VARIATIONS IN VESICLE DENSITIES WITHIN PILLOW BASALTS OF THE ABITIBI REGION: ROUYN-NORANDA, QUEBEC

A variety of eruptive morphologies, including pillowed, massive, and hyaloclastite flows occur in the submarine volcanic rocks of the Blake River Group in Rouyn-Noranda, Quebec. Among all these styles of eruption, differing styles of vesiculation can be observed. This study examines the vesicle distributions in individual pillows and massive flows within this suite of rocks.

The Blake River Group is in the Abitibi Subprovince of the Superior Province of Canada. The rocks in this group are greenschist facies (chlorite-actinolite-clinozoisite assemblage) metavolcanic rocks of ages 2759-2670 Ma. Because of the relatively low grade of metamorphism, primary igneous textures, such as glassy pillow chill margins and rounded vesicles, can be easily observed. This study investigates three areas (one ~2 m², another ~50 m², and the third ~100 m²) of pillow basalts and massive flows in the town of Rouyn-Noranda, Quebec. Facing indicators, including pillow morphology and drain-back features were present at all three exposures.

North-south shortening has produced minor folds and subvertical dips. The three exposures examined here cross a syncline – one from each limb of the fold and one near the hinge surface of the fold. Eighteen 1-2 m² areas of pillows were mapped in detail, with vesicle sizes and densities measured from 3 to 17 1 cm² regions on individual pillows. Vesicles are filled with calcite, but appear in the field as open porosity where the calcite has weathered away. Deformation of vesicles has transformed them into slight elliptical strain ellipses that are elongate subparallel to bedding on the limb of the fold where bedding is oriented approximately E-W. Vesicle orientations on the opposite limb of the fold are more riable.

Many pillows, especially those containing drain-back features, exhibit a characteristic vesicle pattern: a near radial symmetry and a high density of vesicles in the center of the pillow. This is surrounded by a region of decreased vesicle density, a ring ~10 cm. away from the pillow rim of high vesicle density and a vesicle-poor chill margin. Vesicle distribution and orientation will be used to investigate models of magma devolatilization and fold formation.