Cas RAF1, Hayman PC2, Squire RJ3, IH Campbell IH4, Wyche S5, Sapkota J6, Smithies H7
1School of Earth, Atmosphere and Environment, Monash University, Vic, 3800, Australia 2Queensland University of Technology, Brisbane, QLD, 4074, Australia 3School of Earth, Atmosphere and Environment, Monash University, Vic, 3800, Australia 4Research School of Earth Sciences, Australian National University, ACT, 4074, Australia 5Geological Survey of Western Australia, Perth and Kalgoorlie, WA, Australia 6Geological Survey of Western Australia, Perth and Kalgoorlie, WA, Australia 7Geological Survey of Western Australia, Perth and Kalgoorlie, WA, Australia
The stratigraphy of the late Archean (>2.7 Ga to 2.658 Ga) Kalgoorlie Terrane Large Igneous Province, in the Eastern Goldfields Superterrane, Yilgarn Craton, Western Australia, preserves an evolution of magmas, eruption processes paleo-environments, sediment provenance, and deformation that are inconsistent with a plate tectonic setting. An initial, deep submarine LIP komatiite and basaltic succession several kms thick (Stage 1 ~2.72-2.69 Ga) of lavas, hyaloclastite breccia and sills, with intercalated chert, black mudstones and minor felsic volcanics, extends > 600km along strike. It represents a widespread mantle plume event during regional extensional (D1) rift volcanism. Eruptions occurred in an open, anoxic, deep-water setting with no evidence of nearby emergent continents. Hydrostatic pressure suppressed all explosive activity.
From ~2.69 to 2.67 Ga (Stage 2) felsic TTG magmatism was dominant, represented by submarine lavas, hyaloclastite, monomictic tuffaceous megaturbidites, polymictic volcanogenic turbidite and conglomeratic mass flow deposits, and contemporaneous high-level granitoid intrusions. A major mafic event at ~ 2.80 Ga indicates mantle heat likely caused felsic crustal magmatism. The felsic volcanics and volcaniclastics represent marine intra-basinal felsic volcanoes (lava domes, stratovolcanoes?), some of which grew into shallow water and became emergent, were explosive under low ambient pressures, and produced the submarine tuffaceous megaturbidites. The Stage 2 volcanic conglomerates, including some granitoid clasts, were not generated by orogenic deformation and uplift, but were derived exclusively from the intra-basinal volcanic centres and sub-volcanic plutons, and from Stage 1 komatiite and mafic stratigraphy that was up-domed by diapiric granitoids. Ongoing plume buoyancy and contemporaneous local granitoid diapirism caused regional and local uplift (D2’) and shallowing of basins during ongoing regional extension (D1).
From ~2.67 Ga a transition into more widespread subaerial paleoenvironments is represented by felsic tuffaceous volcanic sediments deposited in fluvial braid-plain, alluvial fan, shallow marine, and local deep-water settings (Stage 3A). At < 2.658 Ga “late basin” polymictic, alluvial fan – fan delta conglomerates and sandstones represent major, regional crustal compression, uplift (D2/D4’, depending on scheme), emergence of large landmasses, and far-field sediment provenance (Stage 3B). There is no geological evidence of a plate tectonic regime prior to 2.658 Ga (i.e. no ophiolites, accretionary prisms, blueschist belts, uplifted orogenic belts, mountainous continents). A long-lived mantle plume caused partial melting of thick metasomatized mafic crust causing uprise of TTG plutons, leading to plutonic diapiric updoming, and changing paleoenvironments and eruption styles from Stages 1 to 3 (~ 50 Myr). Widespread TTG magmatism does not signify a subduction setting, but widespread anatexis of a lid-like crust/lithosphere.
Ray Cas has interesets in all aspects physical volcanology, including (paleo)environmental influences on eruption styles, geodynamic settings of volcanism, associated mineralisation and Archean paleovolcanology. He leads an annual professional shortcourse on these topics hosted by Monash U, UTasmania and Queensland University of Technology.