EPSL
ELSEVIER
Earth and Planetary Science Letters 142 ( 1996) 501-5 12
Pb isotope constraints on the provenance and diagenesis of
detrital feldspars from the Sudbury Basin, Canada
S.R. Hemming a7*, D.K. McDaniel by1,S.M. McLennan by2,G.N. Hanson bt3
b Department
a Lamont-Doherty Earih Observatory, Palisades, NY 10964, USA
of Earth and Space Sciences, State University ofNew York at Stony Brook. Stony Brook. NY I 1794, USA
Received 22 December 1995; accepted 14 May 1996
Abstract
Residues after HF leaching of detrital feldspar grains from the Early Proterozoic Chelmsford Formation of the Sudbury
Basin, Ontario record isotopic compositions very close to the initial Pb isotopic composition of their Late Archean, Superior
Province source. These grains have been recrystallized to albite and K-feldspar. Residues of Chelmsford feldspar grains lie
within the range of reported 206Pb/ 204Pb compositions of feldspars from plutons of the adjacent Abitibi subprovince of the
Superior Province, with a steep 207Pb/206Pb slope of 0.35 f 0.15 (3.7 * 0.7 Ga). Leaches are more radiogenic than
residues, but leach-residue
pairs do not record in situ U (and Th) decay since either the Late Archean time of feldspar
crystallization in the Superior Province or the Early Proterozoic time of recrystallization
in the Chelmsford Formation.
Instead, leach-residue pairs lie along an average 207Pb/206Pb slope of 0.250 k 0.036, representing mixing trends. Most
whole-rock samples from the Chelmsford Formation lie close to a 2.7 Ga slope, collinear with detrital feldspar compositions.
We interpret the Pb data from the leaches as recording the passing of altering fluids. These fluids likely carried Pb derived
from rocks or minerals with relatively high U/Pb, Th/Pb and Th/U within the sedimentary basin itself. The residues
appear to record very close to initial values because they lie along the very steep trend defined by feldspars of the Superior
Province.
Keywords: Sudbury Basin; Chelmsford Formation; Pb-206/Pb-204; Pb-207/Pb-206; feldspar group
1. Introduction
Feldspar is an important rock-forming mineral in
igneous and metamorphic rocks and is a common
Corresponding author. Fax + 1 914 365 8155. E-mail: [email protected]
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detrital phase in sediments and sedimentary rocks.
Accordingly, natural isotopic and geochemical variations in feldspar can be used to help identify sediment sources. Pb is an important trace element in
feldspars [1,2], and it is well known that the U/Pb
and Th/Pb ratios of feldspar (especially K-feldspar)
are low (for example, [3]). These attributes allow
feldspar to record the isotopic composition of Pb that
was incorporated at the time of its formation. Although multiple intracrustal processes produce scatter in the initial Pb isotopic compositions of rocks
that formed at the same age, the variations
are
Copyright 0 1996 Elsevier Science B.V. All rights reserved.
PII SOOl2-821X(96)00102-1
502
S.R. Hemming et d/Earth
and Planetary Science Letters 142 (1996) 501-512
understandable.
In fact, these natural variations provide a distinctive signature for tracing the provenance of detrital feldspar grains. In many simple
granitic samples there is a predictable trend in the
isotopic composition of progressive leaches such that
the “‘Pb/ 206Pb ages of leach-residue
pairs approximate the crystallization age of the granite (for example, [4,5]). In one example anomalous results were
found for leaches, due to incorporation
of extra
intermediate parent(s) to “‘Pb [4]; however, residues
still provided a reasonable approximation of the initial isotopic composition.
Later plutonism or hydrothermal
alteration
associated with rifting can
complicate feldspar Pb isotope systematics. For example, in Early Proterozoic granites of the southwestern United States, feldspars are found to lie
along slopes that reflect the original crystallization
age (ca. 1.7 Ga) and the ages of disturbance (ca. 1.4
and 0 Ga) [6]. Less is known about the Pb isotope
systematics of feldspars from metamorphic terranes
[7,8], although simple interpretations
based on the
known regional geochronology
(time of formation
and time of metamorphism)
seem invariably to provide an adequate explanation.
In addition to having favorable geochemical characteristics to record the initial Pb isotopic composition of its host, feldspar is relatively susceptible to
chemical alteration during weathering. This tendency
to destruction is in contrast to other commonly applied isotopic provenance tracers. For example, the
U-Pb dating of individual detrital zircons provides a
powerful provenance tracer (some recent references
include [9-121). However, zircon is among the most
resistant minerals to chemical attack and may be
transported great distances and recycled through the
sedimentary system multiple times. Another widely
used and powerful isotopic provenance tool is the
Sm-Nd
system in bulk sediment samples (some
recent references include [ 13-151). Because the rare
earth elements are mostly retained in the terrigenous
elastic fraction during weathering, the Sm-Nd isotope system may reflect great transport distances and
multiple sediment recycling history in some complicated way. Studies of detrital feldspar grains in
young sediment samples have demonstrated that Pb
isotopes provide a robust indication of their sources
[17,18]. Additionally, studies of feldspar grains from
ancient sediments provide promise that the prove-
nance information
may be retained through burial
diagenesis [ 13,19-211.
Few studies exist that allow evaluation
of Pb
isotope systematics during diagenetic alteration of
feldspar. We report Pb isotope data for diagenetically
altered feldspars from the Chelmsford Formation. In
this case, the source and history of both the Pb
originally present in the detrital feldspar and the Pb
introduced during diagenesis can be evaluated. Given
the relatively large time difference between its ca.
2.7 Ga provenance from the Superior Province [13]
and ca. 1.8-1.7 Ga depositional/diagenesis
age (inferred from regional constraints provided by Fairbairn et al. [22], Krogh et al. [23], and Deutch et al.
[24]), the Chelmsford Formation provides a good test
case for the nature of the Pb isotope system in
detrital feldspar during diagenetic or metamorphic
replacement.
2. Geological
background
The northeast-southwest
trending Sudbury Basin
in southern Ontario (Fig. 1) contains faulted and
gently folded Early Proterozoic strata of the Whitewater Group. The formations
of the Whitewater
Group appear to be conformable [25,26]. The bottom
unit is breccia of the Onaping Formation, interpreted
as having formed as a result of a meteorite impact
(reviewed in Deutch et al. [24]). The Onaping Formation is overlain by black shale of the Onwatin
Formation. Dark gray, fine to coarse sandstones and
argillites of the Chelmsford Formation comprise the
top of the Whitewater Group. The Onaping and
Onwatin formations are not related to the impact
event. U-Pb lower intercept ages of shattered zircons in the Onaping Formation, combined with concordant U-Pb ages from the Sudbury Igneous complex, constrain the base of the Whitewater Group to
1850 Ma [23]. The shattering of zircons is interpreted
as a result of meteorite impact and the Sudbury
Igneous complex is intrusive into the Onaping Formation [23]. The Chelmsford Formation is not cut by
any precisely dated feature and thus its depositional
age is not well constrained. However, Deutch et al.
[24] report that the Chelmsford Formation is cut by
gabbroic dikes, and one has an internal Sm-Nd
isochron age of 1648 i 103 Ma. Additionally,
an
S.R. Hemming et al./Earth
and Planetary Science Letters 142 (1996) 501-512
Rb-Sr whole-rock errorchron for samples of the
Onwatin and Chelmsford formations yields an apparent age of 1643 + 130 Ma ([22] recalculated in [ 131
using the decay constants of [27]) that may also be
reasonably interpreted as a minimum age for the
time of deposition.
The Sudbury Basin is in a foreland position relative to any Penokean-age orogen that may have been
located within the present Grenville Province (for
example, [ 13,28-3 11). Any possible erogenic margin
southeast of the Sudbury Basin has been obscured by
middle Proterozoic Grenville deformation and metamorphism, and there is contention
about whether
Early Proterozoic crust exists within the Grenville
Province in this region [8,29,30]. Studies of paleocurrent indicators
in the Chelmsford
Formation
show southwestward
sediment transport along the
long axis of the basin, which suggests deposition in
an elongate trough [32,33]. Because the Sudbury
Basin appears to have been generated by meteorite
impact at 1.85 Ga [23,24,34], the elongate depositional basin shape suggested by the paleocurrent
indicators must imply that the basin was deformed
between impact and deposition of the Chelmsford
Formation.
In contrast to potentially age-correlative rocks in
the Animikie Basin [35,36], there is no evidence for
- (Chcimsfbrd
~Onwar~n
~Otqmg
~Szrdbbu~
503
a juvenile Penokean provenance for the Chelmsford
Formation [I 31. McDaniel et al. [13] reported some
Pb isotope data from detrital feldspar grains and used
this to support evidence from major and trace element compositions and whole rock Pb and Sm-Nd
systematics that the Chelmsford Formation was derived predominantly
from the ca. 2.6-2.7 Ga Superior Province. Based on these geochemical indicators
of Archean provenance, plus the petrographic evidence for a first cycle sediment source, McDaniel et
al. [ 131 interpreted the Chelmsford
Formation
as
having a basement-uplift
tectonic environment,
possibly associated with a bend in the Penokean orogenie margin near the region of the Sudbury Basin.
Thrust imbrication of basement in the eastern part of
the Sudbury Basin seems to be indicated by the
Canadian LITHOPROBE studies [24], and Archean
basement
is clearly preserved east of the basin
[8,29,30]. Such a foreland basin scenario for the
Chelmsford Formation is proposed by Young [31] to
explain the provenance.
McDaniel et al. [13] have previously showed that
Chelmsford feldspars were most likely derived from
the Superior Province and that the major element
compositions most probably reflect burial diagenetic
alteration (Fig. 2). For this study, we have assumed
that the dominant process of feldspar alteration in the
Formatton
’
Formatron
Formatron
Igneous Complex
Grenville
Province
Fig. 1. Map of the Sudbury Basin region showing sampling locations of McDaniel et al. [ 131. 0 = graywacke
samples. Sampling is describedin detail in [ 131. Basement rocks of the Late Archean Superior Province are shown as
Proterozoic rocks of the Southern Province (Huronian) are shown as a light stippled pattern. The legend refers to the
within the Sudhury Basin. The Grenville Province, a middle Proterozoic erogenic belt, is in the southeastern comer
samples; 0 = argillite
the cross pattern, Early
Early Proterozoic units
of the map.
S.R. Hemming et al./ Earth and Planetary Science Letters 142 (1996) Sol-512
504
f
8or--
-80
An
90/
h
90
”
100
90
80
Fig. 2. Ternary plot of Ab-An-Or
for feldspar grains of tire
Chelmsford Formation from McDaniel et at. [13]. These nearly
end-member compositions are taken as evidence for the feldspars
having been replace during burial diagenesis [ 131.
Chelmsford Formation is burial diagenesis. However, there are rocks in the region that have been
pervasively albitized by fluid alteration associated
with gold mineralization
at about 1.7 Ga [37]. The
interpretation of Schandl et al. [37] is that the massive albitization event is related to deep-seated alkalit magmatism in the Southern Province. It would
not be possible with existing data to distinguish Pb
from such sources from mixing along a 2.7-1.8 Ga
isochron; meaning that the Pb isotopes do not allow
us to discriminate between these scenarios. However,
the Chelmsford
Formation
sandstones
are petrographically similar to sandstones where feldspars
were replaced during burial diagenesis (for example,
bined for analysis. Residues and leaches were dried
and then dissolved in 6 N HCl by cooking for 30
min or more at 100°C. The HCl solution was dried,
and the salts dissolved with 0.7 N HBr for anion
exchange chemistry with 0.7 N HBr followed by a 6
N HCl elution. The procedural blank for Pb was
about 90 pg.
Approximately 0.1 g whole-rock sample powders
were dissolved with HF-HNO,
in Krogh bombs for
36 h at 22O”C, dried and redissolved overnight with
12 N HCl in bombs. Column chemistry was on
anion exchange resin with 2.5 N HCl followed by a
6 N HCl elution. This chemistry is not nearly as
effective as the HBr procedure, so column procedures were repeated 3 times. Pb blanks for the
whole-rock procedure are about 300 pg.
Samples were analyzed by the H,PO, silica gel
technique [40]. Feldspar and some whole-rock samples were analyzed on a VG sector 54 mass spectrometer at Lamont-Doherty
Earth Observatory in
14.8
]38,391).
2m
3. Analytical
procedures
and results
204Pb
14.6
Single feldspar grains were picked from a crushed
sample of graywacke and cleaned several times with
water, dilute HCl and dilute HNO,. After hand
picking grains from the > 250 pm fraction, all
handling was conducted in a clean environment
at
Lamont-Doherty
Earth Observatory.
Surfaces of
grains from the Chelmsford Formation are difficult
to see because of a dark, fine-grained coating. Grains
were boiled in a 6 N HCl-7 N HNO, mixture for
30 min or put in an ultrasonic cleaner for 30 min in
the same acid mixture. Following this procedure,
both sets of samples were rinsed and leached with 4
N HF in an ultrasonic cleaner for 5 min. Residues
were also dissolved in 4 N HF. For larger grains,
more than one leaching step is possible. Each leaching step was followed by rinsing three times with
distilled water, and the leach and rinses were com-
.
1AA
XI.
13.0
13.4
13.8
14.2
14.6
15.0
206Pb
204Pb
Fig. 3. Plot of “‘Pb/
*04Pb versus *06Pb/ *04Pb for residues of
detrital feldspar grains from tire Chelmsford Formation. A p. = 8
curve from 4.57 Ga is show for reference. Crosses = data from
late plutons (2.6-2.7
Ga) in the Abitibi greenstone belt [41]
located a few hundred kilometers north of tire Sudbury area. The
Abitibi data are highlighted by the shaded field. Line a, m = 0.35
f0.15
(3.7kO.7
Gal, is the best fit line through residue data
reported in this paper. Line 6, m = 0.26~tO.04 (3.2* 0.2 Ga), is
the best fit line through residue data reported in McDaniel et al.
[ 131. Note that the *“Pb/ *04Pb ratios are relatively high, and the
data lie along the very steep trend defined by least radiogenic
samples from the Abitibi greenstone belt [451.
S.R. Hemming
et al./
Earth and Planetary
single collector, peak switching mode or in static,
multicollector mode (after Faraday cup replacement).
Analyses of NBS Pb standard SRM981 over the
interval of the analysis were used for mass fractionation corrections, and measured ratios, corrected for
mass fractionation using the standard data, are reported in Table 1. Some whole-rock samples were
analyzed in duplicate on a Finnigan MAT 262 in
static multicollection mode at Stony Brook. Results
Table 1
Feldspar and whole-rock
Sample b
Feldspars ’
DKM- 15-W
lR
1L
1L (MS rep.)
2R
3R
3Ll
3L2
5R
5L
6R
6L
7R
7L
8R
;:
DKM-16-90 d
DKM- 17-W d
data from the Chelmsford
?‘J6Pb, ?04Pb
&
formation
Science Letters 142 (1996) 501-512
505
for SRM982 Pb standard over the interval of the
analyses are reported in Table 1.
Pb isotope results for feldspars and whole-rocks
are reported in Table 1. Also included are the data
from McDaniel et al. [13]. Pb isotopic compositions
of the feldspar residues have compositions consistent
with the observations and interpretation of McDaniel
et al. [ 131 that the Chelmsford Formation was derived from Late Archean sources within the nearby
a
207Pb/ *04Pb
+
“*Pb/
‘04Pb
+
13.601
13.954
13.952
13.538
13.528
14.917
13.692
13.610
13.95 1
13.747
14.505
13.561
14.242
13.546
13.780
13.676
13.854
14.143
0.00 I
0.020
0.017
0.009
0.00 1
0.00 I
0.005
0.004
0.012
0.004
0.017
0.003
0.017
0.002
14.640
14.745
14.739
14.635
14.609
15.007
14.676
14.637
14.726
14.684
14.81 1
14.610
14.748
14.63 1
14.647
14.623
14.679
14.743
0.002
0.02 1
0.018
0.009
0.002
0.002
0.006
0.005
0.014
0.004
0.019
0.003
0.021
0.002
33.423
33.805
33.757
33.417
33.355
35.014
33.581
33.429
33.821
33.556
34.091
33.346
34.02 1
33.427
33.523
33.454
33.770
33.945
0.004
0.047
0.041
0.02 1
0.004
0.003
0.014
0.009
0.034
0.01 I
0.05 1
0.007
0.05 1
0.007
20.576
19.700
17.552
18.134
18.587
18.972
18.922
17.957
17.940
18.492
19.066
18.272
0.02 I
15.783
0.018
39.247
39.791
37.198
37.603
38.112
38.005
39.198
37.825
37.779
38.501
38.460
39.436
0.048
Whole rocks
DKM-XX-90
03 e
13 d
15 e
17 d
18 d
21 c
22 c
23 e
23 ’
29 c
30 d
32 ’
0.010
0.011
0.003
0.012
0.003
0.007
0.005
15.600
15.301
15.381
15.477
15.438
15.568
15.373
15.357
15.468
15.552
15.408
0.004
0.016
0.003
0.004
0.003
0.006
0.004
0.039
0.040
0.008
0.005
0.007
0.014
0.012
a Reported ratios are corrected for mass fractionation using measured ratios from NBS standards. b Sample numbers follow [ 131. ’ Feldspar
samples weighed 0.5-l mg. Samples were analyzed at L-DE0 (expect 4 from 1131); SRM 981 (2~ external, n = 13): *“Pb/ ‘06Pb =
0.91344(38),
206Pb/ *04Pb = 16.891(16), 207Pb/204Pb = 15.429(17), ‘08Pb/204Pb = 36.504(38), uncertainty
reported in table is 20
in-run. d Data reported in 1131, NBS 12”. Stony Brook. e Finnigan MAT Stony Brook (static multicollection
analysis): SRM 982 (2~
external for 2 analyses, n = 35): “‘Pb/ 206Pb = 0.46657(19),
208Pb/ *“Pb = 0.99794(77), “‘Pb/
‘04Pb = 36.664(29), uncertainty reported in table is difference between 2 runs.
S.R. Hemming et ul./ Earth and Planetary Science Letters 142 (1996) 501-512
506
Superior Province (Table 1, Fig. 3). The slopes of
the feldspar residue-leach
pairs are steep (Fig. 4),
ranging from 0.168 to 0.409. All leach and residue
14.5
42
2”8pb
38
““Pb
36
33.6
Provmce feldspar
Od
33.4 ;
3
33.2
13.5
‘““Pb
h
13.7
13.9
14.1
14.3
14.5
Fig. 4. Pb isotopic compositions of feldspar leaches and residues
from the Chelmsford Formation. To create the ‘field of wholerocks at 1.8 Ga’, whole-rock samples were calculated back to 1.7
(approximate diagenesis age) and 1.85 Ga (approximate depositional age of the Whitewater Group and age of Penokean plutonic
rocks) using the two-stage model calculations of Gale and Musset
[42]. The time of the first stage, t,, is taken to be 2.6 or 2.8 Ga,
and the initial composition is taken to be estimated by the lowest
206Pb/ *04Pb detrital feldspar. 232’I’h/ ‘04Pb (0) is assumed not
to have changed. (a) Plot of ‘07Pb/ ‘04Pb vs. “‘Pb/
204Pb for
residues measured for this study and reported in McDaniel et al.
[ 131 and for leach-residue
pairs. Shown for reference are lines
representing 2.7- 1.8, 2.7-O and I .8-O Ga Pb evolution from the
lowest ‘06Pb/ 204Pb detrital feldspar residue. The sample with the
shallowest slope for leach residue pair is noticeably steeper than
the I I-0 Ga evolution (Table 2). Due to the nature of the model,
the back-calculated
whole-rock compositions are perfectly co-linear with the 2.7- 1.8 Ga slope through the lowest 206Pb/ *04Pb
feldspar, and the purpose of this plot is merely to show graphically how strong the influence of Pb from a reservoir like these
whole-rocks
could have been. (b) Plot of “*Pb/ *04Pb vs.
206Pb/ ‘04Pb for residues measured for this study and reported in
McDaniel et al. [ 131 and for leach-residue
pairs. Shown for
reference are lines representing K = 3 and K = 5 from the lowest
*06Pb/ *04Pb feldspar residue.
‘““Pb
Fig. 5. Pb isotopic compositions
of whole-rock
samples and
feldspar residues from the Chelmsford
Formation.
Reference
curves are p. = 8, K = 4 from 4.57 Ga, with tics at 1 Ga increments, and the reference field includes reported feldspar compositions from the Abitibi greenstone belt of the Superior Province
[41]. (a) Plot of “‘Pb/
so4Pb vs. 2MPb/ ‘04Pb. Shown for reference are lines through the lowest zo6Pb/Zo’Pb
feldspar residue
with slopes representing evolution from 2.7-1.8 and 1.8-O Ga.
samples
(b) Plot of “*Pb/ ‘04Pb vs. 206Pb/ “‘Pb. Whole-rock
have compositions consistent with K between 3 and 5 since the
time of formation of the feldspars. Feldspar residues spread out
along a trend between K = 3 and K = 5 also.
data reported in this study, taken together, yield a
slope of 0.250 k 0.035. For comparison, evolution
between 2.7 (approximate age of provenance) and
1.8 Ga (approximate age of sedimentation)
corresponds to a slope of 0.293. For each residue-leach
pair, it is possible to calculate a model age of
disturbance (t,) for a given provenance
age (t,).
These values are tabulated in Table 2. Values for t,
are highly scattered, but are reasonable within the
context of the provenance and deposition/diagenetic
ages of the Chelmsford Formation.
507
S.R. Hemming et al./ Earth and Plunetary Science Letters 142 (1996) Sol-512
Table 2
Model calculations
based on detrital feldspar data from the Chelmsford
‘06Pb/
Sample
IL
IR
2R
13.95
13.59
13.54
3Ll
3L2
3R
3 L-L-R ’
14.91
13.68
13.52
5L
5R
“‘Pb
207Pb/ 206 Pb slope
+c
0.313
0.054
353
0.287 a
0.409 b
0.281
0.014
0.108
0.177
13.95
13.61
0.261
6L
6R
14.50
13.75
7L
7R
8L
8R
14.24
13.56
13.72
13.55
Avg. of all
Best tit line ’
RC
Re
DKM-16-90 e
DKM- 17-90 ’
03
13
15
17
18
21
22
23
23
29
30
32
1
S-K model age ’
S-K
p,’
(Ma)
260
1852
3399
3941
3367
077
400
980
1596
0.059
3253
350
1298
0.168
0.028
2538
290
0.202
0.03 1
2842
240
2647
9.00
0.295
0.111
3455
570
1679
2704
9.42
0.277
0.250
0.036 ’
2540
2530
2423
2215
8.40
8.56
8.47
8.19
2641
2724
9.08
9.57
2675
9.07
2636
9.10
2558
9.00
1 and residue. b Calculated from leach 2 and residue. ’ Calculated using ISOPLOT [48]. S-K stands for Stacey and
for 3 L-L-R is 4.42. d Calculated
using ISOPLOT
Most whole-rock samples lie close to a 2.7 Ga
reference line that projects into the field of Superior
Province feldspar compositions (Fig. 5). Using: (1)
Sample
tz (t, = 2.71
(Ma)
+
13.78
13.68
13.85
14.14
a Calculated from leach
Kramers [49]. M.S.W.D.
Table 3
Two-stage
Formation
t,(t,=O)
(Ma)
model calculations
for Chelmsford
‘Initial’ a
whole-rock
1481. M.S.W.D.
is 8.84. e data reported in [13].
the average feldspar composition from Abitibi plutons [41] with 206Pb/ 204Pb less than 14; and (2) the
least radiogenic Chelmsford feldspar as an initial
samples
Referred to average Abitibi feldspar
b
Referred to detrital feldspar
‘OhPb/ *04Pb
207Pb/204Pb
u,
pz
0
pi
)Lz
ILz-IL,
20.58
19.70
17.55
18.13
18.59
18.97
18.92
17.96
17.94
18.49
19.07
18.27
14.42
14.40
14.52
14.48
14.50
14.36
14.54
14.51
14.51
14.51
14.48
14.49
12.4
10.1
8.5
8.9
10.1
8.0
11.7
9.2
9.0
10.3
10.7
9.4
14.2
12.8
7.1
8.7
9.4
11.8
9.5
8.0
8.1
9.0
10.5
8.9
10.2
8.0
6.4
6.8
8.0
5.9
9.5
7.1
6.9
8.1
8.5
7.2
15.6
14.3
8.6
10.1
10.8
13.3
10.9
5.4
9.5
10.4
11.9
10.3
53.1
78.7
64.5
49.3
35.8
125.8
15.1
32.8
38.3
28.2
40.0
42.8
IQ--CL,(%)
15
26
- 16
-3
-7
47
- 19
- 14
-11
- 13
-2
-5
41.3
45.1
27.0
29.8
33.4
32.6
35.4
31.4
31.2
30.5
35.8
37.1
’
(%l
w
41.2
45.0
26.9
29.7
33.3
32.5
40.9
31.3
31.0
36.0
35.7
42.5
a ‘Initial’ *07Pb/ *“Pb is calculated as the intersection between the Abitibi volcanic massive sulfide array 1451 and the 2.7 Ga line that
contains the datum. b P, and P2 (p. =238U/204 Pb) are calculated using the average of Abitibi feldspars with 206Pb/204Pb less than 14
from [41] (3R, Table 1) and t, and tr of 2.7 and 1.8 Ga, respectively. o (w =23’Th/ 204Pb) is calculated assuming no change at I .8 Ga.
’ p,, and p,? are calculated using the least radiogenic feldspar residue from the Chelmsford Formation (3R, Table 1) and t, and t2 of 2.7 and
1.8 Ga, respectively. w is calculated assuming no change at 1.8 Ga.
508
S.R. Hemming et d/Earth
and Planetary Science Letters 142 (1996) 501-512
composition and t, and t, of 2.7 and 1.8 Ga respectively; the calculated p. for first (p ,, p. = *j*U/ *04Pb,
2.6 1.8 Gal and second (pZ. 1.8-O Ga) stages are
reported in Table 3. Two-stage model calculations
were made using the derivation of Gale and Musset
[42]. For most samples, using the average Abitibi
composition, l~r and p2 are similar, but two samples
stand out significantly (Table 3, Fig. 5). Note that
the choices of t , , t, and the initial isotopic compositions are not uniquely constrained by the data. Additionally, the disturbance could have occurred at almost any time early in the history, although the
magnitude of the change in U/Pb would vary accordingly. For instance, if the isotopic composition
of the least radiogenic detrital feldspar is used to
approximate the initial Pb ratios, p,, is uniformly
lower than p2.
4. Discussion
As briefly mentioned in the Introduction, the Pb
isotopic composition of detrital feldspars can provide
important information
on sediment sources. However, to date, this approach has been rarely used. A
notable example of the utility of Pb isotopes in
detrital feldspars is the work of Patterson and Tatsumoto 123. Patterson and Tatsumoto [2] used the Pb
isotopic compositions
of large (1 g) detrital Kfeldspar composites, along with their Rb-Sr model
ages, to demonstrate that there is an excess of 206Pb
and 208Pb that increases as the age of the feldspar
decreases. They modeled this effect to be the result
of transport of U and Th from the interior of the
earth to an outer mantle layer that was the source of
continental
crustal terranes that were formed and
isolated at different times and locations. Further,
they interpreted the data as suggesting that the bulk
of the North American continent was formed during
the interval 3.5-2.5 Ga. This interpretation of continental crustal growth is consistent with later estimates.
In addition to providing powerful constraints on
the evolution of continental
crust, the Pb isotope
system in detrital feldspar has great utility as a
provenance tool for more detailed regional studies.
Only recently have Pb-isotope studies of detrital
feldspars been used to evaluate specific sediment
provenance (for example, [ 13,17-2 1,431) and secondary alteration [13,19,20]. Aleinikoff et al. [19]
demonstrated that authigenic overgrowths of detrital
K-feldspars from Cambrian sandstones had distinct
radiogenic Pb-isotopic compositions compared to the
cores. They were able to identify multiple episodes
of authigenesis in one case and in another case were
able to identify the source of the Pb that was introduced.
4.1. Provenance of the Chelmsford Formation
McDaniel et al. [13] interpreted limited Pb isotope
data for whole rocks and leached feldspars from the
Chelmsford
Formation
to be consistent
with an
Archean Superior Province provenance and, further,
consistent with Sm-Nd
isotopic data from whole
rocks. Data presented here add considerable strength
to this conclusion. Fig. 5 is a plot of “*Pb/ ‘04Pb
and *“Pb/ 204Pb versus *06Pb/ *04Pb for residues of
leached detrital feldspars and whole-rocks from the
Chelmsford Formation. Also shown are the fields for
igneous
feldspars
from the Archean
Superior
Province [41] and the Proterozoic Penokean Province
[441.
Feldspar residues all plot within the range of
*06Pb/ 204Pb values for Superior Province feldspars
and along the trend defined by lowest “‘Pb/
‘04Pb
values, strongly indicating that this terrane is the
provenance of the feldspar grains. However, Chelmsford data are largely outside of the range of
‘07Pb/ *04Pb recorded for plutons of the Abitibi belt
(Fig. 3). Volcanogenic
massive sulfide deposits in
the Abitibi greenstone belt span a larger range [45]
and include the Chelmsford detrital feldspar data.
Vervoort et al. 1451 found a strong correlation between initial Nd and Pb isotopic compositions in the
Superior Province. McDaniel et al. [46] reported
Sm-Nd isotope systematics from the same suite of
Chelmsford Formation samples as we used for this
study. Although a subset of the samples are characterized by low abundances of rare earth elements
(REE) and positive Ce anomalies,
interpreted to
record loss of light REE, all samples have Nd isotopic compositions at the time of deposition that are
consistent
with derivation
from
the Superior
Province. Using the samples that have normal REE
patterns, the calculated range of ENd at 2.6 Ga is
S.R. Hemming
et &/Earth
and Plunetary
14.4
Superior Province
1431
-0.5
I
0
0.5
/
1.0
1.5
I
20
I
25
3.0
I
3.5
Initial E,
Fig. 6. Plot of “‘Pb/
‘04Pb of volcanogenic
massive sulfides
from the Abitibi belt versus initial lNd of host volcanic rocks
from Vervoort et al. [45]. Also shown is the range of calculated
tNd at 2.7 Ga for Chelmsford Formation whole-rock samples
without obvious sedimentary alteration from [ 131 and the range of
detrital feldspar residues from this study. Data from the Chelmsford Formation lie along the curve formed by the volcanic rocks
and massive sulfide deposits of the Abitibi greenstone belt, and
this relationship provides further strength for the interpretation of
both provenance tools.
0.8-2.5
(Fig. 6). Accordingly,
detrital feldspar
residue Pb data and whole rock Sm-Nd data from
the Chelmsford Formation lie on the curve defined
by the Pb isotopic compositions
of volcanogenic
massive sulfides and the initial Nd isotopic compositions of their hosts. Thus, they corroborate a Superior Province source and provide mutual strength to
the interpretation of both provenance tools.
Most whole-rock samples from the Chelmsford
Formation lie close to a 2.7 Ga reference line that
projects into the field of Superior Province feldspar
compositions
(Fig. 5). Although there is a zone
where evolution
from Superior
and Penokean
provinces overlap, the trend (i.e., slope) and position
of the whole-rock data lend strong support to an
interpretation that the whole-rock Pb isotope compositions
reflect
an almost
exclusively
Superior
Province provenance. There are several possible explanations for three whole-rock samples that are not
on the main trend (samples 3, 10 and 2 1). It is
ossible that there was a range in the initial
!?
“Pb/ ‘04Pb ratios of the feldspar-s that translates
into scatter in the whole-rock data. In other words,
all Chelmsford whole-rock samples can be fit to 2.7
Ga slopes that project through the range of measured
Superior Province feldspar compositions
(compare
Science Letters 142 (1996) SOl-Sl2
509
calculated
initial “‘Pb/ *04Pb in Table 3 with
feldspar data in Fig. 3). Another possibility is that
there has been some disturbance of the U/Pb ratio
during the history of these sediments. It is likely that
any disturbance of the U/Pb would have occurred
near the time of sedimentation during weathering or
burial diagenesis. On the plot of *“Pb/ 20’Pb versus
206Pb/ *04Pb (Fig. 5) all of the whole-rock samples
plot in positions consistent with a Th/U ratio (~1
between 3 and 5 and averaging about 4. These are
typical upper crustal values [16], consistent with no
disturbance in the Th/U ratio. One implication of
these data is that any increase in U/Pb of the bulk
sediment samples is more likely due to removal of
Pb.
4.2. Pb isotopes and diagenesis
feldspars
of Chelmsford
McDaniel et al. [ 131 noted that the four feldspar
residues
they analyzed
appeared
to form a
“‘Pb/ *04Pb versus 206Pb/ *OJPb trend consistent
with Pb isotope evolution between 2.7 and 1.8 Ga,
and suggested that this could be due to incorporation
of Pb with an isotopic composition
similar to
Penokean aged rocks. In this study, we have taken
care to leach the feldspars thoroughly and this can be
seen in the lower 206Pb/ *04Pb compared to those
reported in [ 131 (Fig. 3). Nevertheless, several of the
data appear to form a trend that is steeper than
predicted based on the assumption
of Pb isotope
evolution between 2.7 and 0.0 Ga, although uncertainties are large. Thus, even aggressive leaching of
the feldspar has failed to remove all of the altered Pb
in some of the samples. It is noteworthy that the
steep trends defined by these feldspar residues are in
marked contrast to simple leached igneous feldspars,
where the “‘Pb/ 206Pb slopes tend to correspond
approximately to the age of crystallization
(for example, [4,51X
Although appealing because the steepest leachresidue slopes match that of a 2.7- 1.8 Ga paleo-isochron, it is not precisely correct to interpret them
slopes simply as paleo-isochrons.
Instead, these
slopes probably represent mixing lines between Pb
originally present in the detrital feldspar and Pb
introduced at the time of alteration. The shallower
slopes may reflect a component of in situ U decay,
510
S.R. Hemming rt d./Eurth
urd Planetary Scirncr Letters 142 (1996) 501-512
although no measured slopes give less than 2.5 Ga
ages (Table 2, Fig. 4a). In Fig. 4, the field for
Penokean igneous feldspars is plotted, as are the
ranges of Chelmsford whole-rock data at 1.7-l 35
Ga, a model-dependent
calculation [42], assuming a
two-stage evolution, with t, = 2.6 and 2.8 Ga and
initial Pb isotope composition estimated from the
detrital feldspar data. The large range in ages was
chosen to create the largest reasonable
field.
Penokean igneous ages are ca. 1.85 Ga, but there is
evidence of some pervasive. low-grade regional event
that overprinted the Rb-Sr whole-rock systems in
many rocks (for example, [22,47]). It is reasonable to
suspect that this event could have been related to
some of the disturbance found in whole-rock samples and to the diagenetic recrystallization
of the
feldspar
grains.
In both
207Pb/ “‘Pb
and
lo8Pb/ *“Pb versus *06Pb/ “‘Pb diagrams. three
lines through feldspar leach-residue
pairs project
into the region defined by calculated compositions of
the Chelmsford Formation whole rocks in the Early
Proterozoic (Fig. 41. The other two pairs lie on
shallower slopes but are still steeper than a 1.8 Ga
slope. These results can be explained by addition of
radiogenic Pb due to in situ decay of U within the
cracks in feldspar grains.
It is a reasonable interpretation that the altering
fluid obtained its Pb directly from the sedimentary
basin itself. In other words, the leach-residue
chords
could have been produced by incorporation of radiogenie whole-rock
Pb during diagenesis.
Altematively, mixing with Early Proterozoic Pb, as represented by Penokean feldspars. or from the surrounding basement of the Superior Province cannot be
eliminated by the Pb data. However, it would be
fortuitous if the addition of Pb from external sources
would yield whole-rock data that lie along a Late
Archean slope co-linear with feldspars from the Superior Province.
5. Conclusion
Residues of detrital feldspars from the Chelmsford Formation preserve the initial Pb isotopic composition, indicative
of derivation
from the Late
Archean Superior Province; however, radiogenic Pb
was added to the feldspars during burial diagenesis.
The result is that progressive leaching of the feldspars
yields progressively less radiogenic Pb isotopic compositions. Leaches and residues lie along a steep
slope that precludes the leaches’ Pb from having
been produced by in situ U (and Thl decay. The
source of the Pb that was diagenetically introduced
into the detrital feldspars is best interpreted as the
host sedimentary rocks themselves. In future provenance studies of ancient sandstones, it will be useful
to evaluate routinely the Pb isotope systematics of
residues and leaches from detrital feldspars in order
to understand more clearly the terrigenous elastic
source and diagenetic components.
Acknowledgements
This work was partially supported by EAR957784
(to SMM). The comments and discussions of Troy
Rasbury and Gary Hemming are appreciated. Thanks
go to Grant Young, John Aleinikoff and an anonymous reviewer for their journal reviews. This is
LDEO contribution number 5524. [CL]
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