Paleomagnetism of the Alkaline Coldwell Complex: New Results, New Insights
Ashley N. Kern, E.V. Kulakov, A.V. Smirnov, J.F. Diehl
Department of Geological Engineering and Sciences, Michigan Technological University
Abstract:
The alkaline Coldwell Complex is a ≈1.1 Ga intrusive complex which is thought to
be emplaced during three different episodes of magmatism. Previous paleomagnetic
work by Lewchuk and Symonds (1990) has shown that rocks representing magmatic
episodes I and III exhibit reversed (R) magnetizations with mean directions that are
significantly different. The rocks representing episode II were characterized by
normal (N) magnetizations which are symmetrical with respect to the R
magnetizations. Lewchuk and Symonds (1990) interpreted these results as indicating
the reversal asymmetry seen in other Keweenawan age rocks is not the effect of a
long-standing non-dipole field but the effect of plate motion. This is a reevaluation
of the previous paleomagnetic study with samples from 43 additional sites.
Characteristic remanent magnetizations (ChRM), in our study, have been isolated by
thermal and alternating field demagnetization techniques. Mean paleomagnetic
directions have been categorized according to the existing model of the three
episodes of magmatism. The mean paleomagnetic directions for episodes I and III
have been found to be statistically similar at 95 percent confidence, making these
episodes indistinguishable with paleomagnetism unlike the Lewchuk and Symons’
study. We will discuss two possible scenarios of the geomagnetic field behavior
during the formation of the Coldwell Complex:
1. Multiple reversals R→N→R, as recorded consecutively by episodes I, II and III;
2. Single reversal from R to N polarity, where episodes I and III were emplaced at
the same time (single episode) or in pulses during a time interval insufficient to
result in any difference between the primary ChRM directions.
Results:
Fig.2 (a,b,c) Examples of alternating field demagnetization of samples from the
Coldwell Complex. Vector endpoint diagrams, normalized magnetization versus
field and equal area projections of the same samples.
Fig.2 (d) The mean paleomagnetic directions for each site were calculated using
standard Fisher (1953) statistics. The solid circles represent the sites mean directions
of normal polarity. The open circles show the site means for the reversely
magnetized rocks. Paleomagnetic data were categorized according to the existing
model of three episodes of magmatism, resulted in emplacement of the Coldwell
Complex, with episode I being the oldest.
Fig.2 (e) The group mean directions with 95% percent confidence. The group means
for each episode were calculated using same Fisher statistics. Each data point
represents a different episode of magmatism.
Figure 2. Paleomagnetic study results.
Figure 1. An aeromagnetic anomaly map of the Coldwell Complex with sampling
locations (black squares). Warm and cool colors represent positive and negative
magnetic anomalies respectively. Top right inset is a sketch of the three proposed
magmatic episodes.
Conclusions:
The group mean paleomagnetic directions for reversely magnetized rocks of
episodes I and III appear statistically indistinguishable as shown in figure 2 (e).
Episodes I and III of the Coldwell Complex magmatism may have been emplaced at
the same time or in pulses with a time gap that was insufficient to result in any
difference in paleomagnetic directions.
Fisher, R.A., Dispersion on a sphere. Proceedings of the Royal Society of London, Series A, 217, 295-305, 1953
Lewchuk M.T., D.T.A. Symons, Paleomagnetism of the Late Precambrian Coldwell Complex, Ontario, Canada. Tectonophysics, 184, pp 73-86, 1990.