New Constraints from Garnetite on the P^T Path of the Khondalite Belt: Implications for the Tectonic Evolution of the North China Craton

JOURNAL OF PETROLOGY VOLUME 54 NUMBER 9 PAGES1725^1758 2013 doi:10.1093/petrology/egt029

1 1 2 SHUJUAN JIAO *, JINGHUI GUO ,SIMONL.HARLEY AND Downloaded from BRIAN F.WINDLEY3

1STATE KEY LABORATORY OF LITHOSPHERIC EVOLUTION, INSTITUTE OF GEOLOGY AND GEOPHYSICS, CHINESE ACADEMY OF SCIENCES, BEIJING, 100029, CHINA 2GRANT INSTITUTE OF EARTH SCIENCE, SCHOOL OF GEOSCIENCES, UNIVERSITY OF EDINBURGH, http://petrology.oxfordjournals.org/ KINGS BUILDINGS, WEST MAINS ROAD, EDINBURGH EH9 3JW, UK 3DEPARTMENT OF GEOLOGY, UNIVERSITY OF LEICESTER, LEICESTER LE1 7RH, UK

RECEIVED MARCH 5, 2012; ACCEPTED APRIL 12, 2013 ADVANCE ACCESS PUBLICATION MAY 21, 2013

Garnetite pods containing 460 mol % garnet in association with at 1890 Ma, following the emplacement of S-type granitoids at Institute of Geology and Geophysics, CAS on August 27, 2013 garnet-bearing quartz-rich lenses are sporadically distributed in the at 1920^1890 Ma and ultrahigh-temperature metamorphism at Khondalite Belt, North China Craton. The mineral assemblages 1920 Ma that resulted from collision of the Yinshan and Ordos and microtextures of garnetite occurring in quartzo-feldspathic silli- Blocks at 1950 Ma. manite^garnet gneiss in the Xiaoshizi area allow definition of the metamorphic history of the area. Formation of garnet poikiloblasts coupled with their enclosing and matrix minerals quartz, sillimanite, KEY WORDS: clockwise P^T path; garnetite; Khondalite Belt; North plagioclase, biotite, rutile and ilmenite defines an M1 assemblage China Craton; pseudosection consistent with partial melting and subsequent melt separation. Thermodynamic modeling of bell-shaped Ca zoning in garnet poikiloblasts confirms that this occurred during decompression. A retro- INTRODUCTION grade M2 assemblage replacing M1 garnet or sillimanite is defined The Khondalite Belt (Fig. 1a) of the North China Craton by two microdomains: coronas of Grt2 þ Crd (M2a) where quartz has been studied intensively in recent years, especially fol- is present, and Spl þ Crd symplectite (M2b) where quartz is lowing the discovery of ultrahigh-temperature (UHT) absent. The final metamorphic stage led to the coronal assemblage metamorphic rocks (Santosh et al., 2007a, 2009a;Guo Grt3 þ Opx þ Crd þ Pl þ Bt (M3)surroundingM1 garnet. et al., 2012).The North China Craton (Fig. 1a) was divided Comparison of the observed mineral proportions and compositions into the Eastern and Western Blocks, separated by the with the predictions of phase equilibria modeling in the Trans-North China Orogen, by Zhao et al.(2001).Zhao NCKFMASHTO system for the effective bulk compositions, in et al. (2005) further divided the Western Block into the combination with Zr-in-rutile thermometry, results in the following Yinshan and Ordos Blocks, separated by the Khondalite P^T conditions for the metamorphic events: 820^8508C (up to Belt, and inferred that the Khondalite Belt was a colli- 9508C) and 8·5^9·5 kbar for M1;850^8658Cand7·4^7·6kbar sional orogen, like the Trans-North China Orogen, based for M2;710^7208Cand6·4^6·6 kbar for M3.The clockwise P^T on the metamorphic P^T paths, lithologies, structure and path thus defined for M1 to M3 evolution is interpreted to reflect geochronology of the high-grade rocks in the belt, and pro- the onset of extension and exhumation of the Khondalite Belt posed that it formed before the amalgamation of the

ß The Author 2013. Published by Oxford University Press. All *Corresponding author. Telephone: þ86 -(0)10 - 82998521. Fax: rights reserved. For Permissions, please e-mail: journals.permissions@ þ 86-(0)10-62010846. E-mail: [email protected] oup.com JOURNAL OF PETROLOGY VOLUME 54 NUMBER 9 SEPTEMBER 2013 Downloaded from http://petrology.oxfordjournals.org/ at Institute of Geology and Geophysics, CAS on August 27, 2013

Fig. 1. (a) Geological and tectonic map of the North China Craton (after Zhao et al., 2005). (b) Tectonic map of China showing the major cra- tons and orogens (after Zhao et al., 2001). CAOB, Central Asian Orogenic Belt; HO, Himalaya Orogen; NCC, North China Craton; SCC, South China Craton; SLO, Su^Lu Orogen; TC, Tarim Craton; QDO, Qinling^Dabie Orogen; QO, Qilianshan Orogen. Metamorphic complexes: AL, Alashan; DU, Daqingshan^Ulashan; GY, Guyang; HL, Helanshan; JN, Jining; QL, Qianlishan; WC, Wuchuan.

North China Craton. In their discussion of the presence of Khondalite Belt, especially its metamorphic P^T^t trajec- anti-clockwise P^T paths, Zhao et al. (1998, 2001,2005) sug- tories, to accurately constrain the tectonic evolution. gested that the metamorphism of the Western (mainly the Furthermore, the peak temperatures calculated from the Yinshan Block, as the Ordos Block is covered by the metamorphic rocks in theJining terrane^eastern part of the Mesozoic to Cenozoic strata of the Ordos Basin) and Khondalite Belt (Figs 1 and 2) have not been well con- Eastern Blocks (Fig. 1a) was related to the intrusion and strained, as they have been largely based on conventional underplating of mantle-derived magmas, whereas the Fe^Mg exchange thermobarometers. The low closure tem- clockwise P^T paths involving near-isothermal decom- peratures of these geothermobarometers (less than 8008C) pression from the Khondalite Belt and the Trans-North overprinted by subsequent cation diffusion (Lu et al., 1992, China Orogen (Fig. 1a) reflected continental collision tec- 1996; Lu & Jin,1993) have consequences for the inferred P^ tonics. Santosh (2010) interpreted the Inner Mongolia T paths not only because of the uncertainties in estimated Suture Zone (i.e. the Khondalite Belt) as a subduction^ temperature, but also because of the impacts on pressure cal- accretion^collision orogen between the Yinshan and culations that result from Fe^Mg feedback effects (Frost & Ordos Blocks. In his model the tonalite^trondhjemite^ Chacko, 1989; Harley, 1989; Fitzsimons & Harley, 1994). granodiorite (TTG) gneiss, charnockite and calc-alkaline Advances in thermodynamic analysis, for example, granite in the Suture Zone (i.e. the Khondalite Belt) repre- THERMOCALC 3.33 (Powell & Holland,1988) and in par- sented an accreted oceanic plate sequence including a con- ticular the updated internally consistent thermodynamic tinental arc built up through subduction from the north. dataset (e.g. Holland & Powell, 1998), provide powerful Santosh (2010) also speculated on the basis of recent seis- quantitative methods to calculate P^T paths with greater mic data that the North China Craton developed by confidence and precision. THERMOCALC makes it pos- double-sided subduction between major crustal blocks. sible to correlate the bulk-rock compositions with their most The existence of these controversial models for its genesis stable mineral assemblages over a range of P^T space (i.e. and geological context necessitates a further study of the P^T pseudosection calculation), and to use the contouring

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Fig. 2. Distribution of the high-grade metamorphic rocks in the eastern segment of the Khondalite Belt (Jining terrane), North China Craton (modified after Guo et al., 2001a). at Institute of Geology and Geophysics, CAS on August 27, 2013

of calculated phase diagrams for cations that have low diffu- Mineral abbreviations are based on those of Whitney & sivities, such asVIAl in orthopyroxene, and Ca in plagioclase Evans (2010). andgarnet, toprovide additionalconstraints onthe P^Tcon- The second garnetite type occurs along with garnet- ditions. In addition, the pseudosection approach not only bearing quartz-rich lenses as irregular pods within gives the peak metamorphic conditions, but also P^T quartzo-feldspathic sillimanite^garnet gneiss (e.g. Xiao- path information, as well as allowing investigation of shizi area; Figs 2 and 3). This garnetite type has a processes such as partial melting and melt loss (Powell & distinctive mineral assemblage consisting of Grt þ Sil þ Holland,2008). Pl þ Qz þ Bt þ Ilm, with minor rutile, spinel, cordierite Garnetite with garnet contents up to 60 mol % is very and orthopyroxene. The nearby garnet-bearing quartz- rare and sporadically distributed in the metasedimentary rich lenses contain abundant quartz and minor gar- rocks of the Khondalite Belt. In detail, two types of garne- net associated with rare biotite, K-feldspar, rutile and tite are identified. The first garnetite type occurs as hori- ilmenite. zons or lenses between gabbro dykes and their host This study focuses on the second garnetite type, from the metapelites (e.g. Tuguishan; Fig. 2), and contains abundant Xiaoshizi area (Jining terrane of the Khondalite Belt; garnet, as well as orthopyroxene, rutile and plagioclase. Figs 1^3), and utilizes its mineral assemblages and micro- Rivalenti et al. (1997) investigated similar garnetites in the textures to deduce the P^Tevolution. The detailed petrog- Ivrea^Verbano Zone (Val Fiorina), NW Alps. The mineral raphy, reaction analyses coupling mineral chemistry with assemblage in that case was Grt þ Bt þ Pl Cpx Hb pseudosection calculations and independent geother- Opx, and Rivalenti et al. (1997) suggested that these mometric estimates (i.e. Zr-in-rutile) are presented, and a garnetites may have formed by two processes: (1) meta- P^T path is derived using a combination of all of these somatism through interstitial melt retention in amphibo- constraints. The metamorphic evolution and petrogenesis lites and metapelites undergoing synchronous anatexis; (2) of the garnetites, and the implications for the tectonic evo- reaction between metapelite-derived anatectic melt and lution of the Khondalite Belt between the Yinshan and phases segregated from a basaltic sill during its intrusion, Ordos Blocks, are discussed in the light of this new which induced partial melting of the country rocks. evidence.

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Fig. 3. A local geological map of the Dashizi and Xiaoshizi areas (modified after Lu et al., 1992).

GEOLOGICAL SETTING suggested that the craton formed by the collision between The North China Craton (Fig. 1b), one of the oldest base- the Eastern and Western Blocks along the Trans-North ment terranes in China, is divisible into the Archean to China Orogen at 1·85 Ga. Before that, the Western Block Paleoproterozoic Eastern and Western Blocks and three formed by collision of the Ordos and Yinshan Blocks Paleoproterozoic orogenic belts, namely the Trans-North along the Khondalite Belt^Inner Mongolia Suture Zone China Orogen, Khondalite Belt, and Jiao-Liao-Ji Belt at 1·95 Ga (Zhao et al., 2005, 2010 a; Santosh et al., 2007b; (Fig. 1a) (Zhao et al., 2001, 2003, 2005). Many researchers Yin et al., 2009, 2011; Santosh, 2010). (e.g. Guo et al., 2001a,2005;Zhaoet al., 2002, 2007, 2008a, The east^west-trending Khondalite Belt, which separates 2008b, 2010b; Wilde & Zhao, 2005; Kro« ner et al., 2006; the northern Yinshan Block from the southern Ordos Zhang et al., 2006, 2007, 2009; Lu et al., 2008) have Block, is divisible into three terranes from west to east:

1728 JIAO et al. KHONDALITE BELT GARNETITE

Helanshan^Qianlishan (HL^QL), Daqingshan^Ulashan 2010c).The younger metamorphic zircon ages, mostly from (DU) and Jining (JN) (Fig. 1a). The belt is mainly com- the Jining terrane, have been interpreted to constrain the posed of granulite-facies metasedimentary rocks (Lu et al., timing of exhumation subsequent to collision (Guochun 1992, 1996; Z hao et al., 1999, 2005). The predominant rock Zhao, personal communication).The UHT metamorphism types are quartzo-feldspathic gneisses, garnet- and silli- inthe centralJining terrane is consideredto have occurred at manite-bearing plagioclase gneisses, feldspathic quartzites, 1·92 Gabased on U^Pb ages of zircon and monazite chem- marbles and calc-silicate rocks, the protoliths of which ical dating (Santosh et al., 2006, 2007a, 2007b, 2009b). The were deposited on a passive continental margin (Condie Liangcheng S-type granitoids were emplaced over the1·92^ et al., 1992; Lu et al., 1992, 1996).These lithologies are exten- 1·89 Ga time interval based on zircon U^Pb geochronology sively exposed in the Jining terrane, near the junction of (Guo etal.,2001b;Zhongetal.,2007). the Khondalite Belt and the Trans-North China Orogen Samples for this study were collected in the Xiaoshizi Downloaded from (Fig. 2), which mainly consists of sillimanite^garnet^K-feld- area, about 25 km SW of Zhuozi Town (Fig. 2). Figure 3 is spar gneisses and quartzo-feldspathic gneisses that contain a geological sketch map of the Dashizi to Xiaoshizi area horizons of marble, calc-magnesium silicate rocks, and that shows the major lithologies and structures. Garnetites rare graphitic gneiss (Lu et al., 1992).The metasedimentary were sampled from a small hill close to Xiaoshizi (Fig. 3), rocks are associated with meta-gabbronorites that docu- where they form an irregular pod (c.7mlong, c.5m http://petrology.oxfordjournals.org/ ment the intrusion of mantle-derived magmas, and abun- wide) associated with garnet-bearing quartz-rich lenses dant S-type granitoids (Peng et al., 2010). The S-type (Fig. 4a^c) within quartzo-feldspathic sillimanite^garnet granitoids are particularly abundant in the Liangcheng gneisses (Fig. 4d). Figure 4e shows a sharp contact between area (Fig. 2), where they are interpreted to have formed the garnetite and a garnet-bearing quartz-rich lens, but in through extensive partial melting of the metasediments most exposures the contact is transitional. Figure 4f shows (Peng et al., 2012). Ultrahigh-temperature assemblages have the appearance of typical garnetite in the field. been identified from several localities, mainly situated in the eastern Daqingshan^Ulashan terrane (Dongpo) and the central Jining terrane (Tuguiwula, Tuguishan and MICROTEXTURES AND

Xuwujia) (Santosh et al., 2006, 2007a;Jiaoet al., 2011; Jiao at Institute of Geology and Geophysics, CAS on August 27, 2013 &Guo,2011;Guoet al., 2012), where gabbroic and noritic REACTION HISTORY to dioritic intrusive bodies are also common, and metamor- As shown in Fig. 5, the garnetite contains 460 mol % phosed under granulite-facies conditions (Guo et al., 2001b; garnet poikiloblasts, locally separated by an interstitial Peng et al., 2010). matrix containing sillimanite (7^10%), quartz (5^7%), Four episodes of Precambrian deformation have been biotite (2^3%), plagioclase (1^2%), ilmenite (1^2%), and identified by previous researchers in the Jining terrane rutile (c. 1%). Large garnet poikiloblasts (44mm)typic- (Lu et al., 1992; Lu & Jin, 1993). D1 caused complex plastic ally contain various inclusions, with abundant quartz and deformation, particularly in mica schists and thin-bedded minor biotite, plagioclase, sillimanite and rutile (Fig. 6a), marbles. D2, the most important deformation in this and occasionally rare spinel and orthopyroxene in their region, is represented by tight, isoclinal to recumbent rims (Fig. 6b). In contrast, medium and small garnet poi- folds (Fig. 3) which produced a pervasive S2,ENE^WSW- kiloblasts (53 mm) contain few inclusions, but occasionally striking schistosity. D3 is characterized by open folds in S2 acicular sillimanite forms linear inclusion fabrics (Fig. 6c). that have subvertical, east^west-trending axial planes, and Sillimanite and biotite inclusions are in places replaced by has created dome and basin structures. D4 produced a spinel-bearing assemblage (Fig. 6d and e). Garnet poiki- quartz^feldspar ribbon fabrics in ductile high-strain zones loblasts and sillimanite show varying degrees of resorption that have been attributed to strike-slip movements during associated with coronal or symplectitic reaction texture crustal transtension. microdomains. For example, garnet forms newly grown DetritalzirconsfromthemetasedimentsoftheKhondalite aggregates and cordierite partially replaces garnet poiki- Belt indicate protolith ages in the range of 2·3^1·9Ga, loblasts and sillimanite in quartz-bearing domains whereas metamorphic zircons have ages of 1·95 Ga and (Fig. 6f^g), whereas spinel and cordierite partially replace 1·88^1·81Ga (Wu et al., 1998; Wan et al., 2006; Xia et al., sillimanite in quartz-absent domains (Fig. 7a^d). Ortho- 2006, 2008;Yin et al., 2009,2011;Zhao et al.,2010c).The 1·95 pyroxene, plagioclase and cordierite occur around garnet Ga ages, which are mainly preserved in zircons from the poikiloblasts in sillimanite-absent microdomains (Figs 7e, metasediments in the Helanshan^Qianlishan terrane far f and 8a^c), and retrograde biotite occurs around garnet removed from the junction of the Khondalite belt and the poikiloblasts and sillimanite (Figs 7a and 8d). Pseudo- Trans-North China Orogen (Fig.1a), have been interpreted morphs after a melt phase are also recognized between to date the time of collision of theYinshan and Ordos Blocks garnet poikiloblasts (Fig. 8e and f). Zircon, rutile, ilmenite, along the Khondalite Belt (Yin et al., 2009, 2011; Zhao et al., monazite and apatite occur in both garnet poikiloblasts

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Fig. 4. Field photographs of the outcrop of garnetites in the Jining terrane. (a^c) The outcrop of the garnetite pod and garnet-bearing quartz- rich lens near Xiaoshizi. (d) The quartzo-feldspathic sillimanite^garnet gneiss close to the garnetite. (e) The garnetite and the garnet-bearing quartz-rich lens in this location. (f) The garnetite in this location. and the matrix. Chlorite and sericite occasionally replace Major mineral assemblage in garnetite at cordierite, biotite and plagioclase in the matrix. stage M1 Three metamorphic stages have been distinguished in Sillimanite, quartz, plagioclase, biotite, ilmenite and rutile these garnetites based on the observed microtextures and occur as inclusions in both garnet poikiloblasts and the their overprinting relationships (Table 1). matrix (Fig. 6a and c). These minerals associated with the

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Fig. 5. (a) Plane-polarized light photomicrograph showing the mineral distribution in the garnetite (DSZ20). (b) A summary sketch showing the distribution of minerals in the garnetite. formation of garnet poikiloblasts (mainly their core and consisting of Grt2 aggregates, ilmenite and cordierite in mantle, as their rims are generally resorbed or overgrown) the presence of quartz (Fig. 6f^h). Biotite is also a represent metamorphic stage M1 Grt1 þSil þ Qz þ Pl1 þ common phase (Fig. 6f). Thus, the mineral assemblage at Bt þ Ilm þ Rt, which is representative of the granulite-facies stage M2a comprises Grt2 þ Crd þ Ilm þ Sil þ Qz þ Bt. metamorphism of typical pelitic bulk-rock compositions. Based on the microtexture (Fig. 6f^h), the local domain has experienced a reaction consuming Grt1, sillimanite Coronal or symplectitic association at and quartz, and producing Grt2 aggregates, cordierite stage M2 and ilmenite. Biotite may be a reactant in this case, leading Formation of newly grown garnet (Grt2) and/or spinel after to the formation of hydrous mineral cordierite and facili- garnet, sillimanite and biotite marks the onset of M2,and tating production of melt. Thus, the proposed reaction is is usually preserved in the form of composite inclusions in Grt1 þ Sil þ Qz þ Bt ¼ Grt2 þ Crd þ Ilm þ Liq: ð1Þ garnet poikiloblasts (Fig. 6d and e). In some microtextures minor K-feldspar also occurs with spinel replacing biotite, which suggests a biotite dehydration reaction to form Stage M2b Spl þ Crd symplectites spinel, quartz, and/or garnet, and/or K-feldspar. However, Fine-grained spinel and cordierite are ubiquitous in inter- in most cases within the matrix, Grt2 and Spl are generally growths replacing sillimanite amongst dominant Grt1 localized to different microdomains dependent on the pres- in microdomains where quartz is absent (0·5 mm in size, ence of quartz (Fig.7f), so that they occur with coronal cor- Fig. 7a^d). Biotite is usually present, and spinel coexists dierite in separate sites (Figs 6f^h and 7a^d). Thus, M2 is with ilmenite (Fig. 7b^d). This results in the M2b mineral divided into two substages to distinguish the quartz-present assemblage Spl þ Crd þ Sil þ Grt þ Bt þ Ilm. No new (M2a) from quartz-absent (M2b)microdomains. garnet is identified from the petrography, but a change to more Fe-rich compositions on Grt1 rims is considered to re- Stage M2a Grt þ Crd coronas flect equilibrium with Spl þ Crd, as spinel inclusions occa- Grt1 and sillimanite grains are extensively replaced, in sionally occur in these garnet rims. This assemblage is local microtextures (c.0·3 mm in size), by coronas interpreted to result from reaction of Grt1 and sillimanite

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Fig. 6. Photomicrographs and back-scattered electron images showing the mineral assemblages and microtextures of the garnetites. (a) Inclusions occurring in Grt1. (b) Orthopyroxene occurring as inclusion in the rim of garnet (Grt1 rim^Grt3), or surrounding garnet associated with plagioclase (Pl3) and cordierite. (c) Acicular sillimanite in Grt1. (d) Spinel and Grt2 in contact with sillimanite and plagioclase in a composite inclusion in Grt1. (e) Spinel coexisting with rutile, zircon and quartz, along with biotite as a composite inclusion in Grt1. (f^h) Regrown Grt2 aggregate coexisting with cordierite and quartz around sillimanite and Grt1.

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Fig. 7. Photomicrographs and back-scattered electron images showing mineral assemblages and microtextures in garnetite. (a) Grt1 coexisting with sillimanite, which is partly replaced by spinel and cordierite, and retrograde biotite occurring around garnet and sillimanite. (b, c) Symplectitic spinel and cordierite growing between sillimanite and Grt1. (d) Symplectitic spinel and cordierite growing between sillimanite and Grt1, and Grt3 þ Opx þ Crd among Grt1. (e) Orthopyroxene-bearing coronas around Grt1. (f) Symplectitic orthopyroxene and cordierite occur between garnet and quartz. Sillimanite is separated from orthopyroxene by cordierite. Between sillimanite and quartz there are small garnet grains (Grt2), whereas some spinel grains have grown between sillimanite and Grt1. and/or biotite to form the Spl þ Crd þ Ilm association, Grt3 þ Opx þ Crd þ Qz þ Pl3 þ Bt. Formation of retro- with the involvement of biotite promoting the production grade biotite requires the addition of a melt or fluid phase. of melt: This microtexture is interpreted to reflect operation of the following multivariant reaction: Grt1 þ Sil þ Bt ¼ Spl þ Crd þ Ilm þ Liq: ð2Þ Grt1 þ Qz þ Pl1 þ Liq=fluid ¼ Grt3 þ Opx Stage M Grt3 þ Opx þ Crd þ Pl coronas ð3Þ 3 þ Crd þ Pl3 þ Bt The replacement of Grt1 rims by subhedral Grt3, orthopyroxene, plagioclase, cordierite, and biotite is identified As shown in Figs 7f and 8a, b, both orthopyroxene and in localized domains (0·5 mm in size) (Figs 7e, f and 8a^ sillimanite occur amongst the garnet grains, but never in d). Thus, the M3 mineral assemblage is taken to be contact with each other. In the absence of detailed

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Fig. 8. Photomicrographs and back-scattered electron images showing mineral assemblages and microtextures in garnetite. (a) Inclusions of monazite, quartz and biotite occurring in the core of Grt1, and symplectitic orthopyroxene, Grt3, plagioclase and cordierite growing among Grt1. (b) Grt3 coexisting with cordierite, orthopyroxene and quartz around Grt1. (c) Orthopyroxene coexisting with plagioclase (Pl3), biotite, quartz and cordierite among Grt1. (d) Retrograde biotite growing around Grt1. (e, f) Melt pseudomorphs occurring between Grt1.

petrographic observations this could be used to infer the surviving from the Grt1 þSil M1/M2 assemblage (Fig. 9). presence of coexisting Opx þ Sil, a diagnostic mineral as- Consistent with this, the Al content of the orthopyroxene semblage of UHT metamorphism [in the presence of Qz is low compared with that in typical UHTrocks worldwide and K-feldspar; see Harley (1998a, 1998b, 2004, 2008)]. (Harley, 1998a, 1998b, 2004, 2008; Osanai et al., 2004; However, these two minerals are always separated by cor- Brandt et al., 2007; Santosh et al., 2007a; Kelsey, 2008). dierite, and no K-feldspar exists in the matrix. In Fig. 7f, Pseudosection calculations (see later section) support the orthopyroxene has grown until it is almost in contact with petrographic interpretation that orthopyroxene and silli- sillimanite, but these phases do not form a stable paragen- manite did not coexist in the garnetite at any stage of the esis as orthopyroxene is produced with cordierite in a metamorphic history. domain formed at the expense of former garnet, whereas In conclusion, stage M1 is represented by garnet poikilo- sillimanite is simply a relict grain situated on garnet, blast (Grt1) formation coupled with its enclosing and

1734 JIAO et al. KHONDALITE BELT GARNETITE

Table 1: Mineral assemblages in the various metamorphic stages of the garnetite

Mineral M1 M2a M2b M3

Grt þþ þ þ Qz þþ þ Sil þþ þ Pl þþ

Bt þþ þ þ Downloaded from Ilm þþ þ þ Spl þþ Opx þ Crd þþþ Rt þ http://petrology.oxfordjournals.org/

þ, mineral is present in the assemblage.

þ matrix minerals of Qz þ Sil þ Pl1 þBt þ Ilm þ Rt. M2 is Fig. 9. (MF)AS (MgO Fe O ^ A l 2O3^SiO2) diagram projected characterized by the coronas of garnet (Grt2) þ Crd and from the feldspar components, showing the bulk compositions from Table 9 and analyzed mineral assemblages. (Refer to the text for Spl þ Crd symplectites formed after sillimanite and explanation.) garnet, in quartz-present (M2a) and -absent (M2b) micro- textural settings, respectively. Stage M3 is characterized by the Opx þ Crd þ Grt3 þ Pl3 þ Bt coronal association. The rutile grains were analyzed with a JEOL JXA-8230 As shown in Fig. 9, a simplified (MF)AS (MgO þFe O ^ at Institute of Geology and Geophysics, CAS on August 27, 2013 EMP at the Institute of Mineral Resources, Chinese Al O ^SiO ) diagram (the calculations of the bulk com- 2 3 2 Academy of Geological Sciences. The analytical method is positions of M ^M are described in the ‘Determining 1 3 thesameasthatofJiaoet al. (2011). The chemical compos- bulk compositions’ section), the peak assemblage of M is 1 itions of rutile grains are listed inTable 8. Grt þ Sil þ Qz, with the close proximity of the projected bulk composition of M1 to garnet compatible with its high modal abundance. Cordierite is added to stage M2a ac- Garnet cording to its bulk composition in the (MF)AS diagram, Poikiloblastic garnet (usually 42 mm diameter) is a Prp^ and Grt þ Crd þ Qz represents the typical assemblage Alm solid solution with XMg ¼ 0·26^0·40 [XMg ¼ Mg/ occurring during M2a. The assemblage Grt þ Sil þ Crd or (Mg þ Fe)], with minor grossular and spessartine Sil þ Spl þ Crd represents stage M2b, where quartz is (Table 2; Fig. 10). Garnet cores þ mantles (XMg ¼ 0·32^ absent. The Sil-absent assemblage Grt þ Opx þ Qz or 0·40) contain 30·7^39·0mol % pyrope,57·6^65·7mol % Crd þ Opx þ Qz corresponds to stage M3 (Fig. 9). almandine, 3·0^3·7 mol % grossular, and 0·5^0·8mol% spessartine, whereas rims (XMg ¼ 0·27^0·35) have 25·9^ 33·9 mol % pyrope, 62·4^69·7 mol % almandine, 2·8^ MINERAL CHEMISTRY 3·2 mol % grossular, and 0·5^0·8 mol % spessartine. All mineral analyses, except for rutile, were performed Garnet poikiloblasts generally show asymmetrical com- with a JEOL JXA-8100 electron microprobe (EMP) at the positional zoning in terms of almandine (and spessartine) Institute of Geology and Geophysics, Chinese Academy of and pyrope, which increase and decrease respectively to- Sciences in Beijing, China. Operating conditions were 15 wards (and in) the rims (Fig. 11), reflecting post-peak Fe^ kV and 10 nA with a 3 or 5 mm beam size. Count times Mg exchange with adjacent Fe^Mg minerals. In contrast, were 20 s on peaks and 10 s on each background. Natural grossular shows concentric zoning with a bell-shaped pro- and synthetic phases were used as standards. Minerals file (Fig.11), with contents decreasing continuously towards were routinely analyzed for Na, Mg, Al, Si, K, Ca, Ti, the rim. Mn, Fe, Zn/Ba, Cr, and Ni. Biotite was also analyzed for Grt2 shows broadly similar chemical compositions to the F and Cl. The data were processed with an online ZAF- rims of garnet poikiloblasts, with XMg of 0·31 ^ 0 ·33, type correction. Representative analyses of garnet, biotite, pyrope of 29·8^31·6 mol %, almandine of 63·4^64·7mol plagioclase, spinel, orthopyroxene, and cordierite are %, and spessartine of 0·6^0·8 mol %, but has higher gros- given inTables 2^7 and illustrated in Figs 10^13. sular contents of 4·2^5·0mol % (Table 2;Fig.10). XMg

1735 JOURNAL OF PETROLOGY VOLUME 54 NUMBER 9 SEPTEMBER 2013

Table 2: Representative compositions of garnet

Texture: Grt1-core Grt1-core Grt1-core Grt1-mantle Grt1-mantle Grt1-rim Grt1-rim Grt2-core Grt2-rim Grt3 Grt3 Sample: 21-3GRT2 23-3GRT1 23-3GRT2 23-7GRT1 23-9GRT1 23-1GRT1 23-2GRT1 23A-1GRT 23A-1GRT 20-8GRT6 20-8GRT7

SiO2 39·25 38·66 39·07 38·99 39·35 38·86 38·83 38·91 38·81 38·04 38·06

TiO2 b.d. 0·03 b.d. b.d. b.d. 0·04 b.d. b.d. 0·03 0·13 0·06

Al2O3 22·41 22·32 22·36 22·17 22·50 22·31 22·17 22·16 22·26 21·52 21·95

Cr2O3 0·05 0·08 0·08 0·10 0·04 0·11 0·06 0·10 0·06 0·04 0·03 FeOT 29·30 29·15 28·34 28·60 28·77 29·98 29·57 29·90 29·47 32·66 31·34 Downloaded from MnO 0·33 0·23 0·31 0·36 0·26 0·33 0·33 0·30 0·27 0·40 0·32 MgO 8·61 8·76 9·12 9·24 8·89 8·25 8·46 8·14 7·89 6·56 7·27 CaO 1·36 1·25 1·26 1·23 1·21 1·12 1·17 1·60 1·68 0·94 0·96 Total 101·29 100·48 100·53 100·70 101 ·02 101·00 100·59 101·11 100·47 100·29 99·99 Structural formulae normalized to 12 oxygens http://petrology.oxfordjournals.org/ Si 2·994 2·976 2·992 2·987 3·000 2·985 2·991 2·989 2·995 2·987 2·977 Ti b.d. 0·002 b.d. b.d. b.d. 0·002 b.d. b.d. 0·002 0·008 0·004 Al 2·015 2·025 2·018 2·002 2·022 2·020 2·012 2·006 2·024 1·992 2·024 Cr 0·003 0·005 0·005 0·006 0·002 0·007 0·004 0·006 0·004 0·002 0·002 Fe 1·869 1·876 1·815 1·832 1·834 1·926 1·905 1·920 1·901 2·145 2·050 Mn 0·021 0·015 0·020 0·024 0·017 0·021 0·022 0·020 0·018 0·027 0·021 Mg 0·979 1·005 1·041 1·056 1·011 0·945 0·971 0·932 0·908 0·768 0·848 Ca 0·111 0·103 0·103 0·101 0·099 0·092 0·097 0·132 0·139 0·079 0·080

XMg 0·341 0·350 0·362 0·366 0·353 0·329 0·338 0·327 0·323 0·264 0·293

Prp 0·328 0·338 0·349 0·354 0·341 0·317 0·324 0·310 0·306 0·254 0·283 at Institute of Geology and Geophysics, CAS on August 27, 2013 Alm 0·627 0·622 0·609 0·604 0·620 0·645 0·636 0·639 0·641 0·711 0·683 Grs 0·037 0·035 0·035 0·034 0·033 0·031 0·032 0·044 0·047 0·026 0·027 Sps 0·007 0·005 0·007 0·008 0·006 0·007 0·007 0·006 0·006 0·009 0·007 b.d., below detection limit. FeOT is total FeO.

(from 0·33 to 0·31), pyrope (from 31·3to29·8 mol %), biotite grains (Fig. 12b and c). However, XMg^F shows and spessartine (from 0·8to0·6 mol %) decrease slightly positive correlations, which are distinct for the inclusion from core to rim, whereas almandine (from 63·4to biotite grains when compared with matrix grains 64·7 mol %) and grossular (from 4·2to4·9 mol %) in- (Fig. 12d). In the case of single biotite grains, those grains crease slightly. Grt3 has the lowest XMg (0·26^0·31) and present as inclusions in garnet are too small to see the dif- pyrope (24·9^29·8 mol %), but the highest spessartine ference (50·25 mm), but XMg and Ti (p.f.u.) decrease (0·7^1·0 mol %) and almandine (66·2^71·1%), with a gros- slightly towards the rim in the matrix grains. sular content of 2·6^3·4mol%. Matrix biotite grains lack chemical differences that could be ascribed to the different metamorphic stages Biotite (M1,M2, and M3). However, as most of the grains occur As shown in Table 3 and Fig. 12, biotite grains enclosed in around garnet poikiloblasts as retrograde phases, they may all have been produced during M . Pre-existing bio- garnet poikiloblasts (Bt1) are richer in Mg and F [XMg 3 0·71 ^0·85; F (p.f.u.) 0·256^2·578] than the matrix grains tite that may have been involved in melt-forming reactions [reactions (1) and (2), M ] is preserved only as inclusions [XMg 0·53^0·80; F (p.f.u.) 0·010^1·594], and have lower Cl 2 contents [Cl (p.f.u.) 0·021^0·317 vs 0 ·021^1·104].Ti contents (i.e. Bt1) in garnet poikiloblasts. of these two biotite types extend over a wide range, with inclusion biotite grains ranging to slightly higher Ti con- Plagioclase tents [Ti (p.f.u.) 0·250^0·566] than the matrix ones [Ti Plagioclase (Pl1) in Grt1 preserves the lowest anorthite (p.f.u.) 0·089^0·517] (Table 3; Fig. 12). Both of Mg þ Fe þ contents (An20^35). Plagioclase present in the matrix to- Mn (cations, p.f.u.)^Ti (cations, p.f.u.) and XMg^Cl (cat- gether with Grt1, sillimanite and quartz has similar ions, p.f.u.) plots show negative correlations for all types of anorthite contents of An29^32, suggesting that they formed

1736 JIAO et al. KHONDALITE BELT GARNETITE

Table 3: Representative compositions of biotite

Texture: Grt mantle, Grt core Grt mantle Grt mantle Matrix: Bt þ Matrix: Bt þ Matrix: Bt þ Matrix: Bt þ Bt þ Ilm Sil þ Crd Grt þ Ilm Grt þ Ilm Sil þ Crd Sample: 20-15BI(GRT)1 22-3BI(GRT)1 30-2BI(GRT)1 31’-4BI(GRT)1 20-8BI1 22-6BI3 29-6BI2 31’-1BI3

SiO2 40·37 38·77 38·46 37·78 38·25 38·45 38·30 36·35

TiO2 4·03 3·26 3·43 4·10 4·35 2·89 3·18 0·79

Al2O3 15·43 14·21 14·49 14·79 14·13 12·98 13·96 15·46

Cr2O3 0·09 0·10 0·08 0·07 0·38 0·07 0·07 0·13 Downloaded from FeOT 6·71 8·04 7·64 11·511·15 8·99 13·71 18·02 MgO 20·08 19·22 18·97 16·18 16·68 18·49 15·89 13·53 CaO 0·03 b.d. b.d. 0·04 b.d. 0·03 b.d. b.d. NiO 0·05 b.d. b.d. 0·05 b.d. b.d. b.d. b.d. ZnO b.d. 0·11 0·11 0·13 b.d. 0·12 b.d. b.d. http://petrology.oxfordjournals.org/

Na2O0·05 0·26 0·19 0·14 0·20 0·28 0·21 0·20

K2O10·14 9·40 9·26 9·35 9·53 9·16 9·44 9·50 F2·06 1·57 0·82 0·31 1·18 1·48 0·58 0·16 Cl 0·11 0·19 0·12 0·46 0·20 0·35 1·62 2·56 Total 99·15 95·13 93·57 94·90 96·05 93·29 96·96 96·71 O–F–Cl 0·89 0·70 0·37 0·23 0·54 0·70 0·61 0·64 CTotal 98·26 94·43 93·20 94·67 95·51 92·59 96·35 96·07 Structural formulae normalized to 24(O,OH,F,Cl) Si 5·939 5·968 5·950 5·872 5·907 6·074 5·971 5·849

Ti 0·446 0·377 0·399 0·479 0·505 0·343 0·373 0·096 at Institute of Geology and Geophysics, CAS on August 27, 2013 AlIV 2·061 2·032 2·050 2·128 2·093 1·926 2·029 2·151 AlVI 0·612 0·544 0·590 0·579 0·477 0·489 0·534 0·779 AlTotal 2·673 2·576 2·640 2·707 2·570 2·415 2·563 2·930 Cr 0·010 0·012 0·010 0·009 0·046 0·009 0·009 0·017 Fe 0·826 1·035 0·988 1·495 1·440 1·188 1·787 2·425 Mg 4·404 4·410 4·375 3·749 3·840 4·354 3·693 3·246 Ca 0·005 b.d. b.d. 0·007 b.d. 0·005 b.d. b.d. Ni 0·010 b.d. b.d. 0·010 b.d. b.d. b.d. b.d. Zn b.d. 0·010 0·010 0·010 b.d. 0·010 b.d. b.d. Na 0·014 0·078 0·057 0·042 0·060 0·086 0·063 0·062 K1·903 1·846 1·828 1·854 1·877 1·846 1·877 1·950 F1·917 1·528 0·802 0·305 1·152 1·479 0·572 0·163 Cl 0·055 0·099 0·063 0·242 0·105 0·187 0·856 1·397

XMg 0·842 0·810 0·816 0·715 0·727 0·786 0·674 0·572 b.l., below detection limit. FeOT is total FeO.

during the same stage M1. Plagioclase grains (Pl3) coexist- Hc (23·9^66·2mol %), and Ghn (11·9^49·0mol %) 3þ total ing with orthopyroxene and/or cordierite in M3 textures (Table 5, Fig. 13). Fe /Fe is less than 0·10. Spinel grains are the most calcic preserved (An35^56:Table4).Noplagio- are highly variable in their chromite contents, containing be- clase grains are identified during the M2 stage. Within tween 0·14 and 3·66 wt % Cr2O3. There is a broad positive single plagioclase grains, Ca contents increase slightly to- correlation between XMg and Zn (Fig. 13), with spinel inclu- wards those rims adjacent to garnet. sions in Grt1 showing higher XMg (0·40^0·53) and Zn cations (0·355^0·478 p.f.u.) than grains coexisting with Spinel cordierite in the matrix [XMg 0·21^0·43; Zn (p.f.u.) 0·123 ^ Spinel grains in the garnetites are ternary Spl^Hc^Ghn solid 0·412]. This essentially reflects an inverse covariation be- solutions, with variations in terms of Spl (18·0^32·4 mol %), tween the Hc and Ghn components of spinel grains, whereas

1737 JOURNAL OF PETROLOGY VOLUME 54 NUMBER 9 SEPTEMBER 2013

Table 4: Representative compositions of plagioclase

Texture: Included in Grt1 core Included in Grt1 mantle Matrix: Pl þ Grt þ Sil Symplectite: Symplectite: Pl þ Opx þ Grt3 Qz Pl þ Opx þ Grt3 þ Crd þ Qz

Sample: 20-13PL 20-13PL 20-13PL 29’-3PL 20’-7PL 20-9PL1 29-4PL2 29-4PL1 22’-5PL1 22’-5PL3 20-13PL5 20-4PL1 20-4PL2 23-1PL3 (IN)2 (IN)3 (IN)4 (IN)2 (IN)1

SiO2 60·55 60·61 60·30 59·29 59·92 60·98 60·76 60·35 58·44 57·60 58·23 56·02 56·58 55·08

TiO2 0·05 b.d. 0·04 b.d. 0·05 0·06 b.d. 0·04 b.d. b.d. b.d. b.d. b.d. b.d.

Al2O3 25·18 25·17 24·90 25·50 25·22 24·36 25·00 25·17 25·81 26·01 25·51 27·48 27·22 28·89 Downloaded from

Cr2O3 b.d. b.d. b.d. b.d. b.d. 0·04 b.d. b.d. b.d. 0·05 b.d. b.d. b.d. b.d. FeOT 0·08 0·12 0·10 0·33 0·17 0·06 0·05 0·16 0·15 0·20 0·36 0·07 0·21 0·10 CaO 6·40 6·21 6·31 7·34 6·74 6·30 6·37 6·64 8·48 9·00 8·13 9·75 9·47 11·04 NiO b.d. b.d. b.d. b.d. b.d. b.d. b.d. b.d. b.d. b.d. b.d. 0·04 0·07 b.d. ZnO b.d. b.d. b.d. 0·10 0·06 n.d. 0·04 b.d. b.d. b.d. n.d. b.d. b.d. 0·05 http://petrology.oxfordjournals.org/

Na2O7·98 8·07 7·91 7·34 7·76 8·33 7·93 7·82 6·85 6·51 6·84 6·06 6·21 5·41

K2O0·13 0·11 0·06 0·09 0·12 0·06 0·15 0·09 0·09 0·06 0·11 0·07 0·05 0·12 Total 100·37 100·29 99·62 99·99 100·04 100·19 100·3 100·27 99·82 99·43 99·18 99·49 99·81 100·69 Structural formulae normalized to 8 oxygens Si 2·685 2·688 2·692 2·650 2·671 2·709 2·695 2·680 2·620 2·598 2·628 2·531 2·546 2·469 Ti 0·002 b.d. 0·001 b.d. 0·002 0·002 b.d. 0·001 b.d. b.d. b.d. b.d. b.d. b.d. Al 1·316 11·316 ·310 1·343 1·325 1·275 1·307 1·317 1·364 1·382 1·357 1·463 1·444 1·526 Cr b.d. b.d. b.d. b.d. b.d. 0·001 b.d. b.d. b.d. 0·002 b.d. b.d. b.d. b.d. Fe 0·003 0·004 0·004 0·012 0·006 0·002 0·002 0·006 0·006 0·008 0·014 0·003 0·008 0·004

Ca 0·304 0·295 0·302 0·352 0·322 0·300 0·303 0·316 0·407 0·435 0·393 0·472 0·457 0·530 at Institute of Geology and Geophysics, CAS on August 27, 2013 Ni b.d. b.d. b.d. b.d. b.d. b.d. b.d. b.d. b.d. b.d. b.d. 0·001 0·002 b.d. Zn b.d. b.d. b.d. 0·003 0·002 b.d. 0·001 b.d. b.d. b.d. b.d. b.d. b.d. 0·002 Na 0·686 0·694 0·684 0·636 0·671 0·717 0·682 0·673 0·595 0·569 0·598 0·531 0·542 0·470 K0·007 0·006 0·003 0·005 0·007 0·003 0·008 0·005 0·005 0·003 0·006 0·004 0·003 0·007 An 0·305 0·296 0·305 0·354 0·322 0·294 0·305 0·318 0·404 0·432 0·394 0·469 0·456 0·526 Ab 0·688 0·697 0·692 0·641 0·671 0·703 0·687 0·677 0·591 0·565 0·600 0·527 0·541 0·467 Or 0·007 0·006 0·003 0·005 0·007 0·003 0·009 0·005 0·005 0·003 0·006 0·004 0·003 0·007 b.d., below detection limit. nd: not detected. FeOT is total FeO. the Spl component remains reasonably constant at 20^ between them. Fe3þ is low (Fe3þ/Fetotal50·08). There are 30 mol % (Fig. 13). Single spinel grains do not exhibit any no pronounced compositional variations within single zoning or compositional variation. As inferred above from grains (usually51mm). The orthopyroxene grains are consideration of the reaction textures and composite inclu- interpreted to have been produced (with cordierite) at sion features, both spinel grains in Grt1 and the grains in stage M3. the matrix, which generally occur in intergrowths with cordierite, were produced during stage M2, but in different Cordierite microdomains. Cordierites in the garnetites can be divided into four textural types, coexisting with Grt2, Sil þ Spl in symplectites, Orthopyroxene Opx þ Qz þ Grt3 Pl, or alone surrounding Grt1 or silli- The Al2O3 contents of orthopyroxene are lower than 5 wt manite. In all these cases their XMg is similar (Table 7). %. This reflects low formation temperatures in cases However, cordierite grains close to ilmenite, magnetite where the reactant garnets are low in Ca and the product and/or Grt2 show slightly higher XMg (up to 0·85), whereas orthopyroxenes are intermediate in XMg (e.g. Harley, grains close to biotite have lower XMg (0·77). Cordierite VI 1984, 1998b), as is the case here. XMg and Al cations of XMg increases slightly from core to rim when adjacent to orthopyroxene are in the range of 0·55^0·65 and 0·04^0·10 other Fe^Mg minerals, notably garnet. Overall the range p.f.u., respectively (Table 6), with no obvious correlation of XMg in cordierite is from 0·77 to 0·86.

1738 JIAO et al. KHONDALITE BELT GARNETITE

Table 5: Representative compositions of spinel

Texture: Included in Included in Included in Matrix: Spl þ Matrix: Spl þ Matrix: Spl þ Grt1: Splþ Grt1: Splþ Grt1: Sil þ Ilm þ Sil þ Ilm þ Sil þ Grt þ Crd Bt þ Pl Bt Pl þ Grt þ Spl Crd Crd Sample: 20-15SPL- 29-6SPL- 29-3SPL- 23-9SPL2 23-9SPL3 22’-4SPL1 (GRT)1 (GRT)1 (GRT)2

SiO2 0·06 b.d. b.d. 0·07 0·14 b.d.

TiO2 b.d. b.d. b.d. b.d. 0·11 b.d. Downloaded from

Al2O3 61·03 60·46 59·60 58·48 57·81 59·44

Cr2O3 1·11 0·35 0·23 1·66 2·78 0·34 FeOT 11·84 13·55 15·69 23·26 20·85 18·53 MgO 7·45 6·90 5·85 4·96 5·92 6·43 NiO 0·04 b.d. b.d. 0·07 b.d. 0·07 http://petrology.oxfordjournals.org/ ZnO 19·24 18·85 18·14 11·68 12·64 15·76 Total 100·77 100·11 99·51 100·18 100·25 100·57 Structural formulae normalized to 4 oxygens Al 1·990 1·996 1·995 1·955 1·926 1·970 Cr 0·024 0·008 0·005 0·037 0·062 0·008 Fe2þ 0·264 0·313 0·367 0·507 0·418 0·406 Fe3þ 0·010 0·004 0·005 0·045 0·074 0·030 Mg 0·307 0·288 0·248 0·210 0·249 0·270 Zn 0·393 0·390 0·380 0·245 0·264 0·327

XMg 0·529 0·476 0·399 0·275 0·336 0·382 at Institute of Geology and Geophysics, CAS on August 27, 2013 Spl 0·315 0·290 0·248 0·209 0·248 0·261 Hc 0·281 0·319 0·372 0·548 0·490 0·422 Ghn 0·403 0·392 0·380 0·243 0·262 0·317

b.d., below detection limit. FeOT is total FeO. Fe3þ ¼ 2 – Al, Fe2þ ¼ Fe – Fe3þ.

Other minerals corresponding activity^composition (a-x) models are Sillimanite contains minor FeOtotal (0·21^0·54 wt%) and garnet, biotite and silicate melt (White et al., 2007), spinel, 3þ Cr2O3 (0·10^0·32 wt%). Fe is calculated to be lower orthopyroxene and magnetite (White et al., 2002), ilmenite than 0·013 cations (p.f.u.). Ilmenite usually contains minor (White et al., 2000), cordierite (Holland & Powell, 1998), MgO (c.0·15 w t % ), M n O (0 ·12 ^0·17wt %), and Cr2O3 and plagioclase (Holland & Powell, 2003). Aluminosili- (0·13 ^ 0 ·17wt %). Rutile exhibits a range of compositions cates and quartz are modeled as pure phases. in terms of Zr (c. 500^5000 ppm), Nb (c. 2000^5000 ppm), and Cr (c. 400^600 ppm) (Table 8). Determining bulk compositions Whole-rock bulk compositions were determined by Axils X-ray fluorescence spectrometry (XRF) using a PHASE EQUILIBRIA AND CLAISSE M4 Fluxer at the Institute of Geology and Geophysics, Chinese Academy of Sciences. The detected THERMOMETRY CALCULATIONS 3þ elements are Si, Ti, Al, TFe 3 þ (total Fe ), M n, C a, Na, K Phase equilibria calculations and P. FeO was obtained by colorimetry (chemical titra- P^T pseudosections for each stage involving M1,M2a, tion with potassium permanganate), preceded by digestion M2b, and M3 have been modeled using THERMOCALC in a multi-acid mixture (HF^H2SO4^H3BO4)inanon- 3.33 (Powell & Holland, 1988; updated in October 2009) oxidizing environment. The whole-rock composition of together with the internally consistent thermodynamic sample dsz20 is used for P^T pseudosection modeling of dataset (created in November 2003) of Holland & Powell the major mineral assemblage at stage M1 (Table 9). (1998) (Figs 14^17). The calculations were preformed in During post-peak processes, especially cooling, the ef- the NCKFMASHTO (Na2O^CaO^K2O^FeO^MgO^ fective bulk composition of the equilibrium volume (the Al2O3^SiO2^H2O^TiO2^Fe2O3) system, and the volume of rock that, at a given P and T, reacts to be in

1739 JOURNAL OF PETROLOGY VOLUME 54 NUMBER 9 SEPTEMBER 2013

Table 6: Representative compositions of orthopyroxene Table 7: Representative compositions of cordierite

Sample: 23-1OPX65 23-15OPX7 23-1OPX7 23-1OPX8 23-7OPX2 Sample: 20-12CRD2 20-8CRD1 20-11CRD6 31-1CRD2

SiO2 51·10 50·30 51·22 51·67 50·31 SiO2 50·18 49·38 50·22 50·01

TiO2 0·05 0·05 0·04 0·06 0·04 TiO2 0·07 b.d. b.d. b.d.

Al2O3 3·85 4·59 4·00 3·36 4·81 Al2O3 34·04 33·46 33·45 33·73 T Cr2O3 0·18 0·08 0·11 0·06 0·25 FeO 4·35 4·77 4·82 4·65 FeOT 24·30 23·75 24·56 24·28 24·90 MnO b.d. b.d. b.d. 0·03 MnO 0·05 0·05 0·07 0·07 0·07 MgO 10·61 10·18 10·46 10·62 Downloaded from MgO 20·40 20·35 20·55 21·09 19·62 CaO b.d. 0·02 b.d. b.d. CaO 0·08 0·10 0·08 0·08 0·08 NiO b.d. 0·03 0·03 b.d. ZnO 0·18 0·16 0·28 0·03 0·13 ZnO 0·04 b.d. 0·03 b.d.

Total 100·19 99·43 100·91 100·7100·21 Na2O0·08 0·03 0·04 0·05 Structural formulae normalized to 6 oxygens Total 99·37 97·87 99·05 99·09 http://petrology.oxfordjournals.org/ Si 1·913 1·894 1·906 1·921 1·889 Structural formulae normalized to 18 oxygens Ti 0·001 0·001 0·001 0·002 0·001 Si 5·012 5·018 5·042 5·017 AlIV 0·087 0·106 0·094 0·079 0·111 Ti 0·005 b.d. b.d. b.d. AlVI 0·083 0·098 0·082 0·069 0·102 Al 4·007 4·007 3·958 3·987 Cr 0·005 0·002 0·003 0·002 0·007 Fe 0·363 0·405 0·405 0·390 Fe 0·761 0·748 0·764 0·755 0·782 Mn b.d. b.d. b.d. 0·003 Mn 0·002 0·002 0·002 0·002 0·002 Mg 1·580 1·542 1·566 1·588 Mg 1·138 1·142 1·140 1·169 1·098 Ca b.d. 0·002 b.d. b.d. Ca 0·003 0·004 0·003 0·003 0·003 Ni b.d. 0·002 0·002 b.d.

Zn 0·005 0·004 0·008 0·001 0·004 Zn 0·003 b.d. 0·002 b.d. at Institute of Geology and Geophysics, CAS on August 27, 2013

XMg 0·599 0·604 0·599 0·608 0·584 Na 0·015 0·006 0·008 0·010

XMg 0·813 0·792 0·795 0·803 FeOT is total FeO. b.d., below detection limit. FeOT is total FeO. chemical equilibrium) will change as a result of decreasing diffusion rates (Stu« we, 1997). Furthermore, in the pelitic 7.0 imaging (Table 9). Proportions of Fe3þ to Fe2þ for each system, garnet is usually the slowest diffusing phase, de- microdomain composition were retrieved from Fe-bearing veloping zoning profiles during cooling. Hence, the re- minerals for which Fe3þ/Fe2þ was estimated using stoichio- moval of garnet core compositions from the reacting metric charge balance (Spear & Kimball, 1984). C-Fig. 15 equilibrium volume will cause changes to the effective inTable 9 refers to the composition used for the calculation bulk composition (Stu« we, 1997). In the present case of the of Fig. 15, and the corresponding local domain for this is garnetite, the large grain size (usually42 mm) and strong the dotted rectangle in Fig. 6g, which is calculated to repre- compositional zoning in the garnet poikiloblasts (Fig. 9) sent the stage M2a equilibrium microdomain. C-Fig. 16 suggest that successively greater volumes of the M1 garnet in Table 9 refers to the composition used for calculating have been effectively removed from the reacting equilib- Fig. 16, and the local area for this is that inside the dotted rium volume of the rock. Thus, the whole-rock bulk com- rectangle of Fig. 7d, which represents the equilibrium position is not appropriate for the modeling of every stage microdomain for M2b. The local area for modeling stage (i.e. M2a,M2b and M3) of the metamorphic history M3 P^Tconditions (Fig. 17) is that inside the dotted rect- (Powell & Holland, 2008).We have therefore taken the ap- angle in Fig. 8a, labeled as C-Fig. 17 inTable 9. proach of using the local bulk compositions of single Calculation of meaningful P^T pseudosections requires microdomains to model each stage separately (e.g. Warren estimation of the water content in the bulk composition. & Waters, 2006; Groppo et al., 2009a, 2009b;A¤ lvarez- This is not a trivial issue, as in a melt-bearing system Valero & Waters, 2010). it will depend upon the amount of melt gained, lost or Microdomain chemical compositions were estimated transferred through the domain of interest during its equi- utilizing the mineral compositions of the phases involved librium. A series of T^X (H2O mol % in rock) pseudosec- (from EMP) and their volume proportions using area esti- tions (Figs 14a, 15a, 16a and 17a) were calculated for each mation software by Jinhui Pan, coupled with Photoshop stage at different pressures, to determine water contents in

1740 JIAO et al. KHONDALITE BELT GARNETITE

the P^T conditions of different metamorphic stages (e.g. Warren & Waters, 2006; Groppo et al., 2009a, 2009b; A¤ lvarez-Valero & Waters, 2010).This is because the hetero- geneous nature of the rock, especially high-grade rocks with variably coarse grain sizes, makes it unlikely that the whole-rock composition can reflect the whole metamorphic evolution if local equilibrium has occurred and growth-zoned minerals with relict cores are present. As discussed by Stu« we (1997), it is impossible to identify the realistic equilibrium volume and therefore the effective bulk composition exactly. The equilibrium volume will Downloaded from generally be of different sizes for different elements, and some elements may be derived from outside the physically defined microdomain. For example, the equilibrium volume for Fe and Mg, with their faster cation diffusion rates, would be larger than that of Ca. When selecting the http://petrology.oxfordjournals.org/ Fig. 10. Ternary Prp^Alm þ Sps^Grs diagram showing the variation equilibrium volume, if Ca equilibrium can be demon- in composition of garnet grains during the stages of garnetite strated on the basis of a lack of Ca zoning in the partici- formation. pant phases, then Fe^Mg will most probably be in equilibrium too. Thus the kinetics of elements shared by the coexisting minerals is controlled by the slowest diffus- the systems. Taking into account both the chemical com- ing species in the system (Berger et al., 2005). As grain positions and volume percentages of the minerals analyzed boundary diffusion is much more rapid than volume diffu- in thin section, water contents were then determined to sion, it is appropriate to include those phases far removed ensure good comparability between the observed and cal- from the reaction site that can easily be accessed along culated mineral data (the shaded areas in Figs 14a, 15a,

grain boundaries (Stu« we, 1997). Moreover, fluids and de- at Institute of Geology and Geophysics, CAS on August 27, 2013 16a and 17a), which are discussed in detail below. Finally, formation may also affect the size of the equilibrium the water contents in each system were constrained using volume. Fluid infiltration will enlarge the equilibrium a different approach as a test of consistency and sensitivity. volume, but is subject to cooling rate like diffusion. As The water contents in the residual solids can be regarded long as no mass transfer occurs associated with fluid infil- as being mainly preserved in cordierite and biotite, as tration, the system can still be regarded as closed on the little former liquid now exists (51·0 mol %; Figs 15 and scale of the equilibrium volume under consideration 17) in the garnetite, shown by the bulk-rock composition (Stu« we, 1997). In addition, the lack of knowledge about (SiO -poor and FeO-, MgO-, Al O -rich; Table 9). 2 2 3 three-dimensional grain size and shape in the analyzed Furthermore, White & Powell (2002) demonstrated that domain will add to the uncertainties in the estimate of the the removal of nearly all of the melt at a temperature bulk composition. above the breakdown of biotite was required for the preser- In this study, the products of the reaction in all the se- vation of the peak mineral assemblage in an aluminous lected local domains are unzoned or only weakly zoned, pelite. In the present case, biotite is secondary and shows and the relict core of the poikiloblastic M garnet is a small volume percentage in each modeling approach 1 excluded from the local domain (Figs 6g, 7d and 8a). To (Table 9). Harley et al. (2002) proposed that cordierite in account for grain boundary diffusion, the rims of the sur- equilibrium with granitic melt contains 0·4^1·6wt % rounding grains (e.g. garnet) are regarded as part of the H O. Thus, we have used the cordierite volume percent- 2 equilibrium volume. As the garnetite preserved a granu- ages and their water contents from Harley et al.(2002)to lite-facies mineral assemblage such as Grt þ Sil þ Qz þ estimate minimum water contents and compare them Pl þ Bt at stage M , the high-grade metamorphism pre- with those calculated using the T^X (H Omol%in 1 2 cluded the existence of a free fluid along grain boundaries, rock) pseudosections to constrain the final water contents although melt might be present. In addition, no deform- to be used in the P^T pseudosections (Figs 14b, 15b, 16b ation structures have been identified. and 17b). As emphasized by A¤ lvarez-Valero & Waters (2010), a somewhat arbitrary choice of the boundary of the Evaluation of the effective bulk local domain does not in most cases significantly affect composition of the microdomains the P^T conditions, which mainly rely on matching the Several recent studies have demonstrated that it is essential observed and analyzed phase compositions and modes in to use different microdomain compositions to constrain the calculated phase equilibria, no matter what volume

1741 JOURNAL OF PETROLOGY VOLUME 54 NUMBER 9 SEPTEMBER 2013 Downloaded from http://petrology.oxfordjournals.org/ at Institute of Geology and Geophysics, CAS on August 27, 2013

Fig. 11. A chemical zoning profile of Grt1in garnetite, showing the variation of pyrope, almandine, grossular and spessartine along the marked traverse.

has been defined. A¤ lvarez-Valero & Waters (2010) also contribution. Based on their calculations of the sensitivity showed that the area measurement of the local domain of field boundaries in the phase equilibra to uncertainty may exaggerate the compositional contribution from a in the bulk composition estimation, they concluded that porphyroblast core, and under-represent the matrix enve- low-variance assemblages were insensitive, but high-vari- lope contribution compared with its likely volume ance assemblage boundaries were more uncertain.

1742 JIAO et al. KHONDALITE BELT GARNETITE Downloaded from http://petrology.oxfordjournals.org/ at Institute of Geology and Geophysics, CAS on August 27, 2013

Fig. 12. The chemical compositions of biotite from garnetites plotted in terms of (a) XMg^Ti (p.f.u.), (b) Mg þ Fe þ Mn (p.f.u.)^Ti (p.f.u.), (c) XMg^Cl (p.f.u.), and (d) XMg^F (p.f.u.).

When the prograde history of a rock has involved melt change significantly in different scenarios of melt loss be- loss, it is necessary to add the melt composition back into cause they are largely controlled by the solid phases. This the observed rock composition before attempting model- general result was also emphasized by White & Powell ling by pseudosections (White & Powell, 2002; White (2002). Based on this, we consider that the precise compos- et al., 2004), if the objective is to constrain the prograde ition of the lost melt and timing of its loss will have only a P^T conditions of the rock prior to reaching the melting small effect on the P^Testimates for this suprasolidus as- interval (e.g. White et al., 2004; Indares et al., 2008; semblage (M1). Groppo et al., 2012). As shown in Table 9, the garnetite has aSiO2-poor and FeO-, MgO-, Al2O3-rich refractory com- P^T conditions from phase equilibria position, consistent with it being a residue after the re- calculations moval of considerable melt rich in K2O and Na2O. Determination of the P^T conditions of granulite-facies However, it is difficult to evaluate when the melt loss rocks is difficult owing to the fact that diffusion is fast at occurred; for example, whether during or after formation high temperatures, and the compositions of minerals that of the garnet poikiloblasts (i.e. M1). Nevertheless, it is evi- are interpreted to have once been in equilibrium will con- dent from both the textural features of the garnetite and tinue to be affected by cation exchange (e.g. Fe^Mg in bio- its structural setting that the M1 assemblage was suprasoli- tite) until a closure temperature is reached (Powell & dus and coexisted with a melt phase (Fig. 8e and f). Holland, 2008). Thus, the minerals formed during granu- Indares et al. (2008) have shown from forward modeling lite-facies metamorphism, especially the Fe^Mg minerals, of melt loss that the main suprasolidus topologies do not do not in general record their peak metamorphic

1743 JOURNAL OF PETROLOGY VOLUME 54 NUMBER 9 SEPTEMBER 2013

the water contents in the system. Contours of Ca cations in garnet are shown in the Grt þ Sil þ Qz þ Pl þ Bt field. Figure 14b shows the P^T pseudosection of sample dsz20 over the range 750^10508C and 6^13kbar, which is used to constrain the P^T conditions of stage M1. Isopleths of garnet volume fraction and Ca (cations, p.f.u.) contents in garnet are shown in the fields of Grt þ Sil þ Qz þ Pl þ Bt and/or Grt þ Sil þ Qz þ Pl þ Bt þ Kfs. Comparing the observed mineral assemblages with the calculated results, the M1 stage is in the field of Grt þ Sil þ Qz þ Pl þ Bt in both figures, with garnet volume fractions greater than 70% (Fig.14b). Based on the calculated average Ca cations Downloaded from in Grt1, the M1 stage would locate in a field at 800^8608C and 0·7^1·0mol% H2O(at9·0 kbar, Fig. 14a), and 800^ 8508C and 8·5^9·5 kbar (at 0·794 mol % H2O, Fig. 14b), ignoring the inherent uncertainties on the isopleth pos- http://petrology.oxfordjournals.org/ Fig. 13. The chemical compositions of spinel in garnetite plotted in itions in P^T space. The influence of water content on the term of XMg^Zn (p.f.u.), and (inset) a ternary Spl^Ghn^Hc diagram system is small (Fig. 14a). Based on the measured grossular (in molar fraction). content of the Grt1 cores (Ca cations c.0·035; Fi g. 11) it would appear that Grt1 started to form at c.10kbar. compositions. Garnet is regarded as the most resistant mineral to diffusion (Florence & Spear, 1991; Stu« we, 1997). P^Tconditions of stage M2a As shown in Fig. 11, the compositional zoning of garnet (c. Figure 15a shows the T^X (H2O mol % in rock) pseudo- 3·5 mm) shows clear signs of diffusional re-equilibration, section for stage M2a over the range 0·1^5·0mol% H2O with almandine increasing and pyrope decreasing near and 750^9508C, used to determine precise water contents the rim. Ca diffusivities are several orders of magnitude in the system. Figure 15b presents the calculated P^T pseu- slower than those of Fe and Mg (Tuccillo et al., 1990; dosection over the range 700^10008C and 6^10kbar, used at Institute of Geology and Geophysics, CAS on August 27, 2013 Schwandt et al., 1996; Vielzeuf et al., 2007), and conse- to constrain P^T conditions of stage M2a in a local quently the more distinct zoning pattern in grossular con- domain shown by the dotted rectangle in Fig. 6g. Isopleths tent in the garnet, decreasing continuously and of melt, garnet, and cordierite volume fractions and Ca significantly from the core towards the rim, is unlikely to cations in garnet are shown for the multivariant field result from diffusion, but rather is due to growth (Fig. 11) Grt2 þ Sil þ Bt þ Crd that corresponds to the M2a mineral with smoothing by later diffusion. This is consistent with assemblage in both figures. When considering the pro- calcium variations in plagioclase inclusions, which increase portions of the major minerals Grt2 and cordierite (see towards garnet rims (Table 4) and hence suggest garnet^ Table 9), as well the Ca cations in garnet in this corona, plagioclase equilibrium during garnet growth. In addition, the field for this M2a assemblage is calculated as 850^ the limited length-scales for diffusion are also supported 8608C and 1·7^2·0mol%H2O(at7·6 kbar; Fig. 15a) and by the regularly spaced symplectite or corona nature 850^8658C and 7·4^7·6kbar (at 1·768 mol % H2O; of the replacement products after garnet or sillimanite Fig. 15b). These P^T estimates will retract to lower tem- (Figs 6^8). peratures, and more melt will be present in the system, if Taking these factors into account, thermobarometric de- higher H2O contents are assumed. However, as shown in termination of the P^T conditions of garnetites in this Fig. 15a, the water contents constrained using the propor- study mainly relies on slower-diffusing cations, such as Al- tions and chemical compositions of the participating min- in-orthopyroxene and Ca-in-garnet, and plagioclase, as erals are rather restricted, so the effect of different well the proportions of the analyzed phases in the calcu- estimated water contents within the available range is lim- lated pseudosections, but avoids use of the faster-diffusing ited and can be ignored. cations such as Fe and Mg in ferromagnesian minerals, especially biotite. The estimated P^T conditions of each P^Tconditions of stage M2b stage presented in the following section are regarded as Figure 16a shows the T^X (H2O mol % in rock) pseudo- minima. section for stage M2b over the range 0·1^8·0mol% H2O and 750^9508C, used to determine precise water contents P^Tconditions of stage M1 in the system. Figure 16b presents the P^T pseudosection Figure 14a shows the T^X (H2O mol % in rock) pseudo- over the range 800^11008C and 5^9 kbar, which has been section for stage M1 over the range 0·1^5·0mol% H2O used to constrain the P^T conditions of stage M2b. and 750^10508C. This has been used to closely constrain The local domain is the dotted rectangle area shown in

1744 JIAO et al. KHONDALITE BELT GARNETITE

Table 8: Representative compositions of rutile and the calculated Zr-in-rutile temperatures

Composition: wt % ppm Temperature (8C)

Sample TiO2 FeO Cr2O3 Nb2O5 ZrO2 Total Zr Nb Cr Tomkins et al. Ferry & (2007) (9 kbar) Watson (2007)

09DSZ19-4-3 98·52 0·13 0·06 0·51 0·23 99·46 1678 3568 437 808 807 09DSZ19-8-1 98·37 0·14 0·08 0·37 0·57 99·54 4209 2601 570 920 920 09DSZ19-8-2 98·44 0·12 0·08 0·37 0·56 99·58 4155 2593 565 918 919 09DSZ19-8-3 98·16 0·14 0·08 0·38 0·56 99·32 4126 2645 568 917 918 Downloaded from 09DSZ19-6-1 98·66 0·13 0·08 0·34 0·25 99·47 1841 2409 546 818 817 09DSZ19-6-2 98·55 0·07 0·08 0·35 0·29 99·34 2145 2466 516 835 835 09DSZ19-6-3 98·30·13 0·08 0·57 0·30 99·38 2210 4009 543 839 838 09DSZ19-12-1 98·78 0·07 00·08 ·39 0·24 99·56 1779 2696 553 814 813

09DSZ19-12-2 98·78 0·18 0·08 0·43 0·20 99·66 1445 2977 521 791 790 http://petrology.oxfordjournals.org/ 09DSZ20’-2-3 98·58 0·39 0·07 0·40 0·07 99·53 545 2797 473 696 694 09DSZ20’-3-1 98·49 0·17 0·07 0·54 0·27 99·57 2016 3803 497 828 828 09DSZ20’-3-2 98·47 0·16 0·08 0·50 0·33 99·57 2473 3501 519 852 852 09DSZ20’-3-3 98·24 0·15 0·08 0·60 0·14 99·23 1029 4168 536 756 755 09DSZ20’-5-1 98·71 0·15 0·06 0·31 0·44 99·69 3247 2152 418 886 886 09DSZ20’-5-2 98·66 0·40 0·06 0·36 0·37 99·89 2756 2525 432 865 865 09DSZ22-5-1 98·25 0·10 0·10 0·64 0·12 99·23 922 4481 715 745 744 09DSZ22-5-2 98·01 0·13 0·09 0·68 0·17 99·11 1250 4737 636 776 775 09DSZ22-5-3 98·22 0·15 0·09 0·60 0·17 99·25 1284 4172 624 779 778 at Institute of Geology and Geophysics, CAS on August 27, 2013 09DSZ22-4-2 98·93 0·07 0·08 0·45 0·14 99·69 1019 3140 536 755 754 09DSZ22-4-3 98·82 0·12 0·07 0·41 0·08 99·53 626 2854 457 708 707 09DSZ22-4-4 98·26 0·08 0·08 0·50 0·17 99·14 1271 3493 537 778 777 09DSZ22-4-5 98·58 0·08 0·08 0·56 0·22 99·54 1595 3923 530 802 801 09DSZ22-4-6 98·27 0·23 0·08 0·73 0·07 99·41 513 5119 566 690 689 09DSZ22-12-4 98·31 0·08 0·07 0·39 0·19 99·07 1390 2757 463 787 786 09DSZ22-12-5 98·83 0·07 0·07 0·38 0·15 99·52 1115 2669 454 764 763 09DSZ22-12-6 98·54 0·09 0·06 0·40·09 99·22 655 2806 411 712 711 09DSZ29’-11-1 98·17 0·40·06 0·46 0·799·82 5176 3236 430 949 949 09DSZ29’-11-2 98·18 0·23 0·06 0·49 0·61 99·59 4497 3434 398 929 930 09DSZ29’-11-3 97·92 0·44 0·06 0·49 0·799·65 5165 3439 417 948 949

Fig.7d. Isopleths of Ca cations in garnet, and volume frac- the phases involved in this local domain constrain the tions of sillimanite, cordierite, garnet and spinel are water content to between 2·2 and 3·1mol %. shown in the relevant M2b assemblage field of Grt þ Sil þ Crd þ Bt þ Spl þ Liq in both figures. Estimates of P^Tconditions of stage M3 900^9308C and 2·2^3·1mol % H2Oat7·0 kbar, and Figure 17a shows the T^X (H2O mol % in rock) pseudo- 910^9258C and 6·8^7·1kbar at an H2O content of 2·53 mol section for stage M3 over the range 0·1^5·0mol% H2O % are obtained respectively from the pseudosection calcu- and 650^8008C, used to determine the water contents in lations (Fig. 16a and b), based on comparison of modeled the system. Figure 17b shows the P^T pseudosection over and observed volume fractions of the participating phases the range 700^8508C and 5^8 kbar, which has been used (Table 9). As shown in Fig. 16a, the assemblage of to constrain the P^T conditions of stage M3, relevant to Grt þ Sil þ Crd þ Bt þ Spl þ Liq would be restricted to the local domain shown by the dotted rectangle in Fig. 8a. higher temperatures (49008C) if a lower water content is Isopleths of melt, orthopyroxene, quartz and/or cordierite estimated in the system. However, the volume fractions of and garnet volume proportions, Ca cations in plagioclase,

1745 JOURNAL OF PETROLOGY VOLUME 54 NUMBER 9 SEPTEMBER 2013 Downloaded from http://petrology.oxfordjournals.org/ at Institute of Geology and Geophysics, CAS on August 27, 2013

Fig. 14. (a) T^X (H2O mol % in rock) pseudosection calculated to determine the water content in the system at 9·0 kbar, for the same bulk composition as in (b), except that the H2O mol % is from 0·1to5·0mol%.(b) P^T pseudosection calculated for the M1 mineral assemblage of the garnetite with the bulk composition in molar fractions: H2O ¼ 0·794, SiO2 ¼45·992, Al2O3 ¼15 ·377, CaO ¼1·240, MgO ¼12·917, Fe O ¼ 21 ·181, K 2O ¼ 0·466, Na2O ¼ 0·188, TiO 2 ¼1·012, O ¼ 0·833 in the NCKFMASHTO system. Also shown are isopleths of Ca cations in garnet [Ca in Grt ¼ Ca/(Ca þ Fe þ Mg) in mol], and garnet volume fraction.

1746 JIAO et al. KHONDALITE BELT GARNETITE Downloaded from http://petrology.oxfordjournals.org/ at Institute of Geology and Geophysics, CAS on August 27, 2013

Fig. 15. (a) T^X (H2O mol % in rock) pseudosection calculated to determine the water content in the system at 7·6 kbar, for the same bulk composition as in (b), except that the H2Omol%isfrom0·1to5·0 mol %. (b) P^T pseudosection calculated for the M2a mineral assemblage of the garnetite with a bulk composition in molar fractions: H2O ¼1·768, SiO2 ¼67·203, Al2O3 ¼14·614, CaO ¼ 0·489, MgO ¼ 7·431, Fe O ¼ 8·072, K2O ¼ 0·103, Na 2O ¼ 0·028, TiO2 ¼0·240, O ¼ 0·052 in the NCKFMASHTO system. Also shown are isopleths of garnet, cordierite and melt volume fractions and Ca cations in garnet.

1747 JOURNAL OF PETROLOGY VOLUME 54 NUMBER 9 SEPTEMBER 2013 Downloaded from http://petrology.oxfordjournals.org/ at Institute of Geology and Geophysics, CAS on August 27, 2013

Fig. 16. (a) T^X (H2O mol % in rock) pseudosection calculated to determine the water content in the system at 7·0 kbar, for the same bulk composition as in (b), except that the H2Omol%isfrom0·1to8·0 mol %. (b) P^T pseudosection calculated for the M2b mineral assemblage of the garnetite with a bulk composition in molar fractions: H2O ¼ 2·534, SiO2 ¼47·183, Al 2O3 ¼24·820, CaO ¼ 0·361, MgO ¼12·910, Fe O ¼11 ·231, K2O ¼ 0·108, Na 2O ¼ 0·010, TiO 2 ¼0·778, O ¼ 0·065 in the NCKFMASHTO system. Isopleths of Ca cations in garnet, and garnet, spinel, cordierite, sillimanite volume fractions are shown.

1748 JIAO et al. KHONDALITE BELT GARNETITE Downloaded from http://petrology.oxfordjournals.org/ at Institute of Geology and Geophysics, CAS on August 27, 2013

Fig. 17. (a) T^X (H2O mol % in rock) pseudosection calculated to determine the water content in the system at 6·6 kbar, for the same bulk composition as in (b), except that the H2Omol%isfrom0·1to5·0 mol %. (b) P^T pseudosection calculated for the M3 mineral assemblage of the garnetite with a bulk composition in molar fractions: H2O ¼1·768, SiO2 ¼55·353, Al2O3 ¼9·027, CaO ¼1·328, MgO ¼15 ·576, Fe O ¼15 ·974, K2O ¼ 0·072, Na2O ¼ 0·678, TiO2 ¼0·038, O ¼ 0·186 in the NCKFMASHTO system. Also shown are isopleths of garnet, cordierite, orthopyroxene, quartz and melt volume fractions, Ca cations in plagioclase and VIAl cations in Opx.

1749 JOURNAL OF PETROLOGY VOLUME 54 NUMBER 9 SEPTEMBER 2013

Table 9: Chemical compositions of garnetite isopleth, the temperature of stage M3 could be lower than 6808C given the range in the chemical compositions of the orthopyroxene (Table 6). Variations within the allow- Sample: 09DSZ20- C-Fig. 15 C-Fig. 16 C-Fig. 17 able range of water contents have little impact on the cal- Fig. 14 culated conditions, which instead are limited by VIAl in Stage: M1 M2a M2b M3 orthopyroxene of 0·10 in Fig. 17a. (wt %) Whole-rock Local Local Local domain domain domain Thermometry estimates Results from Zr-in-rutile thermometerça further constraint SiO2 40·82 61·97 41·03 54·07 on M1 conditions TiO2 1·19 0·29 0·90 0·05 The Zr-in-rutile thermometers of the Ferry & Watson Downloaded from Al O 23·16 22·87 36·63 14·96 2 3 (2007) and Tomkins et al. (2007) models (at 9 kbar) were Fe O 1 95 0 13 0 15 0 48 2 3 · · · · applied to the rutile grains in the garnetites (Table 8, FeO 20·63 8·90 11·68 18·66 Fig. 18). We prefer the Tomkins et al. (2007) model as it in- MnO 0·20 0·08 0·08 0·16 cludes a pressure correction term, consistent with the MgO 7·66 4·60 7·53 10·21

volume change of the governing reaction between rutile, http://petrology.oxfordjournals.org/ CaO 1·02 0·42 0·29 1·21 zircon, and quartz. The rutile grains in garnetites are ZnO n.d. 0·00 0·61 0·00 mostly present as inclusions in Grt1 or in contact with Na O0·17 0·03 0·01 0·68 2 Grt1 in the matrix, and hence are attributed to stage M1. K2O0·65 0·15 0·15 0·11 The estimate of 9 kbar for rutile formation is based on the P O 0·09 n.d. n.d. n.d. 2 5 Ca cations isopleth for Grt1 in the M1 assemblage (Fig. 14). LOI –0·48 n.d. n.d. n.d. For this pressure the Zr temperatures of the measured Total 97·06 99·44 99·07 100·58 rutile grains are in the range 690^9498C. The point-to- Mg/(Mg þ FeTotal)0·38 0·48 0·53 0·49 point uncertainty on the temperature estimate is less than Al/(Al þ FeTotal þ Mg) 0·48 0·65 0·67 0·36 58C, as the precision on the measured Zr contents in

Mineral (mol) rutile is less than 20 ppm. Rutile grains that are neither at Institute of Geology and Geophysics, CAS on August 27, 2013 Grt 0·259 0·269 0·339 replaced by ilmenite nor grown adjacent to zircon grains Sil 0·170 0·210 yield high temperatures (Fig. 18a^c). One rutile included Qz 0·351 0·002 0·139 in garnet (Fig. 18b) has a Zr content of c. 4200 ppm and Bt 0·015 0·015 0·010 thus records a temperature of c. 9208C. Another grain Pl 0·098 included in garnet (Fig. 18c) has the highest Zr content Ilm 0·004 0·016 (up to c. 5200 ppm) and gives the highest temperature of Crd 0·200 0·446 0·117 9498C. In other textural settings, and in particular where Spl 0·041 other oxides have formed, the rutile grains yield lower Opx 0·298 temperatures owing to the effects of retrograde net-transfer of Zr cations and nucleation of late zircon. An example of (See text for an explanation of C-Fig.xx.) n.d., not deter- this is seen in Fig. 18d, in which rutile located between mined. LOI, loss on ignition. Grt1 and plagioclase has been partly replaced by ilmenite accompanied by the production of fine-grained zircon. In summary, the Zr-in-rutile temperatures obtained for the IV and Al cations in orthopyroxene are shown for the rele- least altered, usually isolated rutile grains reflect the peak vant M3 assemblage fields in both figures. The measured or near-peak temperature conditions at stage M1,which mineral volume fractions and compositions at this stage were c. 820^9498C at 9 kbar. These thermal conditions (IVAl in orthopyroxene50·10; average Ca in Pl3 0·43; overlap with those obtained from pseudosection modeling, Table 9) lead to estimates of 710^7208C and 1·6^3·1mol % but are shifted to higher temperature in the case of the H2Oat6·6 kbar (Fig. 17a), and P^T estimates of highest rutile Zr content analyses (Fig. 19). 710^7208C and 6·4^6·6 kbar at the assumed water content As shown in the modeled P^T pseudosection for M1 of 1·768 mol % used in the calculations of Fig. 17b. (Fig. 14b), the rutile-in line does not appear until the pres- As the highest IVAl in orthopyroxene value detected by sure exceeds 11 kbar in the kyanite-bearing field, which is EMP has been used, the P^T conditions obtained consti- inconsistent with our observations that rutile was one of tute the maxima for stage M3. Below 7108C the domain is phases coexisting with M1 garnet poikiloblasts and that H2O-saturated for the assumed water content, meaning no kyanite was present in M1. The rutile-in line is also in- that fluid would be a phase in excess in the system. As compatible with the measured garnet composition, in shown in Fig. 17a, based on the 0·09 IVAl in orthopyroxene which Ca cations in garnet are less than 0·035. This

1750 JIAO et al. KHONDALITE BELT GARNETITE Downloaded from http://petrology.oxfordjournals.org/ at Institute of Geology and Geophysics, CAS on August 27, 2013

Fig. 18. Photomicrographs and back-scattered electron images showing the textural setting of rutile grains and the calculated Zr-in-rutile temperatures. The number before * is the concentration of Zr in rutile (in ppm); the number after the * is the calculated temperature (in 8C) fromTomkins et al. (2007) at 9 kbar. contradiction might result from the high trace element profiles of Grt1 (Jiao et al., unpublished data) also suggest concentrations (Fe, Zr, Nb and Cr) in the rutile (Table 8), that it formed in a melt-bearing environment (Rubatto, which may have enhanced its stability to lower-P and 2002; Hermann & Rubatto, 2003; Whitehouse & Platt, higher-T conditions compared with those estimated by 2003). Moreover, extensive partial melting of metasedi- phase equilibria modeling in the NCKFMASHTO mentary rocks has taken place in this area to produce system. Kelsey & Powell (2011) have built an a^x model S-type granitoids (Peng et al., 2012). Recognition of the for zircon as well as Zr-bearing silicate melt, garnet and former presence of melt provides the rationale for calcula- rutile. Thus, in principle phase diagrams that explicitly in- tion of suprasolidus pseudosections for all the metamorphic clude ZrO2 in the bulk composition can be calculated to stages of the garnetite (Figs 14^17). investigate the stability of rutile in the NCKFMASHTO(ZrO2) system. We expect that this type Effect of Zn in spinel of work will be extended to allow calculation of rutile sta- The high Zn content in spinel grains from the garnetites bility in more realistic systems that incorporate the effects will undoubtedly affect the P^T conditions displayed in of those impurities that may be significant in this phase. pseudosections constructed for spinel-bearing assemblage domains (i.e. Fig. 16). Unfortunately, at present there is no valid ZnKFMASHTO system in which to consider the DISCUSSION AND CONCLUSIONS Zn-bearing bulk composition quantitatively. However, the Evidence for syn-M1 melting experimental ZnFMASH system and Schreinemaker’s Melt pseudomorphs occurring in the garnetite suggest the constraints from the qualitative ZnKFMASHT system in- existence of melt during the formation of the garnet poiki- dicate that the stability fields of all spinel-bearing mineral loblast (Fig. 8e and f). The bell-shaped Y and HREE assemblages move to lower temperatures by addition of

1751 JOURNAL OF PETROLOGY VOLUME 54 NUMBER 9 SEPTEMBER 2013 Downloaded from http://petrology.oxfordjournals.org/ at Institute of Geology and Geophysics, CAS on August 27, 2013

Fig. 19. P^T projection showing the P^T path determined in this study for garnetite (rectangles indicating P^Testimates and black bold lines), and previous metamorphic P^T paths depicted by other researchers from the Khondalite Belt. 1, High-pressure metapelites from the Helanshan terrane (Yin et al., unpublished data); 2, high-pressure metapelites from the Qianlishan terrane (Yin et al., 2007); 3, high-pressure metapelites from the Jining terrane (Wang et al., 2011); 4, silica-undersaturated sapphirine-bearing granulites from the Daqingshan terrane (Guo et al., 2012); 5, sapphirine-bearing granulites from the Jining terrane (Santosh et al., 2009a).

Zn, because of the strong partitioning of Zn into spinel the Zn effect in the pseudosection. According to the spinel over all other phases in the absence of staurolite (Nichols chemical compositions analyzed by EMP (Table 5), it ap- et al., 1992; Hand et al., 1994). Nichols et al. (1992) further pears that spinel with ZnO2 contents of c. 13wt % will noted that Zn effectively stabilized spinel to higher pres- shift the temperature to 608C higher, and the pressure c. sures in the garnet field and to lower pressures in the cor- 0·5 kbar lower. dierite field. Given the likely effects of Zn, the P^T As noted in several previous reviews (Waters,1991;Harley, conditions of stage M2b, which on the pseudosections 1998a), t h e Sp l þ Qz assemblage in metapelites maybe an in- (Fig. 16) are shifted to higher temperature and lower pres- dicator of UHT metamorphism (higher than 9008C). sure in comparison with the P^T conditions of M2a However, the presence of Zn may stabilize the Spl þ Qz as- (Fig. 15), must be regarded with some caution. M2a and semblage to lower granulite-facies conditions (Stoddard, M2b are two substages of M2 occurring in different local 1979; Shulters & Bohlen, 1989; Dasgupta et al., 1995) and domains, and hence might form contemporaneously makes it difficult to evaluate the thermal significance of the rather than in sequence. Thus, we suggest that the P^T Zn-bearing Spl þ Qz assemblages enclosed by Grt1 in the conditions of M2 experienced by the garnetite were garnetites considered inthis study (Fig.6d ande). 850^8658C and 7·4^7·6 kbar based on the modeling applied to M2a, whereas the higher temperatures and P^T path of the garnetites lower pressures (910^9258C and 6·8^7·1kbar) obtained According to the quantitative phase equilibria and Zr-in- from modeling of M2b are erroneous because of neglect of rutile thermometry calculations, the P^Tconditions of the

1752 JIAO et al. KHONDALITE BELT GARNETITE

stages (from M1 to M3) experienced by the garnetite are or decompression under granulite-facies conditions. 820^8508C (even up to 9508C) and 8·5^9·5 kbar, 850^ Hence, the garnetites are likely to have been generated 8658C and 7·4^7·6kbar,and710^7208C and 6·4^6·6 kbar, through the incongruent dehydration melting of biotite in respectively. These results allow the construction of an ex- the protolith (Patin‹ o Douce & Johnston, 1991). Based on tensive segment of its P^T path. Consequently, a clockwise the preserved mineral assemblage, the garnetite might be P^T path involving an initial post-peak near-isothermal generated through reaction of Bt þ Sil þ Qz þ Pl ¼ Grt þ decompression and subsequent cooling is deduced for the Ilm/Rt þ Kfs þ Liq, and Kfs þ Liq were removed from the garnetites of the Khondalite Belt (Fig. 19). During M1,the system during melt loss, which resulted in the depletion of rocks mainly experienced decompression from c. 10kbar K and Na in the garnetite and the associated garnet-bear- to 8·5 kbar at temperatures of 820^8508C (even up to ing quartz-rich lens. The separation of the garnetite and 9508C). Rutile became unstable and was partially replaced garnet-bearing quartz-rich lens occurred during or after Downloaded from by ilmenite, and Ca in plagioclase increased, whereas Ca the generation of peritectic garnet (Grt1). The obtained in garnet decreased with decompression. The growth of Zr-in-rutile temperatures, up to 9508C (Table 8; Figs 18 Grt2 or Spl coupled with Crd during M2 resulted from re- and 19), suggest that the protoliths were heated sufficiently actions involving Grt1, Sil and Bt in quartz-present or to produce high volumes of granitic melt and peritectic -absent microdomains. During M3, Grt1 was extensively garnet. Recent parallel studies that have applied two-feld- http://petrology.oxfordjournals.org/ replaced by a Grt3 þ Opx þ Crd þ Pl þ Bt association in spar and Zr-in-rutile thermometers to the metasedimen- sillimanite-absent microdomains. At this stage, Ca cations tary rocks of the Khondalite Belt have obtained in plagioclase show the highest values, and retrograde bio- comparably high temperatures of c. 820^10208C and c. tite grains show lower XMg and F contents, but higher Cl 800^10008C, respectively (Jiao & Guo, 2011; Jiao et al., contents compared with those biotite grains surviving as 2011).These results, along with the localized occurrence of inclusions in Grt1. some diagnostic UHT mineral assemblages, such as No kyanite was found in our samples, though Lu et al. Spr þ Qz and Opx þ Sil Qz (Santosh et al., 2007a), indi- (1992,1996), Lu & Jin (1993), and Wang et al. (2011) r e p o r te d cate that the Khondalite Belt protoliths had experienced kyanite-bearing aluminous gneisses near Zhuozi and very high temperatures before or during their extensive Tuguiwula in the Jining terrane. In those occurrences

melting to produce S-type granitoids and garnetite at Institute of Geology and Geophysics, CAS on August 27, 2013 kyanite occurrs as inclusions in garnet and in the matrix, residua. and the textural relations between kyanite and sillimanite The production of the garnetite as a residue from partial suggest that the relict kyanite-bearing assemblages formed melting is supported by a number of experimental studies prior to the peak stage (Cui, 1987) (i.e. pre-M ). 1 on the melting of synthetic or natural pelitic compositions. Petrogenesis of the garnetites Green (1976) presented experimental melting studies on a The bulk-rock composition of the garnetite listed inTable 9 synthetic glass close to the average pelitic composition is depleted in silica and enriched in aluminium and iron with 2 and 5 wt% added water, and demonstrated that compared with typical subsolidus pelites, and suggests garnet, quartz, biotite, sillimanite, and plagioclase are the that the garnetites are fragments of a residue after melt ex- important restitic phases coexisting with granitic liquid at traction, which is a common process during crustal ana- 10kbar. This is consistent with the mineral assemblage texis and preservation of granulite-facies rocks (White & Grt1 þSil þ Qz þ Pl þ Bt ( þ Ilm þ Rt) present at stage Powell, 2002; White et al., 2004). The association of garnet M1 of our garnetites. Moreover, the compositions of with sillimanite argues against the garnetites being accu- garnet in the above experimental study are similar to mulations from a melt (Stevens et al., 1997). Furthermore, those in the garnetite studied here. Patin‹ o Douce & the high proportion of garnet suggests that there was a Johnston (1991) experimentally demonstrated that garnet, considerable flux or loss of melt, as mass-balance calcula- aluminosilicate, quartz, rutile, and ilmenite were the prin- tions on garnet-producing dehydration-melting reactions cipal restitic phases at temperatures of c. 9008C, with in typical pelites such as those in the Khondalite Belt re- Ti-rich biotite and calcic plagioclase also present depend- quire that the volume of melt is greater than or equal to ing on the bulk composition of the protolith, but alkali that of peritectic garnet produced in the reaction. For ex- feldspar is always absent. Again, this is consistent with the ample, in the Neogene Volcanic Province of SE Spain, en- M1 mineral assemblage in the garnetite. The starting ma- claves of restitic bulk composition formed after up to 60% terial used by Patin‹ o Douce & Johnston (1991) was a nat- melt extraction (Cesare et al., 1997; A¤ lvarez-Valero & ural metapelitic rock from northern Idaho (USA), the Waters, 2010). In addition, the widespread distribution of composition of which is similar to the quartzo-feldspathic S-type granitoids (garnet granites) in the metasediments sillimanite^garnet gneisses along the Dashizi^Xiaoshizi of the Liangcheng^Zhuozi area (Fig. 2) (Peng et al., 2012) section in the Khondalite Belt (Fig. 3). Patin‹ o Douce & indicates that the sedimentary rocks in the Khondalite Johnston (1991) further suggested that fractionation or sep- Belt experienced extensive crustal anatexis during heating aration of restitic garnet from peraluminous anatectic

1753 JOURNAL OF PETROLOGY VOLUME 54 NUMBER 9 SEPTEMBER 2013

magmas is an essential process prior to the crystallization North China Craton exhibit a syn-metamorphic geother- of S-type granitoids, as this fractionation prevents much mal gradient of 22^288Ckm^1, much higher than the of the orthoclase component in the melt from recombin- P^T gradients associated with subduction-related orogens ing as biotite during cooling. In the case of the in Phanerozoic collisional belts, and also show a much Khondalite Belt the steep, heavy rare earth element slower exhumation rate. Zhai & Santosh (2011) further (HREE)-depleted REE patterns of several strongly per- speculated that the HT^UHT metamorphism developed aluminous granitoids are consistent with the formation of along the collisional sutures at 1·95^1·82 Ga, enhanced substantial peritectic garnet during melting of typical flat either by a thermal input from the sub-lithospheric HREE crustal protoliths, followed by the separation of mantle resulting from slab-break off and asthenospheric that garnet from the melt, which crystallized elsewhere injection, or by magmas in a subduction regime, the cryp- as an HREE-depleted granitoid (Peng et al., 2012). Thus, tic pathways of which were defined by the present-day ex- Downloaded from we conclude that the garnetite, and the associated posures of metagabbros and metamorphosed gabbro- garnet-bearing quartz-rich lenses that yield the same norites. zircon U^Pb ages (Jiao et al., unpublished data), are the Our clockwise P^T path from the garnetite in the result (residuum) of crustal anatexis controlled by the in- Jining terrane is similar to those deduced by previous re- congruent dehydration-melting of biotite þ sillimanite- searchers (Fig. 19, paths 1, 2 and 3), which show near-iso- bearing metasediments beginning at c. 10 kbar and 820^ thermal decompression. It is consistent with the http://petrology.oxfordjournals.org/ 8508C (even up to 9508C). collisional characteristics of the Khondalite Belt suggested Implications for the tectonic evolution of by Zhao et al. (2005). The peak conditions of the garnetite are higher than those for the normal metasedimentary the orogenic belt rocks (paths 1^3), but lower than those for the UHT As shown in Fig. 19, Yin et al.(2007;unpublisheddata) metamorphic rocks (paths 4 and 5) in the Khondalite investigated high-pressure pelitic granulites from the Belt. Moreover, zircon U^Pb data from the garnetites Qianlishan and Helanshan terranes in the Khondalite and the associated garnet-bearing quartz-rich lenses, and Belt, and demonstrated clockwise P^T paths that involved the adjacent quartzo-feldspathic sillimanite^garnet near-isothermal decompression (paths 1 and 2; Fig. 19).

gneisses, indicate a similar age of 1890Ma for zircon at Institute of Geology and Geophysics, CAS on August 27, 2013 Wa ng et al. (2011) investigated the aluminous gneisses in grains and overgrowths formed during stage M (Jiao the Jining terrane, determining a clockwise P^T path that 1 et al., unpublished data). As discussed above, the garne- involves isothermal decompression from peak conditions tites are considered to have been generated as residua, of 10kbar and 8258C (path 3; Fig. 19). These results, when composed mainly of peritectic minerals formed in associ- combined with available isotopic age data (Santosh et al., ation with granitic melts that segregated and crystallized 2007b, 2009b;Yinet al., 2009, 2011), support the model of elsewhere to produce at least some of the S-type granit- Zhao et al. (1999, 2005), who suggested that the Khondalite Belt formed by the collision of the Yinshan oids present in the Khondalite Belt over the time period and Ordos Blocks at 1·95 Ga. 1920^1890 Ma (Guo et al., 2001b;Zhonget al., 2007). Several UHTgranulites have been discovered within the Chemical and isotopic evidences that mantle-derived Khondalite Belt, as described by Santosh et al.(2007a, magmas provided negligible contributions to these S-type 2009a) and Guo et al. (2012). The P^T paths determined granitic melts (Peng et al., 2012), suggests that their gener- for these granulites by those researchers all have an iso- ation, and that of the garnetite, is unlikely to be directly thermal decompression segment (paths 4 and 5; Fig. 19), related to mantle-derived gabbronorite intrusions or albeit at higher temperatures than those deduced here mantle plume impact at 1·92 Ga, as in models proposed and in one case preceded by a phase of near-isobaric cool- by previous workers (Santosh & Kusky, 2010; Guo et al., ing (path 5). Those researchers preferred a metamorphic 2012) to explain the 1·92 Ga UHT metamorphism in the age of 1920 Ma for the UHT rocks, and proposed that Khondalite Belt. Instead, the clockwise P^T path invol- the UHT metamorphism was caused by the emplacement ving near-isothermal decompression depicted here is con- of coeval mantle-derived gabbronorite intrusions during sidered to reflect exhumation associated with orogenic either ridge subduction (Guo et al., 2012) or mantle plume extension that initiated at 1890 Ma. This followed HT impact (Santosh & Kusky, 2010). Zhao (2009) argued that and UHT metamorphism, potentially as early as 1920 the 1·92 Ga UHT metamorphism occurred later than Ma, and the emplacement of S-type granitoid magmas at the collision of the Yinshan and Ordos Blocks, and in the 1920^1890 Ma, which were the natural consequences of light of this timing, suggested that the UHT metamorphic the prolonged deep burial of high-heat-producing crustal event was related to the emplacement of mantle-derived material in the large, long-lived, hot orogen (Beaumont magmas in a post-collisional extensional setting. Zhai et al., 2006; Clark et al., 2011; Jamieson & Beaumont, 2011) et al. (2010) remarked that the high-temperature^ultra- that formed as a consequence of collision between the high-temperature (HT^UHT) granulites within the Yinshan and Ordos Blocks at 1·95 Ga.

1754 JIAO et al. KHONDALITE BELT GARNETITE

ACKNOWLEDGEMENTS Fitzsimons, I. C.W. & Harley, S. L. (1994).The influence of retrograde cation-exchange on granulite P^T estimates and a convergence We sincerely thank Professor Chunjing Wei and Dr Yi technique for the recovery of peak metamorphic conditions. Chen for their advice with the THERMOCALC calcula- Journal of Petrology 35, 543^576. tions, and Dr Changqing Yin for his help with the calcula- Florence, F. P. & Spear, F. S. (1991). Effects of diffusional modification tions. Professor Guochun Zhao provided useful advice of garnet growth zoning on P^T path calculations. Contributions to and encouragement. Reviewers Antonio M. A¤ lvarez- Mineralogy and Petrology 107, 487^500. Valero, A. Christy and A. Indares, and Editor Jo« rg Frost, B. R. & Chacko, T. (1989). The granulite uncertainty principle: Hermann are thanked for their constructive suggestions limitations on thermobarometry in granulites. Journal of Geology about this paper. We also thank Zhenyu Chen, Qian Mao, 97,435^450. Green, T. H. (1976). Experimental generation of cordierite- or garnet- and Yuguang Ma for their assistance with the electron bearing granitic liquids from a pelitic composition. Geology 4, microprobe analysis, and Peng Peng, Fu Liu, and Luojuan 85^88. Downloaded from Wang for their help in the fieldwork. Groppo, C., Beltrando, M. & Compagnoni, R. (2009a).The P^T path of the ultra-high pressure Lago Di Cignana and adjoining high- pressure meta-ophiolitic units: insights into the evolution of the FUNDING subducting Tethyan slab. Journal of Metamorphic Geology 27, 207^231. Groppo, C., Forster, M., Lister, G. & Compagnoni, R. (2009b). This work was supported financially by research grant No.

Glaucophane schists and associated rocks from Sifnos (Cyclades, http://petrology.oxfordjournals.org/ 41023009 and No. 40730315 from the NSFC in China, and Greece): New constraints on the P^Tevolution from oxidized sys- the State Key Development Program for Basic Research tems. Lithos 109, 254^273. of China (Grant No. 2012CB416601). Groppo, C., Rolfo, F. & Indares, A. (2012). Partial melting in the higher Himalayan Crystallines of Eastern Nepal: the effect of decompression and implications for the ‘channel flow’ model. Journal of Petrology 53, 1057^1088. REFERENCES Guo, J. H., Wang, S. S., Sang, H. Q. & Zhai, M. G. (2001a). 40Ar^39Ar A¤ lvarez-Valero, A. M. & Waters, D. J. (2010). Partially melted crustal age spectra of garnet porphyroblast: Implications for metamorphic xenoliths as a window into sub-volcanic processes: evidence from age of high-pressure granulite in the North China Craton. Acta the Neogene Magmatic Province of the Betic Cordillera, SE Petrologica Sinica 17, 436^442. Spain. Journal of Petrology 51, 973^991. Guo,J.H.,Zhai,M.G.&Xu,R.H.(2001b).Timing of the granulite Beaumont, C., Nguyen, M. H., Jamieson, R. A. & Ellis, S. (2006). facies metamorphism in the Sanggan area, North China craton: at Institute of Geology and Geophysics, CAS on August 27, 2013 Crustal flow modes in large hot orogens. In: Law, R. D., zircon U^Pb geochronology. Science in China Series DçEarth Searle, M. P. & Godin, L. (eds) Channel Flow, Ductile Extrusion and Sciences 44, 1010^ 1018. Exhumation in Continental Collision Zones. Geological Society, London, Guo, J. H., Sun, M., Chen, F. K. & Zhai, M. G. (2005). Sm^Nd Special Publications 268,91^145. and SHRIMP U^Pb zircon geochronology of high-pressure granu- Berger, A., Scherrer, N. C. & Bussy, F. (2005). Equilibration and lites in the Sanggan area, North China Craton: timing of Paleopro- disequilibration between monazite and garnet: indication from terozoic continental collision. Journal of Asian Earth Sciences 24, phase-composition and quantitative texture analysis. Journal of 629^642. Metamorphic Geology 23, 865^880. Guo, J. H., Peng, P.,Windley, B. F., Chen,Y. & Jiao, S. J. (2012). UHT Brandt, S., Will, T. M. & Klemd, R. (2007). Magmatic loading in the sapphirine granulite metamorphism at 1·93^1·92 Ga caused by gab- Proterozoic Epupa Complex, NW Namibia, as evidenced by ultra- bronorite intrusions: Implications for tectonic evolution of the high-temperature sapphirine-bearing orthopyroxene^sillimanite^ northern margin of the North China Craton. Precambrian Research quartz granulites. Precambrian Research 153, 143^178. 222^223, 124^142. Cesare, B., Mariani, E. S. & Venturelli, G. (1997). Crustal anatexis Hand, M., Scrimgeour, I., Powell, R., Stu« we,K.&Wilson,C.J.L. and melt extraction during deformation in the restitic xenoliths at (1994). Metapelitic granulites from Jetty Peninsula, east El Joyazo (SE Spain). Mineralogical Magazine 61,15^27. Antarctica: Formation during a single event or by polymetamorph- Clark, C., Healy, D., Fitzsimons, I. C. W. & Harley, S. L. (2011). How ism? Journal of Metamorphic Geology 12,557^573. does the crust get really hot? Elements 7, 235^240. Harley, S. L. (1984).The solubility of alumina in orthopyroxene coex-

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