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Conference Papers Year : 2014

Mica crystallisation as a marker of pegmatite composition


Micas are ubiquitous minerals within granitic and pegmatitic magmas. The presence and the composition of micas are believed to depend strongly on the magma they crystallised from. The Upper Carboniferous Variscan St Sylvestre Leucogranite Complex (SSLC), located in the western Massif Central (Limousin, France) is composed 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-mica facies (Châteauponsac) emplaced as sills within the Brame facies; finally a muscovite-only facies (Sagne). Pegmatitic bodies within the SSLC are enriched in rare-elements (e.g. Li, Be, Nb, Ta) and most of them are intrusive within the St-Sylvestre facies. Pegmatites in the SSLC can be divided in six groups depending on their 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 rich phases; 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 this classification. We analysed major and trace elements using both electron microprobe and Laser Ablation ICP-MS, within primary micas (biotite, muscovite and lepidolite) from both granite and pegmatite within the St Sylvestre leucogranitic complex. Within the granite, biotite and muscovite do not vary significantly in composition in regard of granite facies. Interestingly, biotite within pegmatite records a considerable change in composition from biotite in granite towards lithium-rich biotite in regard of pegmatite facies, changing from annite-phlogopite composition to zinnwaldite (in type-V pegmatite). On the contrary, muscovite is very homogenous and its composition has only a slightly higher Li-mica content than granitic muscovite. Experimental studies of mica crystallisation from a pegmatitic magma suggest that compositional changes observed in biotite can be directly related to the differentiation of pegmatitic magmas. Behind the apparent compositional homogeneity provided by major elements within muscovite, trace element contents reveal compositional changes. These changes are very similar to the variation of trace elements contents observed in biotite in agreement with the pegmatite type, yet with significantly lower concentration for most elements considered here. It appears that both biotite and muscovite record compositional variability within pegmatitic magmas. Both biotite and muscovite record a subsequent decrease of Ba, Sr and Sc during differentiation while contents of Li, Be, Nb, Rb, Ta, Cs and Zn increase significantly with differentiation degree within both biotite and muscovite. Differences of trace element concentration between biotite and muscovite 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 in regard of magmatic differentiation.
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insu-01143289 , version 1 (17-04-2015)


  • HAL Id : insu-01143289 , version 1


Arnaud Villaros, Sarah Deveaud, Michel Pichavant. Mica crystallisation as a marker of pegmatite composition. 21 st meeting of the International Mineralogical Association, Sep 2014, Johannesburg, South Africa. pp.270. ⟨insu-01143289⟩
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