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Journal of Petrology | Volume 43 | Number 4 | Pages 725-747 | 2002
© Oxford University Press 2002

Metamorphic Conditions and Fluid Compositions of Scapolite-Bearing Rocks from the Lapis Lazuli Deposit at Sare Sang, Afghanistan

SHAH WALI FARYAD^,*

INSTITUT FÜR GEOLOGIE, MINERALOGIE UND GEOPHYSIK, RUHR UNIVERSITÄT BOCHUM, GERMANY

Received January 20, 2001; Revised typescript accepted October 30, 2001

	ABSTRACT

TOP ABSTRACT INTRODUCTION GEOLOGICAL BACKGROUND SAMPLE DESCRIPTION MINERAL CHEMISTRY FLUID COMPOSITION CONCLUSIONS REFERENCES

 
Scapolite and other halogen-rich minerals (phlogopite, amphibole, apatite, titanite and clinohumite) occur in some high-pressure amphibolite facies calc-silicates and orthopyroxene-bearing rocks at Sare Sang (Sar e Sang or Sar-e-Sang), NE Afghanistan. The calc-silicates are subdivided into two groups: garnet-bearing and garnet-free, phlogopite-bearing. Besides garnet and/or phlogopite, the amphibolite facies mineral assemblages in the calc-silicates include clinopyroxene, calcite, quartz and one or more of the minerals scapolite, plagioclase, K-feldspar, titanite, apatite and rarely olivine. Orthopyroxene-bearing rocks consist of clinopyroxene, garnet, plagioclase, scapolite, amphibole, quartz, calcite and accessory dolomite and alumosilicate (kyanite?). Retrograde phases in the rocks are plagioclase, scapolite, calcite, amphibole, sodalite, haüyne, lazurite, biotite, apatite and dolomite. The clinopyroxene is mostly diopside and rarely also hedenbergite. Aegirine and omphacite with a maximum jadeite content of 29 mol % were also found. Garnet from the calc-silicates is Grs_45–95Py_0–2 and from the orthopyroxene-bearing rocks is Grs_10–15Py_36–43. Peak P–T metamorphic conditions, calculated using available exchange thermobarometers and the TWQ program, are 750°C and 1·3–1·4 GPa. Depending on the rock type, the scapolite exhibits a wide range of composition (from Eq_An = 0·07, X_Cl =0·99 to Eq_An = 0·61, X_Cl =0·07). Equilibria calculated for scapolite and coexisting phases at peak metamorphic conditions yield X_CO2 = 0·03–0·15. X_NaCl (fluid), obtained for scapolite, ranges between 0·04 and 0·99. Partitioning of F and Cl between coexisting phases was calculated for apatite–biotite and amphibole–biotite. Fluorapatite is present in calc-silicates, but orthopyroxene-bearing rocks contain chlorapatite. Cl preferentially partitions into amphibole with respect to biotite. All these rocks have suffered various degrees of retrogression, which resulted in removal of halogens, CO₂ and S. Halogen- and S-bearing minerals formed during retrogression and metasomatism are fluorapatite, sodalite, amphibole, scapolite, clinohumite, haüyne, pyrite, and lazurite, which either form veins or replace earlier formed phases.

KEY WORDS: scapolite; fluid composition; high-pressure; amphibolite facies; Western Hindukush; Afghanistan

	INTRODUCTION

TOP ABSTRACT INTRODUCTION GEOLOGICAL BACKGROUND SAMPLE DESCRIPTION MINERAL CHEMISTRY FLUID COMPOSITION CONCLUSIONS REFERENCES

 
The fluid evolution in metamorphic terrains may be recorded by fluid inclusions and mineral chemical equilibria. The scapolite group of minerals has received considerable attention for their ability to act as either a source or a sink for a fluid phase (CO₃, SO₄, Cl) in a wide variety of metamorphic environments (Vanko & Bishop, 1982; Oterdoom & Wenk, 1983; Mora & Valley, 1989; Moecher & Essene, 1991; Harley et al., 1994). Fluorine and chlorine are ubiquitous in fluids from all crustal levels in amounts from ppm to, in the case of Cl, tens of wt % (Markl & Piazolo, 1998). Because they are incorporated into many common rock-forming minerals such as amphiboles, micas, apatite or scapolite, halogens can easily be monitored and their abundances in metamorphic fluids can be quantitatively estimated (Munoz & Swenson, 1981; Zhu & Sverjensky, 1991; Harley & Buick, 1992; Markl & Bucher, 1998). The world-famous lapis lazuli locality at Sare Sang (known in the literature as Sar e Sang or Sar-e-Sang) provides a unique opportunity to study fluid compositions from a large number of minerals, which involve halogens, S and CO₂. Besides lazurite and haüyne, which are the result of the circulation of S-rich fluids, relatively large concentrations of fluorine, chlorine and CO₂ in the metamorphic fluid can be inferred from the composition of phlogopite, amphibole, apatite and scapolite occurring in calc-silicate rocks from this locality.

Despite a number of studies, related to the mineralogy and metamorphic conditions (Blaise & Cesbron, 1966; Schreyer & Abraham, 1976; Grew, 1988; Massonne, 1989; Grew et al., 1994; Faryad, 1999), the origin of the lapis lazuli and related mineralization remains unclear. According to Yurgenson & Sukharev (1984), the lapis lazuli deposits resulted from interaction between granite-related hydrothermal fluids (Na-metasomatism) and marbles with high amounts of pyrite. A metamorphic origin for lapis lazuli deposits and whiteschists from primary evaporites and mudstones was proposed by Kulke (1976) and Schreyer (1977). P–T conditions estimated for the Sare Sang locality indicated high-pressure amphibolite facies (Schreyer & Abraham, 1976; Faryad, 1999) followed by retrograde metamorphism. Because of the high concentration of S and halogens, retrograde reactions were extensive in the lapis lazuli bodies. Investigation of the fluid composition in the surrounding rocks may help to understand fluid evolution in the Sare Sang locality and lapis lazuli formation.

This paper reports analyses of scapolite and various halogen-bearing phases and focuses on mineral reactions and textures observed in scapolite-bearing rocks at Sare Sang. Because the calc-silicates and halogen-bearing minerals are most sensitive to fluid composition, those rocks have been studied to establish whether fluid was present during prograde and/or retrograde metamorphism. P–T–X conditions were estimated using mineral composition and various thermobarometric calibrations and equilibrium reactions. The present contribution demonstrates that the peak-metamorphic fluids were essentially H₂O–CO₂–NaCl–HF mixtures. F- and Cl-bearing phases in scapolite-bearing rocks are used to evaluate whether the minerals crystallized in a closed system or if fluid infiltration occurred as well. The results of this work provide additional constraints for the role of fluids in regional metamorphism. The estimated conditions of formation for the various mineral assemblages are used to model the evolution of these unusual, fluid-rich rocks and to compare them with other halogen-rich rocks. For this study a series of samples collected by H. Kulke and F. Seifert during the late 1970s and forming part of the collection of Institut für Mineralogie, Geologie und Geophysik, Ruhr Universität, were used.

	GEOLOGICAL BACKGROUND

TOP ABSTRACT INTRODUCTION GEOLOGICAL BACKGROUND SAMPLE DESCRIPTION MINERAL CHEMISTRY FLUID COMPOSITION CONCLUSIONS REFERENCES

 
Basic information on the geological setting of the Sare Sang locality was given by Kulke & Schreyer (1973) and Faryad (1999). The lapis lazuli deposits and surrounding rocks investigated here belong to the oldest Sare Sang series, of Archaean(?) age, that is exposed along the north-flowing Kokcha river valley (100 km SSE from Paiz Abad in the Badakhshan Province, Fig. 1). The Archaean age of this series is assumed from U–Pb-isotopic data (2700–2400 Ma) from Pamir (Horeva et al., 1971). Hubbard et al. (1999) recently obtained a Neogene cooling age (⁴⁰Ar/³⁹Ar data from biotite, phlogopite and hornblende) for upper amphibolite facies whiteschists in the Pamir (Tadjikistan). The common rocks of the Sare Sang series are calcite and dolomite marbles and schists intercalated with biotite–amphibole (± garnet, ± diopside) gneisses, amphibolites and quartzites. Granites and pegmatite are also present. Schreyer & Abraham (1976) investigated the metamorphic history of the South Badakhshan block (NE Afghanistan) by studying mineral assemblages in whiteschists at Sare Sang (Fig. 1). They assumed peak pressure and temperature of 1·1 GPa and 800°C for the whiteschists. Pressures of 0·7 GPa and 650–680°C were estimated by Massonne (1989) and Grew (1988) for chlorite–talc–cordierite- and kornerupine–phlogopite–sapphirine-bearing assemblages in the Sare Sang rocks. Thermobarometric calculations for mineral assemblages in metapelites and metabasites (Faryad, 1999) indicated a minimum pressure of 1·2 GPa at 780°C for the Sare Sang locality.

View larger version (88K): [in this window] [in a new window]   Fig. 1. Schematic geological map of South Badakhshan block (Western Hindukush), based on the Geological Map of Afghanistan (Kafarsky et al., 1975; Dronov et al., 1980). Box indicates location of investigated area at Sare Sang lapis lazuli deposit. KL, Kohe Lal (Kohi Lal). Bold lines are faults: HS, Hazrate Sultan fault; CB, Central Badakhshan fault.

 

The Sare Sang lapis lazuli deposits, formed mainly of lazurite with various amounts of haüyne, sodalite, scapolite, afghanite and phlogopite, are the main reservoir of S and halogens. They form lenses or layers in marbles and occur at contacts with granites and pegmatite. The lenses, of up to 40 m length and 2 m thickness, consist usually of three zones. The central part is formed either of fine-grained alkali feldspar and quartz or of coarse-grained calcic plagioclase. Small or accessory amounts of clinopyroxene, titanite, phlogopite, pyrite, epidote, titanite, apatite and scapolite may be present. The second zone is formed of diopside (40–90 wt %) with forsterite, scapolite, phlogopite, tremolite, titanite, zoisite and garnet, and rarely nepheline, haüyne and lazurite. The outer zone contains calcite, diopside and lazurite, haüyne, afghanite and nepheline. Extensive lapis lazuli deposits form layers, which are 6 m thick and reach a length of several hundred metres (maximum 450 m). In most cases a band of calcite–diopside–lazurite is rimmed by a diopside-rich zone on one side and a phlogopite–diopside–calc-silicate zone on the other side. Such bands or zones may alternate several times in a vertical section and they may reflect a rhythmic sedimentation sequence. The minerals present in the layers are similar to those in the lenses. Lazurite, together with scapolite, nepheline, afghanite and haüyne, replaces diopside and amphibole. Lazurite may exhibit characteristic zoning from a blue–purple or blue core to a dark blue rim. The third variety of lapis lazuli deposit is developed at contacts between marble and granite or pegmatite. These have lens-shaped forms of up to 2 m x 0·30 m and are formed mainly of lazurite and haüyne, rarely of afghanite, diopside, calcite, tremolite, phlogopite, pyrite, forsterite, nepheline and scapolite. In some cases, the lapis lazuli body is separated from the granite by a 0·4–1 cm zone of diopside (75 wt %), plagioclase, forsterite and muscovite. Diopside forms symplectite with plagioclase and it is partly replaced by tremolite. Pyroxene is usually replaced by lazurite and haüyne. On the outer side of the lapis lazuli body at the contact with the marble, a zone formed of phlogopite, diopside, albite, sodalite and nepheline can be present.

	SAMPLE DESCRIPTION

TOP ABSTRACT INTRODUCTION GEOLOGICAL BACKGROUND SAMPLE DESCRIPTION MINERAL CHEMISTRY FLUID COMPOSITION CONCLUSIONS REFERENCES

 
The scapolite-bearing rocks selected for this study come from calc-silicate rocks without or with minor amounts of sodalite group minerals and exhibit relatively weak retrogression compared with those occurring in close contact with the lapis lazuli bodies. On the basis of the presence of garnet and orthopyroxene they are subdivided into three groups: (1) garnet-free and (2) garnet-bearing calc-silicates, and (3) garnet- and orthopyroxene-bearing rocks. The first group is characterized by the presence of phlogopite and alkali-feldspar, whereas phlogopite is absent in the second group. The third group has the bulk-rock composition of basalt. Clinopyroxene is present in all the investigated scapolite-bearing samples.

Garnet-free calc-silicates
The garnet-free rocks are characterized by compositionally different layers that range from Na-rich to Ca-rich varieties. The sodium-rich variety (sample SSG-458) represents a thin-layered rock, where layers of 1–10 mm scale have different grain sizes and mineral assemblages. At least four layers with different constituents can be observed in a single thin section:

Olivine is widespread in scapolite-free calc-silicates (not discussed here); in scapolite-bearing rocks it was found only as inclusions in phlogopite. Besides occurring as individual grains, clinopyroxene also forms inclusions in phlogopite. In assemblage (4), representing an alkali feldspar- and quartz-rich layer, clinopyroxene forms only isolated clusters, where columnar crystals of clinopyroxene are usually separated from each other by albite and K-feldspar with accessory danburite. Although prograde danburite within amphibolite facies assemblages (Dnb + Cpx + Kfs + Qtz + Scp, 650–700°C, 7 kbar) is known from the literature (Grew, 1996), in the case of the Sare Sang rocks it is not clear if it coexists with clinopyroxene or, together with albite and K-feldspar, belongs to a later formed mineral assemblage (Table 1). Accessory amounts of nepheline in assemblage (1) occur at contacts between albite, scapolite and sodalite or form inclusions in scapolite. Sodalite usually occurs between feldspar grains, which rim phlogopite and scapolite. Some phlogopite-rich parts contain aggregates of scapolite and calcite with accessory quartz and dolomite. Pyrite mostly occurs at contacts with sodalite and phlogopite or with scapolite, but it may occur also in scapolite.

View this table: [in this window] [in a new window]   Table 1: Prograde and retrograde minerals in scapolite-bearing rocks

 

The main constituents in sample SSG-46 are phlogopite (50 wt %), clinopyroxene (30%), scapolite (15%) and calcite (10%). Some clinopyroxene grains are optically zoned, with a green core of aegirine and a colourless rim of diopside. Clinopyroxene encloses calcite, titanite, phlogopite, K-feldspar and rarely albite. The colourless clinopyroxene is overgrown or rimmed by phlogopite, scapolite and partly by calcite. Phlogopite is usually fine grained, but in calcite- and scapolite-rich parts may form crystals of up to 1 mm which enclose clinopyroxene.

Sample SSG-655 consists mainly of scapolite (40 vol. %), clinopyroxene (40 vol. %), plagioclase (10 vol. %), phlogopite (5 vol. %) and calcite (5 vol. %). Clinopyroxene forms euhedral crystals mainly in contact with plagioclase, rarely with calcite, but has irregular boundaries with scapolite. The clinopyroxene encloses scapolite, quartz, plagioclase and calcite. At least two or three textural varieties of scapolite are present: scapolite enclosed by clinopyroxene, occurring as fine-grained aggregates in the matrix and forming porphyroblasts up to 3 cm in maximum diameter. The porphyroblasts have inclusions of phlogopite, plagioclase, calcite, quartz and rectangular-shaped pseudomorphs of radial chlorite after phlogopite. The large scapolite crystals are partly crossed by calcite veins. A scapolite- and garnet-free sample (SSG-11) is characterized by the presence of clinopyroxene, phlogopite and amphibole with veins of haüyne and apatite. The bluish pale green haüyne replaces pyroxene and forms a corona around apatite. Although some amphibole grains are crossed by haüyne, mostly they form idioblasts, which have sharp contacts with haüyne. The sky blue lazurite was studied in two samples: SSG-55 (Ol + Do + Cc + Phl) and SSG-86 (Ol + Cpx + Phl + Do + Cc). In the first sample, small amounts of lazurite occur as irregular spots, replacing clinopyroxene. In the second sample, lazurite forms veins and aggregates that replace olivine, clinopyroxene and calcite, but phlogopite remains as relics in lazurite. The lazurite aggregates are cross-cut by thin veins of calcite. Clinohumite, rimming olivine, was found in sample SSG-572, containing clinopyroxene, dolomite, calcite, phlogopite and sapphirine.

Garnet-bearing calc-silicates
The rocks consist of garnet (20–35 vol. %), clinopyroxene (15–35 vol. %), scapolite (5–20 vol. %), quartz (20–30 vol. %), calcite (2–5 vol. %), plagioclase (0–5 vol. %), epidote (0–10 vol. %) and accessory titanite, apatite and K-feldspar. They are mostly medium to coarse grained, with large (6 mm) porphyroblasts or poikiloblasts (up to 1 cm in size) of clinopyroxene. In sample SSG-536, clinopyroxene contains inclusions of quartz, calcite and rarely of scapolite. Garnet encloses scapolite or clinopyroxene with overgrowths of scapolite, quartz and calcite (Fig. 2a). Amphibole replaces pyroxene or forms individual grains in contact with pyroxene and scapolite. Epidote occurs in plagioclase-rich parts or forms mono- or bimineralic aggregates with coexisting calcite; however, some calcite grains contain inclusions of epidote. In sample SSG-590, scapolite encloses garnet, pyroxene and epidote, and seems to be a younger phase (Fig. 2b), but in samples SSG-571 and SSG-536, it overgrows apatite enclosed in garnet (Fig. 2c) or forms inclusions in garnet and clinopyroxene (Fig. 2d) and vice versa, which suggest their equilibrium formation. At least three textural types of calcite are present. The texturally early type forms inclusions in garnet and clinopyroxene, a later-formed type is intergrown with scapolite and epidote, and the last variety of calcite forms veins in garnet. In sample SSG-571, garnet contains inclusions of scapolite, clinopyroxene, titanite, calcite, quartz, K-feldspar and albite. Accessory chlorite in contact with scapolite was also observed.

View larger version (208K): [in this window] [in a new window]   Fig. 2. Photomicrographs depicting reaction textures in garnet-bearing calc-silicates. (a) Garnet enclosing scapolite, clinopyroxene, quartz and calcite (SSG-571). (b) Scapolite with inclusions of epidote and pyroxene occurs in garnet (SSG-590). (c) Scapolite corona around apatite in garnet (SSG-536). (d) Scapolite forms inclusions or rims clinopyroxene in garnet (SSG-536).

 

Orthopyroxene-bearing rocks
Sample SSG-531 consists of a large number of minerals (Table 1), but the main constituents are orthopyroxene (40 vol. %), amphibole (25 vol. %), clinopyroxene (15 vol. %), plagioclase (10 vol. %), garnet (10 vol. %), biotite (5 vol. %) and scapolite (3 vol. %). The whole-rock chemical composition (wt %) is characterized by high MgO and CaO, and low Na₂O and K₂O contents [SiO₂ 50·17, TiO₂ 0·40, Al₂O₃ 10·08, Fe₂O₃(tot) 12·71, MnO 0·21, MgO 15·23, CaO 9·02, Na₂O 0·79, K₂O 0·39, CO₂ 0·40], which is comparable with basalt. Ortho- and clinopyroxenes mostly form relatively large crystals up to 5 mm in size. In detail, several textural varieties of orthopyroxene and clinopyroxene can be distinguished: (1) relict orthopyroxene, partly replaced by Mg-actinolite–tremolite and dolomite; (2) large orthopyroxene crystals with exsolution lamellae of clinopyroxene; (3) small orthopyroxene grains in clinopyroxene and scapolite. Some orthopyroxene crystals enclose amphibole (Amph) and plagioclase (Pl) (Fig. 3a). Similarly, at least three varieties of clinopyroxene are present. Some coarse-grained clinopyroxene crystals (Cpx₁) enclose only orthopyroxene, others (Cpx₂) are crowded with inclusions of scapolite (Scp₁), amphibole and orthopyroxene (Fig. 3b). In Fig. 3c, amphibole grains, replacing clinopyroxene, have parallel orientations to the cleavage in clinopyroxene. The last variety of clinopyroxene (Cpx₃) forms a symplectite with amphibole and plagioclase and contains small orthopyroxene grains. From textural relations is not always clear if orthopyroxene is in equilibrium with clinopyroxene, or if it belongs to an earlier (magmatic or metamorphic) generation.

View larger version (190K): [in this window] [in a new window]   Fig. 3. Back-scattered electron image of orthopyroxene-bearing rocks: (a) orthopyroxene with inclusions of plagioclase, clinopyroxene and amphibole; (b) clinopyroxene, enclosing orthopyroxene, scapolite and amphibole; (c) clinopyroxene enclosing orthopyroxene is along schistosity replaced by amphibole; (d) garnet with inclusions of plagioclase, amphibole and quartz; (e) orthopyroxene with corona of plagioclase and scapolite in garnet; (f) inclusions of aluminosilicate, dolomite and calcite in garnet.

 

Garnet occurs in the intergranular areas between pyroxenes, but it may also form relatively large grains. It is rich in inclusions of columnar grains of plagioclase and rarely also of scapolite, amphibole, orthopyroxene, quartz (Fig. 3d and e), dolomite and idioblastic biotite. Besides plagioclase, some garnets contain very fine-grained inclusions of aluminosilicate (Al₂SiO₅); because of its very fine grained nature (Fig. 3f), it was not possible to determine whether it is sillimanite or kyanite. Some aluminosilicate inclusions in garnet are rimmed by plagioclase. Intergranular spaces between pyroxene grains are filled by plagioclase, scapolite (Scp₂), biotite, amphibole (amph₂), quartz and by symplectites of Pl + Cpx + Amph. Coronas between garnet and pyroxene and garnet and amphibole are composed of plagioclase and scapolite. The scapolite coronas mostly occur between amphibole and biotite. Some garnet grains have irregular boundaries that are overgrown by plagioclase, amphibole and biotite. Besides symplectites with plagioclase, amphibole may form individual grains that contain inclusions of plagioclase, clinopyroxene and quartz. Accessory phases are graphite, apatite, rutile, ilmenite, pyrrhotite, pentlandite, chalcopyrite and haematite. Graphite forms elongated grains, rarely with radial orientation, mostly occurring as inclusions in coarse-grained pyroxenes. Rutile rimming ilmenite occurs with amphibole, biotite, scapolite and plagioclase.

	MINERAL CHEMISTRY

TOP ABSTRACT INTRODUCTION GEOLOGICAL BACKGROUND SAMPLE DESCRIPTION MINERAL CHEMISTRY FLUID COMPOSITION CONCLUSIONS REFERENCES

 
Chemical analyses were carried out with a CAMECA Camebax microprobe at the ‘Zentrale Elektronen-Mikrosonde’, Ruhr Universität Bochum, which is equipped with three wavelength-dispersive spectrometers. The following synthetic standards were used: pyrope (Si, Al, Mg), andradite (Ca, Fe), jadeite (Na), spessartine (Mn), K-silicate glass (K), Ba-silicate glass (Ba), NaCl (Cl) as well as natural rutile (Ti), and topaz (F). Operating voltage was 15 kV using beam currents between 10 and 15 nA. The beam was focused to 1–2 µm diameter, except for micas, for which the diameter was 8–10 µm. The peak counting times were 20 s. Apatites were measured using a CAMECA SX 50 electron microprobe. A PC0 pseudocrystal was used to determine the F concentration, thus avoiding the overlaps of third-order PKa on first-order FKa. PKa was discriminated to prevent the overlaps with second-order CaKb. The matrix correction procedure used in all cases was PAP (Pouchou & Pichoir, 1984). Mineral analyses and structural formulae are given in Tables 2 –8.

View this table: [in this window] [in a new window]   Table 2: Selected pyroxene compositions from scapolite-bearing rocks at Sare Sang

 

View this table: [in this window] [in a new window]   Table 3: Microprobe analyses of scapolites from Sare Sang

 

View this table: [in this window] [in a new window]   Table 4: Representative phlogopite and biotite analyses from scapolite-bearing rocks from Sare Sang

 

View this table: [in this window] [in a new window]   Table 5: Representative garnet analyses from scapolite-bearing rocks

 

View this table: [in this window] [in a new window]   Table 6: Representative amphibole analyses from scapolite-bearing rocks

 

View this table: [in this window] [in a new window]   Table 7: Microprobe analyses of feldspars from scapolite-bearing rocks

 

View this table: [in this window] [in a new window]   Table 8: Microprobe analyses of apatite from scapolite-bearing rocks at Sare Sang

 

Pyroxene
The most common clinopyroxene in the investigated rocks is diopside, but diopside–hedenbergite solid solutions (samples SSG-536 and SSG-571), aegirine augite (SSG 46) and rarely omphacite (SSG-458 and SSG-11) are also present (Fig. 4). Mineral formulae and the ferric and ferrous iron ratios in pyroxene were calculated on the basis of six oxygens, four cations and Fe³⁺ = aegirine. The remaining Na was used for jadeite calculation, when enough Al was present. Sample SSG-458 contains three different compositional varieties of clinopyroxene (Table 2): (1) the most common large clinopyroxene crystals and those occurring in phlogopite, feldspar and scapolite are diopside; (2) clinopyroxene intergrown with feldspars and danburite in feldspars and quartz-rich bands (layer 4) corresponds to manganoan diopside with 7 % MnO; (3) some small clinopyroxene grains enclosed by K-feldspars and albite (layer 3) have high jadeite contents (maximum Jd 29 mol %) and plot in the field of omphacite (Fig. 4). Na-rich clinopyroxene (maximum Jd 19 mol %) is present also in the scapolite-free sample SSG-11. With the exception of manganoan diopside, the MnO content is usually low in clinopyroxene (maximum 0·08% in diopside and 0·69% in hedenbergite). The optically zoned clinopyroxene in sample SSG-46 corresponds to aegirine–augite in the core and diopside at the rim.

View larger version (26K): [in this window] [in a new window]   Fig 4. Composition of clinopyroxene from scapolite-bearing and haüyne-bearing (SSG-11) rocks in Q:Jd:Aeg and Ca:Mg:(Fe + Mn) diagrams (Morimoto, 1988). Parallel arrows indicate orthopyroxene composition in sample SSG-531.

 

The augite analyses in orthopyroxene-bearing rock come from clinopyroxenes with exsolution lamellae of orthopyroxene. They are rich in Cr (up to 0·04 a.f.u.) and following the classification of Morimoto (1988) they correspond to chromian diopside (Cr > 0·01 a.f.u.). Most clinopyroxene analyses from the symplectites or those enclosing orthopyroxene have a relatively large jadeite content (maximum 17 mol %, Fig. 4). Orthopyroxene corresponds to enstatite with X_Mg = 0·76–0·78. Two compositional varieties of orthopyroxene can be distinguished in the orthopyroxene-bearing rock. The first variety, with abundant graphite inclusions, has less Al₂O₃ (<0·9), CaO (<0·3), Cr₂O₃ (<0·07 wt %) than the second, more common variety with Al₂O₃ 1·5–2·7, CaO 1·2–1·5, Cr₂O₃ 0·3–0·5 wt %.

Scapolite
Scapolite compositions are commonly reported in terms of equivalent anorthite content (Eq_An = 100(Al – 3)/3, where Al is calculated on the basis of 16 cations (Teertstra & Sherriff, 1997) and mole fractions of X_Cl = Cl p.f.u, and X_CO2 = 1 – Cl – S a.f.u. The scapolite from the Sare Sang rocks exhibits a range of composition from Eq_An = 0·07, X_Cl = 0·99 to Eq_An = 0·61, X_Cl = 0·07 (Fig. 5). With the exception of sample SSG-655, SO₄ is absent in scapolite (Table 3). The meionite-poor scapolites with Eq_An = 0·07–0·13 come from garnet-free rocks (sample SSG-458) with alkali feldspars and nepheline. K contents are small, varying from 0·04 to 0·2 a.f.u. and F values are <0·05 a.f.u. Scapolite with a high meionite content (Eq_An = 0·61) occurs in some garnet-bearing calc-silicate rocks (SSG-590). Generally, the scapolite grains are homogeneous within the expected analytical errors. Some optically zoned scapolite porphyroblasts (sample SSG-655) with Cl 0·31, Na 1·43 and Ca 2·03 a.f.u. are rimmed by relatively marialite-rich scapolite having Cl 0·40, Na 1·67 and Ca 1·69 a.f.u. All scapolite compositions fall close to the binary joins of the marialite–meionite and marialite–mizzonite. No significant compositional variation occurs between the different textural varieties of scapolite in the orthopyroxene-bearing rock (SSG-531). Generally, scapolite inclusions in clinopyroxene have somewhat lower Eq_An (0·27–0·44) compared with individual grains (Eq_An = 0·42–0·46).

View larger version (23K): [in this window] [in a new window]   Fig. 5. Range of scapolite compositions from the Sare Sang rocks. Lines bracket the stoichiometric range of scapolites proposed by Evans et al. (1969). Plagioclase compositions plot on the x-axis where XCl = 0. Continuous lines connect coexisting, and dashed lines non-coexisting scapolite and plagioclase.

 

Biotite
Phlogopite from metaevaporite–metacarbonate rocks has X_Mg = 0·96–1·00 and high F (0·6–1·55 a.f.u., calculated based on 22 oxygens) (Fig. 6); it is poor in Cl (Table 4). The greatest F contents in phlogopite are found in scapolite- and garnet-free rocks (SSG-11), which contain veins of fluorapatite and haüyne. Biotite with 30% annite end-member occurs in the orthopyroxene-bearing rock. In contrast to the phlogopite, it has a little fluorine, but considerable chlorine (0·29 a.f.u.). For comparison, F and Cl contents from biotite in metabasites and metapelites (Faryad, 1999) are also plotted in Fig. 6. They have F and Cl values similar to biotite from orthopyroxene-bearing rock, but biotite from metapelites contains somewhat more Cl.

View larger version (18K): [in this window] [in a new window]   Fig. 6. F and Cl contents vs XMg in biotite and phlogopite from scapolite-bearing and haüyne-bearing (SSG-11) calc-silicate rock. F and Cl contents in biotite from metapelites and metabasites (Faryad, 1999) are also shown for comparison.

 

Garnet
Garnet composition is rather homogeneous in every single sample. Mineral formulae and Fe³⁺/Fe²⁺ ratios were calculated on the basis of 12 oxygens and eight cations. Garnets from scapolite-bearing calc-silicates are rich in grossularite (45–96 mol %, Table 5) with low contents of pyrope (0·1–2·5 mol %) and spessartine (0·5–7·9 mol %) (Fig. 7). Almandine content is low (<0·9 mol %) in samples SSG-571 and SSG-590, but high (41 mol %) in sample SSG-536. Garnet associated with epidote (sample SSG-590) has a relatively large andradite (25 mol %) content. In contrast to garnet-bearing rocks, the orthopyroxene-bearing rock (SSG-531) is characterized by pyrope-rich garnet (Py_36–43Alm_41–50Grs_10–15Sps_1–2Adr_0–3). A weak zonation from core to rim, observed in large crystals, is characterized by an increase of almandine and spessartine and a decrease of pyrope and grossularite content. A relatively Fe-rich garnet (Alm₄₈Py₃₂) was found as an inclusion in pyrrhotite. High pyrope (Py₄₃), but low grossularite (Grs₁₀) contents occur in garnet enclosing amphibole and plagioclase in the orthopyroxene-bearing rock.

View larger version (11K): [in this window] [in a new window]   Fig. 7. Garnet compositions from scapolite-bearing rocks at Sare Sang. Alm, almandine; Py, pyrope; Grs, grossularite; Adr, andradite.

 

Amphibole
Mineral formulae and the ferric and ferrous iron ratios were calculated on the basis of 23 oxygens and averaged Fe³⁺ from 13 and 15 cations (Table 6). Amphibole cross-cutting the haüyne veins in scapolite-free sample SSG-11 is pargasite in composition with Na_A 0·9 a.f.u. and X_Mg = 0·99, and similar to phlogopite has a large F content (F 0·65 a.f.u., Fig. 8). Amphibole in calc-silicate rock SSG-536 is a potassium chlorian hastingsite (0·47 a.f.u. K and 0·32 a.f.u. Cl) with X_Mg = 0·13–0·19, but little F (<0·2 a.f.u.). Amphibole in orthopyroxene-bearing rock SSG-531 corresponds, based on the Leake et al. (1997) classification, to pargasite–edenite and tschermakite (Si 6·4–6·7 a.f.u., X_Mg = 0·64–0·80), but amphiboles in the symplectites and the rims of large amphibole grains are magnesiohornblende. Amphibole included in garnet has generally low Si, and high Al^IV (2·3 a.f.u.) and Na (0·53 a.f.u.). Mg-rich actinolite and tremolite with X_Mg = 0·89–0·92 replace orthopyroxene with graphite inclusions. Cr content in amphibole is usually low (<0·05 a.f.u.), but pargasite and magnesiohornblende occurring in clinopyroxene may have Cr 0·22 a.f.u. The M₄ position in tschermakite is occupied by a maximum of 0·45 Na a.f.u. With the exception of actinolite and tremolite, amphibole from sample SSG-531 has high Cl (up to 0·39 a.f.u.), but low F (<0·2 a.f.u.). Figure 8 also shows Cl contents from metabasites in this area, which have relatively low (<0·1 a.f.u.) Cl contents, except for green Al-rich amphiboles at contacts with garnet (Cl 0·3–0·8 a.f.u.).

View larger version (18K): [in this window] [in a new window]   Fig. 8. F and Cl contents vs XMg in amphiboles from scapolite-bearing rocks and from metabasites (Faryad, 1999).

 

Feldspars
Garnet-free rocks contain alkali feldspar. In sample SSG-458, Na-rich feldspar (Ab₆₉Or₃₀) is associated with perthite (Ab₃₀Or₇₀). Albite and orthoclase inclusions were found in clinopyroxene in sample SSG-46. In garnet-bearing calc-silicate rock SSG-590, plagioclase has a composition of An₃₂. Plagioclase of this composition forms inclusions together with K-feldspar (Ab_9–13An_0–1Or_85–89) in garnet. Plagioclase associated with scapolite in orthopyroxene-bearing rock (SSG-531) has An_39–41, but An-poor plagioclase (An_24–28) occurs as inclusions in garnet. Plagioclase forming symplectites with clinopyroxene in orthopyroxene-bearing rock is oligoclase with An₁₄. Anorthite (An₉₅) forms coronas around aluminosilicate inclusions in garnet, and labradorite (An₆₄) occurs with scapolite and amphibole along the margins of some garnets. Except for An-rich plagioclase in sample SSG-531, scapolite is Ca- and Al-rich relative to plagioclase. Consistent with the data of Rebbert & Rice (1997), parallel tie-lines between scapolite and plagioclase (Fig. 5, SSG-458, SSG-531) may indicate its equilibrium with plagioclase near the peristerite gap (SSG-655). The Ca-rich scapolite in sample SSG-590 is interpreted as a disequilibrium phase in relation to Ca-poor plagioclase, consistent with textural observations.

Apatite
Mineral formulae of apatite, calculated on the basis of 12·5 oxygens, indicate up to 0·36 excess F, which could replace O as C replaces P (Gulbrandsen et al., 1966; Deer et al., 1997). Because C was not analysed, it is not clear if this apatite corresponds to francolite. Ca/P values in apatite range between 1·53 and 1·67. Apatite from both garnet-free and garnet-bearing calc-silicate rocks (Table 8) is low in Cl (<0·5 wt %). Fluorine contents indicate large variations (3·0–4·9 wt %), the F-rich apatite coming from garnet-bearing rocks. Large apatite crystals reveal irregular zoning, which can be easily observed in back-scattered electron images. The F contents in such apatite decrease towards the rims. Some apatites from garnet-bearing calc-silicates contain excess fluorine. The presence of halogens in excess of one stoichiometric atom per formula unit has been reported for natural carbonate apatites (McConnell, 1970; Binder & Troll, 1989) as well as for synthetic, foreign ion-free, apatites (Prener, 1971; Mackie & Young, 1974). In contrast to calc-silicate rocks, apatite from the orthopyroxene-bearing rock has very low F (<0·5 wt %), but high chlorine contents (6·85 wt %). Here, the hydroxyl site is occupied by 80–99% Cl.

Titanite
Variation of Al, Fe and F in titanite is in the range of 1·0–3·5 wt % Al₂O₃, 0·5–1·3 wt % Fe₂O₃ and 0·6–1·4 wt % F. Generally, there is a good correlation between (Al + Fe³⁺)/(Ti + Al + Fe³⁺) and F contents, the former indicating substitution of (Al, Fe³⁺) + (F,OH)^- = Ti⁴⁺ + O^2- (Coombs et al., 1976; Markl & Piazole, 1999). Some large titanite crystals show a weak zonation with a decrease of Al and F and increase of Ti towards the rim.

Sodalite-group minerals
Sodalite in sample SSG-458 has a composition close to the ideal end-member (Na₄Al₆Si₆O₁₂Cl), with Cl = 1·0 a.f.u. and Ca < 0·015 a.f.u., calculated on the basis of 25 oxygens. Maximum SO₄ value in analysed sodalite is 0·1 wt %. The bluish pale green haüyne (sample SSG-11) and sky blue lazurite have composition of Na_5·1Ca_1·2Al₆Si₆S_1·6Cl_0·2O₂₉ and Na_5·5Ca_0·9Al_5·8Si₆S_1·7Cl_0·1O₂₉, respectively. The potassium content is low both in haüyne and lazurite (<0·05 a.f.u.).

Carbonates
In most samples, calcite with Mg < 0·005 a.f.u. and Fe < 0·015 a.f.u. is present. Relatively Mg-rich calcite (Mg 0·04 a.f.u.) occurs in dolomite-bearing sample (SSG-11). Dolomite has Mg 0·45–0·50 a.f.u. and Fe 0·051 a.f.u. The high Mg content come from dolomite in olivine-bearing sample (SSG-55). Magnesite associated with olivine, phlogopite, amphibole and dolomite has Mg 0·993 a.f.u. and Fe 0·007 a.f.u.

Epidote
Analyses of epidote from samples SSG-590 and SSG-536 indicate an irregular zonation with Al₂Fe {100[Fe_tot/(–2 + Al_tot + Fe_tot)]} = 50–58%. Mn and Ti contents are very small (<0·15 wt %).

Other minerals
Nepheline occurring with sodalite in sample SSG-448 has 6 wt % K₂O and Na/(Na + K) 70%. Olivine from all analysed samples corresponds to forsterite with Mg/(Mg + Fe²⁺) = 0·98–1·00. Danburite from sample SSG-458 has a composition corresponding to the ideal member CaB₂Si₂O₈; contents of other elements (MgO, FeO, Al₂O₃, BaO) are in the range of analytic uncertainty (<0·05 wt %). Ilmenite from sample SSG-531 has low MnO (0·39 wt %), but relatively high MgO (2·32 vol. %) contents, indicating 8·5 mol % geikielite solid solution. It is in contact with rutile containing 0·52 wt % Cr₂O₃ and 0·11 wt % Al₂O₃. Clinohumite rimming olivine in sample SSG-572 has Mg 8·76 a.f.u., Ti 0·15 a.f.u., Fe 0·09 a.f.u., Si 3·99 a.f.u. and F 1·1 a.f.u.

P–T–X CONSTRAINTS IN SCAPOLITE-BEARING ROCKS
With the exception of aegirine rimmed by diopside that indicates an increase of metamorphic conditions, textural evidence to reconstruct prograde P–T paths of metamorphism is lacking in these rocks. Although several textural varieties of orthopyroxene are present in sample SSG-531, their possible equilibrium with clinopyroxene or with other phases is not clear. The presence of graphite in orthopyroxene could be interpreted as a result of either reduction of oxidized carbon-bearing species, such as CO₂, or inheritance from a former carbonate (dolomite, calcite). In the latter case, the precursor of the orthopyroxene-bearing rocks could have been basaltic tuffs with mixtures of sedimentary material.

Peak P–T conditions
Most textural relations in the studied rocks indicate retrogression; however, the assemblages with clinopyroxene and garnet can be used for peak pressure and temperature estimates. Available exchange thermobarometers and the TWQ (Berman, 1996) programs for the system CMASH were used to calculate the P–T–X conditions in scapolite-bearing assemblages. To avoid problems arising from uncertainties over the fluid composition during metamorphism, in the first step of the P–T calculation only fluid-free phases were used. In garnet-free calc-silicate, clinopyroxene with maximum Jd content (29%) gives, based on the reaction albite = jadeite + quartz (Holland & Powell, 1996), information on the minimum pressure (Fig. 9a). Alkali feldspar pairs with Or₇₀Ab₃₀ and Or₃₀Ab₆₉An_0·5 or Or₁₅Ab₈₃An_1·3 that occur with Na-rich clinopyroxene in sample SSG-458 yield, using the calibration of Fuhrman & Lindsley (1988), a temperature of 750°C. P–T calculations, using Fe–Mg exchange thermometry between clinopyroxene and garnet (Ganguly et al., 1996) and garnet–clinopyroxene–plagioclase barometry (Powell & Holland, 1988; Eckert et al., 1991), yield average temperatures of 690 and 750°C for samples SSG-571 and SSG-536, respectively (Table 9, Fig. 9b). Pressures and temperature of 1·17–1·35 GPa and 698°C were obtained for sample SSG-590. Similar pressures of 1·25–1·4 GPa in the temperature range of 600–800°C for this sample were obtained by the equilibrium reaction for plagioclase, clinopyroxene and garnet, calculated using the TWQ program [reaction (1), Fig. 9b]. Exchange thermobarometry between garnet, clinopyroxene and plagioclase for sample SSG-531 yielded temperatures and pressures of 780°C and 1·41–1·45 GPa.

View larger version (21K): [in this window] [in a new window]   Fig. 9. P–T diagrams showing results of exchange thermobarometries and equilibrium reactions, calculated based on mineral compositions and TWQ program for the scapolite-bearing rocks at Sare Sang. Continuous lines indicate reactions with fluid-free phases and dashed lines indicate reactions with scapolite, calcite or CO2. (a) Garnet-free rocks. Isopleth of reaction albite = jadeite + quartz after Holland & Powell (1996). Fsp refers to two-feldspar thermometry (Fuhrman & Lindsley, 1988). (b) Garnet-bearing rocks. Box shows pressure and temperature, obtained by Grt–Cpx–Pl thermobarometry (Table 9) for sample SSG-590. GrtCpx indicates average temperature, calculated by garnet–clinopyroxene thermometry. Shaded pentagons are intersections of reactions (7)–(10) for sample 571, calculated using XCO2 = 0·03 and aMe = 0·01 and aMe = 0·07; , sample SSG-536 (at XCO2 = 0·15, aMe = 0·13); , intersection of reactions (1), (8), (11)–(13) for sample SSG-590 (at XCO2 = 0·08 and aMe = 0·37): (c) Orthopyroxene-bearing rock (sample SSG-531). Box represents pressure and temperature, calculated by Grt–Cpx–Pl thermobarometry (Table 9). Star is intersection of reactions (2)–(6) for plagioclase An28. Triangle indicates intersection of reactions (1), (10), (14)–(16) calculated for XCO2 = 0·1, aMe 0·02 and Pl (An40): (d) Summary of temperature and pressure data from (a) and (c) and inferred P–T path for the Sare Sang rocks.

 

View this table: [in this window] [in a new window]   Table 9: Grt–Cpx thermometry and Cpx–Grt–Pl barometry T(°C), P(GPa)

 

Because equilibrium of orthopyroxene with other phases in sample SSG-531 cannot be demonstrated, it was not used for P–T estimation. Considering that aluminosilicate (Al₂SiO₅) inclusions coexist with isolated inclusions of plagioclase (An_26–28) in garnet, the equilibrium reaction calculated using the TWQ program and the CAS system reaction (2) (Fig. 9c) occurs within the kyanite stability field (1·3 GPa and 600°C to 1·6 GPa and 800°C). Slightly lower pressures of 1·1 GPa and 600°C to 1·5 GPa and 800°C are obtained for this reaction using plagioclase with An₄₀. Addition of clinopyroxene to the assemblages and using the CMAS system, for reactions (3)–(6) (Fig. 9c) can be calculated that produce an intersection with reaction (2) at 730°C and 1·3 GPa.

The above P–T results allow one to estimate the CO₂ content of a fluid in equilibrium with scapolite and calcite. Using the thermodynamic data of Berman (1996) and the TWQ program, X_CO2 values were calculated in the P–T range of 700–800°C and 1·0–1·4 GPa. The X_CO2 values are different for each sample but in general they vary between 0·03 and 0·15. Meionite activity was calculated by the method of Moecher & Essene (1991) and Baker & Newton (1995). The first method gave very low values, which generally result in relatively higher pressures compared with that obtained by CO₂-free assemblages. As discussed by Markl & Piazolo (1999), the activity model of Baker & Newton (1995) is virtually independent of temperature, but is valid only for temperature above 750°C; hence, in our calculations we have used the model of Moecher & Essene (1991).

Formation of garnet from scapolite in granulite facies rocks is usually interpreted as a retrograde reaction (Fitzsimons & Harley, 1994; Stephenson & Cook, 1997; Satish-Kumar & Harley, 1998). In our case garnet probably belongs to the peak P–T mineral assemblage and coexists with scapolite in samples SSG-571, SSG-536 and SSG-531. The equilibrium reactions involving garnet, clinopyroxene, scapolite, calcite and quartz [reactions (7)–(10), Fig. 9b], calculated for sample SSG-571 yield temperatures of 690°C and 1·42 GPa at X_CO2 = 0·03 and a_Me = 0·07, and 730°C and 1·5 GPa at X_CO2 = 0·1 and a_Me = 0·07. These temperatures are in agreement with that obtained by garnet–clinopyroxene thermometry in this sample. Lower temperature and pressure of 660°C and 1·14 GPa for these reactions, calculated at X_CO2 = 0·15 and a_Me = 0·13, were obtained for sample SSG-536. On the basis of textural and compositional relations in sample SSG-590, scapolite does not belong to the equilibrium phases in the assemblage, but the reactions involving plagioclase, garnet and calcite [reactions (8) and (11)–(13), Fig. 9b), calculated for X_CO2 = 0·08 and a_Me = 0·37, form an intersection with reaction (1) at 690°C and 1·3 GPa. Under the assumption that plagioclase (An₄₀) coexisted with scapolite, clinopyroxene and garnet in sample SSG-531, reactions (1), (10), (14)–(16) cross-cut each other at 708 and 1·35 GPa for a_Me = 0·019, X_CO2 = 0·1.

Retrogression
Textural relations summarized in Table 1 indicated that a large number of minerals were formed or re-equilibrated during retrograde metamorphism. Several stages of mineral growth and extensive retrogression, which led to subsequent relatively low-grade assemblages, can be distinguished. The first stage is well documented in the orthopyroxene-bearing rock by the presence of clinopyroxene + scapolite and clinopyroxene + amphibole symplectites and by anorthite coronas around aluminosilicate inclusions in garnet. The appearance of amphibole and biotite, mainly at the margins of some garnets, suggests addition of H₂O to the system. If it is assumed that plagioclase (An₄₀ and An₉₅) coronas around aluminosilicate enclosed in garnet were formed as a result of breakdown of aluminosilicate and garnet, then a pressure decrease from 1·3 GPa (An₂₈) through 1·2 GPa (An₄₀) to 1·0 GPa (An₉₅) at 750°C (Fig. 9c) can be calculated. Temperatures calculated on the basis of amphibole–plagioclase thermometry of Holland & Blundy (1994) are 740–750°C at 0·5–1·0 GPa for plagioclase An₄₀.

The most common retrograde textures in both calc-silicates are the replacement of clinopyroxene and garnet by calcite and amphibole. In some scapolite-free calc-silicate rocks clinopyroxene is replaced by lazurite and olivine by clinohumite and antigorite.

	FLUID COMPOSITION

TOP ABSTRACT INTRODUCTION GEOLOGICAL BACKGROUND SAMPLE DESCRIPTION MINERAL CHEMISTRY FLUID COMPOSITION CONCLUSIONS REFERENCES

 
Mineral compositions and the reactions described above imply the presence of fluid phases with high concentrations of CO₂, halogens and S during metamorphism of the rocks. The X_CO2 = 0·03–0·15 at peak P–T conditions was estimated using scapolite-bearing reactions. Halogens and S are incorporated in the following minerals: F in apatite, biotite, amphibole, titanite and clinohumite; Cl in scapolite, sodalite, biotite, amphibole and apatite; S in haüyne, lazurite, scapolite, pyrite and pyrrhotite. Blaise & Cesbron (1966) reported the presence of gypsum and galena, associated with calcite and sodalite. With the exception of accessory pyrrhotite, pentlandite and some scapolites, the S-bearing minerals originated during retrogression and metasomatism. There are several ways to estimate the halogen content of fluids in equilibrium with various minerals. The scapolite composition as well as Cl and F content in amphibole, biotite, apatite and titanite can be used as monitors for the composition of fluid phases (Ellis, 1978; Munoz & Swenson, 1981; Binder & Troll, 1989; Mora & Valley, 1989; Kullerud, 1996; Markl & Bucher, 1998; Yardley et al., 2000; Svensen et al., 2001). Danburite is the only boron mineral in the studied calc-silicates; however, kornerupine and tourmaline from Sare Sang whiteschists were reported by Blaise & Cesbron (1966) and Grew (1988).

Composition of scapolite
Experimental studies on the stability of scapolite in the system Ab–An–CaCO₃–NaCl (Orville, 1975; Ellis, 1978; Vanko & Bishop, 1982; Aitken, 1983) indicate that increasing NaCl contents greatly expands the stability field of scapolite + plagioclase. Because SO₄ contents are very small in the studied scapolites, for calculation of the fluid composition the exchange equilibrium investigated by Ellis (1978) has been used:

The X_NaCl (fluid) = NaCl/(NaCl + H₂O) was calculated from the K_D formulation:

The calculated X_NaCl (fluid) ranges from 0·04 (sample SSG-590) to 0·99 (marialite-rich scapolite in sample SSG-458, Table 3). The X_NaCl (fluid) values indicate generally negative correlation with Eq_An contents in different samples. Relatively wide X_NaCl (fluid) variation (0·5–0·7) occurs in scapolites from the orthopyroxene-bearing rock. The largest X_NaCl (fluid) values were calculated for scapolite enclosed in clinopyroxene.

Halogens in biotite
Biotite is most commonly used for estimating halogen content of fluids, where the F and Cl exchange with a coexisting fluid occurs according to the reaction (Munoz & Swenson, 1981)

where X is a halogen. Experimental studies indicate that F is preferentially incorporated into biotite with high Mg/Fe ratios. Fe–F avoidance has been extensively documented in natural systems (Ekstrom, 1972; Valley et al., 1982). On the basis of experimental data on the temperature and compositional (Fe–Mg–Al) dependence of biotite OH–halogen exchange, the following expression was derived (Ludington & Munoz, 1975; Munoz & Swenson, 1981; Munoz, 1984):

The equation permits evaluation of the halogen content of biotite as a function of biotite composition, equilibration temperature, and fluid composition.

As Munoz (1984) and Mora & Valley (1989) showed, biotites with varying X_Mg, which equilibrated at the same temperature and with the same fluid composition (f_HF/f_HCl = constant), form an approximately linear array in the log(X_F/X_Cl)–X_Mg diagram (Fig. 10). Following Mora & Valley (1989), higher temperature shifts the isopleths upwards. Figure 10 shows log(X_F/X_Cl)–X_Mg values for phlogopite and biotite from scapolite-bearing rocks at Sare Sang. Biotite compositions from metapelites and metabasites (Faryad, 1999) are also plotted for comparison. With the exception of the orthopyroxene-bearing rock, all biotites and phlogopites plot close to the isopleth with log(X_F/X_Cl) = –2·46. Of particular interest are biotites from the orthopyroxene-bearing rock, which plot near the isopleth of –4·12. The bimodal distribution of log(X_F/X_Cl) from scapolite-bearing rocks is probably a result from different relative halogen activity.

View larger version (19K): [in this window] [in a new window]   Fig. 10. Log(XFe/XCl) in biotite plotted against the mole fraction of Mg. Diagonal contours are log(fHF/fHCl) in fluid calculated from ln KD (Mora & Valley (1989)

 

Amphibole
Figure 8 indicates that amphiboles from scapolite-bearing samples (SSG-531 and SSG-536) are rich in Cl and their F content is below 0·2 a.f.u. In the first sample, amphibole coexists with peak P–T phases as apatite, biotite and scapolite, which are also rich in Cl. In sample SSG-336, amphibole replaces clinopyroxene and apatite, coexisting with peak P–T phases, and is rich in fluorine. This observation suggests introduction of Cl from scapolite into the amphibole structure during retrogression. Amphiboles from orthopyroxene-bearing rocks have generally higher F (ranging to 0·2 a.f.u.) than Cl contents (ranging to 0·1 a.f.u.). Extremely high F contents (0·4–0·8 a.f.u.) occur in amphibole from haüyne-bearing and scapolite-free sample SSG-11, where amphibole occurs together with fluorapatite and haüyne, which are later-formed phases in the rock. There is no regular relation between X_Mg and halogen content in the studied amphiboles.

Partitioning of F and Cl between coexisting phases
Apatite–biotite
F–OH and Cl–OH partitioning coefficients for apatites with F + Cl < 1 a.f.u. (Table 8) were calculated using the method of Zhu & Sverjensky (1992). The results given in Table 10 are consistent with data from Zhu & Sverjensky (1992) and Nijland & Maijer (1993), indicating preferential partitioning of F into apatite with respect to biotite. The extent of preference is dependent on temperature and Fe concentration in biotite. Partitioning of F between apatite and biotite has been utilized as a geothermometer (Stormer & Carmichael, 1971; Ludington, 1978; from Zhu & Sverjensky, 1992). This relationship between F content in apatite and biotite and X_Fe values in biotite was used here as a potential thermometer. The results of apatite–biotite thermometry, obtained by the method of Zhu & Sverjensky (1992), range between 713 and 780°C for the Sare Sang rocks.

View this table: [in this window] [in a new window]   Table 10: Henders–Koracek partitioning coefficients of F–OH and Cl–OH between apatite and biotite and temperature calculated by the methods of Zhu & Sverjensky (1992)

 

Biotite–amphibole
According to Markl & Piazolo (1998), no correlation between Cl in biotite and amphibole is found in most calc-silicates. The lack of correlation suggests either a different dependence on cation chemistry or that they are influenced by different factors (e.g. Al rather than X_Fe). To estimate partitioning of F and Cl, I calculated log(X_F/X_OH) and log(X_Cl/X_OH) for coexisting biotite and amphibole in orthopyroxene-bearing rocks (SSG-531) and biotite-bearing metabasites (SSG-449, SSG-589), and also in haüyne-bearing rocks (SSG-11, Table 11). The log(X_Cl/X_OH) values show, consistent with data from Nijland & Maijer (1993), preferential partitioning of Cl into amphibole and F into biotite. In scapolite-free sample (SSG-11), amphibole that cuts fluorapatite and haüyne veins has higher log(X_Cl/X_OH) compared with phlogopite in host rock. This suggests that F content in fluids increased during fluorapatite and haüyne formation.

View this table: [in this window] [in a new window]   Table 11: log(XF/OH) and log(XCl/OH) for coexisting (except sample SSG-11) amphibole and biotite

 

Origin of scapolite
Textural relations indicate the presence of several varieties of scapolite in the Sare Sang rocks. Compositional variation in this high-grade scapolite from different samples is related to rock type. This can be explained by the relatively large marialite content in scapolite from orthopyroxene-bearing rocks, where the coexisting phases apatite, amphibole and biotite are rich in Cl. The calculated equilibrium reactions in the Sare Sang rocks indicate that scapolite coexists with pyroxene and/or garnet, hence belongs to the earlier-formed phases in the rocks, and it was partly affected by later metamorphic events. The presence of NaCl-rich scapolite in metamorphosed rocks is generally thought to imply an evaporitic source with the NaCl provided from slate-rich layers (Ellis, 1978; Mora & Valley, 1989; Oliver et al., 1994). A sedimentary origin for the NaCl in the Sare Sang rocks is assumed by the stratigraphic control on the occurrence of carbonate and Na-, Al- or Mg-rich minerals that follow bedding in the rocks. Presumably the scapolite crystallized during regional metamorphism in rock layers that originally contained halite, limestone and dolomite. Additionally, the occurrence of phlogopite or Mg- and Al-rich biotite with only minor K-feldspar may indicate metamorphism of evaporitic argillites (Moine et al., 1989). There is no significant compositional variation between textural varieties of scapolite in one sample. This suggests mostly a closed system for recrystallization or formation of scapolite during later metamorphic history. Textural and compositional equilibrium between plagioclase and scapolite in sample SSG-531 suggests that the scapolite-forming reaction was anorthite + calcite = scapolite, which is a pressure-dependent equilibrium. Increasing pressures decrease An in plagioclase and Eq_An in scapolite likewise (Harley et al., 1994). NaCl-rich scapolite was synthesized at low to high pressure and high temperature (>700–800°C, Eugster & Prostka, 1960; Newton & Goldsmith, 1975). Vanko & Bishop (1982) synthesized scapolite with An₁₈ at 0·17–0·28 GPa and 600–750°C. The large value of X_NaCl (fluid) in scapolite enclosed in clinopyroxene (SSG-531) compared with that occurring in intergranular spaces with amphibole and biotite may suggest high X_NaCl (fluid) during peak P–T conditions. The high concentration of F in apatite and phlogopite and the presence of NaCl-rich scapolite in sample SSG-458 suggest infiltration of Cl and localization of NaCl-rich scapolite and sodalite in thin bands parallel to bedding. Because thermodynamic data for NaCl-rich scapolite and sodalite are not well known, no equilibrium reaction involving scapolite was calculated for sample SSG-458.

	CONCLUSIONS

TOP ABSTRACT INTRODUCTION GEOLOGICAL BACKGROUND SAMPLE DESCRIPTION MINERAL CHEMISTRY FLUID COMPOSITION CONCLUSIONS REFERENCES

 
There is little doubt that the metamorphosed scapolite-bearing rocks were originally an evaporite–carbonate sequence, which indicates a shallow marine or continental environment. Thermobarometry here indicates a remarkable agreement among P–T estimates in metapelites and metabasites (Faryad, 1999), whiteschists (Schreyer & Abraham, 1976) and scapolite-bearing rocks, suggesting that they experienced the same thermotectonic events. Integrating these thermobarometric data, reaction textures and mineral compositions reveal that the high-pressure amphibolite facies assemblages were formed at 1·3–1·4 GPa and 750°C. On the basis of these data, the South Badakhshan block was at depth of 35–40 km when it experienced amphibolite facies conditions at 700–750°C. This reflects a geothermal gradient of about 18–20°C/km, which is somewhat smaller than normal cratonic values (20–30°C/km).

Textures in some weakly retrogressed samples prove that halogen- and CO₂-bearing phases were formed during a high-grade tectonothermal event, but the fluid composition partly changed during retrogression and decompression. The scapolite crystallized during regional metamorphism in rocks, which had originally contained halite and carbonates. It still belongs to stable peak metamorphic assemblages in some samples and it was formed as a result of transformation of calcite–dolomite and plagioclase. X_CO2 levels in the fluid developed from a CO₂-rich fluid during amphibolite facies conditions. The halogen chemistry in the investigated minerals indicates that the rock systems did not evolve in a simple way. Whereas some behaved as closed systems, others evolved as open systems in which new fluids were introduced. Consistent with textural relations and partitioning of F between apatite and biotite, apatite was formed at peak P–T conditions. The weak rimwards decrease of F in apatite and titanite suggests equilibrium of rock with rock fluid in a closed system as F was taken up by phlogopite. This observation, consistent with high X_NaCl (fluid) values in scapolite enclosed in pyroxene, suggests that in the closed system the halogen contents of the fluid phase (and the ratio ) increase as a function of the progress of hydration reactions between fluid and rock. The consequence of this is that the last minerals to form in equilibrium with the halogen-bearing fluid are the most enriched in F and Cl (Munoz & Swenson, 1981; Mora & Valley, 1989; Kullerud, 1996; Markl & Piazolo, 1998).

In addition to the bulk composition role and influence of a closed system, the F and Cl behave differently during chemical processes that are relevant for geological considerations. The most important feature is that F is preferably partitioned into apatite or phlogopite, whereas Cl is overwhelmingly partitioned into scapolite. With the exception of sample SSG-531, scapolite is the only phasé recording the early high-Cl fluids in the studied samples. In the chemical closed system only minor amounts of scapolite reacted to form Cl-rich amphibole during retrogression. Having made the case for infiltration of extremely F-, S- and Cl-rich fluids during retrogression, two alternative exist: (1) fluids released by prograde devolatization of rocks were infiltrated during decompression; (2) they were produced by crystallization of far-travelled hydrous granitoid magma. In the Sare Sang locality both alternatives were probably operative. The presence of F- or Cl-rich apatite, amphibole and scapolite that occur with sodalite group minerals in veins or form bands suggests infiltration from the primary evaporite layers, which are now represented by lapis lazuli, to surrounding rocks. In the second alternative, granitoid magma probably contributed heat, and it is reasonable to assume that it also contributed at least some fluid that infiltrated the surrounding rocks. The crystallization of the numerous minor discordant intrusions of post-metamorphic granite would have released water, which permeated the carbonate–evaporite sequence and enriched it in S, Cl and CO₂. The best evidence for this model is accumulation of lapis lazuli near contact of granite.

	ACKNOWLEDGEMENTS

 
Microprobe analyses were provided during my stay as Humboldt Fellow in 1999 at Institut für Mineralogie, Geologie and Geophysik, Ruhr Universität Bochum. Dr. H. J. Bernhardt helped with analysis of apatite and scapolite. The earlier version of this manuscript has been improved by discussions and constructive criticism of W. V. Maresch and W. Schreyer (Bochum). G. Goles (Oregon) helped with English correction. Subsequent reviews by E. Grew and G. Markl assisted greatly in removing discrepancies and suggesting alternatives. S. L. Harley provided helpful editorial comments.

	FOOTNOTES

 
^*Present address: Department of Mineralogy and Geology, Technical University, Letná 9, 04 084 Koice, Slovakia. E-mail: wali.faryad{at}tuke.sk

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