Abstract

Zircon and apatite crystals from the western Canadian shield yield (U-Th)/He dates that are the oldest yet reported for terrestrial rocks. Zircon dates from 1.73 to 1.58 Ga are consistent with independent geological and thermochronological constraints, and indicate that the rocks were at temperatures ≤180 °C and crustal depths ≤7–10 km since ca. 1.7 Ga. Apatite dates from 0.95 to 0.55 Ga suggest residence of rocks at temperatures ≤40–50 °C and crustal depths ≤1.5–2 km for ∼1.0–0.6 b.y., when interpreted using conventionally accepted apatite He diffusion kinetics and considering the proposed effect of radiation damage on apatite He retentivity. Our analysis implies long-term integrated unroofing rates of ≤2.5 μm/yr since ca. 1.7 Ga. These rates are significantly lower than the long-term rates suggested by previous thermochronological data sets in continental interiors, but are within the range of short-term erosional estimates. The results are consistent with the extreme stability of this region since the Proterozoic.

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