The Rayner Complex in MacRobertson Land, east Antarctica Daniel Joseph Dunkley Submitted in fulfilment of the requirements for the degree of Doctor of Philosophy Department of Geology and Geophysics The University of Sydney NSW Australia 2006 January 1998 Declaration I declare that this thesis is less than 100,000 words in length, exclusive of tables, figures, references and appendices, and that the work contained in this thesis has not been submitted for a higher degree at any other university or institution. Crevasse near Rumdoodle Peak, Masson Range, January 1993 Abstract The Rayner Complex in MacRobertson Land, east Antarctica, is a Proterozoic mobile belt composed of granulite facies gneisses that were metamorphosed at mid-crustal levels during the c. 1000-900 Ma Rayner Structural Episode. At Cape Bruce on the Mawson Coast, gneisses are present that represent a metamorphosed supercrustal sequence of probable Proterozoic age. Felsic and pelitic sedimentary rocks were buried at mid-crustal levels prior to the emplacement of charnockite before c. 1000 Ma. These lithologies were metamorphosed, migmatized and deformed into a gneissic fabric by Di-M] during the early stages of the Rayner Structural Episode. Granitoids and a 995±7 Ma biotite-rich orthogneiss were intruded after D l. Peak metamorphic conditions of 6-7 kbar and >750°C during Mi were followed by cooling at near-constant pressures, which produced hydrous M2 mineral assemblages. M 2 occurred before or during intense D2 deformation, that produced a strong foliation in the gneisses. At 944±10 Ma, felsic dykes and pegmatites were intruded that may have derived from partial melting during Dj and D2 . Lithologies were subsequently deformed into upright north-trending folds by D3, which represented the waning stage of continuous Di to D3 deformation. Felsic pegmatites were emplaced after D3, and were deformed into upright east-trending folds during D4. The growth of M3 minerals at c. 910Ma occurred after D3 and before or during the D4 event. M3 occurred at lower temperatures than and similar pressures to Mi and M 2. Narrow east-trending D5 shear zones, containing mylonite and pseudotachylite, formed after D4. These shear zones contain M4 mineral assemblages that grew at approximately 6.5 kbar and 550-800°C. Whereas Dj to D4 occurred during the c. 1000-900 Ma Rayner Structural Episode, D5 may have occurred at a much later time. On the basis of geochronological data from Cape Bruce and other localities, terranes with distinct ages of formation can be recognised in Kemp and MacRobertson Land. Archaean gneisses are exposed on the Mawson Coast west of the Stillwell Hills. In MacRobertson Land, the protoliths of exposed gneisses were formed within the period 1800-1000 Ma. At Cape Bruce, isotopic data provide evidence for unexposed Archaean crust in the region, whereas isotopic data from the northern Prince Charles Mountains and the Framnes Mountains yield inherited ages of 2200-1800 Ma. If the isotopic data represent inherited crustal material from the lower crust, the Rayner Complex in MacRobertson Land was underlain by mostly Paleoproterozoic crust during the Rayner Structural Episode. By contrast, the Cape Bruce area was underlain in part by Archaean crust. 1 During the peak of metamorphism in the Rayner Structural Episode, pressures and temperatures were sufficiently high to cause ionic diffusion on a cm scale. At Rumdoodle Peak in the Framnes Mountains, a xenolith of metasedimentary granulite is enclosed in Mawson Charnockite. The xenolith contains a disrupted layer of black spinel-orthopyroxene-phlogopite gneiss, that was truncated by quartz-rich pegmatite. A reaction band that developed between these units is composed of a sequence of mineral zones that contain spinel, orthopyroxene, sapphirine, cordierite and plagioclase. The sequence of mineral zones approximately matches that predicted by a model of closed system diffusion metasomatism, involving the exchange of Si for Fe and Mg. The sequence differs from that predicted by the model in the presence of micro-scale disequilibrium textures, that include ‘double coronas’ composed of cordierite surrounding sapphirine and sapphirine surrounding spinel. The growth of the reaction band was controlled by diffusion along intergranular pathways, with local equilibrium maintained adjacent to grain boundaries. The presence of corona textures was a result of slow reaction rates, due to limited diffusive exchange of Si and A1 across mineral grains. Similarities between the structural histories of various localities in MacRobertson Land indicate the operation of regional scale processes during the Rayner Structural Episode. Tectonism involved the compression of Proterozoic crust against Archaean continental crust in Enderby Land and the Indian peninsula. The orogeny occurred in an oblique-collisional setting. The early stages of the Rayner Structural Episode involved high-strain deformation associated with a large component of east-west compression, which produced east-over-west thrust structures in the Stillwell Hills and upright north-trending structures in the Framnes Mountains. Charnockitic magmas were emplaced in the MacRobertson during or after this period of compression. In the late stages of the Rayner Structural Episode, there was a regionally significant transition from a contraction-dominated regime to a transcurrent tectonic regime. The latter produced lateral movement along sub-vertical east-trending high-strain zones, and north-south compression in domains of lower strain. The transition was a result of either: 1) progressive deformation during east-west or southeast-northwest convergence at an oblique angle to the continental margin, or 2) a change in the direction of tectonic convergence, from east-west to north-south. Similar transitions have been commonly observed in oblique-collisional orogens. East-trending semi- brittle shear zones, that had a predominantly south-over-north sense of movement, were generated by north-south compression at some time after the Rayner Structural Episode. 11 Contents Abstract...........................................................................................................................................................i C ontents...................................................................................................................................................Hi Lists of Figures, Plates and Tables................................................................................................. m Acknowledgem ents.............................................................................................................................xii Introduction Aims of this study.................................................................................................................. 2 Thesis outline...........................................................................................................................3 O riginality of thesis and published components........................................................3 1. Regional Geology 1.1 The Rayner Complex............................................................................................................. 7 1.2 Geological work on the Mawson Coast.................................................................9 1.3 Geochronology of the Rayner Complex a review...........................................................................................................................14 1.4 A general history of the Rayner Complex....................................................................23 2. Geology of the Cape Bruce area Introduction...........................................................................................................................26 2.1 lithologies..............................................................................................................................27 Proclamation gneiss Metapelitic gneiss Felsic paragneiss Charnockitic orthogneiss Biotite-pyroxene orthogneiss White granitic orthogneiss Red granitic orthogneiss Felsic dykes and pegmatites 2.2 Structural relationships...................................................................................................... 39 D j event D2 event D $ event D 4 event D 5 event 2.3 The sequence of geological events at Cape Bruce.................................................. 49 2.4 Correlations with the Colbeck Archipelago................................................... 32 2.5 Comparisons with other parts of the Mawson Coast................................. 53 3. Metamorphism Introduction...........................................................................................................................56 3.1 Previous W ork............................................................................................................... 56 32 Petrography............................................................................................................................57 3.2.1 Orthogneiss...............................................................................................................................................5 8 3.2.2 Paragneiss.............................................................................................................................................. 6 2 3.3 Sequence of Metamorphism.............................................................................................71 3.4 Mineral Chemistry...............................................................................................................74