Chemical Geology 340 (2013) 76–93
Contents lists available at SciVerse ScienceDirect
Chemical Geology
journal homepage: www.elsevier.com/locate/chemgeo
Timing of incremental pluton construction and magmatic activity in a
back-arc setting revealed by ID-TIMS U/Pb and Hf isotopes on complex
zircon grains
Mélanie Barboni a, c,⁎, Blair Schoene a, Maria Ovtcharova b, François Bussy c, Urs Schaltegger b, Axel Gerdes d
a
Department of Geosciences, Princeton University, NJ, USA
Earth Sciences, University of Geneva, Switzerland
c
Institute of Earth Sciences, University of Lausanne, Switzerland
d
Goethe University Frankfurt, Institut für Geowissenschaften, Frankfurt am Main, Germany
b
a r t i c l e
i n f o
Article history:
Received 8 November 2011
Received in revised form 12 December 2012
Accepted 13 December 2012
Available online 27 December 2012
Editor: K. Mezger
Keywords:
U–Pb geochronology
Zircon
Hafnium isotopes
Zircon residence time
Zircon antecryst
Variscan
a b s t r a c t
The lifetimes and thermal histories of upper crustal plutons are increasingly determined using geochronology, but
complex growth of datable minerals in magmas impedes simple age interpretations. Careful field observation
helps constrain zircon U–Pb dates in terms of timing of magma injection, for example because relative ages of
successive magma pulses must be honored. We use ID-TIMS U/Pb zircon geochronology and field geology to
construct timescales of incremental pluton assembly in the St-Jean-du-Doigt (SJDD) bimodal layered intrusion
(Brittany, France). Field evidence suggests that early pulses were injected into a cold environment with little
supersolidus interaction among successive magma pulses. Later injections occurred in a progressively hotter
environment with protracted mafic and felsic magma interaction. Zircon dates show that early activity ca.
347 Ma predates the thermally mature episode by about 1 Ma, which terminated at ca. 345 Ma. Dates from samples displaying core-rim zircon overgrowths span about 5 Ma (351–346 Ma), which we interpret to represent
two distinct crystallization events. Hf isotopic data from cores and rims are homogeneous, precluding zircon
inheritance from basement rocks. These textures and dates could instead reflect zircon saturation fluctuations
at the emplacement depth, or antecrystic zircon grains recording pre-emplacement magmatic growth.
© 2012 Elsevier B.V. All rights reserved.
1. Introduction
A growing body of evidence suggests that pluton emplacement occurs by amalgamation of numerous pulses of magma that accumulate
over thousands to millions of years, both in the middle-to-upper crust
(e.g., Hill et al., 1985; McNulty et al., 2000; Miller and Paterson, 2001;
Mahan et al., 2003; Miller and Miller, 2003; Glazner et al., 2004;
Michel et al., 2008; Schaltegger et al., 2009) and in the subvolcanic
environment (e.g., Bacon et al., 2007; Charlier et al., 2008; Claiborne
et al., 2010). Growth models for incremental upper-crustal laccoliths
often involve a succession of accreted sills emplaced through feeder
dykes that initially spread laterally along a horizon separating an
upper, more rigid layer and a lower, less rigid layer (e.g. Cruden et
al., 1999; de Saint-blanquat et al., 2006; Kavanagh et al., 2006; Michel
et al., 2008; Menand, 2011). Pluton assembly proceeds through a
succession of high flux periods, where magma injections occur
much faster than the average pluton construction rate, interrupted
by repose periods (with no or few magma injections) (e.g. Matzel et
al., 2006; de Saint-Blanquat et al., 2011). In such a model, plutons
represent a time-integrated accumulation of magma pulses with little
⁎ Corresponding author.
E-mail address: [email protected] (M. Barboni).
0009-2541/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.chemgeo.2012.12.011
liquid at any one time. Assessing the validity of such models requires
robust constraints on the rates of magma injection, the geometry of
successive pulses, and the ambient thermal regime (Annen, 2010),
which nominally requires combining high-precision geochronology
with careful field observation.
U/Pb geochronology of zircon is a widely utilized method to
reconstruct the timescales of pluton assembly because Pb diffusion
in zircon is negligible at magmatic temperatures (Cherniak and
Watson, 2001), and therefore can retain age information from crystallization at different stages of magmatic evolution (source, ascent and
emplacement level). Isotope dilution thermal ionization mass spectrometry (ID-TIMS) U/Pb analyses of single zircon crystals can attain
precision of better than ca. 0.1% for a single analysis (e.g., Sláma et
al., 2008; Davydov et al., 2010; Schoene et al., 2010a), or ±300 ka
for ca. 300 Ma zircon. Increased precision now often results in complex spreads in dates within zircon populations, illustrating that zircon grains can crystallize over 10 4 to 10 6 years in many magmatic
systems (e.g. Charlier et al., 2005; Matzel et al., 2006; Bachmann et
al., 2007; Schaltegger et al., 2009; Memeti et al., 2010; Schoene et
al., 2012).
Identification of individual magma pulses in incrementally assembled plutons is complicated by the lack of contrast between the different magma injections (e.g., Glazner et al., 2004). In this paper, we
M. Barboni et al. / Chemical Geology 340 (2013) 76–93
present ID-TIMS U/Pb zircon crystallization dates that document the
magmatic history of the Saint-Jean-du-Doigt (SJDD) bimodal layered intrusion (Brittany, France). This Carboniferous pluton preserves sill-like
emplacement of mafic–felsic–hybrid magmas at shallow crustal levels.
The bimodal nature of the rocks, shallow emplacement level, and preservation of the pluton roof offer a rare opportunity to identify and date various magma pulses. Despite prolonged zircon crystallization in each hand
sample, we use a combination of field observation, geochronology, and
zircon Hf isotopic data to arrive at a self-consistent time-frame for the
emplacement duration of the SJDD pluton. These results build on and
refine efforts using ID-TIMS U/Pb zircon geochronology as a timekeeper of multipulse plutons, and provide crucial inputs for thermal
models of the crust during magmatic and orogenic episodes.
microcontinents and a late Paleozoic (Devonian to Late Carboniferous)
cycle corresponding to a continental collision, after closure of the
Rheno-Hercynian ocean by southward subduction under the northern
margin of Gondwana. This subduction induced calc-alkaline (diorites)
arc magmatism and back-arc extension gave rise to the SJDD gabbro
and related magmatic rocks.
The SJDD complex is a 200-km2 heterogeneous layered body
emplaced in Precambrian basement rocks and displays complex magmatic interactions (gabbroic and intermediate to granitic compositions)
as mapped in broad outline at 1: 50,000 (Chantraine et al., 1986). These
characteristics were interpreted as representing magma mingling.
Al-in-hornblende geobarometry points to an emplacement at shallow
crustal level (6–9 km, 0.3 ±0.06 GPa, Barboni et al., 2009b; Barboni
and Bussy, 2010). Based on gravity data, the SJDD intrusion is tabular
in shape and extends at least to depths of 1.5 km below the presentday erosion level. The SJDD mafic facies display chemical characteristics
falling between the tholeiitic and calk-alkaline trends, with a dominantly tholeiitic affinity and similar features to Back-Arc Basin Basalts
(BABB; Barboni et al., 2007). Preliminary trace-element geochemistry
suggests several sources for the associated felsic rocks, including partial
melting of quartzofeldspathic continental crust (A-type granites;
Barboni and Bussy, 2011) and subsequent mixing with the mafic
magma (intermediate rocks; Barboni et al., 2007). The SJDD pluton
was built by sill underaccretion (in the sense of Annen et al., 2006) at
upper crustal levels in a continental rift or a pull-apart basin, presumably during an extensional or transtensional tectonic phase (see review
2. Geological setting and field relationships
2.1. Overview of SJDD complex
The St. Jean du Doigt (SJDD) massif is located along the seashore
close to the city of Morlaix in northern Brittany, France (Fig. 1A).
Geologically, it is a sector of the North Armorican massif. Growth and
modification of the Armorican massif spans from the Precambrian to
the late Paleozoic, characterized by Cadomian events (620 to 540 Ma)
and a polyphase Variscan evolution (440 to 290 Ma). Faure et al.
(2005) propose a polycyclic history characterized by an Early Paleozoic
(Cambrian to Early Devonian) cycle of rifting and convergence of
A
B
439 000 E
453 000 E
206
77
238
Pb/
U age (Ma)
354
The Channel
A
France
N
352
348.0±0.3
B
St. Jean-du-Doigt
352.3±0.9
347.9±0.3
350
347.7±0.3
347 Ma 2 Ga
347.7±0.3
347.7±0.3
300 Ma
347.5±0.5
348
BEG
Lanmeur
MB292
538 1540 N
MB57
347.2±0.4
346.2±0.3
346.7±0.5
Silurian-Devonian metamorphic formations
Red granites of Morlaix (300 Ma)
Barnénez metadolerites (?)
MB222
346.6±0.3
345.5±0.5
Post-intrusion fold
axis direction
Brioverian metamorphic formations
(660-540 Ma)
Sector III
Sector I
B (SE)
E - SE
MB48
MB57
BEG
MB218
A-type granite
Mafic facies
Leucogranite
Banded gabbros
Quaternary
Red granite of
Morlaix (300 Ma)
Basaltic dykes
346.3±0.3
Pseudo-adakites (347 Ma)
Icartian orthogneiss (2.02 Ga)
MB100
Sector III
346.7±0.3
A-type granites (347 Ma)
Sector II
A (NW)
MB48
Sector II
344
St. Jean-du-Doigt intrusion (347 Ma)
MB218
MB100
Sector I
346
5 km
MB222
Tonalitic facies
Faults
Granodioritic
tonalitic hybrid
facies
MB292
Host rock
Banded gabbros
breccias
Sinistral shear zone
1 km
Vertical thickness is ca. 1.5 km. Vertical exaggeration of the sill thickness
Fig. 1. A) Simplified geological map for the SJDD intrusion and host rocks (modified after Chantraine et al., 1986) and seashore cross-section. Location of the dated samples is indicated on the cross-section. B) Oldest (antecryst) and youngest (autocryst) fractions within each dated sample.
78
M. Barboni et al. / Chemical Geology 340 (2013) 76–93
in Faure et al., 1997). The SJDD complex was finally intruded by
late-Variscan red granite stocks (Red granites of Morlaix, Fig. 1A)
dated at 290–300 Ma (Leutwein, 1969; Vidal, 1980) and linked to the
Red Granite Suite of Brittany further west.
Very few dates are available for the SJDD complex and neighboring
lithologies. An age of 350 Ma is mentioned in Chantraine et al. (1986)
for the SJDD gabbro, obtained by Deutsch (unpublished work) using the
U/Pb method on zircon. Similar ages were obtained for the peripheral
granite of Runiou (340 Ma, Rb/Sr date, Leutwein, 1969), which points to
a late Devonian–early Carboniferous age for the SJDD intrusion. However,
Coint et al. (2008) questioned the 350 Ma date on the basis of field relationships, linking the SJDD intrusion to the late-Variscan Red Granite
Suite of Brittany, dated at 290–300 Ma (Leutwein, 1969; Vidal, 1980).
They report comagmatic relationships between small granite bodies ascribed to the 300 Ma granite suite (A-type granite on Fig. 1a) and the
main SJDD gabbroic facies (eastern sector of the seashore). However,
we obtained a mean U/Pb LA-ICPMS date of 350±4 Ma on magmatic zircon grains from the small granite bodies (Barboni et al., 2008), confirming
the Early Carboniferous age of the SJDD intrusion.
2.2. Intrusion chronology of the pluton based on field relationships
The internal structure of the SJDD pluton exhibits a subhorizontal
sill-on-sill assemblage with subordinate cross-cutting dikes (cf.
Barboni et al., 2009a, 2009b; Barboni and Bussy, 2010), typical of a
“Christmas-tree laccolith” architecture (e.g. Hunt, 1953; Westerman
et al., 2004). Because outcrops are lacking in the central sector of
the intrusion, fieldwork and sampling were conducted along the shoreline in a continuous 15 km long cross-section perpendicular to the
main structures (Fig. 1A). Field relationships indicate that magma
pulses young to the west in a progressively warmer environment as
documented by the rheological evolution of the magmas (Barboni et
al., 2009a, 2009b; Barboni and Bussy, 2010). To the east (Sector I,
Fig. 1A), stacks of bimodal sills show little interaction between the
two magma types (Fig. 2B). The central and western sectors (Sectors
III and II, Fig. 1A), however, exhibit contacts between felsic and mafic
sills indicative of magma mingling (Fig. 2E). Internal differentiation
within the mafic sills is widely documented, along with many magmatic flow-banding processes (Fig. 2G). All these features suggest a slower
crystallization rate within the intruding sills in a warmer environment,
as expected in a thermally more mature magmatic system.
We distinguished three main sampling areas for zircon geochronology along the SJDD pluton cross-section based on field relationships
(Fig. 1A):
Sector I is the eastern sector of the pluton; it is composed mainly of
interlayered bimodal sills of gabbroic and granitic compositions
(A-type granite). Sill contacts are flat and subhorizontal without
evidence of magma mingling (Fig. 2B). Absence of contact metamorphism in the surrounding country rock suggests relatively fast
cooling of the sills in cool crustal conditions. The host rock is
observed in contact with the upper sills (Fig. 2A), leading to the
interpretation that this sector represents the intrusion roof and an
early magmatic event within the system. An A-type granite sample
Fig. 2. Field relationships: A) basaltic sill injection (b) within the fractured Precambrian host rock roof (hr) in Sector I; B) sharp contact between a granitic (g) and basaltic (b) sill in
Sector I bimodal sill association; C) sharp contact between the SJDD A-type granite (g, sample BEG) + basaltic pillows (b; sample MB292) with the Precambrian host rock (hr);
D) brecciation of Sector I mafic sills by Sector III hybrid granodiorite/tonalite (sample MB218); E) load-cast and diapir structures in the Sector III sill associations; F) pegmatitic
gabbro (sample MB100) crosscutting the main coarse-grain gabbro facies (sample MB222) in Sector II; G) example of subvertical shear-flow structures in sector III feeding area
(microgabbro MB48), H) breccias of Sector II coarse-grained gabbros within Sector III subvertical associations; I) brecciation of Sector III mafic sills by late tonalitic injections
(sample MB57).
Table 1
U–Th–Pb isotopic data.
Description
+ fraction N°
(a)
(b)
Wt.
U
mg
ppm
(c)
(d)
Th/U
Pb
Radiogenic isotope ratios
Pb*/Pbc
Pbc
206
Pb/204Pb
208
Pb/206Pb
207
Pb/206Pb
ppm
207
Pb/235U
% err
206
Dates (Ma)
Pb/238U
% err
corr. coef.
(d)
(f)
(f)
(g)
(h)
(h)
(i)
(h)
(i)
(h)
(i)
0.60
0.44
0.53
0.45
0.45
0.61
37.3
10.1
13.3
10.0
11.5
16.2
51
15
71
10
34
9.4
0.7
1.1
0.6
1.4
0.8
3.3
3002
918
4253
634
2060
566
0.189
0.139
0.167
0.143
0.143
0.194
0.05353
0.05362
0.05355
0.05391
0.05367
0.05365
0.31
0.54
0.14
0.76
0.27
0.76
0.4084
0.4093
0.4089
0.4116
0.4103
0.4102
0.34
0.59
0.19
0.82
0.39
0.82
0.05533
0.05536
0.05538
0.05538
0.05545
0.05546
0.13
0.12
0.082
0.10
0.24
0.098
MB292 fine-grained gabbro E cross-section (Sector 1)
9
clrls frags pr 0.004 (est.) 53
0.60
10
clrls frags pr 0.003 (est.) 137
0.64
13
clrls frags pr 0.004 (est.) 208
0.72
7
clrls frags pr 0.005 (est.) 64
0.69
11
clrls frags pr 0.005 (est.) 102
0.73
12
clrls frags pr 0.004 (est.) 184
0.75
8
clrls frags pr 0.005 (est.) 83
0.65
3.2
8.2
12.7
3.9
6.3
11.3
5.0
28
112
311
52
163
723
80
5.5
3.6
2.0
3.7
1.9
0.8
3.1
1673
6491
17652
2965
9257
40731
4639
0.188
0.203
0.228
0.217
0.230
0.238
0.204
0.05341
0.05350
0.05344
0.05344
0.05354
0.05348
0.05350
0.27
0.09
0.07
0.16
0.08
0.07
0.17
0.4069
0.4080
0.4077
0.4078
0.4091
0.4086
0.4090
0.34
0.17
0.14
0.21
0.15
0.14
0.21
0.05525
0.05530
0.05534
0.05534
0.05542
0.05541
0.05544
MB100 pegmatitic gabbro cumulate W cross-section (Sector
18
mlk frags
0.0196
1415 0.49
15
clrls frags
0.0361
516
0.53
16
mlk frags
0.0163
832
0.27
17
mlk frags
0.0150
466
1.14
14
clrls frags
0.0251
397
0.47
2)
81.1
30.0
45.1
31.5
22.8
1351
622
381
514
369
1.2
1.7
1.9
0.9
1.5
81382
37122
24400
26412
22371
0.154
0.166
0.084
0.359
0.148
0.05341
0.05345
0.05347
0.05346
0.05344
0.06
0.06
0.07
0.07
0.07
0.4063
0.4078
0.4080
0.4081
0.4083
0.15
0.14
0.15
0.14
0.14
6.6
4.6
4.4
6.6
5.3
4.9
2.4
647
291
1561
683
538
1204
729
0.220
0.324
0.171
0.200
0.208
0.212
0.171
0.05385
0.05406
0.05350
0.05366
0.05388
0.05361
0.05383
0.62
1.56
0.29
0.58
0.80
0.36
0.61
0.4102
0.4120
0.4080
0.4091
0.4109
0.4091
0.4113
0.68
1.66
0.33
0.64
0.87
0.41
0.66
MB222 coarse-grained gabbro
22
clrls frags
19
clrls frags
20
clrls frags
23
clrls frags
24
clrls frags
21
clrls frags
25
clrls frags
cumulate W cross-section (Sector 2)
0.0055
219
0.69 14.5 11
0.0053
69
1.02 5.4
5.2
0.0045
437
0.54 26.4 26
0.0050
254
0.63 16.4 11
0.0015
527
0.66 35.2 9.0
0.0020
845
0.67 53.4 21
0.0012
417
0.54 26.4 12
Pb/206U
% err
(e)
BEG A-type granite E cross-section (Sector 1)
1
clrls frags pr 0.0010
619
2
clrls spr
0.0017
167
4
mlk spr
0.0030
225
5
mlk spr
0.0016
160
6
clrls frags
0.0025
195
3
mlk spr
0.0021
246
207
207
Pb/235U
±
206
Pb/238U
±
±
±
(j)
(i)
(j)
(i)
(j)
(i)
0.42
0.48
0.77
0.66
0.72
0.69
351.4
355
352.2
367
357.4
356
7.1
12
3.1
17
6.1
17
347.7
348.4
348.1
350.1
349.1
349.0
1.0
1.7
0.6
2.4
1.1
2.4
347.2
347.3
347.5
347.5
347.9
348.0
0.4
0.4
0.3
0.3
0.8
0.3
0.15
0.11
0.083
0.070
0.087
0.083
0.087
0.62
0.86
0.94
0.74
0.89
0.91
0.61
346.1
350.1
347.4
347.6
351.9
349.2
349.9
6.1
2.1
1.5
3.7
1.9
1.6
3.9
346.6
347.4
347.2
347.3
348.3
347.9
348.1
1.0
0.5
0.4
0.6
0.4
0.4
0.6
346.7
347.0
347.2
347.2
347.7
347.7
347.9
0.5
0.4
0.3
0.2
0.3
0.3
0.3
0.05517
0.05533
0.05534
0.05536
0.05541
0.10
0.091
0.11
0.092
0.090
0.94
0.94
0.93
0.93
0.93
346.3
348.0
348.6
348.5
347.5
1.4
1.4
1.5
1.5
1.5
346.2
347.3
347.4
347.5
347.6
0.4
0.4
0.4
0.4
0.4
346.2
347.2
347.2
347.3
347.6
0.3
0.3
0.4
0.3
0.3
0.05525
0.05528
0.05531
0.05531
0.05531
0.05534
0.05541
0.088
0.14
0.081
0.083
0.091
0.083
0.092
0.66
0.78
0.62
0.66
0.71
0.61
0.61
365
374
350.0
357
366
354.6
364
14
35
6.5
13
18
8.2
14
349.1
350.3
347.4
348.3
349.5
348.2
349.8
2.0
4.9
1.0
1.9
2.6
1.2
2.0
346.7
346.8
347.0
347.0
347.1
347.2
347.7
0.3
0.5
0.3
0.3
0.3
0.3
0.3
79
(continued on next page)
M. Barboni et al. / Chemical Geology 340 (2013) 76–93
Compositional parameters
Sample
Compositional parameters
Radiogenic isotope ratios
(d)
(f)
(f)
(g)
(h)
(h)
(i)
(h)
(i)
(h)
(i)
MB48 fine-grained gabbro central cross-section (Sector 3)
28
clrls frags
0.004 (est.) 215
0.52
29
clrls frags
0.0043
176
0.51
26
clrls frags
0.004 (est.) 382
0.71
27
clrls frags
0.004 (est.) 364
0.54
31
clrls frags
0.0053
65
0.66
30
clrls frags
0.0050
405
0.54
12.6
11.3
23.3
21.3
4.3
24.2
91
9.0
209
153
11
43
1.4
4.9
1.1
1.4
1.9
2.8
5421
553
11907
9123
660
2556
0.165
0.162
0.225
0.170
0.211
0.170
0.05349
0.05395
0.05344
0.05345
0.05378
0.05355
0.11
0.85
0.08
0.08
0.73
0.20
0.40736
0.41129
0.40740
0.40756
0.41052
0.40920
0.17
0.91
0.15
0.16
0.78
0.24
0.05523
0.05529
0.05529
0.05530
0.05537
0.05543
0.09
0.15
0.08
0.11
0.12
0.08
MB218 diorite central cross section (Sector 3)
33
clrls frags
0.05 (est.)
140
32
clrls frags
0.05 (est.)
82
34
clrls frags
0.04 (est.)
350
35
clrls frags
0.03 (est.)
170
37
clrls frags
0.03 (est.)
521
42
clrls frags
0.0040
181
39
clrls frags
0.03 (est.)
504
41
clrls frags
0.0023
140
40
clrls frags
0.0033
200
36
clrls frags
0.02 (est.)
155
38
clrls frags
0.04 (est.)
448
0.49
0.55
0.72
0.65
0.59
0.60
0.63
0.50
0.76
0.67
0.70
8.3
5.0
21.5
10.4
31.9
11.2
30.6
8.7
15.4
10.0
28.0
30
26
106
64
28
24
66
14
4.2
16
79
1.3
0.9
1.0
0.8
5.5
1.8
2.3
1.3
9.7
2.9
1.7
1815
1570
6010
3702
1652
1425
3841
867
256
938
4521
0.156
0.173
0.227
0.205
0.188
0.189
0.200
0.158
0.242
0.211
0.221
0.05368
0.05317
0.05349
0.05362
0.05374
0.05357
0.05350
0.05373
0.05401
0.05369
0.05335
0.32
0.44
0.11
0.14
0.24
0.34
0.17
0.54
1.12
0.48
0.15
0.40847
0.40521
0.40760
0.40910
0.41027
0.40891
0.40918
0.41098
0.41380
0.41322
0.41323
0.37
0.51
0.17
0.20
0.29
0.38
0.21
0.61
1.20
0.53
0.32
0.05519
0.05528
0.05527
0.05533
0.05538
0.05537
0.05547
0.05548
0.05557
0.05582
0.05618
MB57 tonalite central cross-section (Sector 3)
43
clrls spr
0.003
161
44
clrls frags
0.001
426
47
clrls spr
0.001
437
45
clrls spr
0.002
140
46
clrls spr
0.001
238
0.50
0.56
0.51
0.57
0.53
10.8
31.0
38.4
13.3
16.6
6.2
4.1
1.9
1.6
5.1
3.4
2.4
5.2
5.0
2.7
392
260
134
115
318
0.159
0.179
0.165
0.186
0.170
0.05382
0.05380
0.05476
0.05529
0.05412
1.14
1.82
3.54
4.22
1.42
0.40855
0.40861
0.41731
0.42198
0.41323
1.23
1.94
3.77
4.49
1.52
0.05506
0.05508
0.05527
0.05536
0.05538
mg
ppm
(b)
(c)
(d)
+ fraction N°
(a)
U
Th/U
Pb
208
Pb/206Pb
207
Pb/206Pb
ppm
(e)
207
Pb/235U
% err
206
Pb/238U
% err
corr. coef.
207
Pb/206U
% err
207
Pb/235U
±
206
Pb/238U
±
±
±
(j)
(i)
(j)
(i)
(j)
(i)
0.83
0.49
0.90
0.89
0.51
0.65
349.8
369
347.6
347.9
362
352.0
2.4
19
1.8
1.9
16
4.5
347.0
349.8
347.0
347.1
349.3
348.3
0.5
2.7
0.4
0.5
2.3
0.7
346.6
346.9
346.9
347.0
347.4
347.7
0.3
0.5
0.3
0.4
0.4
0.3
0.10
0.17
0.08
0.09
0.09
0.09
0.09
0.19
0.12
0.10
0.26
0.58
0.52
0.83
0.76
0.73
0.55
0.65
0.50
0.74
0.57
0.89
357.5
336
349.5
355.3
360.0
352.8
350.1
360
371
358
343.7
7.3
10
2.5
3.2
5.3
7.8
3.9
12
25
11
3.4
347.8
345.4
347.1
348.2
349.1
348.1
348.3
349.6
351.6
351.2
351.2
1.1
1.5
0.5
0.6
0.9
1.1
0.6
1.8
3.6
1.6
0.9
346.3
346.8
346.8
347.2
347.4
347.4
348.0
348.1
348.6
350.2
352.3
0.3
0.6
0.3
0.3
0.3
0.3
0.3
0.6
0.4
0.4
0.9
0.14
0.16
0.25
0.31
0.14
0.66
0.81
0.92
0.90
0.73
363
363
402
424
376
26
41
79
94
32
347.8
347.9
354
357
351.2
3.6
5.7
11
14
4.5
345.5
345.6
346.8
347.3
347.5
0.5
0.5
0.8
1.0
0.5
(a) All fractions annealed and chemically abraded after Mattinson (2005).
(b) Grain description. Clrls = colorless-transparent; pr = prisms; frags = fragments; spr = short prismatic; mlk = milky.
(c) Est. = nominal fraction weights estimated after chemical abrasion.
(d) Nominal U and total Pb concentrations subject to uncertainty in estimation of weight after partial dissolution during chemical abrasion.
(e) Model Th/U ratio calculated from radiogenic 208Pb/206Pb ratio and 207Pb/235U age.
(f) Pb* and Pbc represent radiogenic and common Pb, respectively; mol% 206Pb* with respect to radiogenic, blank and initial common Pb.
(g) Measured ratio corrected for spike and fractionation.
(h) Corrected for fractionation, spike, and common Pb; all common Pb was assumed to be procedural blank: 206Pb/204Pb = 18.30 ± 0.13; 207Pb/204Pb = 15.47 ± 0.16; 208Pb/204Pb = 37.60 ± 0.37 (all uncertainties abs. 1-sigma).
(i) Errors are 2-sigma, propagated using the algorithms of Schmitz and Schoene (2007) and Crowley et al. (2007).
(j) Calculations are based on the decay constants of Jaffey et al. (1971). 206Pb/238U and 207Pb/206Pb ages corrected for initial disequilibrium in 230Th/238U using Th/U [magma] = 4.
M. Barboni et al. / Chemical Geology 340 (2013) 76–93
206
Wt.
80
Pbc
Description
Pb/204Pb
Dates (Ma)
Pb*/Pbc
Sample
M. Barboni et al. / Chemical Geology 340 (2013) 76–93
(BEG) and a fine-grained gabbro (MB292) showing co-magmatic
features were collected here (Fig. 2C).
Sector II is located at the western limit of the pluton (Fig. 1A). It is
composed of subvertical coarse-grained gabbros (Fig. 2F), brecciated and wrapped in later injections belonging to Sector III along the
contact between the two sectors (Fig. 2H). The coarse-grained gabbro locally develops a pegmatitic texture. The pegmatite seems to
develop late in the system because it crosscuts the main
coarse-grained gabbro body (Fig. 2F). Samples of pegmatitic gabbro
(MB100) and coarse-grained gabbro (MB222) were collected.
Sector III is located in the central sector of the cross-section,
in-between Sectors I and II. The contact with Sector I is characterized by a transitional, highly chaotic area where early sills are brecciated and included within Sector III hybrid, mafic microgranular
enclave-rich tonalite/granodiorite (sample MB218; Fig. 2D). Bimodal sills have highly mobile contacts between adjacent sills, arguing
for sill accretion in a warm environment (Fig. 2E). Here the felsic
sills consist of albite-rich leucogranite instead of orthoclase-rich granite. Both sill under-accretion and random-accretion are recorded.
Subvertical layering of bimodal dyke swarms injected within a
tonalitic mush, associated with flow-banding structures and vertically
stretched schlieren, document multiple near contemporaneous vertical injections of magma. (Fig. 2G). They are interpreted as feeder
zones of the sills within Sector III (fine-grained gabbro sample
MB48). The last event within Sector III is documented by subvertical
injections of banded tonalite, which brecciated the bimodal sills
(tonalite sample MB57; Fig. 2I).
3. Analytical procedures
3.1. ID-TIMS U/Pb data
3.1.1. Sample preparation
Zircon grains were prepared by standard mineral separation and
purification methods (crushing and milling; concentration via Wilfley
table or hand panning; magnetic separation; heavy liquids). A selection of least-magnetic zircon grains from each sample was mounted
in epoxy resin and imaged by cathodoluminescence to check for potential inherited cores.
Annealing was performed by loading ca. 20 to 100 zircon grains of
each sample in quartz crucibles, which were heated at 900 °C for ca.
48 h. Zircon grains were subsequently transferred into 3-ml
screw-top Savillex vials together with ca. 500-μl concentrated HF
and 20-μl 7N HNO3 for the leaching step (chemical abrasion,
Mattinson, 2005). Savillex vials were arranged into a Teflon Parr™
vessel with 2-ml concentrated HF, and placed in an oven at 180°C
for 12–15 h. After leaching, the leachate was pipetted out and the
remaining zircon grains were rinsed in ultrapure water and then
fluxed for several hours in 6N HCl on a hotplate at a temperature of
ca. 80 °C. The acid solution was removed and the grains were again
rinsed several times in ultra-pure water and acetone in an ultrasonic
bath. Single zircon grains were selected, weighed and loaded for dissolution into pre-cleaned miniaturized Teflon vessels. After adding a
mixed 205Pb– 233U– 235U tracer (EARTHTIME, www.earth-time.org)
zircon grains were dissolved in ca. 60-μl concentrated HF with a 1-μl
7N HNO3 at 206 °C for 6 days, evaporated and redissolved overnight
in 40-μl 3N HCl at 206 °C. Pb and U were separated by HCl based
anion exchange chemistry (Krogh, 1973) in 50-μl columns and dried
down with 3 μl of 0.06N H3PO4.
3.1.2. Mass-spectrometry and blank
The isotopic analyses were performed at the University of Geneva
on a Thermo Triton thermal ionization mass spectrometer equipped
81
with a MasCom electron multiplier. The linearity of the MasCom multiplier was calibrated using U500, Sr SRM987, and Pb SRM982 and
SRM983 solutions, the latter two solutions of which were used to calibrate deadtime for Pb measurements. The mass fractionation of Pb
was controlled by repeated SRM981 and SRM982 measurements
(0.13 ± 0.02 1σ%/amu). U mass fractionation was calculated in
real-time using the 233U– 235U double spike, assuming a 238U/ 235U of
137.88 for sample U. The Pb and U isotopic composition of the tracer
used in this study is identical to that reported in Schoene et al.
(2010a). Both Pb and U were loaded with 1 μl of silica gel–phosphoric
acid mixture (Gerstenberger and Haase, 1997) on outgassed single
Re-filaments. Pb isotopes were measured on the electron multiplier,
while U (as UO2) isotopic measurements were made in static Faraday
mode or, in case of very low-U samples – on the electron multiplier.
All common Pb for the zircon analyses was attributed to procedural
blank and corrected with the following isotopic composition: 206Pb/
204
Pb: 18.30 ± 0.13, 207Pb/ 204Pb: 15.47 ± 0.16, 208Pb/ 204Pb: 37.60 ±
0.37 (all 1σ). U blanks are b0.1 pg and do not influence the degree
of discordance at the age range of the studied samples, therefore a
value of 0.05 pg +/−50% was used in all data reduction.
3.1.3. Data reduction, reporting ages and errors
As pointed out in several recent publications, transparent error
propagation and reporting of errors at each level (internal and external) are important for intercalibrating geochronologic data between
different laboratories and different methods (Renne et al., 1998a,
1998b; Schoene et al., 2006). Initial isotope ratio measurements
were screened using the TRIPOLI program (Bowring et al., 2011)
followed by data reduction and age calculation using the algorithms
and spreadsheet of Schmitz and Schoene (2007). Generation of
concordia plots and weighted means were done with the Isoplot/Ex
v.3 program of Ludwig (2005). All uncertainties reported are at the
2-sigma level. All data are reported in Table 1 with internal errors
only, which include counting statistics, uncertainties in mass discrimination and the common Pb composition. Concordia diagrams for all
samples are shown in Fig. 4, and U–Pb data are summarized in
Table 1. Accuracy and internal reproducibility of the data were
assessed by repeated analysis of the Plesovice standard zircon
(Sláma et al., 2008), which was pre-treated by annealing–leaching
and gave a weighted mean 206Pb/ 238U age of 337.17 ± 0.05 Ma
(N = 13; MSWD = 1.3) (Appendix B).
3.2. Zircon Hf isotope data
3.2.1. Hf on zircon grains (solid mode analysis)
Lutetium–hafnium isotopes were analyzed using a Thermo-Finnigan
Neptune multi-collector ICP-MS at Goethe University, Frankfurt, coupled
to a New Wave UP213 ultraviolet laser system, with a teardrop-shaped,
low volume laser cell. The MC-ICPMS was equipped with 9 Faraday detectors and amplifiers with 1011-Ω resistors. Data were collected in static mode (172Yb, 173Yb, 175Lu, 176Hf–Yb–Lu, 177Hf, 178Hf, 179Hf, 180Hf)
during 58 s of laser ablation. Laser spot size was 40 or 80 μm in diameter, repetition rate 10 Hz and the output energy about 5 Jcm−2. Nitrogen (~0.005 l/min) was introduced via a Cetac Aridus into the Ar
sample carrier gas (1 l/min) to enhance sensitivity (~20–30%) and to
reduce oxide formation. The ablated material was transported in a
0.25 l/min He stream and mixed using a Y-connector with the N–Ar
mixtures directly after the cell. The faraday cup configuration was set
to enable detection of the 4 Hf isotopes as well as potentially interfering
ions: L4–172Yb, L3–173Yb, L2–175Lu, L1–176Hf, C–177Hf, H1–178Hf,
H2–179Hf, H3–180Hf, H4–181Ta. Data were corrected for gas blank and
isobaric interferences of Yb and Lu on 176Hf using 176Yb/173Yb=
0.7952 and 176Lu/175Lu=0.02658 following the method described by
Gerdes and Zeh (2009). The correction for instrumental mass bias
used an exponential law and a 179Hf/177Hf value of 0.7325 (Patchett et
al., 1981) for correction of Hf isotopic ratios. A solution of Hf isotopic
82
M. Barboni et al. / Chemical Geology 340 (2013) 76–93
standard JMC 475 (20 ppb) was used as a monitor of data quality over
the period of laser ablation measurements. All zircon LA-MC-ICPMS
analyses were adjusted relative to the JMC 475 176Hf/ 177Hf ratio of
0.282160 (Gerdes and Zeh, 2009). Repeated analyses of the GJ-1
(n = 30) and Plesovice (n = 60) reference zircon during the course
of this study yielded a 176Hf/177Hf of 0.282012±0.000021 (±2SD) and
0.282478±0.000022, respectively, which is in perfect agreement with
solution mode analyses of these zircon standards (Gerdes and Zeh,
2006; Sláma et al., 2008)
3.2.2. Hf on zircon grains (liquid mode analysis)
Analytical protocols were the same for laser ablation and solution
mode analyses. Solution mode data were acquired with 60 integration cycles over a period of 2 min, followed by 8 min of washout
with a mixture of 2% HNO3–0.5 N HF. Data were corrected and normalized following the procedure of the laser ablation analyses (see
above).
3.2.3. Zircon Ti concentrations
Ti contents in zircon were obtained by laser-ablation ICP-MS mass
spectrometry on a Thermo Scientific element XR quadrupole mass
spectrometer coupled to a New Wave Research AR-F 193 nm Laser
ablation system at the Institute of Mineralogy and Geochemistry of
the University of Lausanne using a 20-μm ablation pit and laser output energy of 0.26 mJ/pulse and 5-Hz frequency. Measurements
were bracketed between analyses of the NIST 612 glass standard
and Si was used as an internal standard. Data reduction was done
using the Lamtrace software (Jackson et al., 1992; Longerich et al.,
1996).
4. Zircon characteristics, Hf signatures and U/Pb results
Sample location, description and chemical composition are given
in Appendix A and summarized in Table 2. Representative Hf isotopic
data are given in Fig. 3 and Table 3. All uncertainties are 2σ. Concordia
and weight mean plots are presented in Fig. 4.
Table 2
Sample location and description.
Sample
Rock type
U.T.M. coordinates
Mineralogical composition
BEG
Microgranular
A-type granite
447,202 E
5,394,742 N
MB292
Fine-grained
gabbro (sill)
443,085 E
5,394,771 N
MB100
Pegmatitic
gabbro
cumulate
Leucocratic
tonalite
439,801 E
5,395,246 N
Coarse-grained
gabbro
cumulate
Fine-grained
gabbro (sill)
441,158 E
5,395,600 N
Fine-grained
diorite
443,124 E
5,394,816 N
PM: Ab, Kfs, Qz, Bt
AM: Ep, Se, Chl
ACC: Zrn, Ap, Ox
PM: Lab (very rare), Hbl
AM: Act, Se, Prn, Ab, Ep
ACC: Zrn, Ap, Tnt, Ox
PM: Hbl (very rare), Lab
AM: Se, Ep, Act, Ab
ACC: Zrn, Ap, Tnt, Ox
PM: Kfs, Olg, Qz, Bt
AM: Se, Chl, Ab
ACC: Zrn, Tnt, Ap, Ox
PM: Lab (very rare)
AM: Act, Se, Ep, Zo, Ab, Qz, Prn
ACC: Zrn, Ap, Ox, Tnt
PM: Aug, Hbl, Lab
AM: Act, Se, Ep, Prn, Ab,
ACC: Zrn, Ap, Ox
PM: And, Bt, Qz
AM: Se, Ep, Ab, Chl
ACC: Zrn, Ap, Ox, Tnt
MB57
MB222
MB48
MB218
443,004 E
5,395,055 N
441,580 E
5,395,039 N
Abbreviations: PM = main primary minerals, AM = alteration mineral
ACC = accessory minerals (magmatic), Ab = albite, Act = actinolite, And = andesine,
Aug = augite, Ap = apatite
Bt = biotite, Chl = chlorite, Ep = epidote, Hbl = hornblende, Lab = labradorite, Kfs =
K-feldspar, Olg = oligoclase, Ox = oxides, Prn = prenite, Qtz = quartz, Se = sericite;
Tnt = titanite, Zrn = zircon, Zo = zoisite
BEG, A-type granite, E cross-section (Sector I):
Zircon grains from sample BEG are small (50–70 μm), clear, pale
yellow to orange, euhedral sharp faceted squatted prisms. P subtypes
are largely dominant (lack of {211} pyramid). They plot in the alkaline granite field of Pupin's (1980) classification. Some grains
displayed cloudy sectors before chemical abrasion, interpreted as
metamict areas. Oxide inclusions were observed in some grains. The
internal CL structure is simple with a small and rounded high-U
dark core and oscillatory zoning towards the rim (see Fig. 3).
Forty-two in-situ 40-μm spots were measured for Hf isotopes, yielding a weighted average εHf value of 2.86 ± 0.77 (MSWD = 4),
2.79 ± 0.56 (MSWD = 1.9) with 4 out of 42 analyses excluded. Values
range from − 1.3 ± 3.8 to + 4.5 ± 1.3 and show no correlation with
the relatively variable 176Yb/ 177Hf ratio of 0.04 to 0.24. There is no
distinction between black cores and lighter rims (Fig. 3, Table 3).
Six zircon grains (1–6, assuming undisturbed oscillatory zoning,
Table 1) were analyzed for U/Pb isotopes. The data are concordant within analytical error and scatter between 347.19±0.43 and 347.96±0.33
Ma. Five grains define a cluster yielding a weighted-mean 206Pb/238U
date of 347.41±0.17 Ma (MSWD=0.71) (Fig. 4), including the
youngest zircon grain (2) (206Pb/238U date of 347.19±0.43 Ma).
MB292, Fine-grained gabbro, E cross-section (Sector I):
Most zircon grains are large (100–200 μm), clear and colorless
fragments, which are sometimes prismatic with slightly smoothed
tips, developing {100} and {101} crystallographic forms. Some grains
contain small channel-like melt inclusions and opaque phases. The internal structure imaged by CL (Fig. 3) shows well-defined and regular
sector and oscillatory magmatic zoning without cores or patchy
zones. Twelve in-situ 8-μm spots were measured for Hf isotopes,
yielding a weighted-average εHf of 8.62 ± 0.19 (MSWD = 1.1). Values
range from +8.2 ± 0.6 to + 10.6 ± 1.9, overlapping within errors;
they are characterized by moderate 176Yb/ 177Hf of 0.017 to 0.093
(Fig. 3, Table 3).
Seven single zircon grains were analyzed (7 to 13, Table 1) for
U/Pb isotopes. The data are concordant within analytical error and
span over 1.6 Ma, from 346.70 ± 0.49 to 347.86 ± 0.29 Ma. The zircon
population defines two clusters. The youngest one (n = 4) yields a
weighted-mean 206Pb/ 238U date of 347.12 ± 0.30 Ma (MSWD = 1.5)
(Fig. 4), including the youngest zircon grain (9) ( 206Pb/ 238U date of
346.70 ± 0.49 Ma).
MB100, Pegmatitic gabbro, W cross-section (Sector II)
MB100 yielded very abundant large (200–450 μm), colorless and
mostly transparent (although some may contain milky areas)
anhedral fragments. CL images reveal simple magmatic oscillatory
zoning (Fig. 3). Oxides and small channel-like melt inclusions are frequent. Twenty in-situ 80-μm spots were measured for Hf isotopes,
yielding a weighted average εHf of 8.17 ± 0.43 (MSWD = 3.0). Values
range from + 7.1 ± 0.9 to + 9.7 ± 1.5, mostly overlapping within errors and their 176Yb/ 177Hf vary from 0.023 to 0.060 (Fig. 3, Table 3).
Five single grains were analyzed (fractions 14 to 18, Table 1) for
U/Pb isotopes. The data are concordant within analytical error and
span ca. 1.46 Ma, from 346.17± 0.34 to 347.64± 0.30 Ma. Four fractions yield a weight-mean 206Pb/238U date of 347.36 ±0.35 Ma
(MSWD = 1.9) (Fig. 4), while fraction (18) gave a younger 206Pb/238U
date of 346.17± 0.34 Ma.
MB222, Coarse-grained gabbro, W cross-section (Sector II)
Zircon grains are ca. 100 μm transparent, colorless and anhedral
fragments, which sometimes show {100} and {101} crystallographic
forms and pitted surfaces. Inclusions are rare. The internal structures
imaged by CL show typical magmatic concentric oscillatory zoning
without complex structures, such as cores or growth discontinuities
(Fig. 3). Sixteen in-situ 80-μm spots were measured for Hf isotopes,
yielding an average εHf of 7.97 ± 0.32 (MSWD = 1.9). Values range
from +7.1 ± 0.7 to + 8.8 ± 0.7, overlapping within errors; they are
characterized by moderate 176Yb/ 177Hf of 0.017 to 0.066 (Fig. 3,
Table 3).
M. Barboni et al. / Chemical Geology 340 (2013) 76–93
BEG A-type granite, sector I, east cross-section
a
b
εHf: 0.8±2.2
83
MB48 fine-grained gabbro, sector III, central cross-sesction
c
εHf: 3.8±2.2
εHf: 2.0±1.4
m
εHf: 8.3±0.6
n
εHf: 8.3±0.6
o
εHf: 8.3±0.6
εHf: 8.5±0.8
εHf: 2.4±2.9
εHf: 2.8±2.6
50 µm
50 µm
εHf: 8.2±1.3
εHf: 1.8±1.9 50 µm
200µm
MB292 fine-grained gabbro, sector I, east cross-section
d
e
εHf: 8.5±0.9
MB218 tonalite, sector III, central cross-section
εHf: 8.2±0.6
f
εHf: 8.6±0.6
εHf: 8.5±0.8
200µm
200µm
o εHf: 4.7±1.1
p
q
εHf: 4.3±1.0
εHf: 5.1±1.2
εHf: 3.6±0.8
εHf: 6.0±1.1
εHf: 5.3±1.1
εHf: 4.4±1.1
100 µm
εHf: 6.2±1.0
200µm
100 µm
100 µm
MB100 pegmatitic gabbro cumulate, sector II, west cross-section
MB57 tonalite, sector III, central cross-section
g
r
h
εHf: 9.0±0.7
i
s
εHf: 5.1±1.5
εHf: 4.0±1.0
100µm
100µm
εHf: 7.2±1.7
t
εHf: 8.9±0.6
εHf: 8.4±0.6
εHf: 5.5±2.1
εHf: 5.5±2.1
100µm
200µm
εHf: 8.5±0.8
k
50µm
50µm
50µm
m
MB222 coarse-grained gabbro cumulate, sector II, west cross-section
j
εHf: 4.9±1.8
εHf: 7.1±1.5
εHf: 5.4±1.3
εHf: 8.7±0.5
εHf: 8.2±1.5
100µm
l
εHf: 7.6±0.6
εHf: 7.1±0.7
100µm
100µm
100µm
Fig. 3. Representative cathodoluminescence (CL) images for each dated samples showing LA-MC-ICP-MS spots for Hf isotopes (εHf values reported).
Seven single grains were analyzed (19 to 25, Table 1) for U/Pb
isotopes. The data are concordant within analytical error and scatter
between 346.67 ± 0.30 and 347.66 ± 0.31 Ma. Six zircon grains yield
a weighted-mean 206Pb/ 238U date of 347.00 ± 0.21 Ma (MSWD =
1.7) (Fig. 4), including the youngest zircon grain (23) ( 206Pb/ 238U
date = 346.67 ± 0.30 Ma).
MB48, Fine-grained gabbro, Central cross-section (Sector III)
MB48 yielded very abundant zircon grains, which are large (200–
400 μm) colorless, transparent angular fragments that sometimes display prismatic tips. They have few mineral inclusions or cracks. CL imaging revealed simple magmatic oscillatory zoning and sector zoning
(Fig. 3), without cores or multiple growth zones. Twenty in-situ 80-μm
spots were measured for Hf isotopes, yielding a weighted-mean εHf of
8.66±0.19 (MSWD=0.8). Values range from +7.6±1.2 to +10.6±
1.9, overlapping within errors; they have 176Yb/177Hf of 0.02 to 0.15
(Fig. 3, Table 3).
Six large fragments (>150 μm) were analyzed (26–31, Table 1) for
U/Pb isotopes. The data are concordant within analytical error and scatter between 346.55 ±0.30 and 347.75± 0.28 Ma. Four fragments yield
a weighted-mean 206Pb/ 238U date of 346.82± 0.34 Ma (MSWD= 1.6)
(Fig. 4), including the youngest fraction (28) ( 206Pb/ 238U date of
346.55 ± 0.30 Ma).
MB218, Tonalite, Central cross section (Sector III)
Zircon grains from sample MB218 are large (150–200 μm), transparent, colorless prismatic fragments. The best developed crystallographic forms are {110} and {101} (G1 subtype, Pupin, 1980). CL
imaging reveals complex internal structures (Fig. 3). Most zircon
grains display a large (>50 vol.%), resorbed central area showing
concentric magmatic oscillatory zoning (Fig. 3), surrounded by clearer magmatic growth rims. 119 in-situ 40-μm spots were measured for
Hf isotopes, yielding a weighted average εHf of 4.86 ± 0.16 (MSWD =
2.4). Values range from + 3.3 ± 0.9 to + 7.1 ± 1.6 and the 176Yb/ 177Hf
varies from 0.02 to 0.25. The highest εHf values are generally
recorded in the CL gray sectors, while the lowest are recorded in the
black areas (Fig. 3, Table 3).
Eleven single grains were analyzed (32 to 42, Table 1) for U/Pb isotopes. The data are concordant within analytical error and span ca.
6 Ma, from 346.31±0.32 to 352.34±0.90 Ma. Five zircon grains form a
statistically insignificant cluster yielding a weighted-mean 206Pb/238U
date of 347.16±0.37 Ma (MSWD=3.5) (Fig. 4). One zircon (33) gave a
slightly younger 206Pb/238U date of 346.31±0.32 Ma, which is not
overlapping with the cluster.
From these eleven grains, eight were analyzed for Hf isotopes by
solution MC-ICP-MS. εHf values range from 5.28 ± 0.45 to 7.38 ±
0.33 and are positively correlated with the dates from the same grains
(Fig. 7).
MB57, Tonalite, Central cross-section (sector III)
This sample yielded abundant zircons. Grains are clear, pale yellowish, 50–100-μm euhedral prisms displaying {100} and {101} pyramids, with S25 and P5 morphologies (Pupin, 1980). Oxide inclusions
are frequent. CL imaging reveals complex internal structures with
rounded cores (30 to 50 vol.%) that show concentric oscillatory zoning,
surrounded by zoned euhedral rims. A low U (bright CL) zone occurs
systematically along the core-rim boundary (Fig. 3). The oscillatory zoning in the rim follows the same orientation as that in the core (Fig. 3).
Thirty-six in-situ 40-μm spots were measured for Hf isotopes, yielding
84
M. Barboni et al. / Chemical Geology 340 (2013) 76–93
Table 3
Representative LA-MC-ICPMS Lu-Hf isotope data of zircon from SJDD pluton samples.
Grain/spot
176
Yb/177Hf
a
±2 s
176
Lu/177Hf
a
±2 s
c
178
Hf/177Hf
180
Hf/177Hf
SigHf
(V)
b
176
Hf/177Hf
±2 s
c
176
Hf/177Hf(t)
d
εHf(t)
d
±2 s
c
TDM2
(Ga)
Sample BEG (40 μm spot) - LA-ICP-MS solid mode
BEG 7.1
0.2393
251
0.0051
BEG 9.2
0.0434
39
0.0011
BEG 15.2
0.1555
135
0.0033
BEG 17.1
0.1269
110
0.0027
BEG 17.2
0.1100
90
0.0024
BEG 19.1
0.1357
135
0.0030
BEG 28.1
0.0539
55
0.0015
BEG 28.2
0.1362
117
0.0029
BEG 49.1
0.1658
140
0.0037
BEG 49.2
0.1060
91
0.0023
41
7
24
19
16
25
13
19
23
15
1.46712
1.46716
1.46719
1.46714
1.46720
1.46713
1.46702
1.46729
1.46719
1.46726
1.88666
1.88634
1.88642
1.88659
1.88649
1.88645
1.88621
1.88681
1.88668
1.88666
7
10
10
8
8
10
14
10
9
7
0.282605
0.282705
0.282658
0.282611
0.282667
0.282554
0.282632
0.282645
0.282662
0.282693
235
48
61
74
83
119
64
51
92
74
0.282572
0.282698
0.282636
0.282594
0.282651
0.282534
0.282622
0.282626
0.282638
0.282678
0.1
4.5
2.3
0.8
2.8
−1.3
1.8
2.0
2.4
3.8
7.9
1.3
1.8
2.2
2.6
3.8
1.9
1.4
2.9
2.2
1.03
0.78
0.90
0.95
0.86
1.03
0.89
0.91
0.90
0.82
Sample MB292 (80 μm spot) - LA-ICP-MS solid mode
MB292 3.1
0.0189
17
0.00059
MB292 7.1
0.0622
54
0.00213
MB292 16.1
0.0175
16
0.00060
MB292 17.1
0.0318
28
0.00102
MB292 20.1
0.0597
48
0.00130
MB292 23.2
0.0406
33
0.00105
MB292 45.1
0.0227
18
0.00067
MB292 46.1
0.0394
32
0.00109
MB292 30.2
0.0575
46
0.00150
MB292 37.2
0.0936
78
0.00241
4
13
4
6
8
6
4
7
9
17
1.46726
1.46727
1.46728
1.46726
1.46721
1.46726
1.46728
1.46725
1.46727
1.46728
1.88668
1.88660
1.88677
1.88666
1.88657
1.88672
1.88684
1.88662
1.88675
1.88685
12
20
19
17
13
15
15
19
13
12
0.282805
0.282826
0.282796
0.282783
0.282810
0.282793
0.282820
0.282808
0.282829
0.282885
27
37
23
37
52
38
28
43
43
63
0.282802
0.282813
0.282792
0.282776
0.282802
0.282787
0.282815
0.282801
0.282820
0.282870
8.2
8.5
7.8
7.3
8.2
7.6
8.6
8.1
8.8
10.6
0.6
0.9
0.4
0.9
1.5
1.0
0.6
1.1
1.2
1.9
0.63
0.62
0.64
0.67
0.63
0.65
0.61
0.63
0.61
0.54
Sample MB100 (60 μm spot) - LA-ICP-MS solid mode
MB100 8.1
0.0309
25
0.0011
MB100 8.2
0.0597
48
0.0013
MB100 11.1
0.0370
30
0.0015
MB100 12.1
0.0365
35
0.0014
MB100 13.1
0.0514
46
0.0013
MB100 15.2
0.0273
25
0.0008
MB100 28.2
0.0439
35
0.0016
MB100 31.2
0.0227
18
0.0007
MB100 44.1
0.0280
23
0.0008
MB100 44.2
0.0401
38
0.0011
7
8
9
10
10
5
10
4
5
7
1.46695
1.46721
1.46707
1.46699
1.46718
1.46720
1.46698
1.46728
1.46701
1.46701
1.88662
1.88657
1.88675
1.88649
1.88649
1.88654
1.88662
1.88684
1.88665
1.88656
14
13
10
12
11
21
16
15
19
17
0.282834
0.282810
0.282804
0.282829
0.282853
0.282776
0.282796
0.282820
0.282828
0.282824
20
52
20
20
53
36
20
28
17
15
0.282826
0.282802
0.282795
0.282820
0.282845
0.282771
0.282786
0.282815
0.282823
0.282817
9.0
8.2
7.9
8.8
9.7
7.1
7.6
8.6
8.9
8.7
0.7
1.5
0.7
0.7
1.5
0.9
0.7
0.6
0.6
0.5
0.69
0.63
0.75
0.71
0.57
0.67
0.77
0.61
0.70
0.71
Sample MB222 (80 μm spot) - LA-ICP-MS solid mode
MB222 2.1
0.0472
39
0.00127
MB222 3.1
0.0189
17
0.00059
MB222 7.1
0.0279
25
0.00095
MB222 14.1
0.0306
26
0.00094
MB222 15.1
0.0241
21
0.00079
MB222 16.1
0.0175
16
0.00060
MB222 17.1
0.0318
28
0.00102
MB222 23.1
0.0277
25
0.00103
MB222 26.1
0.0234
20
0.00076
MB222 29.1
0.0659
53
0.00198
8
4
6
6
5
4
6
6
5
12
1.46724
1.46726
1.46729
1.46718
1.46726
1.46728
1.46726
1.46719
1.46725
1.46718
1.88668
1.88668
1.88676
1.88655
1.88668
1.88677
1.88666
1.88667
1.88674
1.88645
19
12
12
14
17
19
17
14
17
13
0.282820
0.282805
0.282825
0.282778
0.282808
0.282796
0.282783
0.282770
0.282789
0.282785
32
27
31
29
26
23
37
32
27
55
0.282812
0.282802
0.282819
0.282772
0.282803
0.282792
0.282776
0.282764
0.282784
0.282772
8.5
8.2
8.8
7.1
8.2
7.8
7.3
6.8
7.6
7.1
0.8
0.6
0.7
0.7
0.5
0.4
0.9
0.7
0.6
1.6
0.62
0.63
0.61
0.67
0.63
0.64
0.67
0.69
0.65
0.68
Sample MB48 (80 μm spot) - LA-ICP-MS solid mode
MB48 1.1
0.0633
54
0.00175
MB48 3.1
0.0230
19
0.00064
MB48 3.2
0.0472
39
0.00127
MB48 25.1
0.0273
24
0.00086
MB48 25.2
0.0866
71
0.00258
MB48 28.1
0.1521
128
0.00369
MB48 33.1
0.0228
18
0.00063
MB48 33.2
0.0319
28
0.00084
MB48 37.1
0.0261
21
0.00079
MB48 37.2
0.0936
78
0.00241
13
4
8
5
17
22
4
5
5
17
1.46721
1.46729
1.46724
1.46725
1.46723
1.46728
1.46724
1.46723
1.46725
1.46728
1.886575
1.886725
1.886679
1.886655
1.886820
1.886711
1.886732
1.886729
1.886577
1.886850
15
19
19
21
19
8
18
11
17
12
0.282796
0.282810
0.282820
0.282811
0.282820
0.282864
0.282809
0.282818
0.282817
0.282885
45
26
32
28
46
125
22
37
27
63
0.28278
0.28281
0.28281
0.28281
0.28280
0.28284
0.28281
0.28281
0.28281
0.28287
7.6
8.3
8.5
8.3
8.2
9.5
8.3
8.5
8.5
10.6
1.2
0.6
0.8
0.6
1.3
4.0
0.4
0.9
0.6
1.9
0.66
0.62
0.62
0.62
0.64
0.59
0.62
0.61
0.61
0.54
18
55
37
6
7
17
30
24
9
49
1.46687
1.46686
1.46705
1.46708
1.46708
1.46708
1.46697
1.46705
1.46707
1.46707
1.886536
1.886424
1.886629
1.886442
1.886776
1.886709
1.886534
1.886711
1.886530
1.886721
5
4
4
4
5
4
4
3
4
4
0.282702
0.282722
0.282731
0.282712
0.282728
0.282698
0.282704
0.282701
0.282734
0.282751
30
26
34
34
30
39
23
37
35
45
0.28268
0.28266
0.28272
0.282707
0.282721
0.28268
0.28267
0.28268
0.28272
0.28270
3.9
3.3
5.1
4.8
5.3
3.9
3.6
3.9
5.4
4.6
1.1
0.9
1.2
1.2
1.1
1.4
0.8
1.3
1.2
1.6
0.98
1.01
0.91
0.80
0.85
0.98
0.99
0.97
0.89
0.94
Sample MB218-cores
MB218 1.1
MB218 7.1
MB218 14.1
MB218 15.1
MB218 22.1
MB218 26.2
MB218 28.1
MB218 30.1
MB218 42.3
MB218 48.1
(40 μm spot) - LA-ICP-MS solid mode
0.0871
70
0.00305
0.2809
233
0.00914
0.0781
133
0.00233
0.0193
18
0.00078
0.0308
25
0.00118
0.0849
69
0.00276
0.1503
121
0.00489
0.0849
87
0.00290
0.0505
43
0.00154
0.2584
216
0.00767
e
(continued on next page)
M. Barboni et al. / Chemical Geology 340 (2013) 76–93
85
Table 3 (continued)
Grain/spot
176
Yb/177Hf
a
±2 s
176
Lu/177Hf
a
±2 s
c
178
Hf/177Hf
180
Hf/177Hf
SigHf
(V)
b
176
Hf/177Hf
±2 s
c
176
Hf/177Hf(t)
d
εHf(t)
d
±2 s
c
TDM2
(Ga)
Sample MB218-rims
MB218 6.3
MB218 14.3
MB218 20.1
MB218 22.2
MB218 22.3
MB218 22.4
MB218 28.3
MB218 28.4
MB218 46.2
MB218 48.3
(40 μm spot) - LA-ICP-MS solid mode
0.107
86
0.00321
0.020
20
0.00080
0.040
34
0.00146
0.014
14
0.00053
0.019
22
0.00065
0.015
12
0.00062
0.040
59
0.00141
0.026
24
0.00092
0.052
71
0.00177
0.025
21
0.00093
20
6
9
4
6
4
19
7
22
7
1.46716
1.46692
1.46708
1.46710
1.46698
1.46698
1.46698
1.46709
1.46698
1.46689
1.886816
1.886599
1.886800
1.886750
1.886551
1.886419
1.886525
1.886708
1.886542
1.886445
4
6
4
6
5
5
4
6
4
4
0.282793
0.282751
0.282736
0.282700
0.282745
0.282707
0.282702
0.282691
0.282691
0.282716
46
29
31
30
32
32
29
29
36
36
0.28277
0.28275
0.282726
0.282696
0.282741
0.282703
0.282693
0.282685
0.282679
0.282710
7.1
6.2
5.5
4.4
6.0
4.7
4.3
4.0
3.8
4.9
1.6
1.0
1.1
1.1
1.1
1.1
1.0
1.0
1.3
1.3
0.93
0.90
0.89
0.95
0.86
0.94
0.95
0.97
0.98
0.92
Sample MB57-cores
MB57 10.1
MB57 13.1
MB57 16.1
MB57 18.1
MB57 20.1
MB57 23.1
MB57 28.1
MB57 32.1
MB57 34.1
MB57 45.1
(40 μm spot) - LA-ICP-MS solid mode
0.0745
63
0.00166
0.0783
67
0.00171
0.0518
62
0.00122
0.0836
69
0.00193
0.0801
70
0.00193
0.0852
76
0.00189
0.0731
60
0.00175
0.0592
53
0.00143
0.0953
80
0.00219
0.0818
67
0.00197
10
12
10
12
12
12
11
14
13
13
1.46725
1.46718
1.46708
1.46720
1.46725
1.46723
1.46714
1.46714
1.46725
1.46714
1.886601
1.886717
1.886536
1.886757
1.886597
1.886671
1.886684
1.886401
1.886788
1.886428
7
7
6
7
7
7
7
7
6
6
0.282781
0.282748
0.282719
0.282739
0.282790
0.282762
0.282737
0.282693
0.282735
0.282758
54
68
54
69
62
69
71
72
59
61
0.282770
0.282737
0.282711
0.282726
0.282778
0.282750
0.282726
0.282684
0.282721
0.282745
7.1
5.9
5.0
5.5
7.3
6.3
5.5
4.0
5.3
6.2
1.5
2.0
1.6
2.1
1.8
2.1
2.1
2.2
1.7
1.8
0.68
0.73
0.76
0.75
0.67
0.71
0.75
0.80
0.76
0.72
Sample MB57-rims (40 μm spot) - LA-ICP-MS solid mode
MB57 4.2
0.0134
12
0.00043
MB57 10.2
0.0450
39
0.00104
MB57 11.1
0.0447
37
0.00104
MB57 18.3
0.0482
42
0.00123
MB57 18.2
0.0468
38
0.00140
MB57 20.2
0.0464
39
0.00135
MB57 23.2
0.0233
20
0.00075
MB57 28.2
0.0298
28
0.00084
MB57 29.1
0.0382
31
0.00095
MB57 31.1
0.0454
37
0.00113
3
8
9
8
9
8
5
7
7
8
1.46706
1.46725
1.46715
1.46717
1.46713
1.46711
1.46704
1.46702
1.46719
1.46711
1.886255
1.886812
1.886530
1.886534
1.886313
1.886474
1.886302
1.886055
1.886454
1.886418
6
7
7
7
7
8
6
9
8
8
0.282721
0.282781
0.282743
0.282717
0.282723
0.282711
0.282715
0.282728
0.282769
0.282716
44
58
47
60
53
56
46
48
41
55
0.282718
0.282775
0.282737
0.282709
0.282714
0.282702
0.282710
0.282723
0.282763
0.282709
5.2
7.2
5.9
4.9
5.1
4.6
4.9
5.4
6.8
4.9
1.2
1.7
1.3
1.8
1.5
1.6
1.2
1.3
1.1
1.6
0.74
0.67
0.72
0.77
0.76
0.78
0.76
0.74
0.69
0.76
Precambrian host rock (60 μm spot) - LA-ICP-MS solid mode
MB141 1
0.0249
21
0.00072
MB141 2
0.0172
15
0.00066
MB141 3
0.0197
24
0.00062
5
6
6
1.46727
1.46723
1.46722
1.886734
1.886546
1.886411
13
11
13
0.281791
0.282027
0.281419
38
41
42
0.281786
0.282023
0.281415
−27.6
−19.2
−40.8
1.0
1.1
1.1
2.05
1.72
2.55
5
7
17
11
13
5
10
9
9
1.46722
1.46721
1.46722
1.46723
1.46725
1.46719
1.46718
1.46718
1.46718
1.886568
1.886563
1.886546
1.886579
1.886547
1.886495
1.886487
1.886497
1.886523
11
19
16
19
16
64
40
54
53
0.282722
0.282735
0.282755
0.282753
0.282765
0.282770
0.282777
0.282782
0.282778
13
11
14
14
14
9
10
10
10
0.282716
0.282727
0.282736
0.282741
0.282751
0.282765
0.282766
0.282772
0.282768
5.3
5.7
6.0
6.2
6.6
7.1
7.1
7.3
7.1
0.4
0.4
0.5
0.5
0.5
0.3
0.4
0.3
0.4
0.91
0.88
0.87
0.86
0.84
0.81
0.81
0.80
0.80
8
8
8
8
7
1.46721
1.46724
1.46711
1.46717
1.46712
1.886522
1.886508
1.886612
1.886558
1.886596
7
3
4
6
4
0.282746
0.282738
0.282736
0.282737
0.282741
17
16
15
11
13
0.282738
0.282729
0.282728
0.282728
0.282734
6.0
5.7
5.7
5.7
5.9
0.6
0.6
0.5
0.4
0.5
0.86
0.88
0.88
0.88
0.87
2
4
1.46710
1.46720
1.886431
1.886646
15
20
0.282020
0.282483
26
16
0.282018
0.282483
−19.6
−2.8
0.6
0.6
1.71
1.36
Sample MB218 - LA-ICP-MS liquid mode on TIMS fractions
MB218-1 (32)
0.0390
31
0.00091
MB218-2 (33)
0.0569
46
0.00123
MB218-3 (34)
0.1266
101
0.00287
MB218-4 (35)
0.0791
63
0.00181
MB218-5 (36)
0.0912
73
0.00212
MB218-7 (38)
0.0418
33
0.00086
MB218-8 (39)
0.0835
67
0.00173
MB218-9 (41)
0.0769
62
0.00149
MB218-10 (42)
0.0804
64
0.00153
Sample MB57 - LA-ICP-MS liquid mode on TIMS fractions
MB57-2 (43)
0.0477
38
0.00129
MB57-3 (44)
0.0482
39
0.00139
MB57-4 (45)
0.0448
36
0.00126
MB57-5 (46)
0.0483
39
0.00130
MB57-6 (47)
0.0404
32
0.00114
Average standard
GJ1 (n = 19)
Plesovice (n = 11)
0.0074
0.0098
6
28
0.00027
0.00016
e
g
g
The effect of the inter-element fractionation on the Lu/Hf was estimated to be about 6 % or less based on analyses of the GJ-1, 91500 and Plesovice zircons.
a 176
Yb/177Hf = (176Yb/173Yb)true × 173Yb/177Hf)meas × (M173(Yb)/M177(Hf))b(Hf). The 176Lu/177Hf were calculated in a similar way by using the 175Lu/177Hf.
b
Mean Hf signal in volt.
c
Uncertainties are quadratic additions of the within-run error and the daily reproducibility of the 40 ppb-JMC475 solution. Uncertainties for the JMC475, GJ-1 and Plesovice are
2SD.
d
Initial 176Hf/177Hf and eHf calculated using the age determined by ID-TIMS dating (samples = 347 Ma; GJ1 standard = 606 Ma; Plesovice standard = 337 Ma).
e
Two stage model age in billion years using the measured 176Lu/177Lu of each spot (first stage = age of zircon), a value of 0.0113 for the average continental crust (second stage),
and a depleted mantle 176Lu/177Lu and 176Hf/177Hf of 0.0384 and 0.28325, respectively (see Gerdes and Zeh (2006) for details and references).
a weighted average εHf of 5.88± 0.29 (MSWD = 1.2). εHf values range
from +4.0 ±2.2 to +7.3± 1.8 and 176Yb/177Hf from 0.013 to 0.095,
without difference between cores and rims (Fig. 3, Table 3).
Five zircon grains were analyzed (43 to 47, Table 1) for U/Pb isotopes. The data are concordant within analytical error and span
1.97 Ma, from 345.49 ± 0.46 to 347.46 ± 0.48 Ma. Three zircon grains
86
M. Barboni et al. / Chemical Geology 340 (2013) 76–93
are clearly older (weighted mean= 347.30± 0.76 Ma; MSWD = 0.23).
Two young grains yield a weighted mean 206Pb/238U date of 345.55±
0.34 Ma (MSWD= 0.16) (Fig. 4), including the youngest zircon (43)
(206Pb/238U date of 345.49± 0.46 Ma).
These five grains were analyzed by MC-ICP-MS in liquid mode for
Hf isotopes (Table 3). εHf values range from + 5.65 ± 0.56 to
+ 6.03 ± 0.60, perfectly overlapping and without correlation with
ages (Figure not shown).
5. Discussion
5.1. Interpreting magma pulse emplacement dates
Each sample from our study yields multiple U/Pb zircon dates
with greater variability than can be explained by the analytical scatter alone. Weighted means on these populations give unacceptably
high MSWDs (e.g., Wendt and Carl, 1991), and thus require another
source for the observed spread. Two options are that either there is
an unrecognized source of error in our analyses, or that geologic
phenomena caused real isotopic variability. We can rule out analytical issues in the lab as the source of scatter for the following reasons. Repeated measurements on the ca. 337 Ma Plesovice zircon
standard (Appendix B) and the ca. 201 Ma North Mountain Basalt
(Schoene et al., 2010a) performed at the University of Geneva during the study period show excellent reproducibility, illustrating that
sample preparation and mass spectrometry are unlikely sources of
error in our analyses. Though common Pb contents range from 0.5
to 6.6 pg (with one outlier), with corresponding radiogenic Pb to
common Pb ratios between 1300 and 4 (though typically > 10), a
sensitivity test shows that the spread in zircon dates cannot be
resolved by varying the measured common Pb composition within
a reasonable range. Other potential sources of error in calculated
ages, such as U blank and 230Th disequilibria (Mattinson, 1973;
Schärer, 1984), are negligible in zircon grains of this U content
and age.
We therefore assume that the scatter in dates is geologically
meaningful and indicates either a) post-crystallization Pb-loss, b) incorporation of xenocrystic zircon cores, c) post-emplacement zircon
growth, or d) incorporation of slightly older zircon grains from different magma batches of the same system (antecrystic grains; Miller et
al., 2007).
Although Pb-loss (option a) cannot be ruled out for all zircon
grains from this study, two lines of evidence suggest that it is not
the cause for the observed spread in U–Pb dates. First, the chemical
abrasion technique was employed on all zircons from this study,
which has been shown to have a dramatic effect on remediating
Pb-loss in even the most discordant zircon grains (Mattinson, 2005;
Schoene and Bowring, 2010). The reproducibility of the Plesoviçe zircon analyses also suggests that Pb-loss is not an issue (Appendix B),
because that standard is the same age as our samples, but has
higher-U contents, and therefore should result in higher-degrees of
Pb-loss. Second, and very importantly, the correlation in zircon
dates and solution εHf data from the same dissolved zircons from
sample MB218 completely rule out Pb-loss as a source of the spread
in dates for that sample.
Despite numerous cores observed in CL images in zircon from this
study, xenocrystic components from Precambrian basement rocks
(option b) can be ruled out as a source of scatter in U/Pb dates. This
is because our in situ LA-ICPMS εHf data from both cores and rims
of all the SJDD zircon grains are uniformly positive (values are ~ 3–
7; Table 3), whereas epsilon values from the Precambrian host rocks
are uniformly very negative (Table 3).
Post-emplacement zircon growth (option c) is unlikely in the eastern Sector I of the intrusion where magma pulses solidified very
quickly, as documented by sharp contacts between successive sills
(Fig. 2B). Slow cooling accompanying zircon growth is more likely
in the thermally mature Sector III, where magma mush could have
been remobilized prior to complete solidification (as documented
by magma mingling, Fig. 2E). Nonetheless, thermal models mimicking upper crustal sill-on-sill intrusions like the SJDD (e.g., Annen,
2010) show that it is unlikely that sills from the SJDD remained
supersolidus for the millions of years that would be required given
the spread in zircon dates. Furthermore, the positive correlation between U/Pb dates and εHf in sample MB218 (Fig. 7) from Sector III,
discussed above, is also in contradiction with grain post-crystallization
because post-emplacement magma mixing and/or assimilation is not
observed in outcrop. We therefore conclude that option (d), incorporation of antecrystic zircon, is the most likely source for spread in zircon
dates from the SJDD samples, a conclusion that is supported by complex
internal structures revealed by CL imaging (Fig. 3) and both in situ
and solution Hf isotopes of zircon. This will be discussed in detail
in Section 5.3.
Because we are confident that antecrystic zircons are the cause
of the spread in zircon dates, it is reasonable to use the youngest
zircon date as the best estimate of intrusion time for successive
magma pulses. However, this approach ignores anticipated analytical scatter in measurements and may bias our age interpretations
too young. We thus explore two approaches to interpreting intrusion ages and show that our preferred interpretation does not affect
the end result. The first approach is to find the most statistically
equivalent population of zircon and apply the weighted-mean statistic (e.g. Coleman et al., 2004; Ovtcharova et al., 2006; Michel et
al., 2008; Peytcheva et al., 2008; Memeti et al., 2010), which should
account for analytical scatter and arrive at the best estimate of the
crystallization age – assuming instantaneous zircon growth and absence of Pb-loss. In all but two of our samples, the most coherent
population includes the youngest grain from the sample (Fig. 4).
Our preferred approach, following the discussion above, is to consider that the scatter in dates reflects antecrystic grains that crystallized prior to emplacement. In this case, the youngest zircon should
be the best estimate for the magma emplacement (e.g. Schaltegger
et al., 2009). Nonetheless, we present both approaches for our
datasets (Figs. 4 and 5; Table 4) and evaluate the resulting intrusion
ages in the context of field relationships and Hf isotopes. With the
exception of sample MB218, illustrated in Fig. 5, the weightedmean and youngest date approaches lead to very similar interpretations in terms of emplacement duration and both agree well with
relative ages dictated by field relationships. Concerning Sector III
sample MB218, the weighted-mean suggests an emplacement contemporaneous with Sector I A-type granite. However, the positive
correlation between dates and Hf signatures show that the
weighted-mean population is probably not equivalent and rather
point to antecrystic zircon grains (see Section 5.3) within the
sample.
5.2. Timing of emplacement of the SJDD pluton
The composite, incrementally built SJDD pluton exhibits a
subhorizontal sill-on-sill assemblage with subordinate crosscutting
dikes (see Section 2.2) and is reminiscent of a Christmas-tree laccolith
geometry (e.g. Hunt, 1953, Westerman et al., 2004). The age difference between the oldest and youngest magma pulses yields a first
order approximation for the total duration of SJDD pluton construction. As only the top 100 m out of the 1500 m intrusion thickness
are exposed, one might expect significantly younger sill injections in
the lower sector of the laccolith if the latter was built only by sill
underaccretion. We do not favor this interpretation because the
youngest dated sample was collected in a major vertical brecciated
fault zone, which we interpret as a late magma pulse injected vertically throughout the laccolith. The calculated duration based on
outcropping samples is nevertheless a minimum.
M. Barboni et al. / Chemical Geology 340 (2013) 76–93
In the discussion below, we interpret the youngest zircon 206Pb/ 238U
date as our best estimate of the age of successive sill emplacement (see
discussion in Section 5.1). According to field relationships and confirmed by the high-precision dates (Fig. 5), Sector I represents an
early phase of pluton construction, with contemporaneous emplacement of the A-type granite stocks and the eastern bimodal rocks
at 347.19 ± 0.43 Ma (sample BEG) and 346.70 ± 0.49 Ma (sample
MB292), respectively (Figs. 1A and 4). The western coarse-grained
gabbro Sector II was built simultaneously within age uncertainties,
Fig. 4. Concordia diagrams and
206
87
as documented by samples MB222 (346.67 ± 0.30 Ma), while a Sector II gabbro pegmatite (MB100) is younger (346.17 ± 0.34 Ma).
The pegmatites may represent late crystallization in H2O-rich residual melts. The central Sector III records the youngest emplacement
ages as suggested by field observations (Figs. 1A and 2), with crystallization of the younger bimodal sills and their potential feeding
zone at 346.31 ± 0.32 Ma (sample MB218) and 346.55 ± 0.30 Ma
(sample MB48), respectively. The last emplacement event, also consistent with cross-cutting relationships (Fig. 2I), is recorded by a
Pb/238U age plots containing the results of zircon U–Pb dating of the samples from the SJDD intrusion.
88
M. Barboni et al. / Chemical Geology 340 (2013) 76–93
Fig. 4 (continued).
tonalite in the brecciated area at 345.49 ± 0.46 Ma (sample MB57).
Using the oldest and youngest intrusion ages yields an emplacement sequence lasting 1.70 ± 0.63 Ma, taking age uncertainties
into account. Note that using the weighted-mean intrusion age interpretation (Fig. 5; Section 5.1) leads to a similar duration
(1.86 ± 0.38 Ma). Such a duration is realistic as million year emplacement times have been reported in the literature for composite
shallow level plutons, such as the Mount Stuart batholith, Washington (ca. 5.5 Ma, Matzel et al., 2006), the Half Dome granodiorite,
Tuolumne suites, California (ca. 4 Ma, Coleman et al., 2004) and
the southern Adamello batholith (ca. 2.6 Ma, Schaltegger et al.,
2009).
5.3. Sources and implications of antecrystic zircon grains
Complex spreads in dates are observed in analyzed zircon grains
from most SJDD samples, ranging from ca. 1 Ma to more than 7 Ma
(Fig. 1B). As discussed above, we assume that these spreads are
M. Barboni et al. / Chemical Geology 340 (2013) 76–93
89
Fig. 4 (continued).
geologically meaningful and cannot be ascribed to Pb-loss or analytical error. Such a scatter is also too big to simply represent prolonged
growth of autocrystic zircon in an individual magma batch, as demonstrated by Annen (2010) modeling of sills cooling rate in the upper
crust. We will consider sample MB218, which shows the biggest scatter (ca. 7 Ma) as an example for the following discussion. CL images of
zircon grains from this sample (Fig. 3) display a complex geometry
recording at least three growth events. Most zircon grains have a
large U-rich core (2/3 of the volume of the grain), which sometimes
hosts an inner resorbed core, surrounded by low U rims. As mentioned earlier, since the SJDD host rocks are mainly of Precambrian
age (600 Ma to 2200 Ma), incorporation of host rock xenocrystic zircon components would result in a much bigger age scatter, considering the large core to rim volume ratio of the crystals. In-situ Hf isotope
measurement within the various sectors also rules out any
xenocrystic components because εHf values in xenocrystic zircon
from the Precambrian host rock range between − 19 and −40
whereas SJDD zircon cores gave uniformly positive εHf (+ 3 to + 6,
Fig. 3; Table 3). Similarly, there is no record of other magmatic
rocks of the same age in the area that could have provided
zircon grains. Therefore, external inheritance is not likely for sample
MB218.
We therefore infer that the age spread observed in our sample
might result from recycling of antecrystic grains crystallized earlier
in the same magmatic system (internal inheritance). We measured
Hf isotopes on MB218 zircon grains by LA-MC-ICP-MS within the
cores and rims to track any differences within the two and by solution MC-ICP-MS on the TIMS washes in order to track correlations
with the high-precision dates (Figs. 6 and 7). LA-ICP-MS results
display similar ratios within cores and rims, with initial weightedmean εHfcores = 5.26 ± 0.36 and initial εHfrims = 5.08 ± 0.26, respectively (all 2 sigma; Fig. 6, Table 3). Analyses performed on TIMS
washes by liquid mode LA-MC-ICP-MS (Fig. 7) provided more precise results (Table 3) and show a positive correlation between the
U/Pb zircon dates and their respective Hf isotope ratios. Initial εHf
values decrease from 7.38 to 5.66 with decreasing date. As all grains
348
Sector I
Sector II
Sector III
347
MB292
MB222
MB100
MB218
MB48
206
238
Pb/ U date (Ma)
BEG
346
Youngest dates
MB57
Weighted-mean dates
345
Emplacement sequence (oldest to youngest events) according to field relationships
Fig. 5. Compilation of the weighted-mean dates and youngest dates plotted in the order required by field relationships (see text for details).
90
M. Barboni et al. / Chemical Geology 340 (2013) 76–93
8.0
Table 4
Comparison between the youngest zircon date and the weight-mean dates.
Emplacement dates
Youngest
Weighted-mean
7.5
Date
±
N
Date
±
MSWD
1
1
1
1
1
1
1
347.2
346.7
346.2
346.7
346.6
346.3
345.5
0.4
0.5
0.3
0.3
0.3
0.3
0.5
5
4
4
6
4
5
2
347.41
347.12
347.36
347.00
346.82
347.16
345.55
0.17
0.30
0.35
0.21
0.34
0.37
0.34
0.7
1.5
1.9
1.7
1.6
3.5
0.2
7.0
6.5
εHf
BEG
MB292
MB100
MB222
MB48
MB218
MB57
N
All dates are 206Pb/238U dates in millions of years. All uncertainties are at the 95%
confidence level.
N = number of data points used in the weighted-mean calculation; MSWD = mean
square of weighted deviates for weighted-mean.
6.0
5.5
display complex core-rim structures, the results are mixtures between two or more end-members with isotopic composition possibly ranging between 3 and 9. Considering that the antecrystic
hypothesis remains the best explanation for MB218 zircon grains,
then chemical characteristics of the zircon antecrysts could be
used as a tool to decipher potential magma sources, magma hybridization and/or magma differentiation processes in the deep levels of
the magmatic system (e.g. Schoene et al., 2010b). Regarding the
positive correlation between the U/Pb dates and εHf, if we consider
that the youngest single grains contain a larger autocrystic component (predominance of the rim) than the oldest grains (predominance of the cores), then we may infer a contamination of mafic
magmas derived from a relatively depleted mantle source (εHf> 7.38)
by a more enriched component (εHfb 5.66), presumably of crustal origin. This hybridization is corroborated by a spread in whole-rock εNd
values from +4.4 to +6.7 among the mafic sills of the SJDD intrusion
(Barboni et al., 2011).
Potential survival of zircon antecrysts implies that the magma
reached Zr saturation early enough to avoid their complete dissolution. This could be the case for tonalite MB218, in which Zr behaved
as an incompatible element during crystallization of pyroxene, amphibole and plagioclase. Ti-in zircon thermometry (Watson and
5.0
4.5
345
347
349
351
353
TIMS U/Pb date (Ma)
Fig. 7. εHf versus
206
Pb/238U age diagram for sample MB218 zircons.
Harrison, 2005; Watson et al., 2006), can be used in order to determine melt temperatures at time of zircon crystallization (TTi). The
early calibration of this thermometer requires the presence of quartz
and rutile in order to buffer the Si and Ti activity (aSiO2 ≈ 1;
aTiO2 ≈ 1). Rutile is absent in sample MB218, although other Ti-rich
phases such as ilmenite and sphene are present. A more recent calibration (Ferry and Watson, 2007) proposes a reliable Ti-in-zircon
thermometer that can be applied to rocks without rutile, provided
that aSiO2 and aTiO2 are estimated. Quartz is an abundant phase within sample MB218 (whole-rock = 61 wt.% SiO2). Hence aSiO2 could be
estimated to be ≈ 1. aTiO2 was estimated at ca. 0.77 using the
9
8
7
6
Hf
5
4
3
2
1
Rims
Cores
0
0
5
10
15
20
Zircon grain number
Fig. 6. LA-MC-ICP-MS εHf values for cores and rims within MB218 zircons.
25
M. Barboni et al. / Chemical Geology 340 (2013) 76–93
Table 5
Zr-saturation T°C for MB218 antecrystic sample.
Samples
MB218
Zr corrected for an
80% volume core
Composition (wt.%)
SiO2
Al2O3
CaO
K2O
Na2O
Equivalent oxides weight
SiO2
Al2O3
CaO
K2O
Na2O
Total
Cation fractions
Si
Al
Ca
K
Na
M
Zr zircon (ppm)
Zr rocks (ppm)
lnDZr
T (K)
T (°C)
Zircon solubility (Csat, ppm)
61.38
19.20
4.60
7.73
0.77
1.02
0.32
0.08
0.13
0.01
1.56
0.66
0.20
0.05
0.08
0.01
1.41
497,643
190
7.87
1073
800
175
Calculations after Watson and Harrison (1983)
Volumes of antecrystic core estimated by CL Imaging
Whole-rock Zr content corrected for the antecrystic component
Assuming that all the Zr is in zircon
methodology of Hayden and Watson (2007). Such a value is realistic
considering the presence of sphene and ilmenite and is in the typical
range expected for silicic melts that are not saturated in rutile (0.6
and 0.9; Watson et al., 2006; Hayden and Watson, 2007). We
Table 6
Ti concentrations and Ti-temperatures for MB218.
MB218-2 (black cores)
MB218-3 (black cores)
MB218-10 (black core)
MB218-13 (black core)
MB218-25 (black core)
MB218- (black core)
MB218-33 (black core)
MB218-32 (black core)
MB218-46 (black core)
MB218-25 (white rim)
MB218-35 (white rim)
MB218- 33 (white rim)
MB218-32 (white rim)
MB218-44 (white rim)
MB218-46 (white rim)
Average black Cores
Average white rims
Esimated aSiO2
Estimated aTiO2
Ti (ppm)
Ti-temperature in zircons (°C)
6.9
10.6
16.0
8.2
15.9
14.0
9.0
29.9
15.5
13.7
11.3
13.1
17.1
18.0
2.5
14.0
12.6
1
0.77
735
776
819
752
818
805
760
891
815
803
783
798
827
832
650
797 (σX = 47)
782 (σX = 67)
T°C was calculated using the Ferry and Watson (2007) calibration.
aSiO2 is estimated to be 1 (Quartz in equilibrium with zircon).
aTiO2 was estimated using Hayden and Watson (2007) equations.
σX is the standard deviation for the averages.
91
obtained a mean TTi-inherited-core of 797 °C and a TTi-rim of 782 °C for
MB218, respectively (Table 6).
Experimental studies by Watson and Harrison (1983) constrain the
temperature at which zircon crystallizes as a function of melt composition and Zr content (Crock). They also allow a better understanding
of the nature and survival of inherited zircon in magmatic
systems (Miller and Miller, 2003, Miller et al., 2007; Peytcheva et al.,
2008). We therefore investigated the potential survival of MB218
antecrystic grains using this method to estimate the saturation temperature (Tsat) and zirconium concentration in zircon-saturated melt
(Csat). Inheritance of antecryst zircon cores must be taken into account
and subtracted from the whole-rock Zr content before determining the
zircon saturation temperature (Hanchar and Watson, 2003). We used
CL imaging to estimate the volume of antecrystic zircon grains (assuming that the cores are ellipsoidal) and subtracted an average amount of
Zr from the measured bulk rock Zr content (950 ppm) to correct for
the inherited sector, assuming that all the Zr is in zircon (no pyroxene
and hornblende in the sample). Antecrystic cores usually represent as
much as 80% of the zircon total volume in sample MB218 (Fig. 3). We
obtained a corrected Zr content (Crock) of 190 ppm. According to
Watson and Harrison (1983), the tonalitic melt would reach Zr saturation at ca. 800 °C (Tsat, M = 1.4, within the calibration range; Table 5)
and a maximum solubility of 175 ppm (Csat). The Zr concentration in
this sample is thus higher than that required for zircon saturation. Considering the high Tsat, early zircon crystallization is expected at depth
and most of these early crystallized grains may survive during ascent
and emplacement of the oversaturated melt into the shallow crust
(Watson and Harrison, 1983). Note that Tsat (800 °C) is similar to TTi
(797 °C), while Harrison et al. (2007) predicted crystallization temperatures beginning well above the calculated bulk zircon saturation temperature. We assumed in our calculations that the crushed whole-rock
Zr content is representative of that in the melt. In reality, given the
effect of fractionation of major phases on the zirconium content of
the melt (Harrison et al., 2007), the original melt Zr content may
have been overestimated, leading to higher saturation temperatures.
However, quantifying such an effect would require a better knowledge
of the crystallization sequence prior to emplacement.
In summary, zircon from MB218 may record incorporation of older
antecrystic grains (up to several Ma) within new magma inputs, probably within deeper-seated reservoirs located beneath the SJDD intrusion. Such a process would require convection on some scale in order
to mechanically mix the magma and may be linked to the successive
magmatic injections within the reservoirs (thermal waxing and waning as defined e.g. by Annen et al., 2006). Incorporation may also be
driven by crystal remobilization during interstitial melt escape, as proposed by Hildreth (2007) or by mixing between largely crystallized
mush with crystal-poor magmas (Miller et al., 2007). Such processes
require that the mush retains liquid. Scaillet et al. (1996, 1998) have
shown that water-rich magmatic systems (as inferred in SJDD) could
stay mobile even close to their solidus temperature (70–80% crystals).
Magma mixing processes in deeper reservoirs are documented within
the SJDD intrusion by hybrid, microgranular mafic enclave-bearing
tonalite/granodiorite (Barboni et al., 2007). Antecrystic cores may be
formed and incorporated in new batches at the same level and rims
may crystallize during pulse ascent and/or when pulses arrive at
emplacement level (e.g. Bergantz, 2000).
5.4. Timing of overall magmatic activity and magma evolution recorded
by zircon antecrysts
If some zircon grains from sample MB218 predate emplacement of
the magma in the upper crust, we might consider the oldest measured date as recording protracted magmatic activity somewhere
below the final emplacement level, and the youngest date as having
recorded in-situ crystallization. Antecrysts formed at deeper levels
are remobilized and brought to emplacement level (ca. 9 km) by
92
M. Barboni et al. / Chemical Geology 340 (2013) 76–93
new magma pulses, where autocryst rims are formed. Zircon may
crystallize during magma transport to the level of emplacement, leading to the observed spread of ages. Thus the minimum duration of the
overall SJDD magmatic activity can be approximated as the difference
between the oldest antecryst age (352.54 ± 0.90 Ma) and the youngest autocryst age (345.49 ± 0.46 Ma), keeping in mind that the latter
is a maximum age, as all zircon autocrysts might potentially contain a
small antecrystic fraction. This yields an overall lifespan between 5.69
to 8.41 Ma accounting for analytical uncertainties. Dates between 351
and 345 Ma might record mixed ages, or correspond to intermediate
antecrystic growth. Because the SJDD geodynamic environment is
interpreted as a back-arc setting (Faure et al., 1997, 2005; Barboni
et al., 2008), its growth period potentially corresponds to the duration
of the tectonic–magmatic activity linked to such a transtensional to
extensional context.
6. Conclusions
(1) The SJDD pluton is a well-documented example of an incrementally built shallow intrusion formed by discrete magma pulses,
similarly to the Tuolumne pluton, California (e.g. Coleman et al.,
2004) or the Torres del Paine laccolith, Patagonia (Michel et al.,
2008). Relative ages determined by field observations are consistent with, and aid in interpreting, complex high-precision U/Pb
zircon dates. Based on ID-TIMS U/Pb dates on single zircon
grains, we have estimated that the SJDD intrusion was emplaced
over a time span of 1.70 ±0.63 Ma, with the sills becoming
younger from east to west as suggested by field observation.
(2) High precision data sets from single hand samples within the
SJDD commonly show spreads in single zircon dates spanning
from 1 to 7 Ma. We interpret these spreads as incorporation of
antecrystic zircon from the same magmatic system on the
basis of Hf isotopes within cores and rims of complex zircon
grains. We consider that the oldest measured date on a single
batch as having recorded a protracted magmatic activity somewhere below the final emplacement level, and the youngest
date as most closely representing the emplacement age. Taking
the biggest spread within one magma batch in the SJDD case, we
can estimate a minimum lifespan of between 5.69 and 8.41 Ma
for the SJDD magmatic system. Hf isotopes of individual dated zircon grains show decreasing εHf through time. We infer this trend
as being a mixture between mantle derived mafic magmas
(εHf>7.38) and a more enriched component (εHfb 5.66), presumably of crustal origin.
Acknowledgment
We appreciate the stimulating discussions and support in the
writing of this paper offered by C. Annen. Helpful reviews from
two anonymous reviewers improved an early version of this paper.
This study was funded by the Swiss National Science Foundation,
grant 200021–116705, as well as by the Société Académique
Vaudoise (SAV).
Appendix A and B. Supplementary data
Supplementary data to this article can be found online at http://
dx.doi.org/10.1016/j.chemgeo.2012.12.011.
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