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U-Pb datering av zirkon från migmatitisk gnejs i Delsjöområdet, Idefjordenterrängen

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Author

  • Mimmi Ingered

Summary, in English

During the Sveconorwegian orogenesis, parts of southwestern Sweden was recrystallized and formed metamorphic migmatites. In the Idefjorden terrane, west of the Mylonite zone, the migmatites have not been dated and since knowledge of the metamorphic processes in the area is lacking, the migmatites can provide better constraints of when metamorphosis occurred. A migmatitic gneiss from the Delsjö area in eastern Gothenburg has been dated to investigate whether migmatite formation in the Idefjorden terrane is of Sveconorwegian age. U-Pb dating of zircon from the migmatite has been done with Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) after selecting points for analyses in scanning electron microscopy (SEM) with cathodoluminescence (CL) and backscattered electron imaging (BSE). In CL, complex cores and clear growths zones could be distinguished in the zircon crystals, which have different dates and Th/U ratios. Weighted mean values for concordant data yield 207Pb/ 206Pb = 1598 ± 6.6 Ma (MSWD = 0.89) for magmatically zoned cores with Th/U > 0.2 and 1002 ± 9 Ma (MSWD = 0.09) for rims as well as for individual grains from leucosome. The younger population zircon has Th/U < 0.01 and is interpreted to have a metamorphic origin. The old population is interpreted as the magmatic crystallization age of the protolith and the younger populations as the age of migmatization in conjunction with the Sveconorwegian orogenesis. A few zircons yielded 207Pb/ 206Pb dates of 1700 Ma and may represent inherited cores.

Publishing year

2017

Language

Swedish

Publication/Series

Examensarbeten i geologi vid Lunds universitet

Document type

Student publication for Bachelor's degree

Topic

  • Earth and Environmental Sciences

Keywords

  • Svekonorvegiska orogenesen
  • Idefjordenterrängen
  • U-Pb
  • Migmatit
  • Zirkon
  • LA-ICP-MS

Report number

512

Supervisor

  • Anders Scherstén (Dr.)

Scientific presentation