Paleomagnetism of pre-Grenvillian mafic rocks from the northeast Grenville Province, Labrador: implications for the Grenville Track

Abstract

A paleomagnetic study of mafic plutonic rocks in the northeast Grenville Province found five magnetizations, interpreted as Grenvillian and later overprints. Components A, B, and C, with the highest unblocking temperatures and distinct regional distributions, have paleopoles that appear to lie on the Grenville Track. Component C, with pole at 348°E, 11°S (δp, δm = 2°, 4°) and mostly distributed in lower-grade mafic rocks in the Groswater Bay terrane near the Grenville Front, is interpreted as a post ca. 970 Ma Grenville overprint, predating components A and B. The latter components, nearly coextensive in distribution in the Groswater Bay and Sandwich Bay regions farther south, have respective poles at 326°E, 02°N (N = 9 sites; δp, δm = 8°, 11°) and 348°E, 13°N (N = 13 sites; δp, δm = 6°, 8°). Component B could be interpreted either as a Grenville overprint or, more likely, as a later overprint of late Neoproterozoic (615 Ma) to Cambrian age due to rifting in the Lake Melville region. If so, many other so-called "Grenville" overprints from rock units within or near the St. Lawrence rift system may have been erroneously attributed to Grenvillian thermal effects and, in fact, be younger. If component B is post-Grenville and component C, a Grenville overprint, then component A is probably latest Grenvillian or later Neoproterozoic in age. These results strongly suggest that the Grenville Track needs reinterpretation.