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IGNEOUS ROCKS OF THE MOUNT PAINTER BELT. 



By D. Mawson, Kt., D.Sc, F.R.S. 



[Read October 11, 1923.] 



Plates XXXIII. and XXXIV. 



As a result of two brief visits undertaken some years ago to the neigh- 

 bourhood of Yudanamutana and Mount Painter, at the north-eastern extremity 

 of the FHnders Range, a short note was pubHshed in the Proceedings of the 

 Australasian Association for the Advancement of Science *^^^ defining a belt of 

 Pre-Cambrian age in that portion of the State. 



It was intended to follow up that investigation, but up to the present, owing 

 principally to its remote situation, no opportunity has presented itself of revisiting 

 the area. Nevertheless, the great importance of that locality both petrologically 

 and mineralogically, calls for the publication of such notes as are already to 

 hand. 



The salient feature of the Mount Painter belt is the abundance of igneous 

 rocks, in part normal igneous types, at other times metamorphosed to gneisses 

 and schists. To a less extent metamorphic rocks of sedimentary origin are 

 discernible in this complex. 



General Features of the Central Zone. 



In the more central zone of this Pre-Cambrian belt, namely, in the neigh- 

 bourhood of Mounts Pitts and Painter, the outcropping rocks are possibly more 

 largely of igneous origin than in the outer region. Here are dense felsitic and 

 granitic rocks recalling the Pre-Cambrian porphyry and granite of the Gawler 

 Ranges and Moonta ; also a considerable development of syenite-porphyry, quartz- 

 felspar-porphyry, felspar-porphyry, and acidic gneisses sometimes exhibiting 

 augen structure. Crossing both the igneous rocks and the included meta- 

 morphosed sediments are numerous pegmatite reefs, often composed of very 

 coarsely crystalline quartz and red felspar, at other times consisting only of 

 quartz. Pink and red felspars are a notable feature of the rocks of this portion 

 of the field. Micaceous iron, ilmenite, sphene, and rutile are frequently observed 

 to be present in notable quantities. 



All these rocks have suffered metamorphism to some extent, others are very 

 far changed, including both chemical reconstruction in the minerals themselves 

 and mechanical crush. 



Besides these obviously igneous forms, there is in this central belt scattered 

 irregular patches of specially interesting schist formations, *^^^ notably corundum- 

 mica-schist, pleonast-mica-schist, pleonast-tourmaline-mica-schist, cordierite- 

 corundum-schist, cordierite-sillimanite-corundum-schist, cordierite-sillimanite- 

 schist, etc. In certain localities these schists are notably rich in accessory 

 tourmaline, monazite, and apatite. These remarkable patches of schist are fre- 

 quently banded in such a manner as to suggest a former sedimentary origin, but 

 they frequently merge into normal igneous rock. At the principal corundum- 

 schist locality, that lying directly in the line between Mount Pitts and Mount 

 Painter, the dominant rock neighbouring the schists is a quartz-f elspar-muscovite- 

 gneiss. Beyond the fact that pneumatolitic action with the introduction of certain 



(1) Sydney Meeting, 1911, p. 118. 



(2) "Mineral Notes," by D. Mawson, Proc. Roy. Soc. S. Austr., vol. xl., p. 262. 



