#boudinage

The history of these rocks begins over 2.5 billion years ago, in the Archaean. The rocks of this area are ancient, with many sections first having formed 2.8 to 3.0 billion years ago as igneous rocks, and occasional bits possibly being as old as 3.7 billion years old – some of the oldest rocks on Earth. All of them were first metamorphosed about 2.5 billion years ago, heating and stressing them so much that the original igneous textures and relationships were removed. The dark rocks used to be dikes of basaltic igneous rock. About 2 billion years ago, a suite of igneous rocks known as the Kangâmiut dyke swarm intruded this area. Dikes are fractures in rock formed when areas are pulled apart and molten rock swarms in. This area, therefore, was being pulled apart about 2 billion years ago, telling part of the story of this area. Most likely, the earlier metamorphism is related to an early collision between landmasses – pieces of what are today Greenland and North America ran into each other, triggering mountain building and metamorphosing rocks at the bottom. Then, the rocks began to pull apart – creating fractures and allowing the dikes to enter the rocks. Finally, about 1.8 billion years ago, another large metamorphic event happened. After the opening and rifting, mountain building resumed in several pulses, including generation of new igneous rocks. At this spot, we had igneous dikes cross cutting metamorphic rocks. As they were heated and sheared, the dikes remained stronger than the surrounding metamorphic rocks. The dikes fractured into blocks, while the metamorphic rocks around them smeared out and filled in the gaps. This structure is called “Boudinage”, literally referring to the strong rocks being stretched out into blobs similar to the making of sausage. The 1.8 billion year old metamorphic rocks seems to have related metamorphic rocks in Canada, so this mountain building event was likely the docking of the North Atlantic Craton to the growing continent called Laurentia. These continents stayed hooked together until about 60 million years ago, when the Labrador Sea opened along with the spreading of the North Atlantic. -JBB Image credit: https://commons.wikimedia.org/wiki/File:Boudinage_near_Kangerlussuaq.JPG References: http://trigon-gs.com/Wittig_et_2010.pdf http://www.eng.geus.dk/media/10932/nr28_p57-60.pdf http://www.geus.dk/media/12311/nr11_p061-086.pdf https://www.nrcresearchpress.com/doi/10.1139/e02-027