Geology of the Flat Hill area, Otago, New Zealand.


Tenney, Stephen Michael.


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The Flat Hill area covers approximately 115 km2, and is located northeast of Hyde, Otago. The schist basement in this area constitutes part of the eastern portion of the Triassic to Middle Jurassic Otago schists, and is sporadically overlain unconformably by a Tertiary transgressive- regressive sequence. This sequence is in turn unconformably overlain and intruded by members of the Waipiata Volcanic Formation.
Quartzofeldspathic lithologies constitute the bulk of the schist types in the mapped area. Thin horizons of metachert interbedded with the quartzofeldspathic rocks comprise the only other distinctive lithology. Pistacite content of epidotes from schists in the present area range from Ps 19.2 to 29.6. Muscovites are all 2M polymorphs, and silica contents (SiiV cation contents 6.52-7.01) indicate that they are phengitic members of the muscovite-celadonite solid solution series. Chlorites are typically brunsvigite and ripodolite. All newly formed amphibole is actinolite, and garnet in metacherts is zoned with relatively spessartine-rich cores and increasing grossular and almandine components towards the rim. Curious micaceous minerals referred to as green and brown "biotites" occur in some quartzofeldspathic lithologies from the Flat Hill area. Optically and geochemically similar minerals were investigated in schists from Brighton, and a metachert from West Blackstone Hill. The brown "biotite" may represent a transitional, interlayered chlorite-biotite phase formed during progressive metamorphism. X-ray studies and geochemistry indicate the green "biotite" may be a unique> submicroscopically interlayered mi;neral comprised of chlorite and phengite components. In the light of these curious minerals, it remains unclear as to whether the present area and certain other parts of eastern Otago are in the chlorite or biotite zones.
The schists in the present area were subject to three synmetamorphic and one post-metamorphic folding phase generations. Phase I is correlated I with the deformation period which produced large scale recumbent folds throughout the Haast Schist terrain. No macroscopic Phase I structures ( were recognized in the present area owing to the scarcity of mesoscopic structures associated with this phase, and also because of subsequent disruption by Phase II folding. Macroscopic, antiformal Phase II folds are inferred on the basis of overturning of Phase I lineations, and symmetry and vergence of mesoscopic Phase II folds. These large structures may be refolding limbs of earlier Phase I macrostructures. Phase Ill structures are limited to mesoscopic folds and lineations, and Phase IV structures consist of fault-related kink folds and flexures.
Tertiary sediments are comprised of the marine Tiroiti Formation and the terrestrial Wedderburn Formation. The Tiroiti Formation consists of unconsolidated, variably glauconitic, bioturbated, scarcely fossiliferous, well-sorted sands and silty sands. Boulders of biocalcarenite (one algaebearing) were found, and are considered to be associated with the upper part of the Tiroiti Formation. An alternating intratidal nearshore and a deeper water, marginal marine environment is inferred for the Tiroiti Formation. A lagoonal-shoreface environment is envisaged as a likely depositional environment for the biocalcarenites. The Wedderburn Formation consists of a variety of alternating quartz sands, muddy lenses, lignites, and conglomerates ranging from very fine, well-sorted sands to gravels. The silicification of Wedderburn sediments appears to be intimately associated with overlying flow remnants. The Wedderburn Formation is thought to represent a retreating shoreline with both fluvial and shoreline processes playing important simultaneous roles in the sediment deposition.
Remnants of the Waipiata Volcanic Formation cover an area of approximately 14 km2 in the area ┬Ěstudied. Included are: ohawaiite, mafic phonelite, nepheline trachybasalt, mafic nepheline trachyandesite, nepheline hawaiite, and alkaline olivine diabase. Four new analyses from the Flat Hill area are compiled along with published analyses of roe~ from the Waipiata Volcanic Formation. The following lineages are recognized on the basis of geochemistry: mildly undersaturated, sodic lineages; moderately undersaturated, sodic lineages; relatively potassic, variably undersaturated ( lineages; and a highly undersaturated,sodic lineage." A distinctly alkali- poor olivine basalt also occurs, and may result from a unique lineage. Due to the ubiquity of lherzolite nodules in rocks of the Waipiata Volcanic Formation, they are considered to represent high pressure lineages of mantle derivation as distinguished from the more diverse Dunedin magmas which have been at least partially derived as a result of crustal level differentiation processes. Examination of Mg-values and variation diagrams for lherzolite-bearing alkaline rocks of the Waipiata Volcanic Formation has revealed examples of magmas representing both possible primary melts or slightly fractionated derivations of such liquids, and related, more highly evolved high pressure derivatives.
The mineralogy of a lherzolite nodule from the Flat Hill mafic phonolite suggests latest equilibration temperature and pressure at 870-991 ┬░C and 12-16 kilobars, respectively. These conditions should serve to establish a minimum depth of fractionation and derivation for the host mafic phonolite.
The frequent association of megacrysts with the magmas of the Waipiata Volcanic Formation is important when considering the fractionation of the different high pressure lineages which have been recognized. The presence of titaniferous ferroan paragasite megacrysts in the Flat Hill mafic phonolite implies an amphibole-dominated fractionation model for the high pressure derivation of the evolved members of the moderately undersaturated, sodic lineages. Removal of titanomagnetite may play a subordtpate role in the fractionation processes controlling the derivation of both the mildly and the moderately undersaturated,sodic lineages of the Waipiata Volcanic Formation.

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156 leaves : illus. map(in pocket) ; 30 cm.


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Tenney, Stephen Michael., “Geology of the Flat Hill area, Otago, New Zealand.,” Otago Geology Theses, accessed August 20, 2019,