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Abstract Enabling the build-up of continental crust is a vital step in the stabilization of cratonic lithosphere. However, these initial crustal nuclei are commonly either destroyed by recycling or buried by younger rocks. In the Fennoscandian Shield, the oldest rocks are ca. 3.5 Ga, but ca. 3.7 Ga inherited and detrital zircons suggest the presence of an older, unexposed crustal substrate. We present U-Pb, O, and Hf isotope data from detrital zircons of three major Finnish rivers as well as zircon O and Hf isotope data from previously dated rocks of the Archean Suomujärvi and Pudasjärvi complexes, central Finland. Combined, these data indicate a previously unidentified ca. 3.75 Ga crustal nucleus in the Fennoscandian Shield. This adds to the growing number of Eoarchean nuclei recognized in Archean terranes around the globe, highlighting the importance of such nuclei in enabling the growth of continental crust. The isotope signatures of the Fennoscandian nucleus correlate with equivalent-aged rocks in Greenland, consistent with a common Eoarchean evolution for Fennoscandia and the North Atlantic craton.
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This content will become publicly available on March 1, 2026
USA’s Oldest Rock? A Simple Question with a Complex Answer
Superlatives—whether tallest, longest, or fastest—are more interesting than averages. This characteristic applies to many aspects of the geosciences, where scales of time and space are beyond human experience. The deepest trench, the highest mountain, and the most expansive desert are much more interesting than average ones. Interest in superlatives also applies to the oldest rocks. In this essay, we show that the oldest rocks in the United States are 3.62–3.45 billion years old (Ga) and are found in three different states. These localities define an east-west−trending belt in the upper midcontinent that stretches ~3000 km from Wyoming through Minnesota and into the Upper Peninsula of Michigan. Complex U-Pb zircon systematics are observed in the oldest rocks from all three areas, complicating efforts to distinguish zircons that crystallized in the magma(s) that made the host rock from xenocrystic zircons incorporated by assimilating older rocks. Within these uncertainties, the oldest rock in the United States is 3.62 Ga (Eoarchean to Paleoarchean), but older, 3.8 Ga zirconbearing felsic crust existed and may be identified by future investigations.
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- Award ID(s):
- 2042243
- PAR ID:
- 10590055
- Publisher / Repository:
- Geological Society of America
- Date Published:
- Journal Name:
- GSA Today
- Volume:
- 35
- Issue:
- 3
- ISSN:
- 1052-5173
- Page Range / eLocation ID:
- 4 to 10
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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