PETROGRAPHIC STUDY OF THE OTTERTAIL PLUTON, SUPERIOR PROVINCE, NORTHWESTERN ONTARIO

STURM, Claire L., CZECK, Dyanna M. and FEIN, Elizabeth, Oberlin College Geology Department, Oberlin, OH 44074. claire.sturm@oberlin.edu

INTRODUCTION

This study concentrates on the Ottertail Pluton at the Wabigoon-Quetico subprovince boundary in the western Superior Province near Mine Centre, Ontario. The Ottertail is one of the Algoman plutons dated at 2686 Ma (Davis et. al, 1989), which have been used to constrain the termination of deformation at the subprovince boundary (e. g. Davis et. al, 1989; Poulsen, 2000). Our goal is to study the mineralogy and microstructures of the pluton in thin section. At the macroscopic level the pluton seems largely undeformed because of the general lack of macroscopic deformation fabrics. This has led many researchers to interpret the Ottertail Pluton to be post-tectonic. However, it has been shown that determining the relationship between pluton emplacement and deformation requires more detailed analysis (Paterson and Tobisch 1988). We are using petrographic analysis combined with a magnetic fabric study to better determine the relationship between the pluton emplacement and deformation at the Wabigoon-Quetico boundary.

ANALYSIS

Using optical petrography and a vibrating sample magnetometer (VSM), we determined the mineralogy and magnetic mineralogy of the pluton. In general, the pluton ranges from quartz monzonite to granite to granodiorite. Mineralogy consists of: quartz, plagioclase, microcline, microcline perthite, hornblende, myrmekite, and some zircon. There are also minor amounts of sericite, clinozoisite, magnetite (and other opaque minerals) in some portions of the pluton. The grain sizes vary. Many quartz crystals have undulose extinction and some have subgrains, which suggest slight deformation by dislocation processes. Minor metamorphism is indicated by sericite in plagioclase crystals, and microcline perthite. At the macroscopic level, the pluton seems largely undeformed, but our study of anisotropy of magnetic susceptibility (AMS) and microstructures show that there is some evidence for deformation.

REFERENCES:

Davis, D. W., Poulsen, K. H., Kamo, S. L., 1989. New insights into Archean crustal development from geochronology in the Rainy Lake area, Superior Province, Canada. Journal of Geology 97, 379-398.

Paterson, S. R., Tobisch, O. T., 1988. Using pluton ages to date regional deformations: problems with commonly used criteria. Geology 16, 1108-1111.

Poulsen, K. H., 2000. Archean metallogeny of the Mine Centre - Fort Frances area. Ontario Geological Survey Report 266, 121.





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