Mineralogy and geochemistry of the ultrabasic-basic rocks of Mt. Luxmore, Fiordland, New Zealand.
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Mapping has shown that the igneous rocks of Mt. Luxmore form a complex, roughly elliptically shaped intrusion cutting the Fiordland gneisses. Distribution of the main rock types, which range from plagioclase-bearing dunite to leuco-gabbro and norite, is irregular in detail, but outcrops of each rock type within the complex form linear, north~east striking belts, parallel to a prominent aerial photographically defined lineament. The ultramafic portion of the suite has been intruded by various hornblendebearing dykes and veins which represent injection of late-stage hydrous differentiates. The eucritic and gabbroic rocks were severely uralitized by the postmagmatic introduction of calcium-rich fluids.
Electron microprobe analyses of the primary phases indicate distinct though slightly irregular cryptic variation, and iron-enrichment trends for the olivines and pyroxenes.
Plagioclase is found in all igneous rocks and ranges in composition from (An 84-An9z) in the ultramafic-mafic assemblages to An 54 in the more leucocratic norites. Olivine shows a cryptic variation from Fo69 to Foal. The chrysolitic olivine-calcic plagioclase assemblage links plagioclase-bearing dunite on the one hand and anorthosite on the other.
Clinopyroxenes in the eucrites have moderately high Alz0 3 content; the jadeite and Ca-Tschermak's components 2 indicate crystallization at around 8-io kb. Temperature estimates using the Wood and Banno two pyroxene geothermometer (905°-982°) are lower than those for similar ultrabasic- basic igneous complexes and suggest continued equilibration during slow cooling in a regional metamorphic environment.
Amphiboles are chemically the most varied of the essential mineral constituents; they occur as primary poikilitic igneous phases, and as secondary uralitic aggregates replacing pyroxene and olivine.
Analyses of magnetite-ilmeriite pairs demonstrate that all the ilmenite is of secondary origin and temperature estimates indicate the range over which oxidation of magnetite-ulvospinel solid solution- ceased, rather than the temperature of crystallization of the igneous host rocks.
Coronas between adjacent olivine and plagioclase grains are spectacularly developed in Mt. Luxmore rocks. The presence of orthopyroxene in the inner shell adjacent to olivine, and a vermicular intergrowth of amphibole and green spinel in the outer shell adjacent to plagioclase, has been confirmed by electron microprobe analyses. It is considered that the coronas have formed during slow isobaric cooling at pressures above 6 kb .1
Whole rock analyses of representative rock types by electron microprobe suggest that all igneous rocks of the Mt. Luxmore intrusion are comagmatic, and that the parental 3. magma was sub-alkaline, high alumina in character and having an overall tholeiitic affinity.
The Mt. Luxmore intrusion lacks the structural and mineralogical characteristics of stratiform and zoned complexes and more closely resembles alpine-type intrusions, at least in tectonic setting. The mineralogical indications are that the rocks cooled slowly at a depth of approximately 20 km. They may represent the root zone of a volcano which experienced periodic injection of partly crystalline mush from depth during its history, to give the irregular appearance of the complex. At some stage before the Tertiary the igneous rocks were uplifted and then exposed on the sea bed during the Tertiary.
Mt. Luxmore is one of a series of broadly similar basic plutonic bodies which form magnetically disinguishable belts in eastern and central Fiordland. A process of crustal accretion during a prolonged and intermittent period of subduction is suggested as a possible mechanism by which these plutonic zones might have originated.
Electron microprobe analyses of the primary phases indicate distinct though slightly irregular cryptic variation, and iron-enrichment trends for the olivines and pyroxenes.
Plagioclase is found in all igneous rocks and ranges in composition from (An 84-An9z) in the ultramafic-mafic assemblages to An 54 in the more leucocratic norites. Olivine shows a cryptic variation from Fo69 to Foal. The chrysolitic olivine-calcic plagioclase assemblage links plagioclase-bearing dunite on the one hand and anorthosite on the other.
Clinopyroxenes in the eucrites have moderately high Alz0 3 content; the jadeite and Ca-Tschermak's components 2 indicate crystallization at around 8-io kb. Temperature estimates using the Wood and Banno two pyroxene geothermometer (905°-982°) are lower than those for similar ultrabasic- basic igneous complexes and suggest continued equilibration during slow cooling in a regional metamorphic environment.
Amphiboles are chemically the most varied of the essential mineral constituents; they occur as primary poikilitic igneous phases, and as secondary uralitic aggregates replacing pyroxene and olivine.
Analyses of magnetite-ilmeriite pairs demonstrate that all the ilmenite is of secondary origin and temperature estimates indicate the range over which oxidation of magnetite-ulvospinel solid solution- ceased, rather than the temperature of crystallization of the igneous host rocks.
Coronas between adjacent olivine and plagioclase grains are spectacularly developed in Mt. Luxmore rocks. The presence of orthopyroxene in the inner shell adjacent to olivine, and a vermicular intergrowth of amphibole and green spinel in the outer shell adjacent to plagioclase, has been confirmed by electron microprobe analyses. It is considered that the coronas have formed during slow isobaric cooling at pressures above 6 kb .1
Whole rock analyses of representative rock types by electron microprobe suggest that all igneous rocks of the Mt. Luxmore intrusion are comagmatic, and that the parental 3. magma was sub-alkaline, high alumina in character and having an overall tholeiitic affinity.
The Mt. Luxmore intrusion lacks the structural and mineralogical characteristics of stratiform and zoned complexes and more closely resembles alpine-type intrusions, at least in tectonic setting. The mineralogical indications are that the rocks cooled slowly at a depth of approximately 20 km. They may represent the root zone of a volcano which experienced periodic injection of partly crystalline mush from depth during its history, to give the irregular appearance of the complex. At some stage before the Tertiary the igneous rocks were uplifted and then exposed on the sea bed during the Tertiary.
Mt. Luxmore is one of a series of broadly similar basic plutonic bodies which form magnetically disinguishable belts in eastern and central Fiordland. A process of crustal accretion during a prolonged and intermittent period of subduction is suggested as a possible mechanism by which these plutonic zones might have originated.
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218 leaves : illus. (part col.), 2 maps in pocket ; 30 cm.
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1976Sise
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POLYGON ((167.570686954000053 -45.398933611999951,167.569121879000022 -45.398032611999952,167.564841430000115 -45.395568132999983,167.560881846000029 -45.393288058999985,167.565383946000111 -45.389815632999955,167.576939889000073 -45.380899862999968,167.612995018000106 -45.382905839999978,167.633587769000087 -45.384046363999971,167.634579418000044 -45.384101190999957,167.633503704000077 -45.3849494899999,167.614253964000113 -45.400123657999984,167.613339691000078 -45.400844082999981,167.603757158000121 -45.408393326999942,167.596736330000113 -45.413922624999941,167.570686954000053 -45.398933611999951))
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Sise, John Reginald., “Mineralogy and geochemistry of the ultrabasic-basic rocks of Mt. Luxmore, Fiordland, New Zealand.,” Otago Geology Theses, accessed March 23, 2025, https://theses.otagogeology.org.nz/items/show/96.