The petrology and geochemistry of peralkaline granite and volcanic rocks near Davis Inlet, Labrador

White, Charlotte Anne (1980) The petrology and geochemistry of peralkaline granite and volcanic rocks near Davis Inlet, Labrador. Masters thesis, Memorial University of Newfoundland.

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Abstract

The Davis Inlet area is underlain by Archean gneisses and younger igneous rocks which range in composition from gabbroic to granitic. Two granites and a volcanic unit were sampled during the field season of 1978. -- Detailed petrographic and geochemical studies indicate the non-peralkaline nature of the clinopyroxene-fayalite granite and the peralkaline character of the arfvedsonite-riebeckite granite and its associated volcanics. Mineralogical characteristics such as the presence of poikolitic aegirine-augite and arfvedsonite-riebeckite with needles of riebeckite nucleating on grain boundaries, and extensive albitization reflect the peralkaline nature and intense volatile activity in the peralkaline granite. -- Chemical characteristics such as agpaitic indices (Molecular Na₂O+ K₂O/A1₂O₃), normative acmite and sodium metasilicate and trace element behaviour can be used to separate the granitic suites according to their peralkaline and non-peralkaline natures. -- The enigma of the Nuiklavick volcanic suite has been dealt with primarily on the basis of trace (including RE) element data. Extreme enrichment of Zr, Nb, Y, Th, Zn, Ni and the REE along with severely depleted Ba, Sr, Ca, and Mg are a strong indication of a very peralkaline character. Trace element trends and concentrations observed in samples examined cannot be explained by conventional differentiation models. Systematic trace element enrichment patterns indicate a high degree of continuity between the peralkaline granite and volcanic samples reflecting transport of elements to the top of the magma chamber as cationic complexes in a fluid/volatile phase. From the data observed in this study and in others it is possible to suggest that over saturated peralkaline rocks reflect intense volatile activity which produces their characteristic petrography and geochemistry.

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