Petrochemistry of mafic rocks from the Harbour Main Group (Western Block), Conception Bay, Avalon Peninsula, Newfoundland

Cameron, Kevin J. (1986) Petrochemistry of mafic rocks from the Harbour Main Group (Western Block), Conception Bay, Avalon Peninsula, Newfoundland. Masters thesis, Memorial University of Newfoundland.

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Abstract

Volcanic rocks and associated igneous intrusions outcropping in southern Conception Bay, central Avalon Peninsula, represent the upper portion of the late Precambrian (Hadrynian) Harbour Main Group. The Group here consists of three ash-flow tuff (ignimbrite) sequences, associated terrestrial sediments, and minor intrusive rocks, overlain by a thick succession of subaerial mafic lavas. Although the rocks have undergone moderate deformation, pristine igneous textures (eg. glass shard outlines and ophitic texture) are well preserved. Metamorphism has reached only lower greenschist (prehnite-pumpellyite) facies resulting primarily in a reconstitution of most of the primary phases. Plagioclase is replaced by epidote and prehnite and olivine by serpentine, chlorite, and iddingsite while pyroxene remains as a relict phase in most of the rock units. -- The stratigraphy of the mafic sequence can be divided into three main rock units. The lower portion consists of lavas rich in clinopyroxene (augite) phenocrysts and displays subalkaline trends on various discrimination diagrams. The main part of the sequence is made up of flows containing pseudomorphed olivine and plagioclase phenocrysts in addition to plagiophyric mafic sills. The chemical affinities displayed here are subalkaline to transitional trends. The stratigraphically highest lavas outcrop just south of the main map area and are characterised by a lack of phenocrystic phases and mildly alkaline chemical trends. Variations in selected major and trace element analysis indicate that these mafic rock units may be related to one another by some process of differentiation, probably involving fractionation of olivine, clinopyroxene, and plagioclase from an alkali basalt parental magma. Careful use of discrimination diagrams and field relations of associated rock units strongly indicate that these mafic rocks were erupted in a non-orogenic tensional tectonic environment such as an area undergoing strike-slip faulting (pull-apart basin), block-faulting, or possibly a combination of both of these (ie. a transtension regime). A modern day analogue may be the Basin and Range Province of the southwestern United States.

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