|Title:||Geodynamic evolution of the central Appalachian orogen: geochronology and compositional diversity of magmatism from Ordovician through Devonian|
|Authors:||K. Sinha, W. A. Thomas, R. D. Hatcher, and T. M. Harrison|
|Publication:||AJS, v. 312, p. 907‐966.|
The spatial and temporal distributions of igneous rocks provide significant limits on the geodynamic and thermal record associated with the growth of continents through different tectonic processes. In the central Appalachian orogen, both published and new ion microprobe U/Pb zircon ages coupled with geochemical data from igneous rocks provide a window into the thermal and temporal evolution of the mid‐Ordovician Taconic orogeny, as well as younger subduction, delamination and accretionary events. New Ion microprobe U/Pb ages of zircons screened for inheritance and lead loss, yield five discrete groupings of igneous activity: (1) pre‐collision arc stage: 470 to 489 Ma (2) syntectonic: 459 to 472 Ma (3) arc magmatism after change in subduction polarity: 441 to 459 Ma (4) delamination induced extension related plutons: 423 to 438 Ma and (5) Neo‐Acadian plutons related to collision of AvalonCarolina superterrane with Laurentian elements: 362 to 381 Ma. Geochemical signature of mafic and felsic rocks of all age groups are dominated by arc‐type attributes for primitive mantle normalized signatures for depletion in Nb, P, and Ti, and enrichment in Pb. Differences in alkalinity and a well‐developed positive gadolinium anomaly for Group III rocks is interpreted to be the result of remelting of high to intermediate potassic basalts and their plutonic equivalents during the development of a late Ordovician calc‐alkaline arc draped over an earlier (Group I) low‐K tholeiitic arc assemblage. We utilize these magmatic events, including their geochemical signatures to suggest a tectonic model which includes: (1) arc‐continent collision followed by change in subduction polarity, and (2) development of a second arc followed by slab delamination resulting in extension, as well as post‐closure shortening related to docking of peri‐Gondwanan tracts. The proposed model enables a more robust coupling of the opening and closing of the Iapetus ocean basin with the preserved igneous record, but we recognize that lack of detailed mapping over the area of study, accompanied by modern geochemical/geochronologic data to place igneous rocks in a regional deformation and metamorphic framework, precludes a complete endorsement of our preferred model.