THE METAMORPHIC ROCKS OP ADELIE LAND STILLAVELL. 89 



2. THE APLITE GNEISSES. 



Associated with the granodiorite gneiss are quartz felspar gneisses which are the 

 metamorphosed equivalents of pegmatite and aplite veins, which were most probably 

 connected with the intrusion of the original granodiorite magma. These gneisses are 

 red or white or grey, and appear as small bosses or veins in the granodiorite gneiss. 

 The bosses may be a dozen yards or more in width and the foliation cuts through them 

 independently of the outline of the boss or of the trend of the vein. As the surface 

 outcrop of the rocks is perfectly fresh and uncovered, it could be observed in the field 

 that the boundary between the aplite gneiss and the granodiorite gneiss was often 

 indistinct, and there was frequently a gradual transition between the two. A set of 

 specimens was accordingly collected across such a boundary and show a gradual change 

 from the pure white aplite gneiss through pale grey shades to the darker grey granodiorite 

 gneiss. In this we have another example of metamorphic diffusion. No field evidence 

 is available concerning the relation of the aplite gneisses to the amphibolites. The small 

 quartz veins that cut the amphibolites may be correlated with quartz veins which fill 

 fractures in the granodiorite gneiss definitely subsequent to the development of the 

 foliation. 



No. 10A. Specimen No. 10A is an example of the aplite gneiss and was collected 

 from a vein about 18in. wide, close by the southern magnetic hut. The trend of the vein 

 is approximately parallel to the direction of the foliation but was observed in section 

 to cross it horizontally. In the hand specimen the rock has a pale-grey colour and a 

 fine granulitic appearance. Quartz and felspar are the chief minerals, but small biotites 

 are evenly distributed through the rock and produce perceptible schistosity. Occasional 

 large crystals of allanite appear in the vein and have formed a centre of crystallisation 

 around which felspar crystals radiate. 



In thin section the rock is even grained with granoblastic structure and with 

 abundant evidence of mashing and granulation. It is composed chiefly of interlocking 

 crystals of quartz and felspar with smaller amounts of sericite, muscovite, and biotite, 

 while magnetite, apatite, allanite, monazite appear as accessories. 



The quartz appears in rounded, indented, and interlocking grains, and shows 

 considerable cataclasis. Some of the granular aggregates of quartz have developed 

 from the primary individuals of the pegmatite. At times there is a partial drawing 

 out into lenticles and layers. Clear orthoclase is present, but the bulk of the potash 

 felspar is microcline. Some of the microcline is quite clear and transparent and has 

 developed from orthoclase as in the granodiorite gneiss. The microcline may appear 

 as rounded blebs within the quartz crystal. Part may be relics of the original pegmatite 

 because microcline is a common constituent of such, and some microcline crystals show 

 strain polarisation and incipient granulitisation. Perthite is present. A small portion 

 of the felspar has been sericitised and some of the sericite has passed over into muscovite. 



