The Yilgarn Craton is one of the largest fragments of Archean crust and is critical to understanding the geodynamic processes that operated on early Earth. A study of c. 2820–2738 Ma magmatic rocks in the northwestern Youanmi Terrane of the Yilgarn Craton (Lowrey, 2022) identified several lines of evidence that require a geodynamic reappraisal. (1) The distribution and abundance of komatiites reported in previous studies is grossly overstated and the necessity for anomalously high mantle temperatures of a mantle plume is therefore questionable. (2) Volcanic stratigraphy comprises mafic volcanic rocks with boninite-like compositions and mineralogy, MORB-like tholeiitic basalts, siliceous high-Mg basalts and high-Mg intermediate–felsic rocks; these lithologies could collectively be considered diagnostic of a subduction setting, if documented in the modern geologic record. (3) Th/Yb–Nb/Yb systematics for volcanic rocks define several distinct trends, parallel to the modern mantle-array, that overlap with trends from modern subduction settings. (4) Coeval mafic–ultramafic sills and granitic rocks that intrude the greenstone sequence contain locally abundant hornblende and indicate widespread hydrous metasomatised mantle sources throughout the northwestern Youanmi Terrane from c. 2810 Ma. Using new geochemical data and existing geochronological data, Lowrey (2022) defined a chemo-stratigraphic framework with 16 units that can be correlated between structurally separated greenstone belts and, on a regional scale, define discontinuous, broadly NNE trends that extend ≤ 350 km long and ≤ 50 km wide. Neodymium isotope and trace element systematics of magmatic rocks implicate increasing incorporation of metasomatised mantle from c. 2820 to 2738 Ma and variable recycling of a ≥ 3050–2920 Ma crustal component. Mantle sources to multiple magmatic units aged c. 2820–2782 Ma are interpreted to have been metasomatised by fluids from hydrous mafic crust whereas mantle sources to c. 2754 Ma shoshonitic rocks (hornblende-biotite rich lamprophyres and granitic plutons) are interpreted to have mixed with and been metasomatised by ancient sedimentary components. A preliminary geodynamic framework is proposed whereby the first magmatic episode from c. 2820–2782 Ma, involving lithospheric thinning and upwelling asthenosphere, as well as depletion and metasomatism of mafic magma sources, is attributed to subduction of relatively isotopically juvenile crust. Following a c. 20 Ma hiatus, the second magmatic episode from c. 2761–2738 Ma, which included crustal-derived transitional TTGs and mantle-derived shoshonitic and sanukitoid rocks, is interpreted to represent a post-orogenic event related to delamination of arc-crust or slab breakoff.

Reference(s)

Lowrey JR 2022. Petrogenesis of c. 2820–2738 Ma magmatism in the northwestern Youanmi Terrane, Yilgarn Craton. Unpublished PhD Thesis, University of Sydney.