Group D: Little Port Island Arc Complex
GeoSite Group D includes the Little Port Island Arc Complex of unseparated plutonic and volcanic rocks that form narrow northeast-trending coastal slices between Fox Island River and Bonne Bay. The Complex overlies the Humber Arm Supergroup in most places, but between Trout River and Bonne Bay it overlies the Skinner Cove and Old Man Cove formations. It is separated by the Bay of Islands Complex by a wide valley, except at Lewis Hills where deformed gabbros, peridotites and tonalite gneisses of the Mount Barren Complex occur between the Little Port Complex to the west and the Bay of lslands Complex to the east, all part of the Lewis Hills massif. Generally considered an island arc, the only remaining islands of the Complex are found in the outer Bay of Islands, and include Guernsey, Tweed and Pearl Islands.
Rock types are late Cambrian to middle Orodvician, with the oldest being foliated layered gabbros, amphibolites and minor peridotites. Quartz diorites or tonalites cut deformed amphibolites and produce intrusion breccia on Big Island of the Bay of Islands. The granitic rocks vary from massive to well-foliated and occur as northeast-trending bodies that parallel the form of the Little Port structural slices. On the south side of the Bay of Islands, the succession of Little Port slices trends northwest and the long dimension of granitic bodies and main foliations in gabbroic rocks all trend in the same northwest direction.
Massive mafic dykes cut foliated gabbros, amphibolites and granitic rocks and the dykes are inseparable from mafic volcanic rocks that are also part of the complex. The dykes trend northeast in most places and form thin sheeted sets that separate deformed plutonic rocks from nearby, relatively un deformed volcanic rocks. The dykes are almost everywhere brecciated Volcanic rocks of the Little Port Complex are mainly green and red mafic pillow lavas and pillow breccias. Porphyritic dacite and grey silicic flows occur within the northern slice between Chimney Cove and Bonne Bay. Volcanic boulder conglomerate and sandstone are included in the complex at Little Port. Locally, the volcanic rocks contain prehnite, pumpellyite and analcime.
GeoSite Group D has the components of an ophiolite suite. However, the proportions of its rock types, especially an abundance of tonalite, and intricate internal structures, contrast with the orderly succession and proportions of pristine units in the nearby GeoSite Group C, Bay of Islands Ophiolite Complex. The group records the development of an island arc created by tectonic subduction during the closing of the Iapetus Ocean.