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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



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

123



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

123



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


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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)


123



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 )


123



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


123



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)


123



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)


123



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)


123



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)


123



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


123



(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)


123



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


123



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


123



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)


123



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


123



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:


123



(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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