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
JOURNAL OF PETROLOGY VOLUME 39 NUMBER 3 MARCH 1998
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
CHESNER PETROGENESIS OF THE TOBA TUFFS, SUMATRA, INDONESIA
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
JOURNAL OF PETROLOGY VOLUME 39 NUMBER 3 MARCH 1998
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
CHESNER PETROGENESIS OF THE TOBA TUFFS, SUMATRA, INDONESIA
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
JOURNAL OF PETROLOGY VOLUME 39 NUMBER 3 MARCH 1998
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
CHESNER PETROGENESIS OF THE TOBA TUFFS, SUMATRA, INDONESIA
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
JOURNAL OF PETROLOGY VOLUME 39 NUMBER 3 MARCH 1998
406
CHESNER PETROGENESIS OF THE TOBA TUFFS, SUMATRA, INDONESIA
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
JOURNAL OF PETROLOGY VOLUME 39 NUMBER 3 MARCH 1998
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
CHESNER PETROGENESIS OF THE TOBA TUFFS, SUMATRA, INDONESIA
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
JOURNAL OF PETROLOGY VOLUME 39 NUMBER 3 MARCH 1998
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
CHESNER PETROGENESIS OF THE TOBA TUFFS, SUMATRA, INDONESIA
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
JOURNAL OF PETROLOGY VOLUME 39 NUMBER 3 MARCH 1998
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
CHESNER PETROGENESIS OF THE TOBA TUFFS, SUMATRA, INDONESIA
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
JOURNAL OF PETROLOGY VOLUME 39 NUMBER 3 MARCH 1998
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
CHESNER PETROGENESIS OF THE TOBA TUFFS, SUMATRA, INDONESIA
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
JOURNAL OF PETROLOGY VOLUME 39 NUMBER 3 MARCH 1998
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
CHESNER PETROGENESIS OF THE TOBA TUFFS, SUMATRA, INDONESIA
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
JOURNAL OF PETROLOGY VOLUME 39 NUMBER 3 MARCH 1998
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
CHESNER PETROGENESIS OF THE TOBA TUFFS, SUMATRA, INDONESIA
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
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JOURNAL OF PETROLOGY VOLUME 39 NUMBER 3 MARCH 1998
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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CHESNER PETROGENESIS OF THE TOBA TUFFS, SUMATRA, INDONESIA
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JOURNAL OF PETROLOGY VOLUME 39 NUMBER 3 MARCH 1998
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.
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CHESNER PETROGENESIS OF THE TOBA TUFFS, SUMATRA, INDONESIA
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
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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.
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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
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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,
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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
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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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CHESNER PETROGENESIS OF THE TOBA TUFFS, SUMATRA, INDONESIA
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).
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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
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CHESNER PETROGENESIS OF THE TOBA TUFFS, SUMATRA, INDONESIA
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.
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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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CHESNER PETROGENESIS OF THE TOBA TUFFS, SUMATRA, INDONESIA
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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