Direct dating of mid-crustal shear zones with synkinematic allanite: New in situ U-Th-Pb geochronological approaches applied to the Mont Blanc massif

Benedicte Cenki-Tok*, James R. Darling, Yann Rolland, Bruno Dhuime, Craig D. Storey

*Corresponding author for this work

Research output: Contribution to journalArticle (Academic Journal)peer-review

36 Citations (Scopus)

Abstract

Dating the timing of motion on crustal shear zones is of tremendous importance for understanding the assembly of orogenic terranes. This objective is achieved in this paper by combining petrological and structural observations with novel developments in in situ U-Th-Pb geochronology of allanite. A greenschist facies shear zone within the Mont Blanc Massif is documented. Allanite is synkinematic and belongs to the mylonitic assemblage. LA-ICP-MS U-Th-Pb isotope analyses of allanite reveal high contents and highly radiogenic isotopic compositions of the common-Pb component. The use of measured Pb-isotope compositions of associated minerals (feldspars and chlorite) is critical for accurate common-Pb correction, and provides a powerful mechanism for linking allanite growth to the metamorphic assemblage. A mean 208Pb/232Th age of 29.44 ± 0.95 Ma is accordingly taken for synkinematic allanite crystallisation under greenschist facies conditions. This age reflects the timing of the Mont Blanc underthrusting below the Penninic Front and highlights the potential of directly dating deformation with allanite. © 2013 John Wiley & Sons Ltd.

Original languageEnglish
Pages (from-to)29-37
Number of pages9
JournalTerra Nova
Volume26
Issue number1
DOIs
Publication statusPublished - 1 Feb 2014

Keywords

  • ALPINE BERGELL INTRUSION
  • TRACE-ELEMENT
  • RB-SR
  • ACCESSORY MINERALS
  • WESTERN ALPS
  • ITALIAN ALPS
  • RARE-EARTH
  • DEFORMATION
  • CONSTRAINTS
  • DISEQUILIBRIUM

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