https://theses.otagogeology.org.nz/files/original/cd18af8da48980ebf5fc45780e4ecac7.pdf 0863272a18403846a2244fb956817da6 Dublin Core The Dublin Core metadata element set is common to all Omeka records, including items, files, and collections. For more information see, http://dublincore.org/documents/dces/. Title A name given to the resource Geology theses OU Geology thesis Thesis or dissertation completed by University of Otago Geology students Author last name Last name of the Author Johnstone Project type Is it an MSc, PhD, BSc(Hons) or PGDipSci? MSc Advisers Who supervised/advised this student Reay, A. Abstract The Abstract for this thesis Ata is the most southerly island in a chain of young volcanic islands in the Tonga Group lying to the west of the Tonga trench. It is presently inactive and now consists of a small remnant (1.8 km2) of a large composite volcano which had at least four main centres of eruption. The lavas are mainly porphyritic basaltic andesites, with minor phenocryst - poor basalts, and are dominated by phenocrysts of calcic plagioclase. Clinopyroxene, mainly augite,is also a significant phenocryst phase and orthopyroxene, olivine and microphenocrysts of titanomagnetite occur in small amounts. The groundmass textures are intergranular and hyalo-ophitic and consist of plagioclase, augite, pigeonite, titanomagnetite, cristobalite, devitrification products after glass, brown glass, and two coexisting glasses which represent immiscible silicate liquids. Analyses of the coexisting glasses show that they are very similar to coexisting glasses found in lunar basalts. Some very interesting blocks of medium and coarsegrained, mafic to silicic rocks and some small andesite pumice blocks occur in a block breccia. The basalts and basaltic andesites have a very restricted range of compositions (50-55% Sio2 ) with a pumice block sample being the most siliceous (62.25% Si02). Chemically the rocks are similar to rocks from the other Tonga Islands and are more similar to those from the Kermadec Islands. Most of the chemical features are indicative of the island arc tholeiitic series. Although there is only a mild iron enrichment and the K/Rb ratios are more like those from calc-alkaline series rocks. Low pressure crystallisation ( < 2-3 kb) prior to eruption is suggested by the mineralogy and mineral chemistry. Crystal fractionation, which produced the variation in the rock compositions, and crystal accumulation, to form the cumulate rocks, are also considered to have occured at this stage. The p H20 was low, probably less than saturated, and the fo2 is likely to have been close to, or slightly higher than, values shown by the quartz-fayalite-magnetite buffer curve, The evidence suggests plagioclase was the liquidus phase and crystallised at temperatures of about ll50-ll900C with pyroxene crystallising at 20-JQOC lower. The chemical evidence indicates a mantle origin for the magmas but the mineralogical evidence suggests a high temperature, very low p H2o magma, and thus a model of anhydrous melting of subducted oceanic crust followed by intermixing and equilibration with mantle peridotite is prefered. Department The department where the student is studying primarily. Geology Named locality Named locality describing the field area location. Ata Tonga Thesis description Number of pages, maps, CDs, etc. 187 p. : ill. (some col.), maps ; 30 cm. Dublin Core The Dublin Core metadata element set is common to all Omeka records, including items, files, and collections. For more information see, http://dublincore.org/documents/dces/. Identifier An unambiguous reference to the resource within a given context 1980Johnstone Creator An entity primarily responsible for making the resource Johnstone, Roy David. Date A point or period of time associated with an event in the lifecycle of the resource 1980 Title A name given to the resource Geology, mineralogy, petrology and geochemistry of the volcanic island of 'Ata, Tonga. Subject The topic of the resource Map Geochemistry Igneous petrology Mineralogy basaltic andesite calcic plagioclase