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A thermal history of the Proterozoic East Alligator River Terrain, N.T., Australia: a fission track study
101
Tecronophysics, 145 (1988) 101-111 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
A thermal history of the Proterozoic East Alligator River Terrain, N.T., Australia: a fission track study SOHAN
L. KOUL I, AR. WILDE
* and AWTAR
K. TICK00
**
’ Division of Materials Science and Technolog), C.S.I.R.O., Locked Bag 33, Clayton, Vie. 3168 (Awtraliaj 2 Department of Earth Sciences, Monash University, Clayton, Vie. 3168 (Australia} (Received June 30,1986; revised version accepted May 81987)
Abstract Koul, S.L., Wilde, A.R. and Tickoo, A.K., 1988. A thermal history of the Proterozoic East Alligator River Terrain, N.T., Australia: a fission track study. Tectonophysics, 145: 101-111. Radiometric data indicate a major therma.l event in Proterozoic rocks of the East Alligator River Terrain, at 1870 Ma. These data, together with metamorphic mineral assemblages, demonstrate peak temperatures in excess of 600 o C, close to the melting temperature of more deeply buried rocks. A cooling rate following peak metamorphism of 3”C/Ma is suggested. Fission-track dates of peak metamorphic phases, however, reveal a thermal event (or events), after 1650 Ma, rather than the peak metamorphic event. This rise in temperature was the result of thermal bl~eting of the metamorphic basement by Carpentarian sediments and anomalous radiogenic heat flow from underlying granitoid gneiss. The temperatures so generated ( 2 175 o C) were insufficient to reset Rb-Sr and K-Ar systems, but are clearly in excess of F.T. annealing temperatures for a.ll the phases investigated. A cooling history, extending over 1000 m.y. and reflecting gradual erosion of the sedimentary cover, is revealed. This history is consistent with the extraordinary tectonic stability of the region. The importance of F.T. studies in establishing a thermal history is underscored, particularly when maximum temperatures experienced were less than those required to reset Rb-Sr and K-Ar systems.
Introduction The fission-track method has won increasing acceptance in geochronology, not only for the straightforward dating of igneous and metamorphic events but also for s~ati~ap~c studies of ash layers, studies of hydrothe~~ alteration and in archaeology (Naeser et al., 1973; Koul et al., 1981; Nishimura, 1971). Perhaps of greater significance than the straightforward dating of a geological sample is the determination of its thermal history. The track registration in minerals is, in fact, both temperature and time dependent and partial annealing serves to shorten the length of
* Professor of physics, J&K, India. 0040-1951/88/$03.50
0 1988 Elsevier Science Publishers B.V.
the tracks. Thus, the thermal history of the rock formation can be obtained by analysing not only the density of tracks but also their length distribution. Most of such studies to date have employed the accessory minerals apatite, sphene and zircon, whose track-retentive properties are now well documented and suitable for the applications mentioned above. The present work continues along these lines and we report here reconnaissance studies of metamorphic minerals from the East Alligator River Terrain, including zircon, garnet, epidote, apatite, muscovite, phlogopite, biotite and chlorite. The Past Alligator River Terrain @ART), of Australia’s Northern Territory is part of a large expanse of Archaean and Proterozoic rocks, collectively referred to as the Pine Creek geosyncline
102
R
EAST
ALLfQATOR
RIVER
RE@OR
’
”
$ Katherine 9
PNANEROZOtC COVER ROCKS CARPENTARIAN
SANDSTONE
LOWER PROTEROZOIC
:
h VOLCANIC3
VOLCANlCS GRANITIC
INTRUSIVES
METASEDIMENTS
ARCNAEAN WEISS
Fig. 1. Regional geology of the Pine Creek geosynche.
(Fig. 1) which contains one of the largest areas of Proterozoic rocks in the world. In this paper the early Proterozoic history (1900-1650 Ma) of the EART is documented using existing radiometric isotope data (of Page et al., 1980, and Riley et al., 1980). This paper presents new fission track data, which are used to examine the late Proterozoic and Phanerozoic evolution of the EART. This later time period has additional significance in
(From Needham
and Stuart-Smith,
1985.)
that it encompasses the development of important uranium deposits of unconfo~ty-relate type. Tectonostrati~phy The outcrop of the Proterozoic and Archaean rocks within the EART is poor; much of the area being blanketed by alluvium, or weathered to considerable depths. The overall distribution of units
103
depicted in Fig. 2. Delineation of a detailed stratigraphy is hampered by lack of outcrop and polyphase deformation, which has resulted in the superimposition of upright, open folds on one or more episodes of isoclinal and recumbent structures (Johnston, 1984). Localized folding associated with reverse faulting is further superimposed is as
CARPENTARIAN
(-155OMa)
Sandstone (Kombolgie
a
on these events, resulting in considerable structural complexity. The oldest rocks are Archaean gneisses which are unconformably overlain by metasediments and amphibolite of Lower Proterozoic age (Needham, 1984). These rocks were intruded by post-tectonic granitoids and tholeiitic sills of Oenpelli Dolerite.
& volcanics Formation)
LOWER PROTEROZOIC
( > 2000Ma)
cl
Amphibolite facies metamorphic rocks (includes Cahill Formation, Myra Falls Metamorphic81
1+‘+1
Granulite facies metamorphic rocks (Nimbuwah complex and syn-tectonic
granite)
ARCHAEAN m
Gneisa & migmatlte
/
Fault 0
(Nanambu
Complex)
Sample locations
Fig. 2. Geology of the East Alligator River Terrain (with alluvium removed for darity). Modified from Needhn (1984).
(1984) and Johnston
104
Unconformably overlying these rocks is a 2.0 km thick sequence of alluvial sandstones and mafic Nungbalgbarri volcanic rocks of the Kombolgie Formation. With the exception of rare Proterozoic dykes and faults, the period following Kombolgie Formation deposition was one of inactivity. By the late Cretaceous, erosion of the Kombolgie Formation was advanced and unroofing of the Lower Proterozoic and Archaean basement rocks had commenced.
+ 6 Ma (Page et al., 1980). Rb-Sr and K-Ar dates on micas, however, are considerably younger than this age and fall in the range 1850-1780 Ma (Page et al., 1980). Both biotite and muscovite show textural evidence of development during the peak thermal event (Johnston, 1984). The corresponding data are interpreted as indicative of cooling, rather than crystalization ages. The range of Rb-Sr and K-Ar mica ages is depicted in Fig. 3 together with possible ranges for the respective closure temperatures. A best fit cooling path is drawn through the data points. The large spread in biotite ages is believed to be an artifact of later alteration, since in thin-section and in back-scattered-electron images there is frequently chlorite interlayered with biotite. Indeed a biotite from close to the Nabarlek uranium deposit gave a K-Ar age of 1583 + 32 Ma (Page et al., 1980) which is almost certainly attributable to the effects of hydrothermal alteration associated with uranium mineralization. Thus only the older ages were used in construction of the curve. Further constraints on the curve can be imposed from the presence of post-tectonic granite intrusives and the Oenpelli Dolerite sill. Since both these intrusive rock types have metamorphic aureoles (Ferguson and Needham, 1978) their host rocks must have been relatively cool at the time of emplacement, approximately 1750 Ma in the case of the granitoids and 1688 t 13 Ma in the case of
1870 Ma peak metamorphic event
Peak metamorphism was synchronous with early deformation (isoclinal folding). Rocks to the west of the EART reached amphibolite facies conditions of less than 650” C at over 4-6 kbar. In the eastern part of the EART (Fig. 2) rocks of the Nimbuwah complex were buried somewhat deeper, and attained granulite-facies metamorphic conditions. These rocks would in some cases have been close to their water-saturated solidus temperature. Ferguson (1980) has estimated that peak temperatures of 700-800 o C at pressures of 8-10 kbar were experienced. Recovery history from isotopic data
Zircons extracted from syntectonic granitoids allow the dating of the metamorphic peak at 1866
‘I’
600 /-
I
a
\
n
RILEY
ET ALh980)
l
PAGE
ET AL.(1980)
I 500 -
‘i;
POST-TECTONIC
400 I 2 if I ?
K-Ar MUSCOVITE 300 -
200 -
K-Ar,
POST-TECTONIC KOMBOLGIE
100 t
0
L
I
1900
I
MILLIONS
Fig. 3. Cooling history from peak metamorphic
YEARS
conditions,
I
1600
1700
1800 BEFORE
PRESENT
e
utilising Rb-Sr and K-Ar isotopic data of Page et al. (1980) and Riley et
al. (1980). Biotite data is probably unreliable; see text for discussion.
105
the dolerite. Deposition of the Kombolgie Formation at prior to 1650 Ma places a m~mum constraint on the cooling period, since the basement rocks must obviously have cooled to ambient temperatures at this time (Koul et al., 1984). Thus the
cooling history (as depicted in Fig. 3) is protracted, lasting for up to 200 Ma. Using the 200 Ma approximation the mean cooling rate was of the order of 3 o C/Ma. This is somewhat faster than the 0.4”C/Ma. calculated
TABLE 1 Description of samples selected * Sample
Location
Depth
Mineral
Etching conditions
Description
Nabarlek Mine
SK/AT/15
N29(pit)
zircon
NaOH: KOH eutectic mixture for 7-25 hr.
From intensly altered semi-pehtic schist, close to uranium mineralization.
SK/AT/17
N29(pit)
zircon
As above
As above.
SK/AW/9
DHlll
109 m
garnet
SON NaOH boiling for l-8 hr.
From weakly altered schist, circa 1 km from mineralization, garnet is peak metamorphic (pre-Dz) phase.
SK/AT/33
DH2
39 m
muscovite
48% HF at 23°C for 40-70 min.
Peak metamorphic mica, residual phase in intensly altered, semi-pelitic schist.
SK/AT/35
DH178
115 m
chlorite
40% HF at 23’C for 5-15 min.
Chlorite after biotite, hydrothermal alteration product.
SK/AT/3
DH120
191 ft
epidote
50N NaOH at 160 OC for 1-3 hr.
From vein in amphibohte, with dolomite.
SK/AW/3
DH120
191 ft
epidote
As above
As above.
SK/AW/33
DH120
221 ft
muscovite
As above
Peak metamorphic mica, defining S, foliation in semi-pelitic schist.
SK/AT/10
BHK-10
biotite
4O%HFat23“C for 1-4 mm.
Peak metamorphic biotite, defining Sz foliation, circa 1 km from uranium mineralization.
SK/AW/SI
DH120
221 ft
chlorite
As above
Pseudomorphous replacement of metamorphic biotite, due to hydrothermal alteration.
SK/AT/70
DH85
227; ft
apatite
20% HN03 at 23 o C for lo-40 s,
Brecciated and sihcified Komboigie sandstone. Apatite is a gangue phase, associated with hydrothermal alteration and uranium deposition.
SK/AW/lO
DHl
386 ft
biotite
As above
Peak metamorphic biotite, Archaean gneiss, defining S, foliation.
Anomaly H SK/AW/25
DH3
1065 ft
phlogopite
4856HFat23OC for 4-10 nun.
From dolomite and calcite marble, defining S, foliation.
Koangarra deposit
Anomaly Q
* The constants used in the age calculations were hr (spontaneous decay constant 238U) = 6.8 x lo-l7 y-‘, A, (total decay constant of uranium) = 1.55 x lo-” y-l, I (isotopic abundance of 235U with respect to 238U) = 7.253 X 10s3 and er (cross section for thermal neutron induced fission of 235U) = 5.802 x lo-** cm*.
106
for Archaean gneisses of Scotland (Q’Hara, 1977) and 2” C/Ma for Hercynian gneisses of the Southern Alps (Wilde, 1983). It is much slower than for the Alpine orogeny, in contrast. This probably reflects intermediate rates of erosion, and presumably uplift, operating in the East Alligator River Terrain during this time. The fact that granulite-facies rocks are only exposed to the northeast suggests that uplift and erosion was somewhat greater to the east of the region.
muscovite this is supported by Rb-Sr and K-Ar studies (Page et al., 1980; Riley et al., 1980). Chlorite and epidote are alteration products associated with uranium mineralization. The specimen preparation technique was similar with minor modification to that described by Koul et al. (1983). The optimum etchant for epidote, garnet, muscovite, chlorite and apatite was established by observing the degree and nature of the track etching which resulted from the use of various chemical reagents shown in Table 2. The effectiveness of various etchants of differing concentration was studied as a function of time and temperature. Figure 4 shows a plot of etching efficiency (the optimum etching time corresponding to maximum track density) against etching time in different etchants for epidote, garnet, muscovite and chlorite. Zircon, epidote, and garnet separates were dated by external detector method and samples of muscovite, biotite, phlogophite and chlorite were all dated by the internal method (Naeser et al., 1973; Koul et al., 1983). All the irradiations were performed in the HIFAR (Australia) reactor. Neutron fluences were monitored using the National Bureau of Standards SRM962 and SRM963 glasses calibrated against Au values. The etchants used in
Material selection and preparation Thirteen .samples of Lower Proterozoic rocks were selected from the vicinity of the Nabarlek and Koongarra uranium deposits and anomaly H and anomaly Q prospects (Fig. 2), in order to study the fission-track ages of various minerals. Descriptions of these samples are given in Table 1. Owing to poor outcrop and deep weathering all the samples used are from diamond drillcore. The samples were selected to be representative of the major Lower Proterozoic and Archaean lithologies present. The minerals phlogopite, muscovite, biotite and garnet have textural evidence of development during the metamorphic event. For biotite and
TABLE 2 Fission-track results for the minerals from Earuf, N.T. Mineral Zircon Zircon Epidote Epidote Garnet Pblogopite Musco&e Muscovite Biotite Biotite Chlorite Chlorite Apatite
a
Sample location
Lab. symbol
No. counted
PS
Nabarlek Nabarlek Nabarlek Nabarlek Nabarlek Anomaly H. Nabarlek Koongarra Koongarra Anomaly Q Nabarlek Koongarra Koongarra
SK/AT/l SK/AT/17 SK/AT/3 SK/AW/4 SK/AW/O SK/AW/ZS SK/AT/33 SK/AW/34 SK/AT,‘10 SK/AW SK/AT/35 SK/AW/36 SK/AW-l
8 6 3 4 11 18 4 9 7 9 3 5 13
1.23 x 5.87 x 6.16 x 6.68 x 7.65 x 6.15 x 4.25 x 5.41 x 4.72 x 5.41 x 4.65 x 5.21 x 2.47 x
a ps is the spontaneous track density. ’ pi is the induced track density. ’ + is the thermal neutron dose (n cmm2).
PI
(cmp2)
h
(cms2) 10’ 10’ lo6 106 lo5 lo5 IO5 lo5 lo4
104 lo3 lo3 lo6
2.13 x 1.72 x 1.85 x 2.25 x 1.16 x 2.43 x 1.22 x 1.62 x 2.01 x 2.12 x 1.27 x 1.57 x 1.45 x
10’ lo7 106 lo6 lo6 10” 10” 10’ 10s 10s lo4 104 10h
+”
F.T. age
(n cm-‘)
(Ma)
7.55 x 7.55 x 7.55 x 7.55 x 3.65 x 6.31 x 5.47 x 5.41 x 6.31 x 6.31 x 3.65 x 3.65 x 4.85 x
1416.7 + 89 1423.6 + 94 1392.4 rf-82 1255.3 + 78 1242.3 It 65 968.6 + 52 1082.0 + 59 1040.7 & 62 856.6 +- 46 925.7 i 47 763.2 + 43 708.5 * 41 491.5 + 32
lOI 10’5 10’5 10’5 lOI6 lOI 10’5 10’5 1Or6 toI 10lh 1Or6 1015
107
80
2
0
120
80
180
ETCHING TIME (mid Fig. 4. Etching efficiency
of epidote, garnet, muscovite
the present study are listed in Table 1. Fossil and induced fission tracks were counted using conventional optical microscopy and the interactive image analysis technique (Fig. 5). Tracks having a random distribution and with a typical track length of 12.5 pm for chlorite (Fig. 5a) were observed. The tracks could easily be distinguished from surface dislocations. Fission-track
evidence
on Carpentarian
thermal
history
Annealing The possibility of annealing of radiation damage in minerals and hence of track fading, always exists. This effect can be in response to a short time, high-temperature event, or can be caused by a prolonged annealing, at only a slightly elevated temperature during the geothermal history of the earth’s crust. Experiments of the kind detailed are very useful, since they provide vital information from the geological past on the thermal history and characteristic closing temperature of a mineral. Moreover, knowledge of the fission-track annealing behaviour for the mineral also is vital if proper interpretation of detailed observations is expected to yield a reliable fission-track age. It is because of the thermal fading of fission tracks in minerals that the fission-track dating technique can be used
240
-----w
and chlorite for treatments by the etchants indicated.
to provide a sensitive geothermometer for assessing the intensity and timing of the thermal events in the history of rocks. The specific mineral samples selected for the annealing experiment included zircon, epidote, garnet, muscovite, phlogopite, chlorite, biotite and apatite. All the minerals were separated from crushed rock using common laboratory techniques, and samples polished by the usual polishing procedures. After annealing spontaneous fission fragment tracks in zircon at 670 ’ C, epidote at 620 ’ C, garnet at 600 o C, muscovite at 590 o C, chlorite at 450 ’ C and apatite at 390’ C, samples were irradiated by thermal neutrons (10’5-10’6 n cmm2) in the HIFAR reactor to create induced fission tracks. The induced fission-track density was then determined by etching aliquots of mineral sample. The irradiated samples of each mineral were then heated in a furnace at different temperatures at intervals of 20-30 OC, and for times varying from few minutes to several hours. The annealed samples were repolished and etched and scanned for any change of track density and compared with their corresponding unamrealed values of track density. The percentage of tracks faded after each run are thus evaluated and are depicted in a manner as shown for chlorite in Fig. 6. In general the annealing of fission tracks in minerals is a typical activated rate process which is best represented by the Arrhenius relationship:
108
c
CHLORITE
10’ ANNEALING TIME (mid +
Fig.
6. Experimental
versus annealing
Fig. 5. Spontaneous induced
fission fragment
fission fragment
active image analysis
tracks
tracks
in (a) chlorite
in (b) epidote
and
using the inter-
results
for reduction
time of fission tracks
in track
density
in chlorite.
this graph represent 100% track losses determined at various times and temperatures. The data have been extrapolated to geologically meaningful times and temperatures. Extrapolation of the experimentally determined temperatures suggest that temperatures of 175-70°C in lo9 years will erase all the tracks in minerals listed in Fig. 7. Annealing studies of minerals from the EART confirmed that radiation damage of fossil fission fragment tracks in these minerals can be erased during intense metamorphic episodes, thereby resetting the geological clock. It is also clear, that the annealing experiments (Fig. 7) confirm the instability of fission tracks in zircon, muscovite, apatite, garnet, biotite and chlorite. As with other radiometric methods, the F.T. age of the selected minerals (Table 2) determine neither the age of peak metamorphism nor the depositional age of the rock. It is the cooling age of the minerals which is established.
system.
Fission-track ages log,t = {log,a
+ (E/U-)}
0)
where a is a characteristic constant 1 and T are the annealing time and temperature respectively, E is the activation energy and K is Boltzmann’s constant. Figure 7 shows that the annealing behaviour of the minerals, zircon, epidote, granite, muscovite, phlogopite, chlorite and apatite also is well described by eqn. (1). Individual points on
The fission track ages presented here (Table 2, Fig. S), augment and expand earlier data, presented by Koul et al. (1984). The ages are among the oldest fission-track ages yet recorded. Results for the Nabarlek area are comparable with those from Koongarra and anomalies Q and H. It is evident that the minerals which are clearly of peak metamorphic origin have fission-track ages which
109 TEMPERATURE(k)
_,
c
$6
rp
lo8
4
106
a 4 t y lo* i=
10 +I
I
I
1.2
1.6
1 2.0
-
EXPERIMENTAL
-----
EXTRAPOLATED
I
I
1
I
2.4
2.0
3.2
3.6
TEMPERATURE lOOO/TF’K) Fig. 7. Annealing extrapolated
times
as a function
to geologically
meaningful
of annealing
temperatures
for track
minimum exchange,
This
plotted
is consistent Ma
200
with
or older,
a thermal
which
reductions
(100%).
Experimental
results
are
times and temperatures.
are substantially younger than their Rb-Sr and K-Ar ages (Riley et al., 1980; Page et al., 1980). 1428
-,
density
event
did
not
at circa
exceed
the
closure temperature for Rb-Sr i.e. < 350 o C. The fission-track
or K-Ar data are
in Fig. 8. It can be seen that the ages and
relevant
annealing
temperatures
define
a smooth
Deposition of the Kombolgie Formation Mudginberri Phonolite 8 Wurogoii Dolerite
150 ij e E 3 -G t E” :
100
Rare dolerite
\
I
50
I
I
1.7
I
I
I
1
0.9
1.3
0.5
Age (yrs x 10’)
Fig. 8. Plot showing temperatures
the inferred
cooling
history
are given in text. The segment
of the older basement
of rocks from the EART,
of the curve from 1650-1600
rocks by a thick pile of sediments
at 1650 Ma.
during
the period
Ma assumes
of 1600-400
rapid temperature
Ma. Sources increase
of annealing
as a result of burial
110
cooling path Phanerozoic.
from 1428 Ma to well into the
TEMPERATURE 0
100
200
[“C) 300
400
Discussion and conclusions
Temperature in the basement rocks exceeded the bung temperature of zircon (2 175 a C for a lo9 year cooling history) following deposition of the Kombolgie Formation (Fig. 8). Evidently this thermal event did not reach temperatures high enough to reset Rb-Sr and K-Ar mica ages. Furthermore this thermal event peaked at some time between 1650 Ma (the depositional age of the Kombolgie formation) and 1428 Ma (the zircon F.T age). A possible expl~atio~ for this event is heat exchange between mafic extrusives (Nungb~g~ri Volcanics) within the Kombolgie Formation and surrounding rocks. Since the volcanics are dated at 1648 f 29 Ma (Page et al., 1980), the fissiontrack age would imply an unreasonably long cooling history (- 220 Ma) for a sequence which is less than 200 m in thickness. For the same reason anomalous heat flow from the mantle, which could be reflected in volcanism, is ruled out. It is necessary, therefore, to postulate either a highly anomalous geothermal gradient or a greater thickness of superincumbent sediments than that which now remains in the EART, or indeed a combination of both factors. It is apparent that if temperatures in excess of 175 ’ C were achieved in the basement rocks with a cover of 2.0 km of sediments and volcanics, an anomalous geothermal gradient comparable with modern geothermal systems is implied {Fig. 9). If a gradient typical of modern sedimentary basins is postulated, then basement rocks may have been buried beneath sediments up to 5 km thick. This hypothesis requires that all vestiges of the uppermost 3.0 km of cover have been completely eroded (Koul et al.. 1984). Archaean gneiss and post-tectonic Lower Proterozoic granite intrusives which underlie much of EART contain anomalous concentrations of uranium (up to 35 ppm-Ferguson et al., 1980). Such high concentrations of uranium (and potassium and thorium) would result in high heat flow (probably in excess of 13 cal cm3 s-l). Thus we
SALTON SEA (ELMOREal) GEOTHERMAL
SASS0 c 1 GEOTHERMAL
4ooo, Fig. 9. Extreme geothermal gradients, from typical sedimentary basins and present day geothermal systems. The gradient developed following Kombolgie Formation deposition is thought to be cl&e to the upperbounds of the range for sedimentary basins, given evidence for higher heat flow in the Proterozoic relative to the present day.
conclude that the thermal event indicated by the zircon F.T. age is the result of burial of the basement by at least 2.0 km of alluvial sandstone and volcanics and probably another 3 km of sediments coupled with radiogenic heat flow. The remaining data can not readily disting~sh between whether the EART cooled slowly and steady over the period 1420-491 Ma, as a result of declining radiogenic heat production and gradual stripping of the insulating sedimentary cover, or whether there were several relatively short-term low magnitude thermal events. Known geological events during this time are Wurogoij Dolerite dykes at 1370 + 30 Ma and Mudginberri Phonolite dykes at 1316 f 48 Ma (Page et al., 1980). The 491 f 32 Ma apatite age appro~mates to rare 522 Ma dolerite dykes (Page et al., 1980). It is intuitively unreasonable that such small additions of magma could raise the temperature of the host rocks anything but locally and for a relatively short time. These events may however be a mani-
111
festation of increased heat flow from the mantle. There is no reason to suppose that the other F.T. data represents other than gradual diminution of temperatures as the result of erosion of a thick sedimentary cover. The F.T. data have significance with respect to the genesis of unconformity-related uranium deposits. Recent Nd-Sm dating of primary Nabarlek ore (R. Maas, pers. commun, 1987) and reinterpretation of existing Rb-Sr and K-Ar ages (of Page et al., 1980) suggest primary mineralization at 1560 Ma. Thus the deposits would have formed in host rocks which already had achieved temperatures of 180 o C as a result of burial by the overlying sedimentary rocks. This is consistent with fluid inclusion data on breccia-cementing quartz which indicate temperatures of this order (Wilde, unpubl. data; Ypma and Fuzikawa, 1980). In conclusion, this study demonstrates the importance of fission-track studies in elucidating the thermal history of a rock. This utility is enhanced if peak temperatures were, as in this case, below the closure temperature for Rb-Sr and K-Ar exchange, and such that recrystallization did not occur. The fission track method has the ability to detect low temperature events which could not be recognized by conventional radiometric isotopic techniques. This is clearly demonstrated by the case of biotite and muscovite, both peak metamorphic phases, with Rb-Sr and K-Ar ages in excess of 1750 Ma, but whose F.T. ages are less than 1100 and 950 Ma respectively, a radical difference in apparent age.
Ferguson,
J. and Needham,
lower
Proterozoic
from the Northern Ferguson,
of economic
Alligator
uranium
River
Goleby
(Editors),
IAEA, Vienna, Johnston, inlier
J., 1980. Metamorphism
Ferguson
and A. Goleby
Creek Geosyncline.
in the Pine Creek Geosyn-
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I.A.E.A.,
Vienna,
correlations. Uranium
In: J.
in the Pine
pp. 91-100.
a
suite
T.H., 1980. Model for mineralisation
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in the
J. Ferguson
and
A.
in the Pine Creek Geosyncline.
pp. 563-574.
and
mineralisation Ph.D
thesis,
evolution therein,
Monash
of the Pine Creek Northern
University,
Territory, Clayton,
Vie.
(unpubl.). Koul,
S.L., Chadderton,
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S.L., Chadderton,
Brooks,
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of the East Alligator
em Territory,
C.K.,
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Dan. Vid. Selsk., 40 (13) l-37.
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C.K., 1981. Fission
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D.J., 1984. Fission
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Abstr. Geol.
Sot. Aust., 12: 311-313. Naeser.
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G.A.
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Vienna,
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S.J., 1980. Rb-Sr
In: J. Ferguson
chro-
and A. Goleby
in the Pine Creek geosyncline.
I.A.E.A.,
pp. 457-468.
A.R.,
1983.
covery history Italy).
and
Uranium
In: J. Ferguson
J. Geol.
R.S., 1980. Geo-
of basement Rivers
of Archaean
crust.
pp. 39-68.
Riley, G.H.,
Vienna,
Territory.
J., 9: 233-238.
London, 134: 185-200. Page, R.W., Compston, W. and Needham, chronology
strati-
of early Proterozoic
region,
Nature,
M.J., 1977. Thermal
gneisses
Territory. Series, Bur.
44 pp.
Aus., Geol. Geophys.
S., 1971.
materials
Zircon
1985. Revised
and correlation
rocks of the Darwin-Katherine Nishimura,
1973.
notes to the 1: 100,000 Geological
Miner. Resour. Needham,
R.E.,
1: 93-95.
1984.
Explanatory
M.Sc.
Geothennometry,
of the Finer0 thesis,
Univ.
geobarometry
ultramafic-mafic Coll, Wales,
and
complex
Aberystwyth
re(N. (un-
publ.). P.J. and Fuzikawa,
isotope
and its bearing
Uranium
1984. Structural
studies
deposits,
cline
dolerite:
tholeiitic
J. Geol. Sot. Aust.,
uranium
field.
D.J.,
Australia.
Ypma,
Ferguson,
Australia.
J., Ewers, G.R. and Donnelly,
tors),
References
Territory,
the development
Territory,
We thank Queensland Mines Ltd. and Denison Australia Pty. Ltd. for access to the Nabarlek and Koongarra deposits. The project was partly supported by Queensland Mines Ltd. We are grateful to Dr. M.J. Kenny for the help during sample irradiations. We would like to thank Mr. J.S. Noakes for cooperation and interest in this work.
continental
25: 309-322.
rocks
Acknowledgements
R.S., 1978. The Zamu
pre-erogenic
A. Goleby
Northern
Territory,
(Editors),
cline. I.A.E.A.,
K., 1980. Fluid inclusion
of the Nabarlek Uranium
and
Australia.
Jabiluka
and oxygen uranium
In: J. Ferguson
and
in the Pine Creek Geosyn-
Viemta, pp. 375-395.
Tecronophysics, 145 (1988) 101-111 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
A thermal history of the Proterozoic East Alligator River Terrain, N.T., Australia: a fission track study SOHAN
L. KOUL I, AR. WILDE
* and AWTAR
K. TICK00
**
’ Division of Materials Science and Technolog), C.S.I.R.O., Locked Bag 33, Clayton, Vie. 3168 (Awtraliaj 2 Department of Earth Sciences, Monash University, Clayton, Vie. 3168 (Australia} (Received June 30,1986; revised version accepted May 81987)
Abstract Koul, S.L., Wilde, A.R. and Tickoo, A.K., 1988. A thermal history of the Proterozoic East Alligator River Terrain, N.T., Australia: a fission track study. Tectonophysics, 145: 101-111. Radiometric data indicate a major therma.l event in Proterozoic rocks of the East Alligator River Terrain, at 1870 Ma. These data, together with metamorphic mineral assemblages, demonstrate peak temperatures in excess of 600 o C, close to the melting temperature of more deeply buried rocks. A cooling rate following peak metamorphism of 3”C/Ma is suggested. Fission-track dates of peak metamorphic phases, however, reveal a thermal event (or events), after 1650 Ma, rather than the peak metamorphic event. This rise in temperature was the result of thermal bl~eting of the metamorphic basement by Carpentarian sediments and anomalous radiogenic heat flow from underlying granitoid gneiss. The temperatures so generated ( 2 175 o C) were insufficient to reset Rb-Sr and K-Ar systems, but are clearly in excess of F.T. annealing temperatures for a.ll the phases investigated. A cooling history, extending over 1000 m.y. and reflecting gradual erosion of the sedimentary cover, is revealed. This history is consistent with the extraordinary tectonic stability of the region. The importance of F.T. studies in establishing a thermal history is underscored, particularly when maximum temperatures experienced were less than those required to reset Rb-Sr and K-Ar systems.
Introduction The fission-track method has won increasing acceptance in geochronology, not only for the straightforward dating of igneous and metamorphic events but also for s~ati~ap~c studies of ash layers, studies of hydrothe~~ alteration and in archaeology (Naeser et al., 1973; Koul et al., 1981; Nishimura, 1971). Perhaps of greater significance than the straightforward dating of a geological sample is the determination of its thermal history. The track registration in minerals is, in fact, both temperature and time dependent and partial annealing serves to shorten the length of
* Professor of physics, J&K, India. 0040-1951/88/$03.50
0 1988 Elsevier Science Publishers B.V.
the tracks. Thus, the thermal history of the rock formation can be obtained by analysing not only the density of tracks but also their length distribution. Most of such studies to date have employed the accessory minerals apatite, sphene and zircon, whose track-retentive properties are now well documented and suitable for the applications mentioned above. The present work continues along these lines and we report here reconnaissance studies of metamorphic minerals from the East Alligator River Terrain, including zircon, garnet, epidote, apatite, muscovite, phlogopite, biotite and chlorite. The Past Alligator River Terrain @ART), of Australia’s Northern Territory is part of a large expanse of Archaean and Proterozoic rocks, collectively referred to as the Pine Creek geosyncline
102
R
EAST
ALLfQATOR
RIVER
RE@OR
’
”
$ Katherine 9
PNANEROZOtC COVER ROCKS CARPENTARIAN
SANDSTONE
LOWER PROTEROZOIC
:
h VOLCANIC3
VOLCANlCS GRANITIC
INTRUSIVES
METASEDIMENTS
ARCNAEAN WEISS
Fig. 1. Regional geology of the Pine Creek geosynche.
(Fig. 1) which contains one of the largest areas of Proterozoic rocks in the world. In this paper the early Proterozoic history (1900-1650 Ma) of the EART is documented using existing radiometric isotope data (of Page et al., 1980, and Riley et al., 1980). This paper presents new fission track data, which are used to examine the late Proterozoic and Phanerozoic evolution of the EART. This later time period has additional significance in
(From Needham
and Stuart-Smith,
1985.)
that it encompasses the development of important uranium deposits of unconfo~ty-relate type. Tectonostrati~phy The outcrop of the Proterozoic and Archaean rocks within the EART is poor; much of the area being blanketed by alluvium, or weathered to considerable depths. The overall distribution of units
103
depicted in Fig. 2. Delineation of a detailed stratigraphy is hampered by lack of outcrop and polyphase deformation, which has resulted in the superimposition of upright, open folds on one or more episodes of isoclinal and recumbent structures (Johnston, 1984). Localized folding associated with reverse faulting is further superimposed is as
CARPENTARIAN
(-155OMa)
Sandstone (Kombolgie
a
on these events, resulting in considerable structural complexity. The oldest rocks are Archaean gneisses which are unconformably overlain by metasediments and amphibolite of Lower Proterozoic age (Needham, 1984). These rocks were intruded by post-tectonic granitoids and tholeiitic sills of Oenpelli Dolerite.
& volcanics Formation)
LOWER PROTEROZOIC
( > 2000Ma)
cl
Amphibolite facies metamorphic rocks (includes Cahill Formation, Myra Falls Metamorphic81
1+‘+1
Granulite facies metamorphic rocks (Nimbuwah complex and syn-tectonic
granite)
ARCHAEAN m
Gneisa & migmatlte
/
Fault 0
(Nanambu
Complex)
Sample locations
Fig. 2. Geology of the East Alligator River Terrain (with alluvium removed for darity). Modified from Needhn (1984).
(1984) and Johnston
104
Unconformably overlying these rocks is a 2.0 km thick sequence of alluvial sandstones and mafic Nungbalgbarri volcanic rocks of the Kombolgie Formation. With the exception of rare Proterozoic dykes and faults, the period following Kombolgie Formation deposition was one of inactivity. By the late Cretaceous, erosion of the Kombolgie Formation was advanced and unroofing of the Lower Proterozoic and Archaean basement rocks had commenced.
+ 6 Ma (Page et al., 1980). Rb-Sr and K-Ar dates on micas, however, are considerably younger than this age and fall in the range 1850-1780 Ma (Page et al., 1980). Both biotite and muscovite show textural evidence of development during the peak thermal event (Johnston, 1984). The corresponding data are interpreted as indicative of cooling, rather than crystalization ages. The range of Rb-Sr and K-Ar mica ages is depicted in Fig. 3 together with possible ranges for the respective closure temperatures. A best fit cooling path is drawn through the data points. The large spread in biotite ages is believed to be an artifact of later alteration, since in thin-section and in back-scattered-electron images there is frequently chlorite interlayered with biotite. Indeed a biotite from close to the Nabarlek uranium deposit gave a K-Ar age of 1583 + 32 Ma (Page et al., 1980) which is almost certainly attributable to the effects of hydrothermal alteration associated with uranium mineralization. Thus only the older ages were used in construction of the curve. Further constraints on the curve can be imposed from the presence of post-tectonic granite intrusives and the Oenpelli Dolerite sill. Since both these intrusive rock types have metamorphic aureoles (Ferguson and Needham, 1978) their host rocks must have been relatively cool at the time of emplacement, approximately 1750 Ma in the case of the granitoids and 1688 t 13 Ma in the case of
1870 Ma peak metamorphic event
Peak metamorphism was synchronous with early deformation (isoclinal folding). Rocks to the west of the EART reached amphibolite facies conditions of less than 650” C at over 4-6 kbar. In the eastern part of the EART (Fig. 2) rocks of the Nimbuwah complex were buried somewhat deeper, and attained granulite-facies metamorphic conditions. These rocks would in some cases have been close to their water-saturated solidus temperature. Ferguson (1980) has estimated that peak temperatures of 700-800 o C at pressures of 8-10 kbar were experienced. Recovery history from isotopic data
Zircons extracted from syntectonic granitoids allow the dating of the metamorphic peak at 1866
‘I’
600 /-
I
a
\
n
RILEY
ET ALh980)
l
PAGE
ET AL.(1980)
I 500 -
‘i;
POST-TECTONIC
400 I 2 if I ?
K-Ar MUSCOVITE 300 -
200 -
K-Ar,
POST-TECTONIC KOMBOLGIE
100 t
0
L
I
1900
I
MILLIONS
Fig. 3. Cooling history from peak metamorphic
YEARS
conditions,
I
1600
1700
1800 BEFORE
PRESENT
e
utilising Rb-Sr and K-Ar isotopic data of Page et al. (1980) and Riley et
al. (1980). Biotite data is probably unreliable; see text for discussion.
105
the dolerite. Deposition of the Kombolgie Formation at prior to 1650 Ma places a m~mum constraint on the cooling period, since the basement rocks must obviously have cooled to ambient temperatures at this time (Koul et al., 1984). Thus the
cooling history (as depicted in Fig. 3) is protracted, lasting for up to 200 Ma. Using the 200 Ma approximation the mean cooling rate was of the order of 3 o C/Ma. This is somewhat faster than the 0.4”C/Ma. calculated
TABLE 1 Description of samples selected * Sample
Location
Depth
Mineral
Etching conditions
Description
Nabarlek Mine
SK/AT/15
N29(pit)
zircon
NaOH: KOH eutectic mixture for 7-25 hr.
From intensly altered semi-pehtic schist, close to uranium mineralization.
SK/AT/17
N29(pit)
zircon
As above
As above.
SK/AW/9
DHlll
109 m
garnet
SON NaOH boiling for l-8 hr.
From weakly altered schist, circa 1 km from mineralization, garnet is peak metamorphic (pre-Dz) phase.
SK/AT/33
DH2
39 m
muscovite
48% HF at 23°C for 40-70 min.
Peak metamorphic mica, residual phase in intensly altered, semi-pelitic schist.
SK/AT/35
DH178
115 m
chlorite
40% HF at 23’C for 5-15 min.
Chlorite after biotite, hydrothermal alteration product.
SK/AT/3
DH120
191 ft
epidote
50N NaOH at 160 OC for 1-3 hr.
From vein in amphibohte, with dolomite.
SK/AW/3
DH120
191 ft
epidote
As above
As above.
SK/AW/33
DH120
221 ft
muscovite
As above
Peak metamorphic mica, defining S, foliation in semi-pelitic schist.
SK/AT/10
BHK-10
biotite
4O%HFat23“C for 1-4 mm.
Peak metamorphic biotite, defining Sz foliation, circa 1 km from uranium mineralization.
SK/AW/SI
DH120
221 ft
chlorite
As above
Pseudomorphous replacement of metamorphic biotite, due to hydrothermal alteration.
SK/AT/70
DH85
227; ft
apatite
20% HN03 at 23 o C for lo-40 s,
Brecciated and sihcified Komboigie sandstone. Apatite is a gangue phase, associated with hydrothermal alteration and uranium deposition.
SK/AW/lO
DHl
386 ft
biotite
As above
Peak metamorphic biotite, Archaean gneiss, defining S, foliation.
Anomaly H SK/AW/25
DH3
1065 ft
phlogopite
4856HFat23OC for 4-10 nun.
From dolomite and calcite marble, defining S, foliation.
Koangarra deposit
Anomaly Q
* The constants used in the age calculations were hr (spontaneous decay constant 238U) = 6.8 x lo-l7 y-‘, A, (total decay constant of uranium) = 1.55 x lo-” y-l, I (isotopic abundance of 235U with respect to 238U) = 7.253 X 10s3 and er (cross section for thermal neutron induced fission of 235U) = 5.802 x lo-** cm*.
106
for Archaean gneisses of Scotland (Q’Hara, 1977) and 2” C/Ma for Hercynian gneisses of the Southern Alps (Wilde, 1983). It is much slower than for the Alpine orogeny, in contrast. This probably reflects intermediate rates of erosion, and presumably uplift, operating in the East Alligator River Terrain during this time. The fact that granulite-facies rocks are only exposed to the northeast suggests that uplift and erosion was somewhat greater to the east of the region.
muscovite this is supported by Rb-Sr and K-Ar studies (Page et al., 1980; Riley et al., 1980). Chlorite and epidote are alteration products associated with uranium mineralization. The specimen preparation technique was similar with minor modification to that described by Koul et al. (1983). The optimum etchant for epidote, garnet, muscovite, chlorite and apatite was established by observing the degree and nature of the track etching which resulted from the use of various chemical reagents shown in Table 2. The effectiveness of various etchants of differing concentration was studied as a function of time and temperature. Figure 4 shows a plot of etching efficiency (the optimum etching time corresponding to maximum track density) against etching time in different etchants for epidote, garnet, muscovite and chlorite. Zircon, epidote, and garnet separates were dated by external detector method and samples of muscovite, biotite, phlogophite and chlorite were all dated by the internal method (Naeser et al., 1973; Koul et al., 1983). All the irradiations were performed in the HIFAR (Australia) reactor. Neutron fluences were monitored using the National Bureau of Standards SRM962 and SRM963 glasses calibrated against Au values. The etchants used in
Material selection and preparation Thirteen .samples of Lower Proterozoic rocks were selected from the vicinity of the Nabarlek and Koongarra uranium deposits and anomaly H and anomaly Q prospects (Fig. 2), in order to study the fission-track ages of various minerals. Descriptions of these samples are given in Table 1. Owing to poor outcrop and deep weathering all the samples used are from diamond drillcore. The samples were selected to be representative of the major Lower Proterozoic and Archaean lithologies present. The minerals phlogopite, muscovite, biotite and garnet have textural evidence of development during the metamorphic event. For biotite and
TABLE 2 Fission-track results for the minerals from Earuf, N.T. Mineral Zircon Zircon Epidote Epidote Garnet Pblogopite Musco&e Muscovite Biotite Biotite Chlorite Chlorite Apatite
a
Sample location
Lab. symbol
No. counted
PS
Nabarlek Nabarlek Nabarlek Nabarlek Nabarlek Anomaly H. Nabarlek Koongarra Koongarra Anomaly Q Nabarlek Koongarra Koongarra
SK/AT/l SK/AT/17 SK/AT/3 SK/AW/4 SK/AW/O SK/AW/ZS SK/AT/33 SK/AW/34 SK/AT,‘10 SK/AW SK/AT/35 SK/AW/36 SK/AW-l
8 6 3 4 11 18 4 9 7 9 3 5 13
1.23 x 5.87 x 6.16 x 6.68 x 7.65 x 6.15 x 4.25 x 5.41 x 4.72 x 5.41 x 4.65 x 5.21 x 2.47 x
a ps is the spontaneous track density. ’ pi is the induced track density. ’ + is the thermal neutron dose (n cmm2).
PI
(cmp2)
h
(cms2) 10’ 10’ lo6 106 lo5 lo5 IO5 lo5 lo4
104 lo3 lo3 lo6
2.13 x 1.72 x 1.85 x 2.25 x 1.16 x 2.43 x 1.22 x 1.62 x 2.01 x 2.12 x 1.27 x 1.57 x 1.45 x
10’ lo7 106 lo6 lo6 10” 10” 10’ 10s 10s lo4 104 10h
+”
F.T. age
(n cm-‘)
(Ma)
7.55 x 7.55 x 7.55 x 7.55 x 3.65 x 6.31 x 5.47 x 5.41 x 6.31 x 6.31 x 3.65 x 3.65 x 4.85 x
1416.7 + 89 1423.6 + 94 1392.4 rf-82 1255.3 + 78 1242.3 It 65 968.6 + 52 1082.0 + 59 1040.7 & 62 856.6 +- 46 925.7 i 47 763.2 + 43 708.5 * 41 491.5 + 32
lOI 10’5 10’5 10’5 lOI6 lOI 10’5 10’5 1Or6 toI 10lh 1Or6 1015
107
80
2
0
120
80
180
ETCHING TIME (mid Fig. 4. Etching efficiency
of epidote, garnet, muscovite
the present study are listed in Table 1. Fossil and induced fission tracks were counted using conventional optical microscopy and the interactive image analysis technique (Fig. 5). Tracks having a random distribution and with a typical track length of 12.5 pm for chlorite (Fig. 5a) were observed. The tracks could easily be distinguished from surface dislocations. Fission-track
evidence
on Carpentarian
thermal
history
Annealing The possibility of annealing of radiation damage in minerals and hence of track fading, always exists. This effect can be in response to a short time, high-temperature event, or can be caused by a prolonged annealing, at only a slightly elevated temperature during the geothermal history of the earth’s crust. Experiments of the kind detailed are very useful, since they provide vital information from the geological past on the thermal history and characteristic closing temperature of a mineral. Moreover, knowledge of the fission-track annealing behaviour for the mineral also is vital if proper interpretation of detailed observations is expected to yield a reliable fission-track age. It is because of the thermal fading of fission tracks in minerals that the fission-track dating technique can be used
240
-----w
and chlorite for treatments by the etchants indicated.
to provide a sensitive geothermometer for assessing the intensity and timing of the thermal events in the history of rocks. The specific mineral samples selected for the annealing experiment included zircon, epidote, garnet, muscovite, phlogopite, chlorite, biotite and apatite. All the minerals were separated from crushed rock using common laboratory techniques, and samples polished by the usual polishing procedures. After annealing spontaneous fission fragment tracks in zircon at 670 ’ C, epidote at 620 ’ C, garnet at 600 o C, muscovite at 590 o C, chlorite at 450 ’ C and apatite at 390’ C, samples were irradiated by thermal neutrons (10’5-10’6 n cmm2) in the HIFAR reactor to create induced fission tracks. The induced fission-track density was then determined by etching aliquots of mineral sample. The irradiated samples of each mineral were then heated in a furnace at different temperatures at intervals of 20-30 OC, and for times varying from few minutes to several hours. The annealed samples were repolished and etched and scanned for any change of track density and compared with their corresponding unamrealed values of track density. The percentage of tracks faded after each run are thus evaluated and are depicted in a manner as shown for chlorite in Fig. 6. In general the annealing of fission tracks in minerals is a typical activated rate process which is best represented by the Arrhenius relationship:
108
c
CHLORITE
10’ ANNEALING TIME (mid +
Fig.
6. Experimental
versus annealing
Fig. 5. Spontaneous induced
fission fragment
fission fragment
active image analysis
tracks
tracks
in (a) chlorite
in (b) epidote
and
using the inter-
results
for reduction
time of fission tracks
in track
density
in chlorite.
this graph represent 100% track losses determined at various times and temperatures. The data have been extrapolated to geologically meaningful times and temperatures. Extrapolation of the experimentally determined temperatures suggest that temperatures of 175-70°C in lo9 years will erase all the tracks in minerals listed in Fig. 7. Annealing studies of minerals from the EART confirmed that radiation damage of fossil fission fragment tracks in these minerals can be erased during intense metamorphic episodes, thereby resetting the geological clock. It is also clear, that the annealing experiments (Fig. 7) confirm the instability of fission tracks in zircon, muscovite, apatite, garnet, biotite and chlorite. As with other radiometric methods, the F.T. age of the selected minerals (Table 2) determine neither the age of peak metamorphism nor the depositional age of the rock. It is the cooling age of the minerals which is established.
system.
Fission-track ages log,t = {log,a
+ (E/U-)}
0)
where a is a characteristic constant 1 and T are the annealing time and temperature respectively, E is the activation energy and K is Boltzmann’s constant. Figure 7 shows that the annealing behaviour of the minerals, zircon, epidote, granite, muscovite, phlogopite, chlorite and apatite also is well described by eqn. (1). Individual points on
The fission track ages presented here (Table 2, Fig. S), augment and expand earlier data, presented by Koul et al. (1984). The ages are among the oldest fission-track ages yet recorded. Results for the Nabarlek area are comparable with those from Koongarra and anomalies Q and H. It is evident that the minerals which are clearly of peak metamorphic origin have fission-track ages which
109 TEMPERATURE(k)
_,
c
$6
rp
lo8
4
106
a 4 t y lo* i=
10 +I
I
I
1.2
1.6
1 2.0
-
EXPERIMENTAL
-----
EXTRAPOLATED
I
I
1
I
2.4
2.0
3.2
3.6
TEMPERATURE lOOO/TF’K) Fig. 7. Annealing extrapolated
times
as a function
to geologically
meaningful
of annealing
temperatures
for track
minimum exchange,
This
plotted
is consistent Ma
200
with
or older,
a thermal
which
reductions
(100%).
Experimental
results
are
times and temperatures.
are substantially younger than their Rb-Sr and K-Ar ages (Riley et al., 1980; Page et al., 1980). 1428
-,
density
event
did
not
at circa
exceed
the
closure temperature for Rb-Sr i.e. < 350 o C. The fission-track
or K-Ar data are
in Fig. 8. It can be seen that the ages and
relevant
annealing
temperatures
define
a smooth
Deposition of the Kombolgie Formation Mudginberri Phonolite 8 Wurogoii Dolerite
150 ij e E 3 -G t E” :
100
Rare dolerite
\
I
50
I
I
1.7
I
I
I
1
0.9
1.3
0.5
Age (yrs x 10’)
Fig. 8. Plot showing temperatures
the inferred
cooling
history
are given in text. The segment
of the older basement
of rocks from the EART,
of the curve from 1650-1600
rocks by a thick pile of sediments
at 1650 Ma.
during
the period
Ma assumes
of 1600-400
rapid temperature
Ma. Sources increase
of annealing
as a result of burial
110
cooling path Phanerozoic.
from 1428 Ma to well into the
TEMPERATURE 0
100
200
[“C) 300
400
Discussion and conclusions
Temperature in the basement rocks exceeded the bung temperature of zircon (2 175 a C for a lo9 year cooling history) following deposition of the Kombolgie Formation (Fig. 8). Evidently this thermal event did not reach temperatures high enough to reset Rb-Sr and K-Ar mica ages. Furthermore this thermal event peaked at some time between 1650 Ma (the depositional age of the Kombolgie formation) and 1428 Ma (the zircon F.T age). A possible expl~atio~ for this event is heat exchange between mafic extrusives (Nungb~g~ri Volcanics) within the Kombolgie Formation and surrounding rocks. Since the volcanics are dated at 1648 f 29 Ma (Page et al., 1980), the fissiontrack age would imply an unreasonably long cooling history (- 220 Ma) for a sequence which is less than 200 m in thickness. For the same reason anomalous heat flow from the mantle, which could be reflected in volcanism, is ruled out. It is necessary, therefore, to postulate either a highly anomalous geothermal gradient or a greater thickness of superincumbent sediments than that which now remains in the EART, or indeed a combination of both factors. It is apparent that if temperatures in excess of 175 ’ C were achieved in the basement rocks with a cover of 2.0 km of sediments and volcanics, an anomalous geothermal gradient comparable with modern geothermal systems is implied {Fig. 9). If a gradient typical of modern sedimentary basins is postulated, then basement rocks may have been buried beneath sediments up to 5 km thick. This hypothesis requires that all vestiges of the uppermost 3.0 km of cover have been completely eroded (Koul et al.. 1984). Archaean gneiss and post-tectonic Lower Proterozoic granite intrusives which underlie much of EART contain anomalous concentrations of uranium (up to 35 ppm-Ferguson et al., 1980). Such high concentrations of uranium (and potassium and thorium) would result in high heat flow (probably in excess of 13 cal cm3 s-l). Thus we
SALTON SEA (ELMOREal) GEOTHERMAL
SASS0 c 1 GEOTHERMAL
4ooo, Fig. 9. Extreme geothermal gradients, from typical sedimentary basins and present day geothermal systems. The gradient developed following Kombolgie Formation deposition is thought to be cl&e to the upperbounds of the range for sedimentary basins, given evidence for higher heat flow in the Proterozoic relative to the present day.
conclude that the thermal event indicated by the zircon F.T. age is the result of burial of the basement by at least 2.0 km of alluvial sandstone and volcanics and probably another 3 km of sediments coupled with radiogenic heat flow. The remaining data can not readily disting~sh between whether the EART cooled slowly and steady over the period 1420-491 Ma, as a result of declining radiogenic heat production and gradual stripping of the insulating sedimentary cover, or whether there were several relatively short-term low magnitude thermal events. Known geological events during this time are Wurogoij Dolerite dykes at 1370 + 30 Ma and Mudginberri Phonolite dykes at 1316 f 48 Ma (Page et al., 1980). The 491 f 32 Ma apatite age appro~mates to rare 522 Ma dolerite dykes (Page et al., 1980). It is intuitively unreasonable that such small additions of magma could raise the temperature of the host rocks anything but locally and for a relatively short time. These events may however be a mani-
111
festation of increased heat flow from the mantle. There is no reason to suppose that the other F.T. data represents other than gradual diminution of temperatures as the result of erosion of a thick sedimentary cover. The F.T. data have significance with respect to the genesis of unconformity-related uranium deposits. Recent Nd-Sm dating of primary Nabarlek ore (R. Maas, pers. commun, 1987) and reinterpretation of existing Rb-Sr and K-Ar ages (of Page et al., 1980) suggest primary mineralization at 1560 Ma. Thus the deposits would have formed in host rocks which already had achieved temperatures of 180 o C as a result of burial by the overlying sedimentary rocks. This is consistent with fluid inclusion data on breccia-cementing quartz which indicate temperatures of this order (Wilde, unpubl. data; Ypma and Fuzikawa, 1980). In conclusion, this study demonstrates the importance of fission-track studies in elucidating the thermal history of a rock. This utility is enhanced if peak temperatures were, as in this case, below the closure temperature for Rb-Sr and K-Ar exchange, and such that recrystallization did not occur. The fission track method has the ability to detect low temperature events which could not be recognized by conventional radiometric isotopic techniques. This is clearly demonstrated by the case of biotite and muscovite, both peak metamorphic phases, with Rb-Sr and K-Ar ages in excess of 1750 Ma, but whose F.T. ages are less than 1100 and 950 Ma respectively, a radical difference in apparent age.
Ferguson,
J. and Needham,
lower
Proterozoic
from the Northern Ferguson,
of economic
Alligator
uranium
River
Goleby
(Editors),
IAEA, Vienna, Johnston, inlier
J., 1980. Metamorphism
Ferguson
and A. Goleby
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