IDENTICAL TWINS, FRATERNAL TWINS, KISSING COUSINS, OR NO RELATION? A COMPARISON OF THE CHRONOLOGICAL AND ISOTOPIC CHARACTERISTICS OF CIRCUM-ATLANTIC NEOPROTEROZOIC TERRANES

Although significant advances have been made in our understanding of the evolution of major circum-Atlantic Neoproterozoic terranes, there is still considerable uncertainty about the possible relationships of these terranes to one another and their possible paleogeographic positions.

In the past decade there has been a dramatic increase in the amount of new chronological and isotopic data for the terranes, allowing for unprecedented comparisons between them. Much of these new potent databases have come from U-Pb geochronological studies focusing on both crystallization ages of igneous units within the terranes as well as ages of detrital zircons from clastic sedimentary sequences within the terranes. The former chronological data set aids in comparing the magmatic history of a given terrane along with possible host continents while the latter data set aids in the reconstruction of the possible provenance of the dispersed sediment.

For the Carolina terrane, the main episodes of magmatism have now been established to have occurred between 633–547 Ma, and the majority of the rocks are isotopically very juvenile. For the Cadomia terrane there appears to be three discrete magmatic events, a syn-tectonic event occurring between 615–600 Ma, a post-tectonic magmatic event between 585-570 Ma, and a second discrete post-tectonic event that appears to have occurred at ~ 540 Ma. The older rocks are isotopically very evolved, but become less so with time. The Avalon terrane is broadly characterized by two main periods of magmatism, 630–600 Ma and 570–550 Ma, and Nd isotopic compositions are between those of Carolina and Cadomia.

U-Pb dates of detrital zircons from the terranes help constrain possible paleogeographic positions. are also extremely useful in inter-terrane comparisons. Mesoproterozoic zircons dominate Neoproterozoic rocks in the Carolina terrane, whereas 2.3–2.0 Ga zircons dominate Cambrian sandstones. No Mesoproterozoic zircons occur in Neoproterozoic sedimentary units within Cadomia, but 2.4–2.0 Ga zircons predominate. Within Avalon there is an absence of 2.4–2.0 Ga ages, but 1.65–1.5 Ga and 1.25–1.15 Ga zircons are dominant.

Thus although there are general similarities between these major terranes they appear to have been built on different basements and resided in different regions of Gondwana.