Potassic intrusions into the Takitimu Group, Wether Hill

Douglas, Anne, 1976-

Two suites of potassic igneous rocks intrude the Takitimu Group in the Wether Hill area. The first suite, the Wether Hill olivine monzonite, comprises a 9km long linear dyke and small stock intrusion .. The Wether Hill olivine monzonite dyke is characterised mineralogically by distinctive glomeroporphyritic of labradorite feldspar. Potassium feldspar occurs in the groundmass in equal proportions to plagioclase. The Wether Hill olivine monzonite suite is remarkably unaltered in comparison to the countryrock which it intrudes. A second suite, the Razorback monzonite, is made up of a series of dykes which are intruded at similar orientations to the Wether Hill olivine monzonite. Mineralogically this suite is characterised by alteration of plagioclase feldspar to albite, while potassium feldspar remains pristine. Olivine is absent and biotite is a major phase in the groundmass of this rock indicating that the magma was hydrous when the dykes were emplaced. The Razorback monzonite has undergone deuteric metamorphism causing it to be altered while the Wether Hill olivine monzonite has not. Geochemically both suites fall in a very narrow compositional range. They have anomalously high potassium contents (>4%) at a basaltic silica range and are part of the shoshonite association. Despite the alteration of the Razorback monzonite, its geochemistry is consistent between samples and very similar to the Wether Hill olivine monzonite dyke with which it is associated. The Takitimu Group forms the basement in the area of study. It is largely made up of volcanic arc derived volcaniclastic material, largely deposited in a marine environment by mass flows. Small penecontemporaneous intrusions occur within the sediments of the Takitimu Group. The Takitimu Group formed in an oceanic arc basin setting within a dynamic sedimentary setting where intrusive and extrusive volcanism occurred as sedimentation progressed. The Takitimu Group sediments were tilted or folded in the Late Permian prior to the deposition of the Productus Creek Group. Limestone of the Pro ductus Creek Group rests unconformably on the Takitimu Group rocks at Mt Wilanda. The time of emplacement of the shoshonitic intrusions is constrained by crosscutting relationships with the Triassic MacKinnon Peak Intrusive suite and by the unconformably overlying Early-Middle Jurassic Barretts Formation. They are believed to represent a period of stabilisation in a late stage island arc setting or in a postcollisional arc. This is supported by the erosion of a continental landmass to form the Barretts Formation in the Early-Middle Jurassic.

iii, 88 p. : ill., maps (some col.) ; 30 cm.

1997Douglas

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