Mineral compositions and magmatic evolution of the calcalkaline rocks of northwestern Sardinia,...

An International Journal ofMINERALOGY, CRYSTALLOGRAPHY, GEOCHEMISTRY,ORE DEPOSITS, PETROLOGY, VOLCANOLOGYand applied topics on Environment, Archeometry and Cultural Heritage

DOI: 10.2451/2011PM0033Periodico di Mineralogia (2011), 80, 3 (Spec. Issue), 517-545

perIodIco di MInerAlogIAestablished in 1930

Mineral compositions and magmatic evolution of the calcalkaline rocksof northwestern Sardinia, Italy

Vincenza Guarino1,*, Lorenzo Fedele1, Luigi Franciosi1, Roberto Lonis2, Michele Lustrino3,4,Marianna Marrazzo1, Leone Melluso1, Vincenzo Morra1, Ivana Rocco1 and Fiorenzo Ronga1

1 Dipartimento di Scienze della Terra, Università degli Studi di Napoli Federico II,Via Mezzocannone 8, 80134 Napoli, Italy

2 Progemisa S.p.A., Via Contivecchi 7, 09122 Cagliari, Italy3 Dipartimento di Scienze della Terra, Università degli Studi di Roma La Sapienza,

P.le A. Moro 5, 00185 Roma, Italy4 CNR - Istituto di Geologia Ambientale e Geoingegneria (IGAG), Roma, Italy

*Corresponding author: [email protected]

Abstract

During Early Miocene northwestern Sardinia was interested by widespread volcanic activitybelonging to the calcalkaline to high-K calcalkaline “orogenic” Sardinian cycle. The peak ofactivity is recorded in the ~ 22-18 Ma time span and it is mostly concentrated along the FossaSarda graben, an Oligo-Miocene rift system cutting the western Sardinia from north to south.The northwestern Sardinia volcanic rocks crop out in the districts of Bosa-Alghero, Anglona,Logudoro, Mulargia-Macomer and Ottana graben, ranging in composition from rare highalumina basalt lavas to rhyolitic ignimbrites (welded and unwelded) and lava flows, throughbasaltic andesites, andesites and dacitic domes, with the most evolved rocks (dacites andrhyolites) being volumetrically predominant.

Here we report the chemical composition of the main rock-forming minerals (clinopyroxene,orthopyroxene, plagioclase, sanidine, magnetite, ilmenite, olivine, brown mica and amphibole)and whole-rock data covering the entire spectrum of occurring lithotypes.

The bulk-rock major and trace element variations are consistent with an evolution mainlydriven by fractional crystallization of the observed mineral phases. This was confirmed bymass balance calculations and MELTS-based modelling, which allowed reconstructing liquidlines of descent for each district. Crystallization conditions took place at low pressures (1kbar) and low H2O contents (< 2 wt.%), with oxygen fugacities close to the QFM and Ni-NiO synthetic buffers and temperature values ranging from ~ 900 to ~ 1150 °C for maficrocks and from ~ 800 to ~ 1000 °C for the silicic ones. Open-system processes may also havehad a role in the petrogenesis of the silicic rocks.

Key words: ignimbrites; mineral chemistry; orogenic magmatism; Sardinia.

guarino sardegna2_periodico 19/12/11 11:21 Pagina 517

518 V. Guarino et al.Periodico di Mineralogia (2011), 80, 3 (Spec. Issue), 517-545

Introduction

During the Cenozoic an intense igneousactivity with calcalkaline to high-K calcalkalineproducts took place in Sardinia (Coulon et al.,1974; Montigny et al., 1981; Beccaluva et al.,1985; Morra et al., 1994; 1997; Brotzu et al.,1997a; 1997b; Lustrino et al., 2004; 2009; 2011;Conte et al., 2010, and references therein). Thevolcanic products comprise pyroclastic flowdeposits, lava flows and domes; theircompositions are dominated by dacites andrhyolites, with subordinate andesites and veryscarce basalts.

Despite the interest raised by the Sardinianigneous rocks in the last ~ 15-20 years, some keyaspects remain to be investigated in detail.Indeed, most recent literature is focused onprimitive magmatic compositions and theirrelationships with a subduction-modified mantlesource (Brotzu et al., 1997a; 1997b; Morra et al.,1997; Mattioli et al., 2000; Downes et al., 2001;Franciosi et al., 2003; Conte et al., 2010),whereas the petrogenesis of the more widespreadsilicic products is basically that produced during‘60s and ‘70s (e.g., Deriu, 1962; Coulon et al.,1973; 1978; Lonis et al., 1997). This study isintended to fill this gap by presenting a new setof data set for the calcalkaline volcanic rockscropping out in the northwestern sector ofSardinia, where silicic lithologies are particularlyabundant. This investigation is focused on themineralogical characterization and on theevaluation of the physico-chemical environmentof crystallization.

geodynamic and geological background

Up to Early Oligocene times the Sardinia-Corsica micro-plate was contiguous with thesouthern European margin (Provence, France,and Catalonia, Spain; e.g., Cherchi et al., 2008;Dieni et al., 2008; Lustrino et al., 2009;Carminati et al., 2010, and references therein).

During the Late Oligocene, with the opening ofLigurian-Provencal Basin, the Sardinia-Corsicamicro-plate rifted toward SE and then driftedcounter-clockwise away from the southernEurope paleo-margin, with a rotation polelocated around the present gulf of Genoa (NWItaly; Carminati et al., 1998; 2010; Jolivet et al.,1998; Speranza et al., 2002; Gattacceca et al.,2007; Lustrino et al., 2009). The continentalrifting phase started ~ 30 Ma, whereas thedrifting phase started around ~ 22 Ma until ~ 15Ma, after a counter clock-wise rotation of ~ 60degrees (Gueguen et al., 1998; Speranza et al.,2002; Vigliotti and Langenheim, 1995; Schettinoand Turco, 2006; Lustrino et al., 2009; Carminatiet al., 2010). From Langhian (~ 15 Ma) theSardinia-Corsica micro-plate reached its presentposition and the extensional tectonics shiftedeastwards, leading to the development ofTyrrhenian Sea and the Apennine fold-and-thrustbelt (Lustrino et al., 2009; Carminati et al., 2010,and references therein).

In this complex geodynamic scenario, the islandof Sardinia experienced an intense magmatism(Lecca et al., 1997; Lustrino et al., 2004; 2009;2011) which can be chronologically andgeochemically divided into two distinct phases.The products of the oldest phase (Late Eocene-Middle Miocene activity, or LEMM; ~ 38-13 Ma;Lustrino et al., 2009, and references therein) show“orogenic” (i.e., subduction-related) geochemicalsignature, whereas the products of the youngestphase (Late Miocene-Quaternary, hereafter LMQ;~ 12-0.1 Ma) show “anorogenic” (within plate)geochemical characteristics (e.g., Beccaluva et al.,1985; Lustrino et al., 2004; 2007a; 2007b; 2009;2011; Lustrino and Wilson, 2007, and referencestherein).

The volume of the erupted magma amounts to~ 1000-2500 km3, excluding the volume of rocksdeposited along the submerged margins of theisland, hypothesizing an originally continuouspyroclastic flow cover in northern Sardinia (withan areal extent of ~ 4800 km2 and an average


thickness in the range of 200-500 m). Such alarge amount of magma was produced, in arelatively short time span, and may have hadsignificant effects on marine ecosystem of thearea (e.g., Brandano et al., 2010; Brandano andPolicicchio, 2011). As for the products emplacedin the northwestern sector, Deriu (1962) firstproposed a stratigraphic and minero-petrographic subdivision taking into account fourstratigraphic sequences: 1) the andesitoideinferiore (lower andesitic) series; 2) thetrachitoide inferiore (lower trachytic) series, thefirst evidence of acid volcanism with ignimbriticfacies; 3) the andesitoide superiore (upperandesitic) series, characterized by theintercalation of fluvial-lacustrine sediments andpyroclastic materials; 4) the trachitoidesuperiore (upper trachytic) series, characterizedby erosive features and intercalations of fluvial-lacustrine sediments. Coulon (1977) provided adetailed volcano-stratigraphic, mineralogical andpetrologic description (from base to top): 1) theandesitica inferiore (lower andesitic) series,characterized by products with basalt andbasaltic andesite composition with minor dacitesand rhyolites; 2) the ignimbritica inferiore (lowerignimbritic) series; 3) the andesitica superiore(upper andesitic) series; 4) the dactic domes andrhyolitic lava flows; 5) the ignimbritica superiore(upper ignimbritic) series and 6) the andesiticaterminale (upper andesitic) series, with basalticandesites, dacites and rhyolites. More recentstudies (i.e., Lecca et al., 1997) still rely on thisstratigraphic schemes.

The LEMM products range in compositionfrom dacites to rhyolites, with subordinatebasaltic andesites, andesites, high-Al basalts andrare high-Mg basalts and belong to thecalcalkaline and high-K calcalkaline series (e.g.,Brotzu et al., 1997a; 1997b; Lecca et al., 1997;Lonis et al., 1997; Morra et al., 1997; Downes etal., 2001; Lustrino et al., 2004; 2009; Conte et al.,2010). Rare peralkaline products (comendites)are found in southwestern Sardinia (Sulcis; Araña

et al., 1974; Morra et al., 1994).The mantle source of the mafic “orogenic”

volcanic rocks is in the wedge above a west-directed oceanic lithosphere subducted below thesouthern European continental margin, enrichedin large ion lithophile elements by fluids/meltsderived from the subducted slab (Franciosi et al.,2003; Lustrino et al., 2004; 2009; 2011).

Analytical techniques

The samples were cut with a saw and crushedin a jaw crusher. The obtained chips werewashed in distilled water and then ground in alow-blank agate mortar and analyzed for majorand trace elements were obtained with XRF (X-Ray fluorescence) on pressed powder pellets atCISAG (Centro Interdipartimentale di Serviziper Analisi Geomineralogiche), University ofNapoli Federico II (see Melluso et al., 2005 fordetails). Mineral compositions were obtained atIGAG-CNR, Rome, using a Cameca SX50electron microprobe equipped with fivespectrometers and at CISAG, with an OxfordInstruments Microanalysis Unit, equipped withan INCA X-act detector and a JEOL JSM-5310microscope operating at 15 kV primary beamvoltage, 50-100 μA filament current, variablespot size and 50 s net acquisition time (seeMelluso et al., 2010 and Guarino et al., 2011 forfull details).

classification and geochemical features

The Early Miocene volcanic rocks subject ofthis study were collected in five districts in thenorthwestern sector of Sardinia (Figure 1a, b).The district, the number of samples, thelithologies and the absolute ages for each of themare summarized in Table 1.

Table 2 shows major and trace elementcomposition of representative Early Miocenerocks from northwestern Sardinia. According tothe TAS diagram (Figure 2a), the rocks are

519Mineral compositions and magmatic evolution...Periodico di Mineralogia (2011), 80, 3 (Spec. Issue), 517-545


mainly dacites and rhyolites, with subordinatebasalts, basaltic andesites and andesites. Abimodal composition is indicated by a Daly gap,with andesitic compositions (SiO2 = 57-63 wt.%)being rare.

Among the mafic lithotypes, the few basalts,from the Bosa-Alghero and Logudoro districts,show high Al2O3 (17.8-19.4 wt.%) and relativelylow MgO (3.4-7.2 wt.%), Mg# [0.41-0.60; Mg#= molar MgO/(MgO+FeO+MnO)], Cr (16-52.4ppm) and Ni (9.0-19 ppm). These basalts are

classified as high-Al basalts (HAB) according toKersting and Arculus (1994). The basaltic rocksat Montresta (Figure 1b), are classified as high-Mg basalts (HMB; Morra et al., 1997; Franciosiet al., 2003), due to high MgO (up to 11 wt.%)and relatively low Al2O3 (< 16 wt.%).

Metaluminous [Al2O3/(CaO+Na2O+K2O) < 1]and peraluminous [Al2O3/(CaO+Na2O+K2O) >1] dacites and rhyolites are present (Figure 2b);no peralkaline rhyolites or trachytes have beenfound in northern Sardinia.


Figure 1. a) Geological sketch map (modified after Sau et al., 2005) for the studied districts of the Late Eocene-Middle Miocene Sardinia magmatic phase. 1 = Palaeozoic basement; 2 = Early Miocene andesites; 3 = EarlyMiocene ignimbrites; 4 = rhyodacitic lavas; 5 = fluvial-lacustrine and epiclastic complex of Miocene basins; 6= marine sequences of Miocene basins; 7 = anorogenic volcanites of the Late Miocene-Quaternary cycle; 8 =Quaternary sedimentary deposits; 9 = fault zones; 10 = main transcurrent movements. b) Simplified geologicalsketch map of Sardinia (modified after Lustrino et al., 2009).



The analyzed rocks display smooth and linearvariation trends (Figure 3) characterized bydecreasing TiO2, Al2O3, Fe2O3t, MgO and CaOwith increasing SiO2. The trace elements Rb, Ba,Y and Zr, increase with silica, then theirconcentrations suddenly decrease at ~70% SiO2.Sr decreases with increasing SiO2 (Figure 4).

petrographic features

The main petrographic features of EarlyMiocene rocks of northwestern Sardinia arereported in Figure 5 and are briefly summarizedbelow.

Basalts. The rocks (from the Bosa-Algheroand Logudoro districts) are porphyritic. Themain phenocrysts are subhedral to euhedralzoned plagioclase (sometimes with roundedrims), and clinopyroxene, generally showingrounded rims, only sporadically subhedral inshape. Olivine is quite common and showseuhedral shapes often corroded or iddingsitized.Opaque oxides mainly occur as inclusions or inthe groundmass. The groundmass ismicrocrystalline and it is made of the samephases observed as phenocrysts (except olivine)plus rare orthopyroxene.

Basaltic Andesites. The rocks (Bosa-Alghero,

District Numberof samples Lithology Age

Bosa-Alghero district

10 basalts, basaltic andesite lava flows ~ 28-16 Ma(Montigny et al.,1981;Beccaluva et al., 1985;

Lecca et al., 1997;Deino et al., 2001;

Gattacceca et al., 2007)

1 andesitic lava flows

39 welded and unwelded ignimbrites

Anglona district2 andesitic lava flows ~ 21-18 Ma

(Montigny et al., 1981;Lecca et al., 1997;Oudet et al., 2010)5 welded and unwelded ignimbrites

Logudoro district

6 basalts and basaltic andesite lava flows

~ 25-17 Ma(Coulon et al., 1974;

Beccaluva et al., 1985)

1 andesitic lava flows5 dacitic domes1 rhyolitic lava flows26 welded and unwelded ignimbrites

Mulargia-Macomerdistrict

1 rhyolitic lava flows ~ 21.8-21.6 Ma;(Lecca et al., 1997 and

references therein)5 welded and unwelded ignimbrites

Ottana graben district 3 welded and unwelded ignimbrites~ 21-19.4 Ma

(Lecca et al., 1997 andreferences therein)

Table 1. Synoptic table showing district, number of samples, main lithologies and absolute ages of the studiedEarly Miocene rocks from northwestern Sardinia.


Tabl

e 2.

Maj

or (w

t.%) a

nd tr

ace

elem

ent (

ppm

) con

cent

ratio

ns, a

nd C

IPW

nor

mat

ive

min

eral

s fo

r rep

rese

ntat

ive

sam

ples

of E

arly

Mio

cene

rock

sfr

om n

orth

wes

tern

Sar

dini

a.

Bos

a - A

lghe

ro d

istr

ict

Loca

lity

M.te

Ruj

u-M

.te M

ajor

eM

ontre

sta

Mt.

Farr

eP.

taTr

ipid

esM

t. Fa

rre

SW M

t.M

annu

Mt.

Min

erva

Mt.

Farr

eM

t. La

duM

t. M

iner

vaSE

Mia

leIs

pina

Bos

aM

t.M

iner

va

G.P.S.

coordinates

N 4

0°25

'25"

N 4

0°23

'12"

N 4

0°21

'39"

N 4

0°21

'45"

N 4

0°21

'58"

N 4

0°26

'12"

N 4

0°21

'48"

N 4

0°30

'22"

N 4

0°26

'36"

N 4

0°37

'38"

N 4

0°22

'5.4

"

E 08

°25'

02"

E 08

°30'

19"

E 08

°25'

54"

E 08

°25'

56"

E 08

°24'

17"

E 08

°33'

06"

E 08

°25'

52"

E 08

°22'

37"

E 08

°32'

35"

E 08

°25'

47"

E 08

°24'

27"

Rock type

HA

BH

AB

HA

BH

AB

BA

BA

AD

RR

RR

RR

XRF (wt.%

)M

A23

MA

RA

21M

AR

A25

MA

RA

23M

AR

A34

MA

RA

33M

AR

A36

MA

RA

13M

AR

A22

MA

29B

MA

RA

12M

A35

MA

RA

38M

AR

A15

SiO

247

.98

48.1

251

.27

51.9

353

.34

53.9

060

.87

65.2

869

.37

70.3

471

.51

73.6

176

.30

76.6

4Ti

O2

1.24

1.11

0.99

0.99

0.91

0.96

0.70

0.46

0.66

0.64

0.41

0.30

0.28

0.21

Al 2O

318

.85

17.7

818

.26

18.7

419

.27

16.3

715

.85

14.3

513

.59

14.5

013

.22

13.1

812

.40

11.3

9Fe

2O3t

10.6

911

.27

10.2

310

.22

9.29

10.0

96.

734.

935.

413.

883.

862.

922.

702.

06M

nO0.

180.

200.

200.

210.

190.

190.

130.

060.

120.

060.

100.

050.

030.

03M

gO7.

155.

904.

344.

022.

875.

484.

583.

750.

770.

760.

751.

380.

310.

46C

aO10

.42

11.8

710

.56

9.83

9.65

8.67

6.74

4.75

2.29

2.80

2.58

1.98

1.82

1.74

Na 2

O2.

112.

322.

632.

883.

122.

612.

132.

763.

973.

313.

092.

632.

281.

47K

2O1.

141.

221.

261.

011.

121.

512.

113.

493.

663.

564.

363.

893.

785.

95P 2

O5

0.23

0.20

0.26

0.16

0.23

0.22

0.15

0.18

0.17

0.14

0.12

0.05

0.08

0.04

sum

100

100

100

100

100

100

100

100

100

100

100

100

100

100

Mg#

0.60

0.53

0.48

0.46

LOI

1.64

2.59

0.57

0.86

0.43

1.48

4.40

2.46

1.81

3.60

1.33

5.77

1.81

6.86

XRF (ppm)

Sc40

3234

3529

3223

2319

1313

117

8V

375

313

266

290

251

231

159

6274

3436

479

17C

r52

4529

1621

294

111

Ni

919

911

1111

95

37

35

11

Rb

3335

3031

2939

4512

711

713

516

115

212

525

5Sr

492

366

329

318

327

300

291

286

202

187

194

144

186

153

Y29

2624

2624

2930

3255

3231

4524

37Zr

8973

7184

7410

315

823

725

120

222

927

318

720

7N

b1.

64.

94.

54.

94.

17.

213

.69.

715

.212

.312

.614

.410

.114

.2B

a13

713

417

720

620

221

981

960

764

953

368

089

245

355

3CIPW norms:

quartz

0.56

1.64

3.75

3.95

16.8

819

.14

24.1

728

.71

28.9

535

.62

42.9

240

.18

corundum

0.43

1.16

1.43

orthoclase

6.75

7.22

7.45

5.97

6.63

8.93

12.4

920

.59

21.6

021

.07

25.7

422

.97

22.3

635

.15

albite

17.8

819

.65

22.2

724

.39

26.4

322

.11

18.0

523

.39

33.5

528

.05

26.1

822

.29

19.3

112

.45

anorthite

38.5

734

.48

34.2

735

.20

35.2

528

.47

27.4

216

.46

8.48

12.9

99.

319.

518.

526.

92diopside

9.53

19.0

913

.65

10.4

49.

3510

.81

4.13

4.82

1.53

2.32

1.24

hypersthene

9.82

0.02

16.6

517

.44

13.9

120

.77

17.6

013

.01

7.61

6.21

5.43

7.01

4.04

3.02

olivine

11.7

814

.03

magnetite

1.84

1.94

1.76

1.76

1.60

1.74

1.16

0.85

0.93

0.67

0.66

0.50

0.47

0.36

ilmenite

2.36

2.11

1.88

1.88

1.73

1.83

1.33

0.87

1.26

1.22

0.78

0.57

0.53

0.40

apatite

0.55

0.47

0.62

0.38

0.55

0.52

0.36

0.43

0.40

0.33

0.28

0.12

0.19

0.09

sum

99.1

99.0

99.1

99.1

99.2

99.1

99.4

99.6

99.5

99.7

99.7

99.7

99.8

99.8

The

maj

or o

xide

ana

lyse

s are

reca

lcul

ated

to 1

00 w

t.% L

OI-

free

. HA

B =

hig

h al

umin

a ba

salt;

BA

= b

asal

tic a

ndes

ite; A

= a

ndes

ite; D

= d

acite

; R =

rhyo

lite.

Wei

ght l

oss o

n ig

nitio

n (L

OI,

in w

t.%) w

asde

term

ined

by

stan

dard

ther

mo-

grav

imet

ric m

etho

d.




Tabl

e 2.

Con

tinue

d...

log

udor

o di

stri

ct

Loca

lity

Thie

siItt

iriTh

iesi

S. M

t. Fr

usci

uItt

iredd

uM

.te T

raes

su-

Cos

soin

eTh

iesi

G.P.S.

coordinates

N 4

0°33

'01"

N 4

0°23

'49"

N 4

0°33

'01"

N 4

0°28

'09"

N 4

0°28

'51"

N 4

0°29

'53"

N 4

0°30

'28"

N 4

0°30

'28"

N 4

0°31

'57"

N 4

0°28

'35"

N 4

0°28

'34"

N 4

0°33

'01"

E 08

° 39'

16"

E 08

° 37'

38"

E 08

° 39'

16"

E 08

° 38'

13"

E 08

° 37'

05"

E 08

° 36'

09"

E 08

° 36'

05"

E 08

° 36'

05"

E 08

° 54'

23"

E 08

° 40'

48"

E 08

°41'

41"

E 08

° 39'

16"

Rock type

HA

BH

AB

HA

BB

AB

AA

DR

RR

RR

RXRF (wt.%

)M

A72

CM

A38

MA

19M

A72

AM

A12

MA

11M

AR

A7

MA

13A

MA

13B

MA

50M

A37

MA

36M

A71

SiO

249

.93

51.5

451

.81

54.7

955

.70

61.2

369

.73

70.0

672

.42

73.2

875

.93

76.6

677

.42

TiO

20.

931.

010.

831.

040.

810.

770.

460.

420.

360.

260.

180.

170.

17A

l 2O3

19.4

317

.82

18.7

819

.09

16.5

116

.55

14.0

514

.10

13.2

013

.30

12.0

911

.75

11.8

2Fe

2O3t

10.7

210

.76

10.3

29.

748.

467.

253.

563.

422.

712.

702.

162.

052.

04M

nO0.

130.

190.

200.

060.

150.

090.

070.

080.

040.

020.

070.

050.

04M

gO3.

394.

494.

660.

864.

481.

101.

481.

440.

860.

250.

530.

221.

16C

aO11

.15

9.40

9.58

9.73

8.28

6.31

3.47

3.34

3.04

0.98

1.33

0.97

1.86

Na 2

O2.

773.

022.

652.

302.

743.

032.

933.

093.

022.

142.

923.

181.

61K

2O1.

321.

501.

022.

212.

663.

474.

113.

924.

226.

994.

724.

903.

83P 2

O5

0.21

0.26

0.15

0.16

0.20

0.19

0.14

0.12

0.12

0.07

0.06

0.05

0.04

sum

100

100

100

100

100

100

100

100

100

100

100

100

100

Mg#

0.41

0.48

0.50

LOI

1.89

1.86

1.84

1.34

1.27

1.74

2.56

2.45

0.72

0.80

3.45

0.56

6.20

XRF (ppm)

Sc30

2732

2925

2511

79

92

44

V31

726

623

941

519

519

037

3825

2516

Cr

3324

1523

3024

45

51

3N

i18

1613

613

120

32

21

Rb

4839

3484

8512

614

115

114

414

417

818

014

0Sr

462

540

357

389

450

434

269

256

237

237

144

114

206

Y24

2421

1824

3130

3029

2929

2713

Zr72

9166

8215

020

523

422

721

021

018

618

011

1N

b1.

10.

61.

82.

44.

46.

212

.111

.611

.111

.114

.814

.68.

2B

a15

524

318

423

528

838

863

360

164

764

748

450

956

9CIPW norms:

quartz

1.71

8.54

4.17

13.6

126

.50

26.8

330

.73

30.3

036

.20

36.2

146

.59

corundum

0.59

1.73

orthoclase

7.81

8.88

6.03

13.0

815

.74

20.4

824

.27

23.1

424

.91

41.3

027

.87

28.9

322

.66

albite

23.4

625

.59

22.4

419

.49

23.2

125

.68

24.8

326

.18

25.5

918

.13

24.7

426

.94

13.6

4anorthite

36.6

730

.62

36.3

135

.23

24.8

721

.32

13.0

413

.02

9.99

4.41

5.95

3.29

8.98

diopside

14.5

211

.92

8.49

10.4

412

.30

7.51

2.69

2.32

3.63

0.22

1.04

hypersthene

6.90

15.2

320

.40

8.34

15.5

07.

636.

556.

543.

493.

903.

982.

615.

46olivine

5.59

2.44

magnetite

1.85

1.86

1.78

1.68

1.46

1.25

0.61

0.59

0.47

0.47

0.37

0.35

0.35

ilmenite

1.77

1.92

1.58

1.98

1.54

1.46

0.88

0.80

0.68

0.49

0.34

0.32

0.32

apatite

0.50

0.62

0.36

0.38

0.47

0.45

0.33

0.28

0.28

0.17

0.14

0.12

0.09

sum

99.1

99.1

99.1

99.2

99.3

99.4

99.7

99.7

99.8

99.8

99.8

99.8

99.8

The

maj

or o

xide

ana

lyse

s are

reca

lcul

ated

to 1

00 w

t.% L

OI-

free

. HA

B =

hig

h al

umin

a ba

salt;

BA

= b

asal

tic a

ndes

ite; A

= a

ndes

ite; D

= d

acite

; R =

rhyo

lite.

Wei

ght l

oss o

n ig

nitio

n (L

OI,

inw

t.%) w

as d

eter

min

ed b

y st

anda

rd th

erm

o-gr

avim

etric

met

hod.



Tabl

e 2.

Con

tinue

d...

Ang

lona

dis

tric

tM

ular

gia

- Mac

omer

dis

tric

to

ttana

gra

ben

dist

rict

Loca

lity

Sedi

niC

aste

lsar

doM

t. S.

Pad

reA

b. M

ular

gia

Mt.

S. P

adre

Otta

na

G.P.S.

coordinates

N 4

0°50

'54"

N 4

0°51

'42"

N 4

0°54

'47"

N 4

0°17

'06"

N 4

0°17

'53"

N 4

0°17

'52"

N 4

0°17

'20"

N 4

0°14

'19"

N 4

0°13

'34"

E 08

°44'

06"

E 08

°47'

42"

E 08

°41'

58"

E 08

°49'

28"

E 08

°48'

36"

E 08

°50'

30"

E 09

°02'

46"

E 09

°03'

27"

E 09

°01'

54"

Rock type

AD

RR

RR

DR

RXRF (wt.%

)M

A82

MA

84B

MA

6AM

A65

MA

61M

A62

AM

A69

MA

67M

A66

SiO

256

.58

66.8

675

.96

69.2

171

.73

72.9

368

.84

72.6

076

.75

TiO

20.

830.

570.

400.

560.

480.

370.

720.

470.

34A

l 2O3

16.5

114

.80

11.6

314

.90

13.4

813

.10

13.7

112

.85

10.9

0Fe

2O3t

8.93

5.31

2.55

4.35

4.10

3.67

5.57

4.15

3.19

MnO

0.15

0.11

0.04

0.12

0.07

0.06

0.04

0.04

0.03

MgO

2.99

1.95

0.19

0.97

0.25

0.53

0.81

0.44

0.19

CaO

8.65

2.91

1.33

1.92

1.14

1.29

2.81

1.84

1.18

Na 2

O2.

643.

363.

493.

863.

623.

573.

593.

523.

23K

2O2.

513.

984.

324.

005.

064.

383.

733.

984.

09P 2

O5

0.20

0.15

0.08

0.12

0.07

0.09

0.18

0.11

0.08

sum

100

100

100

100

100

100

100

100

100

Mg#

LOI

1.17

5.13

0.64

2.08

1.00

1.38

2.95

2.21

1.25

XRF (ppm)

Sc31

1510

1612

1522

1710

V24

539

140

348

9623

13C

r16

22

1N

i9

81

55

31

Rb

104

306

147

130

148

155

116

140

146

Sr36

425

811

916

412

011

623

814

410

0Y

2552

3841

3447

4337

43Zr

119

283

243

221

244

279

229

260

254

Nb

4.7

14.4

16.1

13.5

15.1

17.5

13.8

16.5

16.2

Ba

372

604

502

496

515

571

1269

551

554

CIPW norms:

quartz

7.55

20.4

535

.50

23.9

126

.45

30.0

524

.59

30.5

638

.41

corundum

0.04

1.03

0.13

0.35

orthoclase

14.8

523

.50

25.5

023

.61

29.8

825

.86

22.0

323

.49

24.2

0albite

22.3

628

.39

29.5

632

.62

30.6

730

.25

30.3

529

.82

27.3

6anorthite

25.7

613

.48

3.32

8.75

5.21

5.82

10.2

97.

493.

14diopside

13.2

82.

412.

100.

801.

92hypersthene

11.8

211

.33

2.10

7.62

5.52

5.81

7.39

5.61

3.30

olivine

magnetite

1.54

0.92

0.44

0.75

0.71

0.63

0.96

0.71

0.55

ilmenite

1.58

1.08

0.76

1.06

0.91

0.70

1.37

0.89

0.65

apatite

0.47

0.36

0.19

0.28

0.17

0.21

0.43

0.26

0.19

sum

99.2

99.5

99.8

99.6

99.6

99.7

99.5

99.6

99.7

The

maj

or o

xide

ana

lyse

s are

reca

lcul

ated

to 1

00 w

t.% L

OI-

free

. HA

B =

hig

h al

umin

a ba

salt;

BA

= b

asal

tic a

ndes

ite; A

= a

ndes

ite; D

= d

acite

; R =

rhyo

lite.

Wei

ght l

oss o

n ig

nitio

n (L

OI,

in w

t.%) w

asde

term

ined

by

stan

dard

ther

mo-

grav

imet

ric m

etho

d.


Mineral compositions and magmatic evolution... 525

Anglona and Logudoro districts) are porphyriticto glomeroporphyritic, with plagioclase beingthe predominant phenocryst. Clinopyroxenegenerally occurs as subhedral pheno- tomicrophenocrysts with rounded rims, sometimescompletely altered. Olivine crystals are rare,often altered into haematite or iddingsite.Orthopyroxene is represented by rare euhedraland quite fractured crystals. Opaque oxidesoccur as inclusions within mafic minerals or inthe groundmass. The groundmass ismicrocrystalline to intersertal, more rarelyhypocrystalline, with microlites of plagioclase,clinopyroxene and opaque oxides.

Andesites. These rocks (Bosa-Alghero andLogudoro districts) are quite altered, withplagioclase and euhedral clinopyroxenephenocrysts. The groundmass is often argillified.

Dacitic domes. The dacites from Logudorodistrict are porphyritic for phenocrysts ofplagioclase, clinopyroxene and occasionallyopaque oxides. Clusters of plagioclase,clinopyroxene and oxides are sometimes noted.Brown mica (often altered), and euhedral greenamphibole are common microlites. Orthopyroxene

is a rare microlite. Accessory apatite is found inthe groundmass.

Rhyolitic lavas. The Logudoro rhyolites areporphyritic, with euhedral to subhedralplagioclase with altered cores. Abundant alkalifeldspar, oxidized brown mica and minoramphibole plus opaque oxide microcrysts arealso observed, set into a completely glassygroundmass with no evidences of devitrification.The Mulargia-Macomer rhyolites are porphyriticwith subhedral zoned phenocrysts of plagioclase,microcrysts of opaque oxides, quartz and deeplyaltered mafic phases set into a microcrystallinegroundmass mainly made of alkali feldspar,quartz and rare apatite crystals.

Ignimbrites outcrop in the Bosa-Alghero,Anglona, Logudoro, Mulargia-Macomer andOttana graben districts. These ignimbrites(dacitic to rhyolitic in composition) can besubdivided into two types: a) unweldedignimbrites and b) welded ignimbrites.a) Unwelded ignimbrites. These samples are

made up of glassy shards, pumices and scoriafragments. The observed mineral assemblagescomprise rounded and variably zoned

Periodico di Mineralogia (2011), 80, 3 (Spec. Issue), 517-545

Figure 2. a) Total Alkali vs. Silica (TAS; Le Bas et al., 1986) classification diagram for the studied Early Miocenevolcanic rocks from north western Sardinia. b) Molar Al2O3/(Na2O+K2O) vs. Al2O3/(CaO+Na2O+K2O) for thestudied Early Miocene volcanic rocks from northwestern Sardinia.


V. Guarino et al.526

plagioclase, euhedral brown mica microcrysts,corroded clinopyroxene, quartz, rare subhedralgreen-brown amphibole and orthopyroxene, plusalkali feldspar and opaque oxides. Commonaccessories are zircon and apatite. Occasionally,secondary minerals such as zeolites, glauconite

and chlorite are observed.b) Welded ignimbrites. These rocks show

textures varying from eutaxitic to spherulitic,more rarely vitrophyric. The observed mineralphases are euhedral to subhedral plagioclase,subhedral clinopyroxene, rare rounded


Figure 3. Selected major element variation diagrams for the studied Early Miocene volcanic rocks fromnorthwestern Sardinia. The best-fit results of MELTS modelling for each volcanic district are also reported (seetext for further explanations).



orthopyroxene, apatite and opaque oxidesincluded in mafic minerals or in isolatedmicrocrysts. Subordinate microcrysts of alkalifeldspar and quartz are also observed. Thewelded ignimbrites are characterized by thepresence of volcanic fragments made principallyof microcrystalline plagioclase, scorias anddeeply altered subhedral minerals (probablymafic phases).

Mineral and glass chemistry

Pyroxenes. The composition of the pyroxenesare reported in Table 3 and Figure 6. Bosa-Algheroand Anglona clinopyroxenes are essentially augitic(Ca36-46Mg40-42Fe14-22 and Ca38-43Mg30-43Fe19-27,respectively) with no substantial chemicaldifferences between basaltic andesites and dacitic-rhyolitic welded ignimbrites. Logudoro basaltic


Figure 4. Selected trace element variation diagrams for the studied Early Miocene volcanic rocks fromnorthwestern Sardinia.


V. Guarino et al.528 Periodico di Mineralogia (2011), 80, 3 (Spec. Issue), 517-545

Figure 5. Thin section microphotographs for some representative samples of the studied Early Miocene volcanicrocks from northwestern Sardinia. ol = olivine; cpx = clinopyroxene; pl = plagioclase; kfs = alkali feldspar.HAB = high alumina basalt; R = rhyolite.



andesites have slightly Ca-richer and Fe-poorerclinopyroxenes (Ca42-48Mg37-41Fe15-17). Mg#[Mg# = Mg/(Mg+Fe)] varies from 0.56 to 0.86.Orthopyroxene from the Bosa-Alghero dacites hasvariable Mg# (0.54 to 0.73; Ca3Mg53-70Fe27-44),whereas orthopyroxene of the Anglona daciteshave a narrower compositional spectrum and agenerally Fe-richer composition (Mg# = 0.53-0.68; Ca3-4Mg50-64Fe32-46). Orthopyroxenes of theLogudoro basaltic andesites and rhyolites have arather homogeneous composition (Mg# = 0.59-0.65; Ca2-3Mg57-63Fe34-41).

Feldspars. Plagioclase from the basalts andbasaltic andesites of the Bosa-Alghero districtshow a wide compositional range fromAn94Ab6Or0 to An22Ab74Or4 (phenocrysts coreand groundmass, respectively; Figure 7a; Table4). Plagioclase of the Logudoro basaltic andesiteshows a compositional range from An88Ab12Or0(phenocryst core) to An52Ab44Or3 (groundmass).The andesites of the Anglona district arecharacterized by plagioclase with a moderatecore-to-rim compositional variations (An71-

81Ab19-29Or0). Plagioclases from the moredifferentiated dacites and rhyolites show a widercompositional spectrum, ranging fromlabradorite to andesine and rare oligoclase (An28-

53Ab45-67Or2-5). Alkali feldspar occurs only in themost differentiated rocks (dacitic and rhyolitic)of the Bosa-Alghero, Anglona, Logudoro,Mulargia-Macomer and Ottana graben districtsand range from K2O-rich sanidine toanorthoclase (Ab1-69An1-16Or15-97; Figure 7a).

Minor phases. Opaque oxides are present asmagnetite and Ti-magnetite groundmassmicrocrysts showing variable ulvöspinelcontents (mol. ulvöspinel = 0.13-0.41), and lowilmenite (mol. ilmenite = 0.81-0.91; Table 5).Olivine crystals were analyzed in basalts and

basaltic andesites from the Bosa-Alghero districtand in basalts from the Logudoro district (Table5). The analyses show a moderate compositionalvariation (e.g., Fo54-71), high FeO (26.4-39.1wt.%) and very low CaO (0.2-0.4 wt.%)

contents. More Mg-rich olivine compositionshave been analyzed in the basalts of Montresta(Fo87-Fo71; Morra et al., 1997).Mica belongs to the phlogopite-annite series

(Figure 7b; Table 6) and have a relatively lowMg# (0.34-0.52). Micas show relatively highFe2+ (2.44-3.39 apfu) and low Ti contents (0.42-0.66 apfu) resulting in a reddish-brown colour ofthe crystals. Most micas are characterized byFe3+ in the tetrahedral site, with values up to 0.27apfu, but several have Al excess in the tetrahedralsite so the remaining Al filled the octahedral site,with values up to 0.33 apfu (Figure 7b; Guarinoet al., 2011).Amphiboles, analyzed in rhyolitic rocks of the

Logudoro district (Table 6), are thus classified astschermakite plus rarer ferrohornblende andmagnesiohornblende (Leake et al., 1997; Figure7c) showing high Ca (1.69-1.83 apfu) contents.The Fe2+ ranges from 1.62 to 2.91 apfu and isassociated with low Ti (0.17-0.31 apfu) andvariable Mg# values [Mg/(Mg+Fe2+) = 0.38-0.65].Glass. The analyzed glasses of rhyolites from

the Bosa-Alghero and Logudoro are rhyoltic.The Anglona glasses are basaltic andesites (inandesitic rocks) and rhyolites (in dacites; Figure7d; Table 7).

discussion and conclusions

The Early Miocene volcanic rocks ofnorthwestern Sardinia have a bimodal chemicalcomposition, with dacitic/rhyolitic rocks beingdefinitely predominant over basalts andandesites. The high alumina basalts (HABs) area common lithotype of “orogenic” magmatismworldwide, whose genesis has been variouslyascribed to high-degree partial melting of thesubducted oceanic slab or low-pressurefractional crystallization of a high magnesiumbasalt (HMB) primitive magma (e.g., Kuno,1960; 1968; Crawford et al., 1987; Myers, 1988;Sisson and Grove, 1993; Kersting and Arculus,




Bos

a-A

lghe

ro d

istr

ict

Ang

lona

dis

tric

tl

ogud

oro

dist

rict

Rock

type

BA

BA

BA

BA

RR

AD

DD

DA

DB

AB

AR

BA

RR

MA

RA

33M

AR

A33

MA

RA

33M

AR

A34

MA

29B

MA

29B

MA

82M

A84

BM

A84

BM

A84

BM

A84

BM

A82

MA

84B

MA

12M

A12

MA

13A

MA

12M

A13

AM

A13

Acp

xcp

xcp

xcp

xop

xop

xcp

xcp

xcp

xcp

xcp

xop

xop

xcp

xcp

xop

xop

xop

xop

xrim

core

rimgm

core

rimrim

rimco

regm

gmrim

core

rimrim

rimco

reco

regm

SiO

251

.19

49.4

349

.11

49.2

552

.45

54.1

151

.23

51.4

251

.23

51.5

851

.27

53.1

351

.55

52.4

247

.01

53.9

053

.40

51.5

451

.81

TiO

20.

700.

790.

660.

550.

220.

320.

570.

450.

410.

360.

160.

240.

200.

15A

l 2O3

1.61

4.52

3.82

1.41

0.60

1.23

2.32

1.19

1.08

1.22

2.16

1.22

0.61

1.40

7.61

1.26

0.69

0.72

0.63

FeO

13.0

98.

368.

6815

.23

25.5

916

.63

10.4

916

.08

16.5

911

.86

10.1

820

.01

27.9

59.

808.

6422

.72

20.6

323

.97

24.7

5M

nO0.

510.

200.

260.

581.

550.

770.

690.

690.

660.

870.

560.

151.

200.

951.

281.

27M

gO14

.49

14.0

814

.11

12.1

718

.35

25.1

714

.90

10.7

610

.59

15.3

215

.12

23.3

117

.53

14.1

612

.02

19.0

422

.35

20.8

020

.43

CaO

17.5

922

.10

22.1

017

.06

1.51

1.50

20.2

921

.03

20.7

518

.64

19.9

51.

721.

8520

.47

22.0

01.

281.

371.

271.

18N

a 2O

0.21

0.23

0.32

0.28

0.07

0.03

0.27

0.22

0.18

0.02

0.03

0.02

Cr 2

O3

0.03

0.06

0.04

0.05

0.05

0.02

sum

99.4

99.7

99.1

96.5

100.4

99.8

99.8

101.2

100.9

99.1

99.1

100.1

100.4

99.4

97.7

99.7

99.7

99.8

100.2

Si1.

934

1.83

71.

836

1.94

01.

995

1.97

51.

913

1.95

11.

952

1.94

51.

925

1.96

01.

973

1.96

81.

769

2.00

01.

991

1.94

31.

952

IVA

l0.

066

0.16

30.

164

0.06

00.

005

0.02

50.

087

0.04

90.

048

0.05

40.

075

0.04

00.

027

0.03

20.

231

0.00

90.

032

0.02

8Fe

3+0.

001

0.02

50.

021

Ti0.

020

0.02

20.

019

0.01

60.

006

0.00

90.

016

0.01

30.

012

0.01

00.

038

0.00

50.

007

0.00

60.

004

VI A

l0.

006

0.03

50.

005

0.00

60.

022

0.02

80.

015

0.00

40.

000

0.02

00.

013

0.00

10.

029

0.10

70.

056

0.02

1Fe

2+0.

378

0.15

90.

128

0.46

00.

814

0.50

80.

288

0.46

50.

481

0.34

50.

288

0.58

90.

869

0.30

50.

209

0.71

90.

643

0.73

10.

759

Fe3+

0.03

50.

100

0.14

30.

042

0.04

00.

045

0.04

80.

028

0.03

20.

028

0.02

60.

003

0.06

30.

000

Mn

0.01

60.

006

0.00

80.

019

0.05

00.

024

0.02

20.

022

0.02

10.

028

0.01

80.

005

0.03

80.

030

0.04

10.

041

Mg

0.81

60.

780

0.78

60.

715

1.04

11.

370

0.82

90.

609

0.60

10.

861

0.84

61.

282

1.00

00.

792

0.67

41.

075

1.24

21.

169

1.14

7C

a0.

712

0.88

00.

886

0.72

00.

061

0.05

90.

812

0.85

50.

847

0.75

30.

802

0.06

80.

076

0.82

30.

887

0.05

20.

055

0.05

10.

047

Na

0.01

60.

017

0.02

30.

021

0.00

50.

002

0.02

00.

016

0.01

30.

001

0.00

20.

001

Cr

0.00

10.

002

0.00

10.

001

0.00

20.

001

Ca

3646

4537

33

4143

4238

413

442

483

33

2M

g42

4040

3653

7042

3030

4343

6450

4137

5763

5857

Fe22

1414

2744

2717

2728

1916

3246

1715

4034

3941

Mg#

0.68

0.83

0.86

0.61

0.56

0.73

0.74

0.57

0.56

0.71

0.75

0.69

0.54

0.72

0.76

0.60

0.66

0.62

0.59

Mg#

= M

g/M

g+Fe

2+gm

= g

roun

dmas

s.

Tabl

e 3. R

epre

sent

ativ

e ana

lyse

s and

site

occ

upan

cies

(stru

ctur

al fo

rmul

as ca

lcul

ated

on

6 ox

ygen

s) fo

r orth

opyr

oxen

e (op

x) an

d cl

inop

yrox

ene (

cpx)

crys

tals

of E

arly

Mio

cene

rock

s fro

m n

orth

wes

tern

Sar

dini

a.


Mineral compositions and magmatic evolution... 531Periodico di Mineralogia (2011), 80, 3 (Spec. Issue), 517-545

Figure 6. Classification of pyroxenes according to IMA rules (Morimoto et al., 1988).

Figure 7. Classification diagrams for a) feldspar; b) brown mica (Deer et al., 1966); c) amphibole (Leake et al.,1997) and d) glasses (TAS, Le Bas et al., 1986) of the studied Early Miocene volcanic rocks from northwesternSardinia.



Tabl

e 4.

Rep

rese

ntat

ive

anal

yses

for f

elds

pars

of E

arly

Mio

cene

rock

s fro

m n

orth

wes

tern

Sar

dini

a.

Bos

a - A

lghe

ro d

istr

ict

Mul

argi

a-M

acom

ero

ttana

gra

ben

dist

rict

Rock

type

HA

BH

AB

BA

RR

RR

RR

RR

RR

R

MA

RA

23M

AR

A23

MA

RA

33M

A29

BM

A35

MA

RA

15M

AR

A27

MA

RA

38M

A62

AM

A62

AM

A62

AM

A67

MA

67M

A67

rimgm

core

core

core

gmm

antle

core

core

man

tlegm

core

gmgm

SiO

257

.72

63.7

144

.01

56.3

757

.91

67.7

757

.37

70.0

957

.78

58.1

866

.84

60.0

459

.65

66.9

1A

l 2O3

26.0

722

.79

34.0

727

.15

26.1

916

.72

26.2

317

.64

26.6

26.1

519

.32

24.6

124

.83

17.8

6Fe

O0.

410.

370.

450.

410.

330.

350.

400.

280.

521.

32C

aO9.

114.

6319

.15

10.1

79.

030.

229.

282.

619.

378.

90.

866.

857.

292.

57N

a 2O

6.12

8.55

0.66

5.48

6.12

0.12

5.65

6.10

6.06

6.49

5.09

7.11

6.92

6.5

K2O

0.47

0.74

0.00

0.38

0.35

14.1

90.

403.

260.

580.

578.

890.

650.

692.

2sum

99.9

100.8

98.3

99.9

99.9

99.4

99.3

99.9

100.4

100.3

101.0

99.3

99.9

97.4

An%

4422

9450

441

4615

4542

433

3515

Ab%

5374

648

541

5163

5255

4563

6169

Or%

34

02

297

222

33

514

416

log

udor

o di

stri

ctA

nglo

na d

istr

ict

Rock

type

BA

BA

BA

BA

RR

RR

AA

DD

DD

MA

12M

A12

MA

12M

A12

MA

13A

MA

37M

A37

MA

71M

A82

MA

82M

A84

BM

A84

BM

A84

BM

A84

Brim

core

gmgm

core

rimrim

core

rimrim

core

man

tleco

reco

reSi

O2

46.2

246

.02

49.9

654

.79

55.5

362

.40

65.3

644

.847

.53

49.6

759

.457

.02

57.2

659

.15

Al 2O

333

.91

33.6

229

.85

27.2

827

.59

23.4

018

.60

35.8

632

.36

30.7

625

.19

26.9

125

.84

25.2

3Fe

O0.

470.

481.

080.

390.

350.

240.

150.

490.

650.

80.

860.

31C

aO17

.60

17.6

214

.31

10.8

310

.88

5.66

0.18

19.8

916

.65

14.4

67.

539.

458.

788.

03N

a 2O

1.28

1.33

3.10

5.07

5.11

7.50

3.03

0.68

2.2

3.24

6.49

5.91

6.60

6.78

K2O

0.05

0.07

0.22

0.57

0.40

0.93

11.7

81.

240.

810.

370.

61sum

99.5

99.1

98.5

98.9

99.9

100.1

99.1

101.7

99.4

98.9

99.9

100.1

99.7

100.1

An%

8888

7152

5328

194

8171

3645

4138

Ab%

1212

2844

4567

286

1929

5751

5658

Or%

00

13

25

710

00

75

23

gm =

gro

undm

ass A

n% =

ano

rthite

mol

.% A

b% =

alb

ite m

ol.%

Or%

= o

rthoc

lase

mol

.%.



1994; Lopez-Escobar et al., 1997; Schiano et al.,2003). Dostal et al. (1982) were the first to dealwith the genesis of northwestern Sardinia HABs,arguing for their non-primitive nature (mainly onthe basis of their low Cr and Ni concentrationsand Mg# values), but still considering them as alikely parental magma for the rocks of thecalcalkaline suite. Morra et al. (1997) pointedthat the Montresta HMBs represent the mostprimitive magma of northwestern Sardinia (i.e.,Mg# between 0.64 and 0.71, Cr ~ 294-737 ppmand Ni ~ 47-205 ppm), and the neighbouringHABs are related to HMBs through closed-system fractional crystallization processes. Thechemical composition of the Sardinia HABs(e.g., Al2O3 > 17.8 wt.%, MgO < 7.2 wt.%, Mg#< 0.60, low Cr and Ni) can be similarlyinterpreted as unlikely primary meltcompositions, but, rather, as melts derived froma more primitive HMB-like magmas. As aconsequence, the HABs cannot be used to obtaininformation on mantle source characteristics. Onthe other hand, their possible parental linkagewith the evolved rocks should be investigated inorder to shed light on the low-pressure evolutionprocesses in the area.

On the whole, major- and trace-elementcompositional trends, as well as mineralcompositions, are quite coherent among thevarious districts, suggesting, at least qualitatively,a cogenetic relationship. Fractional crystallizationof the mineral phases observed in thin sectioncan, indeed, explain the gross features observedin Figures 3 and 4. The decrease of Al2O3, MgOand CaO with increasing SiO2 is compatible withremoval of plagioclase and clinopyroxene plusolivine in the less evolved rocks. Orthopyroxeneand alkali feldspar join the previous phases in theintermediate and evolved melts. This isparticularly evident by the decrease of Rb and Baat SiO2 ~ 70 wt.%, a feature that suggests theonset of alkali feldspar fractionation roughly atthe dacite-rhyolite transition. Similarly, thedecrease of Zr and Y around SiO2 ~ 70 wt.%

marks the onset of zircon fractionation. Scatter ofdata can be due to local differences between thevarious districts, the porphyritic character and/orto post-emplacement processes that may havemodified mobile element contents. In addition,some role should have been also played by open-system differentiation processes, which areknown to have acted in many other LEMMSardinian districts (e.g., Narcao, Brotzu et al.,1997b; Sarroch, Conte, 1997; Sindia, Lonis et al.,1997; Montresta, Morra et al., 1997;Sant’Antioco, Conte et al., 2010; Figure 1b).

Quantitative models of fractionalcrystallization processes were tested by means ofmass balance calculations, performed using theStormer and Nicholls (1978) method. The resultsare summarized in Table 8. The transition from aHMB-type primitive magma (sample KB13;Morra et al., 1997) to the HABs of the Bosa-Alghero (MARA21) and Logudoro districts(MA72C) was tested, yielding satisfactory results(ΣR2 < 0.3 for both; ΣR2 = sum of squareresiduals) involving ~ 30-36% removal of agabbroic assemblage made of 64% clinopyroxene(Ca35Mg46Fe19), 22.8% plagioclase (An91Ab9O0)and 13.2% olivine (Fo87) for the Bosa-Algherodistrict, and of 60% clinopyroxene (Ca45Mg42Fe13), 13.2% plagioclase (An91Ab9O0) and 26.5%olivine (Fo71) for the Logudoro district.

In the Bosa-Alghero district, the transitionfrom HAB to basaltic andesite is modelledassuming ~ 43% of removal of a cumulate madeof ~ 45% plagioclase (An90Ab9Or1), ~ 32%clinopyroxene (Ca41Mg43Fe16), ~ 13% olivine(Fo74) and ~ 9% magnetite (Table 8). Thetransition from basaltic andesite to andesite isobtained after removal of 59.4% of a cumulatemade of ~ 78% plagioclase (An54Ab43Or3), ~ 12% clinopyroxene (Ca39Mg45 Fe16) and ~ 10% magnetite. The evolution from andesiteto dacite was obtained after the removal of ~ 33% of a cumulate made of ~ 62% plagioclase(An68Ab30Or2), ~ 29% orthopyroxene (Ca4Mg50Fe46) and ~ 9% clinopyroxene (Ca43 Mg42Fe15).




Finally, the transition from dacite to a rhyolitewas modelled with removal 55.5% of a solidassemblage made of plagioclase (~ 35%, An45Ab52Or3), alkali feldspar (~ 31%, An6 Ab40Or54),clinopyroxene (~ 17%, Ca39Mg43Fe18),orthopyroxene (~ 15%, Ca3Mg70Fe27) andmagnetite (~ 2%) (Table 8).

As for the Logudoro district, the transition fromHAB to basaltic andesite is modelled withremoval of 50.7% of a cumulate made of ~ 63%plagioclase (An67Ab31Or2), ~ 21% clinopyroxene(Ca40Mg43Fe17), ~ 7% magnetite and ~ 8%olivine (Fo65). The transition from basalticandesite to rhyolite was modelled with removalof ~ 74% of solid assemblage made of ~ 62%plagioclase (An69Ab30Or1), ~ 15% clinopyroxene(Ca30Mg43 Fe27), ~ 10% alkali feldspar (An4Ab27Or69), ~ 11% magnetite and ~ 1% apatite.

The transition from andesite to dacite in theAnglona district was obtained after removal

61.2% of a solid assemblage made of ~ 52%plagioclase (An58Ab39Or3), ~ 29% clinopyroxene(Ca40Mg39Fe21), ~ 11% alkali feldspar (An4Ab27Or69) and 8% magnetite. The evolution fromdacite to rhyolite is obtained removing 44.7% ofa solid assemblage made of ~ 54% plagioclase(An43Ab54Or3), ~ 24% alkali feldspar (An2Ab1Or97), ~ 17% orthopyroxene (Ca4Mg64Fe32) and ~ 6% magnetite.

Finally, the evolution from the dacitic torhyolitic compositions in the Ottana grabendistrict is modelled by 20.3% removal ofplagioclase (52%, An33Ab63Or4), alkali feldspar(22%, An2Ab27Or71), clinopyroxene (13.3%,Ca43Mg30Fe27), magnetite (9.8%) andorthopyroxene (2.9%, Ca3Mg55Fe42). Theevolution from two slightly different rhyoliticcomposition in the Mulargia-Macomer district isobtained through 24.8% of a solid assemblagemade of 55.5% plagioclase (An35Ab60Or5),

Table 5. Representative analyses for olivine (ol), magnetite (mt) and ilmenite (ilm) crystals of Early Miocenerocks from northwestern Sardinia.

Bosa - Alghero district Anglona district

Rock type HAB HAB BA BA BA BA R R A D D

MARA25 MARA25 MARA 33 MARA 33 MARA34 MARA34 MARA22 MA29B MA82 MA84B MA3ol core ol rim ol core ol rim ol ol mt ilm mt mt ilm

SiO2 35.88 35.61 37.81 38.00 35.07 35.19 0.06 0.45TiO2 17.81 46.46 11.59 7.01 41.01Al2O3 1.94 0.13 3.79 4.25 0.98Fe2O3 30.06 9.93 41.77 50.47 16.99FeO 38.08 39.06 26.39 26.52 37.46 37.68 42.81 37.63 38.54 36.13 32.92MnO 0.80 0.56 0.70 0.47 0.96 0.96 4.07 0.76 0.54 1.91MgO 26.17 25.24 36.29 35.96 24.38 24.59 1.95 2.12 0.92 1.44CaO 0.32 0.34 0.16 0.38 0.24 0.16

sum 101.3 100.8 101.3 101.3 98.1 98.6 96.7 96.9 97.8 99.3 95.7

Fo 55 54 71 71 54 54ulvöspinel 0.35 0.22 0.13ilmenite 0.90 0.81

Fo% = forsterite mol%



30.1% alkali feldspar (An4Ab45Or51), 8.5%orthopyroxene (Ca3Mg70Fe27) and 5.9%magnetite.

Further informations on the evolutionaryprocesses of Early Miocene rocks fromnorthwestern Sardinia can be obtained from theestimation of T and fO2 based on the mineralchemistry data (Table 9). The olivine-liquidgeothermometer (Putirka, 2008) applied toolivine in the basalts and basaltic andesites fromthe Bosa-Alghero district gave temperatures inthe range of 1071-1140 °C and of 1114 °C forthe olivine in the basalts from the Logudorodistrict. The clinopyroxene-orthopyroxenegeothermometer (Putirka, 2008) applied for pairsin equilibrium with the host rocks (i.e.,Fe/MgKd

cpx-liq = 0.27 ± 0.03; Putirka, 2008; Figure8a), yielded temperatures between 931 and 964°C for the Bosa-Alghero rhyolites, between 958and 979 °C for the Anglona andesites and

between 934 and 1010 °C for the Anglonadacites.

Temperature and oxygen fugacity values forthe Logudoro rhyolites were estimated onamphibole minerals following the method ofRidolfi et al. (2010). Temperature and oxygenfugacity (logfO2) range from 821 to 923 °C andfrom -10.7 to -13.6 log units, respectively.Similar T-fO2 estimates were obtained by usingcoexisting magnetite-ilmenite pairs for samplesfrom the Bosa-Alghero, Logudoro and Mulargia-Macomer districts. The Bosa-Alghero rhyolitesgave temperature estimates spanning from 872to 1035 °C and oxygen fugacity values rangingfrom -12.8 to -10.4 log units. As for theLogudoro rocks, basaltic andesites gave T valuesof 853-918 °C and oxygen fugacity of -12 to -11.1 log units, whereas rhyolites yieldedtemperature estimates spanning from 777 to841.3 °C and oxygen fugacity from -14.8 to


Table 5. Continued...

logudoro districts Mulargia-Macomerdistrict

ottana grabendistrict

Rock type HAB BA BA R R R R R R R R

MA38 MA72A MA72A MA13A MA13A MA13A MA50 MA62A MA62A MA67 MA67ol mt ilm mt ilm ilm mt ilm mt mt mt

SiO2 36.88 0.32 0.67 0.19 0.3 0.56TiO2 8.52 40.69 10.59 48.29 47.15 9.35 44.87 6.93 21.09 19.71Al2O3 1.49 1.76 1.94 0.17 0.17 1.43 0.87 1.35 1.2 1.44Fe2O3 46.99 16.49 44.73 8.20 10.60 47.18 6.90 53.93 24.89 24.61FeO 29.28 36.62 30.77 40.49 38.71 36.79 37.53 39.95 37.68 48.89 46.65MnO 0.46 0.01 0.77 1.13 3.29 0.7 1.07 1.08 0.89MgO 31.91 0.93 3.27 0.49 2.14 1.49 0.46CaO 0.37

sum 98.9 94.9 93.0 99.7 98.8 99.8 96.6 94.2 99.9 97.2 93.3

Fo 66ulvöspinel 0.17 0.20 0.18 0.13 0.41 0.40ilmenite 0.81 0.91 0.89 0.90

Fo% = forsterite mol%



Bos

a-A

lghe

ro d

istr

ict

log

udor

o di

stri

ct

Rock

type

RR

RR

RR

RR

RR

RR

RR

MA

RA

15M

AR

A38

MA

RA

38M

AR

A38

MA

37M

A37

MA

13A

MA

50M

A48

BM

A71

MA

13A

MA

13B

MA

37M

A37

mic

am

ica

mic

am

ica

mic

am

ica

mic

am

ica

mic

am

ica

amph

amph

amph

amph

SiO

236

.53

36.9

635

.45

36.4

834

.84

34.4

735

.72

36.2

037

.11

35.2

142

.18

43.6

542

.943

.38

TiO

24.

953.

914.

269

5.27

4.15

3.76

4.52

5.70

3.55

3.53

2.73

2.44

1.45

2.35

Al 2O

312

.38

15.1

013

.583

13.8

913

.43

12.6

413

.13

13.4

113

.15

13.7

69.

949.

569.

4610

.78

FeO

22.4

120

.82

23.3

20.7

124

.99

24.4

420

.25

21.1

523

.29

25.8

813

.82

16.8

722

.74

13.2

2M

nO0.

260.

240.

310.

220.

580.

470.

350.

570.

500.

480.

311.

170.

43M

gO9.

249.

008.

663

10.3

57.

557.

0311

.37

10.1

77.

408.

2313

11.3

97.

8113

.57

BaO

0.76

0.43

0.34

80.

540.

350.

380.

41C

aO0.

070.

160.

098

0.01

0.09

0.09

0.05

11.1

511

.46

10.2

711

.15

Na 2

O0.

470.

460.

416

0.68

0.42

0.44

0.52

0.55

0.58

1.95

1.89

1.79

2.3

K2O

8.17

7.76

7.96

68.

608.

417.

888.

738.

788.

078.

710.

961.

050.

850.

73F

0.55

1.01

2.12

0.75

0.37

0.47

0.53

0.22

0.3

0.52

sum

95.8

95.9

96.5

97.5

95.2

92.1

95.6

96.0

93.1

96.4

96.4

98.6

98.7

98.4

Mg#

0.43

0.44

0.42

0.48

0.35

0.34

0.52

0.46

0.36

0.36

0.63

0.55

0.38

0.65

Si5.

624

5.55

25.

327

5.45

75.

496

5.60

05.

473

5.54

45.

876

5.51

36.

375

6.51

96.

573

6.37

7IV

Al

2.24

62.

448

2.40

62.

448

2.49

62.

400

2.37

12.

420

2.12

42.

487

1.62

51.

481

1.42

71.

623

Fe3+

0.13

00.

268

0.09

40.

008

0.15

60.

036

VI A

l0.

225

0.02

00.

330

0.05

20.

146

0.20

20.

282

0.24

4Ti

0.57

30.

442

0.48

20.

593

0.49

20.

460

0.52

10.

656

0.42

30.

416

0.31

00.

274

0.16

70.

260

Fe3+

Fe2+

2.75

52.

616

2.65

82.

496

3.28

83.

320

2.43

82.

673

3.08

43.

389

1.74

72.

107

2.91

41.

625

Mn

0.03

40.

030

0.03

90.

027

0.07

70.

065

0.04

50.

076

0.06

60.

061

0.03

90.

152

0.05

4M

g2.

119

2.01

51.

940

2.30

71.

775

1.70

22.

596

2.32

11.

746

1.92

12.

929

2.53

61.

784

2.97

4

Ba

0.04

50.

025

0.02

00.

031

0.02

20.

024

0.02

5C

a0.

012

0.02

60.

016

0.00

10.

016

0.01

50.

008

1.80

51.

834

1.68

61.

756

Na

0.14

00.

135

0.12

10.

197

0.13

00.

139

0.15

40.

163

0.17

60.

571

0.54

70.

532

0.65

5K

1.60

51.

486

1.52

71.

641

1.69

11.

633

1.70

61.

715

1.63

01.

740

0.18

50.

200

0.16

60.

137

OH

3.73

23.

519

2.99

33.

648

3.81

63.

761

3.74

34

44

1.89

52

1.85

51.

758

F0.

268

0.48

11.

007

0.35

20.

184

0.23

90.

257

0.10

50.

145

0.24

2

Mg#

= M

g/(M

g+Fe

2+).

Tabl

e 6.

Rep

rese

ntat

ive

anal

yses

for m

ica

and

amph

ibol

e (a

mph

) cry

stal

s of E

arly

Mio

cene

rock

s fro

m n

orth

wes

tern

Sar

dini

a. A

lso

repo

rted

are

site

occu

panc

ies (

stru

ctur

al fo

rmul

as c

alcu

late

d on

22

oxyg

ens f

or m

ica,

23

for a

mph

ibol

es).


Mineral compositions and magmatic evolution... 537Periodico di Mineralogia (2011), 80, 3 (Spec. Issue), 517-545

Tabl

e 7.

Rep

rese

ntat

ive

glas

s ana

lyse

s for

Ear

ly M

ioce

ne ro

cks f

rom

nor

thw

este

rn S

ardi

nia.

Bos

a - A

lghe

ro d

istr

ict

Rock

type

RR

RR

RR

RR

RR

RR

RR

MA

35M

A35

MA

35M

A35

MA

29B

MA

29B

MA

29B

MA

29B

MA

RA

38M

AR

A38

MA

RA

12

MA

RA

12

MA

RA

12

MA

RA

12Si

O2

76.2

476

.42

74.3

875

.82

74.5

075

.34

72.6

573

.01

81.2

182

.83

76.6

176

.78

72.8

175

.82

TiO

20.

150.

150.

040.

130.

350.

090.

410.

090.

130.

000.

190.

160.

310.

20A

l 2O3

13.6

013

.68

14.2

113

.54

14.3

813

.57

14.1

514

.80

9.25

8.83

12.9

412

.61

15.6

013

.79

FeO

1.65

1.44

2.11

1.58

0.83

0.27

2.41

0.49

3.08

0.16

0.43

0.39

0.82

0.78

MnO

0.07

0.06

0.00

0.08

0.00

0.07

0.10

0.00

0.02

0.00

0.00

0.00

0.00

0.00

MgO

0.17

0.14

0.24

0.18

0.01

0.00

0.44

0.03

0.05

0.00

0.00

0.00

0.01

0.03

CaO

1.29

1.21

1.42

1.15

1.10

0.64

1.93

0.56

1.35

0.42

0.72

0.62

2.03

1.41

Na 2

O2.

452.

412.

602.

373.

253.

373.

633.

342.

931.

953.

022.

794.

273.

90K

2O4.

394.

494.

985.

105.

586.

594.

287.

651.

975.

826.

066.

534.

143.

86P 2

O5

0.00

0.00

0.02

0.04

0.00

0.06

0.00

0.03

0.00

0.00

0.03

0.12

0.02

0.22

sum

100

100

100

100

100

100

100

100

100

100

100

100

100

100

Ang

lona

dis

tric

tl

ogud

oro

dist

rict

Rock

type

AD

DR

RR

RR

RR

RR

RR

MA

82M

A84

BM

A84

BM

A13

AM

A13

AM

A13

AM

A13

AM

A13

AM

A71

M

A71

M

A71

M

A37

M

A37

M

A37

SiO

253

.15

70.3

169

.18

75.1

775

.93

76.3

376

.35

75.3

378

.47

78.3

878

.35

77.4

877

.63

77.0

6Ti

O2

0.00

0.47

0.54

0.12

0.14

0.00

0.14

0.20

0.00

0.00

0.00

0.04

0.00

0.18

Al 2O

328

.53

16.0

315

.62

13.3

313

.66

13.8

513

.63

13.7

712

.68

12.3

712

.59

12.6

212

.57

12.8

0Fe

O1.

102.

303.

041.

100.

740.

600.

790.

710.

540.

650.

421.

060.

850.

96M

nO0.

000.

190.

050.

070.

070.

000.

070.

060.

000.

000.

000.

160.

000.

03M

gO0.

000.

100.

460.

120.

000.

000.

000.

000.

000.

000.

010.

060.

020.

03C

aO12

.25

1.11

1.45

0.71

0.78

0.75

0.79

0.75

0.83

0.96

0.89

0.70

0.66

0.77

Na 2

O4.

564.

433.

912.

992.

862.

762.

492.

842.

442.

582.

432.

712.

992.

91K

2O0.

425.

065.

686.

415.

685.

715.

736.

285.

045.

065.

155.

055.

115.

18P 2

O5

0.00

0.00

0.07

0.00

0.15

0.00

0.00

0.05

0.00

0.00

0.16

0.12

0.18

0.07

sum

100

100

100

100

100

100

100

100

100

100

100

100

100

100



subtra

cted

solid

usre

sidua

l liqui

d

from

tool

plcp

xop

xm

tkf

sap

%f

∑r

2

Bos

a-A

lghe

ro d

istr

ict

KB

13(H

MB

, Mor

ra e

t al.,

199

7)M

AR

A21

(HA

B)

13.2

22.8

64.0

71.2

0.29

log

udor

o di

stri

ctM

A72

C (H

AB

)26

.513

.260

.364

.00.

30

Bos

a-A

lghe

ro d

istr

ict

MA

RA

21 (H

AB

)M

AR

A34

(BA

)13

.245

.432

.49.

056

.70.

49

MA

RA

34 (B

A)

MA

RA

36 (A

)77

.912

.39.

840

.60.

32

MA

RA

36 (A

)M

AR

A13

(D)

62.1

8.6

29.3

67.5

0.49

MA

RA

13 (D

)M

AR

A38

(R)

34.7

17.0

14.8

1.9

31.5

44.5

0.18

Ang

lona

dis

tric

tM

A82

(A)

MA

84B

(D)

51.9

28.8

8.0

11.3

38.8

0.35

MA

84B

(D)

MA

6A (R

)54

.216

.75.

623

.655

.30.

47

log

udor

o di

stri

ctM

A72

C (H

AB

)M

A72

A (B

A)

8.3

63.3

21.1

7.3

49.3

0.35

MA

72A

(BA

)M

A36

(R)

61.9

15.5

10.2

11.1

1.3

25.8

0.38

Mul

argi

a-M

acom

erdi

stri

ctM

A65

(R)

MA

62A

(R)

55.5

8.5

5.9

30.1

75.2

0.10

otta

na g

rabe

n di

stri

ctM

A69

(D)

MA

67 (R

)52

.013

.32.

99.

822

.079

.70.

05

HM

B =

hig

h m

agne

sium

bas

alt H

AB

= h

igh

alum

ina

basa

lt B

A =

bas

altic

and

esite

A =

and

esite

D =

dac

ite R

= rh

yolit

e %

f = w

t.% o

f res

idua

l liq

uid

ΣR2

= su

m o

fsq

uare

d re

sidu

als o

l = w

t.% o

f sub

tract

ed o

livin

e pl

= w

t.% o

f sub

tract

ed p

lagi

ocla

se c

px =

wt.%

of s

ubtra

cted

clin

opyr

oxen

e m

t = w

t.% o

f sub

tract

ed m

agne

tite

ilm =

wt.%

of s

ubtra

cted

ilm

enite

opx

= w

t.% o

f sub

tract

ed o

rthop

yrox

ene k

fs w

t.% o

f sub

tract

ed al

kali

feld

spar

ap =

wt.%

of s

ubtra

cted

apat

ite. S

ee te

xt fo

r fur

ther

expl

anat

ions

.

Tabl

e 8.

Res

ults

of m

ass b

alan

ce c

alcu

latio

ns fo

r Ear

ly M

ioce

ne d

istri

cts f

rom

nor

thw

este

rn S

ardi

nia.



-12.9 log units. The rhyolites from the Mulargia-Macomer district gave temperature estimates inthe range of 717-902 °C and oxygen fugacityvalues between -16.2 and -13 log units. Somemagnetite-ilmenite pairs giving lowertemperatures (< 800 °C), coupled with lowoxygen fugacity values (up to -16.2 log units),probably represent re-equilibration effects, ratherthan original crystallization conditions. The samecan be held as true for the temperature estimatesobtained for the basaltic andesites from theLogudoro district, which seem to be too low forsuch relatively SiO2-poor magmas.

The results of the temperature and oxygenfugacity calculations presented above show thatmost of the data for the Bosa-Alghero andMulargia-Macomer districts plot close to orslightly above the Quartz-Fayalite-Magnetite(QFM) buffer, whereas the Logudoro rocks plotclose to the Nickel-Nickel Oxide (Ni-NiO)buffer (Figure 8b).

The estimated temperatures are consistent witha genetic linkage among the various rocks. Anegative correlation between temperature andSiO2 can be observed, a feature compatible withthe interpretation as a coherent liquid line ofdescent for each district. Some spread of valuesis present, and this can be due to different causes,including the use of T and fO2 estimates basedon different minerals or mineral pairs. However,we are confident that this does not significantlyaffect the reliability of the proposed model formagma evolution.

The oxygen fugacity and temperature dataobtained on minerals were then used to fix thestarting conditions for the application of theMELTS algorithm (Ghiorso and Sack, 1995) tothe evolution of the studied Early Miocene rocks.The best-fit results of MELTS simulations arereported in Figure 3. The starting conditions arecharacterized by 1 kbar pressure, 0.2 wt.% ofH2O and QFM oxygen buffer for the Bosa-Alghero rocks, Ni-NiO oxygen buffer for theLogudoro and Anglona rocks. Such pressure

values, approximately correspond to a depth of3 km, MELTS simulations performed usinghigher pressure values (2-5 kbar) resulted in thefractionation of a mineral assemblage which didnot match the observed parageneses (e.g., noolivine). It is also to note that the the stronglyevolved composition of the ignimbritic productsmay have been reached only after polybaricevolution.

The obtained best-fit trends (Figure 3) assumethe following fractionating assemblages:

- Bosa-Alghero district: 79.3% removal ofolivine (0.6% Fo75Fa25), clinopyroxene (22%Di81Hd19 to Di50Hd50), plagioclase (40.8%An77Ab23 to An24Ab76), alkali feldspar (1.5%Or), spinel (14.3%) and apatite (0.2%) for thetransition from HAB to “rhyolitic” composition(~ 74 SiO2 wt.%);

- Logudoro district: 73% removal of olivine(2.9% Fo67Fa33), clinopyroxene (11.9% Di78Hd22to Di50Hd50), plagioclase (44.8% An67Ab33 toAn26Ab74), alkali feldspar (0.9% Or), spinel(12.5%) and apatite (0.1%) for the transitionfrom HAB to “rhyolitic” composition (~ 76 SiO2wt.%);

- Anglona district: 69.5% removal ofclinopyroxene (13.5% Di63Hd37 to Di51Hd49),orthopyroxene (4.1% En51Fs49 to En47Fs53),plagioclase (40.1% An54Ab46 to An40Ab60),alkali feldspar (4.4% Or) and spinel (7.4%) forthe transition from andesite to “rhyolitic”composition (~ 76 SiO2 wt.%).

In summary, the fractionating assemblageobtained through MELTS (olivine ±clinopyroxene ± orthopyroxene ± plagioclase ±alkali feldspar ± spinel ± apatite) are generallyconsistent with petrography and mass balancecalculations reported above, even though thedifficulties in defining the exact startingconditions suggest that some parameters (e.g., Pand H2O contents) needs to be better constrained.The estimated evolutionary trends obtained byMELTS simulations are in quite good agreementwith those shown by the studied Sardinian rocks




District Rock type Sample T (°c) logfo2

magnetite-ilmenite pairs(Il-MAT lepage, 2003)

Bosa-Alghero districtR MA29B 872.3 -12.8R MA29B 1035.1 -10.4

Logudoro district

BA MA72A 918.4 -11.1BA MA72A 853.0 -12.0R MA13A 828.7 -13.3R MA13A 816.9 -13.5R MA13A 786.8 -14.6R MA13A 776.7 -14.8R MA13A 841.3 -12.9R MA13A 828.9 -13.1

Mulargia-Macomer districtR MA62A 866.2 -13.6R MA62A 839.4 -14.0R MA62A 901.9 -13.0R MA62A 716.9 -16.2

Amphibole(ridolfi et al., 2010) Logudoro district

R MA13A 856.3 -12.3R MA13A 896.6 -11.4R MA13A 919.0 -10.9R MA13A 894.8 -11.6R MA13B 883.2 -12.0R MA37 832.5 -13.5R MA37 922.7 -10.7R MA37 821.2 -13.6R MA37 890.0 -11.8

olivine-liquidgeothermometer(putirka, 2008)

Bosa-Alghero district

HAB MARA25 1109.8HAB MARA25 1109.8BA MARA33 1139.9BA MARA33 1139.9BA MARA33 1139.9BA MARA33 1139.9BA MARA34 1070.7BA MARA34 1070.7

Logudoro district HAB MA38 1113.7HAB MA38 1113.7

clinopyroxene-orthopyroxenegeothermometer(putirka, 2008)

Bosa-Alghero districtR MA29B 931.0R MA29B 953.1R MA29B 941.8R MA29B 964.3

Anglona district

A MA82 979.5A MA82 957.9D MA84B 967.2D MA84B 967.7D MA84B 983.2D MA84B 951.0D MA84B 963.3D MA84B 1010.1D MA84B 934.3D MA84B 949.3D MA84B 949.8D MA84B 964.6D MA84B 933.8D MA84B 945.6D MA84B 990.3

HAB = high alumina basalt BA = basaltic andesite D = dacite R = rhyolite.

Table 9. Temperature (T, in °C) and oxygen fugacity (logfO2) estimates obtained from different methods for thestudied Early Miocene volcanic rocks from northwestern Sardinia.



for MgO, K2O, Al2O3 (less evidently for theBosa-Alghero district) and partly for the Fe2O3t(except for an anomalous peak at ~ 58-61 wt.%SiO2 for the Bosa-Alghero and Logudorodistricts; Figure 3). On the other hand, the verysteep trend obtained for TiO2 is likely due to thefact that MELTS assumes spinel crystallizationin the initial evolutionary steps. The trends forCaO are remarkably good for the compositionsup to 60 wt.% SiO2, whereas discrepancy existsfor the most evolved rocks (which show CaOvalues close to zero, while MELTS simulationsnever reach concentrations below 2 wt.%).

In conclusion, the postulated genetic linkagebetween mafic and silicic rocks apparently rulesout the early hypothesis of a crustal anatecticorigin for the silicic Early Miocene rocks ofSardinia (e.g., Coulon et al., 1978; Beccaluva etal., 1987). Indeed, as previously observed byMorra et al. (1994) for the Sulcis area rocks, thewidespread occurrence of the evolved rockswould have required unrealistically high degreesof partial melting. Such considerations, along

with the overall reliability of the abovemodelling, make the crustal anatexis modelunlikely, even though a more detailed treatmentof the matter (with isotopic data, geochemicalmodelling of partial melting of a crustalmaterials, and so on) will possibly give muchstronger constraints. At the same time, the highamount of solid required to fractionate anoriginal basaltic melt to reach rhyoliticcompositions (~ 95%) remains hard to explain.Indeed, the huge amounts of dacitic and rhyoliticmelts, if interpreted as the results of fractionalcrystallization would require huge amounts ofcumulates (20,000 to 50,000 km3) at shallowdepths, not easy to be detected by geophysicalstudies. Also, open-system AFC-type processesmust be appropriately taken into account, as theirinterplay seems likely given the local recognitionof evidences of textural (e.g., crystals withcorroded rims) and chemical (e.g., high Caplagioclase in dacites and rhyolites) disequilibriumfeatures.

The obtained results are preliminary basis for


Figure 8. a) 100*Mg#Pyroxene vs. 100*Mg#WR diagram for Fe-Mg partitioning [Kd = (Fe/Mg)px/(Fe/Mg)WR] between

pyroxene and whole rock of the studied Early Miocene rocks from northwestern Sardinia. b) logfO2 vs. T (in°C) diagram for magnetite-ilmenite equilibrium pairs and amphiboles of the studied Early Miocene rocks fromnorthwestern Sardinia.



further mineralogical, petrographic, chemicaland isotopic investigations, which could help toshed new light on the petrogenesis of the silicicrocks in northwestern Sardinia , as well as on thewhole LEMM magmatic cycle of Sardinia.

Acknowledgements

This paper is dedicated to the memory of prof.Enrico Franco, an incomparable professor ofMineralogy who forged generations of students andresearchers with unfailing passion. A special thankgoes to Prof. Pietro Brotzu, who taught andtransmitted us his love and devotion to Sardinia.Thanks to the journal’s scientific editor AntonioGianfagna for the careful handling of the manuscript.The associated editors Maurizio de’ Gennaro, MariaRosaria Ghiara and Giuseppina Balassone (Napoli)are also thanked. The reviewers Michele Mattioli(Urbino University) and Aida Maria Conte (CNR,Rome) provided very constructive comments andsuggestions. Marcello Serracino and Roberto de’Gennaro provided skilled help in the microprobework. This work has been supported by researchgrants to Vincenzo Morra (MIUR-PRIN 2008,research grant#: 2008HMHYFP_003) and MicheleLustrino (University La Sapienza-AST 2008, 2009and MIUR-PRIN 2008, research grant#:2008HMHYFP_005).

references

Araña V., Barberi F. and Santacroce R. (1974) - Somedata on the comendite type area of S. Pietro and S.Antioco islands, Sardinia. Bulletin of Volcanology,38, 725-736.

Beccaluva L., Civetta L., Macciotta G. and Ricci C.A.(1985) - Geochronology in Sardinia: results andproblems. Rendiconti Società Italiana diMineralogia e Petrologia, 40, 57-72.

Beccaluva L., Brotzu P., Macciotta G., Morbidelli L.,Serri G. and Traversa G. (1987) - Cainozoictectono-magmatic evolution and inferred mantlesources in the Sardo-Tyrrhenian area. In: Thelithosphere in Italy: Advances in Earth ScienceResearch. Accademia Nazionale dei Lincei, Roma,229-248.

Brandano M., Brilli M., Corda L. and Lustrino M.

(2010) - Miocene C-isotope signature from thecentral Apennine successions (Italy): Monterey vs.regional controlling factors. Terra Nova, 22, 125-130.

Brandano M. and Policicchio G. (2011) - Strontiumstratigraphy of the Burdigalian transgression in theWestern Mediterranean. Lethaia, in press, doi:10.1111/j.1502-3931.2011.00285.x.

Brotzu P., Lonis R., Melluso L., Morbidelli L.,Traversa G. and Franciosi L. (1997a) - Petrologyand evolution of calcalkaline magmas from theArcuentu volcanic complex (SW Sardinia, Italy).Periodico di Mineralogia, 66, 151-184.

Brotzu P., Callegari E., Morra V. and Ruffini R.(1997b) - The orogenic basalt-andesite suites fromthe Tertiary volcanic complex of Narcao, SW-Sardinia (Italy): petrology, geochemistry andSr-isotope characteristics. Periodico diMineralogia, 66, 101-150.

Carminati E., Wortel M.J.R., Spakman W. andSabadini R. (1998) - The role of slab detachmentprocesses in the opening of the Western-CentralMediterranean basins: some geological andgeophysical evidence. Earth and Planetary ScienceLetters, 160, 651-665.

Carminati E., Lustrino M., Cuffaro M. and DoglioniC. (2010) - Tectonics, magmatism and geodynamicsof Italy: what we know and what we imagine. In:M. Beltrando, A. Peccerillo, M. Mattei, S. Conticelliand C. Doglioni (Eds.) The geology of Italy:tectonics and life along plate margins. Journal ofthe Virtual Explorer, 36, paper 9, doi: 10.3809/jvirtex.2010.00226.

Cherchi A., Mancin N., Montadert L., Murru M., PutzuM.T., Schiavinotto F. and Verrubbi V. (2008) - Thestratigraphic response to the Oligo-Mioceneextension in the western Mediterranean fromobservations on the Sardinia graben system (Italy).Bulletin de la Societé Geologique de France, 179,267-287.

Conte A.M. (1997) - Petrology and geochemistry ofTertiary calcalkaline magmatic rocks from theSarroch district (Sardinia, Italy). Periodico diMineralogia, 66, 63-100.

Conte A.M., Palladino D.M., Perinelli C. and ArgentiE. (2010) - Petrogenesis of the High-AluminaBasalt-Andesite suite from Sant’Antioco Island,SW Sardinia, Italy. Periodico di Mineralogia, 79,27-55.




Coulon C., Baque L. and Dupuy C. (1973) - Lesandésites cénozoïques et les laves associées enSardaigne Nord-Occidentale (Provinces duLogudoro et du Bosano) - Caractèresminéralogiques et chimiques. Contributions toMineralogy and Petrology, 42, 123-139.

Coulon C., Demant A. and Bellon H. (1974) -Premiéres datations par la methode K-Ar dequelques laves cènozoiques et quaternaires deSardaigne nord-occidentale. Tectonophysics, 22, 59-82.

Coulon C. (1977) - Le volcanisme calco-alcalinecénozoique de la Sardaigne (Italie): pétrographie,géochimie et genése des lavas andesitiques et designimbrites. Signification géodynamiques. ThéseUniv. Marseilles, 1-370.

Coulon C., Dostal J. and Dupuy C. (1978) - Petrologyand geochemistry of the ignimbrites and associatedlava domes from NW Sardinia. Contributions toMineralogy and Petrology, 68, 89-98.

Crawford A.J., Falloon T.J. and Eggins S. (1987) - Theorigin of island arc high-alumina basalts.Contributions to Mineralogy and Petrology, 97,417-430.

Deer W.A, Howie R.A. and Zussman J. (1966) - AnIntroduction to the Rock Forming Minerals.Longman, Harlow, Essex, 528 pp.

Deino A.J., Gattacceca J., Rizzo R. and Montanini A.(2001) - 40Ar/39Ar dating and paleomagnetism ofthe Miocene volcanic successions of Monte Furru(western Sardinia): Implications for the rotationhistory of the Corsica-Sardinia microplate.Geophysical Research Letters, 28, 3373-3376.

Deriu M. (1962) - Stratigrafia, cronologia e caratteripetrochimici delle vulcaniti Oligoceniche inSardegna. Memorie Società Geologica Italiana, 3,675-706.

Dieni I., Massari F. and Médus J. (2008) - Age,depositional environment and stratigraphic value ofthe Cuccuru ‘e Flores Conglomerate: insight intothe Paleogene to Early Miocene geodynamicevolution of Sardinia. Bulletin de la SocietéGeologique de France, 179, 51-72.

Dostal J., Coulon C. and Dupuy C. (1982) - Cenozoicandesitic rocks of Sardinia (Italy). In: Thorpe R.S.(ed.), Andesites. John Wiley & Sons, 353-370.

Downes H., Thirlwall M.F. and Trayhorn S.C. (2001) -Miocene subduction-related magmatism inSardinia: Sr-Nd and oxygen isotopic evidence for

mantle source enrichment. Journal of Volcanologyand Geothermal Research, 106, 1-21.

Franciosi L., Lustrino M., Melluso L., Morra V. andD’Antonio M. (2003) - Geochemical characteristicsand mantle sources of the Oligo-Miocene primitivebasalts from Sardinia: the role of subductioncomponents. Ofioliti, 28, 105-114.

Gattacceca J., Deino A., Rizzo R., Jones D.S., HenryB., Beaudoin B. and Vedeboin F. (2007) - Miocenerotation of Sardinia: new paleomagnetic andgeochronological constraints and geodynamicimplications. Earth and Planetary Science Letters,258, 359-377.

Ghiorso M.S. and Sack R.O. (1995) - Chemicaltransfer in magmatic processes IV. A revised andinternally consistent thermodynamic model for theinterpolation and extrapolation of liquid-solidequilibria in magmatic systems at elevatedtemperatures and pressures. Contributions toMineralogy and Petrology, 119, 197-212.

Guarino V., Azzone R.G., Brotzu P., Gomes C.B.,Melluso L., Morbidelli L., Ruberti E., TassinariC.C.G. and Brilli M. (2011) - Magmatism andfenitization in the Cretaceous potassium-alkaline-carbonatitic complex of Ipanema São Paulo State,Brazil. Mineralogy and Petrology, doi:10.1007/s00710-011-0168-4.

Gueguen E., Doglioni C. and Fernandez M. (1998) -On the post-25 Ma geodynamic evolution of thewestern Mediterranean. Tectonophysics, 298, 259-269.

Jolivet L., Faccenna C., Goffé B., Mattei M., RossettiF., Brunet C., Storti F., Funiciello R., Cadet J.P.,D’Agostino N. and Parra T. (1998) - Mid crustalshear zones in post orogenic extension: examplefrom the northern Tyrrhenian Sea. Journal ofGeophysical Research, 103, 12123-12161.

Kersting A.B. and Arculus R.J. (1994) - KlyuchevskoyVolcano, Kamchatka, Russia: The role of high-fluxrecharged, tapped and fractionated magmachamber(s) in the genesis of High-Al2O3 fromHigh-MgO basalt. Journal of Petrology, 35, 1-41.

Kuno H. (1960) - High-Alumina basalt. Journal ofPetrology, 1, 121-145.

Kuno H. (1968) - Differentiation of basalt magmas.In: Hess H.H., Plodevaart A.A., (eds.), Basalts: thePoldervaart treatise on rocks of basalticcomposition, 2, New York, Interscience, 623-688.

Le Bas M.J., Le Maitre R.W., Streckeisen A. and




Zanettin P. (1986) - A chemical classification ofvolcanic rocks based on the Total Alkali-Silicadiagram. Journal of Petrology, 27, 745-750.

Leake B.E., Woolley A.R., Arps C.E.S., Gilbert M.C.,Grice J.D., Hawthorne F.C., Kato A., Kisch A.J.,Krivovichev V.G., Linthout K., Laird J., MandarinoJ.A., Maresch W.V., Nickel E.H., Schumacher J.C.,Smith D.C., Stephenson N.C.N., Whittaker E.J.W.and Youzhi G. (1997) - Nomenclature ofamphiboles: report of the subcommittee onamphiboles of the International MineralogicalAssociation, commission on new minerals andminerals names. Canadian Mineralogist, 35, 219-246.

Lecca L., Lonis R., Luxoro S., Melis F., Secchi F. andBrotzu P. (1997) - Oligo-Miocene volcanicsequences and rifting stages in Sardinia: a review.Periodico di Mineralogia, 66, 7-61.

Lepage L.D. (2003) - ILMAT: An Excel worksheetfor ilmenite-magnetite geothermometry andgeobarometry. Computer and Geosciences, 29,673-678.

Lonis R., Morra V., Lustrino M., Melluso L. andSecchi F. (1997) - Plagioclase textures, mineralogyand petrology of Tertiary orogenic volcanic rocksfrom Sindia (central Sardinia). Periodico diMineralogia, 66, 185-210.

Lopez-Escobar L., Frey F.A. and Vergara M. (1977) -Andesites and high-alumina basalts from thecentral-south Chile high Andes: Geochemicalevidence bearing on their petrogenesis.Contributions to Mineralogy and Petrology, 63,199-228.

Lustrino M., Morra V., Melluso L., Brotzu P.,d’Amelio F., Fedele L., Franciosi L., Lonis R. andPetteruti Liebercknecht A.M. (2004) - TheCenozoic igneous activity of Sardinia. Periodico diMineralogia, 73, 105-134.

Lustrino M. and Wilson M. (2007) - The circum-Mediterranean anorogenic Cenozoic igneousprovince. Earth-Science Reviews, 81, 1-65.

Lustrino M., Melluso L. and Morra V. (2007a) - Thegeochemical peculiarity of “Plio-Quaternary”volcanic rocks of Sardinia in the circum-Mediterranean area. In: Beccaluva L., Bianchini G.,Wilson M. (eds.), Cenozoic Volcanism in theMediterranean Area, Geological Society of AmericaSpecial Paper, 418, 277-301.

Lustrino M., Morra V., Fedele L. and Serracino M.

(2007b) - The transition between “orogenic” and“anorogenic” magmatism in the westernMediterranean area. The Middle Miocene volcanicrocks of Isola del Toro (SW Sardinia, Italy). TerraNova, 19, 148-159.

Lustrino M., Morra V., Fedele L. and Franciosi L.(2009) - The beginning of the Apennine subductionsystem in central-western Mediterranean:constraints from Cenozoic “orogenic” magmaticactivity of Sardinia (Italy). Tectonics, 28, TC5016,doi:10.1029/2008TC002419.

Lustrino M., Duggen S. and Rosenberg C.L. (2011) -The central-western Mediterranean: anomalousigneous activity in an anomalous collisionaltectonic setting. Earth-Science Reviews, 104, 1-40.

Mattioli M., Guerrera F., Tramontana M., Raffaelli G.and D’Atri M. (2000) - High-Mg Tertiary basalts insouthern Sardinia (Italy). Earth and PlanetaryScience Letters, 179, 1-7.

Melluso L., Morra V., Brotzu P., Tommasini S., RennaM.R., Duncan R.A., Franciosi L. and D’Amelio F.(2005) - Geochronology and petrogenesis of theCretaceous Antampombato–Ambatovy complexand associated dyke swarm, Madagascar. Journalof Petrology, 46, 1963-1996.

Melluso L., Srivastava R.K., Guarino V., Zanetti A.and Sinha A.K. (2010) - Mineral compositions andmagmatic evolution of the Sung Valley ultramafic-alkaline-carbonatitic complex (NE India). CanadianMineralogist, 48, 205-229.

Montigny R., Edel J.B. and Thuizat R. (1981) - Oligo-Miocene rotation of Sardinia: K-Ar ages andpaleomagnetic data of Tertiary Volcanism. Earthand Planetary Science Letters, 54, 261-271.

Morimoto N., Fabries J., Ferguson A.K., GinzburgI.V., Ross M., Seifert F.A., Zussman J., Aoki K. andGottardi D. (1988) - Nomenclature of pyroxenes.American Mineralogist, 62, 53-62.

Morra V., Secchi F.A. and Assorgia A. (1994) -Petrogenetic significance of peralkaline rocks fromCenozoic calcalkaline volcanism from SW Sardinia,Italy. Chemical Geology, 118, 109-142.

Morra V., Secchi F.A.G., Melluso L. and Franciosi L.(1997) - High-Mg subduction-related Tertiarybasalts in Sardinia, Italy. Lithos, 40, 69-91.

Myers J.D. (1988) - Possible petrogenetic relationsbetween low- and high-MgO Aleutian basalts.Geological Society of America Bulletin, 100, 1040-1053.




Oudet J., Münch Ph., Verati C., Ferrandini M.,Melinte-Dobrinescu M., Gattacceca J., Cornèe J.J.,Oggiano G., Quillèvèrè F., Borgomano J. andFerrandini J. (2010) - Integrated chronostratigraphyof an intra-arc basin: 40Ar/39Ar datings,micropalaeontology and magnetostratigraphy of theearly Miocene Castelsardo basin (northern Sardinia,Italy). Palaeogeography, Palaeoclimatology,Palaeoecology, 295, 293-306.

Putirka K. (2008) - Thermometers and barometers forvolcanic systems. In: Putirka K, Tepley F (eds.),Minerals, Inclusions and Volcanic Processes.Reviews in Mineralogy and Geochemistry, 69,American Mineralogical Society, Washington, DC,61-120.

Ridolfi F., Renzulli A. and Puerini M. (2010) -Stability and chemical equilibrium of amphibole incalc-alkaline magmas: an overview, newthermobarometric formulations and application tosubduction-related volcanoes. Contributions toMineralogy and Petrology, 160, 45-66.

Sau A., Lecca L., Lonis R., Secchi F. and Fercia M.L.(2005) - La seconda fase del rift Sardo: vulcanismoed evoluzione dei sub-bacini di Ardara-Chilivani eBonorva (Sardegna Settentrionale). BollettinoSocietà Geologica Italiana, 124, 3-20.

Schettino A. and Turco E. (2006) - Plate kinematicsof the Western Mediterranean region during theOligocene and Early Miocene. Geophysical JournalInternational, 166, 1398-1423.

Schiano P., Clocchiatti R., Boivin P. and Medard E.(2003) - The nature of melt inclusions insideminerals in an ultramafic cumulate from Adakvolcanic center, Aleutian arc: implications for theorigin of high-Al basalts. Chemical Geology, 203,169-179.

Sisson T.W. and Grove T.L. (1993) - Temperatures andH2O contents of low-MgO high-alumina basalts.Contributions to Mineralogy and Petrology, 113,167-184.

Speranza F., Villa I.M., Sagnotti L., Florindo F.,Cosentino D., Cipollari P. and Mattei M. (2002) -Age of the Corsica-Sardinia rotation and Liguro-Provençal Basin spreading: new paleomagnetic andAr/Ar evidence. Tectonophysics, 347, 231-251.

Stormer J.C.J. and Nicholls J. (1978) - XLFRAC: Aprogram for the interactive testing of magmaticdifferentiation models. Computer and Geosciences,4, 143-159.

Vigliotti L. and Langenheim V.E. (1995) - When didSardinia stop rotating? New paleomagnetic results.Terra Nova, 7, 424-435.

Submitted, August 2011 - Accepted, November 2011



Mineral compositions and magmatic evolution of the calcalkaline rocks of northwestern Sardinia,...

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