136 / 352
6. PROVENANCE OF THE HP-HT UPPER ALLOCHTHON
120
Paleoproterozoic 2.14
–
1.88 Ga zircon population is
arranged in two ways. First, as a group with
e
Hf
(
t
)
from
~ +
8.3 to 3.5 (average:
+
3.1), composed by
78% (
n
=
104) of this Paleoproterozoic zircon, and
with
T
DM
values, from
c.
2.56 to 2.02 Ga (average:
2.26 Ga). The older zircon in this group (
c.
2.1 Ga)
has higher
e
Hf
(
t
)
values than the younger ones
(
c.
1.9 Ga), showing a steep decrease in their
e
Hf
(
t
)
values as the age of the zircon decreases. The other
way in which zircon with the same time span (
n
=
29,
22%) are arranged is with
e
Hf
(
t
)
values from 6.7 to
18.4 (average:
12.1), and
T
DM
from 3.44 to
2.76 Ga (average: 3.05 Ga). This group seems to be
arranged vertically in the Hf
–
U
–
Pb age diagram
(Figs 7a & 8).
Paleoproterozoic and Archean zircon (
n
=
128)
has crystallization ages between
c.
3.01 and
2.19 Ga. Values range from slightly positive
e
Hf
(
t
)
up to
+
5.5 to negative
e
Hf
(
t
)
down to 13.8, and
have T
DM
from
c.
3.69 to 2.62 Ga (average:
3.04 Ga, Fig. 7a). These zircon values are not clus-
tered but arranged in a linear trend, which is lower
than the average continental crustal evolution trend
of
176
Lu/
177
Hf
=
0.0113 used to calculate
T
DM
in
this study. In fact, this trend corresponds to a
176
Lu/
177
Hf
=
0, as revealed by the horizontal dis-
tribution of this population in the
176
Hf/
177
Hf
v.
age plot (Fig. 8). This population can be subdi-
vided
–
a first group formed by the main Archean
U
–
Pb population (
c.
2.68
–
2.61 Ga) with
e
Hf
(
t
)
between
~ +
5 and 5 and the data following the
mentioned linear trend, with decreasing age and
e
Hf
(
t
)
values, and a second group formed by zircon
with crystallization ages of
c.
3.0 Ga (
e
Hf
(
t
)
from
c.
+
2.2 to 4.8) and zircon with
T
DM
of
c.
3.5 Ga.
Sm
–
Nd results
Whole-rock Sm
–
Nd analyses were performed on five
Banded Gneisses samples, with results (Table 1) plotted
in Fig. 9. Present-day Nd epsilon values (
e
Nd
t
= 0 Ma
)
for four of the samples (not considering GCH-17) vary
from 15.0 to 10.8 and
e
Nd for their maximum sedi-
mentation age (
e
Nd
t
= MDA
) varies from 9.8 to 6.1.
Their depleted mantle model ages (Nd
TDM
) range
between 1.85 and 1.62 Ga (average of 1.74 Ga). These
results are in agreement with previous published data
(Nd
TDM
=
1.70 Ga, Peucat
et al.
, 1990). Sample
GCH
–
17 results are
e
Nd
(
t
= 0 Ma)
=
7.3,
e
Nd
(
t
=
MDA)
=
2.5 and Nd
TDM
=
1.31 Ga.
DISCUSSION
Provenance of the HP
–
HT Upper Allochthon
Concordia (Fig. 3) and density plots (PDP
–
aKDE;
Fig. 4) reveal two main age populations, with pro-
nounced age peaks at
c.
522
–
512 Ma (Palaeozoic
–
Neoproterozoic population: 34.7%) and
c.
2.07 Ga
(Paleoproterozoic population:
39.6%;
Fig. 5).
Archean populations comprise
~
22.8% of the total
Banded Gneisses zircon, and the Mesoproterozoic
population, at only 2.8%, is scarce.
The HP
–
HT Upper Allochthon zircon detrital sig-
nature has been compared in Fig. 10 to cratons and
terranes that could potentially be the source areas for
the studied meta-sedimentary rocks. Data representa-
tive of the Avalonian terranes are those proposed for
Ganderia and Avalonia by Willner
et al.
(2013,
2014), and data from the Laurentian craton are from
Willner
et al.
(2014). It is noteworthy to highlight the
high number of zircon grains with mixing patterns at
c.
1.5 and 1.0 Ga in these data sets, because they are
not present in the HP
–
HT Upper Allochthon spec-
trum. Data representative for the West Africa craton
are from Abati
et al.
(2012), and show compatible
and similar patterns to those shown by the HP
–
HT
Upper Allochthon. Data from a high grade Armori-
can metasedimentary rock, with a probable WAC
provenance (Gerdes & Zeh, 2006), are also plotted
for comparison.
The distal (
>
1 Ga) detrital zircon spectrum
The main Archean U
–
Pb zircon population (34.7%)
in the Banded Gneisses is bracketed at 2.75
–
2.50 Ga
(Fig. 4). The
e
Hf
(
t
)
v.
age pattern for this zircon
population (Fig. 7a) is a linear trend that points to
an intrusion at
c.
2.70 Ga (
e
Hf
(
t
)
from
c.
+
5 to 5)
of juvenile magmas (data with
T
DM
of
c.
3.2
–
2.8 Ga, maximum at 3.0 Ga) into an older crust
triggering limited mixing processes. Its linear evolu-
tion from intrusion at
c
. 2.70 to 2.40 Ga could rep-
resent a long-lasting continental crust reworking
process (and probably other intrusion events at
~
2.51 Ga), or Pb-loss processes triggered by high
Table 1.
Whole-rock Sm-Nd isotope data of the Banded Gneiss samples.
Sample
Sm
Nd
147
Sm/
144
Nd
143
Nd/
144
Nd
(t = 0)
2SE(abs)
9
10
6
e
Nd
(
t
= 0)
e
Nd
(
t
= MDA)
T
DM
GCH-02
4.105
20.736
0.1197
0.511960
2
13.2
8.3
1754
GCH-06
4.371
21.356
0.1237
0.512083
2
10.8
6.1
1624
GCH-17
4.162
20.429
0.1232
0.512263
2
7.3
2.5
1313
GCH-21
3.484
17.993
0.1170
0.511869
1
15.0
9.8
1852
GCH-24
1.157
5.756
0.1215
0.512007
2
12.3
7.3
1711
Average
1651
©
2015 John Wiley & Sons Ltd
970
R. ALBERT
ET AL.