Petrology of the volcanic rocks of the Whalesback area, Springdale Peninsula, Newfoundland

Fleming, J. M. (1970) Petrology of the volcanic rocks of the Whalesback area, Springdale Peninsula, Newfoundland. Masters thesis, Memorial University of Newfoundland.

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Abstract

Detailed mapping in the vicinity of the Whalesback Mine has established the feasibility of consistently distinguishing two types of metabasalts in a poorly exposed and rather montonous sequence of mainly pillowed volcanics, part of the Lower Ordovician Lush's Bight Group. The two types, called the 'Whalesback' and 'St. Patrick' volcanics, each have a mineral assemblage characteristic of the greenschist facies - albite, epidote, chlorite, actinolite and leucoxene - but are distinguished in the field by their relative abundances of epidote and chlorite, respectively. Intercalated with the flows are discontinuous lenses of pyroclastic rocks. Gabbroic equivalents of the flows are present in dykes, sills and small stocks. The whole sequence is cut by a set of feldspar, amphibole-feldspar, and pyroxene porphyry dykes. -- The poor exposure and the general paucity of primary stratification features make structural analysis very difficult. There is, however, some petrologic indication that the St. Patrick volcanics are stratigraphically above the Whalesback volcanics and the general outcrop pattern suggests an overall anticlinal structure. The area is cut by two major, northeasterly-trending faults, the Davis Pond and Little Deer Pond faults. Minor faulting and shearing has affected all parts of the area but is particularly prevalent in the north block which is also characterized by a relative abundance of porphyritic dykes. -- The Whalesback and Little Deer sulfide orebodies are contained in adjacent, east-west trending shear zones, to the north of and subsidiary to the Little Deer Pond fault. The host rocks are chlorite and chlorite-sericite schists which were derived, at least in part, from pyroclastic rocks. -- A detailed petrographic and chemical study was carried out, concentrating mainly on the flow rocks. Seventeen complete and ten partial chemical analyses were obtained. These show that the Whalesback and St. Patrick volcanics are tholeiitic basalts and basaltic andesites, respectively, that have been affected by metasomatism, mainly addition of soda. The two flow types appear to form part of a differentiation series that is characterized by consistently low potash suggesting a correlation with oceanic tholeiites.

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