U-Pb geochronological constraints on Precambrian stratified units in the Avalon Composite Terrane of Nova Scotia, Canada: tectonic implications

摘要

Concordant U-Pb ages of detrital zircons in several Proterozoic units in the Avalon Composite Terrane of Nova Scotia, along with previously published data, indicate that deposition of low- and high-grade stratified units in the Cobequid Highlands (Gamble Brook Formation) and central Cape Breton island (variously assigned to the George River Group or George River Metamorphic Suite and the Bras d'Or Gneiss) started after similar to 977 Ma, extended through a period of volcanic activity between similar to 694 and 637 Ma, and ended before the similar to 629-612-600 Ma volcanic activity associated with deposition of the Georgeville and Jeffers groups in the Antigonish and Cobequid highlands, and the East Bay Hills and Coxheath groups in southern Cape Breton Island. These age data, along with U-Pb analyses of metamorphic and igneous zircon and monazite, also constrain three Late Proterozoic tectonothermal events: (i) oblique sinistral ductile shear deformation and greenschist-amphibolite-facies metamorphism probably occurred between similar to 694 and 629 Ma; (ii) oblique dextral deformation and greenschist-facies metamorphism is bracketed between similar to 612 and 607 +/- 2 Ma; and (iii) low-angle, normal shear deformation associated with low-pressure, high-temperature metamorphism took place at similar to 553-550 Ma. These three events may record (i) opening and (ii) closing of the Antigonish-Cobequid back-are basin, followed by (iii) gravitational collapse of the central Cape Breton magmatic are following cessation of subduction, respectively. The combination of similar to 700-550 Ma are magmatism and similar to 1.3-1.0 Ga concordant detrital zircon ages in the Avalon Composite Terrane suggests provenance in the Amazon craton or Oaxaquia in Mexico. [References: 62]