They associate extensive plutonic activity with this orogeny that uplifted and metamorphosed the rocks of the New Zealand Geosyncline. A few crystalline rocks in the Voorland province north of the Alpine fault with measured ages between 340 and 120 million years have been found. In New Zealand, help in the field was offered by the students and staff of Victoria University and by the staff of New Zealand.
Landis of the University of Otago has generously informed me of his discovery of glaucophane facies metamorphism in the marginal zone of the New Zealand geosyncline.
INTRODUCTION AND PURPOSE
BRIEF INTRODUCTION TO THE GEOLOGICAL FRAMEWORK OF
GENERAL ASPECTS OF GEOCHRONO- LOGI CAL RESULTS AND CONCLUSIONS
The ages of the plutonic and metamorphic rocks of New Zealand are generally known only within broad limits, and some have been tentatively mapped as Precambrian. This study has so far been limited to New Zealand, the region's most continental land. Much of this time was spent on the South Island and Stewart Island, where nearly all of New Zealand's plutonic and metamorphic rocks are located.
The Foreland Province crops out only in the South Island and southern Stewart Island, west of the Geosynclinal Province and is of major interest in this study because it contains most of the main bodies of acid plutonic rocks found in New Zealand.
NORTH ISLAND
FORELA.ND PROVINCE
SOUTH ISLAND
STEWAAT ISLAND
FORELAND PROVINCE 1. Introduction
PRECAMBRIAN IN NEW ZEALAND 1. North Island
The samples examined in this study were taken from areas widely distributed in the South Island of the Foreland Province. Five samples from the Geosynclinal Province were also examined, of which three from the Marginal Belt and two from the Alpine Belt. The overall error assignment is considered very liberal and the reader is referred to the data and discussion presented in the appendix regarding analytical reproducibility.
The Rb-Sr results together with zircon results are discussed in detail below, starting with the Geosynclinal Province and followed by the Foreland Province.
Na1'11nal lOM
In the long, narrow, faulted part of the syncline near the Alpine Fault both north and. All of these occur on the western flank .. of the marginal syncline south of the Alpine fault. The central part of the Alpine Zone has been regionally transformed into the Otago and Alpine Schists.
A sample of the Otago Schists was collected from Cromwell, 120 km east of Alpine Gas.
GEOSYNCL\NAL PROVINCE
PROVINCE
The Charleston Gneiss, which occurs in the middle part of the North Block on the west coast and just inland (Figure 1 O), has been mapped as Precambrian. All the results for the Foreland Province are shown in summary form as a histogram in Figure 9. Along the western boundary of the eastern belt (separation point) of granitic rocks occurs a discontinuous narrow belt of basal plutonic rocks and associated volcanics which are partially metamorphosed (Figure 10).
Quartz Hill is one of several isolated small stocks of the southern part of this block. 8 km west of the Alpine fault, movements along the fault may have caused this discordance. One of the granites intrusive in the Greenland series is a coarse tourmaline-bearing pegmatitic granite and the other is a medium-.
Here an adamellite from this Middle Belt near the west coast, just north of Karamea and north of the Charleston Gneiss, was examined. 715 is indicated and this suggests that the age of 100 million years may represent a secondary event in the history of the Charleston Gneiss. The biotite age of 102 million years agrees well with the biotites from the above-mentioned adjacent samples of the.
These occur along the western border of the eastern band (separation point) of granites (Fig. 10). Summary of the distribution of two age groups of crystalline rocks in the North Block. Inland from the west coast, the middle part of the Middle Belt (Karamea) consists partly of rocks of a similar age.
Much of the middle and southern parts of the Middle Belt (Karamea) is also of this age.
SOUTH BLOCK
1960) has noted the conformable nature of the granite intrusions throughout the complex. Thus there is some suggestion that (1) plutonic and metamorphic rocks are synchronous in development throughout the crystalline complex and (2) that metamorphic units occur continuously throughout much of the region. A portion of the errors shown in Table 5 for the above two ages is the estimated error in the initial ratio Sr 87 I Sr 8 6.
North of Lake Manapouri, an amphibolitic gneiss was studied from the mouth of the south arm of Lake Te Anau (Figure 24). Due to the metamorphic nature of this rock, the assumed initial Sr 87 I Sr 86 may be too low. In the northern part of the North Island on the north-west side of the Coromandel Peninsula (Figure 28) a small body of fine-grained hypersthene tonalite occurs in the Moehau area.
This study has shown that a probably major part of the plutonic and metamorphic rock now exposed in the Foreland Province is Creta-. On the other hand, if the biotite age is a minimum age, then the source of the conglomerates could be a. As previously mentioned, the Greenland series, widely allied to the southern part of the North Block, are considered probably of Precambrian age.
It is important to try to determine the age of the deposition of the Greenland Series, certainly one of New Zealand's oldest sedimentary rocks. Because of the low spread in the Rb/Sr ratio for the whole rocks in the two samples.
GREENLAND 5ER\E5
GENERAL ASPECTS OF GEOCHRONOLOGICAL RESULTS AND CONCLUSIONS
The results of this study reveal key aspects of the geochronology of plutonic and metamorphic activity in New Zealand. In addition, plutonic activity occurred in the marginal zone of the New Zealand geosyncline during the Permian and probably extended into the Mesozoic. This suggests that these rocks cannot be as old as the 345-370 million year old rocks common in the Northern Block of the Foreland Province.
The Cretaceous comes from the Otago Schist muscovite of the Geosynclinal Province, at 116 million years. The results of this study show that the Rangitata Orogeny is also similar to Japan. In discussing the micro-floral dating of these late Mesozoic terrestrial beds in the Foreland Province north of the Alpine Fault, .
Hence the lack of Fox River Hawks Crag Breccia formed from the North Island. To connect this period of plutonism, for which evidence has been found only in the Northern Block of the For eland Province, with the known. The western p.ortion of these rocks in the Carpentaria region of the Northern and Northwest Territories.
A narrow channel 2500 meters deep separates the Coral Sea platform from the northern end of the Lord Howe Rise (Standard, 1961), and the Coral Sea platform is separated from the Australian shelf by a narrow saddle between 2000 and 3000 meters deep, as shown by the Menard and Smith contours ( 1964). This terrain is separated from the coast by Paleozoic sedimentary rocks of the Tasman Geosyncline and intrusive Permian granites. Other workers in the past have discussed the extension of the Australian continent to the Marshall "Andesite Line" (Gutenberg and Richter, 1954).
A possible in situ Precambrian base for the Greenland Series should be sought in the field, preferably in the very southern part of the northern block.
SAMPLE PROCESSING
- EXPERIMENTAL PROCEDURES
SAMPLE LOCATION AND DESCRIPTION
In the first case, the mineral purity for all minerals except feldspar was assessed by eye under a binocular microscope as being greater than about 95% pure. By staining, the inguinal remains of some individual potassium feldspar grains were shown to consist of more than 99% potassium feldspar grains, but intergrown perthitic plogioclose, as indicated by X-ray diffraction, was always present. The feldspar-quartz fraction that yielded the optimal compromise of maximum strontium content and minimum Rb/Sr ratio by X-ray fluorescence was usually seen as sufficiently plagioclosely separated and only occasionally further verified by index oils.
All binary biotite and muscovite impurities in the feldspars were removed by slow runs through the Frantz at minimum side tilt and maximum amperage. Total rock aliquots were selected from large quantities of crushed sample, which generally weighed from 10-40 kg. At the beginning of the study, aliquots of the aggregated rock were selected using jointless sample splitters.
EXPERIMENTAL PROCEDURES
Aliquots of strontium were collected in 2.5 N HCl and loaded onto conditioned Dowex 50 columns 15-20 cm long and 1 cm in diameter. Aliquots of strontium were collected in glass beakers, evaporated to dryness and loaded onto a mass spectrometer filament. This is probably just due to the lack of precision in measuring the rubidium concentration, as discussed below.
The zircons were rinsed in 1-1 hot nitric acid until all the pyrite dissolved, then rinsed several times with clean water. The solution was divided into two parts by weighing or volume, one containing approximately 90% for lead composition and the other 10% for lead, uranium and thorium concentration. This eluate was collected from the spiked aliquot for uranium and thorium post-treatment.
The lead was further purified by standard dithizone procedures which were extracted in chloroform from a pH 9, cyanide-citrot solution, and back extraction into dilute nitric acid. The eluate containing the spiked uranium and thorium was co-precipitated again with Zr(OH). This was dissolved in 6N nitric acid and loaded onto a new Dowex 17 x l cm Dowex anion exchange column, previously conditioned with nitric acid.
Uranium was eluted with water, and thorium with 6N hydrochloric acid in the same trap. The uranium and thorium in the final vaporization were picked up in nitric acid and loaded onto two side filaments of a triple filament source.
