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Geochemical modification of plastically deformed zircon
A522
Goldschmidt Conference Abstracts 2006
Precambrian terranes in West Australia and East Antarctica: Seismic structure and implications for continent formation and evolution A.M. READING Research School of Earth Sciences, Australian National University, Canberra, ACT 0200, Australia ([email protected]) The crustal thickness and seismic S-wave velocity profile of the upper lithosphere are found using energy from distant earthquakes recorded at remote, temporary seismic stations (‘receiver function’ analysis). Such methods can be used to constrain major structural and compositional characteristics of the continental crust, and delineate the extent of cratonic and orogenic terranes in regions where geological exposure of the surface is limited. They provide an effective alternative to active source seismic techniques for deep crustal targets. A new, near-comprehensive survey is presented which highlights variations in the structure of the West Australian Craton at the scale of the main terrane groups. Seismic structure is very consistent within several of the individual Archaean terrane groups, most notably the Pilbara, Murchison and Southern Cross. They are underlain by lower crust of low seismic velocity and show a sharp seismic Moho. Significant contrasts are seen between neighbouring terranes, thus, major tectonic units have a velocity profile that is a signature of that terrane or terrane group. We infer that the seismic structure is fixed before craton assembly and preserved through the subsequent collision and accretion of the tectonic units forming the West Australian Craton. Archaean terranes younger than 2.8 Ga do not show the low velocities in the lower crust or the very sharp Moho that is characteristic of the oldest terranes. One explanation is that less reworking of the lithosphere took place after this time. Reconnaissance-scale determinations of seismic structure have been made in East Antarctica in the Lambert Glacier region, which encompasses the proposed boundary between three of the ancient continents that formed East Gondwana. The only area of extensive rock exposure in central East Antarctica, it uniquely allows the seismic structure to be linked to surface geology. Data was recorded at stations of the SSCUA deployment in remote regions as far south as 75 °S. Seismic structure and Moho depth are determined across the region, providing constraints on its tectonic evolution. A significant contrast in crustal depth is found between the Northern and Southern Prince Charles Mountains that may indicate the location of a major tectonic boundary. Baseline seismic receiver structures have established for the Rayner, Fisher and Lambert Terranes that may be used to map the crust beneath the Antarctic ice sheet in future work. doi:10.1016/j.gca.2006.06.960
Geochemical modification of plastically deformed zircon S.M. REDDY, N.E. TIMMS, P.D. KINNY Department of Applied Geology, Curtin University, GPO Box U1987, Perth, WA 6845, Australia ([email protected]; [email protected]; [email protected]) Electron backscatter diffraction (EBSD) analysis of Indian Ocean zircon deformed at amphibolite facies reveals variations in intragrain crystallographic orientations of up to 20°. Quantitative orientation maps show this variation to reflect progressive localised bending of the zircon and the formation of discrete low-angle orientation boundaries accommodating between 0.5 and 3° misorientations. Analysis of crystallographic pole figures and minimum misorientation angle/axis pairs shows that these variations are associated with rotations around low index crystallographic axes. The distribution of orientations and misorientation axes are consistent with crystal plastic deformation of zircon associated with the formation and migration of [100] dislocations into low-angle tilt boundaries by <001>{100} glide. Panchromatic cathodoluminescence (CL) studies of the deformed zircon indicates homogenisation of primary growth zoning and a decrease in emittance in areas of high-dislocation density. Compositional maps derived from wavelength CL data show that this variation corresponds to relative changes in rare earth element (REE) geochemistry (specifically a change in Er/ Tb) that is spatially identical to plastic strain mapped by EBSD. The change in REE is quantified by ion microprobe (SHRIMP) analyses that show both a systematic increase in REE abundance in deformed zones and a relative enrichment in lighter/heavier REE elements. These data confirm the modification of zircon REE chemistry within the areas of crystal plasticity and indicate that the enhanced diffusion of REEs into the zircon is spatially linked to the presence of dislocations. These dislocations behave as high-diffusivity pathways, increasing bulk diffusion rates and effective diffusion distances in the zircon by 105 compared to theoretical migration distances calculated from empirically derived volume diffusion data. The presence of deformation-related crystal-plastic microstructures in zircon, and their role in modifying elemental diffusion, questions the commonly made assumption of chemical robustness and has fundamental implications for diffusion and dissolution of zircon in high-grade rocks and therefore the interpretation of zircon trace-element and isotopic data. doi:10.1016/j.gca.2006.06.961
Goldschmidt Conference Abstracts 2006
Precambrian terranes in West Australia and East Antarctica: Seismic structure and implications for continent formation and evolution A.M. READING Research School of Earth Sciences, Australian National University, Canberra, ACT 0200, Australia ([email protected]) The crustal thickness and seismic S-wave velocity profile of the upper lithosphere are found using energy from distant earthquakes recorded at remote, temporary seismic stations (‘receiver function’ analysis). Such methods can be used to constrain major structural and compositional characteristics of the continental crust, and delineate the extent of cratonic and orogenic terranes in regions where geological exposure of the surface is limited. They provide an effective alternative to active source seismic techniques for deep crustal targets. A new, near-comprehensive survey is presented which highlights variations in the structure of the West Australian Craton at the scale of the main terrane groups. Seismic structure is very consistent within several of the individual Archaean terrane groups, most notably the Pilbara, Murchison and Southern Cross. They are underlain by lower crust of low seismic velocity and show a sharp seismic Moho. Significant contrasts are seen between neighbouring terranes, thus, major tectonic units have a velocity profile that is a signature of that terrane or terrane group. We infer that the seismic structure is fixed before craton assembly and preserved through the subsequent collision and accretion of the tectonic units forming the West Australian Craton. Archaean terranes younger than 2.8 Ga do not show the low velocities in the lower crust or the very sharp Moho that is characteristic of the oldest terranes. One explanation is that less reworking of the lithosphere took place after this time. Reconnaissance-scale determinations of seismic structure have been made in East Antarctica in the Lambert Glacier region, which encompasses the proposed boundary between three of the ancient continents that formed East Gondwana. The only area of extensive rock exposure in central East Antarctica, it uniquely allows the seismic structure to be linked to surface geology. Data was recorded at stations of the SSCUA deployment in remote regions as far south as 75 °S. Seismic structure and Moho depth are determined across the region, providing constraints on its tectonic evolution. A significant contrast in crustal depth is found between the Northern and Southern Prince Charles Mountains that may indicate the location of a major tectonic boundary. Baseline seismic receiver structures have established for the Rayner, Fisher and Lambert Terranes that may be used to map the crust beneath the Antarctic ice sheet in future work. doi:10.1016/j.gca.2006.06.960
Geochemical modification of plastically deformed zircon S.M. REDDY, N.E. TIMMS, P.D. KINNY Department of Applied Geology, Curtin University, GPO Box U1987, Perth, WA 6845, Australia ([email protected]; [email protected]; [email protected]) Electron backscatter diffraction (EBSD) analysis of Indian Ocean zircon deformed at amphibolite facies reveals variations in intragrain crystallographic orientations of up to 20°. Quantitative orientation maps show this variation to reflect progressive localised bending of the zircon and the formation of discrete low-angle orientation boundaries accommodating between 0.5 and 3° misorientations. Analysis of crystallographic pole figures and minimum misorientation angle/axis pairs shows that these variations are associated with rotations around low index crystallographic axes. The distribution of orientations and misorientation axes are consistent with crystal plastic deformation of zircon associated with the formation and migration of [100] dislocations into low-angle tilt boundaries by <001>{100} glide. Panchromatic cathodoluminescence (CL) studies of the deformed zircon indicates homogenisation of primary growth zoning and a decrease in emittance in areas of high-dislocation density. Compositional maps derived from wavelength CL data show that this variation corresponds to relative changes in rare earth element (REE) geochemistry (specifically a change in Er/ Tb) that is spatially identical to plastic strain mapped by EBSD. The change in REE is quantified by ion microprobe (SHRIMP) analyses that show both a systematic increase in REE abundance in deformed zones and a relative enrichment in lighter/heavier REE elements. These data confirm the modification of zircon REE chemistry within the areas of crystal plasticity and indicate that the enhanced diffusion of REEs into the zircon is spatially linked to the presence of dislocations. These dislocations behave as high-diffusivity pathways, increasing bulk diffusion rates and effective diffusion distances in the zircon by 105 compared to theoretical migration distances calculated from empirically derived volume diffusion data. The presence of deformation-related crystal-plastic microstructures in zircon, and their role in modifying elemental diffusion, questions the commonly made assumption of chemical robustness and has fundamental implications for diffusion and dissolution of zircon in high-grade rocks and therefore the interpretation of zircon trace-element and isotopic data. doi:10.1016/j.gca.2006.06.961