GEOLOGY AND TECTONICS OF PERI-GONDWANAN TERRANES IN COASTAL MAINE

Pre-Silurian terranes of non-Laurentian affinity are found along the eastern margin of the northern Appalachians and preserve a complex history of tectonic evolution prior to their accretion with Laurentia. Coastal Maine is particularly well endowed as a number of distinct peri-Gondwanan belts ranging in age from Late Proterozoic to Late Ordovician are exposed. All of these fault-bounded terranes lie inboard of the peri-Gondwanan New River, Kingston and Brookville terranes in southeastern New Brunswick. This abstract will focus on the more extensive of the peri-Gondwanan terranes in the Penobscot Bay region of Maine and the discussion will proceed in chronological order.

In Penobscot Bay, two Late Proterozoic platformal sequences are exposed in the Islesboro block. The Seven Hundred Acre Island Fm. (~ 650 Ma minimum age) consists of metamorphosed platform sediments and minor basaltic rocks that have Pb isotopic compositions similar to peri-Gondwanan basement rocks in Atlantic Canada. The younger Islesboro Fm. (perhaps as young as Cambrian) is a sequence of shallow water metasedimentary rocks most likely originally deposited unconformably on the Seven Hundred Acre Island Fm.

East of the Islesboro block lies an extensive belt of Middle to Late Cambrian bimodal metavolcanic and associated metasedimentary rocks collectively referred to as the Ellsworth terrane. Composed largely of the Ellsworth and Castine Formations, rocks within this terrane have geochemical characteristics that are most similar to an oceanic extensional setting.

The St. Croix terrane (west of the Islesboro block) of Late Cambrian to Middle Ordovician age is an extensive belt of rocks extending over 200 km from western Penobscot Bay through eastern Maine and into New Brunswick. This dominantly metasedimentary belt has been interpreted to represent slope and rise sediments deposited along the eastern margin of the Iapetus Ocean basin (Gander margin).

Finally, significantly west of the St. Croix terrane (& separated by the younger Fredericton belt) lies the Middle to Late Ordovician Liberty-Orrington belt. This belt, dominated by metavolcanic and metavolcanogenic sedimentary rocks, is correlative with the Bathurst Supergroup in the Miramichi belt of New Brunswick. Geochemistry suggests an arc to backarc setting along the Gander continental margin for these rocks.