May
16

Background: The Geologic History of the Lofoten Islands, Norway

Since I will be traveling to the Lofoten Islands, northern Norway this month, (from May 19 – June 1), it is important that the geologic history of the islands is well-understood. Not only will this put what we find today into its geologic context, but it will also help me understand the types of kinematic processes that were at work in this area of the world back when these rocks and structures were forming.

The Lofoten Islands are an archipelago off of the coast of northern Norway at a latitude of 67.5° N, and are a part of the Scandinavian Caledonides, a mountain range that stretches from the tip of northern Norway all the way to southern Norway. The geologic history of the Lofoten Islands is complex and includes a series of deformational and metamorphic events that reclassify, or alter, many of Lofoten’s rock units and structures. Lofoten’s history began approximately 2.7 billion years ago (Ga) with the formation of Archaean basement plutons which intruded a collection of supracrustals, and at 2.3 Ga many of these rocks were migmatized, or partially melted and recrystallized. At 2.1 Ga, more supracrustal rocks were deposited, and shortly after this, starting at roughly 1.9 Ga, there was a granulite-facies metamorphic event that overprinted them.

During the latter half of this event, first at 1.87-1.86 Ga then again at 1.8-1.79 Ga, large mangeritic and charnockitic plutons as well as smaller gabbroic, anorthositic, and granitic plutons were emplaced into this series of basement rocks. This set of emplaced magma bodies is known as the Anorthosite-Mangerite-Charnockite-Granite (AMCG) Suite. Some of the protoliths for the eclogites in Lofoten today are from this AMCG Suite, while other protoliths derive from the original country rock.

Next, the Leknes Group, a previously eclogite-facies, allochthonous group of metasedimentary and metavolcanic rocks with an unknown origin, were thrust upon the Lofoten basement rocks at 469-461 million years ago (Ma) under amphibolite-facies conditions. As this emplacement happened, Lofoten basement rocks continued to subduct for approximately 17 million years into the mid-lower crust before the rocks experienced a pulse of rapid uplift, and then remained at upper-mid crustal depths for 100-190 million years. This was followed by slow uplift to the surface, where we can find the rocks today.

Over the next two weeks, I will be conducting extensive field work in the Lofoten Islands, and I will get to see many of the rocks I have described above. Eclogitization of some of these rocks is postulated to have occurred ~51 km beneath the surface sometime around 478 Ma, and under pressure-temperature (P-T) conditions of 15 kbar and 680–780 °C. We hope to find evidence of this out in the field in addition to our future laboratory analysis of the rocks. The field work I will conduct will include mapping the eclogite-facies shear zones, collecting samples, and taking structural measurements along the shear zones. All of the measurements and samples I take will be further analyzed back in the United States.

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