Abstracts
Abstract
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 d£crit les caract€ristiqucs g£ologiques 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'abondancc de'eroissante): (1) des monzogranites et leucomonzogranites a m6gacristaux ou 6quigranulaires, (2) des monzonites porphyriques a biotite, (3) des leucomonzogranites 6quigranulaires ct (4) des microgranites. Les indications de litage magmatique observers en forage sont: (1) la presence d'un aggrfigat de plagioclase semblable a un cumulat, (2) des lits centim€triqucs enrichis en biotite et en apatite et (3) des contacts graduels entre les types de granite. L*aggr£gat de plagioclase contient de 50 a 90% de cristaux de plagioclase zonfis et gfine'ralement trapus qui ont commun£mcnt des contacts engine's et une texture poecilitiquc avec d'abondantes inclusions de biotite. Les contacts del'aggr^gatde plagioclase avec les autrcs facies du pluton sont gendralement graduels el sont souligne's par un enrichissement progressif en biotite ct un appauvrissement en cristaux de plagioclase eu6driqucs.
La proportion d'aggnSgat riche en plagioclase d£croit 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'alteralion ct de la mineralisation. L'abondance de pegmatite au scin du microgranite en contact avec les leucomonzogranites 6quigranulaires suggere que lc microgranite a cristallise- a partir d'un magma tres diffcrencie' et sature1 en eau. Le microgranite pourrait reprfisenter un leucomonzogranite dont la texture et la min^ralogie ont 6l6 modifiers plutot qu'un produit magmatique primaire.
Le presence dc cc qui est apparemment un litage magmatique primaire dans le pluton du lac Big Indian semble confirmer la pertinence des modeles de cristallisation fractionn£e pour le batholite du mont South.
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