Tomasz Werner MSc thesis abstract

Thesis Title: 
Paleomagnetism, structure and magnetic fabrics in a traverse of the Quetico Subprovince between Atikokan and Kashabowie, NW Ontario

The metasedimentary rocks along the traverse reveal low to high grade metamorphism from chlorite schist near Atikokan, through biotite schist to migmatites in the west within the centre of the Quetico Belt.  Continuing toward the ESE, near Huronian Lake, the metamorphic grade decreases symmetrically but with somewhat less steep gradients to Kahabowie.  The metamorphism was syn- to late-tectonic.

Only one pervasive tectonic fabric was observed in the interior of the belt, with NE-SW striking S1 foliations (mean direction:  256/85°NW, n=121) and extension lineations L1 plunging shallowly to the NE (mean direction:  70-20°, n=52).  In migmatite and pegmatite zones foliation was often less steep.  It was probably deflected due to the intrusion of pegmatite or granitoid bodies.

Minerals contributing to ferromagnetic properties are mostly monoclinic ferromagnetic pyrrhotite within the belt with some magnetite in medium grade amphibolite - rich outcrops in metavolcanics of Shebandowan Belt.

Anistropy of magnetic susceptibility fabric (AMS) is mainly controlled by paramagnetic biotite or chlorite and subordinate ferromagnetic pyrrhotite in metasediments of the Quetico Belt.  In some outcrops of mafic metavolcanics near Kashabowie, the high magnetite content controls bulk susceptibility more than its anisotropy.  Variations in AMS fabric are largely due to variations in relative composition of magnetite, pyrrhotite and paramagnetic sheet silicates, not due to changes in strain along the traverse.

AMS foliations and lineations within the belt and on its south-east margin are generally coaxial with tectonic fabric, with AMS foliation less steep (dip of 70°) in the interior of the belt.  However a slight (2-5°) anticlockwise offset of the mean AMS foliation from S1 can be postulated in the centre of the belt and at the southern margin.  When fabric data is studied separately for each sample, such offsets are not significant.  In the Atikokan area, AMS fabric is partly of sedimentary origin.  The offset of mean AMS lineations with respect to L1 was confirmed for the Kashabowie - Huronian Lake area (also reported by Borradaile and Spark, 1990) but it is not prominent in the centre of the belt.

Anisotropy of remanence based on acquisition of anhysteretic remanence (AARM) is controlled by preferred crystallographic orientation of pyrrhotite in metasediments and partly by preferred dimensional orientation of magnetite in metavolcanics.  It is substantially higher than AMS fabric (mean P' of order 3 to 5).  AARM foliation is usually oriented closer to the tectonic fabric than AMS foliation; it is vertical at the margins and less steep in the centre of the Belt.  AARM lineations usually plunge more gently than tectonic and AMS lineations in the centre of the Belt and at the southern margin in the Kashabowie - Huronian Lake area.  At the northern margin, AARM fabric is highly oblate, so that fabric lineations are difficult to define. 

At the belt margins, a slight anticlockwise offset of AARM foliations with respect to the tectonic fabric exists, when fabrics for individual samples are examined.  However, the relative orientation of tectonic and magnetic fabric (AMS and AARM) are not a region - wide consistent kinematic indicator of dextral transpression.

Correlation studies between magnetic susceptibility and ARM intensity indicate that a monotonic correlation between ARM and MS exists only in the Calm Lake - Perch Lake area, in which pyrrhotite controls ARM.  In other areas a higher content of magnetite increases magnetic susceptibility in several samples, but not ARM.

Both magnetic fabric (AMS and ARM) are usually oblate in low grade metasedimentary rock (at the Belt margins), but in metavolcanic rocks magnetite can produce prolate ARM, but in metavolcanic rocks magnetite can produce prolate ARM, whereas ARS is oblate.  In the interior of the Belt, 50% of samples have prolate AARM, but most of them are pyrrhotite-bearing.

The AARM fabric is believed to have arisen by the growth of ferromagnetic grains (at least in case of pyrrhotite) in the later stages of transpressive penetrative deformation with a dominant dextral shear component of deformation along an ENE - SWS, vertical plane.  The preferred orientation of pyrrhotite grains was probably controlled by the older biotite - chlorite matrix fabric.  This would explain the lack of a more significant deflection of the youngest ARM fabric from the oldest tectonic fabric.  However, the accuracy of determinations of ARM axes (at least 5-10°) limits the validity of these conclusions.

Characteristics directions of magnetic remanence obtained during thermal or AF step-wise demagnetization form a weakly developed girdle along the basal plane of ARM anisotropy.  Therefore natural remanent magnetization is probably entirely of chemical origin and controlled mostly by high anisotropy of pyrrhotite and its preferred crystallographic orientation.  The orientation of NRM toward basal plane of ARM is believed to be mostly an effect of deflection from the true Earth's magnetic field direction due to anisotropy of pyrrhotite (P' of values of 3-5, Fuller, 1963), and not the effect of grain rotation.  Stress control of post - metamorphic remagnetization of  primary ChRM is also a possibility.  Therefore, it is suggested that NRM was acquired in the same late stage of deformation as AARM, when sizes of individual ferromagnetic grains were suitable to carry a stable remanence.  Any primary magnetization acquired at the time of rock formation was not preserved.

The model of different ages of development of paramagnetic and ferromagnetic fabric is consistent with the dextral transpression model of the deformation for both margins and the fabric of the Quetico Belt.  The latest ferromagnetic sub-fabric formed along the shear plane when the paramagnetic matrix was already oriented subparallel to this plane and mimetically controlled the orientation of crystallization of ferromagnetic phase.  The ferromagnetic fabric is not therefore useful as a kinematic indicator of shear component in those areas.

A copy of the thesis can be downloaded here