Paleomagnetism of the Late Precambrian Coldwell Complex, Ontario, Canada

Paleomagnetism of the ~1.1 Ga Coldwell Complex (Ontario, Canada): Implications for Proterozoic geomagnetic field morphology and plate velocities

“…All major lithologies of the Coldwell Complex except for granites were sampled (Table ). Our sites SL5‐SL33 and CLD1‐CLD6 generally replicate the sampling localities of Lewchuk and Symons []. However, we did not find any traces of the original sampling which could have been destroyed by widening the highway after 1990.…”

“…Early geological studies considered the Coldwell Complex as a single intrusion that underwent extensive fractional crystallization of magma [ Lilley , ; Puskas , ]. However, subsequent geochemical, petrological, and field relationship analyses resulted in a model in which the complex was emplaced in three magmatic episodes separated by quiescent periods as the magma source deepened and migrated to the western side of the complex [e.g., Currie , ; Mitchell et al ., ; Lewchuk and Symons , ; Walker et al ., ]. However, many details of the complex's geological history such as the extent and order of the intrusive episodes have been debated [e.g., Currie , ; Mitchell and Platt , , ; Lewchuk and Symons , ].…”

“…In the absence of reliable radiometric ages, paleomagnetism can provide time constraints on the number and order of intrusive episodes. The only published paleomagnetic investigation of the Coldwell Complex by Lewchuk and Symons [] that also incorporated an unpublished paleomagnetic data set obtained in 1972 by William A. Robertson identified three distinct directions (named A, B, and C) of the primary characteristic remanent magnetization (ChRM). The reversed polarity direction A with the mean of D = 121.0°, I = −70.9° ( α 95 = 3.3°, N = 19) was exclusively observed from the eastern gabbro and ferroaugite syenite.…”

“…Because the reversed mean directions A and C were statistically different, Lewchuk and Symons [] concluded that the three ChRM directions corresponded to three separate intrusive episodes during two periods of reversed geomagnetic field polarity (directions A and C) separated by a period of normal field polarity (direction B), hence recording two geomagnetic reversals (CR1 → CN1 and CN1 → CR2). Despite the fact that the components A and C mean directions are statistically different, both directions were combined in constructing the reversal test versus the component B mean direction, which marginally passes with a “C” classification [ McFadden and Lowes , ; McFadden and McElhinny , ].…”

“…Unfortunately, direct testing of the reversal asymmetry is impossible because of the lack of known MCR sequences that record transitional field directions between the two stable asymmetric directions. However, there are two MCR locations that presumably record multiple reversals which makes them crucial for evaluating the reversal asymmetry: the Mamainse Point volcanics [ Palmer , ; Robertson and Fahrig , ; Robertson , ; Swanson‐Hysell et al ., ] and the intrusive Coldwell Complex [ Lewchuk and Symons , ], both located in Ontario, Canada.…”