Insight into Archean crustal growth and mantle evolution from multi-isotope U-Pb and Lu-Hf analysis of detrital zircon grains from the Abitibi and Pontiac subprovinces, Canada

Abstract

Lu-Hf laser ablation – multi-collector – inductively coupled plasma – mass spectrometry (LA-MC-ICP-MS) analysis was conducted on ~1800 detrital zircon grains from successor basins of the Archean Abitibi and Pontiac subprovinces of Ontario and Quebec, Canada, and paired with previous U-Pb LA-MC-ICP-MS analyses of the same grains. Results are used to constrain the isotopic character of magmatic source domains of the zircon grains to establish the sedimentary provenance of the ~2690–2670 Ma successor basins, to provide constraints on terrane configurations and amalgamations at the time of basin formation, and to assess their significance for the record of crust-mantle growth in the region. The majority of results (95%) yield ɛHf values of + 1 to + 10 for ~2850–2675 Ma zircon, and clusters along compositions of the Archean depleted mantle (DM), which is based on projections of modern MORB compositions. Subordinate results, comprising ~2% of the data set, yielded values (ɛHf > +10) corresponding to extremely depleted mantle compositions, reflecting anomalously depleted sources in the ~2950–2670 Ma age range. The remaining 3% correspond to chondritic uniform reservoir (CHUR)-like to negative ɛHf values that reflect primitive sources and/or evolved magmas in zircon that crystallized in the ~3250–3050 Ma and ~2950–2670 Ma age ranges. While Neoarchean grains dominate the data set (~88%), approximately 12% are Mesoarchean. The Lu-Hf data collected on these zircon grains, when compared with published isotopic results, preserve signatures indicative of derivation from exotic crustal domains juxtaposed during ~2690–2670 Ma amalgamation of the southern Superior Province. Since depleted compositions are characteristic of Neoarchean and Mesoarchean zircon groups in the southern Superior Province, and sources include local and distal domains that were likely separated by many 100s of kilometers prior to amalgamation, it is inferred that a depleted upper mantle reservoir was not only well-established, but prevalent in the mantle below each of these areas during their construction. Based on the predominant Hf isotope signatures in the detrital zircon results and predicted isotopic trends produced by probable geodynamic mechanisms, crustal growth by direct differentiation from a depleted mantle reservoir is likely to have been moderated by subduction-accretion processes.