Abstract: Petrogenesis, age and economic potential of gabbroic intrusions in southern New Brunswick and southeastern Cape Breton Island

The Mechanic Settlement and Duck Lake plutons in southern New Brunswick are layered intrusions that range in composition from olivine pyroxenite to diorite; the smaller Devine Corner pluton is composed entirely of gabbro. The relationship of the Hamilton Lake diorite exposed southwest of the main body of the Mechanic Settlement pluton is not clear due to poor exposure. However, the diorite may compose the upper part of the pluton. Layering in the Mechanic Settlement pluton trends northeast and dips southeast and consists of mineral layering on the hand-specimen scale and interlayering of rock types on outcrop scale. Cumulate textures are well deveJoped in most samples in both the Mechanic Settlement and Duck Lake plutons with cumulate phases including spinet (only in Duck Lake), chromite, olivine, clinopyroxene, orthopyroxene, and plagioclase. Abundance of hydrous minerals indicates that crystallization probably took place under relatively high water pressure. Majorand trace-element data show that olivine and pyroxene dominated the crystallization sequence and that the plutons are tholeiitic. Gabbroic rocks of the St. Peters area in Cape BrelDn Island vary in composition from olivine gabbro to gabbro ID hornblende-bearing gabbro. Other gabbroic plutons in southeastern Cape Breton Island occur near Baleine, Cape Breton, Louisbourg, Black Rock, and Gabarus. They consist of clinopyroxene and plagioclase with minor amphibole, biotite, quartz, apatite, and magnetite. Olivine occurs only in the St. Peters gabbros. Majorand trace-element data indicate that the St. Peters gabbros ar~ alkaline and formed in a withinplate setting. In contrast, the other gabbros are tholeiitic but their tectonic setting is uncertain. The age (or ages) of these plutons is still uncertain but may be Devonian.