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Paleoproterozoic crustal history of the southwestern Grenville Province: evidence from Nd isotopic mapping

1995; NRC Research Press; Volume: 32; Issue: 4 Linguagem: Inglês

10.1139/e95-040

1480-3313

Christopher Holmden, Alan P. Dickin,

earthquake and tectonic studies

Nd isotopic mapping in the North Bay area of the Central Gneiss Belt, southwestern Grenville Province, has revealed the precise trend of a T DM model age line developed between the uplifted southern margin of the Archean Superior craton (T DM = 2.7 Ga) and a Paleoproterozoic allochthon (T DM = 1.9 Ga). Separating these two crustal blocks is a narrow zone of gneisses with intermediate T DM ages. These transitional gneisses are interpreted to reflect a remnant fault or ductile shear zone, of uncertain age, along which crustal material from both blocks mechanically mixed during their juxtaposition. Accordingly, the nature of the T DM line in the North Bay area is interpreted to be tectonic. In the Temiscaming area, widespread exposures of mature metasedimentary gneisses are shown by their T DM ages to be dominantly of Paleoproterozoic provenance. These results are consistent with the existing detrital zircon geochronology, inferring a maximum depositional age of ~1.7 Ga. The anorogenic chemistry of the North Bay orthogneiss and mixed calc-alkaline–alkaline chemistry of the Temiscaming gneisses suggest a connection between Paleoproterozoic anorogenic magmatism and synsedimentary quartzite deposition, which is a common association in 1.9–1.6 Ga accretionary orogens of southern Laurentia. The relatively close correspondence between widespread 1.9 Ga T DM ages and U–Pb crystallization ages as old as 1.74 Ga implies that rocks of the Central Gneiss Belt were originally the juvenile products of Paleoproterozoic orogenesis.