Nd isotope mapping of the Dysart gneiss complex: Evidence for a rifted block within the Central Metasedimentary Belt of the Grenville Province

More than 65 new Nd isotope analyses are presented for the vicinity of the Central Metasedimentary Belt Boundary Zone (CMBBZ) near Haliburton in the Grenville Province of Ontario, Canada. They fall into two well-defined groups, with distinct ranges of TDM model ages. These two groups comprise the CMBBZ with an average TDM age=1.46Ga, and rocks of the Central Metasedimentary Belt (CMB) with an average TDM age of 1.28Ga. However, a distinct block of crust within the CMB comprising the Dysart gneiss complex has an older model age signature (TDM age=1.40Ga) resembling the CMBBZ rather than the rift zone. The Dysart complex is surrounded by marble tectonites interspersed with elongate bodies of orthogneiss that yield young Nd model ages typical of the CMB. However, it is argued that the development of the Dysart complex as a distinct crustal block predates ductile deformation during the Grenville orogeny. Instead, the Dysart Block is thought to have rifted away from the Central Gneiss Belt to the northwest during the formation of the CMB as an ensimatic back-arc rift zone. Isotopic mapping of the Dysart block, along with the previously discovered Elzevir block, provides evidence for the geometry of the rifting process, and also has important implications for the structure of Grenville basement in eastern North America.