An overview of recent bedrock mapping and follow-up petrological studies of the South Mountain Batholith, southwestern Nova Scotia, Canada

Recent detailed geological mapping has provided insight into the evolution of the South Mountain Batholith of southwestern Nova Scotia. Mapping has resulted in the delineation of 49 map units consisting of: biotitegranodiorite, biotite monzogranite, muscovite-biotitc monzogranite, coarse- and fine-grained leucomonzogranite and leucogranite; 49 map units have been assigned to thirteen plutons which have been grouped into two stages, including early stage 1 (mostly granodiorite and monzogranite) and late stage 2 (mostly monzogranite, leucomonzogranite, leucogranite). Follow-up petrographic and geochemical studies indicate a continuous sequence from least evolved biotite granodiorite to most evolved leucogranite. In spite of a definitive sequence of emplacement for the plutons and their units, an evaluation of published geochronological data indicates that all plutons were intruded and crystallized during a very short time interval (<5 Ma) at ca. 370 Ma. Various structural characteristics, including the shape and distribution of plutons, the coincidence of several stage 2 plutons with major fault zones, and the orientation of primary and secondary structural features (e.g., megacryst alignment, joints, veins), indicate that the batholith was subjected to regional stresses associated with the waning stages of the Acadian Orogeny during intrusion. The various rock types within the plutons have broadly similar compositions; however, detailed petrographic and geochemical studies have revealed unique compositional characteristics. These differences are explained by variations in protolith composition. Similarly, the style of mineralization in the sundry plutons is interpreted as reflecting the protolith composition and the physio-chemical conditions that prevailed during their crystallization. Accordingly, the economic potential of the thirteen plutons must be evaluated individually. RESUME Une nouvelle cartographie geologique detaillee a fourni des donnees sur l’evolution du batholite du mont South du sud-ouest de la Nouvelle-Ecosse. La cartographie a permis de suivre 49 unites qui consistent en: granodiorite a biotite, monzogranite a biotite, monzogranite a biotite et muscovite, leucomonzogranite et leucogranite a grain grossier et fin; les 49 unites ont ete assignees a treize plutons qui sont regroupes en deux stades: le stade 1, preeoce (principalement granodiorite et monzogranite), et le stade 2, tardif (principalement monzogranite, leucomonzogranite et leucogranite). Les etudes petrographiques et geochimiqucs d'appoint indiquent une suite continue allant des granodio rites a biotite, peu evoluees, aux leucogranitcs, les plus evolues. En depot d'une sequence d'intrusion bien definie pour les plutons et leurs unites, une evaluation des donnees geochronologiques pubiees indique que tous les plutons se sont mis en place et ont cristallise pendant un interval le tres court (<5 Ma), il y a environ 370 Ma. Diverses caracteristiques structurales, dont la forme et la distribution des plutons, la coincidence de plusieurs plutons de stade 2 avec des zones de failles majeures et 1 'orientation des structures primaires et secondares (e.g., alignement des megacristaux, joints, veines) indiquent que le batholite etait soumis, lors de sa mise en place, aux contraintes regionales associees aux stades finaux de l'orogenese acadienne. Les diverses lithologies a l'interieur des plutons ont des compositions proches. Cependant, des etudes petrogra-phiques et geochimiques detaillees ont reveles des compositions particulieres. Ces differences sont expliquees par des variations dans la composition des protolites. De la meme maniere, les patrons de mineralisations associees aux divers plutons sont interprets comme refletanl la composition du protolite et les conditions physico-chimiques lors de la cristallisation. En consequence, le potentiel economique des 13 plutons doit Q;tre evalue individuellement. [Traduit par le journal]