ORIGINAL PAPER

Garnet-bearing rhyolite from Deh-Salm area, Lut block,Eastern Iran: anatexis of deep crustal rocks

H. Mirnejad & G. H. Blourian & M. Kheirkhah &

M. A. Akrami & F. Tutti

Received: 21 November 2007 /Accepted: 24 June 2008 / Published online: 22 July 2008# Springer-Verlag 2008

Abstract Rhyolite from the Deh Salm area, Lut block,Iran, is a 10-km-long flow unit that crops out as a 6-m thicksub-horizontal sheet (<5° dip) underlain by agglomerateand overlain by crystal lithic tuff. The ryholite containsphenocrysts of garnet, plagioclase and amphibole phasesresiding in a cryptocrystalline groundmass consisting ofplagioclase, K-feldspar and quartz. Euhedral garnet phasesrange in size from 0.2 to 3.0 mm, have inclusions ofilmenite, rutile, apatite and zircon, and show no reaction/resorption microtextures. Electron microprobe analysisreveals that the garnet is homogeneous, lacks substantialchemical zoning and is dominantly almandine with subor-dinate pyrope, grossular, spessartine and andradite. Theabsence of inclusions of metamorphic minerals withingarnet as well as the lack of included metamorphic rockfragments in the host rhyolite rule out a xenocrystic originfor the garnet. The garnet phases have low CaO content andvariable MnO concentration, resembling those formed fromS-type magma. The host rhyolite contains quartz, K-feldspar, and amphibole, has normative corundum, and isperaluminous. Inclusions within garnet as well as thechemistry of the garnet and the host rhyolite suggest thatthe garnet is a phenocryst phase crystallized from a deep-

rooted (>25 km) crustal-derived peraluminous magma. It islikely that the peraluminous feature of the magma, inheritedfrom partial melting of metapelitic source rocks, hasfavoured crystallization of garnet. Upon rising to higherlevels, the garnet phases were in equilibrium with the hostmagma and thus were preserved as relatively large euhedralcrystals in the Deh-Salm rhyolite.

Introduction

Garnet is commonly developed at pressure-temperatureconditions existing in metamorphic terrains, and often asan accessory phase in granites and acid volcanic rocks.Garnet in the volcanic rocks, however, can have either arestite/xenocyrstic (a residual phase of partially meltedmetamorphic rocks or a phase entered the melt followingassimilation of the country rocks) or a phenocrystic(primary phase crystallized from a magma) origin. Greenand Ringwood (1968, 1972) demonstrated that almandine-rich garnet in igneous rocks could crystallize from anAl-rich silicic magma at high pressures (9–18 kbar) as anear-liquidus phase. It has also been suggested that thecomposition of magmas can be inferred from the chemistryof garnet (Harangi et al. 2001; René and Stelling 2007). Forexample, garnet crystallized from peraluminous (S-type)magmas has lower CaO content (CaO <4 wt.%) comparedwith that occurring in metaluminous (I-type) volcanic rocks(Green 1992; Harangi et al. 2001). The presence ofeuhedral garnet crystals in volcanic rocks may indicateeither rapid ascend of magma or that the garnet was inequilibrium with the host magma (Gilbert and Rogers 1989;Day et al. 1992; Kawabata and Takafuji 2005).

Garnet-bearing volcanic rock with a thickness of ∼6 moccurs northwest of the village of Deh Salm, central part of

Miner Petrol (2008) 94:259–269DOI 10.1007/s00710-008-0015-4

Editorial handling: K. Stüwe

H. Mirnejad (*) : F. TuttiDepartment of Geology, Faculty of Science, University of Tehran,Tehran, Irane-mail: mirnejad@khayam.ut.ac.ir

G. H. Blourian :M. A. AkramiGeological Survey of Iran, Mashhad Branch,Mashhad, Iran

M. KheirkhahResearch Institute of Earth Sciences, Geological Survey of Iran,Tehran, Iran

Lut block, eastern Iran (Fig. 1). This volcanic unit isbounded by garnet-free volcanic–volcanoclastic rocks tothe east and west. Although the Lut block and its associatedvolcanism have been studied (e.g., Kluyver et al. 1983;Jung et al. 1983), no report exists on the garnet-bearingvolcanic rocks. The aims of this study are to describe thepetrography and major and trace element geochemistry ofthe garnet and its host rock, to identify the ultimate sourceof the garnet (i.e., primary vs. secondary phase), and toestimate the crystallization environment of the host magma.

Geology

The study area is situated 30 km northwest of Deh Salmvillage, in the central part of the Lut block in Eastern Iran(Fig. 1). The rocks of the Lut block are part of the centraleast Iranian microcontinent (Takin 1972), which alsoincludes the Tabas, Yazd, and Posht-Badam blocks.According to Davoudzadeh and Schmidt (1984), the Iranplate fragmented following collision with Turan plate, andthe central east Iranian microcontinent is one of those

fragments. Although the lithology of the Iran plate andparticularly that of the central east Iranian microcontinent isnot well characterized, it is likely that numerous orgoenicactivities (e.g., Pan-African, Hercynian, Cimmerian, andAlpine) have resulted in the creation of a metamorphicbasement (Stöcklin et al. 1971).

The Lut block, extending over some 900 km in NS and200 km in EW directions, is defined at its eastern andwestern margins by deep-reaching fault systems andintensively fold and thrust belts. The northern terminationof the Lut block is the Kavir-e Namak depression and thesouthern edge is defined by the Bazman-Shahsavaranvolcanic complex (Jung et al. 1983). The structural geologyof Lut block is not complicated and gentle faulting, tiltingand folding predominate. Strike–slip faults are welldeveloped throughout the Lut block that may have beendeveloped due to both the confinement of the blockbetween two large fault systems of Nehbandan on the eastand Nayband on the west side, and the compression forcesexerting on the Lut block (Nogol-Sadat 1978). Davoudza-deh et al. (1981) suggest that the Lut block has undergonean anticlockwise rotational movement, by as much as 135°,

Fig. 1 (a) Map showing the position of the Lut block in Iran relative to the adjacent blocks. (b) Geological map of the Deh Salm region

260 H. Mirnejad et al.

relative to its current position since the Triassic. Themagmatic activity in the Lut block that led to thedevelopment of a variety of volcanic, volcanoclastic,subvolcanic and intrusive rocks, started in the late Jurassicwith the intrusion of Shah-Kuh batholith and continueduntil the Quaternary (Esmaiely et al. 2005). Tertiaryvolcanic activity, composed dominantly of basic to inter-mediate volcanic rocks, cover much parts of the Lut block(Stöcklin et al. 1971). Tirrul et al. (1983) suggest that theoceanic plate between the Lut block and the Afghan blocksubducted under the Lut block in the Tertiary. Although thegeochemical data from the Lut block are insufficient todecipher its geotectonic environment, data from volcanicrocks from the northern parts of the Lut block also point tocalc-alkaline magmatism in a convergent tectonic environ-ment (Jung et al. 1983).

The sedimentary strata in Lut block are all younger thanPermian and consist of shallow marine carbonates, detritaldeposits, shales and sandstones. Quaternary sand dunes,salt flats and alluvial fans also cover large areas of the Lutblock. In the eastern part of the Lut block, varioussuccessions of low- to medium-grade metamorphic rocks(e.g., phylite, schist, marble, amphibolite) with possiblePaleozoic age crop out that may also represent the basementof the Lut block (Stöcklin et al. 1971).

The main lithological units in the study area are volcanicand volcanoclatic rocks of various compositions. Garnet-bearing volcanic rocks crop out as a single flow unit that is10 km-long and 1 km-wide with a thickness of ∼6 m thatdips <5° W and trends in north–south direction (Fig. 1).This unit rests conformably over agglomerate and isunderlain by crystal lithic tuff. The association of garnet-bearing rhyolite with lithic tuff may indicate that thevolcanic activity in the area was more or less of explosivenature. A number of non-coeval strike–slip NW–SE faultscut through the area. In the north and west of the study area,andesite and basaltic andesite are the dominant volcanicrocks while in southwest, lithic tuffs and devitrified glasstuffs occur. No radiometric dates are available to indicatethe age of these rocks. However, the Deh-Salm garnet-bearing volcanic rocks are reported as mid-Eocene in a1:100,000 geology map (Akrami and Naderi 2005),based on the regional geology of the Lut block and therelative positions of the volcanic rocks in the stratigraphicsequence.

Sample description and petrography

In hand specimen, the garnet-bearing rocks (DS-3, -5, -32)have a porphyritic texture with phenocrysts of abundantchalky white plagioclase laths, altered greenish amphiboleneedles and deep- to brownish-red garnet crystals set in a

light-green cryptocrystalline groundmass (Fig. 2a). Garnetis euhedral and contains fine-grained equant and elongatedinclusions. Garnet sizes vary from 0.2 to 3.0 mm and itsmodal abundance ranges from 3% to 8%. Some separatedgarnet grains show well-formed crystal faces which areclassified as trapezohedron and combinations of trapezohe-dron and dodecahedron. Rounded to sub-rounded maficxenoliths with a maximum dimension of 0.5 cm occur ingarnet-bearing rocks. One of the garnet-barren volcanicunits (Ea) has a microcrystalline porphyritic texture, is grayto greenish gray in color and contains feldspar andamphibole (DS-23) or pyroxene (DS-22) residing in afine-grained matrix. The other unit is a crystal lithic tuffwhich contains crystal fragments of quartz and feldspar(DS-37).

Microscopic studies show that garnet phenocrysts areeuhedral and contain numerous inclusions of ilmenite,rutile, apatite and zircon (Fig. 2b). Plagioclase phenocrystsin the garnet-bearing samples are subhedral and inclusion-free. Amphibole is mostly altered to chlorite. The fine-grained groundmass minerals consist of euhedral toanhedral plagioclase, K-feldspar and quartz ranging in sizefrom <5 to 50 μm (Fig. 2c). Ilmenite as an accessory phaseis present throughout the matrix. Small angular fragmentsof garnet are found in some samples. Based on themicroscopic observation, this rock is designated as agarnet-bearing rhyolite.

Sample DS-22 has a porphyritic texture and consists ofeuhedral to subhedral pyroxene and plagioclase phenocrystsset in a cryptocrystalline groundmass of plagioclase,opaque and glass. Plagioclase phenocrysts show occasionalzonation and in some parts of thin section have been alteredto sericite. Plagioclase microlites in the groundmass arepartly oriented. A few pyroxene grains have been altered tochlorite and have developed Fe-oxide veinlets along micro-fractures. This sample is classified as basaltic andesite.

In sample DS-23, euhedral to subhedral phenocrysts ofplagioclase slightly altered to sericite, and euhedral amphi-bole with a dark brown to creamy poleochroism covered bydark-colored oxide at the margins, are set in a groundmassof microycrystalline plagioclase and glass. The rock showsflow texture and has an overall volcanoclastic texture. Onthe basis of mineral contents and texture, this sample isclassified as andesite.

Sample DS-37 has a volcanocalstic texture and consistsof lithic fragments and anhedral to subhedral quartz,plagioclase and mica set in a glassy matrix. A number offeldspar crystals show albite-carlsbad twining and havebeen altered to clays minerals along microfractures. Thelithic fragments are composed of volcanic glass and rockparticles. The groundmass has been altered to Fe-oxide andin some areas has been recrystallized to quartz and feldspar.This sample is classified as crystal lithic tuff.

Garnet-bearing rhyolite from Deh-Salm area, Lut block, Eastern Iran 261

Analytical techniques

In order to determine the chemical compositions ofminerals, electron microprobe (EMP) analysis was carriedout to obtain detailed compositional profiles of mineralphases. EMP imaging and quantitative analyses wereperformed by a wavelength-dispersive technique (WDS)using a Camebax MBX electron microbeam in theDepartment of Earth Sciences, Carleton University, Ottawa,Canada. Mineral compositions are accurate to 1–2% of thequoted values for major elements and 3–5% for minorelements. FeO and Fe2O3 were estimated in garnetassuming eight cations, and then adjusting Fe3+/Fe2+ tocharge-balance 12 oxygen atoms.

Fresh whole rocks spanning the complete range of rocktypes were selected for chemical analysis. Whole-rockmajor and trace elements for samples DS-3, -5, -32, -33,-22, -23, -37 were analyzed using inductively plasmaoptical emission spectrometry and inductively plasma massspectrometry (ICP-OES and ICP-MS) facilities, respective-ly, at ACME laboratory, Vancouver, British Columbia.Hand-picked garnet separates from two samples (DS-3,-32) with no attachments from surrounding materials werealso analyzed for their refractory and rare earth elementsusing ICP-MS facility at ACME laboratory. All samples for

ICP analysis were dissolved using an alkali fusion (LiBO2/LiB4O7) method. Reproducibility, based on repeat analyses,for the major elements is ±1.0% of the quoted values, whilemost trace elements have an uncertainty of ±5.0% if theconcentrations are >100 ppm and ±10% for those with<100 ppm.

Mineral chemistry and whole-rock geochemistry

Table 1 lists the chemical composition and unit formula ofrepresentative mineral phases in the garnet-bearing rhyolite.Feldspar phenocrysts are plagioclase and show slightzonation from the core (An38) to the rim (An36). Plagioclasein the matrix is euhedral to subhedral and is less calcic(An22–29) than the phenocryst. Orthoclase (Or97) occurssolely as an anhedral groundmass phase. Fe-rich chlorite(FeO 36%) is classified as ripidolite–brunsvigite. End-member garnet compositions were calculated usingRickwood’s (1968) method. All the studied garnet grainsconsists of almandine (Fe2Al2Si3O12: 72–76%), pyrope(Mg2Al2Si3O12: 7–12%), grossular (Ca3Al2Si3O12: 8–14%), spessartine (Mn2Al2Si3O12: 3–4%) and andradite(Ca3Fe2Si3O12: 0–3%). Garnet grains are homogeneous anddo not show any substantial chemical zoning. The oxide

Fig. 2 (a) Garnet grains (dark)and plagioclase laths (white) in amicrocrystalline groundmass.Field of view 6 cm. (b) Euhedralgarnet with inclusions of ilmen-ite (dark) and rutile (colorless).(c) Back-scattered image show-ing groundmass of a garnet-bearing rhyolite containingquartz (QTZ), plagioclase (x1and x2), and K-feldspar (x3). (d)Back-scattered image of zirconinclusions in garnet

262 H. Mirnejad et al.

mineral inclusions in the garnet crystals are either ilmenite(DS3-1–8) or rutile (DS3-1–4).

Table 2 gives the major and trace element abundances ofthe whole rocks and garnet separates from Deh Salm area.The garnet-bearing samples are acidic (SiO2 65–70 wt.%)while the garnet-free rocks are intermediate (SiO2 54–61 wt.%). A garnet-free crystal lithic tuff that overliesthe garnet-bearing unit is also acidic (SiO2 70 wt.%). Ona discrimination diagram of Na2O+K2O versus SiO2

(Le Maitre 2002) garnet-bearing samples are classified asrhyolite and the garnet-free samples are andesite (DS-23)and basaltic andesite (DS-22). The crystal lithic tuff (DS-37) also falls in the rhyolite field.

To understand the effect of garnet grains on thechemistry of the host rock, trace element contents of bothare reported here. Figure 3a. shows the REE distributionpatterns of the whole rocks and garnet separates from DehSalm, normalized to chondritic values (McDonough andSun 1995). Garnet has a rough V-shaped chondrite-normalized pattern with distinctive HREE enrichment.One other feature characteristic of REE patterns in garnetseparates is that both LREE and HREE display relativelyflat distribution patterns, which is similar to igneous garnetreported elsewhere. The whole rock garnet-bearing and -barren samples display similar enrichments in LREE butstart diverging at Eu, with the former plotting at lower HREElevels than the latter (Fig. 3a). The varying degrees of HREEdepletion is also reflected in (La/Yb)N ratios (N denoteschondrite-normalized values) which range from 7.5 in thegarnet-free samples to 19.3 in the garnet-bearing rhyolite.

Trace element data, normalized to primitive mantle values(McDonough and Sun 1995), are plotted on a conventionalspider diagram in order of decreasing chemical incompati-bility from left to right (Fig. 3b). The garnet separates showdepletions in Rb, Ba, Nb, and Sr and enrichments in Ta andother high field strength elements such as Th, U, Zr, and Hf.The whole rock garnet-bearing rhyolite trace elementpatterns display somehow different trends such as the lackof strong negative anomalies in Cs, Rb, and positiveanomaly in Ta. With the exception of a few notable features,both the garnet-free samples and the garnet-bearing rhyolitesdisplay roughly similar mantle-normalized trace elementpatterns. Both show negative anomalies in Ba, Nb and Ti,and enrichments in U and Th (excluding DS-22, a basalticandesite). It is notable that a negative Sr anomaly prevalentin the garnet-bearing rhyolites does not exist in the andesite,basaltic andesite and crystal lithic tuff.

Discussion

The main concern over the study of garnet in the volcanicrocks is whether it is the residual phase that has remained

during partial melting of a metamorphic source (Hensenand Green 1973) or whether it crystallized directly from asilicate magma at high pressures and as a near-liquidusphase (Fitton 1972; Lantai 1991; Mitropolous et al. 1999).In general, the presence of certain mineral inclusions (e.g.,sillimanite, staurolite, muscovite, and quartz) in the garnetand the occurrence of metamorphic crustal xenoliths in thehost rock are suggestive of a secondary (xenocrystic) originfor the garnet. However, the garnet crystals from Deh Salmdo not contain any metamorphic mineral inclusions, nordoes the host ryholite have any country rock xenoliths to beindicative of a xenocrystic origin for the garnet. A back-scattered image analysis on zircon inclusions revealed nozoning patterns (Fig. 2d) and it is thus quite likely that thezircon inclusions in garnet were crystallized from therhyolitic magma. The studied garnet cannot be a refractoryrestite phase that survived partial melting of the protolith. Ifgarnet has a restitic origin, some form of garnet-bearingenclaves should be present in the rhyolite. For example,Zeck (1970) suggested that almandine-rich garnet in dacitefrom Spain was a restite phase derived by anatexis of apelitic source because of the presence of xenoliths ofquartz-cordierite gneiss and garnet-biotite-sillimanite gneissin dacite. No metamorphic enclaves exist in the Deh Salmrhyolite and the only recovered xenoliths are cognate andhave igneous origin. Furthermore, there is the question as towhether garnet can occur as a restitic refractory residuum ina rhyolitic magma, especially if extensive partial meltinghas taken place at high temperatures. These, in addition tothe lack of reaction rims around garnet, rule out a restitemodel but are indicative of the magmatic nature.

If a garnet has a primary magmatic origin, its occurrenceplaces important constraints on the P–T–X conditions ofmagma crystallization. Igneous garnet with distinct chem-ical composition can crystallize from either an I-/M- typemagma (i.e., melts derived by partial melting of igneousrocks in the crust and melts from the upper mantle,respectively) or a S-type magma (Chappell and White1974; Green 1992; Harangi et al. 2001). Almandine garnetfrom the former is Ca-rich (CaO >5 wt.%) and Mn-poor(MnO <2 wt.%) and occurs with hornblende ± epidote(Green 1992) while the latter type of garnet has low CaOcontent (<4 wt.%) and variable MnO concentration(Harangi et al. 2001). It is notable that xenocrystic garnetfrom metapelites partially overlaps the S-type phenocrysticgarnet on the CaO vs. MnO diagram (Fig. 4). Moreimportantly, the composition of almandine garnet fromDeh Salm rhyolite plots dominantly in the field of garnetfrom S-type magma and slightly extends towards the fieldof garnet from metapelites (Fig. 4).

For a magmatic origin, the peraluminous feature of themagma inherited from partial melting of metapelitic sourcerocks would favour crystallization of garnet. While I- or M-

Garnet-bearing rhyolite from Deh-Salm area, Lut block, Eastern Iran 263

Table 1 Representative mineral chemistry of garnet-bearing rhyodacite from the Deh Salm area, Lut block

Sample DS3 DS3 DS3 DS3 DS3 Sample DS3 DS5Mineral Plagioclse Plagioclse Plagioclse Plagioclse Plagioclse Mineral Chlorite Chlorite

Phenocryst Phenocryst Groundmass GroundmassCore Rim

SiO2 59.17 58.54 61.83 61.36 65.36 SiO2 25.4 25.16Al2O3 26.57 25.63 23.68 24.89 18.59 TiO2 0.2 0.11Fe2O3 0.07 0.01 0.92 0.17 0.16 Al2O3 20.04 20.26BaO 0 0 0 0 0.15 FeO 35.91 36.56CaO 8.07 7.47 4.59 6.08 0.03 MnO 0.15 0.14Na2O 7.07 7.14 8.59 8.04 0.29 MgO 5.62 5.98K2O 0.29 0.29 0.44 0.47 15.86 CaO 0.22 0.08Total 101.24 99.08 100.05 101.01 100.44 Total 87.54 88.29Si 10.46 10.55 10.99 10.82 12.00 Si 3.19 3.12Al 5.53 5.44 4.96 5.17 4.02 AlIV 2.81 2.88Fe3 0.01 0.00 0.12 0.02 0.02 Sum_T 6.00 6.00Ba 0.00 0.00 0.00 0.00 0.01 AlVI 0.15 0.08Ca 1.53 1.44 0.87 1.15 0.01 Ti 0.02 0.01Na 2.42 2.50 2.96 2.75 0.10 Fe3 0.00 0.00K 0.07 0.07 0.10 0.11 3.72 Fe2 3.77 3.79Cations 20.01 20.00 20.00 20.01 19.89 Cr 0.00 0.00X 15.99 15.99 16.06 16.01 16.04 Mn 0.02 0.02Z 4.02 4.01 3.93 4.00 3.84 Mg 1.05 1.11Ab 60.3 62.3 75.2 68.7 2.7 Ca 0.03 0.01An 38.1 36 22.2 28.7 0.2 Cations 11.03 11.01Or 1.6 1.7 2.5 2.6 97.1 Fe_FeMg 0.78 0.77

Mg_FeMg 0.22 0.23

Sample DS3 DS3 DS3 DS3 DS3 DS3 DS5 DS5 DS5Mineral Garnet Garnet Garnet Garnet Garnet Garnet Garnet Garnet Garnet

Core Between Rim Core Between Rim Core Between Rim

SiO2 37.51 37.49 37.28 36.68 37.32 36.9 37.1 37.28 37.81TiO2 0.32 0.36 0.47 0.74 0.27 0.36 0.51 0.47 0.43Al2O3 21.05 20.95 21.07 20.35 20.92 20.61 20.83 20.75 21.33Cr2O3 0.05 0 0 0 0.01 0 0 0 0FeO 34.07 34.5 34.35 32.85 33.95 33.7 34.74 34.43 34.05MnO 1.34 1.43 1.23 1.98 1.26 1.25 1.24 1.65 1.23MgO 3.02 3.12 3.13 1.82 3.10 3.18 2.75 2.28 2.99CaO 3.51 3.26 3.05 5.66 3.30 3.29 3.77 4.15 3.74Total 100.87 101.11 100.58 100.08 100.13 99.29 100.94 101.01 101.58FeOcalc 33.99 34.05 34.35 32.11 33.93 33.38 33.92 34.04 34.05Fe2O3calc 0.09 0.50 0.00 0.82 0.03 0.35 0.91 0.44 0.00TSi 2.99 2.98 2.98 2.96 2.99 2.98 2.96 2.98 2.99TAl 0.01 0.02 0.02 0.04 0.01 0.02 0.04 0.02 0.01Sum_T 3.00 3.00 3.00 3.00 3.00 3.00 3.00 3.00 3.00AlVI 1.96 1.94 1.96 1.89 1.97 1.95 1.92 1.93 1.97Fe3 0.01 0.03 0.00 0.05 0.00 0.02 0.06 0.03 0.00Ti 0.02 0.02 0.03 0.05 0.02 0.02 0.03 0.03 0.03Sum_A 1.99 1.99 1.99 1.99 1.99 1.99 2.00 1.99 2.00Fe2 2.26 2.26 2.30 2.17 2.28 2.26 2.26 2.28 2.25Mg 0.36 0.37 0.37 0.22 0.37 0.38 0.33 0.27 0.35Mn 0.09 0.10 0.08 0.14 0.09 0.09 0.08 0.11 0.08Ca 0.30 0.28 0.26 0.49 0.28 0.29 0.32 0.36 0.32Sum_B 3.01 3.01 3.01 3.01 3.01 3.01 3.00 3.01 3.00Sum_cat 8 8 8 8 8 8 8 8 8O 12 12 12 12 12 12 12 12 12Alm 75 75 76 72 75 75 76 75 75

264 H. Mirnejad et al.

type melts are normally associated with amphibole, biotite,orthopyroxene, they have diopside in their normativecomposition, and are metaluminous (Al2O3/CaO + Na2O +K2O <1 (molar)) (Green 1992). The garnet-bearing rhyolitefrom Deh Salm contains quartz, K-feldspar, and amphibole,has normative corundum, and is peraluminous (Al2O3/CaO +Na2O + K2O >1 (molar)). These features are consideredcharacteristics of S-type magmas (Chappell and White1974). Although it is possible that an I- or M-type magmabecomes acidic and can obtain an S-type geochemicalcharacteristics as a result of extensive assimilation of crustalmaterials, the following evidence negates such a model forthe Deh Salm rhyolite. First, there is a lack of any garnet-bearing crustal xenoliths in the Deh Salm rhyolite. The onlyxenolith observed in the rhyolite consists of coarse-grainedquartz–plagioclase–biotite phases which may represent crys-tal aggregations from the magma at depth. Second, thegarnet-bearing rhyolite has similar LREE and lower HREEcontents than the garnet-free samples (Fig. 3a). Any crustallycontaminated magma would be expected to have higher

LREE and HREE contents relative to the more basic melt.Third, the composition of the studied garnet resemblesalmandine in S-type volcanic rocks (Fig. 4). Finally, garnetxenocrysts released from disaggregating country rockswould likely be out of chemical equilibrium with the hostrock, and would thus be expected to develop reaction rims. Itis therefore probable that the Deh Salm rhyolitic magma wasgenerated by anatexis of lower-crustal metapelites and thatgarnet was preserved as a phenocryst phase that crystallizedfrom a peraluminous melt.

As to the ultimate origin of the Deh Salm rhyoliticmagma, partial melting of upper crustal materials is notplausible. This is because, other than spessartine-richcompositions, melting experiments for intermediate toacidic volcanic rocks suggest that garnet cannot crystallizeat P<5–7 kbar (Green and Ringwood 1968; Green 1977;Green 1992). For example, Green (1976, 1977) showed thatin an acidic melt, the grossular and spessartine contents ofgarnet increase and decrease, respectively, with increasingpressure of formation. Garnet in the Deh Salm rhyolite,

Table 1 (continued)

Sample DS3 DS3 DS3 DS3 DS3 Sample DS3 DS5Mineral Plagioclse Plagioclse Plagioclse Plagioclse Plagioclse Mineral Chlorite Chlorite

Phenocryst Phenocryst Groundmass GroundmassCore Rim

And 0 1 0 3 0 1 3 1 0Gross 10 8 9 14 9 8 8 10 11Pyrope 12 12 12 7 12 13 11 9 12Spess 3 3 3 4 3 3 3 4 3XCagnt 0 0 0 0 0 0 0 0 0XFegnt 1 1 1 1 1 1 1 1 1XMggnt 0 0 0 0 0 0 0 0 0Fe_Mggnt 6 6 6 10 6 6 7 8 6

Sample DS3 DS3Mineral Ilmenite Rutile

SiO2 0.29 1.38Al2O3 0.28 0.25TiO2 51.63 96.15FeO 45.89 1.35MgO 0.71 0MnO 0.34 0.1CaO 0.09 0.84Total 99.24 100.07Si 0.08 0.29Al 0.09 0.06Ti 10.48 15.43Fe 10.36 0.24Mg 0.29 0.00Mn 0.08 0.02Ca 0.03 0.19SubTotal 21.40 16.24

Sample DS3 DS3 DS3 DS3 DS3 DS3 DS5 DS5 DS5Mineral Garnet Garnet Garnet Garnet Garnet Garnet Garnet Garnet Garnet

Core Between Rim Core Between Rim Core Between Rim

Garnet-bearing rhyolite from Deh-Salm area, Lut block, Eastern Iran 265

nevertheless, is depleted in the spessarine end membercompared to grossular content. Plus, the chemical similarityof the Deh Salm garnet with igneous-type garnet inignimbrites of Violet Town volcanic rocks, Victoria,Australia (Clemens and Wall 1984) and its inclusioncontents including ilmenite, apatite, and zircon wouldstrongly suggest that garnet crystals in the study area areearly magmatic phenocrysts and the product of medium- tohigh-pressure crystallization of a peraluminous magma.

Direct crystallization of garnet from a peralumious magmahas also been reported from Peñón Rosado garnet-bearinggranites (Dahlquist et al. 2007). High pressure andtemperature (P–T) studies in the system FeO–(Fe2O3)–Al2O3–SiO2–(H2O) shows that elevated pressures arerequired to stabilize garnet at magmatic temperatures(Keesmann et al. 1971). The results of melting experimentson model pelitic compositions by Birch and Gleadow(1974), and Green and Ringwood (1968) also demonstrate

Table 2 Major element (wt.%) and trace element (ppm) concentrations of garnet separates, garnet-bearing rhyolite and surrounding rocks fromDeh Salm, Lut block

Sample: DS-3 DS-5 DS-32 DS-33 DS-3 DS-32 DS-22 DS-23 DS-37

Rock Type: Garnet-bearing rock Garnet separates Garnet-barren rock Lithic tuff

SiO2 69.27 70.46 69.18 53.87 61.29 69.64Al2O3 15.77 15.63 15.84 14.99 15.00 12.84Fe2O3T 3.43 3.03 3.22 10.81 6.74 2.20MgO 0.50 0.48 0.47 4.90 4.08 2.22CaO 2.24 2.43 1.80 10.44 5.69 2.10Na2O 4.02 4.05 4.50 1.86 2.60 3.66K2O 2.78 2.71 2.85 1.89 3.76 6.74TiO2 0.20 0.18 0.20 0.75 0.52 0.28P2O5 0.13 0.12 0.12 0.04 0.08 0.01MnO 0.06 0.05 0.05 0.30 0.07 0.07LOI 1.60 0.90 1.80 1.26 0.35 1.00SUM 100.00 100.04 100.03 101.11 100.18 100.76Ba 374 301.9 273.1 81.4 7.8 21.8 385.1 504.3 678.2Co 3.4 15.9 9.7 6.4 291.8 374.7 27.4 19 2.4Cs 3.6 3.3 2.8 4.9 0.1 0.1 2.2 3.4 4.6Ga 20.3 20.2 19.7 18.5 14.2 14.3 16.2 14.8 13.8Hf 3.4 3.4 3.3 3.5 15.6 15.8 2.9 4.1 5.2Nb 7.9 7.6 7.5 7.9 9.1 9.8 7.2 7.8 15.8Rb 94.5 98.3 92.8 118.6 0.6 0.7 39.3 130.2 185.3Sn 2 2 2 3 1 1 2 2 2Sr 448.8 237.5 244.9 282.5 11.6 14.8 441.5 371.2 335Ta 0.8 1.2 0.9 0.7 7.4 10.5 0.6 0.9 1.2Th 7.5 8.1 8.4 8.3 6.6 5.5 4.2 16.1 17.4U 2.1 1.9 1.8 1.9 1.4 1.5 1 3.4 4.2V 11 10 8 8 147 152 198 107 19W 5.3 176.8 70.6 35.8 2762.6 3295.2 42.4 66 4.6Zr 117.8 117.9 114.6 114.9 668.1 671.1 104.7 150.5 172.1Y 10.1 7.8 8.6 9.3 324.6 328.7 20.3 22.5 24.1La 27.3 26.8 24.7 22.8 20.6 22.1 15.2 22.9 37.2Ce 58.1 57.8 54.3 50.2 49.6 52.6 32.7 46.5 75.3Pr 6.04 6.24 5.81 5.82 6.48 6.49 3.97 5.3 8.35Nd 23.4 22.5 21.3 21.2 31.8 31.2 16.5 20 31Sm 4 3.4 3.5 3.1 14.6 15 3.2 3.8 4.9Eu 1.11 1.01 0.99 0.72 1.13 1.22 0.91 0.84 0.82Gd 2.01 1.88 1.98 1.99 30.72 31.43 3.3 3.52 3.87Tb 0.32 0.27 0.27 0.31 7.71 7.99 0.6 0.64 0.71Dy 1.98 1.41 1.7 1.57 49.09 50.06 3.21 3.53 3.81Ho 0.33 0.24 0.29 0.27 11.47 11.69 0.7 0.71 0.74Er 0.99 0.82 0.85 0.89 33.23 34.47 2.1 2.2 2.45Tm 0.18 0.11 0.15 0.13 5.17 5.42 0.34 0.33 0.37Yb 1.06 0.76 0.92 0.88 36.98 37.43 2 2.15 2.49Lu 0.18 0.12 0.14 0.14 5.68 5.86 0.3 0.33 0.37

266 H. Mirnejad et al.

that almandine-rich garnet can crystallize from rhyodaciticmelt at pressures between 9 and 18 kbar, but is no longerpresent as a near-liquidus phase at pressure <9 kbar. Wood(1974) calculated that the temperature of garnet crystalliza-tion is near-liquidus in a garnet rhyolite and is about 1,000–1,050°C. These P–T conditions, corresponding to depths of

>25 km, point to garnet crystallization in the deep parts ofthe crust. Experimental works (e.g., Green and Ringwood1972; Green 1976; Clemens and Wall 1984; Conrad et al.1988) confirm that the almandine-rich garnet in silicicigneous rocks can crystallize from a magma generated bypartial melting of sedimentary materials at lower crustallevels. Garnet-bearing rhyolites from Canterbury, NewZealand, are similar to those from Deh Salm in terms ofmineralogy and geochemistry and taking into account theconditions of garnet crystallization in the former (i.e., highpressure regimes), Wood (1974) proposed anatexis of thelower continental crust as a plausible origin of thegarnetiferrous rhyolite magma. Gilbert and Rogers (1989)also suggest that almandine-rich garnet phenocrysts inrhyolite from Pyrenees crystallized from S-type magmasby partial melting of metasedimentary materials in deepcrust. Garnet crystallization during hydrous melting exper-iment of peraluminous greywacke at 10 kbar was alsoreported by Conrad et al. (1988). Fitton (1972) suggestedthat the crystallization of garnet requires a deep-seatedmagma chamber, at the mantle-crust boundary. Consideringthe chemistry of the Deh Salm garnet, in which the

Fig. 3 REE patterns (a) andextended trace element patterns(b) of garnet separates, andwhole-rock samples from DehSalm, normalized to chondriticvalues (McDonough and Sun1995). Garnet separates: DS-3,DS-32; Garnet-bearing rhyolite:DS-3, DS-5, DS-32, DS-33;Basaltic-andesite: DS-22;Andesite: DS-23; Lithic tuff:DS-37

Fig. 4 CaO vs. MnO for Deh Salm garnet (filled square). Fields ofgarnet from different environments are from Harangi et al. (2001)

Garnet-bearing rhyolite from Deh-Salm area, Lut block, Eastern Iran 267

grossular content (8–14 mol%) is higher than the spessar-tine content (3–4 mol%), and the antipathetic relationshipbetween these two components from experimental studies(Green 1977), the depth of garnet crystallization must be>25 km if the garnet was in equilibrium with a silicic melt.Because garnet in the Deh Salm area is an earlycrystallizing phase and is only confined to rhyolite, it isinferred that the garnet did not crystallize in more basicmagmas and that the rhyolite could not have been derivedfrom a basic magma by crystal fractionation.

As to the mechanism for preserving euhedral to subhedralgarnet crystals in Deh Salm ryholite, evidence such as thelack of zonation, resorptoin and reaction coronas in thegarnet, and the microcrystalline texture of the groundmassmay indicate a rapid ascent and eruption of the host rhyoliticmagma. Alternatively, it is likely that upon rising to higherlevels, the garnet phases were in equilibrium with the hostmagma and thus have been preserved as relatively largeeuhedral crystals in the Deh-Salm rhyolite.

Conclusions

The peraluminous garnet-bearing rhyolite from Deh Salmdoes not contain any metamorphic xenoliths. This, inaddition to the lack of inclusions of aluminosilicateminerals in the garnet crystals, rules out a metamorphicorigin for the garnet. However, the presence of ilmenite,rutile and zircon inclusions and the chemistry of garnet(mainly almandine with subordinate pyrope, grossular, andspessartine) suggest that the garnet is a primary phasecrystallized from an aluminum-rich magma. Based onmelting experimental results and comparison with garnetfrom other localities, the garnet crystals are consideredearly magmatic phenocrysts crystallized at depths (>25 km)from a peraluminus magma. Such conditions are bestprovided by partial melting of metasedimentary materialsin the lower crust. To preserve the euheral garnet crystalsfrom resorption, the magma must have been in equilibriumwith the host rhyolite magma.

Acknowledgments Keith Bell is thanked for reading an earlierversion of this paper. Help by Peter Jone for microprobe analysis isappreciated. We would like to thank Kurt Stuewe, and the tworeviewers, Eugenio Fazio and Tim Johnson, whose comments addedgreatly to this manuscript. This work was financially supported by theresearch council of the University of Tehran, grant number 27838/1/10to H. Mirnejad.

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