GEOCHEMISTRY OF PRECAMBRIAN BASEMENT ALONG THE TRANS-HUDSON FOLDBELT/SUPERIOR CRATON BOUNDARY ZONE, CENTRAL NORTH DAKOTA, USA

Two Precambrian basement cores from Oliver County, North Dakota, USA were recently cut into the proposed Trans-Hudson foldbelt and Superior craton boundary zone. The J-ROC1 intersected basement at 2969.7 m (9,743 ft) and the J-LOC 1 at 3132.9 m (10,278.5 ft) depths in the Williston Basin.

Petrography and geochemistry (FMW weathering plot; K₂O ~ 11 wt.%) of J-ROC1 show the top 2 m of granitic gneiss and thin granitoid intrusion are heavily altered, perhaps by weathering processes. Granitic gneiss contains ~2.68 Ga Archean zircons while the intrusive granitoid contains both Archean and Paleoproterozoic (1.78 Ga) zircons. The Archean components have ɛHf(i) ranging from +3.6 to +4.1 indicating derivations from a relatively homogeneous depleted mantle. LaN/YbN are 59-70 with Eu/Eu* = 0.75-0.98. They show negative Nb-Ta anomalies and trace elements plotting in the VAG field. Unaltered granitic gneiss plots as metaluminous.

The J-LOC 1 mafic gneiss (50.42 wt.% SiO2) is mostly foliated fine to coarse grained hornblende-plagioclase-biotite. It contains thin quartz and cc veins, and cm to 60 cm, med. grained to pegmatitic granitoid intrusions. Foliated gneiss plots as alkaline, has LaN/YbN= 41, Eu/Eu*=1.58, and no Nb-Ta anomaly. It contains 1.83 Ga zircons while the intrusive granitoids (SiO2 = 72-73 wt.%) yielded slightly younger ages of 1.75 Ga, or mixed Paleoproterozoic-Archean zircons. Granitoids (72-73 wt.% SiO2) are peraluminous, with normative Corundum values of 0.8-1.7. However, they plot as VAG on the Y+Nb vs Rb diagram. The Paleoproterozoic samples represent either a depleted mantle component (ɛHf(i) = +6.6 ± 1.1) or a mixture of depleted mantle and older (Neoarchean) crustal components, (ɛHf(i) = +2.2 ± 4.3 and -1.7 ± 3.0) consistent with the existence of mixed ~2.68 Ga and ~1.78 Ga components in these samples.

Together, these cores contain a mix of Paleoproterozoic and Archean crustal components, consistent with the regional interpretation that they sample the Superior/Trans-Hudson boundary zone.