Iveland Wall, Iveland, Agder, Norwayi

Regional Level Types
Iveland Wall	Road Cutting
Iveland	Municipality
Agder	County
Norway	Country

The Iveland Wall is a cross cut through pegmatite containing amphibolite. Geologist Axel Müller (2017) writes the following about this fantastic geological phenomenon:

"At the junction of Ivelandsveien and Frikstadveien in thecentre of Iveland a spectacular road cut has been exposed in 2009.

The exposed cross-section, which is c. 25 m long and up to 6 m high, shows the three most typical rocks of the Evje-Iveland area: (1) the banded amphibole gneiss (1459 ± 8 Ma Vånne banded gneiss), (2) massive, dark green, gabbroic amphibolite of the Iveland-Gautestad mafic intrusion (1285 ± 8 to 1271 ± 12 Ma) and (3) coarse-grained pegmatites.

The protoliths of the banded gneiss were sediments deposited about 1.46 Ga.

At about 1280 Ma the sediments were intruded by gabbroic melt of the Iveland-Gautestad mafic suite.

During the Sveconorwegian orogeny the rocks of the Evje-Iveland area, being part of the Rogaland-Hardangervidda-Telemark Sector, were multiple metamorphosed.

The first metamorphic event comprises amphibolite to medium-pressure granulite conditions between 1035 and 970 Ma, called M1 stage and the second event high-temperature-low-pressure amphibolite to granulite facies conditions between 930 to 920 Ma (e.g.Bingen et al. 2008b).

The M2 stage took place at the end of a period of voluminous late orogenic magmatism during Sveconorwegian dilatation and relaxation.

The grade and structures observed in the Iveland Wall correspond to the M2 stage.

Applying PT estimates for M2 in southwestern Telemark by Bingen & van Breemen (1998) and Bingen et al. (1998), the interpolated pressure-temperature conditions during emplacement of the Evje-Iveland pegmatites (924 to 896 Ma) were close to 600° to 550°C at 4 to 5 kbars (corresponding to depths of 13.2 to 16.5 km).

The banded gneisses show migmatitic textures with leucocratic schlieren (neosomes; partially pegmatitic)flowing preferentially parallel to the ductile folded foliation and collect in larger, irregular batches of coarse-grained to megacrystic pegmatites.

Almost all types of migmatite structures, like diktyonitic, schollen, phlebitis, stromatic, surreitic, schlieren and folded structures (Mehnert 1968) can be observed in the wall.

Pegmatites in the northern part (left) of the wall are unzoned and contain euhedral magnetite (up to 4 cm) as major mafic mineral.

In addition a single, 3-cm large rutile crystal is exposed.

In the southern part (right) the pegmatites show simple zoning with granitic to blocky wall zone and quartz core.

The major mafic mineral is ‘biotite’, indicating increasing water content in the melt compared to the magnetite-bearing pegmatites.

There is a gradual transition between both pegmatite types.

The Iveland Wall is a unique exposure were the differentiation of the anatectic-formed Evje-Iveland pegmatites can be observed in meter-scale.

The heat responsible for this M2 stage metamorphism and the widespread and voluminous late orogenic magmatism in the Telemark lithotectonic domain can be explained by large-scale and long-lasting mafic underplating generating high heat flows (Hansen et al. 1996; Andersen et al. 2002b; Vander Auwera et al. 2011).

The underplating of the lower crust east of the Evje-Iveland area is evident from the emplacement of juvenile, mantle-derived material, such as the 940 Ma Tovdal granite, and from the presence of distinct, mantle-derived components in the other granites (Andersen et al. 2002a, b).

In the area of Evje-Iveland the underplating caused melting of the amphibolites and banded gneisses.

Field observations and geochemical modeling by Snook (2014) indicate that the pegmatites formed by 15 to 30% partial melting of Iveland-Gautestad amphibolites.

Figure 2.14 (in the paper) shows the modelling result for the rocks of the Iveland Wall performed by Snook (2014).

The REE composition of pegmatites exposed in the Iveland Wall (‘Target’) is achieved with 30% melting (approximately that observed in the wall) of the amphibolite gneiss (‘Source’). The estimated quartz crystallization temperature of Evje-Iveland pegmatites (613° ± 70°C; Müller et al. (2015), which represents the minimum temperature of partial melting, is in the range of fluid-present partial melting of crustal rocks (650°–700°C; e.g., Brown & Korhonen 2009).

However, the estimated regional temperature (600° to 550°C) in southwest Telemark is slightly below the fluid-present partial melting temperature.

That might be due to the fact that precise PT estimates for the M2 stage are missing for the Evje-Iveland area.

It is concluded that the Evje-Iveland pegmatites formed due to anatexis of metamorphosed intermediate and mafic rocks (amphibolites) of the Iveland-Gautestad intrusion (Müller et al. 2015).

The water-rich pegmatite melts were probably too viscous to coalesce into large granitic intrusions and/or the subhorizontal compressive stress regime did not allow for large-scale melt segregation, coalescence, and transport."


NOTE 2019:

During the student excursion of the University in Bonn, August 2019, the wall has been extensively mapped by XRF and Raman.

A brown mineral that initially was believed to be rutile, seems to be titanite in stead.

Pending the final result of the excursion, the list of minerals will not be updated with this data yet; only pyrite has been added since this identification is beyond doubt.

Other Regions, Features and Areas containing this locality

References