Testing the `terrane-boundary' concept and geodynamics in the Neoarchean: A case study of the stratigraphy from the West and East Laverton Greenstone Belts

PhD thesis abstract: Archean (>2.5 billion years: Ga) cratons consist of numerous crustal blocks that are separated by crustal-scale structures. Knowledge of how these crustal blocks came to be in their current configuration has implications for geodynamic models, links to other cratons and mineral exploration. Largely on account of poor 3D exposure, the study of Archean structures is insufficient to fully address questions of their origin. The Hootanui Shear Zone (HSZ) in the Eastern Goldfields Superterrane (Yilgarn Craton, Western Australia) is ascribed a variety of origins, including a suture between two tectonic plates, an intra-arc structure, or as the margin of a rift. In this study the well-preserved volcano-sedimentary sequences from either side of the HSZ (the West and East Laverton Greenstone belts), are characterised from field traverses and drillcore, whole-rock geochemistry (XRF, LA-ICP-MS) and U-Pb zircon (and Lu-Hf), titanite and monazite isotope chemistry (LA-ICP-MS, SIMS). These belts are investigated for similarities and differences to constrain the geological history, focussing on the evolving tectonic system, depositional environments and origins of the HSZ. The study area records 300 million years of history. The oldest rocks sit at the base to the WLGB (West Laverton Greenstone Belt) and comprise <3.02 Ga chert-pebble conglomerates, which reflect the uplift and erosion of granitic crust. After ca. 2.85 Ga, crustal thinning led to basin formation and the deposition of turbidites. The first event linked across the HSZ is a 2.81 - 2.79 Ga subaqueous event marked by mafic-ultramafic rocks. An episode of volcano-quiescence followed from <2.79 to 2.76 Ga, during which time, the deposition of deep-water sediments (chert, banded iron formation and rare carbonate) occurred in the WLGB, whereas shallow-water sedimentation (sandstone dominated) occurred in the ELGB (East Laverton Greenstone Belt). This was followed by renewed tholeiitic magmatism until 2.71 Ga, then intermediate magmatism and epiclastic sedimentation in both belts, between 2.71 - 2.70 Ga. These data are consistent with formation at an Archean rift-margin, with the WLGB representing rift-infill and the ELGB a rift-shoulder; temporal and stratigraphic similarities cannot be explained by allochthonous models. Differences in stratigraphy and depositional environment reflect the different tectonic settings – the WLGB is well preserved in a subaqueous basin, whereas the ELGB likely experienced erosion and/or non-deposition on account of topographic control at the rift shoulder. Integration with Yilgarn-wide data supports a (7-part) regional framework spanning the 3.02 - 2.71 Ga time-interval, across the EGST (Eastern Goldfields Superterrane) and into the Youanmi Terrane. The regional geodynamic setting is interpreted to reflect a failed continental rift, which developed outboard (NE) of the Yilgarn proto-craton between 2.81 - 2.71 Ga.