THE TECTONIC ORIGINS OF DEVONIAN MAGMATISM IN NORTHEASTERN VERMONT: A SPECULATIVE MODEL

Magmatic activity associated with the Taconic orogeny in Vermont was dominated first by Shelburne Falls arc formation (~485-470 Ma), and then by development of the Bronson Hill arc slightly to the east (~455-440 Ma). Discussion persists about locations of sutures and directions of subduction. The next tectonic stage, initiated by cessation of subduction, involved splitting the arcs during opening and filling of the Connecticut Valley Trough (CVT) starting ~440-435Ma and ending ~395-390 Ma. Proposed mechanisms for extension call for delamination of subducted lithosphere, accompanied by mafic magma production by decompression melting. This magmatism is most visibly expressed by mantle-derived Standing Pond Volcanics that flooded the CVT basin in southern Vermont at the stratigraphic position marked by the close of carbonate sedimentation (~425-420 Ma). In northern Vermont those magmas have very limited expression, occurring near the same stratigraphic horizon, and their temporal and spatial distribution at depth is unconstrained.

Extension and basin filling gave way directly to deformation, regional metamorphism (~395-375 Ma) and magmatism (~390-370 Ma) as the northwest-advancing Acadian orogenic front approached its western limit. Middle Devonian magmatic rocks in Vermont occur almost exclusively in the northern half of the state where plutons with a wide range of composition (gabbro to granite) reflect hybridization due to mingling with mafic magmas, and contamination by assimilation of carbonate-rich crust. These relationships, coupled with (1) similar ages of plutons and regional metamorphism, and (2) superposition of contact aureoles with isograds concentrically arranged within the regional isograds, suggest the following model: i) the lower crust was heated through the extensional period by intraplating of mantle magmas, preparing the crust for Acadian magma production, ii) extension stopped and transpressional shortening began, with mantle-derived melts and anatectic crustal products producing hybridized Acadian magmas, iii) transpression facilitated magma flow into the thickened crustal sediments where it contributed heat and promoted regional metamorphism, and iv) plutons locally domed the regional isograd surfaces to produce the pattern of metamorphism now exposed by erosion.