Mica crystallisation as a marker of pegmatite composition

International audienceMicas are ubiquitous minerals within granitic and pegmatitic magmas. The presenceand the composition of micas are believed to depend strongly on the magma theycrystallised from. The Upper Carboniferous Variscan St Sylvestre LeucograniteComplex (SSLC), located in the western Massif Central (Limousin, France) iscomposed of five main granitic facies: a biotite-only facies (Brame), a two-mica facies(St Sylvestre); a coarse grained two-mica facies (St Goussaud); a fine grained two-micafacies (Châteauponsac) emplaced as sills within the Brame facies; finally amuscovite-only facies (Sagne). Pegmatitic bodies within the SSLC are enriched inrare-elements (e.g. Li, Be, Nb, Ta) and most of them are intrusive within theSt-Sylvestre facies. Pegmatites in the SSLC can be divided in six groups depending ontheir chemistry, mineralogy and internal structuration. I.) No rare-element-rich phase(not sampled); II.) Be bearing phase only; III.) Be and Nb rich; IV.)Be, Nb and Ta richphases; V.) Be, Nb, Ta-rich phases and Li-enrichment; VI.) Li-rich phase (lepidolite,petalite). From I to VI the volume ratio between aplitic and pegmatitic texture increases.Sampling of micas within the pegmatites have been realised accordingly to thisclassification.We analysed major and trace elements using both electron microprobe and LaserAblation ICP-MS, within primary micas (biotite, muscovite and lepidolite) from bothgranite and pegmatite within the St Sylvestre leucogranitic complex. Within the granite,biotite and muscovite do not vary significantly in composition in regard of granitefacies. Interestingly, biotite within pegmatite records a considerable change incomposition from biotite in granite towards lithium-rich biotite in regard of pegmatitefacies, changing from annite-phlogopite composition to zinnwaldite (in type-Vpegmatite). On the contrary, muscovite is very homogenous and its composition hasonly a slightly higher Li-mica content than granitic muscovite. Experimental studies ofmica crystallisation from a pegmatitic magma suggest that compositional changesobserved in biotite can be directly related to the differentiation of pegmatitic magmas.Behind the apparent compositional homogeneity provided by major elements withinmuscovite, trace element contents reveal compositional changes. These changes arevery similar to the variation of trace elements contents observed in biotite in agreementwith the pegmatite type, yet with significantly lower concentration for most elementsconsidered here. It appears that both biotite and muscovite record compositionalvariability within pegmatitic magmas. Both biotite and muscovite record a subsequentdecrease of Ba, Sr and Sc during differentiation while contents of Li, Be, Nb, Rb, Ta, Csand Zn increase significantly with differentiation degree within both biotite andmuscovite.Differences of trace element concentration between biotite and muscovite can either bethe consequence of partition coefficient between biotite and muscovite or a diachronouscrystallisation of muscovite and biotite. Additionally the reduced variability inmuscovite structural formulae emphasizes a lack of sensitivity of muscovite in regard ofmagmatic differentiation