Middle Devonian to Permian plutonism and volcanism in the N American Appalachians

Abstract

Summary Mid-Devonian to late Palaeozoic igneous activity in the N American Appalachians occurs in the eastern portions of the orogen. Represented radiometric ages are almost continuous from 250 to 370 Ma and older, thus overlapping Acadian and Alleghanian orogenic events. In many instances igneous activity is generally similar to that initiated earlier in the Palaeozoic. Plutonic rocks consist of granitoids with subordinate intermediate to mafic rocks. Plutonism is mainly of ‘Caledonian’ or alkalic variety and is dissimilar to subduction-related magmatism of the Andean type. Regional synthesis indicates that some general distinctions of plutonism occur across strike as well as along strike from S to N within the orogen in eastern N America. In the southern Appalachians granites and minor contemporaneous gabbros occur in all lithotectonic zones between the Blue Ridge and the Coastal Plain. These plutons are commonly syn-kinematic in the eastern Piedmont where Alleghanian deformation and metamorphism are best exhibited, but are late- to post-kinematic to the W. The rocks include metaluminous biotite and megacrystic granites and lesser peraluminous two-mica granites. Late Palaeozoic igneous activity is sparse to absent in the central Appalachians. In New England several kinds of mid-Devonian to Carboniferous igneous activity are represented. Biotite and biotite + muscovite granitoids dominate the central terranes, whereas the more easterly Avalon zone is intruded by alkalic-peralkalic plutonic rocks and contains minor bimodal volcanics. Final Permian plutonism includes intrusion of homogeneous metaluminous to peraluminous granite. Within the Canadian Maritimes biotite and megacrystic granites followed by mildly alkalic varieties dominate the Devonian-Carboniferous plutons of New Brunswick and Newfoundland, whereas muscovite + biotite rocks are prominent in the Meguma terrane of Nova Scotia. Volcanism, which is bimodal, is best documented in New Brunswick and northern Nova Scotia. Igneous activity of Permian age is not recognized in the Canadian Maritimes. The petrological diversity together with the considerable span of time represented by the late Palaeozoic igneous activity in the N American Appalachians indicates repeated and locally prolonged anatexis of a variety of heterogeneous crustal sources. The alkalic and bimodal suites are characteristic of extensional or anorogenic tectonics. The more aluminous rocks are generally compositionally similar to the Caledonian and, to a lesser extent, the Hercynian suites of Europe. Overall the igneous activity may reflect relatively independent rifting and accretion of small crustal plates of diverse thicknesses and compositions in which magmatism was not related to a simple subduction island-arc setting.