RbSb age determinations on Precambrian carbonate rocks of the Carpentarian McArthur basin, Northern Territories, Australia

Precambrian Research, 18 (1982) 157--170

157

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

Rb--Sb AGE DETERMINATIONS ON PRECAMBRIAN CARBONATE ROCKS OF THE CARPENTARIAN McARTHUR BASIN, NORTHERN TERRITORIES, AUSTRALIA

M. KRALIK Research School o f Earth Sciences, Australian National University, P.O. B o x 4, Canberra, A. C.T. 2600 (Australia) *

(Accepted for publication December 28, 1981)

ABSTRACT Kralik, M., 1982. Rb--Sr determinations on Precambrian carbonate rocks of the Carpentarian McArthur basin, Northern Territories, Australia. Precambrian Res., 18: 157--170. The applicability and the uncertainty range of Rb--Sr age determinations on carbonate rocks are discussed briefly. A study of clay minerals isolated from carbonate rocks of the McMinn Formation provides an isochron age of 1429 ±31 Ma and thus the age for cessation of the Proterozoic McArthur basin sedimentation. The eukaryots recently described in this area are therefore the oldest occurrences found in Earth history. A second sequence of measurements on a homogeneous dolomite siltstone ca. 23 km southeast of the McArthur River (H.Y.C.) Pb--Zn deposits yields an Rb--Sr age of 1537 ±52 Ma for the Upper Barney Creek Formation in broad agreement with recent age determinations on feldspar beds of this horizon. These data are compared with previous isotopic results. From data presently available it can be concluded that the sediments in the McArthur basin (the type locality of the Carpentarian) were deposited between 1800--1400 Ma.

INTRODUCTION I s o t o p i c m e t h o d s o f d a t i n g s e d i m e n t a r y r o c k s are o f p a r t i c u l a r i m p o r t a n c e w h e n dealing w i t h t h e P r e c a m b r i a n , since placing P r e c a m b r i a n s e q u e n c e s in t h e geological t i m e scale b y o t h e r m e a n s is m o s t l y i m p o s s i b l e . K - - A t a n d R b - - S r are t h e t w o m e t h o d s m o s t c o m m o n l y u s e d w h i c h h a v e b e e n e m p l o y e d t o d a t e P r e c a m b r i a n s e d i m e n t a r y events. M a n y o f t h e reliable d a t a f r o m s e d i m e n t a r y r o c k s h a v e b e e n o b t a i n e d o n l y f r o m m i n e r a l s o f m i n e r a l o g i c a U y w e l l ~ o n t r o l l e d shales. T h e a p p l i c a b i l i t y a n d t h e u n c e r t a i n t y r a n g e o f t h e R b - - S r m e t h o d o n shales has b e e n discussed adeq u a t e l y b y Claue~ ( 1 9 7 6 , 1 9 7 9 ) , C o r d a n i et al. ( 1 9 7 8 ) a n d B o n h o m m e ( 1 9 8 2 ) . The separation of undisturbed clay minerals from those affected by weather ing, a n d also o f c l a y m i n e r a l s f r o m c a r b o n a t e s , w h i c h f r e q u e n t l y o c c u r in *Present address: Geochronologisches Labor, Geologisches Institut, Universit~tsstrasse 7, A-1010 Wien, Austria.

0301-9268/82/0000--0000/$02.75 © 1982 Elsevier Scientific Publishing Company

158 shales, are the two greatest inherent difficulties in obtaining reliable isotopic data. The problem of excessive carbonate has discouraged geochronologists from dating such sedimentary rocks as impure carbonates. Recently, Kralik (1979) and Kralik and Compston (1982) have developed a method of separating clay minerals from carbonate rocks, which has considerably raised the potential for successful dating of clay minerals by the Rb--Sr method. They have adopted an ion-exchange method, following a procedure given by Ray et al. (1957), to dissolve carbonate rocks and separate the clay minerals with minimal disturbance to the chemical systems of these minerals. This technique has permitted, for the first time, the estimation of isotopic Rb--Sr data by which reliable ages for the end of diagenetic consolidation in non-glauconitic consolidation in non-giauconitic carbonate rocks can be calculated. Carbonate environments are much more suitable for the formation of clay minerals and their equilibration with their surrounding environments (Millot, 1978) then they are for the more detrital shales. In addition, consolidation progresses much faster in carbonate rocks, especially where meteoric water influence is evident (Gavish and Friedman, 1969). A similar observation was made by Zankl (1969), who observed a positive correlation between the purity and the lithification rate of carbonate rocks in ancient carbonate enVironments. Therefore an early closed system behaviour was assumed, which minimized the time-lapse between sedimentation and the end of diagenetic recrystalli~ations. In contrast, several studies on more or less carbonate-free shales by the K--At and Rb--Sr methods indicated that diagenetic changes were slow in this sort of environment and could last as long as 18--50 Ma before a closed stable system was obtained (Garrels and MacKenzie, 1974; Perry and Turekian, 1974; Aronson and Hower, 1976). The sediments of the McArthur basin in the Northern Territory (Fig. 1) were chosen as they represent the type section of the Middle Proterozoic Carpentarian unit in Australia (Dunn et al., 1966). Fortunately, they lack any sign of metamorphism, but on the other hand they are composed mainly of carbonate rocks and sandstones in wide areas. Sandstones are generally unsuitable as their composition of mineral grains is overwhelmingly detrital and they therefore provide an age of the source area rather than one of the time of deposition. In the carbonate rocks the clay minerals can provide ages close to the time of deposition as they contain Rb and Sr in a sufficiently high ratio and have the potential capability to equilibrate with their environment at low temperatures. To the begin with, the ion-exchange separation method was applied on wavy laminated silty dolomites collected from a drill core through the Kyalla Member (Fig. 1) of the McMinn Formation, to get an upper age limit of the Proterozoic sedimentation in the MacArthur Basin. Additional stratigraphic age information was obtained from a homogeneous silty dolomite in the upper part of the Barney Creek Formation to allow a comparison with the recorded Rb--Sr and U--Pb ages from feldspar beds in the H.Y.C. Pb--Zn deposits.

159

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Fig. 1. Carpentarian rock units in the McArthur basin, slightly modified according to Plumb et al. (1980, Fig. 5). Only the highest age reported in the literature is attributed to each horizon, since most determinations can be regarded only as minimum (min.) values. Max. = maximum, TR = total rock, Fsp. = feldspar, Bi = biotite, Zr = zircon.

INDIRECT EVIDENCE F O R THE ESTIMATE OF DEPOSITIONAL TIMES OF THE McMINN AND BARNEY C R E E K FORMATIONS

The geology and previous isotopic age determinations in the McArthur basin were adequately covered by Plumb et al. (1980) and Page (1981). Up to now the cessation of the Precambrian sedimentation was referred as minimum ages 1160--1289 Ma (McDougall et al., 1965), from K--At measurements on total rocks of quartz dolerites, intruding into the McMinn Formation (Fig. 1). The same authors investigated glauconites of sandstones in the Crawford

160

Formation (Fig. 1). Seven Rb--Sr analyses gave various model ages in the range of 1243--1370 Ma and six K--At determinations gave even younger ages between 1095--1270 Ma. As these ages are all minima and weathering phenomena and impurities in the glauconite concentrates are obvious (see discussion), the age of the McMinn Formation is most likely older than these glauconite ages but must be younger than the ages of the feldspar beds in the underlying McArthur Group (1690 +29/25 Ma, U--Pb zircon, Page, 1981; 1589 +14 Ma Rb--Sr, Kralik et al., 1982, see Fig. 1). These feldspar beds are thin layers interbedded in the shaly~arbonate sequence and containing mostly K-feldspar or albite. According to Croxford and Jephcott {1972) they have been interpreted as tuff beds, due to sporadic observations of shard-like structures. The age of the Barney Creek Formation (Fig. 1) should be in reasonable agreement with the above ages of the feldspar beds in these series (Fig. 1). The real age can be expected to be closer to the Rb--Sr value (1589 -+14 Ma) as the age of the zircons could be misleading if they are of detrital origin from nearby, slightly older magmatic bodies. Magmatic bodies interlayering and intruding the sequences of the Tawallah Group are the Hobblechain Volcanics and the Packsaddle Granite. McDougaU et al. (1965) referred to Rb--Sr model ages between 1458--1512 Ma for the fine-grained Packsaddle microgranite (Fig. 1). Webb (1974) constructed from total rock samples of the Hobblechain Volcanics an isochron with a slope corresponding to an age of 1542 + 117 Ma. These data represent minimum ages, though the reasons for this lowering in age cannot be unequivocally stated. Lower brackets for the ages of the McArthur Group are the oldest glauconites from the Aquarium Formation (Fig. 1, K--At 1606 Ma, Webb et al., 1963).or even better recent Rb--Sr total rock ages (quartzarenite/basic volcanics) of the Kombolgie Formation (1650 -+30 Ma, Page et al., 1980; see Fig. 1) in the Katherine River Group (Arnhem Shelf), which has been correlated with the Tawallah Group by Plumb et al., (1980). In any case, the upper part of the Barney Creek Formation must be older than the oldest measurements of the overlying Roper Group (1429 -+31 Ma), but equal to or younger than the zircons (1690 -+29/25 Ma) of the stratigraphically more or less equivalent feldspar beds. ANALYTICAL TECHNIQUES

Drill-core samples ranging in weight from 100--500 g were crushed and the carbonate phases eliminated by a modified ion~exchange treatment (Ray et al., 1957) as described in Kralik (1979) and Kralik and Compston (1982). The total rocks and the different fractions have been studied by means of thin sections and X-rays of orientated and unorientated samples following the methods of Larqu~ and Weber (1975). The Rb--Sr chemistry and measurements were carried out as described by Kralik and Compston (1982).

161

All strontium measurements were corrected for variable mass discrimination in 87Sr/S6Sr by normalizing 86Sr/SSSr to 0.1194. Analyses of NBS 70a Kfeldspar, gave values of 519.1 tag g-' Rb and 65.5 tag g-i total Sr. During the course of this study three measurements of the SRM 987 Sr were carried out, averaging 0.71033 +6 (2~). Mean Rb and Sr chemistry blanks are 0.3 and 2.3 ng, respectively. All isochrons were treated in the combined computer program of York's (1969) regression with McIntyre et al., (1966) statistical assessment of variances of X and Y. The decay constant of STRb is taken as 1.42 × 10-1~y-~. Ages cited in the literature are recalculated with the new decay constants (Steiger and J~iger, 1977). RESULTS

Kyalla Member, McMinn Formation The mineralogy and the Rb--Sr data of different separates of wavy laminated dolomite siltstone (see Appendix) from the Kyalla Member are presented in Table I and Fig. 2. The separated mineral fraction <2 tam contains mostly 1 M iUite and quartz. The illite crystallinity, according Kubler (1968), ranges between 5.2 and 6.5. The intensity ratios of 1002/1001 of the first two illite peaks in the X-ray charts have a value of about 0.25--0.23, indicating a higher Fe and Mg content in the illite composition (Esquevin, 1969). Mr MINN Fm. K~LLa Mb. Dolomite.siltstone,

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One leached dolomite and four illite fractions ( < 2 #m), all treated by ionexchange resin, were analyzed for Rb and Sr isotopes. They define an isochron with a calculated age (model 2 according McIntyre et al., 1966) of 1429 +31 Ma, with an initial STSr/S6Srratio of 0.7136 +2. An isochron fitted through > 2 # m (mainly quartz, feldspars and illites) gives a similar age of 1419 -'159 Ma (model 3, McIntyre et al., 1966) with a characteristic high initial STSr/S6Srratio of 0.745.

Barney Creek Formation Mineralogical and Rb--Sr data obtained from a homogeneous dolomite (see Appendix) are shown in Table II and Fig. 3. The fractions < 2 # m contain only 1M illite, quartz and traces of feldspar. The mites with a crystallinity index of 7.5 have a ratio of I002/I001 -- 0.65 in the diagenetic field of the Esquevin (1969) diagram, indicating more aluminium-rich illites than in the samples of the Kyalla Member. The calculated age (model 3 according McIntyre et al., 1966) of the isochron formed by three < 2 # m illite fractions (19 < 2, 20 < 2, 21 < 2) and by the carbonate leachate (19C), all treated with ion~xchange resin, is 1537 +52 Ma (initial SVSr/S6Srratio of 0.7106 +12). The total rock samples and the fractions > 2 # m (quartz and feldspar) plot just above this isochron. ,~

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DISCUSSION Even though we lack many measurements on resin-treated samples we have data from test experiments on pure blends of illite and carbonate carried out by Kralik (1979) and Kralik and Compston (1982). They show, in contrast to the treatment with diluted HC1, that carbonates are dissolved by ion-exchange resin (Amberlite IRC-50) without leaching radiogenic 87Sr* out of the crystal lattice of iUites. The changes in the isotopic composition of illites are minute and may be cause by the removal of Rb and Sr from the basal surfaces and edge interlayer sites. The main silicate components of the carbonate rocks (~2 pm in size) analyzed here are mainly 1M illites. These, by equilibrating in their carbonate environments, acquire the same S~Sr/S68r during diagenesis as the carbonate. Therefore clay minerals newly formed and transformed in carbonate environments are supposed to be well suited to give early diagenetic age information, especially in unfossiliferous and Precambrian sediments where ages can only be obtained by isotopic age determinations. The crystaUinity index of the illites (5.2--6.0) and the exclusive presence of 1M indicate that the wavy laminated dolomite siltstone of the KyaUa Member has experienced much diagenetic influence, that it is close to the threshold between diagenesis and anchimetamorphism (according to Clauer, 1979, the border-line between the diagenetic and the metamorphic was drawn at an illite crystaUinity value of 5.7. This differs from the value of 4 assigned by Kubler, 1968). On the other hand there are still traces of feldspar in the fraction ~ 2 pm, which may partly have been formed authogenically. The initial ratio (STSr/S6Sr = 0.7136 -+2) of the isochron formed by the fraction ~ 2 ~m (1429 +31 Ma) exceeds the value of contemporary seawater (0.705--0.708, Veizer and Compston, 1976) and can be explained in three ways: (1) by the presence of detrital material, (2) by contemporaneous continental water influx and (3) by metamorphic recrystallization. Explanations (1) or (2) may be valid since the value of the carbonate leachate (9C = 0.71621 -+6) supports the idea that the carbonate has isotopically exchanged with continental waters. This conclusion is strengthened by Peat et al. (1978), who observed lamination and dessication features in these fine-grained and wellbedded carbonate rocks of the KyAll~ Member. They interpreted these features in context with other observations as restricted and transitional marine environments (barred basins, lagoons and tidal flats). Intertidal and supratidal carbonate environments which are frequently or temporarily exposed, and therefore potentially exposed to meteoric waters, are nearly always thinly bedded, laminated and characterized by dessication features. Interpretation (3) for example the thermal influence from the crosscutting dykes, can be excluded for these samples because of the poor crystallinity of the illites. The fraction ~ 2 ~m lines up on an isochron in good agreement with the

166 age obtained from the smaller fractions. This indicates a new formation or the equilibration during diagenesis of a large proportion of the feldspars and illites. Certainly, without a point close to the ordinate, the uncertainty range for the age and the initial ratio must be high (+159 Ma, initial STSr/S6Sr = 0.74). These ages, from the t o p of the sequence (Fig. 1), are regarded as good estimates of the younger limit of the Precambrian sedimentation in the McArthur basin. This time limit is of particular importance as the sedimentation of the Carpentarian unit, proposed b y Dunn et al. (1966) for this sequence, could be limited at a b o u t 1400 Ma. This fits well and without overlap to the Australian upper Proterozoic Adelaidian unit (Dunn et al., 1966). With this minimum age, the fossils in the Kyalla Member are confirmed to be the oldest convincing occurrences of eukaryotes found in Earth history (Peat et al., 1978). The discrepancy of a b o u t 70 Ma between the above mentioned ages and the oldest Rb--Sr values on glauconites from the underlying Crawford Formation (Fig. 1) analyzed b y McDougall et al. (1965) m a y be explained by weathered impurities in the glauconite concentrates. They report values of 5.14 --5.77% K whereas good glauconites should have as a minimum 6.2% K (Hunziker, 1979). Also the values of 1160--1289 Ma obtained from the dolerite dykes intruding the Roper Group should be regarded as a minimum age as the authors themselves (McDougall et al., 1965) reported sericitization of plagioclase and transformation of pyroxene to amphibolites. The age of 1537 -+52 Ma (Fig. 3) obtained from the Barney Creek Formation may be regarded as an approximate age of deposition characterized b y a large standard error. However, several facts suggest that this age could be close to the real one. An environment suitable for primary dolomite formation is confirmed b y the relatively small grain size (mean = 11 pm) and the agreement of the Sr-content (82 pg g-l) with values c o m m o n l y found in primary dolomites (Weber, 1964). These sorts of environments have the necessary preconditions for the new formation and reconstitution of illites due to sufficiently high concentrations of K and Mg and K/Mg ratios in the pore waters (Harder, 1974). As already pointed out, consolidation and cementation generally occur early during diagenesis. X-ray diffractometry studies on orientated and unorientated patterns reveal only more aluminium-rich illites, which plot well in the diagenetic realm (IC = 7.3; I O 0 2 / I O 0 1 = 0.65). However, traces of orthoclase, albite and probably microcline unveil a small detrital influence reaching d o w n even to the fraction <: 2 pm. The assumption that the source area of the detrital fraction is only slightly older than the time of deposition of the sequence is supported b y the fact that total rock samples and fractions > 2 ~m, which contain mainly detrital quartz and feldspar, plot just above the isochron. If the feldspars would have totally preserved their primary igneous age then the age of the source area would be a b o u t 1610 Ma. Nevertheless, the sedimentation must have occurred after this date.

167 The age of 1537 +52 Ma is somewhat younger than that of the feldspar beds in the adjacent H.Y.C. (McArthur River) Pb--Zn deposits (1589 +28 Ma, Kralik et al., 1982). While the feldspar beds are interpreted as being early diagenetic, there are some indications that the zircons separated from the albitic feldspar bed underlying the H.Y.C. Pb--Zn deposit are detrital from nearby magmatic bodies. However, Page (1981) has interpreted his U--Pb age of 1690 +-29/25 Ma as that of synsedimentary volcanically-expelled and water-laid deposits at a more or less unique point in time. In any case the feld. spar beds, at least in the H.Y.C. deposit are therefore formed in between 1590--1690 Ma (Kralik et al., 1982). The dolomite siltstone from the upper part of the BMR drill hole No. 2 (ca. 23 km SE of the H.Y.C. deposit) is supposed to be drilled through the Reward Dolomite and the upper part of the Barney Creek Formation. However, the lithologies are not typical of Barney Creek and the possibility that they may belong to the even higher Lynott Formation cannot be excluded (M. Muir, personal communication, 1981). The Rb--Sr ages of the Hobblechain Rhyolites and the Packsaddle Microgranites (Fig. 1) obtained by Webb (1974) and McDougall et al. (1965} cannot be regarded as reliable lower brackets as all the authors report weathering problems on their samples. Estimates of the true ages in the Tawallah Group might be the Rb--Sr age from the Kombolgie Formation (1650 +30 Ma, Page et al., 1980), stratigraphically correlated with the Twallah Group. The oldest glauconite of 1606 Ma (K--At) from the Aquarium Formation, reported by Webb et al. (1963), may be the best minimum age available as these authors analyzed discrete dark green pellets with a K content of 6.43%, which is characteristic for good glauconites. For similar reasons, as in the case of the Hobblechain Volcanics and the Packsaddle Granites, the reported ages from the basement (Cliffdale Volcanics and Nicholson granite complex} seem somewhat too low and the real ages can be expected to be older than 1800 Ma at least. Even with these additional age determinations the time scale of the Precambrian McArthur basin sequences rests still in the preliminary stages. However, due to the lack of any sign of metamorphism and the availability of investigations such as direct isotopic age determinations on carbonate rocks, the McArthur basin is well suited for further investigations on the middle Precambrian chronostratigraphy. CONCLUSIONS Clay minerals separated out of carbonate rocks by the less aggressive cationexchange method enable us to obtain at least approximate ages of stratigraphic horizons. With this sort of investigation two new data have been obtained from the dolomite horizons; one, with an age of 1429 +31 Ma, lies on the top of the Precambrian sequence in the McArthur basin (Kyalla Member, McMinn For-

168 mation) and a second one lies more or less in the middle with an age of 1537 +52 Ma (at the top of the unmineralized Barney Creek Formation). The first one is of special interest as it allows us to put the upper limit of the Carpentarian unit at about 1400 Ma. In addition the eukaryotes described from the Kyalla Member by Peat et al. (1978) are confirmed to be the oldest found in Earth history. From the second age determination we deduce that the Barney Creek Formation is older than 1500 Ma. Even the analyzed dolomite could already belong to the next higher Reward Dolomite or the L y n o t t Formation. The age of 1690 +29/25 Ma yielded by U--Pb zircon data from a feldspar bed in the Barney Creek Formation (Page, 1981) is regarded as the upper limit, regardless of the simultaneously expelled or inherited, detrital nature of the zircons. ACKNOWLEDGEMENTS I wish to thank the Bureau of Mineral Resources, Canberra, and the Northern Territory Department of Mines, Darwin, for letting me sample the drill cores. This work was supported by an Australian National University Scholarship. I am particularly indebted to W. Compston for his guidance and the use of the facilities in his laboratory. I am grateful to R. Page, K. Plumb, J. Richards and N. Williams (Canberra) for stimulating discussions, S. Chaudhuri and M. Muir for improving the paper by critical comments and review, I. McLennan for polishing the English, L. Leitner for drafting the figures and M. Stelzhammer for typing the manuscript. APPENDIX 78/17 Drill core of the Kyalla Member (McMinn Formation), Sherwin Creek coreshed Roper River, Northern Territory (134°17'1"E, 14°13'13"S. The drill core was presented to the author by Dr. M. Muir. Medium till, light grevy, wavy parallel, sial-feldspathic dolomite siltstone. Coarser and f i n e , r a i n e d laminae alternate with a variable content of organic carbon. 78/16 Drill core, 59.6 m, BMR No. 2 (135056'26 '' E, 16034'22 '' S) ca. 23 km SE of the McArthur River Pb--Zn deposit (Northern Territory, Barney Creek Formation or L y n o t t Formation), Black homogeneous till, even parallel discontinuous laminated sialfeldspathic dolomite siltstone, Fine-grained dolomite (mean = 11 pm) and angular quartz and feldspar grains. Pyrite is almost exclusively present as framboidal pyrite. REFERENCES Aronson, J.L. and Hower, J., 1976. Mechanism of burial metamorphism of argillaceous sediments. 2. Radiogenic argon evidence. Geol. Soc. Am. Bull., 87: 738--744.

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