Geometry of Intrusive Impact Melt Rock Bodies in Central Uplifts: A New Geological Map of the Yarrabubba Impact Structure, Western Australia

Introduction: Melting is an important characteristic of hypervelocity impact and the cratering process that both generates and recycles planetary crust, e.g. [1]. Intrusive melt rock has been studied from both relatively small terrestrial impact structures (<4 km diameter, e.g., Brent [2]) and giant (>200 km diameter) impact structures like Sudbury, Vredefort, and Chicxulub, e.g. [3]. Numerical simulations have advanced understanding of the mechanisms of melt generation in impacts, e.g., [4]. Yet, many aspects of the distribution, geometry, timing, and intrusion mechanisms of impact melt rocks in mid- to large-scale terrestrial impact structures remain unknown. Here we present new detailed field mapping in the central uplift of Earth’s oldest precisely dated and preserved impact structure – Yarrabubba, Western Australia [5-6]. The eroded remnants of a ~70 km diameter, 2229 ± 5 Ma Yarrabubba impact structure currently expose shocked Archean monzogranite target rock injected by impact melt bodies (e.g., Barlangi Granophyre; [5-6]). The exposed rocks occur within the central 5 km of a ~12 km wide aeromagnetic anomaly assumed to represent the central uplift [5-6]. Neither the crater's topographic feature nor impact breccia remains. Target rock granitoid exposed at Yarrabubba preserves shatter cones (which require at least 2 GPa), prevalent planar deformation features (PDFs) in quartz (indicating at least 10 GPa), and {112} twinned zircon. Barlangi Granophyre preserves granular zircon with evidence of former reidite, and dissociated rims (indicating at least 20 GPa and 1673 ºC, respectively) [6].

Methods and approach: Geological field mapping of outcrops containing impact melt bodies in the apparent central uplift at Yarrabubba was performed at 1:2,000 and 1:500 scales, with basemap photomosaic imagery collected via a DJI M300 drone equipped with a Zenmuse P1 48 MP camera [7-8]. Automated survey flights were flown at altitudes ranging from 100 m to 120 m, resulting in photogrammetric orthomosaic and DEM data at a resolution of ~5 cm/pixel. Structural data were visualized with stereographic projections, and the azimuthal traces of geological boundaries shown on rose diagrams produced via FracPaQ [7-9].

Results and Discussion: Field mapping reveals a complex network of subvertical ~0.2 to ~6 m wide green impact melt rock dykes and sheets and granitic granophyre bodies up to ~500 m wide in map view [7-8]. The dykes trend in a wide range of directions, with larger dykes radiating from the centre of the impact and in some cases forming en echelon arrays. Boundaries of the granophyre bodies appear complex and segmented, with steps and apophyses that mimic the general dyke orientations. The 3D shape of the granophyre bodies is poorly constrained, but outcrops coincide with a ring-like aeromagnetic anomaly [5, 8]. Granite/granophyre hybrid with textures indicative of partial
melting occur along some margins of granophyre bodies. These domains underwent variable degrees of static (i.e., not mobilised) incipient partial melting. Similar textures are observed in ‘xenoliths’ within granophyre [10]. This texture is consistent with shock-release melting [11] rather than frictional melting. Shock-release melting is also consistent with microstructures in zircon from the Barlangi granophyre, which indicate both high shock pressures and superheated post-shock temperatures [6]. Petrologic, petrographic, and temporal relationships among the shocked target granite, xenoliths, and granophyre are consistent with the Yarrabubba monzogranite being the source of the impact melt rocks [6, 10]. Faults with steeply dipping shatter cone surfaces and monomict breccia are spatially related to melt intrusions, extending from jogs and terminations of the dykes. Impact melt rock dykes and granophyre bodies exploited these pre-existing damage structures which likely formed during the shock-rise phase of the impact event.

Conclusions: Field mapping at Yarrrabubba shows green impact melt rock dykes and granophyre bodies with wide ranging orientations within the central uplift, including concentric vertical dykes, cone sheets, radial dykes, and subhorizontal to gently outward dipping sheets. Melt for both types of intrusion was sourced locally from target granite via shock-release melting. Contribution from frictional melting to the Barlangi granophyre was minor.

References: [1] Marchi S. et al. (2014) Nature 511(7511): 578-582. [2] Grieve R. (1978) Proc. LPSC: 2579-2608.
[3] Prevec S. A. and Büttner S. H. (2018) MAPS 53(7): 1301-1322. [4] Manske L. et al. (2022) JGR Planets 127(12):
e2022JE007426. [5] MacDonald F. A. et al. (2003) EPSL 213(3-4): 235-247. [6] Erickson T. M. et al. (2020) Nature
Comms. 11(1): 300. [7] Beetge I. (2022) Hons. Thesis unpubl: 58 p. [8] Timms N. E., et al. (2023) LPSC: Abstract
1812. [9] Healy D. et al. (2017) JSG 95: 1-16. [10] Maillot W. M. A. (2021) Hons. Thesis unpubl: 68 p. [11] Ahrens
T. J. and O’Keefe J. D. (1972) The Moon 4: 214-428.