Modelling U-Pb discordance in the Acasta Gneiss: Implications for fluid–rock interaction in Earth's oldest dated crust
© 2019 International Association for Gondwana Research The U–Pb isotopic system in zircon is the tool of choice to interrogate high-temperature geological processes, yet this system has potential to investigate lower temperature fluid–rock interaction as well. In some cases, removal of radiogeni...
| Main Authors: | , , , |
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| Format: | Journal Article |
| Language: | English |
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ELSEVIER
2020
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| Online Access: | http://hdl.handle.net/20.500.11937/80262 |
| _version_ | 1848764192180404224 |
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| author | Kirkland, Chris Johnson, Tim Kinny, Pete Kapitany, T. |
| author_facet | Kirkland, Chris Johnson, Tim Kinny, Pete Kapitany, T. |
| author_sort | Kirkland, Chris |
| building | Curtin Institutional Repository |
| collection | Online Access |
| description | © 2019 International Association for Gondwana Research
The U–Pb isotopic system in zircon is the tool of choice to interrogate high-temperature geological processes, yet this system has potential to investigate lower temperature fluid–rock interaction as well. In some cases, removal of radiogenic Pb is incomplete, potentially allowing regression of discordant U–Pb data on a concordia diagram to determine both the age of crystallization and the timing of fluid-driven isotopic disturbance. However, in rocks preserving more complex histories, simple regression is not effective at resolving multiple Pb loss events. Here, we use a ‘concordant–discordant comparison’ (CDC) test to establish the times of U–Pb disturbance in the Acasta Gneiss Complex (AGC), Canada. AGC c. 4.03 to c. 3.40 Ga orthogneisses experienced a long and complex post-crystallization history, for which U–Pb zircon data reflects not only the heterogeneous nature of the rock, but also the varying degrees and duration of crustal reworking that inevitably involved open system processes. The CDC test calculates the similarity between the concordant age structure and a modelled age structure, the latter inferred from discordant analyses, over a wide range of potential disturbance times. Our analysis reveals concordant zircon components implying new growth and/or recrystallization at 3992 ± 5, 3501 ± 6, 3442 ± 5 and 3126 ± 6 Ma. In addition, we establish episodes of radiogenic-Pb loss driven by fluid–rock interaction at 3150 ± 50 Ma, and probably at 2875 ± 50 Ma and c. 2590 Ma. These Pb-loss episodes correlate with previously recognised events recording growth of zircon rims during metamorphism, granite emplacement, and unroofing. Pb-loss within the AGC shows an antithetic relationship in different samples that are in close geographic proximity. We suggest that zircon alteration and associated new growth effectively rendered those grains that underwent Pb-loss at a particular time less susceptible to alteration during the next episode of fluid interaction. |
| first_indexed | 2025-11-14T11:15:27Z |
| format | Journal Article |
| id | curtin-20.500.11937-80262 |
| institution | Curtin University Malaysia |
| institution_category | Local University |
| language | English |
| last_indexed | 2025-11-14T11:15:27Z |
| publishDate | 2020 |
| publisher | ELSEVIER |
| recordtype | eprints |
| repository_type | Digital Repository |
| spelling | curtin-20.500.11937-802622021-09-13T07:07:03Z Modelling U-Pb discordance in the Acasta Gneiss: Implications for fluid–rock interaction in Earth's oldest dated crust Kirkland, Chris Johnson, Tim Kinny, Pete Kapitany, T. Science & Technology Physical Sciences Geosciences, Multidisciplinary Geology Zircon Acasta Gneiss Complex Pb-loss U-Pb geochronology Discordance SLAVE BASEMENT-COMPLEX NORTHWESTERN CANADA LU-HF TH-PB ZIRCON GEOCHRONOLOGY TRACE-ELEMENT PROVINCE EVOLUTION METAMICTIZATION RADIATION © 2019 International Association for Gondwana Research The U–Pb isotopic system in zircon is the tool of choice to interrogate high-temperature geological processes, yet this system has potential to investigate lower temperature fluid–rock interaction as well. In some cases, removal of radiogenic Pb is incomplete, potentially allowing regression of discordant U–Pb data on a concordia diagram to determine both the age of crystallization and the timing of fluid-driven isotopic disturbance. However, in rocks preserving more complex histories, simple regression is not effective at resolving multiple Pb loss events. Here, we use a ‘concordant–discordant comparison’ (CDC) test to establish the times of U–Pb disturbance in the Acasta Gneiss Complex (AGC), Canada. AGC c. 4.03 to c. 3.40 Ga orthogneisses experienced a long and complex post-crystallization history, for which U–Pb zircon data reflects not only the heterogeneous nature of the rock, but also the varying degrees and duration of crustal reworking that inevitably involved open system processes. The CDC test calculates the similarity between the concordant age structure and a modelled age structure, the latter inferred from discordant analyses, over a wide range of potential disturbance times. Our analysis reveals concordant zircon components implying new growth and/or recrystallization at 3992 ± 5, 3501 ± 6, 3442 ± 5 and 3126 ± 6 Ma. In addition, we establish episodes of radiogenic-Pb loss driven by fluid–rock interaction at 3150 ± 50 Ma, and probably at 2875 ± 50 Ma and c. 2590 Ma. These Pb-loss episodes correlate with previously recognised events recording growth of zircon rims during metamorphism, granite emplacement, and unroofing. Pb-loss within the AGC shows an antithetic relationship in different samples that are in close geographic proximity. We suggest that zircon alteration and associated new growth effectively rendered those grains that underwent Pb-loss at a particular time less susceptible to alteration during the next episode of fluid interaction. 2020 Journal Article http://hdl.handle.net/20.500.11937/80262 10.1016/j.gr.2019.07.017 English http://creativecommons.org/licenses/by-nc-nd/4.0/ ELSEVIER fulltext |
| spellingShingle | Science & Technology Physical Sciences Geosciences, Multidisciplinary Geology Zircon Acasta Gneiss Complex Pb-loss U-Pb geochronology Discordance SLAVE BASEMENT-COMPLEX NORTHWESTERN CANADA LU-HF TH-PB ZIRCON GEOCHRONOLOGY TRACE-ELEMENT PROVINCE EVOLUTION METAMICTIZATION RADIATION Kirkland, Chris Johnson, Tim Kinny, Pete Kapitany, T. Modelling U-Pb discordance in the Acasta Gneiss: Implications for fluid–rock interaction in Earth's oldest dated crust |
| title | Modelling U-Pb discordance in the Acasta Gneiss: Implications for fluid–rock interaction in Earth's oldest dated crust |
| title_full | Modelling U-Pb discordance in the Acasta Gneiss: Implications for fluid–rock interaction in Earth's oldest dated crust |
| title_fullStr | Modelling U-Pb discordance in the Acasta Gneiss: Implications for fluid–rock interaction in Earth's oldest dated crust |
| title_full_unstemmed | Modelling U-Pb discordance in the Acasta Gneiss: Implications for fluid–rock interaction in Earth's oldest dated crust |
| title_short | Modelling U-Pb discordance in the Acasta Gneiss: Implications for fluid–rock interaction in Earth's oldest dated crust |
| title_sort | modelling u-pb discordance in the acasta gneiss: implications for fluid–rock interaction in earth's oldest dated crust |
| topic | Science & Technology Physical Sciences Geosciences, Multidisciplinary Geology Zircon Acasta Gneiss Complex Pb-loss U-Pb geochronology Discordance SLAVE BASEMENT-COMPLEX NORTHWESTERN CANADA LU-HF TH-PB ZIRCON GEOCHRONOLOGY TRACE-ELEMENT PROVINCE EVOLUTION METAMICTIZATION RADIATION |
| url | http://hdl.handle.net/20.500.11937/80262 |