A SHRIMP ion microprobe study of inherited and magmatic zircons from four Scottish Caledonian granites
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ABSTRACT Using the ion microprobe SHRIMP we have analysed zircons from the Ben Vuirich, Glen Kyllachy, Inchbae and Vagastie Bridge granites from the Scottish Caledonides, in an attempt to resolve the ages of inherited zircons shown to be present in these granites by previous conventional multigrain analyses. Middle Proterozoic age components were found in inherited zircons from all four granites. Late Proterozoic (900–1,100 Ma) components have been identified in zircons from the Glen Kyllachy and Ben Vuirich granites in the Grampian Highlands. A Late Archaean age has only been detected in one zircon from the Glen Kyllachy granite. The distribution of inherited components in the granite zircon populations could reflect fundamental divisions in the age composition of granite source rocks; however, detailed assessment of this possibility must await further ion microprobe analyses on zircons from many more granites. SHRIMP isotopic and U, Th and Pb analyses were made on successive shells of zoned zircon surrounding inherited cores from the Glen Kyllachy granite to monitor chemical changes during magmatic zircon growth. Results show that zircon shells have characteristic but significantly different Th, U and Pb concentrations. Magmatic zircon from the Vagastie Bridge granite also forms as clearly defined oscillatory zoned shells around unzoned nuclei of inherited zircon. However, the distinction between magmatic and inherited zircon in zircons from the Inchbae granite is less clear. Zircons from the Ben Vuirich granite occur as euhedral, magmatic zircons, or as rounded, subhedral, inherited zircon grains. A SHRIMP age of 597 ± 11 (2σ) Ma for euhedral magmatic zircon from this granite is identical, within the uncertainty, to the conventional multigrain zircon age of 590 ± 2 (2σ) Ma reported by Rogers et al. (1989) and confirms the conclusions of those authors that sedimentation of the Dalradian sequence took place in the Precambrian.Keywords:
Geochronology
Abstract. Detrital zircon geochronology by laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) is a widely-used tool for determining maximum depositional ages, sediment provenance, and reconstructing sediment routing pathways. Although the accuracy and precision of U-Pb geochronology measurements has improved over the past two decades, Pb-loss continues to impact the ability to resolve zircon age populations by biasing affected zircon toward younger apparent ages. Chemical abrasion (CA) has been shown to reduce or eliminate the effects of Pb-loss in zircon U-Pb geochronology, but has yet to be widely applied to large-n detrital zircon analyses. Here, we assess the efficacy of the chemical abrasion treatment on zircon prior to analysis by LA-ICP-MS and discuss the advantages and limitations of this technique in relation to detrital zircon geochronology. We show that i) CA does not systematically bias LA-ICP-MS U-Pb dates for thirteen reference materials that span a wide variety of crystallization dates and U concentrations; ii) CA-LA-ICP-MS U-Pb zircon geochronology can reduce, or eliminate, Pb-loss in samples that have experienced significant radiation damage; and iii) bulk CA prior to detrital zircon U-Pb geochronology by LA-ICP-MS improves the resolution of Neoproterozoic to present zircon age populations and the percentage of concordant analyses in Mesoproterozoic and older age populations. The selective dissolution of zircon that has experienced high degrees of radiation damage suggests that some detrital zircon age populations could be destroyed or have their abundance significantly modified during this process. However, we did not identify this potential effect in either of the detrital zircon samples that were analyzed as part of this study. We conclude that pre-treatment of detrital zircon by bulk CA may be useful for applications that require increased resolution of detrital zircon populations.
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Abstract. Detrital zircon geochronology by laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) is a widely-used tool for determining maximum depositional ages, sediment provenance, and reconstructing sediment routing pathways. Although the accuracy and precision of U-Pb geochronology measurements has improved over the past two decades, Pb-loss continues to impact the ability to resolve zircon age populations by biasing affected zircon toward younger apparent ages. Chemical abrasion (CA) has been shown to reduce or eliminate the effects of Pb-loss in zircon U-Pb geochronology, but has yet to be widely applied to large-n detrital zircon analyses. Here, we assess the efficacy of the chemical abrasion treatment on zircon prior to analysis by LA-ICP-MS and discuss the advantages and limitations of this technique in relation to detrital zircon geochronology. We show that i) CA does not systematically bias LA-ICP-MS U-Pb dates for thirteen reference materials that span a wide variety of crystallization dates and U concentrations; ii) CA-LA-ICP-MS U-Pb zircon geochronology can reduce, or eliminate, Pb-loss in samples that have experienced significant radiation damage; and iii) bulk CA prior to detrital zircon U-Pb geochronology by LA-ICP-MS improves the resolution of Neoproterozoic to present zircon age populations and the percentage of concordant analyses in Mesoproterozoic and older age populations. The selective dissolution of zircon that has experienced high degrees of radiation damage suggests that some detrital zircon age populations could be destroyed or have their abundance significantly modified during this process. However, we did not identify this potential effect in either of the detrital zircon samples that were analyzed as part of this study. We conclude that pre-treatment of detrital zircon by bulk CA may be useful for applications that require increased resolution of detrital zircon populations.
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Abstract. Detrital zircon geochronology by laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) is a widely-used tool for determining maximum depositional ages, sediment provenance, and reconstructing sediment routing pathways. Although the accuracy and precision of U-Pb geochronology measurements has improved over the past two decades, Pb-loss continues to impact the ability to resolve zircon age populations by biasing affected zircon toward younger apparent ages. Chemical abrasion (CA) has been shown to reduce or eliminate the effects of Pb-loss in zircon U-Pb geochronology, but has yet to be widely applied to large-n detrital zircon analyses. Here, we assess the efficacy of the chemical abrasion treatment on zircon prior to analysis by LA-ICP-MS and discuss the advantages and limitations of this technique in relation to detrital zircon geochronology. We show that i) CA does not systematically bias LA-ICP-MS U-Pb dates for thirteen reference materials that span a wide variety of crystallization dates and U concentrations; ii) CA-LA-ICP-MS U-Pb zircon geochronology can reduce, or eliminate, Pb-loss in samples that have experienced significant radiation damage; and iii) bulk CA prior to detrital zircon U-Pb geochronology by LA-ICP-MS improves the resolution of Neoproterozoic to present zircon age populations and the percentage of concordant analyses in Mesoproterozoic and older age populations. The selective dissolution of zircon that has experienced high degrees of radiation damage suggests that some detrital zircon age populations could be destroyed or have their abundance significantly modified during this process. However, we did not identify this potential effect in either of the detrital zircon samples that were analyzed as part of this study. We conclude that pre-treatment of detrital zircon by bulk CA may be useful for applications that require increased resolution of detrital zircon populations.
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Abstract. Detrital zircon geochronology by laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) is a widely-used tool for determining maximum depositional ages, sediment provenance, and reconstructing sediment routing pathways. Although the accuracy and precision of U-Pb geochronology measurements has improved over the past two decades, Pb-loss continues to impact the ability to resolve zircon age populations by biasing affected zircon toward younger apparent ages. Chemical abrasion (CA) has been shown to reduce or eliminate the effects of Pb-loss in zircon U-Pb geochronology, but has yet to be widely applied to large-n detrital zircon analyses. Here, we assess the efficacy of the chemical abrasion treatment on zircon prior to analysis by LA-ICP-MS and discuss the advantages and limitations of this technique in relation to detrital zircon geochronology. We show that i) CA does not systematically bias LA-ICP-MS U-Pb dates for thirteen reference materials that span a wide variety of crystallization dates and U concentrations; ii) CA-LA-ICP-MS U-Pb zircon geochronology can reduce, or eliminate, Pb-loss in samples that have experienced significant radiation damage; and iii) bulk CA prior to detrital zircon U-Pb geochronology by LA-ICP-MS improves the resolution of Neoproterozoic to present zircon age populations and the percentage of concordant analyses in Mesoproterozoic and older age populations. The selective dissolution of zircon that has experienced high degrees of radiation damage suggests that some detrital zircon age populations could be destroyed or have their abundance significantly modified during this process. However, we did not identify this potential effect in either of the detrital zircon samples that were analyzed as part of this study. We conclude that pre-treatment of detrital zircon by bulk CA may be useful for applications that require increased resolution of detrital zircon populations.
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Abstract. Detrital zircon geochronology by laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) is a widely-used tool for determining maximum depositional ages, sediment provenance, and reconstructing sediment routing pathways. Although the accuracy and precision of U-Pb geochronology measurements has improved over the past two decades, Pb-loss continues to impact the ability to resolve zircon age populations by biasing affected zircon toward younger apparent ages. Chemical abrasion (CA) has been shown to reduce or eliminate the effects of Pb-loss in zircon U-Pb geochronology, but has yet to be widely applied to large-n detrital zircon analyses. Here, we assess the efficacy of the chemical abrasion treatment on zircon prior to analysis by LA-ICP-MS and discuss the advantages and limitations of this technique in relation to detrital zircon geochronology. We show that i) CA does not systematically bias LA-ICP-MS U-Pb dates for thirteen reference materials that span a wide variety of crystallization dates and U concentrations; ii) CA-LA-ICP-MS U-Pb zircon geochronology can reduce, or eliminate, Pb-loss in samples that have experienced significant radiation damage; and iii) bulk CA prior to detrital zircon U-Pb geochronology by LA-ICP-MS improves the resolution of Neoproterozoic to present zircon age populations and the percentage of concordant analyses in Mesoproterozoic and older age populations. The selective dissolution of zircon that has experienced high degrees of radiation damage suggests that some detrital zircon age populations could be destroyed or have their abundance significantly modified during this process. However, we did not identify this potential effect in either of the detrital zircon samples that were analyzed as part of this study. We conclude that pre-treatment of detrital zircon by bulk CA may be useful for applications that require increased resolution of detrital zircon populations.
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Abstract. Detrital zircon geochronology by laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) is a widely-used tool for determining maximum depositional ages, sediment provenance, and reconstructing sediment routing pathways. Although the accuracy and precision of U-Pb geochronology measurements has improved over the past two decades, Pb-loss continues to impact the ability to resolve zircon age populations by biasing affected zircon toward younger apparent ages. Chemical abrasion (CA) has been shown to reduce or eliminate the effects of Pb-loss in zircon U-Pb geochronology, but has yet to be widely applied to large-n detrital zircon analyses. Here, we assess the efficacy of the chemical abrasion treatment on zircon prior to analysis by LA-ICP-MS and discuss the advantages and limitations of this technique in relation to detrital zircon geochronology. We show that i) CA does not systematically bias LA-ICP-MS U-Pb dates for thirteen reference materials that span a wide variety of crystallization dates and U concentrations; ii) CA-LA-ICP-MS U-Pb zircon geochronology can reduce, or eliminate, Pb-loss in samples that have experienced significant radiation damage; and iii) bulk CA prior to detrital zircon U-Pb geochronology by LA-ICP-MS improves the resolution of Neoproterozoic to present zircon age populations and the percentage of concordant analyses in Mesoproterozoic and older age populations. The selective dissolution of zircon that has experienced high degrees of radiation damage suggests that some detrital zircon age populations could be destroyed or have their abundance significantly modified during this process. However, we did not identify this potential effect in either of the detrital zircon samples that were analyzed as part of this study. We conclude that pre-treatment of detrital zircon by bulk CA may be useful for applications that require increased resolution of detrital zircon populations.
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Abstract. Detrital zircon geochronology by laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) is a widely-used tool for determining maximum depositional ages, sediment provenance, and reconstructing sediment routing pathways. Although the accuracy and precision of U-Pb geochronology measurements has improved over the past two decades, Pb-loss continues to impact the ability to resolve zircon age populations by biasing affected zircon toward younger apparent ages. Chemical abrasion (CA) has been shown to reduce or eliminate the effects of Pb-loss in zircon U-Pb geochronology, but has yet to be widely applied to large-n detrital zircon analyses. Here, we assess the efficacy of the chemical abrasion treatment on zircon prior to analysis by LA-ICP-MS and discuss the advantages and limitations of this technique in relation to detrital zircon geochronology. We show that i) CA does not systematically bias LA-ICP-MS U-Pb dates for thirteen reference materials that span a wide variety of crystallization dates and U concentrations; ii) CA-LA-ICP-MS U-Pb zircon geochronology can reduce, or eliminate, Pb-loss in samples that have experienced significant radiation damage; and iii) bulk CA prior to detrital zircon U-Pb geochronology by LA-ICP-MS improves the resolution of Neoproterozoic to present zircon age populations and the percentage of concordant analyses in Mesoproterozoic and older age populations. The selective dissolution of zircon that has experienced high degrees of radiation damage suggests that some detrital zircon age populations could be destroyed or have their abundance significantly modified during this process. However, we did not identify this potential effect in either of the detrital zircon samples that were analyzed as part of this study. We conclude that pre-treatment of detrital zircon by bulk CA may be useful for applications that require increased resolution of detrital zircon populations.
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Most of central Africa is underlain by Archaean terrains (mostly below a Phanerozoic cover), represented by high-grade gneissic complexes and by low-grade granite-greenstone belts. The lowermost Proterozoic is represented either by gneisses in mobile zones or low-grade supracrustals in forelands. The remaining Lower Proterozoic is made of low-grade supracrustal metasediments in mobile zones. Such zones developed thus almost immediately after the end-Archaean cratonization. The successive mobile zones appear to have developed in a centrifugal pattern during the Lower-Proterozoic. The mineral wealth is unevenly distributed. Only some greenstone belts have given an appreciable gold output, whereas the gneissic Archaean terrains have proven to be almost barren. Iron remains an important resource of the Archaean, as manganese is for the Lower Proterozoic. Uranium and some Cu, Co has been found in the Lower Proterozoic of respectively Gabon and Uganda.
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