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This paper describes geological features observed in diamond drill core recovered from three holes in the Big Indian Lake pluton near the eastern margin of the South Mountain Batholith. The pluton is comprised of four texturally variable granitic facies (order of decreasing abundance): (1) megacrystic to equigranular monzogranite and leuco-monzogranite, (2) porphyritic biotite monzogranite, (3) equigranular leucomonzogranite, and (4) microgranitc. Evidence of igneous layering observed in drill core includes: (1) the presence of a cumulate-like aggregate of plagioclase, (2) centimetre-thick layers modally enriched in biotite and apatite, and (3) gradational contacts between granitic rock-types. The plagioclase aggregate contains SO to 90% zoned and generally equidimensional plagioclase crystals which commonly have mutually interfering grain boundaries and a poikilitic texture characterized by abundant biotite inclusions. Contacts of the plagioclase aggregate with other facies of the pluton are generally gradational, and marked by a progressive increase in biotite and decrease in euhedral plagioclase crystals.  The amount of plagioclase-rich aggregate decreases with depth in the drill core, and granodiorite and biotite monzogranite become more abundant. They grade into predominately leucomonzogranite and microgranite at depth. The microgranite is typically of syenogranite composition and closely associated with several types of alteration and mineralization. The common occurrence of pegmatite within the microgranite at boundaries with equigranular leucomonzogranite suggests that the microgranitc crystallized from a highly fractionated, water-saturated liquid. The microgranite may represent a texturally and mineralogically modified leucomonzogranite rather than a primary magmalic product.  The occurrence of what appears to be primary magmatic layering in the Big Indian Lake pluton appears to confirm the relevance of applying fractional crystallization models to the South Mountain Batholith.   RESUME  Cet article decrit les caracteristiques geologiques de trois forages dans le pluton du lac Big Indian, pres de la bordure est du balholite du mont South. Lc pluton est compose de quatre facies granitiques de diverses textures (en ordre d'abondanee deeroissante): (1) des monzogranites et leucomonzogranites a megacristaux ou equigranulaires, (2) des monzonites porphyriques a biotite, (3) des leucomonzogranites equigranulaires et (4) des microgranites. Les indications de litage magmatique observers en forage sont: (1) la presence d'un aggregat de plagioclase semblable a un cumulat, (2) des lits centimetriques enrichis en biotite et en apatite et (3) des contacts graduels entre les types de granite. L’aggregat de plagioclase contient de 50 a 90% de cristaux de plagioclase zones et generalement trapus qui ont communemwnt des contacts engines et une texture poecilitique avec d'abondantes inclusions de biotite. Les contacts de l'aggregate plagioclase avec les autres facies du pluton sont gendralement graduels el sont soulignes par un enrichissement progressif en biotite et un appauvrissement en cristaux de plagioclase euedriques.   La proportion d'aggregat riche en plagioclase decroit avec la profondeur tandis que la granodiorite et le monzogranite a biotite deviennent plus abondants. Le microgranitc a typiquemenl unc composition de syenogranite et est intimement associe a de l'alteration et de la mineralisation. L'abondance de pegmatite au sein du microgranite en contact avec les leucomonzogranites equigranulaires suggere que le microgranite a cristallise a partir d'un magma tres differencie et sature en eau. Le microgranite pourrait representer un leucomonzogranite dont la texture et la mineralogie ont ete modifiers plutot qu'un produit magmatique primaire.  Le presence de ce qui est apparemment un litage magmatique primaire dans le pluton du lac Big Indian semble confirmer la pertinence des modeles de cristallisation fractionnee pour le batholite du mont South.  [Traduit par le journal]

Geological features within the Big Indian Lake pluton, Hants County, Nova Scotia: evidence of igneous layering from recent diamond drilling