MESOPROTEROZOIC TECTONIC PROCESSES BETWEEN 1520 AND 1230 MA IN THE EASTERN GRENVILLE PROVINCE AND ITS NORTHERN BORDERLAND

The evolution of the eastern Grenville Province can be divided into three distinct stages. The first (1800-1600 Ma) involved Labradorian arc accretion and arrested subduction. The second (1600-1230 Ma) was one of continental margin processes and their inboard expression. The third (1230-955 Ma) involved compressional processes culminating in Grenvillian (1080-980 Ma) orogenesis.

This presentation is focused on the second stage. After Labradorian subduction ceased, a passive continental margin developed. This lasted until Pinwarian continental-arc orogenesis (1520-1460 Ma). Activity then shifted farther north with two contrasting, coeval events between 1460 and 1420 Ma (Early Elsonian). Gabbro sheets were emplaced in the northern Grenville Province and the AMCG suites farther north. The pattern continued with the emplacement of the Nain Plutonic Suite between 1350 and 1290 Ma (Middle Elsonian) even farther north. Along with its northward advance and younging, the overall shape of magmatically affected regions at any given time changed from elongate east-west to elongate north-south, a feature accompanied by similar change in orientation of coeval mafic dykes. After 1290 Ma (Late Elsonian; 1290-1230 Ma), northward migration ceased and was replaced by a central zone of rift-related mafic magmatism, with, to the north and south, inner flanking swarms of mafic dykes, and outer flanking zones of dominantly alkalic felsic magmatism.

The above magmatic pattern is related to funneled flat-subduction, possibly also including an over-ridden oceanic spreading centre. The funneling is caused by subducted oceanic crust being driven deeper by Early Paleoproterozoic orogenic roots between the Superior and Nain Archean cratons. Post-1290 Ma bimodal magmatism began as a consequence of limitations on inboard incursion of subducted crust, forcing back-arc basin development.