Stratigraphy and sedimentology of an early Archean felsic volcanic sequence, eastern Pilbara Block, Western Australia, with special reference to the Duffer Formation and implications for crustal evolution

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Abstract A sequence of early Archean felsic volcanic and volcaniclastic rocks and chert is present in the upper half of the Warrawoona Group throughout greenstone belts in the eastern Pilbara Block, Western Australia. It includes three formations. The Duffer Formation, which makes up the bulk of the sequence, consists of up to 5 km of metamorphosed dacitic to rhyolitic volcaniclastic breccia and lava and lesser amounts of tuffaceous rocks and mafic lava. It is overlain in several sections by the sandy Panorama Formation, and both are capped by silicified orthochemical and biogenic deposits of the Towers Formation, including both the Strelley Pool and Marble Bar Cherts. These units are identifiable in at least six greenstone belts in the eastern Pilbara Block. The Duffer Formation is divided into four facies: (A) structureless and graded breccia, (B) complexly organized conglomerate, (C) turbiditic tuff-breccia and tuff, and (D) lava. These facies represent deposition on predominantly subaqueous volcaniclastic aprons flanking felsic volcanic centers. A three-stage model of the evolution of these aprons includes (1) an early stage of felsic and mafic eruptions and subaqueous deposition, (2) a middle stage, during which the bulk of the sequence was deposited, involving shallow-water to subaerial felsic volcanism and subaqueous deposition, and (3) a terminal stage of subaerial felsic eruptions and subaerial deposition. Post-Duffer felsic volcanism was marked by explosive pyroclastic activity, which resulted in widespread deposition of ash in surrounding shallow-water and subaerial systems. This phase of activity is represented by the Panorama Formation. After the cessation of volcanism, erosion of the emergent vents and regional subsidence led to an interval of orthochemical and biogenic sedimentation, represented by the Towers Formation. Paleocurrent and facies patterns in the Duffer and Panorama Formations support earlier proposals that the intrusion of granitoid rocks in the eastern Pilbara Block was coeval with felsic volcanism. It seems likely that this volcanism was the surficial expression of plutonism that represented early stages of cratonization in the eastern Pilbara. The calc-alkaline volcanism and depositional styles represented by this sequence are similar to those associated with many Phanerozoic arcs. The abundance of shallow-water facies, closely spaced and possibly non-linear distribution of felsic centers, close association of felsic units with thick sequences of basalt erupted at or near sea level, sparse syn-volcanic deformation, and lack of granitic detritus suggest, however, that the setting of this Archean felsic volcanism was in many details, unlike that of most modern arcs.