Insights into early continental crust formation from the most ancient heart of Scotland.
basic_science · Level V
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- Record sourced from PubMed, PMID 42014690.
- Also identified by DOI 10.1038/s41467-026-72076-6.
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Abstract
The paucity of rocks from Earth's first billion years (4.5-3.5 Ga) limits understanding of early felsic (continental) crust formation and craton development. We present zircon U-Pb, O- and Hf-isotope and whole-rock geochemical data from deformed and metamorphosed (ultra)mafic and felsic rocks of the Archaean Lewisian Gneiss Complex, NW Scotland. The felsic, MgO-rich hornblende-bearing tonalite gneisses contain magmatic zircon populations at c. 3.6, 3.5, and 2.8 Ga, as do enclosed hornblendite pods (>95 vol.% hornblende). Zircons from hbl-tonalite preserve median δ¹<sup>8</sup>O<sub>(zircon)</sub> of ~5.8‰, indicating limited supracrustal input. The hornblendites have high-MgO (~18 wt%), Cr (up to 5500 µg/g), and Ni (up to 2800 µg/g) concentrations and record slightly higher δ¹<sup>8</sup>O<sub>(zircon)</sub> median values (~6.3‰). Zircon εHf<sub>(zircon)</sub> from both lithologies implies a Hadean to early Archaean (c. 4.1-3.9 Ga) depleted mantle source variably modified by low-temperature surface-derived fluids and/or incorporation of Hadean continental crust. We argue that the hornblendites represent vestiges of a hydrated (proto)crust that contributed to the growth of the earliest Archaean continental nuclei.