SM - ND ISOTOPIC GEOCHEMISTRY OF PRECAMBRIAN TO PALEOZOIC GRANITOID SUITES AND THE DEEP-CRUSTAL STRUCTURE OF THE SOUTHEAST MARGIN OF THE NEWFOUNDLAND APPALACHIANS

In the Eastern Central Mobile Belt of the Newfoundland Appalachians, l ate Precambrian basement inliers have epsilon(Nd) from -3 to +2, but C ambro-Ordovician metasedimentary rocks have initial epsilon(Nd) below -7. This region is inferred to have an ''inverted'' crustal residence structure, which influenced subsequent Appalachian-cycle magmatism. Or dovician and Silurian granitoid suites have epsilon(Nd) of -8 to -2, b racketing both basement and cover, but peraluminous, ''S-type'' granit es have the lowest epsilon(Nd) Devonian granites have initial epsilon( Nd) values from -5 to +1, and low epsilon(Nd) is associated with peral uminous character. These Paleozoic granites show geographic trends, wi th lowest epsilon(Nd) values in areas where metasedimentary rocks are abundant. They are suggested to contain anatectic material from both P recambrian basement and metasedimentary cover, but some ''I-type'' sui tes probably also include a mantle-derived component. In the adjacent Avalon Zone, Precambrian plutonic suites mostly have epsilon(Nd) from +1 to +6, but there are negative epsilon(Nd) values (-8 to -4) in the westernmost Avalon Zone. Devonian plutonic suites mostly have epsilon( Nd) from +2 to +5. Thus, the Precambrian crust of the Avalon Zone is l argely ''juvenile,'' except at its westernmost edge. Contrasts across the Eastern Central Mobile Belt - Avalon Zone boundary, defined by the Dover - Hermitage Bay fault system, indicate a major, crustal-scale s tructure, and suggest an isotopically distinct ''central block'' benea th the central Appalachian Orogen, rather than a simple extension of ' 'Avalonian'' crust. Similar geographic-isotopic patterns have been rep orted in Nova Scotia and New Brunswick, suggesting that this pattern r epresents a first-order deep-crustal subdivision of the northern Appal achian Orogen.