WHAT CAUSED THE ACADIAN OROGENY, NEW ENGLAND APPALACHIANS? A PROVENANCE STUDY OF THE CARRABASSETT FORMATION, CENTRAL MAINE BASIN

The Early Devonian Carrabassett Formation of the Central Maine Basin has major and trace element contents indicative of a continental margin provenance, not unlike the older formations of the Central Maine Basin that are thought to have been derived from Laurentian sources. However, the Carrabassett Formation contains detrital zircons that best match the ages of the peri-Gondwanan Gander and Avalon terranes. The Gander terrane was accreted to Laurentia prior to the deposition of the Carrabassett, and in many models, was basement to the sediments of the Central Maine Basin which should eliminate Gander as a source of the Carrabassett. Positive εNd values are characteristic of Avalonian plutonic rocks and many researchers think that the plutons reflect the isotopic characteristics of Avalonian basement. Gander has more negative εNd values that are a better match for the negative εNd values of the Carrabassett Formation, but Avalonian supracrustal rocks also have negative εNd values. Thus the best overall match for the Carrabassett Formation appears to be Avalon. An Avalonian provenance of these easterly derived Carrabassett Formation sediments indicates the approach of the colliding element in the Early Devonian and identifies Avalon as the cause of the Acadian orogeny. The lack of Acadian metamorphism in Avalon may be explained by the intermediate sedimentary wedge having absorbed the thermotectonic effects of the collision and by subsequent underthrusting of Avalon beneath the inboard terranes during the Alleghanian orogeny that buried those portions of Avalon involved in the Acadian orogeny. Exposed portions of Avalon represent the trailing edge of the colliding terrane.