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6. PROVENANCE OF THE HP-HT UPPER ALLOCHTHON
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rocks with those previously obtained for the
metasedimentary rock series of the IP upper units is
the best method known to find out if the upper units
are a single or a composite terrane. Additionally, this
comparison should also provide clear information
about the Gondwanan or Laurussian provenance of
the HP–HT upper units.
SAMPLE DESCRIPTIONS
Five metasedimentary rock samples were chosen from
the Banded Gneiss formation, whose location is
shown in Fig. 2. Sample GCH–02 (ref. 112974) is
from an outcrop south of Cari~no beach (43°43046.1″
N, 7°520 08.5″W). Sample GCH–06 (ref. 112978) is
from Figueiroa beach (43°42039.6″ N, 7°510 48.6″W),
10 m east of the Figueiroa beach geological section
presented by Albert et al. (2012). GCH– 17 (ref.
112989) is from the Area da Vaca beach (43°43031.2″
N, 7°51044.6″W), at the 270 m point of the Area da
Vaca geological section presented by Albert et al.
(2012). Sample GCH–21 (ref. 113164) is from
Sismundi (43 °42025.0″N, 7°520 10.4″W) and sample
GCH–24 (ref. 113170) is from Punta Promontorio
(43°41016.3″N, 7°52039.7″W; in all cases using Simple
Cylindrical projection, WGS84 datum; ref: rock col-
lection reference, UCM).
All samples are variably fresh (not altered) fine-
grained migmatitic para-gneisses with grano-
lepidoblastic texture. To avoid possible problems
derived from leucosome generation and the presence
of new metamorphic zircon, only the most massive
layers, with no evident generation of leucosomatic
bands along the foliation, were sampled. They con-
tain a main mineral assemblage formed at the meta-
morphic peak P–T conditions or in the first stages of
retrogression, constituted by Qz + Grt + Bt + Pl + Rt
Ky Afs, with Ap + Zrn + Ilm + Py + Gr as
accessory phases and Chl + Ms as common retrogres-
sive minerals (mineral abbreviations after Whitney &
Evans, 2010).
SAMPLE PREPARATION AND ANALYTICAL
PROCEDURES
Zircon sample preparation
Zircon crystals were separated from bulk samples
using conventional mineral separation techniques at
the Facultad de Ciencias Geologicas, Universidad
Complutense de Madrid (UCM). Samples were
cleaned and crushed in a jaw crusher and in a tung-
sten disc mill. The light fraction was removed using a
Wilfley table. The magnetic fraction was separated
with a hand magnet and with a Franz model mag-
netic separator to remove those minerals susceptible
to a magnetic field induced by an electric current up
to 1.7 A. Minerals with a density below 3325 kg m 3
were removed using CH2I2 (diiodomethane). Zircon
hand picking, mounting, imaging and analysis were
performed at the Goethe University of Frankfurt am
Main (GUF). Hand-picked zircon grains of all sizes
and morphologies were mounted in epoxy filled
mounts depending on their size and polished to
~50% of their thickness. All grains were documented
by back-scattered electron and cathodoluminiscence
images using a JSM 6490 scanning electron micro-
scope to study their internal structure to choose the
best areas for laser ablation.
U–Pb zircon analyses
Zircon was analysed for U, Th and Pb isotopes at
the GUF with a ThermoScientific Element 2 sector
field ICP –MS coupled with a RESOlution M–50
(ASI) 193 nm ArF excimer laser system (Com-
pexPro 102, Coherent), using a slightly modified
method as described in Gerdes & Zeh (2006, 2009)
and Zeh & Gerdes (2012). Laser spot-size was
23–33 lm for unknowns, 15 lm for Plesovice,
33 lm for GJ1 and 91500, and 50 l m for Felix
standard zircon. Sample surface was cleaned by
four pre-ablation laser pulses. Ablation was per-
formed in a 0.6 l min 1 He stream, 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 torch. The sensitivity achieved was in the
range of 8000–12,000 cps lg g 1 for 238 U with a
23 lm spot size, at 5.5 Hz and 4–5 J cm 2 using
GJ1 zircon. All analyses were common-Pb corrected
following the method described in Millonig et al.
(2012). The 204Hg during the analytical session was
~200 cps. For the analysed sample, the common
204Pb contents were mostly near or below the
detection limit, and thus a 208Pb-based common Pb
correction has usually been applied. The results are
presented in Tables S1–S5. The accuracy of the
method was verified by repeated analyses of refer-
ence zircon 91500 (Wiedenbeck et al., 1995),
Plesovice (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 five samples, a total of 729 zircon cores
were dated (Tables S1–S5), from which 613 are con-
sidered valid analysis (15.9% rejected) in terms of
concordance (up to 10% discordance accepted).
More than 117 zircon grains were analysed in each
sample to achieve statistical adequacy (Vermeesch,
2004). Data have been represented for visualization
as Wetherill concordia diagrams for each sample
(Fig. 3). Data have also been plotted as adaptive
Kernel Density Estimates (aKDEs) and probability
density plots (PDPs) in Fig. 4, using DensityPlotter
5.0 software (Vermeesch, 2012). The age assigned to
each zircon core was chosen depending on
207Pb/206Pb age. If the 207Pb/ 206Pb age was <1 Ga,
the 206Pb/ 238U age was chosen, if not 207Pb/206Pb age
was used. Bar diagrams, are presented in Fig. 5
© 2015 John Wiley & Sons Ltd
964 R. ALBERT ET AL.