The Geochemistry of the Waipapa terrane metabasalts
Abstract (Summary)Restricted Item. Print thesis available in the University of Auckland Library or available through Inter-Library Loan. The Waipapa terrane consists of imbricated terrigeneous clastics, red and green argillites, cherts and metabasalts. The metabasalts, red and green argillites, and cherts represent slices of ocean floor. Petrographical and geochemical differences between the metabasalts, different field associations and probable age differences, characterize three subgroups of the Waipapa terrane. The northern Waipapa terrane metabasalts are Permian in age and are dominated by transitional to alkaline basalts. The central Waipapa terrane is probably Triassic-Jurassic in age and shows a variation in metabasalt types from tholeiitic-ferrotholeiitic associations, through N-type and E-type MORB associations typical of mid-ocean ridge environments. The Puketi Forest volcanic unit encompasses a tholeiitic-ferrotholeiitic-oceanic andesite association assigned, at present, a broad Permian-Jurassic age range. The northern Waipapa terrane metabasalts were derived from predominantly within-plate sources, either as off-axis seamounts or as oceanic islands. The alkali and evolved metabasalts from Whangaroa and Russell Peninsula and the transitional metabasalts from Purerua Peninsula have been derived by low degrees of partial melting of a garnetlherzolite source, similar to that postulated to give rise to ‘mantle plumes’ or ocean island magma sources. Subsequent crystal fractionation by predominantly olivine and clinopyroxene has modified the rocks. The tholeiitic and transitional metabasalts from Whangaroa and Russell Peninsula were derived by the mixing of these enriched OIB-like sources with depleted N-type MORB sources. The Puketi Forest volcanic unit tholeiitic-ferrotholeiitic-andesitic association was derived by extensive (>25%) and variable degrees of melting of a depleted heterogeneous N-type MORB source, similar to that that gave rise to the LIL element and LREE depleted N-type MORB that occur in the Galapagos spreading centre today. Some magma mixing and within group clinopyroxene + plagioclase ± olivine fractionation has subsequently modified the rocks. The meta-andesite has been derived by plagiclase + Fe-Ti oxide fractionation. The meta-andesite is also compositionally similar to andesites occurring within the Galapagos spreading centre. The central Waipapa terrane metabasalts range from tholeiitic-ferrotholeiitic associations at Tutukaka Heads, to N- and E-type MORB at Waipu, Eyres Point, Tawharanui Peninsula, Kawau Island and Waiheke Island. The majority of metabasalts show petrographical and chemical similarities to tholeiites occurring at mid-ocean ridge spreading centres. The central Waipapa terrane metabasalts petrogenesis can be modelled along processes postulated to give rise to the diversity of rock types at mid-ocean ridges, for example, the Tutukaka Heads ferrotholeiitic metabasalts were derived by clinopyroxene and plagioclase fractionation from a N-type MORB source, whereas the E-type MORB from Tawharanui Peninsula, Kawau Island and Eyres Point were derived by the mixing of enriched source material with depleted N-type MORB source material. The various mantle source rocks that gave rise to the central Waipapa terrane metabasalts can be modelled by 20-10% partial melting of a N-type MORB source. There are significant compositional differences among the clinopyroxene between and within the rock types from each of the three Waipapa terrane metabasalt groups. The composition of the clinopyroxene is primarily related to the host magma type. However, care needs to be taken in using clinopyroxene compositions to discriminate their host magmas original tectonic setting. The Waipapa terrane metabasalts have been derived from a variety of oceanic sources. The northern Waipapa metabasalts represent seamount/ocean island setting. The Puketi Forest volcanic unit metabasalts represent a oceanic rifting setting similar to that of the present day Galapagos Rift. The central Waipapa terrane metabasalts are similar to basalts occurring predominantly at mid-oceanic ridge tectonic environments.
School Location:New Zealand
Source Type:Master's Thesis
Date of Publication:01/01/1991