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zircons studied were documented by back-scattered electron (BSE)
and cathodoluminiscence (CL) images using a JSM 6490 scanning
electron microscope to study their internal structure in order to
choose the best areas for laser ablation.
4.2. U–Pb zircon analyses
Zircons were analysed for U, Th and Pb isotopes at the GUF using a
ThermoScientific Element 2 sector field ICP-MS coupled to a Resolution
M-50 (Resonetics) 193 nm ArF excimer laser (CompexPro 102,
Coherent) system using a slightly modified method as described in
Gerdes and Zeh (2006, 2009); Zeh and Gerdes (2012). Laser spot-size
was 23 to 33 μm for unknowns, 15 μ m for Plešovice, 33 μm for GJ1
and 91500, and 50 μm for Felix standard zircons. Sample surface was
cleaned directly before each analysis by four pre-ablation pulses.
Ablation was performed in a 0.6 L·min−1 He stream, which was
mixed directly after the ablation cell with 0.07 L·min− 1 N2 and
0.68 L·min− 1 Ar prior introduction into the Ar plasma of the SF-
ICP-MS. The sensitivity achieved was in the range of 8000 –12,000
cps/ μg·g−1 for 238U with a 23 μm spot size, at 5.5 Hz and 4–
5 J·cm−2 using GJ1 zircon. The two-volume ablation cell (Laurin
Technic, Australia) of the M50 laser enables detection and sequential
sampling of heterogeneous grains (e.g., growth zones) during time
resolved data acquisition, due to its quick response time of b1 s
(time until maximum signal strength was achieved) and wash-out
(b99% of previous signal) time of b3 s. All analyses were common-
Pb corrected following the method described in Millonig et al.
(2012). 204Hg during the analytical session was about 200 cps. For
the analysed sample the common 204Pb contents were mostly near
or below the detection limit, and thus a 208 Pb-based common Pb correc-
tionhasbeenusuallyapplied.Theanalytical resultsarepresentedassup-
plementary electronic material (Suppl. Electr. Mat., Tables 1 to 6). The
accuracy of the method was verified by repeated analyses of reference
zircon 91500 ( Wiedenbeck et al., 1995), Plešovice (Slama et al., 2008)
and in-house standard Felix ( Millonig et al., 2012). Data were plotted
using Isoplot 3.75 software (Ludwig, 2012).
From the six samples studied a total of 889 zircon cores were dated
(Suppl. Electr. Mat., Tables 1 to 6), from which 839 are considered valid
analysis (5.6% rejected) in terms of concordance (up to 10% discordance
accepted). Following recommendations made by Vermeesch (2004)
more than 117 zircons were analysed in each sample to achieve statisti-
cal adequacy. Data have been represented for visualization in complete
and partial conventional concordia diagrams (from 450 Ma to 750 Ma)
for each sample (Fig. 3). Data have also been plotted as adaptive Kernel
Density Estimates (KDEs) and Probability Density Plots (PDPs) in Fig. 4,
using DesityPlotter5.0 software ( Vermeesch, 2012). KDEs were built
with bandwidth = 15 Ma and histograms with binwith = 25 Ma. The
age assigned to each zircon core was chosen depending on 207 Pb/206Pb
age. If 207Pb/206Pb age b 1 Ga, 206Pb/238U age was chosen, if not
207 Pb/206 Pb age was preferred.
Maximum depositional ages (MDAs) for each sample were calculat-
ed following the most conservative method (YC2σ(3+)) reported by
DickinsonandGehrels(2009)withsomemodifications.MDAswerecal-
culated as the weightedmean of theyoungest cluster of zircon ages that
can beused to calculatea concordia age withIsoplots normal “ConcAge”
tool (i.e. probability of data-point equivalence higher than 0.001),
choosing the first (younger) zircon age of the cluster as to have less
than 1% age difference with the next zircon age.
No major differences have been found between the studied samples.
As all six samples were collected from the same formation it is assumed
that the original detritus was derived from the same source area. To
support this assumption a comparison between distributions of detrital
zircon ageshas been performed usinga Kolmogorov–Smirnov nonpara-
metric test ( Fig. 5 a), in a similar way as had been used previously to
establishcommonprovenance(Fernández-Suárezetal.,2013andrefer-
encestherein),anda Cumulative DistributionPlot (CDP)witherrorshas
also been reported ( Fig. 5b). This test and plot were performed with a
MS Excel© spreadsheet downloaded from the Arizona Laserchron
Center webpage .
4.3. Lu–Hf zircon analyses
Hafnium isotope measurements (Suppl. Electr. Mat., Tables 7 to 11)
were performed with a Thermo-Finnigan Neptune multicollector
ICP-MS at GUF coupled to the same laser as described in the U–Pb
method. Laser spots with diameter mostly of 40 μm were drilled
with a repetition rate of 5.5 Hz and an energy density of 6 J/cm2 dur-
ing 55 s of data acquisition. All data were adjusted relative to the
JMC475 of 176Hf/177Hf ratio = 0.282160 and quoted uncertainties
are quadratic additions of the within run precision of each analysis
and the reproducibility of the JMC475 (2 SD = 0.0033%, n = 16).
Accuracy and external reproducibility of the method were verified
by repeated analysis of reference zircon GJ-1 and Ple šovice, (see
Suppl. Electr. Mat., Table 12) which are well within the range of solu-
tion mode data (Woodhead and Hergt, 2005; Gerdes and Zeh, 2006).
For calculation of epsilon Hf (εHft ) chosen values for chondritic uni-
form reservoir (CHUR) are 176Lu/ 177Hf = 0.0336 and 176Hf/177Hf =
0.282785 (Bouvier et al., 2008), and a 176Lu decay constant of
1.867 × 10−11 a− 1 (average of Scherer et al. (2001) and Soderlund
et al. (2004) calculated from terrestrial mineral isochrones). Initial
176 Hf/177 Hft and εHft for all analysed zircon domains were calculated
using the preferred U–Pb ages.
Depleted mantle hafnium model ages (TDM) were calculated using
present day 176Hf/177Hf = 0.283164 value for average MORB (Chauvel
et al., 2008) which is assumed to be present day depleted mantle com-
position. This value corresponds to an initial εHf(t = 0Ga) = 13.4.
DM evolution trend was propagated to εHf(t = 4Ga) = 0, because
the existence of a voluminous depleted mantle reservoir during
Hadean –Early Archean is highly speculative (see Hawkesworth
et al. (2010) ; Kemp et al. (2010) and discussions of Zeh et al.
(2011)). TDM values for all data were calculated using a mean
176Lu/177Hf of 0.0113 (average continental crust; Rudnick and Gao,
2003) for the zircon crystallization age.
MORB 176Hf/177 Hf intervals were taken from the Atlantic, Pacific and
Indian MORB values (excepting three unusual low values from the
Indian Ocean) reported by Chauvel and Blichert-Toft (2001) consider-
ing a minimum 176 Hf/177 Hf = 0.28302 (εHf(t = 0Ma) = 8.3) and a max-
imum 176Hf/177Hf = 0.28337 (εHf(t = 0Ma) = 20.7). These values are
propagated to εHf(t = 4Ga) = 0 defining a field (blue discontinuous
lines) around the DM-evolution trend where the real DM composition
should lie (i.e. error field for DM-evolution line).
4.4. Sm–Nd whole rock analyses
Samplepreparationandanalyseswereperformedatthelaboratoryof
Geocronología y Geoquímica Isotópica at the Universidad Complutense
de Madrid.
Whole rock samples were dissolved by oven digestion in ultra-pure
HF and HNO 3acids together with the 149 Sm/150 Nd spike in sealed teflon
microreactors. Once the samples were dissolved and dried HNO3 was
added to eliminate silica fluorides and after that HCl was added to
formchlorine molecules.Then thesamplewassubjectedto acentrifugal
process and to a two stage conventional ion-exchange chromatography
to concentrate and separate REEs with DOVEX AG-50x12 (200–400
mesh) resin, and to separate Sm from Nd with HEDHP resin. The
fractions where Sm and Nd are present in high concentrations are
dried and loaded with H3PO4on rhenium filaments in triple disposition,
and analysed in a thermal ionization mass spectrometer TIMS-Phoenix
HCT040® following a dynamic multicollector method. The measured
143Nd/144Nd isotopic ratios were corrected for possible isobaric
interferences from 142Ce and 144Sm (only for samples with
147Sm/144Sm b0.0001) and normalized to 146Nd/144Nd = 0.7219
1438 R. Albert et al. / Gondwana Research 28 (2015) 1434– 1448
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