Oslo Fjord
An ancient heritage with strange minerals
Oslo Fjord in southern Norway was carved by glaciers over the past two million years or so, but the down-faulted trough (a graben, from German for ditch) that localized the glaciers has a much older origin.
The photos at the top are from near Grundvikveien (Nærsnes), Norway, on the west side of Oslo Fjord (but near the east side of the Oslo Graben). They show three different episodes of tectonic deformation. In the upper photo, my friend Pat Dickerson is pointing out planes within the Precambrian granite and gneiss that forms much of the Scandinavian craton (also called the Baltic shield). It’s a complexly deformed and metamorphosed terrane that last underwent significant pressure and high temperatures about 1000 million years ago (to as young as 920 million years ago) – Episode #1.
The tilted, layered rocks in the background of the upper photo and in the upper left part of the outcrop in the lower photo are Lower Paleozoic rocks, probably Ordovician (around 460 million years old), that were deposited in a foreland basin adjacent to the developing Caledonian Fold Belt. The Caledonian Orogeny (mountain-building event) culminated toward the end of the Silurian Period and in the early Devonian Period, about 390 to 420 million years ago. It resulted from the closing of the Iapetus Ocean when Scandinavia and Greenland collided (the Atlantic Ocean formed when those areas ripped apart, nearly but not exactly along the old line of Caledonian collision). In these photos, that deformation (Episode #2) is recorded in the tilted Ordovician rocks.
There was additional collision a bit later (largely Carboniferous to Permian time, about 320 to 270 million years ago) that was most prominent as the Variscan Orogeny in central Europe, with analogies in the Allegheny Mountains of the United States. Complex collisions don’t always just produce compression and squeezing; the rocks may respond by breaking in other ways. The extensional rift of Lake Baikal in Siberia is an aspect of the collision between India and Eurasia, and during the Variscan Orogeny similar rifts developed in Europe.
One of those Carboniferous to Permian rifts today underlies southern Norway, helping to localize the north-south-trending Oslo Fjord. In the photos, there is a fault between the Precambrian and Ordovician rocks – it’s not a simple unconformity – that developed as part of that Permian rifting about 290 million years ago. The fault is exposed in the lower photo, Episode #3 of deformation here. That faulting formed the downdropped Oslo Graben, a north-south trough set into the older Precambrian rocks.
The faults were pervasive enough that they allowed molten material to rise and partially fill the trough with igneous intrusive rocks (similar to granite) and volcanic rocks, both of which are rich in alkali elements (sodium, potassium), resulting in diverse and unusual minerals.
The map above shows the geometry of the graben; I added localities for the minerals I show below. All the white areas are Precambrian, and the colors are either preserved Paleozoic rocks (blue) or intrusive (red) and volcanic (green) rocks associated with the formation of the graben in Carboniferous to Permian time.
Minerals associated with the Carboniferous-Permian igneous activity and the forming of the graben include strange minerals such as loparite-Ce (sodium-rare-earth titanate), kupletskite (sodium-potassium iron-manganese titano-silicate fluoride!), and ktenasite (hydrous zinc-copper sulfate), as well as aegirine (sodium-iron silicate), the pyroxene typically associated with alkalic magmas.
The Glomsrudkollen deposit containing the blue-green ktenasite is a skarn, a contact metamorphic zone in which there has been chemical exchange between the country rock and an intrusive magmatic body. The location was mined sporadically from 1870 to 1921, mostly for zinc. It’s not visible in the photo, but the ktenasite sits on a substrate of sphalerite, zinc sulfide.
The host rock is probably Silurian limestone (blue on the map above) altered by the intrusion of the Finnemarka Batholith (about 260 million years old, Permian, red on the map). Permian faulting that formed the Oslo Graben complex (locally the Vestfold-Ringerike Graben) may have contributed to the mineralization as well.
According to MinDat, ktenasite was Named by P. Kokkoros in 1950 for Konstantinos I. Ktenas (1884-1935), Greek physician, professor of mineralogy and petrology and Dean of the National and Kapodistrian University of Athens, Director of the Museum of Mineralogy and Paleontology and of the Zoological Museum of the University, Head of the Greek Geological Survey and one of the founding members of the Academy of Athens.
The much older Precambrian metamorphic rocks host a famous locality for octahedral hematite crystals at Dypingdal, where they are associated with lizardite (hydrous magnesium silicate) and other magnesium-rich serpentinite rocks. Dypingdal is also the type locality for hydrotalcite, hydrous magnesium-aluminum carbonate. Many of the magnesium-bearing serpentine rocks are the result of retrograde metamorphism (cooling and hydration, down from a higher peak temperature of metamorphism) of primary magnesium minerals like olivine and enstatite in original ultramafic rocks, which might have been something like oceanic crust involved in the tectonic activity 1000 million years ago (Episode #1 above).
Despite its age, about 300 million years, the Oslo Graben is a persistent weak zone that helped focus glaciation over the past 2 million years or so, creating the modern fjord (100-120 km, 60-75 miles long). The Oslo Graben system is about twice the length of the modern fjord.
For a nice overview of the tectonic history of the Oslo Graben, see Larsen and others, 2008, The Permo-Carboniferous Oslo Rift through six stages and 65 million years.Â
