ABSTRACT
The Utopia Granite is part of the Saint George Plutonic Suite, a
gabbroic to granitic assemblage of plutons of Late Silurian--Late
Devonian age in southwestern New Brunswick. New U-Pb (ID-TIMS) analyses
of five fractions of strongly air-abraded zircon grains define a
discordant array which suggests that all five fractions have suffered at
least some minor degree of post-crystallization Pb-loss and yielded an
imprecise upper intercept age of 426.3 [+ or -] 5.9 Ma. Fifteen zircon
grains analyzed by LA-ICP-MS yielded overlapping concordant analyses,
from which a weighted average of the [sup.206]Pb/[sup.238]U ages,
excluding one younger age, is 428.3 [+ or -] 1.0 Ma (MSWD = 0.66,
probability of fit = 0.81), which we interpret as the best estimate for
the crystallization age of the Utopia Granite. This age suggests that
the Utopia Granite may be the oldest component of the Saint George
batholith, and 5-8 million years older than plutons of the contiguous
coastal Maine magmatic province to the southwest.
RESUME
Le granite Utopia fait partie de la suite plutonique de
Saint-George, un assemblage gabbroique a granitique de plutons du
Silurien tardif et de la fin du Devonien, dans le sud-ouest du
Nouveau-Brunswick. De nouvelles analyses U-Pb (ID-TIMS) de cinq
fractions de grains de zircon fortement alteres par l'air
indiqueraient un assemblage discordant qui porte a croire que les cinq
fractions ont subi au moins en partie une perte de plomb apres la
cristallisation. Les analyses ont permis d'etablir de maniere
imprecise un age geologique de 426,3 [+ or -] 5.9 Ma. Quinze grains de
zircon analyses a l'aide des methode LA-ICP-MS ont produit des
resultats concordants qui se recoupaient, dont on a tire une moyenne
ponderee des ages [sup.206]Pb/[sup.238]P de 428,3 [+ or -] 1,0 Ma,
exception faite d'un age plus jeune (deviation ponderee de la
regression par moindres carres = 0,66, probabilite de congruence =
0,81). Nous croyons qu'il s'agit la de la meilleure estimation
qui soit de la periode de cristallisation du granite Utopia. Ces
analyses de datation suggerent que le granite Utopia pourrait etre la
partie la plus ancienne du batholithe Saint-George, et de 5 a 8 millions
d'annees plus ancienne que les plutons de la province magmatique de
la zone cotiere contigtie du Maine, au sud-ouest.
[Traduit par la redaction]
INTRODUCTION
The Utopia Granite is part of the Saint George Plutonic Suite, also
known informally as the Saint George batholith, in southern New
Brunswick (Fig. 1). According to the current New Brunswick lexicon of
bedrock geology (New Brunswick Department of Natural Resources 2010),
the Saint George Plutonic Suite is a contiguous cluster of mainly
high-level intrusive rocks comprising (in inferred chronological order
from oldest to youngest) the Bocabec Gabbro, Utopia Granite,
Welsford,Jake Lee Mountain, and Parks Brook alkali granites,
Magaguadavic Granite, John Lee Brook Granite, and Mount Douglas Granite.
The age of the Saint George Plutonic Suite is broadly assigned to the
Late Silurian to Late Devonian (New Brunswick Department of Natural
Resources 2010), but the absolute ages of some of the constituent
plutons are not well constrained. The purpose of this paper is to report
new U-Pb zircon ages for the Utopia Granite based on both TIMS and laser
ablation (LA) ICP-MS dating, the latter of which provides a precise age
for the granite.
GEOLOGICAL SETTING AND FIELD RELATIONS
The Saint George Plutonic Suite intruded mainly Silurian
sedimentary and volcanic rocks of the Mascarene terrane, but also
intruded across terrane boundaries into the New River terrane in the
south and the St. Croix terrane and Fredericton belt in the north (Fig.
1). All of these areas are interpreted to be part of Ganderia (Hibbard
et al. 2006; Fyffe et al. 2009).
The Utopia Granite underlies an area of about 90 [km.sup.2] in the
southwestern part of the Saint George Plutonic Suite (Fig. 1).
Ruitenberg and Fyffe (1982) introduced the name Utopia Granite during a
regional metallogenic study of granitic rocks in the province. Along its
southern margin, the Utopia Granite intruded bimodal volcanic and
fossiliferous sedimentary rocks of the Eastport Formation of the
Mascarene Group. Biotite hornfels is developed locally in the contact
zone near Lake Utopia, and equigranular to porphyritic microgranite is
common along the southern contact of the granite, consistent with the
intrusive relationship (McLeod 1990). McLeod (1990) described the
northwestern margin of the granite as intrusive into the Bocabec Gabbro,
but based on the presence of mixing/ mingling textures the two units
were considered to have been more or less coeval. Fyffe (1971) also
described co-mingling of granitic and gabbroic units in the Bocabec
Gabbro, but did not attribute the granitic component to the Utopia
Granite. McLeod (1990) suggested that rounded, partially assimilated
mafic xenoliths that occur sporadically throughout the Utopia Granite
could be related to the Bocabec Gabbro, although that relationship is
not definitive.
[FIGURE 1 OMITTED]
Near its southwestern and southeastern margins, respectively, the
Utopia Granite has been interpreted to have intruded alkali-feldspar
granite of the Parks Brook and Jake Lee Mountain plutons (McLeod 1990;
Currie 2003). These alkalifeldspar granite plutons are considered to be
correlative with the Welsford Granite (e.g., Currie 2003), which has
yielded a U-Pb zircon age of 422 [+ or -] 1 Ma (Bevier 1990), and
collectively these plutons have been termed the Welsford Intrusive Suite
(McLeod 1990). If this correlation is correct, then that age is a
maximum for the Utopia Granite. Contacts of the Utopia Granite with the
ca. 396 Ma Magaguadavic and ca. 366 Ma Mount Douglas granites to the
north have not been observed; however, outcrop patterns are consistent
with the geochronological data (Bevier 1990) which indicate that these
plutons are younger than the Utopia Granite. McLaughlin (2003) suggested
that the Utopia Granite might be a more evolved component of the magma
that formed the Baring Granite in Maine, for which McLaughlin et al.
(2003) reported an age of 421.1 [+ or -] 0.8 Ma.
PREVIOUS DATING OF THE UTOPIA GRANITE
Fyffe et al. (1981) reported a Rb-Sr whole rock age of 406 [+ or -]
7 Ma for the Utopia Granite, and Bevier (1990) reported a preliminary
U-Pb zircon age of 430 [+ or -] 3 Ma. McLeod (1990) pointed out that the
latter age contradicts fossil evidence, which indicates an Early
Devonian age for the Eastport Formation in the contact aureole of the
granite. Consequently, he proposed that the [sup.40]Ar/[sup.39]Ar on
biotite total gas age of 418 [+ or -] 5 Ma is probably a more accurate
indication of the age of the Utopia Granite. However, a discrepancy
relating to the absolute age of the fossils was revealed later when
Silurian U-Pb ages were obtained from volcanic rocks elsewhere in the
Eastport Formation (Van Wagoner and Dadd 2003). Early Silurian U-Pb ages
have also been reported for rhyolite in the Letete and Waweig formations
of the Mascarene Group, further suggesting that the fossil evidence for
the age of the group may not be reliable (Miller and Fyffe 2002).
Re-examination of the U-Pb data of Bevier (1990) resulted in an
amended age for the Utopia Granite of 423 [+ or -] 3 Ma (M.L. Bevier,
cited in McLaughlin 2003), similar to the U-Pb age of the Baring
Granite, which McLaughlin (2003) and McLaughlin et al. (2003) considered
to be co-magmatic with the Utopia Granite based on petrological
similarities.
NEW U-PB DATA
The uncertainties related to the age of the Utopia Granite and its
host rocks motivated the authors to resample the granite for U-Pb dating
at the Pacific Centre for Isotopic and Geochemical Research (PCIGR). The
new sample was collected from the type locality of the Utopia Granite,
from the same location where McLeod (1990) collected the argon dating
sampie, along Route 785 along the east shore of Lake Utopia to the south
of Mill Lake on NTS 21 G/02W. The sample is red, medium-grained,
equigranular syenogranite transitional to monzogranite. Zircon grains
were separated using conventional crushing, grinding and wet shaking
table methods, followed by heavy liquid and magnetic separation. The
sample yielded abundant zircons, comprising clear, pale brown, stubby to
elongate, square prisms with simple terminations. Rare clear
bubble-shaped inclusions were present in some grains; however, no
internal zoning or inherited cores were observed under a binocular
microscope or in transmitted light.
The methodology used for TIMS analyses at the PCIGR was as
described by Mortensen et al. (1995). The outer portions of the grains
were removed using air-abrasion methods prior to dissolution in order to
minimize the effects of post-crystallization Pb-loss. Individual
fractions for analysis ranged from 1 to 11 grains. Zircons were also
dated using laser ablation (LA) ICP-MS methods, using methods as
described by Tafti et al. (2008). Instrumentation employed for LA-ICP-MS
dating of zircons at the PCIGR comprises a New Wave UP-213 laser
ablation system and a ThermoFinnigan Element2 single collector,
double-focusing, magnetic sector ICP-MS. Zircons were handpicked from
the heavy mineral concentrate and mounted in an epoxy puck along with
several grains of the Plesovice zircon standard (Saima et al. 2007),
together with a separate in-house, 197 Ma standard zircon, and brought
to a very high polish. High quality portions of each grain free of
alteration, inclusions, or possible inherited cores were selected for
analysis. The surface of the mount was washed for 10 minutes with dilute
nitric acid and rinsed in ultraclean water prior to analysis. Line scans
were employed in order to minimize elemental fractionation during the
analyses. Backgrounds were measured with the laser shutter closed for
ten seconds, followed by data collection with the laser firing for
approximately 29 seconds. The time-integrated signais were analysed
using GLITTER software (Van Achterbergh et al. 2001; Griffin et al.
2008), which automatically subtracts background measurements, propagates
all analytical errors, and calculates isotopic ratios and ages.
Corrections for mass and elemental fractionation were made by bracketing
analyses of unknown grains with replicate analyses of the Plesovice
zircon standard. A typical analytical session at the PCIGR consists of
four analyses of the standard zircon, followed by four analyses of
unknown zircons, two standard analyses, four unknown analyses, etc., and
finally four standard analyses. The 197 Ma in-house zircon standard was
analysed as an unknown in order to monitor the reproducibility of the
age determinations on a run-to-run basis. Final interpretation and
plotting of the analytical results employed the ISOPLOT software (Ludwig
2003). The final interpreted age for the sample is based on a weighted
average of the individual calculated [sup.206]Pb/[sup.238]U ages.
Five fractions of strongly air-abraded zircon were analyzed using
ID-TIMS methods (Table 1). Four of the analyses are statistically
concordant, but show some scatter along concordia (Fig. 2). A fifth
analysis (G) is discordant, reflecting a significant amount of
post-crystallization Pb-loss that likely relates to the substantially
higher U content of this fraction (Table 1). Together the five analyses
define a discordant array with a calculated upper intercept of 426.3 [+
or -] nd a lower intercept at 21 [+ or -] 260 Ma (MSWD = 0.05;
probability of fit = 0.99). We interpret the upper intercept to give the
crystallization age of the sample, and the lower intercept to reflect
mainly relatively young Pb-loss. The data suggest that all five
fractions have suffered at least some minor degree of
post-crystallization Pb-loss that was not completely removed by the
strong air abrasion.
We also applied LA-ICP-MS methods in an attempt to obtain a more
precise and robust age for the sample. A total of sixteen zircon grains
were analyzed (Table 2), and all but one yielded overlapping concordant
analyses (Fig. 3a). A weighted average of the [sup.206]Pb/[sup.238]U
ages, excluding one younger age, is 428.3 [+ or -] 1.0 Ma (Fig. 3b; MSWD
= 0.66, probability of fit = 0.81). We interpret this age as the best
estimate for the crystallization age of the Utopia Granite. The younger
[sup.206]Pb/[sup.238]U age for the single analysis is interpreted to
reflect subtle alteration of the analyzed zircon that resulted in minor
Pb-loss.
IMPLICATIONS OF THE RESULTS
Silurian-Devonian gabbroic to granitic plutons of the Saint George
Plutonic Suite are contiguous with the coastal Maine magmatic province
of Hogan and Sinha (1989), the northeastern part of which in adjacent
Maine is the Moosehorn Plutonic Suite of McLaughlin et al. (2003) (Fig.
1). The Baring Granite component of the Moosehorn Plutonic Suite yielded
a Late Silurian U-Pb (zircon) age of 421.1 [+ or -] 0.8 Ma, and based on
field relations and petrology, McLaughlin et al. (2003) concluded that
the Baring Granite is approximately contemporaneous with gabbroic and
dioritic components of the suite. Based on petrological comparisons,
they also suggested that the gabbroic and dioritic components of the
Moosehorn Plutonic Suite are correlative with the gabbroic and dioritic
components of the Bocabec Gabbro in the Saint George Plutonic Suite and
that the granitic component of the Bocabec Gabbro is correlative with
the Baring Granite.
[FIGURE 2 OMITTED]
The Utopia Granite has been interpreted to have intruded the
Bocabec Gabbro, but locally to be veined by granodiorite of the Bocabec
Gabbro, suggesting similarity in age (McLeod 1990). The granodiorite was
interpreted by McLeod (1990) as a zone of co-mingling between the two
plutons, and hence he referred to the Bocabec and Utopia plutons
collectively as the Digdeguash Lake Intrusive Suite. However, the age of
428.3 [+ or -] 1.0 Ma reported here for the Utopia Granite causes
problems with these interpretations because it is older than the age of
421.1 [+ or -] 0.8 Ma reported by McLaughlin et al. (2003) for the
Baring Granite. The close petrochemical similarity of the gabbroic and
dioritic components of the Bocabec Gabbro and Moosehorn Plutonic Suite
suggests that all of these plutons were co-magmatic (McLaughlin et al.
2003). However, the granitic component of the Bocabec Gabbro is
generally more similar to the Baring Granite than to the Utopia Granite,
which is chemically more evolved (McLaughlin et al. 2003). Rather than
the Utopia Granite being a younger, more evolved unit, however, our new
age data indicate that it is older than the Baring Granite and hence not
directly related. If so, then the co-magmatic relationship between the
Utopia Granite and the Bocabec Gabbro also becomes questionable--which
pluton is mingled with Bocabec Gabbro, the 421 Ma Baring Granite or the
428 Ma Utopia Granite? Our data do not provide a definitive answer to
that question, which would require more petrological studies and/or
geochronology, but the [sup.40]Ar/[sup.39]Ar cooling age of 421 [+ or -]
4 Ma for phlogopite in a gabbroic unit in the Moosehorn Plutonic Suite
suggests that it is more likely the Baring Granite.
[FIGURE 3 OMITTED]
The age of 428 Ma for the Utopia Granite also brings into question
the relationships among the Parks Brook, Jake Lee Mountain, and Welsford
alkali granites. McLeod (1990) reported that the Utopia Granite intruded
the Parks Brook and Jake Lee Mountain alkali granites, although he did
not provide detailed descriptions of the field relationships. Currie
(2003) considered these plutons to be correlative with the Welsford
Granite. However, the Welsford Granite has a U-Pb zircon age 422 [+ or
-] 1 Ma (Bevier 1990), and hence if the intrusive relationship reported
by McLeod (1990) is correct, then the U-Pb ages are not consistent with
the inferred correlation based on petrological similarities. Fyffe
(1998) noted that the Jake Lee Mountain pluton has a core of medium-to
coarse-grained, equigranlar riebeckite-bearing granite enveloped by a
fine-gained rim of rapakivi granite porphyry. Limited exposure in the
area does not preclude the presence of that fine-grained rim of the Jake
Lee Mountain pluton everywhere against the Utopia Granite (L.R. Fyffe,
written communication 2010), consistent with the latter being older as
indicated by the U-Pb age reported here. Alternatively, the Jake Lee
Mountain and Parks Brook granites might not be correlative with the
Welsford Granite.
The age reported here for the Utopia Granite is older than the ages
of other plutons of the coastal Maine magmatic province, which are
generally similar to the ages of the Baring and Welsford granites. They
include the South Penobscot Pluton (419 [+ or -] 2 Ma, U-Pb, zircon;
Stewart et al. 2001), Spruce Head Pluton (421 [+ or -] 1 Ma, U-Pb,
zircon; Tucker et al. 2001), Cadillac Mountain intrusive complex (424 [+
or -] 2 and 419 [+ or -] 2 Ma, U-Pb, zircon, Seaman et al. 1995), and
Sedgwick Pluton (419.5 [+ or -] 1 Ma, U-Pb, zircon; Stewart et al.
2001). Widespread and voluminous Late Silurian volcanic and plutonic
activity throughout the coastal Maine magmatic province and adjacent New
Brunswick shows that igneous activity was a major characteristic of this
area, but the specific tectonic setting in which it occurred is
uncertain. The span of ages from the Saint George Plutonic Suite from
428 Ma to 366 Ma (this study and Bevier 1990) indicates that these
plutons are not co-magmatic and were not related to a single orogenic
event. Hence the more generic term Saint George batholith seems more
appropriate than Saint George Plutonic Suite, as also suggested by
McLeod (1990).
ACKNOWLEDGEMENTS
Fieldwork by Sandra Barr in southern New Brunswick has been funded
in part by Natural Sciences and Engineering Research Council of Canada
Discovery grants, and by the New Brunswick Department of Natural
Resources through grants in aid of research. We thank the staff of the
Pacific Centre for Isotopic and Geochemical Research, especially Richard
Friedman, for assistance in generating the analytical results presented
here. Journal reviewers Les Fyffe and Malcolm McLeod are thanked for
their helpful comments which led to clarifications and improvements in
the manuscript.
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Editorial responsibility: Simon K. Haslett
S.M. BARR (1) *, J.K. MORTENSEN (2), AND M.L. BEVIER (2)
(1.) Department of Earth and Environmental Science, Acadia
University, Wolfville, Nova Scotia B4P 2R6, Canada
(2.) Department of Earth and Ocean Sciences, University of British
Columbia, Vancouver, British Columbia V6T 1Z4, Canada
* Corresponding author
Date received: 01 January 2010 [paragraph] Date accepted: 23
February 2010
doi: 10.4138/atlgeol.2010.004Table 1. ID-TIMS U-Pb data for zircon from the Utopia Granite.
Sample Wt U Pb (2) [sup.206] Pb/ [sup.204] U
description (1) (mg) (ppm) (ppm) (measured)
A: N1,+134,1 0.031 65 4.5 3460
C: N1,+134,6 0.037 171 13.1 6019
D: N1,+134,10 0.041 184 13.5 3008
E: N1,+134,8 0.058 0.77 5.7 2811
G: N1,+134,7 0.078 639 44.8 2047
Sample total common [sup.208] Pb
description (1) Pb (pg) %
A: N1,+134,1 2 12.6
C: N1,+134,6 4 19.8
D: N1,+134,10 11 16.6
E: N1,+134,8 7 17.3
G: N1,+134,7 98 15.0
Isotopic Ratios (3)
Sample
description (1) [sup.206] Ph/ [sup.238] U error
A: N1,+134,1 0.06715 0.18
C: N1,+134,6 0.06765 0.09
D: N1,+134,10 0.06728 0.18
E: N1,+134,8 0.06774 0.10
G: N1,+134,7 0.06561 0.11
Isotopic Ratios (3)
Sample [sup.207] Pb/ [sup.235]
description (1) U error
A: N1,+134,1 0.5128 0.32
C: N1,+134,6 0.5162 0.18
D: N1,+134,10 0.5135 0.28
E: N1,+134,8 0.5167 0.27
G: N1,+134,7 0.5005 0.21
Isotopic Ratios (3)
Sample [sup.207] Pb/ [sup.206] Correlation
description (1) P error Coefficient
A: N1,+134,1 0.05539 0.29 0.48
C: N1,+134,6 0.05534 0.11 0.87
D: N1,+134,10 0.05536 0.20 0.67
E: N1,+134,8 0.05533 0.23 0.60
G: N1,+134,7 0.05532 0.13 0.84
Isotopic Ages (Ma) (4)
Sample
description (1) [sup.206] Pb/ (238) U error
A: N1,+134,1 419.0 1.5
C: N1,+134,6 422.0 0.7
D: N1,+134,10 419.7 1.5
E: N1,+134,8 422.5 0.8
G: N1,+134,7 409.7 0.9
Isotopic Ages (Ma) (4)
Sample
description (1) [sup.207] Pb/ [sup.206] P error
A: N1,+134,1 428.0 13.0
C: N1,+134,6 426.1 4.9
D: N1,+134,10 426.5 8.9
E: N1,+134,8 425.4 10.1
G: N1,+134,7 425.2 6.0
(1) N1= non-magnetic at 1 degee side slope on Frantz magnetic
separator; grain size given in microns; number of grains; (2)
Radiogenic Pb, corrected for blank, spike, and initial common Pb;
(3) corrected for blank Pb and U, and common Pb; errors given in
percent (1 standard deviation); (4) corrected for blank Pb and U,
and common Ph; errors given in million years (2 sigma).
Table 2. Laser ablation ICP-MS data for zircon from the
Utopia Granite.
Isotopic ratios and 2 sigma errors (absolute)
Analysis [sup.207] Pb/ [sup.206] Pb Error
Utopia-1 0.05459 0.00082
Utopia-2 0.05586 0.00125
Utopia-3 0.05549 0.00078
Utopia-4 0.05462 0.00088
Utopia-5 0.05546 0.0006
Utopia-6 0.0577 0.0005
Utopia-7 0.0548 0.00089
Utopia-8 0.0548 0.00088
Utopia-9 0.05503 0.00056
Utopia-10 0.05525 0.00042
Utopia-11 0.05544 0.00142
Utopia-12 0.05402 0.00055
Utopia 13 0.05542 0.00059
Utopia-14 0.05717 0.00072
Utopia-15 0.05388 0.0004
Utopia-16 0.05583 0.00122
Isotopic ratios and 2 sigma errors (absolute)
Analysis [sup.207] Pb/ [sup.235] U Error
Utopia-1 0.51283 0.00847
Utopia-2 0.53006 0.01316
Utopia-3 0.52348 0.00817
Utopia-4 0.50688 0.009
Utopia-5 0.52731 0.0063
Utopia-6 0.54915 0.00531
Utopia-7 0.5194 0.0094
Utopia-8 0.5269 0.00946
Utopia-9 0.5167 0.00582
Utopia-10 0.52238 0.00445
Utopia-11 0.49207 0.01381
Utopia-12 0.5103 0.00576
Utopia 13 0.5289 0.00621
Utopia-14 0.54226 0.00761
Utopia-15 0.5081 0.00419
Utopia-16 0.53833 0.01298
Isotopic ratios and 2 sigma errors (absolute)
Analysis [sup.206] Pb/ [sup.238] U Error
Utopia-1 0.06853 0.0004
Utopia-2 0.06857 0.00059
Utopia-3 0.06908 0.00039
Utopia-4 0.06819 0.00042
Utopia-5 0.06897 0.00031
Utopia-6 0.06856 0.00026
Utopia-7 0.06887 0.00044
Utopia-8 0.06898 0.00044
Utopia-9 0.06839 0.00029
Utopia-10 0.06844 0.00023
Utopia-11 0.06456 0.00063
Utopia-12 0.06864 0.00029
Utopia 13 0.06879 0.0003
Utopia-14 0.06903 0.00036
Utopia-15 0.069 0.00023
Utopia-16 0.06855 0.00056
Isotopic ages and 2 sigma errors (m.y.)
Analysis [sup.207] Pb/ [sup.206] Pb Error
Utopia-1 395.6 32.97
Utopia-2 446.4 48.91
Utopia-3 431.9 30.45
Utopia-4 396.5 35.53
Utopia-5 430.4 23.72
Utopia-6 518.1 18.57
Utopia-7 403.9 35.82
Utopia-8 404.2 35.38
Utopia-9 413.4 22.27
Utopia-10 422.2 16.89
Utopia-11 429.8 55.7
Utopia-12 371.6 22.89
Utopia 13 429.1 23.33
Utopia-14 497.7 27.66
Utopia-15 365.9 16.71
Utopia-16 445.3 47.45
Isotopic ages and 2 sigma errors (m.y.)
Analysis [sup.207] Pb/ [sup.235] U Error
Utopia-1 420.4 5.69
Utopia-2 431.8 8.74
Utopia-3 427.5 5.44
Utopia-4 416.3 6.06
Utopia-5 430 4.19
Utopia-6 444.4 3.48
Utopia-7 424.7 6.28
Utopia-8 429.8 6.29
Utopia-9 422.9 3.9
Utopia-10 426.7 2.97
Utopia-11 406.3 9.4
Utopia-12 418.7 3.88
Utopia 13 431.1 4.13
Utopia-14 439.9 5.01
Utopia-15 417.2 2.82
Utopia-16 437.3 8.57
Isotopic ages and 2 sigma errors (m.y.)
Analysis [sup.206] Pb/ [sup.238] U Error
Utopia-1 427.3 2.41
Utopia-2 427.5 3.55
Utopia-3 430.6 2.36
Utopia-4 425.3 2.56
Utopia-5 429.9 1.85
Utopia-6 427.5 1.57
Utopia-7 429.3 2.66
Utopia-8 430 2.67
Utopia-9 426.4 1.76
Utopia-10 426.7 1.41
Utopia-11 403.3 3.8
Utopia-12 428 1.76
Utopia 13 428.8 1.82
Utopia-14 430.3 2.18
Utopia-15 430.1 1.37
Utopia-16 427.4 3.41