Fortescue Group: The Neoarchean Breakup of the Pilbara Craton

Abstract

At 2775 Ma, the Neoarchean crust of the Pilbara Craton began to be extended and rifted resulting in the widespread eruption of basaltic lavas. Between c. 2775 and 2710 Ma, mafic–felsic volcanic and intrusive activity continued in stages that were separated by periods of uplift, folding, erosion, and sedimentation. The first basaltic formation deposited across the eroded surface of the craton was the Mount Roe Basalt, up to 2.44 km thick and fed from dolerite dykes intruded into extensional fractures; this was the first regionally extensive formation of the Fortescue Group. Deformation and erosion of the Mount Roe Basalt were followed by clastic deposition and felsic volcanism and intrusion of the 2766–2749 Ma Hardey Formation. The stratigraphic nomenclature of the Fortescue Group from 2749 Ma onwards differs between the North and South Pilbara. Even so, the same magmatic events affected both areas. Almost all volcanic activity ended at c. 2710 Ma following eruption of the Maddina Formation of the North Pilbara (correlated with the Bunjinah Formation in the south). Between c. 2710 and 2630 Ma, mainly clastic sedimentary rocks of the Jeerinah Formation, the upper formation of the Fortescue Group, were deposited in both areas. Because the stratigraphy and sedimentology of the Jeerinah Formation indicates passive margin basin deposition, it is interpreted that extension and rifting of the Pilbara Craton culminated in continental breakup and plate separation at c. 2710 Ma. Most workers have interpreted the mainly volcanic 2775–2710 Ma lower Fortescue Group as a large igneous province formed by one or more mantle plumes. A mantle plume origin is consistent with the crustal extension and rifting of the Pilbara Craton, the continental breakup, and the stratigraphy of the volcanic succession that includes ultramafic–mafic–felsic volcanic cycles.