DETRITAL ZIRCON GEOCHRONOLOGY OF TACONIAN AND ACADIAN FORELAND SEDIMENTARY
ROCKS IN NEW ENGLAND
S.M. MCLENNAN,1,2 B. BOCK,3 W. COMPSTON,2 S.R. HEMMING,4 AND D.K. MCDANIEL1*
1
Department of Geosciences, State University of New York at Stony Brook, Stony Brook, New York 11794-2100, U.S.A.
e-mail: [email protected]
2 Research School of Earth Sciences, Australian National University, Canberra, Australian Capital Territory 2601, Australia
3 GEOMAR, Abt. Marine Umweltgeologie, Wischhofstraße 1-3, D-24148 Kiel, Germany
4 Lamont-Doherty Earth Observatory, Palisades, New York 10964, U.S.A.
ABSTRACT: U-Pb ages of detrital zircons in lower Paleozoic sedimentary rocks from New England permit evaluation of the relationships
between tectonic activity and provenance of foreland sediments deposited in response to the Taconian and Acadian orogenies. The Lower
Cambrian Poughquag Quartzite was deposited on the Laurentian passive margin. Poughquag detrital zircons (n 5 40) are mostly concordant to slightly discordant with 207Pb*/ 206Pb* ages of 1.67–0.87 Ga
and are consistent with a provenance from the nearby Grenville Structural Province. Two Poughquag zircon grains record 206Pb*/ 238U ages
of about 0.64 Ga and 0.55 Ga and could have been derived from riftrelated magmatic rocks. The Middle Ordovician Austin Glen Member
of the Normanskill Formation and Lower Silurian Shawangunk Formation represent clastic-wedge sedimentation in the Laurentian foreland deposited in response to the Taconian orogeny (ca. 470–430 Ma).
Zircons from these units (n 5 41 for Austin Glen, n 5 29 for Shawangunk) are also predominantly of Grenville age ( 207Pb*/ 206Pb* ages
of 1.46–0.94 Ga). One zircon from the Austin Glen records an age of
3.29 Ga, consistent with derivation from North American Archean terranes. No zircon from either unit (or reported in the literature) can be
interpreted as recording an Ordovician (Taconian) age. Devonian Catskill ‘‘redbeds’’ represent synorogenic to postorogenic foreland sedimentation associated with the Acadian orogeny (ca. 400–360 Ma). Zircons from one sample (n 5 45) are all near concordant and have a
bimodal age distribution. Approximately 60% of grains have 207Pb*/
206Pb* ages of 1.23–1.00 Ga and the other 40% have 206Pb*/ 238U ages
of 0.47–0.42 Ga. Very different proportions of Precambrian and Paleozoic ages are seen in the Catskill sample if zircons are selected on
the basis of size or optical clarity, and this observation highlights the
possibility of sampling bias if detrital zircons are not randomly selected
for U-Pb analysis. The 0.47–0.42 Ga population is consistent with derivation from rocks crystallized during the Taconian orogeny. No analyzed zircon in this sample records an unambiguous Devonian (Acadian) age. Accordingly, during early Paleozoic evolution of the Laurentian foreland basins in New England, detrital zircons do not appear
to record contemporaneous orogenic activity but instead are restricted
to recycling of the preexisting continental margin. It is possible that
substantial topography (e.g., accretionary prism) impeded sediment
transport from the active orogen to the foreland basin.
INTRODUCTION
The age distribution of detrital zircons in sandstones is a well established
approach to evaluate the provenance of sedimentary rocks (e.g., Gaudette
et al. 1981; Froude et al. 1983; Ireland 1992; Ross et al. 1992; Miller and
Saleeby 1995; Gehrels and Dickinson 1995; Gehrels et al. 1995; Morton
et al. 1996; Rainbird et al. 1997). The power of this technique is the ability
to assign specific igneous (or metamorphic) crystallization ages to provenance components that contain zircon as a significant phase, and thus help
* Present address: Department of Geology, University of Maryland, College Park,
Maryland 20742-4211, U.S.A.
JOURNAL OF SEDIMENTARY RESEARCH, VOL. 71, NO. 2, MARCH, 2001, P. 305–317
Copyright q 2001, SEPM (Society for Sedimentary Geology) 1073-130X/01/071-305/$03.00
identify some of the geological terranes that make up the provenance. A
first-order difficulty with the approach is that most sediments are themselves derived from sedimentary rocks and the process of sedimentary recycling is largely invisible to an analysis of detrital zircon ages. On the
other hand, this approach is highly complementary to whole rock Sm-Nd
isotope data that provide an estimate of the mean age of mantle extraction
of all provenance components (weighted to their Nd contents) and to a
variety of other isotopic, geochemical, and petrographic provenance indicators (see review in McLennan et al. 1993).
There are a variety of models for the early Paleozoic tectonic evolution
of the New England area of North America. For example, the Ordovician
Taconian orogeny has been variously interpreted as being the result of
collision of an island-arc terrane to eastern North America (e.g., Rowley
and Kidd 1981), development of a continental arc (Coakley and Gurnis
1995), or collision of eastern North America with Gondwana (e.g., Dalziel
et al. 1994). Among the important issues in interpreting the Taconian orogeny has been uncertainty in the interpretation of Nd isotope data for foreland sedimentary rocks derived from the Taconian orogen. Although most
Middle to Late Ordovician foreland sediments have very uniform Nd isotopic compositions (eNd in range 27 to 29 at about 450 Ma), some workers have interpreted this to indicate the possibility of a significant young
mantle-derived component of approximately Ordovician age (e.g., Anderson and Samson 1995) whereas others have argued that such values can
be derived exclusively from pre-Ordovician North American continental
material, mainly from the Grenville Province (Bock et al. 1996, 1998; also
see Patchett et al. 1999).
In this paper we report ion-microprobe ages for detrital zircons from
four lower to middle Paleozoic sandstone samples from the New England
region of North America (Fig. 1). Together these samples represent deposition on the early Paleozoic North American passive margin and in the
Appalachian foreland, in response to the Taconian and Acadian orogenies.
The primary purpose of the study is to evaluate the provenance of these
detrital zircons and comment on the implications for early Paleozoic tectonic evolution of the eastern North American continental margin. A second
issue that will be addressed is the question of possible sampling bias induced during selection of detrital zircons for U-Pb isotopic analysis.
GEOLOGICAL SETTING AND SAMPLES
The Grenville Structural Province is the dominant Precambrian basement
element in eastern North America and incorporates a protracted igneous
and metamorphic history mainly from about 1.6 (and in places $ 1.8 Ga)
to 1.0 Ga (e.g., Easton 1986; McLelland et al. 1996; Corrigan and van
Breeman 1996; Gower 1996; Kamo et al. 1996). A passive margin formed
on the eastern edge of Laurentia during the latest Proterozoic to Early
Cambrian following ca. 760–560 Ma rifting of a Late Proterozoic supercontinent to form the Iapetus Ocean (Fig. 2; Bond et al. 1984; Rankin et
al. 1989; Aleinikoff et al. 1995). During this rift to drift stage, deposition
on the eastern margin of Laurentia consisted of volcanic and coarse riftrelated clastic sedimentary rocks followed in turn by quartzites and carbonates on the developing passive margin (Rowley and Kidd 1981; Stanley
and Ratcliffe 1985; Rodgers 1987; Rankin et al. 1989). During the Cam-
306
S.M. MCLENNAN ET AL.
FIG. 1.—Generalized geological map of
southern New York State showing sample
locations. Adapted from Bock et al. (1998).
brian and Early Ordovician ‘‘starved’’ clastics were deposited on the continental slope and rise to the east of the extensive carbonate platform (Stevens 1970; Rowley and Kidd 1981).
Sedimentation, deformation, metamorphism, and earliest plutonic activity associated with the Ordovician Taconian orogeny appears to have taken
place over a fairly short interval of about 465 (6 10) to 445 Ma (see
reviews of geochronology in Bock et al. 1996; Karabinos et al. 1998; Ratcliffe et al. 1998). As the orogeny proceeded, clastic sediments onlapped
onto the Laurentian shelf, diachronously from east to west (Zen 1967;
Stevens 1970; Rowley and Kidd 1981). This culminated in an accretionary
prism, containing slices of Grenville crust, thrust slices of previously deposited rift and drift sediment of the Laurentian margin, and possibly exotic
material from the east, that in turn acted as the dominant source of foreland
basin sedimentation transported to the west (but ultimately derived from
the west) on Laurentia (Bock et al. 1996, 1998). The exact tectonic setting
is controversial, and currently popular models include collision of an island-arc terrane over an east-dipping subduction zone (Stevens 1970; Rowley and Kidd 1981; Stanley and Ratcliffe 1985), a continental arc on Laurentia formed over a west-dipping subduction zone (Coakley and Gurnis
1995) and a collision between eastern Laurentia and western Gondwana
(Dalziel et al. 1994; also see Thomas and Astini 1996).
The Siluro–Devonian Acadian Orogeny followed the Taconian and in
the New England region is generally regarded as reflecting the collision of
the microcontinent Avalon to eastern Laurentia (Osberg et al. 1989; Robinson et al. 1998). Plutonic activity occurred over a fairly long period of
time, from being essentially synchronous with late Taconian metamorphism
through to about 315 Ma. The dominant episode of Acadian metamorphism
in the New England region extended from about 400 Ma to about 360 Ma
DETRITAL ZIRCON AGES IN TACONIAN AND ACADIAN FORELAND SEDIMENTS
307
FIG. 2.—Highly simplified schematic
stratigraphic relationships of the major Late
Proterozoic and Paleozoic sedimentary
successions in eastern North America. On the
left are possible terranes that collided with
Laurentia to generate the Taconian, Acadian and
Alleghenian orogenies. On the right, the
approximate stratigraphic locations of samples
are also shown.
but in places may have begun as early as about 420 Ma and extended to
about 350 Ma (e.g., Robinson et al. 1998).
Samples analyzed for this study were collected along an approximately
east–west traverse in southern New York State (Fig. 1), from the following
stratigraphic units listed in ascending order (see Fig. 2 for regional stratigraphic setting):
1. Poughquag Quartzite (Lower Cambrian; Sample PQ), a possible correlative of
the more extensive Cheshire Quartzite and Dalton Formation to the north (Zen
1967; Rankin et al. 1989), is a quartz arenite and was selected to be representative
of the Laurentian passive margin prior to initiation of Paleozoic orogenic activity
in eastern North America. Details of this sample were described in Bock et al.
(1996).
2. Austin Glen Member of the Normanskill Formation (Middle Ordovician; Sample BB-1) is a sandstone from a typical turbidite Bouma cycle and is representative of proximal synorogenic deposition in the Taconian foreland basin. The
stratigraphic relationships, location, petrology, geochemistry and radiogenic isotopes of the Austin Glen (including this sample) were described in Bock et al.
(1994, 1998).
3. Shawangunk Formation (Lower Silurian; Sample NP-1), a correlative of the
Tuscarora Sandstone and unconformably overlying Ordovician turbidites (including the Austin Glen and its equivalents), is a quartz pebble conglomerate representing synorogenic to postorogenic sedimentation in the Taconian foreland basin
(Epstein and Epstein 1972). Details of this sample were described in Hemming
(1994).
4. Catskill Clastic Wedge ‘‘redbed’’ (Upper Devonian; Sample 93SM003), taken
from the fluvial facies of the Sonyea Group (418 389 300 N; 748 379 250 W;
lower Walton Formation(?) on the New York State geological map), is a red lithic
wacke (Q:F:R 2 57:9:35) representing syntectonic to posttectonic sedimentation
in the Acadian foreland basin (Sevon and Woodrow 1985; Osberg et al. 1989).
SAMPLE PREPARATION AND ANALYTICAL METHODS
Samples were carefully passed through a jaw crusher followed by a disc
mill with a minimum gap setting at 300 mm. Samples were then passed
over a carefully cleaned single piece molded fiberglass Wilfley table followed by standard separation by heavy liquids and a magnetic separator.
The final concentrate was successively coned and quartered to obtain a
subset of approximately 200–400 zircons, and these were characterized for
size, shape, and color. The zircons were mounted in epoxy, polished, and
gold coated in preparation for ion probe analyses. Zircons were selected at
random for U/Pb analyses and the grain shape and color characteristics of
the analyzed subset were then compared to the 200–400 grain population
to ensure reasonably representative sampling.
U-Th-Pb isotopes and U, Th concentrations were measured by Sensitive
High-mass Resolution Ion Microprobe (SHRIMP) at the Australian National University. SHRIMP-1 (approximately 30 mm spot size) was used
for samples BB-1, NP-1, and 93SM003, and SHRIMP-2 (approximately 15
mm spot size) was used for sample PQ, because of the finer grain size of
zircons in this sample. Analytical methods have been described by Compston et al. (1984) and Williams and Claesson (1987; also see Ireland 1992).
Six mass scans were employed for each analysis, with dwell times on 204Pb
of 20 s, to monitor common lead.
Common lead is typically ascribed to result from surface contamination
during preparation of the ion probe mount. For these zircons, however, that
may not entirely account for all of the common lead, and other sources
intrinsic to the zircon, such as inclusions, are likely. Thus, in the Catskill
sample (93SM003), the two age populations of zircons (see below) have
statistically distinct average 204Pb contents and in general high 204Pb is
commonly associated with high U content. Accordingly, in making corrections, we adopted common lead compositions based on the Pb isotope
evolution model of Cumming and Richards (1975) rather than using the
composition of typical Broken Hill lead as is commonly done for SHRIMP
data obtained in Australia (e.g., Compston and Pidgeon 1986; Ireland 1992;
Carter and Moss 1999). For zircons in the range of 1.0–1.5 Ga, the difference in these two approaches is negligible, and in general the final results
are not very sensitive to the assumed composition of common lead.
The large majority of zircons are corrected for common lead on the basis
of 208Pb content. This correction assumes closed-system Th-U behavior
since the time of crystallization. Such an assumption is increasingly less
secure for zircons with high Th/U ratios, and accordingly a 204Pb common
lead correction was applied to zircons with 232Th/ 238U . 0.6 and to zircons
for which the 208Pb correction predicted negative concentrations of common lead. An exception to this was sample 93SM003, for which 7 zircons
with 232Th/ 238U . 0.6 were corrected using 208Pb because using 204Pb
resulted in obvious overcorrection for a number of zircons in this sample,
suggesting a possible interference.
Ages can be determined from a number of isotopic ratios, most commonly 207Pb*/ 206Pb* and 206Pb*/ 238U (asterisk denotes common-lead-corrected value), and a certain amount of judgment is necessary in assigning
final ages. For relatively young ages 207Pb*/ 206Pb* varies little with time,
and accordingly this ratio is an insensitive estimator of age. We assign ages
using the same criteria as Ireland (1992), who used 207Pb*/ 206Pb* ages for
relatively old zircons (. 800 Ma) and 206Pb*/ 238U ages for relatively
young zircons (, 800 Ma). There is one exception to this approach; in
one case where the 206Pb*/ 238U age of a discordant young zircon is younger
than the age of sedimentation (93SM003; Spots 9.1, 9.2), the 207Pb*/ 206Pb*
age of the most concordant analysis was adopted as the age. It is important
to note that 207Pb*/ 206Pb* ages are best regarded as minimum ages for
discordant zircons because such ages assume that any discordance results
from recent disturbance of the U-Pb system. Nd isotope data for three of
the samples also are considered for this study. Data for PQ and BB-1 were
308
S.M. MCLENNAN ET AL.
TABLE 1.—U-Pb isotopic data for detrital zircons from the Poughquag Quartzite (Sample PQ).
Grain
1.1
2.1
3.1
4.1
5.1
6.1
7.1
8.1
9.1
10.1
11.1
12.1
13.1
14.1
15.1
16.1
17.1
18.1
19.1
20.1
21.1
22.1
23.1
24.1
25.1
26.1
27.1
28.1
29.1
30.1
31.1
32.1
33.1
34.1
35.1
36.1
37.1
38.1
39.1
40.1
Description#
100; 1.5; R; C/M
85; 1.7; SR; C/M
50; 1.8; SA; C
90; 1.3; SA; C
90; 1.2; SR; C
90; 1.2; SR; C/LB
65; 2.2; SA; C/M
65; 1.8; R; LB
75; 2.6; SA; C
70; 1.5; SR; LB
75; 2.0; SR; C
65; 1.5; SR; C
60; 3; E/SA; C
85; 1.2; SA; LB
70; 1.4; SR; C
50; 2.1; SR; C
80; 1.7; SA; LB
80; 1.1; A; C
105; 1.1B; R; C
55; 1.7B; SR; C
50; 1.9; SA; CM
80; 1.4; R; C
70; 2.1B; A; LB
95; 1.4; SR; LB
95; 1.3; R; C/LB
95; 1.1; R; C/LB
55; 2.8; SA; C
55; 2.6; SA; C
60; 1.8B; SR; C
115; 1.0; R; C
180; 1.2; R; C
55; 2.6; SR; C
90; 1.3; SR; LB
70; 1.9; SA; C
65; 1.4; SA; C/LB
80; 1.2; R; C/M
75; 1.5; SR; C/LB
50; 2.7; SA; C
40; 2.0; SA; LB
105; 1.0; R; C/M
Corr’n^
U (ppm)
Th (ppm)
208
204
204
208
204
208
204
208
208
208
204
208
204
208
204
208
204
208
208
208
204
208
208
208
208
208
208
208
208
204
208
208
208
208
208
208
204
208
208
208
64
202
197
138
230
276
578
202
413
1879
70
169
57
168
169
699
454
171
551
63
557
131
352
516
251
487
310
686
115
212
607
103
160
368
706
198
162
110
772
777
35
166
206
70
693
86
350
58
160
252
62
90
45
73
170
316
195
77
47
25
256
59
172
50
37
61
145
159
50
243
216
61
55
178
197
86
167
39
177
170
204
Pb (ppb)
4
12
10
12
32
20
17
11
94
887
27
24
20
9
10
120
23
10
59
11
19
7
15
7
20
54
127
67
19
50
34
29
35
36
29
26
48
25
87
27
f 206 (%)
0.7
0.4
0.4
1.0
1.9
0.7
0.4
0.6
2.5
8.5
3.9
2.0
2.4
0.6
1.3
1.4
0.6
0.6
2.6
1.8
0.3
0.5
0.4
0.2
0.6
1.1
3.5
1.0
1.9
2.5
0.5
2.5
2.0
0.9
0.4
0.9
2.3
2.3
1.0
0.4
206
Pb*/ 238U
0.1649 6 50
0.2601 6 41
0.2486 6 40
0.1691 6 33
0.1482 6 25
0.2028 6 34
0.1396 6 20
0.1766 6 29
0.1764 6 27
0.1049 6 13
0.1889 6 57
0.1357 6 26
0.2714 6 144
0.1879 6 40
0.0885 6 16
0.2287 6 30
0.1826 6 26
0.2030 6 36
0.0785 6 15
0.1920 6 59
0.2142 6 29
0.2155 6 41
0.2185 6 40
0.1769 6 44
0.2406 6 84
0.1875 6 67
0.2119 6 59
0.1898 6 37
0.1650 6 62
0.1890 6 50
0.2173 6 55
0.2170 6 96
0.2035 6 96
0.2203 6 225
0.1943 6 63
0.2603 6 107
0.2383 6 83
0.1831 6 57
0.2196 6 50
0.1712 6 60
207
Pb*/ 235U
1.80 6 10
3.363 6 72
3.421 6 73
1.873 6 68
1.455 6 49
2.532 6 65
1.387 6 33
1.922 6 52
1.906 6 51
0.882 6 66
1.77 6 16
1.327 6 75
3.84 6 27
2.132 6 77
0.722 6 38
3.017 6 78
1.960 6 43
2.457 6 79
0.861 6 31
2.34 6 12
2.683 6 47
2.668 6 86
2.678 6 66
1.887 6 71
3.07 6 15
2.19 6 15
2.66 6 15
1.991 6 56
1.81 6 12
1.79 6 11
2.50 6 10
2.57 6 24
2.40 6 20
2.57 6 35
2.203 6 92
3.42 6 25
3.03 6 20
2.29 6 18
2.45 6 12
1.80 6 10
Pb*/ 206Pb*
Age1 (Ma)
0.0792 6 35
0.0938 6 11
0.0998 6 12
0.0803 6 23
0.0712 6 19
0.0906 6 16
0.0721 6 13
0.0789 6 15
0.0784 6 16
0.0610 6 44
0.0680 6 54
0.0709 6 36
0.1027 6 42
0.0823 6 22
0.0592 6 28
0.0957 6 20
0.0779 6 12
0.0878 6 22
0.0796 6 23
0.0883 6 32
0.0909 6 09
0.0898 6 21
0.0889 6 13
0.0774 6 20
0.0927 6 28
0.0849 6 45
0.0912 6 42
0.0761 6 14
0.0797 6 40
0.0688 6 36
0.0834 6 24
0.0859 6 65
0.0854 6 54
0.0845 6 68
0.0823 6 19
0.0954 6 54
0.0922 6 36
0.0906 6 63
0.0808 6 33
0.0762 6 30
1177 6 91
1504 6 23
1620 6 22
1205 6 57
963 6 57
1437 6 33
988 6 36
1170 6 38
1156 6 40
643 6 08
868 6 173
955 6 108
1673 6 78
1252 6 52
547 6 10
1541 6 40
1143 6 31
1378 6 48
1187 6 59
1389 6 72
1444 6 19
1421 6 46
1401 6 28
1130 6 52
1481 6 58
1312 6 107
1450 6 91
1098 6 37
1190 6 104
894 6 113
1279 6 58
1337 6 155
1326 6 128
1304 6 165
1251 6 45
1535 6 110
1471 6 98
1439 6 140
1216 6 82
1100 6 81
207
# Descriptions are based on the following in order, separated by semi-colons: Width (mm); Length/Width ratio, with ’B’ designating a broken grain; degree of rounding; color.
Key: R-rounded; SR-subrounded; SA-subangular; A-angular; with grains showing crystal faces included in SA/A. C-clear; M-milky; LB-pink/light brown; B-dark pink/brown; DB-dark brown.
1 Accepted ‘‘age’’ is either the 207Pb*/ 206Pb* age or 206Pb*/ 238U age using criteria described in text.
^ Common lead correction method (see text for details).
taken from Bock et al. (1996) and Bock et al. (1994), respectively. Data
for 93SM003 were determined for this study by methods similar to those
described in Bock et al. (1994).
RESULTS
The U-Pb isotopic data for detrital zircons are presented in Tables 1
through 4, and Nd isotopic data for bulk samples are reported in Table 5.
Gray and Zeitler (1997) have reported ion microprobe U-Pb age data for
zircons taken from pebbles in a sample of Shawangunk Formation in Pennsylvania, and these results are considered here. Note that in considering
these data we have assigned ages differently than Gray and Zeitler (1997)
to be consistent with our other results.
Figure 3 is a plot of Nd isotopic composition versus age. Samples from
the passive margin and Taconian foreland all fall on similar evolution lines
that fall entirely within the known evolution of magmatic rocks from the
Grenville Province and are consistent with derivation dominantly from this
terrane. The Catskill sample differs in having a significantly younger model
age and accordingly appears to have a significant component of younger
material compared to the other two samples.
U-Pb isotope results are presented on plots of 207Pb*/ 206Pb* versus 238U/
206Pb* in Fig. 4, and histograms of adopted ages (see Tables 1–4) are
shown in Fig. 5. With only a few exceptions, zircons are concordant to
slightly discordant, as is commonly the case with detrital zircons that have
been abraded by natural processes.
Absence of a zircon of some hypothesized source terrane does not necessarily mean that such a terrane was not part of the provenance. One
possibility is that such a source terrane did not contain lithologies that
contribute significant amounts of zircons (e.g., mafic or carbonate rocks)
or contained zircons that are too small to be sampled by standard separations methods (e.g., shale or slate source rocks). Even in a study such as
ours, where a relatively large number of zircons are analyzed, there is a
statistical chance that a zircon population will be missed. This can be readily quantified (Compston et al. 1985; Dodson et al. 1988). For statistical
sampling with replacement, the probability (P) of missing some component
can be given as
P 5 (1 2 f) n
(1)
where f is the proportion of the component and n is the number of samples
(i.e., number of zircons analyzed). In this study, where between 40 and 50
analyses are available for each formation, the probability of missing a component making up 10% of the zircon population is about 0.005 to 0.015
(i.e., about 1%) and the probability of missing a 5% component is about
0.08 to 0.13 (i.e., about 10%).
Poughquag Quartzite
Detrital zircons from this Cambrian sample are the most discordant of
the four samples analyzed and fall into two general groupings (Figs. 4A,
5A). Thirty-eight of the analyzed zircons form a broad age distribution
between 1.67 and 0.87 Ga. These are 207Pb*/ 206Pb* ages and thus represent
minimum ages. Accordingly, we consider it likely that most or all of these
zircons are derived from the Grenville Structural Province. This could be
either through direct erosion of Grenville crystalline basement or indirectly
DETRITAL ZIRCON AGES IN TACONIAN AND ACADIAN FORELAND SEDIMENTS
309
TABLE 2.—U-Pb isotopic data for zircons from the Austin Glen (Sample BB-1).
Grain
1.1
2.1
3.1
4.1
5.1
6.1
7.1
8.1C
8.2R
9.1
10.1
11.1
12.1
13.1
14.1
15.1
16.1
17.1
18.1
19.1
20.1
21.1
22.1
23.1
24.1
25.1
26.1
27.1
27.2
28.1
29.1
30.1
31.1
32.1
33.1
34.1
35.1
36.1
37.1
38.1
39.1
40.1
41.1
Description#
240; 1.1; SR; B
240; 1.4; SR; DB
165; 2.0; SA; B
175; 1.4; SA; DB
175; 1.0B; SR; LB
185; 1.5; E; DB
115; 2.4B; E; C/LB
150; 2.7; E; B
85; 2.4; SA; C
200; 1.2; R; C/LB
190; 2.3; R; C
250; 1.2; R; C/LB
200; 1.5; R; C
80; 1.8; SR; C
75; 3.1; SR; C
150; 1.0; R; C/LB
235; 1.3B; SA; C/LB
140; 1.6B; R; C
185; 1.1B; SA; C/LB
165; 1.9; SA; C
240; 2.1; A; C
160; 1.9B; A; C
160; 2.7; SA; C
215; 1.3; R; C/LB
235; 1.1; R; C/LB
160; 1.1; SA; C/LB
150; 1.5; SR; LB
185; 1.4; SR; C
250; 1.3; SR; LB
160; 1.3; SR; C
190; 1.2B; A; C/LB
215; 1.3; R; C/LB
85; 2.5; A; C
140; 2.3; SA; C
290; 1.1; A; C/LB
215; 2.1; SA; C/LB
100; 2.2B; SR; C/LB
160; 1.3; SA; C/LB
165; 1.4; A; C/LB
110; 1.5; SR; B
310; 1.5; SA/A; C/M
Corr’n^
U (ppm)
Th (ppm)
208
208
208
208
none
204
204
208
208
208
204
208
204
204
208
204
208
208
208
204
208
204
208
208
204
208
204
208
208
208
208
208
208
208
208
208
204
208
208
208
208
208
208
963
1489
123
871
556
1759
885
1992
1090
104
678
33
73
482
255
136
20
16
92
175
166
93
96
217
302
134
302
98
108
90
214
90
139
225
62
68
90
62
306
322
172
491
175
287
666
26
331
89
1132
720
1080
383
39
336
13
46
371
125
89
0
4
32
118
84
74
29
62
183
44
237
53
61
41
105
39
54
63
22
37
74
18
145
152
82
210
50
204
Pb (ppb)
13
1815
7
18
—
32
46
209
522
4
5
12
8
7
2
2
4
3
4
40
10
4
5
6
4
5
83
11
10
7
3
5
9
5
6
8
7
5
8
9
10
9
10
f 206 (%)
0.2
12.0
0.5
0.3
—
0.2
0.7
1.0
4.7
0.4
0.1
3.0
1.1
0.3
0.1
0.1
2.4
2.5
0.7
2.3
0.6
0.5
0.6
0.4
0.2
0.4
2.7
0.2
0.2
0.6
0.2
0.5
0.6
0.3
1.1
1.2
0.9
0.9
0.3
0.3
0.8
0.2
0.7
206
Pb*/ 238U
0.1697 6 33
0.1738 6 20
0.2359 6 63
0.1644 6 69
0.1970 6 59
0.1792 6 25
0.1550 6 24
0.2028 6 46
0.1879 6 85
0.2170 6 46
0.1820 6 40
0.2332 6 102
0.1935 6 53
0.1199 6 16
0.1986 6 41
0.2598 6 46
0.1750 6 78
0.1580 6 61
0.1181 6 36
0.1816 6 30
0.1860 6 56
0.1691 6 72
0.1641 6 35
0.1586 6 47
0.1777 6 36
0.2038 6 37
0.1918 6 28
0.674 6 39
0.669 6 14
0.2429 6 63
0.1837 6 25
0.2236 6 46
0.1987 6 13
0.1723 6 27
0.1775 6 53
0.1978 6 95
0.1651 6 38
0.1742 6 77
0.2033 6 39
0.1909 6 76
0.1543 6 50
0.1724 6 47
0.1535 6 49
207
Pb*/ 235U
1.718 6 51
1.941 6 46
2.826 6 99
1.645 6 86
2.14 6 12
1.985 6 40
1.544 6 39
2.25 6 11
2.12 6 11
2.34 6 11
1.923 6 59
2.75 6 19
2.079 6 93
1.042 6 21
2.170 6 84
3.286 6 75
1.75 6 13
1.60 6 12
1.084 6 71
1.930 6 85
2.02 6 12
1.68 6 15
1.637 6 77
1.661 6 74
1.811 6 45
2.160 6 56
2.060 6 78
24.96 6 1.86
24.68 6 60
2.85 6 13
1.925 6 43
2.66 6 11
2.12 6 21
1.723 6 44
1.86 6 10
2.05 6 15
1.692 6 63
1.74 6 12
2.228 6 60
2.06 6 13
1.68 6 11
1.816 6 85
1.486 6 90
Pb*/ 206Pb*
Age1 (Ma)
0.0734 6 15
0.0810 6 16
0.0869 6 17
0.0726 6 19
0.0789 6 35
0.0804 6 10
0.0723 6 13
0.0806 6 32
0.0820 6 19
0.0782 6 32
0.0766 6 15
0.0855 6 42
0.0779 6 25
0.0630 6 09
0.0793 6 24
0.0917 6 12
0.0726 6 39
0.0733 6 46
0.0666 6 37
0.0771 6 30
0.0786 6 38
0.0723 6 54
0.0724 6 28
0.0759 6 23
0.0739 6 09
0.0769 6 13
0.0779 6 26
0.2685 6 108
0.2676 6 27
0.0852 6 29
0.0760 6 12
0.0864 6 29
0.0775 6 54
0.0725 6 13
0.0760 6 33
0.0752 6 37
0.0743 6 20
0.0724 6 35
0.0795 6 13
0.0783 6 34
0.0789 6 41
0.0764 6 27
0.0702 6 33
1026 6 42
1222 6 39
1358 6 39
1003 6 55
1169 6 91
1206 6 26
993 6 37
1212 6 79
see 8.1C
1152 6 83
1112 6 39
1326 6 99
1145 6 66
730 6 09
1179 6 61
1462 6 24
1002 6 112
1023 6 131
720 6 21
1124 6 79
1163 6 98
993 6 160
996 6 82
1094 6 62
1039 6 24
1118 6 33
1144 6 67
see 27.2
3292 6 16
1319 6 67
1095 6 33
1347 6 65
1133 6 144
1001 6 37
1095 6 89
1073 6 104
1050 6 55
996 6 102
1184 6 34
1154 6 90
1171 6 108
1105 6 71
935 6 100
207
# see Table 1 footnote for explanation.
1 Accepted ‘‘age’’ is either the 207Pb*/ 206Pb* age or 206Pb*/ 238U age using criteria described in text.
^ Common lead correction method (see text for details).
TABLE 3.—U-Pb isotopic data for detrital zircons from the Shawangunk Formation (Sample NP-1).
Grain
1.1
2.1
3.1
4.1
5.1
6.1
7.1
8.1
9.1
10.1
11.1
12.1
13.1
14.1
15.1
16.1
17.1
18.1
19.1
20.1
21.1
22.1
23.1
24.1
25.1
26.1
27.1
28.1
29.1
Description#
470; 1.1; R; DB
120; 1.6; SR; C/LB
180; 1.4B; SA; C
70; 1.7B; A; C
100; 1.7; SA; C/LB
95; 2.2; SA; C
190; 1.1; SR; C/LB
85; 1.1; SA; C
130; 1.2; SR; C/LB
150; 1.1B; SA; C
170; 1.6; R; C
170; 1.6; SA; C/LB
180; 1.1; SR; C
210; 1.8; SR; C
430; 1.4; SR; C
210; 1.1; R; C
110; 3.2B; SA; C
100; 2.5B; SA; C
105; 2.2; SR; C
110; 1.6B; A; C
115; 2.3; A; C
155; 1.8; SR; C
220; 1.5B; SA; C
145; 1.3; SA; C
125; 1.4; SA; C
160; 1.3; SR; C
195; 1.6; SA; C
170; 1.2; R; C
275; 1.0; R; C
Corr’n^
U (ppm)
Th (ppm)
204
204
204
208
208
208
208
204
208
208
208
204
208
208
208
208
208
208
208
208
208
204
208
208
204
208
204
208
208
326
1328
151
247
904
177
646
139
355
98
100
139
71
193
70
231
121
119
114
130
76
403
94
407
27
152
67
157
151
55
269
83
68
179
87
53
100
85
30
56
138
14
32
19
64
62
38
37
66
26
103
42
128
29
29
43
40
37
204
Pb (ppb)
f 206 (%)
6
24
8
10
1
3
4
4
9
7
4
,0.5
3
10
6
12
2
5
5
6
10
8
9
8
7
12
7
15
11
0.2
0.3
0.6
0.5
0.01
0.2
0.1
0.3
0.3
0.7
0.3
0.02
0.5
0.6
0.8
0.6
0.1
0.4
0.6
0.5
1.7
0.2
1.0
0.2
3.1
0.8
1.3
0.5
0.8
# see Table 1 footnote for explanation.
1 Accepted ‘‘age’’ is either the 207Pb*/ 206Pb* age or 206Pb*/ 238U age using criteria described in text.
^ Common lead correction method (see text for details).
206
Pb*/ 238U
0.1715 6 15
0.1335 6 22
0.1781 6 87
0.1621 6 86
0.1675 6 36
0.1981 6 43
0.1808 6 62
0.1892 6 79
0.1723 6 135
0.1963 6 47
0.2355 6 63
0.1960 6 51
0.1793 6 56
0.1758 6 38
0.202 6 11
0.1732 6 31
0.1913 6 55
0.2013 6 59
0.1668 6 54
0.1647 6 95
0.1569 6 50
0.1880 6 33
0.1705 6 51
0.1982 6 59
0.1656 6 98
0.2011 6 58
0.1677 6 71
0.328 6 23
0.185 6 22
207
Pb*/ 235U
1.79 6 16
1.389 6 37
1.87 6 11
1.590 6 96
1.657 6 42
2.145 6 75
1.846 6 68
2.14 6 12
1.74 6 19
2.078 6 98
2.76 6 14
2.13 6 10
1.82 6 11
1.739 6 58
2.07 6 17
1.660 6 53
2.14 6 13
2.14 6 11
1.681 6 97
1.70 6 14
1.482 6 99
1.051 6 56
1.90 6 14
2.26 6 11
1.63 6 22
2.096 6 99
1.89 6 15
4.98 6 41
1.80 6 27
207
Age1 (Ma)
0.0758 6 16
0.0754 6 14
0.0763 6 17
0.0710 6 17
0.0718 6 08
0.0785 6 20
0.0741 6 07
0.0820 6 29
0.0730 6 46
0.0768 6 29
0.0850 6 35
0.0788 6 29
0.0737 6 35
0.0717 6 17
0.0744 6 43
0.0695 6 17
0.0813 6 42
0.0772 6 32
0.0731 6 32
0.0749 6 39
0.0685 6 38
0.0791 6 15
0.0810 6 49
0.0826 6 31
0.0716 6 83
0.0756 6 26
0.0817 6 53
0.1102 6 36
0.0706 6 58
1089
1081
1104
957
979
1160
1044
1245
1015
1116
1316
1167
1034
978
1054
914
1228
1125
1016
1066
884
1175
1221
1260
973
1084
1238
1802
945
Pb*/ 206Pb*
6 42
6 39
6 46
6 50
6 23
6 51
6 20
6 72
6 135
6 78
6 82
6 74
6 100
6 48
6 121
6 51
6 104
6 85
6 92
6 108
6 119
6 38
6 125
6 75
6 256
6 70
6 132
6 61
6 179
310
S.M. MCLENNAN ET AL.
TABLE 4.—U-Pb isotopic data for detrital zircons from the Catskill ‘‘Redbed’’ (Sample 93SM003).
Grain
1.1
2.1
3.1
4.1
5.1
6.1
7.1
8.1
9.1
9.2
10.1
11.1
12.1
13.1
14.1
15.1
16.1
17.1
18.1
19.1
20.1
21.1
22.1
23.1
24.1
25.1
26.1
27.1
28.1
29.1
30.1
31.1
32.1
33.1
34.1
35.1
36.1
37.1
38.1
39.1
40.1
41.1
42.1
43.1
44.1
45.1
Description#
70; 2.3B; SA; C
180; 1.4; SR; B
80; 2.6; SA; C
150; 2.1; SA; C/M
150; 1.4SA; DB
300; 1.0; SR; DB
180; 1.8; R; LB
380; 1.4B; A; LB/B
180; 2.3; E; DB
90; 2.6B; E; C
260; 1.2B; SR; C/LB
300; 1.0B; A; C
100; 3.3; E; C
190; 1.1B; A; B
190; 1.0B; R; C
100; 1.9B; A; C/LB
180; 1.4; SA; LB
100; 2.3; A; C
120; 2.0B; E; C
130; 1.5B; SA; B
120; 2.1B; A; C
160; 1.5; R; C
270; 1.8; SA; LB
135; 1.1; A; C/M
80; 1.3B; A; C
210; 1.0; SR; LB
190; 1.4; SR; LB
180; 1.1B; SA; B
95; 2.4B; A; C
100; 2.4; E; C
190; 2.3; SR; C
130; 1.7; R; B
150; 1.2B; A; C
250; 1.0; R; B
260; 1.0; R; LB
270; 1.7; R; LB
160; 2.5B; SA; C
70; 2.1; A; C
180; 1.1B; A; C
130; 4.2; E; C/LB
200; 2.0; SR; C/LB
140; 3.2; SA; C
280; 1.1; SA; C/M
380; 1.1; SR/SA; LB
85;1.3; A; C
Corr’n^
U (ppm)
Th (ppm)
208
none
none
208
208
208
none
208
208
208
208
208
208
208
208
208
208
208
208
208
208
208
208
208
208
none
208
208
none
208
208
208
208
208
208
208
208
208
208
208
208
208
208
208
208
208
1069
966
429
248
993
1512
1559
1193
4031
2574
458
87
501
261
2252
30
442
643
188
455
815
516
147
51
548
202
59
522
635
551
474
33
2653
497
1122
244
158
120
404
313
524
343
333
113
85
319
195
489
405
60
141
143
900
226
806
408
219
50
348
150
708
8
261
127
30
182
134
219
45
56
239
89
22
164
195
410
320
12
98
176
168
89
65
45
127
326
318
110
147
55
33
98
204
Pb (ppb)
9
—
—
15
11
19
—
2
463
162
11
4
5
11
9
5
20
4
5
6
7
4
4
6
28
—
5
5
—
8
10
5
10
2
60
4
9
7
9
6
12
13
16
6
8
10
f 206 (%)
0.1
—
—
0.6
0.1
0.2
—
0.02
3.9
2.0
0.7
0.6
0.3
1.2
0.1
1.9
1.4
0.1
0.3
0.4
0.1
0.2
0.3
1.2
1.5
—
1.0
0.1
—
0.4
0.6
1.8
0.04
0.01
0.6
0.2
0.7
0.6
0.7
0.6
1.0
0.5
0.5
0.6
1.0
0.9
206
Pb*/ 238U
0.1751 6 28
0.1779 6 31
0.0691 6 14
0.2035 6 61
0.1870 6 25
0.1694 6 37
0.1996 6 39
0.1709 6 24
0.0601 6 11
0.0659 6 08
0.0740 6 11
0.1776 6 48
0.0724 6 20
0.0749 6 19
0.1661 6 21
0.1838 6 71
0.0708 6 17
0.1760 6 23
0.1726 6 34
0.0738 6 15
0.1671 6 28
0.0719 6 20
0.1778 6 51
0.1727 6 97
0.0702 6 33
0.0723 6 19
0.1668 6 70
0.1712 6 25
0.1923 6 31
0.0671 6 16
0.0724 6 13
0.1815 6 84
0.1790 6 29
0.0709 6 16
0.1706 6 27
0.1801 6 34
0.1617 6 50
0.1774 6 61
0.0695 6 15
0.0688 6 21
0.0725 6 15
0.1637 6 105
0.1674 6 48
0.1663 6 52
0.1680 6 98
0.0695 6 19
207
Pb*/ 235U
Pb*/ 206Pb*
Age1 (Ma)
0.0739 6 16
0.0754 6 05
0.0610 6 13
0.0792 6 20
0.0765 6 10
0.0752 6 09
0.0789 6 06
0.0738 6 10
0.0566 6 14
0.0564 6 10
0.0551 6 18
0.0732 6 34
0.0558 6 29
0.0609 6 38
0.0726 6 07
0.0745 6 46
0.0492 6 36
0.0743 6 09
0.0726 6 20
0.0551 6 22
0.0735 6 13
0.0560 6 19
0.0778 6 20
0.0777 6 90
0.0608 6 25
0.0649 6 22
0.0785 6 41
0.0747 6 15
0.0771 6 07
0.0563 6 30
0.0535 6 25
0.0762 6 68
0.0748 6 09
0.0582 6 23
0.0739 6 18
0.0758 6 19
0.0735 6 29
0.0825 6 29
0.0582 6 25
0.0630 6 47
0.0548 6 28
0.0762 6 30
0.0775 6 21
0.0812 6 54
0.0814 6 64
0.0614 6 39
1038 6 45
1078 6 14
431 6 08
1177 6 52
1107 6 25
1073 6 24
1170 6 15
1037 6 26
see 9.2
467 6 39
460 6 07
1018 6 98
451 6 12
466 6 12
1002 6 20
1056 6 129
441 6 10
1050 6 26
1003 6 58
459 6 09
1028 6 36
447 6 12
1140 6 52
1140 6 250
437 6 20
450 6 12
1161 6 107
1061 6 40
1123 6 19
419 6 10
450 6 08
1099 6 191
1064 6 23
442 6 09
1039 6 49
1090 6 50
1028 6 83
1258 6 70
433 6 09
429 6 13
451 6 09
1101 6 81
1134 6 55
1226 6 138
1231 6 162
433 6 12
207
1.783 6 51
1.848 6 36
0.581 6 18
2.221 6 93
1.971 6 38
1.756 6 45
2.171 6 47
1.740 6 35
0.469 6 15
0.512 6 12
0.563 6 21
1.792 6 102
0.557 6 34
0.629 6 44
1.662 6 28
1.89 6 14
0.480 6 39
1.803 6 34
1.728 6 63
0.560 6 26
1.693 6 44
0.555 6 25
1.905 6 78
1.85 6 25
0.588 6 39
0.647 6 29
1.81 6 13
1.765 6 46
2.043 6 40
0.521 6 31
0.533 6 28
1.91 6 20
1.847 6 38
0.569 6 27
1.738 6 53
1.883 6 62
1.638 6 88
2.02 6 11
0.558 6 28
0.598 6 50
0.548 6 31
1.72 6 14
1.789 6 75
1.86 6 14
1.89 6 19
0.588 6 43
# see Table 1 footnotes for explantion.
1 Accepted ‘‘age’’ is either the 207Pb*/ 206Pb* age or 206Pb*/ 238U age using criteria described in text.
^ Common lead correction method (see text for details).
through sedimentary recycling processes, or both. Although the ‘‘classical’’
Grenvillian orogeny is about 1.1–1.0 Ga, the Grenville Structural Province
has magmatic rocks with zircon crystallization ages that span a broad range,
mostly from 1.70 Ga (and locally with ages as old as Archean) to about
0.97 Ga (e.g., Easton 1986; Goodwin 1991; Gower 1996; Kamo et al. 1996;
McLelland et al. 1996; Corrigan and van Bremen 1997). Ages become
systematically older closer to the northern boundary of the province (i.e.,
nearer to the Grenville Front). Accordingly, a Grenville Province provenance can explain this full range of detrital zircon ages.
The North American Midcontinent Belt also contains abundant anorogenic magmatic rocks with crystallization ages mostly in the range of 1.5
to 1.4 Ga (Goodwin 1991) and this terrane could also potentially be a
source of at least 16 Poughquag detrital zircons with 207Pb*/ 206Pb* ages
in the range 1.54 to 1.30 Ga. Although zircon ages do not distinguish these
sources, we consider that contributions from the Midcontinent are minor
to non-existent. Central North America also has abundant and widely distributed Precambrian magmatic rocks with ages in excess of 1.6 Ga, but
only two Poughquag detrital zircons have older Proterozoic ages, of 1.62
and 1.67 Ga. In addition, the Midcontinent region is farther away from the
depositional site of the Poughquag Quartzite than are regions within the
Grenville Province with similar magmatic ages.
Because of their slightly discordant nature we cannot exclude the possibility that some of the Grenville zircons are in fact much older and underwent extreme Pb loss during the Grenville orogeny at about 1.0 Ga such
that they now lie very close to a lower intercept. Such relationships were
seen for detrital zircons from the Cambrian Potsdam Formation in northern
TABLE 5.—Summary of Sm-Nd isotopic data.
Sample
PQ#
BB-11
93SM003
Formation
Poughquag
Austin Glen
‘‘Catskill’’
Stratigraphic Age
Cambrian
Ordovician
Devonian
147
Sm/144Nd
0.1351
0.1435
0.1122
143
Nd/144Nd
0.512128
0.512091
0.512153
eNd
TDM
(Ga)
29.91
210.63
29.42
1.90
(2.18)
1.49
# Data from Bock et al. (1996).
1 Data from Bock et al. (1994). Several samples in this unit (including BB-1) show evidence of an increase in Sm/Nd ratio during diagenesis, leading to unrealistically high model ages. A model age of ca. 1.8 Ga is
likely more representative of undisturbed Austin Glen samples.
DETRITAL ZIRCON AGES IN TACONIAN AND ACADIAN FORELAND SEDIMENTS
FIG. 3.—Plot of eNd versus age for the samples analyzed in this study (Bock et
al., 1994; Bock et al., 1996; unpublished data) showing evolution of the Sm-Nd
isotopic characteristics. Note that the Austin Glen sample likely had a disturbance
of the Sm-Nd isotope system at about the time of sedimentation and the evolution
path prior to 450 Ma was likely steeper than suggested by the measured 147Sm/
144Nd ratio. Accordingly, in this diagram a two-stage model is adopted for this
sample with average upper crustal Sm/Nd used prior to the age of sedimentation,
leading to a younger model age of about 1.8 Ga (Bock et al. 1994). Also shown is
the field for evolution of Precambrian igneous rocks from the Grenville Province in
this general area (Daly and McLelland 1991; McLelland et al. 1993). Samples collected for this study have Nd isotope evolution that is consistent with derivation
entirely from the Grenville Province, but the range seen for the Grenville is so large
that juvenile components cannot be ruled out. The Devonian Catskill Formation
clearly has a significantly younger model age than the others, indicating a younger
provenance on average.
New York (Gaudette et al. 1981). Any Archean zircons may have been
recycled through sedimentary rocks that were in turn metamorphosed during the Grenville Orogeny (Gaudette et al. 1981).
Two concordant zircons are clearly younger than Grenville and provide
238U/ 206Pb* ages of about 643 and 547 Ma. Such ages are consistent with
the estimated age range of magmatic activity associated with rifting of
Laurentia from the Late Proterozoic supercontinent (e.g., Rankin et al.
1989; Aleinikoff et al. 1995).
Austin Glen Member of the Normanskill Formation
Detrital zircons from the Middle Ordovician Austin Glen form a near
concordant cluster at 207Pb*/ 206Pb* ages in the range of 1.46 to 0.94 Ga,
consistent with a Grenville provenance (Figs. 4B, 5B). A single zircon was
found that is concordant with a 207Pb*/ 206Pb* age of 3292 6 16 Ma (1s).
Although 3.3 Ga is not a common age for North American Archean terranes, such ages are known from the Minnesota River Valley Inlier (see
review of zircon geochronology in Goodwin 1991) and accordingly the
detrital zircon also is consistent with a Laurentian source. The Austin Glen
contains two relatively young zircons, at about 720 and 730 Ma, that are
consistent with late Precambrian rifting of Laurentia.
Shawangunk Formation
Zircons from the Lower Silurian Shawangunk Formation analyzed in this
study are from a bulk sample taken from New York State. In contrast, Gray
and Zeitler (1997) separated zircons from the quartz pebbles contained
within their Shawangunk sample from Pennsylvania. In both cases most
zircons fall within a 207Pb*/ 206Pb* age range of 1.52 to 0.87 Ga, consistent
311
with Grenville sources (Figs. 4C, 5C). There is essentially complete overlap
of the zircon ages for these two samples, and accordingly it would appear
that the pebbles contained within the Shawangunk have a provenance similar to that of zircons in the sandy matrix.
Excluding the two grains 2.1 (highly discordant) and 28.1 (clearly older),
the zircons analyzed in this study form an even tighter cluster of 207Pb*/
206Pb* ages (1.32 to 0.88 Ga), but the range of 206Pb*/ 238U ages is outside
analytical uncertainty, indicating real age differences or possibly some Pb
loss. This cluster of zircons falls near concordia, and the 207Pb*/ 206Pb*
ages are not precise enough to detect any real discordance with certainty.
One zircon (grain 28.1) has a 207Pb*/ 206Pb* age of about 1.80 Ga, an age
that is very common in Precambrian rocks of central North America (Midcontinent Belt, Penokean Orogen, Trans-Hudson Orogen). Accordingly, the
presence of this zircon is also consistent with a Laurentian provenance.
Zircons of late Precambrian age, consistent with late Precambrian rifting
of the Laurentian continental margin, such as those found in the Poughquag
and Austin Glen, are apparently absent from the Shawangunk. This zircon
population represented only a small proportion of the Poughquag and Austin Glen, comprising about 5 6 6% (at 95% confidence) and by Silurian
time these terranes may have been no longer exposed or effectively
‘‘swamped’’ by other sources. One must be cautious interpreting the absence of such small populations because there is at least a 10% statistical
chance of missing this population in the 49 zircon analyses available for
the Shawangunk Formation.
Catskill Clastic Wedge (Lower Walton Formation)
The Walton Formation sample from the Devonian Catskill Clastic
Wedge ‘‘redbeds’’ has a very distinctive distribution of detrital zircons
(Figs. 4D, 5D). Approximately 62% of the analyzed zircons cluster very
tightly with 207Pb*/ 206Pb* ages in the range 1.26 to 1.00 Ga, consistent
with Grenville sources. There is good internal agreement of the 207Pb*/
206Pb* ages among these older zircons, but the age range is probably significant. For example, grains 37.1 and 7.1 with 207Pb*/ 206Pb* ages of 1258
6 70 and 1170 6 15 Ma are likely older than most of the other 26 grains,
which together have a weighted mean age of 1071 6 6.5 Ma. The remaining 38% of the zircons are much younger, with 238U/ 206Pb* ages of
approximately 467 to 419 Ma, and are clearly derived from magmatic rocks
associated with early Paleozoic tectonic activity. Figure 6 is an expanded
histogram showing the Paleozoic-age zircons with the age ranges of earlier
rift magmatism and the Taconian and Acadian orogenies superimposed.
The Paleozoic detrital zircons are all consistent with derivation from Taconian-age sources. Ages of magmatic activity associated with the Taconian and Acadian orogenies substantially overlap (e.g., Sevigny and Hanson
1995). Accordingly, an Acadian source cannot be entirely excluded for at
least a few of the grains but is not required to explain the age range of the
detrital zircons.
Comment on Sampling Bias in Detrital Zircon Studies
It is generally agreed that analysis of individual zircon grains rather than
multi-grain populations is preferable in order to avoid ambiguity about
mixed ages. On the other hand, results for large populations can also be
helpful in constraining the relative importance among the various age populations (e.g., Miller and Saleeby 1995). If sufficient zircon grains are measured it is also possible to constrain the relative proportions of the various
provenance components, but great caution is warranted in interpreting detrital zircon data in this manner because of possible bias introduced in
selecting zircons for analysis.
There are two well established methods of determining the age of detrital
zircons. The first is using conventional thermal ionization mass spectrometry with individual zircon grains. Because of the low levels of Pb involved,
this method relies on very careful sample preparation. The major advantage
312
S.M. MCLENNAN ET AL.
FIG. 4.—Plots of 207Pb*/ 206Pb* versus 238U/ 206Pb* for zircons analyzed in this study (open symbols with 1s error bars) and data for the Shawangunk Formation reported
by Gray and Zeitler (1997; crosses). On each diagram, concordia is plotted with 100 Ma increments marked. For the Poughquag, Austin Glen, and Shawangunk the majority
of zircons are slightly discordant to concordant and plot in an area consistent with derivation from the Grenville Province. The Poughquag and Austin Glen also have a
small number of zircons with ages in the range of 750 to 550 Ma, consistent with magmatic activity on the Laurentian continental margin associated with late Precambrian–
Cambrian rifting. The Austin Glen and Shawangunk also have a small number of old zircons (. 1800 Ma) that are also consistent with derivation from known Precambrian
terranes within Laurentia. The Devonian Catskill ‘‘redbed’’ has two very distinct zircon age populations, one consistent with Grenville ages and the other within a fairly
tight range of 410 to 470 Ma, ages consistent with derivation from magmatic rocks formed during the Taconian Orogeny.
is that extremely precise ages can often be determined for individual grains.
The second method utilizes high mass resolution ion microprobes, as described in this study. Among the advantages of this method are minimal
sample preparation, high sample throughput, and the ability to analyze different parts of different grains. Depending on the exact analytical needs,
either one of the methods may be more useful.
An important question in studies of the provenance of detrital zircons is
whether or not the zircon population analyzed is representative of the provenance or whether some bias is introduced when selecting zircons for analysis. In the ion microprobe technique, it is common to measure zircons at
random to minimize bias. (In fact, there is always the possibility of preferentially excluding or including some zircon fraction during preparation
of a zircon concentrate, but this is common to both methods.) On the other
hand, conventional methods typically select zircons for analysis on the
basis of size (typically zircons . 100 mm are selected; e.g., Gehrels and
Dickinson 1995; Gehrels et al. 1995), to ensure sufficient Pb for analysis
and to allow for abrading away altered rims, or on the basis of optical
characteristics (with clear or only slightly discolored inclusion-free and
crack-free zircons being selected; e.g., Ross et al. 1992; Rainbird et al.
1997), to ensure minimal loss of Pb and thus provide for more concordant
ages.
Sample 93SM003 from the lower Walton Formation is a very useful
sample to evaluate the types of biases that may be introduced in using
various grain characteristics to select detrital zircons for analysis (Fig. 7).
Grains were analyzed at random by ion microprobe and, as part of the
analytical protocol, the characteristics of individual grains were recorded.
The sample contains two unambiguously distinct age populations that are
of nearly equal proportions, making statistical evaluation relatively straightforward. In the bulk sample, the Paleozoic zircon fraction represents 17 of
45 grains or 38 6 14% (at 95% confidence) of the total zircon population.
If zircons are selected by size, with those , 100 mm being excluded, then
34 grains are left and of these, Paleozoic zircons represent only 24 6 14%
of the population. On the other hand, if zircons are selected for optical
clarity with relatively dark-colored zircons being excluded, 27 zircons are
left, and those of Paleozoic age represent 59 6 19% of the population.
Thus, selecting for size seriously biases the population against Paleozoic
zircons whereas sampling for optical characteristics seriously biases the
population in favor of Paleozoic zircons. It also is apparent that such sampling biases could readily result in entirely missing zircon ages that make
up a significant part of the population (10–20%). In this particular sample,
selecting for both criteria would effectively cancel out the bias but in general there is no reason to believe that this would be the case.
DETRITAL ZIRCON AGES IN TACONIAN AND ACADIAN FORELAND SEDIMENTS
FIG. 5.—Histograms of adopted ages of detrital zircons for the four analyzed samples. Note that ages greater than 800 Ma are based on
accordingly are best interpreted as minimum ages. Ages younger than 800 Ma are based on 206Pb*/ 238U ratios.
The major conclusion is that if one is interested in determining the provenance of a sample in anything near a quantitative manner, it is important
to select zircons for analysis as randomly as possible. Selecting zircons on
the basis of grain characteristics should be reserved for problems where
the age of a provenance component needs to be determined with great
precision even at the risk of biasing against or even excluding other components.
DISCUSSION
Provenance of the Cambrian Passive Margin
Quartz-rich sediments, such as those deposited at passive margins, commonly have a long history of sedimentary recycling (Veizer and Jansen
1985), and thus a complex provenance record should be typical (McLennan
et al. 1990, 1993). In a study of detrital zircons from the Cambrian Potsdam
Formation in northern New York, using conventional mass spectrometry
methods, Gaudette et al. (1981) proposed four dominant sources. In that
study, zircons were separated on the basis of shape and color and for the
most part small populations (1–24 grains), rather than individual grains
were analyzed, allowing the possibility of mixing different age populations.
Provenance components with ages of about 2.7, 2.1, 1.3, and 1.2–1.1 Ga
were suggested. All of these zircon populations were discordant, and they
were severely affected by Grenville metamorphism. In the case of the older
populations (2.7 and 2.2 Ga), zircons plotted very close to a lower intercept
of about 985 Ma, and Gaudette et al. (1981) suggested derivation from
metasedimentary rocks contained within the Grenville Province. McDaniel
et al. (1997) studied detrital zircons from the highly metamorphosed and
deformed Trap Falls Formation that they interpreted as likely representing
313
207
Pb*/ 206Pb* ratios and
deposition on the Late Proterozoic–Cambrian passive margin. Abraded
grains (1–4 grains per analysis) yielded near concordant 207Pb*/ 206Pb* ages
of 1113 to 992 Ma suggesting magmatic rocks of the Grenville Province
as the dominant source. Karabinos et al. (1999) recently reinterpreted the
Cavendish Formation (mainly quartzites and marbles) in Vermont to have
been deposited after the end of the Grenville orogeny and probably younger
than 0.49 6 0.10 Ga (age of the youngest detrital zircon). Apart from a
single Paleozoic-age zircon, they found exclusively Grenville-age detrital
zircons ( 207Pb*/ 206Pb* ages of 1.30 to 0.89 Ga) in a quartzite from this
unit.
Zircons from the Poughquag Formation also appear to be dominated by
the adjacent Grenville Province with the added small component of riftrelated rocks. The Grenville component may include some much older
zircons that underwent extreme Pb loss during Grenville metamorphism.
Nevertheless, we find it noteworthy that nearly all of the zircons analyzed
from passive-margin sedimentary rocks are likely derived from the nearby
Grenville Orogen, with very few if any being unequivocally derived from
erosion of older North American terranes, such as the Late Archean Superior Province or Early Proterozoic Penokean Orogen. Such a localized
provenance is surprising for passive-margin quartz arenites, which are normally thought to have a protracted recycling history (e.g., Veizer and Jansen 1985).
Early Paleozoic Orogenic Activity in New England
The most intriguing result of our study is the complete absence of Paleozoic-age zircons in either the Austin Glen Member or Shawangunk Formation. Gray and Zeitler (1997) also noted the absence of Paleozoic zircons
314
S.M. MCLENNAN ET AL.
FIG. 6.—Histogram of zircons from the Catskill Clastic Wedge ‘‘redbed’’ expanded to show the distribution of Paleozoic-age zircons. Also shown are bars representing the age ranges of magmatic activity associated with late Precambrian–
Cambrian rifting of the Laurentian margin and the major phases of the Taconian
and Acadian Orogenies. The range of zircon ages in this sample is consistent with
all being of Taconian heritage, and no zircon has an unambiguous Acadian age.
in their analyses of Shawangunk detrital zircons. Two major lines of evidence have been used to suggest Ordovician-age juvenile contributions to
Taconian foreland sediments. The first is high abundances of Cr (and chromite) in Taconian foreland sediments in more northern regions (e.g., Newfoundland, Quebec, northern New England), which have been used to suggest an ophiolitic component (Hiscott 1984; Garver et al. 1996). A study
of detrital zircons is entirely insensitive to such a provenance component
because ophiolitic rocks are unlikely to contain significant amounts of zircon. On the other hand, Bock et al. (1998) conducted a thorough provenance analysis of the Austin Glen using petrography, major-element and
trace-element geochemistry, and Nd-Pb isotopes and found no evidence for
enriched Cr or for an ophiolitic component in the Austin Glen. They suggested that ophiolitic sources were not important for Taconian forelandbasin sedimentation as far south as southern New England. Instead, the
results of Bock et al. (1998) are consistent with a provenance dominated
by Laurentian sources (mainly Grenville Province) that were recycled
through the accretionary prism and deposited in the Taconian foreland.
The second piece of evidence suggesting juvenile components comes
from Nd isotope evidence. A number of workers have noted that there is
a shift in the average Nd isotopic composition of the late Precambrian to
Cambrian passive margin sequence through the Taconian foreland sequence
from eNd(450 Ma) of about 212 to a value of about 28 (Krogstad et al.
1994; Anderson and Samson 1995; Bock et al. 1996; Patchett et al. 1999).
One explanation for this change calls for introduction of an Ordovicianage juvenile component, such as that expected from a colliding arc terrane
(Anderson and Samson 1995; also see Krogstad et al. 1994). On the other
hand, Bock et al. (1996, 1998) pointed out that the range of Nd isotopic
compositions in Middle to Late Ordovician Taconic foreland sediment was
consistent with a provenance dominated by Laurentia, mainly the Grenville
Province.
The results from this study, and that of Gray and Zeitler (1997), clearly
indicate that the Grenville Structural Province is the dominant source of
detrital zircon for the Taconian foreland in southern New England. What
does this mean for tectonic models of the Taconian orogeny? The Austin
Glen Member turbidites were probably deposited fairly early during the
Taconian orogeny. On the other hand, the Shawangunk Formation unconformably overlies the Taconian turbidite sequence and represents the syntectonic to posttectonic phase (Drake et al. 1989; Robinson et al. 1998).
Accordingly, if an island-arc terrane collided with Laurentia during the
Taconian orogeny, Paleozoic zircon-bearing detritus from the volcanic arc
FIG. 7.—Histograms of zircon ages plotting frequency percent for the Catskill
‘‘redbed’’ A) where all zircons analyzed, B) where certain zircons are excluded on
the basis of being #100mm, and C) where certain zircons are excluded on the basis
of showing evidence of possible radiation damage in being dark colored. Note that
selecting zircons on the basis of size or color results in significant differences in the
relative importance of Grenville-age versus Paleozoic-age zircons.
DETRITAL ZIRCON AGES IN TACONIAN AND ACADIAN FORELAND SEDIMENTS
315
FIG. 8.—Distribution of the mean, standard
deviation (heavy line), and total range (light
line) of 207Pb*/ 206Pb* ages of Grenville-type
zircons in stratigraphic order. There is an
indication that Grenville zircons get younger on
average and the age range becomes narrower in
younger samples.
was not transported far into the foreland basin, even during the later stages
of orogenic development.
The zircon data similarly provide little constraint on the tectonic model
where the Taconian orogeny results from the collision of Laurentia with
South America. Rocks of Grenville age are reasonably common in South
America (e.g., Goodwin, 1991) and such terranes would provide a possible
source of detrital zircons in the Taconic foreland. It does not seem plausible, however, that Grenville-age zircons from South America could be
transported into the Taconic foreland without a significant component of
younger zircons from the intervening arc, and thus we consider it unlikely
that any of the Grenville-age zircons were derived from South America.
On the other hand, the absence of zircons from South America certainly
does not eliminate this model for the origin of the Taconian orogeny.
The provenance model for the Taconian foreland basin of Bock et al.
(1996, 1998) appears to be consistent with the zircon data. The Austin
Glen and Shawangunk were derived directly from the east (Yeakel 1962;
Epstein and Epstein 1972). Sources were likely dominated by recycled
Proterozoic–Cambrian rift (found only in the Austin Glen) and Cambrian
to Lower Ordovician passive-margin sediment that was caught up in the
developing accretionary prism. Thus, Taconian sediment was derived directly from the east by westward transport, but this represents recycled
material. Ultimately, the Taconic foreland has a provenance from Laurentian sources to the west. Although components of contemporaneous juvenile volcanic material cannot be excluded, the detrital zircon geochronology
and other geochemical data (Hemming 1994; Bock et al. 1998) restrict such
a component to being minor (certainly less than 10% and likely less than
5%).
Data from the Devonian Catskill ‘‘redbed’’ represent the synorogenic to
postorogenic phase of the Siluro–Devonian Acadian orogeny. Unlike sedimentary rocks from the Taconic foreland, the sample analyzed here does
have a major component (40%) of Paleozoic zircons. An interesting feature,
however, is that the Paleozoic zircons have a very restricted range of 238U/
206Pb* ages between 467 and 419 Ma and none record an age that is
unambiguously associated with Acadian orogenic activity. All appear best
interpreted as being associated with Taconian magmatic activity (Fig. 6).
This observation is at odds with K-Ar ages for detrital white micas from
both proximal and distal locations in the Catskill clastic wedge. Aronson
and Lewis (1994) found restricted K-Ar ages mostly in the range of 406
to 387 Ma for multi-grain populations from several samples. Micas from
a single Walton Formation sample had a K-Ar age of 401 6 7 Ma. They
concluded that these detrital micas were almost exclusively Acadian in
origin. Such an interpretation is non-unique because of the possibility of a
mixed population but Aronson and Lewis (1994) calculated that no more
than about 30% of a Taconian component and less than about 5% of a
Precambrian component were permitted by the data. In a Pb isotope study
of detrital feldspars from the same sample as studied here (93SM003),
McDaniel and McLennan (1997) found that all analyzed feldspars were
likely crystallized during the Paleozoic but could not differentiate between
the Taconian and Acadian orogenies.
Thus, different detrital components appear to be giving very different
perspectives of the provenance. At one extreme detrital zircons, which are
crystallized during igneous or very high-grade metamorphic processes,
have high closure temperatures, and are resistant and thus readily preserved
during sedimentary recycling processes, indicate a nearly equal mix of Precambrian and Taconian ages with no unambiguous evidence for Acadian
sources. The detrital feldspars, which are far less resistant to physical and
chemical breakdown during sedimentary processes than zircon, indicate
almost exclusively Paleozoic sources (but cannot distinguish Taconian from
Acadian) with no feldspar being unambiguously derived from the Grenville
Province. At the other extreme, K-Ar ages of detrital micas, which form
during igneous or relatively low-grade metamorphic processes, have relatively low closure temperatures for Ar (ca. 3508C), and are also more
readily chemically and mechanically broken down during sedimentary processes than is zircon, can be interpreted as exclusively Acadian.
In summary, during early Paleozoic evolution of the Taconic and Acadian foreland basins in New England, detrital zircons do not appear to
record contemporaneous orogenic activity but instead are restricted to recycling of the preexisting continental margin. One explanation is that substantial topography, such as an emerging accretionary prism, impeded sediment transport from the active orogen to the contemporaneous foreland
basin. An alternative explanation is that contemporaneous zircon-bearing
magmatic rocks (Taconian-age for Austin Glen and Shawangunk; Acadianage for Catskill) were not yet exhumed and exposed at the time of sedimentation. Although such an explanation is quite likely for the early-tectonic to syntectonic Austin Glen turbidites, it seems far less likely for the
Catskill, and especially the Shawangunk samples, which were deposited
much later in the respective orogenic cycles. In either case, this result
clearly indicates that relationships between the tectonic development of an
orogenic belt and the provenance ‘‘signal’’ recorded in contemporaneous
sediments are likely to be complex. Accordingly, caution is warranted in
attempting to constrain the evolution of orogenic belts by examining the
character of nearly contemporaneous foreland sediments.
Stratigraphic Evolution of Grenville-age Sources?
Although there are other possible sources for zircons with ages greater
than about 1.3 Ga, such as the Midcontinent discussed above, zircons in
the age range of about 1.7 to 0.9 Ga have been interpreted here as being
derived mainly from a Grenville Structural Province source. If so, there
appears to be a stratigraphic evolution in ages of the Grenville sources.
316
S.M. MCLENNAN ET AL.
Figure 8 is a plot of 207Pb*/ 206Pb* ages for Grenville-type zircons showing
the mean, standard deviation, and range. There is a suggestion that the
mean age of these zircons becomes younger as one moves stratigraphically
upwards from the passive margin (Poughquag) through the Taconian foreland sedimentary sequence (early, Austin Glen Member; late, Shawangunk
Formation). The greatest contrast in mean age is between the passive-margin samples and the others, with difference in mean age being significant
at the . 99.9% confidence level. The difference in mean age of the Austin
Glen and Shawangunk detrital zircons is less secure, being distinct only at
the 90% confidence level, and there is no significant difference between
the Shawangunk and Catskill zircons. It is interesting to note that the major
break in Nd isotopic composition of whole rocks, from older to younger
model ages, also occurs at this stratigraphic level throughout North America (Gleason et al. 1994; Krogstad et al. 1994; Bock et al. 1996; Patchett
et al. 1999) although Sample PQ is an exception to this general rule and
has younger model age than many pre-Taconian sedimentary rocks (Table
5; Bock et al. 1996).
Although the geology of the Grenville Province is complex in detail,
there is a general progression in the age of overprinted crust from Archean
adjacent to the Grenville Front through to mid-Proterozoic in the south east
along the Grenville-Appalachian boundary (e.g., Goodwin 1991). The detrital zircon record suggests that more distal sources from within the Grenville Province become less important in the sedimentary record during the
evolution of the Taconian and Acadian Orogenies.
CONCLUSIONS
The major conclusions of this study are as follows:
1. A significant bias in the age distribution of detrital zircons from any
given sample may occur if zircons are selected for analysis on the basis of
size and/or optical characteristics.
2. Available sandstone samples from the Cambrian passive margin in
New England contain detrital zircon populations with ages consistent with
derivation predominantly from the adjacent Grenville Structural Province.
Sediments deposited at passive margins are generally thought to have a
protracted sedimentary recycling history, but these results suggest that relatively localized sources dominated the provenance.
3. Detrital zircon populations in samples from both the Taconian and
Acadian foreland basins are characterized by an absence of ages indicative
of the contemporaneous orogeny. For sediments deposited early in the orogenic cycle (e.g., Austin Glen Member) this may simply reflect the lack of
exposure of coeval plutons. For syntectonic to posttectonic sedimentary
rocks, however, it may reflect a topographic impediment between the magmatic rocks and the foreland. Regardless of the cause, this result suggests
that caution is warranted in using sedimentary compositions to interpret
the contemporaneous orogenic history.
4. There may be a stratigraphic evolution in the age distribution of Grenville-type detrital zircons from relatively old and variable in the passive
margin to relatively young with little variability in the Taconian and Acadian foreland sequences in New England. Such an evolution in detrital
zircon ages would be consistent with more distal (northwest) Grenville
Province sources for the passive-margin sample and progressively more
proximal (southeast) Grenville sources for stratigraphically younger Taconian and Acadian foreland sedimentary rocks.
ACKNOWLEDGMENTS
We are grateful to D. Compston, A. Comacho, and R. Reinfrank for assistance
with ion-probe analyses and to Mary Beth Gray, Mark Johnsson, and Jon Patchett
for helpful journal reviews. This research was carried out while the first author was
a Visiting Fellow at the Research School of Earth Sciences and was also supported
in part by National Science Foundation Grants EAR8957784 and EAR9627908 to
SMM.
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