AVALONIA: A NEOPROTEROZOIC LOW-18O TERRANE

The West Avalonian Neoproterozoic igneous rocks of Newfoundland (Avalon zone), Cape Breton Island (Mira terrane), Nova Scotia (Antigonish Highlands), New Brunswick (Caledonia terrane) and New England (Boston Peninsula) are known to be isotopically distinct from the inboard peri-Gondwanan (Ganderian) terranes that comprise the Central Mobile Belt of the Appalachians. Nd and Pb isotopic measurements, for example, have indicated a different source for Neoproterozoic and younger igneous units in Avalonia than for the equivalent units in Ganderia. However, the relative juxtaposition and timing of amalgamation of Avalonia with the other peri-Gondwanan terranes during the Neoproterozoic and early Paleozoic remain controversial. Oxygen isotope studies of Neoproterozoic felsic plutonic and volcanic units in Avalonia reveal regional ¹⁸O-depletion not observed on such a scale in the Paleozoic igneous rocks of Avalonia or in any of the igneous units from other peri-Gondwanan terranes. The δ¹⁸O_WR values for the majority of Neoproterozoic granitoid and rhyolitic units from Avalonia have the following ranges: (i) Avalon zone (NL), +2.8 to +6.3 ‰, (ii) Mira terrane, –1.2 to +6.3 ‰, (iii) Antigonish Highlands, +5.8 to +7.3 ‰, (iv) Caledonia terrane, +1.9 to +6.6 ‰, and (v) Boston Peninsula, –3.1 to +6.1 ‰ (preliminary results). These values are much lower than those of ‘normal' felsic igneous rocks (+7 to +10 ‰). We attribute this ¹⁸O-depletion to pervasive, post-magmatic hydrothermal alteration. This alteration likely occurred during the last stages of Neoproterozoic igneous activity, and was related to rifting as Avalonia separated from the Gondwanan supercontinent at ca. 560-550 Ma. The development of rift-wrench extensional basins allowed large-scale fluid infiltration and hydrothermal alteration of the Avalonian crust. As this systematic ¹⁸O-depletion is not observed in the Neoproterozoic igneous rocks of the other peri-Gondwanan terranes, its occurrence implies that Avalonia remained separate from Ganderia until at least ~550 Ma.