Pinpointing the Altar Stone Origin

The origin of Stonehenge's Altar Stone must be in Laurentian terranes north of the Iapetus Suture (a major geological boundary separating northern Laurentian-derived rocks from southern Gondwanan ones) because the U-Pb ages in detrital minerals like zircon, apatite, and rutile show dominant Mesoproterozoic-Archaean peaks (e.g., ~1,047–1,790 Ma from Grenville, Labrador, and Gothian orogenies) with a mid-Ordovician overprint (~451–462 Ma from Grampian events), which are absent or mismatched south of this suture in regions like the Anglo-Welsh Basin or Dingle Peninsula. It cannot be from the Orkney Islands (Mainland Orkney) because petrographic and mineralogical analyses reveal mismatches, such as abundant detrital K-feldspar in Orkney Old Red Sandstone samples (versus very low in the Altar Stone), absent tosudite clay, and rare baryte cement (present in the Altar Stone). It cannot be from southwest Scotland (e.g., southwest Grampian Highlands or Midland Valley Basin) because, despite broad terrane similarities, statistical tests (Kolmogorov–Smirnov P < 0.05) show zircon U-Pb spectra mismatches (e.g., additional Devonian grains and fewer Archaean-Palaeoproterozoic ones), plus multi-proxy discrepancies in apatite trace elements, rutile ages, Pb isotopes, and petrography (e.g., more metamorphosed Dalradian sequences lacking the Altar Stone's unmetamorphosed, low-K-feldspar, baryte-cemented fabric). Therefore, it must be from the Orcadian Basin in northeast mainland Scotland, specifically areas like the Moray Firth to Caithness or John O'Groats, where the Mid-Devonian Old Red Sandstone matches all signatures as first-cycle detritus from Grampian sources.

Schematic map of Britain, showing outcrops of ORS and other Devonian sedimentary rocks, basement terranes and major faults. 

From: Clarke, A.J.I., Kirkland, C.L., Bevins, R.E. et al. A Scottish provenance for the Altar Stone of Stonehenge. Nature 632, 570–575 (2024). https://doi.org/10.1038/s41586-024-07652-1

See also: Richard E. Bevins, Nick J.G. Pearce, Stephen Hillier, Duncan Pirrie, Rob A. Ixer, Sergio Andò, Marta Barbarano, Matthew Power, Peter Turner, Was the Stonehenge Altar Stone from Orkney? Investigating the mineralogy and geochemistry of Orcadian Old Red sandstones and Neolithic circle monuments. Journal of Archaeological Science: Reports, 58, 104738 (2024). https://doi.org/10.1016/j.jasrep.2024.104738

Signature Type	Altar Stone Characteristics	Matching Source Characteristics	Pinpointing Explanation
Detrital Zircon U-Pb Ages	Concordant ages span 498–2,812 Ma; major peaks at 1,047 Ma, 1,091 Ma, 1,577 Ma, 1,663 Ma, and 1,790 Ma (dominated by Mesoproterozoic and Archaean components; no Carboniferous-Permian grains).	Statistically indistinguishable (Kolmogorov–Smirnov test P > 0.05) from Orcadian Basin Old Red Sandstone (ORS) samples (e.g., Spittal Quarry, Caithness; similar Mesoproterozoic peaks tied to Grenville (1,095–980 Ma), Labrador (1,690–1,590 Ma), and Gothian (1,660–1,520 Ma) orogenies).	Rules out Anglo-Welsh Basin (mid-Palaeozoic zircon maxima, P < 0.05) and New Red Sandstone (lacks Archaean-Mesoproterozoic); matches Laurentian terranes north of the Iapetus Suture, narrowing to Orcadian Basin due to first-cycle detritus from Grampian Terrane. Southwestern Scotland claim (e.g., Kokelaar) over-relies on broad terrane similarities but ignores statistical mismatches (P < 0.05) with Midland Valley Basin (central/southwest Scotland), which has additional Devonian zircons (~402 Ma) and fewer Archaean-Palaeoproterozoic grains; no southwestern samples fit the exact spectra.
Apatite U-Pb Ages	Two groups: Group 1 at 462 ± 4 Ma (mid-Ordovician, n=108); Group 2 at 1,018 ± 24 Ma (Grenville, n=9).	Orcadian Basin apatite: 473 ± 25 Ma and 466 ± 6 Ma (Ordovician), 1,013 ± 35 Ma (Grenville); overlaps within analytical uncertainty.	Ordovician ages reflect Grampian magmatism (466–443 Ma granitoids/gabbros); Grenville peak indicates Laurentian derivation; excludes southern Britain (Neoproterozoic-early Palaeozoic dominance) and Dingle Peninsula (ages <450 Ma). Southwestern sources lack the precise mid-Ordovician overprint and Grenvillian balance seen in Orcadian samples, further distinguishing from broader Grampian Terrane areas south of the Great Glen Fault.
Apatite Trace Elements	61% felsic (La/Nd <0.6, (La + Ce + Pr)/ΣREE <0.5, median Eu/Eu* = 0.59); 35% mafic-intermediate (La/Nd 0.5–1.5, (La + Ce + Pr)/ΣREE 0.5–0.7, median Eu/Eu* = 0.62); 4% alkaline (La/Nd >1.5, (La + Ce + Pr)/ΣREE >0.8, median Eu/Eu* = 0.45). Chondrite-normalised REE patterns show flat to negative gradients; mafic grains REE-enriched (up to 1.25 wt% ΣREEs).	Aligns with Grampian Terrane granitoids (felsic dominance) and Orcadian samples (similar REE profiles and principal component analysis discriminants like Nd and La).	Felsic-mafic mix indicates direct input from Grampian igneous sources; supports northeast Scottish provenance over Anglo-Welsh (different REE signatures and metamorphic overprints). Southwestern/central Scotland sediments show varied REE profiles due to different metamorphic histories and source mixing, not matching the Altar Stone's specific felsic-mafic ratio or REE enrichment.
Apatite Lu-Hf Ages	Ages at 1,496 Ma and 1,151 Ma (Laurentian); Group 1 at 470 ± 29 Ma (Ordovician).	Matches Orcadian Basin (e.g., 470 Ma overprint from Grampian events).	Reinforces Laurentian crust sourcing with mid-Ordovician metamorphic-magmatic overprint unique to regions north of Iapetus Suture. Southwestern claims ignore this overprint's specificity to northeastern basins, where Grampian events align more closely.
Rutile U-Pb Ages	Group 1 at 451 ± 8 Ma (mid-Ordovician, n=83); Group 2 Proterozoic (591–1,724 Ma, peak at 1,607 Ma, overlapping Labrador/Pinwarian orogenies).	Consistent with Laurentian orogenies in Orcadian Basin (Ordovician overprint from Grampian; Proterozoic peaks match basement terranes).	Ordovician group indicates Grampian influence; Proterozoic peak rules out southern Gondwanan terranes (Neoproterozoic rutile dominance); supports first-cycle detritus from northeast Scotland. Rutile signatures in southwestern Scotland include more variable Proterozoic peaks due to Dalradian metamorphism, not fitting the exact Labrador/Pinwarian dominance.
Pb Isotopes (207Pb/206Pb)	Apatite: 0.8603 ± 0.0033; Rutile: 0.8564 ± 0.0014 (initial ratios).	Matches Stacey-Kramers continental crust evolution model at 465 Ma (0.8601).	Consistent with evolved Laurentian crust north of Iapetus Suture; excludes less radiogenic southern British sources. Southwestern sources show similar but not identical ratios, diluted by local crustal variations not present in Orcadian Basin.
Mineral Composition & Petrography	Micaceous sandstone with baryte cement, calcite, clay minerals (including tosudite); very low K-feldspar; heavy mineral bands (zircon, rutile, apatite) with igneous textures (oscillatory zoning, no metamorphic overgrowths); absent marine fossils.	Orcadian Basin (non-Orkney): Low K-feldspar, presence of baryte and tosudite in some sequences; first-cycle magmatic detritus. Differs from Mainland Orkney ORS (abundant detrital K-feldspar in all samples, absent tosudite, baryte rare in only 2 samples).	Rules out Mainland Orkney (petrographic mismatches via X-ray diffraction, Raman, SEM-EDS); indicates continental fluvial-lacustrine deposition in other Orcadian areas (e.g., Moray Firth to John O'Groats); supports Mid-Devonian ORS with Grampian-derived sediments. Southwestern Grampian/Dalradian sequences are more metamorphosed (e.g., poly-deformed with garnets), lacking the unmetamorphosed, low-K-feldspar, baryte-cemented fabric; no exact petrographic match exists there.