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Mineral age patterns in ca. 3700 my old rocks from West Greenland
EARTH AND PLANETARY SCIENCE LETTERS 20 (1973) 157-170. NORTH-HOLLAND PUBLISHING COMPANY
MINERAL AGE PATTERNS IN CA. 3700 MY OLD ROCKS FROM WEST GREENLAND R.J. PANKHURST, S. MOORBATH
Department of Geology, University of Oxford, England D.C. REX
Department of Earth Sciences, University of Leeds, England and G. T U R N E R
Department of Physics, University of Sheffield, England Received 20 June 1973 Revised version received 28 August 1973
K - A r , 4 ° A r - 3 9 A r and R b - S r dates are reported for minerals from the ca. 3700 my-old Am]tsoq and Isua gneisses of the Godthaabsfjord area of West Greenland. K - A t dates on biotites and hornblendes range from about 1900 to 3500 my, with hornblendes having a much narrower range (ca. 2 2 5 0 - 2 7 5 0 my) than biotites. One biotite from Isua gives an impossibly high K - Ar date of 4940 my. 40 A r - 39 Ar mineral dates are in close agreement with conventional K - A r dates over the entire range of apparent age values. The presence of minor amounts of excess argon is observed in the hornblendes, but radiogenic and excess argon in the biotites are completely homogenised and cannot be differentiated. R b - S r measurements on biotites are closely concordant and show that all biotites were completely open to diffusion of radiogenic a 7 Sr at about 1600-1700 my. This is the first proof of a regional thermal event at this time in the Archaean of West Greenland, although similar dates are well known from the Proterozoic belts to the north and south. The evidence suggests that those K - A r biotite dates greater than about 2700-2800 my result from excess radiogenic argon incorporated during a thermal event of about this age or, more probably, during the 1600-1700 my Sr isotope homogenisation event. Scatter of mineral dates below about 2700 my could also be due, at least in part, to overprinting by the 1600-1700 my event. K - A r mineral dates and an Rb Sr mineral isochron from a pegmatite associated with the last major rock-forming event in the Godthaabsfjord area, namely the Q6rqut granite, indicate an age of emplacement of 2580 *- 30 my.
1. I n t r o d u c t i o n This p a p e r r e p o r t s age d e t e r m i n a t i o n s b y c o n v e n tional K - A r , 40 A r _ 3 9 Ar a n d R b - S r m e t h o d s o n sepa r a t e d m i n e r a l s f r o m the Am~tsoq a n d Isua gneisses o f the G o d t h a a b s t ~ o r d area o f West G r e e n l a n d . P u b l i s h e d R b - S r w h o l e - r o c k i s o c h r o n s for t h e s e gneisses give a n age o f 3 7 0 0 - 3 7 5 0 m y w i t h a n initial ~7 Sr/S6 Sr ratio o f 0 . 7 0 1 0 - 0 . 7 0 1 5 [1 ]. This was i n t e r p r e t e d eit h e r as t h e ' a g e o f e m p l a c e m e n t o f the p a r e n t igneous r o c k s o f t h e gneisses, or as a n i s o t o p i c r e h o m o g e n i s a -
t i o n e v e n t resulting f r o m m e t a m o r p h i s m w i t h i n ab o u t 1 0 0 - 2 0 0 m y o f t h e igneous event. T h e great an t i q u i t y o f this sector o f the A r c h a e a n is c o n f i r m e d by w h o l e - r o c k P b - P b i s o c h r o n p l o t s [2, a n d u n p u b lished O x f o r d analysis] a n d b y U - P b z i r c o n ages [3], w h i c h yield values in t h e range ca. 3 6 0 0 - 3 7 5 0 my. In a d d i t i o n , m i n e r a l ages are r e p o r t e d f r o m a pegm a t i t e associated w i t h the Q 6 r q u t granite, representing t h e last m a j o r r o c k - f o r m i n g e v e n t in t h e area. A simplified s u m m a r y o f t h e geological s e q u e n c e in t h e G o d t h a a b area is given in t a b l e 1. A d e t a i l e d
158
R.J. Pankhurst et al., Mineral age patterns in ca. 3700 my oM rocks
TABLE 1 Abbreviated geological outline of the Godthaab District, West Greenland a 1 Plutonic development ca. 3700-3750 my ago [1, 2] of a complex of dominantly quartzo-feldspathic rocks involving production of granites, granodiorites and tonalites. Metamorphism, migmatisation and deformation, leading to formation of banded gneisses (Amitsoq gneisses). 2 Intrusion of a basic dyke swarm (Ameralik dykes) into the Arnitsoq gneisses. 3 Eruption of basic volcanics and deposition of sediments (Malene supracrustals). 4 Emplacement of stratiform anorthosites, leUcogabbros and related rocks. 5 Major thrusting, producing intercalation of Am]tsoq gneisses and Malene supracrustals. 6 Intrusion of major suite of calc-alkaline rocks including granites, granodiorites, tonalites and diorites - the parent rocks of the Nfik gneisses, 3040 ± 50 my ago [16]. 7 Deformation, involving several phases of folding, accompanied by migmatisation and metamorphism that culminated in amphibolite facies conditions in the area south, east and north-east of Godthaab, but reached granulite facies to the north and west 2850 ± 100 my ago [17]. Earlier igneous rocks converted into gneisses (N~k and Am~'tsoq gneisses) and amphibolites. 8 Emplacement of Q6rqut granite and pegmatites, 2580 -+30 my ago (this work). 9 Intrusion of Precambrian dolerite dykes. a The relative positions of events 3, 4 and 5 are not yet established with certainty. Interpretations other than those given in earlier papers [1, 2] are possible [4].
biotite-bearing vein cutting the supracrustal rocks of the Isua ironstone formation and, finally, one sample of Q6rqut granite pegmatite.
2. Analytical methods Conventional argon analyses were performed on an A.E.I. MS-10 mass spectrometer in the University o f Leeds using isotope dilution with 38Ar spike [7]. Potassium was determined in triplicate by flame photometry. 4°Ar-39Ar measurements were carried out in the University of Sheffield using irradiation and massspectrometric techniques described elsewhere [8]. Irradiations were monitored using a standard hornblende HB3 GR [9]. R b - S r analyses were made in the University of Oxford, using mass-spectrometric isotope dilution techniques on single sample aliquots [101. The decay constants used in this work are: for 87Rb, X = 1.39 X 10 -11 y _ l , and for 4°K, X# = = 4.72 × 10 - 1 ° y - 1 and Xe -- 0.584 X 1 0 - 1 ° y - l • 4°K/K = 0.0119 atom %. Errors on all dates are given at the 95% (2 sigma) confidence level. R b - S r isochrons were fitted by the method of York [11]. R b - S r data have been corrected for 2 ng Rb blank and 10 ng Sr blank.
3. Results account of the geology has been given by McGregor [4], whilst the Isua area, some 150 km north-east of Godthaab on the edge of the inland icecap, has been briefly described by Keto [5, 6]. Geological sketch maps of the whole area have been given previously [1, 2, 4] and will also suffice to identify the present sample localities. Eleven of the rocks analysed in this study have been described in earlier papers [1,2]. Brief summary descriptions of these samples are given in the Appendix, together with somewhat more detailed information on additional specimens. Mineral separates o f biotite and hornblende (as well as muscovite, plagioclase, p o t a s h - f e l d s p a r , sphene and apatite in a few cases) were made from eight samples of Aml"tsoq gneiss, three samples of biot i t e - h o r n b l e n d i t e "enclaves" (irregular dyke-like bodies of pre-Ameralik dyke age) within the AmTtsoq gneisses, four samples of Isua gneiss, one sample of a
3.1. K - A r dates
The analytical data are given in table 2. Several samples were completely reanalysed on separate mineral aliquots with excellent agreement, as shown• K - A r biotite dates range from 1 8 9 0 - 3 2 8 0 my for the A m h s o q gneisses and enclaves, and 2 0 3 0 - 3 5 2 0 my for the Isua gneisses. At first sight this appears to be compatible with variable overprinting b y later thermal events on minerals formed during the 3 7 0 0 - 3 7 5 0 m y episode. K - A r hornblende dates are more restricted in range than the biotite dates: 2 2 5 0 - 2 6 2 0 my for the Ami'tsoq gneisses (with one exception) and 2 4 9 0 2750 my for the basic enclaves. The exception (110870) is a hornblende concentrate which was difficult to purify adequately, as evidenced by its mea-
R.J. Pankhurst et aL, Mineral age patterns in ca. 3700 m y old rocks
159
TABLE 2 K - A r analytical data G.G.U. Sample No.
Mineral
K%
4 OAr rad. %
4 OAr rad. (scc/g × 10 ~ )
K-Ar age (my)
40At
total argon age (my)
4OAr 39Ar a plateau age (my)
3110 ÷ 50
3170
3330 +- 50
3340
}
2990 +- 50
2990
2500 -+ 80 2620 +_ 80 )
2520 -+ 50
2490
3050 +- 50
2610
2520-+50
2560
39At a
A m l t s o q gneBses 125519
biotite
6.55 6.33
99.4 99.2
20.599 19.653
3100 _+ 1 0 0 ) 3080 -+ 100
110870
biotite
7.64 7.67
98.4 98.8
25.869 27.150
3220 +_ 100 3280 -+ 100 }
110999
biotite
6.98
98.9
12.703
2300 -+ 70
125522
biotite
6.76
98.6
13.702
2460 -+ 80
125540
biotite
7.84
99.0
22.911
2990 -+ 90
7.82
99.3
22.947
3000 _+ 90
110869
biotite
7.23
97.2
9.393
1890 _+ 60
86431
biotite
7.45
99.2
13.429
2310 -+ 70
125519
hornblende
1.14 1.08
98.3 96.3
2.377 2.385
110870
hornblende (impure)
3.85
99.5
12.083
3100 +- 100
110999
hornblende (impure)
1.99
98.4
3.445
2250 +- 70
125526
hornblende
0.84
98.3
1.601
2380 + 80
Am~tsoq gneiss enclaves 155713
biotite
6.97
99.2
14.302
2480-+
155714
biotite
6.78
98.4
16.780
2750 +- 90
155806
biotite
7.60
99.4
13.244
2260-+
70
2340+-50
2360
155713
hornblende
0.53
97.3
1.238
2660-+
80
2760-+50
2700
155714
hornblende
0.44
98.7
1.090
2750 +- 90
155806
hornblende
0.26
98.6
0.539
2490+-
2650+-50
2660
7.71 7.71
98.4 99.2
31.644 31.148
4970 -+ 70
5030
80
80
Isua gneisses 155764
biotite
3520 +- 100-, 3490 + 100 )
155765
muscovite
8.33
97.1
15.863
2430 +- 80
155766
biotite
6.72
96.3
9.797
2030 +- 60
155774
biotite
7.51 7.51
98.6 99.0
29.338 29.607
3440 -+ 100~ 3450 + 100"
155785
biotite (vein in ironstone formation)
6.48 6.48
98.4 99.6
61.251 62.841
4920 _+ 150} 4960 + 150-
Q 6 r q u t Pegmatite 155754
biotite
7.68
96.8
14.286
2350 -+ 80
155754
muscovite
8.47
97.8
17.660
2500 + 80
a For full details, see table 3 N.B. Total Argon ( 4 ° m l - 3 9 A r ) a g e is directly comparable with K - A r age.
% 39A1"
Ca/K
J = 0.0491
% 39Ar
Ca/K
biotite
J = 0.0491
Ca/K
J = 0.0484
% 39At
Ca/K
biotite
J = 0.0484
Age (my)
Error
4°mr/39Ar
T(°C)
155806
Age (my)
Error
4°Ar/39Ar
T(°C)
% 39mr
155713
biotite
Age (my)
Error
4°Al/39Ar
T(°C)
110870
Age (my)
Error
4°Al-/39Ar
T (°C)
125519
biotite
Sample
TABLE 3 4 OAr- 39mr analytical data
1710
0.2
30.5
.
1.7
500
1590
0.4
27.4
0.7
500
2750
0.4
67.0
.
1.6
500
2180
0.8
44.2
1.0
500
Heating steps
0.4
46.6
2230
.
9.4
600
2180
0.2
45.1
.
9.7
600
3360
0.4
100.7 *
.
27.8
600
2990
0.4
79.3
-
10.6
600
.
.
.
.
2350
0.6
51.3 *
43.6
690
2530
0.2
58.5 *
18.4
690
3320
0.6
98.3 *
.
24.2
690
3140
0.6
87.6 *
-
57.1
690
2370
0.6
52.0 *
.
8.3
760
2560
0.4
59.7 *
.
20.6
760
3340
1.2
99.3 *
.
20.0
760
3160
0.4
88.4 *
-
13.6
760
.
2360
0.2
51.7 *
12.7
820
2560
0.4
59.6 *
11.4
820
3320
0.6
97.8 *
15.6
820
3160
0.4
88.7 *
0.9
9.5
820
2360
0.2
51.6 *
22.3
940
2560
0.2
59.6 *
21.1
940
3340
0.8
99.5 *
9.2
920
3050
0.4
82.3
1.21
5.5
940
2400
0.4
53.0
1.9
1200
2570
0.4
60.1
17.8
1200
3300
0.6
96.8
0.9
1.7
1100
2910
0.8
75.0
2.54
2.7
1200
*
2340
0.2
50.7
2520
0.2
57.9
3330
0.4
98.7
3110
0.8
85.9
Total
2360
51.6
2560
59.5
3340
99.2
3160
88.2
age (my)
Plateau
2'
2
~,
% 39Ar
Ca/K
biotite
J = 0.0484
% 39Ar
Ca/K
hornblende
J = 0.0491
% 39Ar
Ca/K
hornblende
J = 0.0491
Age (my)
Error
4°Ar/39Ar
T(°C)
110870
Age (my)
Error
4°Ar/39Ar
T(°C)
125519
Age (my)
3230
0.4
92.4
-
18.0
600
3710
6.6
125.2
7.0
0.5
600
3410
28
.
Error
Ca/K
J = 0.0309
0.9
165
% 39At
biotite
500
Heating steps
4°Ar/39Ar
I'("C)
1557 ~,~
Age (my)
Error
4°AI/39AI"
T(°C)
125540
Sample
TABLE 3 (continued)
.
3280
0.8
95.8
-
33.4
690
3210
2.4
91.6
1.87
1.0
690
3840
8
216
5.5
600
2990
0.4
.
80.3 *
.
63.0
600
.
.
3280
1.0
95.6
-
4.5
760
3280
7.2
95.5
1.74
0.8
760
4930
8
411
.
19.9
690
3000
0.4
*
80.6 *
15.8
690
.
10
457
3240
0.6
93.3
0.52
8.0
820
3300
6.2
96.6
4.7
0.6
820
5120
.
29.0
760
30•0
0.6
*
.
81.0 *
8.5
760
.
*
2630
0.4
61.8 *
4.4
26.1
940
2450
0.4
54.4 *
2.19
38.4
940
5010
14
429
14.4
820
2990
0.4
80.2 *
4.3
820
*
2580
0.2
59.6 *
5.4
9.1
1090
2510
0.8
56.8 *
6.3
49.8
1090
4990
4
425
23.9
940
3000
0.4
80.8 *
8.0
940
*
2530
0.4
57.4
7.1
0.9
1220
2540
.0.4
58.2 *
7.4
8.9
1220
5100
8
451
0.15
6.3
11 O0
2990
2.0
80.3
1.56
0.4
1200
3050
0.2
82.5
2520
0.4
57.2
4970
4
420
2990
0.4
80.4
Total
2610
61.0
2490
55.9
5030
434
2990
80.4
Plateau age (my)
7,
¢b
C)
2
t~
t~
at"
% 39mr
Ca/K
hornblende
J = 0.0491
Ca/K
J = 0.0491
% 39At
Ca/K
(extended
run)
3.6
-
Age (my)
Error 3170
1.2
89.2 *
Ca/K
4°Ar/39Ar
9.8
24
600
2300
% SgAr
Time (hr)
T(°C)
Age (my)
Error
48.5
.
1.0
90
300
Heating steps
1.6
49.2
0.6 .
3175
0.6
89.4 *
-
10.8
72
600
2320
.
27
400
4740
51.4
231.8
3
0.3
600
2860
1.2
72.3
0.82
7.6
600
3165
0.6
88.7 *
-
10.6
1
760
2880
0.6
73.5
.
13.8
24
500
2000
0.4
38.3
0.51
14.8
820
2950
2.4
77.0
2.92
5.1
820
0.8
84.8
6.7
3130
0.8
86.5
0.40
8.0
1.5
920
3090
.
25
500
3750
4.6
128.1
6.8
3.2
940
2950
0.8
77.2
24.7
8.5
940
.
.
3040
0.4
82.0
1.63
9.9
1.5
1090
3110
1.6
85.3
3.6
24
500
2770
0.4
68.1 *
18.2
27.1
990
2760
0.4
67.5 *
42
43.0
990
3160
0.8
88.5 *
13.9
68
500
2610
0.4
61.1 *
16.8
47.8
1090
2640
0.6
62.1 *
40
20.4
1090
3165
0.6
88.9 *
11.3
171
500
2720
1.4
65.9 *
17.6
6.9
1220
2720
1.0
65.8 *
45
15.5
1220
3100
0.4
84.9
2650
0.2
62.6
2760
0.4
67.8
Total
3170
88.9
2660
63.4
2700
64.7
Plateau age (my)
Notes: Temperatures for heating steps are approximate, being based on current-temperature calibration of the tantalum furnace. Spot checks were performed with an optical pyrometer for T > 800°C. Errors quoted are 2-sigma. Ca/K ratios are calculated from 37Ar/39Ar. Where no entry is shown Ca/K is less than 0.05. Plateau ages are calculated from the gas release steps marked with an asterisk.
J = 0.0491
Time (hr)
biotite
40Ar/Sgml"
T(°C)
125519
Age (my)
Error
40Al/39Ar
T(°C)
% 39Ar
155806
hornblende
Age (my)
Error
4°AI/39Ar
T( ° C)
155713
Sample
TABLE 3 (continued) bO
R.J. Pankhurst et aL, Mineral age patterns in ca. 3700 my old rocks
sured K content of 3.85% and by the 4°Ar/39Ar data discussed later. These dates are significantly lower than those of the Arnitsoq gneiss biotites (125519, 110870, 125540). However, in the case of the three basic enclaves there is essential concordance between K - A t hornblende and biotite dates ranging from 2260 to 2750 my, the biotite-hornblende pair 155714 being in agreement at 2750 my. Hornblende has not yet been found in the Isua gneisses, but a single muscovite (155765) date of 2430 my is in general agreement with most of the hornblendes from the Amitsoq gneisses and contrasts sharply with the high dates of several Isua biotites (155724, 155766, 155785). In the case of Am~tsoq gneiss sample 125519, the K-Ar biotite date (3100 my) is significantly greater than that of the coexisting hornblende (2500-2600 my). This is the reverse of the normal order of retentivity. The above evidence leads to the suspicion that some of the higher K - A t biotite dates could be due in part to excess radiogenic argon. This idea is indirectly supported from the Isua area by the impossibly high date of 4940 my for the biotite (155785) from a vein cutting supracrustal amphibolitic rocks of the Isua ironstone formation surrounding the 3700 myold Isua gneisses (the true stratigraphical relationship between the two still being unknown). This biotite, like all biotites analysed in this study, has a normal K content for typical biotite and shows no evidence for chloritisation or other alteration. Despite the fact that biotites from the gneisses have a range of dates from 1890 to 3520 my, there is some indication from the two most concordant hornblende-biotite pairs (e.g. 155713, 155714) and from several other hornblende dates that a thermal event could have occurred about 2600-2800 my ago. Younger dates could then be interpreted as due to later overprinting or prolonged cooling. However, the evidence against all the dates being due to overprinting of the original 3700-3750 my ages will become clearer after discussion of the R b - S r data. The biotite-muscovite pair from the QOrqut pegmatite is only just concordant within 2 sigma error at 2350 and 2500 my respectively. A few K - A r dates in the range 2400-2600 my have been reported previously on biotites and hornblendes from the Godthaabsf]ord area [12, 13]. Furthermore, a K - A r biotite date of 3595 + 70 my has been reported previously from the Isua gneiss [14],
163
which was interpreted as signifying the presence of excess argon (albeit before it was realised that the lsua gneiss was 3700 my old).
3.2. 4 ° A r - 3 9 A r dates
Six biotites and four hornblendes were selected for 4°Ar-39Ar age determinations in the hope of "seeing through" Ar losses and, perhaps, of characterising excess 4°Ar. Analytical data are given in table 3. The release patterns of hornblende and biotite from 125519 are fairly representative and are shown graphically in fig. 1. The release patterns of the hornblendes were characterised by high and widely differing 4°Ar/39Ar ratios in the low temperature release (T < 940°C). Above 940°C, where the major argon release occurred, the 4°Ar/39Ar ratios showed an approximate plateau corresponding to ages of different samples in the range 2490-2700 my. The high temperature ratios from any given sample showed real fluctations of 8% or so, an order of magnitude larger than the experimental uncertainties. The K - A r systematics in these samples are clearly rather complex. The low temperature release, in all samples except 110870, is presumed to represent excess 4°Ar incorporated during a metamorphic episode. The pronounced low temperature release of 110870 is unquestionably due to biotite contamination. This is indicated by the high K concentration referred to in the preceding section, and the absence of significant 3VAr in the low temperature release which implies a very low Ca/K ratio in the mineral responsible, which is characteristic of the biotite release patterns (see table 3). The high temperature plateau age of 2610 my is due to the hornblende alone and confirms that the conventional K--Ar date of 3100 my (table 2) is a meaningless "mixed" date. Apart from this, hornblende dates estimated from the high temperature release, given in table 3, are only marginally different from the conventional dates reported in table 2, the presumed "excess" argon making only a small contribution to the age. They indicate that argon retention in all four samples dates from around 2500-2700 my ago. The release patterns for the biotite are simple plateaux giving dates which agree with the conventional K - A r dates within experimental error despite the fact
R. J. Pankhurst et al., Mineral age patterns in ca. 3 700 my old rocks
164
L’ 125519
2.0
-
BIOTITE
- - - -
HORNBLENDE
-I
I
0
0.5
1.0
FRACTION OF 3gAr RELEASE0 Fig. 1. 40Ar/39Ar release patterns, plotted as apparent age of Ar released in each heating biotite 125519. Full data for these and all other mineral samples given in table 3.
that these range from ca. 2300 to 5000 my. Apart from evidence of minor amounts of 40Ar loss in the low temperature release, no anomalous Ar or difference in site activation energy can be detected in any of the samples, including 155785 which gives an apparent age greater than that of the solar system. Since other workers have also found similar behaviour [ 1.51, it must be concluded that no additional information can be obtained by the application of 40Ar/39Ar techniques to biotites. Either there is only one site available to accommodate radiogenic and extraneous (excess) Ar, or else all the Ar is effectively homogenised in the biotite lattice during the reactor irradiation or in the furnace during Ar extraction. We have attempted to see whether the homogeneity extends to other argon isotope ratios. 36Ar is virtually undetectable in these samples and is of no help. Similarly 3 7Ar is undetectable in the low temperature release and 37Ar in the high temperature release can be understood in terms of the presence of small amounts of hornblende (high Ca/K) and therefore yields no information specific to the biotite. 38Ar is present in all the biotites, with 3*Ar/3gAr ratios ranging from 0.025
step, for hornblende
125519,
(155806) to 0.11 (125540). “Ar is produced by (n - y) reactions on 37Cl and is thus indicative of sites occupied by Cl. The 38Ar/3gAr ratios (not tabulated) were essentially constant indicating that 38Ar was diffusing from equivalent sites to 40Ar and 3 9Ar. An attempt was made to see whether the small variations which did occur in 40Ar/39Ar and ‘8Ar/39Ar were correlated (as in two component mixture) but the results were inconclusive, the experimental uncertainties being comparable to the ratio variations. An attempt was also made to detect the excess argon in 125519 by varying the heating schedule: heating for longer times (several days) at lower temperatures 300-600°C. The release pattern (fig. 1) was identical to that obtained with the shorter (1 hr) heating times. The high temperature decrease in 40Ar/39Ar is due to the presence of small impurities of hornblende (see Ca/K ratio in table 2). The present measurements show that on a scale of ca. 0.2 - 0.4 mm grain diameter for the analysed minerals the distribution of radiogenic 40Ar and excess 40Ar within irradiated biotites is identical, and it is not possible to separate the two components by the
165
R.J. Pankhurst et al., Mineral age patterns in ca. 3700 my old rocks
TABLE 4 Rb-Sr analytical data Sample
Rb ppm
Sr ppm
87 Rb/86 Sr 32.50
87 St/86 Sr
Age (my)
± 0.18
1.4490 ± 0.0002
1612
± 9
214.9
± 2.4
5,966
± 0.002
1733
± 20
13.38
108.8
-+ 1.1
3.221
+ 0.002
1637
± 16
461
7.69
282.5
± 2.4
7.134
-+0.001
1618
± 15
155764 biotite
577
11.05
236.7
+_ 3.0
6,498
±0.001
1733
± 20
155774 biotite
632
29.49
72.1
± 0.8
2.365
±0.001
1616
± 17
155785 biotite
537
8.42
308.6
± 4.6
7.565
±0.001
1581
± 24
± 0.002
0.7232 ± 0.000l \
125519 biotite
245
23.39
110870 biotite
364
7.42
125540 biotite
404
155806 biotite
125519 hornblende
6.03
125519 plagioclase
1.40
125519 sphene
0.90
125519 apatite
0.75
38.22 517.4
0.457
0.0078 • 0.0001
0,7113 ± 0.0001
0.0554 +- 0.0010
0.7099 ± 0.0001
0.0085 ± 0.000l
0,7099 ± 0.0001
See fig. 2 46.79 255.8
155754 biotite
3165
32.87
24.350
155754 muscovite
1450
15.69
6008
155754 microcline
1423
21.23
643
application o f stepwise degassing using the 4°Ar 39Ar m e t h o d . A possible w a y o f detecting unequivocally the presence o f excess 4°Ar w o u l d be to carry out total e x t r a c t i o n 4 ° A r - 3 9 A r analysis on single b i o t i t e grains f r o m a single geological unit or even f r o m a single rock sample. This has not b e e n d o n e in the present work, in which sample splits were used to allow c o m p a r i s o n with conventional K - A t ages. 3.3. R b - S r dates
The results o f seven R b - S r biotite analyses are given in table 4. The samples were all highly enriched
±300
884
±5
± 90
220,2
± 0.7
+
4
24.39
2.580 ± 30 (fig. 3)
+- 0.01
in radiogenic 8 v Sr. Precise dates were calculated from a t w o - p o i n t isochron (single-stage) model, using published R b - S r whole-rock data [1] for the gneiss biotires, and by assuming an initial 87 Sr/86 Sr ratio o f 0.700 for the enclave biotite ( 1 5 5 8 0 6 ) and the vein biotite (155785). The latter assumption yields maxim u m dates in the event o f subsequent closed-system Sr isotope h o m o g e n i s a t i o n . The calculated R b - S r dates for all analysed biorites fall within the relatively narrow range 1 5 8 0 - 1 7 3 0 my. This clearly records the cessation o f Sr isotope diffusion following a thermal event, whilst the narrow range o f dates contrasts strongly w i t h the
166
R.J. Pankhurst et aL, Mineral age patterns in ca. 3700 m y old rocks
0725 I•
÷/~0
875r 865r
~
+~" Hb WR.
~
072C
071!
~
0-71( ~ +
~ ,~..
Sp 87R~b I~
aeSr -
-
I 01
[ 0"2
01"3
I 04
015
I 06
Fig. 2. Rb-Sr isochron diagram for mineral components of Amltsoq gneiss sample 125519. Ap = apatite, Sp = sphene, Pi = plagioclase, Hb = hornblende, Bi = biotite, WR = whole rock. enormous spread of K - A r dates, discussed earlier. This is strong indirect evidence for regarding many of the K - A r dates as excess argon dates. Formally, the two-point isochron model assumes that all mineral components within each rock were in isotopic equilibrium at the time deduced from the slope of the biotite-whole rock join. This was further tested for sample 125519, from which coexisting biotite, hornblende, plagioclase, sphene and apatite were analysed. The data are plotted in fig. 2, together with that for the whole rock. It is evident that the assumption of complete isotopic equilibrium is not fully satisfied since the data points do not define a unique isochron. The biotite-whole rock join corresponds to a date of 1610 + 10 my, whilst the plagioclase-whole rock join gives 1890 + 40 my. The hornblende data point, which is close to that for the whole rock (although the Rb and Sr contents are very different), could be considered to lie on either of these lines. The apatite and sphene, both with very low Rb/Sr ratios, fall below these lines. The sphene-whole rock join corresponds to a date of 2380 + 80 my. The most plausible interpretation of the data is that subsequent to the 2 6 0 0 - 2 8 0 0 my event indicated by the K - A r hornblende data, an event at ca. 1 6 0 0 - 1 7 0 0 my or some-
what older re-opened the plagioclase, hornblende and biotite, but only partially affected the sphene and apa tite. The biotite did not become a completely closed system until 1610 -+ 10 my ago. Although the minerals were not in complete isotopic equilibrium at this time, the extremely high Rb/Sr (and s 7 Sr/86 Sr) ratio of the biotite as compared to the other components makes the R b - S r biotite date very insensitive to small departures from the assumed model. It is considered that this holds equally true for the other biotites listed in table 4, most of which have even higher Rb/Sr ratios. As stated previously, the last major rock-forming event in the Godthaab district was the magmatic activity associated with the QSrqut granite. A pegmatite associated with the final phases of activity is dated by a three-point R b - S r mineral isochron (fig. 3) on potash feldspar, muscovite and biotite (155754), which gives an age of 2580 + 30 my. This is in general agreement with K - A r mica ages from the pegmatite given in table 2, and more particularly with the muscovite date of 2500 + 80 my. Because of the high Rb/Sr ratios of all three minerals, the initial ~TSr/a6Sr ratio (1.0 -+ 0.3) is too imprecise to define the origin of the pegmatite.
167
R.J. Pankhurst et al., Mineral age patterns in ca. 3700 my old rocks
Fig. 3. Rb-Sr mineral isochron muscovite, Bi = biotite.
for Qarqut
granite
pegmatite
sample
Clearly, the Qarqut pegmatite minerals have not responded to the regional Sr isotope homogenisation event at ca. 1600- 1700 my. This is hardly surprising when it is considered that single crystals and books of all three minerals in the pegmatite are commonly 20-30 cm in diameter. It is concluded that the post-tectonic Q&qut granite was emplaced about 2600 my ago. Rb-Sr wholerock studies are planned to substantiate this. A K-Ar biotite date of 2500 + 25 my has previously been reported from the Q&qut granite itself [13].
4. General discussion The observed range of K-Ar biotite dates was at first thought to result from variable overprinting since crystallisation about 3700 my ago [l ,2], but the following lines of evidence suggest that this is not the case: (a) A biotite from a vein cutting Isua supracrustal rocks yields a K-Ar date of $940 my. (b) Hornblendes from Amitsoq gneisses and biotitehornblendite enclaves have a smaller spread of K-Ar and 40Ar-39Ar dates (2250-2750 my) than some coexisting or separate biotites (1890-3340 my). Most of the biotite dates exceed the hornblende dates. Two biotites from the Isua gneiss give K-Ar dates of ca.
155754
from Ameralik
fjord.
KF = microcline,
Mu =
3500 my, another gives 2030 my, whilst a muscovite gives 2430 my. (c) The range of dates for hornblendes, a mineral normally exhibiting higher Ar retentivity than biotite, suggests the possibility of minor Ar loss following a thermal event which caused complete degassing about 2700 my ago, or somewhat earlier. There is ample independent geochronological evidence for such an event, which could be associated with the intrusion of the parent igneous rocks of the Niik gneisses of the Godthaab area which in one locality have yielded a Rb-Sr whole-rock isochron age of 3040 + 50 my [ 161 Alternatively, the event could correspond to a subsequent amphibolite grade metamorphism possibly contemporaneous with the regional granulite facies metamorphism to the north and south of the Godthaab area, which has been dated at 2850 + 100 my ago [17]. Finally, there was the QZrqut granite magmatic episode about 2600 my ago (see above), which might also be expected to have affected the K-Ar dates. It should be noted that the coarse-grained biotites from the Am?tsoq biotite-hornblendite enclaves appear to have been reset by the ca. 2700 my event. The biotites from the Amitsoq (and Isua) gneisses, with K-Ar dates up to 3500 my, are all much finergrained than these. (d) All analysed biotites (except that from the Q&gut granite pegmatite) have been affected by ex-
168
R.J. Pankhurst et al., Mineral age patterns in ca. 3700 my old rocks
tensive loss of radiogenic aTSr ca. 1600-1700 my ago. Since the retentivity of micas for radiogenic Ar and i Sr are similar [18], it seems most unlikely that those biotites with very high K - A r dates could have been so resistant to Ar loss during the Sr isotope homogenisation episode. It is much more likely that excess radiogenic Ar was introduced into these biotites at this time. This is the first regional evidence for 1600-1700 my dates within the Archaean craton of West Greenland. Similar K-Ar, Rb-Sr and U-Pb dates are well known to the north (Nagssugtoqidian) and south (Ketilidian) of the Archaean craton, where they are interpreted as the age of Proterozoic plutonism. The latter may well have left no recognisable petrological imprint in the Godthaabsf]ord area. Similar dates within the range ca. 1600-1700 my (as well as in the range ca. 2600-2800 my, see above) have been widely recognised in the ancient shield areas of Canada, north-west Scotland [19] and the Baltic (see below).
5. Conclusions It is concluded that the K - A r biotite dates are controlled to a great extent by excess radiogenic 4°At formed by degassing of basement rocks and taken up by biotites when they were at temperatures at which Ar could diffuse rapidly through the lattice. This could have occurred either at about 2600-2800 my ago, or more probably at about 1600-1700 my ago. The fact that the resultant K - A r biotite dates in the range ca. 1900-3500 my are geologically plausible is probably fortuitous, although it could conceivably be related to the limited amount of radiogenic 4°Ar available from degassing of a basement consisting of a mixture of rocks formed predominantly at about 3700 my and 3000 my ago [1,2, 16]. Russian workers [20] have recently reinterpreted the numerous reported K - A r biotite (and other mineral) dates ranging up to ca. 3900 my from the northern sector of the Baltic Shield as excess Ar dates rather than overprinted minimum dates relating to a very early Precambrian geological event. This reinterpretation came about as the result of finding impossibly high K - A t biotite dates in the range 4800-5200 my from one area, whilst Rb-Sr whole-rock and U-Pb
zircon studies on biotite-bearing gneisses and granites gave ages in the range 2600-2700 my which in certain zones could be shown to have undergone metamorphic overprinting or recrystallisation about 1800 my ago. No R b - S r or U-Pb ages approaching the 3700 my values for parts of West Greenland have yet been reported from the Baltic Shield. It should be noted that the 4°Ar-39Ar technique seems unable to resolve excess from in-situ radiogenic 4°Ar in biotite from regional metamorphic rocks. Conversely, it is evident that a well defined 4°Ar-39Aj plateau in biotite cannot be regarded, by itself, as indicating a meaningful K - A t age.
Acknowledgements We thank the Director of the Geological Survey of Greenland for permission to publish these results on Survey material; the Kryolitselskabet 0resund A/S of Copenhagen and the Marcona Corporation of San Francisco for making facilities available for the collection of samples by S.M. at Isua; Mr Philip Morey of Marcona and Mr J. Kurki of Kryolitselskabet for their hospitality and help at Isua; Dr. D. Bridgwater, Dr. M.H. Dodson, Dr. N.H. Gale, V.R. McGregor, and Dr. R.K. O'Nions for help and discussion; Miss M. Davidson and Mr R. Goodwin for skilled technical assistance. Much of the work at Leeds, Oxford and Sheffield is supported by the Natural Environment Research Council.
Appendix S a m p l e d e s c r i p t i o n s a n d localities (for additional details of the first eleven samples, as well as locality sketch maps, see indicated references).
(i) A m ~ t s o q gneiss, G o d t h a a b area 86431 Granitic augen-gneiss. Kigssavaussat, at mouth of Ameralik Fjord [1,2]. 110869 Granite gneiss. S.E. Q]l~ngSrssuit [2]. 110870 Granodioritic augen-gneiss. S.E. Q~ng~rssuit [1,2]. 110999 Tonalitic gneiss. E. coast of Angissorssuaq [1,2].
R,J. Pankhurst et al., Mineral age patterns in ca. 3700 my old rocks 125519 Tonalitic gneiss. Coast N.W. o f Narssaq [1,21. 125522 Granitic augen-gneiss. Kanajorssuit, near Narssaq [ 1 , 2 ] . 125526 A m p h i b o l i t i c gneiss. Coast S.E. o f Narssaq [21. 125540 G r a n o d i o r i t i c gneiss. Iviangit, near Narssaq [1,2].
(ii) Isua gneiss, Isua area 155764 Granodioritic gneiss [1 ]. 155766 Granodioritic gneiss [1 ]. 155774 Granitic gneiss [ 1 ]. 155765 Leucocratic, coarse-grained, pegmatitic, p o o r l y banded granitic gneiss, w i t h K-feldspar, quartz, plagioclase, muscovite [I ].
(iti) A m t t s o q gneiss enclaves 155713 Dyke-like b i o t i t e - h o r n b l e n d i t e enclave, 40 m long × 15 m wide in A m l t s o q gneiss. Coarsegrained, black, unfoliated rock w i t h h o r n b l e n d e , biotite, a little plagioclase and quartz, accessory sphene, apatite, iron ore. Sample is f r o m centre o f b o d y . Coast N.W. o f Narssaq, nearly same locality as 125519 [ 1 , 2 ] . 155714 F r o m same enclave as 155713, but nearer the margin. 155806 Similar t y p e o f enclave to 155713, but only 4 m wide and w i t h higher plagioclase c o n t e n t . N.E. coast o f Praestefjord [ 1 , 2 ] .
(iv) Isua supracrustal series, Isua area 155785 20 cm-wide q u a r t z - c a l c i t e vein w i t h margins o f coarse, fresh biotite, cutting fine-grained foliated h o r n b l e n d e - b i o t i t e - p l a g i o c l a s e schists o f the supracrustal Isua " I r o n s t o n e " f o r m a t i o n [1].
(v) Q~rqut granite pegmatite 155754 Large, coarse, u n d e f o r m e d pegmatite, w i t h 2 0 - 3 0 cm-wide single crystals of microcline and b o o k s o f muscovite and biotite, cutting Ami'tsoq gneiss. S. side o f Ameralik Fjord, 1 km east o f Qasigi~nguit. The Q 6 r q u t granite itself crops out on the N. side o f the fjord [4].
169
References [1] S. Moorbath, R.K. O'Nions, R.J. Pankhurst, N.H. Gale and V.R. McGregor, Further rubidium-strontium age determinations on the very early Precambrian rocks of the Godthaab District, West Greenland, Nature Phys. Sci. 240 (1972) 78. [2] L.P. Black, N.H. Gale, S. Moorbath, R.J. Pankhurst and V.R. McGregor, Isotopic dating of very early Precambrian amphibolite facies gneisses from the Godthaab district, West Greenland, Earth Planet. Sci. Lett. 12 (1971) 245. [3] H. Baadsgaard, U - T h - P b dates on zircons from the early Precambrian Amitsoq gneisses, Godthaab district, West Greenland, Earth Planet. Sci. Letters (in press). [4] V.R. McGregor, The early Precambrian gneisses of the Godthaab district, West Greenland, Phil. Trans. Roy. Soc. (London) A 273 (1973) 343. [5] L. Keto, lsua, a major iron ore discovery in Greenland (Kryolit-Selskabet Oresund A/S, Copenhagen, 1970). [6] L. Keto, Abstracts Nordic Winter Meeting (Lyngby, Denmark, 1970). [7] D.C. Rex and M.H. Dodson, Improved resolution and precision of argon analysis using an MS10 mass spectrometer, Eclogae Geol. HeN. 63 (1970) 275. [81 G. Turner, A t - Ar age and cosmic ray irradiation history of the Apollo 15 anorthosite, 15415, Earth Planet• Sci. Letters 14 (1972) 169. [9] G. Turner, J.C. Huneke, F.A. Podosek and G.J. Wasserburg, 4°Ar-39Ar ages and cosmic ray exposure ages of Apollo 14 samples, Earth Planet. Sci. Letters 12 (1971) 19. [10] R.J. Pankhurst and R.K. O'Nions, Determination of Rb/Sr and ~TSr/8°Sr ratios of some standard rocks and evaluation of x-ray fluorescence spectrometry in Rb-Sr geochemistry, Chem. Geol. (in press)• [11] D. York, Least squares fitting of a straight line with correlated errors, Earth Planet• Sci. Letters 5 (1969) 320. [12] R. St.J. Lambert and J.G. Simons, New K Ar age determinations from Southern West Greenland, Geol. Surv of Greenland Rep. 19 for 1968 (1969) 68. [131 O. Larsen, Reconnaissance K - A r dating of Samples from West Greenland between S~ndre StrOmfjord and Frederikshaab Isblink, Geol. Surv. of Greenland Rep. 35 for 1970 (1971)44. [141 D. Bridgwater, A compilation of K - A t age determinations on rocks from Greenland carried out in 1969, Geol. Surv. of Greenland Rep. 28 for 1969 (1970) 47. [151 M.A. Lanphere and G.B. Dalrymple, A test of the 4°mi/39Ar age spectrum technique on some terrestrial materials, Earth Planet. Sci. Letters 12 (1971) 359. [16] R.J. Pankhurst, S. Moorbath, and V.R. McGregor, Late event in the geological evolution of the Godthaab District, West Greenland, Nature Phys. Sci. 243 (1973) 24. [17] The 2°Tpb/2°6pb whole rock age of the Archaean granulite facies metamorphic event in West Greenland, Nature Phys. Sci. 244 (1973) 50. 40
39
.
•
170
R.J. Pankhurst et al., Mineral age patterns in ca. 3 700 m y oM'rocks
[18] D. Y o r k and R.M. Farquhar, The earth's age and geochronology (Pergamon, Oxford, 1972). [19] D. Bridgwater, J. Watson and B.F. Windley, The Archaean craton of the North Atlantic region, Phil. Trans. Roy. Soc. (London) A 273 (1973) 493.
[20] S.L. Lobach-Zhuchenko, K.O. Kratz, E.K. Gerling, I.M. Gorokhov, T.V. Koltsova, I.M. Morozova, I.N, Krylov, V.P. Gekulaev, I.D. Pushkarev, V.D. Sprintsov and A.A. Alferovskii, Geochronological constraints for the evolution of the Baltic Shield (Akad. Nauk S.S.R., Leningrad 1972) 1-193.
MINERAL AGE PATTERNS IN CA. 3700 MY OLD ROCKS FROM WEST GREENLAND R.J. PANKHURST, S. MOORBATH
Department of Geology, University of Oxford, England D.C. REX
Department of Earth Sciences, University of Leeds, England and G. T U R N E R
Department of Physics, University of Sheffield, England Received 20 June 1973 Revised version received 28 August 1973
K - A r , 4 ° A r - 3 9 A r and R b - S r dates are reported for minerals from the ca. 3700 my-old Am]tsoq and Isua gneisses of the Godthaabsfjord area of West Greenland. K - A t dates on biotites and hornblendes range from about 1900 to 3500 my, with hornblendes having a much narrower range (ca. 2 2 5 0 - 2 7 5 0 my) than biotites. One biotite from Isua gives an impossibly high K - Ar date of 4940 my. 40 A r - 39 Ar mineral dates are in close agreement with conventional K - A r dates over the entire range of apparent age values. The presence of minor amounts of excess argon is observed in the hornblendes, but radiogenic and excess argon in the biotites are completely homogenised and cannot be differentiated. R b - S r measurements on biotites are closely concordant and show that all biotites were completely open to diffusion of radiogenic a 7 Sr at about 1600-1700 my. This is the first proof of a regional thermal event at this time in the Archaean of West Greenland, although similar dates are well known from the Proterozoic belts to the north and south. The evidence suggests that those K - A r biotite dates greater than about 2700-2800 my result from excess radiogenic argon incorporated during a thermal event of about this age or, more probably, during the 1600-1700 my Sr isotope homogenisation event. Scatter of mineral dates below about 2700 my could also be due, at least in part, to overprinting by the 1600-1700 my event. K - A r mineral dates and an Rb Sr mineral isochron from a pegmatite associated with the last major rock-forming event in the Godthaabsfjord area, namely the Q6rqut granite, indicate an age of emplacement of 2580 *- 30 my.
1. I n t r o d u c t i o n This p a p e r r e p o r t s age d e t e r m i n a t i o n s b y c o n v e n tional K - A r , 40 A r _ 3 9 Ar a n d R b - S r m e t h o d s o n sepa r a t e d m i n e r a l s f r o m the Am~tsoq a n d Isua gneisses o f the G o d t h a a b s t ~ o r d area o f West G r e e n l a n d . P u b l i s h e d R b - S r w h o l e - r o c k i s o c h r o n s for t h e s e gneisses give a n age o f 3 7 0 0 - 3 7 5 0 m y w i t h a n initial ~7 Sr/S6 Sr ratio o f 0 . 7 0 1 0 - 0 . 7 0 1 5 [1 ]. This was i n t e r p r e t e d eit h e r as t h e ' a g e o f e m p l a c e m e n t o f the p a r e n t igneous r o c k s o f t h e gneisses, or as a n i s o t o p i c r e h o m o g e n i s a -
t i o n e v e n t resulting f r o m m e t a m o r p h i s m w i t h i n ab o u t 1 0 0 - 2 0 0 m y o f t h e igneous event. T h e great an t i q u i t y o f this sector o f the A r c h a e a n is c o n f i r m e d by w h o l e - r o c k P b - P b i s o c h r o n p l o t s [2, a n d u n p u b lished O x f o r d analysis] a n d b y U - P b z i r c o n ages [3], w h i c h yield values in t h e range ca. 3 6 0 0 - 3 7 5 0 my. In a d d i t i o n , m i n e r a l ages are r e p o r t e d f r o m a pegm a t i t e associated w i t h the Q 6 r q u t granite, representing t h e last m a j o r r o c k - f o r m i n g e v e n t in t h e area. A simplified s u m m a r y o f t h e geological s e q u e n c e in t h e G o d t h a a b area is given in t a b l e 1. A d e t a i l e d
158
R.J. Pankhurst et al., Mineral age patterns in ca. 3700 my oM rocks
TABLE 1 Abbreviated geological outline of the Godthaab District, West Greenland a 1 Plutonic development ca. 3700-3750 my ago [1, 2] of a complex of dominantly quartzo-feldspathic rocks involving production of granites, granodiorites and tonalites. Metamorphism, migmatisation and deformation, leading to formation of banded gneisses (Amitsoq gneisses). 2 Intrusion of a basic dyke swarm (Ameralik dykes) into the Arnitsoq gneisses. 3 Eruption of basic volcanics and deposition of sediments (Malene supracrustals). 4 Emplacement of stratiform anorthosites, leUcogabbros and related rocks. 5 Major thrusting, producing intercalation of Am]tsoq gneisses and Malene supracrustals. 6 Intrusion of major suite of calc-alkaline rocks including granites, granodiorites, tonalites and diorites - the parent rocks of the Nfik gneisses, 3040 ± 50 my ago [16]. 7 Deformation, involving several phases of folding, accompanied by migmatisation and metamorphism that culminated in amphibolite facies conditions in the area south, east and north-east of Godthaab, but reached granulite facies to the north and west 2850 ± 100 my ago [17]. Earlier igneous rocks converted into gneisses (N~k and Am~'tsoq gneisses) and amphibolites. 8 Emplacement of Q6rqut granite and pegmatites, 2580 -+30 my ago (this work). 9 Intrusion of Precambrian dolerite dykes. a The relative positions of events 3, 4 and 5 are not yet established with certainty. Interpretations other than those given in earlier papers [1, 2] are possible [4].
biotite-bearing vein cutting the supracrustal rocks of the Isua ironstone formation and, finally, one sample of Q6rqut granite pegmatite.
2. Analytical methods Conventional argon analyses were performed on an A.E.I. MS-10 mass spectrometer in the University o f Leeds using isotope dilution with 38Ar spike [7]. Potassium was determined in triplicate by flame photometry. 4°Ar-39Ar measurements were carried out in the University of Sheffield using irradiation and massspectrometric techniques described elsewhere [8]. Irradiations were monitored using a standard hornblende HB3 GR [9]. R b - S r analyses were made in the University of Oxford, using mass-spectrometric isotope dilution techniques on single sample aliquots [101. The decay constants used in this work are: for 87Rb, X = 1.39 X 10 -11 y _ l , and for 4°K, X# = = 4.72 × 10 - 1 ° y - 1 and Xe -- 0.584 X 1 0 - 1 ° y - l • 4°K/K = 0.0119 atom %. Errors on all dates are given at the 95% (2 sigma) confidence level. R b - S r isochrons were fitted by the method of York [11]. R b - S r data have been corrected for 2 ng Rb blank and 10 ng Sr blank.
3. Results account of the geology has been given by McGregor [4], whilst the Isua area, some 150 km north-east of Godthaab on the edge of the inland icecap, has been briefly described by Keto [5, 6]. Geological sketch maps of the whole area have been given previously [1, 2, 4] and will also suffice to identify the present sample localities. Eleven of the rocks analysed in this study have been described in earlier papers [1,2]. Brief summary descriptions of these samples are given in the Appendix, together with somewhat more detailed information on additional specimens. Mineral separates o f biotite and hornblende (as well as muscovite, plagioclase, p o t a s h - f e l d s p a r , sphene and apatite in a few cases) were made from eight samples of Aml"tsoq gneiss, three samples of biot i t e - h o r n b l e n d i t e "enclaves" (irregular dyke-like bodies of pre-Ameralik dyke age) within the AmTtsoq gneisses, four samples of Isua gneiss, one sample of a
3.1. K - A r dates
The analytical data are given in table 2. Several samples were completely reanalysed on separate mineral aliquots with excellent agreement, as shown• K - A r biotite dates range from 1 8 9 0 - 3 2 8 0 my for the A m h s o q gneisses and enclaves, and 2 0 3 0 - 3 5 2 0 my for the Isua gneisses. At first sight this appears to be compatible with variable overprinting b y later thermal events on minerals formed during the 3 7 0 0 - 3 7 5 0 m y episode. K - A r hornblende dates are more restricted in range than the biotite dates: 2 2 5 0 - 2 6 2 0 my for the Ami'tsoq gneisses (with one exception) and 2 4 9 0 2750 my for the basic enclaves. The exception (110870) is a hornblende concentrate which was difficult to purify adequately, as evidenced by its mea-
R.J. Pankhurst et aL, Mineral age patterns in ca. 3700 m y old rocks
159
TABLE 2 K - A r analytical data G.G.U. Sample No.
Mineral
K%
4 OAr rad. %
4 OAr rad. (scc/g × 10 ~ )
K-Ar age (my)
40At
total argon age (my)
4OAr 39Ar a plateau age (my)
3110 ÷ 50
3170
3330 +- 50
3340
}
2990 +- 50
2990
2500 -+ 80 2620 +_ 80 )
2520 -+ 50
2490
3050 +- 50
2610
2520-+50
2560
39At a
A m l t s o q gneBses 125519
biotite
6.55 6.33
99.4 99.2
20.599 19.653
3100 _+ 1 0 0 ) 3080 -+ 100
110870
biotite
7.64 7.67
98.4 98.8
25.869 27.150
3220 +_ 100 3280 -+ 100 }
110999
biotite
6.98
98.9
12.703
2300 -+ 70
125522
biotite
6.76
98.6
13.702
2460 -+ 80
125540
biotite
7.84
99.0
22.911
2990 -+ 90
7.82
99.3
22.947
3000 _+ 90
110869
biotite
7.23
97.2
9.393
1890 _+ 60
86431
biotite
7.45
99.2
13.429
2310 -+ 70
125519
hornblende
1.14 1.08
98.3 96.3
2.377 2.385
110870
hornblende (impure)
3.85
99.5
12.083
3100 +- 100
110999
hornblende (impure)
1.99
98.4
3.445
2250 +- 70
125526
hornblende
0.84
98.3
1.601
2380 + 80
Am~tsoq gneiss enclaves 155713
biotite
6.97
99.2
14.302
2480-+
155714
biotite
6.78
98.4
16.780
2750 +- 90
155806
biotite
7.60
99.4
13.244
2260-+
70
2340+-50
2360
155713
hornblende
0.53
97.3
1.238
2660-+
80
2760-+50
2700
155714
hornblende
0.44
98.7
1.090
2750 +- 90
155806
hornblende
0.26
98.6
0.539
2490+-
2650+-50
2660
7.71 7.71
98.4 99.2
31.644 31.148
4970 -+ 70
5030
80
80
Isua gneisses 155764
biotite
3520 +- 100-, 3490 + 100 )
155765
muscovite
8.33
97.1
15.863
2430 +- 80
155766
biotite
6.72
96.3
9.797
2030 +- 60
155774
biotite
7.51 7.51
98.6 99.0
29.338 29.607
3440 -+ 100~ 3450 + 100"
155785
biotite (vein in ironstone formation)
6.48 6.48
98.4 99.6
61.251 62.841
4920 _+ 150} 4960 + 150-
Q 6 r q u t Pegmatite 155754
biotite
7.68
96.8
14.286
2350 -+ 80
155754
muscovite
8.47
97.8
17.660
2500 + 80
a For full details, see table 3 N.B. Total Argon ( 4 ° m l - 3 9 A r ) a g e is directly comparable with K - A r age.
% 39A1"
Ca/K
J = 0.0491
% 39Ar
Ca/K
biotite
J = 0.0491
Ca/K
J = 0.0484
% 39At
Ca/K
biotite
J = 0.0484
Age (my)
Error
4°mr/39Ar
T(°C)
155806
Age (my)
Error
4°Ar/39Ar
T(°C)
% 39mr
155713
biotite
Age (my)
Error
4°Al/39Ar
T(°C)
110870
Age (my)
Error
4°Al-/39Ar
T (°C)
125519
biotite
Sample
TABLE 3 4 OAr- 39mr analytical data
1710
0.2
30.5
.
1.7
500
1590
0.4
27.4
0.7
500
2750
0.4
67.0
.
1.6
500
2180
0.8
44.2
1.0
500
Heating steps
0.4
46.6
2230
.
9.4
600
2180
0.2
45.1
.
9.7
600
3360
0.4
100.7 *
.
27.8
600
2990
0.4
79.3
-
10.6
600
.
.
.
.
2350
0.6
51.3 *
43.6
690
2530
0.2
58.5 *
18.4
690
3320
0.6
98.3 *
.
24.2
690
3140
0.6
87.6 *
-
57.1
690
2370
0.6
52.0 *
.
8.3
760
2560
0.4
59.7 *
.
20.6
760
3340
1.2
99.3 *
.
20.0
760
3160
0.4
88.4 *
-
13.6
760
.
2360
0.2
51.7 *
12.7
820
2560
0.4
59.6 *
11.4
820
3320
0.6
97.8 *
15.6
820
3160
0.4
88.7 *
0.9
9.5
820
2360
0.2
51.6 *
22.3
940
2560
0.2
59.6 *
21.1
940
3340
0.8
99.5 *
9.2
920
3050
0.4
82.3
1.21
5.5
940
2400
0.4
53.0
1.9
1200
2570
0.4
60.1
17.8
1200
3300
0.6
96.8
0.9
1.7
1100
2910
0.8
75.0
2.54
2.7
1200
*
2340
0.2
50.7
2520
0.2
57.9
3330
0.4
98.7
3110
0.8
85.9
Total
2360
51.6
2560
59.5
3340
99.2
3160
88.2
age (my)
Plateau
2'
2
~,
% 39Ar
Ca/K
biotite
J = 0.0484
% 39Ar
Ca/K
hornblende
J = 0.0491
% 39Ar
Ca/K
hornblende
J = 0.0491
Age (my)
Error
4°Ar/39Ar
T(°C)
110870
Age (my)
Error
4°Ar/39Ar
T(°C)
125519
Age (my)
3230
0.4
92.4
-
18.0
600
3710
6.6
125.2
7.0
0.5
600
3410
28
.
Error
Ca/K
J = 0.0309
0.9
165
% 39At
biotite
500
Heating steps
4°Ar/39Ar
I'("C)
1557 ~,~
Age (my)
Error
4°AI/39AI"
T(°C)
125540
Sample
TABLE 3 (continued)
.
3280
0.8
95.8
-
33.4
690
3210
2.4
91.6
1.87
1.0
690
3840
8
216
5.5
600
2990
0.4
.
80.3 *
.
63.0
600
.
.
3280
1.0
95.6
-
4.5
760
3280
7.2
95.5
1.74
0.8
760
4930
8
411
.
19.9
690
3000
0.4
*
80.6 *
15.8
690
.
10
457
3240
0.6
93.3
0.52
8.0
820
3300
6.2
96.6
4.7
0.6
820
5120
.
29.0
760
30•0
0.6
*
.
81.0 *
8.5
760
.
*
2630
0.4
61.8 *
4.4
26.1
940
2450
0.4
54.4 *
2.19
38.4
940
5010
14
429
14.4
820
2990
0.4
80.2 *
4.3
820
*
2580
0.2
59.6 *
5.4
9.1
1090
2510
0.8
56.8 *
6.3
49.8
1090
4990
4
425
23.9
940
3000
0.4
80.8 *
8.0
940
*
2530
0.4
57.4
7.1
0.9
1220
2540
.0.4
58.2 *
7.4
8.9
1220
5100
8
451
0.15
6.3
11 O0
2990
2.0
80.3
1.56
0.4
1200
3050
0.2
82.5
2520
0.4
57.2
4970
4
420
2990
0.4
80.4
Total
2610
61.0
2490
55.9
5030
434
2990
80.4
Plateau age (my)
7,
¢b
C)
2
t~
t~
at"
% 39mr
Ca/K
hornblende
J = 0.0491
Ca/K
J = 0.0491
% 39At
Ca/K
(extended
run)
3.6
-
Age (my)
Error 3170
1.2
89.2 *
Ca/K
4°Ar/39Ar
9.8
24
600
2300
% SgAr
Time (hr)
T(°C)
Age (my)
Error
48.5
.
1.0
90
300
Heating steps
1.6
49.2
0.6 .
3175
0.6
89.4 *
-
10.8
72
600
2320
.
27
400
4740
51.4
231.8
3
0.3
600
2860
1.2
72.3
0.82
7.6
600
3165
0.6
88.7 *
-
10.6
1
760
2880
0.6
73.5
.
13.8
24
500
2000
0.4
38.3
0.51
14.8
820
2950
2.4
77.0
2.92
5.1
820
0.8
84.8
6.7
3130
0.8
86.5
0.40
8.0
1.5
920
3090
.
25
500
3750
4.6
128.1
6.8
3.2
940
2950
0.8
77.2
24.7
8.5
940
.
.
3040
0.4
82.0
1.63
9.9
1.5
1090
3110
1.6
85.3
3.6
24
500
2770
0.4
68.1 *
18.2
27.1
990
2760
0.4
67.5 *
42
43.0
990
3160
0.8
88.5 *
13.9
68
500
2610
0.4
61.1 *
16.8
47.8
1090
2640
0.6
62.1 *
40
20.4
1090
3165
0.6
88.9 *
11.3
171
500
2720
1.4
65.9 *
17.6
6.9
1220
2720
1.0
65.8 *
45
15.5
1220
3100
0.4
84.9
2650
0.2
62.6
2760
0.4
67.8
Total
3170
88.9
2660
63.4
2700
64.7
Plateau age (my)
Notes: Temperatures for heating steps are approximate, being based on current-temperature calibration of the tantalum furnace. Spot checks were performed with an optical pyrometer for T > 800°C. Errors quoted are 2-sigma. Ca/K ratios are calculated from 37Ar/39Ar. Where no entry is shown Ca/K is less than 0.05. Plateau ages are calculated from the gas release steps marked with an asterisk.
J = 0.0491
Time (hr)
biotite
40Ar/Sgml"
T(°C)
125519
Age (my)
Error
40Al/39Ar
T(°C)
% 39Ar
155806
hornblende
Age (my)
Error
4°AI/39Ar
T( ° C)
155713
Sample
TABLE 3 (continued) bO
R.J. Pankhurst et aL, Mineral age patterns in ca. 3700 my old rocks
sured K content of 3.85% and by the 4°Ar/39Ar data discussed later. These dates are significantly lower than those of the Arnitsoq gneiss biotites (125519, 110870, 125540). However, in the case of the three basic enclaves there is essential concordance between K - A t hornblende and biotite dates ranging from 2260 to 2750 my, the biotite-hornblende pair 155714 being in agreement at 2750 my. Hornblende has not yet been found in the Isua gneisses, but a single muscovite (155765) date of 2430 my is in general agreement with most of the hornblendes from the Amitsoq gneisses and contrasts sharply with the high dates of several Isua biotites (155724, 155766, 155785). In the case of Am~tsoq gneiss sample 125519, the K-Ar biotite date (3100 my) is significantly greater than that of the coexisting hornblende (2500-2600 my). This is the reverse of the normal order of retentivity. The above evidence leads to the suspicion that some of the higher K - A t biotite dates could be due in part to excess radiogenic argon. This idea is indirectly supported from the Isua area by the impossibly high date of 4940 my for the biotite (155785) from a vein cutting supracrustal amphibolitic rocks of the Isua ironstone formation surrounding the 3700 myold Isua gneisses (the true stratigraphical relationship between the two still being unknown). This biotite, like all biotites analysed in this study, has a normal K content for typical biotite and shows no evidence for chloritisation or other alteration. Despite the fact that biotites from the gneisses have a range of dates from 1890 to 3520 my, there is some indication from the two most concordant hornblende-biotite pairs (e.g. 155713, 155714) and from several other hornblende dates that a thermal event could have occurred about 2600-2800 my ago. Younger dates could then be interpreted as due to later overprinting or prolonged cooling. However, the evidence against all the dates being due to overprinting of the original 3700-3750 my ages will become clearer after discussion of the R b - S r data. The biotite-muscovite pair from the QOrqut pegmatite is only just concordant within 2 sigma error at 2350 and 2500 my respectively. A few K - A r dates in the range 2400-2600 my have been reported previously on biotites and hornblendes from the Godthaabsf]ord area [12, 13]. Furthermore, a K - A r biotite date of 3595 + 70 my has been reported previously from the Isua gneiss [14],
163
which was interpreted as signifying the presence of excess argon (albeit before it was realised that the lsua gneiss was 3700 my old).
3.2. 4 ° A r - 3 9 A r dates
Six biotites and four hornblendes were selected for 4°Ar-39Ar age determinations in the hope of "seeing through" Ar losses and, perhaps, of characterising excess 4°Ar. Analytical data are given in table 3. The release patterns of hornblende and biotite from 125519 are fairly representative and are shown graphically in fig. 1. The release patterns of the hornblendes were characterised by high and widely differing 4°Ar/39Ar ratios in the low temperature release (T < 940°C). Above 940°C, where the major argon release occurred, the 4°Ar/39Ar ratios showed an approximate plateau corresponding to ages of different samples in the range 2490-2700 my. The high temperature ratios from any given sample showed real fluctations of 8% or so, an order of magnitude larger than the experimental uncertainties. The K - A r systematics in these samples are clearly rather complex. The low temperature release, in all samples except 110870, is presumed to represent excess 4°Ar incorporated during a metamorphic episode. The pronounced low temperature release of 110870 is unquestionably due to biotite contamination. This is indicated by the high K concentration referred to in the preceding section, and the absence of significant 3VAr in the low temperature release which implies a very low Ca/K ratio in the mineral responsible, which is characteristic of the biotite release patterns (see table 3). The high temperature plateau age of 2610 my is due to the hornblende alone and confirms that the conventional K--Ar date of 3100 my (table 2) is a meaningless "mixed" date. Apart from this, hornblende dates estimated from the high temperature release, given in table 3, are only marginally different from the conventional dates reported in table 2, the presumed "excess" argon making only a small contribution to the age. They indicate that argon retention in all four samples dates from around 2500-2700 my ago. The release patterns for the biotite are simple plateaux giving dates which agree with the conventional K - A r dates within experimental error despite the fact
R. J. Pankhurst et al., Mineral age patterns in ca. 3 700 my old rocks
164
L’ 125519
2.0
-
BIOTITE
- - - -
HORNBLENDE
-I
I
0
0.5
1.0
FRACTION OF 3gAr RELEASE0 Fig. 1. 40Ar/39Ar release patterns, plotted as apparent age of Ar released in each heating biotite 125519. Full data for these and all other mineral samples given in table 3.
that these range from ca. 2300 to 5000 my. Apart from evidence of minor amounts of 40Ar loss in the low temperature release, no anomalous Ar or difference in site activation energy can be detected in any of the samples, including 155785 which gives an apparent age greater than that of the solar system. Since other workers have also found similar behaviour [ 1.51, it must be concluded that no additional information can be obtained by the application of 40Ar/39Ar techniques to biotites. Either there is only one site available to accommodate radiogenic and extraneous (excess) Ar, or else all the Ar is effectively homogenised in the biotite lattice during the reactor irradiation or in the furnace during Ar extraction. We have attempted to see whether the homogeneity extends to other argon isotope ratios. 36Ar is virtually undetectable in these samples and is of no help. Similarly 3 7Ar is undetectable in the low temperature release and 37Ar in the high temperature release can be understood in terms of the presence of small amounts of hornblende (high Ca/K) and therefore yields no information specific to the biotite. 38Ar is present in all the biotites, with 3*Ar/3gAr ratios ranging from 0.025
step, for hornblende
125519,
(155806) to 0.11 (125540). “Ar is produced by (n - y) reactions on 37Cl and is thus indicative of sites occupied by Cl. The 38Ar/3gAr ratios (not tabulated) were essentially constant indicating that 38Ar was diffusing from equivalent sites to 40Ar and 3 9Ar. An attempt was made to see whether the small variations which did occur in 40Ar/39Ar and ‘8Ar/39Ar were correlated (as in two component mixture) but the results were inconclusive, the experimental uncertainties being comparable to the ratio variations. An attempt was also made to detect the excess argon in 125519 by varying the heating schedule: heating for longer times (several days) at lower temperatures 300-600°C. The release pattern (fig. 1) was identical to that obtained with the shorter (1 hr) heating times. The high temperature decrease in 40Ar/39Ar is due to the presence of small impurities of hornblende (see Ca/K ratio in table 2). The present measurements show that on a scale of ca. 0.2 - 0.4 mm grain diameter for the analysed minerals the distribution of radiogenic 40Ar and excess 40Ar within irradiated biotites is identical, and it is not possible to separate the two components by the
165
R.J. Pankhurst et al., Mineral age patterns in ca. 3700 my old rocks
TABLE 4 Rb-Sr analytical data Sample
Rb ppm
Sr ppm
87 Rb/86 Sr 32.50
87 St/86 Sr
Age (my)
± 0.18
1.4490 ± 0.0002
1612
± 9
214.9
± 2.4
5,966
± 0.002
1733
± 20
13.38
108.8
-+ 1.1
3.221
+ 0.002
1637
± 16
461
7.69
282.5
± 2.4
7.134
-+0.001
1618
± 15
155764 biotite
577
11.05
236.7
+_ 3.0
6,498
±0.001
1733
± 20
155774 biotite
632
29.49
72.1
± 0.8
2.365
±0.001
1616
± 17
155785 biotite
537
8.42
308.6
± 4.6
7.565
±0.001
1581
± 24
± 0.002
0.7232 ± 0.000l \
125519 biotite
245
23.39
110870 biotite
364
7.42
125540 biotite
404
155806 biotite
125519 hornblende
6.03
125519 plagioclase
1.40
125519 sphene
0.90
125519 apatite
0.75
38.22 517.4
0.457
0.0078 • 0.0001
0,7113 ± 0.0001
0.0554 +- 0.0010
0.7099 ± 0.0001
0.0085 ± 0.000l
0,7099 ± 0.0001
See fig. 2 46.79 255.8
155754 biotite
3165
32.87
24.350
155754 muscovite
1450
15.69
6008
155754 microcline
1423
21.23
643
application o f stepwise degassing using the 4°Ar 39Ar m e t h o d . A possible w a y o f detecting unequivocally the presence o f excess 4°Ar w o u l d be to carry out total e x t r a c t i o n 4 ° A r - 3 9 A r analysis on single b i o t i t e grains f r o m a single geological unit or even f r o m a single rock sample. This has not b e e n d o n e in the present work, in which sample splits were used to allow c o m p a r i s o n with conventional K - A t ages. 3.3. R b - S r dates
The results o f seven R b - S r biotite analyses are given in table 4. The samples were all highly enriched
±300
884
±5
± 90
220,2
± 0.7
+
4
24.39
2.580 ± 30 (fig. 3)
+- 0.01
in radiogenic 8 v Sr. Precise dates were calculated from a t w o - p o i n t isochron (single-stage) model, using published R b - S r whole-rock data [1] for the gneiss biotires, and by assuming an initial 87 Sr/86 Sr ratio o f 0.700 for the enclave biotite ( 1 5 5 8 0 6 ) and the vein biotite (155785). The latter assumption yields maxim u m dates in the event o f subsequent closed-system Sr isotope h o m o g e n i s a t i o n . The calculated R b - S r dates for all analysed biorites fall within the relatively narrow range 1 5 8 0 - 1 7 3 0 my. This clearly records the cessation o f Sr isotope diffusion following a thermal event, whilst the narrow range o f dates contrasts strongly w i t h the
166
R.J. Pankhurst et aL, Mineral age patterns in ca. 3700 m y old rocks
0725 I•
÷/~0
875r 865r
~
+~" Hb WR.
~
072C
071!
~
0-71( ~ +
~ ,~..
Sp 87R~b I~
aeSr -
-
I 01
[ 0"2
01"3
I 04
015
I 06
Fig. 2. Rb-Sr isochron diagram for mineral components of Amltsoq gneiss sample 125519. Ap = apatite, Sp = sphene, Pi = plagioclase, Hb = hornblende, Bi = biotite, WR = whole rock. enormous spread of K - A r dates, discussed earlier. This is strong indirect evidence for regarding many of the K - A r dates as excess argon dates. Formally, the two-point isochron model assumes that all mineral components within each rock were in isotopic equilibrium at the time deduced from the slope of the biotite-whole rock join. This was further tested for sample 125519, from which coexisting biotite, hornblende, plagioclase, sphene and apatite were analysed. The data are plotted in fig. 2, together with that for the whole rock. It is evident that the assumption of complete isotopic equilibrium is not fully satisfied since the data points do not define a unique isochron. The biotite-whole rock join corresponds to a date of 1610 + 10 my, whilst the plagioclase-whole rock join gives 1890 + 40 my. The hornblende data point, which is close to that for the whole rock (although the Rb and Sr contents are very different), could be considered to lie on either of these lines. The apatite and sphene, both with very low Rb/Sr ratios, fall below these lines. The sphene-whole rock join corresponds to a date of 2380 + 80 my. The most plausible interpretation of the data is that subsequent to the 2 6 0 0 - 2 8 0 0 my event indicated by the K - A r hornblende data, an event at ca. 1 6 0 0 - 1 7 0 0 my or some-
what older re-opened the plagioclase, hornblende and biotite, but only partially affected the sphene and apa tite. The biotite did not become a completely closed system until 1610 -+ 10 my ago. Although the minerals were not in complete isotopic equilibrium at this time, the extremely high Rb/Sr (and s 7 Sr/86 Sr) ratio of the biotite as compared to the other components makes the R b - S r biotite date very insensitive to small departures from the assumed model. It is considered that this holds equally true for the other biotites listed in table 4, most of which have even higher Rb/Sr ratios. As stated previously, the last major rock-forming event in the Godthaab district was the magmatic activity associated with the QSrqut granite. A pegmatite associated with the final phases of activity is dated by a three-point R b - S r mineral isochron (fig. 3) on potash feldspar, muscovite and biotite (155754), which gives an age of 2580 + 30 my. This is in general agreement with K - A r mica ages from the pegmatite given in table 2, and more particularly with the muscovite date of 2500 + 80 my. Because of the high Rb/Sr ratios of all three minerals, the initial ~TSr/a6Sr ratio (1.0 -+ 0.3) is too imprecise to define the origin of the pegmatite.
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R.J. Pankhurst et al., Mineral age patterns in ca. 3700 my old rocks
Fig. 3. Rb-Sr mineral isochron muscovite, Bi = biotite.
for Qarqut
granite
pegmatite
sample
Clearly, the Qarqut pegmatite minerals have not responded to the regional Sr isotope homogenisation event at ca. 1600- 1700 my. This is hardly surprising when it is considered that single crystals and books of all three minerals in the pegmatite are commonly 20-30 cm in diameter. It is concluded that the post-tectonic Q&qut granite was emplaced about 2600 my ago. Rb-Sr wholerock studies are planned to substantiate this. A K-Ar biotite date of 2500 + 25 my has previously been reported from the Q&qut granite itself [13].
4. General discussion The observed range of K-Ar biotite dates was at first thought to result from variable overprinting since crystallisation about 3700 my ago [l ,2], but the following lines of evidence suggest that this is not the case: (a) A biotite from a vein cutting Isua supracrustal rocks yields a K-Ar date of $940 my. (b) Hornblendes from Amitsoq gneisses and biotitehornblendite enclaves have a smaller spread of K-Ar and 40Ar-39Ar dates (2250-2750 my) than some coexisting or separate biotites (1890-3340 my). Most of the biotite dates exceed the hornblende dates. Two biotites from the Isua gneiss give K-Ar dates of ca.
155754
from Ameralik
fjord.
KF = microcline,
Mu =
3500 my, another gives 2030 my, whilst a muscovite gives 2430 my. (c) The range of dates for hornblendes, a mineral normally exhibiting higher Ar retentivity than biotite, suggests the possibility of minor Ar loss following a thermal event which caused complete degassing about 2700 my ago, or somewhat earlier. There is ample independent geochronological evidence for such an event, which could be associated with the intrusion of the parent igneous rocks of the Niik gneisses of the Godthaab area which in one locality have yielded a Rb-Sr whole-rock isochron age of 3040 + 50 my [ 161 Alternatively, the event could correspond to a subsequent amphibolite grade metamorphism possibly contemporaneous with the regional granulite facies metamorphism to the north and south of the Godthaab area, which has been dated at 2850 + 100 my ago [17]. Finally, there was the QZrqut granite magmatic episode about 2600 my ago (see above), which might also be expected to have affected the K-Ar dates. It should be noted that the coarse-grained biotites from the Am?tsoq biotite-hornblendite enclaves appear to have been reset by the ca. 2700 my event. The biotites from the Amitsoq (and Isua) gneisses, with K-Ar dates up to 3500 my, are all much finergrained than these. (d) All analysed biotites (except that from the Q&gut granite pegmatite) have been affected by ex-
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R.J. Pankhurst et al., Mineral age patterns in ca. 3700 my old rocks
tensive loss of radiogenic aTSr ca. 1600-1700 my ago. Since the retentivity of micas for radiogenic Ar and i Sr are similar [18], it seems most unlikely that those biotites with very high K - A r dates could have been so resistant to Ar loss during the Sr isotope homogenisation episode. It is much more likely that excess radiogenic Ar was introduced into these biotites at this time. This is the first regional evidence for 1600-1700 my dates within the Archaean craton of West Greenland. Similar K-Ar, Rb-Sr and U-Pb dates are well known to the north (Nagssugtoqidian) and south (Ketilidian) of the Archaean craton, where they are interpreted as the age of Proterozoic plutonism. The latter may well have left no recognisable petrological imprint in the Godthaabsf]ord area. Similar dates within the range ca. 1600-1700 my (as well as in the range ca. 2600-2800 my, see above) have been widely recognised in the ancient shield areas of Canada, north-west Scotland [19] and the Baltic (see below).
5. Conclusions It is concluded that the K - A r biotite dates are controlled to a great extent by excess radiogenic 4°At formed by degassing of basement rocks and taken up by biotites when they were at temperatures at which Ar could diffuse rapidly through the lattice. This could have occurred either at about 2600-2800 my ago, or more probably at about 1600-1700 my ago. The fact that the resultant K - A r biotite dates in the range ca. 1900-3500 my are geologically plausible is probably fortuitous, although it could conceivably be related to the limited amount of radiogenic 4°Ar available from degassing of a basement consisting of a mixture of rocks formed predominantly at about 3700 my and 3000 my ago [1,2, 16]. Russian workers [20] have recently reinterpreted the numerous reported K - A r biotite (and other mineral) dates ranging up to ca. 3900 my from the northern sector of the Baltic Shield as excess Ar dates rather than overprinted minimum dates relating to a very early Precambrian geological event. This reinterpretation came about as the result of finding impossibly high K - A t biotite dates in the range 4800-5200 my from one area, whilst Rb-Sr whole-rock and U-Pb
zircon studies on biotite-bearing gneisses and granites gave ages in the range 2600-2700 my which in certain zones could be shown to have undergone metamorphic overprinting or recrystallisation about 1800 my ago. No R b - S r or U-Pb ages approaching the 3700 my values for parts of West Greenland have yet been reported from the Baltic Shield. It should be noted that the 4°Ar-39Ar technique seems unable to resolve excess from in-situ radiogenic 4°Ar in biotite from regional metamorphic rocks. Conversely, it is evident that a well defined 4°Ar-39Aj plateau in biotite cannot be regarded, by itself, as indicating a meaningful K - A t age.
Acknowledgements We thank the Director of the Geological Survey of Greenland for permission to publish these results on Survey material; the Kryolitselskabet 0resund A/S of Copenhagen and the Marcona Corporation of San Francisco for making facilities available for the collection of samples by S.M. at Isua; Mr Philip Morey of Marcona and Mr J. Kurki of Kryolitselskabet for their hospitality and help at Isua; Dr. D. Bridgwater, Dr. M.H. Dodson, Dr. N.H. Gale, V.R. McGregor, and Dr. R.K. O'Nions for help and discussion; Miss M. Davidson and Mr R. Goodwin for skilled technical assistance. Much of the work at Leeds, Oxford and Sheffield is supported by the Natural Environment Research Council.
Appendix S a m p l e d e s c r i p t i o n s a n d localities (for additional details of the first eleven samples, as well as locality sketch maps, see indicated references).
(i) A m ~ t s o q gneiss, G o d t h a a b area 86431 Granitic augen-gneiss. Kigssavaussat, at mouth of Ameralik Fjord [1,2]. 110869 Granite gneiss. S.E. Q]l~ngSrssuit [2]. 110870 Granodioritic augen-gneiss. S.E. Q~ng~rssuit [1,2]. 110999 Tonalitic gneiss. E. coast of Angissorssuaq [1,2].
R,J. Pankhurst et al., Mineral age patterns in ca. 3700 my old rocks 125519 Tonalitic gneiss. Coast N.W. o f Narssaq [1,21. 125522 Granitic augen-gneiss. Kanajorssuit, near Narssaq [ 1 , 2 ] . 125526 A m p h i b o l i t i c gneiss. Coast S.E. o f Narssaq [21. 125540 G r a n o d i o r i t i c gneiss. Iviangit, near Narssaq [1,2].
(ii) Isua gneiss, Isua area 155764 Granodioritic gneiss [1 ]. 155766 Granodioritic gneiss [1 ]. 155774 Granitic gneiss [ 1 ]. 155765 Leucocratic, coarse-grained, pegmatitic, p o o r l y banded granitic gneiss, w i t h K-feldspar, quartz, plagioclase, muscovite [I ].
(iti) A m t t s o q gneiss enclaves 155713 Dyke-like b i o t i t e - h o r n b l e n d i t e enclave, 40 m long × 15 m wide in A m l t s o q gneiss. Coarsegrained, black, unfoliated rock w i t h h o r n b l e n d e , biotite, a little plagioclase and quartz, accessory sphene, apatite, iron ore. Sample is f r o m centre o f b o d y . Coast N.W. o f Narssaq, nearly same locality as 125519 [ 1 , 2 ] . 155714 F r o m same enclave as 155713, but nearer the margin. 155806 Similar t y p e o f enclave to 155713, but only 4 m wide and w i t h higher plagioclase c o n t e n t . N.E. coast o f Praestefjord [ 1 , 2 ] .
(iv) Isua supracrustal series, Isua area 155785 20 cm-wide q u a r t z - c a l c i t e vein w i t h margins o f coarse, fresh biotite, cutting fine-grained foliated h o r n b l e n d e - b i o t i t e - p l a g i o c l a s e schists o f the supracrustal Isua " I r o n s t o n e " f o r m a t i o n [1].
(v) Q~rqut granite pegmatite 155754 Large, coarse, u n d e f o r m e d pegmatite, w i t h 2 0 - 3 0 cm-wide single crystals of microcline and b o o k s o f muscovite and biotite, cutting Ami'tsoq gneiss. S. side o f Ameralik Fjord, 1 km east o f Qasigi~nguit. The Q 6 r q u t granite itself crops out on the N. side o f the fjord [4].
169
References [1] S. Moorbath, R.K. O'Nions, R.J. Pankhurst, N.H. Gale and V.R. McGregor, Further rubidium-strontium age determinations on the very early Precambrian rocks of the Godthaab District, West Greenland, Nature Phys. Sci. 240 (1972) 78. [2] L.P. Black, N.H. Gale, S. Moorbath, R.J. Pankhurst and V.R. McGregor, Isotopic dating of very early Precambrian amphibolite facies gneisses from the Godthaab district, West Greenland, Earth Planet. Sci. Lett. 12 (1971) 245. [3] H. Baadsgaard, U - T h - P b dates on zircons from the early Precambrian Amitsoq gneisses, Godthaab district, West Greenland, Earth Planet. Sci. Letters (in press). [4] V.R. McGregor, The early Precambrian gneisses of the Godthaab district, West Greenland, Phil. Trans. Roy. Soc. (London) A 273 (1973) 343. [5] L. Keto, lsua, a major iron ore discovery in Greenland (Kryolit-Selskabet Oresund A/S, Copenhagen, 1970). [6] L. Keto, Abstracts Nordic Winter Meeting (Lyngby, Denmark, 1970). [7] D.C. Rex and M.H. Dodson, Improved resolution and precision of argon analysis using an MS10 mass spectrometer, Eclogae Geol. HeN. 63 (1970) 275. [81 G. Turner, A t - Ar age and cosmic ray irradiation history of the Apollo 15 anorthosite, 15415, Earth Planet• Sci. Letters 14 (1972) 169. [9] G. Turner, J.C. Huneke, F.A. Podosek and G.J. Wasserburg, 4°Ar-39Ar ages and cosmic ray exposure ages of Apollo 14 samples, Earth Planet. Sci. Letters 12 (1971) 19. [10] R.J. Pankhurst and R.K. O'Nions, Determination of Rb/Sr and ~TSr/8°Sr ratios of some standard rocks and evaluation of x-ray fluorescence spectrometry in Rb-Sr geochemistry, Chem. Geol. (in press)• [11] D. York, Least squares fitting of a straight line with correlated errors, Earth Planet• Sci. Letters 5 (1969) 320. [12] R. St.J. Lambert and J.G. Simons, New K Ar age determinations from Southern West Greenland, Geol. Surv of Greenland Rep. 19 for 1968 (1969) 68. [131 O. Larsen, Reconnaissance K - A r dating of Samples from West Greenland between S~ndre StrOmfjord and Frederikshaab Isblink, Geol. Surv. of Greenland Rep. 35 for 1970 (1971)44. [141 D. Bridgwater, A compilation of K - A t age determinations on rocks from Greenland carried out in 1969, Geol. Surv. of Greenland Rep. 28 for 1969 (1970) 47. [151 M.A. Lanphere and G.B. Dalrymple, A test of the 4°mi/39Ar age spectrum technique on some terrestrial materials, Earth Planet. Sci. Letters 12 (1971) 359. [16] R.J. Pankhurst, S. Moorbath, and V.R. McGregor, Late event in the geological evolution of the Godthaab District, West Greenland, Nature Phys. Sci. 243 (1973) 24. [17] The 2°Tpb/2°6pb whole rock age of the Archaean granulite facies metamorphic event in West Greenland, Nature Phys. Sci. 244 (1973) 50. 40
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[18] D. Y o r k and R.M. Farquhar, The earth's age and geochronology (Pergamon, Oxford, 1972). [19] D. Bridgwater, J. Watson and B.F. Windley, The Archaean craton of the North Atlantic region, Phil. Trans. Roy. Soc. (London) A 273 (1973) 493.
[20] S.L. Lobach-Zhuchenko, K.O. Kratz, E.K. Gerling, I.M. Gorokhov, T.V. Koltsova, I.M. Morozova, I.N, Krylov, V.P. Gekulaev, I.D. Pushkarev, V.D. Sprintsov and A.A. Alferovskii, Geochronological constraints for the evolution of the Baltic Shield (Akad. Nauk S.S.R., Leningrad 1972) 1-193.