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A peculiar Ms-Pg textural association in a chloritoid-bearing micaschist recording a multistage P-T path

¹ Dip. di Mineralogia e Petrologia, C.so Garibaldi 37, 1–35137 Padova, Italy
² CRMC2, Campus de Luminy, 13288 Marseille Cedex 9, France

^* E-mail: claudio{at}dmp.unipd.it

A peculiar textural association of muscovite, paragonite and chloritoid occurs in a micaschist bearing muscovite and garnet porphyroblasts, from the Greiner Unit (Eastern Alps, Italy). Three different tectonometamorphic stages can be recognised in this rock: 1) the early development of S₁ preserved only as relic within garnet porphyroblasts; 2) a syn-kinematic crystallisation of chloritoid and muscovite along an early foliation S₂; 3) the development of a crenulation cleavage (S₃) associated with the growth of chlorite, muscovite and paragonite along S₃ and porphyroblastic garnet and muscovite. Both garnet and muscovite porphyroblasts show inclusion trails of chloritoid, quartz and ores, which are parallel to S₂. Furthermore, the muscovite porphyroblasts include paragonite crystals which are parallel to S₂ and display a rectangular to elongate "blocky shape" in thin section. The elongation is nearly perpendicular to the muscovite (001) basal plane (but with the same cleavage traces as those of the muscovite).

An HRTEM/SAED/AEM study of the muscovite/paragonite single crystals reveals two different situations: 1) sharp boundaries for the "blocky shaped" paragonite crystal within muscovite porphyroblasts with the angle of the (001) basal plane across the boundary being that required for adjusting the different basal spacing; 2) the presence of K-rich paragonite micro-domains within Na-rich muscovite porphyroblasts; both are 2M₁ polytypes and in structural continuity, with an angle between [00l]^*paragonite and [00l]^*muscovite that varies from 0 to 5° in order to accommodate the layer-thickness misfit. This results in a "tweed texture", suggesting a further decomposition inside the muscovite-paragonite solvus.

This peculiar muscovite-paragonite textural association is a microstructural relic of a multistage metamorphism which affected these rocks with evidence of an isothermal increase of pressure due to the reaction: Fe-rich Cld + Narich Ms + Qtz Phe-rich Ms + Pg + Chl ± Mg-rich Cld + H₂O.

Key-words: muscovite, paragonite, chloritoid, multistage P-T paths, HRTEM, Penninic Basement..

This article has been cited by other articles:

				K. J.T. Livi, G. E. Christidis, P. Arkai, and D. R. Veblen White mica domain formation: A model for paragonite, margarite, and muscovite formation during prograde metamorphism American Mineralogist, April 1, 2008; 93(4): 520 - 527. [Abstract] [Full Text] [PDF]