The 2766–2749 Ma Hardey Formation is a voluminous and widespread lower stratigraphic unit of the Fortescue Basin of the Pilbara region. It shows significant regional variation both in terms of the proportion of clastic and volcaniclastic rocks and in the prevailing composition of magmatic units. In the central and western parts of the basin, siliciclastic rocks including sandstone, siltstone and conglomerate, dominate the formation; igneous units are restricted to the upper part and comprise the mafic–ultramafic Coolajacka Member, Cooya Pooya Dolerite and the overlying volcanoclastic Lyre Creek Member. The latter comprises relatively primitive (Mg#= 55 to 71) mafic to intermediate deposits, including ignimbrites, and shows early Fe-depletion trends reminiscent of calc-alkaline magmatism, a common feature of magmatic rocks of the Hardey and Kylena Formations in the western Pilbara. The Cooya Pooya Dolerite is geochemically extremely similar to the Coolajacka Member, suggesting the units might be genetically related. In the northeast of the basin, at Gregory Range, the Hardey Formation is dominated by felsic volcanic rocks, and comagmatic and cogenetic intrusions form the locally syn-deformational granites marking the north-northwest trending structure that now forms the boundary with Proterozoic and younger sedimentary basins. Magmatic units include the 2763–2757 Ma Gregory Range Suite (granites), dacites and rhyolites of the 2764 Ma Koongaling Volcanic Member, and in the upper part of the formation, mafic and felsic magmas of the 2766–2749 Ma Tanguin Member. Only mafic magmas of the Tanguin Member have equivalents in the western exposures of the Hardey Formation, being geochemically very similar to the Lyre Creek Member. The Gregory Range Suite, Koongaling Volcanic Member and felsic magmas of the Tanguin Member have the ferroan and incompatible trace element-enriched characteristics of hot, dry, A-type magmas. Incompatible trace element patterns for the Tanguin Member and the overlying Kylena Basalt suggest they may be end products of a similar source. The Gregory Range Suite and Koongaling Volcanic Member, however, were most likely derived from a slightly different mafic parental magma – perhaps related to the earlier Mount Roe Basalt? The rhyolites and dacites of the 2766–2755 Ma Bamboo Creek Member lie to the west of Gregory Range. These are compositionally variable, with three regionally distinct groups, likely reflecting variation in bulk source. All have compositional attributes transitional to A-type magmas. Thus, A-type magmatism dominates the Hardey stratigraphy in the Gregory Range region and extends to the west and south in the form of the Bamboo Creek magmas. Elsewhere, clastic rocks dominate the Hardey Formation; previously mapped felsic volcanic rocks are probably weakly fractionated eruptions from mafic sills. The structural and magmatic characteristics of the Gregory Range support a long-lived, north-northwest trending, magma-related rift. It reflects one of the oldest preserved rift structures on Earth and was fundamental to the development of later Proterozoic basins.