QUARTZOFELDSPATHIC GNEISS

By this description, I also hope to provide evidence of the protolith for various subdivisions and demonstrate the nature of the response to various shear zones.  My classification has been modified from Baba (1997) to produce the following subdivisions: Layered (Q), Banded (Qb), Mafic Rich (Qm) and Undifferentiated (Qu) Quartzofeldspathic Gneiss.

 In the field exposure is poor since this unit tends to reside in low-lying loch areas; ridges of Qb and Qm stand out. The subdivisions are related to each other in that there are often gradational contacts and inclusional relationships between the lithologies when describing the lateral and along-strike variations respectively.  Qb and Qm often form lensoid shapes within Q, whereas Q and Qu occupy larger areas that are the hosts to Pelitic and Mafic Gneisses also.  Qu is grouped with Q on the Final Geological Map because the nature of the contacts with other rocks is unclear in many cases.  Thicknesses vary from 1.5 km in the south to only 100m in N Chaipaval and inclusions of other lithologies are often 5-30m thick and 0.1-1 km long parallel to strike. 

 At exposure scale the same, dextral shear of rotated mafic blocks is seen (Photo 9. page 30, locality 6, notebook page 18, 02288534) in Qb  from 50 cm to 5 cm scale.  Complex refolding around the borders of Loch Steisevat was observed to contain at least four phases of deformation that fold numerous quartz veins of at least 3 ages (see figure 8).

 In hand specimen, the finer scale mafic/felsic layering in all lithologies except Qu is seen to vary considerably.  Boudinaged mafic pods (Photo 12, page 30) are sometimes draw out into thin mafic layers and are a probable origin for the layering seen elsewhere.  Additional features include pseudotachylite development across a mylonite zone (Photo 13, locality 25 notebook page 25) and rhythmic layering both on eastern Aird an t-Sruith, the latter picked out by aerial photo (notebook page 93). Small, centimetre scale, layering of mafic/felsic minerals is observed: Qm and Qu are usually homogenous or weakly layered, whereas the foliation of Q and Qb is usually readily discernable.  Qm hybridizes with the Metagabbro on western Roneval producing a 2 mm grain size matrix with 25 mm porphyroblasts of garnet, which are mantled by 10 mm amphiboles.

 Thin section KSP 117 (Figure 9 and Appendix VI) is from a mafic rich layer in the Quartzofeldpathic Gneiss of Strondeval (to the south of my mapping area).







 

 









Figure 9: Thin section KSP 117, PPL, fov 1.5 x 1.5 mm. Strongly antiperthitic feldspar can be seen in PPL as the blebby matrix.  Garnet is retrogressed to biotite and opaques at the rims with inclusions of quartz and opaques.  A degree of mylonitization has occurred in the more turbid areas in the centre.