Paleomagnetism and geochronology of the Ecstall pluton in the Coast Mountains of British Columbia: Evidence for local deformation rather than large‐scale transport

Abstract

[1] Samples for geochronologic, geobarometric, and paleomagnetic analyses were collected across the northern portion of the Ecstall pluton southeast of Prince Rupert, British Columbia. Al-inhornblende geobarometry indicates pressures from 740 ± 10 to 840 ± 30 MPa corresponding to crystallization depths of $25 to $30 km. U/Pb analyses of zircons from western, central, and eastern localities within the pluton yield crystallization ages of 91.5 ± 1.0 Ma, 90.8 ± 1.0 Ma, and 90.5 ± 1.0 Ma, respectively. Rock magnetic experiments, reflected light microscopy, and thermal demagnetization behavior suggest that natural remanent magnetism is carried by low-Ti titanohematite. Unblocking temperatures of the characteristic remanent magnetization (ChRM) are dominantly in the 560°C to 630°C range, with age of magnetization approximated by the 40 Ar/ 39 Ar hornblende ages of 84.2 ± 0.10 Ma on the western margin and 76.4 ± 0.6 Ma in the center of the pluton. Site-mean ChRM directions were isolated for paleomagnetic samples from 23 sites and are distributed along a small circle with subhorizontal axis at $340°azimuth. ChRM directions from the central portion of the pluton are concordant with the expected Cretaceous magnetic field direction, while ChRM directions from the western margin are discordant by >70°. Folding of the Ecstall pluton, either during Late Cretaceous west directed thrust transport above the convex upward Prince Rupert Shear Zone or during younger deformation of the pluton and underlying shear zone, can account for the paleomagnetic data and is consistent with the geochronologic, geobarometric, and structural geologic observations.

Thermal history of the Ecstall pluton from 40Ar/39Ar geochronology and thermal modeling

“…Assuming a depth of emplacement of 27 km (7.4-8.4 kbar, Butler et al, 2002), a depth of reheating of 15 km as used in Hollister et al (2004), and exhumation rates of $0.3 km/Ma except for a 5 Ma interval (58-53 Ma) during and after reheating when depth is held constant, the modeled age trends are broadly consistent with the data (Fig. 9a) using published diffusion parameters for Fig.…”

“…2004), (circles and squares). As discussed in the text, the square symbols are samples 008, and 001, from Butler et al (2002), which are from the same locations as 008, and 001 in this study, but they plot $4 and $3 km further from the thermal boundary due to the different methods of measuring distance to the thermal boundary used in this study vs. Hollister et al (2004). The inset shows only results from this study.…”

“…In this study distance is measured to the nearest edge of the Quottoon plutonic complex, and in Hollister et al (2004) distances are calculated by projecting sample locations onto a linear transect perpendicular to the trend of the Coast Shear Zone. Because of the different measurement techniques, the samples from Butler et al (2002), which are from the same locations as 008, and 001 in this study, plot $4, and $3 km further from the thermal boundary, respectively, in the Hollister et al (2004) compilation than in this study. In the western part of the Ecstall pluton, 40 Ar/ 39 Ar ages for hornblende are between 90 ± 3 and 79.2 ± 0.7 Ma (Table 2).…”

“…Gaber et al, 1988;Lo et al, 2000;Min et al, 2001), biotite ages reported are integrated ages, calculated as the mean of all steps weighted by the amount of 39 Ar released in each step. For hornblende the principal concern was to obviate Hutchison (1982), and structures are after Butler et al (2002), and Crawford et al (1987). age bias due to biotite inclusions and thus plateau ages, generally coinciding with uniform Ca/K, were used. Though low-temperature discordance is not typical in our samples, misinterpretation of young step ages due to inclusions or secondary phases with distinct Ca/K has been shown to produce spurious conclusions about diffusive loss from hornblendes (Onstott and Peacock, 1987).…”

“…The steeper paleomagnetic directions from the eastern part of the Ecstall pluton are concordant with results from cratonic North America. Butler et al (2002) proposed that local scale deformation (i.e. folding) 0016-7037/$ -see front matter Ó 2010 Elsevier Ltd. All rights reserved.…”