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Fibrous nepheline, pectolite and plagioclase from Baffin Island

¹ Department of Earth Sciences, University of Ottawa, Ottawa, ON, Canada, KIN 6N5
² Department of Geology, Arizona State University, Tempe, AZ 85287, U.S.A.
³ Department of Geophysical Sciences, University of Chicago, 5734 South Ellis Ave, Chicago, 1L 60637, U.S.A.

^* e-mail dhogarth{at}science.uottawa.ca

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.

Key-words: fibrous aggregates, nepheline, pectolite, andesine, labradorite, lamproite, Mesoproterozoic, Baffin Island.