T200 Oceanic and Juvenile Continental Crust Formation in the Precambrian

Tuesday, 11 October 2011
1:30 PM - 5:30 PM
Room 200H-J

KARASJOK-TYPE KOMATIITES AND ASSOCIATED VOLCANIC ROCKS IN THE PALEOPROTEROZOIC OF SOUTHEASTERN BAFFIN ISLAND, CANADA: FIELD RELATIONSHIPS AND CONSTRAINTS ON PETROGENESIS

Paper number: T200-6
Program: Technical Sessions
Time: 2:55 PM – 3:10 PM

Authors

Abstract

Through the Canadian government’s GeoMapping for Energy & Minerals (GEM) initiative, a volcanic belt extending for 150km and believed to be part of the Paleoproterozoic Hoare Bay group has been identified on Cumberland Peninsula, Baffin Island. The volcanic rocks, which have been metamorphosed to amphibolite grade, can be divided into two types based on outcrop appearance, petrology and geochemistry. 1) Bright green-weathering komatiites and komatiitic basalts with locally preserved fragmental, pillowed and variolitic textures, containing mainly magnesiohornblende, and with MgO values ranging from 9.88-22.20%. 2) Schistose black-weathering basalts with no primary textures, containing hornblende, plagioclase, and minor quartz, and with MgO values between 5.58-7.90%. Both types are associated with psammitic/pelitic metasedimentary rocks, however, some are also associated with chemical metasedimentary rocks, leading to the field-based interpretation that the volcanic packages may not all be stratigraphic equivalents. Bright green-weathering sills, up to 200 m thick and characterized by cm-size, brown-weathering olivine phenocrysts, have MgO values ranging from 20.50-23.70% and are interpreted as feeders to the ultramafic volcanic rocks.

Totnes Road Fiord, east-central Cumberland Peninsula, affords the best exposure of komatiitic volcanic rocks. Geochemical analyses show that Totnes komatiites and komatiitic basalts are enriched in Ti (0.8-1.8 wt%) and HFSE (Th, Nb, Hf, Zr, Y) with higher (La/Yb)cn (average 2.9) relative to typical komatiites. Their composition, as well as their fragmental character, are compatible with them being Karasjok-type komatiites. This distinctive ultramafic volcanic suite may have been derived by melting of metasomatised sub-continental mantle lithosphere, assimilation of basaltic melts, or through retention of the first melt increment in the source. Their higher volatile content may have aided fragmentation by steam and magmatic explosivity upon eruption.

REE, incompatible element, and tectonic discrimination plots of the komatiitic to basaltic volcanic rocks highlight an affinity to OIBs, which suggests a ca. 2.0-1.9 Ga mantle plume may have been the ultimate source for the magmas and played a role in basin development and deposition of the Hoare Bay group.