Fibrous nepheline, pectolite and plagioclase from Baffin Island

Abstract

At Napoleon Bay, Baffin Island, fibrous aggregates define the contacts of Mesoproterozoic lamproite dykes with feldspathic wall-rock. The aggregates are composed of nepheline, pectolite and andesine, with fibre extensions along c , b and an indeterminate axis, respectively. Optically continuous, polymineralic fibre bundles are 0.1 to 0.2 mm long at the contact (all three minerals), 0.01 to 0.05 mm long in the wall-rock, but were not found beyond 8 mm from the dykes. Strongly alkaline, hydrothermal solutions, emanating from the dykes, reacted with labradorite in the wall-rock producing the fibrous agregates. Nepheline and pectolite were possibly derived from an unidentified, high-temperature phase that decomposed during cooling. Andesine may have been generated independently. Growth was directed across a set of tension fractures and proceeded with the aid of a thermal gradient as heat was removed continuously through the wall-rock.