NEW GEOCHRONOLOGICAL DATA FROM THE BOOLGEEDA BIF AND WOONGARRA RHYOLITE, HAMERSLEY GROUP (WESTERN AUSTRALIA)

The Hamersley Group of Western Australia contains five large banded iron formations (BIFs) deposited as chemical muds in low-energy shelf/slope environments. The age of the Boolgeeda BIF (the youngest) has been bracketed using SHRIMP dates from stratigraphically adjacent units, but an unequivocal depositional age is needed to constrain correlations with broadly coeval strata in South Africa, the depositional rates of large BIFs, and the timing of important events in earth history such as the onset of the Great Oxidation Event (GOE). Here we report new U-Pb zircon dates from a unique event bed in the Boolgeeda and from near the top of the underlying Woongarra Rhyolite. All samples were collected at Woongarra Gorge where the Boolgeeda is 200 m thick and consists of three members stratigraphically: upper and lower BIF members plus a shale-rich member between them. The Boolgeeda event bed is ~23 cm thick and constitutes the only coarse clastic layer of any sort we observed. It is roughly 60 m above the base of the lower BIF member and its primary constituents were largely replaced by K-feldspar and stilpnomelane. While it does not contain any clear traction structures, its upper 17 cm is normally graded with well-sorted coarse/very coarse sand at its base. The sand is rich in unusual composite grains (possibly devitrified glass fragments) and contains detrital K-feldspar and zircon. The crystal facets and internal textures of these grains suggest minimal transport, but the presence of cherty flat pebbles requires at least some resedimentation. Two of the Woongarra samples analyzed are poorly sorted fragmental rocks with pseudomorphs of perlitic fractures in larger fragments; the third is a porphyry from the interior of either a thick lava flow or sill. CA-TIMS analyses of zircon crystals yielded weighted mean 207Pb/206Pb dates (calculated using 238U/235U of 137.818) of ca. 2446 Ma for the event bed and ca. 2449 Ma for the uppermost Woongarra. We interpret the event bed as the product of a single volcanic eruption, although it could have been re-deposited sometime after ca. 2446 Ma. This age still sets a lower limit for the start of the GOE as the Boolgeeda is the last of the large Hamersley BIFs. The new dates also support a volcanic (vs. intrusive) origin for the Woongarra, consistent with the Boolgeeda’s higher stratigraphic position.