• Harvey Thorleifson, Chair
    Minnesota Geological Survey
  • Carrie Jennings, Vice Chair
    Minnesota Geological Survey
  • David Bush, Technical Program Chair
    University of West Georgia
  • Jim Miller, Field Trip Chair
    University of Minnesota Duluth
  • Curtis M. Hudak, Sponsorship Chair
    Foth Infrastructure & Environment, LLC


Paper No. 3
Presentation Time: 9:30 AM


DIDIER, Amélie1, BOSSE, Valérie1, BOULVAIS, Phillipe2, BOULOTON, Jacky1, PAQUETTE, Jean-Louis1 and MONTEL, Jean-Marc3, (1)Clermont Université, Université Blaise Pascal, Laboratoire Magmas et Volcans, UMR 6524, BP 10448, Clermont-Ferrand, 63000, France, (2)Géosciences Rennes, Université de Rennes 1,UMR 6118 CNRS, Observatoire des Sciences de l'Univers de Rennes, Rennes, 35042, France, (3)Ecole nationale Supérieure de Géologie, UMR 7566, G2R, Vandoeuvre-lès-Nancy, 54501, France,

Monazite is extensively used to date crustal processes and usually considered to be resistant to diffusive Pb loss at temperatures as high as 800°C. Nevertheless, fluid-assisted recrystallization is known to be efficient to reset the monazite chronometer. This study focuses on chemical and isotopic disturbances in monazites from two microgranite intrusions representing the final anatectic event of the Velay granitic dome (French Central Massif). Negative Qtz-Kfs oxygen isotope fractionation (- 0,2 ‰<Δ18O(Qz-Fds) <- 3,5‰) shows that both of these intrusions were affected by a late or post-magmatic hydrothermal event. In the first intrusion, Ca,REE-fluorocarbonates, fluorite, calcite and chloritized biotite partially replace the magmatic assemblage suggesting interaction with alkali-F-bearing fluids. Monazites preserved in garnet, quartz and feldspar are chemically homogeneous and display 208Pb/232Th and 206Pb/238U concordant ages at 307 ± 2 Ma. By contrast, groundmass monazites show dissolution-recrystallization features associated with apatite and thorite precipitation and strong chemical reequilibration. 208Pb/232Th ages are disturbed ranging between 270 and 690 Ma. Apparent U-Pb ages are older due to common Pb incorporation yielding a Tera-Wasserburg lower intercept age at 301 ± 17 Ma. These results show that F-rich fluid is responsible for Th mobility and common Pb incorporation during dissolution crystallization processes and thus have strongly disturbed the U-Th-Pb chronometers in the monazite. The second intrusion, located 50 km from the first one, displays little petrological evidence of hydrothermal alteration despite negative Qtz-Kfs oxygen isotope fractionation (Δ18O(Qz-Fds) = -3,5‰) suggesting H2O-rich fluid interaction. Whatever their textural location, monazites are sub-euhedral or euhedral and cover a large domain of composition. U-Pb chronometers, disturbed by slight common Pb contamination, yield a Tera-Wasserburg lower intercept age of 296 ± 3 Ma, consistent with the 208Pb/232Th mean age. An inherited component at 320 Ma is responsible for the scattering of the U-Th-Pb ages and intragrain age variations. Both of these studied examples illustrate the influence of fluid composition on monazite chronometer disturbance.
Meeting Home page GSA Home Page