Eyre Peninsula
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Downloaded from http://jgslegacy.lyellcollection.org/ at University College London on May 31, 2016 part 2~ GRANITE-GNEISSES OF SOUTHERN EYRE PENINSULA. 75 ,5. The GRANITE-GNEISSES of SOUTHERI~ EYRE PENINSULA (SouTH AUSTRALIA)and their ASSOClA'rED AMPItlBOLITES. By CECIL EDGAR TILLEY, B.Sc., A I.C. (Communicated by Dr. H. H. TtIOMAS, M.A., Sec.G.S. Read February 2nd, 1921.) CONTENTS. Page I. Introduction and Previous Literature ........................ 75 rL The Divisions of the Pre-Cambrian of Southern Eyre Peninsula ......................................................... 76 (a) The Hutchison Series. (b) The Flinders Series. (c) The Warrow Series. (d) The Dutton Series. III. The Flinders Series ................................................ 78 (a) Geographical Extent. (b) Constitution. IV. The Relationships of the Amphibolites to the Associated Gneisses ............................................................ 81 V. The Petrography of the Granites and Granite-Gneisses of the Flinders Series ............................................. 84 (a) Gneisses of the Lincoln Area. (b) Gneisses of the Sleaford Area. u The Petrography of the Amphibolites and Allied Types in the Fllnders Series ............................................. 98 (a) The Amphibolites and Hornblende-Schists. (b) The Pyroxene-bearing Types. VII. The Origin of the Amphibolites ................................. 107 VIII. The Metadolerites of the Lincoln Area ........................ 116 (a) General Description. (b) Petrography. (c) Chemical Changes induced by Metamorphism. IX. Comparison of the Flinders Series with the Rocks of other ~ Pre-Cambrian Areas ............................................. 127 X. Bibliography and Summary .................................... 130 I. II~-TRODUCTION. TilE basement platform which forms the foundation of Southern Eyre Peninsula, and on which the much younger Tertiary and Recent sediments repose, consists of a complex of igneous and metamorphic rocks, which in their lithologieal and petrographical characters bear not only a remarkable resemblance to rocks of known Pre-Cambrian age in other parts of the State, but also to the well-known Pre-Cambrian tracts in other regions of the world. The evidences for the Pre-Cambrian age of the Eyre Peninsula rocks cannot rest on palmontological evidence, for there are no Q. J. G. S. :No. 306. tl Downloaded from http://jgslegacy.lyellcollection.org/ at University College London on May 31, 2016 76 MR. C. E. TILLEI" ON TttE (;RANITE-GNEISSES ~vol. lxxvii, known deposits of Lower Palmozoie age that have been recognized within the area. The data from which the Pre-Cambrian age is deduced are, therefore, of a comparative kind. The older rock series of Yorke Peninsula to which these Evrian rocks bear a close lithological and petroglmphieal resemblance are of definite Pre-Cambrian age, as proved by the presence of Cambrian sediments on an eroded platform of the series, in the Moonta and Wallaroo region. Apart from this local correlation, the complex beal~ a notewol~hv similarity to the Pre-Cambrian tracts of the Northern Hemisphere, and to none more than the Laurentian of Canada. Superposed with a marked angular unconformity on the meta- morphosed members of this older complex, and resting horizontally on the deeply-eroded igneous types, are Upper Tertiary ~nds and travel~ine. These deposits form a mantle to a considerable 1)o2"tion of the region, and hinder the study of the older series over large areas of country. Inland, in the hilly regions of the hundreds of Warrow and Hutchison, and to a less extent in the hundreds of Louth and Lincoln, the older rocks are stripped of this mantle, pemnitting a study of the complex. The best exposures, however, are displayed along the coastline, where detailed studies can be made on the wave-swept platforms which fringe the coast. Here the.older rocks are usually within 100 feet of sea-level, and are covered unconformably by consolidated sands of molian origin. Precipitous cliffs are often developed, and occur especially in the hundreds of Flinders and Sleaford, where the coastal cliffs attain a height of from 300 to 400 feet. The older rocks contribute but a small part to this elevation. Previous Literature. The literature bearing on the geological structure of Southern Eyre Peninsula is principally to be found in the publications of the Geological Survey of South Australia. As the Survey investigations have been prilnarily made in order to determine the possibility of the discovery of petroleum in this area, but little detailed work has been done on the older complex, and, so far as petrological investigation is concerned, the field up to the time of the ])resent enquiry can be regarded as untouched. A summary of the available geological literature of the area is appended t'o this paper. II. TIIE DivisioNs OF THE PRE-CAMBRIAN OF SOUTHERN EYRE PE~-I~'SULA. The division of the complex into well-eharaeterized groups or series of rocks is largely hampered bv the fact that sections of these rocks, continuous over large areas, are seldom to be obtained. 'There are, however, despite this dominant mantle of calcareous Downloaded from http://jgslegacy.lyellcollection.org/ at University College London on May 31, 2016 pai~ 2] OF SOUTHERN EYRE PENINSULA. 77 sands and lateritic fi'onstone, certain features which can be singled oat as characterizing types of rocks, and the following divisions or series are recognized :--(a) The Hutehison Series, (b) The Flinders Series, (c) The Warrow Series, and (d) The Dutton Series. The division calling especially for treatment in this paper is the Flinders Series ; but a brief statement is necessary concerning the nature and constitution of the remaining series. (a) The Hutchison Series. Some members of this series have already been described: namely, the dolomites and tale-magnesian silicate-rocks, 1 the para- gneisses,2 and the graphite-roeks.3 This formation is predomin- antly of sedimentary origin. The sequence of rocks, and further subdivision of this series, if ever possible, must await a detailed examination of the hundred of Hutchison, and its northward extension, where these rocks are extensively developed. The Sleaford-Bav area gives some idea of the complexity of this formation, which lms been invaded and cut up by later igneous intrusions. The strata strike in a north-and-south direction, swinging to a north-easterly direction in the northern exposures. This may be appropriately called the' graphite line,' as the occur,cute of gral~hite is, so far as is known, confined to the metamorphosed sediments which are grouped within the Hutehison Series. (b) The Fliuders Series. This is the most widespread member of the group. It consists of a complex of igneous rocks: granites and gneisses with asso- ciated basic rocks, amphibolites, hornblende-schists, and pyroxene- granulites. Its relation to the Hutchison Series is always of the intrusive kind. A section has been given, in the papers to which reference is made above, of the Hutchison Series invaded and metamorphosed by sills of the Flinders gneisses. (c) The Warrow Series. This is a metamorphosed sequence of sediments, principally developed in the hundreds of Warrow and Lake Wangary. The rocks are quartzites, slates, and mica-schists. This group is distinct froln the Hutchison Series, its chief 1 Geol. Mag. 1920, pp. 449-62,492-500. 2 Ibid. 1921, p. 251. 3 , Economic Geology' 1921, pp. 184-98. ~2 Downloaded from http://jgslegacy.lyellcollection.org/ at University College London on May 31, 2016 7S _~ra. c. E. TILLEY ON THE GRANITE-GNEISSES [VO1. lxxvii, characteristic being a great thickness of massive quartzites, rocks which form inconspicuous members of the Hutchison Series. (d) The Dutton Series. The Warrow Series has been metamorphosed by a later extrusion of ~'anites, developed in the hundred of Warrow. These granites are distinguished by the development of abundant tourmaline in their more acid dii~erentiates. A zone of lit-par-lit injection alon.g the contact has resulted in the development of injection- gnelsses from the Warrow schists. The serial sequence given above is one of decreasing geological antiquity, and the oldest group of rocks developed in the area is thus of sedimentary origin. There can be bu:L little doubt as to the Pre-Cambrian age of the Hutchison, Flinders, and Warrow Series, but the inclusion of the Dutton Series within the Pre-Cambrian is tentative only, and with an increasing knowledge of the igneous histolT of South Australia it may be relegated to the Lower Palaeozoic. III. TIIE FLINDERS SERIES. (a) Geographical Extent. The members of this series occupy the most extensive area of any of the Pre-Cambrian groups of rocks in Southern Evre Pen- insula. In the south, they are developed in the hundreds of Flinders and Sleaford. Starting from their westernmost exposure at the Red Banks on the west coast of the hundred of Sleaford, they crop out along the coastline in an easterly direction past Cape Wiles to Sleaford B~S, where the members of the Hutchison Series intervene in a half-mile section. The coastline farther east consists of Recent calcareous sands until the Curta Rocks are reached, adjacent to the coast of the hundred of Flinders, when the gneisses reappear. They are well developed farther south on the fringe near West Point and Cape Catastrophe, and form the eastern coast of the hundred of Flinders. The coastline ~)f the hundred of Lincoln, as also Boston Island, consists of these rocks, and they are pal'ticularly well exposed on the shore-line at Kirton Point, Port Lincoln. The minor peninsula of P~,iut Boston is formed of these gneisses, as also the island of Louth and the promont~)ry of Bolingbroke in the hundred of Louth. In the hundred ()f Hutchison, they crop out along the coast south of Tuml~v Bay and at Cape Eulcr. Th~ in'land eXl)Osures of the gneisses arc but few and isolated. The l)r~,miuent hills behind Memory Cove (hundred of Flinders) consist of these rocks, and in the hundred of Lincoln the S are well exposed in the monadnoeks of C~)bbler's Hill, North Hill, and Winter Hill, occupving also an area behind Port Lincoln itself.