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O RI G I N A L P A P E R
Petrology and Sr–Nd–Hf isotope geochemistryof gabbro xenoliths from the Hyblean Plateau:
a MARID reservoir beneath SE Sicily?
Giovanna T. Sapienza Æ William L. Griffin ÆSuzanne Y. O’Reilly Æ Lauro Morten
Received: 20 February 2008 / Accepted: 28 May 2008 / Published online: 17 June 2008
Ó Springer-Verlag 2008
Abstract In situ trace-element and isotopic (87Sr/ 86Sr)
data and whole-rock Sr–Nd–Hf data on 12 gabbro xenolithsfrom the Hyblean Plateau (south-eastern Sicily) illustrate
the complex petrogenetic evolution of this lithospheric
segment. The gabbros formed by precipitation of plagio-
clase + clinopyroxene from a HIMU-type alkaline melt,
then were cryptically metasomatized by a low-Rb,
high-87Sr/ 86Sr fluid, and finally infiltrated by an exotic, late
Fe–Ti-rich melt with 87Sr/ 86Sr * 0.7055, carrying high
concentrations of Sr, Rb and HFSE. The geochemical and
isotopic features of both the metasomatizing fluid and the
Fe–Ti-rich melt are compatible with their common deri-
vation by the progressive melting of an amphibole–
phlogopite–ilmenite metasomatic domain (MARID-type?)
that probably resided within the subcontinental lithospheric
mantle. Therefore, both the astenosphere and the litho-
sphere underneath the Hyblean Plateau contributed to the
petrogenesis of the gabbros. Sm–Nd dating yields an age of
253 ± 60 Ma for the cumulitic pile, roughly coinciding
with a hydrothermal event recorded by crustal zircons in
the area. We suggest that the Hyblean Plateau suffered a
thermal event—probably related to lithospheric thinning
and upwelling and melting of the asthenosphere—inPermo-Triassic time (the opening of the Ionian Basin?).
The induced perturbation in the lithosphere caused conse-
quent melting of some previously metasomatised portions.
Keywords Gabbro xenoliths Á In situ Sr isotopes Á
Sr–Nd–Hf isotope data Á MARID Á Hyblean Plateau
Introduction
The geochemical and mineralogical features of the lower
crust vary widely in different areas of the Earth, reflecting
episodes of crustal extraction and tectonic modification.
Much of our knowledge of the processes through which
this inaccessible portion of the lithosphere evolves comes
from the study of lower-crustal xenoliths brought to the
surface by ascending magmas. Their rapid ascent after
entrainment helps to preserve the geochemical and minera-
logical features of the deep crust.
In the last decades the discovery of a large number of
lower crustal xenoliths in Miocene tuff-breccia pipes in the
Hyblean Plateau (south-eastern Sicily) has yielded infor-
mation on the petrological nature of the lithosphere in that
area (Scribano 1986, 1988; Tonarini et al. 1996; Sapienza
and Scribano 2000; Scribano et al. 2006; Sapienza et al.
2007). The Hyblean Plateau is located in the Central
Mediterranean, a geodynamically complex area affected by
the collision between the European and African Plates
(Dewey et al. 1989). Within this geological framework, the
Plateau has generally been considered to represent the
northernmost portion of the Pelagian Block (African Plate).
However, doubts have been expressed about the conti-
nental nature of this lithospheric micro-block (Vai 1994;
Lauro Morten deceased, November 18, 2006.
Communicated by J. Hoefs.
G. T. Sapienza (&) Á L. Morten
Dipartimento di Scienze della Terra e Geologico-Ambientali,
Universita di Bologna, Piazza Porta San Donato 1,
40126 Bologna, Italy
e-mail: [email protected]
W. L. Griffin Á S. Y. O’Reilly
ARC National Key Centre for Geochemical Evolution
and Metallogeny of Continents, Department of Earth
and Planetary Sciences, Macquarie University,
Macquarie, NSW 2109, Australia
123
Contrib Mineral Petrol (2009) 157:1–22
DOI 10.1007/s00410-008-0317-x
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Scribano et al. 2006), and suggestions of a possible oceanic
or transitional ocean-continent nature have been put for-
ward. Recently, in situ techniques have provided evidence
to substantiate the Archean origin of the Hyblean lower
crust and upper mantle (Sapienza et al. 2007), but many
aspects of the evolution of the Hyblean crust remain
unconstrained: e.g., the reservoir(s) contributing to crustal
accretion and the processes through which this lithosphericportion evolved.
These observations highlight the need for a better
knowledge of the Hyblean lower crust, and of its
role within the geological framework of the Central
Mediterranean setting. Among the Hyblean lower crustal
lithotypes (e.g., basic granulites, gabbroic rocks; Sapienza
and Scribano 2000; Scribano et al. 2006), the cumulitic
gabbros in particular can provide useful clues. To this
purpose, a geochemical and isotopic study of Hyblean
gabbroic xenoliths has been carried out. Here we present
isotopic and trace-element data for 12 gabbro xenoliths
collected from the Valle Guffari diatreme (Hyblean Pla-teau). We discuss (1) the geochemical and isotopic
characters of the magma source(s) and possible contam-
inant(s), and those of the parental magmas of the Hyblean
gabbros (via whole-rock analysis and in situ LAM-MC-
ICPMS data), (2) a petrogenetic model for the formation
and evolution of these rocks and (3) the significance of
these geochemical results within the geological frame-
work of the Central Mediterranean.
Geological setting
The Hyblean Plateau makes up the south-eastern corner of
Sicily (Fig. 1). It is bordered by the Apennine–Maghre-
bide Chain to the north and west, and by the oceanic
Ionian Basin to the east through the Hybla–Malta
Escarpment. The exposed crust consists of a *10 km-
thick Mesozoic–Cenozoic carbonate sequence, overlain by
Neogene-Quaternary clastics. Several volcanic layers of
different ages interrupt the sedimentary cover (Bianchi
et al. 1987) and show variable geochemical, mineralogi-
cal and volcanological characteristics: Cretaceous alkali-
basaltic lavas (Amore et al. 1988; Sapienza et al. 2008),
Miocene alkali-basaltic and nephelinitic lavas and
tuff-breccia pipes (Bianchini et al. 1998, 1999), and
Plio-Pleistocene lava flows (basalts, basanites and rare
nephelinites, with both tholeiitic and alkaline affinities;
Beccaluva et al. 1998; Trua et al. 1998). The oldest
outcropping volcanics are Cretaceous, but Triassic alkali-
basaltic layers are found in the subsurface (Cristofolini
1966).
The Plateau has traditionally been considered the
northernmost portion of the Pelagian Block (African Plate)
that extends through the thinned Sicily Channel and was
subducted under the European Plate (Burollet et al. 1978;
Ben-Avraham and Grasso 1990). Although the continental
nature of the block is widely accepted, the poor constraintson the lithospheric structure have allowed alternative sug-
gestions, such as oceanic or ocean-continent transitional
affinities. On the basis of the distribution of pelagic
deposits within the Mediterranean area, Vai (1994) pro-
posed that the Hyblean Plateau represented the western part
of the Ionian Permo-Triassic fossil oceanic domain (the so-
called Neo-Tethys). Indeed, even the nature (oceanic vs.
continental) and the age of the Ionian Basin is still matter
of debate (Farrugia and Panza 1981; Catalano et al. 2001;
Argnani 2005 and references therein).
Petrographic similarities between the Hyblean gabbro
xenoliths and oxide gabbros from oceanic settings—e.g.,the late intrusion of Fe–Ti-rich melts into early gabbroic
cumulates and the sheared microstructure along which these
melts were injected (documented in gabbros from the South
West Indian Ridge; Niu et al. 2002)—led Scribano et al.
(2006) to suggest an oceanic or ocean-continent transitional
origin for this lithospheric sector. However, recent dating of
zircons in crustal xenoliths attests to the presence of relic
Archean crust beneath the Hyblean Plateau, and Re-Os data
on sulfides hosted in mantle-derived peridotite xenoliths
Fig. 1 Simplified geological sketch map of the Hyblean Plateau after
Lentini (1984) and Sapienza and Scribano (2000), showing location
of Valle Guffari (the sampling site)
2 Contrib Mineral Petrol (2009) 157:1–22
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testify to the presence of Archean lithospheric mantle roots
(Sapienza et al. 2007). These results reinforce the conti-
nental nature of this lithospheric sector.
Analytical techniques
The 12 samples studied here were selected among thosestudied by Scribano et al. (2006). The mineral major ele-
ment compositions of Hyblean gabbros were determined on
C-coated polished thin section using a Cameca SX50-
electron microprobe installed at the CNR—Istituto di
Geoscienze e Georisorse (Padova) and fitted with four
wavelength-dispersive spectrometers. The operating con-
ditions were: accelerating voltage of 15 kV, beam current
of 15 nA and a nominal spot size of *1 lm. Raw data
collected with the probe were corrected for matrix effects
using the ZAF procedure.
Whole-rock and in situ isotopic data and mineral trace-
element data were acquired in the Geochemical AnalysisUnit at GEMOC, Macquarie University, Australia. Whole-
rock samples were finely powdered in an agate mortar,
dissolved in HClO4 and finally loaded into resin columns.
Strontium was separated from the bulk REE using 2.5 N
HCl as elutant. The separation of Sm and Nd was per-
formed using 6 N HCl as elutant. Hafnium was separated
using firstly 6 N HCl (to collect Ti, Zr and Hf), and sec-
ondly 2.5 N HCl and 0.5 N HF. Sr–Nd–Hf isotope data
were performed by solution analysis, using a Nu Plasma
multi-collector inductively coupled plasma mass spec-
trometry (MC-ICPMS).
Nd, Sr and Hf isotopic data are normalized, respectively,
to 146Nd/ 144Nd = 0.7219, 86Sr/ 88Sr = 0.1194 and179Hf/ 177Hf = 0.7325. During period of measurement the
mean value for 87Sr/ 86Sr of the SRM 987 standard was
0.710264 ± 16 (n = 2), for 143Nd/ 144Nd of the JMC321
standard was 0.511115 ± 4 (n = 3), and for 176Hf/ 177Hf of
JMC475 standard was 0.282178 ± 5 (n = 7). For the cal-
culation of epsilon Hf (eHf ) and Nd (eNd) we use a decay
constant for 176Lu of 1.93 9 10-11 a-1 (Sguigna et al.
1982) and a decay constant for 147Sm of 6.54 9 10-12 a-1
(Steiger and Jager 1977). Epsilon values have been calcu-
lated as present-day values. 147Sm/ 144Nd values have been
calculated using isotopic abundances: 147Sm = 14.99% and144Nd = 23.8%. Maximum errors for 147Sm/ 144Nd ratio are
taken as ±1%. The Sm–Nd isochron calculation was done
using Isoplot (Ludwig 1999).
In situ Sr isotope analyses were carried out on three
samples. The analyses were performed using a New Wave
LUV213 laser ablation microprobe (LAM) attached to a
Nu Plasma multi-collector ICPMS (MC-ICPMS). The
laser spot size was 50–100 lm. The BB-1 standard
(87Sr/ 86Sr = 0.70444 ± 7; n = 6) was analyzed as a
cross-check of analytical accuracy. Each analysis took
*200 s, following the collection of 30 s gas background.
Other analytical details are given by Adams et al. (2005).
The trace-element compositions of minerals from seven
xenoliths were determined in polished sections *100 lm
thick using a LUV266 LAM attached to an Agilent 7500
Inductively Coupled Plasma Mass Spectrometry (LAM-
ICPMS). NIST 610 was used as the external standard;internal standards were Ca for clinopyroxene, plagioclase
and amphibole and Ti for oxides. The spot size was
*40 lm. Samples were analyzed in runs of ca. ten anal-
yses (two analyses of NIST 610 standard, one analysis of
BCR-1, five analyses of unknown points, two analyses of
NIST 610 standard). Each analysis took *180 s, with gas
background measurement of *55 s prior to ablation. Data
were reduced using the in-house GLITTER online (van
Achterbergh et al. 2001). Further analytical details are
given by Belousova et al. (2001). In case of clinopyroxene
and plagioclase grains, the analyses were performed in the
core of the minerals to avoid possible late reaction rims,which in turn are too thin to be analyzed.
Petrology of the Hyblean gabbro xenoliths
Petrographic and geochemical features of the Hyblean
gabbros have been presented elsewhere (Sapienza and
Scribano 2000; Scribano et al. 2006). Here we summarize
the most relevant features. The Hyblean gabbros are med-
ium- to coarse-grained rocks (Fig. 2), mainly composed of
cumulitic plagioclase (An40–80; Table 1) and diopsidic
clinopyroxene (mg# = 72–78; Al2O3 * 4.5 wt%, TiO2 *
0.9 wt%; Al2O3 and TiO2 are higher in samples FB11 and
FB-f7; Table 2). These rocks have suffered more or less
pronounced textural re-equilibration, which sometimes
partially obliterates the primary igneous texture. However,
there is a petrographic continuum from gabbros with clearly
igneous microstructure (euhedral to subhedral, up to 4 mm
plagioclase and up to 6 mm clinopyroxene, PL-I and
CPX-I, with only local recrystallisation into polygonal
neoblasts\0.5 mm, PL-II and CPX-II; Fig. 2a, b) to those
with mainly granuloblastic microstructures (very rare or
absent relict igneous phases set in a polygonal matrix of
PL-II and CPX-II; Fig. 2c). Porphyroclastic samples rep-
resent the intermediate textural type (Fig. 2d).
Fe–Ti oxides are enclosed within silicate phases
(ilmenite and Ti-magnetite, OX-I; Fig. 2b) or occur as
interstitial grains (ilmenite, OX-II; Fig. 2a). Intercumulus
pockets consist of mildly altered brownish crypto- to
micro-crystalline matter enclosing fine-grained clino-
pyroxene, plagioclase, zeolites and large anhedral Fe–Ti
oxides (OX-III B 1 mm, sometime up to 4 mm; Table 3
and Fig. 2e, f). These oxides enclose irregular green
Contrib Mineral Petrol (2009) 157:1–22 3
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hercynitic spinel (SPL; Fig. 2e), and can reach up to
18 vol% (Scribano et al. 2006). The modal abundance of
the intercumulus pockets is generally higher in the more
igneous-textured gabbros than in more metamorphic-tex-
tured ones. Clinopyroxene grains in contact with these
pockets exhibit a thin exsolution-free rim (Fig. 2e),
whereas plagioclase grains are strongly corroded (Fig. 2f);
these features imply disequilibrium with intercumulus
liquid, and almost disappear in the most metamorphic-
textured gabbros (Scribano et al. 2006; Fig. 2c). Clino-
pyroxene often contains exsolution lamellae of ilmenite.
Rare grains and laths of brown amphibole (Ti-pargasite)
occur within clinopyroxene in sample FB70; the Ti-parg-
asite grains show reaction rims of oxides at the contact with
the host clinopyroxene.
The whole-rock C1-normalized rare earth element
(REE) patterns of all samples are similar (Fig. 3a), char-
acterized by relatively unfractionated light REE (LREE)
(LaN /SmN * 1, rarely up to 2), a weak positive Eu
anomaly (Eu/Eu* = 1–2) and depletion in heavy REE
(HREE) relative to the LREE and middle REE (MREE)
(LaN /YbN = 2–8; SmN /YbN = 3–5) (Scribano et al. 2006).
When normalized to primitive mantle (PM) (Fig. 3b), all
these rocks show enrichment of selected large ion litho-
phile elements (LILE; Sr, Rb and Ba) over Th, U and high
field strength elements (HFSE; Zr, Hf, Nb, Ta) (Scribano
et al. 2006). The variably positive Ti anomaly reflects the
modal abundance of Fe–Ti oxides.
Whole-rock Sr–Nd–Hf isotope geochemistry
Present-day Sr–Nd–Hf isotope data are listed in Table 4 and
plotted in Figs. 4 and 5. 143Nd/ 144Nd is very similar in all
samples (average *0.5129 and eNd * +5, except for
gabbro FB-e11 having 143Nd/ 144Nd = 0.513062), whereas
Fig. 2 Photomicrographs
showing the petrographic
features of the studied Hyblean
gabbros. a Foliation in gabbro
FB-f4, defined by oriented
clinopyroxene (CPX-I ) and
plagioclase (PL-I )
porphyroclasts; interstitial
Fe–Ti oxides (OX-II ) are
present. b Local polygonal
plagioclase and clinopyroxene
neoblasts (PL-II and CPX-II )
surround the igneous
porphyroclasts; CPX-I hosts
Fe–Ti oxides (OX-I ).
c Granoblastic-textured gabbro
FB11 characterized by a very
few igneous relics (CPX-I and
PL-I ) set in a matrix of
clinopyroxene and plagioclase
neoblasts (CPX-II and PL-II ).
d Porphyroclastic texture in
gabbro FB13 showing some
relics of igneous plagioclase
(PL-I ) set in a polygonal matrix
of clinopyroxene and
plagioclase neoblasts (CPX-II
and PL-II ). e Fe–Ti-rich
intercumulus pockets enclosing
large amoeboid Fe–Ti oxide
(OX-III ) in gabbro FB80; green
hercynitic spinel (SPL ) occurs
within OX-III; thin, exsolution-
free rim in CPX-I grains at
contact with intercumulus
pockets is visible. f Plagioclase
grains (PL-I ) show corrosion
rims in contact with the
intercumulus pockets in sample
FB80; a OX-III grain occurs
within this pocket
4 Contrib Mineral Petrol (2009) 157:1–22
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87Sr/ 86Sr ranges widely, from 0.703943 to 0.705192
(Fig. 4a). For comparison, fields for xenoliths and lavas
from the Central Mediterranean area are also plotted in the
Sr–Nd space (Fig. 4a). The Nd isotope composition of the
Hyblean gabbros is similar to that of the Hyblean peridotites
and basic granulites and to lavas from Linosa and Pantelleria
Table 1 Representative in situ major- and trace-element analyses for plagioclases from Hyblean gabbros
FB50 FB-f31 VB5 FB-f4 FB70 FB11 FBf7
PL_11 PL_12 PL_1 3 PL_14 PL_13 PL_ 16 PL_12 PL_13 PL_12a PL_16 PL_12a PL_15 PL_11 PL_15
Elements (wt%)
SiO2 49.38 47.95 50.92 51.6 51.67 51.45 48.66 49.02 58.0 57.43 49.85 49.98 53.5 53.35
TiO2 b.d.l. b.d.l. b.d.l. 0.08 b.d.l. b.d.l. 0.01 b.d.l. 0.03 0.04 0.01 0.02 b.d.l. b.d.l.
Al2O3 32.18 32.94 32 31.58 31.5 31.82 32.61 32.26 26.0 26.2 31.51 31.70 30.89 30.61
FeOt 0.04 0.07 0.08 0.2 0.14 0.15 0.08 0.09 0.03 0.03 0.02 0.05 b.d.l. 0.06
MnO 0.01 0.04 b.d.l. 0.08 b.d.l. b.d.l. b.d.l. 0.02 b.d.l. b.d.l. b.d.l. b.d.l. b.d.l. b.d.l.
MgO 0.01 b.d.l. b.d.l. b.d.l. b.d.l. b.d.l. b.d.l. 0.03 0.01 b.d.l. 0.01 b.d.l. b.d.l. b.d.l.
CaO 15.77 16.64 12.17 11.37 11.76 11.73 16.55 15.83 8.38 8.45 15.06 15.01 10.16 10.34
Na2O 3.09 2.31 4.22 4.47 4.42 4.24 2.54 3.02 6.65 6.69 3.14 3.07 4.99 5.1
K 2O 0.04 0.03 0.16 0.21 0.13 0.16 0.04 0.03 0.29 0.24 0.05 0.05 0.16 0.15
Total 100.5 100.0 99.6 99.6 99.6 99.6 100.5 100.3 99.4 99.1 99.6 99.9 99.7 99.6
Ab mol% 26.1 20.1 38.2 41.0 40.2 39.2 21.7 25.6 58.3 58.0 27.1 27.0 46.6 46.7
An mol% 73.7 79.8 60.9 57.7 59.1 59.9 78.1 74.2 40.1 40.6 72.6 72.7 52.4 52.4
Or mol% 0.2 0.1 1.0 1.3 0.8 1.0 0.2 0.2 1.6 1.4 0.3 0.3 1.0 0.9
Elements (ppm)
Rb 0.092 0.546 0.164 0.134 79.1 0.127 0.112 b.d.l. 0.069 0.103 0.109 b.d.l. 4.13 0.356
Ba 99.1 106 134 123 7,688 101 77.6 75.1 464 482 69.0 64.11 474 332
Th 0.141 0.059 0.073 0.020 0.079 0.048 0.062 0.051 b.d.l. b.d.l. b.d.l. b.d.l. b.d.l. b.d.l.
U 0.051 0.019 0.031 b.d.l. 0.019 0.038 b.d.l. 0.051 0.074 b.d.l. b.d.l. b.d.l. b.d.l. b.d.l.
Nb 0.143 0.173 0.446 0.009 0.306 0.629 0.314 0.172 b.d.l. b.d.l. b.d.l. b.d.l. b.d.l. b.d.l.
Ta 0.007 0.011 0.018 b.d.l. 0.016 0.046 0.070 0.041 b.d.l. b.d.l. 0.018 b.d.l. b.d.l. b.d.l.
Sr 2,039 2,062 2,208 1,957 2,922 1,529 2,006 1,922 1,936 1,831 1,688 1,779 2,001 1,815
Zr 1.58 1.63 2.87 0.21 2.08 3.40 0.680 1.34 9.97 b.d.l. b.d.l. b.d.l. b.d.l. b.d.l.
Hf 0.025 0.044 0.047 b.d.l. 0.064 0.076 b.d.l. b.d.l. b.d.l. b.d.l. b.d.l. b.d.l. b.d.l. b.d.l.
Y 0.371 0.370 0.290 0.424 0.550 0.454 0.126 0.498 0.121 0.133 0.087 0.104 0.143 b.d.l.
La 6.18 5.94 4.39 3.04 5.77 3.50 5.08 2.05 11.2 10.9 2.97 3.04 6.99 6.55
Ce 10.4 8.94 7.27 6.02 7.95 6.82 7.98 3.74 13.6 13.8 5.18 5.24 10.5 9.79Pr 0.894 0.862 0.794 0.716 0.785 0.801 0.777 0.442 0.978 1.06 0.563 0.512 0.848 0.689
Nd 2.75 2.84 2.71 2.35 2.45 2.99 2.76 1.61 2.74 2.76 1.71 1.74 2.45 1.89
Sm 0.279 0.277 0.306 0.328 0.278 0.390 0.217 0.242 0.185 0.213 0.181 0.221 0.136 b.d.l.
Eu 0.727 0.866 0.921 0.929 1.34 1.00 0.845 0.918 0.870 1.05 0.526 0.498 1.21 0.961
Gd 0.164 0.234 0.148 0.179 0.285 0.251 b.d.l. 0.217 b.d.l. 0.087 0.139 0.064 b.d.l. b.d.l.
Tb 0.016 0.012 0.018 0.018 0.014 0.023 b.d.l. b.d.l. b.d.l. b.d.l. b.d.l. b.d.l. b.d.l. b.d.l.
Dy 0.084 0.116 0.079 0.110 0.089 0.104 b.d.l. 0.18 b.d.l. b.d.l. 0.05 b.d.l. b.d.l. b.d.l.
Ho 0.015 0.014 0.008 0.015 0.018 0.019 b.d.l. 0.03 b.d.l. b.d.l. b.d.l. b.d.l. b.d.l. b.d.l.
Er 0.049 0.014 0.025 0.035 0.027 0.025 b.d.l. b.d.l. 0.026 b.d.l. b.d.l. b.d.l. b.d.l. b.d.l.
Tm 0.004 0.003 0.005 0.003 0.002 0.004 b.d.l. b.d.l. b.d.l. b.d.l. b.d.l. b.d.l. b.d.l. b.d.l.
Yb 0.036 0.029 b.d.l. 0.032 0.038 0.035 b.d.l. b.d.l. b.d.l. b.d.l. b.d.l. b.d.l. b.d.l. b.d.l.
Lu 0.004 0.004 0.007 0.005 0.006 0.007 b.d.l. b.d.l. b.d.l. b.d.l. b.d.l. b.d.l. b.d.l. b.d.l.Eu/Eua 9.5 10.0 11.6 10.5 14.2 9.1 11.8 19.8 9.6 9.8
LaN /SmN 12.8 12.4 8.3 5.4 12.0 5.2 13.6 4.9 35.0 29.7 9.5 8.0 29.8
Rare earth element ratios are C1-normalized (Anders and Grevesse 1989)
b.d.l. below detection limita Data from Scribano et al. (2006)
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Islands, and is within the ranges of the least enriched
clinopyroxenes from Vulture peridotites and the least
depleted clinopyroxenes from North Africa peridotites. By
contrast, 87Sr/ 86Sr of the Hyblean gabbros is the highest
among the samples showing 143Nd/ 144Nd * 0.5129.87Sr/ 86Sr and Rb content are positively correlated in 7
out of 12 samples (Fig. 4b). The other samples have higher
Rb contents (up to 16 ppm) and variable 87Sr/ 86Sr. Hyblean
peridotites and basic granulites have lower Rb contents and
Sr isotope ratios than the other gabbros.176Hf/ 177Hf ranges from 0.282429 to 0.283032 (eHf =
-6.6 to +14.8). Hf–Nd isotopic values (epsilon notations)
are plotted in Fig. 5a relative to the ‘‘Terrestrial array’’ of
Vervoort et al. (1999). Most samples plot in the upper right
quadrant (suprachondritic 176Hf/ 177Hf and 143Nd/ 144Nd)
and only two samples (gabbros XLP1 and FB50) plot in the
lower right quadrant (subchondritic 176Hf/ 177Hf). A few Hf
data for lavas from Etna volcano and Hyblean Plateau are
Table 3 Representative in situ major-element, rare earth element, Nb
and Ta data for oxides from Hyblean gabbros
FB50_ox9 FB50_ox11 FB70_ox5
OX III OX III OX III
Elements (wt%)
SiO2 0.12 b.d.l. 0.03
TiO2 52.25 54.08 50.15Al2O3 0.54 0.57 0.70
FeOt 39.28 37.63 41.30
MnO 0.37 0.54 0.49
MgO 7.09 8.55 6.21
CaO 0.06 0.07 0.07
Cr2O3 b.d.l. b.d.l. b.d.l.
Total 99.7 101.4 99.0
Elements (ppm)
Nb 9.16 30.6 114
Zr 6.89 11.2 9.40
La 0.005 0.010 0.006
Ce 0.022 0.056 0.017
Pr 0.004 0.010 0.003
Nd 0.023 0.049 0.027
Sm 0.008 0.012 b.d.l.
Eu 0.004 0.005 0.006
Gd 0.009 0.018 0.013
Tb 0.002 0.003 0.007
Dy 0.023 0.024 0.051
Ho 0.006 0.007 0.011
Er 0.023 0.021 0.040
Tm 0.004 0.004 0.008
Yb 0.033 0.036 0.065
Lu 0.007 0.007 0.012
b.d.l. below detection limit
T a b l e 2
c o n t i n u e d
F B 5 0
F B - f 3 1
F B - f 4
V B 5
F B 7 0
F B 1 1
F B - f 7
C P X_
2 a
C P X_
4
C P X_
1 0
C P X_
5
C P X_
6
C P X_ 1
C P X_
4
C P X_
1
C P X_
2
C P X_ 1
a
C P X_
3
A M P H a
C P X_
1 a
C P X
_ 3
C P X_
2
C P X_
3
T m
0 . 1 5 8
0 . 2 1 2
0 . 2 2 0
0 . 1 7 5
0 . 1 8 7
0 . 1 0 7
0 . 2 4 3
0 . 2 8 1
0 . 3 2 4
0 . 4 7 3
0 . 4 2 6
0 . 5 1 4
0 . 2 1 4
0 . 2 0
6
0 . 2 7 8
0 . 2 4 3
Y b
1 . 0 1
1 . 3 3
1 . 2 7
1 . 1 7
1 . 1 3
0 . 6 0 0
1 . 9 1
1 . 5 8
1 . 9 8
3 . 0 3
2 . 6 8
3 . 5 5
1 . 1 8
1 . 3 2
1 . 6 5
1 . 4 7
L u
0 . 1 3 6
0 . 1 8 8
0 . 1 8 5
0 . 1 6 5
0 . 1 5 7
0 . 0 8 9
0 . 2 5 0
0 . 2 2 0
0 . 2 7 7
0 . 4 2 6
0 . 3 9 1
0 . 4 7 3
0 . 1 7 8
0 . 1 7
8
0 . 2 1 5
0 . 2 3 4
L a N / Y b N
2 . 0
1 . 9
1 . 9
1 . 3
1 . 3
3 . 1
1 . 4
1 . 5
1 . 5
2 . 2
2 . 5
3 . 3
1 . 4
1 . 3
2 . 1
2 . 3
S m N / Y b N
3 . 7
3 . 6
3 . 9
3 . 8
3 . 8
6 . 3
3 . 1
3 . 9
3 . 3
3 . 0
3 . 4
3 . 3
4 . 7
4 . 3
3 . 9
4 . 4
E u / E u a
1 . 1
1 . 0
1 . 1
1 . 1
1 . 0
0 . 9
0 . 8
0 . 9
1 . 0
1 . 0
1 . 0
1 . 2
1 . 1
1 . 1
1 . 4
1 . 3
R a r e e a r t h e l e m e n t r a t i o s a r e C 1 - n o r m a l i z e d ( A n d e r s a n d G r e v e s s e 1 9 8 9 )
b . d . l . b e l o w d e t e c t i o n l i m i t , m g # M
g / ( M g +
F e t o t )
a
D a t a f r o m S c r i b a n o e t a l . ( 2 0 0 6 )
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available in the literature (Fig. 5a). They are in the range of
the gabbros with the highest eHf . The field of Hyblean
hydrothermal crustal zircons is also shown; half of the
gabbros fall within this range. Sr and Hf isotope ratios are
not correlated with one another (Fig. 5b).
Rb–Sr and Lu–Hf systematics do not provide age con-
straints; however, Sm–Nd data for nine gabbros yield an
isochron age of 253 ± 60 Ma (Fig. 6).
Trace-element compositions
Plagioclase
Representative trace-element analyses of plagioclases are
listed in Table 1. Their REE patterns show the typical
LREE-enriched shape (Fig. 7), with a strong positive Eu
anomaly (Eu/Eu* = 9–20); HREE are often below detec-
tion limit (b.d.l.). PM-normalized multi-element diagrams
for average plagioclases are characterized by marked
positive Ba, Sr and Eu anomalies (Fig. 8) and variable Rb,
Ba and HFSE abundances. In general, selected LILE and
HFSE in plagioclase grains vary widely (Rb = b.d.l. to79 ppm; Ba = 64–7,688 ppm; Nb = b.d.l. to 0.6 ppm;
Zr = b.d.l. to 10 ppm; Table 1). Sample VB5 shows the
highest average Rb, Ba, Th, Nb, Zr and Hf values. Some
differences also exist among grains from the same sample:
for instance, plagioclase grains in gabbro VB5 have widely
variable contents of Rb (0.13–79 ppm), Ba (101–
7,688 ppm) and Sr (1,529–2,922 ppm).
Clinopyroxene
Representative trace-element analyses of clinopyroxenes
are listed in Table 2. The C1-normalized REE patterns of
all clinopyroxene cores are upward-convex and are nearly
parallel (LaN /YbN = 1.3–3.1 and SmN /YbN = 3–6;
Fig. 7). Eu anomalies are very small (Eu/Eu* = 0.8–1.4).
Clinopyroxenes from sample FB70 show the highest
Fig. 3 a C1-normalized REE patterns and b PM-normalized trace-
element compositions of Hyblean gabbros included in this study (after
Scribano et al. 2006, modified). Normalizations to C1 and PM are
after Anders and Grevesse (1989) and McDonough and Sun (1995),
respectively
Table 4 Sr–Nd–Hf isotope compositions of Hyblean gabbro xenoliths
Sample 87Sr /86Sr 1 SE 143Nd /144Nd 1 SE 147Sm /144Nd 176Hf/ 177Hf 1 SE eNd eHf
XLP1 0.704294 5 0.512896 9 0.1663 0.282579 29 5.04 -1.25
VB5 0.704477 9 0.512901 20 0.1757 0.282913 28 5.12 10.58
FB50 0.704721 6 0.512901 7 0.1642 0.282429 46 5.12 -6.55
FB80 0.70479 8 0.512910 6 0.1776 0.282938 50 5.30 11.46
FB-f4 0.705104 4 0.512914 8 0.1657 0.282756 39 5.38 5.03
FB-f31 0.704431 6 0.512866 9 0.1757 0.282795 38 4.44 6.42
FB-f7 0.704439 5 0.512848 11 0.1447 0.282778 43 4.09 5.81
FB-f32 0.704726 9 – – – 0.282648 40 – 1.22
FB-e11 0.705192 8 0.513062 6 0.2186 0.283032 62 8.27 14.78
FB13 0.704883 9 0.512861 8 0.1465 0.282981 20 4.35 12.98
FB70 0.704141 4 0.512849 7 0.1368 0.282909 3 4.12 10.43
FB11 0.703943 7 0.512901 9 0.1741 0.282818 30 5.14 7.21
8 Contrib Mineral Petrol (2009) 157:1–22
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overall REE content among these gabbros. Figure 8 shows
the PM-normalized average clinopyroxene compositions:LILE, Nb and Ta values show the largest scatter (e.g.,
RbN = 0.1–0.5; BaN = 0.01–1.2; NbN = 0.04–2), whereas
the other elements show similar contents and distribution in
all samples. The patterns are characterized by a marked
negative Sr anomalies (due to co-precipitation of plagio-
clase) and weak negative Zr and Ti anomalies. In the most
metamorphic-textured gabbros (e.g., sample FB11), Rb and
Ba contents are below the detection limit. Some differences
between grains from the same sample have been observed:
for example, clinopyroxene grains from gabbro FB50 show
variable Ba (1.5–20.7 ppm).
Amphibole
The analysis of amphibole in gabbro FB70 is listed in
Table 2. The C1-normalized REE pattern of the amphibole
is parallel to that of the clinopyroxene from the same sample,
but with higher abundances, and shows a weak positive Eu
anomaly (Eu/Eu* = 1.2; Fig. 7). LREE are moderately
enriched relative to the HREE (LaN /YbN = 3.3). When
normalized to PM (Fig. 8), the amphibole shows higher
values than coexisting clinopyroxene: its pattern is
characterized by positive Ba, Nb, and Ta anomalies, withBaN * 100, NbN * 63 and TaN * 72.
Fe–Ti oxides
Representative trace-element analyses of Fe–Ti oxides are
listed in Table 3. Oxides are very poor in REE, with
LREEN\HREEN (Fig. 7). Fe–Ti oxides concentrate vari-
able amounts of HFSE such as Zr (=7–11 ppm) and Nb
(=9–114 ppm).
In situ Sr isotope data
In situ Sr istope data for plagioclase (cores and rims) and
the crypto- to micro-crystalline portions of intercumulus
pockets in samples VB5, FB50 and FB11 are listed in
Table 5 and plotted in Fig. 9. 87Sr/ 86Sr in plagioclase cores
varies from 0.704841 to 0.702775, with a preponderance of
relatively low values (*0.7027), whereas the crypto- to
micro-crystalline portions of intercumulus pockets show
much higher 87Sr/ 86Sr (up to 0.7055 in sample FB50).
Fig. 4 a87Sr/ 86Sr versus 143Nd/ 144Nd plot for Hyblean gabbros.
Fields for Hyblean basic granulites (Tonarini et al. 1996), Hyblean
peridotite xenoliths (D’Orazio 1994), peridotite clinopyroxenes from
Monte Vulture (Downes et al. 2002) and North Africa (Beccaluva
et al. 2008), lavas from Pantelleria Island (Esperanca and Crisci 1995;
Civetta et al. 1998), Linosa Island (Civetta et al. 1998; Del Moro and
Rottura, unpublished data), Vulture Mt (Peccerillo 2005 and refer-
ences therein), Hyblean Plateau (Bianchini et al. 1999) and Etna Mt
(Armienti et al. 2004 and references therein) are shown for
comparison. CMR field is after Lustrino and Wilson (2007).
b87Sr/ 86Sr versus Rb plot for Hyblean gabbros. Fields for Hyblean
basic granulites and peridotites (Tonarini et al. 1996) are shown for
comparison. Three mixing curves are reported between the cumulate
pile (Rb = 0.2 ppm; 87Sr/ 86Sr = 0.7027; clinopyroxene contribution
is considered for Rb content) and hypothetical metasomatic agents
with different Rb–87Sr/ 86Sr characteristics: (a) Rb = 80 ppm,87Sr/ 86Sr = 0.7 05, (b) Rb = 10 ppm, 87Sr/ 86Sr = 0.710; (c)
Rb = 2 ppm, 87Sr/ 86Sr = 0.710. Mixing curves were calculated
using the expression given by Faure (1998)
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Plagioclase rims record varying degrees of isotopic equili-bration between cores and the Fe–Ti rich intercumulus
pools. Rb content varies in the ranges 0.02–77 ppm in
plagioclase cores and 3.5–80 ppm in intercumulus pools.
Sr content varies between 1,152 and 9,714 ppm in pla-
gioclase cores and 1,010–5,524 ppm in the pools (only one
point up to 15,000 ppm). Plagioclase rims have Sr con-
centrations in the range 706–10,667 ppm; 1 point has
Sr = 16,000 ppm. Plagioclase rims have Rb contents in the
range 0.6–69 ppm.
Discussion
Petrogenesis of the Hyblean gabbros: cumulus and
post-cumulus mixing processes
On the basis of textural and geochemical evidences,
Scribano et al. (2006) suggested that Hyblean gabbros
represented MORB-type cumulates infiltrated by late Fe–
Ti rich melts that may have been filter-pressed from a
nearby cumulate mush. However, since no isotopic or in
situ trace-element data were available for these xenoliths,the origin of either the cumulus phases (clinopyrox-
ene + plagioclase) or the late Fe–Ti rich melts could only
be inferred from petrography and the whole-rock and
mineral chemistry. The new data presented here shed more
light on this issue.
The trace-element composition of clinopyroxene raises
questions about the possible co-precipitation with plagio-
clase: the negative Sr anomaly is not accompanied by a
negative Eu anomaly, and thus clinopyroxene might have
precipitated before plagioclase. The Eu–Sr decoupling may
be explained by the fact that cpx/liq DSr\
cpx/liq DEu (mineral/
liquid
D = partition coefficient) in basaltic systems (e.g.,Hart and Dunn 1993; Fujimaki et al. 1984). However, only
very rarely do small clinopyroxene grains occur within
plagioclase, as would be expected if clinopyroxene pre-
cipitated earlier than plagioclase. Therefore we favor the
hypothesis of the simultaneous segregation of these phases.
The crypto- to micro-crystalline intercumulus pockets
bearing large Fe–Ti-rich oxides can be regarded as origi-
nally melt pockets, and related to the late injection of
Fe–Ti-rich melt (Scribano et al. 2006).
Fig. 6 Sm–Nd isochron for the Hyblean gabbros. Gray squares
indicate out of the trend samples, which were not considered in age
calculation. Uncertainties (±1r) are smaller than the symbol size
Fig. 5 a eHf versus eNd and b 87Sr/ 86Sr plots for Hyblean gabbro
xenoliths. Black array and dark gray field indicate the terrestrial Hf–
Nd trend from Depleted Mantle ( DM ) to Crust (Vervoort et al. 1999
and references therein). Stars are data for Etna and Hyblean Plateau
(from Gasperini et al. 2002); pale gray field is that of the Hyblean
hydrothermal zircons from Sapienza et al. (2007); suggested field of
SCLM is from Griffin et al. (2000); HIMU field is from Salters and
White (1998). Hf isotope composition of MARID is from Choukroun
et al. (2005); Sr isotope composition of MARID is from Hawkesworth
et al. (1990)
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An important issue concerns the nature of these late melts,
i.e., residual after the cumulus phases crystallization or
exotic. To unravel the question, we calculated the evolution
of the liquid crystallizing the plagioclase + clinopyroxene
taking the gabbro FB50 as example (Fig. 10; procedure is
described in the figure caption). The comparison between thecomposition of the residual liquid and that of the intercu-
mulus pockets clearly shows that the latter cannot be the
residue after the plagioclase + clinopyroxene crystalliza-
tion, given the significant differences with the modeled REE
abundance and distribution (Fig. 10). Moreover, cumulus
clinopyroxene is rich in thin exsolution lamellae of ilmenite
(Fig. 2a, b, e) and the initial Ti content of clinopyroxene
should be even higher than that measured by microprobe
in exsolution-free micro-volumes (see Table 2). It is
improbable that a liquid segregating such a Ti-rich clino-
pyroxene would evolve toward compositions able to produce
a residual liquid containing abundant Fe–Ti oxides. We
cannot exclude the possibility that small quantities of
residual liquid after plagioclase + clinopyroxene crystalli-
zation were present, but they have not been identified.Afterwards, these gabbroic bodies suffered variable meta-
morphic re-equilibration as attested by the spectrum of
igneous to granoblastic microstructures.
Having reconstructed the sequence of petrological pro-
cesses suffered by the Hyblean gabbros, we can use the
geochemical and isotopic data to constrain the nature of
these processes. Although the whole-rock Nd isotope com-
position remains rather constant (143Nd/ 144Nd * 0.5129),
both Sr and Hf isotopes indicate a complex mixing process.
Fig. 7 REE compositions of
gabbros-forming minerals.
Normalization is to C1
chondrite (Anders and Grevesse
1989)
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The horizontal array for Sr data suggests the presence of at
least two components, one with 87Sr/ 86Sr B 0.7039, and the
other more enriched in radiogenic Sr (87Sr/ 86Sr C 0.7051;
Fig. 4a). In addition, eHf values do not distribute along the
crust–mantle array in Hf–Nd space, but trend vertically
toward negative values (Fig. 5a). These isotopic trends
cannot be explained by crustal contamination, which would
have required a simultaneous change of Nd isotope com-
position. A contribution by more than one component to the
cumulate pile is strongly suggested by the decoupling
between Sr and Hf isotopes (Fig. 5b), which also show that
re-equilibration with the surrounding subcontinental litho-
spheric mantle (SCLM) alone cannot account for the Sr–
Nd–Hf isotope features (Fig. 5).
In situ Sr analyses of plagioclase and the crypto- to micro-
crystalline portions of intercumulus pockets from three
selected gabbros (FB11, FB50 and VB5) can help to unravel
this geochemical puzzle. Plagioclase cores and intercumulus
pools are considered representative of the whole cumulate
pile and the late Fe–Ti-rich melts, respectively. The plot of
87Sr/ 86Sr against Sr and Rb concentrations (Fig. 9) reveals
that a simple binary mixing cannot explain the scatter of
data. Plagioclase cores from all samples tend to have low87Sr/ 86Sr (*0.7027), low Rb (0.1 ppm) and variable Sr, but
show significant within-grain heterogeneity with 87Sr/ 86Sr
up to 0.7048 and highly variable Rb and Sr concentrations.
Within-sample variations were already evident in the trace-
element data for plagioclase and clinopyroxene (Tables 1,2). The within-grain variability is unlikely to depend on
mantle source heterogeneities, but rather testifies to the
interaction between the cumulus phases—derived from a
homogeneous mantle-derived magma—and an isotopically
distinct agent. Such an interaction could result in a patchy
geochemical zoning of plagioclase and clinopyroxene.
The intercumulus pools show also radiogenic Sr-isotope
compositions, up to 0.7055 in gabbro FB50. In this sample,
plots of Rb and Sr contents against 87Sr/ 86Sr show a
complex distribution (Fig. 9). In the Rb–87Sr/ 86Sr plot,
plagioclase cores and rims have low Rb contents
(B10 ppm, except for two rim points with Rb *30 and*60 ppm) and Sr isotope ratios ranging from 0.703 to
0.705. However, analyses of intercumulus pools show a
range of 87Sr/ 86Sr = 0.7044–0.7055, and higher Rb con-
tents (up to*80 ppm). The Sr-87Sr/ 86Sr plot is not so well
defined, since the analyses of plagioclase cores and rims
also have very high Sr contents. Therefore, the plagioclase
data suggest heterogeneities related to interaction between
the cumulate pile and a low-Rb, high-87Sr/ 86Sr metaso-
matic component. Later, the infiltrating Fe–Ti-rich melts,
characterized by 87Sr/ 86Sr * 0.7055, introduced signifi-
cant amounts of Rb and Sr. The whole-rock data are the
expression of such a mixing (Fig. 9).
These geochemical and textural observations, especially
on gabbro FB50, allow us to sketch the relative time
relationships between the metasomatic events. Plagioclase
cores locally record the isotopic-geochemical features due
to the interaction between the cumulate pile and the low-
Rb component. By contrast, the plagioclase rims texturally
and geochemically reflect variable degrees of re-equili-
bration with the late Fe–Ti rich melt. Therefore, the
plagioclase cores indicate that the cumulate pile was firstly
affected by metasomatism driven by the low-Rb agent;
afterward the Fe–Ti-rich melt infiltrated the previously
metasomatized cumulates.
The lowest-87Sr/ 86Sr plagioclase cores and the intercu-
mulus pools constrain the characteristics of the cumulate
pile and of the Fe–Ti-rich melt, respectively, since they
represent their direct crystallization/cooling products. The
situation is more complex for the low-Rb metasomatic
agent, whose geochemical features can only be inferred
from the highest-87Sr/ 86Sr portions of the plagioclase cores.
The geochemistry of these components has been estimated
as follows:
Fig. 8 PM-normalized trace-element distributions for average pla-
gioclases, clinopyroxenes and amphibole for Hyblean gabbro
xenoliths. Normalization to PM after McDonough and Sun (1995)
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T a b l e 5
I n s i t u S r i s o t o p e a n a l y s e s
f o r p l a g i o c l a s e c o r e s ( P L - C o r e s ) a n d r i m s (
P L - R i m s ) a n d m i c r o - t o c r y p t o - c r y s t a l l i n e p
o r t i o n s o f t h e F e – T i - r i c h i n t e r c u m u l u s p o o l s ( m i c r o - c r y p t o I N T - C U )
f r o m H y b l e a n g a b b r o s F B 5 0 , F B 1 1
a n d V B 5
G a b b r o F B 5 0
G a b b r o F B 1 1
G a b b r o V B 5
8 7 S r /
8 6 S r
± 2 r
S r ( p p m )
R b ( p p m )
8 7 S r /
8 6 S r
± 2 r
S r ( p p m )
R b ( p p m )
8 7 S r /
8 6 S r
± 2 r
S r ( p p m )
R b ( p p m )
P L - C o r e s
0 . 7 0 4 7 7 1
0 . 0 0 0 1 9
1 , 3 9 0
2 . 4
P L - C o r e s
0 . 7 0 2 9 6
0 . 0 0 0 1
1 , 5 9 3
0 . 5
P L - C o r e s
0 . 7 0 3 3 7 2
0 . 0 0 0 1 3
1 , 5 8 4
3 . 8
0 . 7 0 4 5 9 8
0 . 0 0 0 0 9
9 , 7 1 4
1 1
0 . 7 0 2 9 4
0 . 0 0 0 1
1 , 5 7 2
2 . 4
0 . 7 0 2 8 5 2
0 . 0 0 0 1 2
1 , 1 8 0
0 . 1
0 . 7 0 3 2 5 2
0 . 0 0 0 0 9
3 , 2 3 8
5 . 5
0 . 7 0 3 0 2
0 . 0 0 0 1
1 , 6 3 1
0 . 0
2
0 . 7 0 4 8 4 1
0 . 0 0 0 1 2
2 , 4 2 7
7 7
0 . 7 0 3 6 8 0
0 . 0 0 0 0 8
5 , 7 1 4
3 . 1
0 . 7 0 2 8 6
0 . 0 0 0 1
2 , 2 0 9
1 . 1
0 . 7 0 3 8 1 5
0 . 0 0 0 1 5
2 , 6 0 6
1 7
0 . 7 0 3 8 7
0 . 0 0 0 2
3 , 7 9 1
6 . 3
0 . 7 0 4 2 6 7
9 . 6 E - 0 5
2 , 8 3 4
2 2
0 . 7 0 2 9
0 . 0 0 0 1
2 , 1 8 6
0 . 1
0 . 7 0 2 7 7 5
9 . 8 E - 0 5
1 , 1 5 2
0 . 1
0 . 7 0 4 0 8 8
0 . 0 0 0 1 1
2 , 7 1 6
1 4
0 . 7 0 3 5 8 7
9 . 6 E - 0 5
1 , 8 5 6
2 0
P L - R i m s
0 . 7 0 4 4 5 9
0 . 0 0 0 0 6
6 , 0 0 0
3 . 8
P L - R i m s
0 . 7 0 3 0 9
0 . 0 0 0 1
1 , 5 5 3
3 . 1
P L - R i m s
0 . 7 0 4 7 4 3
0 . 0 0 0 0 5
7 5 7
1 . 0
0 . 7 0 4 8 7 4
0 . 0 0 0 0 5
1 6 , 0 0 0
5 8
0 . 7 0 3 5 6
0 . 0 0 0 1
7 0 6
5 . 9
0 . 7 0 5 3 0 2
0 . 0 0 0 1
1 , 1 8 0
6 9
0 . 7 0 3 1 3 2
0 . 0 0 0 0 9
1 , 8 6 7
0 . 6
0 . 7 0 5 0 5
0 . 0 0 0 1
5 , 3 7 2
7 . 4
0 . 7 0 4 1 4 8
0 . 0 0 0 1
2 , 8 2 7
2 3
0 . 7 0 4 7 5 1
0 . 0 0 0 0 4
1 0 , 6 6 7
7 . 4
0 . 7 0 3 1 2
0 . 0 0 0 0
1 , 7 9 1
0 . 9
0 . 7 0 4 1 1 0
0 . 0 0 0 1 2
8 , 7 6 2
6 . 5
0 . 7 0 4 2 2
0 . 0 0 0 1
3 , 9 0 7
0 . 8
0 . 7 0 3 4 0 7
0 . 0 0 0 0 7
4 , 3 8 1
0 . 7
0 . 7 0 5 0 5
0 . 0 0 0 1
7 , 9 1 9
6 . 3
0 . 7 0 4 7 8 0
0 . 0 0 0 0 6
5 , 7 1 4
5 . 5
0 . 7 0 3 0 5
0 . 0 0 0 1
1 , 8 6 0
4 . 4
0 . 7 0 4 6 4
0 . 0 0 0 1
6 , 5 8 1
4 . 0
0 . 7 0 3 3 7
0 . 0 0 0 1
2 , 1 8 6
1 . 0
M i c r o - c r y p t o
I N T - C U
0 . 7 0 5 2 2 4
0 . 0 0 0 1 1
9 , 7 1 4
3 1
M i c r o - c r y p t o
I N T - C U
0 . 7 0 4 4 1
0 . 0 0 0 1
4 , 5 9 3
8 0
0 . 7 0 4 5 3 2
0 . 0 0 0 0 8
2 , 9 9 0
3 . 5
0 . 7 0 4 5 9 9
0 . 0 0 0 0 6
3 , 4 6 7
4 . 5
0 . 7 0 5 0 6 3
0 . 0 0 0 0 6
4 , 5 7 1
7 6
0 . 7 0 5 1 3 0
0 . 0 0 0 1 4
1 5 , 0 4 8
6 5
0 . 7 0 5 0 1 4
0 . 0 0 0 1 1
2 , 6 1 0
3 2
0 . 7 0 5 1 2 2
0 . 0 0 0 0 5
4 , 9 5 2
4 4
0 . 7 0 4 9 3 3
0 . 0 0 0 1 2
2 , 4 7 6
1 5
0 . 7 0 5 5 1 4
0 . 0 0 0 1 7
1 , 0 1 0
2 5
0 . 7 0 5 0 7 0
0 . 0 0 0 0 6
3 , 0 4 8
3 2
0 . 7 0 4 3 8 0
0 . 0 0 0 0 7
5 , 5 2 4
1 4
0 . 7 0 5 1 7 2
0 . 0 0 0 1 2
2 , 0 9 5
4 3
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(a) Cumulitic pile (plagioclase + clinopyroxene ± OX-
I ± OXII): constrained by plagioclase cores, with
Sr * 1,200 ppm, Rb* 0.1 ppm and 87Sr/ 86Sr *
0.7027.
(b) Metasomatic agent 1: This metasomatic agent
imposed cryptic metasomatism on the cumulate pile.
Because the patchy geochemical zoning in plagio-
clase is not associated with melt inclusions, we
propose that the low-Rb agent was a separate fluid.
The trace-element data indicate that cumulus phases
from gabbro FB11 are the least contaminated by late
Fe–Ti-rich melts (e.g., elements as HFSE, U and Th
Fig. 9 In situ Rb and Sr
concentrations versus in situ87Sr/ 86Sr for gabbros FB11,
VB5 and FB50. Two mixing
curves are shown between the
cumulate pile and the two
metasomatic agents: the low-Rb
fluid and the Fe–Ti-rich melt
(see Table 7 for the isotopic and
geochemical features of the
three components). Micro-
crypto INT-CU = micro- to
crypto-crystalline portions of
the intercumulus pockets.
Mixing curves were calculated
using the formulation reported
in Faure (1998)
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in cumulus phases are b.d.l.s; Tables 1, 2 and Fig. 8).
This is corroborated by textural and geochemical
evidence: rare modal occurrence of intercumulus
material (Fig. 2c); restriction of interaction with the
intercumulus material to plagioclase rims (Fig. 9).
Thus, this sample is the best candidate to constrain
the geochemical features of the first metasomatizing
agent. LAM-data for sample FB11 show some
variation only for Rb and Ba contents (Tables 1, 2).
We infer that this fluid was solute-poor, and carried
only relatively small concentrations of selected LILE
such as Rb, Ba and Sr (87Sr/ 86Sr C 0.7055).
(c) Metasomatic agent 2 (Fe–Ti-rich melt): Gabbro FB50
was selected to constrain the geochemical character of
this metasomatic agent, because it contains analy-
zable Fe–Ti-rich intercumulus pockets. The Rb–Sr
features of this component are taken as those of the
intercumulus portions of these pockets (Sr up to*15,000 ppm, Rb up to *80 ppm, Rb/Sr * 0.005
and 87Sr/ 86Sr * 0.705). Other geochemical charac-
ters were inferred from a mass-balance calculation
based on 1,000-point modal analyses of thin sections
and the geochemistry of cumulus plagioclase and
clinopyroxene (Table 6). We assumed that the OX-III
are genetically related to the intercumulus pockets,
and their geochemical contribution is included within
them. Hence, the chemical composition of the inter-
cumulus material is defined by the difference between
the whole-rock analysis (Scribano et al. 2006) and the
modally based geochemical contribution of clinopy-roxene and plagioclase (Fig. 11). This approach may
lead to an underestimation of Ti content in clino-
pyroxene, which increases in the overgrowth rims
(Scribano et al. 2006). However, the proportion of the
CPX-I rims is small relative to the whole-rock, and
their Ti contribution is assumed to be negligible. The
uncertainties related to this method (e.g., the within-
grain chemical variation) indicate that the estimate
must be considered only semiquantitative. As a whole
the Fe–Ti-rich melt introduced LILE (*95% of Rb,
*70% of Sr,*80% of Ba) and HFSE (*90% of Nb
and Ta, *60% of Zr, *40% of Hf) and contributed
*40% of the REE (Fig. 11).
We have modeled this three-component mixing process,
calculating the mixing curves between the cumulate pile
and either the low-Rb fluid and the Fe–Ti rich melt. The
geochemical and isotopic parameters of the cumulates and
Fig. 10 PM-normalized REE composition of liquids in equilibrium
with clinopyroxene from gabbro FB50. Thehatched curves represent
the residual liquid after various degrees (numbers in italics) of
plagioclase + clinopyroxene fractional crystallization. The REE
pattern of the initial magma has been calculated using the Dcpx/liquid
of Hart and Dunn (1993); then, the composition of the residual liquidafter different degrees of cumulus phases crystallization has been
calculated taking into account the plagioclase/pyroxene modal ratio
and Dcpx/liquid (Paster et al. 1974; Hart and Dunn 1993) and Dplag/liquid
(Paster et al. 1974; McKenzie and O’Nions 1991; Bindeman et al.
1998 for An77). The composition of the intercumulus pockets
(calculated by subtracting the mode-based chemical contribution of
plagioclase + clinopyroxene from the whole-rock composition;
Scribano et al. 2006) is shown for comparison. Normalization to
PM after McDonough and Sun (1995)
Fig. 11 Trace-element
distribution among the rock-
forming minerals in gabbro
FB50. See Table 6 for modalabundances
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Fe–Ti rich melt were taken directly from plagioclase cores
and the crypto- to micro-crystalline portions of the inter-
cumulus pockets (Table 7). The fluid responsible for the
cryptic metasomatism can be constrained by the whole-rock
data (see below). Figure 4b shows mixing lines between the
cumulitic pile and possible metasomatic fluids with differ-
ent Rb–87Sr/ 86Sr characteristics (a: Rb = 80 ppm,87Sr/ 86Sr = 0.705; b: Rb = 10 ppm, 87Sr/ 86Sr = 0.710; c:
Rb = 2 ppm, 87Sr/ 86Sr = 0.710). The mixing curve b
between the early plagioclase and a fluid with a 10 ppm Rb
and 87Sr/ 86Sr of 0.710 fits 7 out of 12 samples, including
gabbro FB11, which is the sample that seems to have
experienced the minimum degree of metasomatism by the
Fe–Ti-rich melt. A mixing curve for a fluid with87Sr/ 86Sr = 0.7055 does not fit the in situ Sr data (not
shown in Fig. 9); in contrast, low-Rb points from the three
gabbros on Fig. 9 are well explained by*50% mixing with
an end-member having 87Sr/ 86Sr = 0.710 and Sr and Rb
contents = 500 ppm and 10 ppm, respectively (Table 7).
The highest Sr and Rb contents can be related to the late
contribution of the Fe–Ti-rich melt. Thus the whole-rock
compositions can be explained as the result of such a mixing
(see also Fig. 9), even if chemical heterogeneities in both
the metasomatic agents can cause some discrepancies fitting
between the data points and the mixing curves.
Origin of the metasomatic agents: a possible
MARID-type reservoir beneath the Hyblean Plateau
The analysis reported in the previous section raises two
questions: (1) are the two metasomatic agents (fluid and
Fe–Ti-rich melt) related one another? (2) what kind of
mantle reservoirs are required to explain the isotopic andgeochemical features of these agents? The two meta-
somatic agents share a more or less radiogenic Sr isotope
composition and a low Rb/Sr ratio (Table 7), but the fluid
is estimated to be much more ‘‘diluted’’ than the melt,
which carried high levels of both HFSE (Nb, Ta, Zr, Hf)
and LILE (Rb, Sr, Ba) (Fig. 11). The significant Sr and Rb
contents inferred in both metasomatic agents imply high
levels of these elements in their source(s). In the upper
mantle, these elements generally reside in amphibole and
phlogopite, while the high concentrations of Fe and HFSE
in the metasomatizing melt suggest an ilmenite- and rutile-
rich source. Such a mineral assemblage can correspond to
more than one mantle source. For example it is equivalent
to the MARID (mica–amphibole–rutile–ilmenite–diopside)
rocks found as xenoliths in some kimberlites (Dawson and
Smith 1977; Gregoire et al. 2002) as well as to amphibole–
phlogopite–ilmenite associations like those found as veins
in composite mantle xenoliths from the Kerguelen Islands
(Moine et al. 2001). These options are discussed below.
Kramers et al. (1983) reported the trace-element com-
positions of MARID minerals: amphibole has Rb/
Sr * 0.06, phlogopite has Rb/Sr * 1.5, and apatite has
Rb/Sr ( 1. None of these minerals alone can account for
the geochemical features of each metasomatizing agent, but
different contributions from amphibole and phlogopite
(±apatite?) could yield these features. Sweeney et al. (1993)
experimentally investigated the behavior of MARID rocks
during partial melting in the KNFMASH system. They
found that dry melting of these rocks requires a quite high
temperature (T * 1,200°C) at pressure (P) * 30 Kbar; the
addition of 10 wt% H2O lowers the melting point by
*300°C (Fig. 12). In dry experiments, phlogopite behaves
as a restitic phase and completely disappears only above
Table 6 Trace-element mineral analyses of sample FB50 used for
the mass balance calculation
FB50
WRa CPX_10 PL_12 INT-CUb
Elements (ppm) 30.1 41.8 28.1
Rb 9.96 0.09 0.55 9.71
Ba 247 1.96 106 202
Th 0.20 0.16 0.06 0.19
U 0.07 0.05 0.02 0.09
Nb 5.34 2.21 0.17 5.47
Ta 0.49 0.15 0.01 0.44
La 6.03 4.43 5.94 2.23
Ce 14.8 16.9 8.94 6.02
Sr 3,185 67.1 2,087 2,293
Nd 11.6 17.9 2.84 4.98
P 742 180 141 798
Hf 1.35 3.05 0.04 0.68
Zr 40.0 79.8 1.63 35.9
Sm 3.02 5.17 0.28 1.30
Eu 1.37 1.82 0.87 0.45
Tb 0.44 0.72 0.01 0.21
Y 10.7 17.3 0.37 5.37
Yb 0.73 1.27 0.03 0.32
Modal percentage are reported in italicsa Whole-rock data are from Scribano et al. (2006)b INT-CU = intercumulus material composition (including OX-III)
Table 7 Geochemical and isotopic characters of the three compo-
nents used for the calculations of the mixing lines in Fig.9
Cumulus plagioclase Low-Rb fluid Fe–Ti rich melt
87Sr /86Sr 0.7027 0.710 0.705
Rb 0.1 10 80
Sr 1,200 500 15,000
Rb/Sr 0.00008 0.02 0.005
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1,350°C (see also Yoder and Kushiro 1969). Amphibole (K-
richterite in the assemblage studied by Sweeney et al. 1993),
which is unstable already below the solidus, is supposed to
break down or to dehydrate prior to breakdown.
The melting of HFSE- and Fe-rich phases is required to
supply the significant amounts of these elements in the Fe–
Ti-rich melt. Effective repositories for HFSE and Fe in the
mantle are, respectively, titanate phases—such as rutile and
the lindsleyite-mathiasite series (LIMA)—and ilmenite.
Residual rutile in mantle sources can produce negative
HFSE anomalies in the fluid or melt in equilibrium (fluid/ rutile
D and melt/rutile D ? Nb, Ta\Zr, Hf \ other elements;
Jenner et al. 1993; Brenan et al. 1994; Stalder et al. 1998;
Foley et al. 2000). Ilmenite can be assumed to behave
similarly (Ayers and Watson 1993). The solubility of rutile
in aqueous fluids or hydrous melts increases with T and
decreases with P (Ryerson and Watson 1987; Ayers and
Watson 1993). Therefore, the geochemical and textural
evidence can be explained by a thermal perturbation that
initially led to the dehydration/melting of amphibole and
phlogopite (±apatite), and later to the dissolution of rutile
and ilmenite together with melting of amphibole and
phlogopite. The Sr and Hf isotope ratios attest to the der-
ivation of the Hyblean gabbros from isotopically distinct
sources or a very heterogeneous single source. The
MARID source itself is very heterogeneous (e.g., Haw-
kesworth et al. 1990; Choukroun et al. 2005), and different
domains of a MARID-type source may have developedvery different isotopic features due to their complex petro-
genetic history (e.g., Choukroun et al. 2005). The isotopic
variations shown by the Hyblean gabbros are compatible
with the metasomatizing effect of agents derived from such
a source, as indicated by arrows in Fig. 5.
Alternatively, a mineral association made of amphibole–
phlogopite–ilmenite can represent a possible candidate to
generate metasomatic agents able to impart the geochemical
features observed in our gabbros. Moine et al. (2001)
interpreted amphibole-rich, phlogopite–ilmenite-bearing
veins as due to the interaction between the upper mantle and
percolating highly alkaline melts similar to the host rock.Melts with such a composition actually erupted in the
Hyblean Plateau in Cenozoic time, testifying that the
Hyblean lithosphere was actually percolated by those melts.
However, Sr–Hf isotopic data do not fit with an origin
related to the Hyblean Cenozoic volcanics (Figs. 4a, 5a):
they do not account for the high-87Sr/ 86Sr and the low- eHf ,
as would be expected in case of a genetic link. A possible
explanation may be to invoke the co-participation of more
factors: for example, a high 87Rb/ 86Sr in the amphibole–
phlogopite–ilmenite assemblage and/or the possible
re-equilibration with the surrounding environment.
The above discussion does not allow us to categorically
exclude one reservoir in favor of the other. However, the
isotopic constraints and the comparison with the data
available in literature lead us to favor the MARID-type
reservoir.
Parental liquids of Hyblean cumulitic gabbros
With the assumption that metamorphic re-equilibration
processes have not significantly changed the composition
of the cores of cumulus minerals, we have calculated the
composition of the parental liquids in equilibrium with
clinopyroxene from the Hyblean gabbros, using appropri-
ate cpx/liquid D for basaltic liquids (Hart and Dunn 1993). For
this calculation, the average analyses of clinopyroxenes
from each samples have been used. Figure 13 shows the
PM-normalized trace-element distribution of melts in
equilibrium with Hyblean clinopyroxenes. The theoretical
melts are very homogeneous from La to Yb, with marked
enrichment of LREE over HREE (LaN /YbN = 12–23) and
a negative Sr anomaly. Sample FB70 shows the highest
REE contents, in keeping with its slightly more evolved
Fig. 12 P–T space showing the (vertically hatched ) field of crystal-
lization of clinopyroxene+ plagioclase ± ilmenite (Thompson
1972). The obliquely hatched area indicates the field of Hyblean
basic granulites (Punturo et al. 2000). The Moho depth is after
Scarascia et al. (1994). Hyblean paleogeotherm is after Perinelli et al.
(2008). Olivine gabbro/spinel gabbro transition curve is after
Gasparik (1984). Spinel granulite/garnet granulite transition curve is
after Irving (1974). Dry and wet MARID solidus curves are after
Sweeney et al. (1993)
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character (e.g., lower An content in plagioclase; Table 1
and Scribano et al. 2006). The negative Sr and Ti anoma-
lies and the scattered Ba and Nb data shown in Fig. 13
deserve further discussion: Sr and Ti anomalies may be
related to the co-precipitation of plagioclase and oxides and
Nb and Ba distributions probably depend on post-cumulus
mixing with the Fe–Ti-rich melt providing *80% of Ba
and*90% of Nb (Fig. 11). Barium and Nb distributions in
clinopyroxenes from all samples are quite scattered
(BaN = 18–1,700; NbN = 6–260). The lowest Ba and Nb
contents are found in gabbro FB11, which is among thesamples with low modal contents of intercumulus material
and the least contaminated by late Fe–Ti-rich melts. It is
noteworthy that sample FB70, which is in general the
richest in trace-elements, has a relatively low Nb content,
and its Ba content is below detection. However, this sample
carries amphibole, which is relatively rich in Nb and Ba
(NbN * 63 and BaN * 102; Fig. 8). This feature, coupled
with the petrographic evidence of reaction rims at the
contact with the host clinopyroxene and the rough simi-
larity to the clinopyroxene in trace-element distribution,
supports a metasomatic origin for the FB70 amphibole.
The parental melts of the clinopyroxenes have beencompared with (1) the host rocks (Hyblean Upper Mio-
cene lavas; Bianchini et al. 1998; Rocchi et al. 1998;
Perinelli 2000; Scribano et al. 2006) (Fig. 13a), (2) the
liquids in equilibrium with clinopyroxenes from Hyblean
pyroxenites (Nimis and Vannucci 1995) (Fig. 13b), (3)
the liquids metasomatizing the Hyblean peridotites
(Perinelli et al. 2008) (Fig. 13c). In general, the com-
parison shows that the host basalt cannot be regarded as a
possible parental melt for the cumulus phases; the HREE
and Zr–Hf contents in the host basalt are lower than those
of the clinopyroxene parental melt. Closer similarities can
be observed between the calculated liquids and those in
equilibrium with the Hyblean clinopyroxenites, although
the lack of some elements in the data set prevents an
effective comparison. The metasomatic melts in equili-
brium with peridotitic clinopyroxenes are clearly very
different from the parental melts of our gabbros. How-
ever, the pattern of the hawaiitic glass vein cutting the
peridotite HYB40 resembles that of our gabbros, with the
exception of very high Ba and Nb levels (Perinelli et al.
2008), which however may indicate a link with the
metasomatic source of the Fe–Ti-rich melt.
Considering all of these data, we suggest that the melts
that crystallized the cumulus clinopyroxenes show some
similarity with the liquids in equilibrium with Hyblean
clinopyroxenites and the hawaiitic vein cutting the Hyblean
peridotite HYB40. Unfortunately there are no isotopic data
to test the genetic links between these melts.
Geological implications
The geological interpretation of the Hyblean gabbros raises
the questions of the geochemical and isotopic
Fig. 13 Composition of liquids at equilibrium with clinopyroxenes
from analyzed Hyblean gabbros. The fields of a Hyblean Upper
Miocene lavas (Bianchini et al. 1998; Rocchi et al. 1998; Perinelli2000; Scribano et al. 2006), b the theoretical parental liquids of
Hyblean pyroxenites (Nimis and Vannucci 1995), c the theoretical
parental liquids in equilibrium with Hyblean peridotite clinopyrox-
enes and an hawaiitic glass vein cutting the HYB40 peridotite
(Perinelli et al. 2008), are reported for comparison. cpx/liquid D of Hart
and Dunn (1993) have been used for the calculation of all the parental
liquids. Symbols as in Fig. 8
18 Contrib Mineral Petrol (2009) 157:1–22
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characterization of the source(s) within the petrological
context of the Central Mediterranean area and a better
definition of the nature of the Hyblean lithospheric micro-
block.
The Sr–Nd isotopic features of the Hyblean upper
mantle are rather similar to those of lavas from Pantelleria
and Linosa Islands, and partly overlap the data from Etnean
and Hyblean Cenozoic lavas and the clinopyroxenes sep-arated from North African peridotites (Fig. 4a). In the last
decade, many authors have proposed a common mantle
reservoir for the Cenozoic magmatic products in the Cen-
tral Mediterranean area (Granet et al. 1995; Hoernle et al.
1995; Goes et al. 1999; Lustrino and Wilson 2007 and
references therein) whose ‘‘location’’ (lower mantle vs.
Transition Zone) and geochemical-isotopic affinity (e.g.,
HIMU- or FOZO-like) are still debated. However, the
presence of such a common mantle reservoir (CMR in
Fig. 4a; Lustrino and Wilson 2007) is widely accepted, and
the observed isotopic variations are considered to reflect
the contingent geodynamic situations. In this scenario, theHyblean lavas and xenoliths are expected to be among the
least contaminated products, giving the best possibility of
defining the geochemical and isotopic characteristics of
this mantle reservoir (e.g., Sapienza et al. 2005). The
Hyblean clinopyroxene + plagioclase cumulates probably
were derived from this source. These asthenospheric
magmas, in fact, permeate the lithosphere, and probably
were stored at different lithospheric levels, giving rise to
clinopyroxenite veins within the peridotite matrix, to gab-
broic bodies, or even erupting on the surface. By contrast,
the isotopic data show that the Fe–Ti-rich melt that intru-
ded the gabbro pile and formed the intercumulus pockets
cannot be derived from such a source: a MARID-type
source appears to be the best reservoir to fit the geo-
chemical and isotopic features of the metasomatizing
agents. Since the MARID reservoir is considered to reside
within the SCLM, we suggest that the lithosphere, as well
as the asthenosphere, contributed geochemically and iso-
topically to the petrogenesis of the gabbros, and thus to the
Hyblean crustal accretion. Interestingly, the Hf-isotope
variation in the Hyblean gabbros is similar to that found in
hydrothermal, lower crustal zircons from the same locality
(Fig. 5; Sapienza et al. 2007). The U–Pb ages on these
zircons dated this hydrothermal event to Permo-Triassic
time (246 Ma), similar to the Sm–Nd age obtained for nine
gabbro samples (Fig. 6) and probably representing the
gabbro crystallization age; those that lie off this trend
appear to reflect extensive post-cumulus metasomatism
and/or were not cogenetic with the other samples. Such a
convergence suggests that a genetic linkage exists among
these phenomena.
Another key point regards the lithospheric ‘‘location’’ of
the studied gabbros. The mineral assemblage of the
gabbros does not allow P–T estimates. However, the field
for clinopyroxene + plagioclase (±ilmenite) crystalliza-
tion in basaltic systems (Thompson 1972) indicates P–T
conditions of at least *1,100°C and *0.8 GPa, well
above the Hyblean paleogeotherm (Perinelli et al. 2008)
(Fig. 12). The petrographic evidence, namely the lack of
garnet and the presence of hercynitic spinel, suggests that
the gabbro emplacement occurred at T * 1,200°C andP = 0.8–0.9 GPa, corresponding to the crust–mantle
interface. This T is also consistent with that required for
melting for a MARID domain under anhydrous conditions
(Fig. 12). Therefore, we suggest that the gabbros can be
related to magmatic underplating processes. Moreover,
they lie below the presumed depth of the basic granulites of
the lower crust (Punturo et al. 2000) (Fig. 12). These two
xenolith populations are quite different (Scribano et al.
2006; see also Fig. 4): the Fe–Ti-rich gabbros—even the
more granuloblastic—consist of plagioclase + clino-
pyroxene + Fe–Ti oxides (rarely enclosing hercynitic
spinel) ± small amounts of intercumulus material, whereasthe basic granulites are made of plagioclase + clinopy-
roxene + orthopyroxene + green Al-spinel. Furthermore,
remnants of intercumulus pockets and/or anhedral Fe–Ti
oxides have never been reported in the basic granulites.
Another salient question regards the nature of the
Hyblean lithosphere. The suggestion of a MARID-type
reservoir in the Hyblean SCLM strengthens the evidence
for the continental nature of this micro-block. The geo-
chemical and isotopic data clearly indicate that a
significant thermal/magmatic episode in Permo-Triassic
time affected the old Hyblean continental lithosphere
(Sapienza et al. 2007), causing a significant perturbation of
the paleogeotherm. Such an event was related to litho-
sphere thinning, which caused upwelling and melting of the
asthenosphere and produced the gabbro pile; the same
thermal event may have also caused the melting of ancient
MARID-type domains in the progressively thinned SCLM.
The fluids produced by early breakdown of such a domain
may have caused the cryptic metasomatism in the cumulitic
gabbros as well as the lower crustal hydrothermal event
described by Sapienza et al. (2007). The above inferences
are compatible with rifting preceding an ocean opening,
and the age of this thermal event is that often invoked for
the opening of the Ionian Basin. However, more investi-
gations are needed to better define this scenario.
Concluding remarks
The geochemical and isotopic study of selected gabbros
from the Hyblean Plateau helps to unravel the geochemical
and isotopic evolution of the underlying lithosphere. The
main conclusions are summarized below:
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(1) The Hyblean gabbros consist of clinopyroxene + pla-
gioclase ± Fe–Ti oxides precipitated from alkali-
basaltic melts rising from the asthenosphere—ascrib-
able to the CMR of Lustrino and Wilson (2007)—and
emplaced near the crust–mantle interface. The result-
ing cumulitic pile represents episodes of magmatic
underplating. These gabbroic bodies subsequently
underwent metasomatism and more or less pro-nounced re-equilibration.
(2) The combination of geochemical, isotopic and textural
evidence suggests the contribution of two distinct, but
possibly related, metasomatic agents to the cumulate
pile: (1) *50% mixing with a high-87Sr/ 86Sr fluid
carrying relatively small amounts of Sr and Rb, and
(2) infiltration of solute-rich Fe–Ti-rich melts supply-
ing Fe, Ti, Rb, Sr and HFSE to the system. Both
metasomatic events may be related to the progressive
partial melting of a heterogeneous MARID-type
reservoir within the SCLM. Thus the petrogenesis of
the Hyblean gabbros could have involved bothastenospheric and lithospheric reservoirs.
(3) Gabbro emplacement occurred in Permo-Triassic
time, at the crust–mantle interface. At that time a
significant thermal episode related to lithospheric
thinning and asthenospheric upwelling caused the
perturbation of the paleogeotherm. The heating may
have caused melting of previously metasomatised
lithospheric domains, producing fluids and melts that
percolated and enriched the nearby lithosphere. As a
whole, this scenario is consistent with a rifting
preceding the (Ionian?) ocean opening.
Acknowledgments We are grateful to Vittorio Scribano for pro-
viding samples. We thank Norman Pearson for his contributions to the
analytical work and discussions of the results, and Suzy Elhlou, Peter
Wieland and Carol Lawson for their invaluable assistance and guid-
ance in the laboratory. Roberto Braga is thanked for the continuous
and precious advices during preparation of the manuscript. Andrea
Argnani is thanked for interesting discussions on geodynamic situa-
tion of the Central Mediterranean area. Alessandro Rottura and Aldo
Del Moro are thanked for providing unpublished isotopic data of
lavas from Linosa Island. Constructive criticism by Massimo Coltorti
and Michel Gregoire significantly improved the paper. Funding for
this research was provided by a Marco Polo grant from the Universita
di Bologna (GTS), MIUR 60% (LM) and an ARC Discovery Project
(SYOR/WLG). Analytical data were obtained at GEMOC usinginstrumentation funded by ARC LIEF, and DEST Systemic Infra-
structure Grants and Macquarie University. This is contribution 538
from the ARC National Key Centre for Geochemical Evolution and
Metallogeny of Continents (www.es.mq.edu.au/GEMOC).
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