Abstract

Uppermost Silurian–Lower Devonian felsic rocks in the bimodal volcanic suite of the Tobique Group in the northwestern mainland Appalachians (northern New Brunswick, Canada) form part of an overstep sequence deposited across the accreted vestiges of Iapetus Ocean on composite Laurentia. Whereas the mafic rocks of the bimodal volcanic suite are continental tholeiites, the felsic rocks are peraluminous and possess geochemical characteristics of A2-type granites emplaced in post-collisional extensional settings. The major and trace element compositions of the felsic rocks indicate that they were generated by dehydration melting of late Precambrian granitoid rocks triggered by heat derived from the rising mafic magma. Unlike the basalts, which have positive ε_Nd(t) values, the felsic rocks have values close to chondrites (−1.6 to +1.1), which is consistent with derivation from a crustal source. The rapid transition from compressional to extensional magmatism in latest Silurian–Early Devonian times in this part of Ganderia is probably due to Late Silurian Ganderia–Laurentia collision followed by slab breakoff. Based on Sm–Nd isotopic characteristics in their respective igneous rocks, both Ganderia and Avalonia are underlain by similar Neoproterozoic lower–middle crust and subcontinental lithospheric mantle.