Preiswerkite and Högbomite Within Garnets of Aktyuz Eclogite, Northern Tien Shan, Kyrgyzstan

320 R.T.Journal Orozbaev, of Mineralogical K. Yoshida, A.B.and PetrologicalBakirov, T. Hirajima, Sciences, A. Volume Takasu, 106, K.S. page Sakiev 320 and─ 325, M. 2011 Tagiri

LETTER

Preiswerkite and högbomite within garnets of Aktyuz eclogite, Northern Tien Shan, Kyrgyzstan

*,** * ** * *** Rustam T. Orozbaev , Kenta Yoshida , Apas B. Bakirov , Takao Hirajima , Akira Takasu , ** **** Kadyrbek S. Sakiev and Michio Tagiri

* Department of Geology and Mineralogy, Kyoto University, Kitashirakawa Oiwakecho, Sakyo-ku, Kyoto 606-8502, Japan **Institute of Geology, Kyrgyz National Academy of Science, 30 Erkindik Avenue, Bishkek 720481, Kyrgyzstan ***Department of Geosciences, Shimane University, 1060 Nishikawatsu, Matsue 690-8504, Japan **** Hitachi City Museum, Miyatacho 5-2-22, Hitachi 317-0055, Japan

We report the occurrence of preiswerkite and högbomite as inclusion phases within the garnets of eclogite from the Aktyuz area of Northern Tien Shan, Kyrgyzstan. Preiswerkite and högbomite occur both as a constituent of multiphase solid inclusions (MSI) and as single discrete grains in the mantle and rim of the garnets. However, they do not occur in the core of the garnet and in the matrix of the eclogite. Preiswerkite is associated with the minerals paragonite ± staurolite ± Mg-taramite ± Na-biotite ± hematite ± högbomite ± chlorite ± titanite ± phengite ± magnetite, and högbomite is associated with paragonite ± preiswerkite ± staurolite ± hematite ±

chlorite ± Na-biotite ± magnetite in MSI. The average compositions of preiswerkite and högbomite are (Na0.96K 2+ VI IV 2+ 3+ 3+ 0.02Ca0.01)0.99(Mg1.52Fe0.54 Al0.93)2.99( Al1.93Si2.07)4.00O10(OH)2 and (Mg1.47Fe3.02Zn0.04Fe1.45)5.98(Fe0.31Al15.13Ti0.56)16O30 2+ VI IV (OH)2, respectively. Na-biotite, with an average composition of (Na0.89K0.07Ca0.01)0.97(Mg1.66Fe0.69 Al0.63)2.98( Al1.57

Si2.43)4.00O10(OH)2, corresponding to the intermediate composition between preiswerkite and aspidolite (i.e., Na- phlogopite), is also observed. The compositions of the newly found preiswerkite and Na-biotite with similar

XMg values (0.66-0.78) are arrayed along preiswerkite-aspidolite solid solution series. The mode of occurrence of inclusion phases in garnets may suggest that the activity of Na-Al-rich and Si-undersaturated aqueous fluids played a major role in the formation of preiswerkite during the prograde stage of high-pressure eclogitic metamorphism.

Keywords: Preiswerkite, Högbomite, Na-biotite, Aktyuz area, Eclogite, Tien Shan, Kyrgyzstan

INTRODUCTION that preiswerkite mostly occurs as symplectitic or coroni- tic aggregates, thus as constituent of retrograde mineral The trioctahedral Al-rich sodium mica preiswerkite (Prw) assemblages. is a rare naturally occurring mineral with the ideal formu- In this paper, we report on the first occurrence of la Na(Mg2Al)(Al2Si2O10)(OH)2. Ever since it was first de- preiswerkite as an inclusion phase within the garnets of scribed in a metarodingite in the Geisspfad ultramafic eclogite from the Aktyuz area, Northern Tien Shan, Kyr- complex, the Penninic Alps, Switzerland (Keusen and Pe- gyzstan. It occurs as discrete grains and as a constituent ters, 1980), preiswerkite has been reported from several of multiphase solid inclusions (MSI) along with other localities in the world. A summary of the host rock type, minerals such as paragonite (Pg), staurolite (St), hematite mode of occurrence, and mineral assemblage associated (Hem), Na-biotite (Na-Bt), chlorite (Chl), högbomite with preiswerkite from each locality is compiled in sup- (Hgb), magnetite (Mag), Mg-taramite (Mtm), phengite plementary Table S1 (available online http://joi.jlc.jst. (Ph), and titanite (Ttn). We further report that högbomite, go.jp/JST.JSTAGE/jmps/110621c). These data suggest a rare mineral with the general formula (Mg,Fe,Mn,Zn)6 doi:10.2465/jmps.110621c (Fe,Al,Ti)16O30(OH)2 related to the spinel group, is newly R.T. Orozbaev, [email protected] Corresponding author found in the Aktyuz eclogite. Preiswerkite and högbomite in garnets, Aktyuz eclogite 321

The petrography and mineral chemistry of the main 1.74, and Mn = 0.01 p.f.u.), and proposed that mineral in- constituent minerals in the studied sample have been de- clusions in garnets formed at two distinct metamorphic scribed by Orozbaev et al. (2007, 2010). Herein, our pri- events i.e., M1 and M2. For instance, a relic mineral as- mary focus is on the mode of occurrence of preiswerkite semblage of staurolite + Mg-taramite + paragonite ± he- and högbomite inclusions within garnets. We describe matite ± oligoclase (An<16) in garnet cores is referred to their chemical composition, Raman spectroscopy data and be formed under the precursor amphibolite or epidote- discuss their significance in MSI. amphibolite facies metamorphism (M1; 560-650 °C, 4-10 kbar). Another group of mineral inclusions that are repre- GEOLOGICAL SETTING AND PETROGRAPHY sentative of the epidote-blueschist facies conditions (330- 570 °C, 8-16 kbar) during the prograde stage of the eclog- The Aktyuz Formation is located in the Zaili Range in itic metamorphic event (M2) are composed of glaucophane, Northern Tien Shan, Kyrgyzstan, and it consists of pelitic barroisite, Mg-katophorite, epidote, paragonite, phengite, gneisses, gneissose-granites and migmatites, accompa- aegirine-rich omphacite, albite, quartz, rutile, and hema- nied by eclogites, garnet amphibolites and amphibolites tite. This group of mineral inclusions also occurs in the that occur as layers or lenticular bodies (Bakirov, 1978; garnet core and only rarely at garnet rims. Discrete miner- Tagiri et al., 1995; Bakirov et al., 2003; Orozbaev et al., al inclusions of omphacite, phengite, paragonite and rutile 2007, 2010). A detailed description of the geology in the occurring in garnet rims are suggested to have been area and the location of studied sample (KG-426) were formed under the peak conditions of M2 (550-660 °C, previously reported by Orozbaev et al. (2007, 2010). 21-23 kbar; Orozbaev et al., 2010). Eclogites in the Aktyuz area experienced three distinct The sample (KG-426) is medium- to coarse-grained, metamorphic events (M1-M3), namely a precursor medi- and consists mainly of garnet, omphacite, phengite and um-pressure and high-temperature (MP-HT) metamor- paragonite, along with minor rutile and quartz. phic event (M1), a high-pressure and low-temperature Garnets in the eclogite are subhedral, up to 2 mm in (HP-LT) eclogitic event (M2), and a high-pressure and diameter, and contain various types of inclusions (Fig. 1). high-temperature (HP-HT) metamorphic event (M3), Garnets show zoning from core to rim with decreasing whereas the surrounding country-rock gneisses experi- Ca, Fe, and Mn contents and with increasing Mg content enced a single metamorphism of the HP-HT event (M3) (see supplementary material Fig. S2, available online (Orozbaev et al., 2007, 2010). http://joi.jlc.jst.go.jp/JST.JSTAGE/jmps/110621c), and on Orozbaev et al. (2007) divided garnets of eclogites the basis of their chemical compositional zoning they can according to their chemical composition into core [Mg = be divided into core (Mg = 0.17-0.38, Ca = 1.07-0.81, Fe 0.17-0.73, Ca = 1.07-0.78, Fe < 2.16, and Mn < 0.16 per = 2.16-1.86, and Mn = 0.16-0.07 p.f.u.), mantle (Mg = formula unit (p.f.u.)] and rim (Mg = 0.73, Ca = 0.56, Fe = 0.38-0.73, Ca = 0.81-0.66, Fe = 1.86-1.74, and Mn =

Figure 1. Backscattered electron images showing the mode of occurrences of preiswerkite, Na-biotite, and hög- bomite within garnet of Aktyuz eclogite (KG-426). (a) Subhedral crystal of zoned garnet containing discrete and multiphase solid inclusions (MSI). (b) Epidote, Mg-katophorite, rutile, apatite, and ilmenite occurrences with MSI of St + Na-Bt in the garnet core. (c) MSI of Prw + St + Pg + Hem + Na-Bt + Chl and St + Hem + Na-Bt in the garnet mantle. Preiswerkite occurs as discrete grains. (d) MSI consisting of Prw + Pg + Hgb + Mag and Prw + Pg. Zircon grains occur nearby. (e) Paragonite occurs next to the MSI of St + Hgb + Hem + Na-Bt in the rim of the garnet. 322 R.T. Orozbaev, K. Yoshida, A.B. Bakirov, T. Hirajima, A. Takasu, K.S. Sakiev and M. Tagiri

0.07-0.02 p.f.u.), rim (Mg = 0.73-0.81, Ca = 0.66-0.53, dic-calcic amphiboles (glaucophane, Mg-katophorite, Fe = 1.74-1.65, and Mn = 0.02-0.01 p.f.u.), and outer- Mg-taramite, taramite, and barroisite), aegirine-rich om- most rim (Mg = 0.56, Ca = 0.55, Fe = 1.81 and Mn = 0.03 phacite, rutile, ilmenite, hematite, pyrite, plagioclase, and p.f.u.). These differing zones are illustrated in Figure 1a, rare quartz occur mainly in the core and rarely in the in which it is shown that the garnet has a bright core sur- mantle of garnets. In the rim of the garnets, mineral inclu- rounded by a grey mantle and a dark rim. The outermost sions of omphacite (with less aegirine component), para- rim can be seen as bright, very thin layers that lie at the gonite, rutile, apatite, and rare epidote are present. The margins of the garnet rim. These four zones correlate well constituent minerals of MSI, such as preiswerkite, hög- with the compositional zoning of Mg, Ca, and Fe shown bomite, staurolite, Na-biotite, and magnetite are not ob- in the color element map (Fig. S2). Moreover, they corre- served in the matrix of eclogite and they present only as spond to the core (core and mantle in this study) and rim inclusions within garnets. (rim and outermost rim) reported by Orozbaev et al. (2007). MINERAL CHEMISTRY AND RAMAN Preiswerkite occurs both as a member of MSI and as SPECTROSCOPY DATA discrete grains in the mantle and rim of garnets (Figs. 1a, 1c, and 1d). Preiswerkite is either colorless or pale yellow Mineral compositions were determined using electron to pale green in color, and it occurs as small-sized plates probe micro analyzers at Shimane University (JEOL up to 80 µm in length, with a typical mica habit. The MSI JXA-8800M) and Kyoto University (JEOL JXA-8105). associated with preiswerkite are mainly composed of par- Backscattered electron images were obtained using a Hi agonite, staurolite, hematite, Na-biotite, chlorite, hög- tachi S3500H scanning electron microprobe equipped bomite, magnetite, Mg-taramite, phengite and titanite. with an EDAX energy-dispersive X-ray analytical system The following assemblages are observed (Figs. 1c and at Kyoto University. The representative microprobe anal- 1d): Prw + Pg + Hgb ± Mag ± Chl, Prw + Pg + St + Hem yses of the minerals (preiswerkite, Na-biotite, and hög- + Chl ± Na-Bt, Prw + St + Mtm + Na-Bt + Hem, Prw + bomite) are listed in Table 1. Pg + Hem ± Chl, Prw + St + Na-Bt + Chl, Prw + Pg + The average composition of twelve microprobe anal- Mtm + Chl + Ttn + Ph, Prw + St ± Hem, Prw + Pg, and yses of preiswerkite gives a structural formula of (Na0.96 2+ VI IV Prw + Hgb. K0.02Ca0.01)0.99(Mg1.52Fe0.54 Al0.93)2.99( Al1.93Si2.07)4.00O10 Other sodium micas observed in the MSI are parago- (OH)2, based on 11 oxygen, which is close to the theoreti- nite and Na-mica referred to as “Na-biotite”. Paragonite cal end-member formula Na(Mg2Al)(Al2Si2O10)(OH)2 is colorless and occurs as plates of up to 300 µm in (Fig. 2). The obtained formula of preiswerkite exhibits the length, mainly in the mantle and rim zones of garnets interlayer cation >0.85 and octahedral cation >2.5 (Table (Figs. 1a, 1c, 1d, and 1e) but rarely in the core part. It can 1), suggesting that it is a trioctahedral sodium-rich true also be found as discrete grains. On the other hand, Na- mica (Reider et al., 1998). The preiswerkite composition biotite always occurs as a constituent mineral of MSI shows trend toward the aspidolite (i.e., Na-phlogopite) (Figs. 1b, 1c, and 1e). It is pale green to green in color and preiswerkite solid-solution series (Fig. 2). XMg = Mg/ and occurs as platy grains that are up to 100 µm in length. (Mg + Fe + Mn) varies from 0.68 to 0.78, which is the Högbomite is brown in color and forms fine-grained lowest value comparing to those reported in the literature subhedral grains (up to 60 µm in length). It occurs both as (XMg = 0.74-0.96, see Table S1). XNa = Na/(Na + K + Ca) a member of MSI and as discrete grains in the mantle and ranges from 0.92 to 0.99. The Cr2O3, Cl, and F contents rim of garnets. Högbomite-bearing MSI are mainly com- are negligible (<0.03 wt%). posed of the following assemblages: Hgb + Pg + Prw ± The composition of Na-biotite lies between those of Mag ± Chl, Hgb + St + Hem + Na-Bt ± Pg, and Hgb + the preiswerkite and aspidolite solid-solution series, and Prw. plots near the “aspidolite” composition (XMg = 90-91) re- Staurolite is colorless to pale brown in color and ported by Banno et al. (2005) (Fig. 2). The average com- forms prismatic (sometimes idiomorphic) crystals that can position of eleven microprobe analyses of Na-biotite cor- reach 300 µm in length. It also occurs as a member of responds to the structural formula (Na0.89K0.07Ca0.01)0.97 2+ VI IV MSI or as discrete grains, mainly in the mantle and rim of (Mg1.66Fe0.69 Al0.63)2.98( Al1.57Si2.43)4.00O10(OH)2. Na-bio- garnets but rarely in the core. Staurolite-bearing MSI oc- tite differs from preiswerkite in terms of the (Mg + Fe) IV curring in the core of garnets are preiswerkite-free and and Al values (Fig. 2); however, the XMg ratios of Na-Bt are mainly composed of St + Na-Bt (Fig. 1b) and St + and Prw are relatively similar, 0.66-0.73 and 0.68-0.78, Mtm + Pg ± Hem ± Pl (Orozbaev et al., 2007). respectively. The Cr2O3, Cl, and F contents are negligible Other inclusion phases such as epidote, sodic and so- (<0.04 wt%). Preiswerkite and högbomite in garnets, Aktyuz eclogite 323

Table 1. Representative microprobe analyses of preiswerkite, Na-biotite, paragonite, and högbomite in the Aktyuz eclogite

* Average composition of 12 and 11 analyses, respectively. ** Standard deviation. *** Total Fe as FeO.

Högbomite is relatively homogeneous in composi- 2+ tion, with the average structural formula (Mg1.47Fe3.02Zn0.04 3+ 3+ Fe1.45)5.98(Fe0.31Al15.13Ti0.56)16O30(OH)2 (Table 1). It can be

classified as ferrohögbomite (Armbruster, 2002). The XMg values range between 0.22 and 0.25, whereas the ZnO content is less than 0.31 wt%. Högbomite contains ferric iron (up to 1.86 p.f.u.), as calculated assuming the stoichi- ometry (Droop, 1987).

Paragonite has Si = 2.55-2.98 p.f.u. and XNa = 0.72- 0.91, with a high margarite component of up to Ca = 0.27

p.f.u. XMg of staurolite ranges between 0.16 and 0.29, and the ZnO content is as much as 0.32 wt%. Raman spectroscopy analysis of preiswerkite and högbomite was performed with a laser Raman spectro- photometer (JASCO NRS-3100) at Kyoto University, using the 514.5 nm line of an Ar+ laser. Calibration of the instrument was verified using the 520 cm−1 Si band and a Ne spectrum. In the case of preiswerkite, the observed IV −1 Figure 2. Al vs. Mg + Fe composition plot of preiswerkite, Na- peaks at 212, 280, 643, and 911 cm and the OH stretch- biotite, and paragonite in Aktyuz eclogite. The reported composi- ing peak at 3625 cm−1 were identical to those in the refer- tions of preiswerkite (all combined in Table S1) and aspidolite ence preiswerkite spectra (peaks at 216, 292, 648, and from other localities are also shown. 916 cm−1 and the OH stretching peak at 3628 cm−1) re- 324 R.T. Orozbaev, K. Yoshida, A.B. Bakirov, T. Hirajima, A. Takasu, K.S. Sakiev and M. Tagiri ported by Tlili et al. (1989). The Raman spectra of hög- controlled by the unusual bulk chemistry of the rock. bomite, with peaks at 257, 412, 525, 711, 776, and 830 In Aktyuz eclogite, preiswerkite always exists as a cm−1, was similar to the högbomite spectra (peaks at 261, constituent of MSI and as discrete grains in the mantle 421, 529, 723, 782, and 846 cm−1) reported by Tsunogae and rim of prograde zoned garnets, where the Ca and Mg and Santosh (2005). The slight differences in the wave- contents are lower and higher than those in the core, re- −1 numbers (within <16 cm ) may reflect the compositional spectively (Fig. S2). Furthermore, Ca-bearing phases (ep- variations between variety of preiswerkite and högbomite idote and sodic-calcic amphiboles, Figs. 1a and 1b) are considered in this study and those reported in the litera- abundant in the core of the garnet but less in the mantle/ ture. rim, where the amounts of Na-micas and Al-rich phases are increases (Figs. 1c, 1d, and 1e). These facts may sug- DISCUSSION gest that the preiswerkite was formed during prograde metamorphism, prior to or close to the peak eclogite fa- Preiswerkite and högbomite occur as constituents of MSI cies stage, possibly caused by the infiltration of hydrous and as single discrete grains within garnets, while neither Na-Al-rich and Si-undersaturated fluids at the garnet is observed in the matrix of the Aktyuz eclogite. Pre- mantle/rim forming stage. On the other hand, Ferrando et iswerkite is associated with St ± Pg ± Mtm ± Na-Bt ± al. (2005) proposed that the MSI in peak minerals can Hem ± Hgb ± Chl ± Ttn ± Ph ± Mag, and högbomite is represent the remnants of silicate-rich aqueous fluids that associated with Pg ± Prw ± St ± Hem ± Chl ± Na-Bt ± were dominant under prograde or peak metamorphic con- Mag in MSI. ditions. Further detailed studies on the formation mecha- The possible stability field of preiswerkite is sug- nism of MSI in garnets will open a new window in the gested to be within the range P = 0.5-5.0 kbar and T = study of deep fluid activity below plate convergent re- 500-850 °C, according to the experimental study on K- gions. Na-Al-Fe-Mg micas (Hewitt and Wones, 1975; Franz and Althaus, 1976). The natural occurrences of pre- ACKNOWLEDGMENTS iswerkite are reported within the conditions P < 15 kbar and T < 700 °C (Table S1). These previous studies illus- Constructive reviews by Toshiaki Tsunogae and an anony- trate that preiswerkite appears in H2O-saturated, unusual mous referee and editorial handling by Norimasa Shimo- Na-Al-rich, and Si-poor systems (e.g., Godard and Smith, bayashi are greatly acknowledged. We thank Tetsuo 1999; Visser et al., 1999), and the rarity of this mica is not Kawakami for his valuable discussion and help with the related to extreme or unusual P-T conditions. EPMA analyses. We also thank Atsushi Goto for his com- Högbomite has been described from greenschist/am- ments on the early version of the manuscript. This study phibolite facies to ultra-high temperature (UHT) granulite was partly supported by a JSPS Grant-in-Aid for Scien- facies metamorphic rocks (T = 400-800 °C; P = 3-15 tific Research to T.H. (Nos. 21109004 and 22244067) and kbar). It is considered as a product of retrogressive meta- A.T. (No. 17340149). morphism after spinel, mainly caused by the introduction of hydrous and oxidized fluids (Rammlmair et al., 1988; SUPPLEMENTARY MATERIAL Liati and Seidel, 1996; Tsunogae and Santosh, 2005; Ra- kotonandrasana et al., 2010). Some prograde högbomite Table S1 and Figure S2 showing the color element (Mg, experiencing UHT metamorphism with T > 1000 °C has Ca, Fe, and Mn) maps of garnet are available online from also been reported from the granulite facies Mg-Al rock http://joi.jlc.jst.go.jp/JST.JSTAGE/jmps/110621c. 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