Altering Avalonia: oxygen isotopes and terrane distinction in the Appalachian peri-Gondwanan realm

摘要

Distinct O-18 depletion is characteristic of a large majority of the 620-550 Ma felsic igneous rocks of Avalonia in the northern Appalachian orogen. Neoproterozoic rocks in the Boston Avalon terrane have the lowest delta O-18(WR) values (>=-3.1 parts per thousand), followed by the Mira terrane in Cape Breton Island and the Caledonia terrane in New Brunswick (>=-1.2 parts per thousand), the Avalon terrane in Newfoundland (>=+2.8 parts per thousand), and the Antigonish Highlands in Nova Scotia (>=+5.3 parts per thousand). In contrast, this depletion of O-18 is observed in very few of the Paleozoic felsic igneous rocks from these Avalonian terranes, and also in very few of the Neoproterozoic and Paleozoic felsic igneous rocks from the inboard Ganderian terranes. The low-O-18 character of the Neoproterozoic igneous rocks is related to regional pervasive, post-magmatic alteration by predominantly meteoric-hydrothermal fluids (delta O-18(H2O) similar to-6 parts per thousand to -4 parts per thousand) at 200-450 degrees C. The alteration likely occurred during late Neoproterozoic transtensional extension of Avalonia. Large-scale fluid infiltration and circulation within the Avalonian crust accompanied this extension with development of pull-apart sedimentary basins and extension-related magmatism that were the prelude to Cambrian submergence of Avalonia. This regional O-18 depletion provides a geochemical fingerprint by which Avalonia can be distinguished from other peri-Gondwanan terranes. These data suggest that Avalonia existed as a composite terrane on the Gondwanan margin in the Neoproterozoic, separate from Ganderia.