Calcite-Graphite Isotope Thermometry of Marble Mylonites in the Bancroft Shear Zone
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The Bancroft Shear Zone is an extensional collapse structure that formed in the Grenville Province due to the over thickening and resulting crustal weakening during the Ottawan orogeny, ~1030-1070 Ma (Cosca et al., 1995). The shear zone was active late in the orogenic cycle, ~1045 to 1030 Ma (Metzger et al., 1991), forming very fine-grained mylonites with textures indicating that deformation occurred well below peak metamorphic temperatures (van der Pluijm, 1991). These mylonites mainly consist of calcite, along with minor amounts of other minerals including graphite. The 13C/12C ratios of coexisting calcite and graphite are sensitive to temperature. Expressed as δ13C(cal) - δ13C(gr) = Δ13C(cal-gr), the Δ-values correspond to temperature, presumably peak metamorphic temperature. Carbon diffusion rates are relatively fast in calcite, but extraordinarily slow in graphite, such that carbon equilibration appears to require graphite crystallization (Valley, 2001). So unless graphite is recrystallized, the Δ-values provide an estimate of the peak metamorphic temperature (Valley and O’Neil, 1981). In this study marble mylonites from six localities within the Bancroft Shear Zone were analyzed to determine whether the Δ-values were reset from the regional metamorphic temperature down to the lower temperature of shearing. Previous work shows peak regional temperatures to be ~650-700°C (van der Pluijm and Carlson, 1989). The protomylonite Δ-values range from 3.12 to 4.80‰, which correspond to a similar mean temperature of 677°C (n=13) using the calibration of Kitchen and Valley (1995). Two localities for which I have the most data, gave a mean protomylonite temperature of 686°C (n=4), a mean mylonite temperature of 600°C (n=4), and a mean ultramylonite temperature of 642°C (n=5). The data indicated no significant difference in temperature due to the locality (p=0.598) or due to the extent of mylonitization (p=0.326). We therefore conclude that mylonites within the shear zone retain the isotopic signature from the previous peak regional metamorphism, rather than from the subsequent shearing at temperatures lower than graphite can recrystallize.