THE THERMAL STRUCTURE OF THE WESTERN HALF OF THE CENTRAL METASEDIMENTARY BELT, GRENVILLE PROVINCE, ONTARIO, REQUIRES A RE-SIZING OF THE OTTAWAN OROGENIC LID

The Grenville Province of Ontario, Canada includes the Central Gneiss Belt and the Central Metasedimentary Belt (CMB). The CMB consists of several terranes that represent an amalgamation of arc rocks and associated sedimentary basins metamorphosed at relatively low pressure. Calcite-graphite carbon isotope thermometry results from marble samples across the western CMB show a gradual increase in peak metamorphic temperature from under 500°C in the Tudor Township area in the east to over 700°C along the western margin of the CMB. Based on published geochronology, we argue that these temperatures correspond to peak metamorphism during the 1090-1020 Ma Ottawan orogeny. A lack of significant thermal discontinuities at terrane boundaries and shear zones, including the Bancroft shear zone and the CMB boundary zone, indicates that the entire region remained largely intact during and after the Ottawan peak metamorphic event. These results require a reassessment of the geometry of the Ottawan Orogenic Lid, which Rivers (2012, CJES v.49) defined thermomechanically as a cool, upper crustal region that escaped significant ductile deformation and metamorphism during the Ottawan phase of the Grenvillian Orogeny. Much of the western CMB meets the criteria for the allochthonous low-pressure belt rather than the Ottawan Orogenic Lid. Only the Tudor metamorphic low meets the <500°C criterion (during the Ottawan) for the orogenic lid. The exposed orogenic lid in the western CMB in Ontario, then, is apparently a quite small part of the Elzevir terrane, no more than 75 km across in any direction. At the scale of the orogen, we tentatively hypothesize that the actual orogenic lid of the CMB in Ontario and NY state includes the Sharbot Lake terrane, the Frontenac terrane and the Adirondack Lowlands. To the west of the orogenic lid, the western CMB constitutes a wide allochthonous low-pressure belt, which includes a small and relatively superficial orogenic lid outlier at its southern edge (where Paleozoic cover sequences obscure Grenville geology). The boundaries of this small Elzeviran lid are paleo-isotherms and not late, extensional shear zones, such as the boundaries of the orogenic lid to the east. In other words, the Elzeviran lid is discordant to, and appears to post-date, the regional structural grain defined by contacts and foliation.