A study of the igneous intrusive rocks of the Dunnage Melange, Newfoundland

The Dunnage Melange, located in the central mobile belt of the Appalachian Orogen, in north-central Newfoundland, is host to several suites of igneous intrusions. These include mafic through felsic dikes, stocks, and batholiths ranging in age from Early Ordovician through Jurassic. -- The tectonic environment and mode of origin of the Dunnage Melange, as well as the origin and significance of its intrusions, have long been a subject of controversy. The purpose of this study was to examine the field relationships between the intrusions and the host melange, and the geochemistry and petrography of the intrusions, in the context of this controversy. -- It was concluded that the melange and its intrusions were part of a complex and dynamic igneous, sedimentary and tectonic system, the local history of which was characterized by penecontemporaneous sedimentation, block faulting, olistostrome deposition, intrusion, and sediment slumping and sliding in Ordovician time. The earliest intrusions are mafic tholeiites with a chemistry indicative of a tensional environment, interpreted to be related to back-arc basinal rifting. The Dunnage Formation was then intruded, penecontemporaneously with sedimentation and melange formation, by a suite of silicic rocks (the Coaker Porphyry) that have a sedimentary source. These intrusions were followed closely in time by a calc-alkaline suite that was derived from an igneous or mantle source. On a regional scale, melange formation and intrusion of the Coaker Porphyry are interpreted to have taken place in a back-arc setting during the Early Ordovician subduction of the leading edge of the continental margin of North America, with partial melting of sediments from the continental margin giving rise to large quantities of silicic magma that permeated the overlying mantle wedge. Later calc-alkaline magmas were derived from the partial melting of this contaminated mantle