LITHODEMES, STRUCTURAL COMPLEXES AND GNEISS ASSOCIATIONS: NEW APPROACHES TO STRATIGRAPHIC MAPPING OF HIGH-GRADE TERRAINS, WITH EXAMPLES FROM THE GRENVILLE PROVINCE OF ONTARIO

One of the new concepts included in the 1983 North American Stratigraphic Code was the provision for creating formal stratigraphic units in rocks where the laws of superposition do not apply, i.e., igneous, metamorphic and highly deformed rocks. In the past 2 decades, however, these tools have not been widely adopted the geoscience community. In part this reflects the fact that few researchers work in such terrains, but it may also reflect the fact that mapping in high-grade terrains is commonly lithologically and/or structurally, not stratigraphically, focussed. Examples of the naming of lithodemes in a metamorphic belt, of the defining of regional intrusive suites, and of a structural complex, all from the Ontario Grenville Province (a Mesoproterozoic orogenic belt consisting of a wide variety of amphibolite to granulite facies gneisses of varied age, composition and tectonism) will be presented to illustrate how formal stratigraphic nomenclature, as outlined in the 1983 North American Stratigraphic Code, can be used effectively in detailed mapping of high-grade terrains.

In addition, another informal stratigraphic term, “gneiss association”, has also been devised for use within high-grade gneiss terrains and which has also been used successfully with the Ontario Grenville Province. As defined originally by N. Culshaw and co-workers in 1988, a “gneiss association” is an informal stratigraphic term that is primarily lithologic, but which also may contain information with respect to plutonic history, metamorphism and/or mafic diking. If it were decided to formally name a “gneiss association” as a lithodemic unit under the 1983 North American Stratigraphic Code, in most cases, they would correspond to a suite or a complex. For example, the Pardo gneiss association contains metatextite derived from the Archean metasedimentary rocks and is cut by well-preserved diabase dikes of the Matachewan and Sudbury dike swarms. In contrast, the compositionally similar Red Cedar Lake gneiss association consists of more highly deformed Archean metatextite, and only contains remnants of Sudbury swarm diabase dikes, at higher metamorphic grade than those present within the Pardo gneiss association. To date, gneiss associations have been mostly utilized in 1:50 000 to 1:250 000 scale regional mapping programs where large areas need to be mapped in a relatively short period of time.