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Stability and isotopic dating of monazite and allanite in partially molten rocks: examples from the Central Alps

Published inSwiss journal of geosciences, vol. 102, no. 1, p. 15-29
Publication date2009
Abstract

We investigated the stability of monazite and allanite as a function of bulk rock composition within several types of Tertiary Alpine anatexites, characterized by different compositions and melting reactions, but similar P-T conditions of melting. The investigated rocks consist of: (1) orthogneisses in which the melting reaction was triggered by water infiltration from the Bergell pluton; (2) anatectic tonalites, which were affected by water-assisted melting; and (3) metapelitic migmatites, which underwent muscovite dehydration melting. The studied anatexites cover a large range of Ca contents and water activities during partial melting, and allow an assessment of how much these parameters affect the stability of accessory phases. The different melting reactions that affected these rocks generated different water activities during the melt-present stage; they were highest in the water-saturated, contact metamorphic anatexites, and lowest in the metapelitic anatexites that underwent dehydration melting. These differences go together with different accessory phases within the migmatites. Whereas metapelitic anatexites only contain monazite, anatexites derived from tonalitic and granodioritic protoliths mainly contain allanite. This is consistent with observations made on Tertiary Alpine anatexites, suggesting that the growth of specific accessory phases is determined by the water activity and Ca content during melting.

Keywords
  • U-Pb monazite
  • Accessory minerals
  • Hydrothermal alteration
  • Western carpathians
  • Pelitic schists
  • Contact aureole
  • Granitic-rocks
  • Bergell pluton
  • Ivrea-verbano
  • Calc-alkaline
Citation (ISO format)
BERGER, Alfons, ROSENBERG, Claudio, SCHALTEGGER, Urs. Stability and isotopic dating of monazite and allanite in partially molten rocks: examples from the Central Alps. In: Swiss journal of geosciences, 2009, vol. 102, n° 1, p. 15–29. doi: 10.1007/s00015-009-1310-8
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ISSN of the journal1661-8726
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