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4. Locations

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Location No. 04

Roadcut at the main street north of Hallmare. This is one of the most worthwhile locations and hours can be spent detecting every detail. Unfortunately this outcrop adjoins to private property. Please do not use hammers or take any samples!

R: 1556377 - H: 6416861
RT 90: N 57° 52' 22.7" - E 016° 45' 30.4"
WGS 84: N 57° 52' 21.5" - E 016° 45' 19.5"

Keywords: LLDZ, blastomylonites, recrystallization, metasomatism, porphyroclasts, porphyroblasts


Description:

This location offers the excellent chance to examine parts of a big ductile shear zone of different stages of mylonitization which belongs to the Loftahammar-Linköping Deformation Zone. The exposure shows red colored feldspars (mm to cm size) which are embedded in a dark, fine-grained, massive matrix. The feldspars show different indications of ductile deformation from nearly undeformed idiomorphic crystals to sigma-clasts, and completely recrystallized grains which are stretched to form layers of several cm in length. The sense of shear appears to be constantly dextral.

The main fraction in the matrix is made up of light minerals like quartz, feldspar, calcite and rarely white mica and zircon. Thus, the dark color is mainly related to the small grain size. The actual percentage of the mafic fraction is only 15 - 20 %. It mainly consists of chloritized hornblende, layers of chlorite and biotite and rarely epidote (Basse 2002).

According to a description by Basse (2002), the feldspar clasts consist of alkali feldspar with a fine-grained reaction rim of plagioclase and potash feldspar. These rims look rather reddish, but the "cores" are white to pinkish. He described two different feldspar generations:

  1. Primary feldspar clasts as remnants of the granitic source rock.
  2. Porphyroblastic feldspars grown during syntectonic blastesis. All clasts/blasts were superimposed by a late- to post-deformative sodium metasomatism.

Due to visible alteration rims of fine-grained K-feldspar around some plagioclases, a K-metasomatism also has to be considered.

It is discussed whether the K-feldspars should be regarded as porphyroclasts or porphyroblasts. At first sight it might be considered that these feldspar clasts have experienced deformation in varying intensity but it cannot be excluded that early grown porphyroblasts have experienced a stronger and later grown blasts none or only less deformation (Vernon 1990).


Study Questions:
  • Find some significant feldspar clasts on which you can show the sense of shear.
  • Try to detect the different stages of deformation of the feldspar clasts.


Further Reading:

Beunk, F.F., 1996. Geology of the Loftahammar Area: Introduction and Excursion Guide, EUROBRIDGE 1996 Workshop, Oskarshamn, Sweden: Southern Baltic Shield - correlation of shallow and deep structures, 1- 44.

Vernon, R.H., 1990. K-feldspar augen in felsic gneisses and mylonites - deformed phenocrysts or porphyroblasts?, GFF, 112, 157-167.

General information on the development of the LLDZ:
Beunk, F.F. & Page, L.M., 2001. Structural evolution of the accretional continental margin of the Paleoproterozoic Svecofennian orogen in southern Sweden, Tectonophysics, 339, 67-92.



Fig. 4.4a: Overview of the outcrop (Foto: K. Wemmer, pers. comm.)
Fig. 4.4b: Partial view of feldspar clasts varying from semi-rectangular to sigma clasts and completely recrystallized grain layers.
Fig. 4.4c: Close up view on feldspar clasts varying from semi-rectangular to elongated shape, quartz vein at the bottom. (Photo: K. Wemmer, pers. comm.)
Fig. 4.4d: Cathodoluminescence image: relic core of K-feldspar mantled by two generations of recrystallized grains; I: K-feldspar (blue); II: plagioclase (albite, brown to pink); formation of stress-induced myrmekite in areas under compression; dextral sense of shear. (Photo: A. Vollbrecht, pers. comm.)


(top)

Fig. 4.4a
Fig. 4.4a

Fig. 4.4b
Fig. 4.4b

Fig. 4.4c
Fig. 4.4c

Fig. 4.4d
Fig. 4.4d
 

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