CALCITE-GRAPHITE ISOTOPE THERMOMETRY ACROSS THE WESTERN PART OF THE CENTRAL METASEDIMENTARY BELT, GRENVILLE PROVINCE, SOUTHERN ONTARIO

We present over 110 new temperature estimates from marble in the Central Metasedimentary Belt of the Ontario-NY segment of the Grenville Orogen. The temperature-dependent fractionation of carbon isotopes between calcite and graphite is resistant to retrograde resetting due to exceptionally slow diffusion of carbon in graphite. The prevalence of graphitic marble in the Grenville allows this single thermometer to be applied across a large area. Although graphite is not expected to equilibrate with calcite at low T, we find, with the exception of very few outliers, surprisingly consistent fractionations even below 500°C. The lowest grade rocks in the Ontario-NY Grenville are greenschist facies (Dol+Qtz in marble) in the center of the orogen, Belmont domain of the Elzevir terrane, which has been interpreted as an orogenic lid that was let down onto collapsing, over-thickened crust in the Ottawan orogeny (1080-1020 Ma). Grade increases westward to amphibolite facies (first Tr, then Di) within the western Belmont domain and remains amphibolite facies across the Harvey-Cardiff domain and the Bancroft terrane where rocks of the CMB meet the granulite-bearing Central Gneiss Belt. In spite of many thermometric studies of the Grenville, data in the Bancroft terrane and Harvey-Cardiff domain were sparse until now. We document a gradual E to W increase in T from 470°C ±26° (1s, N=18) in the central Belmont domain (Tudor Township), to 524°±33 (N=9) 10km to the SW. The next large marble belt 35 km to the W is a 10km wide belt in the westernmost Belmont domain adjacent to the Harvey-Cardiff domain stretching 50km from Apsley to Bancroft that shows very uniform T of 620°±34° (N=50). Marble within the Harvey-Cardiff domain is modestly warmer, averaging 652°±31 (N=16). T shows no change crossing westward into the Bancroft terrane, yielding 651°±42 (N=17) (excluding one 835° sample next to the Glamorgan metaigneous complex). The westernmost marble belt of the CMB, the Gull Lake to Minden area, is 749°±18 (N=4). These results use the Dunn & Valley 1992 calibration below 600° and Kitchen & Valley 1996 above. The thermal structure is one of gradual T increase westward from 470° to 750°C without noticeable discontinuities across terrane boundaries, indicating the thermal maximum postdates terrane assembly.