STRUCTURAL AND TECTONIC FRAMEWORK OF >4.0 GA DETRITAL ZIRCONS FROM THE JACK HILLS BELT, NARRYER TERRANE, WESTERN AUSTRALIA

The Jack Hills Belt hosts the world’s oldest detrital zircon population, located within the Narryer Terrane and northwestern margin of the Yilgarn Craton. The Narryer Terrane contains Australia’s oldest rocks, which are gneisses up to ~3.7 Ga. So far no potential source rocks for detrital zircons with ages older than this have been located, and stratigraphic relationships and depositional ages of the sedimentary sequence at Jack Hills are problematic. This study provides a structural, temporal, and tectonic framework to allow these problems to be addressed.

The belt is ~100 km long and is hosted by gneisses and granitoids older than ~2.6 Ga. The dominant lithologies are banded iron formation, chert, ultramafics, mafic schists, granitoids, pelitic schist, quartzite and conglomerate, the latter being host to the >4.0 Ga detrital zircons. Much of the sequence occurs as fault lenses on scales up to 10’s of kms in length however in lower strain zones more complete sections are preserved. Detailed fieldwork coupled with satellite imagery and geophysical data interpretation show that the Jack Hills Belt has undergone a complex deformation history, and has been cut by major, predominantly east-west wrench-dominated shearing. The belt as a whole has a pronounced sigmoidal curvature suggestive of dextral kinematics, and the majority of kinematic indicators in the field match this interpretation. This shearing has also affected the host gneisses and granitoids.

Ar-Ar dating of white micas suggests that this shear-related deformation occurred in the Jack Hills during the Capricorn Orogeny (~1830 to 1780 Ma) as the micas record cooling ages mostly around ~1760 to 1740 Ma. The Capricorn Orogeny was a widespread event that recorded the oblique collision of the Yilgarn and Pilbara Cratons. Shearing in the Jack Hills is overprinted by one or more low grade events, possibly related to the intraplate Edmundian Orogeny, loosely constrained between ~1075 and 750 Ma. Thus the Jack Hills records a particularly long geological history spanning at least 3 billion years. This has implications for understanding the source and distribution of ancient detritus, the structural and tectonic evolution of a reworked craton margin, and the evolution of ancient fragments of earth’s crust such as the Narryer Terrane.