ILLITE TO MUSCOVITE TRANSITION: INDICATOR OF A WESTWARD ENCROACHING ACADIAN DEFORMATIONAL FRONT

Clay polytypes and illite crystallinity (IC) data provide a means to assess the spatial distribution of deformation in orogenic belts characterized by broad zones of shortening and metamorphism. In southern Quebec, the Baie Verte – Brompton Line, an ophiolite representing the closure of the Iapetus ocean, separates later Acadian (Late Silurian - Devonian) to the east from earlier Taconic (Mid to Late Ordovician) deformation to the west. Although low-grade metamorphism in the Humber zone has been presumed to be associated with a eastward thickening sedimentary prism, the spatial pattern of low grade metamorphism is suggestive of thermal maturation and heating associated with the emplacement of the Acadian deformational front as demonstrated by the illite to muscovite transition, with the sequence 1Md to 2M1 illite polytypes as an indicator of higher temperatures. This evidence is supported by IC measurements, where lower illite crystallinities represent higher grades of metamorphism. Samples collected along several east-west transects starting in the St Lawrence Lowland through the Humber zone along the St. Lawrence, Etchemin, Chaudiere and St. Francois rivers. Units within the St. Lawrence Lowland (undeformed passive margin) contain 0-37% 2M1 and fall in the diagenic zone (zeolite facies). East of the St. Lawrence Lowland, the percentage of 2M1 from units contained in the Humber Zone (External and Internal; mildly to strongly deformed passive margin) range from 38-79% (External) to 81-100% (Internal) and increase eastwardly from anchizonal (prehnite-pumpellyite facies) to epizonal (greenschist facies) grades of metamorphism. In contrast to the previous interpretation for the eastward increase in metamorphism resulting from the tectonic juxtaposition of higher grade older rocks tectonically over less mature younger rocks during the Taconic orogeny, we propose that the steady eastward increase in 2M1 and metamorphic grade is the result of subsequent Acadian deformation. Based on the distribution of anchizonal grade metamorphism and the transition between low-grade metamorphism and diagenesis, we propose that the Acadian deformational front extends west of the BVBL and continues as far west as the External Humber Zone.