Some aspects of the geology about the Moeraki River, South Westland

Wallace, Robert Cleland.

In the Mataketake Range in South Westland the high grade axis of the Haast Schist abuts the Alpine Fault. A eugeosynclinal suite, now represented by quartzofeldspathic schist and minor amphibolites, metacherts and pegmatite, has been intensely deformed and metamorphosed under conditions of higher grade amphibolite facies, equivalent to sillimanite zone. The amphibolites contain green hornblende and plagioclase (An30-An50), accessory garnet, epidote (Ps 15-24), calcite, potassium feldspar, sphene, opaques and in one case salitic pyroxene (Wo49En34Fs17). In the quartzofeldspathic schist the anorthite content of the plagioclase increases from An 22-26 coexisting with albite in the garnet zone to An36 in the core of the Haast Antiform. Muscovite and biotite are ubiquitous. Staurolite is reported from lower grade amphibolite facies rocks. Metachert horizons have a simple mineralogy of quartz, garnet, tourmaline with accessory rutile and apatite. In the west of the area, pegmatites have been formed by partial melting and have been injected concordantly to foliation s1.
An initial phase of deformation of the schist produced isoclinal folds. The westerly vergence of the mesoscopic structures suggests that the Mataketake Range area lies on the upper limb of a recumbent fold, closing to the north west, with a fold axis plunging moderately steeply south south-west. A second phase of folding produced broad open structures about a subvertical axial plane (Haast Antiform, Thomas Synform, Moeraki Antiform and Mount Clark Synform) with fold axes trending 216° plunging 20°S. The third phase of deformation is restricted to the development of kink folds in a zone adjacent to the Alpine Fault.
A study of the ultracataclastic rock from along the Alpine Fault which forms the western boundary of the region studied was undertaken. Here the Alpine Fault is approximately 500 metres wide and has a foliation which dips 20° south east throughout much of the fault zone. The fault zone is composed of breccias, mylonites, augen mylonites, blastomylonites, ultramylonites and a half metre wide fault pug (which probably represents cataclasis due to recent movement). The ultramylonite is usually opaque but samples were identified which contained radiating microlites (good devitrification textures) and in one case a non vesicular glass (hyalomylonite) was located.
Electron microprobe analysis of the glass indicated that it has Sio2 : Or : Ab ratio similar to that of a granite minimum melting liquid at 500-1000 bars and a comparison between the chemistry of the hyalomylonite and possible rock from which it may be derived indicates that the hyalomylonite differs fro~ these possible source rocks. The conclusion has been reached that the hyalomylonite is the product of a process of fractional melting of the rock in, or adjacent to, the Alpine Fault Zone. Based on the evidence that vesiculation has not occurred, and that there is approximately 3-5% water in the sample, it has been suggested that melting occurred at 750 C for a pressure range of 1/2-2 kb (a depth of 2-7 km).
Using these considerations and estimating that the displacement that produced melting was in the range 0.3 to 6 metres it can be shown that stresses of the order of 200 to 250 bars were involved in faulting.
Members of a basic alkali dike swarm (lamprophyres) were located in the headwaters of the Moeraki River. The dike swarm is composed of kersantites, camptonites, monchiquites, odinites, ouchitites and mica peridotite all of which have been pneumatolytically altered by late stage carbonatitic fluids. Chemical analyses of coexisting mineral phases are presented and an attempt made to show that some of the trachytic members of the dike swarm may have developed from the camptonitic or odinitic members.
One member of the dike swarm is a carbonatitic hornblende-mica peridotite which contains copious amounts of xenolithic material. The xenolithic suite comprises schist, gabbro, harzburgite and lherzolites with megacrysts of clinopyroxene. Analyses of the coexisting phases in the nodules indicates that the lherzolites and harzburgites are stable in the spinel-lherzolite field and that they would be a stable assemblage at pressures equivalent to a depth of lOOkm. The clinopyroxene megacrysts may represent a cognate xenolith from a magma which has fractionated at depth, while the gabbroic nodules represent cumulous material differentiating at shallower levels still (at the base of the crust). The schist xenoliths represent material stoped from the conduit walls as intrusion has occurred. The suggestion has been made that intrusion may be in the form of a fluidised solid-gas system, and the body may be termed a diatreme.

152 leaves : illus. ; 30 cm.

1973Wallace

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