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Lithos 84 (2005

Mafic and ultramafic enclaves in Ustica Island lavas: Inferences on

composition of lower crust and deep magmatic processes

M. Allettia,T, M. Pompiliob, S.G. Rotoloa

aDipartimento di Chimica e Fisica della Terra (CFTA). Universita di Palermo, Via Archirafi 36-90123 Palermo, ItalybIstituto Nazionale di Geofisica e Vulcanologia-Sezione Sismologia e Tettonofisica, Centro per la Modellistica Fisica e Pericolosita dei Processi

Vulcanici. Via della Faggiola, 32 56126 Pisa, Italy

Received 30 August 2004; accepted 23 March 2005

Available online 19 May 2005

Abstract

Ustica Island, southern Tyrrhenian Sea, is constituted of Quaternary alkaline volcanics. Avariety of enclaves representative of

deep to supra-crustal settings were recently found in a hawaiitic lava flow. Enclaves consist of: (i) Ultramafic meta-cumulates, i.e.

clinopyroxenites and wherlites characterized by variably deformed porphyroclastic to granoblastic textures. (ii) Mafic cumulates,

i.e. gabbros (F amphibole) and troctolites, the first often characterized by frequent amphibole breakdown coronas (olivine+Ti-

augite+plagioclase+magnetite+ ilmenite+ rhfnite) in response to an H2O decrease during the ascent, while the troctolites

interpreted as meta-cumulates. (iii) Microsyenites, consist of anorthoclase and Fe-clinopyroxene organized in a granular sub-

ipidiomorphic texture. Amphibole is absent in Ustica lavas and is found only in some old, now exposed, sub-intrusive volcanic

bodies. This evidence suggests a late appearance of amphibole on the liquidus, at a high crystal content that inhibits further ascent

of the magma. The importance of the amphibole as a medium pressure liquidus phase in Ustica mafic magmas is in the bearings on

the geochemistry of lavas e.g. in buffering Na and Ti abundances, in trace elements partitioning, etc.

Density measurements pointed out higher values for clinopyroxenites (3160 to 3300 kg/m3) than for gabbros (ca. 2900 kg/

m3). Given the density contrast between enclaves and host lavas (2790 kg/m3) and assuming appropriate rheological models, we

calculated a minimum ascent rate of 0.01 m/s, corresponding to an ascent time in the range of 5–29 days for a depth of

entrapment of 25 km.

D 2005 Elsevier B.V. All rights reserved.

Keywords: Ustica; Lower crust; Enclaves; Amphibole; Rhfnite; Fassaite; Ascent rate

0024-4937/$ - see front matter D 2005 Elsevier B.V. All rights reserved.

doi:10.1016/j.lithos.2005.03.015

T Corresponding author. Fax: +39 91 6168376.

E-mail address: marina.alletti@tin.it (M. Alletti).

1. Introduction

The study of enclaves in lavas (cognate holocrys-

talline, liquid-bearing crystal mushes, xenoliths)

provides important opportunities to characterize the

magmatic system beneath volcanoes, such as the

) 151–167

M. Alletti et al. / Lithos 84 (2005) 151–167152

source zone of magmatism, the vertical extent of the

plumbing system, the dynamics of magmas within

magmatic reservoirs and the relationships with host

rocks, as well as allowing us to characterize the pre-

eruptive physical–chemical parameters.

The discovery of several kinds of enclaves at

Ustica (Lanzafame and Pompilio, 1997) gives us the

possibility to study the root region of the volcano.

Located in the southern Tyrrhenian Sea (60 km

north of the Sicilian coast), the island of Ustica is

almost entirely composed of volcanic rocks, showing

a distinctive Na-rich alkaline affinity. This alkaline

volcanism, Quaternary in age, is in contrast to the

otherwise widespread calcalkaline volcanism in the

southern Tyrrhenian Sea, which is either coeval

(Aeolian archipelago) or older (e.g. the sea-mount

Anchise, a few km west of Ustica, whose age is 3.6–

5.0 Ma, Savelli, 1988). While subaerial and submarine

volcanism at Ustica is rather well studied petrologi-

cally and geochemically (Romano and Sturiale, 1971;

Cinque et al., 1988; Bellia et al., 2000; Schiano et al.,

2004), the enclaves have not been studied until now.

In this paper we focus on the mineralogy and

petrology of the enclaves recovered from the hawaiitic

Fig. 1. Location of Ustica Island in the southern Tyrrhenian Sea (modified

and older metamorphites and granitoids); (2) Kabylo-Calabrian allochthon

cover); (3) Apenninic-Maghrebian Meso-Cenozoic sedimentary units; (4) H

tectonics and thinned crust (oceanic crust in the Tyrrhenian and Algeria

volcanic areas. Principal thrust fronts are also reported.

lava flow of Contrada Spalmatore area. Some

enclaves show clear deformation patterns while others

are undeformed. Using paragenetic and textural

criteria, enclaves were grouped into three types: (i)

ultramafic meta-cumulates, (ii) mafic cumulates and

(iii) microsyenites.

We focused on amphibole breakdown reactions and

other sub-solidus reactions recorded in high-grade

metamorphic enclaves, as well as on reactions involv-

ing a melt phase. These particular aspects were

developed in order to infer the depth of provenance,

the paths to the surface (including ascent velocities),

and ultimately to improve the knowledge of the nature

of the lower crust beneath Ustica Island.

2. Geological setting

The island of Ustica (Fig. 1) rises up from the

Tyrrhenian seafloor in the transitional zone between a

northern domain characterized by a thinned oceanic

crust (about 8 km thick) and a western–southern

domain characterized by a variably thick continental

crust, ranging from 25 km beneath the Aeolian

from Catalano et al., 1998). (1) Sardinian basement units (Variscan

ous units (Variscan and older metamorphites and their sedimentary

yblean sedimentary foreland; (5) areas characterized by extensional

n basins, continental crust in the Sicily Channel); (6) Quaternary

Fig. 3. Microscope photo of a clinopyroxenitic enclave (ultramafic

meta-cumulates group). Cumulus minerals show triple joints and are

always in direct contact among themselves without any interstitial

material. Sample PUS 30d, crossed polars; scale bar is 100 Am.

M. Alletti et al. / Lithos 84 (2005) 151–167 153

archipelago to 30 km beneath the study area (Sulli,

2000). Southwards, beneath the Apenninian-Maghre-

bian chain, the inferred thickness is as high as 40 km

(Savelli, 1988 and references therein cited; Pepe et al.,

2000).

The subaerial volcanics represent a small portion

(around 6 km2) of a much wider volcanic edifice (about

100 km2 of aerial extent at a depth of 2000 m b.s.l.).

The main tectonic lineaments of the island (N–S,

NE–SW) are coincident with those of the main

regional system and consist of (i) fault systems

characterized by the superposition of extensional to

compressional regimes (Bousquet and Lanzafame,

1992), and (ii) dikes showing N–S, N608E and E–W

orientation and particularly well exposed in the

western side of the island.

Fig. 2. (a) Geological sketchmap of Ustica Island (modified fromCinque et al., 1988; DeVita et al., 1998with unpublished data of the authors); (b)

stratigraphic relationships in the study area. (1) Principal marine terraces (200–350 ka); (2) Capo Falconiera tuff (130 ka); (3) Casa Zacame basaltic

lavas (250 ka); (4) Tramontana and Cala del Camposanto mugearitic lavas (250–400 ka); (5) M. Costa del Fallo hawaiitic lavas (300–500 ka); (6)

Villaggio subaerial hawaiitic lavas (300 ka); (7) Case Alaimo tuff (N 300 ka); (8) Cala Sidoti hawaiitic hyaloclastites and pillow lavas (420 ka); (9)

M. Costa del Fallo tuff (500 ka); (10) Torre S. Maria and Punta Falconiera submarine basaltic lavas (550–735 ka); (11) dikes; (12) faults.

Fig. 4. SEM photo (BSE) of the same sample in Fig. 3, at high

magnification. Textural patterns suggest the spinel forming reaction:

Ca-plagioclase+olivine=hercynite/spinel+clinopyroxene, which is

typical of granulite facies conditions. Sample PUS 30d.

M. Alletti et al. / Lithos 84 (2005) 151–167154

As regards the volcanological evolution, a rough

age estimate of less than 1 Ma was suggested by early

workers (Barberi et al., 1969). More recently, a

detailed geochronological reevaluation of the stratig-

raphy of the island suggests five periods of activity in

the range 735 to 130 ka (De Vita et al., 1998). From

the volcanological point of view, the island is

characterized by the presence of: (i) two main

volcanic edifices, Mt. Guardia dei Turchi and Mt.

Costa del Fallo (ca. 500 ka in age) that produced lava

flows and subordinate pyroclastic fall deposits; (ii) a

tuff cone at Capo Falconiera (130 ka), and (iii) a

number of small submarine/subaerial eruptive centres

(e.g. Case Zacame, Contrada Spalmatore, and Secca

Fig. 5. Pyroxene classification diagram. (a) Clinopyroxenites and wherl

circles—clinopyroxenite; black diamonds—wherlite; black crosses—amph

squares—troctolite; open diamond—microsyenite. Pyroxenes with woll

clinopyroxenites (low-Ti fassaites) and in amphibole breakdown coron

precipitated from a liquid, while the latter are clearly sub-solidus.

Colombara) that produced submarine eruptions

mostly consisting of hyaloclastites and pillow lavas

(Fig. 2), in the time interval 250–300 ka (Romano and

Sturiale, 1971; Cinque et al., 1988).

Erupted lavas range in composition from hawaiites

to mugearites; more acidic products consist of small

volume (areab1 km2) trachytic pumice fall deposits

found at Costa del Fallo, Guardia dei Turchi and a

benmoreitic, amphibole-bearing, sub-intrusive body

exposed at Punta San Paolo (Bellia et al., 2000). More

recently, a trachytic lava body was also found at the top

of the Colombara submarine reef, ca. 2 km NNW of

Punta Gorgo Salato (Bellia et al., 2000). The obvious

consideration is that fractional crystallization played a

role in the generation of the evolved liquids, but their

ascent to the surface could have been inhibited by the

variable tectonic regime which characterized the evo-

lution of the island (Bousquet and Lanzafame, 1992).

As to the source region of magmatism, trace

elements patterns and Sr isotopic ratios (the latter

ranging from 0.70306F1 to 0.70342F1), suggest a

metasomatized and layered upper mantle source, with

amphibole playing a major role in controlling the trace

elements patterns (Cinque et al., 1988). Schiano et al.

(2004), studying trace elements patterns in olivine-

hosted melt inclusions from lavas of the Spalmatore

area, proposed a mantle source characterized by HIMU

isotopic and elemental signatures. This mantle signa-

ture is shared also with the eastern Sicily volcanism

(Etna, Iblean Mts), leading Schiano et al. (2004) to

hypothesize a large mantle plume beneath Sicily.

ites; (b) gabbros; (c) troctolites and microsyenites. Symbols: open

ibole-free gabbro; open triangle—amphibole-bearing gabbro; open

astonite end member N 50% (i.e. fassaites) are common both in

as of gabbros (high-Ti fassaites). The former are considered as

M. Alletti et al. / Lithos 84 (2005) 151–167 155

Enclaves considered in this study are mainly

hosted in lava flows and subordinately in hyaloclas-

tites and dikes, which outcrop along the SW coast of

the island, between Cala Sidoti and Punta Cavazzi.

The stratigraphy of this area is characterized from

bottom to top (Fig. 2b) by: subaerial tuffs

(thicknessb2 m) emitted from the Mt. Costa del

Fallo centre, which are followed by hyaloclastites

and pillow lavas (thicknessb10 m) and by the

enclave-rich, columnar bVillaggio subaerial lavasQ(Cinque et al., 1988), up to 10 m thick. Several

feeding dykes of the bVillaggio subaerial lavasQ

Table 1

Representative chemical analyses of clinopyroxene crystals

Sample Clinopyroxenites

30d/7,

interst

30d/8,

micro

30d/25,

core

30d/27,

core

30p/16,

interst

30p/24,

interst

30p

inte

SiO2 46.57 49.68 45.75 46.77 46.75 43.27 43

TiO2 1.12 1.73 0.97 0.89 0.08 1.36 1

Al2O3 9.11 2.78 9.86 9.98 8.64 13.38 11

FeO 3.77 10.92 4.54 3.86 5.27 7.59 7

MnO 0.27 0.17 0.17 0.14 0.16 0.06 0

MgO 13.64 13.79 13.02 13.46 12.26 10.47 11

CaO 25.36 20.27 25.25 24.95 25.66 24.89 23

Na2O 0.14 0.61 0.39 0.03 0.18 0.46 0

K2O 0.02 0.05 0.05 b.d.l. 0.02 0.04 b.d.

Total 100.00 100.00 100.00 100.08 99.02 101.52 100

Wo 54 42 54 53 55 55 53

En 40 40 39 40 36 32 34

Fs 6 18 8 6 9 13 13

Mg/Mg+Fetot 0.87 0.69 0.84 0.86 0.81 0.71 0

Sample Amph-free gabbros Amph-bearing gabbro

30h/1,

rim

30h/7,

core

30h/20,

interst

23e/3,

core

30a/14,

core

31b/5,

corona

31b

coro

SiO2 46.57 48.82 44.33 49.13 47.29 44.53 40

TiO2 2.42 1.55 2.80 1.29 2.61 5.01 6

Al2O3 6.81 5.14 8.47 5.84 5.92 7.10 10

FeO 8.00 7.30 10.21 7.24 7.87 8.23 7

MnO 0.09 0.14 0.21 0.16 0.15 0.14 0

MgO 12.76 13.68 11.05 13.73 13.51 12.54 10

CaO 22.76 22.62 22.19 23.00 21.98 21.86 22

Na2O 0.62 0.76 0.75 0.52 0.58 0.54 0

K2O b.d.l. b.d.l. b.d.l. b.d.l. 0.02 0.06 0

Total 100.03 100.01 100.01 100.91 99.93 100.01 100

Wo 49 48 49 48 47 48 52

En 38 40 34 40 40 38 34

Fs 13 12 18 12 13 14 14

Mg/Mg+Fetot 0.74 0.77 0.66 0.77 0.75 0.73 0

Interst=anhedral interstitial clinopyroxene occurring among euhedral

breakdown coronas; b.d.l.=below detection limit.

(thickness from 0.3 to 1 m) are now exposed in

the area (Fig. 2a). This sequence is unconformably

covered by the younger Case Zacame lavas (Cinque

et al., 1988), which are totally devoid of enclaves.

3. Analytical techniques

Whole rock chemical analyses were performed by

XRF on pressed powder pellets using a Philips PW

1400 spectrometer and following analytical proce-

dures of Franzini et al. (1972).

Wherlite Troctolites

/25,

rst

30p/26,

interst

31a/4,

interst

31a/14,

interst

30g/10,

core

30g/23,

core

8c/14,

core

8c/15,

rim

.24 43.79 37.84 48.83 46.62 50.30 44.43 47.64

.84 1.14 6.07 1.10 3.93 1.76 3.52 1.78

.92 11.69 13.94 5.48 4.19 2.47 8.47 6.73

.57 7.94 7.84 7.84 12.35 9.63 10.06 8.09

.10 b.d.l. b.d.l. 0.17 0.20 0.23 0.34 b.d.l.

.16 10.58 10.22 13.72 10.14 13.68 10.93 13.26

.73 24.55 23.44 22.23 21.51 21.31 21.39 21.82

.46 0.35 0.72 0.56 0.97 0.64 0.76 0.59

l. 0.07 b.d.l. 0.09 0.08 b.d.l. 0.03 0.06

.02 100.11 100.07 100.02 99.99 100.02 99.93 99.97

54 54 47 48 45 48 47

32 32 40 31 40 34 40

14 14 13 21 16 18 14

.72 0.70 0.76 0.70 0.59 0.72 0.66 0.75

s Microsyenite

/7,

na

31b/13,

corona

30i/3,

corona

30i/21,

corona

30c/1,

core

30c/2,

rim

30c/12,

core

30c/13,

rim

.99 40.22 38.21 44.69 50.47 51.13 49.20 51.08

.52 6.59 6.71 5.05 0.2 0.10 0.11 0.36

.79 11.44 13.61 6.59 0.72 0.60 1.00 0.92

.62 7.87 9.32 10.01 16.06 14.05 20.15 13.18

.17 0.21 0.03 0.26 0.96 0.69 0.92 0.43

.52 10.36 9.20 11.58 7.71 9.64 5.49 10.17

.44 22.53 22.14 21.10 22.77 22.84 21.43 22.75

.85 0.76 0.78 0.75 1.16 1.03 1.66 1.13

.10 0.01 b.d.l. b.d.l. b.d.l. b.d.l. 0.04 b.d.l.

.00 99.99 100.00 100.03 100.05 100.08 100.00 100.02

52 53 47 49 48 48 48

33 30 36 23 28 17 30

14 17 17 27 23 35 22

.71 0.70 0.64 0.67 0.46 0.55 0.33 0.58

grains; corona=small sized crystals occurring in the amphibole

M. Alletti et al. / Lithos 84 (2005) 151–167156

Mineral analyses have been performed using a

LEOk 440 scanning electron microscope coupled to

anOxford-Link EDS. Operating conditions were 20 kV

accelerating voltage and 600 pA beam current. Quan-

titative analyses were obtained using ZAF correction

procedures, with natural standards for calibration.

Density measurements were carried out using a

Berman balance. The weight of several nodule chips

(1–4 cm in diameter) were measured in air and in

water at 20 8C and the measured densities of each chip

were then averaged.

Modal analyses have been carried out on thin

sections with a point counter (c1000 points counted

for each sample) along square grids with length

related to the grain size ranging from 0.16 to 0.48 mm.

4. Host lavas

The enclave-rich bVillaggio lavasQ are porphyritic, palegray rocks ranging in composition from basalt to hawaiite

(phenocryst content=23 vol.%; SiO2=49.0 wt.%,

Na2O=3.9 wt.%, MgO=5.3 wt.%; ne norm=0 to 1, hy

Table 2

Representative chemical analyses of olivine crystals

Sample Clinopyroxenites Wherlite

30d/3,

interst

30d/33,

interst

8b/2,

interst

31a/1,

rim

31a/2,

core

3

co

SiO2 39.88 38.80 38.88 38.25 38.01 3

FeO 10.73 15.97 20.81 21.36 20.97 1

MnO 0.79 0.61 b.d.l. 0.49 0.53

MgO 47.19 43.09 40.14 38.92 39.53 4

CaO 0.41 0.52 0.17 0.21 0.13

Total 99.00 98.99 100.00 99.23 99.17 9

Fo 88 83 77 76 77 7

Sample Troctolites Amph-free gabbros

28a/18,

interst

8c/2,

core

8c/6,

rim

30h/4,

interst

30h/9,

core

3

ri

SiO2 39.18 39.05 38.94 38.34 38.69 3

FeO 16.59 17.32 18.02 21.27 19.95 2

MnO 0.23 0.33 0.23 0.28 0.19

MgO 43.17 42.25 41.54 38.91 40.51 3

CaO 0.30 0.21 0.31 0.54 0.31

Total 99.47 99.16 99.04 99.34 99.65 9

Fo 82 81 80 77 78 7

Interst=anhedral interstitial olivine occurring among euhedral grains; b.d.

norm=3 to 0). Plagioclase is the most abundant

phenocryst (18.1 vol.%), followed by olivine (4.2

vol.%; compositionally Fo76–82), Ti-augite (0.7 vol.%;

Wo47, En37, Fs16; Mg#=0.79) and minor Ti-magnetite.

Phenocrysts are set in a pilotaxitic to locally glassy

groundmass. Microlites are dominated by plagioclase and

followed by olivine, clinopyroxene and Fe–Ti oxides.

5. Petrography and mineral chemistry of the

enclaves

We recovered a total of 43 enclaves in the SW

portion of the island, Punta Spalmatore area. Their

shape is generally sub-angular and they range in size

from 2 to 10 cm; they are variably colored from dark

green to grayish/black. On the basis of their textural

and mineralogical features, enclaves were subdivided

in the following groups:

Ultramafic meta-cumulates: represented by fine- to

medium-grained (0.05–1 mm) clinopyroxenites

(cpx+sp-hercFplg) and wherlites (ol+cpxFplg),

Troctolites

1a/5,

re

31a/15,

core

30g/7,

core

30g/15,

rim

30g/16,

core

30g/22,

interst

8.27 38.11 37.90 36.75 37.88 38.35

9.45 22.22 23.58 28.69 22.61 20.33

0.52 0.37 0.35 0.29 0.33 0.25

0.95 38.55 37.29 33.12 38.58 40.29

0.16 0.16 0.24 0.33 0.14 0.21

9.35 99.41 99.36 99.18 99.54 99.43

9 76 74 67 75 78

Amph-bearing gabbros

0h/17,

m

30h/18,

core

30i/1,

rim

30i/2,

core

30a/8,

core

30a/9,

rim

8.22 37.36 38.57 38.43 38.65 38.6

1.43 26.57 18.79 18.95 20.16 19.18

0.31 0.54 0.29 0.35 0.33 0.22

9.22 34.90 41.50 41.42 40.23 41.38

0.4 0.15 0.33 0.28 0.19 0.23

9.58 99.52 99.48 99.43 99.56 99.61

7 70 80 80 78 79

l.=below detection limit.

M. Alletti et al. / Lithos 84 (2005) 151–167 157

displaying evidence of deformation such as flaser

textures with strong grain size reduction in the more

strained areas, granoblastic textures with triple

jointing and deformation lamellae in olivines. These

cumulates represent the 16% (clinopyroxenites 14%

and wherlites 2%) of total number of enclaves.

Mafic cumulates: consisting of medium- to coarse-

grained (0.5–2.0 mm) gabbros (plg+cpx+olFamph) with clearly magmatic textures virtually

undeformed and troctolites (plg+ol+cpxFglass)

characterized by granoblastic triple junctions. These

cumulates are the most abundant class among the

whole nodule set (77% of the total number of

enclaves) and are composed of: 26% gabbros, 35%

amphibole gabbros, 16% troctolites.

Microsyenites: composed of comagmatic cumulates

lacking any trace of deformation. They are charac-

terized by the presence of fine- to coarse-grained

(0.3–3 mm) anorthoclase and an Fe-rich clinopyrox-

ene. These cumulates sum up to 7% of the whole set.

5.1. Ultramafic meta-cumulates: clinopyroxenites

In the meta-cumulate group, clinopyroxenites are

by far the most abundant type of enclaves. Interaction

Fig. 6. Plagioclase compositional diagram. (a) Clinopyroxenites and whe

Fig. 5. Note the presence of anorthoclase which occur in microsyenitic

with the host lava is generally weak and affects only

the enclave rims. In fact, the contact with the host lava

is sharp, and no relevant textural changes have been

detected. Glassy rims or veins are rare.

Even at the scale of the thin section, these rocks

show rather variable textures, ranging from proto-

granular to porphyroclastic to granoblastic (Mercier

and Nicolas, 1975). The protogranular types are

characterized by a very heterogeneous grain size (50

Am to 1 mm), with clinopyroxene porphyroblasts

showing irregular edges, being either curvilinear or

angular. Clinopyroxene shows frequent wavy extinc-

tion, sieve texture and exsolution lamellae of spinel.

The porphyroclastic types are characterized by

strain-induced grain size reduction: poorly faceted

to lobate cpx porphyroclasts (up to 6 mm across)

are set in a cpx granoblastic matrix of much smaller

size (50 to 70 Am) typically showing triple

junctions (Fig. 3).

Dark green spinels are very common, though their

abundance in nodules is not homogeneous and seems to

be closely linked to the texture typology. In proto-

granular and porphyroclastic portions of the nodules,

euhedral spinel (50 to 250 Am in size) is present mostly

between cpx grain boundaries, with a broad tendency to

rlites; (b) gabbros; (c) troctolites and microsyenites. Symbols as in

enclaves and in a troctolitic crystal mush.

M. Alletti et al. / Lithos 84 (2005) 151–167158

form poorly defined layers within the nodule. The

presence of plagioclase relics (detectable only at high

magnification) in intergranular positions associated

with cpx and spinel, may suggest a spinel forming

reaction such as: Ca-plg+ol=herc-sp+cpx (Fig. 4). At

high magnification spinel frequently shows lobate

shapes (holly-leaf texture, Mercier and Nicolas,

1975). In the fine-grained granoblastic portions of the

nodule spinel is totally lacking. Tiny apatite crystals are

often included in spinels.

Clinopyroxene (96.3 vol.%) displays an anoma-

lously high wollastonite content (i.e. molar [100*Ca /

(Ca+Mg+Fe+Mn)]; Fig. 5, Table 1) in the range 53–

55% and high values of Al2O3 (8.6–13.4 wt.%). These

high-Ca pyroxenes are known in the literature as

fassaites and are rather rare in volcanic rocks, being

present only in few basic lavas from strongly alkaline

districts (Huckenholtz, 1973; Wandij et al., 2000).

Most of them are also characterized by a relatively high

Table 3

Representative chemical analyses of plagiocase crystals

Sample Clinopyroxenites Wherlite

30d/4,

interst

30d/19,

interst

30p/21,

interst

30d/31,

interst

30d/32,

interst

31a/3,

interst

31a/1

inters

SiO2 44.20 43.30 45.12 43.99 44.19 46.60 45.37

Al2O3 35.38 35.64 34.81 35.20 34.84 33.77 34.70

Fe2O3 0.77 0.89 0.73 0.85 0.76 0.79 0.50

CaO 18.20 19.05 18.14 18.35 18.52 16.18 17.88

Na2O 1.19 0.80 1.37 1.10 1.15 2.29 1.52

K2O 0.10 0.01 0.07 0.10 0.14 0.14 b.d.l.

Total 99.84 99.69 100.20 99.59 99.60 99.77 99.97

An 89 93 88 90 89 79 87

Ab 11 7 12 10 10 20 13

Or 0 0 0 1 1 1 0

Sample Troctolites Amph-free gabbros Amp

28a/10,

core

28a/16,

interst

23e/11,

core

30h/3,

core

30h/19,

interst

30h/21,

core

30i/5

coron

SiO2 48.76 52.43 45.86 50.52 51.58 50.01 50.19

Al2O3 31.52 29.60 34.57 31.27 30.26 31.52 31.05

Fe2O3 0.47 0.98 0.29 0.52 0.75 0.54 0.79

CaO 14.02 12.53 17.41 13.58 12.58 13.84 13.49

Na2O 3.37 4.30 1.73 3.97 4.45 3.80 3.93

K2O 0.16 0.25 b.d.l. 0.06 0.16 0.14 0.24

Total 98.30 100.10 99.86 99.92 99.78 99.85 99.69

An 69 61 85 65 60 66 65

Ab 30 38 15 35 39 33 34

Or 1 1 0 0 1 1 1

Interst=plagioclase occurring as interstitial phase; corona=small sized cry

detection limit.

Mg# (molar Mg/Mg+Fetot), clustering in the range

0.81–0.87 and low TiO2 (0.1–1.8 wt.%). Augite (Wo42,

En40, Fs18) is very rare and shows a distinctly lower

Mg# (0.69). Olivine (1.6 vol.%; Table 2) is rather rare

and is characterized by a mean Fo=83%, low CaO

(0.2–0.5 wt.%), and MnO up to 0.8 wt.%. Plagioclase

(0.5 vol.%), associated with spinel as an intercumulus

phase, shows a notably Ca-rich composition (An88–93)

(Fig 6, Table 3). Spinel (1.6 vol.%) is characterized by

Mg-rich compositions (herc10–23, sp82–62). Cr-rich

types, even if rare, have a Cr2O3 concentration up to

25.3 wt.%. SEM images show distinctly the presence of

Al-rich cores (Al2O3 up to 63.3 wt.%) (Fig. 4, Table 4).

In one sample (PUS 30p) a large pool (3 mm across) of

colorless high-silica peraluminous glass (SiO2=up to

80.0 wt.%; A/CNKmolar ratio=1.1–1.4) coexists with

a euhedral green-spinel. On purely textural basis, glass

and spinel are likely to have formed at the expense of

plagioclase and clinopyroxene.

Troctolites

1,

t

31a/12,

interst

30g/5,

core

30g/18,

core

30g/30,

core

28a/1,

core

28a/6,

interst

28a/9,

rim

48.05 46.38 50.92 65.35 46.73 52.00 46.84

32.81 33.98 30.89 20.21 32.97 28.15 33.31

0.61 0.81 0.70 0.59 0.47 1.19 0.48

15.23 16.62 13.27 0.97 15.73 11.90 16.23

3.01 2.08 3.89 7.35 2.45 4.56 2.18

0.11 0.06 0.18 5.52 0.02 0.40 0.07

99.82 99.93 99.85 99.99 98.35 98.20 99.11

73 81 65 5 78 58 80

26 19 34 64 22 40 19

1 0 1 31 0 2 0

h-bearing gabbros Microsyenite

,

a

30i/7,

corona

30a/7,

corona

30a/25,

corona

30a/30,

core

30c/3,

core

30c/5,

core

30c/24,

core

53.85 51.70 52.19 48.43 65.52 63.79 65.81

28.71 30.39 29.90 32.51 19.23 20.99 20.61

0.81 0.84 0.57 0.57 1.13 0.81 0.08

10.50 12.50 11.98 15.36 0.54 2.00 0.88

5.53 4.46 4.63 2.83 7.81 8.43 8.53

0.34 0.19 0.39 0.13 5.45 2.85 3.82

99.74 100.08 99.66 99.83 99.68 98.87 99.73

50 60 58 74 3 10 4

48 39 40 25 67 74 74

2 1 2 1 31 16 22

stals occurring in the amphibole breakdown coronas; b.d.l.=below

Table 4

Representative chemical analyses of spinel crystals

Sample Clinopyroxenites Wherlite Troctolites

30d/5,

core

30d/9,

core

30d/30,

core

30p/12,

core

30p/22,

core

31a/13,

mph

30g/14,

interst

30g/19,

interst

30g/24,

interst

SiO2 0.48 0.19 0.25 0.07 0.02 0.35 0.39 0.38 0.35

TiO2 13.10 1.60 0.10 0.10 0.20 11.38 2.40 1.80 21.40

Al2O3 3.95 25.10 63.29 56.62 60.14 9.05 34.38 43.63 1.53

FeO 69.85 27.92 12.26 17.12 16.02 67.68 40.02 29.77 63.15

MnO 0.71 0.29 0.76 0.40 0.40 0.33 0.17 0.32 0.59

MgO 2.51 10.46 22.82 21.41 22.45 4.27 10.01 13.71 1.84

Cr2O3 b.d.l. 25.30 b.d.l. b.d.l. b.d.l. b.d.l. b.d.l. b.d.l. b.d.l.

Total 90.56 90.85 99.47 95.76 99.25 93.06 87.39 89.56 88.84

Usp 38.1 4.0 0.2 0.3 0.4 31.1 6.0 4.0 65.2

Mg/Mg+Fetot 0.06 0.40 0.77 0.69 0.71 0.10 0.31 0.45 0.05

Sample Amph-free Amph-bearing Microsyenite

30h/5,

interst

30h/22,

interst

23e/4,

interst

30a/21,

interst

30a/29,

interst

30i/14,

interst

30c/4,

interst

30c/17,

interst

30c/21,

interst

SiO2 0.33 0.29 0.47 0.32 0.29 0.32 0.59 0.66 0.81

TiO2 14.40 11.40 13.10 20.30 20.70 15.60 24.15 5.70 20.26

Al2O3 6.29 6.51 6.23 1.00 1.65 8.34 0.08 1.10 0.36

FeO 67.55 68.24 71.43 63.83 62.55 59.73 64.71 89.19 76.24

MnO 0.36 0.27 0.34 0.92 0.76 0.33 1.95 0.73 1.14

MgO 3.73 4.99 4.57 2.74 2.34 5.53 0.06 1.29 0.27

Cr2O3 b.d.l. b.d.l. b.d.l. b.d.l. b.d.l. b.d.l. b.d.l. b.d.l. b.d.l.

Total 92.62 91.68 96.17 89.13 88.29 89.89 91.54 98.67 99.08

Usp 40.20 31.7 35.2 61.4 63.1 44.1 73.5 15.5 56.6

Mg/Mg+Fetot 0.09 0.12 0.10 0.07 0.06 0.14 0.00 0.03 0.01

Interst=anhedral interstitial spinel occurring among euhedral grains; b.d.l.=below detection limit.

M. Alletti et al. / Lithos 84 (2005) 151–167 159

5.2. Ultramafic meta-cumulates: wherlite

The single sample collected within this group shows

a protogranular texture mostly made by olivine (82

vol.%), clinopyroxene (13 vol.%) and minor plagio-

clase (2 vol.%). Olivine and cpx show generally

curvilinear grain boundaries. Locally, where recrystal-

lization is more developed, they show angular grain

boundaries giving rise to truly granoblastic textures.

The olivine size is about 200 Am with mostly equant

grains. More rarely, olivine shows tabular habit with

deformation lamellae. Rims are often affected by

iddingsitic alteration. Intercumulus plagioclase (parti-

ally altered to sericite) and apatite are also present.

Clinopyroxene, which is rare in this group, is salitic

to fassaitic in composition (Wo47–54, En40–32, Fs13–14)

and is characterized by an Mg#=0.70–0.76 and

TiO2=1.1–6.1 wt.%. Olivine covers a very narrow

compositional range (Fo75–79) and is characterized by

low CaO (0.1 to 0.3 wt.%) and MnO=0.5 wt.%.

Intercumulus plagioclase has a compositional range

An73–87. Spinel is Ti magnetite (Usp31).

5.3. Mafic cumulates: gabbros

Among the gabbroic nodules, we distinguished two

subgroups on the basis of the presence or absence of

amphibole. Both groups do not show any deformation

or metamorphic recrystallization.

(a) Amphibole-free gabbros are medium- to coarse-

grained with a granular allotriomorphic texture

made of clinopyroxene (33 vol.%) and plagio-

clase (63 vol.%), both in size range of 0.5–5

mm, plus fine-grained olivine (3 vol.%; 0.25–2

mm) and sparse spinel.

Clinopyroxene displays great compositional

homogeneity, clustering in the salite field

M. Alletti et al. / Lithos 84 (2005) 151–167160

(Wo46–49, En34–42, Fs12–18) with relatively stable

Mg# (mostly in the range 0.74–0.77) and rather

high TiO2 (1.3–2.8 wt.%). Olivine is quite

homogeneous in composition (Fo70–78) showing

just a small reverse core-rim zoning and variable

CaO contents (0.1–0.5 wt.%). The plagioclase

composition covers the wide range An60–85.

Spinel is a titaniferous magnetite (Usp32–40)

and often associated with ilmenite (Ilm90–96).

(b) Amphibole-bearing gabbros are porphyroclastic

heterogranular in texture, characterized by large

amphibole phenocrysts (66 vol.%) up to 1 cm in

diameter, set in a matrix of plg (15 vol.%), ol (4

vol.%), cpx (13 vol.%). Some samples show a

variably thick (0.2 to 2 mm) fine-grained

amphibole breakdown corona around amphib-

oles, which is granoblastic and is composed of

fine grained, pinkish cpx, ol, plg, spinel, and

rhfnite (Fig. 7).

Fig. 7. (a) Amphibole instability during magma ascent produces

variably thick fine-grained breakdown coronas composed of

plagioclase+Ti-Al-clinopyroxene+ rhfniteFFe–Ti oxides. (b)

Detail of rhfnite (light gray) and clinopyroxene (dark gray) which

are clearly substituting amphibole (SEM photo, BSE).

Amphibole is essentially kaersutite and is com-

positionally homogeneous (Mg#=0.66–0.69,

TiO2 content of 4.6–5.4 wt.% and Al2O3 content

ranging between 13.0 and 13.7 wt.%; Table 5).

As regards its crystal chemistry, Al is dominant

in tetrahedral coordination (i.e. IVAl=2.088–

2.182 apfu; VIAl=0.121–0.185 apfu). Two types

of clinopyroxene are present:

– a salitic to diopside (Wo47–50, En35–40,

Fs13–17; Mg#=0.67–0.75) type after amphi-

bole (i.e. in coronas) and also in the matrix,

and

– high-Ti, high-Al fassaite (Wo52–54 En30–34Fs13–17; Mg#=0.64–0.71; TiO2=6.5–6.7

wt.%; Al2O3=10.8–13.6 wt.%) (Fig. 5,

Table 1) after amphibole. The very high Ti

content of this cpx distinguishes it from fassaites

of the clinopyroxenitic group and may be

inherited from a former Ti-rich amphibole.

Olivine is present either in the breakdown

coronas or as larger crystals in the matrix outside

of the coronas and has a similar compositional

range (Fo78–80) and variably low CaO (up to 0.3

wt.%). We also found rhfnite (an aenigmatite

group mineral, Johnston and Stout, 1985) (Table

6) in the breakdown coronas. Plagioclase in

amphibole coronas is less calcic (An50–65) than

into the matrix plagioclase (An68–74). Spinel is a

Ti-magnetite (Usp26–70).

5.4. Mafic cumulates: troctolites

(glass-free/glass-bearing)

Troctolites are characterized by medium- to coarse-

grained crystal sizes (up to 1 mm in diameter).

Plagioclase is the most abundant phase (80 vol.%)

followed by olivine (20 vol.%), organized in a granular

subidiomorphic texture. The presence of glass allows us

to subdivide this group into (i) glass-free holocrystalline

and (ii) glass-bearing enclaves (commonly less than

10%). The latter subgroup is texturally different from the

former essentially by the presence of glass and must be

inferred to represent a crystal mush containing glassy

pools with crystallites (b 20 Am) of plg, ol and opaques.

In glass-bearing enclaves, plagioclase usually

displays granoblastic texture (with frequent defor-

mation twins) as well as a thin overgrowth rim

around euhedral plagioclases (adcumulate type),

Table 5

Representative chemical analyses of amphibole crystals

Sample Amph-bearing gabbros

30i/8, core 30i/32, core 30a/3, core 30a/19, core 31b/2, core 31b/8, rim 31b/9, core 31b/14, rim

SiO2 39.94 40.43 39.75 39.91 40.42 40.49 40.24 40.49

TiO2 4.64 4.91 4.64 4.64 5.45 5.26 5.08 5.19

Al2O3 13.03 13.20 13.31 13.01 13.55 13.40 13.68 13.57

FeO 11.62 11.57 11.53 11.49 11.52 11.52 11.18 11.37

MnO 0.18 0.21 0.24 0.18 0.13 0.23 0.12 0.23

MgO 12.92 13.56 13.10 13.25 13.01 13.13 13.74 13.25

CaO 11.85 12.04 11.96 11.71 12.18 12.10 11.99 12.11

Na2O 3.22 3.46 2.82 3.24 3.12 3.19 3.34 3.17

K2O 0.64 0.70 0.65 0.58 0.62 0.68 0.64 0.62

Total 98.04 100.08 98.00 98.01 100.00 100.00 100.01 100.00

Si 5.912 5.865 5.869 5.894 5.862 5.874 5.818 5.864IVAl 2.088 2.135 2.131 2.106 2.138 2.126 2.182 2.136VIAl 0.185 0.121 0.185 0.158 0.179 0.166 0.149 0.180

Fe2+ 1.395 1.341 1.295 1.307 1.402 1.380 1.250 1.337

Fe3+ 0.043 0.063 0.129 0.111 0.000 0.017 0.102 0.040

Mg 2.850 2.932 2.883 2.917 2.813 2.839 2.961 2.860

Mn 0.023 0.026 0.030 0.023 0.016 0.028 0.015 0.028

Ti 0.516 0.536 0.515 0.515 0.594 0.574 0.552 0.565

Ca 1.879 1.871 1.892 1.853 1.893 1.881 1.857 1.879

NaM4 0.109 0.111 0.071 0.116 0.107 0.114 0.114 0.110

NaA 0.815 0.862 0.736 0.812 0.770 0.783 0.822 0.780

K 0.121 0.130 0.122 0.109 0.115 0.126 0.118 0.115

Total 15.936 15.992 15.858 15.921 15.888 15.909 15.940 15.895

Mg/Mg+Fetot 0.66 0.68 0.67 0.67 0.67 0.67 0.69 0.68

Crystal-chemical formulae calculated on the basis of 23 oxygens.

M. Alletti et al. / Lithos 84 (2005) 151–167 161

whereas glass-free enclaves are almost undeformed.

Plagioclase is medium- to coarse-grained (diame-

terz1 mm), polysynthetically twinned, displays

patchy and concentric zoning, and contains frequent

inclusions of opaques. Inclusions of tiny pink

clinopyroxene microlites are often arranged parallel

to crystal growth edges. More rarely clinopyroxene

is present as microlites in the glassy groundmass.

Olivine ranges in size from 30 to 500 Am and is often

rounded in shape and surrounded by a thin red film of

iddingsitic alteration. Inclusions of opaques and pinkish

clinopyroxenes are frequent. In glass-bearing samples

olivine is skeletal with frequent glass inclusions;

plagioclase and olivine microlites display quench

textures such as swallow tails and hollow centres.

Clinopyroxene composition ranges between calcic

augite and salite (Wo44–48, En31–40, Fs14–21), with an

Mg#=0.59–0.75. The distinctively high TiO2 (1.8–

3.9 wt.%) and Al2O3 (2.5–8.5 wt.%) are peculiar.

Some cpx show coronas of Ti-mt and Al-mt. Olivine

compositions cover a wide range (Fo67–82) often

characterized by normal zoning from core to rim of

about 8% Fo. The CaO content is in the range 0.1–0.3

wt.%. Plagioclase cluster in the compositional range

An65–81 (Fig. 6), showing an increase in calcium

content towards the rims (core=An69, rim=An80).

One sample only (PUS 30 g/30) is characterized by

the presence of anorthoclase (Ab64Or31). In glassy

samples, plagioclase microlites have a composition of

An58–61, Ab38–40, Or1–2. Spinel is present as ulvospi-

nel (Usp65) and Al-rich spinel (Al2O3=34.4–43.6

wt.%, herc-pleon). Ilmenite (Ilm97) is scarce and

mostly occurs as inclusion in plagioclase. Glass is

present mostly in small pools and has a rather mafic

composition (SiO2 wt.%=48–50).

5.5. Microsyenites

The single sample belonging to this group shows

an ipidiomorphic texture, composed mainly of anor-

thoclase and clinopyroxene. There is no evidence of

deformation or recrystallization.

Table 6

Representative chemical analyses of rhfnite crystals occurring in amphibole breakdown coronas

Sample Amph-bearing gabbros

30a/34, corona 30a/5, corona 30i/13, corona 30i/27, corona 30i/29, corona 30i/9, corona 31b/17, corona

SiO2 24.03 25.57 22.63 27.55 28.22 27.30 22.50

TiO2 12.33 9.98 10.98 9.78 9.31 9.62 13.09

Al2O3 16.44 15.71 14.93 15.36 14.89 15.11 16.55

FeO 20.01 17.50 19.33 17.51 17.83 19.52 21.62

MnO 0.00 0.12 0.12 0.31 0.16 0.10 0.14

MgO 13.22 14.19 11.89 15.79 15.99 14.56 12.54

CaO 12.29 11.30 11.47 11.63 11.68 11.78 12.10

Na2O 1.55 1.44 1.25 1.97 1.86 2.00 1.42

K2O 0.02 0.02 b.d.l. 0.02 0.05 0.01 0.06

P2O5 0.13 b.d.l b.d.l. 0.07 0.01 b.d.l. b.d.l.

Total 99.88 95.83 92.60 100.00 99.99 100.00 100.02

Si 3.191 3.496 3.260 3.577 3.663 3.573 3.013IVAl 2.573 2.504 2.535 2.351 2.278 2.331 2.612IVFe3+ 0.236 0.000 0.205 0.072 0.059 0.096 0.375

Ti 1.231 1.026 1.189 0.955 0.909 0.947 1.318VIAl 0.000 0.027 0.000 0.000 0.000 0.000 0.000VIFe3+ 0.705 0.811 0.711 0.989 0.992 1.043 0.729VIFe2+ 1.281 1.190 1.413 0.840 0.885 0.998 1.318

Mn 0.000 0.014 0.015 0.034 0.018 0.011 0.016

Mg 2.617 2.892 2.553 3.057 3.094 2.841 2.504VICa 0.166 0.040 0.119 0.125 0.102 0.160 0.115VIIICa 1.583 1.615 1.651 1.493 1.523 1.492 1.621

Na 0.399 0.382 0.349 0.496 0.468 0.508 0.369

Mg/Mg+Fetot 0.541 0.591 0.523 0.617 0.615 0.571 0.508

Crystal-chemical formulae calculated on the basis of 20 oxygens; b.d.l.=below detection limit.

M. Alletti et al. / Lithos 84 (2005) 151–167162

Anorthoclase (93 vol.%; 100 Am to a few mm in

size) is anhedral, often with a patchy zonation, irregular

rims and a cloudy surface; clinopyroxenes (5 vol.%; up

to 1 mm across) show distinctive pleochroism from

pale to dark green. Olivine is rare (up to 1%) and occurs

as euhedral microlites. Fe–Ti oxides (i.e. magnetite) are

preferentially enclosed in K-feldspar. Titanite is also

abundant. Clinopyroxene shows a salitic to Fe-salitic

composition (FeO: 13.2–20.1 wt.%) with relatively

low Mg# (0.33–0.58), TiO2 (0.1–0.4 wt.%) and Al2O3

(up to 1.0 wt.%). Anorthoclase shows very slight

variations in composition, in the range An1–10 Ab64–74Or16–34, the mean CaO content is 1.1 wt.%. Spinel is a

titaniferous magnetite (Usp16–74).

6. The density of enclaves

Density measurements were carried out on selected

samples and yielded results ranging from 3160 to

3300 kg/m3 in ultramafic xenoliths, with fine-grained

materials having a slight but significantly lower

density and the highest value reported in protogra-

nular coarse-grained nodules (PUS 8b). Gabbros are

characterized by a mean density of 2900 kg/m3. There

are slight differences between the densities of

amphibole-bearing and amphibole-free gabbros. The

amph-bearing gabbro displays a higher density and a

smaller bintra-chipQ variability compared to the amph-

free gabbro, which have lower and more scattered

density values. Gabbros show density values close to,

or slightly higher than, the host hawaiitic rocks.

Conversely, clinopyroxenites have densities up to

20% higher than the host magma. This allowed us to

estimate a minimum ascent rate on the basis of Stoke’s

law. Given the measured size (Table 7) of enclaves,

the density contrast between enclaves and host lavas

(2790 kg/m3), assuming a viscosity of about 102 Pa s

and yield strength of 24 Pa, density contrast, accord-

ing to appropriate rheological properties of the melts

(Spera, 1980), we can calculate a minimum ascent rate

in the range of 0.01–0.09 m/s. Assuming a depth of

Table 7

Density measurements and calculations of ascent rates

Type Mineral assemblage Texture-grain

size

Max size

(cm)

Density

(kg/m3)aAscent rateb (m/s)

Newtonian viscosity

Ascent rateb (m/s)

Bingham viscosity

Hawaiitic lava Plg, Ol, Al-Aug, Amph porphyritic 2790

Amph-bearing gabbro Amph, Plg, Ol, Ap,

Ti-Mt, Al-Di

medium 5 2920F70 (3)

Amph-free gabbro Plg, Ol, Al-Di, Ti-Mt coarse-medium 12 2800F160 (7)

Olivine clinopyroxenite Al-Di, Ol, Sp, glass blebs coarse-medium 10 3300F30 (6) 0.09 0.06

Clinopyroxenite Al-Di, Sp, Plg, Ol, Ap,

Ti-Mt

fine 5 3160F30 (6) 0.03 0.01

Minerals are sorted by modal abundance. Underlined symbols indicate disequilibrium crystals.a Density resulting from several direct measurements (bracketed numbers) performed on rocks chips by Berman balance. The density of the

nodule bearing lava has been estimated following Lange and Carmichael’s (1987) method.b Minimum ascent rate calculated following Spera (1980) considering a Newtonian magma viscosity (60 Pa s) or Bingham viscosity (yield

strength=24 Pa).

M. Alletti et al. / Lithos 84 (2005) 151–167 163

entrapment of 25 km we can propose an ascent time in

the range of 3–29 days.

7. Discussion

7.1. Constraints on the ascent mechanism

The nature and the variability of enclaves brought

to the surface by the magma are related to the nature

of the intruded rocks and to physical and chemical

processes that were active during the entrainment

and the transport of enclaves. The strength of the

rocks, the shear strain along the feeder dykes,

fluidization processes due to early volatile exsolu-

tion, or fragmentation during explosive phenomena,

are all processes that control the nature, the shape

and the size of entrained rocks fragments. The

ascent rate operates a further selection by the size

and lithology of enclaves, on the basis of density

contrast with the host magma. Furthermore, even

assuming a rigorous and statistically significant

sampling, able to avoid idiosyncrasies of individual

collectors, we must realize that xenolith suites

represent, unavoidably, an intrinsically biased pop-

ulation of deep-seated rocks (Griffin and O’Reilly,

1987). On the contrary chemical processes, that

involve breakdown of minerals, partial fusion or

alteration by fluids, will produce textural and

compositional changes which are easily recognizable

by petrological methods. Thus we can discriminate

between primary features pertaining to host rocks

and modifications induced by the chemical reaction

with magma during the ascent.

In Ustica enclaves from a merely textural point of

view, the low degree of interaction between enclaves

and magma suggests a short time-scale for magmatic

ascent path. This is confirmed by the short ascent

times that are derived from the observed sizes of the

nodules and the density contrast with the host rocks.

High ascent velocities imply a tectonic regime

dominated by tensile stresses that allow the persis-

tence of open fissures and feeding dykes throughout

the whole crustal thickness. Alternatively, a high

ascent rate in basaltic magma is favoured by high

volatile content, which can significantly increase the

buoyancy of magma (e.g. Etna or Columbia River

Basalt; Corsaro and Pompilio, 2004; Lange, 2002).

These two observations suggest that the enclaves

can be considered as representative of the status of the

rocks just before the entrapment in the ascending

magma and that physical and chemical modifications

during their journey to the surface are negligible or

only affected the external portion on the nodule. We

can thus make some inferences concerning the

magmatic processes, particularly regarding the depths

of provenance and thermal records of enclaves.

7.2. Thermobarometric constraints

Given the peculiar mineralogy of the enclaves of

this study and the absence of suitable cation exchange

geothermobarometers, P–T calculations are rather

poorly constrained. However, an attempt to evaluate

Fig. 8. Relative Ti and Al contents (calculated on a 23 oxygen basis)

of amphiboles from gabbroic enclaves (open squares) at Ustica.

Hatched area represents kaersutites of shallow crustal origin, the

unpatterned area represents mantle derived amphiboles (following

Vinx and Jung, 1977), the stippled field represents amphibole

phenocrysts in the lavas erupted during the august 2002 eruption of

Mt. Etna. This diagram suggests a deep-seated origin for Ustica

amphiboles, close to the crust–mantle boundary.

M. Alletti et al. / Lithos 84 (2005) 151–167164

these parameters can be made for some nodule

groups.

7.2.1. Clinopyroxenites

The sub-solidus reaction recognized in some clino-

pyroxenites, the formation of spinel plus clinopyroxene

at the expense of plagioclase and olivine (Fig. 4) is

typical of granulite facies conditions. Despite of the

anomalous chemistry of most of the pyroxenes, high

pressure conditions are also confirmed by the Nimis

(1999) geobarometer, whose calculations yield pres-

sures of 0.7–1.1 GPa. Temperatures derived byMg and

Fe cation exchange between olivines and pyroxenes

(Loucks, 1996) cluster tightly at 1000 8C.Textural patterns (i.e. triple junctions and spinel

forming reactions) strongly suggest granulite facies

metamorphism. As a consequence, taking into account

the P–T conditions determined for the other enclaves, the

clinopyroxenites can be considered as the deepest-seated

representative of the studied enclaves. Their original

depth roughly corresponds to that of the Mohorovicic

discontinuity beneath Ustica. This is in agreement with

their density that is comparable with those measured for

the lower crust (Holbrook et al., 1992).

7.2.2. Gabbros, troctolites and amphibole stability

Amphibole, absent in Ustica lavas, has been

observed in gabbroic enclaves of this study for the

first time.

Experimental studies on Ti-rich amphibole stability

suggest that the higher stability limit for kaersutite/Ti-

pargasite is at 1 GPa and temperatures less than 1100

8C (Barclay and Carmichael, 2004; King et al., 2000).

Conversely, the lower pressure limit is set for basaltic

magmas at P=75–100 MPa and T b1000–1050 8C(Pompilio and Rutherford, 2002).

Mantle-derived kaersutites typically contain less Ti

than kaersutites crystallized at shallower level in the

crust (Best, 1974; Cawthorn, 1976; Vinx and Jung,

1977). On these bases, amphiboles from studied

enclaves are mantle-derived (Fig. 8), but close to the

boundary with crustal amphiboles (Vinx and Jung,

1977). Etnean amphiboles, plotted for comparisons

(Pompilio et al., 2001), show a slightly lower Ti content

and a larger Al variability, suggesting a deeper origin

than those of Ustica. The instability of amphibole

during ascent produced breakdown coronas (200 to 800

Am in width) consisting primarily of rhfnite and Ca-Al

rich cpx (fassaite). As to the presence of the rare

rhfnite, it is generally considered to be a low pressure

breakdown product of amphibole under decreasing

fH2O of the magma (Bonaccorsi et al., 1990). Recently,

rhfnite has been reported in lavas outpoured during

the July–August 2001 eruption of Mt. Etna (Pompilio

et al., 2001). Based on substitution mechanism on the

octahedral sites of rhfnite [2(Fe3+, Al) XTi+(Fe2+,

Mg)] and a plot of (Ti+Fe2++Mg) vs. Fe3+(Fig. 9), it

is possible to speculate about the fO2 during this stage.

In fact, comparing our data with the experimental

results of Grapes et al. (2003), coronitic rhfniteshould have formed at an fO2 between IQF and

NNO. The possible reaction involved in the formation

of rhfnite by the breakdown of kaersutite is:

kaersutite= rhfnite+plg+Ti-Al-cpx+melt and occurs

at a temperature of about 1100 8C at very low pressure

(10–45 MPa) (Kunzmann, 1999).

In amphibole-bearing gabbros, cation exchange

geothermometry based on the combined NaSiCa�1

Al�1 exchange between plagioclase and amphibole

(Holland and Blundy, 1994) yielded temperatures

Fig. 9. Plot of Fe3+ vs. Ti+Fe2++Mg contents (analyses recalcu-

lated on the basis of 20 oxygens and 14 cations) of rhfnite after

amphibole, in breakdown coronas (gray triangles). The shaded area

represents microphenocrysts of rhfnite in basanitic melt (Grapes et

al., 2003). This diagram suggests that rhfnite should have formed at

a rather low fO2, close to QFM buffer.

M. Alletti et al. / Lithos 84 (2005) 151–167 165

ranging from 978 to 1029 8C, taking in account

crystal core compositions. Slightly lower temper-

atures (c10 8C) result using compositions of

amphibole rims and plagioclase coronas.

On the contrary, temperatures derived by Mg and

Fe cation exchange between olivine and pyroxenes

(Loucks, 1996) in the same enclaves range from 1160

to 1230 8C. This discrepancy in temperature is in

agreement with textures that indicate early crystal-

lization of pyroxene and olivine and late appearance

of amphibole. Geobarometric calculations based on

Nimis (1999) for amphibole-bearing gabbros yield a

value of 200 MPa, which in this paper is considered as

the lower pressure limit.

According to Loucks (1996) and to the structural

geobarometer of Nimis (1999), amph-free gabbros

crystallized in the range 1045–1100 8C and 400–600

MPa, while troctolites formed from 1168 to 1230 8Cand from 300 MPa to 600 MPa.

7.3. Magmatic processes

Among the studied enclaves, truly mantle-derived

xenoliths were not unambiguously identified. The

majority of enclaves are thought to represent the deep

crust beneath Ustica. Supra-crustal rocks, though few

in number, provide useful insight on magmatic

processes occurring in deep reservoirs, being essen-

tially composed of cognate inclusions (e.g. micro-

syenites, gabbros). In fact, the microsyenitic enclaves

are here considered as intrusive equivalents to the rare

trachytes that crop out in the island or in submarine

structures (Bellia et al., 2000). The evolved character

of these cognate enclaves is also mirrored by the

presence of Fe-rich clinopyroxene and anorthoclase

(both never found in lavas) and emphasize the role of

extended fractional crystallization in producing

evolved liquids at Ustica.

The presence of amphibole in gabbroic enclaves

suggests that amphibole is a liquidus phase in Ustica

mafic magmas at medium-high pressures. Amphibole

stability has an important influence on major element

chemistry of lavas, particularly the buffering of Na

enrichment of the liquid. Furthermore, it could have a

significant effect on the solid/liquid partitioning of

some HFS elements, some of them (e.g. Nb, Ta)

exhibiting slight positive spikes at Ustica (Cinque et

al., 1988; Schiano et al., 2004; Trua et al., 2003). As

to the presence of fassaite in southern Italian

volcanoes, to our knowledge it has been found only

in some crustal enclaves and a few alkali lavas of Mt.

Etna among southern Italy volcanoes (Michaud and

Clocchiatti, 1994; Tanguy, 1980). Michaud and

Clocchiatti (1994) attribute these compositions to

the interaction of basaltic melts with the carbonatic

basement underlying Etna. In contrary, the interaction

with a carbonatic basement is to be excluded in this

part of the Sicily since carbonate nodules have never

been found in Ustica volcanics. Thus the presence of

fassaite in the amphibole breakdown coronas (even if

not exclusively in coronas) is interpreted as mainly a

breakdown product of amphibole.

7.4. Inferences on the composition of the lower crust

beneath Ustica

The above data allow us to propose a model for the

crustal structure beneath Ustica. Density, temperature

and inferred pressure indicate that clinopyroxenites and

minor wherlites may represent the lowest crustal

portion belowUstica, very close to theMoho transition.

Textures show that these rocks underwent a recrystal-

M. Alletti et al. / Lithos 84 (2005) 151–167166

lization and reequilibration at low temperatures

(T ~1000 8C) after the early cooling from magma.

Gabbro and amphibole gabbro may represent cumu-

lates of repeated events of magma intrusion beneath the

volcano. Barclay and Carmichael (2004) showed that

the onset of amphibole crystallization induces amarked

increase in crystallinity and a consequent difficulty in

eruptibility of these mushes. This process may explain

the absence of amphibole in Ustica eruptive products,

in contrast to their abundance in the sub-intrusive

counterparts brought to the surface as enclaves.

8. Conclusions

The studied enclaves represent a natural probe of

the shallow-to-deep crust beneath Ustica. The pres-

ence of ultramafic cumulates showing granulite facies

metamorphic conditions underlines the occurrence of

mafic cumulates at the base of the crust beneath

Ustica. Supra-crustal gabbroic enclaves show wide-

spread development of breakdown coronas around

amphibole (cpx+plg+rhfnite+plg) due to its insta-

bility upon ascent.

Fassaite (a rather rare silica poor, Ca–Al rich

pyroxene) occurs as a low-Ti type in clinopyroxenites

and in a high-Ti type in amphibole gabbros. The

former is considered to have crystallized directly from

a low-silica alkaline magma (in such liquids fassaite is

known to be a liquidus phase, albeit rarely), while the

latter (together with rhfnite) is clearly a sub-solidus

replacement product of amphibole and suggest shal-

lower conditions.

It is worth noting that the abundance of amphibole in

enclaves is in contrast to its scarcity in lavas. This

behaviour is considered to be due to the massive

crystallization that accompanies the onset of amphibole

crystallization (Barclay and Carmichael, 2004) which

dramatically reduces the possibility of eruption of these

crystal-rich mushes. The presence of amphibole in

enclaves also suggest that amphibole is a medium

pressure liquidus phase in Ustica mafic magmas. This

finding has major implications for the geochemistry of

lavas, most importantly on the K/Na ratio and also on

the trace element signatures of the lavas.

Finally, measured densities of enclaves and their

host lavas allowed us to calculate high ascent

velocities, implying highly buoyant magmas and/or

a tectonic regime dominated by tensile stresses that

allows the persistence of open fissures and feeding

dykes throughout the whole crustal thickness.

Acknowledgments

We wish to acknowledge D. Baker for a critical

review of an early version of the manuscript and the

two anonymous reviewers for their helpful comments.

We would also like to thank D. Cellura and R. Corsaro

who gave their help during the fieldwork. Thanks are

due to S. Bellia who gave unpublished whole rock

analyses of Ustica lavas.

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