Petrology of the zoned, peraluminous Halifax Pluton, south-central Nova Scotia
DOI:
https://doi.org/10.4138/1638Abstract
The Halifax Pluton (HP) is a large (approximately 900 km2 ), discrete plutonic body which forms the southeastern end of the South Mountain Batholith (SMB). The HP comprises a zoned sequence of rocfc units into which a second, generally more evolved sequence of rocks was intruded. Rocks of the zoned sequence are predominantly medium- to coarse-grained, have megacrystic textures, are generally monzogranltic In composition (with minor granodiorite) and have biotite (4-15%), muecovite (trace-4%) and cordlerlte (0->4%) as essential constituents. The second sequence is primarily fine-to medium-grained with highly variable textures (e.g., porphyrltic, equigranular, graphic), is generally monzogranltic in composition with minor leucogranite (<2% mafic minerals) and syenogranite, and have muscovite (2-10%), biotite (0-8%) and cordlerlte (0-2%) as essential minerals. Andalusite, generally absent from the zoned sequence, is commonly present as an accessory phase. Contacts between the two sequences are invariably intrusive. Petrographlc and geochemical evidence suggests that the outer segment of the zoned sequence is normally zoned (i.e., more evolved toward the core). This may be explained by sidewall crystallization in a magma chamber with the removal of plagloclase and biotite (with accessory zircon, monazlte. xenotLme, apatite and ilmenite). Conversely, the inner portion of the pluton Is reversely zoned (i.e., less evolved toward core with an increase in biotite) which cannot be explained by fractional crystallization. Therefore, a model involving vertical stratification in the melt is proposed. The presence of the zoned arrangement of rock units coupled with high contents of MgO, P2O5 and normative corundum, enstatite, hypersthene, apatite and high modal amounts of cordlerlte compared with the central SMB suggest that the H? evolved as a discrete pluton. RÉSUMÉ Le pluton de Halifax (PH) est un bâti plutonique distinct de grande taille (environ 900 km 2) qui constitue l'extreraité sud-est du Batholite de South Mountain (BSM). Le PH comprend une série d'unités rocheuses zonée à travers laquelle s'est injectée une seconde série de rocbes généralement plus évoluée. Lea terroes de la série zonee montrent surtout un grain intermédiaire à grossier ainsi que des textures mégacristes et ont généralement une composition monzogranltlque (avec quelque granodiorite). Lea constituents essentiels en sont la biotite (4-15%), la muscovite (trace -4%) et la cordlerlte (0->4%). La seconde série présente un grain surtout fin à intermédiaire. exhibe des textures varices (e.g., porphlrique, grenue, graphlque) et possede une composition généralement monzogranltlque avec quelque leucogranite (minéraux mafiques 2%) et syenogranite. Ses minéraux essentiels sont la muscovite (2-10%), la biotite (0-8%) et la cordlerlte (0-2%). Bien que généralement absente de la éerie zonee, l'andalousite formé éonmunement une phase accessoire. Les contacts entre les deux series sont invariablement intrusifs. À la lumière de la pétrographie et la géochimie, le liseré de la série zonée semble zone normalement (i.e., plus évolue vers le coeur). Ceci peut s'expliquer par la cristallisation des épontes de la chambre raagmatique et l’elimination du plagloclase et de la biotite (avec les zircon, monazite, xénotime, apatite et ilménite). Par contre, la portion interne du pluton montre une zonation inverse (i.e., moins évoluée vers le coeur avec une augmentation de la biotite) qui ne peut s'éxpliquer par la cristallisation frectionnée. On propose done un modèle necessitant une stratification verticale du bain. La présence d'une zonation des unites rocheuses combinée aux contenus élevés de MgO, P2O5 corindon normative, enstatite, hypersthene, apatite ainsi qu'aux valeurs modèles élevées de la cordleite, par rapport à la portion centrale du BSM suggèrent que le PH a évolué en tant que pluton distinct. [Traduit par le journal]Downloads
Published
1988-04-01
How to Cite
MacDonald, H. A., & Home, H. J. (1988). Petrology of the zoned, peraluminous Halifax Pluton, south-central Nova Scotia. Atlantic Geoscience, 24(1). https://doi.org/10.4138/1638
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