U-Pb zircon dating of metasedimentary rocks in the Areachap, Kakamas
and Bushmanland Terranes in Namaqua Province, South Africa.
Maria Fransson, Göteborg University, Departement of Earth Sciences; Geology, Box 460, SE-405 30 Göteborg
Abstract
Ion probe and laser ablation ICPMS zircon U-Pb isotope dating have been applied to metasedimentary rocks
from the Areachap, Kakamas and Bushmanland Terranes in the Namaqua Sector of the Namaqua-Natal
Province, South Africa. The Namaqua-Natal Province makes up the northwest part of South Africa and consist
of igneous and metamorphic rocks that formed or were metamorphosed during the Namaquan Orogeny at ~12001000 Ma.
The Areachap Terrane is considered juvenile, previously suggested to consist of maximum ~1300 Ma old rocks.
In this work, considerably older zircons have been dated from the Bethesda Gneiss which is part of the Areachap
Group. Concordia and Pb-Pb ages for 19 core spots range between 1350 and 2100 Ma. This suggests that
Bethesda Formation is not juvenile and if it is part of the Areachap terrane, that in turn is not juvenile either. The
metamorphic age for the Bethesda Gneiss has been determined at 1190 ± 27 Ma (discordia age).
Two samples of the Goedehoop Quartzite in the Kakamas Terrane, were analyzed. According to Moen (2007),
the provenance rocks should date between 1600 and 2000 Ma. However, well-documented cores from this work
give ages as low as 1160 Ma. Eight concordia and 13 Pb-Pb ages give ages between 1160 and 1350 Ma. There
are also five Pb-Pb ages between 1670 and 2020 Ma, as Moen suggested. The 1241 ± 12 Ma metamorphic age
from this work is older than some of the detrital grains and must thus be inherited from the source.
The Kenhardt Migmatite in Kakamas Terrane hase also been dated. A reliable metamorphic age could be
determined at 1180-1200 Ma from two discordia lines and two concordia ages. Due to equipment failure, only a
few cores could be dated and these overlap with the metamorphic age at 1205 ± 12 Ma, similar to that reported
by Cornell and Pettersson (2007).
The Droëboom Quartzite in Bushmanland Terrane, had 15 cores dated in this work ranging between 1700 and
1900 Ma. There is no younger component present, which distinguishes this Bushmanland sample from the
Kakamas samples. The metamorphic age, determined at 1041 ± 9 Ma Ma, is also considerably younger than
found in the Kakamas Terrane.
The two methods for zircon dating used in this work are NordSIM ion microprobe and GEUS LA ICPMS. There
are problems with the common lead correction with the ICPMS and the results from the two methods do not
correspond completely. However, reducing the data with the new in-house software Zirchron at GEUS and only
using uncorrected spots, the results from the NordSIM and GEUS correspond acceptably.
Keywords: metasediments, zircon dating, Namaqua Province, Areachap Terrane, Kakamas Terrane,
Bushmanland Terrane.
ISSN 1400-3821
B542
2008
2
U-Pb zirkondatering av metasedimentära bergarter från Areachap,
Kakamas and Bushmanland terrängerna, Namaqua-provinsen, Sydafrika.
Maria Fransson, Göteborgs Universitet, Instutitionen för geovetenskaper, Avdelningen för geologi, Box 460, SE405 30 Göteborg
Sammanfattning
Ion probe och LA (Laser Ablation) ICPMS U-Pb-isotopzirkondatering har tillämpats på metasedimentära
bergarter från Areachap-, Kakamas- och Bushmanlandterrängerna i Namaquasektorn av NamaquaNatalprovinsen, Sydafrika. Namaqua-Natalprovinsen utgör den nordvästra delen av Sydafrika och består av
magmatiska och metamorfa bergarter som bildades eller metamorfoserades under Namaqua-orogenesen vid
~1000-1200 Ma.
Areachap-terrängen anses vara juvenil, tidigare föreslagen att bestå av maximalt ~1300 Ma gamla bergarter. I det
här arbetet har avsevärt äldre zirconer daterats från Bethesda-gnejsen vilken hör till Areachap-gruppen.
Concordia och Pb-Pb åldrar för 19 kärnpunkter ligger mellan 1350 och 2100 Ma. Detta antyder att
Bethesdaformationen inte är juvenil och om den är del av Areachap-terrängen, är denna i sin tur inte heller
juvenil. Den metamorfa åldern för Bethesda-gnejsen har bestämts till 1190 ± 27 Ma (discordia-ålder).
Två prover från Goedehoop-kvartsiten från Kakamasterrängen analyserades. Enligt Moen (2007) borde
provenansbergarterna datera mellan 1600 och 2000 Ma. Väldokumenterade kärnor från detta arbetet ger
emellertid åldrar så lågt som 1160 Ma. Åtta concordia och 13 Pb-Pb åldrar ger åldrar mellan 1160 och 1350 Ma.
Det finns även fem Pb-Pb åldrar mellan 1670 och 2020, i enighet med Moen. Den metamorfa åldern 1241 ± 12
Ma från det här arbetet är äldre än några av de detriala kornen och därför torde den vara medärvd från källan.
Kenhardt-migmatiten från Kakamasterrängen har också daterats. En pålitlig metamorf ålder kunde bestämmas
till 1180-1200 Ma från två discordialinjer och två concordiaåldrar. På grund av problem med utrustningen kunde
endast ett fåtal kärnor dateras och dessa överlappar med den metamorfa åldern kring 1205 ± 12 Ma, vilket
tidigare även rapporterats från Cornell och Pettersson (2007).
Droëboom-kvartsiten från Bushmanlandterrängen har 15 kärnor daterade i detta arbetet med åldrar mellan 1700
och 1900 Ma. Det finns ingen yngre component närvarande, vilket särskiljer det här provet från
Bushmanlandterrängen från proverna från Kakamasterrängen. Den metamorfa åldern, bestämd till 1040 ± 11 Ma,
är även avsevärt yngre än den i Kakamasterrängen.
De två metoderna för zirkondatering som använts i detta arbetet är NordSIM ion probe och GEUS LA ICPMS.
Det finns problem med common lead-korrektion för ICPMS:en och resultaten från de två metoderna
överensstämmer inte helt. Men, om man reducerar datan med den nya in-house mjukvaran Zirchron på GEUS
och endast använder punkter som inte behövt common lead-korrigeras, resultaten från NordSIM och GEUS
överensstämmer acceptabelt.
Keywords: metasediments, zircon dating, Namaqua Province, Areachap Terrane, Kakamas Terrane,
Bushmanland Terrane.
ISSN 1400-3821
B542
2008
3
Table of contents
1. INTRODUCTION.................................................................................................................. 5
1.1 Aims with this project ...................................................................................................... 5
1.2 Geological setting............................................................................................................. 5
1.2.1 Namaqua Sector ........................................................................................................ 5
1.2.2 Tectonic evolution of Namaqua-Natal Province ....................................................... 8
2. SAMPLES AND ANALYTICAL METHODS ................................................................... 10
2.1 Methods .......................................................................................................................... 10
2.1.1 Theoretical background to U-Pb zircon dating ....................................................... 12
2.1.2 Problems with the methods and interpreting results ............................................... 12
2.2 Geological background of the samples .......................................................................... 15
2.2.1 Bushmanland Terrane.............................................................................................. 15
2.2.2 Kakamas Terrane..................................................................................................... 15
2.2.3 Areachap Terrane .................................................................................................... 16
3. RESULTS............................................................................................................................. 18
3.1 Bethesda metapelitic gneiss DC0754 ............................................................................. 18
3.2 Goedehoop quartzite DC0760 ........................................................................................ 21
3.3 Goedehoop quartzite DC0781 ........................................................................................ 22
3.4 Kenhardt migmatite DC0767 ......................................................................................... 24
3.5 Droëboom quartzite DC0768 ......................................................................................... 25
3.6 Comparison between NordSIM (ion probe) and GEUS (ICPMS) as methods for zircon
dating .................................................................................................................................... 30
4. DISCUSSION ...................................................................................................................... 32
4.1 Bethesda metapelitic Gneiss DC0754 ............................................................................ 32
4.2 Goedehoop Quartzite DC0760 and DC0781.................................................................. 32
4.3 Kenhardt Migmatite DC0767......................................................................................... 33
4.4 Droëboom quartzite DC0768 ......................................................................................... 33
4.5 The methods ................................................................................................................... 34
5. CONCLUSIONS .................................................................................................................. 35
Acknowledgements .................................................................................................................. 36
APPENDICES.......................................................................................................................... 39
Appendix 1. .......................................................................................................................... 39
Appendix 2. .......................................................................................................................... 44
4
1. INTRODUCTION
1.1 Aims with this project
The aim of this project is to apply zircon dating to metasedimentary rocks from the
Bushmanland, Kakamas and Areachap Terranes in the Namaqua Sector of the Namaqua-Natal
Province in South Africa. This provides further insight into the tectonostratigraphic
subdivision of rocks in this area, where defining terrane boundaries is a major problem and
the subject of controversy. Knowledge about the tectonic evolution is important because of
the economic potential of ore deposits and also gives increased insight into the crustal
evolution of South Africa and its role in the creation of the 1.0 Ga supercontinent Rodinia.
1.2 Geological setting
The Namaqua-Natal Province is a tectonostratigraphic province that stretches 1400 km across
South Africa and Namibia (Fig 1). It is 400 km wide and has borders with the Kaapvaal
Craton to the north and Pan-African (Gariep and Saldania) belts in the west and south. A
tectonostratigraphic province is defined by Stockwell et al., (1970) as a large area of
contiguous structural fabric with well-defined boundaries which formed during a particular,
geochronologically defined, tectono-metamorphic event. The Namaqua-Natal Province is
considered to contain igneous and metamorphic rocks formed or metamorphosed during the
Namaqua Orogeny at ~ 1200-1000 Ma.
Fig 1. Namaqua-Natal Province, after Cornell et al (2006). Geophysical boundaries after De Beer and
Mayer (1984).
1.2.1 Namaqua Sector
The Namaqua sector is the western part of the Namaqua-Natal Province and is separated from
the Kaapvaal Craton by the Kheis Province. The Kheis Province consists predominantly of
low-grade supracrustals in three different age intervals; ~3000, ~2000 and ~1300 Ma. Five
Subprovinces or Terranes are found within the Namaqua sector (Fig 2); from west to east
Richtersveld Subprovince, Bushmanland Terrane, Kakamas Terrane, Areachap Terrane and
Kaaien Terrane (Thomas et al 1994a). The Bushmanland, Kakamas and Areachap Terranes
will be further described below.
5
Fig 2. Tectonic subdivision of the Namaqua Sector, after Cornell et al (2006). Division is largely
based on Thomas et al (1994b) and Hartnady et al (1985). BoSZ: Boven Rugzeer Shear Zone, BSZ:
Brakbosch Shear Zone, DT: Dabep Thrust, GT: Groothoek Thrust, HRT: Hartbees River Thrust, NSZ:
Neusberg Shear Zone, PSZ: Pofadder Shear Zone.
o Bushmanland Terrane
The Bushmanland Terrane lies south of Richtersveld Subprovince, the boundary defined by
the Groothoek Thrust and Wortel Belt. Bushmanland Terrane is made up by three groups of
rocks, different in age and composition. The oldest unit is a Kheisan (2050-1700 Ma) granitic
basement complex. Then there are ~1900, 1600 and 1200 Ma supracrustal sequences of
sedimentary and volcanic origin. The youngest units are syn- and late-tectonic (Namaquan)
suites of intrusive rocks of predominantly granitic and charnockitic composition. The
Bushmanland Terrane was accreted during the Namaquan orogeny and Raith and Cornell
(2000) have dated overgrowths on zircons to 1030 Ma. Monazites give the ages 1038±12 Ma
and 1047±18 Ma (Raith et al 2003). The metamorphic grade in the northern and north-eastern
rocks of Bushmanland is upper amphibolite facies, 650-700° and 4 kbar (Waters, 1986, 1989).
6
o Kakamas Terrane
The Kakamas Terrane lies east and north of Bushmanland Terrane. As it is defined today, the
boundary goes along the Hartbees River Thrust east of 20°E and along the Swartrand fault
west of 20°E (Moen 2007). In the east, it is in contact with Areachap Terrane possibly along
Boven Rugzeer Shear Zone. Neither of these boundaries are confirmed, primarily due to lack
of reliable dating in the area (Cornell et al, 2006). The shear- and thrust-zones that now define
the boundaries might have developed later in the structural history than the sutures that make
up the actual boundary between the two terranes.
In several publications the Kakamas Terrane together with the Areachap Terrane, has been
mentioned as the Gordonia Subprovince which would mean that it consists of more than one
distinguishable terrane (Cornell et al 2006). These terranes would in that case be divided by
the Neusberg Shear Zone, but are yet to be named and correctly distinguished. Inadequate
dating in the region also brings problems in solving the question of whether the Kakamas
Terrane has a Kheisan basement or not. U-Pb zircon dating of the Kenhardt migmatite gives
detrital core ages ranging from ~1600 Ma to ~1300 Ma (Cornell and Pettersson 2007) which
probably originates from older crustal material. Rims were dated in the same work and have a
discordia age of 1194 ± 23 Ma.
The Kakamas Terrane consists of high-grade supracrustal rocks, mostly metasediments
such as metapelitic gneisses, quartzites, calc-silicates and marble. There are also intrusive
rocks, classified as pre- syn- and post-tectonic.
o Areachap Terrane
Areachap Terrane lies east of Boven Rugzeer Shear Zone (BRSZ) which is, according to
Thomas et al (1994a), the terrane boundary between Kakamas and Areachap. Cornell and
Pettersson (2007) highlight strong evidence that the BRSZ should not be considered a terrane
boundary in the south. To the east, the Areachap rocks overlie Kaaien Terrane along the
Brakbosch-Trooilapsan Shear Zone, a major crustal discontinuity. This zone is considered to
be one of the early thrusts related to the Namaqua orogeny that later has been reactivated as a
shear zone.
Distinguished from the Kakamas Terrane, the Areachap Terrane is considered to consist
only of juvenile material, because no Kheisan basement has been found so far. The Areachap
Terrane consists of ~1300 Ma amphibolite-facies metabasic and intermediate supracrustal
gneisses in a NNW-trending belt; the Areachap Group. These are intruded by at least three
types (Geringer et al, 1988, 1994) of granitoids of the Keimoes Suite that intruded across the
boundary of Areachap and Kakamas Terranes. The supracrustals have island-arc signatures
which are supported by the occurrence of Besshi type massive sulphide ores (Besshi type
massive sulphides occur in oceanic or back-arc extensional environments).
o Different divisions of the terranes in the Namaqua sector nomenclature.
In “The geology of the Upington area” (explanation to sheet 2820), Moen (2007) uses a
different subdivision of the Namaqua sector. There are three Subprovinces instead of the five
Terranes that are described by Thomas et al (1994a). From west to east, the Bushmanland
Subprovince consists of what Thomas et al (1994a) calls the Bushmanland Terrane and the
Richtersveld Subprovince. It is bounded to the east by the Gordonia Subprovince along the
Hartbees River thrust (in the south) and Swartrand fault (in the northwest). The Gordonia
Subprovince then stretches to the Brakbosch fault (in the south) and Trooilapspan shear zone
(in the north) and contains Thomas’ Kakamas and Areachap Terranes. The Terrane that
Thomas et al (1994a) mentions as the Kaaien Terrane, Moen (2007) names the Kheis
Subprovince and it borders the Kaapvaal Craton along the Dabep thrust.
7
1.2.2 Tectonic evolution of Namaqua-Natal Province
The following tectonic models were proposed by Cornell et al, (2006) and Cornell and
Pettersson, (2007).
Previous to the Namaquan Wilson cycle, a Kheisan Wilson cycle took place in the Namaqua
sector between ~2000 and 1600 Ma, with an orogenic event at ~1750 Ma. Due to the intensity
of the later Namaquan orogeny, the distribution and characteristics of the Kheisan domains
are hard to determine. Some time between ~1600 and 1350 Ma the Bushmanland and KheisKaapvaal cratons started to rift. Intracratonic basins formed and the supracrustal sequences of
Bushmanland and Korannaland Groups were deposited. The breaking apart of the KheisKaapvaal craton initiated the Areachap Ocean basin. The rifting ceased at ~1300 Ma and
subduction of the Areachap Ocean started. This gave rise to the arc-related juvenile Areachap
Terrane, see Fig 3A. The Kakamas Terrane was a small crustal fragment in the ocean basin
west of Areachap island arc that had formed at 1568 Ma or before. At 1220 Ma the Kakamas
Terrane docked with the Areachap island arc, see Fig 3B. This stopped the arc magmatism in
Areachap and the subduction moved west. Continental magmatism proceeded in the Kakamas
Terrane and it is suggested that Kenhardt Formation formed in this tectonic setting (Cornell
and Pettersson, 2007). The 1300 Ma Wilgenhoutsdrift mafic lavas formed in a back-arc
environment at this time.
The convergence terminated between 1220 and 1150 Ma. The Bushmanland Craton (with its
Namaquan cover) collided with the Kheis-Kaapvaal Craton and squeezed the Areachap arcs
(docked with Kakamas Terrane) in between, see Fig 3C. The collision between Areachap and
Kheis-Kapvaal Craton is dated to have ocurred between 1194±23 Ma and 1158±7 Ma based
on magmatic cores and metamorphic rims on zircons in the Areachap group. This collision led
to intense deformation and medium to high grade metamorphism in both sectors. There was
also widespread crustal melting and generation of voluminous granitoid magmas.
The first collisional event was followed by a ~100 Ma quiet period, long enough to consider
the second phase of the Namaquan orogeny as a separate orogeny. However, the later events
have so many features in common with the first so they are considered to be related.
It appears that the collision continued to a point where no more crustal shortening could
happen through low-angle thrusting and crustal thickening. So, at about 1080 Ma, lateral
escape was initiated and created subvertical shear zones and ductile mylonite belts. Lowpressure, high-temperature metamorphism took place between 1080 and 1020 Ma. The
thermal input is thought to be caused by mantle delamination, suggested by Gibson et al,
1996. During this process, the subducted cold lithosphere slab broke off from the crust and
the mantle underneath was replaced by hot asthenospheric mantle. The Namaqua orogeny as
described above terminated at ~1000 Ma and the province had been uplifted and cooled down
to 350°C by ~950 Ma.
8
Fig 3. Crustal evolution in the Namaqua Section, after Cornell and Pettersson (2007).
9
2. SAMPLES AND ANALYTICAL METHODS
2.1 Methods
The samples listed in Table 1 were collected in the region around Upington and Kenhardt,
South Africa. The sampling spots are shown in Fig 4. Hand specimens were taken and 30micron polished thin sections were made and examined with petrographic microscope, see
Table 1. About 2 kg of each sample was collected to extract zircons for dating. These were
crushed carefully in a jumbo swing mill for 15 seconds and the procedure was repeated until
the whole sample passed through a 400 micron sieve. The sample was then panned by hand
and zircons hand-picked and mounted in epoxy. Care was taken during the entire process to
avoid contamination. Digital zircon images were made using backscattered electrons (BS) and
cathodoluminescence (CL) detectors with a Hitachi S-3400N Scanning Electron Microscope,
(SEM) at the Centre for Earth Science, Gothenburg University, Sweden. A more detailed
description of zircon preparation is given by Cornell and Thomas (2006). U-Pb analysis was
done on all five samples with the NordSIM ion probe at the Swedish Museum of Natural
History in Stockholm, according to the methods described by Whitehouse et al (1999) and
Whitehouse and Kamber (2005). Four of the samples (DC0754, DC0760, DC0767 and
DC0768) were in addition to NordSIM also analysed using the laser ablation ICPMS at GEUS
in Copenhagen, as described by Gerdes and Zeh (2006) and Frei et al, (2006). All data except
DC0767 was reduced by an in-house Excel spreadsheet as the software Zirchron that is now
available was not running properly at the time. DC0767 was reduced using Zirchron and only
data that was not common-lead corrected was used. NordSIM used the 91500 standard for
calibration whereas GEUS used the GJ-1 standard. The data was calculated and displayed
with ISOPLOT ver. 3.00 (beta) by Ludwig (2003). All results are given in the text with 2σ
age errors and excluding decay-constant errors.
10
Fig 4. Sampling spots. The map is based on the 2820 Upington map sheet, mapped by the South African Geological Survey.
11
2.1.1 Theoretical background to U-Pb zircon dating
Uranium (U) and thorium (Th) are high field strength, lithophile elements and are present in
minerals like uranite, monazite and in trace amounts in titanite and zircon. Lead (Pb) initially
occurs at very low levels in zircon but does exist as a radioactive decay product of U and Th.
Ages are calculated from radioactive parent-daughter ratios. 204Pb, is not radiogenic, and can
be used to correct the 'common lead' component of the other three Pb isotopes when
calculating ages.
The most common method of interpreting the data is using a concordia diagram. 207Pb/235U is
plotted along the x-axis and 206Pb/238U along the y-axis. A concordia curve is drawn, along
which the 207Pb/235U age equals the 206Pb/238U age, the curved shape being due to the
different half lives of 235U and 238U (Rollinson, 1993). Discordant zircon data can be plotted
in this diagram, forming a straight line called a discordia line, which may intersect concordia
at two points reflecting lead loss. The upper intercept should represent the age of formation of
the zircon grains and the lower the age of the lead loss event. Lead loss can be recent,
meaning within the past few million years, or ancient, where lead loss occurred earlier in the
geological history, for example an ice age three hundred million years ago. The calculated
MSWD (Mean Sum of Weighted Deviates) should be below 2.0 for a discordia line. The
same applies to a concordia age, where one or more data points lie within error of concordia.
Zircons in a sediment or sedimentary rock may not produce a discordia line because the
zircons are detrital and thus might not have the same time of formation. Ages of individual
grains are then considered instead. In grains where different zones occur, different ages for
cores (e.g. magmatic formation or xenocryst age) and rims (metamorphic growth or resetting)
can be measured. Different age-domains in grains can be seen using a CL detector in an SEM,
where zones with less uranium appear bright and zones with more uranium dark. If a domain
appears dark with both CL and BS detectors it is usually metamict (U-decay having destroyed
the crystal structure) and should not be used for dating. The BS images show zonation less
clearly, but reveal cracks in the grains, which should be avoided to make sure that the
undisturbed isotope composition is being measured with as low common lead as possible.
2.1.2 Problems with the methods and interpreting results
The ion probe is regarded as an accurate method for dating and has a straightforward way of
reducing the data produced. However some problems arise with the LA-ICPMS. The LAICPMS has problems analyzing rims because the laser beam might go through the rim into
other zones which will then be measured, for example a core, which will obviously give the
wrong age for the rim. Further, all the data reduction after the LA-ICPMS analysis had to be
done by hand, since there was not a reliable way of doing it automatically in a computer. This
adds a source of human error. Common lead correction is also a big problem, due to the low
sensitivity and interference by 204Hg in the plasma argon. The rules for how and when this
should be done are not obvious and are best be considered from one case to another. In this
work, when using the spreadsheet for data reduction, we used 0.4% of common lead
expressed as 206Pb (f206) as the limit above which samples should be common-lead corrected.
All samples with f206% values below 0.4% have not been corrected for common lead. This
decision was made by looking at data for the 1067 Ma old standard zircon 91500 that is
known not to contain significant amounts of common lead, and gives too-young ages when
common-Pb corrected. When using Zirchron no data that needed common lead correction was
used.
12
A problem with micro beam zircon dating, irrespective of the method, is to correctly
distinguish age domains (cores, rims and xenocrysts). This is very important in order to
interpret ages correctly. Domains are mainly determined by looking at CL images and when
the results are available the Th/U ratio gives a good clue as to whether a spot is magmatic or
metamorphic. Spots with Th/U values below 0.2 are considered to be metamorphic, whereas
magmatic domains usually have values above 0.4, as discussed by Raith and Cornell, (2000).
13
Table 1. List of samples, classified as they were mapped by Council for Geoscience South Africa (Moen 2007). Petrographic data for the samples is also
included. Minerals are listed in decreasing order of abundance, with abbreviations as follows: Bi=biotite, Co=cordierite, Ksp=k-feldspar, Mu=muscovite
Ore=opaque minerals, Plag=plagioclase, Qz=quartz, Sp=spinel and Zr=zircon.
Sample
name
Description
DC0754
Migmatitic
metapelitic
gneiss
DC0760
Quartzite
DC0767
DC0768
Migmatitic
quartz
feldspar
gneiss
Quartzite
DC0781
Quartzite
Latitude
deg. minutes
28
28
28
28
28
37.068
26.110
44.416
50.294
45.771
Longitude
Formation
Group
Terrane
deg. minutes
21
20
20
20
20
Rock description
Colour
Grain size
Texture
Minerals
Bethesda
Areachap
Areachap
Grey
Medium
grained
Banded
Goedehoop
Korannaland
Kakamas
Pinkish
grey
Medium
grained
Foliated
Kenhardt
Vyfbeker
metamorphic
suite
Kakamas
Grey
Medium
grained
Armoedputs
Droëboom
Bushmanland
Whitegrey
Coarse
grained
Goedehoop
Korannaland
Kakamas
Greyish
pink
Medium
grained
Banded with
plagioclase
lenses 1-3 cm,
migmatitic.
Glassy, no
sign of
bedding
Slightly
banded
8.713
34.205
30.922
18.918
52.126
14
Qz, Mu, Bi,
Plag. Mu, Bi
pseudomorph,
possible Co.
Qz, Mu, Ksp,
accessory ore
and Sp.
Qz, Plag, Ksp,
Bi, minor Mu,
accessory Zr.
Qz
Qz, Mu,
accessory ore
and Zr.
2.2 Geological background of the samples
The samples were taken and classified according to the Council for Geoscience, South Africa
geological map and explanation to Upington sheet 2820 by Moen (2007).
2.2.1 Bushmanland Terrane
• Droëboom Group
The Droëboom Group lies in an area restricted by the Vogelstruisleegte and Swartrand faults.
Praekelt (1984) considered the Droëboom and Arribees Groups as the same, but this division
does not take the stratigraphic significance of the Swartrand fault into consideration. Although
the division by Moen (2007) puts Arribees in the Gordonia Subprovince or Kakamas Terrane,
it is also stated that this has not yet been verified in the mapped area.
The Droëboom Group consists mainly of dark-grey calc-silicate rocks with
interbedded layers and lenses of hornblende-bearing and feldspathic quartzite, paraamphibolite and marble (Praekelt, 1984). The basement is not exposed. Horizons of
conglomerate occur. A significant portion of the group consists of mafic gneiss and
amphibolite. Outcrops of massive quartzites, from which the sample was taken (Fig 5), are
thick-bedded, coarse-grained and contain disseminated magnetite. Some kilometers west of
the sample point, a road cutting contains quartzite with sillimanite nodules similar to those
found in the quartizites of the Bushmanland Sequence at Namiesberg in the Bushmanland Ore
District, described by Moore (1977).
Fig 5. Droëboom Quartzite, sample DC0768, in field.
2.2.2 Kakamas Terrane
• Korannaland Group
The Korannaland Group occurs as a north-west trending belt from an area north of Kenhardt
to the eastern boundary of Riemvasmaak. It consists of metamorphosed psammitic (quartzite)
and semipelitic rocks, many with a calc-silicate affinity, including a significant calc-silicate
and marble horizon. According to Cornell et al (2006), the Korannaland Group was deposited
in an intracratonic setting early in the Namaquan history.
15
o Goedehoop Formation
The Goedehoop Formation consists of muscovite-bearing quartzite with variable feldspar
content, and normally lies above the calc-silicate rocks of Puntsit Formation at the top of the
Korannaland group. These two formations remain associated throughout most of their
distribution. The quartzites are either dominated by mica or feldspar and are usually well
sorted. The micaceous quartzites are well-foliated and can grade into schists. These are fine
grained and light in colour. The foliation is parallel to bedding and sedimentary structures
such as bedding planes, graded bedding and cross-beds are often well preserved. With an
increase in feldspar contents, and hence a decrease in muscovite, the quartzites appear more
massive and gneissic and get a yellowish-brown weathering colour. Near the base of the
formation conglomerate occurs. A rather unsuccessful attempt was previously made to date
detrital zircons from Goedehoop Formation according to Moen (2007). Ages between ~2000
and 1600 Ma probably represent the provenance, whereas the depositional age of the
formation is probably around 1500 Ma. Pb/Pb ages for the formation showed what are thought
to be Namaquan metamorphic resettings at ~1300 and 1100 Ma.
• Hartbees River Complex
This unit consists of high-grade, migmatitic rocks in which the origin and stratigraphic
succession is impossible to determine.
o Kenhardt Migmatite
The Kenhardt Migmatite belongs to the Vyfbeker Metamorphic Suite. In the Upington area
the Kenhardt Migmatite consists of coarse-grained biotite and garnet gneiss with augenshaped porphyroblasts, see Fig 6. Lenses of amphibolite are common and garnet-bearing
leucogneiss lenses occur. The migmatisation is more widespread in the northern parts. Two
parallel belts with thick successions of marble, semipelitic and aluminous gneisses and
amphibolite occur within the porphyroblastic biotite ± garnet migmatite; this suggests a
sedimentary or volcanic origin for the amphibolites in the area.
Fig 6. Kenhardt Migmatite in field. The lines for scale are 1 cm across.
2.2.3 Areachap Terrane
• Areachap Group
The Areachap Group in the Upington area consists of the Ratel Draai, Bethesda, Jannelsepan
and Sprigg Formations. Whether the Copperton Formation should be included in the
16
Areachap group or not is argued about. As the terrane boundary is defined by the Council for
Geoscience, the Copperton Formation belongs to the Kakamas Terrane on the west side of the
Boven Rugzeer Shear Zone. But evidence published by Cornell and Pettersson (2007),
indicates that the Copperton Formation should be included in the Areachap Group as it was
originally defined, because the Boven Rugzeer Shear Zone is not a persistent terrane
boundary. This evidence rests on data from Pb and S isotopes, tectonic discrimination and
reliable dating.
o Bethesda Formation
First recognised and described by Vajner (1978b), the Bethesda Formation lies west of the
associated Jannelsepan Formation. Most rocks in the formation are badly weathered (see Fig
7) and obscured by pegmatitic rubble, calcrete and sand. The most common parageneses are
biotite ± muscovite gneiss and schist, sometimes with sillimanite or garnet. Layers of dark
quartzite, amphibolite, hornblende gneiss and biotite-garnet granoblastite are interbedded in
places.
Fig 7. Badly weathered Bethesda gneiss.
17
3. RESULTS
All dating results are summarized in Table 2 and 3 at the end of this chapter. The data used
can be found in Appendix 1. All the samples are metasediments, thus the zoned cores
represent detrital grains from the different protoliths that make up these rocks. The core ages
should not be expected to give a collective age but rather a time span representing the ages of
the source rocks. The youngest grain gives the maximum age of deposition. The rims could be
inherited, but more typically represent later events which are most likely metamorphic.
Although concordia or discordia ages are preferable, Pb-Pb ages can be used if only recent
lead loss is involved. In this work we apply a limit of 10% discordance for Pb-Pb ages,
samples with higher discordance are considered too discordant to give reliable ages, because
they may have suffered ancient Pb loss. Magmatic rocks in the region commonly conform to
ancient Pb loss discordia lines with lower intercepts 200 to 300 Ma.
3.1 Bethesda metapelitic gneiss DC0754
In CL images the zircon grains are typically subhedral and many are elongate (Fig 8A, spot 5a
& 105a). They show zoned cores and most have a dark grey rim, some too thin to measure but
others thick enough to fit a spot, shown in Fig 8A, spots 1b and 32b.
Results from NordSIM ion probe
Five cores were analyzed. As predicted these spread widely on the concordia diagram and a
discordia age can not be calculated, see Fig 9A. One of the grains is concordant with
concordia age 1516 ± 15 Ma with MSWD=2.1, shown in Fig 9B. The oldest grain has a Pb-Pb
age of 1945±45 Ma (8.9% discordant). The remaining three spots have minimum Pb-Pb ages
ranging between these , but are more discordant.
Six of the eight rims conform to a discordia line with upper intercept at 1190 ± 27 Ma and
lower at 101 ± 150 Ma (MSWD=1.4), see Fig 10. The two that were discarded are discordant
and have slightly younger Pb-Pb ages, suggesting a more complex lead-loss history.
0.36
data-point error ellipses are 68.3% conf.
A.
0.268
0.32
1520
1510
0.264
1500
Pb/238U
U
1530
0.266
0.28
5a
206
Pb/
238
B.
9a
1700
206
data-point error ellipses are 68.3% conf.
0.270
1900
0.24
1a
1300
0.262
1500
0.260 1490
32a
0.20
Core 5a
Concordia Age = 1516 ±15 Ma
MSWD = 2.1,
Probability = 0.15
0.258
86b
1100
0.256
0.16
1.5
2.5
3.5
207
4.5
Pb/
5.5
6.5
235
U
0.254
3.32
3.36
3.40
3.44
207
3.48
Pb/235U
Fig 9. Result for cores; sample DC0754, Bethesda Formation, from NordSIM ion probe data. A. The
detrital cores. B. Concordia age for 5a, 1516±15 Ma.
18
3.52
A. DC0754 Bethesda Gneiss
1
105a
5a
B. DC0760 Goedehoop Quartzite
21b
1b
32b
21a
51a
22a
C. DC0781 Goedehoop Quartzite
38a
28a
64 b
D. DC0767 Kenhardt Migmatite
127c
35b
42b
42 a
38a
E. DC0768 Droëboom Quartzite
115b
33b
12a
14a
41c
Fig 8. CL images of zircons used for dating. A. Sample DC0754, Bethesda Gneiss. B. Sample
DC0760, Goedehoop Quartzite. C. Sample DC0781, Goedehoop Quartzite. D. Sample DC0767,
Kenhardt Migmatite. E. Sample DC0768, Droëboom Quartzite.
19
data-point error ellipses are 68.3% conf.
0.21
1200
0.19
1b
206
Pb/
238
U
1100
0.17
65b
1000
13b
47b
900
0.15
Rims
Spots 35b and 15b excluded
(marked in diagram with □).
Intercepts at
101 ± 150 & 1190 ± 27 Ma
MSWD = 1.8
32b
4b
0.13
0.11
1.2
1.4
1.6
1.8
207
2.0
2.2
2.4
235
Pb/
U
Fig 10. Discordia plot for rims, sample DC0754, Bethesda Formation from NordSIM ion probe data.
The plot gives the discordia age 1190 ± 27 Ma for metamorphic rims.
Results from GEUS LA-ICPMS
Out of the 33 cores analyzed, there are five concordant cores with ages 1339 ± 20 Ma
(MSWD=0.112), 1387 ± 21 Ma (MSWD=0.70), 1532 ± 30 Ma (MSWD=0.81), 1829 ± 17 Ma
(MSWD=0.018) and 1940 ± 26 (MSWD= 1.9). There are also 12 other cores, within 10%
discordance, that have Pb-Pb ages ranging between 1350 and 2100 Ma. All of the above
mentioned cores are plotted in a probability density plot, Fig 11.
Eleven rims were analyzed but scatter about a discordia line. Only one rim is concordant and
has concordia age 1205±16 Ma (MSWD=0.048).
4
Relative probability
Number
3
2
1
0
1100
1300
1500
1700
1900
2100
2300
Minimum Pb-Pb age
Fig 11. Concordant and max. 10% discordant cores from GEUS ICPMS for detrital cores, sample
DC0754, Bethesda formation.
20
3.2 Goedehoop quartzite DC0760
Most of the zircon grains appear brittle and have a lot of cracks and many are only fragments.
Intact grains are usually subhedral. Most cores are zoned, Fig 8B spots 22a and 127c. Not all
grains have a visible rim, but where it exists it is typically CL dark and thin, as shown in Fig
8B spots 51a and 21b.
Results from NordSIM ion probe
One core out of the total ten was good enough to calculate a concordia age; 1305 ± 23
(MSWD=0.63). Two more grains were only slightly discordant and could be used for Pb-Pb
ages; 1732 ± 15 Ma (3.8%) and 1802 ± 11 Ma (4.6%). The cores for the Goedehoop
Quartzite, from samples DC0760 and DC0781, are displayed in Fig 13 (concordia plot) and
Fig 14 (probability density plot) in the next section.
Five of eight rim spots define a discordia line with upper intercept 1241 ± 11 Ma and lower
247 ± 21 Ma (MSWD=0.56), see Fig 12A. One grain on this line had concordia age 1241 ± 12
Ma (MSWD= 0.49), see Fig 12B. The three that were omitted had younger Pb-Pb ages,
suggesting a more complex Pb-loss history.
data-point error ellipses are 68.3% conf.
0.26
A.
0.22
data-point error ellipses are 68.3% conf.
0.222
1300
109a
56a
1100
0.218
B.
1270
U
130a
0.10
500
0.06
Rims
Excluded spots are 56b, 49a and 43a
(marked with square in diagram).
Intercepts at
247 ± 21 & 1241 ± 11 [±13] Ma
MSWD = 0.56
51a
94a
300
Pb/
0.14
700
0.02
0.0
0.4
0.8
1.2
207
1260
238
900
1250
0.214
206
206
Pb/
238
U
0.18
1.6
Pb/
235
2.0
2.4
2.8
1240
0.210
1230
Rim 109a
Concordia Age = 1241 ±12 Ma
MSWD = 0.49,
Probability = 0.48
1220
0.206
2.32
2.36
2.40
207
Pb/
2.44
235
U
U
Fig 12. Results from NordSIM ion probe for metamorphic rims, sample DC0760, Goedehoop
Quartzite. A. Discordia plot with age 1241 ± 11 Ma. B. Concordia age for core 109a, 1241 ± 12 Ma.
Results from GEUS ICPMS
With 31 cores analyzed, six cores are concordant and generate concordia ages: 1198 ± 16 Ma
(MSWD=0.26), 1221 ± 23 (MSWD=0.24), 1227 ± 18 Ma (MSWD=0.031), 1262 ± 22 Ma
(MSWD=1.3), 1280 ± 15 Ma (MSWD=0.27) and 2021 ± 16 Ma (MSWD=1.4). Another core
is 4.1% discordant and its Pb-Pb age 1801 ± 14 Ma is usable. These are shown in Fig 13 and
Fig 14 together with the NordSIM and DC0781 cores. The remaining cores are too discordant
to be correctly interpreted.
Of the nine rims that were analyzed, two could be used for concordia ages; 1114 ± 20 Ma
(MSWD=0.91) and 1220 ± 18 Ma (MSWD=1.6). Two others were less than 10% discordant
with Pb-Pb ages of 1192 ± 18 Ma (8.2%) and 1313 ± 32 Ma (6.6%). All rims for DC0760
(both methods) and DC0781 are displayed in the probability density plot, Fig 15. All rims
have rather high Th/U values, suggesting a magmatic rather than metamorphic origin.
21
2.48
3.3 Goedehoop quartzite DC0781
Another sample of Goedehoop quartzite was analyzed in order to try to confirm the rim age
found in DC0760. All grains are substantially cracked and often have holes. All grains also
have a zoned core which is eroded and covered by a CL bright rim, see Fig 8C spots 38a, 28a,
64b and 35b. Some grains have a dark rim between the core and the bright rim, but the
analyses are not usable because several spots measured on the same rim give totally different
values both in Th/U and Pb-Pb. The Pb-Pb age can vary by as much as 200 Ma so these
values were not used. The rim values accounted for are thus the CL-bright rims.
Results from NordSIM DC0781
Twelve cores were analyzed and two of these are concordant with concordia ages 1166 ± 18
Ma (MSWD=0.115) and 1172 ± 15 Ma (MSWD=0.87). Of the remaining core spots, five are
less than 5% discordant, with Pb-Pb ages ranging between 1200 and 1350 Ma. Three are
between 5-10% discordant and have the following Pb-Pb ages; 1669 ± 25 Ma (9.4%), 1802 ±
30 Ma (8.6%) and 1887 ± 10 Ma (5.0%). These cores are displayed together with the DC0760
Goedehoop cores in Fig 13 and Fig 14.
The eleven rims are too scattered to form a reliable discordia line, but one grain is concordant
with concordia age 1131 ± 18 Ma (MSWD=0.2). Four other rims are between 2 and 9%
discordant and their Pb-Pb ages are between 1075 and 1275 Ma. The rims are plotted in a
probability density plot together with the DC0760 Goedehoop rims in Fig 15.
data-point error ellipses are 68.3% conf.
0.45
2200
0.35
1400
0.25
206
Pb/238U
1800
1000
0.15
600
0.05
0
2
4
207
6
235
Pb/
8
U
Fig 13. The overall distribution for cores from samples DC0760 and DC0781, Goedehoop Quartzite.
22
9
8
7
Relative probability
Number
6
5
4
3
2
1
0
1000
1200
1400
1600
1800
2000
2200
Minimum Pb-Pb age
Fig 14. Probability density plot for cores, samples DC0760 and DC0781, Goedehoop Quartzite, using
both NordSIM ion probe and GEUS ICPMS data.
3
Relative probability
Number
2
1
0
1000
1050
1100
1150
1200
1250
1300
1350
1400
1450
Minimum Pb-Pb ages
Fig 15. Probability density plot for rims from the Goedehoop Quartzite, samples DC0760 and
DC0781, analyzed at both GEUS and NordSIM. Four spots are concordant, the remaining are
maximum 10% discordant.
23
3.4 Kenhardt migmatite DC0767
The Kenhardt Formation grains are euhedral and elongate, typically shown in Fig 8D, grains
38 and 33. In CL images the cores are somewhat zoned but also have a messy, undefined
structure (Fig 8D, spots 42a, 38a and 33). All grains have a grey rim, of varying thickness, see
Fig 8D, spots 42b and 33b.
Results from NordSIM ion probe
Only rims were analyzed because of equipment failure. All five rims form a discordia line
with upper intercept at 1190 ± 15 Ma and lower at 273 ± 45 Ma (MSWD=0.32), see Fig 16A.
One spot gives a concordia age 1200±15 Ma (MSWD=0.17), see Fig 16B.
data-point error ellipses are 68.3% conf.
0.24
data-point error ellipses are 68.3% conf.
0.216
A.
0.20
1200
B.
42b
0.212
1230
33b
U
238
0.16
1220
0.208
1210
Pb/
800
206
206
Pb/
238
U
1000
0.12
1200
0.204
1190
38b
600
All rims, intercepts at
273 ± 45 & 1190 ± 15 [±16] Ma
MSWD = 0.32
11b1
0.08
Rim 42b
Concordia Age = 1200 ±15 Ma
(2σ, decay-const. errs ignored)
MSWD (of concordance) = 0.17,
Probability (of concordance) = 0.68
1180
20b
400
0.200
1170
0.196
2.16
0.04
2.20
2.24
207
0.2
0.6
1.0
1.4
207
1.8
2.2
2.6
2.28
Pb/
2.32
2.36
235
U
235
Pb/ U
Fig 16. Results from NordSIM ion probe for metamorphic rims, sample DC0767, Kenhardt migmatite.
A. Discordia plot for all rims, 1190 ± 15 Ma. B. Concordia age for rim 42b, 1200 ± 15 Ma.
Results from GEUS ICPMS
Different from the other GEUS samples, the DC0767 data was reduced using the new inhouse software, Zirchron. Only spots that did not need common lead correction were used for
further interpretation. Nine cores could be used and their minimum Pb-Pb ages are shown in
Fig 17A. Four of these yield the concordia age 1205 ± 12 Ma, with the MSWD 0,0066 (Fig
17B).
24
data-point error ellipses are 2σ
1300
0.22
A.
38a
data-point error ellipses are 2σ
36a
1200
Cores 13a, 36a, 38a and 42a
Concordia Age = 1205 ±12 Ma
MSWD = 0.0066,
Probability = 0.94
0.216
13a
0.212
1100
1230
1a
B.
Pb/238U
0.208
1000
206
0.16
900
1210
0.204
1190
0.200
47a
1170
80a
0.14
1250
9a
11a1
0.18
206
Pb/238U
0.20
42a
0.196
0.12
1150
0.192
1.2
1.4
1.6
1.8
2.0
207
2.2
2.4
2.0
2.6
2.1
2.2
Pb/
2.3
207
Pb/
235
U
235
2.4
U
Fig 17. Results for detrital cores for DC0767 from GEUS ICPMS. A. shows the overall distribution
and B. shows concordant cores 13a, 36a, 38a and 42a that yield the concordia age 1205 ± 12 Ma.
All seven (common Pb-uncorrected) rims fitted a discordia line which yielded the intercept
age 1182±12 Ma (Fig 18A). The plot has a lower intercept at 16 ± 80 Ma, that is, it has
probably suffered recent lead loss. Two of these rims are concordant and give the age 1203 ±
16 Ma with MSWD 0.69 (Fig 18B).
data-point error ellipses are 2σ
0.26
A.
B.
42b
40b1
0.22
1240
0.212
47b
11b
33b
18b
1220
0.208
Pb/
238
1000
U
0.18
206
206
Pb/238U
1200
data-point error ellipses are 2σ
0.216
1400
0.14
800
600
0.10
1180
Intercepts at
16±80 & 1182±12 [±13] Ma
MSWD = 1.2
15b
1.0
1.4
1.8
207
2.2
2.6
3.0
Pb/235U
Rims 11b and 47b
Concordia Age = 1203 ±16 Ma
MSWD = 0.69,
Probability = 0.41
0.200
0.06
0.6
1200
0.204
0.196
2.14
2.18
2.22
2.26
207
2.30
2.34
2.38
235
Pb/
U
Fig 18. Results for rims DC0767 from GEUS ICPMS data. A. All rims give the discordia age 1182 ±
12 Ma. B. Rims 11 b and 47b give the concordia age 1203 ± 16 Ma.
3.5 Droëboom quartzite DC0768
The zircon grains are subhedral and of varying shape, from stubby to needlelike. About two
thirds of the grains have zoned cores surrounded by a rim, see Fig 8E grains 115 ans 12. There
are both CL grey and CL dark rims present but their ages overlap and separating them into
two different populations showed no significant difference. The difference in CL brightness
could depend on different U contents causing different degrees of lead loss. The CL-bright
25
rims are noticeably less discordant, so the dark rims could possibly have suffered more lead
loss, though still useful in a discordia plot. Sometimes entire grains consist of only the dark
rim material.
Results from NordSIM ion probe
Sixteen core spots were analyzed and one is concordant with concordia age 1813 ± 22 Ma
(MSWD=1.7). There are grains that are 3% discordant; 1878 ± 38 Ma (3.6%), 1885 ± 18 Ma
(3.1%) and 1939 ± 21 Ma (3.2%), all Pb-Pb ages. Another core spot is 8.0% discordant and
has the Pb-Pb age 1758 ± 18 Ma. The remaining 10 are too discordant to be useful, but are
shown together with the cores analyzed at GEUS in a concordia diagram in Fig 20.
Ten of 15 rims analysed form a discordia line with intercepts at 1041 ± 9 Ma and 210 ± 29 Ma
(MSWD=1.5), see Fig 19A. One of the omitted rims is concordant and has a concordia age
997 ± 13 Ma (MSWD=0.50). Two more rims are concordant, both on the discordia line, and
their concordia ages are 1012 ± 13 Ma (MSWD=0.71) and 1048 ± 16 Ma (MSWD=0.00073),
see Fig 19B. Three more rim grains are only 4-5% discordant and their Pb-Pb ages; 1029 ± 12
Ma, 1038 ± 12 Ma and 1048 ± 12 Ma, are within error of the rim discordia age.
data-point error ellipses are 68.3% conf.
0.26
A.
1300
0.22
data-point error ellipses are 68.3% conf.
42c2
0.181
1100
51b
117b4
500
0.06
117b1
58c2
300
1060
U
238
Pb/
41c
116b2
117b3
115b
117b2
206
700
0.10
0.179
43c2
27b30c2
42c 43c
900
0.14
206
Pb/
238
U
0.18
1070
B.
Rims
Excluded 35b, 9b, 7b, 117b and 58c
(marked with square in diagram).
Intercepts at
210 ± 29 & 1040.8 ± 8.5 [±10] Ma
MSWD = 1.5
1050
0.177
1040
0.175
0.173
0.171
1.74
Rim 42c
Concordia Age = 1048 ±16 Ma
MSWD = 0.00073,
Probability = 0.98
1030
1.76
1.78
1.80
207
1.82
0.02
0.2
0.6
1.0
1.4
207
1.8
235
Pb/
2.2
1.84
1.86
235
Pb/
U
2.6
U
Fig 19. Results for DC0768 rims, data from NordSIM ion probe. A. Discordia plot, 5 spots excluded,
yield the metamorphic age 1041 ± 9 Ma. B. Concordia age for rim 42b, 1048 ± 16 Ma.
Results from GEUS ICPMS
Three core grains out of the 20 analyzed are concordant and these have concordia ages 1714 ±
25 Ma (MSWD=0.86), 1727 ± 26 Ma (MSWD=0.35) and 1867 ± 19 Ma (MSWD=1.16). One
grain has the Pb-Pb age 1505 ± 32 Ma and is useful with the discordance of 4.1%. Seven
cores have Pb-Pb ages between 1750 and 1950 Ma with discordances between 4 and 10%.
These are displayed in Fig 20 and Fig 21 below, together with the cores analyzed at NordSIM.
Two out of the four analyzed rims were useful. One gives the concordia age of 1085 ± 15 Ma
(MSWD=0.20) and the other is only 0.8% discordant and have a Pb-Pb age of 1015 ± 20 Ma.
26
data-point error ellipses are 68.3% conf.
2600
0.5
2200
0.4
206
Pb/238U
1800
0.3
1400
0.2
1000
0.1 600
0.0
0
2
4
6
207
8
Pb/
10
12
235
U
Fig 20. The overall distribution of core ages for sample DC0768, from both GEUS ICPMS and
NordSIM ion probe.
5
4
Relative probability
Number
3
2
1
0
1400
1500
1600
1700
1800
1900
2000
2100
Minimum Pb-Pb age
Fig 21. Probability density plot of Pb-Pb ages for cores, sample DC0768, from both GEUS ICPMS
and NordSIM ion probe.
27
Table 2. Ages for U-Pb ion probe zircon data. All Pb-Pb ages have between 0-10% discordance and are considered minimum ages.
Sample Formation, rock type
Areachap terrane
DC0754 Bethesda, metapelitic gneiss
Kakamas terrane
DC0760 Goedehoop, quartzite
DC0781 Goedehoop, quartzite
DC0767 Kenhardt, metapelitic migmatite
Bushmanland terrane
DC0768 Droëboom, quartzite
Age (Ma)
Type of grain
1516±15
1945±45
1190±27
Detrital core
Detrital core
Metamorphic rims
1
1
6
Concordia
Pb-Pb age
Discordia
2.1
1305±23
1180-1280
1732 and 1802
1241±12
1241±11
Detrital core
Detrital cores
Detrital core
Metamorphic rim
Metamorphic rim
1
4
2
1
5
Concordia
Pb-Pb ages
Pb-Pb age
Concordia
Discordia
0.63
1166±18
1172±15
1200-1350 Ma
1669±25
1802±30
1887±10
1131±18
1075-1275
1190±15
1200±15
Detrital core
Detrital core
Detrital core
Detrital core
Detrital core
Detrital core
Metamorphic rim
Metamorphic rim
Metamorphic rim
Metamorphic rim
1
1
5
1
1
1
1
4
5
1
Concordia
Concordia
Pb-Pb
Pb-Pb
Pb-Pb
Pb-Pb
Concordia
Pb-Pb
Discordia
Concordia
0.115
0.87
1813±22
1670-1940
1041±9
1048±16
1012±13
997±13
1029, 1038 and 1048
Detrital core
Detrital cores
Metamorphic rims
Metamorphic rim
Metamorphic rim
Metamorphic rim
Metamorphic rims
1
5
15
1
1
1
3
Concordia
Pb-Pb ages
Discordia
Concordia
Concordia
Concordia
Pb-Pb ages
1.7
28
Spots used Isoplot regression
MSWD
1.4
0.49
0.56
Lower intercept (Ma)
101±82
247±21
0.2
0.32
0.17
273±45
1.5
210±29
0.00073
0.71
0.5
Table 3. Ages for U-Pb ICPMS zircon data. All Pb-Pb ages have between 0-10% discordance and are considered minimum ages.
Sample Formation, rock type
Areachap Terrane
DC0754 Bethesda, metapelitic gneiss
Kakamas Terrane
DC0760 Goedehoop, quartzite
DC0767 Kenhardt, metapelitic migmatite
Bushmanland Terrane
DC0768 Droëboom, quartzite
Age (Ma)
Type of grain
1339±20
1387±21
1532±30
1829±17
1940±26
1350-2100
1205±16
Detrital core
Detrital core
Detrital core
Detrital core
Detrital core
Detrital core
Metamorphic rim
1198±16
1221±23
1227±18
1262±22
1280±15
2021±16
1150-1350
1801±14
1114±20
1220±18
1192±18
1313±32
1205±12
1182±12
1203±16
1714±25
1727±26
1867±19
1505±32
1700-1900
1085±15
1015±20
Isoplot regression
MSWD
1
1
1
1
1
11
1
Concordia
Concordia
Concordia
Concordia
Concordia
Pb-Pb
Concordia
0.112
0.7
0.81
0.018
1.9
Detrital core
Detrital core
Detrital core
Detrital core
Detrital core
Detrital core
Detrital core
Detrital core
Metamorphic rim
Metamorphic rim
Metamorphic rim
Metamorphic rim
Detrital core
Metamorphic rim
Metamorphic rim
1
1
1
1
1
1
8
1
1
1
1
1
4
7
2
Concordia
Concordia
Concordia
Concordia
Concordia
Concordia
Pb-Pb
Pb-Pb
Concordia
Concordia
Pb-Pb
Pb-Pb
Concordia
Discordia
Concordia
0.26
0.2
0.031
1.3
0.27
1.4
Detrital core
Detrital core
Detrital core
Detrital core
Detrital core
Metamorphic rim
Metamorphic rim
1
1
1
1
7
1
1
Concordia
Concordia
Concordia
Pb-Pb
Pb-Pb
Concordia
Pb-Pb
0.86
0.35
1.16
29
Spots used
Lower intercept
(Ma)
0.048
0.91
1.6
0,0066
1,2
0,69
0.2
16±80
3.6 Comparison between NordSIM (ion probe) and GEUS (ICPMS) as
methods for zircon dating
Problems arise when using two different methods to analyze the same samples and the results
don’t entirely correspond. The different equipment has different analytical problems to deal
with which may bias the data in different directions. Unfortunately there is no room for a
detailed discussion about the reasons for differences, but the results of this work are compared
below.
21 spots were measured with both ion probe and ICPMS. Ten of these are less than 10%
discordant (both ion probe and ICPMS data) and can be used for comparing the two methods.
Using the strategy described earlier for whether the ICPMS samples should be corrected for
common lead, the following can be ascertained (see Fig 22). Four spots have significantly
younger Pb-Pb age by ICPMS whereas six spots overlap in age at the two-sigma confidence
level. Eight of the spots have larger errors by ICPMS and only two ion probe spots have
significantly larger errors.
The same comparison was also made with common lead correction for all the spots (see Fig
23). In this case, three spots were younger and one older by ICPMS and six overlap in age.
Two of the spots had the same 2σ errors and the remaining eight were higher by ICPMS.
The difference between Pb-Pb ages, by ICPMS, with and without common lead correction
commonly lies around 20-30 Ma, and a few with up to 80 Ma.
data-point error ellipses are 2σ
1900
NordSIM Pb-Pb ages
1700
1500
1300
1100
900
900
1100
1300
1500
1700
1900
GEUS Pb-Pb ages
Fig 22. NordSIM Pb-Pb ages with 2σ errors plotted against GEUS Pb-Pb ages with 2 σ errors . The
ICPMS data is this diagram is common lead corrected according to the rule stated earlier in this work,
all samples with f206% (common lead) over 0.400 are corrected and those with values below 0.400 are
not corrected. The diagonal line represents points that have the same Pb-Pb ages. The 2 σ errors are
displayed as error ellipses. The data used to make this digram can be found in Appendix 2.
30
data-point error ellipses are 2σ
1900
NordSIM Pb-Pb age
1700
1500
1300
1100
900
900
1100
1300
1500
1700
1900
GEUS Pb-Pb age
Fig 23. NordSIM Pb-Pb ages with 2σ errors (y-axix) plotted against GEUS Pb-Pb ages with 2 σ errors
(x-axix). All data in this diagram is common lead corrected. The diagonal line represents points that
have the same Pb-Pb ages. The 2 σ errors are displayed as error ellipses. The data used to make this
diagram is in Appendix 2.
31
4. DISCUSSION
4.1 Bethesda metapelitic Gneiss DC0754
The Bethesda Formation is regarded as a part of Areachap Group which makes up the
Areachap Terrane. In the work by Cornell et al, (2006), the Areachap Terrane is presented as
a juvenile Terrane consisting of ~1300 Ma arc-related supracrustals and younger granitoids.
Cornell and Pettersson (2007), zircon dated the Jannelsepan Formation, considered related to
Bethesda Formation, and got the following ages for detrital cores; 1254 ± 7 Ma (Pb-Pb age),
1275 ± 7 Ma (discordia) and 1301 ± 14 Ma (concordia age). Three rims in the same
publication give a discordia age of 1204 ± 50 Ma. In an article by Pettersson et al., (2007), a
magmatic core from Jannelsepan migmatite was dated to 1241 ± 12 Ma (concordia age) and a
metamorphic rim to1165 ± 10 Ma (concordia age). Another metamorphic rim for a
Jannelsepan biotite gneiss were dated in the same work to 1158 ± 12 Ma (concordia age).
According to the dating done in this work, the Bethesda Formation metasediment does not
appear to be derived from a 1300 Ma juvenile rock unit. There are six concordia ages that are
older than the previously published data, 1339 ± 20, 1387 ± 21, 1516 ± 15, 1532 ± 30, 1829 ±
17 and1940 ± 26 Ma. In addition, there are 13 spots with less than 10% discordance that have
Pb-Pb ages ranging between 1350-2100 Ma.
There are several possible explanations for this result. The most drastic one is that the
Areachap Terrane and Areachap Group are not all juvenile but were in contact with a craton,
hence the source of old zircons. A second explanation could be that the Bethesda Formation is
not a part of the Areachap Group. The possibility that this specific rock unit is not a part of the
Bethesda Formation seems very unlikely. The second explanation seems most likely as no
older zircons have thus far been foundin other Areachap Group Formations despite several
dating efforts.
The age for metamorphic rim growth is determined in this work at 1190±27 Ma (discordia
line) and 1205±16 Ma (Pb-Pb age). These correspond within error to the interpreted age of
collision in this area, ~1200 Ma (Pettersson et al, 2007) and between 1194±23 Ma and 1158±7
Ma by Cornell and Pettersson, (2007). Although slightly older than the other metamorphic
ages reported for the Areachap Group in this area, the rim age, together with the fact that most
grains show rims, shows that this rock unit formed before and participated in the collision.
4.2 Goedehoop Quartzite DC0760 and DC0781
Moen (2007), states that the Goedehoop quartzite formed as a unit at around 1600 Ma and this
would mean that no detrital zircons should be younger than 1600 Ma. Five Pb-Pb ages from
this work support this thesis; 1669 ± 25, 1732 ± 15, 1802 ± 11, 1802 ± 30 and 1887 ± 10 Ma.
There is also one spot with the concordia age 2021 ± 16 Ma. These detrital cores probably
originate from an old, unidentified basement underlying Kakamas Terrane. However, there
are several detrital cores with ages younger than 1600 Ma. Eight concordia ages on welldocumented detrital cores between 1160 and 1305 Ma and 13 Pb-Pb ages between 1200 and
1350 Ma indicate that there is also a younger component in the detrital material. Moen (2007)
also recognized this younger component, but explained it with isotopic resetting or new zircon
growth during the Namaquan event. It seems unlikely though, that these grains could have
been reset as the metamorphic grade is only amphibolite facies, whereas temperatures around
900ºC are required for resetting by diffusion. The older material might have been redistributed
32
during and mixed with newly formed material in s sedimentary cycle during the Namaquan
collision. According to this work, the Goedehoop Formation must have been deposited later
than 1166±18 Ma, that is after the ~ 1200 Ma collision event.
There is a reliable discordia age and also a concordia age from the ion probe analysis of
sample DC0760 that gives a metamorphic age 1241±12 Ma. Further there are numerous
concordia and Pb-Pb ages that date the rims to ages between 1075 and 1275, as seen in Fig 15.
Sample DC0781 was intended to confirm the 1240 Ma metamorphic age, but unfortunately
there is no consistent rim age in this sample. The ICPMS data for DC0760 could also not
confirm an age of metamorphism. Moen (2007) states that there are two events of
metamorphism at ~1300 Ma and ~1100 Ma, but these can not be distinguished as the spots are
so spread out and do not cluster. The 1240 Ma metamorphic event could have been an early
Namaquan deformation event which was then inherited from source rocks when the unit was
formed, as there are cores that are younger than 1240 Ma. The later rim ages could have
formed by thermal metamorphism due to late-tectonic intrusions in the area after the
formation was deposited.
4.3 Kenhardt Migmatite DC0767
Cores from Kenhardt migmatite samples have previously been dated to 1296 ± 14, 1351 ± 24
and 1568 ± 23 Ma by Cornell and Pettersson (2007). The same authors also got discordia ages
of 1197 ± 5 Ma for cores and 1194 ± 23 Ma for rims, which means there is an overlap
between cores and rims.
In this work five rim spots yielded the discordia age of 1190 ± 15 Ma and one rim spot gave a
concordia age of 1200 ± 15 Ma. The GEUS data, also in this work, dates the metamorphic
event to 1182 ± 12 Ma with support by concordia ages at 1203 ± 16 Ma. This coincides very
well with the previous work and it seems most likely that Namaquan metamorphism affected
this formation between 1190 and 1200 Ma.
Cores were also dated in this work with a cluser of concordia ages around 1205 ± 12 Ma.
These are within error of previously dated cores, and overlap with the metamorphic age,
confirming that metamorphism took place geologically soon after deposition.
4.4 Droëboom quartzite DC0768
Most cores from the Droëboom Quartzite cluster between 1714 and 1939 Ma. One core has a
Pb-Pb age at 1505 Ma. The suggestion that the Droëboom Group could belong to Kakamas
Terrane and be a part of Arribees Group is not supported by this work. Both of the Kakamas
samples have a younger component that the Droëboom quartzite cores lack, see Fig 24 below.
It is therefore more probable that Droëboom belongs to the Bushmanland Group. The
Droëboom Group has similarities to Bushmanland Group and the relationship between these
two could be examined further. If Moen’s (2007) statement that the young component in the
Goedehoop Quartzite is due to isotopic resetting, these would have a somewhat similar age
distribution curve. However this is thought to be unlikely.
A geochemical study by Fransson (2008), shows that there are similarities in geochemical
signatures between a Droëboom amphibolite and Kakamas metabasite samples. However,
very few samples are analyzed and more have to be investigated before drawing any proper
conclusions.
33
9
5
8
A.
B.
7
Number
2
Relative probability
Relative probability
3
6
Number
4
5
4
3
2
1
1
0
1400
1500
1600
1700
1800
1900
2000
2100
0
1000
1200
1400
1600
1800
2000
Minimum Pb-Pb ages
Minimum Pb-Pb age
Fig 24. Comparison between core ages in probability density plots for A. Droëboom Quartzite
(Bushmanland Terrane) and B. Kenhardt Migmatite and Goedehoop Quartzite (Kakamas Terrane).
Metamorphic rims in the Droëboom Quartzite are now dated to 1041 ± 9 Ma (discordia) and
there are a number of concordia and Pb-Pb rim ages ranging between 997 ± 13 Ma and 1085 ±
15. This shows that the metamorphism in this area occurred considerably later than in the
Kakamas and Areachap Terranes. It coincides well with the tectonic model suggested by
Cornell and Pettersson (2007) where the Bushmanland Terrane is the last to dock with the
South African Craton. The metamorphism at this time was low pressure, high temperature
during the late phase of the Namaquan deformation.
The zircon rims of the Droëboom Group also point to it belonging to the Bushmanland
Terrane. The relatively young metamorphic age of 1040 ±11 Ma coincides well with what is
earlier interpreted for Bushmanland group. Raith and Cornell (2000) state that the
Bushmanland Terrane was affected late in the Namaquan metamorphic cycle and gives a
metamorphic age of ~1030 Ma which agree with ~1020 by Robb et al (1999 ). The main
Kakamas Terrane metamorphism occurred earlier, at ~1200 Ma and the 1030 Ma event is not
seen in zircon data.
4.5 The methods
The laser ablation ICPMS method has two problems with zircon dating. Measuring and
correcting for common lead does not work well. The rapid penetration of the laser ablation
hole deep into the sample sometimes yields measurements from more than one age domain.
The latter makes it difficult to get good rim data from an ICPMS.
The GEUS facility is still developing software for data reduction and all the software bugs
have not been fixed at this stage (January 2008). One sample was reduced with the new
software, Zirchron, and when using only spots that did not need common lead correction,
these yield similar ages to NordSIM (compare rim data for Kenhardt migmatite, DC0767).
34
2200
5. CONCLUSIONS
According to this work, the Bethesda Formation does not appear to be a 1300 Ma, juvenile
formation as previously stated (Cornell et al, 2006). This is based on six concordia ages and
13 minimum Pb-Pb ages date older than 1300 Ma, ranging all the way up to 2100 Ma. This
leads either to the conclusion that Bethesda Formation is not a part of the juvenile Areachap
Terrane, or if Bethesda Formation definitely is part of Areachap Terrane, then the Areachap
Terrane can not be juvenile. The metamorphic ages date around 1200 Ma, which corresponds
to Namaquan deformation.
The statement by Moen (2007) that the Goedehoop Quartzite (Kakamas Terrane) formed at
around 1500 Ma, is questioned by the dating of well-documented detrital cores in this work,
much younger than 1500 Ma. Eight concordia ages and 13 minimum Pb-Pb ages date between
1160 and 1350 Ma. According to this work, the Goedehoop Quartzite, as a unit, was deposited
after the ~1200 Ma Namaquan collision. Rims have been dated to 1241 ± 12 (11) Ma, but
there are also a number of rims dating from 1075 to 1275. Rims older than 1160 Ma are
considered inherited from the source rock and the younger rims might have formed in situ by
thermal events.
Kenhardt migmatite (Kakamas Terrane) rims have been dated in this work and give an age of
metamorphism between 1182 ± 12 and 1203 ± 16 Ma. This age coincides well with previous
dating and the age corresponds to the age of Namaquan collision. Cores have also been dated
and overlap with the metamorphic age with concordia ages at 1205 ± 12 Ma.
Most dated cores in the Droëboom Quartzite (Bushmanland Terrane) have ages between
1700-1900 Ma. A few date around 1500 Ma but there is no younger component present. This
disproves the suggestion that Droëboom Group could belong to the Kakamas Terrane which
has a different age distribution. The late metamorphic age of 1041 ± 9 Ma also supports this,
the Kakamas Terrane was affected considerably earlier according to both this and previous
work.
35
Acknowledgements
I want to thank my supervisor David Cornell at Gothenburg University for invaluable help
with both sample preparation, writing the report and all the fundings. Also big thanks for the
fantastic field trip to South Africa in August 2007. Thank you PhD. student Åsa Pettersson for
help with “lite av varje” and good answers to stupid questions. Thank you Ali Firoozan for
help with producing the thin sections.
Thank you to Martin Whitehouse and the staff at the NordSIM facility, Museum of Natural
History, Stockholm, for help with the ion probe.
Thank you to Dirk Frei and Anders Scherstén at GEUS, Copenhagen, for help with the
ICPMS.
36
REFERENCES
Cornell, D.H. and Thomas, R.J., 2006: Age and tectonic significance of the Banana Beach
gneiss, KwaZulu – Natal South Coast, South Africa. South African Journal of Geology 109,
pp 335-340.
Cornell, D.H., Thomas, R.J., Gibson, R., Moen, H.F.G. and Reid, D.L., 2006: Namaqua-Natal
Province in Johnson, M.R., Anhanesser, C.R., Thomas, R.J. (eds) Geology of South Africa.
Geological Society of South Africa and Council Geoscience, Pretoria, pp 325-379.
Cornell, D.H., and Pettersson, Å., 2007: Ion probe zircon dating of metasediments from the
Areachap and Kakamas Terranes, Namaqua-Natal province and the stratigraphic integrity of
the Areachap Group. South African Journal of Geology 110, pp 169-178.
De Beer, J.H. and Meyer, R., 1984: Geophysical characteristics of the Namaqua-Natal Belt
and its boundaries, South Africa. J. Geodyn. 1, pp 473-494.
Fransson, M., 2008: A geochemical investigation of metabasites from Kakamas and
Bushmanland Terranes, Namaqua Province, South Africa. Mater thesis, Göteborgs
Universitet. In prep.
Frei, D., Hollis, J.A., Gerdes, A., Harlov, D., Karlsson, C., Vasques, P., Franz, G., Johansson,
L., Knudsen, C. 2006: Advanced in-situ trace element and geochronological microanalysis of
geomaterials by laser ablation techniques. Geological Survey of Denmark and Greenland
Bulletin 10, 25-28.
Gerdes, A. and Zeh, A., 2006: Combined U-Pb and Hf isotope LA-(MC)-ICP-MS analyses of
detrital zircons: Comparison with SHRIMP and new constraints for the provenance and age of
an Armorican metasediment in Central Germany. Earth and Planetary Science Letters 249, pp
47-61.
Geringer, G.J., Botha, B.J.V. and Slabbert, M.J., 1988: The Keimoes Suite – a composite
granitoid batholith along the eastern margin of the Namaqua mobile belt, South Africa. South
African Journal of Geology 91, pp 465-476.
Geringer, G.J., Humphreys, H.C. and Scheepers, D.J., 1994: Lithostratigraphy , protolithology
and tectonic setting of the Areachap Group along the eastern margin of the Namaqua mobile
belt, South Africa. South African Journal of Geology 97, pp 78-100.
Gibson, R.L., Robb, L.J., Kisters, A.F.M. and Cawthorn, R.G., 1996: Regional setting and
geological evolution of the Okiep Copper District, Namaqualand, South Africa. South African
Journal of Geology 99, pp 107-120.
Hartnady, C.J., Joubert, P. and Stowe, C.W., 1985: Proterozoic crustal evolution in
Southwestern Africa. Episodes 8, pp 236-244.
Moen, H.F.G., 2007: The geology of the Upington area, explanation sheet 2820. !: 250 000.
Council for Geoscience South Africa.
37
Moore, J., M., 1977: The geology of the Namiesberg, Northern Cape. Bullentin of the
Precambrian Research Unit, University of Cape Town, South Africa 20, 69pp.
Praekelt, H.E., 1984: Die geologie van die gebied rondom Augrabies (2820 C). M Sc thesis,
University of the Orange Free State, Bloemfontein, 80 pp. Unpublished.
Raith, C.G. and Cornell, D.H., 2000: The first U-Pb ion probe zircon ages from a migmatitic
metapelite, central zone, western Namaqualand, South Africa. Abstr. 18th Colloq. Afr. Geol. –
J. Afr. Earth Sci. 30 (4A), pp 73.
Raith, J.G., Cornell, D.H., Frimmel, H.E. and De Beer, C.H., 2003: New insights into the
geology of the Namaqua tectonic province, South Africa, from ion probe dating of detrital and
metamorphic zircon. Journal of Geology 111, pp 347-366.
Robb, L.J., Armstrong, R.A. and Waters, D.J., 1999: The history of granulite-facies
metamorphism and crustal growth from single zircon U-Pb geochronology; Namaqualand,
South Africa. Journal of Petrology 40, pp 1747-1770.
Rollinson, H., 1993. Using geochemical data: evaluation, presentation, interpretation. 1st
edition, Pearson Education Ltd, 352pp.
Stockwell, C.H., McGlynn, J.C., Emslie, R.F., Sanford, B.V., Norris, A.W., Donaldson, J.A.,
Fahrig, W.F. and Currie, K.L., 1970: Geology of the Candian Shield. Douglas, R.J.W. (ed.),
Geology and economic minerals of Canada. Econ. Geol. Rep. Geol. Surv. Can. 1, pp 43-150.
Thomas, R.J., Agenbacht, A.L.D., Cornell, D.H., and Moore, J.M., 1994a: The late Kibaran of
southern Africa: tectonic evolution and metallogeny. Ore geology Rev. 9, pp 131-160.
Thomas, R., J., Bühmann, D., Bullen, W.,D., Scogings, A., J. and De Bruin, D., 1994b:
Unusual spodumene pegmatites from the Kiberan of Natal, South Africa. Ore Geology Rev. 9,
pp 161-182.
Vajner, V., 1978b: The Jannelsepan Formation: Annual Report. Precambrian Research Unit,
Univeristy of Cape Town, 14 and 15, pp 26-40.
Waters, D. J., 1986: Metamorphic zonation and thermal history of politic gneisses from
western Namaqualand, South Africa. Trans. Geol. Soc. S. Afr. 89, pp 97-102.
Waters, D.J., 1989: Metamorphic evidence for the heating and cooling path of Namaqualand
granulites. In: Daly, J.S., Cliff, R.A. and Yardley, W.D. (eds.), Evolution of Metamorphic
belts. Geol. Soc. Spec. Publ. 43, pp 357-363.
Whitehouse, M.J., Kamber, B.S. AND Moorbath, S., 1999: Age significance of U-Th-Pb
zircon data from early Archean rocks of West Greenland; a reassessment based on combined
ion-microprobe and imaging studies. Chemical Geology 160, pp 201-224.
Whithouse, M.J., and Kamber, B.S., 2005: Assigning dates to thin gneissic veins in highgrade metamorphic terranes: a cautionary tale from Akilia, Southwest Greenland. Journal of
Petrology 46, pp 291-318.
38
APPENDICES
Appendix 1.
Data for Bethesda Gneiss, Godehoop Quartzite, Kenhardt Migmatite and Droëboom Quartzite
from ion probe and ICPMS zircon isotope dating.
Isotope ratios
Spot
207
Pb
206
Pb
±σ%
Ages (Ma)
207
Pb
235
U
±σ%
206
Pb
238
U
±σ%
Discordance%
207
Pb
206
Pb
±σ
Concentrations
207
Pb
235
U
±σ
206
Pb
238
U
±σ
[U]
[Pb]
Th/U
ppm
ppm
meas.
f206 %
DC0754 Bethesda Gneiss
NordSIM ion probe
1a
0.0999
0.32
3.2050
1.06
0.2326
1.01
-18.73
1623
6
1458
8
1348
12
1468
419
0.382
0.09
5a
0.0948
0.50
3.4249
1.09
0.2621
0.97
-1.71
1524
9
1510
9
1500
13
349
108
0.291
* 0.01
9a
0.1192
1.28
5.2724
1.61
0.3207
0.98
-8.91
1945
23
1864
14
1793
15
99
40
0.471
2.37
32a
0.1139
0.82
3.3581
1.33
0.2138
1.05
-36.20
1863
15
1495
10
1249
12
328
93
0.433
1.92
86b
0.0946
0.86
2.6044
1.87
0.1996
1.66
-24.96
1521
16
1302
14
1173
18
1080
239
0.037
0.10
1b
0.0797
0.31
2.1249
1.10
0.1934
1.06
-4.60
1190
6
1157
8
1140
11
1536
321
0.003
0.23
4b
0.0791
2.41
1.4656
2.60
0.1344
0.99
-32.75
1174
47
916
16
813
8
2016
296
0.007
2.48
32b
0.0789
0.39
1.6169
1.12
0.1486
1.05
-25.28
1170
8
977
7
893
9
1576
254
0.010
0.25
35b
0.0775
0.29
1.6725
1.01
0.1565
0.97
-18.72
1135
6
998
6
937
8
2016
340
0.003
0.04
13b
0.0802
0.69
1.8418
1.77
0.1666
1.63
-18.72
1202
14
1061
12
993
15
1576
286
0.005
0.71
15b
0.0773
6.72
1.7220
7.09
0.1615
2.27
-15.70
1130
128
1017
47
965
20
1546
[ 279]
0.061
2.13
47b
0.0791
0.27
1.7490
1.63
0.1605
1.61
-19.63
1173
5
1027
11
959
14
1861
322
0.002
0.01
65b
0.0788
0.28
1.8250
1.75
0.1680
1.72
-15.32
1167
6
1054
12
1001
16
1885
342
0.001
0.05
0.14
GEUS ICPMS
1a
0.1018
1.06
3.7522
3.20
0.2673
3.02
10.30
1671
20
1583
51
1527
46
571
687
0.645
5a
0.0954
0.87
3.4384
3.29
0.2613
3.17
4.60
1550
16
1513
50
1496
47
127
134
0.859
2.10
4a
0.1219
0.53
5.9260
1.48
0.3525
1.38
3.68
1998
9
1965
29
1946
27
134
692
0.597
1.23
7a
0.0880
0.64
2.9454
1.68
0.2428
1.55
0.28
1396
12
1394
23
1401
22
373
335
0.558
1.04
10a
0.1026
0.69
3.5485
2.77
0.2508
2.68
16.64
1685
13
1538
43
1443
39
335
310
0.900
0.91
14a
0.1016
1.08
3.8230
1.99
0.2729
1.67
8.16
1667
20
1598
32
1556
26
310
328
0.289
0.20
15a
0.0861
0.62
2.7301
1.66
0.2299
1.54
2.45
1355
12
1337
22
1334
21
328
965
1.208
0.19
21a
0.1094
0.58
4.7129
1.56
0.3124
1.45
3.89
1803
11
1770
28
1752
25
100
419
1.395
0.29
30a1
0.0798
1.50
1.9946
2.05
0.1814
1.41
12.43
1205
29
1114
23
1074
15
419
389
3.354
1.18
30a2
0.0800
0.59
2.0517
1.97
0.1860
1.88
10.69
1211
12
1133
22
1100
21
389
415
1.081
0.88
30a3
0.0813
0.61
2.0492
1.84
0.1828
1.74
14.69
1243
12
1132
21
1082
19
415
0
1.101
0.56
33a
0.1017
1.21
4.9833
1.57
0.3555
1.00
19.48
1668
22
1816
29
1961
20
111
124
2.189
0.45
17a
0.1023
1.74
2.6999
2.28
0.1914
1.46
36.19
1680
32
1328
30
1129
17
124
219
0.650
0.75
19a
0.1095
0.68
4.5853
1.95
0.3037
1.83
6.66
1804
12
1747
34
1710
31
219
669
0.361
0.40
34a
0.0791
0.59
1.8052
1.06
0.1656
0.88
18.72
1188
12
1047
11
988
9
669
218
0.671
1.03
69a
0.1261
1.75
5.6344
3.66
0.3241
3.21
14.36
2057
31
1921
70
1810
58
218
1010
0.208
0.33
76a
0.0870
0.39
2.2497
1.06
0.1876
0.99
21.61
1374
7
1197
13
1108
11
651
32
0.741
0.30
77a
0.1162
0.73
6.0488
1.12
0.3775
0.85
8.56
1912
13
1983
22
2065
18
32
188
1.505
0.36
81a
0.1163
1.12
6.4485
1.50
0.4023
1.00
15.65
1912
20
2039
31
2180
22
188
469
0.387
0.05
80a
0.1022
0.82
2.6302
1.33
0.1866
1.05
37.72
1679
15
1309
17
1103
12
429
1347
0.688
2.04
86a
0.1201
1.01
5.7487
1.52
0.3472
1.14
3.56
1970
18
1939
29
1921
22
43
523
1.025
-0.15
88a
0.1106
0.39
4.0234
1.02
0.2638
0.95
19.83
1822
7
1639
17
1509
14
523
126
1.007
0.22
105a
0.1118
0.50
5.0667
1.36
0.3287
1.26
1.34
1842
9
1831
25
1832
23
126
312
0.258
-0.08
100a
0.1131
0.71
5.0707
1.40
0.3251
1.21
3.70
1863
13
1831
26
1814
22
312
616
0.701
1.09
110a
0.0786
0.86
1.1762
1.17
0.1085
0.79
46.25
1177
17
790
9
664
5
631
506
0.154
2.78
111a
0.0794
0.40
2.2986
1.18
0.2100
1.11
2.25
1196
8
1212
14
1229
14
506
120
0.227
0.58
112a
0.1133
0.66
5.6621
1.72
0.3626
1.59
7.25
1866
12
1926
33
1994
32
120
129
1.268
0.18
113a
0.0877
0.52
2.8157
1.12
0.2329
0.99
3.85
1389
10
1360
15
1350
13
129
50
0.636
0.18
130a
0.1091
2.04
4.3389
2.30
0.2884
1.05
11.00
1798
37
1701
39
1633
17
50
50
0.743
12.96
132a
0.1091
2.04
4.3467
2.30
0.2889
1.05
10.84
1798
37
1702
39
1636
17
50
561
0.741
12.96
134a
0.0908
0.44
2.4947
1.59
0.1993
1.53
21.87
1456
8
1271
20
1172
18
561
59
0.339
0.24
139a
0.1303
0.64
7.2439
1.12
0.4033
0.92
3.13
2114
11
2142
24
2184
20
59
63
0.690
0.25
39
Isotope ratios
Spot
207
Pb
206
Pb
±σ%
Ages (Ma)
207
Pb
235
U
±σ%
206
Pb
238
U
±σ%
Discordance%
207
Pb
206
Pb
±σ
Concentrations
207
Pb
235
U
±σ
206
Pb
238
U
±σ
[U]
[Pb]
Th/U
ppm
ppm
meas.
f206 %
140a
0.0810
1.47
2.4261
2.72
0.2171
2.29
1.91
1237
29
1250
34
1267
29
63
918
0.287
2.26
1b
0.0803
0.49
2.2825
1.50
0.2062
1.42
1.63
1219
10
1207
18
1208
17
687
127
0.012
0.10
4b
0.0819
0.64
1.8756
1.47
0.1662
1.32
23.33
1256
13
1072
16
991
13
692
373
0.068
0.50
15b
0.0764
0.40
1.3952
1.66
0.1324
1.61
30.70
1120
8
887
15
802
13
965
100
0.089
0.78
31b
0.0786
0.57
1.7431
1.15
0.1608
0.99
20.25
1177
11
1025
12
961
10
659
111
0.014
0.66
65b
0.0755
0.56
1.3846
1.57
0.1330
1.47
28.79
1097
11
882
14
805
12
1010
781
-0.011
0.60
68b
0.0772
0.49
1.7525
1.65
0.1646
1.58
15.69
1142
10
1028
17
982
15
781
651
0.026
0.60
81b
0.0814
0.42
2.6634
0.96
0.2373
0.86
10.52
1246
8
1318
13
1372
12
469
429
0.042
0.25
80b
0.0719
0.46
1.1550
2.23
0.1166
2.18
30.75
996
9
780
17
711
15
1347
43
0.119
0.54
100b
0.0887
0.83
2.5627
1.30
0.2096
1.00
14.99
1411
16
1290
17
1227
12
616
2958
0.086
0.88
104b
0.0630
1.59
0.3558
2.33
0.0410
1.70
65.66
722
34
309
7
259
4
2958
881
0.067
5.55
106b
0.0746
0.64
1.2877
1.18
0.1251
0.99
31.40
1073
13
840
10
760
7
881
631
0.004
1.24
DC0760 Goedehoop Quartzite
NordSIM ion probe
127a
0.1047
0.37
3.9638
1.10
0.2746
1.04
-9.52
1709
7
1627
9
1564
14
647
198
0.064
0.60
128a
0.1060
0.42
4.3381
1.35
0.2967
1.28
-3.77
1732
8
1701
11
1675
19
539
200
0.510
0.19
124a
0.1075
0.47
3.6308
1.25
0.2449
1.16
-21.89
1758
9
1556
10
1412
15
222
66
0.544
0.17
50a
0.1102
0.29
4.6744
1.13
0.3077
1.09
-4.61
1802
5
1763
9
1729
17
405
163
0.665
0.03
54a
0.0852
1.18
2.6245
1.65
0.2233
1.16
-1.76
1321
23
1308
12
1300
14
55
16
0.893
{0.06}
56c
0.0794
0.73
1.9861
1.33
0.1814
1.11
-9.89
1182
14
1111
9
1075
11
428
88
0.175
0.76
29b
0.0811
0.57
1.9572
1.30
0.1750
1.16
-16.28
1224
11
1101
9
1040
11
904
205
0.822
1.71
127c
0.0836
0.38
2.5947
1.43
0.2251
1.38
2.19
1283
7
1299
11
1309
16
412
103
0.095
{0.03}
127b
0.0842
0.32
2.5284
1.11
0.2179
1.06
-2.14
1296
6
1280
8
1271
12
520
126
0.108
0.02
30a
0.0845
0.95
1.9918
1.42
0.1710
1.06
-23.75
1304
18
1113
10
1017
10
250
52
1.299
0.78
29a
0.0802
1.04
2.3556
1.47
0.2129
1.04
3.77
1203
20
1229
11
1244
12
662
163
0.237
1.11
50b
0.1002
0.82
3.5988
1.36
0.2606
1.08
-9.23
1627
15
1549
11
1493
14
254
81
0.426
0.07
109a
0.0817
0.34
2.4101
1.30
0.2139
1.25
0.98
1239
7
1246
9
1250
14
691
161
0.022
0.04
56a
0.0817
0.35
2.3003
1.16
0.2041
1.11
-3.66
1239
7
1212
8
1198
12
754
174
0.168
0.27
94a
0.0634
1.60
0.5104
2.00
0.0584
1.20
-50.64
721
34
419
7
366
4
4099
286
0.334
11.53
56b
0.0690
1.60
0.8898
1.99
0.0936
1.18
-37.40
898
33
646
10
577
7
2487
262
0.107
6.42
51a
0.0716
1.42
0.8095
1.76
0.0820
1.04
-49.73
974
29
602
8
508
5
2933
282
0.462
6.56
49a
0.0754
0.62
1.7215
1.44
0.1655
1.30
-9.22
1080
12
1017
9
987
12
896
162
0.064
0.30
130a
0.0758
0.70
1.2219
1.94
0.1169
1.81
-36.52
1090
14
811
11
713
12
3009
388
0.056
3.81
43a
0.0785
0.42
2.0593
1.77
0.1902
1.72
-3.55
1160
8
1135
12
1122
18
639
142
0.336
0.19
GEUS ICPMS
2a
0.0808
0.97
2.1080
1.34
0.1893
0.92
10.55
1230
19
1151
15
1118
10
66
721
0.971
0.08
7a
0.0792
0.79
1.9623
1.47
0.1796
1.25
12.20
1192
16
1103
16
1065
13
721
148
0.737
1.41
8a
0.0777
1.09
1.9063
1.73
0.1779
1.34
9.88
1154
22
1083
19
1056
14
148
546
1.767
1.04
10a
0.0795
0.60
1.9257
1.35
0.1758
1.20
14.54
1198
12
1090
15
1044
13
546
192
0.399
1.22
15a
0.0800
1.02
2.5066
1.32
0.2274
0.84
9.32
1210
20
1274
17
1321
11
192
280
0.416
1.60
21a
0.0792
0.68
2.2153
1.78
0.2027
1.64
0.95
1193
13
1186
21
1190
20
427
216
0.214
0.64
22a
0.0836
0.58
2.5252
1.08
0.2190
0.91
2.57
1298
11
1279
14
1276
12
236
0
0.416
0.38
30a1
0.0802
1.02
1.9835
2.04
0.1793
1.76
14.32
1217
20
1110
23
1063
19
437
299
1.455
1.69
30a2
0.0851
0.67
2.2804
2.16
0.1943
2.06
15.94
1332
13
1206
26
1145
24
299
220
0.654
0.31
31a
0.0822
0.54
2.3491
1.80
0.2073
1.72
5.00
1264
11
1227
22
1214
21
220
262
0.481
0.03
32a
0.0830
0.43
2.2155
1.62
0.1937
1.56
12.66
1283
8
1186
19
1141
18
262
136
0.622
0.23
33a
0.0830
0.67
2.4345
1.86
0.2127
1.74
4.17
1284
13
1253
23
1243
22
136
453
0.537
0.68
36a
0.0827
0.51
2.3781
1.64
0.2085
1.56
5.46
1277
10
1236
20
1221
19
453
517
0.592
0.59
37a
0.0809
0.82
2.0759
1.85
0.1861
1.67
12.32
1233
16
1141
21
1100
18
517
215
0.562
2.71
50a
0.1093
0.38
4.6913
1.72
0.3113
1.68
4.12
1801
7
1766
30
1747
29
215
1398
1.163
0.11
54a
0.0840
1.29
2.3726
2.01
0.2049
1.55
9.45
1306
25
1234
25
1201
19
63
226
1.024
0.60
58a
0.0800
0.45
2.2684
1.64
0.2057
1.58
1.10
1210
9
1203
20
1206
19
226
870
0.780
0.11
61a
0.0812
0.52
2.3524
1.60
0.2101
1.51
1.65
1240
10
1228
20
1230
19
189
354
0.644
0.08
76a
0.0819
0.51
2.2182
1.58
0.1965
1.49
9.34
1257
10
1187
19
1157
17
354
266
0.257
0.02
77a
0.1080
0.65
4.1291
2.88
0.2772
2.81
13.47
1780
12
1660
48
1577
44
266
161
0.719
1.44
83a
0.0812
0.81
2.3224
1.74
0.2074
1.54
2.97
1241
16
1219
21
1215
19
161
224
0.884
1.69
90a
0.0845
0.60
2.8101
1.61
0.2411
1.50
5.39
1319
12
1358
22
1392
21
224
378
0.797
0.52
98a
0.0794
1.03
1.6989
4.02
0.1551
3.89
24.52
1197
20
1008
41
930
36
378
187
1.274
2.52
93a
0.0762
1.02
1.3527
1.96
0.1288
1.67
32.21
1114
20
869
17
781
13
517
101
0.185
6.74
40
Isotope ratios
Spot
207
Pb
206
Pb
Ages (Ma)
±σ%
207
Pb
235
U
±σ%
206
Pb
238
U
±σ%
Discordance%
207
Pb
206
Pb
±σ
Concentrations
207
Pb
235
U
±σ
206
Pb
238
U
±σ
[U]
[Pb]
Th/U
ppm
ppm
meas.
f206 %
96a
0.0799
0.79
2.0357
1.69
0.1849
1.50
10.93
1208
16
1128
19
1094
16
101
580
0.692
0.07
99a1
0.0829
0.73
2.1693
1.75
0.1897
1.59
14.34
1281
14
1171
20
1120
18
580
207
1.033
0.39
99a2
0.0809
0.53
2.0973
1.58
0.1880
1.49
11.43
1233
10
1148
18
1111
17
207
323
0.897
0.23
103a
0.0825
1.36
3.0205
2.06
0.2656
1.55
20.94
1271
26
1413
29
1518
24
323
142
0.584
0.40
105a
0.1247
0.48
6.2142
1.71
0.3615
1.64
3.41
2037
9
2006
34
1989
33
142
627
0.842
0.01
111a
0.0791
0.53
1.7572
1.61
0.1611
1.52
21.00
1189
10
1030
17
963
15
627
293
0.399
1.54
0.17
128a
0.1039
0.45
3.7862
1.65
0.2643
1.59
13.49
1708
8
1590
26
1512
24
293
939
0.748
3b
0.0843
0.84
2.4706
1.53
0.2126
1.28
6.62
1314
16
1263
19
1242
16
280
427
0.303
0.33
21b
0.0763
0.81
1.9982
1.46
0.1900
1.22
0.22
1116
16
1115
16
1122
14
216
217
0.415
1.10
19b
0.0814
0.64
2.3136
1.37
0.2061
1.21
3.96
1245
13
1216
17
1208
15
217
236
0.966
0.08
51b
0.0692
1.11
0.6969
2.00
0.0731
1.67
52.66
919
23
537
11
455
8
1398
341
0.213
9.54
52b
0.0704
1.39
1.3571
2.06
0.1398
1.52
13.08
955
28
871
18
843
13
341
332
0.959
7.15
49b
0.0793
0.46
2.0550
1.55
0.1880
1.48
8.16
1193
9
1134
18
1111
16
332
63
0.536
0.23
63b
0.0760
0.96
1.2708
2.53
0.1212
2.34
35.94
1110
19
833
21
738
17
870
189
0.250
8.67
83b
0.0839
0.52
2.2862
1.55
0.1976
1.45
12.51
1304
10
1208
19
1163
17
187
145
0.981
0.24
89b
0.0827
0.71
2.0162
1.69
0.1769
1.53
19.73
1275
14
1121
19
1050
16
145
517
0.779
0.42
0.22
DC0767 Kenhardt Migmatite
NordSIM ion probe
11b1
0.0704
1.26
0.7705
3.24
0.0813
2.94
-45.25
892
28
580
14
504
14
2547
223
0.027
20b
0.0984
1.23
1.0562
2.98
0.1058
1.64
-36.81
998
50
732
16
648
10
1204
143
0.145
3.4
33b
0.0784
0.36
1.8694
1.64
0.1733
1.6
-11.51
1153
7
1070
11
1030 15
2032
405
0.244
0.02
38b
0.0863
3.39
1.0333
6.04
0.1041
1.63
-37.00
986
114
721
32
638
1586
191
0.291
1.87
42b
0.0805
0.4
2.2718
1.7
0.2058
1.64
0.74
1198
8
1204
12
1206 18
818
199
0.336
10
GEUS ICPMS
0.05
*
1a
0.0775
0.00
1.9064
0.07
0.1783
0.01
2.36
1135
29
1083
23
1058 31
550
0.178
9a
0.0817
0.00
2.1428
0.1
0.1903
0.01
3.43
1238
63
1163
31
1123 32
138
0.19
11179
8449
11a1
0.0748
0.00
1.8221
0.07
0.1767
0.01
0.42
1063
29
1053
25
1049 34
323
0.177
7354
13a
0.0805
0.00
2.2605
0.08
0.2037
0.01
0.43
1209
38
1200
24
1195 31
137
0.203
16583
36a
0.0779
0.00
2.1811
0.09
0.2031
0.01
-1.44
1144
50
1175
29
1192 37
373
0.203
933.4
38a
0.0796
0.00
2.2916
0.08
0.2088
0.01
-1.05
1187
42
1210
23
1222 28
129
0.209
31734
42a
0.0811
0.00
2.3018
0.08
0.2059
0.01
0.48
1223
24
1213
24
1207 35
179
0.206
6519
21005
47a
0.0761
0.00
1.5313
0.08
0.1460
0.01
6.83
1097
30
943
34
879
43
867
0.146
11b
0.0802
0.00
2.2821
0.07
0.2063
0.01
-0.19
1203
28
1207
23
1209 32
305
0.206
23206
15b
0.0797
0.00
1.0796
0.09
0.0982
0.01
18.75
1190
61
743
46
604
47
342
0.098
6507
18b
0.0787
0.00
1.9847
0.08
0.1830
0.01
2.43
1164
32
1110
29
1083 39
339
0.183
49407
33b
0.0784
0.00
2.0360
0.07
0.1882
0.01
1.40
1158
32
1128
23
1112 31
653
0.188
22090
40b1
0.0792
0.00
2.3220
0.08
0.2127
0.01
-1.97
1177
34
1219
24
1243 32
397
0.212
32234
42b
0.0796
0.00
2.5910
0.06
0.2360
0.01
-5.21
1188
21
1298
18
1366 26
264
0.236
99999
47b
0.0795
0.00
2.2641
0.08
0.2065
0.01
-0.74
1185
37
1201
25
1210 34
234
0.206
18242
10c
0.0788
0.00
2.3691
0.12
0.2181
0.01
-3.14
1166
40
1233
37
1272 55
610
0.218
15606
18c
0.0655
0.01
0.3074
0.03
0.0340
0
20.77
792
174
272
24
216
2836
0.034
49.99
19c
0.0802
0.00
2.1522
0.09
0.1947
0.01
1.63
1201
45
1166
28
1147 34
442
0.195
4789
52c
*206Pb
0.0572
0.00
0.3941
0.02
0.0500
0
6.81
499
89
337
12
314
4
1654
0.05
359
204
11
Pb
DC0768 Droëboom Quartzite
NordSIM ion probe
49a
0.0677
0.99
1.0007
1.39
0.1072
0.98
-24.77
859
20
704
7.1
657
6.1
3041
361
0.386
1.86
30a
0.0739
0.49
1.8234
1.11
0.1789
1.00
2.24
1039
10
1054
7.3
1061 9.7
2066
401
0.047
0.15
24a
0.0976
0.85
0.9375
1.60
0.0697
1.35
-74.90
1579
16
672
7.9
434
911
83.4
0.449
0.24
14a
0.1011
0.50
3.4318
1.11
0.2463
0.99
-15.21
1644
9
1512
8.8
1419 12.6
5.7
528
151
0.208
0.03
0.02
8a
0.1020
0.56
3.6297
1.12
0.2581
0.98
-12.13
1660
10
1556
9.0
1480 12.9
490
168
0.695
14c
0.1026
0.40
3.8380
1.04
0.2714
0.96
-8.32
1671
7
1601
8.4
1548 13.2
806
248
0.108
0.03
2a
0.1075
0.51
4.2775
1.08
0.2885
0.96
-8.00
1758
9
1689
9.0
1634 13.9
273
95.3
0.293
0.07
11a
0.1100
0.83
4.9754
1.29
0.3280
0.99
1.83
1800
15
1815
11.0
1829 15.8
89.8
41.5
1.017
{0.01}
7a
0.1121
0.56
3.4121
1.11
0.2207
0.95
-32.92
1834
10
1507
8.7
1286 11.1
245
72
0.838
0.67
44a
0.1149
1.38
3.4876
1.68
0.2202
0.96
-34.89
1878
25
1524
13.4
1283 11.1
179
55.6
1.336
0.32
115a
0.1149
1.05
5.1649
1.42
0.3260
0.95
-3.61
1878
19
1847
12.1
1819 15.1
61.8
29.1
1.083
0.09
5a
0.1154
0.51
5.5733
1.20
0.3504
1.08
3.13
1886
9
1912
10.3
1936 18.1
142
69.7
0.960
{0.02}
41
Isotope ratios
Spot
207
Pb
206
Pb
±σ%
Ages (Ma)
207
Pb
235
U
±σ%
206
Pb
238
U
±σ%
Discordance%
207
Pb
206
Pb
±σ
Concentrations
207
Pb
235
U
±σ
206
Pb
238
U
±σ
[U]
[Pb]
Th/U
ppm
ppm
meas.
f206 %
51a
0.1175
1.80
3.2166
2.14
0.1986
1.16
-42.69
1918
32
1461
16.7
1168 12.4
172
46.2
0.869
26a
0.1188
0.59
5.5629
1.17
0.3395
1.01
-3.24
1939
11
1910
10.1
1884 16.5
413
161
0.137
1.05
0.04
16b
0.1673
0.59
9.3880
1.12
0.4071
0.95
-15.31
2530
10
2377
10.3
2202 17.7
601
305
0.296
0.10
1a2
0.1051
0.42
3.4575
1.48
0.2385
1.42
-21.84
1717
8
1518
11.7
1379 17.6
743
234
0.683
0.10
35b
0.0666
0.98
0.9104
1.36
0.0991
0.95
-27.57
827
20
657
6.6
609
5.5
2853
313
0.157
1.65
9b
0.0678
0.92
1.0022
1.43
0.1072
1.09
-25.19
863
19
705
7.3
656
6.8
2593
307
0.143
3.21
7b
0.0690
0.50
1.0698
1.30
0.1124
1.20
-24.96
900
10
739
6.8
687
7.8
2081
258
0.112
1.40
27b
0.0709
0.68
1.0064
1.18
0.1029
0.96
-35.54
955
14
707
6.0
632
5.8
2842
327
0.347
0.22
51b
0.0717
0.90
1.2063
1.31
0.1221
0.96
-25.37
977
18
803
7.3
742
6.7
1878
251
0.121
1.12
115b
0.0720
0.30
1.3226
1.54
0.1332
1.51
-19.51
987
6
856
8.9
806
11.5
2849
419
0.116
0.41
116b2
0.0712
0.38
1.1455
1.39
0.1167
1.34
-27.60
963
8
775
7.6
711
9.0
2104
269
0.094
0.18
117b2
0.0726
0.54
1.3176
1.47
0.1316
1.36
-21.78
1003
11
853
8.5
797
10.2
2415
353
0.148
1.69
117b5
0.0719
0.69
1.0073
1.51
0.1017
1.34
-38.22
982
14
708
7.7
624
8.0
3023
339
0.149
3.55
117b3
0.0677
1.01
0.8700
1.72
0.0932
1.39
-34.76
860
21
636
8.2
574
7.7
2803
289
0.123
6.65
117b4
0.0646
2.10
0.6660
3.60
0.0748
2.93
-40.36
761
44
518
14.7
465
13.1
3239
272
0.164
8.91
117b1
0.0659
1.68
0.6189
4.63
0.0681
4.31
-48.59
802
35
489
18.1
425
17.8
3821
292
0.142
8.41
58c
0.0726
0.44
1.6656
1.10
0.1665
1.00
-0.98
1002
9
996
7.0
993
9.2
2455
451
0.143
0.05
58c2
0.0729
0.62
1.5089
1.13
0.1500
0.95
-11.76
1012
12
934
6.9
901
8.0
1923
317
0.120
0.23
41c
0.0731
0.47
1.7051
1.08
0.1691
0.97
-1.18
1018
10
1010
6.9
1007 9.0
2157
402
0.131
0.14
43c
0.0736
0.31
1.6691
1.07
0.1645
1.02
-4.98
1030
6
997
6.8
982
9.3
1677
305
0.106
0.17
42c
0.0742
0.73
1.8061
1.20
0.1765
0.95
0.05
1047
15
1048
7.9
1048 9.2
2221
434
0.137
0.88
43c2
0.0743
0.31
1.7342
1.66
0.1694
1.64
-4.08
1048
6
1021
10.8
1009 15.3
1761
325
0.048
0.15
30c2
0.0748
0.30
2.2636
1.33
0.2194
1.29
22.24
1064
6
1201
9.4
1279 15.0
1485
360
0.096
0.52
42c2
0.0739
0.29
1.7056
1.64
0.1675
1.62
-4.14
1038
6
1011
10.6
998
15.0
2124
391
0.082
0.11
1a
0.0838
0.28
2.4075
1.04
0.2084
1.00
-5.79
1288
5
1245
7.5
1220 11.1
1222
294
0.171
0.27
GEUS ICPMS
1a2
0.0777
1.01
1.9109
1.80
0.1783
1.50
9.70
1154
20
1085
20
1058 16
864
81.8
0.128
0.49
2a2
0.1117
0.67
4.6164
1.82
0.2997
1.69
9.94
1840
12
1752
32
1690 29
81.8
90
0.613
-0.02
5a
0.1136
0.58
4.9076
1.38
0.3132
1.25
7.68
1871
10
1804
25
1757 22
90
220
1.395
0.18
5a2
0.0940
1.35
3.1631
3.10
0.2440
2.79
9.05
1522
25
1448
45
1407 39
220
99.2
1.467
1.83
6a
0.1084
0.67
4.3748
1.21
0.2928
1.01
8.91
1785
12
1708
21
1655 17
99.2
452
1.586
0.20
8a2
0.0932
0.85
3.5457
1.39
0.2759
1.10
4.11
1506
16
1537
21
1571 17
452
1380
0.594
0.08
11a
0.1133
0.74
5.0462
1.54
0.3230
1.35
4.49
1866
13
1827
28
1804 24
55.7
462
1.352
0*
12a
0.1045
0.82
4.4425
1.74
0.3082
1.54
0.12
1719
15
1720
30
1732 27
462
335
0.539
0.13
14a
0.1061
0.88
4.4609
1.75
0.3050
1.52
2.65
1746
16
1724
30
1716 26
335
416
0.317
0.02
13a
0.0915
1.24
1.6289
1.48
0.1292
0.80
49.93
1470
23
981
14
783
416
156
0.294
1.00
16a2
0.1402
0.94
5.9673
2.30
0.3087
2.10
26.34
2242
16
1971
45
1734 36
156
146
0.471
0.46
6
17a
0.1032
1.07
3.7920
2.10
0.2664
1.81
12.12
1696
20
1591
33
1523 28
146
306
0.884
-0.01
26a2
0.1178
0.71
5.1764
1.55
0.3187
1.38
9.70
1936
13
1849
29
1783 25
306
1242
0.167
0.01
50a
0.0880
2.57
1.6380
11.42
0.1350
11.12
44.59
1396
49
985
112
816
864
79
0.321
0.12
51a2
0.1140
0.86
4.2677
1.73
0.2716
1.50
20.22
1877
16
1687
29
1549 23
91
79
1017
1.135
0.95
70a
0.1062
0.71
4.1725
1.34
0.2848
1.13
9.28
1749
13
1669
22
1616 18
243
49.9
0.287
0.10
96a
0.1137
0.64
5.3172
1.24
0.3391
1.06
0.15
1873
12
1872
23
1882 20
49.9
335
1.300
0.09
97a
0.0979
1.20
3.3674
2.09
0.2493
1.71
12.05
1599
22
1497
31
1435 25
335
134
0.802
0.26
110a
0.1154
1.13
4.5444
2.85
0.2857
2.62
17.20
1899
20
1739
50
1620 42
134
180
0.433
0.47
111a
0.0964
1.79
2.2329
4.15
0.1680
3.74
39.45
1569
34
1191
49
1001 37
180
46.8
0.540
0.37
9b
0.0670
0.98
0.9870
1.55
0.1068
1.20
25.14
854
20
697
11
654
1380
55.7
0.158
3.26
41c
0.0781
1.05
1.9132
1.90
0.1777
1.58
10.79
1163
21
1086
21
1054 17
1242
1440
0.155
0.39
42c
0.0999
1.51
1.7752
2.11
0.1288
1.48
55.74
1636
28
1036
22
781
12
1440
1076
0.133
0.91
43c
0.0758
0.58
1.9100
1.14
0.1828
0.98
2.79
1104
12
1085
12
1082 11
1076
864
0.153
0.14
58c2
0.0725
0.48
1.7338
1.16
0.1733
1.05
0.85
1016
10
1021
12
1031 11
1017
243
0.154
0.01
* 0.02
8
DC0781 Goedehoop Quartzite
NordSIM ion probe
24a
0.0849
0.86
2.5428
1.22
0.2172
0.87
-3.86
1313
17
1284
9
1267
10
371
101
0.589
25a
0.1102
0.84
4.4812
1.16
0.2949
0.81
-8.59
1803
15
1727
10
1666
12
62
31
1.923
0.15
38a
0.0791
1.43
2.1614
1.68
0.1981
0.87
-0.94
1175
28
1169
12
1165
9
554
155
1.128
1.98
41a
0.0811
0.62
2.2147
1.10
0.1982
0.91
-5.12
1223
12
1186
8
1165
10
640
160
0.622
0.14
46a
0.0854
0.69
2.6312
1.12
0.2233
0.87
-2.17
1326
13
1309
8
1300
10
404
105
0.284
* 0.06
50a
0.0867
1.27
2.6574
1.57
0.2223
0.94
-4.88
1354
24
1317
12
1294
11
156
42
0.467
* 0.16
42
Isotope ratios
Spot
207
Pb
206
Pb
±σ%
Ages (Ma)
207
Pb
235
U
±σ%
206
Pb
238
U
±σ%
Discordance%
207
Pb
206
Pb
±σ
Concentrations
207
Pb
235
U
±σ
206
Pb
238
U
±σ
[U]
[Pb]
Th/U
ppm
ppm
meas.
f206 %
52a
0.0813
1.11
2.2347
1.42
0.1993
0.90
-5.14
1229
22
1192
10
1172
10
145
38
0.730
* 0.05
67a
0.1025
0.68
3.7814
1.08
0.2677
0.84
-9.41
1669
12
1589
9
1529
11
116
44
0.993
0.08
97a
0.1155
0.27
5.1430
0.86
0.3230
0.82
-5.03
1887
5
1843
7
1804
13
729
278
0.230
0.01
98a
0.1135
1.24
3.2525
4.08
0.2078
3.89
-37.72
1856
22
1470
32
1217
43
1441
348
0.183
2.02
28a
0.0794
0.64
2.1696
1.11
0.1983
0.91
-1.37
1181
13
1171
8
1166
10
479
126
0.864
0.12
69a
0.0772
1.23
1.7077
1.52
0.1605
0.89
-15.91
1126
24
1011
10
959
8
1030
209
0.765
2.02
18b
0.0834
0.87
2.3034
1.19
0.2003
0.80
-8.69
1279
17
1213
8
1177
9
111
27
0.477
0.11
1b
0.0749
0.82
1.8025
1.16
0.1746
0.82
-2.89
1066
16
1046
8
1037
8
196
41
0.453
0.11
31b
0.0822
1.24
2.2069
1.48
0.1946
0.80
-9.12
1251
24
1183
10
1146
8
87
24
1.081
0.17
35b
0.0767
0.84
1.6342
1.17
0.1546
0.81
-17.93
1113
17
983
7
927
7
176
32
0.414
0.25
51b
0.0743
6.56
1.5879
6.64
0.1550
1.04
-12.27
1049
127
965
42
929
9
156
28
0.326
3.55
64b
0.0776
0.77
2.0478
1.29
0.1914
1.04
-0.79
1137
15
1132
9
1129
11
315
69
0.275
0.3
67b
0.0871
0.59
1.9457
1.26
0.1620
1.11
-31.15
1363
11
1097
8
968
10
464
89
0.389
1.08
74b
0.0751
1.19
1.6425
1.43
0.1585
0.80
-12.40
1072
24
987
9
949
7
165
31
0.455
1.14
80b
0.0685
1.14
0.6214
1.53
0.0658
1.03
-55.16
883
23
491
6
411
4
3345
246
0.217
5.86
8b
0.0804
0.68
2.0981
1.05
0.1892
0.80
-8.10
1207
13
1148
7
1117
8
199
46
0.564
0.11
97b
0.0756
0.89
1.7990
1.20
0.1727
0.80
-5.63
1083
18
1045
8
1027
8
195
39
0.508
0.27
14d
0.0784
1.13
1.6291
1.48
0.1507
0.96
-23.30
1157
22
982
9
905
8
940
162
0.297
2.72
25d
0.0808
0.45
2.0486
0.94
0.1838
0.82
-11.55
1217
9
1132
6
1088
8
494
102
0.146
0.17
25d2
0.0829
0.82
1.9478
1.32
0.1704
1.04
-21.52
1267
16
1098
9
1014
10
558
111
0.309
0.74
25d4
0.0924
1.30
2.7474
1.57
0.2155
0.89
-16.26
1477
24
1341
12
1258
10
348
97
0.580
1.57
65d
0.0908
0.93
1.5336
1.66
0.1225
1.37
-51.12
1442
18
944
10
745
10
2570
358
0.155
3.01
65d2
0.1129
0.40
4.7404
1.04
0.3046
0.97
-8.12
1846
7
1774
9
1714
15
699
259
0.353
0.11
65d3
0.1009
0.70
2.1775
1.36
0.1565
1.17
-45.99
1641
13
1174
10
937
10
2402
433
0.171
1.88
73d
0.1028
0.34
2.5622
1.05
0.1808
0.99
-39.03
1675
6
1290
8
1072
10
1835
370
0.056
0.8
60b
0.1045
0.77
3.2367
1.40
0.2246
1.18
-25.86
1706
14
1466
11
1306
14
1290
341
0.303
2.22
85b
0.1138
2.96
4.4234
3.08
0.2818
0.82
-15.81
1861
53
1717
26
1601
12
93
33
0.498
0.63
43
Appendix 2.
Data used for comparing NordSIM ion probe and GEUS ICPMS as methods for zircon dating.
Sample
DC0754 5a
DC0760 50a
DC0760 49b
DC0760 128a
DC0767 33b
DC076742b
DC0768 5a
DC0768 11a
DC0768 41c
DC0768 43c
Nordsim
GEUS
Pb-Pb age
2 σ Pb-Pb age
1523
19
1550
1802
11
1801*
1079
25
1193*
1732
15
1708*
1153
14
1174*
1198
16
1227*
1885
18
1871*
1800
30
1866*
1001
18
1163*
1048
12
1104*
* Without common lead correction
2σ
33
14
18
17
19
34
21
27
42
23
Data used in Fig 22.
Nordsim
Sample
Pb-Pb age
DC0754 5a
1523
DC0760 50a
1802
DC0760 49b
1079
DC0760 128a
1732
DC0767 33b
1153
DC0767 42b
1198
DC0768 5a
1885
DC0768 11a
1800
DC0768 41c
1001
DC0768 43c
1048
All with common lead correction
2σ
19
11
25
15
14
16
18
30
18
12
GEUS
Pb-Pb age
1550
1788
1149
1686
1162
1182
1851
1868
1088
1076
2σ
33
15
25
15
18
22
31
31
36
24
Data used in Fig 23.
44