Petrogenesis of andalusite-kyanite-sillimanite veins and host rocks, Sanandaj-Sirjan metamorphic belt, Hamadan, Iran

Petrogenesis of andalusite–kyanite–sillimanite veins andhost rocks, Sanandaj-Sirjan metamorphic belt, Hamadan, Iran

A. A. SEPAHI1 , D. L . WHITNEY2 AND A. A. BAHARIFAR3

1Department of Geology, Bu Ali Sina University, Hamadan, Iran2Department of Geology and Geophysics, University of Minnesota, Minneapolis, Minnesota 55455 USA ([email protected])3Department of Geology, Tarbiat-Moallem University, Tehran, Iran

ABSTRACT Quartz-rich veins in metapelitic schists of the Sanandaj-Sirjan belt, Hamadan region, Iran, commonlycontain two Al2SiO5 polymorphs, and, more rarely, three coexisting Al2SiO5 polymorphs. In mostandalusite and sillimanite schists, the types of polymorphs in veins correlate with Al2SiO5 polymorph(s)in the host rocks, although vein polymorphs are texturally and compositionally distinct from those inadjacent host rocks; e.g. vein andalusite is enriched in Fe2O3 relative to host rock andalusite. Low-graderocks contain andalusite + quartz veins, medium-grade rocks contain andalusite + sillim-anite + quartz ± plagioclase veins, and high-grade rocks contain sillimanite + quartz + plagioclaseveins ⁄ leucosomes. Although most andalusite and sillimanite-bearing veins occur in host rocks that alsocontain Al2SiO5, kyanite-quartz veins crosscut rocks that lack Al2SiO5 (e.g. staurolite schist, granite).A quartz vein containing andalusite + kyanite + sillimanite + staurolite + muscovite occurs inandalusite–sillimanite host rocks. Textural relationships in this vein indicate the crystallization sequenceandalusite to kyanite to sillimanite. This crystallization sequence conflicts with the observation thatkyanite-quartz veins post-date andalusite–sillimanite veins and at least one intrusive phase of a granitethat produced a low-pressure–high-temperature contact aureole; these relationships imply a sequence ofandalusite to sillimanite to kyanite. Varying crystallization sequences for rocks in a largely coherentmetamorphic belt can be explained by P–T paths of different rocks passing near (slightly above, slightlybelow) the Al2SiO5 triple point, and by overprinting of multiple metamorphic events in a terrane thatevolved from a continental arc to a collisional orogen.

Key words: andalusite; Iran; kyanite; quartz veins; sillimanite.

INTRODUCTION

Quartz veins and leucosomes containing Al2SiO5polymorphs occur in regional metamorphic terranes(Read, 1932; Yardley et al., 1980; Stout et al., 1986;Lang & Dunn, 1990; Nabelek, 1997; Whitney & Dilek,2000; Widmer & Thompson, 2001; McLelland et al.,2002) and contact aureoles (Speer, 1982; Cesare, 1994;Okuyama-Kusunose, 1994; Larson & Sharp, 2003).The presence of Al2SiO5 phases gives informationabout the P–T conditions of vein formation andpossibly also the conditions of metamorphism, thesource(s) of vein-forming materials, and the mech-anisms of vein formation. These aspects of vein petro-genesis and Al2SiO5 crystallization are of interestbecause they provide insight into fluid–rock inter-actions during orogenic processes such as metamor-phism, magmatism and deformation, and informationabout the influence of chemical and kinetic factors onpolymorph crystallization.Various combinations of two Al2SiO5 polymorphs

are common in pelitic schist and quartzite: for exam-ple, andalusite + sillimanite (Leake & Skirrow, 1960;

Okrusch & Evans, 1970; Rumble, 1973; Pattison,1992; Cavosie et al., 2002) and andalusite + kyanite(Grambling, 1981; Evans & Berti, 1986; Kerrick,1988). In cases where veins and host rocks contain thesame polymorph(s), the veins likely formed at similarP–T conditions as the host rocks, and formation of theveins may have involved chemical and ⁄ or physicalinteraction with the host rock; e.g. transport of com-ponents from the host rock to the vein or mechanicalincorporation of host rock phases into the veins. Theopposite case (e.g. Sauniac & Touret, 1983; Whitney &Dilek, 2000) is more difficult to interpret because thepresence of different polymorphs in veins compared tohost rocks does not necessarily imply lack of vein–hostinteraction during crystallization of the vein nor anentirely external source of vein-forming materials.Variation in vein vs. host rock mineralogy mightindicate that different kinetic or chemical factorsaffected one environment relative to the other, such asvariation in abundance of fluid or nature of deforma-tion regimes. For example, andalusite might formduring decompression in veins within host rocks con-taining kyanite or sillimanite (Stout et al., 1986), but

J. metamorphic Geol., 2004, 22, 119–134 doi:10.1111/j.1525-1314.2004.00502.x

� 2004 Blackwell Publishing Ltd 11 9

might not form in the host rock owing to a morefavourable kinetic or chemical environment in theveins. Vein assemblages may therefore provide addi-tional, new information about metamorphic condi-tions and paths.In most rocks containing multiple Al2SiO5 poly-

morphs, the polymorphs formed during sequentialcrystallization during progressive regional metamor-phism or a combination of regional and contactmetamorphism, and one or more may persist meta-stably relative to the peak metamorphic (equilibrium)assemblage in a rock (Loomis, 1972; Kerrick, 1990).The sequence of crystallization may be deduced fromtextural relationships among the polymorphs, provi-ding information about the conditions and mech-anisms of vein formation. An ideal situation forinvestigating Al2SiO5 crystallization conditions andsequence is a field site containing rocks with multiplepolymorphs and regional variation in polymorphtype, so that the crystallization sequence can beevaluated in the context of different P–T conditionsand the overall tectonic setting. In this paper, wedescribe a suite of quartz veins and host rocks fromHamadan, Iran, that contain andalusite, kyanite, sil-limanite, andalusite–sillimanite, andalusite–kyanite,sillimanite–kyanite, or andalusite–sillimanite–kyanite(that is, all possible combinations of Al2SiO5 poly-morphs), and the Al2SiO5 crystallization conditions,sequences and relationship to country rock evolutionare discussed.

GEOLOGICAL SETTING

The study area is a part of the 1500-km long Sanandaj–Sirjan metamorphic belt of the Zagros orogen ofwestern Iran (Fig. 1a). The Zagros formed duringsubduction of a Neo-Tethyan seaway and subsequentoblique collision of Afro-Arabia (Gondwana) with theIranian microcontinent in the Late Cretaceous–earlyTertiary (Berberian & King, 1981; Alavi, 1994; Moh-ajjel & Fergusson, 2000). Crustal shortening associatedwith subduction and collision metamorphosed anddeformed Late Palaeozoic and Mesozoic sedimentaryrocks, including a sequence of pelitic, psammitic,mafic, calc-pelitic, and calc-silicate rocks nearHamadan (Fig. 1b).The tectonic evolution of the Sanandaj–Sirjan belt

involved continental arc magmatism followed bycollision. Mafic to intermediate plutonic bodies(olivine gabbro, gabbro, gabbro-norite, diorite, quartzdiorite and tonalite) (Valizadeh & Cantagrel, 1975)are older than crustally derived granitic plutons in theregion (Alvand Plutonic Complex), but all intrusionsformed during Cretaceous-Tertiary subduction andcollision (Baharifar et al., 2004). The plutons, inclu-ding the granites, are commonly associated withcontact aureoles defined by hornfelsic textures andmineral assemblages that overprint earlier mineralsand fabrics.

FIELD OBSERVATIONS

In the field area near Hamadan, metapelitic rocks are the mostabundant rock type, and are interlayered with minor metabasalticrocks (amphibole schist and amphibolite), metacarbonate and calc-silicate rocks. Metapelitic rocks occur as slate, phyllite, mica schist,garnet schist, garnet–andalusite (± sillimanite or kyanite) schist,garnet–staurolite schist and garnet–sillimanite (± kyanite) schist, andhornfelsic or migmatitic rocks (near the Alvand Plutonic Complex).These contact zone rocks include cordierite + K-feldspar (± andalu-site, fibrolite) and garnet–staurolite (± kyanite) hornfels. In the

+ + +++

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Hamadan

2.5 km

1

1

1

2

2

2

2

1

3

3

32

3

4a

44

5

67a

7

8

8

8

crd

bt

bt

bt

bt

bt

crd

gt

bt

gt

gt

gt

gt

and

and

and

st

st

st

st

sil+and

sil+and

st

sil+kfs

crd+kfs

crd+kfs

KVKV

KV

KV

ASV

ASKV

AV

Alvand Plutonic Complex

9

9

1

N

b

TehranHamadanSanandaj-Sirjan belt

IRAN

CaspianSea

Turkmenistan

Iraq

Persian Gulf

SaudiArabia Oman

Qatar

Afghanistan

45° 60°

30°

Turk

ey

a

1bt

AVH

ASVH

ASKVHSschH

KVHKSVH

SVH

7b

4b

crd+ and + sil

531

KV

4a

KV

KV

KV

KV

KAV

KAV

AV

AV

SV

KSV

+ +++

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235A

Fig. 1. (a) Location of the Sanandaj–Sirjan belt in Iran.(b) Generalized geological map of the study area near Hamadan.Isograds (dashed lines) and metamorphic zones are shown, aswell as the locations of Al2SiO5-bearing quartz veins (blackellipses). Kyanite-bearing schists and hornfels are within units 6and 7 but are not mappable. Some faults are shown (heavy lineswithin unit 4); contacts between zones 4a–4b, 4a–5, 2–5, andpossibly 4–7 are faults. Locations of vein samples used in thisstudy are designated with white stars; host rock samplesare designated by black stars. In the sample numbers:A ¼ andalusite; H ¼ host rock; K ¼ kyanite; S ¼ sillimanite;V ¼ vein. Locations of samples 235 A (kyanite host rock) and531 (sillimanite vein) are also shown. Zones: 1 ¼ biotite zone;2 ¼ garnet zone; 3 ¼ andalusite zone (± fibrolite); 4 ¼ stauro-lite zone (a ¼ staurolite + andalusite ± fibrolite; b ¼ stauro-lite + garnet, no andalusite); 5 ¼ sillimanite-andalusite zone;6 ¼ sillimanite-kfs zone; 7 ¼ cordierite zone (a ¼ cordieritewith no new growth of Al2SiO5; b ¼ cordierite + neoblasticandalusite + sillimanite); 8 ¼ cordierite kfs zone; 9 ¼ spottedschist.

1 20 A . A . S E P A H I E T A L .

� 2004 Blackwell Publishing Ltd

Hamadan metamorphic sequence, cordierite and K-feldspar are onlyfound near (<3 km from) the Alvand granite.The general trend of increasing metamorphic grade is towards the

pluton, with a succession of isograds in metapelitic rocks: biotite-in,garnet-in, andalusite-in, staurolite-in, cordierite-in, and K-feldspar-in. In some parts of the field area, the complete sequence occurs, butin other areas, some of the lower grade zones are missing (Fig. 1b).Faulting has disrupted the sequences in places, and some apparentisograds may actually be faults. The most obvious example of dis-ruption of the sequence by faulting is the fault-bounded panel ofandalusite–sillimanite schists (zone 5) that occurs between theandalusite and staurolite zones (Fig. 1b).Abundant coarse-grained, quartz-rich veins and pods occur in the

metamorphic rocks of the study area (Fig. 1b). Veins range inthickness from centimetres to 3 m and strike NW–SE, sub-parallel toisograds and to regional structures such as axial planes of folds,foliation, and major directions of joint sets.Veins contain quartz and one, two, or three Al2SiO5 polymorphs

(andalusite, sillimanite, kyanite) as major minerals. The mode isextremely variable, from Al2SiO5-dominated (up to 75% of themode) to quartz-dominated. The size of Al2SiO5 crystals varies frommm-scale up to 30 cm, and most polymorphs can be identified inoutcrop (Fig. 2), although identification may be complicated bypseudomorph textures. Although grain size varies from vein to vein,most veins are very coarse grained (cm-scale). Some host rocks (e.g.sample ASVH) also contain large porphyroblasts (Fig. 2), but mosttypically Al2SiO5 in host rocks is finer-grained than in associatedveins (Fig. 3a–c).In outcrop, the veins are isolated and do not form interconnecting

networks with other veins. Some veins have been boudinaged andare therefore discontinuous in outcrop; others can be traced inoutcrop for up to 20 m. Contacts with host rocks are typicallysharp. However, some veins are associated with metre-scale zones(up to 3 m wide; referred to here as vein marginal zones) thatcontain less quartz and more muscovite than the main part of theadjacent vein. These vein marginal zones contain Al2SiO5 poly-morph types and textures that have characteristics of both veins andhost rocks. One sample described in this study is from a veinmarginal zone (andalusite vein sample AV2); all other samplesdesignated with a �V� are from the interior regions of veins and aredominated by quartz or Al2SiO5.The most abundant vein types identified in the field are: andalu-

site–quartz veins in garnet–andalusite (chiastolite) schists (zone 3;Fig. 3a–c); andalusite–sillimanite–quartz veins in garnet–andalusite–sillimanite schists (zone 5; Fig. 3d); andalusite–kyanite–quartz veinsin staurolite schist and garnet–andalusite–sillimanite schist (zone 4;Fig. 3g); sillimanite–quartz–plagioclase veins in garnet–sillimaniteschist, including migmatitic rocks (zone 6); and kyanite–quartz veinsin many lithologies (schist, hornfels and granite) (zones 4–7 and thegranite; Fig. 3h). The zone designations in this list are keyed to themap in Fig. 1(b). Other quartz-Al2SiO5 vein types such as andalu-site–sillimanite–kyanite veins (Fig. 3e) and kyanite–sillimanite veins(Fig. 3f) occur but are less abundant.

PETROGRAPHY AND MINERAL CHEMISTRY

Al2SiO5-bearing veins of the study area are composed of quartz +Al2SiO5 polymorphs, ± plagioclase, garnet, muscovite, biotite andstaurolite (Table 1). The naming of zones (Fig. 1b, Table 1) is basedon the dominant assemblage in metapelitic rocks. For example, zone3 is comprised of andalusite schists and andalusite–quartz veins, butsome rocks in this zone contain accessory fibrolite and others containrare crystals of kyanite. Despite the significance of these otherAl2SiO5 polymorphs for the P–T path of the rocks, the zone isdesignated by, and the rocks are named for, the presence of abun-dant, coarse-grained andalusite.The petrography of the metamorphic rocks and plutonic complex

is reported in one M.Sc. thesis (Baharifar, 1997) and one Ph.D. thesis(Sepahi, 1999), both in Farsi, and in Forghani (1975). Based oninformation in these previous studies and the results of this work,we discuss the mineral assemblages, mineral compositions, and

textures of major phases, as these are relevant for understanding theconditions and mechanisms of vein formation, and the relationshipof veins to host rocks.Mineral compositions and major element distribution maps of

garnet, Al2SiO5 polymorphs, and staurolite were obtained using aJEOL JXA-8900 electron microprobe at the University of Minne-sota. Operating conditions for quantitative analysis (WDS) were15 kV accelerating voltage, 15–25 nA beam current, and a rangeof beam diameters (higher current, <1 lm beam for garnet,Al2SiO5, and staurolite; lower current, beam defocused to 5–20 lmfor micas and plagioclase). X-ray maps were determined for Fe, Mn,Mg, Ca and either Al or Si using a beam current of 100 nA, 50 msdwell-time, and 2–10 lm beam diameters. Most grains that weremapped were also analyzed quantitatively along traverses from core

Fig. 2. Photographs of outcrops. (a) Andalusite in fine-grainedgraphitic schist ⁄ phyllite. (b) Sillimanite in migmatite (diatexite).Sillimanite is inferred to have replaced (pseudomorphed) anda-lusite. (c) Andalusite-sillimanite schist (host to andalusite-sillimanite veins) with two large andalusite crystals that havebeen partially replaced by prismatic sillimanite (sample ASVH).

A N D A L U S I T E – K Y A N I T E – S I L L I M A N I T E V E I N S 12 1


1 cm

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Fig.3.PhotographsofhandsamplesofAl 2SiO5-bearingrocks.(a)Andalusite(chiastolite)schist(AVH),hostrocktoandalusitevein.(b)Andalusiteveinmarginalzone;

intersticesbetweenandalusitegrainsaregraphitic(AV2).(c)Andalusite-quartzvein.Pink(zoneddarktolightpink)areandalusite;allwhiteregionsarequartz(AV1).

(d)Andalusite-sillimanitevein(ASV):pinkisandalusite,whiteissillimanite.(e)Andalusite-sillimanite-kyanitevein(ASKV):blueiskyanite(someofitisverypalebutcanbe

seentooccurasbladedcrystals),pink⁄redisandalusite,whiteismostlyquartz.Sillimaniteisdifficulttoseeinhandsampleinthisrock.(f)Kyanite-sillimanitevein(KSV):

75modal%

ofthisrockiskyanite;mostofthewhitecrystalsvisibleinhandsamplearequartz.Sillimaniteoccursasprismaticandfibrouscrystalsthataredifficulttoseein

handsample.(g)Andalusite-kyanitevein(AKV):photographis15cm

intheverticaldimension.(h)Kyanite-quartzvein(KV).

1 22 A . A . S E P A H I E T A L .


to rim. Natural mineral standards and the ZAF matrix correctionroutine were used. Representative analyses for Al2SiO5, garnet, andstaurolite are given in Tables 2–4.

The following discussion is organized by metamorphic grade, fromlowest (chlorite, biotite zones) to highest (sillimanite–K-feldsparzone).

Table 1. Characteristics of veins and host rocks.

Vein type Zone (Fig. 1b) Strike Vein minerals Host rock Host rock minerals Grade

Andalusite (AV) 3 N10)20W And, Qz, Ms, Bt, Ilm ± Gr ± Pl ±

Grt ± Ky (rare), Chl (s), Dsp (s)

Grt-And schist (AVH) Qz, Bt, Ms, Gr, Grt,

And, Pl, Fi, Chl, Opq

low

Andalusite-Kyanite (AKV) 4 N10)20W And, Ky, Qz, ± Ms ± Grt St schist, Grt-And-Sil (Fi) schist Qz, Bt, Ms, Pl, Gr, Grt,

St ± and ± Fi ± Chl

medium

Andalusite-Sillimanite (ASV) 5 N10-20 W And, Sil (prism + Fi), Ms, Qz, Opq Grt-And-Sil schist (ASVH) Qz, Bt, Ms, Gr, Grt,

And, Sil, St, Pl ± Ky*

medium

Andalusite-Sillimanite-

Kyanite (ASKV)

5 N20)30W And, Sil (prism + Fi), Ky, Qz, Ms, St,

Opq ± Grt**

Grt-And-Sil ± Ky schist (ASKVH) Qz, Bt, Ms, Gr, Grt,

And, Sil, Ky, St, Pl, Opq

medium

Sillimanite (SV) 6 N20)30W Sil (prism + Fi), Qz, Pl, Bt ± Grt ± St Grt-Sil-Kfs schist, migmatite (SVH) Qz, Bt, Ms, Grt, Sil, Pl,

Kfs, Opq

medium-high

Kyanite-Sillimanite (KSV) 6 N20)30W Ky, Sil (prism + Fi), Qz, Ms Grt-Sil-Kfs ± Ky schist,

migmatite (KSVH)

Qz, Bt, Ms, Grt, Sil, Ky,

Pl, Kfs, Opq

medium-high

Kyanite (KV) 4–8 + granite N10)20W Ky, Qz, Ms, Opq, Chl (s) ± Dsp (s) various; e.g. Grt-St schist (KVH),

hornfels, granite

various various

And ¼ andalusite, Bt ¼ biotite, Chl ¼ chlorite, Dsp ¼ diaspore, Fi ¼ fibrolite, Gr ¼ graphite, Grt ¼ garnet, Ilm ¼ ilmenite, Kfs ¼ K-feldspar, Ky ¼ kyanite, Ms ¼ muscovite,

Opq ¼ opaques; Pl ¼ plagioclase; prism ¼ prismatic sillimanite; Qz ¼ quartz; (s) ¼ secondary, Sil ¼ sillimanite, St ¼ staurolite.

* Relict kyanite was observed in one sample of andalusite-sillimanite-staurolite schist.

** Garnet occurs at the margin of the vein; may be part of the host schist.

Table 2. Representative Al2SiO5 analyses from quartz veins and host rocks.*

And

schist

And

vein

And

vein

And vein ⁄marginAV2

And-Sil vein

ASV

And-Sil schist

ASVH

And-Sil-Ky vein ⁄marginASKV

KV-Sil

vein

Ky-Sil

schist

Ky

vein

Sil

vein

AVH

And

AV1

And-white

AV1

And-pink

And

core

And

rim And Sil** Sil** And And Sil Ky

KSV

Ky

KSVH

Ky

KV

Ky

SV

Sil

SiO2 36.93 37.09 37.23 36.54 37.36 36.70 37.09 36.85 37.13 37.73 36.92 37.74 37.02 37.06 37.22 36.47

TiO2 - - - - - - 0.01 0.04 0.01 0.08 0.01 - - - 0.01 0.01 - - - - - - 0.06 - - - - - - - - -

Al2O3 63.11 63.18 63.31 62.06 63.48 63.13 63.56 62.83 63.03 62.36 62.97 62.18 63.47 63.02 62.88 62.83

Fe2O3 0.15 0.13 0.22 0.73 0.20 0.55 0.11 0.10 0.24 0.44 0.15 0.15 0.10 0.18 0.25 0.16

MnO - - - - - - - - - 0.01 - - - - - - - - - - - - - - - 0.04 0.02 0.02 - - - - - - 0.06 - - -

MgO 0.03 0.02 0.03 0.17 0.02 0.11 - - - 0.02 0.03 0.02 0.01 - - - - - - - - - 0.02 - - -

Cr2O3 0.07 - - - - - - 0.08 - - - 0.05 0.07 0.05 0.08 0.02 0.01 0.03 - - - 0.03 0.08 0.03

Total 100.28 100.43 100.79 99.63 101.08 100.62 100.83 99.85 100.52 100.61 100.08 100.12 100.65 100.30 100.51 99.49

mol% Fe 0.10 0.09 0.15 0.50 0.14 0.37 0.07 0.07 0.16 0.30 0.10 0.11 0.07 0.12 0.17 0.11

* Representative means that these are actual analyses (not averages) and are typical compositions observed within each sample (vein, host) or domain (core, rim).

** Prismatic sillimanite replacing andalusite.

Key to abbreviations and symbols: And ¼ andalusite, Ky ¼ kyanite, Sil ¼ sillimanite; - - - ¼ analyzed but element not detected.

Table 3. Representative garnet analyses.

Gt-St

schist

SschH core

Gt-St

schist

SschH rim

And

v ⁄marginAV2 core

And

v ⁄marginAV2 rim

And-Sil

schist

ASVH core

And-Sil

schist

ASVH rim

And-Sil-Ky

vein

ASKV core

And-Sil-Ky

vein

ASKV rim

Ky-St

schist

KVH core

Ky-St

schist

KVH rim

Ky-Sil

schist

KSVH rim

Sil

vein

SV rim

Sil

schist

SVH core

Sil

schist

SVH rim

SiO2 37.23 37.25 31.13 37.38 37.38 37.36 37.23 37.40 37.36 37.30 37.19 37.34 37.33 37.43

TiO2 0.06 0.03 0.30 0.33 – 0.01 – – 0.04 0.02 – 0.32 0.01 –

Al2O3 21.04 21.07 21.10 21.15 20.89 21.19 21.16 21.11 21.24 21.21 21.07 21.30 20.95 21.12

FeO 27.93 36.18 31.24 35.02 21.29 34.36 30.47 33.14 31.61 34.31 34.86 33.73 31.10 34.66

MnO 9.56 1.12 6.39 2.63 5.44 4.39 8.56 5.39 7.38 4.65 2.77 2.70 6.15 2.79

MgO 1.00 1.95 2.07 1.71 2.38 1.80 1.86 2.32 2.22 2.33 2.44 2.88 2.76 2.68

CaO 2.81 2.42 1.76 2.57 1.38 1.01 0.69 0.86 0.82 0.64 1.26 1.86 1.74 1.88

Total 99.64 100.02 99.99 100.80 99.76 100.12 99.97 100.22 100.67 100.47 99.59 100.12 100.04 100.56

Si 3.02 3.01 3.00 3.00 3.02 3.02 3.02 3.02 3.00 3.01 3.01 3.00 3.01 3.00

Ti 0.00 0.00 0.02 0.02 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.02 0.00 0.00

Al 2.01 2.01 2.01 2.00 1.99 2.02 2.02 2.01 2.01 2.01 2.01 2.01 1.99 2.00

Fe 1.90 2.45 2.11 2.35 2.18 2.32 2.06 2.24 2.13 2.31 2.36 2.26 2.10 2.33

Mn 0.66 0.08 0.44 0.18 0.37 0.30 0.59 0.37 0.50 0.32 0.19 0.18 0.42 0.19

Mg 0.12 0.23 0.25 0.20 0.29 0.22 0.22 0.28 0.27 0.28 0.29 0.34 0.33 0.32

Ca 0.24 0.21 0.15 0.22 0.12 0.09 0.06 0.07 0.07 0.06 0.11 0.16 0.15 0.16

XAlm 0.65 0.82 0.72 0.80 0.74 0.79 0.70 0.76 0.72 0.78 0.80 0.77 0.70 0.78

XSps 0.23 0.03 0.15 0.06 0.13 0.10 0.20 0.12 0.17 0.11 0.06 0.06 0.14 0.06

XPrp 0.04 0.08 0.08 0.07 0.10 0.07 0.08 0.09 0.09 0.09 0.10 0.12 0.11 0.11

XGrs 0.08 0.07 0.05 0.07 0.04 0.03 0.02 0.03 0.02 0.02 0.04 0.05 0.05 0.05



Low-grade rocks (zones 1 and 2)

The lowest-grade metapelitic rocks in the field area are very fine-grained slates and phyllites that are interlayered with carbonaterocks and quartzite. Rare quartz veins with accessory iron oxides andrutile cut through the slates, but are more common in the phyllites;these veins lack Al2SiO5. The slates are chlorite zone rocks (notshown in Fig. 1b) and the phyllites are biotite zone rocks (zone 1,Fig. 1b).

Mica schists, some containing garnet, also occur in the sequence(zone 2, Fig. 1b). In some schists, garnet is partially to completelyreplaced by chlorite and white mica. Garnet crystals have complexrelationships to deformation, and may have had a protracted growthhistory in relation to deformation. These schists contain moreabundant quartz veins than the lower-grade rocks, and the veins lackAl2SiO5 polymorphs.

Andalusite-bearing schists and veins (zone 3)

Andalusite-bearing schists are medium to coarse grained, with anaverage ground-mass mineral size of 3–6 mm, porphyroblasts ofgarnet up to 1 cm in diameter, and andalusite crystals up to 20 cm inlength (Fig. 2a). Common minerals are quartz, biotite, andalusite(chiastolite), garnet and muscovite. Minor minerals are graphite,chlorite, plagioclase, tourmaline, ilmenite, rutile and fibrolite (insome rocks). Garnet has been partially to completely pseudomor-phed by white mica ± chlorite in andalusite schists near veins. Someandalusite grains, particularly those near faults and veins, are par-tially replaced at their margins and along fractures by fine-grainedwhite mica and coarse chlorite. Andalusite porphyroblasts, includingchiastolite (Fig. 3a), in the schists are colourless in plane light(Fig. 4a) and contain low abundances of trace elements (¼ 0.15 wt%Fe2O3, or ¼ 0.1 mol% FeAlSiO5; sample AVH; Table 2). Elongatecrystals display boudinage.

Table 4. Representative staurolite analyses.

Gt-St schist

SschH

And-Sil schist

ASVH

And-Sil-Ky vein

ASKV

Ky-St schist

KVH

Ky-Sil schist

KSVH

SiO2 26.70 26.46 28.08 27.73 27.68

TiO2 0.43 0.59 0.29 0.67 0.87

Al2O3 55.68 55.32 54.58 53.33 55.18

FeO 11.71 14.16 13.16 14.36 12.39

MnO 0.02 0.41 0.39 0.29 0.41

MgO 0.93 1.57 1.45 1.38 0.98

ZnO 2.18 0.45 0.36 0.09 0.68

Total 97.63 98.95 98.32 97.84 98.18

XFe-St 0.78 0.69 0.69 0.72 0.74

0

0.2

0.4

0.6

0.8

Distance (mm)

wei

ght p

erce

nt o

xide

s

Fe203

MgO

Cr2O3

0 0.5 1.0 1.5 2.0 2.5 3.01 mm

1 mm

a

c

b

d

And

And

And

And

Ms

Chl

ChlQz + Gr + Ms

Dsp

And1 mm

1 mm

And

AV2

AVH AV1

pink

pink

pinkclear

clear

clear

pink

clear

clear

clear

Dsp

Fig. 4. Andalusite-bearing schists and veins. (a) Andalusite from the host schist (AVH), see Fig. 3(a); (b) Vein andalusite with bothpink (Fe-richer) and white (Fe-poorer) regions and texturally late diaspore (inset); both photographs are from sample AV1, seeFig. 3(c). (c) Vein marginal zone andalusite with pink core inside graphite-rich inclusion band and clear rim (sample AV2, see Fig. 3b).Colour zoning corresponds to higher trace element concentration in the core (d).

1 24 A . A . S E P A H I E T A L .


Andalusite-bearing schists are cut by boudinaged quartz veins.Some veins contain only accessory iron oxides in addition to quartz,but others contain andalusite (Fig. 4b), with minor muscovite,plagioclase, biotite, ilmenite, almandine-rich garnet and chlorite(retrograde). Andalusite is largely inclusion-free except for radiatingcrystals of secondary diaspore (Fig. 4b, inset) and rare euhedralkyanite that appears to be texturally later than the andalusite (i.e. itnucleated within andalusite). The vein andalusite exhibits patchycolour zoning, varying from pink to white, where the pink regionscorrespond to slightly higher transition metal (Fe, Cr, Ti) contentsthan the white ⁄ colourless zones (Fig. 4b, Table 2, sample AV1).The margins of some andalusite–quartz veins are characterized by

andalusite-rich regions that are distinct from the host rock in theirgreater abundance of andalusite (60–70% of the mode) and finerandalusite grain size (cf. Fig. 3a,b). The andalusite in the vein mar-ginal zone is similar to host rock andalusite in that it displays achiastolite texture (Figs 3b & 4c), and inclusions in andalusite arephases found in the host rock [graphite, ilmenite, quartz andplagioclase (An42)]. Andalusite in the vein marginal zone differsfrom host rock andalusite, however, in that it is zoned in traceelements, whereas host rock andalusite is typically homogeneous andnear-end member Al2SiO5. In vein margin andalusite, the boundarybetween a pink, inclusion-rich core zone and a colourless, inclusion-free rim region (Fig. 4c) corresponds to an abrupt decrease in minorelement content of the andalusite (e.g. 0.46–0.74 wt% Fe2O3 inthe cores vs. <0.20 wt% near the rim; sample AV2, Fig. 4d,Table 2); cores are also enriched in Cr, Mg, Mn and Ti relative torims. The cores contain higher Fe than the pink (Fe-bearing) regionsof unambiguous vein andalusite, and the rims are similarly low in Feas host rock andalusite.In the andalusite-rich vein marginal zones, the interstices between

andalusite grains are very graphite-rich (Figs 3b & 4c), containmuscovite + ilmenite, and are enriched in apatite + monazite rel-ative to the host schist In these graphitic interstices, accessorymonazite occurs in clusters with zoned allanite, apatite and musco-vite. Ilmenite inclusions in andalusite have a higher pyrophanite(Pyr) component, that is, are more Mn-rich, than ilmenite in thegraphitic interstices (inclusions: Ilm91)93Pyr6)7; matrix: Ilm96)97Pyr2). Euhedral to subhedral garnet occurs in andalusite and in thegraphitic interstitial regions. Garnet displays typical growth zoningtrends of increasing Fe and decreasing Mn from core to rim (core:Alm72Sps15Prp8Grs5; rim: Alm80Sps7Prp7Grs6) (Table 3). It is notpossible to compare the composition of these garnet with those in thehost rock because host rock garnet has been completely pseudo-morphed by white mica, quartz and plagioclase. Nevertheless, theseobservations and mineral composition data suggest that garnet andandalusite have crystallized ⁄ grown in vein marginal zones rather

than being mechanically incorporated from the groundmass, andthat these vein marginal zones may represent transition zones wherethe host rocks have extensively interacted with vein-forming fluids.

Staurolite schists and associated quartz veins(andalusite-kyanite; zone 4)

Staurolite schists are composed of quartz, staurolite, garnet,biotite, muscovite, chlorite, plagioclase, graphite and tourmaline.Porphyroblasts of garnet are typically small (<1 cm), but staurolitecrystals are up to 20 cm long. Staurolite porphyroblasts have beenpartially to completely replaced by chlorite and muscovite nearfault zones, veins, and intrusive bodies. Relict staurolite in the coresof pseudomorphs are Fe- and Zn-rich (XFe-St ¼ 0.8; 2 wt% ZnO)(Table 4). In these schists, plagioclase has an intermediate compo-sition and is unzoned or slightly normally zoned, from An36)29cores to An29)32 rims. Garnet occurs in the matrix and withinpseudomorphed staurolite, and is strongly growth zoned(core: Alm65Sps23Prp4Grs8; rim: Alm82Sps3Prp8Grs7) (sampleSschH; Fig. 5; Table 3). Garnet is euhedral and unaltered, withno evidence for retrograde zoning, even in schists in which staur-olite has been extensively replaced (Fig. 5a). These featuressuggest new crystallization of garnet following formation andreplacement of staurolite. The presence of pressure shadows ongarnet indicates deformation continued during and possibly aftergarnet growth.Quartz veins are common in these rocks, and some contain kyanite

or kyanite + andalusite. In veins containing both andalusite andkyanite, tabular kyanite surrounds large euhedral crystals of anda-lusite, and also occurs within andalusite, suggesting that they grew atthe same time or that kyanite post-dated andalusite; i.e. kyanite mayhave nucleated within andalusite.Zone 4 has been divided into two sub-regions (Fig. 1b). Both

regions are characterized by staurolite–garnet schists, so are includedas one zone, but the western part of the zone contains andalusite(kyanite–andalusite veins and andalusite–sillimanite schist), whereasthe eastern part of the zone lacks andalusite. The 4a)4b boundary isat least in part a fault (Fig. 1b).

Andalusite–sillimanite schists and veins (zone 5)

Andalusite-sillimanite schists contain large (3–20 cm long) porphy-roblasts of andalusite partially replaced by prismatic and fibro-lite (sample ASVH1) (Fig. 2c). These large porphyroblasts occurin a fine-grained matrix of biotite + garnet (1 mm) + quartz +

0

0.25

0.5

0.75

1.0

Distance (mm)

GrsPrpSpsAlm

mol

e fr

actio

n

0 0.2 0.4 0.6 0.8 1.0 1.2

c

GrtChl + Ms

Gr + Ms + Chl + Qtz + Pl

a

1 mm

relictSt

Mnb

Fig. 5. Staurolite-garnet schist. (a) Euhedral garnet crystal adjacent to a pseudomorphed staurolite porphyroblast. Relict stauroliteoccurs in the core of the grain, now largely comprised of chlorite + muscovite. (b) X-ray map of Mn zoning in garnet from thematrix of the same sample. Line marks the location and trajectory of a microprobe traverse, with data shown in (c). The traversestarted in the upper right and ended at the lower left end of the line.



plagioclase. Accessory minerals are graphite, tourmaline, andilmenite. Garnet is slightly zoned; Mn decreases and Fe increasesfrom core to rim (core: Alm73Sps14Prp10Grs3; rim: Alm78Sps11-Prp9Grs3; Table 3). Small (<500 lm) garnet also occurs as inclu-sions in andalusite, and has similar compositions and zoning trendsas matrix garnet. Clusters of euhedral staurolite crystals occur at themargins of sillimanite-andalusite porphyroblasts near veins (e.g.sample ASVH). These staurolite-rich vein marginal zones also con-tain abundant, coarse muscovite and rare, relict kyanite. This kyaniteis surrounded by muscovite and is not in contact with andalusite orsillimanite.Quartz-rich veins that crosscut andalusite-sillimanite schists also

contain andalusite + sillimanite (Fig. 3d), with minor muscovite,plagioclase, biotite, chlorite, graphite, tourmaline and ilmenite. As inthe host schists, andalusite has been partially replaced by prismaticsillimanite (Fig. 6b,c). Andalusite is pink and enriched in Fe relativeto sillimanite (sample ASV; Fig. 6d; Table 2).The three-Al2SiO5 vein (sample ASKV, Fig. 3e) occurs in the

andalusite–sillimanite zone. All three polymorphs are in mutualcontact with each other. The vein is dominated by Al2SiO5 poly-morphs in a matrix of coarse-grained quartz. Euhedral staurolite isalso relatively abundant in the vein (Fig. 7a-c), and muscovite occursas a minor phase. Pink andalusite (0.4–0.5 wt% Fe2O3) and

kyanite are the most abundant polymorphs in the vein. Euhedralkyanite occurs within and adjacent to partially resorbed andalusite(Fig. 7a,c,d); this texture suggests that kyanite grew later thanandalusite, nucleating within the aluminous domain of the andalusiteand likely forming by polymorphic transformation. Both kyanite andandalusite are crosscut by prismatic sillimanite (Fig. 7c). Sillimanitealso occurs as small, oriented prisms in staurolite (Fig. 7a,b) and asmats of fibrolite. The texture of sillimanite crosscutting andalusiteand kyanite conflicts with other observations in the Hamadan seq-uence, in which kyanite is typically the texturally latest polymorph.Garnet occurs at the margins of the vein, most likely as part of thehost rock assemblage.

Kyanite-bearing rocks and veins

Kyanite-bearing rocks are most common in zone 7 of the map(Fig. 1b), but kyanite also occurs in schists in zones 3–8. Kyaniteschists (e.g. sample KVH in zone 7) contain biotite + plagio-clase + quartz + kyanite ± garnet ± staurolite. Garnet is typic-ally almandine-rich (Alm72)78) and is not strongly zoned (Table 3).Quartz veins with kyanite as a major phase occur in zones 4–8.

Kyanite occurs in veins with andalusite (zone 4), andalusite-

And

Sil

1 mm

c And

Sil

Sil

Sil

And

1 mm

b

And

And

Sil

Sil

1 mm

0

0.1

0.2

0.3

0.4

0.5

Distance (µm)0 1 2 3 4 5 6 7

Fe 2

O3

(wei

ght

%)

S A S A A A S

A

S S S

d

ASVH

ASV

ASV

Sil

Sil

Sil

Sil

Sil

AndAnd

And

a

Fig. 6. Andalusite-sillimanite schist and vein. (a) Andalusite partially replaced by prismatic sillimanite in a garnet-biotite-andalusite (sillimanite) schist, see Fig. 2(c). Andalusite is rimmed by muscovite. (b) Intergrown andalusite (pink) and sillimanite(colourless) in a vein; see Fig. 3(d). (c) Sillimanite cross-cutting andalusite in a vein. (d) Variation in Fe2O3 in intergrown andalusiteand sillimanite (sample ASKV, an andalusite-kyanite-sillimanite vein). A ¼ andalusite; S ¼ sillimanite.

1 26 A . A . S E P A H I E T A L .


sillimanite (zone 5), sillimanite (zone 6), and alone (zones 4, 7, and inthe granite). Kyanite-quartz veins occur in the contact aureole of thepluton, within the granite (Fig. 8), and in low-grade rocks that donot otherwise contain Al2SiO5 polymorphs. Kyanite crystals in veinsare typically large blue bladed crystals (Fig. 3f-h); kyanite in someveins (e.g. sample KV, Fig. 3g) is deformed (folded ⁄ kinked; Fig. 9).Fe is the most abundant trace element in both vein and host kyanite,with mol% FeAlSi2O5 ¼ 0.07–0.17.

Sillimanite ± kyanite schists, migmatites, and veins(zones 6–8)

The highest-grade rocks in the Hamadan region contain sillim-anite+quartz+biotite+muscovite+garnet+plagioclase+K-feld-spar (perthitic orthoclase) + ilmenite ± staurolite ± andalusite orkyanite. Sillimanite schists grade into migmatitic rocks with meso-some mineralogy similar to the mineral assemblages in the schists.The schists are cut by abundant granitic dykes and sillimanite-bearing quartz–plagioclase veins ⁄ leucosomes.Close to the granite (<3 km), schists have a hornfels texture but

preserve their primary regional metamorphic assemblages, such asthose containing staurolite or andalusite. Texturally late cordieriteoccurs in these rocks (hornfelsed schists), but cordierite has a com-plex history: in zone 7b (Fig. 1b), neoblastic andalusite crystals occurin rocks in which cordierite has broken down to form sillim-anite + biotite ± garnet. Closer to the granite, cordierite has beenpseudomorphed by fibrolite + biotite + garnet ± K-feldspar. Lessthan 1 km from the Alvand Plutonic Complex (granite), rocks havebeen extensively hornfelsed and contain unaltered cordierite inhornfels and in leucosomes. Rocks in this inner contact zone includecordierite ± andalusite ± garnet hornfels, cordierite – K-feldspar ±garnet hornfels, and sillimanite – K-feldspar ± garnet hornfels. Insome rocks, sillimanite has been partially replaced by symplectiticspinel + plagioclase.Some kyanite schists ⁄ hornfels in the contact zone have textures

that suggest there may have been two distinct generations of kyaniteformation. In these rocks, staurolite is ragged and partially replacedby plagioclase + quartz + biotite (Fig. 10a), and some kyanite isdeformed (bent, broken), and also partially replaced by these phases(Fig. 10b). These rocks also contain euhedral, randomly orientedkyanite (Fig. 10b) that is typically less blocky (more elongate) thanthe broken kyanite, and which crosscuts the relict foliation. Thesecond generation of kyanite may be related to the event that formedthe late kyanite veins.Sillimanite–quartz veins are the most plagioclase-rich of the

quartz vein suite in the field area. These veins consist of pris-matic and fibrolite + plagioclase (An39)42) + quartz ± garnet(Alm77Sps6Prp12Grs5) ± staurolite ± andalusite or kyanite. Sillim-anite-bearing veins also contain minor muscovite, biotite, chloriteand ilmenite. Vein garnet is distinct from host rock garnet; the lattercommonly has complex textural and compositional zoning(Fig. 11a–d, Table 3) whereas vein garnet is homogeneous andinclusion-free. In some sillimanite-rich veins, small inclusion-freegarnet occurs within prismatic sillimanite. In other sillimaniteveins, garnet is rimmed by, and in some grains, pseudomorphed byplagioclase that is typically twinned (Fig. 11e), and some atoll garnetis cored by plagioclase. Sample SV, from zone 6 (<0.5 km from theAlvand granite, Fig. 1b), contains radiating sprays of prismatic andfibrolite (Fig. 11f) in a matrix of plagioclase + quartz, with rareeuhedral garnet.

SUMMARY OF MINERAL COMPOSITIONRELATIONS AND TEXTURES

Vein andalusite has a different trace element compo-sition ⁄ zoning compared to host rock andalusite, andvein garnet has different compositions, zoning patternsand textures compared to host rock garnet. Veinandalusite and the core regions of andalusite in vein

marginal zones is Fe-richer than andalusite in hostschists. Vein andalusite typically contains 0.11–0.70 wt% Fe2O3 (assuming all Fe is Fe3+), andandalusite in the host rocks contain 0.07–0.16 wt%Fe2O3. In vein andalusite, the lower values are fromthe rims and the higher values from the cores of grains.Higher Fe2O3 values correlate with higher abundancefor other transition elements (Cr, Ti).Vein and host rock sillimanite typically contain

0.05–0.18 wt% Fe2O3. Fe2O3 concentrations in kya-nite in both veins and host rocks are also similar;vein kyanite has Fe2O3 concentrations of 0.16–0.25wt% (Table 2), and kyanite in kyanite-sillimaniteschists contains 0.10–0.30 wt%. These low traceelement abundances will displace the Al2SiO5equilibria an insignificant amount (Kerrick & Speer,1988).Garnet has been analyzed from andalusite, andalu-

site–sillimanite, and sillimanite veins to comparecompositions with garnet in the host rocks and toexamine zoning patterns for information about themetamorphic evolution of the veins. Garnet composi-tions are almandine-rich and growth zoned in hostrocks and veins, but some rocks (e.g. zone 6 sillimaniteschist) have texturally and compositionally complexgarnet, whereas vein garnet exhibits only simplegrowth zoning. Garnet in veins is typically small(<1 mm), euhedral, and commonly included withinAl2SiO5 polymorphs; host rock garnet is larger (mm-to cm-scale) and occurs as porphyroblasts in the schistmatrix, with the exception of small garnet includedwithin extremely large Al2SiO5 crystals in somesamples (e.g. within 10–20 cm long andalusite crystals,sample ASVH).Vein staurolite has been analyzed from only the

andalusite–kyanite–sillimanite vein; these have similarcomposition to staurolite within the host schist (XFe-St ¼0.70 ± 0.05 in both; Table 4). Staurolite in garnet–staurolite ± kyanite schists and hornfels is moreFe-rich (XFe-St ¼ 0.74–0.78). Zinc content of stauroliteis variable, with the highest amounts (2 wt% ZnO) invein staurolite and in relict (partially pseudomorphed)staurolite in schist.Plagioclase compositions are sodic in both veins

(andesine) and hosts (oligoclase–andesine) but arevariable. Staurolite schists and hornfelsed kyanite-staurolite schists contain plagioclase that is more sodicthan plagioclase in sillimanite schists.

PRESSURE–TEMPERATURE CONDITIONS &REACTION HISTORY

The Hamadan metamorphic rocks have experiencedmultiple episodes of metamorphism driven by burialand heating during arc construction and collision, andthese events are associated with local partial melting(at high grades, near the pluton) and infiltration ofaqueous fluids. Determining the peak P–T conditionsand the paths for these various events using vein



granite

granite

Ky

Ky

Fig. 8. Kyanite-quartz vein crosscuttinggranite in the Alvand Plutonic Complex.Field of view is 40 cm.

a

And

And

Ky

KyKy

St

Qz blebsin St

Sil prismsin St

c

And

And

Ky

Ky

St

Sil

AndAnd

Ky

b

0.5 mm

StQzblebs

Sil

1 mm

Ky

And

Ky

d

1 mm

Ky

1 mm

Fig. 7. Photomicrographs of the andalusite-kyanite-sillimanite vein (ASKV), see Fig. 3(e). (a) Zoned pink to colourless andalusiteencloses euhedral kyanite and staurolite. Staurolite contains blebs of quartz and prisms of staurolite. (b) Close up view of staurolitefrom (a), shown under crossed polars. Arrows point to sillimanite crystals in staurolite. (c) Euhedral kyanite and staurolite in partiallyresorbed andalusite. Both andalusite and kyanite are crosscut by prismatic sillimanite. The staurolite crystal contains quartz blebsas in (b), and (d) Euhedral kyanite in andalusite.

1 28 A . A . S E P A H I E T A L .


assemblages and associated host rocks is in most casesnot possible because of difficulty in determining whichminerals (and mineral compositions) represent equi-librium assemblages, and because some key mineralsfor P–T determinations have been pseudomorphedor extensively resorbed in rocks located near veins.Several metres from veins, textures are more straight-forward (more consistent with an assumption ofequilibrium), and a study is in progress by A. Baharifarto characterize P–T conditions of these rocks.Textures suggesting disequilibrium are common in

both veins and associated host rocks. For example,in many rocks containing the pressure-sensitiveassemblage garnet–Al2SiO5–quartz–plagioclase, coex-isting Al2SiO5 polymorphs represent a crystallizationsequence of uncertain relationship to the (zoned)

St

Grt

Ky

Ky

Ky

Ky

Ky

a b

c d0.5 mm 0.5 mm

Crd

Grt

Fibrolite + Bt

Bt

BtBt

Ky

0.5 mm 1 mm

Fig. 10. Photomicrographs of rocks in the contact zone of the granite. (a, b) Kyanite-bearing rock in the contact zone of thepluton (sample 235 A). Ragged staurolite (a), partially resorbed by plagioclase + quartz + biotite; garnet + kyanite (b) (deformed,lower left of garnet; and euhedral, twinned, randomly oriented (right of garnet). (c) Cordierite partially pseudomorphed by biotiteand fibrolite. (d) Cordierite partially pseudomorphed by garnet + biotite.

0.25 mm

Ky

Fig. 9. Deformed kyanite from kyanite-quartz vein (sample KV);see Fig. 3(g).



garnet and plagioclase. Our approach to describing theP–T history of this region therefore relies largely on apetrogenetic grid approach, supplemented by thermo-barometry for rocks with apparently simple and well-preserved mineral assemblages. P–T path inferencesare informed by field relations (e.g. the pluton andits contact aureole) and thin section observations(e.g. textures indicating the crystallization sequence ofAl2SiO5 polymorphs).The absence of primary pyrophyllite, kaolinite

and ⁄ or diaspore from low-grade rocks provides auseful minimum temperature estimate (350–375 �C,Fig. 12) only in Al2SiO5-bearing rocks in which theAl2SiO5 formed at the expense of these phases. Thismay have been the case for the low-grade andalusiteschists (e.g. zone 3, sample AVH), but there is nodirect evidence for the andalusite-forming reactionsin these rocks. These minimum temperatures wouldimply high geothermal gradients at the pressureconditions required for andalusite crystallization(>50 �C km)1; Fig. 12).The metamorphic conditions of the low-grade

andalusite-bearing rocks are therefore not well definedby mineral assemblages, but peak temperatures andpressures were likely in the range of 375–500�C and2–3 kbar based on petrogenetic grid considerations,

estimates of geothermal gradients, and calculationsand inferences for rocks in higher grade zones(Fig. 12). For example, garnet-staurolite schist in zone4 records garnet-biotite Fe-Mg exchange temperaturesof 520–570�C (Baharifar, 1997; this study) and apressure of 3 kbar (Baharifar, 1997).In zone 5 rocks, sillimanite clearly replaces anda-

lusite in schists (ASVH) and veins (ASV). SampleASKV is also from zone 5, and in this vein, sillim-anite post-dates andalusite, kyanite and staurolite(Fig. 7). Although ASKV contains kyanite, the lackof kyanite from other rocks (veins and schists) in thesame zone is puzzling. Relict kyanite was found inone sample of andalusite–sillimanite–staurolite schist,indicating that kyanite may have been more wide-spread earlier in the metamorphic history but waslargely replaced by other minerals (including non-Al2SiO5 phases). Based on these observations, wehave drawn two generalized P–T paths for zone 5rocks as segments of clockwise loops: a path thataccounts for the sequence andalusite fi kya-nite fi sillimanite (ASKV), and a path that accountsfor the sequence (kyanite) fi andalusite fi sillim-anite + staurolite (Fig. 12). Andalusite–kyanite veins(zone 4) may have formed along a path that followedthe and ¼ ky equilibrium. The proximity of samples

Fig. 11. (a–b) X-ray maps of Mn and Ca concentration in garnet from a sillimanite schist. Composition in mol% spessartine (a)and grossular (b) shown for selected points. Data from a complete rim-rim traverse are shown in (c) and (d). Dashed lines on theCa map show the outlines of compositional zones: intermediate Ca core, low Ca outer core, higher Ca rim. (e) Garnet rimmed ⁄pseudomorphed by plagioclase ± sillimanite in sillimanite-quartz-plagioclase vein (sample 531); and (d) Prismatic and fibrolite insillimanite-quartz-plagioclase vein (SV). The entire field of view is comprised of sillimanite (fibrolite is darker and mostly in lowerright of photo).

1 30 A . A . S E P A H I E T A L .


ASKV and ASV to each other in zone 5, and the lackof recognized structures that could account for asignificant variation in their tectonic histories, sug-gests that factors related to kinetics or fluid ⁄mineralcompositions more likely accounted for the differ-ence in the assemblages and inferred crystallizationsequences.An unresolved question is the relationship between

the staurolite-forming reaction(s) and reactions thatform Al2SiO5 polymorphs. Possible explanations toaccount for the formation of staurolite before anda-lusite (zone 4a) and before sillimanite (zone 5) include:(1) staurolite formed by a different, lower temperaturereaction than the garnet + chlorite equilibrium shownin Fig. 12 (perhaps a reaction involving pyrophyllite orother low-T Al-rich phase), (2) the Al2SiO5 triple pointof Holdaway (1971) is not applicable to these rocks, or(3) staurolite and andalusite ⁄ sillimanite formed duringentirely different metamorphic events (separated intime).The Alvand Plutonic Complex was emplaced at

shallow crustal levels and is associated with a low-pressure–high-temperature contact aureole (Baharifar,1997; Sepahi, 1999) (zones 6–8; Fig. 1b). Cordierite

occurs as a texturally late phase near the pluton(<3 km) in rocks with a hornfels texture, and over-prints an earlier andalusite ± sillimanite assemblage.Migmatitic rocks and the assemblage K-feldspar +sillimanite also occur only near the pluton, implyingthat the highest temperatures were associated withintrusion of the granite. Mineral assemblages inhornfelsic rocks and the presence of granitic leuco-somes suggest that the stability of muscovite inquartz-bearing rocks was exceeded, and that contactmetamorphic P–T conditions were therefore in excessof 650�C at moderate to low pressures (¼ 4 kbar)(Fig. 12).Contact metamorphism texturally overprinted

regional metamorphic assemblages and fabrics, butsome phases of the granite and the contact aureole arecrosscut by kyanite-bearing veins, suggesting a complexP–T path of increasing pressure and ⁄or decreasingtemperature following intrusion of at least part of theplutonic complex. In some zones near the granite,cordierite is replaced (pseudomorphed) by sillim-anite + biotite, implying back reaction of cordierite(Fig. 12), perhaps during the post-migmatite ⁄ granitepath that accounts for growth of late kyanite (Figs 8

Pre

ssur

e (k

bar)

0

2

4

6

8

Prl

Kln

200 300 400 500 600 700 800

Temperature (°C)

Ky

+Q

z+

H2O

Ky

+Q

z+

H2O

PrlAnd

Dsp

+ Q

z

20°C/km

40°C/km

60°C/km

AndSt

St +Ky

contactmeta-morphism

Ky

+H

2O

Sil

+K

fs+

H2O

Ms

+Q

z

Grt

+C

hlS

t +B

t

St +Sil

Sil+

Bt +Q

z

Fe-C

d+

Kfs+

H 2O

ASV/HASKV

St +

Qz

Sil

+G

rt+

H2O

or L

H2O saturatedgranite solidus

Fig. 12. Pressure–temperature diagram. Al2SiO5 triple point from Holdaway (1971); other equilibria calculated using databaseand software of Berman (1991). P–T conditions and paths are shown for selected samples. Dsp ¼ diaspore; Kln ¼ kaolinite; Prl ¼pyrophyllite. The shaded field labelled And (andalusite) shows the conditions at which the andalusite schists and andalusite-quartzveins may have equilibrated. The field labelled St (staurolite) was calculated for the staurolite-garnet schist (SschH) using garnet-biotitethermometry, brackets from lower and higher grade rocks, and results of Baharifar (1997). The field labelled St + Ky and St + Sil arepartially defined by equilibria (e.g. the upper stability of staurolite + quartz), but the maximum pressure for St + Ky is not known.The path for sample ASKV (3 Al2SiO5 polymorph vein) is schematic, but shows a possible path that would result in the inferredcrystallization sequence (andalusite fi kyanite fi sillimanite). This path does not account for the appearance of staurolite beforesillimanite. The path for samples ASV and ASVH is similarly schematic, but accounts for relict kyanite fi andalusite fi sillim-anite + staurolite. Kyanite veins appear to be late in the metamorphic and magmatic history, so a path is drawn from the contactmetamorphism field (grey box) to the kyanite stability field; this represents a decrease in temperature, with or without an increase inpressure (and therefore decreasing geothermal gradient); this path could be isobaric.



& 10). Whether the kyanite-bearing rocks within andbeyond the obvious contact aureole experiencedP >4 kbar before and ⁄ or after intrusion of the plutondepends on how the different generations of kyaniteformed.The P–T diagram (Fig. 12) illustrates the following

implications of our data. Some rocks (ASKV, ASV,ASVH) record a prograde heating path that isclockwise on a P–T diagram. Other rocks that wereburied and heated along apparently similar paths asASV ⁄H, or at slightly higher pressures (kyanite zone),experienced low-pressure–high-temperature contactmetamorphism near the Alvand granite, reachingconditions sufficient to break down muscovite andinitiate partial melting in pelitic rocks. The contactmetamorphosed rocks, the granite itself, and somerocks that are beyond the contact aureole, experi-enced kyanite zone conditions following intrusion ofthe granite. Whether these rocks were buried andheated (e.g. during contraction associated with colli-sion and suturing) or whether the P–T path wasdominated by cooling is not defined by our observa-tions and analyses.The P–T diagram and inferred paths do not account

for the decompression history of the terrane. Thebreakdown of garnet to plagioclase + sillimanite,dehydration melting and the formation of spinel-plagioclase symplectite on sillimanite could occurduring decompression or heating; these textures arelimited to the contact aureole, so heating is perhaps themore likely explanation, but, as noted earlier, the P–Tconditions and paths associated with the late kyanitevein-forming event are not known.

PETROGENESIS

In the suite of NW–SE trending quartz-rich veins nearHamadan, andalusite-bearing quartz veins occur pri-marily in andalusite-bearing rocks, andalusite-sillim-anite veins occur in andalusite-sillimanite schist, andsillimanite-bearing quartz veins occur primarily inhigh-grade, migmatitic sillimanite-bearing rocks.Kyanite-quartz veins, however, are not correlated withhost rock mineralogy or metamorphic grade (Fig. 1b).These observations indicate that different vein-formingmechanisms may have operated at different timesduring dynamothermal metamorphism.Veins and host rocks containing two Al2SiO5 phases

show a similar crystallization sequence for the poly-morphs: that is, in most veins and hosts, the inferredsequence of polymorphic transitions is andalusite fisillimanite, andalusite fi kyanite, or sillimanite fikyanite. A prominent exception to this trend is theandalusite–kyanite–sillimanite vein (ASKV), whichshows evidence for the sequence andalusite fi kya-nite fi sillimanite. In the following sections, we discussthese inferred crystallization sequences in the contextof P–T conditions and paths and possible vein-formingmechanisms.

Petrogenesis of the Al2SiO5-bearing veins

The large size (up to 3 m) of the andalusite-bearingveins, their coarse quartz and Al2SiO5 crystals con-taining abundant aqueous fluid inclusions, the growthof Al2SiO5 perpendicular to vein walls, and the pres-ence of cm- to m-scale reaction zones with host rockssuggest crystallization from an aqueous fluid thatinteracted with the host rock.In all andalusite veins that also contain other

polymorphs (sillimanite ± kyanite), andalusite is tex-turally earliest and partially replaced by the other(s)(e.g. Figs 6 & 7). Sillimanite and kyanite that occurin andalusite veins may have formed by polymorphictransformations, as they nucleated in and aroundtexturally earlier andalusite crystals. In the hostrocks, there is evidence for polymorphic transforma-tion of andalusite to sillimanite in large porphyro-blasts (e.g. sample ASVH, Figs 2b & 6a), but Al2SiO5phases may also have formed in the host rocks frombreakdown of other aluminous minerals. For example,in the high-T (K-feldspar-bearing) rocks near thegranite, some sillimanite in veins and host rocks mayhave formed from breakdown of muscovite + quartz,with or without the production of granitic melt(Fig. 12); some of the sillimanite + plagioclase-bear-ing segregations may be leucosomes (cf. Nabelek,1997).The formation of the kyanite–quartz veins may not

have involved the same type of interaction with thehost rocks as inferred for the andalusite and sillimaniteveins, as there is no correlation between kyanite-quartzveins and host rock type or grade. The rocks in theHamadan region may have experienced a significantchange in P–T conditions following generation of theandalusite and sillimanite-bearing veins. Late growthof kyanite may have occurred in response to anincrease in pressure and ⁄ or decrease in temperature(Fig. 12), but the maximum pressures and the pathsare not known.

Metamorphic–tectonic history of the region

The observations and data presented in this paper areconsistent with a thermal-tectonic history in whichearly regional metamorphism (greenschist to amphi-bolite facies) occurred during initial collision ⁄contraction of continental margin basins and develop-ment of a continental margin arc and associated highgeothermal gradient. Intermediate stages of contract-ional history and arc evolution were associated withintrusion of the Alvand Plutonic Complex, includingthe granite in the Hamadan area that created a low-pressure–high-temperature contact aureole. Fluidsrelated to the intrusion or regional dehydration ofhydrous minerals in pelitic rocks may have generatedthe quartz ± andalusite ± sillimanite vein system.Continued collision associated with final closure ofthe Neo-Tethyan seaway may have driven further

1 32 A . A . S E P A H I E T A L .


burial of the rocks (creating the kyanite veins athigher pressures), or continued fluid infiltration dur-ing cooling may have generated the kyanite-quartzveins at lower temperatures without an increase inpressure.

Significance of andalusite–kyanite–sillimanite veins

Veins or leucosomes that contain all three Al2SiO5polymorphs are extremely rare. Three coexisting poly-morphs are known from metapelitic rocks (Hietanen,1956; Garcıa-Casco & Torres-Roldan, 1996) andquartzites (Holdaway, 1978; Grambling, 1981; Whit-ney, 2002), but reports of Al2SiO5–bearing veins typ-ically describe at most two polymorphs (Cesare, 1994;Whitney & Dilek, 2000; Cavosie et al., 2002). Thecomplexity of the tectonic and metamorphic history ofthe Hamadan region may have contributed to theabundance of Al2SiO5 polymorphs in the region, pro-viding suitable P–T conditions for all three polymorphsto be created at different times. The presence of dif-ferent crystallization sequences within a relatively smallregion may also indicate that the P–T paths passed nearthe Al2SiO5 triple point (some slightly above, someslightly below), producing different P–T paths in rocksthat experienced overall similar P–T conditions.Alternatively, the complexity of regional vs. contactmetamorphic events, combined with fluid–rock inter-action associated with infiltration of aqueous fluids,may have created a variety of chemical ⁄mechanicalenvironments in which Al2SiO5 nucleation variedgreatly in neighbouring rocks.Once created, the multiple Al2SiO5 polymorph

assemblages survived, most notably in quartz veins, fora similar reason that two and three Al2SiO5 polymorphassemblages occur in quartzites in other terranes: i.e. ina simple chemical system, there is a better chance forthe metastable persistence of the polymorphs duringprogressive metamorphism because the polymorphs arenot destroyed by reactions involving other aluminousphases (biotite, muscovite, plagioclase, staurolite, gar-net). In addition, if the P–T paths did not stray far fromthe Al2SiO5 triple point, nontectonic factors such askinetics and chemistry could have resulted in the varietyof Al2SiO5 assemblages without significant differencesin tectonic histories.

ACKNOWLEDGEMENTS

We thank S. Khodabakhsh, C. Manning, B. Cesare,and T. Larson for their reviews.

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Received 2 December 2002; revision accepted 7 November 2003.

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Petrogenesis of andalusite-kyanite-sillimanite veins and host rocks, Sanandaj-Sirjan metamorphic belt, Hamadan, Iran

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