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Spinel, olivine, and pyroxene chemistry of the Eoarchaean Manfred Complex (Yilgarn Craton, Western Australia), with implications for the tectonic setting of Archaean layered mafic intrusions and the stabilisation of continental nuclei

Anthony I.S. Kemp, Matthew L. Rowe

Anorthosite complexes are distinctive components of Archaean cratons and their petrogenesis has implications for ancient mantle evolution, early Earth geodynamics, and growth of continental crust. However, the origin and tectonic setting of Archaean anorthosites and their complementary mafic and ultramafic cumulates is uncertain due to metamorphism and deformation. Here, we report spinel, olivine, and pyroxene chemical compositions from rare, well preserved ultramafic cumulates of the Eoarchaean Manfred Complex in the Yilgarn Craton of Western Australia, which contains the oldest anorthosites on Earth. Relict magmatic spinel in peridotite and pyroxenite enclaves define a distinctive high Al, low Ti-Fe3+ layered mafic intrusion trend that establishes a petrogenetic link to the ca. 3730 Ma anorthosites. Olivine chemistry (Fo85), Ni concentrations, FeO/MgO ratio trends for coexisting olivine and liquid, and spinel Mg# identify the parental magma as picritic to tholeiitic. Unique spinel Cr#, Fe#, and Ti contents are attributed to derivation of the parental magmas from a spinel peridotite source, in contrast to younger Archaean anorthosite complexes, reflecting an evolution of Archaean mantle. Magmatic spinel Al3+, Fe3+, and Cr3+ systematics are consistent with the emplacement and differentiation of the ca. 3730 Ma Manfred Complex parental magma within a thickened oceanic plateau. This implies a similar, intraplate tectonic environment for formation of the surrounding, coeval tonalite-trondhjemite-granodiorite gneisses in the Narryer Terrane. We therefore infer that early Archaean oceanic plateaus may be important tectonic settings for producing and preserving ancient continental crust, as well as the variety of supracrustal rocks commonly observed in gneiss complexes of Archaean cratons.