Nature and evolution of the eastern margin of lapetus: geochemical and isotopic constraints from Siluro-Devonian granitoid plutons in the New Brunswick Appalachians

Abstract

In the Gander Zone, Ordovician to Devonian, metaluminous to slightly peraluminous plutons, with within-plate affinities, intrude a lower Paleozoic metasedimentary package. During the Siluro-Devonian, εNd(T) and δ18O values of the metasedimentary rocks would have been about −7 to −8 and +10 to +14[Formula: see text], respectively. Granitoid εNd(T) values range from −0.3 to −4.2, δ18O values lie between +6.6 and +10.4[Formula: see text], and published Pb isotopic compositions are radiogenic (207Pb/204Pb > 15.65). εNd(T) values of associated Silurian gabbros are somewhat positive (+1.2 to +2.3); some plutons exhibit evidence for comingling with this mafic component. Potential source materials include enriched-mantle, supracrustal, and infracrustal sources. Although some constraints can be put on the first two sources, the nature of infracrustal material is difficult to evaluate. Heterogeneous data obtained from Silurian plutons can be explained by crust–mantle mixing. If some plutons contain a significant mantle component, then Mesoproterozoic or older crust is required. However, mixing is potentially complex and significant Neoproterozoic contributions cannot be excluded. The more compositionally homogeneous Devonian plutons probably represent partial melts of hybridized lower crust. The crustal component is interpreted as reflecting the seismically defined Central Crustal Block. The Gander Zone was probably deposited on Avalonian Brookville belt basement when it formed part of the Amazonian craton. Gander Zone clastic sediments, which are characterized by pre-Neoproterozoic isotopic signatures, were derived from this composite Gondwanan craton. We attribute Siluro-Devonian plutonism to collision and lithospheric delamination which accompanied, and followed, accretion of Avalonian belts to Laurentia during its collision with Gondwana.