Clasts in Archean conglomerates and implications for uplift: Evidence from the 2.7 Ga Agnew Greenstone Belt (Western Australia)

Abstract

Archean greenstone belts commonly consist of a stratigraphy that records subaqueous deposition capped by subaerial sedimentation, yet it remains unclear if the controls on this change were driven by horizontal or vertical processes. Clasts in conglomerates can provide important constraints on this question because they inform on the changing nature of sources. The 2.7 Ga Agnew Greenstone Belt, Yilgarn Craton, Australia, provides the ideal opportunity for such a study because it contains abundant and diverse conglomerate facies and its basal stratigraphy is well understood. This study focusses on the texture and composition of lithic clasts derived from three stratigraphic conglomerate units and their host successions. The lowermost conglomerate-bearing unit, hosted in the Vivien Formation, unconformably overlies the mafic-ultramafic lavas and intrusions of the Kalgoorlie Group (ca. 2720–2690 Ma) and is dominated by aphyric to porphyritic mafic and felsic volcanic clasts. The conformably overlying Maria Mine Formation conglomerates contain a higher proportion of felsic clasts with medium- and plutonic groundmass textures. The Vivien and Maria Mine Formations (ca. 2690–2670 Ma) are dominated by cherts, turbidites and conglomerates, and represent subaqueous sediments formed in the ring-plain surrounding an emergent volcanic island. The unconformably overlying Scotty Creek Formation (ca. 2665–2655 Ma) contains conglomerates with many plutonic felsic clasts, and is dominated by bedded and cross-bedded sandstones formed in a subaerial environment. Whole-rock geochemistry of mafic and ultramafic clasts matches the local Kalgoorlie Group rocks and indicates progressive erosion of the underlying supracrustal sequence. Undeformed conglomerates lack clasts with metamorphic foliations, indicating uplift by horizontal compression could not have been significant before ca. 2655 Ma, which is considered as the time of the first major compressional event in many published structural frameworks for the region. The systematic erosion of a felsic stratovolcano, its associated plumbing system, and any resultant upwarping of underlying sequences through vertically-driven diapirism, better explains the progressive changes observed in clast texture and composition, as opposed to horizontally-driven uplift, such as compression associated with subduction.