Graphitization and coarsening of organic material in carbonate-bearing metasedimentary rocks is accompanied by carbon isotope exchange which is the basis of a refractory, pressure-independent geothermometer. Comparison of observed isotopic fractionations between calcite and graphite (A13Ccal-Gr) with independent petrological thermometers provides the following empirical calibration over the range 400-800" C: A13CCal-Gr = 5.81 x lo6 x T-*(K) -2.61. This system has its greatest potential in marbles where calcite + graphite is a common assemblage and other geothermometers are often unavailable. The temperature dependency of this empirical calibration differs from theoretical calibrations; reasons for this are unclear but the new empirical calibration yields temperature estimates in better agreement with independent thermometry from several terranes and is preferred for geological applications.Both calcite-graphite isotopic thermometry and calcite-dolomite solvus thermometry are applied to marble adjacent to the Tudor gabbro in the Grenville Province of Ontario, Canada. The marble has undergone two metamorphic episodes, early contact metamorphism and later regional metamorphism. Values of A13CCa,-Gr decrease regularly from c. 8%0 in samples over 2 km from the pluton to values of 3-4% within 200m of the contact. These samples appear to preserve fractionations from the early thermal aureole with the empirical geothermometer, and indicate temperatures of 450-500" C away from the intrusion and 700-750" C near the gabbro. This thermal profile around the gabbro is consistent with conductive heat flow models. In contrast, the distribution of Mg between calcite and dolomite has been completely reset during later regional metamorphism and yields uniform temperatures of c. 500" C, even at the contact.Graphite textures are important for interpreting the results of the calcite-graphite thermometer. Coarsening of graphite approaching the Tudor gabbro correlates with the decrease in isotopic fractionations and provides textural evidence that graphite crystallization took place at the time of intrusion. In contrast to isotopic exchange during prograde metamorphism, which is facilitated by graphitization, retrogressive carbon isotopic exchange appears to require recrystallization of graphite which is sluggish and easily recognized texturally. Resistance of the calcite-graphite system to resetting permits thermometry in polymetamorphic settings to see through later events that have disturbed other systems. 13 13 A~a~r = ccalc,te -Cgraphute. Textural type on a scale of I-IV relative to samples in Fig. 6 (see text) 'Long dimension of graphite flakes. Minerals in addition to calcite and graphite, minerals in parentheses appear texturally to be secondary. Am = amphibole, Bi =biotite, Chl = chlorite, Cpx = clinopyroxene, Ep = epidote, Gr = graphite, Ilm = ilmenite, Kfs = K-feldspar, Mu = muscovite, 0 1 =olivine, PI = plagioclase, Po = pyrrhotite, Py = pyrite, Qtz =quartz, Rt = rutile, Srp = serpentine, Tc = talc, Ttn = titanite, Tur = tou...

This study presents calcite-graphite carbon isotope fractionations for 32 samples from marble in the northern Elzevir terrane of the Central Metasedimentary Belt, Grenville Province, southern Ontario, Canada. These results are compared with temperatures calculated by calcite-dolomite thermometry (15 samples), garnet-biotite thermometry (four samples) and garnet-hornblende thermometry (three samples). D cal-gr values vary regularly across the area from >6.5& in the south to 4.0& in the north, which corresponds to temperatures of 525°C in the south to 650°C in the north. Previous empirical calibration of the calcite-graphite thermometer agrees very well with calcite-dolomite, garnet-biotite and garnet-hornblende thermometry, whereas, theoretical calibrations compare less well with the independent thermometry. Isograds in marble based on the reactions rutile + calcite + quartz ¼ titanite and tremolite + calcite + quartz ¼ diopside, span temperatures of 525-600°C and are consistent with calculated temperature-X(CO 2 ) relations. Results of this study compare favourably with large-scale regional isotherms, however, local variation is greater than that revealed by large-scale sampling strategies. It remains unclear whether the temperature-D cal-gr relationship observed in natural materials below 650°C represents equilibrium fractionations or not, but the regularity and consistency apparent in this study demonstrate its utility for thermometry in amphibolite facies marble.

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The Central Metasedimentary Belt boundary zone (CMBbz) is a crustal-scale shear zone that juxtaposes the Central Gneiss Belt and the Central Metasedimentary Belt of the Grenville Province. Geochronological work on the timing of deformation and metamorphism in the CMBbz is ambiguous, and the questions that motivate our study are: how many episodes of shear zone activity did the CMBbz experience, and what is the tectonic significance of each episode? We present electron microprobe data from monazite (the U–Th–Pb chemical method) to directly date deformation and metamorphism recorded in five garnet–biotite gneiss samples collected from three localities of the CMBbz of Ontario (West Guilford, Fishtail Lake, and Killaloe). All three localities yield youngest monazite dates ca. 1045 Ma; most of the monazite domains that yield these dates are high-Y rims. In comparison with this common late Ottawan history, the earlier history of the three CMBbz localities is less clearly shared. The West Guilford samples have monazite grain cores that show older high-Y domains and younger low-Y domains; these cores yield a prograde early Ottawan (1100–1075 Ma) history. The Killaloe samples yield a well-defined prograde, pre- to early Shawinigan history (i.e., 1220–1160 Ma) in addition to some evidence for a second early Ottawan event. In other words, the answers to our research questions are: three events; a Shawinigan event possibly associated with crustal thickening, an Ottawan event possibly associated with another round of crustal thickening, and a late Ottawan event that resists simple interpretation in terms of metamorphic history but that coincides chronologically with crustal thinning at the base of an orogenic lid.

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