NATURE AND EVOLUTION OF THE EASTERN MARGIN OF LAPETUS - GEOCHEMICAL AND ISOTOPIC CONSTRAINTS FROM SILURO DEVONIAN GRANITOID PLUTONS IN THE NEW-BRUNSWICK APPALACHIANS

In the Gander Zone, Ordovician to Devonian, metaluminous to slightly p eraluminous plutons, with within-plate affinities, intrude a lower Pal eozoic metasedimentary package. During the Siluro-Devonian, epsilon(Nd )(T) and delta(18)O values of the metasedimentary rocks would have bee n about -7 to -8 and +10 to +14 parts per thousand, respectively. Gran itoid epsilon(Nd)(T) values range from -0.3 to -4.2, delta(18)O values lie between +6.6 and +10.4 parts per thousand, and published Pb isoto pic compositions are radiogenic (Pb-207/Pb-204 > 15.65). epsilon(Nd)(T ) values of associated Silurian gabbros are somewhat positive (+1.2 to +2.3); some plutons exhibit evidence for comingling with this mafic c omponent. Potential source materials include enriched-mantle, supracru stal, and infracrustal sources. Although some constraints can be put o n the first two sources, the nature of infracrustal material is diffic ult to evaluate. Heterogeneous data obtained from Silurian plutons can be explained by crust-mantle mixing. If some plutons contain a signif icant mantle component, then Mesoproterozoic or older crust is require d. However, mixing is potentially complex and significant Neoproterozo ic contributions cannot be excluded. The more compositionally homogene ous Devonian plutons probably represent partial melts of hybridized lo wer crust. The crustal component is interpreted as reflecting the seis mically defined Central Crustal Block. The Gander Zone was probably de posited on Avalonian Brookville belt basement when it formed part of t he Amazonian craton. Gander Zone elastic sediments, which are characte rized by pre-Neoproterozoic isotopic signatures, were derived from thi s composite Gondwanan craton. We attribute Siluro-Devonian plutonism t o collision and lithospheric delamination which accompanied, and follo wed, accretion of Avalonian belts to Laurentia during its collision wi th Gondwana.