EGU21-2909, updated on 03 Mar 2021
https://doi.org/10.5194/egusphere-egu21-2909
EGU General Assembly 2021
© Author(s) 2021. This work is distributed under
the Creative Commons Attribution 4.0 License.

Mg-K-Fe fluid producing mineral reactions, metasomatism and microfabric development during formation of nodular sillimanite-gneiss in the middle crust

Ane K. Engvik1, Claudia A. Trepmann2, and Håkon Austrheim3
Ane K. Engvik et al.
  • 1Geological Survey of Norway, Trondheim, Norway (ane.engvik@ngu.no)
  • 2Department of Earth and Environmental Sciences, Ludwig-Maximilians-University Munich, Germany (Claudia.Trepmann@lmu.de)
  • 3Physics of Geological Processes (PGP), The NJORD Centre, Department of Geosciences, University of Oslo, Norway (h.o.austrheim@geo.uio.no)

The Proterozoic gneisses of the Bamble lithotectonic domain (south Norway) underwent intense scapolitisation caused by K- and Mg-rich fluids and extensive albitisation with formation of numerous ore deposits.

By detailed studies of mineral reaction fabrics we document release of the chemical active Mg, K and Fe-components forming the metasomatic fluid: Breakdown of biotite to muscovite releases K, Mg, Fe, Si and H2O. As reaction products tiny Fe-oxide needles are present in the transforming rock. H2O is reacting with K-feldspar to produce additional amounts of white mica and quartz. During a subsequent reaction muscovite is replaced to sillimanite again releasing quartz and a K-rich fluid. The reactions form the peculiar sillimanite-nodular quartzite, but also well-foliated sillimanite-mica gneiss.

Optical and EBSD microfabric studies reveal a shape preferred orientation for quartz, but despite of a pronounced foliation, quartz does not show a crystallographic preferred orientation. A crystallographic preferred orientation is present for mica and sillimanite. Coarse micas show sutured boundaries to quartz, implying low nucleation rates, no crystallographic or surface-energy control during growth and no obvious crystallographic relationship to quartz.

Our study illustrates the transformation of a quartzofeldspatic lithology into sillimanite-bearing quartzite. The mineral replacement and deformation show ongoing metamorphic reactions during deformation. The microfabric data indicates reaction at non-isostatic stress condition. The deduced mineral replacement reactions document a source of K-, Mg- and Fe-rich metasomatic fluids necessary to cause the pervasive scapolitisation and Fe-deposition in the area. The mineral reactions and deformation produce rocks with a new mineralogy and structure; an increased understanding of these processes is important for the modelling of crustal building and geological history.

How to cite: Engvik, A. K., Trepmann, C. A., and Austrheim, H.: Mg-K-Fe fluid producing mineral reactions, metasomatism and microfabric development during formation of nodular sillimanite-gneiss in the middle crust, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2909, https://doi.org/10.5194/egusphere-egu21-2909, 2021.

Display materials

Display file

Comments on the display material

to access the discussion