Petrographic, mineral and pressure–temperature constraints on phyllites from the Variscan basement at Punta Bianca, Northern Apennines, Italy

Deborah Lo Po' (1), Roberto Braga (1) & Hans-Joachim Massonne (2)
(1) Dipartimento di Scienze Biologiche, Geologiche e Ambientali, Università di Bologna, Piazza di Porta San Donato 1, I-40126, Bologna, Italy.
(2) Institut für Mineralogie und Kristallchemie, Universität Stuttgart, Azenbergstr. 18, D-70174 Stuttgart, Germany.

We present a petrological study of phyllites from the Variscan basement at Punta Bianca (Northern Apennines). According to the literature the studied rocks are believed to be the result of Alpine and Variscan metamorphism of pre-Carboniferous pelites, which consist of Fe-chlorite, potassic white mica, quartz, minor paragonite, and accessory hematite, rutile, monazite, zircon, xenotime, and florencite. Microstructural observations indicate a main S2 foliation that had overprinted an earlier S1 foliation. The mineral assemblage can be related to both S2 and S1.
Large flakes of relatively Mg-rich potassic white-mica along the two foliations are probably detrital grains that preserved their composition. The calculation of isochemical phase diagrams, their contouring by various mineral chemical parameters and the deformation microstructures of quartz indicate intermediate pressure-low temperature conditions (5-7 kbar and 300-400 °C). The anhedral habit and the very low Pb contents (< 50 ppm) in monazite point to dissolution of this mineral probably during Tertiary times.

Northern Apennines, Alpine metamorphism, white mica, monazite, thermodynamic modelling.