MIGMATITES OF THE MUSKOKA DOMAIN, GRENVILLE PROVINCE, ONTARIO

Abstract

The Muskoka Domain, Central Gneiss Belt, Grenville Province, Ontario, is characterized by widespread highly migmatitic orthogneisses. The purpose of this study is to describe a variety of migmatites and associated rocks from a small section representative of the Muskoka Domain, using detailed petrographic descriptions and microprobe analyses. The amount of leucosome in outcrop ranges from 10 to 35%. Thirty-eight samples were classified into 4 types based on the amount and shape of leucosome, and the presence of mafic porphyroblasts in the leucosome. Non-migmatitic fine-grained mafic rocks from discrete layers in the migmatites comprise Type 1. Type 2 migmatites are characterized by thin ( ~ 1 em wide), stringy leucosome patches, parallel to or cross-cutting the foliation. Type 3 migmatites are stromatic, with variable leucosome width. Type 4 migmatites are characterized by hornblende and/or biotite porphyroblasts in the coarse-grained leucosomes, typically parallel to and crosscutting the foliation. Deformation in Type 4 migmatites is illustrated by large quartz ribbons (- 2 em), myrmekite, mosaic texture in quartz and felspars, cross-hatch twinned microcline, and finer-grained recrystallized matrix in leucosomes. Allleucosomes are enriched in potassium feldspar relative to mesosome and/ or melanosome; Type 4 leucosomes are also quartz-rich. The typical mineral assemblage of a mesosome is hb+pl+bt+kf+opq+qz. Microprobe analyses for each group of minerals are fairly uniform. Some samples have slightly more calcic plagioclase and more potassic alkali feldspars in the mesosome, and biotite and hornblende are more magnesian in some samples. These data are consistent with the formation of leucosome by partial melting. However, general similarity of feldspar compositions between leucosome and host, and abundance of leucosome in outcrop, suggest extensive equilibration of melt with host and limited migration.

Key Words: migmatite, petrogenesis, melt migration, anatexis, stromatic, orthogneiss, Muskoka Domian, Central Gneiss Belt