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Metamorphic record of monazite in aluminous migmatitic gneisses at Stensjöstrand, Sveconorwegian orogen

Olsson, Andreas LU (2016) In Dissertations in Geology at Lund University GEOR02 20152
Department of Geology
Abstract
Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) has been utilised to investigate the age and chemical composition of two texturally different types of monazite in leucosome and mesosome of polymetamorphic aluminous migmatitic gneisses at Stensjöstrand, Halland. Results of radiometric dating are compared with previously published zircon dates. Monazite grains were documented and investigated in three thin sections using polarization microscopy and Scanning Electron Microscopy coupled with Energy Dispersive X-ray (SEM-EDX). Backscatter Electron (BSE) images show that monazite grains in the mesosome have anhedral morphology, embayments, and complex internal zonation. Monazite grains in the leucosome are relatively... (More)
Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) has been utilised to investigate the age and chemical composition of two texturally different types of monazite in leucosome and mesosome of polymetamorphic aluminous migmatitic gneisses at Stensjöstrand, Halland. Results of radiometric dating are compared with previously published zircon dates. Monazite grains were documented and investigated in three thin sections using polarization microscopy and Scanning Electron Microscopy coupled with Energy Dispersive X-ray (SEM-EDX). Backscatter Electron (BSE) images show that monazite grains in the mesosome have anhedral morphology, embayments, and complex internal zonation. Monazite grains in the leucosome are relatively small, euhedral to subhedral or ovoid, and have relatively distinct internal zoning. Anhedral and ovoid morphologies, embayments, and complex zoning indicate recrystallisation and resorption processes having affected monazite grains during metamorphism. Documented monazite grains were analysed for trace elements Y, Lu, Th, and U, rare earth elements (REE), and radiometrically dated. Results reveal two age groups, c. 1.43 Ga and c. 0.99 Ga, that relate to the Hallandian and Sveconorwegian orogenies, respectively. Both monazite generations occur in leucosome and mesosome, either as single- or double-generation grains. Weighted average calculations of concordant 207Pb/206Pb isotopic dates yield 1434 ± 14 Ma and 988 ± 11 Ma, which can be compared to previously published zircon dates of 1386 ± 7 Ma and 970 ± 5 Ma (Piñán-Llamas et al. 2015). The analytical results may have been affected by data drift owing to downhole fractionation, most distinctly observed in results from Thompson Mine Monazite (TMM) standard as steadily increasing dates throughout the day of analysis. Trends in trace element analyses show relative U enrichment and depletion of Y, Lu, and Th in the 0.99 Ga monazite generation compared to the 1.43 Ga generation, indicating that the bulk of REE content in the rock may have been locked into the Hallandian generation preventing incorporation into the Sveconorwegian generation. In conclusion, obtained U-Pb monazite dates correlate broadly with U-Pb zircon dates, but are c. 50 Ma and c. 18 Ma older, for the Hallandian and Sveconorwegian generations, respectively. (Less)
Abstract (Swedish)
Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) har använts för att undersöka ålder och kemisk komposition av två texturellt skilda typer av monazit i leucosom och mesosom i polymetamorf aluminös migmatitgnejs vid Stensjöstrand, Halland. Dateringsresultat jämförs med tidigare publicerade zirkondateringar. Monazitkorn dokumenterades och undersöktes i tre tunnslip med hjälp av polarisationsmik-roskopi och Scanning Electron Mikroskopy kopplat med Energy Dispersive X-ray (SEM-EDX). Backscatter Electron (BSE) bilder visar att monasitkorn i mesosomen har anhedral morfologi med inskärningar och komplex intern zonering. Monazitkorn i leucosomen är relativt små, euhedrala till subhedrala eller äggformiga och har relativt... (More)
Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) har använts för att undersöka ålder och kemisk komposition av två texturellt skilda typer av monazit i leucosom och mesosom i polymetamorf aluminös migmatitgnejs vid Stensjöstrand, Halland. Dateringsresultat jämförs med tidigare publicerade zirkondateringar. Monazitkorn dokumenterades och undersöktes i tre tunnslip med hjälp av polarisationsmik-roskopi och Scanning Electron Mikroskopy kopplat med Energy Dispersive X-ray (SEM-EDX). Backscatter Electron (BSE) bilder visar att monasitkorn i mesosomen har anhedral morfologi med inskärningar och komplex intern zonering. Monazitkorn i leucosomen är relativt små, euhedrala till subhedrala eller äggformiga och har relativt distinkt intern zonering. Anhedrala och äggformiga morfologier, inskärningar samt komplex zonering indikerar att monazitkornen har utsatts för omkristallations- och resorptionsprocesser vid metamorfism. Dokumenterade monazitkorn analyserades för spårelementen Y, Lu, Th och U, sällsynta jordartsmetaller (REE) samt utfördes ratdiometrisk datering. Dateringar visar två åldersgrupper, ca 1.43 Ga och ca 0.99 Ga, som relaterar till den Hallandiska respektive den Sveconorvegiska orogenesen. Båda generationer av monazit förekommer i leucosom samt mesosom, antingen som enstaka eller dubbelgenerations korn. Viktade medelvärdesberäkningar från konkordant 207Pb/206Pb isotopiska datum gav 1434 ± 14 Ma och 988 ± 11 Ma som kan jämföras med tidigare publicerade zirkondata, 1386 ± 7 Ma och 970 ± 5 Ma (Piñán-Llamas et al. 2015). De analytiska resultaten kan ha påverkats av datadrift på grund av s.k. downhole fractionation. Detta syns tydligast i resultat från standarden Thompson Mine Monazite (TMM) i form av stadigt ökande dateringar under dagen som analyserna genomfördes. Trender i spårelementanalyser visar relativ U anriktning samt utarmning av Y, Lu och Th i 0.99 Ga monazitgenerationen jämfört med 1.43 Ga generationen. Detta indikerar att bulkmängden av REE i bergarten kan ha blivit låst i den Hallandiska generationen vilket till följd har förhindrat inkorporering i den Svekonorvegiska generationen. Sammanfattningsvis korrelerar erhållna U-Pb monazitdateringar brett med U-Pb zirkondateringar, men är ca 50 Ma och ca 18 Ma äldre för Hallandiska respektive Svekonorvegiska generationer. (Less)
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author
Olsson, Andreas LU
supervisor
organization
course
GEOR02 20152
year
type
H2 - Master's Degree (Two Years)
subject
keywords
Monazite, LA-ICP-MS, U-Th-Pb Geochronology, Migmatite, Stensjöstrand
publication/series
Dissertations in Geology at Lund University
report number
467
language
English
id
8872040
date added to LUP
2016-05-04 16:23:15
date last changed
2017-02-27 10:46:27
@misc{8872040,
  abstract     = {Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) has been utilised to investigate the age and chemical composition of two texturally different types of monazite in leucosome and mesosome of polymetamorphic aluminous migmatitic gneisses at Stensjöstrand, Halland. Results of radiometric dating are compared with previously published zircon dates. Monazite grains were documented and investigated in three thin sections using polarization microscopy and Scanning Electron Microscopy coupled with Energy Dispersive X-ray (SEM-EDX). Backscatter Electron (BSE) images show that monazite grains in the mesosome have anhedral morphology, embayments, and complex internal zonation. Monazite grains in the leucosome are relatively small, euhedral to subhedral or ovoid, and have relatively distinct internal zoning. Anhedral and ovoid morphologies, embayments, and complex zoning indicate recrystallisation and resorption processes having affected monazite grains during metamorphism. Documented monazite grains were analysed for trace elements Y, Lu, Th, and U, rare earth elements (REE), and radiometrically dated. Results reveal two age groups, c. 1.43 Ga and c. 0.99 Ga, that relate to the Hallandian and Sveconorwegian orogenies, respectively. Both monazite generations occur in leucosome and mesosome, either as single- or double-generation grains. Weighted average calculations of concordant 207Pb/206Pb isotopic dates yield 1434 ± 14 Ma and 988 ± 11 Ma, which can be compared to previously published zircon dates of 1386 ± 7 Ma and 970 ± 5 Ma (Piñán-Llamas et al. 2015). The analytical results may have been affected by data drift owing to downhole fractionation, most distinctly observed in results from Thompson Mine Monazite (TMM) standard as steadily increasing dates throughout the day of analysis. Trends in trace element analyses show relative U enrichment and depletion of Y, Lu, and Th in the 0.99 Ga monazite generation compared to the 1.43 Ga generation, indicating that the bulk of REE content in the rock may have been locked into the Hallandian generation preventing incorporation into the Sveconorwegian generation. In conclusion, obtained U-Pb monazite dates correlate broadly with U-Pb zircon dates, but are c. 50 Ma and c. 18 Ma older, for the Hallandian and Sveconorwegian generations, respectively.},
  author       = {Olsson, Andreas},
  keyword      = {Monazite,LA-ICP-MS,U-Th-Pb Geochronology,Migmatite,Stensjöstrand},
  language     = {eng},
  note         = {Student Paper},
  series       = {Dissertations in Geology at Lund University},
  title        = {Metamorphic record of monazite in aluminous migmatitic gneisses at Stensjöstrand, Sveconorwegian orogen},
  year         = {2016},
}