SHRIMP zircon ages constraining the depositional chronology of the Hamersley Group, Western Australia

The Mt Bruce Supergroup of Western Australia was laid down between ca 2.8 Ga and ca 2.2 Ga in the Hamersley Basin, unconformably over a basement of the older, granite‐greenstone, component of the Pilbara Craton. The Mt Bruce Supergroup consists of three groups: the Fortescue Group, Hamersley Group and Turee Creek Group in upward sequence. The Hamersley Group, which is divided into eight formations, has a general thickness of ∼2.5 km, and is characterised by major banded iron‐formation (BIF) units. Reported here are SHRIMP U–Pb zircon results (406 grain analyses) from 13 samples taken from the Hamersley Group and near the top of the underlying Fortescue Group. In combination with SHRIMP results previously published from 12 Hamersley Group samples, the present results provide significant new constraints on the depositional chronology of the group, and suggest that the average (compacted) depositional rates of each of the main depositional lithologies (BIF, carbonate, shale) were ∼180 m per million years, 12 m per million years and 5 m per million years, respectively. Some recently published SHRIMP ages from the Joffre Member differ slightly from those that are interpreted from the present data, and it is suggested that the two datasets may be reconciled if non‐zero‐age Pb loss is taken into account. The total body of zircon U–Pb age data from the Fortescue and Hamersley Groups is consistent with a model involving continuous accumulation of basin‐fill for at least 330 million years, from ca 2780 Ma to the top of the Hamersley Group at ca 2449 Ma. The word ‘continuous’ in this context means that there may have been no breaks in deposition longer than 1 million years. However, this model is not proven, and a major challenge for future work is to measure the length of any proposed non‐depositional intervals.

Keywords:

• depositional chronology
• Fortescue Group
• Hamersley Group
• SHRIMP
• uranium‐lead dating
• zircon