Lu–Hf isotopic memory of plume–lithosphere interaction in the source of layered mafic intrusions, Windimurra Igneous Complex, Yilgarn Craton, Australia

Abstract

Most layered mafic intrusions (LMI) are formed via multiple magma injections into crustal magma chambers. These magmas are originally sourced from the mantle, likely via plume activity, but may interact with the overriding lithosphere during ascent and emplacement in the crust. The magma injections lead to the establishment of different layers and zones with complex macroscopic, microscopic and cryptic compositional layering through magmatic differentiation and associated cumulate formation, sometimes accompanied by crustal assimilation. These complex mineralogical and petrological processes obscure the nature of the mantle sources of LMI, and typically have limited the degree to which parental liquids can be fully characterised. Here, we present Lu–Hf isotope data for samples from distinct layers of the Upper Zone of the Windimurra Igneous Complex (WIC), an immense late-Archean LMI in the West Australian Yilgarn Craton. Lu–Hf isotope systematics of whole rocks are well correlated (MSWD=5.6, n=17) with an age of ∼3.05±0.05 Ga and initial εHf∼+8. This age, however, is older than whole rock Sm–Nd and zircon U–Pb ages of the intrusion, both of which are ca. 2.8 Ga. Stratigraphically-controlled initial Hf isotope variations (associated with multiple episodes of emplacement at ca. 2.8 Ga) indicate isotope mixing between a near-chondritic and an ultra-radiogenic component, the latter with εHf[2.8 Ga]>+15. This Hf isotope mixing creates a pseudochron-relationship at the time of intrusion of ∼250 Myr that is superimposed on subsequent radiogenic ingrowth after crystallisation, generating an age that predates the actual emplacement event. Mixing between late-stage crystallisation products (melt + crystals) from the Middle Zone and replenishing, plume-derived liquids was followed by crystal accumulation in a chemically evolving magma chamber. The ultra-radiogenic Hf isotope endmember in the WIC mantle source requires parent–daughter ratios consistent with very early formation in Earth history, akin to early Archean komatiitic plume sources. We propose that plume-derived melts that formed the Windimurra LMI reacted with ancient refractory lithospheric keels already underpinning ancient cratons, creating a melt with extremely high εHf[t]. Melting a refractory component with super-chondritic, time-integrated high Lu/Hf, in this case by plume–lithosphere interaction, simultaneously accounts for the extreme Hf isotope signals, Hf–Nd isotope decoupling, and difference in radiometric Lu–Hf and Sm–Nd ages.