petrogenesis of the toba tuffs, sumatra, indonesia - journal of

JOURNAL OF PETROLOGY VOLUME 39 NUMBER 3 PAGES 397–438 1998

Petrogenesis of the Toba Tuffs, Sumatra,Indonesia

CRAIG A. CHESNERDEPARTMENT OF GEOLOGY & GEOGRAPHY, EASTERN ILLINOIS UNIVERSITY, CHARLESTON, IL 61920, USA

RECEIVED NOVEMBER 3, 1995; REVISED TYPESCRIPT ACCEPTED SEPTEMBER 19, 1997

Earth (Smith & Bailey, 1968). It is elongated in a NW–SEDuring the past 1·2 my, at least 3400 km3 of magma have beendirection parallel to the active volcanic front of Sumatra,erupted in four ash flow tuff units from the Toba Caldera Complex.and it measures 100 km by 30 km (Fig. 1). Over the pastThis activity culminated at 74 ka with the fourth eruption, which1·2 my, there have been four ash flow tuff eruptions fromproduced 2800 km3 of magma and formed the 100 km× 30 kmthe caldera complex (Chesner & Rose, 1991; Chesner etcaldera visible today. A relatively homogeneous two-pyroxene daciteal., 1991). The youngest tuff was erupted at 74 kawas erupted during the first phase of activity. Magma erupted during(Ninkovich et al., 1978; Chesner et al., 1991) and has aeach successive eruption was compositionally zoned, generally rangingminimum volume of 2800 km3 (Rose & Chesner, 1987).from rhyodacite to rhyolite. The youngest three tuff deposits containEruption of the Youngest Toba Tuff (YTT) is responsibleup to 40 wt % crystals of quartz, sanidine, plagioclase, biotite,for the collapse structure visible today (Van Bemmelen,and amphibole. Minor minerals are magnetite, ilmenite, allanite,1949). It consists of an extensive, mostly non-weldedzircon, fayalite, and orthopyroxene; inclusions of apatite and pyrrho-outflow sheet with abundant pumice blocks (<80 cm),tite are common. Extensive fractional crystallization in double-and it covers 20 000–30 000 km2 (Van Bemmelen, 1949;diffusive convecting magma bodies is suggested for creation ofAldiss & Ghazali, 1984). Where deep exposures arethe compositional variation. Although similar in composition andavailable, the YTT is seen to be incipiently to denselymineralogy, the tuffs can be distinguished by subtle variations in theirwelded towards its base. Samosir Island, forming part ofmineral, whole-rock, and glass compositions. Intensive parameterthe post-YTT resurgent dome, consists of densely weldeddeterminations suggest that much of the crystallization of the quartz-

YTT caldera fill (Chesner et al., 1991). The older Tobabearing tuffs occurred between 700 and 760°C at depths of 10 km.

Water pressure estimates indicate that Ptot > PH2O; thus volatile units are the Middle Toba Tuff (MTT) (40Ar/39Ar ageoversaturation probably did not initiate the eruptions. Individual 0·50 Ma, Chesner et al., 1991), the Oldest Toba Tuffpumice blocks and fiamme collected from the youngest three units (OTT) (40Ar/39Ar age 0·84 Ma, Diehl et al., 1987), andrecord simultaneous eruption across compositional boundaries. Low- the Haranggoal Dacite Tuff (HDT) (fission track age 1·2energy ring fracture eruptions resulted in dense welding of all units Ma, Nishimura et al., 1977). They were erupted al-except for the top of the youngest unit, and thick accumulations of ternately from north and south vent areas in the presentrhyodacitic magma in the collapsing calderas. caldera, but they are generally exposed only in the steep

caldera walls, and are densely welded (Chesner & Rose,1991).

Collectively, the YTT, MTT, and OTT are referredto as the quartz-bearing Toba tuffs. They typically containKEY WORDS: composition; crystal fractionation; magma chamber gradients;up to 40 wt % phenocrysts, mostly of quartz, sanidine,intensive parameters; Toba Calderaplagioclase, biotite, and amphibole, with minor Fe–Tioxides, allanite, zircon, fayalite, and orthopyroxene. In-clusions of apatite and pyrrhotite occur in most mineral

INTRODUCTION phases. The HDT contains plagioclase, orthopyroxene,clinopyroxene and Fe–Ti oxides, plus accessory apatiteThe Toba Caldera, located in northern Sumatra, In-

donesia, is the largest resurgent Quaternary caldera on and zircon.

Oxford University Press 1998

JOURNAL OF PETROLOGY VOLUME 39 NUMBER 3 MARCH 1998

Fig. 1. Tectonic setting and location maps of the Toba Caldera Complex. Recent areas of updoming along the western lake shore are associatedwith hydrothermal and fumarolic activity. Stratigraphic sections were sampled near Haranggoal, Silalahi, Pangururan, Bakara, and Siguragura.In inset, I.R. is Investigator Ridge fracture zone.

Despite the imposing size of Toba and its fame as a processes of magmatic evolution that occurred in themagma chambers.resurgent caldera, a comprehensive petrogenetic study

has not previously been attempted. Such work is nowfacilitated by a stratigraphic framework developedby Knight et al. (1986) and Chesner & Rose (1991). Toba

GEOLOGIC SETTING AND HISTORYis a good subject for study because:(1) The development and geochemical evolution of The Toba Caldera lies in a complex geologic setting

the batholithic magma chamber can be deduced by that exhibits features of both island and continental arccombining time constraints on the various tuff eruptions volcanism (Rock et al., 1982). Sumatra and western Javawith their geographical distributions. are composed mostly of continental crust (Westerveld,

(2) The freshness and variety of phenocrysts in the 1952; Schlater & Fisher, 1974; Hamilton, 1979; Kieck-quartz-bearing tuffs yield information on intensive para- hefer, 1980) whereas eastern Java and islands of the eastmeters in the magma bodies. Sunda arc overlie oceanic crust (Ben Avraham & Emery,

(3) Most exposures of the YTT feature pumice blocks 1973; Hamilton, 1979). The tectonic regime is also com-that appear to represent the composition of the magma plicated by the low-angle oblique subduction of thebefore its eruption. Therefore, whole-rock and mineral Indian–Australian Plate beneath the Southeast-Asian

Plate and subduction of the Investigator Ridge fracturechemistry should yield abundant information on the

398

CHESNER PETROGENESIS OF THE TOBA TUFFS, SUMATRA, INDONESIA

zone (Fig. 1). The Sumatran Benioff Zone is ~100 kmbeneath Toba and is traceable to only 183 km (Kieck-hefer, 1980). The rate of volcanism is low (Rock et al.,1982) and the right-lateral Sumatran Fault ( just to thewest of Toba) has had at least 400 km of offset in thepast 19 my (Haile, 1979).

Development of the Toba Caldera Complex (Fig. 2)(Chesner & Rose, 1991) occurred in an area of broaduplift, called the Batak Tumor by Van Bemmelen (1939),that began to form in the Miocene (Clarke et al., 1982).Toba is situated along the crest of this regional feature.By 1·3 Ma, a large stratovolcano had grown on the BatakTumor within the northern boundaries of the presentcaldera (Fig. 2a). Outcrops of weathered andesites in thesouthern caldera walls suggest there was some volcanicactivity focused there also. The first pyroclastic eruptionknown at Toba produced the 35 km3 HDT. It wasapparently erupted from a caldera within the northernstratovolcano, at 1·2 Ma (Fig. 2b) (Chesner & Rose,1991). At 0·84 Ma the 500 km3 OTT (which is the firstquartz-bearing Toba tuff ) was erupted from the PorseaCaldera in the southern half of Toba (Fig. 2c). Activityresumed in northern Toba with eruption of the 60 km3

MTT, at 0·50 Ma, possibly from the same caldera thaterupted the HDT (Fig. 2d). Finally, at 74 ka, the YTTerupted during the collapse of the structure visible today(Fig. 2e). Subsequently, resurgence of Samosir Island hasoccurred, exposing YTT caldera fill and the OTT calderafill of the Uluan Block (Fig. 2f ). During resurgence,rhyolite domes have erupted along the Samosir Faultscarp (Fig. 1). Domes were also erupted along the south-western ring fracture (Pardepur dome complex and Pu-sikbukit Volcano, Fig. 1) after the YTT eruption. Giantpumice blocks (<1 m) were rafted into position alongsouthern Samosir Island and Uluan (Fig. 1) presumablyfollowing a subaqueous dome eruption. Characteristicsof the Toba tuffs are summarized in Table 1.

SAMPLINGExposures suitable for stratigraphic sampling of the vol-canic rocks occur only in the caldera walls, except alongthe Siguragura section, exposed where the Asahan Riverhas cut through the YTT outflow sheet (Fig. 1). Whereverpossible, fiamme were sampled from the welded tuffseither by chiselling or with a rock drill. The non-weldedtuffs of the extensive YTT outflow sheet (Fig. 2g) weresampled at many road cuts and at the top of stratigraphicsections exposed in the caldera walls (Fig. 1). Multiplepumice blocks (4–10) were collected at each samplingsite along with the ignimbrite matrix. These procedureswere adopted to insure sampling of juvenile magma

Tab

le1:

Cha

ract

eris

tics

ofth

eT

oba

tuffs

Un

it/c

ald

era

Ag

e(M

a)T

hic

knes

s(m

)Vo

lum

e(k

m3 )

Mag

net

icp

ola

rity

Co

mp

osi

tio

nM

iner

alo

gy

SiO

2ra

ng

e(w

t%)

YT

T/

0·07

4<4

0028

00N

Rh

yod

acit

e–rh

yolit

eQ

z+P

l+S

a+B

i+A

m+

Mg+

68–7

6

Tob

aex

cep

to

nS

amo

sir

Il+A

l+Fa+

Zi+

Op

x

MT

T/

0·50

1>1

4060

NR

hyo

lite

Qz+

Pl+

Sa+

Bi+

Am+

Op

x+72

–76

No

rth

ern

Tob

aM

g+

Il+A

l+Fa+

Zi

OT

T/

0·84

>300

500

RA

nd

esit

e–rh

yolit

eQ

z+P

l+S

a+B

i+A

m+

Mg+

61–7

4

Po

rsea

Cal

der

aIl+

Al+

Zi+

Op

x

HD

T/

1·2

<200

35R

Dac

ite

Pl+

Op

x+C

px+

Mg

63–6

6

No

rth

ern

Tob

a

Qz,

qu

artz

;Pl,

pla

gio

clas

e;S

a,sa

nid

ine;

Mg

,mag

net

ite;

Il,ilm

enit

e;A

p,a

pat

ite;

Am

,am

ph

ibo

le;B

i,b

ioti

te;Z

i,zi

rco

n;A

l,al

lan

ite;

Fa,f

ayal

ite;

Op

x,o

rth

op

yro

xen

e;C

px,

clin

op

yro

xen

e.N

,n

orm

al;

R,

reve

rsed

.

compositions which existed immediately before eruption(Lipman, 1967; Walker, 1972). The young lava flows

399


Fig. 2. (a–f ) Eruptive and resurgent history and inferred magma chamber development of the Toba Caldera Complex. (g) Distribution of theYoungest Toba Tuff (YTT) (from Aldiss & Ghazali, 1984).

400


and volcanoes located inside and outside of the caldera PETROGRAPHY AND MINERALOGYwere also sampled. In total, 261 samples were collected Toba rocks contain as much as 40 wt % phenocrystsfrom 100 sites. The collection comprises 63 samples of (Aldiss & Ghazali, 1984), which are generally euhedralwelded tuff, 165 of pumice, 14 of ignimbrite matrix, and in pumices but usually broken in the bulk tuffs. Common19 of lava. petrographic features of the different units can be sum-

marized as follows.The HDT contains plagioclase, orthopyroxene, clino-

pyroxene, magnetite, and ilmenite (Table 3); matrix glassis devitrified to a dark brown color. Glass occurs inANALYTICAL TECHNIQUESflattened fiamme that contain microlites exhibiting

Minerals were separated by heavy liquid and magnetic spherulitic and dendritic quench textures. Alignment oftechniques. Polished mounts were made from splits of these microlites, sometimes into folds, suggests secondaryall samples for wavelength dispersive microprobe analysis flowage. Fiamme have higher plagioclase/pyroxene ratiosat Washington State University. Microprobe analyses than the ignimbrite matrix, suggesting concentration ofwere performed on mineral concentrates to obtain ad- denser minerals in the matrix during emplacement.equate numbers of analyses and to make more efficient The OTT, MTT, and YTT are dominated by quartz,use of machine time. This practice was especially im- plagioclase, sanidine, biotite, and amphibole (Table 3).portant for minor minerals. Orthopyroxene is present in all units, but is more abund-

Whole-rock samples and glass separates from selected ant in MTT. Magnetite, allanite, zircon, and ilmenitesamples were analyzed for major and trace elements are minor or accessory phases in all three units. Extremely(Table 2) by the automated X-ray fluorescence (XRF) oxidized fayalite has been identified in some YTT andtechniques of Rose et al. (1986) following the ratio method MTT mineral separates. As SiO2 content increases: (1)of Leake et al. (1969). Oxide totals ranging from 95 to phenocryst content decreases; (2) sanidine abundance100% and total LOI (loss on ignition) of 0·3–4·0 wt % increases; (3) biotite/amphibole ratios increase fromindicate that the rocks containing volcanic glass, especially about 50/50 to 95/5; (4) orthopyroxene decreases fromporous pumice, have been strongly hydrated. 1% to zero. Welded samples show different degrees

of groundmass devitrification from vitric to thoroughlydevitrified. The most devitrified groundmass typically ischaracterized by intergrown spherulites. In some cases,the devitrified groundmass appears granophyric, and canCLASSIFICATION OF TOBA ROCKSbe accompanied by carbonate and layer silicate (probably

The YTT ranges from high-K rhyolite to rhyodacite. biotite) alteration products. Welding ranges from in-More than half of the analyzed YTT pumice samples cipient with no deformation of the glass shards, to intensecontain 75–77% SiO2 and three-quarters have >73% with glass shards elongated almost beyond recognition.SiO2. The remainder have 68–72% SiO2; most of these Lithic fragments of metasedimentary country rocks com-contain 68–70% SiO2. In this study, I have used 73% monly occur in the welded tuffs. Pumice blocks from theSiO2 as a cutoff between compositional populations of non-welded YTT usually have densities >1 g/cm3 and‘high-SiO2’ and ‘low-SiO2’ YTT samples. All MTT contain a variety of vesicle types from elongate to spher-samples are characterized as high-K rhyolites (SiO2 = ical.72–76%). The OTT predominantly ranges between high-K rhyolite and rhyodacite; however, at one samplinglocality a few fiamme of dacite and andesite compositions

Quartzwere collected. Consequently, the OTT displays thelargest chemical variation of the Toba tuffs (SiO2 = Quartz phenocrysts occur in all rocks from the OTT,61–74%). Separation into ‘high-SiO2’ and ‘low-SiO2’ MTT, and YTT and commonly are as long as 2 cm.compositional populations for the OTT is at ~72·5% Usually the quartz is clear and resorbed (Fig. 3a). QuartzSiO2. The HDT is classified as a high-K dacite (SiO2 = in the OTT is commonly pink, probably because it63–66%). Separated glasses from the quartz-bearing units contains MnO and (or) TiO2 (Holden, 1924; Deer etare all high-K rhyolites. The Toba eruptive units have al., 1963). Many quartz phenocrysts have been highlyalkali–lime indices between 62 and 63·5, hence the fractured and shattered, perhaps as a result of quenchingsuite is calcic (Peacock, 1931). Toba rocks are mostly or bursting of overpressured melt inclusions during orperaluminous, although the least silicic samples from the after eruption (Anderson et al., 1989; Tait, 1992). Abund-YTT, OTT, and HDT are metaluminous. All Toba ant melt inclusions (<0·2 mm) usually appear trigonal orrocks are hypersthene normative. Occasionally, some of rectangular in cross-section (Fig. 3b and c). Daughter

minerals may be present as tiny microlites. The inclusionthe low-SiO2 rocks show normative diopside.

401


Tab

le2

Rep

rese

ntat

ive

XR

Fw

hole

-roc

kan

alys

esan

dC

IPW

norm

sof

the

Tob

atu

ffs

HD

TO

TT

MT

TY

TT

WF

Wel

ded

Fiam

me

Gla

ssW

eld

edFi

amm

eG

lass

Wel

ded

Pu

mic

e

Sam

ple

:9

46A

2715

A74

76A

186

A1

74A

774

A2

15A

-G74

-G7

841

A41

B7-

G8-

G54

1170

5B3

17A

1

SiO

263

·85

65·5

372

·41

73·5

169

·94

74·3

970

·39

65·3

661

·44

76·4

371

·99

76·1

974

·17

75·4

272

·52

75·2

376

·94

75·0

372

·97

70·3

676

·67

76·2

6Ti

O2

0·67

0·73

0·22

0·26

0·38

0·18

0·44

0·75

0·81

0·08

0·23

0·13

0·21

0·14

0·33

0·06

0·10

0·19

0·33

0·46

0·06

0·14

Al 2

O3

16·2

916

·42

14·1

213

·98

14·8

013

·55

14·4

915

·91

18·1

813

·27

14·4

412

·98

13·7

713

·35

14·1

813

·98

12·6

313

·24

13·8

315

·09

13·2

712

·71

Fe2O

36·

065·

032·

521·

873·

511·

443·

534·

425·

780·

612·

581·

582·

092·

173·

821·

431·

201·

942·

493·

321·

211·

71M

nO

0·11

0·08

0·07

0·04

0·07

0·02

0·08

0·08

0·10

0·02

0·06

0·02

0·05

0·05

0·06

0·03

0·05

0·06

0·07

0·08

0·08

0·08

Mg

O1·

611·

220·

580·

520·

790·

500·

771·

931·

250·

170·

560·

210·

390·

220·

410·

140·

180·

390·

700·

740·

180·

36C

aO5·

444·

731·

941·

932·

811·

432·

864·

484·

671·

041·

911·

021·

760·

931·

590·

750·

821·

562·

142·

800·

671·

23N

a 2O

2·79

2·68

3·10

3·66

3·50

3·36

3·67

3·49

4·11

3·56

3·44

3·56

3·22

3·19

2·94

3·51

2·98

2·90

3·35

3·45

2·93

2·66

K2O

3·00

3·44

4·98

4·17

4·08

5·09

3·66

3·38

3·45

4·79

4·71

4·30

4·29

4·50

4·13

4·85

5·08

4·67

4·04

3·61

4·91

4·83

P2O

50·

170·

140·

060·

060·

120·

040·

110·

200·

210·

030·

080·

010·

050·

030·

020·

020·

020·

020·

080·

090·

020·

02

Sc

19—

66

7—

——

——

——

6—

——

—3

5—

—3

V12

510

517

2141

1339

8388

—22

421

426

——

1623

44—

9C

r16

10—

—13

7—

616

——

—10

——

——

——

17—

—M

n84

864

553

328

051

816

263

163

773

817

443

018

440

136

246

221

136

743

354

062

760

958

8N

i6

32

42

——

4—

73

36

52

46

35

—6

—C

u24

3225

1612

2012

—6

5527

1812

113

2823

916

1421

8Z

n65

6046

4655

2057

8177

3143

3643

4651

4232

4150

5134

31R

b11

415

521

518

215

018

217

011

112

226

226

714

512

814

112

2—

171

191

166

131

260

245

Sr

214

191

113

121

180

111

167

252

297

4712

166

139

5714

139

5410

814

221

525

74Y

3236

3330

3024

3433

3634

3126

2223

2331

2428

2827

3628

Zr

167

205

132

131

177

112

187

286

331

7599

114

133

8915

976

8110

512

519

067

110

Nb

1011

2121

1719

1713

923

1928

1419

1120

1919

1815

2524

Ba

553

586

519

573

736

557

619

939

895

278

580

611

1147

659

1253

489

820

501

749

944

247

290

La33

4525

3638

2644

6950

2616

2845

2750

2333

3137

6019

29C

e64

6959

6880

5567

104

9837

5862

101

6310

949

7356

7685

3744

AN

49·4

49·9

26·0

22·9

29·9

19·4

28·3

37·6

37·6

14·2

23·5

14·2

23·6

14·1

23·8

10·8

13·5

23·7

26·3

31·3

11·4

21·0

Q21

·523

·829

·631

·026

·131

·227

·319

·712

·034

·828

·436

·034

·136

·734

·434

·037

·835

·831

·928

·738

·738

·6o

r17

·720

·329

·424

·624

·130

·121

·620

·020

·428

·327

·825

·425

·426

·624

·428

·730

·027

·623

·921

·329

·028

·5ab

23·6

22·7

26·2

31·0

29·6

28·4

31·1

29·5

34·8

30·1

29·1

30·1

27·3

27·0

24·9

29·7

25·2

24·5

28·4

29·2

24·8

22·5

an23

·122

·69·

29·

212

·66·

812

·317

·821

·05·

09·

05·

08·

44·

47·

83·

63·

97·

610

·113

·33·

26·

0C

——

0·3

0·1

——

——

—0·

40·

40·

60·

81·

62·

01·

60·

80·

60·

30·

62·

00·

9d

i2·

3—

——

0·4

—1·

02·

60·

7—

——

——

——

——

——

——

hy

6·6

6·1

3·3

2·5

4·2

2·2

3·8

6·1

5·5

0·8

3·3

1·7

2·5

2·2

3·8

1·5

1·4

2·4

3·4

4·0

1·5

2·2

mt

3·2

2·4

1·2

0·9

1·7

0·7

1·7

2·1

3·4

0·3

1·3

0·8

1·0

1·1

1·8

0·7

0·6

0·9

1·2

1·6

0·6

0·8

il1·

31·

40·

40·

50·

70·

30·

81·

41·

50·

20·

40·

30·

40·

30·

60·

10·

20·

40·

60·

90·

10·

3ap

0·4

0·3

0·1

0·1

0·3

0·1

0·3

0·5

0·5

0·1

0·2

—0·

10·

10·

10·

10·

10·

10·

20·

20·

10·

1

402


Table 2: continued

Pumice Glass

Sample: 94A5 51A5 51A1 51A2 6A2 40A1 23A2 57A2 57A3 40B1 94A5-G 51A5-G 6A2-G 63A1-G

SiO2 75·83 75·19 74·24 73·72 72·11 71·16 70·86 69·59 68·88 68·84 78·12 78·20 77·60 76·82

TiO2 0·18 0·24 0·26 0·33 0·30 0·50 0·36 0·56 0·61 0·42 0·04 0·06 0·08 0·08

Al2O3 12·81 12·91 13·45 13·42 14·57 14·27 14·79 14·71 14·82 15·24 12·04 11·90 12·78 12·93

Fe2O3 1·82 2·12 2·21 2·64 2·96 3·87 3·47 3·94 4·28 4·10 0·92 1·02 0·84 1·21

MnO 0·07 0·07 0·07 0·07 0·08 0·09 0·08 0·09 0·09 0·13 0·06 0·07 0·06 0·07

MgO 0·33 0·40 0·45 0·53 0·58 0·82 0·72 0·96 1·11 0·85 0·15 0·17 0·20 0·18

CaO 1·20 1·51 1·70 2·03 2·17 2·65 2·94 3·28 3·49 3·06 0·62 0·69 0·80 0·94

Na2O 2·77 2·86 2·99 2·98 2·96 2·91 3·14 3·31 3·32 3·22 2·79 2·68 2·60 2·94

K2O 4·97 4·69 4·61 4·26 4·25 3·71 3·62 3·52 3·36 4·12 5·26 5·20 5·03 4·82

P2O5 0·02 0·01 0·02 0·02 0·02 0·02 0·02 0·04 0·04 0·02 0·01 0·01 0·01 0·01

Sc 2 3 4 5 6 7 7 8 8 7 — — — —

V 3 12 17 29 35 46 50 52 58 41 — — 1 —

Cr — — — — — — — — — — — — — —

Mn 531 575 550 556 610 713 630 669 720 991 457 542 484 512

Ni 2 3 — 3 2 4 — — — — 6 2 3 3

Cu 19 9 14 5 7 10 5 10 11 7 29 22 98 45

Zn 38 45 45 48 53 57 50 57 66 64 31 29 39 48

Rb 234 203 195 155 161 135 127 125 116 157 344 252 212 209

Sr 79 97 119 160 171 184 202 228 241 180 21 32 61 69

Y 33 30 28 25 27 27 25 29 30 31 37 32 27 25

Zr 97 115 124 139 162 185 173 195 226 166 65 72 88 83

Nb 21 22 20 19 18 17 16 16 17 21 28 25 21 20

Ba 394 487 677 854 706 881 887 908 880 658 182 412 700 674

La 35 34 39 40 45 39 57 45 47 23 20 30 34 33

Ce 46 57 65 78 83 86 88 90 92 65 27 51 68 66

AN 19·9 23·5 24·7 28·3 29·8 34·6 35·2 34·7 35·7 35·5 11·3 12·9 15·1 15·6

Q 37·1 36·1 34·3 34·2 32·1 32·0 30·2 27·6 26·8 25·3 39·9 40·6 40·9 38·6

or 29·4 27·7 27·2 25·2 25·1 21·9 21·4 20·8 19·9 24·4 31·0 30·7 29·7 28·5

ab 23·4 24·2 25·3 25·2 25·1 24·6 26·6 28·0 28·1 27·3 23·6 22·7 22·0 24·9

an 5·8 7·4 8·3 9·9 10·6 13·0 14·5 14·9 15·6 15·0 3·0 3·4 3·9 4·6

C 0·7 0·4 0·5 0·3 1·2 0·7 0·4 — — — 0·7 0·6 1·6 1·2

di — — — — — — — 0·9 1·2 0·1 — — — —

hy 2·2 2·5 2·7 3·1 3·6 4·6 4·2 4·4 4·9 5·0 1·2 1·3 1·2 1·5

mt 0·9 1·0 1·1 1·3 1·4 1·9 1·7 1·9 2·1 2·0 0·4 0·5 0·4 0·6

il 0·3 0·5 0·5 0·6 0·6 1·0 0·7 1·1 1·2 0·8 0·1 0·1 0·2 0·2

ap 0·1 — 0·1 0·1 0·1 0·1 0·1 0·1 0·1 0·1 — — — —

Analyses normalized to 100%, water-free, with all Fe as Fe2O3. Rubidium data for glass separates may be suspect becauseof spectral interferences with Br absorbed from bromoform. W and Welded, whole-rock welded tuff; F and Fiamme, whole-rock fiamme; Pumice, whole-rock pumice; Glass, glass separates from welded tuff samples in OTT and MTT, and pumicesin YTT.

403


Tab

le3:

Pet

rogr

aphi

cch

arac

teri

stic

sof

Tob

am

iner

als

Min

eral

Co

lor

Form

Siz

e(m

m)

Mel

tin

clu

sio

n(m

m)

Cry

stal

incl

usi

on

Co

mp

osi

tio

nR

eact

ion

Qu

artz

clea

reu

hed

ral

<20·

0n

egat

ive

——

com

mo

n

pin

k(O

TT

)cr

ysta

l<0

·2re

sorp

tio

n

Pla

gio

clas

ecl

ear

euh

edra

l<2

·0ab

un

dan

t—

and

esin

eo

ccas

ion

al

tab

ula

rn

egat

ive

crys

tal

and

reso

rpti

on

zon

edfo

rmle

ss<0

·2

San

idin

ecl

ear

euh

edra

l<2

·0n

egat

ive

—O

r 65–

75o

ccas

ion

al

tab

ula

rcr

ysta

l<0

·2re

sorp

tio

n

zon

ed

Bio

tite

yello

w/b

row

neu

hed

ral

<3·0

rare

<0·0

05ap

,zi

,m

g-n

o.=

0·35

–0·4

6o

xid

ized

tab

ula

rm

ag,

feld

bio→

mag

Am

ph

ibo

leg

reen

/bro

wn

euh

edra

l<2

·0co

mm

on

<0·0

2ap

,zi

,b

i,fe

rro

-ed

enit

icam

ph→

bri

gh

tg

reen

pl,

ox

ho

rnb

len

de

ox+

bi+

feld

Ort

ho

pyr

oxe

ne

ligh

tb

row

neu

hed

ral,

<1·5

com

mo

n<0

·1ap

,o

xfe

rro

hyp

erst

hen

e,o

px→

ligh

tg

reen

sub

hed

ral

eulit

e,h

yper

sth

ene

amp

h+

cpx

Clin

op

yro

xen

elig

ht

gre

ensu

bh

edra

l<2

·0co

mm

on

<0·0

5ap

,o

x,p

lau

git

e—

Faya

lite

op

aqu

eeu

hed

ral

<0·5

??

Fa94

Te6

oxi

diz

ed

Zir

con

clea

r,p

ink

(OT

T)

euh

edra

l<0

·3n

egat

ive

crys

tal

and

ap,

mag

,zi

Hf/

Zr=

0·03

—

pri

smat

icsh

apel

ess

<0·2

Alla

nit

ere

d–b

row

neu

hed

ral

<0·5

neg

ativ

eap

,o

x,zi

RE

E=

23w

t%—

ligh

tg

reen

twin

ned

crys

tal

<0·5

Mag

net

ite

op

aqu

esu

bh

edra

l<0

·4?

?u

lvo

spin

el=

18–3

6%o

xid

ized

,ex

solu

tio

n

ovo

id

Ilmen

ite

op

aqu

esu

bh

edra

l<0

·2?

?ilm

enit

e=85

–94%

—

ovo

id

Ap

atit

ecl

ear

nee

dle

s<0

·15×

0·01

??

?—

Pyr

rho

tite

op

aqu

eb

leb

s<0

·04

?cc

Po

a=1

—

ap,

apat

ite;

zi,

zirc

on

;m

ag,

mag

net

ite;

feld

,fe

ldsp

ars;

bi,

bio

tite

;o

x,o

xid

es;

amp

h,

amp

hib

ole

;cp

x,cl

ino

pyr

oxe

ne;

op

x,o

rth

op

yro

xen

e;p

l,p

lag

iocl

ase;

cc,

chal

cop

yrit

e;Fa

,fa

yalit

e;Te

,te

ph

roit

e;O

r,o

rth

ocl

ase;

Po

,p

yrrh

oti

te.

404


glass is usually clear to light brown and commonly Biotiteencompasses vapor bubbles. Re-entrants formed during Biotite is common to the quartz-bearing tuffs and theresorption have rounded walls and contain devitrified crystals are euhedral (<3 mm). Kinked biotite is commonglass with no vapor bubble. Some elongate (up to 1 mm) in both welded and non-welded samples. Inclusions ofmelt inclusions are connected to the phenocryst rim by rods and needles of apatite are ubiquitous. Other in-a narrow neck (Fig. 3d). Anderson (1991) has named clusions are zircon, magnetite, and feldspar. Rare meltthese ‘hourglass’ inclusions, and suggested that they de- inclusions (<0·005 mm) contain vapor bubbles.velop only in decompressing, gas saturated magmas As a group, the YTT biotites exhibit the greatest range(Fig. 3d). Beddoe-Stephens et al. (1983) obtained com- in chemical composition; the MTT and OTT biotitespositions (wt %) for melt inclusion glass from Toba quartz have more restricted compositions (Table 5, Fig. 5).(SiO2 73·12, Al2O3 12·42, FeO 0·84, CaO 0·69, Na2O Generally, as the silica content of its host rocks increase,3·13, K2O 4·75). biotite becomes richer in FeO and MnO, and poorer in

MgO and TiO2. Biotites in OTT and MTT containmore Fe than those in the YTT. The MTT and OTTbiotites can be distinguished by their TiO2 and MnOcontents. A few biotite analyses were markedly differentPlagioclasefrom the rest and are interpreted as xenocrysts or severelyPlagioclase is ubiquitous, usually occurring as tabular,altered biotites.euhedral phenocrysts (<2 mm), exhibiting little resorption

Toba biotites have mg-numbers [Mg/(Mg+ Fe), alland normal zoning; oscillatory zoning occurs also. MostFe as Fe2+] of 0·35–0·46. As oxidation of the Tobaindividual plagioclase analyses range from An25 to An43biotites was noted petrographically, it can be assumed(Table 4, Fig. 4). Only the HDT has labradorite. Thethat a considerable amount of Fe is present as Fe3+.MTT has the greatest range, from An16 to An43. MeltOxidation is apparent in the chemical analyses, whichinclusions are more abundant than in quartz and usuallytotal 97–99% using all Fe as Fe2+. These low totals couldappear as elongate rectangles parallel to the plagioclasebe explained by variable amounts of Fe3+. The proportionc-axis. They commonly occur in parallel sets that ap-of OH in the hydroxyl site is generally 0·76–0·95, al-parently mark growth surfaces. Plagioclase in the leastthough some welded OTT samples have lower values.silicic rocks, such as the HDT, are riddled with suchHigh F contents in biotite occur only in welded tuffinclusions and also contain large formless melt inclusionssamples. Fluorine–OH exchange at the hydroxyl sitewith vapor bubbles. These inclusions impart a ‘wormy’possibly took place as a result of low-temperature post-appearance to the host. Occasionally, this texture occursemplacement processes such as exchange with a fluidin plagioclase in other units, especially the MTT. Ac-phase during welding.cording to Carter et al. (1986), the plagioclase phenocrysts

Reaction of biotite to magnetite is common in manyfrom the YTT exhibit evidence of shock stress levels inOTT welded tuffs and some from the YTT, particularlyexcess of 10 GPa, such as planar features, shock mo-in devitrified caldera fill of the resurgent dome. Thissaicism, incipient recrystallization, and possibly partialreaction occurs around the edges of phenocrysts andmelting. Sharpton & Schuraytz (1989) disagreed, andpenetrates along basal cleavage planes inside the pheno-suggested that these features developed during a complexcryst (Fig. 3e), suggestive of a post-emplacement reaction.crystallization history and are thus a signature of mag-In some samples from both the outflow sheet and calderamatic processes, not a shock wave.fill, biotite is very dark to almost opaque and lacks Fe-oxides.

SanidineSanidine occurs in the quartz-bearing tuffs, except in the

Amphiboleleast silicic OTT and YTT rocks. When not fragmented,the sanidine crystals are tabular and euhedral (<2 mm); Amphibole occurs in all Toba rocks except the HDT.resorption is uncommon. Melt inclusions are less common Phenocrysts are euhedral, <2 mm in length, and containin sanidine than in plagioclase and quartz, and commonly abundant acicular apatite inclusions along with inclusionshave negative crystal shapes. Sanidine compositions range of Fe-oxides, zircon, biotite, plagioclase, and negative-from Or65 to Or75 (Table 4, Fig. 4) and compositional crystal melt inclusions (<0·02 mm) with vapor bubbles.zoning is evidently small. The MTT sanidine shows the Commonly, the amphibole is partly to completely re-greatest variation. In general, rocks with the most evolved placed by an assemblage of Fe-oxides, a fine-grainedcompositions contain the most sodic plagioclase and most light brown mica (possibly biotite), and feldspar, especially

in highly devitrified samples of the OTT. This alterationsodic sanidine.

405


406


Fig

.3.

Phot

omic

rogr

aphs

ofso

me

Tob

am

iner

als.

(a)

Part

lyre

sorb

edqu

artz

phen

ocry

st.

(b,c

)M

elt

incl

usio

nsw

ithva

por

bubb

les

inqu

artz

.(d

)H

ourg

lass

mel

tin

clus

ion

inqu

artz

.(e

)B

iotit

epa

rtly

repl

aced

byFe

-oxi

des

onri

mof

aph

enoc

ryst

and

alon

gcl

eava

gepl

anes

.(f

)O

rtho

pyro

xene

part

lyre

sorb

edan

dri

mm

edby

amph

ibol

e.(g

)O

xidi

zed

faya

lite

with

min

orfr

esh

inte

rior

.(h

)Apa

tite

and/

orzi

rcon

incl

usio

nsin

zirc

onph

enoc

ryst

.

407


Tab

le4:

Rep

rese

ntat

ive

anal

yses

ofT

oba

plag

iocl

ase

and

sani

dine

Pla

gio

clas

eS

anid

ine

HD

TO

TT

MT

TY

TT

OT

TM

TT

YT

T

Sam

ple

:9-

29-

332

B-1

60-2

85-4

7-3

99-1

99-2

94A

5-1

63A

1-1

89A

2-4

85-1

60-1

32B

-499

-17-

17-

294

A5-

189

A2-

263

A1-

2

SiO

254

·23

50·9

161

·46

61·1

060

·62

64·8

360

·09

58·1

363

·14

58·9

658

·78

65·1

064

·24

65·3

165

·61

66·6

367

·10

65·8

564

·49

65·1

6

TiO

20·

030·

020·

010·

02—

——

—0·

020·

02—

—0·

08—

0·12

0·04

——

0·01

—

Al 2

O3

28·5

930

·95

23·6

223

·64

23·8

621

·96

24·9

826

·69

23·5

024

·85

25·3

818

·26

18·6

718

·46

18·8

418

·94

18·7

418

·40

18·6

618

·46

FeO

0·43

0·41

0·11

0·20

0·21

0·17

0·15

0·09

0·14

0·23

0·19

0·02

0·03

0·13

0·05

0·10

0·13

0·07

0·04

0·08

Mn

O—

—0·

010·

070·

07—

0·10

0·02

0·03

0·02

—0·

010·

060·

020·

050·

01—

—0·

01—

Mg

O0·

050·

030·

010·

010·

01—

0·03

0·03

0·02

0·04

0·03

0·01

0·05

0·03

—0·

020·

020·

06—

—

CaO

11·8

613

·94

5·59

5·67

6·17

3·54

6·97

8·78

5·29

7·28

7·91

0·18

0·24

0·22

0·24

0·15

0·14

0·14

0·19

0·16

Na 2

O4·

873·

327·

617·

617·

449·

047·

146·

067·

956·

736·

743·

292·

922·

903·

683·

443·

012·

922·

852·

71

K2O

0·45

0·23

0·97

0·98

0·81

1·37

0·75

0·46

0·94

0·70

0·61

11·6

712

·19

12·5

711

·22

11·6

112

·33

12·6

212

·65

12·3

5

Tota

l10

0·51

99·8

199

·39

99·3

099

·19

100·

9110

0·21

100·

2610

1·03

98·8

399

·64

98·5

498

·48

99·6

499

·81

100·

9410

1·47

100·

0698

·90

98·9

2

Ab

0·41

50·

297

0·67

10·

668

0·65

40·

760

0·62

20·

541

0·69

20·

600

0·58

60·

297

0·26

40·

257

0·32

90·

308

0·26

90·

258

0·25

30·

248

Or

0·02

60·

014

0·05

70·

057

0·04

70·

076

0·04

30·

027

0·05

40·

041

0·03

50·

694

0·72

50·

733

0·65

90·

684

0·72

40·

735

0·73

80·

744

An

0·55

90·

689

0·27

20·

275

0·29

90·

164

0·33

50·

432

0·25

40·

359

0·38

00·

009

0·01

20·

011

0·01

20·

008

0·00

70·

007

0·00

90·

008

All

Feas

FeO

.

408


Fig. 4. Ternary feldspar plots of plagioclase and sanidine from Toba rocks.

is most abundant in the rocks that have altered biotite, distinctive in having the highest TiO2 and lowest MnOof all units.and it is thought to be a post-emplacement effect that

occurred during slow cooling of the thick caldera-fill.Amphiboles are noticeably brighter green in thin sectionin the least silicic caldera-fill than in the earlier erupted Orthopyroxenematerial, where they are green–brown. Orthopyroxene was found in small amounts in all mineral

The Toba amphiboles are calcic (Table 6) and most separates from both the welded tuffs and the pumices.can be classified as ‘ferro-edenitic hornblende’ using the Rare orthopyroxene phenocrysts observed in thin sectionnomenclature of Leake (1978). Fluorine and Cl are predominantly euhedral to subhedral (<1·5 mm) andconcentrations are relatively constant. The analyses prob- contain Fe-oxide inclusions and negative crystal meltably have slightly low totals because of unanalyzed ele- inclusions (<0·1 mm) with vapor bubbles. Apatite in-ments and oxidation of Fe. Compositional variation of clusions are present but are not as abundant as inthe amphiboles (Fig. 6) is similar to that described in the other minerals. Orthopyroxene in mineral separates isbiotites. As the silica content of the host rocks increase, commonly partly replaced by uralitic amphibole andMgO and TiO2 contents of the amphibole decrease, and contains exsolution lamellae, presumably of clino-FeO and MnO increase. This variation is greatest in the pyroxene (Veblen & Buseck, 1981) (Fig. 3f ). ManyYTT, and the amphiboles in the OTT contain the most samples had both textural types of orthopyroxene, im-

plying both a stable and an unstable population wereFeO and the least MgO. The MTT amphiboles are

409


Table 5: Microprobe analyses of Toba biotite

OTT MTT YTT

Sample: 60-3 32B-3 85-3 7-1 99-1 99-4 94A5-3 63A1-4 89A2-2

SiO2 36·25 35·31 35·05 35·16 34·05 34·76 35·13 35·31 35·32

TiO2 3·55 3·70 3·52 4·50 4·69 4·41 3·51 3·88 4·49

Al2O3 12·95 13·06 12·64 13·46 13·48 13·41 12·95 13·79 13·81

FeO 25·37 24·43 25·20 24·68 24·17 23·76 22·78 21·40 20·65

MnO 0·59 0·38 0·44 0·24 0·27 0·28 0·51 0·34 0·30

MgO 8·02 7·77 8·25 7·84 7·86 8·35 8·83 9·22 9·88

CaO 0·02 0·06 0·02 0·02 0·04 0·02 0·01 0·06 0·05

Na2O 0·43 0·48 0·42 0·40 0·57 0·48 0·35 0·41 0·46

K2O 8·99 9·01 8·98 8·80 8·64 8·79 8·64 8·75 8·73

F 1·50 1·04 1·87 0·82 0·60 0·28 0·45 0·66 0·64

Cl 0·28 0·27 0·29 0·23 0·21 0·22 0·30 0·23 0·19

O=F+Cl 0·69 0·50 0·85 0·40 0·30 0·17 0·26 0·33 0·32

H2O∗ 3·05 3·19 2·79 3·35 3·40 3·58 3·44 3·41 3·46

Total 100·30 98·20 98·62 99·10 97·68 98·18 96·64 97·13 97·67

Si 2·836 2·817 2·801 2·774 2·729 2·760 2·822 2·798 2·773

Al(IV) 1·164 1·183 1·191 1·226 1·271 1·240 1·178 1·202 1·227

Total 4·000 4·000 3·992 4·000 4·000 4·000 4·000 4·000 4·000

Al(VI) 0·032 0·046 — 0·026 0·003 0·017 0·049 0·087 0·051

Fe2+ 1·660 1·630 1·684 1·628 1·620 1·578 1·530 1·418 1·356

Mn 0·039 0·026 0·030 0·016 0·018 0·019 0·035 0·023 0·020

Mg 0·935 0·924 0·983 0·922 0·939 0·988 1·057 1·089 1·156

Ti 0·209 0·222 0·212 0·267 0·283 0·263 0·212 0·231 0·265

Total 2·875 2·847 2·908 2·859 2·863 2·865 2·883 2·848 2·848

Ca 0·002 0·005 0·002 0·002 0·003 0·002 0·001 0·005 0·004

Na 0·065 0·074 0·065 0·061 0·089 0·074 0·055 0·063 0·070

K 0·897 0·917 0·916 0·886 0·883 0·891 0·885 0·885 0·874

Total 0·964 0·996 0·982 0·949 0·975 0·966 0·941 0·953 0·949

F 0·371 0·263 0·473 0·205 0·152 0·070 0·114 0·165 0·159

Cl 0·037 0·037 0·039 0·031 0·029 0·030 0·041 0·031 0·025

OH 1·591 1·700 1·488 1·765 1·819 1·900 1·845 1·804 1·816

Total 2·000 2·000 2·000 2·000 2·000 2·000 2·000 2·000 2·000

mg-no. 0·360 0·362 0·368 0·361 0·367 0·385 0·409 0·434 0·460

X(OH) 0·796 0·850 0·744 0·882 0·910 0·950 0·922 0·902 0·908

Ann (1) 0·577 0·572 0·579 0·570 0·566 0·551 0·531 0·498 0·476

Ann (2) 0·193 0·188 0·194 0·185 0·181 0·167 0·150 0·123 0·108

Ann (3) 0·122 0·136 0·108 0·144 0·150 0·151 0·127 0·100 0·089

Ann (4) 0·113 0·125 0·100 0·134 0·136 0·139 0·120 0·093 0·082

Structural formulae calculated on the basis of 12 anions. All Fe as Fe2+, and OH contents such that the hydroxyl site iscompletely filled by OH, F, and Cl. Annite activities calculated as: (1) molecular solid solution (Wones & Eugster, 1965); (2)ionic solid solution (Mueller, 1972); (3) multi-site ionic mixing (Hildreth, 1977); (4) multi-site ionic mixing (Czamanske &Wones, 1973). Ideal behavior and all Fe as Fe2+ are assumed in all calculations.

410


Fig. 5. Toba tuff biotite variation diagrams. All Fe as FeO.

present. Euhedral orthopyroxene characterizes the HDT, orthopyroxenes, MnO increases with the SiO2 contentof the host rocks.the least silicic OTT and YTT, and the MTT. Ap-

parently, orthopyroxene phenocrysts react to amphiboleas the melts evolve. Most orthopyroxene in the high-silica rocks occurs as cores in amphibole or as parts of

Clinopyroxenecrystal clots, both of which protect it from reacting withthe melt. Some orthopyroxene in the welded tuff separates Phenocrysts of clinopyroxene were observed in themay also be xenocrystic. fiamme and the matrix of the HDT. Although mineral

Orthopyroxene compositions in the tuffs range from separates from virtually all the quartz-bearing tuffs con-Wo2En50Fs48 to Wo2En24Fs74. The HDT contains the most tain a few grains of clinopyroxene, it does not appear tomagnesian orthopyroxene whereas the MTT and OTT have been stable. In these rocks it occurs entirely within

crystal clots, as inclusions in other minerals, and asare enriched in FeO (Table 7, Figs 7 and 8). In the YTT

411


Table 6: Microprobe analyses and structural formulae of Toba amphibole

OTT MTT YTT

Sample: 16-3 16-1 60-3 7-4 7-2 99-1 94A5-3 97A7-2 23A4-3

SiO2 43·54 43·18 43·43 43·37 42·96 44·25 43·66 43·93 44·54

TiO2 1·16 1·39 1·60 1·64 2·22 1·39 1·17 1·43 1·67

Al2O3 8·30 8·73 9·14 8·63 9·25 8·10 8·14 8·59 8·52

FeO 23·23 21·66 20·49 22·02 21·40 19·72 21·28 19·46 18·08

MnO 0·96 0·85 0·74 0·59 0·45 0·52 1·14 0·66 0·38

MgO 7·20 7·88 8·79 7·69 7·98 9·29 8·30 9·56 10·81

CaO 10·42 10·50 10·65 10·63 10·41 10·70 10·47 10·67 10·55

Na2O 1·98 1·83 1·84 1·70 1·90 1·80 1·80 2·12 1·77

K2O 1·04 0·99 1·12 0·92 0·73 0·78 0·92 0·91 0·71

F 0·52 0·62 0·58 0·31 — 0·11 0·45 0·60 0·40

Cl 0·28 0·19 0·23 0·15 0·11 0·08 0·25 0·22 0·10

O=F+Cl 0·28 0·30 0·30 0·16 0·02 0·06 0·25 0·30 0·19

H2O∗ 1·61 1·59 1·62 1·75 1·92 1·87 1·65 1·62 1·76

Total 99·96 99·10 99·94 99·23 99·30 98·55 98·99 99·47 99·10

Si 6·756 6·705 6·650 6·719 6·620 6·812 6·778 6·720 6·754

Al(IV) 1·244 1·295 1·350 1·281 1·380 1·188 1·222 1·280 1·246

Total 8·000 8·000 8·000 8·000 8·000 8·000 8·000 8·000 8·000

Al(VI) 0·275 0·304 0·301 0·296 0·302 0·283 0·268 0·270 0·277

Ti 0·135 0·162 0·184 0·191 0·257 0·161 0·137 0·165 0·190

Mg 1·665 1·824 2·006 1·775 1·833 2·131 1·920 2·180 2·443

Fe2+ 2·924 2·710 2·509 2·738 2·608 2·425 2·675 2·386 2·089

Total 5·000 5·000 5·000 5·000 5·000 5·000 5·000 5·000 5·000

Fe2+ 0·090 0·102 0·115 0·115 0·150 0·114 0·088 0·104 0·203

Mn 0·126 0·112 0·096 0·077 0·059 0·068 0·150 0·086 0·049

Ca 1·733 1·747 1·747 1·764 1·719 1·765 1·742 1·749 1·714

Na 0·050 0·039 0·041 0·043 0·073 0·053 0·021 0·062 0·034

Total 2·000 2·000 2·000 2·000 2·000 2·000 2·000 2·000 2·000

Na 0·545 0·512 0·505 0·467 0·495 0·485 0·521 0·567 0·487

K 0·206 0·196 0·219 0·182 0·144 0·153 0·182 0·178 0·137

Total 0·751 0·708 0·724 0·649 0·639 0·638 0·704 0·745 0·624

F 0·255 0·305 0·281 0·152 — 0·054 0·221 0·290 0·192

Cl 0·074 0·050 0·060 0·039 0·029 0·021 0·066 0·057 0·026

OH 1·671 1·645 1·659 1·809 1·971 1·926 1·713 1·653 1·782

Total 2·000 2·000 2·000 2·000 2·000 2·000 2·000 2·000 2·000

mg-no. 0·346 0·384 0·424 0·377 0·394 0·450 0·397 0·458 0·511

fe-no. 0·654 0·616 0·576 0·623 0·606 0·550 0·603 0·542 0·489

X(OH) 0·836 0·823 0·830 0·904 0·986 0·963 0·857 0·826 0·891

Ptot(kbar) 2·97 3·30 3·52 3·21 3·65 2·76 2·84 3·10 2·98

Structural formulae calculated on the basis of 23 oxygens, all Fe as FeO, and hydroxyl site assumed to contain 2(OH+F+Cl).Ptot estimates from technique of Hammarstrom & Zen (1986) and Johnson & Rutherford (1989a). ∗H2O determined from OH.

412


Fig. 6. Toba tuff amphibole variation diagram. All Fe as FeO.

exsolution lamellae in orthopyroxene. The clino- sometimes encountered. Melt inclusions of various typespyroxenes are generally subhedral (<2 mm) and typically are abundant and include (1) negative crystal shapes, (2)contain inclusions of apatite, Fe-oxides, and plagioclase. rounded equidimensional, and (3) very narrow elongateNegative crystal melt inclusions (<0·05 mm) are common, inclusions that parallel the c-axis and can be almost asand some contain vapor bubbles. Their compositions are long as the host phenocryst. Vapor bubbles are commonapproximately Wo43En40Fs17 (Table 7, Fig. 7). in all inclusion types. The YTT zircons appear to contain

more melt inclusions than in other units. Pink zirconsoccur only in the OTT. Fielding (1970) attributed thepink–red color of zircon to Nb4+ ions produced byFayaliteradiation-induced reduction of Nb5+.Euhedral, often doubly terminated fayalite phenocrysts

Zircons in the YTT and OTT are generally highly(<0·5 mm) occur in most of the YTT and in the mostelongate with aspect ratios up to 20:1. By contrast, zirconssilicic MTT rocks. They have been observed only inin MTT, and especially in HDT, are stubby with aspectmineral separates and are rare. Most of the phenocrystsratios of only about 2:1, and their melt inclusions are alsoare oxidized to a fine-grained, opaque intergrowth ofmore equidimensional. Some observations concerningiron-oxides and quartz (Fig. 3g). Larger grains retain

fresh cores, and the oxidation aggregates have rod-like zircon morphology are: (1) rapid crystallization favorsshapes penetrating toward these cores, perpendicular to long prismatic crystals (Larsen & Poldervaart, 1958;the c-axis. Inclusions were not seen. Analyses of fayalite Kostov, 1977); (2) lower crystallization temperatures favorgrains from one sample show almost no variation. An (110) prisms over (100) prisms (Pupin & Turco, 1972,average composition is: Fa 0·938, Fo 0·001, Te 0·061, 1975); (3) crystallization from hydrous magmas is likely[aFeSiO4 = 0·878]. to yield (110) prisms (Pupin et al., 1978). Toba zircons

are consistent with these observations in that (110) prismsoccur only in rocks with the most evolved compositions.

Electron microprobe analyses of zircons show littleZirconcompositional variation among the different eruptiveZircon is present in all Toba rocks and is most abundantunits. Hafnium contents range from 1·36 to 3·52%;in the quartz-bearing tuffs. It occurs as elongate, doubly-HfO2/ZrO2 ratios range from 0·020 to 0·055, averagingterminated, euhedral (<0·3 mm) phenocrysts. Zircon is0·031. The YTT has the highest ratios, and the MTTa common inclusion in biotite, amphibole, and allanite.the lowest, but no correlation with whole-rock chemistryNeedle-shaped to rod-like apatite inclusions are common

(Fig. 3h); magnetite and small zircon inclusions are also is discernible.

413


Table 7: Microprobe analyses and structural formulae of Toba pyroxenes

OPX CPX

HDT OTT MTT YTT HDT

Sample: 9-1 9-7 74-3 74-2 7-5 99-3 99-1 94A5-2 63A1-2 89A2-4 9-2 9-3

SiO2 52·03 51·77 50·49 50·48 48·93 49·14 50·11 50·45 50·65 50·39 52·86 52·59

TiO2 0·01 0·22 0·09 0·10 0·02 0·11 0·10 0·05 0·07 0·04 0·31 0·19

Al2O3 0·85 1·35 0·44 0·50 0·26 0·31 0·41 0·27 0·35 0·39 1·35 0·66

FeO 24·56 22·69 34·94 33·11 40·52 39·44 32·61 31·78 32·54 30·79 11·36 9·70

MnO 0·68 0·61 1·82 1·91 2·64 2·51 1·57 4·19 3·31 1·85 0·35 0·29

MgO 20·33 20·95 10·81 12·38 7·31 7·88 13·27 11·68 12·72 14·74 13·56 13·94

CaO 1·26 1·27 1·16 1·02 1·02 1·06 1·01 1·04 1·05 0·99 20·30 21·25

Na2O — — 0·04 0·02 0·05 — 0·01 — — 0·02 0·28 0·23

K2O — — — — — — — — 0·01 — — —

Total 99·72 98·86 99·79 99·52 100·75 100·45 99·09 99·46 100·70 99·21 100·37 98·85

Si 1·972 1·962 2·019 2·008 2·004 2·006 1·997 2·017 1·998 1·990 1·975 1·988

Al(IV) 0·028 0·038 — — — — 0·003 — 0·002 0·001 0·025 0·012

Total 2·000 2·000 2·019 2·008 2·004 2·006 2·000 2·017 2·000 2·000 2·000 2·000

Al(VI) 0·010 0·022 0·040 0·031 0·016 0·021 0·016 0·029 0·014 0·009 0·034 0·017

Ti — 0·006 0·003 0·003 0·001 0·003 0·003 0·002 0·002 0·001 0·009 0·005

Mg 1·148 1·183 0·644 0·734 0·446 0·479 0·788 0·696 0·748 0·868 0·755 0·785

Fe2+ 0·778 0·719 1·168 1·101 1·388 1·346 1·087 1·062 1·073 1·017 0·355 0·307

Mn 0·022 0·020 0·062 0·064 0·092 0·087 0·053 0·142 0·111 0·062 0·011 0·009

Ca 0·051 0·052 0·050 0·043 0·045 0·046 0·043 0·045 0·044 0·042 0·813 0·861

Na — — 0·003 0·002 0·004 — 0·001 — — 0·002 0·020 0·017

K — — — — — — — — 0·001 — — —

Total 2·009 2·002 1·970 1·978 1·991 1·983 1·991 1·976 1·992 2·000 1·997 2·001

F/FM 0·411 0·384 0·656 0·614 0·768 0·749 0·591 0·634 0·613 0·554 0·327 0·287

aFeSiO3(1) 0·387 0·359 0·593 0·557 0·697 0·679 0·546 0·538 0·539 0·509 — —

aFeSiO3(2) 0·468 0·442 0·645 0·615 0·730 0·715 0·606 0·599 0·600 0·575 — —

Wo 0·026 0·026 0·027 0·023 0·024 0·025 0·022 0·025 0·024 0·022 0·423 0·441

En 0·580 0·606 0·346 0·391 0·237 0·256 0·411 0·386 0·401 0·450 0·393 0·402

Fs 0·394 0·368 0·627 0·586 0·739 0·719 0·567 0·589 0·575 0·528 0·185 0·157

Structural formulae calculated on the basis of six oxygens, with all Fe as FeO. Ferrosilite calculated as (1) ideal mixing, and(2) non-ideal mixing (Williams, 1971).

low-SiO2 pumices was attributed to 0·052% allaniteAllanitefractionation (Fig. 9). Chesner & Ettlinger also determinedThis light rare earth element (LREE)-rich accessory phasethat allanite La and Ce concentrations decline withis present in thin sections of all samples from the quartz-increasing magmatic SiO2 contents and decreasing tem-bearing tuffs. It is commonly euhedral (<0·5 mm) andperatures, whereas Pr, Nd, Sm, and Th increase withtwinned, and it is pleochroic from deep red–brown toincreasing SiO2. In addition, they concluded that allanitelight green. Apatite rods and needles are abundant asoccurs only in volcanic rocks with pre-eruptive tem-inclusions; magnetite and zircon are less common. Neg-peratures below 800°C.ative crystal melt inclusions (<0·05 mm) are prevalent.

Chesner & Ettlinger (1989) used Rayleigh calculations toestimate allanite modes of 0·035 and 0·016 wt % for Fe–Ti oxideslow- and high-SiO2 YTT pumices, respectively. A REE Ovoid to subhedral phenocrysts of titanomagnetite

(<0·4 mm) and ilmenite (<0·2 mm) occur in all Toba‘cross-over’ pattern for glasses separated from high- and

414


Fig. 7. Stable Toba orthopyroxene and clinopyroxene compositions plotted on the pyroxene projection.

Fig. 8. Toba tuff orthopyroxene variation diagram. All Fe as FeO.

rocks and are most abundant in those with the least minerals, parts of crystal clots in some of the less silicicrocks, and reaction products after biotite and amphibole.evolved compositions. Ilmenite is rare. Fe–Ti oxides

occur as discrete phenocrysts, inclusions in all other Magnetite separates from pumices show little sign of

415


Fig. 9. REE plot of Toba allanite separates and associated glasses and whole-rock samples, normalized to chondrite. LG and HG representglasses separated from low- and high-SiO2 YTT pumice samples, respectively; LWR is a low-SiO2 whole-rock pumice sample (from Chesner &Ettlinger, 1989).

oxidation, but those from welded tuffs commonly exhibit inclusions in most minerals, but especially in biotite,amphibole, zircon, and allanite. Apatite occurs only asoxidation and (or) exsolution features similar to those

described by Haggerty (1976). These oxidation reactions inclusions in all Toba rocks except the HDT, wheretabular phenocrysts (<0·1 mm× 0·05 mm) occur. Ap-may stem from slow cooling during welding. No ex-

solution or oxidation was observed in ilmenite. atite has not been analyzed.Ulvospinel mole fraction contents of magnetite range

from 0·365 to 0·185, ilmenite contents of ilmenite from0·856 to 0·944 (Table 8). The most silicic rocks contain Exotic phasesmagnetite and ilmenite with the lowest Al2O3 and MgO, Microprobe analyses of the heaviest non-magnetic min-and the highest MnO concentrations. Compositions of erals separated from pumices and welded tuffs revealedFe–Ti oxides from different tuffs cannot be distinguished the presence of sphalerite, galena, pyrite, a Cu–Zn–Snon variation diagrams, with the exception of the HDT sulfide, and a Cu–Zn–As sulfide mineral. Cassiterite(Fig. 10). and a Cu–Zn–Ni oxide were also encountered, as were

epidote, staurolite, orthoferrosilite, perovskite, and mon-azite. All these minerals are extremely rare, typically

Pyrrhotite occurring in amounts of only one or two grains persample. It is not clear whether these phases are partRounded blebs of pyrrhotite (<0·04 mm) are common

in titanomagnetite, and occur less frequently in ilmenite, of Toba’s primary mineralogy, vapor phase minerals,xenocrysts from assimilated country rocks, or some com-orthopyroxene, amphibole, allanite, and zircon. Four or

five separate blebs may occur in a single magnetite bination thereof.phenocryst. Because they exist as rounded blebs andoccur in several phases, their shape is thought to representblebs of immiscible sulfide liquid, and not a reaction Calculated modes (YTT pumice)with the host (Whitney, 1984). Occasionally, the Toba The crystal content of YTT pumice has been estimatedpyrrhotite blebs contain a smaller bleb of chalcopyrite. by Caress (1985) at 35 wt %, whereas Aldiss & Ghazali

(1984) reported a range of 0–40% crystals. Besides theseestimates, no systematic determination of modal min-

Apatite eralogy has been reported. In this study, modes forindividual pumice samples were calculated by least-Elongate rods and needles of apatite up to 0·15 mm in

length with aspect ratios of 20:1 or greater occur as squares mixing calculations using whole-rock, glass, and

416


Tab

le8:

Mic

ropr

obe

anal

yses

and

ulvo

spin

elan

dilm

enite

cont

ents

for

unox

idiz

edT

oba

mag

netite

and

ilm

enites

Mag

net

ite

HD

TO

TT

MT

TY

TT

Sam

ple

:9-

19-

39-

232

B-2

60-4

85-4

74-1

7-3

8-8

99-3

99-2

94A

5-1

63A

1-3

30-2

89A

2-1

SiO

20·

070·

070·

110·

130·

170·

130·

100·

160·

090·

160·

120·

090·

130·

160·

13

TiO

210

·94

11·2

011

·67

6·76

6·87

7·71

10·0

38·

589·

229·

199·

596·

386·

518·

438·

70

Al 2

O3

2·04

2·33

2·64

1·39

1·52

2·04

1·69

1·59

1·56

1·63

1·45

1·40

1·58

1·89

1·96

Fe2O

344

·60

44·0

242

·09

54·0

653

·75

51·3

145

·12

49·5

948

·71

48·4

047

·45

54·3

254

·32

48·6

748

·82

FeO

39·2

138

·94

39·6

636

·19

36·7

837

·40

38·6

438

·22

38·4

338

·67

38·8

035

·44

35·9

537

·62

37·6

3

Mn

O0·

330·

360·

310·

890·

790·

740·

540·

720·

680·

650·

671·

070·

900·

320·

67

Mg

O1·

061·

431·

220·

390·

260·

320·

330·

170·

390·

300·

240·

340·

410·

480·

57

CaO

0·18

0·21

0·21

0·04

0·06

0·04

0·02

0·02

—0·

060·

010·

030·

040·

050·

05

Tota

l98

·43

98·5

697

·91

99·8

510

0·20

99·6

996

·47

99·0

599

·08

99·0

698

·33

99·0

799

·84

97·6

298

·53

Sto

%U

lv0·

333

0·34

00·

365

0·19

90·

205

0·23

60·

314

0·26

10·

277

0·28

00·

292

0·18

90·

193

0·26

30·

266

An

d%

Ulv

0·31

80·

320

0·34

70·

192

0·20

20·

231

0·30

70·

257

0·27

00·

275

0·28

70·

180

0·18

60·

257

0·25

8

Car

%U

lv0·

315

0·32

00·

337

0·19

80·

202

0·22

40·

298

0·25

20·

267

0·26

90·

282

0·18

70·

190

0·25

00·

255

Lin

%U

lv0·

325

0·33

30·

353

0·18

80·

193

0·22

10·

301

0·24

80·

266

0·26

70·

280

0·17

50·

181

0·25

30·

254

417


Ilmen

ite

HD

TO

TT

MT

TY

TT

Sam

ple

:9-

39-

19-

260

-385

-374

-432

B-1

7-2

99-2

8-4

63A

1-2

30-1

89A

2-1

94A

5-5

SiO

20·

05—

—0·

010·

06—

0·05

0·04

0·06

0·01

——

0·04

0·01

TiO

244

·71

44·3

744

·60

48·3

647

·69

48·3

347

·63

48·4

048

·19

48·4

547

·07

48·5

047

·23

46·3

7

Al 2

O3

0·41

0·33

0·37

0·23

0·22

0·11

0·26

0·18

0·17

0·17

0·20

0·26

0·26

0·11

Fe2O

313

·88

14·3

614

·37

7·00

7·21

7·48

7·75

6·83

7·43

7·97

8·43

8·97

8·97

10·6

4

FeO

35·5

437

·04

36·3

140

·36

40·4

239

·36

40·3

640

·59

40·5

040

·85

38·7

739

·99

39·0

538

·19

Mn

O0·

370·

500·

391·

301·

401·

781·

601·

551·

371·

381·

821·

101·

881·

94

Mg

O2·

441·

321·

911·

020·

631·

290·

510·

790·

850·

750·

961·

410·

880·

87

CaO

0·05

0·07

0·06

0·03

——

—0·

02—

—0·

01—

0·03

0·02

Tota

l97

·45

97·9

998

·01

98·3

197

·63

98·3

598

·16

98·4

098

·57

99·5

897

·26

100·

2398

·34

98·1

5

Sto

%Il

0·85

80·

856

0·85

50·

930

0·92

80·

925

0·92

30·

932

0·92

60·

922

0·91

40·

912

0·91

00·

893

An

d%

Il0·

851

0·85

10·

849

0·92

80·

926

0·92

10·

920

0·93

00·

924

0·91

90·

911

0·90

80·

906

0·88

9

Car

%Il

0·86

00·

856

0·85

60·

929

0·92

70·

927

0·92

10·

931

0·92

60·

922

0·91

50·

912

0·90

90·

895

Nin

%Il

0·86

50·

860

0·86

10·

931

0·92

80·

925

0·92

20·

932

0·92

60·

922

0·91

50·

914

0·91

00·

893

Co

mp

osi

tio

ns

reca

lcu

late

du

sin

gth

ete

chn

iqu

eso

fC

arm

ich

ael

&N

ich

olls

(196

7),

An

der

son

(196

8),

Lin

dsl

ey&

Sp

ence

r(1

982)

,an

dS

torm

er(1

982)

.

418


Fig. 10. (a) Toba tuff magnetite variation diagram. (b) Toba tuff ilmenite variation diagram. FeO–Fe2O3 calculated following Stormer (1983).

mineral compositions (quartz, sanidine, plagioclase, bio- Modes calculated by this technique indicate that thecrystal content for the YTT pumices ranges from 12 totite, amphibole, magnetite, and ilmenite). Other phases

present in the Toba rocks, such as allanite, zircon, 40 wt %, consistent with previous estimates. Generally,the low-SiO2 pumices contain 30–40 wt % crystals,pyrrhotite, orthopyroxene, and fayalite, occur in trace

amounts; therefore their modes could not be calculated whereas the high-SiO2 pumices have only 12–25 wt %.Systematic variations in mineral proportions with magmain this manner. The least sodic plagioclase and sanidine

compositions (representing core compositions), were used composition are distinct. The proportions of felsic andmafic minerals change from about 3:1 in the least silicicin the mixing calculations and produced excellent results

(R2 < 0·1). samples to 10:1 in the most silicic samples. A decrease

419


in plagioclase (from 23·1 to 4·4 wt %), biotite (from 4·0 the YTT range from 68 to 77% SiO2. The maximumcompositional variation among pumices at any one out-to 0·6 wt %), amphibole (from 4·0 to 0·1 wt %), magnetite

(from 1·4 to 0·2 wt %), and ilmenite (from 0·4 wt % to crop is ~5% SiO2. About half of the YTT outcropscontain only one pumice composition (high or low SiO2),trace) also accompanies the change to more silicic magma

compositions. Sanidine is absent in the least silicic pum- whereas in the other half there is a subordinate secondcomposition. Single composition outcrops are more com-ices or occurs only in trace amounts but its concentration

increases to 6% in more silicic samples. Quartz ranges mon outside the caldera, whereas multiple compositionoutcrops usually occur inside the caldera. The YTTfrom 2 to 8% and is variable throughout the suite.

Relationships among phases in the calculated modes are welded tuffs have more restricted compositions, 70–75%SiO2, and those on Samosir Island have only 70–72%consistent with petrographic observations.SiO2. Geographical distribution maps of YTT magmacompositions (Fig. 14b and c) indicate that the outflowsheet and tuffs within the Prapat Graben are pre-

WHOLE-ROCK CHEMISTRY dominantly composed of high-SiO2 magma (>73% SiO2).Magmas with <73% SiO2 occur almost exclusively onCompositional variation of the Toba TuffsSamosir Island and the caldera rim. The only area whereWhole-rock compositions of pumice, fiamme, weldedlow-SiO2 tuff is abundant outside the caldera is south oftuffs, and glass from the four Toba tuffs show similar,Muara (Fig. 1). It would appear, therefore, that the leastoverlapping trends on variation diagrams (Table 2,silicic magma was erupted during the waning stages ofFig. 11). In the quartz-bearing tuffs, all trace elementsthe OTT and YTT eruptions. Similar relationships occuranalyzed behave compatibly except Rb, Nb, and perhapsin many San Juan calderas (Lipman, 1984). In contrast,Cu, Ni, and Y, although their concentrations are nearMTT became steadily more silicic and less magnesiandetection limits (Table 2). Incompatible elements in theduring eruption (Fig. 15).HDT are Zr, Ba, La, and Ce.

To compare the compositions of the Toba tuffs, onlyPumices and fiamme are generally more silicic thananalyses on welded tuff samples were used because in-welded tuff samples, but they show the same trends.dividual pumices in the HDT, OTT, and MTT wereGaps are present in the compositional trends of the OTTrare. Generally, the YTT, MTT, and OTT are difficultand MTT welded tuffs and fiamme (Fig. 12). Althoughto distinguish on chemical variation diagrams, but subtlecompositional gaps in YTT welded tuffs and pumicedifferences do exist (Fig. 16). Although YTT and MTTsamples are debatable, the samples appear to split intoare both normally magnetized, they are distinct in handtwo chemical populations (Fig. 12). Glass separates usuallysample (Chesner & Rose, 1991), and MTT is dis-have the most evolved compositions for any unit, typicallytinguishable by lower Sr and Rb, and higher Ba, Ce,plotting at the ends of the whole-rock trend lines (Fig. 11).and SiO2. In contrast, YTT and OTT have differentOnly on the plot of Sr–Ba for the YTT (Fig. 13) domagnetic polarities, but they are very similar in ap-glasses define their own trend. This trend diverges frompearance and composition (Fig. 16). The OTT is, how-the whole-rock trend in less evolved samples.ever, slightly higher in Rb and lower in Ba and Sr atLimited REE data on YTT glass separates (Fig. 9)equivalent SiO2. On major element plots, the HDTshow similar patterns with two important exceptions.appears to define a lower-SiO2 extension of the OTTHigh-SiO2 glasses have lower LREE but higher HREEchemical trends (Fig. 11). However, some trace elementscontents than low-SiO2 glasses, producing a ‘cross-over’in the HDT are offset from the OTT trends (Fig. 11).pattern. In addition, they have a strong Eu anomalyThe OTT, MTT, and YTT also have overlapping glasscompared with the subtle one of the low-SiO2 glasses.compositions; however, they can be distinguished by theirREE patterns for the MTT and OTT glasses show theBa, Sr, and CaO contents (Fig. 17).same relationships and overlap those of the YTT. The

REE pattern of a low-SiO2 YTT pumice is also plottedin Fig. 9. It too demonstrates the ‘cross-over’ pattern andhas the smallest Eu anomaly. The pattern of a high-SiO2 DISCUSSIONYTT pumice falls between the two glass samples and

Magma chamber conditionswas not plotted.In the OTT and MTT, compositions of fiamme vary Mineral chemistry, whole-rock geochemistry, and geo-

graphical distribution of magma compositions imply thatby as much as 11 and 4 wt % SiO2 in the same outcrop,respectively. This type of variation is observed mainly the Toba magma bodies that erupted the OTT, MTT,

and YTT were compositionally zoned before eruption.near the top of the OTT and MTT. All low-SiO2

(<72·5% SiO2) OTT welded tuff samples occur on the To further constrain the conditions under which the Tobamagmas evolved, intensive parameters were determinedUluan Block; none were found in the caldera walls or

the outflow sheet (Fig. 14a). Pumice compositions in from mineral and whole-rock chemistry (Fig. 18).

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421


Fig. 12. Compositional gaps in the OTT, MTT, and possibly the YTT. For each unit, gaps are most apparent in the fiamme–pumice plots,but are also apparent in the welded tuff plots.

422


Fig. 13. YTT pumices and their corresponding glass separates. Tie lines connect pumice and glass analyses from the same sample.

in this study. Sommer (1977) attempted homogenizationTemperature and oxygen fugacitydeterminations on melt inclusions in quartz from theTemperature and oxygen fugacities were calculated usingBandelier Tuff and also concluded that the temperaturesmicroprobe compositions of coexisting magnetite andare much higher than those calculated from Fe–Ti oxides.ilmenite following the techniques of Stormer (1983) andThe discrepancy between Fe–Ti oxide temperatures andAnderson & Lindsley (1988) (Table 9, Fig. 19). Equi-melt inclusion homogenization temperatures could belibrium between magnetite and ilmenite pairs was evalu-related to timing of crystallization. Van den Bogaard &ated using the Mg–Mn partitioning test (Bacon &Schirnick (1995) have demonstrated that quartz pheno-Hirschmann, 1988). The OTT and YTT both showcrysts in the Bishop Tuff crystallized ~1·3 my beforesimilar temperature zonation; the values for the MTTeruption. Thus, crystallization of quartz may have takenindicate a narrow temperature range and slightly lowerplace earlier and at a higher temperature than Fe–Tif (O2) at similar temperatures, whereas the HDT showedoxides in some silicic magmas.much higher temperatures. In quantitative terms, the

HDT was erupted at 847°C. The OTT temperaturesPressurerange from 704 to 759°C, except for a sample of vitro-

phyre (No. 27), which gave a value that seems an- Using Johnson & Rutherford’s (1989a) modifications toomalously low, 681°C. Temperatures for the MTT range the amphibole geobarometer defined by Hammarstromfrom 743 to 751°C; those for the YTT pumices and & Zen (1986), the Toba samples show a range from 2·6welded tuffs range from 701 to 780°C. Temperatures to 3·4 kbar with most samples close to 3 kbar (Table 9).for individual units range from 8 to 79°C and vary The Johnson & Rutherford (1989a) calibration was usedsystematically with silica and phenocryst contents. Oxy- in this study because it was determined over a temperaturegen fugacities generally range between –16 and –14 log range (720–780°C) that overlaps with temperatures de-units in the quartz-bearing tuffs, with lower values in the termined for the Toba tuffs. Modifications to the am-more silicic magmas. phibole geobarometer by Schmidt (1992) (determined at

Beddoe-Stephens et al. (1983) found that vapor bubbles cooler conditions than Toba magmas; 655–700°C) andhomogenize with melt at ~960°C in Toba quartz, san- Anderson & Smith (1995), yielded total pressures ~1idine, and plagioclase. This inclusion trapping tem- kbar higher than those determined from the Johnson &perature is ~200°C hotter than their estimated eruption Rutherford (1989a) calibration. Toba rocks contain all

the minerals required for application of the amphiboletemperatures and those determined from Fe–Ti oxides

423


Fig. 14. (a) Geographic distribution of high- and low-SiO2 welded tuffs of the OTT. Boundary between high- and low-SiO2 samples is 72·5%SiO2. (b) Geographical distribution of high- and low-SiO2 welded tuffs of the YTT. Distinction between high- and low-SiO2 compositionalvarieties is ~73% SiO2. (c) Geographical distributions of YTT pumice compositions. Division into high- and low-SiO2 compositional types is at73% SiO2.

424


Fig. 15. Reverse compositional zoning in an MTT composite stratigraphic section. Samples 1, 2, 4, 5, 6, and 7 are from the MTT section atHaranggoal. Sample 8 is from the top of the MTT section near Sipisupisu. Sample 3 is partially vitrophyric MTT exposed near Silalahi, andis placed stratigraphically between dense vitrophyre (sample 2) and devitrified welded tuff (sample 4).

geobarometer except sphene. As the TiO2 content of the determined for low- and high-SiO2 YTT pumices, re-Toba amphiboles is similar to that of the amphiboles used spectively (Newman & Chesner, 1989). These data, inin the experimental calibrations, and the melt–amphibole conjunction with CO2 contents of 0–200 ppm (NewmanTiO2 content is buffered by coexisting Fe–Ti oxides, the & Chesner, 1989), were used to estimate gas saturationabsence of sphene in the Toba mineral assemblage should pressures of 1·5–2 kbar for these samples (Stolper et al.,have a negligible effect on the pressure estimates ( Johnson 1987; Newman et al., 1988; Anderson et al., 1989; Silver& Rutherford, 1989a). Glass compositions plotted on pres- et al., 1990). Gas saturation pressures are lower than totalsure-calibrated An–Ab–Or and Ab–Or–Q phase diagrams pressure estimates (3 kbar) and reflect minimum confiningof the granite system (Tuttle & Bowen, 1958; Luth et al., pressures, as the melts may not have been saturated1964; James & Hamilton, 1969; Whitney, 1972, 1975) with respect to H2O and CO2. However, according toalso suggest confining pressures between 2 and 4 kbar. Anderson (1991), the formation of ‘hourglass’ inclusions

Assuming 3 kbar as the confining pressure, the Toba (which occur in YTT quartz) is contingent upon gasmagmas may have resided in the crust at a depth of saturation.~10 km. Similar pressure estimates suggest that all Toba The fugacity of H2O in the Toba magmas was es-magmas equilibrated at approximately the same depth timated by the methods of Wones & Eugster (1965) andin the crust, implying broadly stationary or recurrent Wones (1972), because biotite and sanidine are present.magma chamber roof depths for the last 1·2 my. Magma Annite contents were estimated by four different methodschamber depths inferred from total pressure calculations (Table 5). Annite determined by the molecularfor the Toba tuffs are greater than those reported for solid-solution model of Wones & Eugster (1965) yieldedthe Bishop Tuff (5–6 km; Anderson et al., 1989; Johnson

f (H2O) typically greater than total pressures calculated& Rutherford, 1989a) and Fish Canyon Tuff (7–9 km;

from the amphibole geobarometer and is consideredJohnson & Rutherford, 1989b) but similar to those ofunrealistic. The ionic-solution of Mueller (1972) and thethe tuffs from the Southwestern Nevada Volcanic Fieldmulti-site mixing models of Czamanske & Wones (1973)(10 km; Warren et al., 1989).and Hildreth (1977) gave similar values in the range of0·5–1·0 kbar (equivalent water pressures of 0·6–1·4 kbar;

Water Burnham et al., 1969). Both these values and the gassaturation pressures estimated from fluid inclusions sug-Using IR spectroscopic techniques on melt inclusions

in quartz, water concentrations of 4·9–5·7 wt % were gest that water oversaturation was unlikely to have been

425


Fig. 16. Comparison of YTT, MTT, and OTT welded tuff samples.

a triggering mechanism for the Toba eruptions, if the (8–20 bar); others have fugacities lower than 10–1 bar.magmas equilibrated at 3 kbar. However, if the total The fugacities of all sulfur species are expected to decreasepressure estimates are in error, then volatile with dropping temperature in both closed and openoversaturation could have initiated the eruptions from 5 systems from degassing to the surface and possibly theto 6·5 km below the surface. crystallization of pyrrhotite (Whitney, 1984). The Toba

magmas are on the low end of the temperature spectrumfor rhyolitic magmas and thus have very low sulfur

Fugacities of sulfurous gases and H2 fugacities. It is not apparent which of the above-men-tioned mechanisms is most responsible for the low sulfurThe fugacities of S2, SO2, and H2S in the Toba magmasfugacities of Toba magmas (an order of magnitude smaller(Table 9) were calculated following the method of Whit-than those of the Bishop Tuff; Whitney, 1984). Theney (1984). Free energy data from Robie et al. (1979) wasfugacity of H2 was estimated from water and oxygenused to estimate the fugacities of SO3 and HS (Table 9).

The dominant sulfur species in the Toba magmas is H2S fugacities and free energy data (Robie & Waldbaum,

426


Fig. 17. Comparison of YTT, MTT, and OTT glass separates. Glasses from the MTT and OTT are from welded tuff samples, whereas YTTglasses are from both pumice and welded tuff samples.

1968) to be in the range of 3–5 bar for Toba magmas of Lange & Carmichael (1989) and Lange (1994).(Table 9). Anhydrous rock densities range from 2·70 to 2·63 g/cm3

with increasing magmatic evolution, whereas hydrous-liquid densities range from 2·33 to 2·25 g/cm3.

Density and viscosity Magmatic densities lie between the hydrous-liquid andanhydrous-rock densities because the magmas are partlyDensities for melts and magmas represented by some

of the YTT pumices were calculated using the method crystalline. Thus, anhydrous-liquid density estimates

427


Fig. 18. Schematic summary of physical conditions within the magma chambers that erupted the Toba tuffs. Qt, quartz; Pl, plagioclase;Sa, sanidine; Bi, biotite; Am, amphibole; Al, allanite; Zi, zircon; Mg, magnetite; Il, ilmenite; Fa, fayalite; Ap, apatite; Opx, orthopyroxene;Cpx, clinopyroxene; xtals, crystals.

(2·49–2·41 g/cm3) are used to approximate the mag- for the HDT, the Toba tuffs evolved under similarconditions at depths of ~10 km. These magmas werematic density.

Viscosities for YTT magmas were calculated by the zoned with gradients in composition, mineral contentand chemistry, temperature, f (O2), water contents, dens-method of Shaw (1972). Using an estimated mean crystal

size of 1 mm, the method of Sherman (1968) as modified ity, and possibly f (H2S) (Fig. 18). Viscosity gradients werenot strong within the erupted part of the magma bodies.by McBirney & Murase (1984) gave unrealistic effective

viscosities of 1020 poise. Therefore, the Einstein–Roscoe These gradients may have developed in 0·34–0·43 my,the repose periods between tuff eruptions (Chesner &equation (Roscoe, 1953) was applied using an R value

of 1·67 as suggested by Marsh (1981). Thus, viscosities are Rose, 1991).calculated dependent upon temperature (Fe–Ti oxides),pressure (amphibole), water content (melt inclusions), andcrystal content (calculated modes). Viscosities of the pre- Origin of compositional zonationeruptive hydrous melt phase increases from ~106 to 107

Numerous ignimbrites appear to have erupted frompoise with magma evolution, but the effective viscosityshallow magma bodies that were compositionally zonedof the crystal–liquid mush remains almost constant at(Smith, 1979; Hildreth, 1981; Ferriz & Mahood, 1987;~107 poise. According to these calculations, the coun-Grunder & Mahood, 1988). Many of these occurrencesteracting effects of gradients in composition, temperature,have shown evidence of compositional zonation at-H2O and crystallinity result in no apparent viscositytributable to crystal fractionation, notably the Bishopgradients in the YTT magma chamber.Tuff (Michael, 1983; Cameron, 1984) and the Los Hum-eros tuffs (Ferriz & Mahood, 1987). In this section,

Gradients evidence for crystal fractionation as the major causeof chemical zonation in Toba’s magma bodies will beEstimates of ranges in intensive parameters within the

pre-eruptive Toba magma chambers indicate that, except presented.

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Table 9: Intensive parameters for Toba samples

HDT OTT MTT

Sample: 9 27 16 60 32B 85 15A 74 7 99 8

T(°C) 847 681 705 704 720 708 722 759 743 743 751

f(O2) (log) −12·5 −17·6 −16·1 −16·2 −15·5 −16·0 −15·4 −15·1 −15·4 −15·4 −15·1

Ptot(kbar) — — 3·0 2·8 3·1 2·9 — 3·4 3·0 2·8 —

f(H2O) (bar) — — 700 600 900 700 — — 700 800 —

f(H2) (bar) — — 3·3 3·1 3·5 3·5 — — 5·0 5·2 —

f(S2) (log) — — −3·8 −3·8 −3·2 −3·6 — — — −3·7 —

f(SO2) (log) — — −2·7 −2·7 −1·9 −2·4 — — — −2·4 —

f(H2S) (bar) — — 9·3 8·1 14·0 10·8 — — — 11·6 —

f(HS) (log) — — −5·2 −5·2 −4·8 −5·1 — — — −4·4 —

f(SO3) (log) — — −10·2 −10·3 −9·3 −10·0 — — — −9·5 —

YTT—Pumices

Sample: 20A2 94A5 22A2 51A5 23A4 97AA7 63A1 84A4 90A7 6A2 21A5 89A2

T(°C) 701 710 715 737 724 716 736 742 730 739 756 780

f(O2) (log) −16·0 −15·7 −15·4 −14·8 −15·2 −15·5 −15·4 −14·6 −14·9 −14·6 −14·4 −14·0

Ptot(kbar) — 2·6 — 3·0 3·0 2·6 2·9 2·8 2·9 — — 3·0

f(H2O) (bar) — 700 — 1100 900 800 1000 — 1300 — — 1000

f(H2) (bar) — 2·5 — 3·2 2·8 2·7 2·6 — 3·3 — — 3·8

f(S2) (log) — — — −2·5 — −2·9 −2·5 — — — — −2·6

f(SO2) (log) — — — −1·3 — −1·2 −1·1 — — — — −1·2

f(H2S) (bar) — — — 19·5 — 12·1 18·5 — — — — 19·1

f(HS) (log) — — — −4·4 — −4·8 −4·5 — — — — −4·2

f(SO3) (log) — — — −8·4 — −9·1 −8·2 — — — — −9·2

YTT—welded

Sample: 18 94B 11 87 98 32 100 30

T (°C) 717 719 713 716 740 740 754 761

f(O2) (log) −15·6 −15·6 −15·6 −15·5 −14·9 −14·7 −14·9 −14·6

Ptot(kbar) — 2·9 2·9 2·6 2·9 — 2·8 —

f(H2O) (bar) — 700 800 800 1100 — 800 —

f(H2) (bar) — 2·8 2·8 2·8 3·7 — 4·4 —

f(S2) (log) — −2·9 −2·9 −2·9 −2·5 — −3·0 —

f(SO2) (log) — −1·2 −1·2 −1·2 −1·6 — −1·6 —

f(H2S) (bar) — 10·6 12·5 12·7 18·7 — 13·9 —

f(HS) (log) — −4·7 −4·8 −4·8 −4·4 — −4·5 —

f(SO3) (log) — −9·3 −9·2 −9·1 −8·6 — −9·1 —

T–f(O2) from Fe–Ti oxides following Anderson & Lindsley (1988) and Stormer (1983), errors range between 20 and 30°C; Ptot

from Al in amphibole geobarometer (Hammarstrom & Zen, 1986; Johnson & Rutherford, 1989a), errors±0·5kbar; f(H2O) isfrom annite in biotite (Mueller, 1972) following Wones & Eugster (1965) and Wones (1972); f(S2), f(SO2), and f(H2S) followingWhitney (1984).

429


Fig. 19. Fe–Ti oxide equilibrium temperatures and oxygen fugacities of the Toba tuffs determined following Stormer (1983) and Anderson &Lindsley (1988).

Implicit in this interpretation is that the mineralogyCrystal fractionationand compositions of fractionating phases would be rep-Hypothetical models suggest that compositional zonationresented by minerals in the least evolved, deeper levelsof silicic magma bodies can develop from double diffusiveof the chamber. Another important implication is thatconvection resulting from crystal fractionation (Huppertthe fractionated crystals which produced the zonation& Sparks, 1984; Baker & McBirney, 1985; McBirney etneed not be restricted to crystallization on the walls ofal., 1985; Nilson et al., 1985; Turner & Campbell, 1986).the chamber, but can be partially or wholly containedMcBirney et al. (1985) and Turner & Campbell (1986)within the magma body (Michael, 1984). When modelingenvisioned crystal fractionation as a process that occurstrace elements, distribution coefficients of minerals inmostly in the deeper parts of a magma chamber, eitherequilibrium with deeper level magmas should be used,on the walls or within the magma body itself. Theseas this is the composition from which the bulk of frac-studies imply that compositional zonation begins by crys-tionation occurs, not from the roof zone magmas. Suchtallization in the lower parts or sidewalls of a magma body,reasoning is predicated on the roof of silicic magmaproducing less dense, evolved liquids which buoyantly risechambers becoming zoned before any appreciable crys-to the roof zone and stratify. Successive evolved liquidstallization occurs there (Lipman et al., 1966; Hildreth,stratify at different levels according to their density. Each1979).layer may then convect independently. Chemical and

The above view appears appropriate at Toba for thethermal diffusion occurs across the boundaries betweenfollowing reasons:them, thus allowing for transfer of heat and chemical

(1) Compositional and thermal zoning is apparent fromcomponents upward in the magma chamber. In thisgeochemical plots, compositional geographic distributionmanner, a compositionally zoned silicic cap, consistingof samples, and intensive parameter calculations.of several distinct layers, can develop. Below the layered

(2) The low-silica rocks (presumably from the deepestcap, a non-stratified pool of more mafic residual magmaerupted portion of the magma body) were estimated toresides. This deeper portion of the magma body iscontain ~40 wt % crystals, whereas the high-silica rockscompositionally homogeneous and convects turbulently.(presumably representative of magma near the chamber’sA compositional gap is possible across the interfaceroof ) contain only 12 wt % crystals. This is consistentbetween the silicic cap and more mafic magma. Baconwith the concept that compositional zoning by crystal& Druitt (1988) demonstrated that some aspects of thisfractionation results mostly from crystallization in themodel were important in development of the rhyodacite

magma chamber at Mt Mazama. lower portions of the magma chamber, possibly even

430


forming a ‘cumulate layer’ by loss of light melt upwards whole-rock parent–glass daughter model, which indicates(Bacon & Druitt, 1988). that the glasses from the high-SiO2 pumices can be

(3) Compositional gaps occur in all three quartz- derived by ~50% fractionation from the low-SiO2 parentbearing tuffs, indicating individually convecting layers. pumice (Table 10). This suggests that after the derivative

(4) Rayleigh numbers calculated for the YTT magma liquids were extracted from the parent, they underwentchamber predict that turbulent convection would occur ~10% further crystallization; proportions that are similarin layer thicknesses >600 m, whereas non-turbulent con- to the observed modes of the high-SiO2 rocks. Modelingvection would occur in layers between 100 and 600 m assimilation with a bulk sample of lithics collected fromin thickness (Bartlett, 1969). Thus, several thin convecting the non-welded YTT produced low residuals, but is notlayers could have existed in the silicic cap overlying considered reasonable because of the generally higha thick turbulently convecting portion of the magma fractionation (up to 60%) and assimilation (up to 12%)chamber, consistent with the model. required.

The glass parent–daughter model did not producereasonable results in the major element modeling. This

Modeling crystal fractionationcould be due to similarities in major element con-

Trends of chemical variation diagrams of all Toba units centrations of the high- and low-SiO2 samples. Selectedare suggestive of crystal fractionation of a dacitic parental trace element plots for the glass separates (Figs 13 andmagma towards rhyodacite and rhyolitic compositions. 17) and REE patterns (Fig. 9) are, however, consistentTo test this hypothesis, least-squares fractionation cal- with crystal fractionation. These plots demonstrate theculations on selected samples spanning the compositional importance of plagioclase, sanidine, and allanite frac-range of each unit were done. Various crystal frac-

tionation in evolution of the glass compositions. In ad-tionation models were evaluated, including the derivationdition, other trace element data for glasses are consistentof whole-rock compositions from whole-rock parents,with fractionation of amphibole, biotite, Fe–Ti oxides,glass compositions from glass parents, and glass com-and zircon.positions from whole-rock parents. In addition, as-

Rayleigh trace element fractionations were calculatedsimilation in conjunction with crystal fractionation wason some of the more silicic YTT pumices using frac-tested for each model. Whole-rock and glass compositionstionated proportions estimated from mixing calculations.of the least silicic rocks from each eruption were chosenIn these calculations, distribution coefficients of daciteas parent compositions to derive the more evolved rocksand rhyodacite were generally chosen and varied withinand glasses. The least evolved mineral compositions fromtheir published ranges, until acceptable results were ob-the parent or similar rock samples were used to representtained (Mahood & Hildreth, 1983; Nash & Crecraft,the fractionating assemblage. This modeling is probably1985). The best results were obtained using the as-a simplification of a far more complex process.similation model; this finding suggests that minor as-Whole-rock and glass compositions from pumices weresimilation coupled with crystal fractionation cannot beused in modeling the YTT because they were the freshestdismissed. One complexity difficult to model is the pres-of all rocks and were not subject to enrichments orence of multiple zones of crystal fractionation throughoutdepletions in crystals, glass, or lithics as were the weldedthe entire magma body, each containing distinct mineraltuffs. The best results were obtained from the whole-compositions. Minerals of each zone would thus haverock parent–daughter model. Virtually all pumices withunique distribution coefficients. The KD values chosen forSiO2 >74% could be derived by fractionating quartz,modeling the high-SiO2 pumices did not effectively modelplagioclase, sanidine, ±biotite, amphibole, magnetite,the less silicic samples, thus implying that KD values areand ilmenite, with insignificant residuals. The percentagesvariable throughout the zoned magma body, each zoneof all phases fractionated are similar to the calculatedprobably having its own distinct set.modes presented above and also reflect the observed

Mixing calculations on MTT and OTT welded tuffmodal proportions (Table 10). Less fractionation wascompositions also suggested that compositional zonationrequired to derive pumice compositions between 72 andwithin these tuffs could be produced by crystal frac-74% SiO2 (Table 10). Fractionation percentages maytionation of quartz, plagioclase, sanidine, biotite, am-appear high, but considering that the least silicic YTTphibole, and magnetite. About 20–40 wt % fractionationpumices contain ~40 wt % crystals, most of the frac-was required to describe the compositional zonation oftionation can occur in situ, in deeper levels of the magmathe MTT; some 50–65 wt % fractionation was necessarychamber, without formation of a significant cumulate onfor the OTT. The reliability of these calculations onthe floor or walls of the chamber. Derivative liquidswelded tuff samples is somewhat compromised by em-separated from this zone of extensive crystallization andplacement and post-emplacement processes that affectrose to the roof of the chamber, where fractionation

continued. This process is supported by the results of the the whole-rock and glass compositions.

431


Table 10: Summary of crystal fractionation modeling using least-squares mixing calculations

with major element chemistry

% fractionation for 72–74% SiO2 % fractionation for >74% SiO2 % fractionation for glass

whole-rock derivatives whole-rock derivatives derivatives

n: 6 20 4

Quartz 4·5 (3·1) 8·4 (2·3) 10·6 (1·2)

Plagioclase 14·1 (2·2) 22·2 (1·9) 26·9 (1·5)

Sanidine 3·1 (1·2) 5·1 (2·5) 6·1 (1·3)

Biotite — 0·8 (0·5) 1·6 (0·7)

Amphibole 4·8 (0·7) 5·8 (0·8) 6·1 (0·6)

Magnetite 0·6 (0·2) 1·1 (0·1) 1·4 (0·1)

Ilmenite 0·4 (<0·1) 0·4 (0·1) 0·4 (<0·1)

Total 27·5 43·8 53·1

Fractionated proportions represent averages from several similar samples; n, number of samples; values in parenthesesare 1 SD.

Given that the OTT is temporally closest to the HDT, had similar sources, but the MTT’s source may havebeen slightly different (Chesner, 1988).calculations were made to test whether OTT fiamme

could be derived from the HDT. Major element modelingsuggests that all OTT fiamme could be produced byfractionating between 50 and 60 wt % quartz, plagioclase,sanidine, biotite, amphibole, magnetite, and ilmenite

Crustal sourcefrom the HDT. However, trace element modeling pro-duced poor results. Barium, Zr, La, and Ce, which are Based upon compositional relations of the entire Tobacompatible in the OTT, behave incompatibly in the suite, the parental magmas that evolved into the TobaHDT, consistent with the absence of sanidine, zircon, tuffs were probably dacitic magmas containing ~65%and allanite from the HDT mineral assemblage. These SiO2. It is unlikely that these magmas were emplaced aselements are also lower in concentration in the HDT pure liquids, as trace amounts of exotic minerals suggestthan in the least silicic OTT rocks. Varying proportions the presence of a restitic component. The HDT and aof plagioclase and magnetite fractionation between the few fiamme from the OTT may be representative of thisHDT and OTT may cause differences among Sr, Na2O, parent composition. All Toba tuff samples have initialand V concentrations also. Thus, the HDT evolved by 87Sr/86Sr ratios that range between 0·71333 and 0·71521means of a different fractionation assemblage, presumably (Chesner, 1988). Such ratios restrict the source of theat different conditions. This however, does not preclude parental Toba magmas to the crust, precluding origin bysimilar parental magmas for the HDT and younger Toba differentiation of basalt. Although there is no petrographictuffs. evidence of a basaltic component in any of the Toba

The high crystal contents, variation diagram trends, tuffs, it occurs as a mixing component in post-calderaREE patterns, major element mixing calculations, and andesites from the Pusikbukit Volcano (Fig. 1) (Chesner

& Hester, 1996). This implies the role of basalt inRayleigh trace element modeling all suggest that crystalfractionation of quartz, plagioclase, sanidine, biotite, am- providing heat required for large-scale crustal melting as

invoked in several other caldera studies (Smith, 1979;phibole, Fe–Ti oxides, allanite, and zircon can accountfor the chemical zonation of Toba’s magma bodies. Slight Hildreth, 1981; Marsh, 1984; Wyborn & Chappell, 1986;

Huppert & Sparks, 1988; de Silva, 1989; Boden, 1994).but perceptible chemical differences between the OTT,MTT, and YTT could result from variations in intensive Chesner (1988) suggested that the source rocks of Toba

magmas were metavolcanics and metasediments, and thatparameters, proportions and compositions of frac-tionating phases, and volumes of assimilant. Alternatively, a continental sedimentary source is required. Xenocrystic

epidote, staurolite, perovskite, and monazite discovereddifferent parental magma compositions inherited fromtheir source regions could have the same effect. Strontium in mineral separates may represent restite minerals from

such a source.isotopic data imply that the HDT, OTT, and YTT all

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et al., 1986). Blake & Ivey (1986) have determined thatEruptive styles and mechanismsdraw-up heights at vents are influenced mostly by erup-Because of its geographical distribution and its relationtion rates. To erupt magmas of diverse compositionsto underlying andesites, the 35 km3 HDT appears tosimultaneously, either the thickness of the silicic cap musthave erupted from a large stratovolcano in northernbe small (<100–500 m) or eruption rates must be extremeToba (Chesner & Rose, 1991). Analogous ‘Crater Lake-(107 m3/s). Eruption rates for the YTT can be estimatedType’ caldera eruptions might be those of Mt Mazamaby using eruptive volume (2800 km3) and eruption dur-(Williams, 1942; Bacon, 1983) and Tambora (Sigurdssonation (14 days; Ninkovich et al., 1978) to be about 106

& Carey, 1989). However, unlike these classic examples,m3/s. If 1800 km3 of high-SiO2 magma was erupted,the HDT eruption apparently lacked a significant Plinianthen the silicic cap was ~900 m thick. For the abovephase, and erupted less evolved magma. Reasons for theparameters, Blake & Ivey’s (1986) data predict that duringHDT erupting at high temperatures (866°C) and notthe early parts of the eruption, only the silicic cap wouldevolving to more silicic compositions and lower tem-be tapped. This is consistent with field observations ofperatures, like the other Toba tuffs, may be related toseveral outcrops outside the caldera containing only high-its ascent into a pre-existing volcano and eruption beforeSiO2 pumices. As the eruption continued, and the silicicany significant magmatic evolution could occur.cap thinned to ~500 m, low-SiO2 magma was tappedAs intensive parameters and fluid inclusions suggestsimultaneously with high-SiO2 magma. The same effectthat the YTT, MTT, and OTT were not oversaturatedcould result if discharge rates increased as the eruptionwith respect to H2O (XH2O= 0·4–0·6), at depths suggestedprogressed.by the amphibole geobarometer, then water over-

The geographical distribution of the predominant YTTsaturation and exsolution cannot account for triggering pumice compositions suggests that most of the outflowtheir eruptions. Instead, foundering of the magma body sheet consists of high-SiO2 magma (Fig. 14c). Low-SiO2roof, either as a result of extensive crystal fractionation pumices generally occur only within the caldera and atcreating density contrasts or by extensional faulting dur- its rim, suggesting that this magma erupted with lowing updoming, is proposed as a mechanism to allow energy and did not flow far from the rim when it didmagma migration to shallower levels, from which it surmount it. The YTT and OTT welded tuffs haveerupted. In this model, the magma chambers were located similar distribution patterns. The least evolved YTT~10 km below the surface, and extensive crystal frac- welded tuffs occur only on Samosir Island and in thetionation took place there over time spans of 0·34–0·43 caldera wall near Pangururan (Fig. 14b), whereas evolvedmy (Chesner & Rose, 1991), during which time strong compositions are found both inside and outside thecompositional zoning developed. Eventually, roof found- caldera. In the OTT, no low-SiO2 rocks occur outsideering commenced, which allowed magma to migrate the confines or in the walls of the Porsea Calderaupwards until Ptot = PH2O resulting in vesiculation, and (Fig. 14a). These distributions suggest that most YTTeruption. Saturation pressures determined from CO2 and and OTT caldera collapse occurred coincidentally withH2O contents in melt inclusions of 1·5–2·0 kbar suggest eruption of low-SiO2 magma, which would imply that,eruption from ~5–6·5 km below the surface. This by the time lower levels of magma were tapped, calderasequence of emplacement, fractionation, and eruption collapse was already occurring (Druitt & Sparks, 1984;took place four times over the past 1·2 my as one large, Hildreth et al., 1984). The high-temperature, least evolvedcontinuous magma chamber developed at Toba (Fig. 2). magma, forming the least energetic eruptions, pref-This time frame is consistent with the findings of Van erentially flowed into the collapsing caldera instead ofden Bogaard & Schirnick (1995), who suggested that a joining the outflow sheet; therefore it ponded and ac-single large magma body formed and erupted periodically cumulated to great thicknesses within the caldera. Thisfor 1·3 my before the eruption of the Bishop Tuff at type of relationship is common to large calderas (Mahood,Long Valley Caldera, CA. 1981; Lipman, 1984; Hildreth & Mahood, 1986).

During the OTT, MTT, and YTT eruptions, diverse The MTT is the only tuff in which sampling of amagma compositions were erupted simultaneously from complete stratigraphic section was possible. A peculiaritythe same vent sites, as indicated by outcrops that contain of the sampled MTT section is its reverse compositionalmultiple pumice–fiamme compositions (SiO2 ranges of and density zonation (Fig. 15). The most plausible way11, 4, and 5%, respectively). Such mixed compositions to explain a reversely zoned ash-flow tuff is by tappingat the same outcrop have been documented in several of deeper levels of the magma body first, then the rooftuffs, especially in the Topapah Spring Tuff (Schuraytz zone. Eruption along ring fractures that intersect deeper,et al., 1989). The fluid dynamics of magma draw-up at less evolved portions of the magma body is compatiblean erupting vent result in multiple compositional layers with this observation. Gardner et al. (1991) demonstratedof a zoned magma chamber eventually being tapped that deeper, less evolved magma was tapped during the

initial stages of the Plinian eruption phase at Long Valleysimultaneously (Blake, 1981; Blake & Ivey, 1986; Spera

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Caldera. The Plinian vent at Long Valley was located OTT eruptions were characterized predominantly byring fracture volcanism, not central vent type eruptions.along the incipient ring fracture of the caldera. Al-

ternatively, Blake & Ivey (1986) explained how various Features indicative of ring fracture volcanism for theYTT include: (1) symmetrical distribution of YTT outfloweruption rates and thicknesses of the evolved cap can

result in draw-up and eruption of less silicic magma around the elongate caldera (Fig. 2g); (2) virtually sym-metrical maximum lithic and pumice size distributionsthrough the evolved cap, resulting in reversely zoned

tuffs. around Toba (Caress, 1985); (3) absence of any significantPlinian fall deposits (Rose & Chesner, 1987); (4) lowThe dense welding of the HDT, OTT, MTT, and

parts of the YTT (caldera fill and lower parts of thick emplacement velocity and high thickness/area ratio of theignimbrite deposits (Caress, 1985), (5) magma residence ataccumulations in the outflow sheets) suggest that the tuffs

were emplaced at relatively high temperatures (>600°C). depths of ~10 km, (6) apparent water undersaturationof the magmas, (7) thick caldera fill consisting of the leastThis can be achieved by eruption at high temperatures or,

more importantly, retention of heat during emplacement, evolved magma that implies caldera collapse occurredduring eruption, and (8) linear alignment of post-calderaaccumulation, and cooling (Streck & Grunder, 1995).

Low-height eruption columns and/or high eruptive rates volcanism and uplift along the southwestern lake shorewithin the caldera.would promote heat retention and thus welding. These

conditions are implied for eruption of all four tuffs by The best evidence for MTT ring fracture eruption isthat a compositionally reverse-zoned magma is mostthe absence of any significant Plinian fall deposits atlikely to erupt from ring faults that intersected the deeperproximal basal exposures; no distal basal exposures wereparts of the magma body. The distributions of the differ-observed. In the case of the HDT, low water contentsent magma compositions and of the thick caldera fill alsoimplied by the absence of hydrous phases and relativelysuggest that the OTT erupted from ring fractures. Densehigh eruption temperatures (847°C) suggest low-heightwelding of both the low-temperature MTT and OTTeruptions with low gas contents and velocities. Theseare also consistent with ring fracture eruptions ratherconditions would favor high-temperature emplacement.than high columns associated with central vents.Because the OTT and MTT were erupted at much lower

temperatures, 704–759°C and 743–751°C, respectively,emplacement processes that retain heat are essential fordense welding to occur. Again, low eruption columnsand/or high eruption rates are suggested. The outflow

CONCLUSIONSof the YTT is mostly nonwelded and therefore a more(1) The Oldest, Middle, and Youngest Toba tuffs rep-energetic eruption probably occurred, allowing greaterresent crustal melts that became compositionally zonedheat loss from the pyroclastic flows. No significant airfallthrough extensive crystal fractionation of quartz, plagio-deposits occur near the caldera but an extensive distalclase, sanidine, biotite, and amphibole. In addition, minorash blanket, perhaps entirely of coignimbrite origin, isamounts of Fe–Ti oxides, allanite, and zircon were alsocorrelated with the YTT (Rose & Chesner, 1987). Denselyfractionated.welded YTT does occur, especially on Samosir Island,

(2) The fractionation occurred at pressures of ~3 kbar,suggesting that thick accumulations of tuff retainedor depths of 10 km. The magmas were not saturatedenough heat to weld. This welded magma was also thewith water (PH2O = 0·6–1·4 kbar).hottest, least evolved, and last erupted. Although all other

(3) Temperatures of crystallization for the Oldest,factors are the same, the YTT did not weld as pervasivelyMiddle, and Youngest Toba tuffs ranged from 701 toas the OTT and MTT. Perhaps a more energetic eruption780°C; the Haranggoal Dacite Tuff was hotter, ~847°C.ensued from ingestion of water from a previous caldera

(4) Although similar in composition, the Oldest,lake into the erupting YTT, allowing for greater heatMiddle, and Youngest Toba tuffs can be distinguishedloss, ignimbrite inflation, and an extensive ash blanket.by subtle variations in their mineral, whole-rock, andglass compositions.

(5) Mechanical mixing of distinct pumice compositionsEvidence for ring fracture eruptions indicates that multiple compositional layers were tapped

simultaneously during the eruptions.Eruptive styles at calderas range from predominantlyring fracture eruptions (Smith & Bailey, 1968) to central (6) Low-energy ring fracture eruptions were responsible

for emplacement of the Oldest, Middle, and Youngestvent eruptions (Wilson & Walker, 1985). Transitionaleruptions, from central vent to ring fracture, have also Toba tuffs. Collapse of the Oldest Toba Tuff and Young-

est Toba Tuff calderas was in progress when ‘low-SiO2’been documented (Bacon, 1983; Hildreth & Mahood,1986; Self et al., 1986). Eruption mechanisms invoked in magma from deeper levels of the magma chamber was

erupted.the above discussion imply that the YTT, MTT and

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calc-alkaline magmas in the central Great Basin, U.S.A. ContributionsACKNOWLEDGEMENTSto Mineralogy and Petrology 116, 247–276.

The guidance and encouragement offered by Bill Rose Burnham, C. W., Holloway, J. R. & Davis, N. F. (1969). Thermo-to this study were paramount. I thank Jimmy Diehl dynamic properties of water to 1000°C and 10,000 bars. Geological

Society of America, Special Papers 132, 96.(Michigan Technological University), Michael Knight,Cameron, K. L. (1984). The Bishop Tuff revisited: new rare earthPeter Hehanussa (Indonesian Institute of Sciences), and

element data consistent with crystal fractionation. Science 224, 1338–Antonius Silalahi for assistance in sampling the Toba1340.tuffs. Art Ettlinger provided microprobe access, cali-

Caress, M. E. (1985). Volcanology of the youngest Toba Tuff, Sumatra.bration, and analyses. This research was supported by M.S. Thesis, University of Hawaii, Manoa, 150 pp.National Science Foundation Grants EAR 82-06685 Carmichael, I. S. E. & Nicholls, J. (1967). Iron–titanium oxides andand EAR 85-11914. Reviews by Charles Bacon, Anita oxygen fugacities in volcanic rocks. Journal of Geophysical Research 72,Grunder, and Neil Irvine were very helpful. 4665–4687.

Carter, N. L., Officer, C. B., Chesner, C. A. & Rose, W. I. (1986).Dynamic deformation of volcanic ejecta from the Toba Caldera:possible relevance to Cretaceous/Tertiary boundary phenomena.Geology 14, 380–383.

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