Discovery of Precambrian rocks in New Zealand: Age relations of the Greenland Group and Constant Gneiss, West Coast, South Island

Earth and Planetary Science Letters, 28 (1975) 98-104

© Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands

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DISCOVERY OF PRECAMBRIAN ROCKS IN NEW ZEALAND: AGE RELATIONS OF THE GREENLAND GROUP A N D C O N S T A N T GNEISS, WEST COAST, SOUTH ISLAND* CJ.D. ADAMS Institute of Nuclear Sciences, D.S.I.R., Lower Ilutt (New Zealand)

Received April 4, 1975 Revised version received September 8, 1975

Rb-Sr whole-rock analyses yield a Cambro-Ordovician (495 -+ 11 m.y.) sedimentation age for the supposed Precambrian Greenland Group and a late Precambrian age, 680 +- 21 m.y., for parts of the Constant Gneiss, the first confirmation of Precambrian rocks in New Zealand. A Precambrian age for the Greenland Group is thus unlikely and the large area of Upper Cambrian-Lower Ordovician rocks now established can be considered as a lateral equivalent of the fossiliferous Lower Palaeozoic succession of northwest Nelson to the'east. The Greenland Group, especially in the Paparoa Range has been affected subsequently by a thermal metamorphic overprint about 360 m.y. ago during the Tuhuan Orogeny. Although the Constant Gneiss must form the local basement to the Greenland Group in north Westland, the former does not appear to be the source of the sediments and the true provenance must lie elsewhere.

1. Introduction In recent years several paleogeographic reconstructions of the southwest Pacific region have attempted to correlate the Palaeozoic histories of New Zealand, southeast Australia and Victoria Land, Antarctica [1,2]. In many of these, the Palaeozoic reconstructions of the New Zealand region in relation to Australi~ and Antarctica have been particularly speculative, being based on data from only a relatively small segment of the total New Zealand region, in the northwestern part of the South Island. Suggested areas of "Precambrian crustal rocks" have included supposed Precambrian sediments in Westland and gneissic complexes in Fiordland but in neither case are there firm data to support such conclusions. Lower Palaeozoic rocks have long been known in two main areas of the South Island of New Zealand: (1) Nelson, Buller and Westland, (2) the southern part of Fiordland, both being part of the Foreland Province [3] but separated by about 480 km dextral displacement on the Alpine Fault. Within these areas, gneisses such as those of Fiordland and the Charles* I.N.S. Contribution No. 706.

ton area (Fig. 1) and unfossiliferous sediments such as the Greenland Group (Fig. 1) have been tentatively assigned to the Precambrian although direct p r o o f is lacking [4,5]. Indeed recent work has tended to assign Carboniferbus or Cretaceous ages to at least some of the gneisses [3,6] and the unfossiliferous sediments might extend into the Lower Palaeozoic [7]. This study applies the R b - S r whole-rock dating method to the supposed Precambrian rocks of Westland and Buller in order to establish the age of deposition of the Greenland Group sediments and the age of formation of the Constant Gneiss on the West Coast near Charleston. The application of the R b - S r dating method to the dating of sediments [8,9] has met with mixed success, but in the best geological circumstances realistic sedimentation ages can be obtained [10]. In this respect, it is usually found that fine-grained argillites lacking any coarse detrital component and deposited slowly in a marine environment are the best materials for these studies, providing that they have not been too affected by subsequent deep burial, tectonic stress or metamorphism. The Greenland Group certainly satisfy most of these criteria, although in most cases the degree of metamorphic recrystallisation is fairly

99 complete and might imply some strontium isotopic homogenisation. It seems usual that where anomalous ages arise they are invariably related to post-depositional disturbances such as regional metamorphism rather than pre-depositional or inherited effects from the provenance area [11]. The dating of gneissose rocks is more straightforward providing that complete strontium isotope homogenisation occurs during formation and that they remain closed systems with respect to Rb and Sr subsequently. This may not necessarily be true of the Constant Gneiss which is a complex mixture of ortho- and paragneisses that have undergone several subsequent metamorphic events.

2. Experimental methods Rb-Sr whole-rock analyses were done at the Research School of Earth Sciences, Australian National University, using standard techniques of chemical preparation and mass-spectrometry. All samples were run on the high-transmission "MSZ" mass-spectrometel [ 12] using techniques for computer control of magnet field switching, isotope measurement and correction for fractionation similar to those discussed by Arriens and Compston [13]. The analytical results are listed in Table 1 and presented on Rb-Sr isochron diagrams in Figs. 2 and 3. Rb-Sr isochron ages have been corn-

West /

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Greymouti 3528 "~

i ;

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~

~,~_,._r J 3123 7 / 312R, 31 3i638 , / ~~3~

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Charleston~16/e'~~e

I 331 3177 y 78~,Sj ~loeraki JacksonBa '~

TuhuanGranites

~ ~/A GreenlandGroup ~ ConstantGne,ss

~1kin'

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Fig. 1. Geological sketch map of the Westland and Buller, South Island, New Zealand, and larger-scale insets of the southern Paparoa Range (A) and Charleston (B) areas showing location of analysed samples.

100

puted using the least squares regression equations of York [14]. The mean value for 20 measurements of the NBS 987 strontium standard on the MSZ instrument yield at mean aVSr/a6Sr value of 0.71031 + 0.00006 (standard error of mean). Rb/Sr ratio measurements were measured by a XRF spectrometric method using a primary molybdenum 2-kW X-ray tube operated at 75 kV, 20 mA; a LiF110 diffracting crystal; fine 160/1 collimation. Samples were compared with the U.S.G.S. standard granodiorite GSP-1 (Rb: 255.0 ppm, Sr: 233.0 ppm) and mass absorption effects were approximately related inversely to the height of the X-ray background at the strontium background region. In this way approximate rubidium and strontium concentrations were calculated and the results listed in Table 1 have a 1% error for Rb (one standard deviation) and 2% for St. The Rb/Sr ratio measurement avoids the error associated with mass absorPtion corrections and its error is generally less than 2% (one standard deviation). The fractional experimental errors designated for aTSr/a6Sr and STRb/a6Sr ratios in the isochron age calculations are, 0.0002 and 0.02, respectively.

1.0

Greenland Group

0.9

B'Sr 86Sr

The Greenland Group comprises an unfossiliferous turbiditic succession of greywacke and argillite [7] outcropping in two N - S belts, a western belt close to the coast extending from Jackson Bay to the north of the Buller River and an eastern belt centred mainly on the two of Reefton (Fig. 1). In both areas, fossiliferous Lower Palaeozoic rocks are present to the east but are unconnected with the Greenland Group sediments. The Greenland Group are usually regionally metamorphosed to lowest greenschist grade rocks and in the argillaceous members slates are common. Structurally, the sediments are in simple open folds and Suggate [4] has distinguished between the Greenland Group sensu s t r i c t o occurring mainly in the western belt and with E S E WNW strike, from the Waiuta Group occurring mostly in the eastern belt and having a N - S strike. Apart from this there are no important differences between the groups and Laird [7] considered that the division was not really justified. The source area for the Greenland Group must

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3. The Greenland Group

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Fig. 2. Rb-Sr isochron diagram of Greenland Group samples (hRb ~- 1.39 × 10 -Iz year-Z). contain components at least 1480 m.y. old, based on 2°7Pb/2°6pb detrital zircon data [3] and which accordingly place an older age limit upon the sedimentation. However, source rocks of this age are quite unknown in New Zealand. A younger age limit for the Greenland Group is provided by 365 m.y. (midDevonian) "Tuhuan" granites [3] which intrude the Greenland Group and K - A r whole-rock ages for the Greenland Group up to 438 m.y. (Middle/Upper Ordovician) which are interpreted as the time of lowgrade metamorphism and cleavage formation [ 15]. This leaves a wide time-range from late Precambrian to mid-Ordovician for deposition of the Greenland Group sediments and their previous assignment to the Precambrian rests entirely on their unfossiliferous nature.* * Lower Ordovician fossils have recently been reported from a single locality in Waiuta Group sediments in the Waitahu Valley [23].

101 TABLE 1 R b - S r whole-rock data for the Greenland Group and Constant Gneiss

Sample No.

Locality

(1) 3047* 3174 3175" 3176 3177 3178 3179 3184" 3185" 3187 3188 3189 3527 3528 3530 3531 3532 3533 3534

GreenlandGroup Haku-1 borehole, Tasman Sea Moonlight Creek, nr Blackball Beach, 17 ml bluff, nr Greymouth Beach, 17 ml bluff, nr Greymouth Beach, east of Jackson Bay Beach, east of Jackson Bay Hwy 6, nr Lake Moeraki Mt William Track, Cascade Creek Mt William Track, Cascade Creek Beach, 14 ml bluff, nr Greymouth Railway, nr Stillwater station Railway, nr Stillwater station Totara Valley track, nr Ross Donnelly Creek track, nr Ross Snowy River road, nr Reefton Blacks Pt, Hwy 7, nr Reefton Moonlight Creek, nr Blackball Moonlight Creek, nr Blackball Moonlight Creek, nr Blackball

(G?) (W) (G) (G) (G) (G) (G) (G) (G) (G) (G) (G) (G) (G) (W) (W) (G) (G) (W)

Grid ref. 1

Rb(ppm)

Sr(ppm)

87Rb/86Sr

87Sr/86Sr

$37/749177 $44/970094 $44/805064 $44/815089 $97/47-97$97/47-97$77/012303 $24/325724 $24/325724 $44/806063 $44/853894 $44/853894 $57/433320 $57/388314 $45/230090 $38/346276 $37/949105 $37/952104 $44/964098

223 204 185 251 241 244

-

263 267 110 200 231 197 189

32 49 117 37 33 28 37 19 57 65 51 45 80

2.122 20.711 12.480 4.590 20.123 16.615 24.422 31.340 19.582 11.608 23.710 23.515 20.452 39.502 5.742 8.922 13.305 12.781 6.853

0.7341 0.8426 0.7810 0.7451 0.8444 0.8267 0.8797 0.8868 0.8172 0.7834 0.8736 0.8655 0.8528 0.9861 0.7475 0.7709 0.7916 0.7914 0.7571

142 76 91 159 175

169 508 212 160 187

2.464 0.415 1.240 2.890 2.722

0.7288 0.7100 0.7188 0.7335 0.7318

-

-

-

-

Isochron age 2 = 494.6 + 11.0 m.y. (95% confidence level) Initial 87Sr/86Sr = 0.7087 -+ 0.0018 (95% confidence level) [SUMS/(N - 2)] 1/2 1.5 =

(2) 3122 3123 3136 3138 3139

Constant Gneiss Cliffs, 1 km NW of Charleston Cliffs, 1 km NW of Charleston Nile River mouth nr Charleston Nile River mouth nr Charleston Hwy 6, nr 4 ml creek

$30/944554 $30/944554 $30/700553 $30/699550 $30/937473

Isochron age 2 = 679.5 -+ 21.4 m.y. (95% confidence level) Initial 87Sr/86Sr = 0.7062 +- 0.0002 (95% confidence level) [ S U M S / ( N - 2) 11/2 = 2.1 I New Zealand Map Series NZMS1, 1:63,360. 2 hR b = 1.39 × 10 -11 year-I ; errors 87Rb/86 Sr = -+1.5%, 87Sr/86Sr = -+0.03% (standard deviation). * Not included in calculation of isochron age. G = Greenland Group, W = Waiuta Group.

In T a b l e 1 sample details, a n a l y t i c a l results a n d a p p r o x i m a t e R b a n d Sr c o n c e n t r a t i o n s are listed. F o r e a c h d a t a set, i s o c h r o n ages, initial 87Sr/a6Sr ratios ( a n d t h e i r e r r o r s ) a n d t h e statistical p a r a m e t e r s [ S U M S / ( N - 2)] 1/2 devised b y Y o r k [14] are also given. This last q u a n t i t y p r o v i d e s a n a s s e s s m e n t o f t h e v a l i d i t y o f t h e i s o c h r o n age.

In Fig. 2 a R b - S r i s o c h r o n diagram is s h o w n for 19 G r e e n l a n d G r o u p slate samples t a k e n f r o m widely s e p a r a t e d localities t h r o u g h o u t W e s t l a n d a n d BuUer; in d o i n g this a g o o d s p r e a d o f R b / S r ratios is p r o v i d e d a n d t h e p r e c i s i o n o f t h e i s o c h r o n age i m p r o v e d . It s h o u l d be realised t h a t in t h e a b s e n c e o f g o o d stratigraphic m a r k e r h o r i z o n s t h r o u g h o u t t h e t h i c k Green-

102 land Group sequence it is impossible to select samples that are demonstrably of the same stratigraphic horizon; indeed, those chosen might span a substantial time period and in these circumstances 50 m.y. would not seem an unreasonable amount. It is also possible that small initial strontium isotopic variations (+0.005, for example) existed at the time of sedimentation and over the large area covered by the samples. For these reasons it is important to recognise that in such geological circumstances the isochron method will, at best, average the sedimentation ages of the individual samples. It can be seen from Fig. 2 that, with some exceptions discussed below, 15 of the slates define a general isochron, yielding an age of 495 +- 11 m.y. but for the reasons mentioned above, the scatter about the isochron line is a little more than would be anticipated by consideration of the experimental errors alone, i.e., SUMS is significantly greater than unity [14]. In determining the isochron age four samples are excluded from the calculation since even making allowances for some age variation, R3175, R3184 and R3185 fall significantly below the isochron defined by the remainder of the samples in Fig. 2 and have clearly been affected by some post-depositional event. Maximum model ages for these samples, calculated by assuming the minimum possible initial strontium 87Sr/S6Sr ratio (0.699) range from 480 to 370 whilst using the more realistic initial 87Sr/a6Sr value, 0.709 + 0.002, provided by the main isochron, the model ages fall between 350 and 370 m.y., quite younger than the known minimum age for the Greenland Group itself. Such ages probably indicate a post-sedimentation metamorphic effect of the Tuhuan orogeny and are roughly similar to the ages of the "Tuhuan" granites themselves (360-370 m.y.). The exceptions mentioned above all come from the north and western parts of the Greenland Group where the thermal overprint of younger granites of the Paparoa Range and Lower Buller Gorge is noticeable [3]. In the southern part of the Paparoa Range for example, some of the sediments (R3175, R3176) have developed biotite porphyroblasts during contact metamorphism. A slate sample from the Haku-1 borehole in the Tasman Sea, resembles the Greenland Group on the adjacent coast, yet it does not lie on the main isochron line and the suggested correlation is not proven. The main isochron age of the Greenland Group,

495 + I 1 m.y., thus falls in the Lower Ordovician or Upper Cambrian and it is probable that the sediments at least span this range. There is certainly no evidence in the Palaeozoic stratigraphic or structural history to suggest that this isochron age reflects a post-depositional, metamorphic event, either contact or regional, at about this time. The initial S7Sr/S6Sr ratio, 0.709 +0.002, is typical of sediments of this age, whereas a metamorphic isochron may often be recognised by an unusually high initial s 7Sr/S6Sr ratio [11]. Similarly, in view of the detrital zircon data [3] and the Lower Palaeozoic stratigraphy of the region, the isochron age cannot reflect that of the source area. No age differences between the Greenland and Waiuta sediments [4] are readily apparent. If the samples used in this present study are representative, then it seems unlikely that the Greenland Group extends into the Precambrian. Indeed, the CambroOrdovician age obtained fits into the regional distribution of Lower palaeozoic rocks in this region which generally increase in age westward [16,17]. Fossiliferous Ordovician sediments of Tremadocian age occur in the extreme northwestern part of this area and it is probable that the Greenland Group is a lateral equivalent of these Lower Palaeozoic sediments which form a shallower water belt to the east.

4. The Constant Gneiss

The Lower Ordovician age of the Greenland Group has important implications for that of the Constant Gneiss. The geological contact between the two is rather inaccessible and only poorly exposed in the southern part of the Paparoa Ranges and previous workers have suggested intrusive [ 18], metamorphic [19] and unconformable relationships [20]. It is important to recognise that the Constant Gneiss as previously mapped comprises a mixture of para- and orthogneisses and/or foliated granites of several generations, in particular mid- to late-Devonian (Tuhuan Orogeny) and mid-Cretaceous (Rangitata Orogeny). It is only in the coastal areas near Cape Foulwind and the village of Charleston (Fig. 1, inset B) that structural and metamorphic histories have been studied in detail [18,21,22] and unfortunately it appears that the wellexposed granite-gneiss complex at Cape Foulwind is quite atypical and unlike the Constant Gneiss else-

103 where [22]. Unpublished Rb-Sr data by the writer confirms this and tends to suggest a Tuhuan age for much of the Cape Foulwind complex. For these reasons, preliminary analyses have only been made on gneisses from the Charleston district. The Constant Gneiss here is heterogeneous, varying from highly deformed and injected mica-schists, through porphyroblastic banded granite-granodiorite gneisses to massive, slightly foliated granodiorites and leucogranites of igneous origin. This kind of petrographic variation almost certainly implies some kind of initial strontium isotopic variation at the time of gneiss formation and consequently a single simple isochron is not necessarily to be expected, unless conditions of gneiss formation were sufficient to produce complete strontium isotope rehomogenisation. To minimise the problem, samples were chosen from the orthogneiss components which should have been isotopically, more homogeneous at the time of formation. The Rb-Sr isochron shown in Fig. 3 shows that is essentially so and the age 680 -+ 21 m.y. (initial STSr/S6Sr = 0.7062 + 0.0002) falls into the Late Precambrian, indicating the existence of Precambrian rocks in New Zealand for the first time. With the R b - S r age of the two groups established it is probable that the relationship between the Constant Gneiss and the Greenland Group is an unconfor-

87Si.

v/

BeSr

0.73

~ ' /

mable one but judging from the mineral age data gained from them [3], both groups have been regionally metamorphosed subsequently and then considerably injected by younger granites. To some extent these younger events have obscured the true nature of the contact.

5. Regional implications On a regional scale it can now be seen that a large area of unfossiliferous Cambro-Ordovician sediments extends over 500 km N - S along the west coast of the South Island and lying to the west of the fossiliferous Palaeozoic belt of northwest Nelson. However, it is surprising that over this large area only one small area (10 km X 5 km) of Precambrian basement is exposed near the western margin (although this is substantially enlarged if the Constant Gneiss of the Paparoa Range is included). It is curious that near the Constant Gneiss, the Greenland Group contains no basal conglomerate (indeed throughout the succession conglomerates are rare) nor does the sedimentary facies change particularly [7,10]. Similarly, even close to the Gneiss, sedimentary structures in the Greenland Group point to a source area in quite the opposite direction, to the south and/or east [7]. Finally, the U-Pb ages of detrital zircons in these sediments are completely older than any R b - S r ages obtained so far from the Constant Gneiss. We can conclude that the main source area for the Greenland Group is not the Constant Gneiss and one must look elsewhere for yet older rocks. At the present time no such area has been found and the source of the Greenland Group remains uncertain.

0.72

6. Conclusions

0.71 Constant

Gneiss

o 70<

$TRb/'B6Sr 0.7(~

0

' 1

J 2

3

Fig. 3. Rb-Sr isochron diagram of Constant Gneiss samples (?'Rb = 1.39 × 10-11 year-l).

R b - S r whole-rock analyses yield a CambroOrdovician age 495 -+ 11 m.y. of sedimentation for the Greenland Group and a late Precambrian age 680 + 21 m.y. for the Constant Gneiss. Parts of the Greenland Group, especially in northern Westland and Buller have been affected by a subsequent thermal metamorphic overprint about 370 m.y. ago during the Tuhuan Orogeny. A Precambrian age for the Greenland Group is thus disproved and the large area of Cambro-Ordovi-

104 cian rocks is considered a deeper water equivalent to the shallower water fossiliferous Ordovician succession o f n o r t h w e s t Nelson to the east. A l t h o u g h the Constant Gneiss must f o r m the local b a s e m e n t to the Greenland Group, this is probably not the m a j o r source o f the sediments and the true provenance lies elsewhere.

Acknowledgements I wish to particularly express m y gratitude to Dr W. C o m p s t r o n and P. Arriens for making mass spectrometer time and on-line c o m p u t e r facilities available to me at the Australian National University, Canberra. A financial grant and study leave from the Departm e n t o f Scientific and Industrial Research, Wellington, is gratefully acknowledged. W. C o m p s t o n , P. Suggate, and R. C o o p e r made i m p o r t a n t i m p r o v e m e n t s to the manuscript and their c o m m e n t s are gratefully acknowledged.

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9 P.R. Whitney and P.M. Hurley, The problem of inherited radiogenic strontium in sedimentary age determinations, Geochim. Cosmochim. Acta 28 (1964) 425. 10 S. Moorbath, Evidence for the age of deposition of the Torridonian sediments of northwest Scotland, Scott. J. Geol. 5 (1969) 154. 11 D. Gebauer and M. Grunenfelder, Rb-Sr whole-rock datin~ of late diagenetic to anchimetamorphic, Palaeozoic sediments in southern France (Montagne Notre), Contrib. Mineral. Petrol. 47 (1974) 113. 12 S. Clement and W. Compston, The design and performance of a mass spectrometer using beam transport theory, Int. J. Mass Spectrom. Ion Phys. 10 (1973) 323. 13 P.A. Arriens and W. Compston, A new method for isotopic ratio measurement by voltage peak switching, and its application with digital output. Int. J. Mass Spectrom. Ion Phys. 1 (1968)471. 14 D. York, Least-squares fitting of a straight line with correlated errors. Earth Planet. Sci. Lett. 5 (1969) 320. 15 C.J. Adams, C.T. Harper, M.G. Laird, K--Ar ages of lowgrade metasediments of the Greenland and Waiuta Groups in Westland and Buller, New Zealand, N.Z.J. Geol. Geophys. 18 (1975) 39. 16 D.G. Bishop, Sheet 32, Kahurangi (lst ed.), Geological Map of New Zealand 1:63,360 (Dep. of Scientific and Industrial Research, Wellington, 1971). 17 G.W. Grindley, Sheet 13, Golden Bay (lst ed.), Geological Map of New Zealand 1:250,000 (Dep. of Scientific and Industrial Research, Wellington, 1961 ). 18 D. Shelley, Structure of the Constant Gneiss near Cape Foulwind, southwest Nelson, and its bearing on the regional tectonics of the West Coast, N.Z.J. Geol. Geophys 15 (1972) 33. 19 M.G. Laird, Field relations of the Constant Gneiss and Greenland Group in the central Paparoa Range, West Coast South Island, N.Z.J. Geol. Geophys. 10 (1967) 247. 20 B. Hume, Geology of the central Paparoa Range, north Westland, Unpublished B.Sc. Thesis, University of Otago, Dunedin (1972) 106. 21 D. Shelley, The structure and petrography of the Constant Gneiss near Charleston, southwest Nelson, N.Z.J. Geol. Geophys. (1970) 370. 22 S. Nathan, Sheet $30, Charleston (lst ed.), Geological Map of New Zealand 1:63,360 (Dep. of Scientific and Industrial Research, Wellington (in preparation). 23 R.A. Cooper, Age of the Greenland and Waiuta Groups, South Island, New Zealand, N.Z.J. Geol. Geophys. 17 (1975) 955.