The petrochemistry of the Bull Arm Formation near Rantam Station, southeast Newfoundland

Malpas, John Graham (1971) The petrochemistry of the Bull Arm Formation near Rantam Station, southeast Newfoundland. Masters thesis, Memorial University of Newfoundland.

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Abstract

The Bull Arm Formation of late Precambrian volcanic rocks crops out on the Avalon Peninsula and on the Isthmus of Avalon, S.E. Newfoundland. An area of approximately nine square miles on the Isthmus was mapped and sampled in the summer of 1970. The area was chosen because it showed a complete cross section of rock types exposed in fresh roadside cuttings and therefore easily accessible. -- The rock types are mainly rhyolitic flows, ignimbrites and tuffs, minor late basaltic flows and scoria, and associated volcanogenic sediments. A relative lack of intermediate derivatives makes the distribution essentially bimodal. -- Features such as well preserved igneous flow structures, and the presence of ignimbrites and red sediments, suggest a subaerial environment of deposition for the volcanics, with local reworking of deposits in superficial ponds and lakes to give graded and cross-bedded sediments. -- The rocks have undergone mild deformation into open folds with associated axial planar cleavage visible in some tuffaceous and sedimentary deposits. This deformation appears to be Acadian in age and was accompanied by metamorphism to low greenschist grade. In the tuffs and some flows there is a noticeable development of epidote and zoisite as matrix alteration products. -- Metasomatic effects mask the original identity of the rocks and both chemically and mineralogically they may now be ascribed to a spilite/keratophyre suite. The development of secondary albite 'phenocrysts' visibly accompanies epidotisation in thin section. These are thought to be a product of metasomatism. In one area, this metasomatism is overprinted by a (contemporaneous?) potassium enrichment which appears to be structurally controlled. Here, rhyolites with increased potassium contents have low sodium and show a progressive development of orthoclase at the expense of albite, in the phenocrysts. The metasomatism explains the anomalously young age date of 467 ± 30 m.y. and the high Rb/Sr ratios obtained for the Bull Arm Formation by previous workers. It may also account for a local barite/galena mineralisation. -- Two lines of evidence, Zr values and bulk analyses, suggest an original calc-alkaline affinity for the rocks, but the bimodal distribution of rock types remains difficult to interpret.

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