Early crustal evolution of the Superior craton – A U–Pb, Hf and O isotope study of zircon from the Assean lake complex and a comparison to early crust in other cratons

Abstract

We report U–Pb/Hf/O isotopic measurements of zircon extracted from meta-igneous rocks exposed in the Assean Lake crustal complex, northwestern Superior craton (Canada). With crystallization ages of ca. 3.22 Ga, zircon from the Assean Lake crustal complex are among the oldest reported in the Superior craton. The Hf and O isotope compositions of zircon indicate that these meta-igneous rocks derive from a mantle component with a relatively primitive Hf isotope composition, similar to chondrites, that was uncontaminated by elevated δ18O material (εHf,initial ~ 0 and δ18OVSMOW ~ 5.5‰). These features are shared by the large majority of the zircon extracted from the oldest (meta)-igneous rocks exposed in Archean terranes. Positive εHf deviations in Archean zircons are typically observed in magmas that post-date the oldest magmatic activity in a given terrain. This temporal variability in magma source regions may reflect a sequence of events comprising: (i) felsic magmatism derived from undepleted mantle components followed by, (ii) re-melting of the same mantle component producing the depleted Hf signatures in zircon and then, (iii) accretion of the proto-craton and reworking of the whole-system enabling stabilization.