Variable water pressure metamorphic assemblages in the Meguma Group, Nova Scotia

Abstract

Amphibolite grade pelites from the Buchan type metamorphism of the Meguma Group in southwestern Nova Scotia contain the assemblage And+Grt+Bt+Q+Pl+Ms+Ilm+Mgt±St±Crd±Sil±Po±Py. Grt-Bt geothermometry and Pl-Grt-And-Q geobarometry on samples from three widely separated 3-8 km traverses yield temperatures and pressures that cluster in two groups; (1) two traverses near Shelburne with T=500-510°C and P=3.5 kbar; and (2) one traverse on the West Pubnico peninsula, 40 km to the southwest, with T=530-560°C and P=1.5-2. 5 kbar. PH₂O estimates based on Ms-and-Pl-V assemblages indicate that PH₂O = .4-.9 PTotal for rocks from the Shelburne area and PH₂O=PTotal for the rocks south of West Pubnico. Whole rock chemical analyses on samples from the three traverses show no systematic variations over the region. In the Pubnico traverse, the presence or absence of staurolite is dependent both on slight variations in the Fe/Mg ratios of the rocks and on temperature differences. In the staurolite-bearing rocks the assemblages and mineral chemistries indicate that the systems represent divariant continuous reactions and contain cordierite only where PH₂O < PTotal. Thus the large number of phases is neither the result of disequilibrium nor representative of a low variance assemblage, but can be described as a divariant assemblage involving 8 to 9 major components.