The Grenville Orogen, the Mirovoi Ocean and Their Paleozoic Legacies

The development of the Grenville orogen is widely thought to have been accompanied by the amalgamation of the supercontinent Rodinia, which formed as a result of a number of continent-continent collisions between 1.2 and 1.0 Ga, and dispersed between 0.75 and 0.6 Ga. The existence of Rodinia implies the presence of a Panthalassa-like peri-Rodinian ocean (known as the Mirovoi Ocean) between ca. 1.0–0.75 Ga within which juvenile crust developed. Although the vast majority of this crust was later subducted, vestiges are preserved in terranes that accreted to the leading edges of the dispersing continents following the breakup of Rodinia. These terranes are recognized by their ca. 1.2 to 0.75 Ga Sm-Nd TDM model ages that are coeval with the existence of Rodinia. They include ca. 0.9 to 0.8 Ga ophiolites and ensimatic arc complexes, and ca. 0.8 to 0.6 Ga recycled mafic to felsic arc complexes. Formed within the Mirovoi Ocean, the terranes were accreted to their respective continental margins following the breakup of Rodinia in the mid-Neoproterozoic. Along the northern margin of Gondwana, for example, rift-related Late Neoproterozoic and Early Paleozoic mafic complexes derived from a subcontinental lithospheric mantle have 1.2 to 0.75 Ga TDM ages, suggesting that they inherited this signature from an earlier history in the Mirovoi Ocean. This situation is analogous to the Mesozoic-Cenozoic evolution of the western Cordillera in which the accretion of juvenile terranes to the leading edge of North America during the breakup of Pangea influenced the composition of post-accretionary magmatism along that margin. More generally, crustal formation in Panthalassa-type oceans and the subsequent recycling of this crust can be recognized by Sm-Nd depleted mantle (TDM) model ages that overlap the life-span of the associated supercontinent, and their location provides constraints on the geometry of supercontinent breakup.