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

Highlights

• Clasts derived from Archean conglomerates support diapirism and sagduction models.

• Felsic and mafic clasts indicate systematic erosion of supracrustal sequence.

• Orogenesis (>2655 Ma) unsupported as clasts lack pre-emplacement tectonic fabrics.

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.

Introduction

Archean granite-greenstone terranes typically consist of a supracrustal sequence that can be divided into a lower mafic-ultramafic package and an overlying package of coarse clastic successions, all of which typically form thick greenstone units between large granitoid bodies (e.g., 2.7 Ga Kalgoorlie Terrane, Australia: Myers, 1997; 2.7 Ga Blake River Group, Canada: Ayer et al., 2002; 2.7 Ga Belingwe, Zimbabwe: Hofmann and Kusky, 2004). The lower supracrustal package is dominated by subaqueous lavas and associated intrusions, whereas the upper package can be further divided into a lower subaqueous and an upper subaerial association. An unresolved question for Archean greenstone belts is the mechanism that drives uplift: is the main force horizontal or vertical?

Far-field compressive forces have been used to explain deformation features, unconformities and the change from subaqueous to subaerial sedimentation in many greenstone belts in the Yilgarn Craton in Australia (Agnew Greenstone Belt: Blewett et al., 2010, Thébaud et al., 2018; Kambalda Greenstone Belt: Miller et al., 2010), and Superior (Abitibi Greenstone Belts: Mueller et al., 1994, Davis et al., 1995, Ayer et al., 2002), and Slave Provinces in Canada (Fyson and Helmstaedt, 1988, Corcoran et al., 1998). However, diapiric uplift, sagduction and contemporaneous sedimentation can also explain many of these features (Campbell and Hill, 1988, Bleeker, 2002, Parmenter et al., 2006, Squire et al., 2010, Lin et al., 2013). Using cross-cutting relationships, U-Pb geochronology and isotopic studies of granitic rocks in the Kalgoorlie-Norseman region, Campbell and Hill (1988) demonstrate that the voluminous granitoids are younger than the mafic-ultramafic sequence; and this has since been documented in all granite-greenstone belts. They propose that large degrees of crustal melting led to diapirism and the formation of topographic highs, where granitic magmas intrude the upper crust, and lows where the withdrawal of granitic magmas leads to the sinking of greenstones. These ideas have since been further advanced using detrital geochronological data (Squire et al., 2010, Lin et al., 2013). For example, Lin et al. (2013) demonstrate that zircon populations from the Island Lake Greenstone Belt, Superior Province, become progressively older from the base to the top of the sedimentary package, and interpret this as evidence for diapirism and sagduction. They argue that horizontal compression and thrusting would result in a stacked source region that, when eroded, would not exhibit such a progressive younging trend in the derived sediments.

The clasts contained within conglomerates provide important information about the nature of the source region at the time of erosion (Boggs, 1987, Wandres et al., 2004, Haque and Uddin, 2017), but are seldom used to inform on Archean tectonics (with notable exceptions: Corcoran et al., 1999, Hofmann, 2005, McGoldrick et al., 2013, Agangi et al., 2018). Unlike zircon crystals, which represent a fraction of only the felsic source rocks, clasts preserve texture, mineralogy, composition and, in some cases the age, of source rocks that were uplifted during erosion. Where the local and regional stratigraphy, ages, compositions and spatial distributions are well understood, it may be possible to place tight constraints on the nature and distribution of sources that were uplifted and available for erosion by linking source characteristics with clasts. And where conglomerates have formed at different times, additional insight into the evolving nature of the erosion of source regions may be revealed.

The 2.7 Ga Agnew Greenstone Belt (AGB), Yilgarn Craton, Western Australia, is well suited to investigate the primary mechanism of uplift because the stratigraphy, structure, geochemistry and geochronology of the region are reasonably well understood compared to many other greenstone belts (Platt et al., 1978, Duuring et al., 2010, Hayman et al., 2015, Beardsmore, 2017, Thébaud et al., 2018). In this study, we examine lithic clasts contained within conglomeratic beds in the Agnew Greenstone Belt to assess the utility of clast texture and composition in Archean belts for constraining the uplift history. These data are integrated with other new results (lithology, sedimentary structures and provenance), as well with published data, especially compositional data on potential source rocks from the broader region, to provide further constraints on sources. Because exposure of sedimentary rocks is generally poor, this study relies mainly on the large amount of diamond drill core collected during the long gold-mining history.