Pre-Cenozoic geology of Lake Hauroko, Southeast Fiordland, New Zealand

Ladley, Edwyn Barry, 1973-

This thesis describes the pre-Cenozoic geology of Lake Hauroko, southeastern Fiordland, New Zealand, as observed from the detailed mapping of approximately 50 km of lake shore, and study of petrographic, structural, major and trace element geochernistry, and hand specimen magnetic susceptibility. The country rocks in the area are retrogressed amphibolite fades metasediments, classified as the Princess Formation, within which two lithological fades are differentiated. The eastern Caroline fades is characterised by coarse to medium-grained, quartz-rich (averaging 80%) paragneisses, which are gradational into more feldspathic horizons. The western Waterfall fades is predominantly fine-grained, quartzofeldspathic gneisses to welllaminated schists that have quartz contents averaging 35%. Subordinate metasediments in the area include calcic-gneisses, metabasites, and marble, with the latter only present in the Waterfall fades. Lithologicallayering is likely to reflect relic depositional features, and is parallelled by an apparent foliation. The Princess Formation is correlated to the Western Province Goodyear Terrane of Fiordland, and the Takaka Terrane in NW Nelson. The Caroline fades is tentatively correlated to the Silurian Hailes Quartzite of NW Nelson. Three igneous suites intrude the metasediments. Intrusion of all suites appears to have been isogradic with the host metasediments: 1) The earliest igneous suite informally named the 'Western Province intrusives' consists primarily of the c. 360 Ma Hauroko Granite, intruded syn-plutonically by mafic and net-veined composite diorite-trondhjernite dykes. A range of marginal morphologies for the dykes attests to their intrusion throughout the crystallisation of the Hauroko Granite. Correlation is made to the Straight River Granite (Oliver, 1980) in western Fiordland. The exhibition of both I- and S-type characteristics by the Hauroko Granite is likely to reflect the partial assimilation of sedimentary crust upon intrusion of a mafic magma above a subduction zone. Fractional melting of a common parent is the most likely origin for the coeval felsic-mafic portions of the composite dykes. 2) The second suite occurs in the east of the Lake, and consists of a series of four Jurassic to Early Cretaceous, calc-alkaline, Cordilleran I-type granitoid to dioritoid orthogneissic units: the Albert Edward Granite, Mary Orthogneiss, Ardeer Orthogneiss, and Bunty Orthogneiss. Derivation of these rocks from hydrous melting of mantle above a subducting oceanic plate is inferred. This suite is correlated to the Median Tectonic Zone- a N-trending, <100 km wide belt along the eastern edge of the Western Province in Nelson, Fiordland, and Stewart Island, which is composed predominantly of early Mesozoic to Early Cretaceous, and minor Carboniferous, plutonics and scattered volcanogenic sediments. Intrusive relationships between the Bunty Orthogneiss and the Caroline fades suggest that the MTZ arc was constructed on the paleo-Pacific Gondwana margin. 3) A third suite of leucocratic, generally undeformed, granitoids which is correlated to the Early Cretaceous Separation Point Suite, intrudes rafted Waterfall fades rocks lit par lit style. High Na20, Al20 3, Sr, and low Y contents reflect melting of a garnet-bearing, essentially plagioclase free, mafic underplate beneath an overthickened crust. That these lithologies were the last evolutionary step in the Mesozoic MTZ arc magrnatism is suggested by recent modelling (eg. Muir et al., 1998) that allows their generation if mafic rocks equivalent to those exposed in the Median Tectonic Zone where the mafic underplate. Three metamorphic and six deformational events are differentiated at Lake Hauroko. The first metamorphism at amphibolite fades, coincided with the first deformation, generating a foliation in the metasediments parallel to the lithologicallayering. This occurred prior to the intrusion of the Hauroko Granite in the mid-Paleozoic. The second deformation affected only the western-most Waterfall fades rocks. The third deformation, accompanied by the second metamorphism at amphibolite fades, occurred in the Early Cretaceous producing a foliation in the MTZ orthogneisses, and most likely overprinted the Paleozoic foliations of the Caroline fades. After intrusion of the Separation Point Suite granitoids compression tectonics developed highly partitioned steeply eastward dipping ductile strain zones, with movement of top to the NW. The largest strain zone, the ~ 1 km wide Grebe Shear Zone, forms the Waterfall-Caroline fades boundary. The third metamorphism crystallised porphyroblastic muscovites in the Hauroko Granite, metasediments, and western SPS localities. High angle N-S trending faults resulted from the fifth deformation, and were most likely reactivated by compression from the Cenozoic Kaikoura Orogeny, the sixth deformation, which caused tilting of the pre-Cenozoic structures in the Western Province and MTZ orthogneisses.

xvi, 338 leaves : ill. (some col.), maps (some col., one folded) ; 30 cm.

1999Ladley

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