Fibrous nepheline, pectolite and plagioclase from Baffin Island

At Napoleon Bay, Baffin Island, fibrous aggregates define the contacts of M esoproterozoic lamproite dykes with feldspathic wall-rock. The aggregates a re composed of nepheline, pectolite and andesine, with fibre extensions alo ng c, b and an indeterminate axis, respectively. Optically continuous, poly mineralic fibre bundles are 0.1 to 0.2 mm long at the contact (all three mi nerals), 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, reacte d with labradorite in the wall-rock producing the fibrous agregates. Nephel ine and pectolite were possibly derived from an unidentified, high-temperat ure phase that decomposed during cooling. Andesine may have been generated independently. Growth was directed across a set of tension fractures and pr oceeded with the aid of a thermal gradient as heat was removed continuously through the wall-rock.