CONSTRAINTS ON THE EVOLUTION OF THE AVALONIAN MANTLE AND TECTONIC IMPLICATIONS

Important insights into the genesis of continental rift-related basalts and the evolutionary history of its mantle source can be gained by focusing on terranes in which rift-related magmatism has occurred repeatedly over a long period of time. The Antigonish Highlands, Nova Scotia lie within the Avalon terrane of the northern Appalachians and contain four distinct episodes of rift-related magmatism: Neoproterozoic, Cambrian, Middle Ordovician and Late Devonian. All four magmatic suites are composed of basalts and subordinate crustally-derived felsic rocks. The mafic rocks of these suites, which do not appear to be significantly contaminated by continental crust, display similar geochemical and Sm-Nd isotopic characteristics indicating a remarkable degree of inheritance, and are consistent with a mantle source metasomatically enriched prior to the oldest rifting event, probably between 0.8 and 1.1 Ga. These data also imply that the Avalonian crust and its sub-continental lithospheric mantle remained coupled during four magmatic events. As the earliest phases of magmatism occurred when Avalonia was located along the Gondwanan margin, and the latest phases after Avalonia accreted to Laurentia, the coupling of crust and mantle in the Antigonish Highlands suggests that the migration of Avalonia did not completely detach its lithospheric mantle from its crustal cover. More generally, models for the petrogenesis of continental rift-related basaltic rocks typically involve a combination of partial melting of the sub-continental lithospheric mantle, low pressure fractional crystallization, and chemical contamination by either continental crust or coeval felsic magmas. These data show that the sub-continental lithospheric mantle can underlie a region for a long period of time and the chemical and isotopic imprint of the mantle source on rift-related basalts can be inherited from earlier tectonothermal events. The study indicates the value of comparing the compositions of basalts of the different ages in the same terrane.