Late Precambrian (Hadrynian) ash-flow tuffs and associated rocks of the Harbour Main Group near Colliers, Avalon Peninsula, S. E. Newfoundland

Nixon, Graham T. (Graham Tom) (1974) Late Precambrian (Hadrynian) ash-flow tuffs and associated rocks of the Harbour Main Group near Colliers, Avalon Peninsula, S. E. Newfoundland. Masters thesis, Memorial University of Newfoundland.

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Abstract

Late Precambrian (Hadrynian) volcanic rocks of the western part of the Harbour Main Group near Colliers comprise a thick succession of chiefly rhyolitic ash-flow tuffs overlain by basaltic lavas and intruded by porphyritic sills of intermediate composition. The tuffs form three distinct ash-flow sequences, each composed of a number of simple cooling units. The Bacon Cove sequence at the base of the section, the Weavers Hill sequence in the middle, and the Finn Hill sequence at the top, are each characterised by contrasting phenocryst assemblages consisting of albite + quartz + biotite, albite + quartz, and albite + minor quartz and clinopyroxene, respectively. Concentration of crystals and xenoliths near the base of individual cooling units is a direct result of gravitative settling of the suspended load during ash-flow emplacement together with concomitant winnowing of the fine ash fraction. The majority of the Colliers ash-flew tuffs probably represent the distal deposits of a much larger ash-flow field originally situated to the north of the map area. -- Subsequent devitrification and hydration of volcanic glass have preserved vitroclastic textures with remarkable fidelity. The devitrification textures so formed locally resemble structures produced during the experimental devitrification of natural rhyolite glass. -- Mild structural deformation dated as Late Hadrynian and post-Cambro-Ordovician involved block-faulting and two distinct periods of folding and weak penetrative deformation. Metasomatic alteration largely preceded a regional metamorphism of prehnite-pumpellyite grade which may have accompanied Late Precanbrian folding. -- Electron microprobe analysis of feldspar compositions throughout the complete range of Harbour Main volcanic rocks revealed predominantly albite and K-feldspar of metamorphic origin, and rare igneous anorthoclase and calcic plagioclase. -- The chemical effects of alteration processes on the whole-rock analyses have been examined quantitatively where possible prior to classification of magmatic trends and affinities. Localised metasomatism involved mobility of essentially Na₂O, K₂O, CaO, Rb, and Ba. The least mobile constituents are Al₂O₃, TiO₂, total Fe, Zr, Cr, and Ni. -- The silicic ash-flow tuffs and granitoid rocks of the Harbour Main Group are very similar in composition and characterised by high total alkalies, Ba, and K/Rb, and low CaO and Rb/Sr. Ba/Sr and Rb/Sr ratios indicate that plagioclase was precipitated early in the differentiation history of rhyolitic magmas and that biotite was a comparatively late phase to arrive on the Liquidus. The porphyritic sills or "porphyrites" have relatively high K₂O, Ba, and Rb, and low CaO and Sr, and are very similar in composition to continental interior "andesites". The basaltic rocks are "transitional" to "mildly alkaline" chemical types with typically high Al₂O₃ and low TiO₂. The main rock-types of the Harbour Main Group cannot be related simply by fractional crystallisation of a parental silicate melt. -- The accumulated chemical data and prominent bimodal association strongly suggest that the Harbour Main voltanic suite was emplaced in an environment characterised by rifting and distension of continental lithosphere. It is suggested that the ash-flow tuffs and porphyrites originated by partial fusion of the lower continental crust and that the basaltic rocks were derived by partial melting of an upper mantle source region.

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