Re-equilibration of detrital muscovite and the formation of interleaved phyllosilicate grains in low temperature metamorphism,northern Apennines,Italy |
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| Authors: | Marcello Franceschelli Isabella Memmi Giovanni Gianelli |
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| Institution: | (1) Istituto di Giacimenti Minerari, Geofisica e Scienze Geologiche University of Cagliari, Piazza d'Armi, 09100 Cagliari, Italy;(2) Dipartimento di Scienze della Terra, Università di Siena, Via delle Cerchia 3, I-53100 Siena, Italy;(3) Istituto Internazionale per le Ricerche Geotermiche C.N.R., Piazza Solferino 2, Pisa, Italy |
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| Abstract: | Low temperature metamorphosed clastic rocks of northern Apennines (Verrucano) contain detrital and metamorphic muscovite along with aggregates of interleaved phyllosilicate grains. The extent of celadonite substitution in the metamorphic muscovite progressively increases from Al-rich pyrophyllite-bearing samples to Al-poor K-feldspar-bearing samples and appears to be compatible with the phase relation in the AKF and AKNa diagrams. Some detrital muscovite grains, not equilibrated with the present metamorphic mineral assemblages, seem to be unreacted grains retaining their premetamorphic composition. The chemistry of the re-equilibrated  detrital muscovite is mainly controlled by the activity of Al2O3 that is implied by mineral assemblages in the host rocks and is independent of the original composition of detrital mica. Four types of phyllosilicate associations in the interleaved phyllosilicate grains were recognized: 1) muscovite- pyrophyllite-sudoite; 2) muscovite-pyrophyllite-chlorite; 3) muscovite-paragonite-chlorite; 4) muscovite-chlorite. A microstructural and petrological model is proposed for the origin of interleaved phyllosilicate grains in the Verrucano rocks. The model supports the idea that the interleaved phyllosilicate grains are the result of the trend towards equilibrium between detrital muscovite, metamorphic mineral assemblages and the fluid phase. |
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