SILICATE AND SULPHIDE MINERAL ASSEMBLAGES AND METAMORPHIC FABRICS FROM PELITES IN THE CONTACT AUREOLE OF THE SOUTH MOUNTAIN BATHOLITH, HALIFAX AREA, NOVA SCOTIA

Abstract

Contact metamorphism resulting from intrusion of the South Mountain Batholith (SMB) at 372 Ma produced textural and mineralogical changes in rocks of the Meguma Supergroup. A detailed petrographic study of the contact aureole of the Halifax Group was conducted to determine the relationship between sulphide and silicate mineral assemblages, and the relationship of these assemblages to metamorphic fabrics. Field relations indicate that rocks in the contact aureole of the SMB contain structures formed during regional deformation which were overprinted by contact metamorphism. The samples collected were grouped according to silicate porphyroblast assemblage into six groups which show systematic variation with distance from the contact. Sulphide mineral assemblages show little to no variation with distance from the contact. Microprobe analyses of both sulphides and silicates show little compositional variation throughout the study area. Petrographic analyses show· that regional cleavage (S 1) developed before cordierite growth and was reactivated to form a syn-emplacement fabric (S2) prior to andalusite growth. Maximum P-T conditions are estimated at 2.5-3.0 kbar and 590-620°C, using Pattison and Tracy's (1991) phase diagram for pelites. The silicate mineral assemblage of Halifax Group rocks in the contact aureole of the SMB varies systematically with distance from the contact and is therefore thermally controlled. The sulphide mineral assemblage, as determined from petrography, mineral chemistry, and field data is lithologically controlled in the contact aureole and therefore has no direct relationship to contact metamorphism. In conclusion, the complicated relationships of porphyroblasts and fabrics in the contact aureole of the SMB result from deformation and thermal effects superimposed on lithological variations.

KEYWORDS: South Mountain Batholith, Meguma Supergroup, contact metamorphism, pelites, silicates, sulphides.