Convergent margin on southeastern Laurentia during the Mesoproterozoic: tectonic implications

A continental-margin magmatic arc is inferred to have existed on the southe astern (present coordinates) margin of Laurentia from Labrador to Texas fro m similar to 1500-1230 Ma, with part of the arc subsequently being incorpor ated into the 1190-990 Ma collisional Grenville Orogen. Outside the Grenvil le Province, where the arc is known as the Granite-Rhyolite Belt, it is und eformed, whereas within the Grenville Province it is deformed and metamorph osed. The arc comprises two igneous suites, an inboard, principally quartz monzonitic to granodioritic suite, and an outboard tonalitic to granodiorit ic suite. The quartz monzonite-granodiorite suite was largely derived from continental crust, whereas the tonalitic-granodiorite suite is calc-alkalin e and has a juvenile isotopic signature. Available evidence from the Grenvi lle Province suggests that the arc oscillated between extensional and compr essional settings several times during the Mesoproterozoic. Back-arc deposi ts of several ages, that formed during relatively brief periods of extensio n, include (1) mafic dyke swarms subparallel to the arc; (2) continental se diments, bimodal volcanics and plateau basalts; (3) marine sediments and vo lcanics formed on stretched continental crust; and (4) ocean crust in a mar ginal basin. Closure of the back-arc basins occurred during the accretionar y Pinwarian (similar to 1495-1445 Ma) and Elzevirian (similar to 1250-1190 Ma) orogenies, as well as during three pulses of crustal shortening associa ted with the 1190-990 Ma collisional Grenvillian Orogeny. During the Elzevi rian Orogeny, closure of the Central Metasedimentary Belt marginal basin in the southeastern Grenville Province was marked by subduction-related magma tism as well as by imbrication of back-arc deposits. The presence of a cont inental-margin magmatic arc on southeastern Laurentia during the Mesoproter ozoic implies that other coeval magmatism inboard from the arc took place i n a back-arc setting. Such magmatism was widespread and chemically diverse and included large volume "anorogenic" anorthosite-mangerite-charnockite-gr anite (AMCG) complexes as well as small volume alkaline, quartz-saturated a nd -undersaturated "within-plate" granitoids. Recognition of the similar to 300 million year duration of the Mesoproterozoic convergent margin of sout heastern Laurentia suggests that there may be useful parallels with the evo lution of the Andes, which has been a convergent margin since the early Pal eozoic.