Cyclic continental mafic tuff and flood basalt volcanism in the Late Archaean Nullagine and Mount Jope Supersequences in the eastern Pilbara, Western Australia

Abstract

A late Archaean (ca. 2775–2715 Ma) continental flood basalt and extensional basin succession in the eastern Pilbara of Western Australia has been divided into 11 unconformity-bounded packages of rock, and four mafic dyke suites have been correlated chemically with subaerial flood basalts in the succession. The stratigraphic divisions follow unconformity-based mapping, whole-rock geochemical, palaeomagnetic, volcanological, sedimentological, petrographic and airborne geophysical studies. The 6 km thick lower Chichester Range Megasequence (Fortescue Group minus the Jeerinah Formation) in the Nullagine Synclinorium comprises lower and upper subaerial flood basalt units separated by a thick succession of terrestrial clastic sedimentary and felsic volcanic rocks. A repeated pattern of mafic volcanism is apparent in the upper part of the succession comprising six tuff to basalt cycles, each representing a flood basalt event. Subaerial flood basalt volcanism in each cycle was preceded by mafic hydrovolcanism resulting in a mafic tuff horizon beneath each subaerial flood basalt pile. Mafic tuff horizons were deposited and resedimented in mostly terrestrial settings. Initial pyroclastic volcanism is attributed to interaction between rising mafic magma batches and shallow ground water or surface water. Overlying flood basalt piles represent relatively rapid outpourings of mafic lava that complete the volcanic cycle. Most geological time in this part of the succession is represented either by boundaries between tuff/basalt cycles or by the resedimented tuff horizons. This study supports a rift-related geotectonic model for the succession as a whole, though it is shown to be more complex than previously proposed.