GEMOC ARC National Key CentreM.A. Lackie1, P.W. Schmidt2 and D.A. Clark2
1. GEMOC Macquarie, 2. CSIRO Exploration and Mining
Gondwana's early Palaeozoic apparent polar wander path remains ill-defined particularly for the Siluro-Ordovician segment. Rocks of this age in the Lachlan Fold Belt tend to be magnetically overprinted and most do not appear to contain magnetic grains of optimum stability to retain primary remanence directions. To address this problem we have started a study of Siluro-Ordovician volcanics, intrusions, associated hornfels and mineralisation/alteration systems in central-western NSW. Often haematite and/or fine-grained magnetite are alteration products and it is the palaeomagnetic signature of these minerals we seek.
Pilot samples were collected from fresh and altered rocks of the Late Silurian Wondalga Granodiorite, the Late Ordovician Gidginbung Volcanics at Temora and gabbroic diorites and monzodiorites of the Early Silurian Wallundry Suite. As well, samples were collected from the Late Ordovician Tettenhall Monzodiorite and Tallwood Monzonite and their associated hornfels, and basalt's within the Forest Reefs Volcanics. Samples were also taken from oriented exploration core, the samples being from intrusions, hornfels and haematite altered volcanics close to mineralisation systems.
The palaeomagnetic components from the Adelong and Temora alteration systems were unstable. All oriented drillcore sampled from the Adelong system, was overprinted by the initial drilling, with poor palaeomagnetic data obtained from the surface samples from the Adelong system. Encouraging results were obtained from monzonite samples from oriented core from a drill hole from the Orange region but only a few samples were not strongly overprinted by the initial drilling.
Samples from diorite, gabbro and norite intrusions from Wyalong, Fifield and Hylea have been studied by us and students over a number of years and generally yield directions north-east and shallow, reminiscent of (anti-podal) directions found in the Early Ordovician Black Hill Norite, SA. These units are being re-examined to yield more precise palaeomagnetic results. These older results and new results will be combined to give an overall synthesis of Siluro-Ordovician palaeomagnetic results from the Lachlan Fold Belt.