Magnetic characteristics of the Hiltaba Suite Granitoids and Volcanics: Late Devonian overprinting and related thermal history of the Gawler Craton

Abstract

The ∼1590 Ma Gawler Range Volcanics (GRV) and 1595–1575 Ma Hiltaba Suite Granitoids (HSG), from the central Gawler Craton of South Australia, can be divided into an oxidised Roxby Downs-type containing hematite and magnetite, and a less oxidised Kokatha-type that commonly contains ilmenite and titanomagnetite. In both types the predominant magnetic phase is pure end-member magnetite, except for a minor amount of titanomagnetite present in the Kokatha-type Black Yardea Dacite. The characteristic remanence observed in the GRV is directed upward to the northeast. A fold test is strongly negative, indicating that this characteristic remanence is a post-folding overprint although the age of folding is uncertain. Rock magnetic properties identify magnetically unstable multidomain magnetite particles. The magnetisation has been blocked as the crust cooled from above multidomain (MD) unblocking temperatures probably during a mid-phase of the Alice Springs Orogeny (ASO). The in situ mean direction, not corrected for tilt, from 69 samples is declination = 22.6°, inclination = −44.9°, α ₉₅ = 6.9° and the corresponding pole position is latitude λ_p = 69.5°S, longitude ϕ_p = 36.1°E, dp = 5.5°, dm = 8.7°. No directions of opposite polarity were found and no consistent remanence was found in either type of the HSG. The pole position from GRV is consistent with other Australian paleopoles from rocks of about 360–370 Ma. Apatite fission track ages (AFTA) for the Gawler Craton have yielded a mean age of 331 ± 30 Ma. It is likely that the remanence and fission tracks have been reset during exhumation/cooling after the ASO. The younger AFTA of ∼330 Ma are consistent with the track retention temperature (100 ± 20°C) being attained after the higher remanence blocking temperature of ∼>200°C. The apatite ages are consistent with thermal modelling of ⁴⁰Ar/³⁹Ar data from K-feldspars from the nearby Mount Painter Province.

Key Words:

• South Australia
• Gawler Range Volcanics
• Hiltaba Suite Granitoids
• Alice Springs Orogeny
• paleomagnetism
• fission track dating

Acknowledgements

We are grateful for discussions with John Parker and Terry Crabb in 1990 when fieldwork was first undertaken as part of AMIRA project 78/P96C. The authors are grateful to George Williams for providing unpublished material on AFTA from the Acraman meltrock and for many discussions. Dr Chris Klootwijk and DrRobert Musgrave are thanked for their critical comments on the MS.