JOURNAL OF PETROLOGY
VOLUME 45
NUMBER 6
PAGES 1109–1124
2004
DOI: 10.1093/petrology/egh006
Origin of the Gabbro---Peridotite Association
from the Northern Apennine Ophiolites (Italy)
RICCARDO TRIBUZIO1,2*, MATTHEW F. THIRLWALL3 AND
RICCARDO VANNUCCI1,2
1
DIPARTIMENTO DI SCIENZE DELLA TERRA, UNIVERSITÀ DI PAVIA, VIA FERRATA 1, 27100 PAVIA, ITALY
2
ISTITUTO DI GEOSCIENZE E GEORISORSE, CNR, SEZIONE DI PAVIA, VIA FERRATA 1, 27100 PAVIA, ITALY
3
DEPARTMENT OF GEOLOGY, ROYAL HOLLOWAY, UNIVERSITY OF LONDON, EGHAM TW20 0EX, UK
RECEIVED MARCH 4, 2002; ACCEPTED NOVEMBER 18, 2003
The Northern Apennine ophiolites are remnants of the Middle
Jurassic---Early Cretaceous lithosphere from the Ligurian Tethys.
New trace element and Nd---Sr isotope investigations were performed
on: (1) the rare gabbros associated with the subcontinental mantle
rocks from the External Liguride ophiolites; (2) the gabbro--peridotite association from the poorly known ophiolitic bodies from
Cecina valley (Southern Tuscany). Clinopyroxenes from the
External Liguride and Cecina valley gabbros have similar trace
element compositions, which are consistent with formation from
normal mid-ocean ridge basalt (N-MORB) magmas. Sm---Nd
mineral isochron ages are 179 9 Ma for an External Liguride
gabbro and 170 13 Ma and 1735 48 Ma for two different
gabbroic bodies from the Cecina valley ophiolites. These ages are
interpreted to date the igneous crystallization of the gabbros and are
slightly older than the oldest pelagic sediments of the Ligurian
Tethys. Initial eNd (þ85 to þ89) and 87 Sr/86 Sr of clinopyroxene
are consistent with the interpretation that the studied gabbros
were derived from N-MORB magmas. The least serpentinized
mantle rocks from the Cecina valley ophiolites are porphyroclastic
spinel lherzolites displaying a residual geochemical signature. They
are similar to the least depleted residual peridotites from modern
oceans. Nd and Sr isotopic ratios for separated mantle clinopyroxene
are respectively higher (e.g. eNd ¼ þ11) and lower than those of
clinopyroxene from associated gabbros at the time of the gabbro
intrusion. The gabbro---peridotite associations from the Northern
Apennine ophiolites record the progression of the rifting process that
led to opening of the Ligurian Tethys.
INTRODUCTION
lithosphere
The Middle Jurassic---Early Cretaceous Ligurian Tethys
is considered to have developed in conjunction with the
opening of the Central Atlantic Ocean (Lemoine et al.,
1987; Bill et al., 2001). Lithospheric remnants of this basin
are represented by ophiolites from the Northern Apennines,
Western Alps and Corsica (Fig. 1). These ophiolites are
characterized by a substrate of serpentinized mantle
rocks and gabbroic plutons, which is discontinuously
overlain by pillow lavas, sedimentary breccias or pelagic
sediments (Abbate et al., 1980; Cortesogno et al., 1987).
Similar lithostratigraphic features were reported for the
Mid-Atlantic Ridge, for instance at the intersection with
the Kane Fracture Zone (i.e. the MARK area) and from
the 15 N region (Cannat et al., 1997a, 1997b). The resemblance between the Ligurian Tethys ophiolites and the
MARK area lithosphere is supported by the petrological
and geochemical similarities of the gabbroic sequences
(Tribuzio et al., 2000a).
Rampone et al. (1998) showed that crustal and mantle
rocks of the Ligurian Tethys ophiolites locally have contrasting Nd isotope values. In particular, gabbros and
basaltic rocks from the so-called Internal Liguride ophiolites
(Northern Apennine) display typical normal mid-ocean
ridge basalt (N-MORB) Nd isotope values, whereas
associated mantle lherzolites are characterized by an
extremely depleted Nd isotopic signature (see also
Rampone et al., 1996). On the basis of such isotopic
contrast, it was argued that: (1) a melt-residue genetic
relation does not exist for this crust---mantle association;
(2) the lherzolites differ from the mantle rocks of modern
oceans, despite the mineralogical, major and trace
*Corresponding author. Telephone: 0382 505874. Fax: 0382 505890.
E-mail: [email protected]
Journal of Petrology 45(6) # Oxford University Press 2004; all rights
reserved
KEY WORDS:
gabbro; peridotite; ophiolite; Northern Apennine; oceanic
JOURNAL OF PETROLOGY
VOLUME 45
Milano
External Liguride
ophiolites
Torino
Internal Liguride
ophiolites
Firenze
Cecina valley
ophiolites
Corsica
Fig. 1. Index map of the main ophiolitic bodies (black areas) from the
Northern Apennines, Western Alps and Corsica. Horizontally ruled
area is Helvetic domain (palaeo-European continental margin); dotted
area is Western Alps and Corsica; vertically ruled area is Northern
Apennines; oblique ruled area (close line net) is Sudalpine domain
(palaeo-African continental margin).
element similarities (Rampone et al., 1996, 1998). The
Ligurian Tethys was thus considered as an embryonic
oceanic basin similar to the Red Sea, or as an ocean--continent transition similar to the non-volcanic continental
margin of Western Iberia (see also Rampone & Piccardo,
2000; Piccardo et al., 2002).
To unravel the nature of the crust---mantle link in the
Ligurian Tethys, we have carried out new trace element
and Nd---Sr isotope analyses of the gabbro---peridotite
association from the ophiolitic bodies cropping out
along the Cecina valley in Southern Tuscany (Northern
Apennines). In addition, to provide a comprehensive
overview of the gabbro---peridotite association from
the Northern Apennine ophiolites, we have analysed the
rare gabbros from the so-called External Liguride ophiolites,
which are characterized by mantle material of subcontinental nature (Beccaluva et al., 1984; Ottonello
et al., 1984; Rampone et al., 1995). Combining the data
presented with literature information on the Northern
Apennine ophiolites and Atlantic Ocean lithosphere, we
conclude that the gabbro---peridotite associations of the
Ligurian Tethys record the progression of a rifting process
that resulted in a magma-poor slow-spreading centre.
GEOLOGICAL FRAMEWORK AND
SELECTED SAMPLES
The ophiolitic bodies from the Northern Apennines may
be assembled into three groups: (1) Internal Ligurides;
NUMBER 6
JUNE 2004
(2) External Ligurides; (3) Southern Tuscany (Fig. 1). The
Internal Liguride ophiolites are stratigraphically associated with a turbiditic sequence of Late Cretaceous to
Early Paleocene age, which evolved over time from distal
to proximal (Marroni et al., 1992). These ophiolites preserve close lithostratigraphic similarities to slow-spreading
centres from the Mid-Atlantic Ridge and are thus considered to represent an intra-oceanic domain of the
Ligurian Tethys (Barrett & Spooner, 1977; Cortesogno
et al., 1987; Tribuzio et al., 2000a; Williams et al., 2002).
The External Liguride ophiolites occur as huge olistoliths
within Late Cretaceous sedimentary melanges, together
with sparse bodies of continental origin (Marroni et al.,
1998). The continental crust rocks are mainly granulitefacies metagabbros and peraluminous granitoids, which
both have Late Carboniferous---Early Permian age (see
also Montanini & Tribuzio, 2001). The association of the
External Liguride ophiolites with continental crust rocks
was related to an ocean---continent transition, which fits
well with non-volcanic continental margins (Marroni
et al., 1998).
The ophiolitic bodies from Southern Tuscany crop out
as huge olistoliths in Late Cretaceous to Eocene flyschoid
sequences and are not associated with continental crust
material (Abbate et al., 1980). The palaeogeography pertaining to these ophiolites has not been accurately established yet. The selected ophiolitic bodies (Cecina valley)
are related to a heterogeneous flyschoid sequence of
Middle Paleocene---Early Eocene age (Mazzanti, 1966).
The External Liguride ophiolites
The mantle ultramafics from the External Liguride
ophiolites are mainly fertile spinel lherzolites containing
sporadic pyroxenite layers and disseminated titanian
pargasite (Beccaluva et al., 1984; Ottonello et al., 1984).
Temperature estimates for the spinel-facies equilibration
(1000---1050 C) and Nd---Sr isotope compositions have
been related to a lithospheric subcontinental origin (see
also Rampone et al., 1995). The lherzolites commonly
show late growth of plagioclase, which is locally associated with the development of mylonite fabric (Beccaluva
et al., 1984). The plagioclase growth has been ascribed to
subsolidus decompression in Middle to Late Jurassic
times (Rampone et al., 1993, 1995).
The gabbros are volumetrically subordinate to the
mantle peridotites and basaltic rocks (Terranova &
Zanzucchi, 1982). They crop out as dykes and metrescale lenticular bodies within the mantle ultramafics and,
rarely, as huge olistoliths (up to hundreds of metres in
size). The selected samples for this study were collected
from an olistolith [see Marroni et al. (1998) for location]
made of coarse-grained, subophitic olivine-bearing
gabbro and layered troctolite. Preliminary geochemical
data on olivine-bearing gabbros selected for this study
1110
TRIBUZIO et al.
NORTHERN APENNINE GABBRO---PERIDOTITE ASSOCIATION
(GEL3 and GEL9) indicate that they are cumulates
derived from MORB-type melts (Marroni et al., 1998;
Tribuzio et al., 2000a). New petrographic investigations
have revealed that the igneous clinopyroxene from sample GEL3 is locally overgrown by secondary clinopyroxene (almost pure diopside) and tremolite.
The Cecina valley ophiolites
The petrological and geochemical features of the Cecina
valley mantle rocks are poorly known. On the other
hand, there are many studies of the associated gabbros,
which are coarse-grained subophitic olivine-bearing
gabbros to minor layered troctolites, both of cumulus
origin (Serri, 1980; Hebert et al., 1989; Tiepolo et al.,
1997). The samples selected for this study are olivinebearing gabbros collected from the ophiolitic bodies of
Riparbella and Montecastelli [see Tiepolo et al. (1997) for
location]. The Riparbella samples (CBC3, CB17 and
CB16) were collected in a breccia ( 100 m 30 m in
size) that stratigraphically covers a large mass of mantlederived serpentinites. Tribuzio et al. (1999) showed that
the outermost rims of clinopyroxene and plagioclase from
these rocks are enriched in incompatible trace elements
relative to the cores. These chemical variations were
related to a process of post-cumulus migration of a highly
evolved trondhjemitic liquid in the gabbroic crystal mush
(see also Tribuzio et al., 2000b).
The Montecastelli gabbros (MF1 and MF2) were
sampled from a kilometre-scale pluton that mainly consists of olivine-bearing gabbro (Tiepolo et al., 1997). The
associated kilometre-scale mantle peridotite (Mazzanti,
1966) is made of variably serpentinized rocks, which
locally include small dykes (thickness 502 m) of
rodingitized gabbro. The least serpentinized peridotites
are spinel lherzolites displaying porphyroclastic texture
and rather constant primary modal compositions. In the
selected samples (TP1, TP2 and TP9), the original grains
of olivine and orthopyroxene are 05 cm in size and late
serpentine makes up 45% of the rock volume. Clinopyroxene and spinel occur in minor modal amounts
(5---10% and 55%, respectively) and their grain size is
smaller than that of olivine and orthopyroxene. Spinel
is generally interstitial to olivine and orthopyroxene.
Commonly, orthopyroxene displays thin exsolution
lamellae of clinopyroxene, and vice versa. The orthopyroxene exsolution lamellae in clinopyroxene are locally
replaced by serpentine. Both orthopyroxene and clinopyroxene porphyroclasts are in places bent and rimmed
by fine-grained aggregates of neoblastic pyroxene, associated with minor spinel and, locally, olivine.
ANALYTICAL TECHNIQUES
Clinopyroxene cores were analysed for rare earth
elements (REE) and selected trace elements (Table 1)
by secondary ion mass spectrometry at Istituto di
Geoscienze e Georisorse, Sezione di Pavia, according to
the method described by Bottazzi et al. (1994). The energy
filtering technique (Shimizu et al., 1978) was applied to
remove molecular ion interferences by applying 100 V
voltage offset. In addition, 45 Sc and 52 Cr signals were
corrected for the presence of 29 Si16 O þ and 24 Mg28 Si þ
interferences, respectively. Precision and accuracy are
estimated to be better than 10% above 1 ppm concentration. Below 1 ppm concentration, precision is constrained by counting statistics to be in the range 10---30%.
Major element analyses of minerals from the Cecina
valley mantle peridotites were carried out using a JEOL
JXA-840A electron microprobe, located at Centro
Grandi Strumenti (Università di Pavia). Three JEOL
wavelength-dispersive spectrometers were employed.
Operating conditions were 20 kV and 20 nA, and minerals were used as standards. X-ray intensities were corrected to oxide percent concentration by the procedure
of Bence & Albee (1968), on the basis of semi-empirical
a- and b-factors (Albee & Ray, 1970). Representative
electron microprobe analyses are reported in Table 2.
The formula units of olivine and spinel were calculated
assuming three cations and four oxygens. The formula
units of pyroxenes were obtained by the method of
Cawthorn & Collerson (1974), on the basis of six oxygens.
Mineral concentrates were extracted using a magnetic
separator, and purified by careful handpicking under
a binocular microscope. Clinopyroxene separates were
leached at 150 C for 40 min in 6 M HCl. Nd and Sr
were separated for isotope analyses (Table 3) using conventional ion exchange technique after dissolution in
Savillex capsules and FEP beakers. Nd (as NdO þ ) and
Sr isotope compositions were determined on a fivecollector VG354 mass spectrometer at Royal Holloway
using multidynamic modes (Thirlwall, 1991a, 1991b)
and normalized to 146 Nd/144 Nd ¼ 07219 and 86 Sr/
88
Sr ¼ 01194. 143 Nd/144 Nd and 87 Sr/86 Sr analyses
were carried out in two standardization periods. The
means of the laboratory 143 Nd/144 Nd standard for
these periods were 0511422 8 (2 SD, n ¼ 69)
and 0511413 7 (2 SD, n ¼ 54). The latter analyses
were adjusted to the usual standard value of 0511418,
equivalent to La Jolla 143 Nd/144 Nd ¼ 0511860. The
means for laboratory 87 Sr/86 Sr standard SRM987 were
0710246 21 (2 SD, n ¼ 58) and 0710220 15 (2 SD,
n 4 100). The 87 Sr/86 Sr analyses performed in the
second standardization period were corrected to the
long-term standard mean of 0710248. Typical blank
levels were 250 pg Nd and 700 pg Sr.
Sm and Nd concentrations were determined by isotope
dilution. Reproducibility of isotope dilution Sm/Nd
ratios is 01%. Sm and Nd contents in clinopyroxene
are consistent with the ion microprobe data, with differences 520%. The Sm/Nd values obtained by the two
1111
1112
0.79
2.88
3.29
2.07
2.02
0.71
2.66
3.44
2.18
2.25
Dy
Yb
21.5
0.85
17.4
0.86
18.1
16.6
6630
352
3635
111
1.70
2.84
1.72
0.66
2.36
3.50
1.77
0.37
2.20
MF1
133
2.29
4.35
2.62
0.97
3.45
5.85
2.78
0.92
3.42
0.83
26.1
29.5
834
17.2
368
4210
1
Gb
MC
MF1
128
2.31
3.98
2.32
0.97
3.67
5.30
2.66
0.92
3.55
29.9
0.83
17.2
24.8
1120
366
4155
2
Gb
MC
MF2
111
1.49
2.61
1.68
0.61
2.10
2.98
1.79
0.43
1.67
15.5
0.86
22.2
16.7
2510
309
2850
1
Gb
MC
MF2
110
1.43
2.77
1.43
0.61
2.62
2.82
1.64
0.41
1.84
15.0
0.86
20.0
16.3
1730
331
2940
2
Gb
MC
100
1.54
2.56
1.55
0.43
2.13
1.95
1.12
0.82
15.9
10.1
8 .0
1350
320
2550
1
Gb
0.18
1.15
CBC3y
RP
90
1.38
2.25
1.35
0.47
1.51
1.80
1.24
0.17
1.18
0.82
15.6
9.1
7.7
1490
319
2590
2
Gb
RP
CBC3y
118
2.28
4.08
2.34
0.74
3.25
4.74
2.57
0.84
3.16
0.81
9.6
24.6
15.8
175
392
3400
1
Gb
RP
CB17y
113
2.20
3.94
2.32
0.68
3.17
4.57
2.41
0.76
3.00
0.81
9.1
22.7
15.6
181
380
3420
2
Gb
RP
CB17y
95
2.30
3.46
2.06
0.68
3.04
4.08
1.97
0.72
3.38
16.6
0.77
10.5
21.9
1080
307
2800
1
Gb
RP
CB16y
0.91
12.4
2.58
2.42
6120
235
1500
0.91
13.5
2.74
2.53
6110
246
1540
1.14
51.9
1.09
2.20
1.38
0.40
1.58
0.91
0.76
50.03
0.16
2
Lh
MC
TP1
49.9
2.01
1.32
0.30
1.28
0.93
0.76
50.03
0.13
1
Lh
MC
TP1
0.92
2.39
11.7
2.94
6750
239
1650
1.10
54.4
1.90
1.19
0.29
1.28
0.72
0.69
50.03
0.10
1
Lh
MC
TP2
0.91
2.42
11.3
2.39
7800
280
1640
1.40
48.0
2.03
1.45
0.37
1.30
0.93
0.90
50.03
0.13
1
Lh
MC
TP9
NUMBER 6
*Data after Tribuzio et al. (2000a).
yData after Tribuzio et al. (1999).
EL, External Liguride ophiolites; MC, Montecastelli---Cecina valley ophiolites; RP, Riparbella---Cecina valley ophiolites; Gb, gabbro; Lh, lherzolite (porphyroclasts);
mg number ¼ Mg/(Mg þ Fe2 þ þ Fe3 þ ). (See text for the source of mg number data.)
mg no.
Y
21.7
0.85
7560
Zr
5960
Cr
314
3470
15.4
19.5
347
V
21.0
21.9
3750
Ti
1
Gb
EL
GEL3
VOLUME 45
Sr
109
Sc
Er
Gd
Eu
Sm
103
4.29
2.05
3.65
1.97
Nd
Ce
La
0.40
2.41
2
Gb
Gb
1
GEL9
EL
GEL9
EL
0.41
2.54
Grain:
Sample:
Table 1: Ion microprobe trace element analyses (ppm) of clinopyroxene cores from selected gabbros and mantle lherzolites
JOURNAL OF PETROLOGY
JUNE 2004
TRIBUZIO et al.
NORTHERN APENNINE GABBRO---PERIDOTITE ASSOCIATION
Table 2: Representative electron microprobe analyses (wt %) of minerals (core portions of grains) from Cecina
valley mantle lherzolites
Mineral
SiO2
TiO2
Al2O3
Cr2O3
FeO
MnO
MgO
CaO
Na2O
Total
mg no.
Sample TP1
40.70
54.14
0.05
0.16
0.04
5.09
50.01
0.62
9.19
6.00
0.15
0.15
49.40
33.26
50.01
1.11
50.01
50.01
Cpx
50.93
12.85
0.11
0.19
15.94
19.17
21.98
50.46
1.21
16.70
2.70
50.01
0.34
0.20
6.45
Spn
50.01
0.31
50.01
40.76
54.13
0.01
0.12
0.03
5.30
50.01
0.63
9.62
6.22
0.16
0.16
49.62
32.98
50.01
0.87
55.49
50.67
0.13
0.27
3.12
6.26
0.29
1.18
6.45
2.79
0.16
0.07
34.55
17.31
Spn
51.72
50.01
0.25
0.23
4.79
50.93
0.88
16.32
2.29
12.31
0.07
0.17
Spn II
50.01
0.16
53.47
13.54
12.13
40.67
53.86
50.01
0.17
0.02
5.09
50.01
0.66
54.27
49.94
0.18
0.35
4.20
6.57
50.84
0.35
0.21
5.03
Ol
Opx
95.53
100.53
99.97
0.91
0.91
99.57
0.91
0.73
50.01
50.01
100.19
100.41
0.90
0.90
0.68
20.86
50.01
0.31
100.87
99.72
0.91
0.92
16.49
19.92
22.69
50.01
0.36
50.01
99.54
99.88
0.93
0.74
0.13
20.09
50.01
50.01
99.52
0.75
9.58
5.94
0.13
0.12
49.32
31.78
0.01
2.31
50.01
50.01
99.73
99.93
0.90
0.91
0.60
1.18
6.40
3.10
0.10
0.07
32.85
16.73
0.93
50.01
0.43
99.53
99.34
0.90
0.91
0.86
18.02
13.47
2.94
11.93
12.82
0.08
0.12
16.75
18.95
99.57
99.08
0.91
0.74
0.11
18.57
0.43
50.01
50.01
99.33
0.72
Sample TP2
Ol
Opx
Opx II
Cpx
Cpx II
Sample TP9
Ol
Opx
Opx II
Cpx
Cpx II
Spn
Spn II
50.01
50.01
0.16
49.85
54.20
20.97
22.29
50.01
50.01
*Total
Fe as FeO.
Ol, porphyroclastic olivine; Opx, porphyroclastic orthopyroxene, Cpx, porphyroclastic clinopyroxene; Spn, porphyroclastic
spinel; Opx II, neoblastic orthopyroxene; Cpx II, neoblastic clinopyroxene; Spn II, neoblastic spinel; mg number ¼ Mg/(Mg þ
Fe2 þ þ Fe3 þ ).
methods vary within 14%. Rb and Sr concentrations of
clinopyroxenes were not determined by isotope dilution,
because the low Rb/Sr values of clinopyroxenes do not
require an age correction for Sr isotope compositions.
have the highest Cr and Sr contents (Table 1). The
concentrations of Sc and V cover a narrow range
(95---133 ppm and 309---392 ppm, respectively).
Isotope compositions and geochronology
GEOCHEMICAL RESULTS
The gabbros
Clinopyroxene trace element compositions
Clinopyroxenes from the selected gabbros of External
Liguride and Cecina valley ophiolites have similar chondrite-normalized, incompatible element patterns (Fig. 2),
which resemble those of clinopyroxenes from Internal
Liguride gabbros (Tribuzio et al., 1995). They are characterized by depletion in light rare earth elements
(LREE, CeN/SmN ¼ 02---04) and nearly flat middle to
heavy rare earth element patterns (MREE and HREE,
respectively) at 8---16 times chondrites. Sr, Zr and Ti
are depleted relative to neighbouring REE. The clinopyroxenes with the lowest REE abundances commonly
To derive Sm---Nd mineral isochron ages for External
Liguride and Cecina valley gabbros, we carried out
Sm---Nd analyses of whole rocks and separates of clinopyroxene and plagioclase (Table 3). Isochron ages were
calculated using a York-type regression (York, 1969) and
are interpreted to date the igneous crystallization event.
This is consistent with the high closure temperature for
the Sm---Nd exchange in clinopyroxene and plagioclase
from igneous mafic rocks (Van Orman et al., 2001).
Errors quoted for the isochron ages are based on 2s
analytical uncertainties multiplied by the square root of
the mean squares weighted deviates (MSWD).
The clinopyroxene---plagioclase---whole-rock isochron
(MSWD ¼ 097) of gabbro GEL3 from the External
Liguride ophiolites yields an age of 179 9 Ma and an
1113
JOURNAL OF PETROLOGY
VOLUME 45
NUMBER 6
JUNE 2004
Table 3: Sm and Nd isotope dilution data (ppm) and Nd and Sr isotopic ratios of selected gabbros and
mantle lherzolites
Rock sample
Location
Sm
Nd
147
WR
0.48999
Pl
0.0925
1.4367
1.4157
0.5759
0.209319
0.097135
3.2577
2.1000
0.266719
0.222902
0.69275
5.4014
0.112396
0.263168
1.4905
2.9284
0.198736
0.231552
0.70105
5.5965
0.106373
0.280051
0.513191 40
0.513126 6
0.513006 20
0.702538 12
0.220958
0.107386
0.524298
0.533101
0.513570 7
0.513586 5
0.702035 11
0.702484 12
Material
Sm/144 Nd
143
Nd/144 Nd 2s
87
Sr/86 Sr 2s
Gabbros
GEL3
EL
Cpx
MF1
MC
WR
Pl
Cpx
MF2
MC
WR
CB17
RP
WR
Pl
RP
2.3504
0.4898
1.1212
0.1233
WR
2.5915
1.4654
Pl
0.1931
4.0109
1.0873
0.6425
0.58087
0.7412
0.65904
Cpx
CB16
0.7740
0.12875
0.513096 5
0.512968 6
0.513168 4
0.704468 11
0.513104 4
0.512986 11
0.513152 6
0.702853 18
0.513076 5
0.513133 5
0.512993 4
Lherzolites
TP1
MC
Cpx
TP2
MC
Cpx
EL, External Liguride ophiolites; MC, Montecastelli---Cecina valley ophiolites; RP, Riparbella---Cecina valley ophiolites; WR,
whole rock; Pl, plagioclase; Cpx, clinopyroxene.
initial eNd of þ869 022 (Fig. 3). The gabbro MF1
from the Montecastelli pluton (Cecina valley ophiolites)
gives a clinopyroxene---plagioclase---whole-rock isochron
(MSWD ¼ 203) of 169 18 Ma and initial eNd of
þ854 053. We have also analysed the whole-rock
Sm---Nd compositions of sample MF2, which has nearly
the same initial Nd isotope ratio as MF1. The four
Montecastelli data points yield an isochron of 170 13 Ma (MSWD ¼ 121, initial eNd ¼ þ852 036).
Plagioclase, clinopyroxene and whole-rock compositions of olivine-bearing gabbro CB17 from the Riparbella
body (Cecina valley ophiolites) give an isochron age of
1737 48 Ma (MSWD ¼ 084, initial eNd ¼ þ893 014). Plagioclase and whole-rock Sm---Nd compositions
of Riparbella sample CB16 were also determined. The
five Riparbella data points define an isochron (MSWD ¼
084), corresponding to an age of 1735 48 Ma and an
initial eNd of 894 014. It is noteworthy that, although
the gabbros from the Montecastelli and Riparbella bodies
of the Cecina valley ophiolites give almost coincident
ages, the calculation of a nine-point regression yields a
high MSWD (420). This suggests that the Montecastelli
and Riparbella plutons formed from parental liquids with
slightly different Nd isotope compositions.
The clinopyroxene separates from the External Liguride
and Cecina valley gabbros were also analysed for Sr
isotope compositions. The clinopyroxene from the External
Liguride sample GEL3 is characterized by high 87 Sr/86 Sr
(07045). The clinopyroxenes from Montecastelli (MF1)
and Riparbella (CB17) samples gave 87 Sr/86 Sr values of
07028 and 07025, respectively.
The Cecina valley mantle lherzolites
Major element mineral compositions
Olivine has 90---91 mol % of forsterite component. The
porphyroclastic orthopyroxene has an Mg/(Mg þ Fetot)
value between 090 and 091, and Al and Ca contents of
020---024 and 003---009 atoms per formula unit
(a.p.f.u.), respectively. The associated clinopyroxene displays Mg/(Mg þ Fetot) values of 090---092, and Al and
Na concentrations of 028 a.p.f.u. and 002---003
a.p.f.u., respectively (Fig. 4). Interstitial spinel has high
Mg/(Mg þ Fe2 þ ) and low Cr/(Cr þ Al) and Ti values
(Fig. 5).
Two-pyroxene geothermometry on the basis of the
Wells’ method (1977) yields temperature estimates of
1060 60 C for the porphyroclastic assemblage. These
values are consistent with those obtained (T ¼ 1080 60 C) by the two-pyroxene geothermometer of Brey &
Kohler (1990) assuming P ¼ 15 GPa. However, the local
finding of high Ca contents in orthopyroxene indicates
higher crystallization temperatures (up to 1310 C)
according to the single orthopyroxene method of
1114
TRIBUZIO et al.
NORTHERN APENNINE GABBRO---PERIDOTITE ASSOCIATION
100
0.5133
Nd/144Nd
External Ligurides
143
Cpx/C1
10
Cpx
0.5131
W.R.
0.5130
0.5129
1
0.5128
GEL3
GEL9
External Ligurides
0.5132
Pl
εNd(i) = 8.69 ± 0.22
0.5133
179 ± 10 Ma
(MSWD = 0.97)
Cecina valley (Montecastelli)
0.5132
Nd/144Nd
0.1
100
143
Cecina valley (Montecastelli)
Cpx/C1
10
0.5128
Pl
εNd(i) = 8.52 ± 0.36
0.5133
1
Nd/144Nd
MF1
MF2
0.1
143
100
Cecina valley (Riparbella)
(MSWD = 1.21)
Cecina valley (Riparbella)
0.5131
CB17
CB16
CB16
W.R.
Cpx
0.5130
Pl CB17
εNd(i) = 8.94 ± 0.14 173.5 ± 4.5 Ma
0.5128
0.0
Cpx/C1
170 ± 13 Ma
0.5132
0.5129
10
Cpx
W.R.
0.5130
0.5129
MF2
MF1
0.5131
(MSWD = 0.84)
0.1
0.2
0.3
147Sm/144Nd
1
Fig. 3. Sm---Nd isochrons for the olivine-bearing gabbros from External Liguride ophiolites (sample GEL3) and Montecastelli (samples MF1
and MF2) and Riparbella (samples CB16 and C17) bodies of the Cecina
valley ophiolites. Pl, plagioclase; Cpx, clinopyroxene; W.R., whole-rock.
Error bars on x-axis are smaller than the symbol size.
CBC3
CB17
CB16
0.1
La Ce Sr Nd Zr Sm Eu Gd Ti Dy Y Er Yb
Fig. 2. Chondrite-normalized incompatible trace elements in clinopyroxenes of gabbros from External Liguride and Cecina valley
ophiolites. C1 chondrite from Anders & Ebihara (1982).
Brey & Kohler (1990). This suggests that the estimates
obtained on the basis of the two-pyroxene geothermometry suffer from pyroxene exsolution, i.e. they probably
document the cessation of the exsolution process
recorded by porphyroclastic pyroxenes.
Neoblastic pyroxenes are slightly Al and Cr depleted
relative to porphyroclasts. The Al and Cr decrease in
neoblastic orthopyroxene and clinopyroxene is probably
due to their development in association with newly
formed spinel, which incorporates a high amount of
these elements. Neoblastic orthopyroxene has invariably
low Ca concentrations (002---004 a.p.f.u.), which
imply crystallization temperature of 890---1040 C
assuming P ¼ 15 GPa (Brey & Kohler, 1990). Twopyroxene geothermometry (Wells, 1977; Brey & Kohler,
1990) applied to neoblastic pyroxene grains gives consistent estimates of 960 60 C.
Trace element and Nd---Sr isotope compositions of
porphyroclastic clinopyroxene
Porphyroclastic clinopyroxene displays a severe depletion
of LREE relative to MREE (CeN/SmN 004) and
HREE, which are nearly flat at 8 times chondrite
(Fig. 6). Normalization to chondrite also reveals that Sr,
Zr and Ti are depleted relative to neighbouring REE.
The concentrations of Cr, V and Sc are 6110---7800 ppm,
235---280 ppm and 48---54 ppm, respectively.
Two clinopyroxene separates were analysed for Nd
and Sr isotope compositions (Table 3). The 143 Nd/
144
Nd and 147 Sm/144 Nd ratios are rather high and yield
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0.4
VOLUME 45
NUMBER 6
0.9
Al
JUNE 2004
2+
Mg/(Mg+Fe )
External
Ligurides
0.8
0.3
External
Ligurides
0.2
0.7
0.6
Cecina valley (porphyroclastic)
Cecina valley (interstitial)
Cecina valley (neoblastic)
Cecina valley (neoblastic)
Internal Ligurides
0.1
0.00
0.05
0.10
Internal Ligurides
Na
0.5
20
Ti x 1000
0.15
Fig. 4. Plot of Na vs Al (a.p.f.u.) in porphyroclastic and neoblastic
clinopyroxene from the Cecina valley lherzolites. The compositions of
porphyroclastic clinopyroxene from the Internal Liguride lherzolites
(Beccaluva et al., 1984; Rampone et al., 1996, 1997) are also reported.
The enclosed area refers to porphyroclastic clinopyroxenes from the
External Liguride lherzolites (Beccaluva et al., 1984; Rampone et al.,
1993, 1995). The wide compositional field of External Liguride clinopyroxenes was ascribed to subsolidus decompression from spinel- to
plagioclase-facies conditions, which was associated with a decrease of
Na and Al concentrations in porphyroclastic clinopyroxenes (Rampone
et al., 1993, 1995).
15
10
5
Cr/(Cr+Al)
0
0.0
eNd values of þ109 and þ111 at the time of the gabbro
intrusion (Fig. 7). The 87 Sr/86 Sr values are remarkably
variable (07020---07025). Wide variations in Sr isotope
composition are typically observed for clinopyroxene
separates from ophiolitic and abyssal peridotites, and
attributed to seawater-related alteration (e.g. Bodinier
et al. 1991; Snow et al., 1994). The 87 Sr/86 Sr variations
observed for the Cecina valley lherzolites may therefore
be due to the local replacement of orthopyroxene exsolution lamellae by serpentine that was not completely
removed by the leaching procedure. This implies that
the analysed clinopyroxenes provide an upper limit on
their primary Sr isotope ratio.
DISCUSSION
The geochemical data reported here concerning the
gabbros from the Northern Apennine ophiolites allow
us to determine the compositions of their parental liquids
and provide constraints on the duration of plutonic crystallization in the Ligurian Tethys lithosphere. The nature
and evolution of the Cecina valley mantle lherzolites
are also discussed on the basis of their petrological and
geochemical features. A comparison between the Cecina
valley and Internal Liguride (Rampone et al., 1996)
mantle lherzolites has been carried out, as they are both
characterized by a refractory signature, similar to the
abyssal peridotites from modern oceans, but show
significant geochemical differences. We finally consider
the origin of the gabbro---peridotite association from the
External Liguride units, in conjunction with literature
0.2
0.4
0.6
Fig. 5. Major element compositions (atoms per formula unit on the
basis of four oxygens) of interstial and neoblastic spinel from the Cecina
valley lherzolites: Mg/(Mg þ Fe2 þ ) and (Ti 1000) vs Cr/(Cr þ Al).
The compositions of spinel from Internal Liguride lherzolites (Rampone
et al., 1996) are also reported. The enclosed area refers to spinels from
the External Liguride lherzolites (Rampone et al., 1993, 1995). The wide
compositional field of External Liguride spinels was ascribed to partial
re-equilibration under plagioclase-facies conditions, which was associated with a decrease of Mg/(Mg þ Fe2 þ ) ratio and an increase of
Cr/(Cr þ Al) and Ti values in spinels (Rampone et al., 1993, 1995).
data for the peridotites (Rampone et al., 1995). In particular, recent geochronological and geochemical studies
of gabbros and peridotites from the Western Iberian
margin (Charpentier et al., 1999; Sch€arer et al., 2000;
Abe, 2001; Beard et al., 2002) allow us to verify the
inferred analogy between the External Liguride units
and non-volcanic continental margins (Marroni et al.,
1998). The implications for the rifting process that led
to the opening of the Ligurian Tethys are reported in the
conclusions section.
Evidence for crystallization of gabbros
from N-MORB parental magmas
The trace element compositions of the liquids in equilibrium with clinopyroxene cores from the least evolved
gabbros of the External and Cecina valley ophiolites are
reported in Fig. 8. A calculated liquid composition for a
Mg-rich gabbro of the Internal Liguride ophiolites (data
from Tribuzio et al., 1995) is also shown. The comparison
supports the idea that the different gabbroic plutons from
the Northern Apennine ophiolites formed from similar
1116
TRIBUZIO et al.
NORTHERN APENNINE GABBRO---PERIDOTITE ASSOCIATION
100
(a)
18
εNd(170)
Lanzo
lherzolites
TP1
TP1
0.1
TP2
10
TP9
peridotites
14
1
Atlantis II
Cpx/C1
10
0.01
100
(b)
147
Sm/
144
Nd
6
Cpx/C1
10
0.2
0.3
0.4
0.5
0.6
Cecina valley lherzolite
1
0.1
Internal Liguride lherzolite
External Liguride lherzolite
Cecina valley gabbro
Internal Liguride gabbro
External Liguride gabbro
Cecina valley
Internal Ligurides
External Ligurides (Pl-poor)
External Ligurides (Pl-rich)
0.01
Ce Sr Nd Zr Sm Eu Gd Ti Dy Y Er Yb
Fig. 6. Chondrite-normalized incompatible trace element contents
of porphyroclastic clinopyroxene from lherzolites of the Northern
Apennine ophiolites. C1 chondrite from Anders & Ebihara (1982).
(a) Clinopyroxenes from Cecina valley lherzolites. The field enclosed
by dotted lines represents the compositional range for clinopyroxenes
from residual abyssal peridotites of the Atlantis II fracture zone (Southwest Indian Ridge; Johnson & Dick, 1992). (b) Average compositions of
porphyroclastic clinopyroxenes from Cecina valley (this study), Internal
Liguride lherzolites (core analyses; Rampone et al., 1996, 1997), and
plagioclase-poor and plagioclase-rich External Liguride lherzolites (core
analyses; Rampone et al., 1995). The field enclosed by dotted lines
represents the compositional range for clinopyroxenes from Galicia
Bank lherzolites (Western Iberian margin); the enclosed grey area refers
to clinopyroxenes from amphibole-bearing samples (Charpentier et al.,
1999; Chazot et al., in preparation).
primitive N-MORB-type liquids, which followed similar
igneous differentiation processes (Tribuzio et al., 2000a).
The N-MORB origin is consistent with the initial eNd
(þ85 to þ89) of Sm---Nd isochrons reported in this
study for the Cecina valley and External Liguride
gabbros, and by Rampone et al. (1998) for the Internal
Liguride gabbros.
The low 87 Sr/86 Sr ratios of clinopyroxene from Cecina
valley gabbros (07025---07028) are consistent with an
N-MORB origin. The clinopyroxene from the External
Liguride sample has a higher 87 Sr/86 Sr value (07045),
which approaches the Sr isotope composition of seawater in Middle to Late Jurassic times (Veizer, 1997).
Relatively high 87 Sr/86 Sr values were also found for the
clinopyroxenes from the Internal Liguride gabbros
Fig. 7. eNd at 170 Ma vs 147 Sm/144 Nd for the clinopyroxenes from the
lherzolites of the Cecina valley ophiolites (this work), Internal and
External Liguride ophiolites (Rampone et al., 1995, 1996). Clinopyroxene
compositions of Cecina valley and External Liguride gabbros (this
work) and Internal Liguride gabbros (Rampone et al., 1998) are also
shown. The enclosed area refers to clinopyroxenes from Lanzo
lherzolites (Western Alps ophiolites; Bodinier et al., 1991). The double
arrow shows the present-day eNd range for clinopyroxenes from
abyssal peridotites of the Atlantis II fracture zone (Southwest Indian
Ridge; Salters & Dick (2002).
(07031---07034 relative to 07024---07030 for the enclosing rocks) and ascribed to contamination by tremolite
crystallized during low-temperature interaction with seawater-derived fluids (Rampone et al., 1998). We conclude
that the high 87 Sr/86 Sr ratio of the External Liguride
gabbro is related to subsolidus contamination in the
separate by secondary clinopyroxene and tremolite that
developed in response to ocean-floor alteration.
Timing of gabbro crystallization in the
Ligurian Tethys
The uncertainties on the Sm---Nd isochrons reported in
this study for the External Liguride and Cecina valley
gabbros, and by Rampone et al. (1998) for the Internal
Liguride gabbros (164 14 Ma), do not allow us to
distinguish any significant age differences among the
plutonic bodies of the Northern Apennine ophiolites.
However, the U---Pb zircon age of 153 1 Ma reported
for a late leucocratic dyke from the Internal Liguride
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JOURNAL OF PETROLOGY
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NUMBER 6
JUNE 2004
100
100
N-MORB
Cpx/C1
liquid/C1
10
10
1
0.1
External Ligurides
Cecina valley (Montecastelli)
Cecina valley (Riparbella)
Internal Ligurides
0.01
Ce Sr Nd Zr Sm Eu Dy Er Yb
1
La Ce Sr Nd Zr Sm Gd Ti Dy Y
source
F = 0.02
F = 0.04
F = 0.06
F = 0.08
Cecina valley
Er Yb
Fig. 8. Chondrite-normalized incompatible trace elements in liquids in
equilibrium with clinopyroxene (average compositions) from the least
evolved gabbros of the External Liguride (sample GEL9) and Cecina
valley (samples MF2 and CBC3) ophiolites. A liquid calculated to be in
equilibrium with clinopyroxene from gabbro AP3/3 of Internal Liguride
ophiolites (Tribuzio et al., 1995) is also reported. Liquid compositions
were calculated using the clinopyroxene/liquid partition coefficients
determined by secondary ion mass spectrometry for alkaline olivine
basalt at 10 GPa (Skulski et al., 1994). Cpx=Liq D values are: 007 for La,
012 for Ce, 008 for Sr, 026 for Nd, 014 for Zr, 042 for Sm, 036
for Ti, 057 for Dy, 052 for Y and 049 for Yb. Cpx=Liq D values for Gd
and Er are 049 and 053, respectively, and have been determined by
interpolation on the basis of neighbouring REE values. The average
N-MORB composition (Hofmann, 1988) is shown as a bold line. C1
chondrite from Anders & Ebihara (1982).
ophiolites (Borsi et al., 1996) suggests a younger age for
the Internal Liguride gabbros relative to the External
Liguride and Cecina valley counterparts. An older age
of the External Liguride and Cecina valley gabbros is also
indicated by comparison with U---Pb zircon ages (161 1
to 166 2 Ma) of gabbros from the Western Alps ophiolites (Bill et al., 1997; Rubatto et al., 1998; Schaltegger et al.,
2002; Rubatto & Hermann, 2003).
The Sm---Nd isochron ages of the External Liguride
and Riparbella (Cecina valley) gabbros are also older
than the oldest pelagic sediments (radiolarites) associated
with the Ligurian Tethys ophiolites, which are so far
dated as Bathonian (Bill et al., 2001). The base of the
Bathonian corresponds to 1640 2 Ma and 1660
þ38/---56 Ma, according to the recent revisions of the
Middle Jurassic time scale by Odin (1994) and Palfy et al.
(2000), respectively. Hence, the External Liguride and
Riparbella (Cecina valley) gabbros formed before the
onset of radiolarite sedimentation, which is inferred to
be temporally associated with the outpouring of basalt
flows (e.g. Abbate et al., 1980; Cortesogno et al., 1987;
Molli, 1996). We conclude that formation of riftingrelated gabbroic bodies pre-dated continental break-up,
similar to what is inferred for the ocean---continent transition of the Western Iberian margin (Sch€arer et al., 2000;
Beard et al., 2002). In particular, the ‘syn-rift’ gabbros of
Fig. 9. Chondrite-normalized incompatible trace elements in clinopyroxenes from peridotites subjected to progressive fractional melting
degrees (F ¼ 002, 004, 006, 008), calculated on the basis of the model
of Johnson et al. (1990). The trace element composition of clinopyroxene in the mantle source is from Yang et al. (1998) (see text for
details about calculations). The bold continuous line shows the average
composition of clinopyroxene from the Cecina valley lherzolite (this
study).
the Ligurian Tethys are coeval with pelagic sedimentation over the continental margin (see Bill et al., 2001),
which followed the disruption of shallow-water carbonate
platforms of Early Jurassic age.
The Cecina valley mantle lherzolite:
evidence for a refractory origin
Clinopyroxene from the Cecina valley lherzolites shows a
marked depletion of LREE, Sr and Zr relative to HREE,
similar to residual abyssal peridotites (Fig. 6). We have
calculated the extent to which clinopyroxene has been
depleted using the non-modal fractional melting model
of Johnson et al. (1990), assuming a slightly depleted spinel
lherzolite source. Mineral proportions and trace element
compositions of clinopyroxene in such a mantle source
are those reported by Yang et al. (1998). Calculations
were carried out using the melting stoichiometry for
spinel lherzolites and trace element partition coefficients
reported by Johnson et al. (1990) and Ionov et al. (2002),
respectively.
The model shows that the trace element compositions
of clinopyroxene may be related to 6% fractional melting
of a slightly depleted spinel lherzolite source (Fig. 9). The
Ce concentrations are slightly higher in the Cecina valley
clinopyroxene than in computed clinopyroxene resulting
from 6% fractional melting, possibly in relation to a small
fraction of aggregated melt retained in the porous
residue ( Johnson & Dick, 1992). It is noteworthy, however, that a slight increase of adopted Cpx=Liq D value for
Ce (i.e. from 0086 to 0100) yields a Ce concentration
in calculated clinopyroxene that is consistent with the
observed composition.
The refractory origin of the Cecina valley lherzolites
is supported by the spinel compositions. In particular,
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TRIBUZIO et al.
NORTHERN APENNINE GABBRO---PERIDOTITE ASSOCIATION
in Cpx
10
N
Dy
abyssal
peridotites
1
Cecina valley
Internal Ligurides
0.0
0.1
0.2
0.3
0.4
0.5
0.6
Cr/(Cr+Al) in spinel
Fig. 10. Plot of Cr/(Cr þ Al) in spinel vs DyN concentration in coexisting clinopyroxene of Cecina valley (this work) and Internal Liguride
(Rampone et al., 1996) lherzolites. All points are sample averages. The
enclosed area refers to compositions of residual abyssal peridotites from
Hellebrand et al. (2001).
the Cr/(Cr þ Al) value of spinel and the REE compositions of clinopyroxene plot at the least depleted end
of the trend defined by residual abyssal peridotites
(Fig. 10). According to the fractional melting model of
Hellebrand et al. (2001) based on spinel compositions, the Cecina valley lherzolites are related to
6---8% partial melting, in good agreement with the modelling results obtained from clinopyroxene trace element
compositions.
Comparison between the Cecina valley
and Internal Liguride lherzolites
Although the REE and high field strength element
(HFSE) compositions of clinopyroxene from the Cecina
valley and Internal Liguride lherzolites point to a similar
refractory origin (Fig. 6), these rocks display significant
geochemical differences. In particular, the spinels from
the Internal Liguride lherzolites are poorer in Al and Mg
and richer in Ti than spinels from the Cecina valley
lherzolites (Fig. 5). According to the peridotite partial
melting model of Hellebrand et al. (2001), the high
Cr/(Cr þ Al) value of spinel from the Internal Liguride
lherzolites (Fig. 10) corresponds to a high degree of fractional melting (14---16%), which seems inconsistent with
the trace element chemistry of the associated clinopyroxene.
Because these lherzolites were subjected to late low
melt-fraction percolation associated with partial dissolution of spinel þ clinopyroxene and crystallization of
plagioclase þ orthopyroxene (Rampone et al., 1997), we
propose that the Internal Liguride spinels record a process of chemical modification (Al and Mg depletion and
Ti enrichment). Similar relations have been described
for impregnated peridotites from modern oceans and
ascribed to reaction between original spinel and melt to
produce plagioclase and secondary spinel (e.g. Dick &
Bullen, 1984; Cannat & Casey, 1995).
Close inspection of porphyroclastic clinopyroxene
compositions shows that most clinopyroxenes from the
Internal Liguride lherzolites, even considering core portions of grains, have slightly lower Al, Na and Sr contents
than those from the Cecina valley lherzolites (Figs 4 and
6). Remarkably, spinel and porphyroclastic clinopyroxene
from External Liguride lherzolites show large chemical variations, attributed to late development of plagioclase (Rampone et al., 1993, 1995), which are qualitatively
similar to those observed between the Cecina valley and
Internal Liguride counterparts. In particular, porphyroclastic clinopyroxenes from External Liguride lherzolites
have lower Al, Na and Sr, and slightly higher REE
and HFSE concentrations in plagioclase-rich than in
plagioclase-poor samples (see also Fig. 6). We thus suggest
that the low modal proportion of newly formed plagioclase (55%, Rampone et al., 1997) in Internal Liguride
lherzolites resulted in a slight depletion of Al, Na and Sr
concentrations in porphyroclastic clinopyroxenes, with
little modification of REE and HFSE abundances.
To test whether the chemical variations between
Cecina valley and Internal Liguride clinopyroxenes can
be related to late development of plagioclase, we have
carried out a series of mass balance calculations. These
calculations have been performed for Na and Sr, because
the abundances of these elements in the other peridotite
minerals are negligible. The average compositions
of Cecina valley clinopyroxenes (Na2O 04 wt %, Sr
24 ppm) are assumed to represent original mantle clinopyroxene. The average compositions of Internal Liguride
clinopyroxenes (Na2O 02 wt %, Sr 07 ppm) and plagioclases (Na2O 09 wt %, Sr 43 ppm) are considered
as the products of the impregnation process (data after
Beccaluva et al., 1984; Rampone et al., 1996, 1997). The
possible whole-rock changes of Na and Sr concentrations
in response to melt impregnation have been neglected.
The variations in Na and Sr contents of porphyroclastic
clinopyroxene can be reproduced by assuming that the
initial proportion of original clinopyroxene was 10 wt %
and that the melt impregnation process gave rise to 3 wt %
plagioclase crystallization and 2 wt % clinopyroxene dissolution, in accordance with petrographic observations
(Rampone et al., 1997).
We therefore conclude that the Cecina valley and
Internal Liguride lherzolites had a similar spinel-facies
refractory signature. After having undergone fractional
melting, the Cecina valley lherzolites were locally
deformed under spinel-facies conditions, as indicated by
recrystallization of pyroxene porphyroclasts, probably in
relation to uplift towards shallower levels. On the other
1119
JOURNAL OF PETROLOGY
VOLUME 45
hand, the Internal Liguride lherzolites were subjected to
late-stage melt impregnation under plagioclase-facies
conditions (Rampone et al., 1997), which resulted in significant chemical modifications of spinel and, to a lesser
extent, porphyroclastic clinopyroxene. Both Cecina valley and Internal Liguride lherzolites were subsequently
intruded by N-MORB-type gabbros.
Origin of the refractory signature of the
Cecina valley and Internal Liguride
lherzolites
The initial Nd isotope ratios of Cecina valley gabbros are
lower than the Nd isotope values of associated mantle
lherzolites at the time of intrusion (Fig. 7). 87 Sr/86 Sr
ratios seem to confirm the isotopic depletion of lherzolites
relative to gabbros, in spite of the uncertainties associated
with potential seawater-related contamination. The
87
Sr/86 Sr value of a Cecina valley lherzolite (sample
TP1) is markedly lower than the lowest 87 Sr/86 Sr
value of associated gabbros (07020 and 07025, respectively) and is inconsistent with typical N-MORB values.
A similar isotopic contrast between mantle and crustal
sections is observed for Internal Liguride ophiolites
(Rampone et al., 1996, 1998), as the mantle lherzolites
have higher eNd values and lower 87 Sr/86 Sr ratios (þ116 to
þ146 at 170 Ma, and 07022) than associated gabbros.
The average Nd isotope composition of clinopyroxenes
from Internal Liguride lherzolites yields an Early
Permian model age (t ¼ 274 Ma), assuming a depleted
mantle source (Rampone et al., 1996). This age has been
interpreted as the time of the depletion event recorded by
the Internal Liguride lherzolites, and has been related
to asthenosphere accretion to continental lithosphere
during the post-collisional phase (270---300 Ma) of the
Variscan orogeny (see also Rampone et al., 1998). This
interpretation is supported by the fact that the Nd isotope
ratios of Internal Liguride lherzolites at post-Variscan
times cluster near the initial Nd isotope values of some
post-Variscan gabbros that at present crop out in the
Alpine belt (Th€
oni & Jagoutz, 1992; Montanini &
Tribuzio, 2001). However, the genetic link between
Internal Liguride lherzolites and post-Variscan basic
magmatism (Rampone et al., 1996; Montanini &
Tribuzio, 2001) seems inconsistent with the high initial
87
Sr/86 Sr values of the gabbros (407025).
The same depleted mantle source used by Rampone
et al. (1996) yields a Nd model age of 234 Ma for the Cecina
valley lherzolites, which is inconsistent with a postVariscan depletion event. We have also calculated the
Sr model age of Cecina valley lherzolite TP1, assuming a
depleted mantle with 87 Sr/86 Sr ¼ 07026 and 87 Rb/
86
Sr ¼ 00509 (Ito et al., 1987). The result obtained
(t ¼ 777 Ma) suggests an ancient evolution for the Cecina
NUMBER 6
JUNE 2004
valley mantle section. Remarkably, low 87 Sr/86 Sr values
(07017---07018) were also found for a few lherzolites
of the External Liguride and Lanzo (Western Alps)
ophiolites, and ascribed to MORB-source material that
was isolated from the convective mantle in the Proterozoic
(Bodinier et al., 1991; Rampone et al., 1995). Similarly, the
Sr model age resulting from Cecina valley lherzolite TP1
could be considered as a minimum age of differentiation
from the asthenosphere.
However, Salters & Dick (2002) have recently shown
that abyssal peridotites from the Southwest Indian Ridge
are characterized by extreme Nd isotope heterogeneity.
In particular, present-day eNd values for clinopyroxenes
from abyssal peridotites of the Atlantis II fracture zone
and 10---16 E range from þ86 to þ150 and from þ77
to þ131, respectively. At both locations, the range of
143
Nd/144 Nd ratios of mantle clinopyroxenes extends to
higher values than those of associated basaltic rocks, thus
implying that the isotope compositions of the crustal
rocks require the presence of a component not observed
in the mantle sequences at the ridge. This component
could be represented by pyroxenite or eclogite, as these
rock types have a lower solidus than peridotite and would
thus be exhausted before melting of the peridotite stops
(Salters & Dick, 2002).
Mantle isotope heterogeneity in modern slowspreading ridges is also indicated by melt inclusions
within olivine from basalts of the FAMOUS area. Pb
isotope microanalyses of these inclusions show an extremely wide Pb isotope compositional range; they are
thus interpreted to be incompletely aggregated mantle
melts (Shimizu, 1998; Shimizu & Layne, 1998). This
indicates that the relatively homogeneous isotope compositions of basalts and gabbros result from shallow-level
homogenization among melts generated from a compositionally heterogeneous mantle (Shimizu & Layne,
1998). A similar process of melt aggregation was also
suggested by Kempton & Hunter (1997) and Coogan et
al. (2000) for the gabbros of the MARK area, on the basis
of minor isotopic disequilibrium between clinopyroxene
and plagioclase, and trace element zoning in plagioclase,
respectively.
Both the Cecina valley and Internal Liguride
lherzolites are isotopically depleted relative to associated
crustal rocks, similar to what is observed for modern slowspreading ridges (Salters & Dick, 2002). This confirms the
petrological and trace element resemblances between
the gabbro---peridotite associations from modern slowspreading centres and those from both Cecina valley
and Internal Liguride ophiolites (see also Tribuzio et al.,
2000a). In addition, the isotopic heterogeneity of abyssal
peridotites (Salters & Dick, 2002) suggests that isotope
model ages of Cecina valley and Internal Liguride
lherzolites have little geochronological meaning. If
we assume that the residual signature of these lherzolites
1120
TRIBUZIO et al.
NORTHERN APENNINE GABBRO---PERIDOTITE ASSOCIATION
developed in conjunction with the formation of the
Ligurian Tethys, in the Middle Jurassic, then: (1) the
Cecina valley and Internal Liguride mantle sections
originally contained Nd- and Sr-enriched material;
(2) the history of Cecina valley and Internal Liguride
lherzolite protoliths is mostly unknown.
The gabbro---peridotite associations from the
External Liguride ophiolites and Western
Iberian margin: are they analogues?
The non-volcanic continental margin of Western Iberia
is characterized by large bodies of mantle peridotites,
emplaced from the deep continental lithosphere during
the Early Cretaceous, in response to the extensional
processes that led to the opening of the North Atlantic
Ocean (Whitmarsh & Wallace, 2001, and references
therein). The peridotites are spinel and plagioclase
bearing, and most have a fertile geochemical signature
(Kornprobst & Tabit, 1988; Charpentier et al., 1999;
Abe, 2001; Hebert et al., 2001). They locally contain
pyroxenite layers and small bodies of gabbro and dolerite,
and are covered by basalt flows, post-rift sediments or
continental material (Boillot et al., 1995; Fuegenschuh
et al., 1998). These lithostratigraphic and mantle petrological features have been recognized in the External
Liguride units, which could be considered as fossil remnants of an ocean---continent transition (see also Marroni
et al., 1998; Rampone & Piccardo, 2000).
The correspondence between External Liguride units
and the Western Iberian margin is confirmed by the fact
that in both sequences gabbro crystallization began
before continental break-up [see Sch€arer et al. (2000)
and Beard et al. (2002), for the Western Iberian margin].
The gabbros and associated basaltic rocks from the
Western Iberian margin have incompatible trace element
concentrations indicating normal- to transitional-MORB
affinity and initial eNd values ranging from þ88 to þ22
(Kornprobst et al., 1988; Seifert et al., 1997; Charpentier
et al., 1998). The initial eNd of External Liguride gabbros
(this study) and basalts ( Rampone et al., 1998) are within
the range reported for the igneous mafic rocks of the
Western Iberian margin, although mafic rocks with a
slightly enriched geochemical signature have not been
found in the External Liguride ophiolites (see also
Marroni et al., 1998).
The inferred analogy between External Liguride units
and non-volcanic continental margins is nevertheless
reinforced by recent geochemical studies of the mantle
peridotites from the Western Iberian margin (Charpentier
et al., 1999; Abe, 2001; Chazot et al., in preparation). The
Western Iberian lherzolites are isotopically heterogeneous, with eNd values at the time of the gabbro
intrusion ranging from þ38 to þ132, and have
clinopyroxenes (Fig. 6) with trace element compositions
showing a fertile geochemical signature. Overall, these
features are consistent with the isotope and trace element
signatures of External Liguride lherzolites (Rampone
et al., 1995). In particular, lherzolites with disseminated
Ti-rich amphibole have been collected at the Galicia
Bank (Charpentier et al., 1999; Chazot et al., in preparation) and these rocks have petrological and trace element
features (see also Fig. 6) that are closely similar to those of
the External Liguride lherzolites (Rampone et al., 1995).
Remarkably, mantle clinopyroxenes with a highly
depleted trace element signature, similar to that of Cecina
valley and Internal Liguride mantle clinopyroxenes,
have never been found in the Western Iberian margin
(Charpentier et al., 1999; Abe, 2001; Chazot et al., in
preparation).
GEODYNAMIC IMPLICATIONS AND
CONCLUSIONS
The gabbro---peridotite association of the Northern
Apennine ophiolites formed by intrusion of N-MORBtype melts into a heterogeneous mantle. The External
Liguride gabbros crystallized within a subcontinental
mantle section (Beccaluva et al., 1984; Ottonello et al.,
1984; Rampone et al., 1995) at 179 9 Ma, thus giving
rise to a gabbro---peridotite association similar to that of
non-volcanic continental margins (i.e. the Western
Iberian margin). The External Liguride gabbros testify
to mantle melting prior to continental break-up and
probably record the onset of the magmatism related to
opening of the Ligurian Tethys.
The Cecina valley and Internal Liguride (Rampone
et al., 1996) lherzolites are similar to residual abyssal
peridotites. In particular, although the Cecina valley
and Internal Liguride lherzolites were subjected to different processes after the partial melting event, they can be
modelled as having formed by the same degree of fractional melting ( 6% of a slightly depleted spinel lherzolite
source). In addition, both Cecina valley and Internal
Liguride (Rampone et al., 1996) lherzolites are isotopically depleted relative to associated crustal rocks, similar
to what is observed for the modern oceanic lithosphere
(Salters & Dick, 2002).
The Cecina valley lherzolites reached shallow levels
prior to oceanization, as indicated by the Sm---Nd
mineral isochron of associated Riparbella gabbros
(1735 48 Ma). The development of the Cecina valley
gabbro---peridotite association is interpreted as an intermediate stage of the rifting process that led to the opening
of the Ligurian Tethys. Such a rifting process probably
resulted in a magma-poor slow-spreading centre, whose
remnants are represented by the Internal Liguride ophiolites
(see also Tribuzio et al., 2000a).
1121
JOURNAL OF PETROLOGY
VOLUME 45
ACKNOWLEDGEMENTS
M. Tiepolo is thanked for his precious help and the useful
suggestions. A. Montanini, G. B. Piccardo, E. Rampone
and A. Zanetti are acknowledged for friendly and stimulating discussions. Thorough reviews by L. A. Coogan,
P. D. Kempton, O. M€
untener and J. Shervais have been
greatly appreciated. This work was supported by FAR,
PRIN and CNR funds.
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