The granite-pegmatite connection : insight from mica trace element chemistry

Abstract : Micas are ubiquitous minerals within granitic and pegmatitic rocks. The presence and the composition of micas are believed to depend strongly on the magma they crystallised from. We studied major and trace elements, using both electron microprobe and Laser Ablation ICP-MS, within primary micas (biotite, muscovite and lepidolite) from the different granitic phases of the St Sylvestre leucogranite complex (Western French Massif Central), and associated pegmatitic plugs intrusive in the granite. The granite can be biotite-only, biotite-muscovite or muscoviteonly. Compositions of granitic micas do not vary significantly except for the muscovite-only facies that has a slightly higher F-rich content. Pegmatitic biotite is quite homogenous within single bodies but records a considerable compositional change within the entire pegmatitic field ranging from F-poor (similar to granite) towards F-rich. Pegmatitic muscovite appears rather homogeneous in major element composition with only a slightly higher Li-mica content than most granitic muscovite. Trace elements contents in muscovite are lower than in biotite but compositional trends are very similar to trends observed in biotite contents. While the variation observed within micas from the pegmatite can be related to a magmatic differentiation, facies variation within the granite may be related to changes in magma parameters such as f(O2) and thus are probably not related to differentiation processes. From here it appears that granite and pegmatite, even though spatially associated, may follow different evolutions and may not be genetically linked despite common characters. Differences of trace element concentration between biotite and muscovite in pegmatite can either be the consequence of partition coefficient between biotite and muscovite or a diachronous crystallisation of muscovite and biotite. Additionally the reduced variability in muscovite structural formulae emphasizes a lack of sensitivity of muscovite with respect to magmatic differentiation.
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Arnaud Villaros, Sarah Deveaud, Michel Pichavant. The granite-pegmatite connection : insight from mica trace element chemistry. 24ème Réunion des Sciences de la Terre, Oct 2014, Pau, France. ⟨insu-01291773⟩

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