NEW GEOCHRONOLOGICAL CONSTRAINTS ON THE TECTONIC EVOLUTION OF THE HAMERSLEY AND ASHBURTON PROVINCES IN WESTERN AUSTRALIA BETWEEN 2.4 AND 2.0 GA AND THE TECTONIC SETTING OF IRON-ORE FORMATION

Banded iron formations (BIF) of the 2.77 to 2.41 Ga Mount Bruce Supergroup in the Hamersley Province of Western Australia were locally upgraded to form giant high-grade hematite deposits during the early Paleoproterozoic by a combination of hypogene and supergene processes following the initial rise of atmospheric oxygen. Ore genesis was associated with the stratigraphic break between the Lower and Upper Wyloo Groups and has been linked to the compressional Ophthalmian Orogeny, late-orogenic extension, and post-orogenic continental extension by different workers. Small spot dating of tiny baddeleyite crystals by SHRIMP has resolved the ages of two key suites of mafic intrusions constraining the tectonic evolution of the Province and the setting of ore formation. Sills intruding the Turee Creek Group of the Mount Bruce Supergroup that are deformed by the Ophthalmian Orogeny and cut by the unconformity at the base of the Wyloo Group are ~2210 Ma. A dyke swarm that intrudes the Lower Wyloo Group and has a close genetic relationship to iron ore mineralisation is ~2010 Ma, slightly younger than a syn-eruptive 2031 ± 6 Ma zircon age for the upper part of the Lower Wyloo Group. These new dates constrain the Ophthalmian Orogeny to the period <2.22 to >2.03 Ga, prior to Lower Wyloo Group extension, sedimentation and flood basalt volcanism. The 2.01 Ga dykes provide a new maximum age for iron-ore genesis in the Hamersley Province and deposition of the upper Wyloo Group, clearly linking ore genesis to a 2.1 to 2.0 Ga period of extensional tectonics that is recorded by Paleoproterozoic terranes worldwide well after the initial oxidation of the atmosphere at ~2.32 Ga.