Geology and Age of some Plutons in North-Central Gaspe, Canada

Abstract

North-central Gaspe is characterized by three conspicuous leucocratic stocks intruded into Cambro-Ordovician and Siluro-Devonian formations: Monts McGerrigle, Hog's Back, and Mont Vallieres-de-St, Real. The Monts McGerrigle pluton, the largest of these, is a discordant multiple intrusion surrounded by a striking 1–2 mi (1.6–3.2 km) wide aureole of hornfels and skarn, reaching a prevalent hornblende–hornfels facies near the contact. A quartz-poor, possibly contaminated acid magma invaded an early gabbroic phase resulting in hybrid rocks laden with xenoliths. Subsequently, the mass was intruded by red granite and granite porphyry. Depth of intrusion was probably shallow corresponding to a total pressure of approximately 2 kb. Field evidence indicates that Hog's Back and Mont Vallieres-de-St. Real are satellitc quartz–sodic feldspar porphyries connected in depth to the Monts McGerrigle pluton. While the latter intruded post-kinematically into Cambro-Ordovician rocks, the porphyries were emplaced in the form of semi-concordant hypabyssal sheets in Siluro-Devonian formations. Radiometric dating suggests a late Devonian intrusion. Patchy distribution of thermally altered rocks in the area between the stocks indicates an irregular profile of a large igneous mass not far below surface.