POLYMETAMORPHISM IN THE ARCHEAN HEMLO - HERON BAY GREENSTONE-BELT, SUPERIOR PROVINCE - P-T VARIATIONS AND IMPLICATIONS FOR TECTONIC EVOLUTION

The tectono-metamorphic history of the late Archean (2800-2600 Ma) Hem lo - Heron Bay greenstone belt in the Superior Province has been delin eated from textural relationships, mineral chemistry, and P - T paths in metapelites, cordierite - orthoamphibole rocks, and metabasites fro m the White River exploration property, Hemlo area, Ontario. An early low-temperature, medium-pressure metamorphism (about 500-degrees-C and 6 - 6.5 kbar (1 kbar = 100 MPa)) is indicated by the occurrence of re lict kyanite and staurolite porphyroblasts and zoned garnet porphyrobl asts in metapelites and the presence of zoned calcic amphiboles in met abasites. This early metamorphism appears to have been coeval with the previously documented D1 deformation that is associated with, for exa mple, low-angle thrusts. A second regional metamorphism predominates i n the Hemlo - Heron Bay greenstone belt and is generally of relatively low grade, at about 510-530-degrees-C and 3.2-3.5 kbar, over most of the study area and increases to medium grade (550 - 650-degrees-C and 4 - 5 kbar) towards the southern margin with the Pukaskwa Gneissic Com plex and along the central axis enclosing the Hemlo Shear Zone. The se cond regional metamorphism was contemporaneous with the D3 deformation and was probably related to plutonism. This type of polymetamorphism in the Hemlo - Heron Bay greenstone belt may be equivalent to those in Phanerozoic subduction complexes and therefore supports the arc-arc a ccretion model for the development of the southern Superior Province. Although the Hemlo - Heron Bay greenstone belt most likely represents a single tectonic environment (an oceanic island arc), the restricted occurrence of the relict kyanite and staurolite indicates that the cen tral portion of this Archean greenstone belt probably was at a deeper crustal level at the time of the first metamorphic event.