Investigation of accessory Allanite, Monazite and Rutile of the Barrovian sequence of the Central Alps (Switzerland)
Creators: name: Boston, Katherine
Publisher: name: The Australian National University
Year: 2014
Subjects: subject: Metamorphism (Geology) Switzerland
subject: Metamorphic rocks Switzerland
subject: Allanite
subject: Monazite
subject: Rutile

description: Accessory Allanite, Monazite and Rutile from amphibolite facies rocks across the Barrovian sequence of the Central Alps (Switzerland) were investigated for textural relationships, composition and U-Th-Pb ages. The crystallisation ages of these minerals record multiple stages of prograde and peak metamorphism, which are notoriously difficult to date at these relatively low temperatures. Synkinematic allanite preserves prograde ages of 26-33 Ma, determined by ion microprobe (SHRIMP). This age is interpreted to record a pressure-dominated phase of metamorphism related to nappe stacking in the Central Alps on the basis of textural relationships, trace element composition, Zr-in-rutile thermometry of rutiles associated with allanite, and likely stability of Allanite in metapelites. The timing of amphibolite facies metamorphism (575-650 degC) is recorded in monazite, rutile and, in one sample, allanite rims. Two samples yield monazite ages of 22.1 +/- 0.4 Ma and 21.6 +/- 0.3 Ma. Monazite grew at the expense of allanite and after the first stage of garnet growth. 19.7 +/- 1.3 Ma Allanite rims in one metapelite crystallised on older (26.8 +/- 1.3 Ma) cores, and are interpreted to reflect the timing of maximum temperature to retrograde conditions. Isotope dilution Rutile ages that contribute to this metamorphic history are the result of an improved Pb separation protocol developed specifically for rutile geochronology. The need for a new protocol was highlighted by poor chemistry yields of standard HBr-based procedures. The new HBr+HF protocol consistently returns higher chemistry yields, suggesting a more complete Pb separation by this procedure. The high chemistry yields are especially important for young samples or small fractions. The potential of Allanite to record fluid-rock interaction has been explored using oxygen isotope analysis of distinct growth stages by ion microprobe (SHRIMP). Magmatic Allanite yielded delta 18O values between 6.8 and 7.8 per mil, when standardised against a reference allanite. Metamorphic allanite show a variation in delta 18O from 4.3 to 13 per mil but much of the variation observed is likely to be biased by matrix effects. A preliminary study into matrix effects in oxygen isotope analysis by SHRIMP suggests an increase in delta 18O with decreasing Allanitic component (Th + LREE) in allanite. Oxygen isotopes in mosaic allanite from an orthogneiss show variation (delta 18O between 4.3 and 7.6) that is larger than is expected from matrix effects, is not correlated with composition and may indicate equilibration with a low delta 18O fluid. The unique mosaic Allanite, which are present in one sample from the Central Alps, are aggregates of multiple subgrains that formed due to deformation and dissolution-precipitation of the high-Th pre-Alpine Allanite grains (preserved as relics in some subgrains). Subgrains have core-rim zoning and subgrain cores record an age of 31.3 +/- 1.1 Ma; subgrain rims range from 26 to 29 Ma. Regularly-zoned allanite from the same sample returns allanite dates between 27 and 32 Ma. The overlap of ages suggests that both mosaic and regularly-zoned allanite grew either at the same time or at different times but within the precision of age determination.