Australia: The Land Where Time Began

A biography of the Australian continent 

Jack Hills, Western Australia – Events from the Proterozoic Recorded in Quartzite Cobbles, New Restraints and Source of 4.0 Ga Zircons 

In the Jack Hills belt, Western Australia, rare heavy mineral bands were identified in cobbles of quartzite in 2 conglomerate units by Grange et al. They analysed 7 cobbles containing zircon from site 152 and 3 from site 154, both of the sites being about 1 km to the west of the site where zircons dated to more than 4 Ga are abundant (W74 ‘discovery’ site). There were 3 distinctive features that were revealed in the pebbles from site 152, containing either zircons of >3.0 Ga, of <1.9 Ga age, or ages ranging from ~1.2 to ~3.6 Ga. Those recovered from site 154 are more uniform, containing only zircons with ages between 3.1-3.9 Ga. A single zircon that was 1,220 ± 42 years old is the youngest grain that has been reported so far from the sedimentary rocks of Jack Hills. This zircon implies at least 2 cycles of sedimentation during the Proterozoic, as it shows magnetic oscillatory zoning, and for it to be hosted in the conglomerate requires erosion of an igneous precursor, being incorporated in a clastic sediment, induration and subsequent erosion and transport. The Bangemall Supergroup in the Collier Basin, which is ~100 km to the northeast in the Capricorn Orogen, is the nearest source of rocks of this age. This would require tectonic interleaving of Bangemall rocks that were originally more extensive, possibly being related to activity along the Cargarah Shear Zone that traverses the Jack Hills belt. The lack of zircons with an age of >4.1 Ga in the pebbles is highly significant, as it suggests the immediate source of ancient zircons was no longer present at the surface of the Earth. As noted in rocks containing Proterozoic zircons from previous studies, this equated with a general lack of ancient crystals, and implies that the numbers of such grains diminished as earlier sedimentary rocks were successively recycled.

Sources & Further reading

1. Grange, M. L., S. A. Wilde, A. A. Nemchin and R. T. Pidgeon (2010). "Proterozoic events recorded in quartzite cobbles at Jack Hills, Western Australia: New constraints on sedimentation and source of &gt;&#xa0;4&#xa0;Ga zircons." Earth and Planetary Science Letters 292(1–2): 158-169.