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 meta- morphism. 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 eclog- ite (KG-426). (a) Subhedral crystal of zoned garnet containing discrete and multiphase solid inclusions (MSI). (b) Epidote, Mg-katophorite, rutile, apa- tite, 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.