OXYGEN-ISOTOPE MAPPING OF THE PERI-GONDWANAN TERRANES IN THE APPALACHIAN OROGEN: AN INSIGHT INTO ROCK-WATER INTERACTION AND TECTONICS

The pre-accretion history of peri-Gondwanan terranes in the Appalachian orogen can control certain geochemical characteristics of individual terranes components. In the best case, these traits can be used to trace the relative positioning of individual terranes along the Gondwanan margin and to determine the timing of their rift-drift history and subsequent accretion to Laurentia. Ganderia, Avalonia and Carolinia form a sizeable fraction of the Appalachians and they share many gross similarities in their pre-Paleozoic magmatic histories; consequently they were initially interpreted to constitute a single, coherent Neoproterozoic microcontinent. More recent detailed fieldwork, precise radiometric dating and further geochemical analysis demonstrate that these terranes have distinctly different crust-building, rifting, and collisional histories.

We have used whole-rock and mineral oxygen-isotope compositions to obtain unique information on the source characteristics and regional hydrothermal processes that affected each of these terranes. Previous oxygen-isotope studies showed the Neoproterozoic rocks in West Avalonia to be significantly depleted of ¹⁸O (as low as –3‰). These δ¹⁸O results have been attributed to regional-scale hydrothermal meteoric alteration during late-Neoproterozoic rifting and extension (~590-550Ma). No signs of regional ¹⁸O-depletion have been observed in Ganderia. New oxygen-isotope data for Carolinia, however, reveal significant ¹⁸O-depletion in the 630-615 Ma Hyco magmatic arc (+0.4 to +5.4‰). That said, the lower δ¹⁸O_WR values of the Hyco magmatic arc are succeeded by normal-high δ¹⁸O_WR values in the younger ~545-540 Ma Albermarle magmatic arc (+8.6 to +11.4‰), a transition not observed in West Avalonia. Different processes must have been active in Carolinia at the time that West Avalonia underwent extensive rock-water interaction and alteration.