Palaeomagnetism of Proterozoic mafic intrusions and host rocks of the Mount Isa Inlier, Australia: revisited

Abstract

A palaeomagnetic study was carried out on Proterozoic dolerite dykes, plugs and some of their host rocks in the intrusion-rich Mount Isa Inlier for which only one other similar study has been reported (Duff and Embleton, 1976). After rejection of intrusions with unstable remanences and internally inconsistent directions, the results from eleven dykes and five plugs remain. These yield two main palaeomagnetic directions: a direction with both polarities and moderate northerly upward (southerly downward) inclination, obtained from metamorphosed dykes and plugs; and a near-vertical downward direction, obtained from unmetamorphosed dykes. The first appears to be associated with a period of regional metamorphism in the Mount Isa Inlier, estimated at ∼ 1620-1500 Ma, and having the last major metamorphic peak near 1554 Ma. A moderate directional dispersion suggests that this magnetization post-dates periods of major deformation of the region, estimated to have ended at around 1550 Ma. Thus the likely age of the magnetization is 1550-1500 Ma or younger, depending on the rate of cooling. Its pole (IM) plots at 110.6°E, 79.0°S (A95=8.4°). The second direction is concordant with the IA group of Duff and Embleton (1976), which includes results from the unmetamorphosed Lakeview Dolerite dyke dated at 1116 Ma. The combined results from previous and present studies on the unmetamorphosed dykes give a pole (IAR) at 311.1°E, 9.5°N (A95=17.4°). Poles IAR and IM suggest very high and middle palaeolatitudes at around 1100 and 1500 Ma, respectively. The new results agree with the most recent version of the Australian pole path by Idnurm and Giddings (1988), and do not challenge the concept of a single path for the continent.