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

International audienceMicas are ubiquitous minerals within granitic and pegmatitic rocks. Thepresence and the composition of micas are believed to depend stronglyon the magma they crystallised from. We studied major and traceelements, using both electron microprobe and Laser Ablation ICP-MS,within primary micas (biotite, muscovite and lepidolite) from the differentgranitic phases of the St Sylvestre leucogranite complex (WesternFrench Massif Central), and associated pegmatitic plugs intrusive in thegranite. The granite can be biotite-only, biotite-muscovite or muscoviteonly.Compositions of granitic micas do not vary significantly exceptfor the muscovite-only facies that has a slightly higher F-rich content.Pegmatitic biotite is quite homogenous within single bodies but recordsa considerable compositional change within the entire pegmatitic fieldranging from F-poor (similar to granite) towards F-rich. Pegmatitic muscoviteappears rather homogeneous in major element composition withonly a slightly higher Li-mica content than most granitic muscovite.Trace elements contents in muscovite are lower than in biotite but compositionaltrends are very similar to trends observed in biotite contents.While the variation observed within micas from the pegmatite can berelated to a magmatic differentiation, facies variation within the granitemay be related to changes in magma parameters such as f(O2) and thusare probably not related to differentiation processes. From here it appearsthat granite and pegmatite, even though spatially associated, mayfollow different evolutions and may not be genetically linked despitecommon characters. Differences of trace element concentration betweenbiotite and muscovite in pegmatite can either be the consequence of partitioncoefficient between biotite and muscovite or a diachronous crystallisationof muscovite and biotite. Additionally the reduced variabilityin muscovite structural formulae emphasizes a lack of sensitivity ofmuscovite with respect to magmatic differentiation