Attempting to Pierce the Metamorphic Veil in the Grenville Orogen of the Canadian Shield

English Abstract:

The Grenville Province of the Canadian Precambrian Shield is a mélange of tectonic domains and terranes, containing Archean as well as Proterozoic rocks and assembled in a series of collisions with successive margins of the main Laurentian craton. The details of the accretionary events are obscured by the deep burial and consequent metamorphism of most presently exposed domains and terranes – the Grenvillian metamorphic veil. Post-metamorphic erosional uplift and cooling were extremely slow: cooling from 550⁰C to 250⁰C typically required 200-300 million years. Although it has proven extremely difficult to “see through” the Grenvillian orogeny to the magnetic record of pre-metamorphic events, partial thermoremanent magnetizations (pTRMs) acquired as different grain-size fractions of magnetite (and on occasion hematite) cooled through their blocking temperatures provide a record of post-metamorphic apparent polar wander (APW). Time calibration of this post-metamorphic Grenvillian APW path is provided by ⁴⁰Ar/³⁹Ar plateau ages of minerals like hornblende, actinolite, biotite, muscovite, plagioclase and K-feldspar with known closure temperatures to Ar diffusion and on occasion also by U/Pb ages of sphene. The partnership between isotopic thermochronology and paleomagnetism has been very successful in delineating the 1000-800 Ma APW of “Grenvillia” – and by implication the rest of Laurentia, since the accreted terranes were firmly welded to the craton prior to uplift. Despite quibbles about the details of the Grenvillian polar track, this part of the Laurentian APW path remains better documented than that of almost any other Precambrian time interval, younger or older. The multiple pTRM overprints are also amenable to determination of paleofield intensities (occasionally two paleointensities of different ages for a single rock), the main uncertainty being the correction to be applied for slow cooling in nature compared to that in laboratory heatings and coolings.