The Saglek Block of northern Labrador hosts the Nulliak supracrustal assemblage — metamorphosed basaltic and ultramafic lavas, banded iron formations, marbles and pelites — claimed to be the oldest supracrustal (surface-derived) rocks on Earth at >3.95 Ga. The claim rests almost entirely on secondary-ion-mass-spectrometry (SIMS) U–Pb zircon ages of the intrusive Iqaluk tonalitic gneiss (six oldest concordant spots: 3920 ± 49 Ma; oldest spot: 3953 ± 54 Ma), which brackets the Nulliak rocks only loosely, while the Nulliak assemblage itself carries no direct depositional age. Shimojo et al. (2016) therefore called for chemical-abrasion ID-TIMS or higher-spatial-resolution ion-microprobe dating to refine the Iqaluk crystallization age and the multi-generational magmatic history between ca. 3.9 and 3.87 Ga, and for a direct age of the Nulliak supracrustal rocks. A decade of follow-up work — in-situ U–Pb–Hf zircon studies that deconvolve an 'Iqaluk' magmatic event at ca. 3857 Ma, detrital-zircon constraints showing Nulliak deposition before ca. 3.65 Ga, and challenges to the field relationships underpinning the >3.95 Ga claim — has sharpened but not resolved the question. The open problem is to establish a high-precision age framework for the Iqaluk Gneiss and a direct, or tightly bracketed, depositional age for the Nulliak assemblage, sufficient to decide whether supracrustal rocks older than ca. 3.9 Ga are actually preserved.

Background

The Saglek Block of northern Labrador (Canada) forms the northwestern part of the North Atlantic Craton, a fragment of Archean continental crust later rifted apart into Labrador and southern West Greenland. Its oldest components are the Uivak I orthogneisses — strongly deformed intrusive rocks of tonalite–trondhjemite–granodiorite (TTG) composition, the dominant crust type of the early Archean — which enclose meter- to kilometer-scale enclaves of the Nulliak supracrustal assemblage. 'Supracrustal' rocks form at or near Earth's surface (lavas and sediments), in contrast to the plutonic rocks that intrude them; the Nulliak assemblage comprises amphibolite (metamorphosed basaltic lava), ultramafic cumulates, banded iron formation (a chemical sediment of alternating iron-oxide- and silica-rich layers), marble and pelitic schist, first defined as a unit by Nutman, Fryer and Bridgwater (1989). Because supracrustal rocks record surface environments — seawater chemistry, weathering, possibly life — the age of the oldest preserved examples is a first-order fact of Earth history: before the Labrador claims, the oldest widely accepted supracrustal rocks were the ≥3.7–3.8 Ga sequences of the Isua region of southern West Greenland, with the ca. 3.83 Ga Akilia association the next-oldest candidate. The standard chronometer for such rocks is U–Pb dating of zircon (ZrSiO4), a uranium-bearing mineral that crystallizes in silicic magmas and survives later metamorphism. In secondary-ion mass spectrometry (SIMS, or ion microprobe) dating, small spots 10–30 µm across are sputtered from polished zircon sections, giving individual Eoarchean ages with uncertainties of tens of millions of years. In isotope-dilution thermal-ionization mass-spectrometry (ID-TIMS) dating, single grains or fragments are dissolved and their isotope ratios measured to roughly an order of magnitude higher precision; chemical abrasion (CA-ID-TIMS), a pre-treatment that anneals the crystal and then dissolves away radiation-damaged zones prone to loss of radiogenic lead, is the current highest-precision technique. Analyses are 'concordant' when the two independent U–Pb decay systems (238U–206Pb and 235U–207Pb) agree, the usual test that a zircon age has not been disturbed. Komiya et al. (2015) mapped an outcrop at St. John's Harbour South in which six generations of mafic and felsic magmatism intrude Nulliak supracrustal rocks, and Shimojo et al. (2016) dated the third generation — the Iqaluk Gneiss, a tonalitic orthogneiss — by SIMS U–Pb on zircon: the six oldest concordant spots average 3920 ± 49 Ma and the single oldest spot gives 3953 ± 54 Ma. Because the Iqaluk Gneiss intrudes the supracrustal enclave, the Nulliak rocks must be older than about 3.9 Ga — older than any then-known supracrustal rocks — and the same research group extended this age framework to a claim of >3.95 Ga biogenic graphite in Nulliak metasediments (Tashiro et al., 2017). The available precision is, however, marginal for the questions it must carry: an uncertainty of ±49 Ma cannot cleanly separate the successive magmatic generations emplaced between ca. 3.9 and 3.87 Ga, distinguish a magmatic crystallization age from inherited cores or ancient lead loss, or pin the Nulliak minimum age tightly enough for comparison with the Greenland record. Precision demonstrably matters in this very terrane: Krogh and Kamo (2006) showed that precise ID-TIMS zircon ages of northern Labrador gneisses overturned earlier ion-microprobe interpretations of the same rocks. The unresolved target is a high-precision age framework for the oldest Saglek orthogneisses and a direct age for the Nulliak supracrustal assemblage itself.

Problem Statement

Establish a defensible age framework for Earth's oldest candidate supracrustal rock package in the Saglek Block, northern Labrador, by answering two linked questions posed by Shimojo et al. (2016): (1) Determine the crystallization age(s) of the Iqaluk Gneiss at high precision so as to refine the currently imprecise age (six oldest concordant spots: 3920 ± 49 Ma; oldest spot: 3953 ± 54 Ma) and to resolve the duration and multiplicity of the magmatic generations emplaced between ca. 3.9 and 3.87 Ga at St. John's Harbour South and correlative localities. (2) Obtain a more precise and direct age for the Nulliak supracrustal rocks themselves. The two deliverables serve one overarching objective: to decide, on high-precision geochronological and field evidence, whether supracrustal (volcanic and sedimentary) rocks older than ca. 3.9 Ga — as claimed, >3.95 Ga — are actually preserved in the Saglek Block, or whether the apparent great ages reflect analytical scatter, zircon inheritance, ancient lead loss, or misassignment of the dated units.

References

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