Abstract

In the Moretons Harbour area, at the eastern end of the Lushs Bight terrane of central Newfoundland, the volcanic rocks of the "Lushs Bight Supergroup" are divided into two new groups, viz, the Moretons Harbour Group and the Chanceport Group. The former is separable into four formations, consisting primarily of variable proportions of basaltic pillow lavas and volcanoclastic sediments, with a composite thickness in excess of 6 km, or around 8 km including an extensive area of 'sheeted' diabase dikes. These formations are steeply dipping and face southwest; they are separated by the Chanceport fault from the Chanceport Group to the south. The latter consists of interbedded basaltic pillow lavas with graywackes and banded red and green cherts, all facing north and steeply dipping to overturned, with a composite thickness of approximately 3 km.The Moretons Harbour Group has been intruded by the Twillingate trondhjemitic granite–granodiorite pluton and abundant basic dikes intrude the granite, indicating that the mafic and felsic magmatism were coeval. Both have undergone intense deformation and the volcanics show a change from greenschist to amphibolite facies mineralogy within a distance of 2 km on approaching the pluton, a result of buttressing by the pluton during deformation, and not an intrusive effect.Base metal sulfides are common throughout the area, but the main occurrences of Cu, As, Sb, and Au are concentrated in the Little Harbour Formation, a 2600 m thick sequence of volcanoclastic rocks within the Moretons Harbour Group.The great thickness of volcanic rocks is interpreted as having formed in an island arc environment, although it is possible that the lowermost parts of the sequence represent oceanic crust. It is unlikely that the sheeted diabases of the Moretons Harbour area were produced by sea-floor spreading.

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