PELITIC GNEISS

The Pelitic Gneiss provides a window into the varying, early metamorphic conditions that existed in the suite of units present.  Unlike Baba (1998), I have not distinguished between Na & K rich and Fe & Mg rich varieties, but focused the distinction on modal abundances of aluminous minerals.  Except for these, the Pelitic Gneiss has a similar mineralogy, geological and inclusion relationships to the Quartzofeldspathic Gneiss. 

 The field map contains symbols relating to a subdivision based on the colour of the minerals present (P and Pr), but this did not add any weight to the geology and the distinction does not appear on the Final Geological Map.  Gradational contacts with Quartzofeldspathic and Leucocratic Gneiss: My distinctions were determined by map clarity.

Due to the garnet and kyanite content, this rock is more resistant to weathering than its neighbours (mainly the Amphibolite and Quartzofeldspathic Gneiss) and forms the ridges adjacent to the coast (e.g. Bideinan).  An 800m thickness at Chaipaval thins to a minimum of 80m in the continuous layer of Pelitic rock down the coastline in the NW-SE trend where a dextral mylonitic contact to the Amphibolite is maintained when adjacent.

Both kyanite and sillimanite (Photo 14) zones are present within the Pelitic and Quartzofeldspathic Gneiss, kyanite zones being volumetrically much greater.  There is a 400m of overlap around Bideinan (see Final Geological Map). 

Hand specimens revealed a variety of crystallization conditions and aluminous mineral content and common biotite development as foliation and rims to garnet.  A restite was found at locality 106 (Figure 10).

 













Figure 10: Extreme concentrations of resistant minerals such as the above may have resulted from the melting of a pelitic rock.  Specimen from locality 106, 00278636, notebook page 62.  XRD of T61 indicates presence of garnet and kyanite and additional cordierite (Appendix VIII)

Photo 15 illustrates the exposure scale observations associated with a mylonite zone where a particularly garnetiferous layer is affected as it grades into a Leucocratic horizon. Plagioclase rims (M5) to garnet (M2) are conspicuous, biotite foliation (M6) is also apparent (more clearly seen in Photo 16).

Hand specimen scale features are summarized on page 34: T29 demonstrates a quartz rich variety from The Obbe, Leverburgh with retrogressive biotite, T26 is from a more mylonitized area with more retrogression to biotite (M6), T43 highlights the concentrations of kyanite possible with its organization into layers and T44 demonstrates the involvements of narrow mylonite zones (Appendix VI for a thin section of the mylonitic fabric).

Thin sections T44 and T48a show at least 12 mineral species present (See Appendix VI).  Spinel is observed in the Pelitic Gneiss T48a mantled by ilmenite, possibly an M1 to M2 cooling transition. T48a also demonstrates retrogressive sillimanite (M5) (see also XRD 12 from the same locality).  T44 especially demonstrates the inclusion relationships of sillimanite in the outer regions of garnet (Figure 11, also observed by Baba (1997) indicating an early high temperature history (M1) superimposed by high pressures of kyanite formation (M5) that overgrow the main foliation (M2). 


























Figure 11: Thin Section T44, XPL, fov = 3 x 2 mm
Sillimanite inclusions (laths) occur at the margins of the garnet (black) with rounded quartz inclusions toward the centre. Kyanite (brown grey) mantles part of the lower left rim of the garnet with a recrystallized quartz and feldspar matrix.  Biotite fills cracks in the garnet.