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TitleQuantitative elemental mapping of granulite-facies monazite: textural insights and implications for petrochronology
AuthorWeller, O M; Jackson, SORCID logo; Miller, W G R; St-Onge, M R; Rayner, NORCID logo
SourceJournal of Metamorphic Geology vol. 38, issue 8, 2020 p. 853-880, Open Access logo Open Access
Alt SeriesNatural Resources Canada, Contribution Series 20200020
PublisherJohn Wiley & Sons Ltd
Mediapaper; on-line; digital
File formatpdf; html
NTS26A; 26B; 26C; 26D; 26E; 26F; 26G; 36A; 36B; 36C; 36D; 36E; 36F; 36G; 36H; 25I; 25J; 25K; 25L; 25M; 25N; 25O; 25P; 35P
AreaBaffin Island
Lat/Long WENS -75.0000 -65.0000 65.0000 62.0000
Subjectsigneous and metamorphic petrology; geochemistry; geochronology; Nature and Environment; Science and Technology; tectonic history; metamorphism; thermal history; radiogenic heat; granulite facies; monazite; petrography; orogenies; geochemical analyses; trace element geochemistry; radiometric dating; uranium lead dates; pressure-temperature conditions; geochemical anomalies; europium; uranium thorium ratios; yttrium geochemistry; Trans-Hudson Orogen; Paleoproterozoic; Precambrian; Proterozoic
Illustrationslocation maps; photographs; photomicrographs; pseudo sections; Concordia diagrams; graphs; figures
ProgramGEM2: Geo-mapping for Energy and Minerals Baffin Bedrock Mapping
Released2020 06 12
AbstractTexturally complex monazite grains contained in two granulite-facies pelitic migmatites from southern Baffin Island, Arctic Canada, were mapped by laser ablation- inductively coupled plasma-mass spectrometry (using spot sizes less than or equal to 5 micrometres) to quantitatively determine the spatial variation in trace element chemistry (with up to 1,883 analyses per grain). The maps highlight the chemical complexity of monazite grains that have experienced multiple episodes of growth, resorption and chemical modification by dissolution-precipitation during high-grade metamorphism. Following detailed chemical characterization of monazite compositional zones, a related U-Pb data set is re-interpreted, allowing petrologically significant ages to be extracted from a continuum of concordant data. Synthesis of these data with pseudosection modelling of prograde and peak conditions allows for the temporal evolution of monazite trace element chemistry to be placed in the context of the evolving P-T conditions and major phase assemblage. This approach enables a critical evaluation of three commonly used petrochronological indicators: linking Y to garnet abundance, the Eu anomaly to feldspar content and Th/U to anatectic processes. Europium anomalies and Th/U behave in a relatively systematic fashion, suggesting that they are reliable petrochronological witnesses. However, Y systematics are variable, both within domains interpreted to have grown in a single event, between grains interpreted to be part of the same age population, and between samples that experienced similar metamorphic conditions and mineral assemblages. These observations caution against generalized petrological interpretations on the basis of Y content, as it suggests Y concentrations in monazite are controlled by domainal equilibria. The results reveal a c. 45 Myr interval between prograde metamorphism and retrograde melt crystallization in the study area, emphasizing the long-lived nature of heat flow in high-grade metamorphic terranes. Such long timescales of metamorphism would be assisted by the growth, retention and dominance of high-Th suprasolidus monazite, as observed in this study, contributing to the radiogenic heating budget of mid to lower-crustal environments. Careful characterization of monazite grains suggests that continuum-style U-Pb data sets can be decoded to provide insights into the duration of metamorphic processes.
Summary(Plain Language Summary, not published)
Monazite is a mineral that is frequently used to date the timing of metamorphism in rock samples. However, monazite grains can feature several episodes of growth within one single grain, which makes them complicated to analyse, particularly when the growth zones are indistinct using conventional imaging. Here, we applied a new technique of using a laser to map the variation in chemistry across monazite grains, which helped to demarcate distinct growth zones. By integrating these data with other isotopic data of the age(s) of the grain, we were then able to document a metamorphic event on southern Baffin Island that lasted for 45 million years. Ultimately, this study has improved our understanding of the tectonic history of southern Baffin Island.

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