Calibration of Grenvillian palaeopoles by ⁴⁰Ar/³⁹Ar dating

Abstract

PALAEOPOLES from the exposed Grenville Province of the Canadian Shield are displaced from those of the remaining shield. It has been proposed that these poles record either a Precambrian plate collision ∼1,000 Myr (refs 1–3) or manifest a large rotation and translation of the entire extant North American crust^4–6. Geological evidence in support of the collisional hypothesis is largely lacking; for example, an acceptable suture zone has not been located. The critical evidence for a choice between the two-plate and one-plate tectonic hypotheses must, therefore, come from dating these palaeopoles because if the magnetisations were acquired contemporaneously with those of the Logan Loop (≃1,000–1,200 Myr), then the two-plate or collisional hypothesis is probably correct. We present here an empirically derived cooling curve for the Haliburton basic intrusions within the Grenville Province. These intrusions are located in a region of high-grade metamorphism, upper amphibolite to granulite facies. Their emplacement times are not known, but in the Haliburton Highlands cooling from the culmination of regional metamorphism, not contemporaneous throughout the Grenville Province, began ∼1,050 Myr ago⁷. This is the first cooling curve for an orogen to be derived solely from isotopic data; that is, the first for which both temperature and time have been determined from the same samples. Using this as a calibration curve, we show here that the thermal remanent magnetisation (TRM) components from the Haliburton basic intrusions were acquired between ∼980 and 820 Myr ago during slow cooling and that, consequently, the two-plate hypothesis is improbable.