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Evolution of Granulites from Macrobertson Land, East Antarctica

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t^Õ{-qt Evolution of granulites from MacRobertson Land, East Antarctica by Ian Scrimgeour Thesis submitted for the degree of Doctor of Philosophy in The University of Adelaide (Faculty of Science) April, 1994 ií-ì .,\. .1 .i fi i, '¿' ¡ ¡i\ \ rl Abstract and Granulite facies rocks in the northern Prince charles N{ountains an extensive Mid surrounding regions of MacRobertson Land form part of of the proterozoic (1000 Ma) regional granulite facies terrain which dominates much granulite terrains in the coast of East Antarctica. As one of the largest exposed processes which result in world, it provides an excellent opportunity to examine the of granulite formation, yet much of the metamorphic and Structural evolution to areas to the MacRobertson Land has remained relatively unknown in comparison east and west. rocks on Jetty In the northeastern Prince Charles l\{ountlins granulite flcies and post-tectonic Peninsula consist of a supracrustal sequence intruded by syn- andT -7 granites, and preserve evidence for peak metamorphism (Ml) at -800oc '5 were kbars. In metapelitic rocks, M1 garnet- and sillimanite-bearing assemblages and symplectites which overprinted by the development of cordierite-bearing coronas dipping thrusts formed during the development of tight upright fol<ls and steeply cordierite- (Dz). cordierite-spinel symplectites between gíìfnet and sillimanite and of 1 - 2 orthopyroxene symplectites between garnet and biotite imply decompression assemblages kbars following M1. However, the stabilisarion of biotite-sillimanite were separated by a between the M1 and syn-D2 assemblages suggests that they Fine period of cooling, and that decompression was therefore not near-isothermal' 80 km to the grained reaction textures in pelites from Trost Rocks, a remote outclop A Sm-Nd north on an island in the Amery Ice Shelf, plesorve a similar history. agrees garnet-whole rock age of c.940 Ma for a syn-D2 granite on Jetty Peninsula well with other geochronological constraints wlrich suggest that the entire metamorphic history occurred between c'1000-930 Ma' northern Reaction textures in wollastonite-scapolite calc-silicates from the the prince charles Mountains imply that following peak metamorphism at c.800oc of scapolite to terrain underwent near isobaric cooling, resulting in the breakdown aS the formation calcite and anorthite, and wollastonite to calcite and quartz, as well from of grossular coronas between wollastonite, scapolite and calcite. calc-silicates which is replaced by the Framnes Mountains on the Mawson Coast contain scapolite, and wollastonite-anorthite, anorthite-grossular and calcite-quartz symplectites' reaction textures grossular coronas occur between wollastonite and anorthite. These partially replaced by also imply a cooling-dominated history. Peak wollastonite is forms coronas around calcite and quartz, and then a second generation of wollastonite the calcite and quartz, suggesting that fluids may have played an important role in the evolution the Framnes Mountains. On Fox Ridge, 60 km southwest of Jetty Peninsula, late steeply dipping D2 high strain zones postdate the development of cordierite coronas, and are progressively defined by garnet-cordierite, sillimanite, and biotite-sillimanite assemblages, suggesting that the terrain isobarically cooled late in D2 during ongoing convergen t deform ation. The most plausible explanation for this evolution is that the terrain underwent at least two transient thermal events during a long-lived tectonic event between 1000 and 900 Ma. These relatively transient events were followed by isobaric cooling, and occurred at progressively lower pressures during ongoing convergent deformation, implying that the region underwent decompression during crustal thickening. The extent of decompression during high grade metamorphism decreases from east to west across the Prince Charles Mountains. The region appears to form the southern margin of a broad mountain belt which may have resulted from northeast-directed thrusting of the Proterozoic terrain over the Archean Vestfold Block during the Mid proterozoic. General implications of this study include the apparent transience of the thermal perturbations resulting in low to medium pressure granulite metamorphism, and the likely weakness of the lower crust during granulite metamorphism, which raises the possibility that upper and lower crustal strains may be significantly decoupled during high temperature metamorphism. Contents Abstract Contents Figures Tables. Acknowledgments ... Mineral abbreviations CHAPTER l: Intoductíon...... I 1.1 General 1 1.2 P-T paths in granuliæ tenains 3 1. 3 Proterozoic granulites : geodynamic considerations 4 1.4 The northern Prince Cha¡les Mountains: regional geological setting, previous work and relevant problems. 6 1.5 Thesis outline. 9 CHAPTER 2: The geology and structure of the Jetty Peninsula region, northern Princ e Charle s Mountains 11 2.I Introduction.... 11 2.2 Regional geological context..... 15 2.3 Lithologies..... 15 2.3.1Layered gneiss. 18 2.3.2Intrtsive rocks. 2l 2.3.3 Late Palaeozoic to Mesozoic dykes and sediments . .. 29 2.4 Structure 3T 2.4.1DL 31 2.4.2 D2 .. 35 243D3 38 2.4.4 D4 shear zones....... 40 2.4.5 Late brittle structures . 4l 2.5 Sm-Nd geochronology of the garnet granite...... 42 2.6 Geological history and age constraints 44 2.6.1 Mid Proterozoic deformation and metamorphism .. .. .. .. 44 2.6.2 Pan- Afican thermal event. 46 2.6.3 Lambert Graben development............. 47 2.7 Conelations and discussion 48 CHAPTER 3: Metamorphic evolution of pelitic granulites from Jetty Peninsula, northern Prince Charles Mountains'.... 53 3. 1 Introduction 53 54 3.2 Petrography of Pelites 3.2.1 Assemblage 1 ...'.. 55 3.2.2 Assemblage 2 ..... 59 3.2.3 Assemblage 3 ...... 60 3.2.4 Assemblage 4 ..... 60 3.2.5 Mineral chemistry .... 62 3.3 Interpretation of mineral parageneses............ 63 3.3.1 Assemblage 3 ...... 64 3.3.2 Assemblage 4 ...... 66 3.3.3 Assemblage 1 ...... 67 3.3.4 Assemblage 2 ..... 68 69 3.3.5 P-T estimates for Jetty Peninsula... ' 3.4 Discussion.. 70 CHAPTER 4: Metamorphic evolution of pelites from Trost Rocks, Amery lce Shelf 74 4.1 Introduction....... 74 4.2 Regional setting and geological relationships...'........ 74 4.3 Petrography 77 4.3. 1 Corundum-absent Pelites 78 4.3.2 Corundum-bearing pelites. ... 83 4.3.3 Semi-pelitic assemblages ....... 83 4.3.4 Mineral chemistry ...... 84 4.4 Interpretation of reaction textures 8s 4.4.1 Reactions in the KFMASH and KFMASHTO systems 85 4.4.2 Conelations with Jetty Peninsula. 88 4.4.3T\erole of aÍlz0 in the stabilisation of biotite at Trost Rocks and Jetty Peninsula 89 4.4.4P-T evolution 92 4.5 Discussion .. 94 CHAPTER 5:Wollastonite-scapolite calc-silicates as indicators of the P-T-fluid history in MacRobertson Land .........96 5.1 Introduction.. ..........96 5.2 Calc-silicate gneisses on Else Platform, Jetty Peninsula.. .........99 5.2.1 Geological setting .. 99 5.2.2 Petrography 99 5.2.3 Mineral chemistry .. 103 5.2.4Interpretation of reaction textures and P-T history .... 104 5.3 Calc-silicates from Mt. McCarthy, Porthos Range 108 5.4 Barium silicates in NPCM calc-silicates. 109 5.4.1 Origins of NPCM calc-silicates...... Lt2 5.5 Calc-silicates from Rumdoodle, Framnes Mountains .. '. 113 5.5.1 Geological setting ..... 113 5.5.2 Petrography IT4 5.5,3 Interpretation of reaction textures........ tt7 5.6 The role of the fluid phase t23 5.7 Discussion... ..124 CHAPTER 6: Metamorphic evolution of pelitic shear zones at Fox Ridge, Mcl-oed Massif ...t28 6.I Introduction r28 6.2 Geological setting and structural framework ... t29 6.3 Petrography............. 131 6.3.l Metapeliæs 13T 6.3.2 Biotite-sillimanite rocks... t37 6.3.3 Highly aluminous assemblages... r37 6.3.4 Orthopyroxene-bearing assembalges. 138 6.3.5 Mineral chemistry ... ... 139 6.4 Mineral parageneses and a P-T history for Fox Ridge t4L 6.4.1 Interpretation of mineral parageneses . L4L 6.4.2 Pressure-temperature conditions of metamorphism 142 6.4.3 Summary... r44 6.5 Conelation of metamorphism with other areas. t45 6.6 Discussion..... r46 CHAPTER 7: Some geoldynatnic and thermal considerations in granulite terrains .. .148 7 .l Introduction.... .148 7.2 The duration of granulite metamorphism .t49 7.3 Geodynamic implications of decompressional P-T paths in granulite terrams 153 7. Lower crustal channel flow parameters in granulite tenains. r57 7.4.lEvídence for channel flow in modern terrains 158 7 . .ZlJonzontal structures in the mid to lower crust ... 161 7.4.3 Importance of the rheological structure of the crust... 162 7.4.4 Cltannel flow in granulite terrains: some expectations ... 163 7.5 Summary... ... 168 CHAPTER 8: Discussion t70 8. 1 Introduction.... ... t70 8.2 Summa¡y of the metamorphic evolution of the NPCM and sunounding regions. ...17r 8.3 Evidence for the tectonic setting of the 1000 Ma event ...t74 8.4 Thermal evolution of the NPCM. ...175 8.5 Constraints on the timing and duration of the 1000 Ma event ...t77 8.6 Geodynamic implications ... 178 REFERENCES 181 APPENDD( l: Geological obserttations from other regions visited in the northern P rinc e Charles Mountains 41.1 Mount Lanyon.... I1 41.2 MountCollins t-2 A1.3 Mt. Bechervaise / Hunt Nunataks, Athos Range. I-3 A1.4 Stinear Nunataks...... T4 41.5 Mt. McCarthy, Porthos Range r-5 41.6 Crohn Massif, Porthos Range............ I-6 A1.7 Farley Massif and Mt. Jacklyn, Athos Range I6 APPENDX II: Datafor Sm-Nd geochronology . II-1 APPENDD( fr: Mineral aralyses .. ...rn-r-22 APPENDIX IV: P-T estimates using Thermocalc rv-t-22 Figures CHAPTER 1 1.1 Regional geological setting of the northern Prince Charles Mountains 2 CHAPTER 2 2.1 Photos of the terrain in the northern Prince Charles Mountains .... T2 2.2Location of Jetty Peninsula l3 2.3 Geological map of Else Platform.... r6 2.4 Geological map of Kamenistaya Platform.
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