Paleozoic orogens in New England, USA

Stratigraphy and isotope geochronology in the crystalline core of the Appal achians suggest revised interpretations of the extent, nature and timing of Paleozoic orogens in New England. Five major episodes of magmatism, deform ation, and high-grade regional metamorphism are recognized: Taconian (455-4 42 Ma), Acadian (423-385 Ma), Neo-Acadian (366-350 Ma), Late Pennsylvanian (300-290 Ma) and Alleghanian (280-260 Ma). In the Taconian, the passive mar gin of Laurentia was subducted below a complex magmatic are lasting from 48 0 to 442 Ma, founded in part on continental crust of a Medial New England t errane with possible affinities with Amazonia. Questions about Medial New E ngland involve its coherence as a single plate, and the nature of its under lying crust. The Acadian began in Late Silurian as a collision between the amalgamated Laurentia-Medial New England and outer belts of Composite Avalo n along a cryptic suture in coastal Maine, and progressed northwestward to the Connecticut Valley basin by mid-Devonian. Tonalitic-granitic magmatism and up to granulite-facies metamorphism culminated in Early Devonian, possi bly tied to lithospheric detachment below the subducting northwestern plate and consequent asthenosphere upwelling. Newly discovered Neo-Acadian Late Devonian to Early Mississippian tonalitic-granitic magmatism, up to granuli te-facies metamorphism, and severe deformation in central Massachusetts too k place in a plate context poorly understood. Late Pennsylvanian effects in clude magmatism, metamorphism, and deformation near south New England gneis s domes and the Sebago batholith, and development of the right-lateral Noru mbega fault system. Permian Alleghanian effects include penetrative deforma tion, granitic intrusions and up to sillimanite-grade metamorphism of Penns ylvanian beds in southeastern New England. These last two episodes relate t o the arrival of Africa.