Abstract
The Takitimu Group consists of Lower Permian basaltic, andesitic and rhyodacitic lava flows, primary and redeposited pyroclastic rocks and epiclastic sediments. Two suites of intrusive rocks cut the Takitimu Group strata in the central Takitimu Mountains. These are informally named here the White Hill and MacKinnon Peak Intrusive suites.
This thesis is principally a study of sedimentation, volcanism, mineralogy and petrology of the Takitimu Group, mineralogy and petrology of the intrusive rocks and low grade metamorphism.
The Takitimu Group forms the oldest beds exposed on the western limb of the Southland Syncline. The Takitimu Group crops out as a homoclinal sequence in the central Takitimu Mountains. Strata strike north-south and are partly overturned, dipping predominantly between 80°W and 85°E.
These strata are subdivided here into five formations described in ascending stratigraphic order. The Brunel Formation consists of predominantly red-brown mudstone and silt-laminite with minor intercalated tuff, litharenite, hyaloclastite and near the top of the formation thin basaltic and rhyodacitic lava flows. The Chimney Peaks Formation is made up of interbedded volcaniclastic arenites and rudites and basaltic and rhyodacitic lava flows. Basaltic lava flows, sills and pillow lavas predominate in the Heartbreak Formation. The MacLean Peaks Formation is a sequence of volcaniclastic sediments and interbedded basaltic and andesitic lava flows. The Elbow Formation consists of predominantly volcaniclastic breccias and conglomerates and subordinate andesitic and basaltic lava flows or sills, and litharenites.
Stratigraphic columns are provided for each formation and a detailed column is provided for a 1 200 m sequence in the upper MacLean Peaks Formation. Fourteen lithological groupings (or lithofacies) were recognised in the Takitimu Group within four broad categories: (1) massive and autobrecciated lava flows, (2) pyroclastic and hyaloclastic rocks, (3) epiclastic and reworked pyroclastic arenites and rudites, (4) lutites.
Lava flows form slightly less than one quarter of the 14 km sequence in the central Takitimu Mountains. Brecciated lava flows and pillow lavas are common. Individual flows rarely exceed 100 m in thickness but may be separated by as little as 2 m of sediment.
Category two consists of basic tuffs and lapilli tuffs, rhyodacitic vitric tuffs, agglomerates, hyaloclastites and massive tuff breccias.
The volcaniclastic arenites and rudites show typical sedimentary structures described from classical turbidite terrains. Normal graded- and reverse graded-arenites are recorded. The former closely resemble nonvolcaniclastic turbidites. Normal graded, reverse-to-normally graded and massive rudites are present.
Category four is subdivided into silt-laminites and red-brown mudstones. The former are principally distal turbidites, the latter are the products of 'background' sedimentation.
The lithofacies occur in six associations which are related to a model for sedimentation and volcanism in modern island-arc systems. In this model the active volcanic chain is flanked by volcaniclastic aprons on the margins of fore-arc and marginal basins. Submarine and subaerial volcanism produces thick sequences of lavas and pyroclastic rocks near the vents. Lahars, avalanches, pyroclastic flows and epiclastic mass flows transport clastic material onto the apron where it is redistributed by mass flows and particularly turbidity currents. Pyroclastic fall deposits and wedges of hyaloclastite and pillow lava also accumulate on the aprons.
The volcanic rocks form a reasonably continuous series of calcalkaline rocks (with a possible break in composition at 62-66% silica). They show weak iron-enrichment on an AFM diagram. The suite is subdivided here on the basis of silica content with the boundaries taken to coincide with major mineralogical changes. Basalts and basaltic andesites range in texture from uniformally fine-grained to porphyritic and holocrystalline to near hyaline but strongly plagioclase-phyric basalts predominate. Basalts contain combinations of plagioclase, diopsitic augite, olivine, magnetite and volcanic glass. Andesitic lavas are typically weakly porphyritic and contain phenocrysts of plagioclase, augite, hypersthene and rarely magnetite in a groundmass of plagioclase, augite, pigeonite, magnetite and occasionally volcanic glass. A single hornblende andesite is recorded from the top of the Elbow Formation. Rhyodacites contain plagioclase phenocrysts set in intergranular quartz, alkali feldspar, magnetite and ilmenite.
Twenty-five new chemical analyses are presented here as are representative full and partial analyses of plagioclase and pyroxenes and full analyses of magnetites. The Takitimu Group shows a number of features typical of calcalkaline suites namely it is high in alumina, low in titania, shows little or no absolute iron enrichment and the majority of the analysed specimens are quartz-normative. Harker variation diagrams reveal Al₂O₃, FeO*, MgO, CaO and MnO decrease with increasing silica, Na₂O and K₂O increase and TiO₂ and P₂O₅ remain approximately constant. The chemistry of aphanitic and porphyritic basalts is widely different. The aphanitic rocks are characterised by high total iron and Na₂O and the porphyritic rocks by high Al₂O₃ and CaO.
Some trace element abundance in ten rocks were determined by spark source spectroscopy; other trace elements were determined by X-ray fluorescence in eleven rocks. The analysed specimens are preferentially enriched in light REE and show low abundances of nickel and chromium relative to alkaline and tholeiitic suites.
The porphyritic basalts lack positive europium anomalies suggesting they are not partly cumulus.
A crystal fractionation model is presented for basaltic to andesitic lavas. The chemistry of rhyodacitic lavas cannot be generated by removal of observed phenocryst minerals from the andesites and an independent origin is favoured for the acidic rocks.
The White Hill Intrusives consist of coarsely-crystalline predominantly concordant bodies of gabbro, diorite, basic granophyre and aplite. The suite was probably emplaced over a wide time interval from Middle Permian to Upper Triassic and may have formed from several distinct but similar magma batches. Differentiation of the sills has taken place partly in situ and partly prior to emplacement.
Sixteen new chemical analyses of specimens are presented here together with electron microprobe analyses of plagioclase, pyroxenes, magnetite, hornblende and olivine. The White Hill Intrusives show strong similarity in chemistry and mineralogy to the volcanic rocks of the Takitimu Group. The White Hill Intrusives also closely resemble basic intrusive rocks of the Longwood Complex, MacKay and Tasman Intrusives which intrude strata correlative of the Takitimu Group. The period of time in which the White Hill Intrusives were emplaced was therefore a period of extensive intrusive activity throughout the Brook Street terrain.
The MacKinnon Peak Intrusive suite is mineralogically distinct from all other intrusive rocks in the Takitimu Mountains. The suite consists of a swarm of high-alumina basaltic dykes charged with cognate xenoliths and phenocrysts of anorthite, salitic pyroxene and tschermakitic hornblende. The phenocryst mineralogy suggests these phases crystallised under conditions of high P(H₂O). Chemical analyses of four dykes and a xenolith are presented here, together with representative full and partial analyses of all mineral phases. The model for the formation of the MacKinnon Peak dykes presented in Chapter 6 postulates that crystal fractionation in a shallow-level magma chamber led to the formation of adcumulate layers which were overlain by a transitional zone of close-packed crystals with interstitial fluid. Disruption of the chamber and cumulus layers followed and the dykes were emplaced as 'crystal-mush' probably with the aid of retrogressive boiling and volatile streaming. Four hornblende concentrates from the dykes yield K/Ar cooling ages of 231-242 my.
The Takitimu Group is subdivided into two metamorphic zones. The upper zone contains zeolite facies assemblages. Analcime, albite, chlorite, epistilbite, heulandite, laumontite, natrolite, stilbite, thomsonite and yugawaralite have been recorded from this zone. Prehnite-quartz occurs in shear zones and fractures in the upper zone. The lower zone contains prehnite-pumpellyite facies assemblages. Actinolite, albite, chlorite, epidote, prehnite and pumpellyite have widespread distribution in this zone. Hematite, sphene and andradite also occur in some rocks. Chemical analyses of most of the phases listed above are presented in Chapter 7. The degree of metamorphic reconstitution is variable and is nowhere complete. Textural evidence suggests reaction of volcanic glass with entrapped seawater has occurred forming chlorite and/or montmorillonite and presumably leaving fluid enriched in SiO₂, Al₂O₃, Na₂O and CaO. Ultimately the fluid chemistry evolved to a stage where albite and hydrated calcium aluminosilicate phases formed. Contrasting assemblages in and adjacent to shear zones and in the host rocks, in the central Takitimu Mountains, are a function of either reduced P(H₂O)/P(total) or higher temperatures in the shear zones. The White Hill Intrusives were probably a major source of heat during metamorphism. The Takitimu Group probably experienced a variety of thermal gradients with an initial but short-lived period of high temperature gradients during plutonism followed by a longer period of burial under a moderate geothermal gradient.
In the final chapter a synthesis and geological history for the volcanic and associated intrusive rocks is presented and comparison made with published models for the evolution of Tertiary-Recent sequences formed in the vicinity of present day island arcs.