TECTONIC EVOLUTION OF THE DASHWOOD MICROCONTINENT AND ITS ACCRETION TO THE LAURENTIAN MARGIN

The earliest orogenesis in the Dashwoods microcontinent involves west-directed obduction by suprasubduction zone ophiolites and associated melanges of the Baie Verte Oceanic Tract (BVOT). Shortly thereafter the BVOT is intruded by Tremadoc Notre Dame arc plutons (c. 489-483 Ma), which also intrude a coeval volcano-sedimentary sequence. The arc volcanic and plutonic rocks together with associated older sedimentary rocks were subsequently buried and deformed during HP amphibolite to granulite facies metamorphism (e.g. cpx-gar-hnbl) prior to intrusion of voluminous c. 460-456 Ma syntectonic arc tonalites and granodiorites. The granulite facies metamorphic volcanic and sedimentary rocks together with a charnockite are preserved in the Cormacks Lake Complex, previously thought to represent Grenville basement. U-Pb dating of all its major components, including the charnockite, leaves no doubt that the Cormacks Lake Complex represents the deepest buried part of the Ordovician Notre Dame arc. Available ages suggest there is at least a gap of 10 Ma (480-470 Ma) in arc magmatism immediately preceding and/or during high grade metamorphism. The earliest phase of Notre Dame magmatism took place after obduction of the BVOT onto the Dashwoods microcontinent. However, this process cannot easily explain the subsequent deep burial of the suprastructure of the Dashwoods microcontinent prior to intrusion of the voluminous tonalities, because the latter dictates that the Dashwoods has an upper plate setting. We suggest that the burial of the Notre Dame arc is due to a tectonic thickening induced by a hard collision between the Dashwoods and the adjacent Laurentian margin following closure of the Humber seaway. Resumption of arc magmatism during the Late Ordovician to Early Silurian. 450-435 Ma is due to west-directed subduction and convergence of outboard terranes (e.g. Exploits backarc elements) to Laurentia. This process caused intermittent shortening in the Dashwoods and adjacent Humber Zone. This period was followed by rapid exhumation of granulite facies rocks in the Dashwoods, which led to formation of decompression coronas, during the late Early Silurian (c. 430 Ma) and is also coeval with the onset of bimodal magmatism. We propose that the local rapid exhumation and bimodal magmatism is related to slab break-off.