ANISOTROPY OF MAGNETIC SUSCEPTIBILITY IN THE OTTERTAIL PLUTON, NORTHERN ONTARIO.

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

INTRODUCTION

In this project, we studied the structural and magnetic fabrics of the Ottertail Pluton, a small, granitic body within the Superior Province of Northern Ontario. The Ottertail, part of the Algoman suite of quartz monzonites and granodiorites, intruded surrounding rocks along the Wabigoon- Quetico subprovince boundary around 2.7 Ga (Davis et. al, 1989). From field observation, the emplacement of the Ottertail Pluton has been considered to be post-deformational due to the lack of obvious deformation fabrics evident in other types of rocks at the subprovince boundary (e. g. Davis et. al, 1989; Poulsen, 2000) . This interpretation has been used to constrain the timing of final deformation along the Wabigoon- Quetico boundary associated with microplate collision.

PROBLEM

On a much smaller scale, within deformed conglomerates, such as the nearby Seine River conglomerates, rigid clasts often display evidence of much less deformation than do more yielding clasts. Could a similar situation have occurred on a much larger scale in the case of the Ottertail Pluton? The relatively rigid granite body could appear to be relatively undeformed in the field, even if the timing of its emplacement was pre- or syn- tectonic. This situation, where a pluton appears undeformed despite being present during deformation, has been shown to occur in some modern tectonic regimes (e. g. Paterson & Tobisch, 1988). A more detailed study of plutonic fabrics and relationships is needed to conclusively determine whether a pluton is truly "post deformational."

METHOD

In August of last year, we sampled an E-W (and a shorter N-S) transect across the Ottertail Pluton traveling along Highway 11, west of Mine Centre, Ontario. Thirty-one sites were sampled resulting in a total of 218 oriented cores. The anisotropy of magnetic susceptibility (AMS) of these granitic cores was measured to learn about the history of pluton emplacement. The measurements were conducted at the Institute of Rock Magnetism at the University of Minnesota in Minneapolis in January of this year. In general, the samples have a significant AMS signal contributed by multi-domain magnetite. The AMS data has been complied to describe the attitude of the variations in magnetic fabric within the Ottertail Pluton. To determine the relative timing of pluton emplacement with respect to the deformation, we need to show whether the magnetic fabric is concordant with preserved, regional patterns of strain. The AMS study will be combined with our field observations and a detailed petrographic study.

RESULTS

Regional deformation fabrics have already been well documented in the literature (e. g. Poulsen, 1986; Poulsen, 2000; Czeck, 2001). Preliminary results of our study suggest that the magnetic fabrics preserved in the Ottertail are related to the pattern of regional strain fabrics in the surrounding rocks. Further work will be conducted to analyze the magnetic fabrics to determine if, indeed, the Ottertail Pluton is pre- or syn- tectonic. If it can be shown to be pre- or syn- tectonic, this will have major implications for the relative timing of larger, regional, tectonic events. In particular, if the Ottertail is representative of other Algoman plutons and the deformation is truly regional, it may not be possible to constrain the end of regional deformation using the date of the intrusion of Algoman granites. the magnetic fabric is concordant with preserved, regional patterns of strain. The AMS study will be combined with our field observations and a detailed petrographic study.

REFERENCES

Czeck, D. M., 2001. Strain analysis, rheological constraints, and tectonic model for an Archean polymictic conglomerate: Superior province, Ontario, Canada. Unpublished Ph. D. Thesis, University of Minnesota, 245 p.

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., 1986. Rainy Lake Wrench Zone: An example of an Archean Subprovince boundary in Northwestern Ontario. In: de Wit, M. J., Ashwal, L. D. (Eds.), Tectonic evolution of greenstone belts Technical Report 86-10, pp. 177-179.

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





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