Quantifying the diagenetic impact in the late Ediacaran and early Palaeozoic of the Avalon Peninsula using illite 'crystallinity'

The Avalon Peninsula on the island of Newfoundland, Canada, defined as the type zone of Avalonia, is believed to have been impacted by several orogenetic and deformation events since the Neoproterozoic. Previous studies have determined that the lowest degree of metamorphism reached in the successions was of the prehnite--pumpellyite or greenschist facies. We sampled and measured 13 mainly clastic sedimentary sections ranging from the late Ediacaran to the Early Ordovician and analyzed the illite 'crystallinity' of 331 samples using the Kubler index. Our results show that the occurrence of diagenetic zones relates to lithology, age and burial depth, and regional setting. Samples adjacent to the fault zones bounding the Holyrood Horst experienced the highest degree of metamorphism (anchizone) in the study area. The lowest degree of thermal alteration occurs in the high stratigraphic sections at the centre of the horst structure where shallow diagenetic conditions are preserved. Fault zones, which were probably active during at least the Acadian Orogeny, may have served as potential paths for hot fluids in bounding areas of the horst, whereas the centre of the horst remained almost unaffected by any metamorphic overprint. The thermal impact decreases from the Bonavista Peninsula west of the study area from greenschist facies to anchizonal and diagenetic. The study area experienced lower metamorphic conditions than those in the major regions of Avalonia south of the study area, namely the mainland of New Brunswick and Maine and eastward in Europe, but is in part consistent with a few other areas of Avalonia, such as the Mira Terrane and the Antigonish Highlands in Nova Scotia. Key words: illite 'crystallinity', Kubler index, diagenesis, Holyrood Horst, Avalon Peninsula, southeastern Newfoundland. La peninsule dAvalon (Terre-Neuve, Canada), definie comme etant la region type de lAvalonie, aurait subi les effets de plusieurs episodes orogeniques et de deformation depuis le Neoproterozoique. Des etudes passees ont determine que le metamorphisme le plus faible atteint durant ces episodes est au facies prehnite-pumpellyite ou des schistes verts. Nous avons echantillonne et mesure 13 coupes sedimentaires principalement clastiques allant de l'Ediacarien tardif a l'Ordovicien precoce et analyse la << cristallinite >> de l'illite de 331 echantillons a l'aide de l'indice de Kubler. Nos resultats demontrent que la presence de zones diagenetiques est reliee au type de roche, a l'age et a la profondeur d'enfouissement, respectivement, et au contexte regional. Des echantillons attenants aux zones de faille bordant le horst de Holyrood ont subi le metamorphisme le plus intense (anchizone) dans la region d'etude. Le degre le plus faible d'alteration thermique est observe dans les coupes stratigraphiques hautes situees au centre de la structure de horst, ou des conditions de diagenese peu profonde sont preservees. Des zones de faille, a tout le moins probablement actives durant l'orogenese acadienne, pourraient avoir servi de voies d'ecoulement de fluides chauds dans les zones bordant le horst, alors que le centre de ce dernier est demeure presque exempt de tout effet du metamorphisme. L'impact thermique diminue a partir de la peninsule de Bonavista, a l'ouest de la region d'etude, passant du facies des schistes verts a l'anchizone a la zone diagenetique. La region d'etude a subi des conditions de metamorphisme plus faibles que de grands secteurs de l'Avalonie au sud, dans la partie continentale du Nouveau-Brunswick et du Maine et plus a lest en Europe, mais qui concordent du moins partiellement avec celles de quelques autres regions de l'Avalonie, comme le terrane de Mira et les hautes terres d'Antigonish en Nouvelle-Ecosse. [Traduit par la Redaction] Mots-cles: cristallinite de l'illite, indice de Kubler, diagenese, horst de Holyrood, peninsule d'Avalon, sud-est de Terre-Neuve.
Introduction The upper Ediacaran and lower Palaeozoic rocks of the Avalon Peninsula on the island of Newfoundland, Canada, have been studied for decades (e.g., Howell 1925; Hutchinson 1962; Fletcher 1972, [...]