4°, dm = 9.1°) and for the Mount Jope Volcanics, latitude = 40.5°S, longitude = 128.7°E (dp = 19.9°, dm = 20.8°). The overprint magnetization in the Mount Jope Volcanics yields a pole position, latitude = 35.0°S, longitude = 211.5°E (dp = 2.8°, dm = 5.5°) and is interpreted as having been acquired during folding relating to the formation of the Ophthalmian Fold Belt.