Tectonic evolution of the Late Archaean to Early Proterozoic Mount Bruce Megasequence Set, western Australia

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Abstract
The Mount Bruce Megasequence Set (formerly the Mount Bruce Supergroup) was deposited on the Pilbara Craton during the late Archaean and early Proterozoic. It comprises two megasequences, the Chichester Range Megasequence and the overlying Hamersley Range Megasequence. Each megasequence comprises three supersequences or supersequence packages whose tectonic history can be explained in terms of Phanerozoic‐style tectonics. The basal Nullagine Supersequence, which comprises mostly subaerial basalt and terrigenous sedimentary rock, was deposited during an episode of WNW‐ESE directed crustal extension that was initiated at about 2770 Ma. The overlying Mount Jope Supersequence, which comprises mostly basalt and tuff, was deposited during the subsequent development of a WNW‐ESE trending rift before about 2690 Ma. Mudrock, carbonate sedimentary rock and iron formation in the overlying circa 2690 to circa 2600 Ma Marra Mamba Supersequence Package were deposited on the ensuing divergent craton margin. An unconformity or condensed succession covering a time span of as much as 130 m.y. separates the top of the Chichester Range Megasequence from the Hamersley Range Megasequence during which time an oceanic island arc is interpreted to have collided with the southern Pilbara Craton margin. Subsequently, the subducting oceanic plate flipped from southwest dipping to northeast dipping, and convergence resulted in the formation of a marginal subduction‐related orogen with associated arc and backarc volcanism. Iron formation and distal arc‐derived tuffs of the Brockman Supersequence Package were deposited on a siliciclastic‐starved platform in a neutral continental backarc setting at around 2470 Ma. The overlying circa 2440 Ma Woongarra Supersequence was also deposited in a backarc continental setting and comprises iron formation, felsic tuffs, and probably coeval mafic rocks. The latter two rocks types were derived from a backarc magmatic belt. Shortening of the southern Pilbara Craton margin resulted in the development of a backarc compressive cratonic basin in which iron formation and siliciclastic and carbonate sedimentary rock of the Turee Creek Supersequence were deposited, and ultimately resulted in a continent‐continent collision. The Mount Bruce Megasequence Set and its contained iron formations are preserved in the relatively distal hinterland of this collisional orogen.
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