MULTIPLE DEFORMATION AND CLEAVAGE DEVELOPMENT BY DISSOLUTION-PRECIPITATION IN THE METASEDIMENTARY ROCKS OF THE CHESTER SHEAR ZONE, CENTRAL MAINE, USA

Optical and backscattered-electron (BSE) imaging provide evidence that deformation and cleavage development in the meta-sedimentary rocks of the Chester Shear Zone developed by a dissolution-precipitation mechanism. BSE images show that muscovite and chlorite belong to four populations: 1) detrital muscovite up to 75 µm long distributed among fine to medium silt-size quartz and albite grains; 2) authigenic muscovite finely intergrown with chlorite to form stacks up to 100 µm in diameter; 3) muscovite intergrown with chlorite defining a cleavage (S1) sub-parallel to bedding (S0); and 4) muscovite and chlorite intergrowths defining a cleavage (S2) oblique to S0 and S1. Micas in the S1 fabric truncate the detrital and authigenic micas, and micas in the S2 fabric truncate all other populations. The S2 fabric is also folded by a late phase of deformation (D3) that did not results in recrystallization. Truncations provide evidence for dissolution of earlier micas oriented at a high angle to the shortening direction and for precipitation of cleavage-forming micas (S1 and S2). Further evidence of this recrystallization comes from ⁴⁰Ar/³⁹Ar isotopic data. Samples with low S1 and S2 content but high S0 content have Middle Ordovician to Early Silurian apparent ages, whereas samples with high S2 contents have Late Silurian to Middle Devonian ages. The younger apparent ages of samples dominated by S2 micas are interpreted to be the result of the dissolution of S0 micas and crystallization of S2 micas in the Middle Devonian or later. These results suggest that the deformation forming S2 fabric is associated with Acadian transpression that is more strongly developed along coastal New England.