COMMONWEALTH OF AUSTRALIA.
DEPARTMENT OF NATIONAL DEVELOPMENT.
BUREAU OF MINERAL RESOURCES GEOLOGY AND GEOPHYSICS. RECORDS: 1963/144
THE HIGH-GRADE METAMORPHIC ROCKS OF THE MAC-ROBERTSON LAND AND KEMP LAND COAST by W.R. McCarthy (Australian Mineral Development Laboratories) and D.S. Trail
The information contained in this report has been obtained by the Department of National Development, as part of the policy of the Commonwealth Government, to assist in the exploration and development of mineral resources. It may not be published in any form or used in a company prospectus without the permission in writing of the Director, Bureau of Mineral Resources, Geology and Geophysics.
THE HIGH-GRADE METAMORPHIC ROCKS OF THE MAC-ROBERTSON LAND AND IMP LAND COAST by W.R. McCarthy (Australian Mineral Development Laboratories) and D.S. Trail Records 1963/144
CONTENTS Page
ABSTRACT^ INTRODUCTION^ PETROLOGY^ Basic Charnockites^ Intermediate Charnockites^ Banded Gneisses^ Metasediments^ Hybrid Rocks^ Basic Dyke Rocks^ CONCLUSIONS^
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ACKNOWLEDGEMENTS ^
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REFERENCES^
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FIGURES: ' Figure 1: Location of Mac-Robertson Land and Kemp Land Coast Figure 2: Reconnaissance Geological Map of the Mac-Robertson Land and Kemp Land Coast, Figure 3: Part of the Framnes Mountains. Figure 4: Mount Henderson. Figure 5: Banded Gneiss. Figure 6: Banded Gneiss. The information contained in this report has been obtained by the Department of National Development, as part of the policy of the Commonwealth Government, to assist in the exploration and development of mineral resources. It may not be published in any form or used in a company prospectus without the permission in writing of the Director, Bureau of Mineral Resources, Geology and Geophysics.
Records 1963/144 The High-Grade Metamorphic Rocks of the Mac-Robertson Land and Kemp Land Coast by W.R. McCarthy, M.Sc. 1 and D.S. Trail, B0Sc. 2 Australia. ABSTRACT Rocks collected from coastal exposures and several, nearby, N-S trending ranges of nunataks in Greater Antarctica, during the 1961 mapping programme of the Australian Bureau of Mineral Resources Geology and Geophysics, have been grouped into : - Basic and Intermediate Charnockites; Banded Gneisses; and Matasediments, Hybrid Rocks, Basic Dyke Rocks. The petrology of each group is described. The charnockites are compared to the basic and intermediate varieties of Holland's charnockite series. ^The basic charnockites are thought to be metamorphosed basic igneous rocks.^The intermediate charnockites are of uncertain petrogenesis. Metasediments and hybrid (?) rocks occur within, and at the western extremity of, the outcrop area of the intermediate charnockites. The sediments have been metamorphosed under granulite facies conditions. ^Interpretation of the metasedimenbs and charnockites is discussed in the Conclusions of the paper. The banded gneisses, consist of alternating felsic and mafic bands and have been intensely folded. Banding is irregular and mafic bands contain hypersthene. Metamorphosed impure calcareous sediments, which occur within the gneisses, appear to have bean metamorphosed under amphibolite facies conditions.^Differences in composition (e.g., water content) of the calcareous rocks are thought to be responsible for the lower facies assemblage. Metamorphism, tectonism, and igneous activity followed the regional metamorphism but the sequence of the events is uncertain. The later metamorphism, perhaps under amphibolite facies conditions, is thought to have partially recrystallised the charnockites and basic dykes in the area. ^Two general ranges of ages, 1475-1070 m.y. and 650-490 m.y., have been recorded for rocks of this coast but we are not now able to correlate them with any of the episodes of crystallisation (e.g. regional metamorphism, etc.) described in this paper. 1. 2.
The Australian Mineral Development Laboratories, Adelaide. The Bureau of Mineral Resources, Canberra, A.C.T.
2.
INTRODUCTION This paper was presented at the SCAR Symposium on Antarctic Geology, held in Cape Town, South Africa, in September, 1963.^It is mainly the work of W.R. McCarthy of Australian Mineral Development Laboratories, Adelaide, incorporating the results of his investigation of samples collected by D.S. Trail. The paper is reproduced as a Record of the Bureau of Mineral Resources, with the permission of the Director of the Australian Mineral Development Laboratories. The area described here is in East Antarctica (Greater Antarctica), extending 300 kilometers along tlao coast of Mac-Robertson and Kemp Lands (longitudes 57 0 E to 64'E)(fig.1). Rock exposures are limited, probably forming leos than 1 percent of the total area, and occur along the coast, as fringing rocky islands, and as coastal nunataks (the Framnes Mountains) which pierce the continental ice cap (figs. 2 and 3). Geological collections made by earlier expeditions to the area in 1931 and 1936 have boon described by Tilley (1937) and Rayner and Tilley (1940). ^Reconnaissance geological mapping of the Pica has bop undertaken since the establishment of Mawson (at 67 36 1 S., 62 53'E.) in 1954 by the Australian National Antarctic Research Expeditions. ^The results of those investigations have been described by Stinear (1956), Crohn(1959) and McLeod (1959).^A mapping programme (at 1,250,000 scale) of this coast and the northern Framnes Mountains, which extend 10 to 50 kilometres south of Mawson, was begun in 1961. ^The paper describes results of the first part of the mapping programme, carried out by D.S. Trail, and preliminary petrographic investigations of 200 samples undertaken by W.R. McCarthy during 1962 and 1963. The complete results of this investigation will be incorporated in a forthcoming Bulletin of the Bureau of Mineral Resources, Australia. PETROLOGY The outcrops in this area are of basement rocks of probable Pre-Cambrian age. ^Crohn (1959) grouped the rocks of the area into metasediments, igneous rocks (granitic gneisses and hybrid gneiss with minor aplite, pegmatite, gabbro, norite, pyroxenite and hypersthenite), and intrusive charnockitic granite bodies which he regarded as having a palingenetic origin. In this paper we discuss the reeks under the following headings :Basic Charnockites; Intermediate Charnockites; Banded Gneisses; and Metasediments, Hybrid Rocks and Basic Dyke Rocks. ^The . sampling of the area and the mineralogical detail is still of a survey nature and we therefore do not consider the classification to be final, but chosen only to convoy our present knowledge of these rocks to others particularly concerned with Antarctic geology. Because of the varied genetic interpretations associated with charnockite and charnockite series since the introduction of these terms into the literature by Holland .(1900), most writers today find it necessary, at the least, to explain their use of the terms. ^The basic charnockites we describe are similar in colour and general mineralogy to those described by Holland.^The intermediate charnockites are
massive, dark coloured, contain orthopyroxene and are of adamellitic composition (Hatch,et al., 1961).^Crohn (1959) thought that our intermediate charnockite which he termed charnockite granite" (the Mawson Granite), was intrusive and regarded it as a product of palingenesis.^In contrast to Crohn and Holland we use charnockite purely in a descriptive sense • (without petrogenetic inference) in this paper.
Basic Charnockites Rocks of this group form many of the islands from the eastern extremity of the area westwards to Allison Bay and Byrd Head. In outcrop they are dark brown to black, medium, grained, and poorly foliated. ^Where foliation is locally present it is imparted by small and large scale lens-like, felsic bodies, with or without garnet, and small basic schlieren. In places, large insets of feldspar (orthoclase and plagioclase) are present and sometimes show preferred orientation.^These elements of foliation are mutually parallel when seen together. Some larger concordant, light-grey bodies of garnet, quartz and feldspar are sometimes present within the basic charnockites. The rocks are composed of biotite, hypersthene, quartz and plagioclase. Both garnet and orthoclase also occur in some specimens. Felsic minerals dominate, and ferromagnesians comprise 15 to 25 percent of the rocks. ^The quartz content ranges from 5 to 15 percent.^Apatite, zircon and more abundant opaques occur in accessory amounts. ^Some rocks show little structure in thin-section.^Others show orientation of minerals (e.g., quartz as lenticular bodies and linear concentrations of biotite and hypersthene).^Igneous textures are not apparent, and occasional, irregularly-shaped, coarse insets of plagioclase and more rarely orthoclase occur amongst the generally mediumgrained constituents.^Plagioclase is generally of labradoritic composition but andesine and more rarely bytownite occur. Several generations of plagioclase may be present in the same rock; this is indicated by the replacement nature of the contacts between twinned and untwinned plagioclase. The medium-grained plagioclase is generally not antiperthitic but coarse plagioclase often has perthitic inclusions of the patch, rod or flame style. Deformation of grains resulting in broken crystals and bent twin lamellae is a common feature of the medium-grained plagioclase.^These grains are often partially rimmed by biotite flakes. Biotite, in many specimens, shows a preferred location amongst finer grained feldspar. Some of the garnet also occurs in these finer grained areas and in places partially rims hypersthene grains. Hypersthene is the dominant pyroxene of the rocks and occurs as fine to coarser, ragged and sometimes apparently broken grains.^It shows moderate to strong pleochroism. The basic charnockites are thought to be metamorphosed basic igneous rocks (i.e., gabbros, dolerites, etc.).
Intermediate Charnockites Rocks of this group are present at a number of localities (viz., Ufs Island ? Mawson and nearby islands, and in the Framnes Mountains, fig.4). ^At the Mawson locality, they have been referred to as the Mawson Granite by Crohn (1959). They are yellowish-brown coloured rocks in outcrop, which typically have large insets of orthoclase.^The insets vary in abundance and show a preferred orientation.^Small, irregular schlieren of biotite and garnet and small, medium-grained, light coloured, vein-like lenses of garnet, quartz and feldspar occur within the charnockites. ^Basic schlieren, felsic lenses, and oriented insets occasionally impart a foliation to the rock but generally it is massive. The intermediate charnockites are coarse- to medium,. grained, hypersthene-quartz-plagioclase-orthoclase rocks of adamellitic cemposition.Garnet, biotite, and more rarely hornblende are also present. Feldspar dominates the mineral assemblage and ferromagnesians (5 to 15 percent) and quartz (greater than 10 percent) are less important constituents. Apatite, zircon, and opaques occur as accessories. ^Orthoclase occurs as large insets amongst the medium-grained plagioclase and quartz grains. It is generally more abundant than plagioclase.^Some insets show Carlsbad twinning and most are microperthitic with inclusions of the fusiform, granular or red type.^Finer grained orthoclase occurs much more rarely in the matrix.^Plagioclase is generally andesine (An A0 or greater) and, less commonly, labradorite. ^Often grains'are antiperthitic with alkali feldspar inclusions of the flame or patch type.^Some of the plagioclase is deformed, and broken grains and bent twin lamellae are common. ^Twinned, untwinned and poorly twinned plagioclase is present. ^Quartz occurs as elongated grains with sutured boundaries or as subhedral, mediumgrained crystals. Commonly the grains have needle-like, granular, crypto- and micro-crystalline, transparent and opaque inclusions. Biotite is present as rims or partial rims around feldspar grains, in aggregates with garnet, and as rims around magnetite; it does not appear to be primary. ^Hypersthene occurs as coarse- to medium-grained, irregular, and apparently broken grains.^It shows moderate to strong pleochroism. Hornblende, dark green to olive-brown, occurs as kelyphitic rims around hypersthene and opaques, and as individual grains in a specimen from Ufs Island. It is present in several other specimens but its retrogressive nature is not as evident as in the Ufs Island sample. Banded Gneisses Rocks of this group are in genera4 confined to o coastal exposures and islands between longitudes 60' 30 1 E and 57 OtE. Some specimens collected from Cape Bruce and the Stanton Group are also assigned to this group. ^These are gneisses with alternating felsic- and mafic-rich bands varying from several centimetres to metres in thickness (figs. 5 and 6).^Along strike, the dark bands either lens out abruptly or gradually grade into lighter bands; the contacts between dark and light bands are in general indistinct.^The banded gneisses have been folded into recumbent, isoclinal, and other less pronounced structures.
There are not enough data at present to suggest to what degree the banding in the gneisses may result from original compositional differences in the parent rock rather than or in addition to their tectonic history. ^However, several green, biotite-pargasite-bytownite amphibolites (metamorphosed calcareous sediments) have been found interbedded with the gneisses, which suggests that there were compositional differences in the parent rock before metamorphism. The limited number of specimens collected does not allow a complete description of the variations seen in the field. However, the dark bands are comprised of hornblende, clino-and ortho-pyroxene and plagioclase, felsic and mafic minerals occur in about equal quantities.^Garnet, biotite and quartz are also present.^Apatite, and more abundant opaques occur as accessories. The bands generally have a hornfelsic texture and the minerals are medium, to coarse-grained. ELEiioclase occurs as twinned and untwinned grains and is generally of labradoritic, or less often, bytownite composition.^Antiperthite was rarely observed. Pyroxene is generally more abundant than hornblende and occurs in intimate association with it,^Hypersthene, showing moderate pleochroism, is the major pyroxene present. Clinopyroxene is probably diopsidic augite. Hornblende, green to olive in colour, has in several specimens partial symplekitic rims of pyroxene and plagioclase. Light coloured, felsic bands show a more varied composition.^They are, in general, biotite-garnet-orthoclasequartz-plagioclase gneisses.^Pyroxene and, less commonly, hornblende are sometimes present. ^Accessories are apatite, zircon and, more abundant, opaques. ^Biotite is commonly foliated and quartz often shows a crystallization foliation. Plagioclase ranges in composition from An n to An An and is often antiperthitic, with alkali feldspar includiMs of the patch, string and rod style. ^Orthoclase i generally microperthitic with granular or fusiform inclusions, often forms the coarsest grains of the assemblage.^Plagioclase is generally more abundant than orthoclase.^Quartz ranges in abundance from 5 to 15 percent. Biotite and almaI are minor constituents and occur in close association. Pyroxene, when present, is generally hypersthene. brid Rocks and Basic kes Metaserra_ Metamorphosed sediments are found at Mawson, in the Framnes Mountains, and at a number of ;kocalitie in coastal and island exposures between longitudes 62'E and 60'E. ^Before considering some of these rocks in detail, certain general aspects of their field relations are noted (see map, fig.2). Bodies of hybrid gneiss in the David Range and paragneiss in the Masson Range all dip eastwards or southwards into intermediate charnockite which forms the major-part of both ranges. ^This relatiolgp suggests that the hybrid rocks and paragneisses may beiff^stable representatives of a metamorphosed succession of sedimentary and igneous rocks.
6. Metasediments In the North Masson Range, in the Framnes Mountains, . a 100 metre wide sequence of interlayered lime-silicate gneisses, marble, quartzite, metamorphosed,hybassl basic rocks, hybrid acid gneisses and pegmatites dip 40' to 50 south-eastwards.^Intermediate charnockite crops out east and southeast of these rocks and in this zone the contacts with the charnockite are irregular or gradational; ^some charnockite is interbedded with the gneisses.^The banded lima-silicate gneisses are sphene-plagioclase (bytownite or labradorite)^diopside rocks with hornfelsic or granulitic textures or sphene-diopsidebytownite-scapolite gneisses, with occasional grossularite, that have granulitic textures.^The scapolite-bearing gneisses appear to have recrystallized, since scapolite and grosuularite replace plagioclase, and sphene appears to be an unstable constituent with the scapolite and grossularite rocks.^The hybrid gneisses are garnet-plagioclase-quartz-orthoclase rocks in which orthoclase and quartz are the major constituents.^Quartz and orthoclase show a crystallisation foliation and garnet is often concentrated in linear aggregates. ^Basic rocks, with the assemblage biotite-garnet-hypersthene-labradorite, compare with the basic charnockites.^The sediments and hypabyssal rocks appear to have been metamorphosed under granulite facies conditions, and the pegmatitic and acid gneisses are believed to have formed by granitization following or accompanying the regional metamorphism, Several distinctive rocks were collected by Stinear in 1954 and described by Bayly and Dallwitz (in Roberts, 1961) and by Segnit (1957).^These rocks crop out as small bodies at Mawson and at Departure Rocks (about 3 kilometres WSW of Mawson).^At Mawson they are present as a 7 by 5 metre, oval body of enstatite-sapphire-spinel and enstatite-sapphirecordierite rock described by Segnit (1957).^At Departure Rocks they are sphene-bytownite-scapolite-diopside rocks (vesuvianite, wollastonite, and grossularite has also been identified by Bayly in these rocks).^Dallwitz (Roberts, 1961) suggests that the lime silicate rocks are granulites derived by high grade metamorphism of impure calcareous sediments. We support that interpretation. Yellow-brown, aluminous paragneisses are common in coastal outcrops and islands between Byrd Head and Campbell Head. They also crop out at Low Tongue, and on Nora Island in the Stanton Group.^Crohn (1959) records similar rocks in the Casey Range.^They are sillimanite-cordierite-garnet-plagioclasequartz-orthoclase gneisses.^Orthoclase and quartz generally dominate the mineral assemblage. ^The rocks generally show hornfelsic or granulitic textures but in some specimens felsic minerals are concentrated into linear aggregates separated by garnet, biotite, sillimanite and accessory opaques. ^Spinel is a rare accessory found in a few specimens.^A number of red, garnet-plagioclase-quartz-orthoclase pegthatitic bodies, from one to ton of metres in width are present as generally concordant bodies within the paragneisses. ^The gneisses are considered to represent original aluminous sediments which have been regionally metamorphosed, under granulite facies conditions, and the pegmatitic bodies to be hybrid rocks.
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Hybrid Rocks At Mount Henderson(fig.4), in the Framnes Mountains, garnet-andesine-quartz rocks and garnet-biotite-quartzplagioclase-orthoclase gneisses form concordant bodies 10 to 35 tetres in width within the intermediate charnockite. At this stage of the investigation, it is believed that they are metamorphic pegmatites and partially granitised charnockite. Gneisses present on the western side of Mount (in the David Range of the Framnes Mountains) dip at about 70' eastwards under the main ridge of intermediate charnockite. Three other bodies of gneiss of similar appearance and attitude were observed in nunataks of the David Range. ^The M ount Elliott rocks are medium-grained, hornblende-hyparsthene-quartzplagioclase-orthoclase gneisses of adamellitic composition. Gneissic structure is imparted by linear bodies of ferromagnesians.^Orthoclase generally occurs as coarse augen and in general is more abundant than plagioclase.^Plagioclase, showing albite twinning, is of labradorite composition, but much of it occurs as untwinned cloudy appearing grains. In general comparison, these gneisses differ from the intermediate charnockites primarily in their structure, colour, and abundance of green hornblende. Lighter coloured concordant or locally transgressive bodies of granitic composition occur within the gneiss and are comprised of garnet, plagioclase, quartz, and orthoclase. Because sampling of these rocks has . been random and limited we now prefer to view them as hybrid rocks which may prove with further data to be granitised gneisses of a metamorphic origin. On the easternmost island of the Austskjera Group a narrow zone of sheared basic charnockite widens, along strike within 30 metres, to a 15 metre wide zone of gneiss. ^Darker portions of the gneiss are comprised of fragments of basic charnockite in a matrix of fine-grained, foliated biotite, hypersthene and feldspar while lighter portions are medium- to Coarse-grained and composed of orthoclase and quartz. ^The darker portions are considered to be recrystallized mylonite of basic charnockite and the lighter felsic portions formed by granitisation (sensu stricto) or possibly by injection of granitic magma. Basic Dyke Rocks At Mawson cross-cutting, dyke-like bodies mineralogically similar to the basic charnockites occur. At a number of other localities transgressive bodies composed of biotite, hypersthene, amphibole, and plagioclase (labradorite or bytownite) are present.^They range from 2 to 10 metres in width, and are thought to be metamorphosed basic dykes. ^One dolerite dyke, within the banded gneisses, is unmetamorphosed. CONCLUSIONS At this stage of the investigation, petrogenetic interpretation of the charnockites must be based primarily on the general field relations and petrographic data, because chemical analyses and more detailed mineralogical data, which Groves (1935), Quensel (1949), and others used for this purpose, are not available.
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The metamorphosed sediments associated with the intermediate charnockites would probably be interpreted as xenoliths (at Mawson), and as roof pendants and underlying basement rocks (in the Framnes Mountains) by adherents of magmatic evolution of charnockites.^Proponents of "plutonic metamorphism" (Groves, 1935) would probably consider them as relatively stable representatives (more in the nature of Eskola's "Durchlaufer") of an older interbedded complex of sedimentary, volcanic, basic-intrusive rocks, etc., which have all been metamorphosed under granulite facies conditions. We believe our data to be more conformable with the latter interpretation of charnockite evolution but do not consider this opinion final until more complete field, mineralogical and chemical information can be obtained. Our knowledge of the history of the area is still of a preliminary nature; however, there is evidence for additional tectonism, metamorphism, and igneous activity following the regional metamorphism. The number and sequence of these episodes is uncertain. ^The events are recorded as recrystallized mylonites (in the Basic Charnockites), metamorphosed and unmetamorphosed basic dykes (in the Banded Gneisses, etc.) secondary growth of hornblende and biotite and perhaps orthoclase (in the Intermediate Charnockites) and granitisation or pegmatitic activity at a number of localities. The recrystallisation seen in some of the charnockites and the metamorphism of basic dyke rocks suggests that a later regional metamorphism is possible in the area - perhaps under amphibolite facies conditions. ^Two general ranges of time, 1475 m.y. to 1070 m.y. and 650 m.y. to 490 m.y. 1 are indicated for rocks dated from Davis, Mawson and the Oygarden Group of this coast (Stank, et al., 1960). Absolute correlation of our rocks with the dated samples from the area is not certain.^Thus the dates are only a general indication of two periods of crystallisation and cannot now be assigned to any particular metamorphic, igneous, or tectonic, episode. ACKNOWLEDGMENTS The authors wish to thank Mr. D.E. Ayres, and Mr. H.W. Pander who critically discussed some aspects of the mineralogy and read the manuscript, and Mr. I.R. McLeod who discussed field and regional aspects of the geology with us. The Bulletin of Mr. P.W. Crohn proved to be of great assistance to us and is recommended to readers wishing additional geological and other data of this and nearby areas. We also thank personnel of the Australian National Antarctic Research Expeditions, particularly Dr. R. Pardoe and Messrs. G.D.P. Smith and D.O. Keyser, for their support and assistance to geologists in the field. The paper is published with the permission of the Director of the Bureau of Mineral Resources, Geology and Geophysics of Australia, and the Director of the Australian Mineral Development Laboratories.
REFERENCES CROHN, P.W.,
1959 - A contribution to the geology and glaciology of the western part of Australian Antarctic Territory, Bur.Min.Resour.Aust.Bull., 52
ESKOLA, P.J.,
1957 - On the mineral fades of charnockites Jour. Madras Univ.,B, 27 No.L., TETErial7 No.
GROVES, A.W. 1^1935 - The Charnockite Series of Uganda, British East Africa, Quart.Jour. Geol.Soc.Lond., vol. 91, pp.150-207. HATCH, R.E.
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WELLS, A.K., and WELLS, LK., 1961 - PETROLOGY OF THE IGNEOUS ROCKS, Thomas Murby and Co., London, Twelfth Edition, p 189.
HOLLAND, T.H.,^1900 - The charnockite series, a group of Archaean hypersthenic rocks in Peninsular India. Mem.Geol.Surv. India 28 (2). McLEOD, I.R.,^1959 - Geological and glaciological work by the 1958 Australian National Antarctic Research Expedition, Bur.Min.Resour.Aust.Rec., 1959/131 (unpubl.). QUENSEL, P.,^1949-54 The charnockite series of the Varberg district on the south-west coast of Sweden. Arkiv for Mineralogi Och Geologi, I. RAYNER, G.W., and TILLEY, C.E., 1940 MaeRobertson Land and Kemp Land, Discovery Reports, 19. ROBERTS, W.H.B.,(compiler) 1961 - Miscellaneous petrographic and mineragraphic investigations for the period Jan.-Mar. 1958. Bur.Min. Resour.Aust. Rec. 1961/139(ungubl.). SEGNIT, E.R.,^1957 - Sapphirine bearing rocks from MacRobertson Land, Antarctica. Min. Mag., 31, 690-697. STARIK, I.E. I RAYICH, M.G., KRYLOV, A.Ya., SILIN Yu.I.,^ATRASHENOK, L.Ya., and LOVTSYUS, A.V., 1960 - New data on the absolute age of rocks of the eastern Antarctic continent. Dok. Akad, Nauk. S.S.S.R., 134, (6)7 1441-3. ,
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STINEAR, B.H.,
1956 - Preliminary Report on operations from Mawson Base, Australian National Antarctic Research Expedition 1954-55, Bur.Min.Resour. Aust.Rec., 1956/44 (unpubl.).
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Figure 6: Banded Gneiss, Oygarden Group. Interrupted Band.
