The Jack Hills greenstone belt, Western Australia: Part 2: Lithological relationships and implications for the deposition of ≥4.0 Ga detrital zircons

Abstract

The Jack Hills greenstone belt is situated along the southern margin of the Narryer Terrane in the northwest of the Yilgarn Craton, in fault contact with Archean granitic gneiss and granitic rocks. The belt has endured a long deformation history and the effects of recrystallisation, alteration, and the occurrence of a substantial proportion of lithologically similar metasedimentary rocks makes it difficult to correlate units and assign a stratigraphy. We have therefore divided the lithological units into four associations. These are: (1) an association of banded iron formation, chert, quartzite, mafic and ultramafic rocks; (2) an association of pelitic and semi-pelitic schist, quartzite, and mafic schist; (3) an association of mature clastic rocks including pebble metaconglomerate; (4) an association of Proterozoic metasedimentary rocks. Some of the second association was probably part of the same succession as association 1. The first two associations are of Archean age, were probably deposited at c. 3000 Ma, and have been intruded by Neoarchean granitic rocks. The mature clastic rocks of association 3 host ≥4.0 Ga detrital zircons that have been the focus of most previous work on the belt because they provide insight into early Earth processes. The mature clastic association may also have been deposited at c. 3000 Ma, but there is no direct evidence of intrusion by the Neoarchean granites, and it has a different, simpler, structural history than the first two associations. The maximum depositional age and deformation history of the Proterozoic metasedimentary rocks of association 4 suggest it may have been deposited during the late stages of the Capricorn Orogeny. This could have occurred in small transpressional pull-aparts within the Cargarah Shear Zone, which is a major, dextral, transpressional east-trending shear zone that was active during the Capricorn Orogeny between c. 1830 and c. 1780 Ma. Reactivation of the shear zone and overprinting by semi-brittle faulting has produced a complex network of fault slivers, and further sedimentation may have occurred after the Capricorn Orogeny.