Zircon geochronology and Hf–O isotopes of the Nulliak supracrustal assemblage (Saglek Block–Canada): Constraints on deposition age and setting, metamorphic age and environments of zircon crystallization

Abstract

The Archean Saglek Block, North Atlantic craton, is one of the oldest crustal segments preserved on Earth. This area records two granulite facies metamorphic events: one in the late-Eoarchean at ca. 3.7 Ga and one in the Neoarchean, at ca. 2.75 Ga. Here, we report the results of combined U–Pb/Hf/O in-situ isotope analyses of detrital and metamorphic zircon domains from two meta-sedimentary samples, one psammitic, and one pelitic. Both samples are components of the oldest supracrustal assemblage of the Saglek Block: the Eoarchean Nulliak supracrustal assemblage. The isotopic data presented in this study: (i) confirm deposition age of the Nulliak supracrustal assemblage before ca. 3.65 Ga, (ii) indicate local provenance of the detrital zircon, (iii) suggest deposition of the sedimentary protoliths in a convergent setting, such as an arc, (iv) crystallization of the oldest zircon during the early Eoarchean in a protolith derived from a depleted mantle component, (v) involvement of material that experienced prolonged low temperature alteration during the late Eoarchean granulite facies metamorphism and (vi) selective metamorphic overprinting during the Neoarchean granulite facies metamorphism.The new zircon U–Pb/Hf/O isotope data presented here in conjunction with previously published analyses suggest that a depleted mantle domain existed in the early Archean, as reflected in the positive εHf signature of zircon from the Saglek Block. Yet, such positive Hf isotope deviations are not the norm, but rather the exception in the preserved early Earth zircon record. In addition, mildly elevated zircon δ18O values, i.e. those ≥ 5.9 ‰, are as common in the early Archean as they are in the late-Archean zircon record, implying that crustal reworking was an active and efficient process all through the Archean. Hence, no significant variation in the nature of the reworking and broader geodynamic processes occurred during this period of time.