A REVIEW OF THE PRE-ACCRETIONARY HISTORY OF COASTAL MAINE PERI-GONDWANAN TERRANES

Several pre-Silurian lithotectonic terranes of peri-Gondwanan affinity are found interspersed along the length of the Maine coast. The rocks that comprise these individual terranes formed over hundreds of millions of years in a variety of tectonic settings well outboard of the Laurentian continental margin. They were subsequently accreted onto eastern North America as the Iapetus and Rheic Ocean basins progressively closed during Late Ordovician-Early Devonian orogenic events. The rocks within these terranes in Maine provide a piecemeal view of the evolution of the Ganderian microcontinent from near the time of its inception in Neoproterozoic time to its eventual accretion with the Laurentian margin in the Silurian.

The Islesboro-Rockport belt includes a poorly understood sequence of pre-670 Ma retrograded amphibolite facies metamorphic rocks that are unconformably overlain by a Late Proterozoic to Cambrian platform sequence. These are the oldest known rocks in Maine and may correlate with portions of the Brookville terrane in southeastern New Brunswick and thus represent Ganderian basement and an overlying passive margin sequence that developed off the Gondwanan margin. The younger Ellsworth-Castine-North Haven composite belt includes abundant Middle Cambrian volcanic rocks with rift-related trace element geochemical signatures. These rocks likely reflect the initial opening of the Rheic Ocean between Ganderia and the Gondwanan margin. Slightly younger (Late Cambrian) volcanic rocks within the St. Croix belt represent magmatism associated with the opening and closing of a backarc basin within the Ganderian microcontinent (Penobscot orogeny). Finally, Middle to Late Ordovician rocks of the Casco Bay/Falmouth-Brunswick sequence represent a southern extension of the Tetagouche-Exploits backarc basin that was located near the leading edge of Ganderia prior to its collision with Laurentian during the Silurian Salinic orogeny.

Several enigmatic pre-Silurian belts also exist within this region, including the Ordovician Benner Hill Sequence, the composite Jam Brook Complex of likely Neoproterozoic to Ordovician age, and the Passagassawakeag Gneiss Complex of uncertain protolith age. How these rocks fit into the history of the Ganderian microcontinent is unknown and they beg for additional study.