Zn-Rich Spinel in Association with Quartz in the Al-Rich Metapelites from the Mashan Khondalite Series, NE China

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1 Journal of Earth Science, Vol. 25, No. 2, p , April 2014 ISSN X Printed in China DOI: /s Zn-Rich Spinel in Association with Quartz in the Al-Rich Metapelites from the Mashan Khondalite Series, NE China Xinzhuan Guo* 1, Akira Takasu 2, Yongjiang Liu 3, Weimin Li 3 1. Institute for Study of the Earth s Interior, Okayama University, Misasa, Tottori-ken , Japan 2. Department of Geoscience, Shimane University, Matsue , Japan 3. College of Earth Sciences, Jilin University, Changchun , China ABSTRACT: Al-rich metapelites from the Mashan khondalite series are characterized by the assemblage Spl+Grt+Sil+Crd+Bt+Pl (An 72 )+Kfs+Quartz+graphite. Large amounts of spinel+quartz assemblages occur as inclusions in garnet and prismatic sillimanite in the Al-rich metapelites of the Mashan complex, NE China. The chemical composition of spinel is characterized by Zn-rich (X Zn = X Zn =Zn/Zn+Mg+Fe * ) (Fe * =Fe 2+ +Fe 3+ ) and Fe 3+ rich (up to 0.31 p.f.u.). The characteristic chemical composition and the mineral association indicated that the formation of spinel and quartz assemblage may be due to the breakdown of Zn-rich staurolite. The geothermobarometers studies show that the peak temperature of the Mashan complex is around 820 and the peak pressures is 8.0 kbar. The Mashan complex shows a typical orogen style P-T path. KEY WORDS: Al-rich metapelites, Mashan complex, khondalite, spinel+quartz, granulite. 1 INTRODUCTION The Mashan khondalite series in the Mashan complex, NE China, are characterized by graphite-bearing, Al-rich and phosphorus-bearing, and are mainly composed of Al-rich pelitic rocks, felsic rocks and calc-silicate rocks. These rocks have been intruded by syn- and post-tectonic granitic rocks. Partial melting and migmatization are widely developed in the Mashan khondalite series. Previous petrological studies showed that the Mashan khondalite series underwent granulite facies metamorphism and were characterized by a tight clockwise P-T path with peak temperatures of 850 and pressures of 7.4 kbar (Jiang, 1992). In this study, we focused on the Al-rich metapelites of the Mashan khondalite series. Large amounts of spinels together with quartz occurred as inclusions in peak mineral assemblages, such as garnet, sillimanite and K-feldspar in the Al-rich metapelites. Spinel+quartz mineral assemblage is potential as the ultra-high-temperature (UHT) metamorphism indicator. A lot of studies in the past two decades have reported such mineral assemblage in the granulite-facies rocks (Sato et al., 2009; Shimizu et al., 2009; Barbosa et al., 2006; Morimoto et al., 2004; Sarkar et al., 2003; Sengupta et al., 1999; Waters, 1991). Here, the genesis of the spinel+quartz and the metamorphic evolution were investigated through detailed chemical *Corresponding author:gxzhuan@misasa.okayama-u.ac.jp China University of Geosciences and Springer-Verlag Berlin Heidelberg 2014 Manuscript received July 21, Manuscript accepted January 12, composition analysis and petrographical studies. The temperature of the peak metamorphism was confirmed to be much lower than that of the UHT metamorphism (usually higher than 900 ). 2 GEOLOGICAL SETTINGS The Jiamusi Massif lies east of the Paleozoic Central Asian Orogenic belt (CAOB), located between the Siberian and the North China cratons (Sengör et al., 1993) (Fig. 1), limited by the Mudanjiang fault to the west, the Jiayi fault to the northwest, the Dunmi fault to the southeast and the Tongjiang fault to the east. The Jiamusi Massif is composed of two complexes: the Mashan complex of ~500 Ma in metamorphic age (Wilde et al., 2000) and metamorphosed Jurassic accretionary Heilongjiang complex (Wu et al., 2007). Most geologists believe the Mashan complex is the basement and the Heilongjiang complex is the cover. SHRIMP-U/Pb zircon ages of the Mashan complex reported by Wilde et al. (2003, 2000, 1999), Wilde and Wu (2001) suggested that the Mashan complex was once associated with a Late Pan-African orogenic belt, which included parts of East Antarctica, Western Australia, Sri Lanka and India, and possibly also portions of Madagascar, Tanzania and Kenya (Wilde et al., 1999), within the reconstructed Gondwana land. Two prograde metamorphic zones are recognized in the Mashan complex: the granulite facies belt and the amphibolites belt. Al-rich metapelites of the granulite facies belt is located in the Xi Mashan, Liumao, Sandaogou and Didao. The distribution of the metamorphic belt is independent of the depth. Spinel+quartz bearing metapelites were sampled from Guo, X. Z., Takasu, A., Liu, Y. J., et al., Zn-Rich Spinel in Association with Quartz in the Al-Rich Metapelites from the Mashan Khondalite Series, NE China. Journal of Earth Science, 25(2): , doi: /s

2 208 Xinzhuan Guo, Akira Takasu, Yongjiang Liu and Weimin Li Figure 1. Geological sketch map of the Jiamusi Massif (a), modified from Wilde et al. (2003). (b) Geotectonic map of the eastern Asia. CAOB, Central Asia Orogenic belt. Al-rich pelitic rocks (located in Xi Mashan, Sandaogou and Liumao) (Fig. 2). Foliation of the Al-rich pelitic rocks is clearly defined by compositional layes of various rock types, and lineation is defined by biotite grains and prismatic sillimanite. 3 PETROGRAPHY The Al-rich pelitic rocks are sub-divided into Spl-bearing Grt-Sil schist, Spl-free Grt-Sil gneiss, Grt-free Sil schist and Grt-Crd-Sil schist. All the pelitic schists show strong lepidoblastic foliations defined by prismatic sillimanite. Weak foliations are developed by biotite in the pelitic gneiss. The mineral assemblages in the Al-rich metapelites are summarized in Table 1. Mineral abbreviations are referred from Kretz (1983). 3.1 Spl-Bearing Grt-Sil Schist This type of rock outcrops in Sandaogou and Xi Mashan. The differences between samples MS3-3 and SL06-4 are: (1) the volume fractions of biotite and sillimanite in the MS3-3 are much lower than those of the latter; (2) biotites in the former only occur as the retrograde minerals. The major minerals include sillimanite, K-feldspar, garnet, plagioclase, and quartz and the minor minerals include biotite, muscovite, chlorite, ilmenite and spinel. Strong lepidoblastic foliations are defined by prismatic sillimanite. The garnet porphyroblasts are coarse-grained (2 6 mm), and contain fibrous sillimanite, fine-grained sillimanite, plagioclase, quartz, biotite, spinel and ilmenite. The fibrous sillimante inclusions in garnet and the prismatic sillimanite in the matrix belong to different generations. Spinels occur as inclusions in garnet, sillimanite, K-feldspar and biotite (Figs. 3a, 3b, 3f, and 3h). Spinels always contacts with the biotite and the quartz (Figs. 3a, 3c, 3d, 3e and 3g). In some case, some fine-grained spinels directly contact with the quartz with a sharp boundary (Figs. 3c, 3d and 3g). In some case, biotite grains occur as inclusions in the spinel (Fig. 3c). Myrmekitic and granophyric intergrowths are common along the boundary between K-feldspar and plagioclase, with quartz present as vermicular blebs within plagioclase, which could be elucidated as in situ partial melting at the peak

3 Zn-Rich Spinel in Association with Quartz in the Al-Rich Metapelites from the Mashan Khondalite Series, NE China 'E N Liumao 45 11'E Xi Mashan Mashan 0 2 km Sandaogou Quaternary Tertiary Cretaceous Jurassic Faults Sample localities Graphite schist Sillimanite gneiss Marble Cordierite gneiss Gneiss/ granulite Granitoid/ migmatite Figure 2. Simplified geological map of Xi Mashan, Sandaogou and Liumao. Sample localities are marked by stars. This map is modified after Wilde et al. (1999). Table 1 Mineral assemblages in the metapelites from the Mashan complex Classification Sample No. Mineral assemblage Spl-bearing Grt-Sil schist MS3-3, SL06-4 Grt+Sil+Bt+Spl+Kfs+Pl+Ms+Chl+Qtz±Il± Rt Spl-free Grt-Sil gneiss LG25-3, SL06-7 Grt+Sil+Bt+Kfs+Ms+Pl+Qtz+Il±Rt Grt-free Sil schist SL06-6 Sil+Bt+Kfs+Ms+Qtz±Il±Rt Grt-Crd-Sil schist SL06-1, SL06-2 Grt+Crd+Sil+Pl+Kfs+Bt+Ms+Qtz+Il+Rt Note: MS. Xi Mashan; LG. Liumao; SL. Sandaogou. conditions. This type of rock shows mesoperthitic texture with exsolution lamellae of Na-feldspar within the core of host K-feldspar. But the exsolution texture is rare in the rim of alkali feldspar. Some garnet porphyroblasts have been pulled apart and partly surrounded by biotite, which is partly altered to chlorite and muscovite. The altered biotite also grows along the cracks of the garnet. The late retrograde metamorphism, replacement of sillimanite by muscovite and the chloritization of the matrix minerals, is common in this type of rock. 3.2 Spl-Free Grt-Sil Gneiss Spinel-free garnet-sillimanite gneiss (sampled from Liumao) is characterized by less amount of sillimanite than spinel-bearing garnet-sillimanite schist. The major minerals include quartz, K-feldspar, graphite, garnet, biotite and sillimanite. Ilmenite, rutile and monazite occur as accessory minerals. Weak foliation is developed by biotite. The peak metamorphic assemblage is defined by garnet, biotite, K-feldspar and sillimanite. The garnet, subhedral-euhedral, size up to 2 cm, poikiloblastically encloses fine grains of quartz, biotite and ilmenite. The garnets have been pulled apart, with retrograde biotite growing along the cracks. Some redish biotites contact with the garnet with a sharp boundary. Retrograde muscovite is common, which has overgrown some sillimanite grains. 3.3 Grt-Free Sillimanite Schist The garnet-free sillimanite schists (ubiquitously outcrop in the Sandaogou) display a lepidoblastic fabric defined by prismatic sillimanite. The major minerals include K-feldspar, plagioclase, quartz, graphite and prismatic sillimanite. The minor minerals include muscovite, biotite, and illmenite. The foliation is well developed by prismatic sillimanite. K-feldspar, prismatic sillimanite and plagioclase define the peak metamorphic assemblage. This type of rock is typically different from the other metapelites by a lack of garnet. The retrograde textures in this type of rock are common. Some plagioclase grains have almost been overprinted by muscovite. Some sillimanite grains are pulled apart, with retrograde muscovite growing along the cracks. 3.4 Grt-Crd-Sil Schist Grt-Crd-Sil schist sampled from the Sandaogou sillimanite mine, is composed of garnet, sillimanite, K-feldspar, cordierite, biotite, quartz, graphite and ilmenite. Cordierite is the characteristic mineral in this type of rock. Garnet and cordierite porphyroblasts contain sillimante, quartz and biotite inclusions. In some cordierite grains, there are fine-grained garnet relicts. Some cordierite porphyroblasts exhibit sector twinning. This type of rock displays alternate domains defined by cordierite and sillimanite, respectively. The garnet occurs as porphyroblast, which is stretched into tabular shape. For the sample SL06-1, the matrix biotite is Ti-rich, exhibiting a

4 210 Xinzhuan Guo, Akira Takasu, Yongjiang Liu and Weimin Li Figure 3. Photomicrographs showing detailed textures of spinel inclusions in garnet, sillimanite and K-feldspar for the Spl-bearing Grt-Sil schist. Figs. (a) (e), (g): spinel, coexisting with Bt and Qtz, occur as inclusions in porphyroblastic garnet, (a) also shows that some spinels occur as inclusions in biotite. In the case of (c), (d) and (g), spinel contact with quartz directly. (f) Spinel occurs as inclusion in sillimanite; (h) spinel, partly surrounded by retrograde Bt+Chl, occurs as inclusion in K-feldspar. pleochroic feature from straw to red. Large sillimanite grains are pulled apart with vermicular intergrowths of quartz andbiotite along the cracks. Some coarse-grained sillimanites show a good cleavage parallel to the elongation of the sillimanite. In the sample SL06-2, cordierite was mostly replaced by muscovite, and biotite was retrograded to both chlorite and muscovite. Moreover, chlorite grows along the boundary between adjacent garnet grains. 4 MINERAL CHEMISTRY Chemical compositions of the minerals have been analyzed using a JEOL JXA-8800M electron probe micro-analyzer housed at Shimane University, Japan. The analysis accelerating voltage, probe current and beam size are 15 kv, A and 5 μm, respectively. The data reduction was performed using Oxide-ZAF model corrections for the analysis. Electron microprobe analysis of the representative minerals of the metapelites from the Mashan complex are given in Tables 2 9.

5 Zn-Rich Spinel in Association with Quartz in the Al-Rich Metapelites from the Mashan Khondalite Series, NE China 211 Table 2 Mineral chemistry data on garnets from the Mashan complex Sample No. Lg25-3 MS3-3 SL06-2 SL06-4 Core Core Rim Rim Core Core Core Core Rim Rim Core Core Rim Rim Core Core Core SiO TiO Al 2 O FeO MnO MgO CaO Na 2 O K 2 O Cr 2 O Total (wt.%) Oxygen Si Ti Al Fe Mn Mg Ca Na K Cr Total Almandine Pyrope Grossular Spessartine X Mg X mg =Mg/(Fe 2+ +Mg). Table 3 Mineral chemistry data on biotites from the Mashan complex Sample LG25-3 MS3-3 SL06-1 SL06-2 SL06-4 SL06-7 No. BIG BIG BIG MB MB MB RB BIG RB MB BIG MB MB BIG MB MB BIG RB BIG MB SiO TiO Al 2 O FeO MnO MgO CaO Na 2 O K 2 O Cr 2 O ZnO Total (wt.%) Oxygen Si Ti

6 212 Xinzhuan Guo, Akira Takasu, Yongjiang Liu and Weimin Li Continued Sample LG25-3 MS3-3 SL06-1 SL06-2 SL06-4 SL06-7 No. BIG BIG BIG MB MB MB RB BIG RB MB BIG MB MB BIG MB MB BIG RB BIG MB Al Fe Mn Mg Ca Na K Cr Zn Total X Mg BIG. Biotite inclusion in garnet; MB. biotite in the matrix; RB. retrograde biotite; X mg =Mg/(Fe 2+ +Mg). Table 4 Mineral chemistry data on cordierites from the Mashan complex Sample No. SL06-2 SL06-1 CM CCS SiO TiO Al 2 O FeO MnO MgO CaO Na 2 O K 2 O Cr 2 O Total (wt.%) Oxygen Si Ti Al Fe Mn Mg Ca Na K Cr Total X Mg CM. Retrograded to muscovite; CCS. cordierite contacts with sillimanite.

7 Zn-Rich Spinel in Association with Quartz in the Al-Rich Metapelites from the Mashan Khondalite Series, NE China 213 Table 5 Mineral chemistry data on spinels from the Mashan complex Sample No. MS3-3 SL06-4 SIM SIG SIB SCB SCBQ SiO TiO Al 2 O Fe 2 O FeO MnO MgO ZnO Na 2 O K 2 O Cr 2 O Total (wt.%) Oxygen Si Ti Al Fe Fe Mn Mg Zn Na K Cr Total X Zn * X Mg Fe 2 O 3 was calculated based on stoichiometry; SIM. Spinel in matrix; SIG. spinel inclusion in garnet; SIB. spinel inclusion in biotite; SCB. spinel contacts with biotite; SCBQ. spinel contacts with biotite and quartz; X zn *=Zn/(Zn+Mg+Fe 2+ ); X Mg =Mg/(Fe 2+ +Mg). Table 6 Mineral chemistry data on sillimanites from the Mashan complex Sample No. LG25-3 MS3-3 SL06-2 SiO TiO Al 2 O FeO MnO MgO CaO Na 2 O K 2 O Cr 2 O Total (wt.%) Oxygen Si Ti Al Fe Mn Mg Ca Na K Cr Total

8 214 Xinzhuan Guo, Akira Takasu, Yongjiang Liu and Weimin Li Table 7 Mineral chemistry data on plagioclases from the Mashan complex Sample No. LG25-3 MS3-3 SiO TiO Al 2 O FeO MnO MgO CaO Na 2 O K 2 O Cr 2 O Total (wt.%) Oxygen Si Ti Al Fe Mn Mg Ca Na K Cr Total Albite Anorthite Orthoclase Table 8 Mineral chemistry data on muscovites from the Mashan complex Sample No. LG25-3 MS3-3 SL06-2 SiO TiO Al 2 O FeO MnO MgO CaO Na 2 O K 2 O Cr 2 O Total (wt.%) Oxygen Si Ti Al Fe Mn Mg Ca Na K Cr Total

9 Zn-Rich Spinel in Association with Quartz in the Al-Rich Metapelites from the Mashan Khondalite Series, NE China 215 Table 9 Mineral chemistry data on ilmenites from the Mashan complex Sample No. LG25-3 SL06-2 SiO TiO Al 2 O FeO MnO MgO CaO Na 2 O K 2 O Cr 2 O Total (wt.%) Oxygen Si Ti Al Fe Mn Mg Ca Na K Cr Total Garnet Garnets in all the samples are almost homogeneous and almandine-rich (Alm Sps 1-10 Grs 2-10 Py ). In general, the X Mg values of the cores are relatively higher ( ) than that of the rims ( ). Garnets in samples SL06-2 and SL06-4 are slightly Mg-rich (X Mg = ). 4.2 Biotite Prograde (occur as inclusions) and peak (coexist with other peak metamorphic minerals) metamorphic biotites are Ti-rich ( p.f.u), whereas, retrograde biotites show the lowest Ti ( p.f.u.). X Mg [X Mg =Mg/(Mg+Fe 2+ )] of biotites shows a value of Zn component in the biotites that coexists with Zn-rich Spl are negligible (less than 0.13 wt.%), possibly indicates that there are no genetic relationship between Zn-rich spinel and biotite. The biotites show low probe totals (92.4 wt.% to 96.6 wt.%), suggesting the presence of the volatiles, such as fluorine. 4.3 Cordierite There are no compositional zones in cordierites in Sil-rich metapelites. X Mg [X Mg =Mg/(Mg+ Fe 2+ )] varies between 0.36 and Whereas, X Mg ( ) of the cordierites that partly retrograded to muscovite is much lower than that ( ) of the cordierites coexisting with sillimanites. The analytical totals of the retrograde cordierite are low (88.1% 90.0%), probably indicating the presence of channel-filling volatiles, such as CO 2 and/or H 2 O. Although no direct measurement of the volatiles in cordierite were performed in this study, a previous investigation on channel and inclusion fluids in cordierites from Chittikara, South India, confirmed the common presence of CO 2 (Santosh and Wada, 1993). 4.4 Spinel All the spinels are of gahnite-rich varieties [X Zn = , X Zn =Zn/(Zn+Mg+Fe*)] with low X Mg = (X Mg =Mg/Zn+Mg+Fe*) and low Cr 2 O 3 (less than 0.35 wt.%). Al in spinel varies from 1.78 to 1.94 p.f.u.. For the spinel that contacts with biotite, X Zn decreases from the core to the rim (Fig. 4). Spinels inclusions in biotite show higher X Zn values ( ). The ferric iron of spinels is up to 0.31 p.f.u Other Minerals The chemical composition of sillimanite is close to the ideal chemistry (Al 2 SiO 5 ), although it contains a small amount of FeO (up to 0.56 wt.%). The plagioclase has compositional variation of Ab An Or The plagioclase in Spl-bearing Grt-Sil schist is much more Ab-rich than those in other types of rocks. Analysis of the alkali feldspar and plagioclase components within mesoperthites in the sample MS3-3 is shown in Table 10. The TiO 2 and MgO compositions of muscovite in the sample LG25-3 are higher than those in MS3-3 and SL06-2. Fe-Ti oxides occurring as opaque minerals mainly present as ilmenite and rutile. The stoichiometryof ilmenite and rutile are almost FeTiO 3, and TiO 2, respectively.

10 216 Xinzhuan Guo, Akira Takasu, Yongjiang Liu and Weimin Li Figure 4. In sample SL06-4, Zn-rich green spinel coexists with brown biotite. From the core to rim, the spinel show decreasing trend in component Zn. Different analyzed points are marked by black solid circles and labeled by numbers. Table 10 Electron microprobe analyses of K-feldspar and plagioclase phases in mesoperthites from the Mashan complex Sample No. MS3-3 Pl1 Kfs1 Pl2 Kfs2 Pl3 Kfs3 Pl4 Kfs4 Pl5 Kfs5 Pl6 Kfs6 Pl7 Kfs7 SiO TiO Al 2 O FeO MnO MgO CaO Na 2 O K 2 O Cr 2 O Total (wt.%) Oxygen Si Ti Al Fe Mn Mg Ca Na K Cr Total Ca/(Ca+Na) Albite Anorthite Orthoclase DISCUSSION 5.1 Revolution of the Mineral Assemblages It has been widely observed that the overprints of the high-grade metamorphic conditions on the prograde mineral assemblages. However, it is also known that the core of coarse-grained garnet is shielded against such overprints, and thus mineral assemblages preserved as inclusions in garnet cores from granulite rocks can provide robust information on the prograde history (Tsunogae and van Reenen, 2006). Some interpretation, based on the peak mineral assemblages and their inclusions, is necessary to ascertain the relevant prograde reactions. The reactions shown in this section are not balanced in the stoichiometry but are presented in a general form. In all the metapelite samples, there are two types of sillimanite, fine fibrous and coarse prismatic sillimanite. Sil-rich rocks in the Sandaogou region exhibit a strong foliation defined by coarse prismatic sillimanite, and in some case, prismatic sillimanite are strongly deformed, growing along the margins

11 Zn-Rich Spinel in Association with Quartz in the Al-Rich Metapelites from the Mashan Khondalite Series, NE China 217 of the garnet porphyroblasts. Prismatic and fibrous sillimanite also occur as the inclusions in the K-feldspar or coexist with K-feldspar (Fig. 5a). The original composition of the K-feldspar-bearing sillimanite schist contains large amounts of hydromica clays. Considering that there are very small amount of matrix muscovite in all the samples, we deduce that the formation of large quantities of prismatic sillimanite and K-feldspar are partly caused by the breakdown of muscovite. Ms+Pl+Qtz=Als+Kfs+Liq (1) The fibrous sillimanite might also form during this process. Reaction (1) represents the transition from the lower amphibolite facies to the upper amphibolite facies. The sillimanites formed during above process would recrystalize and grain growth occurred in the following granulite metamorphism. Considering the Al-rich feature of the metapelite, the sedimentary protolish likely consists of pyrophyllite, quartz and mica. With the increase of the burial depth, the preserved water-rich volatile phase distributing along the grain boundaries will induce the initial small amount of melts during the ( a ) ( b) 0.2 mm 1 mm ( c ) ( d) 1 mm 0.5 mm () f ( e) 0.5 mm 0.5 mm Figure 5. Textures in different types of rocks in the Mashan complex. (a) Prismatic sillimanite and fibrous sillimanite occur in Kfs or coexist with K-feldspar. Exsolution texture can be seen in the K-feldspar (Spl-bearing Grt-Sil schist); (b) porphyroblastic garnet (inclusions include: quartz, plagioclase, biotite and sillimanite) was separated by retrograde biotite from K-feldspar (Spl-free Grt-Sil gneiss); (c) large quantities of aggregates of prismatic sillimanite, fibrous sillimanite, quartz and biotite in the garnet poikiloblasts (Spl-free Grt-Sil gneiss); (d) cordierite porphyroblasts show intergrowth with K-feldspar and ilmenite, and there are also biotite and sillimanite inclusions in cordierite (Grt-Crd-Sil schist); (e) cordierite porphyroblast include biotites, prismatic sillimanite and fine-grained garnet aggregates (Grt-Crd-Sil schist); (f) typical compositional zoning in plagioclase and exsolution quartz in the rim of the plagioclase (Grt-Crd-Sil schist). Different analyzed points are marked by open circles and labeled by numbers. The labeled number is the analytical point. (a) (e) are taken by polarized light, and (f) is back-scattered electron image.

12 218 Xinzhuan Guo, Akira Takasu, Yongjiang Liu and Weimin Li prograde metamorphism. This is consistent with thin quartz-feldspar leucosomes distributed in the sillimanite-rich rocks, and the further anatectic melting would occur during the initial formation of sillimanite and K-feldspar via reaction (1). Generally, there are two types of garnet in the metapelites of the Mashan complex. The first type, subhedral, size up to 2 mm, is charactered by: 1) rare inclusions in garnet grains; and 2) retrograde minerals (e.g., biotite, muscovite and chlorite) growing along the boundary or the cracks of the garnet. This kind of garnet and matrix biotite could have formed via continuous reactions Chl+Ms=Grt+Bt+Qtz+H 2 O±melt (2) The other type garnet, size up to 2 cm, coexists with K-feldspar (seperated from garnet by retrograde biotite, Fig. 5b) and contains large quantities of aggregates of prismatic sillimanite, fibrous sillimanite, quartz and biotite in garnet, showing the genetic relationship between this type of garnet and fibrous sillmanite and prismatic sillimanite (Fig. 5c). This texture suggests the reaction Bt+Sil+Qtz=Kfs+Grt+Liq (3) For the Crd-Grt-Sil schist, cordierite porphyroblasts show intergrowth with K-feldspar and ilmenite. There are sillimanite and biotite inclusions (Fig. 4d) in the cordierite porphyroblast. This texture suggests the reaction Bt+Sil+Qtz=Crd+kfs+Liq+Il (4) For the sample SL06-2, cordierite coexists with sillimanite and garnet, which can be explained by reaction Bt+Sil+Qtz=Crd+Grt+Kfs+H 2 O (5) This discontinuous reaction ultimately produces the assemblages of cordierite+garnet+k-feldspar+biotite and cordierite+garnet+k-feldspar+sillimaite. The reaction (5) indicates the transition from the upper-amphibolite to the granulite facies conditions. At the peak metamorphic conditions, perthite textures (Fig. 6a) in metapelite samples (MS3-3) with the host mineral of Kfs and the exsolution lamella of plagioclase (An ) are widely developed. For the Crd-Grt-Sil schist, some cordierite porphyroblast includes biotites, prismatic sillimanite and fine-grained garnet aggregates (Fig. 5e), which is a typical decompression texture, indicating the decompression process Sil+Grt+Qtz=Crd (6) With the depletion of garnet by this reaction, in some layers, garnets were consumed completely and Grt-free Sil-Crd schist was formed. Another decompression proof in the metapelites of the Mashan complex is the compositional zone in the plagioclases (Ca component decreases from the core to the rim, core, An 68 72, rim, An ). We should note that in this process the temperature might not decrease because of the exsolution texture in the rim of plagioclase (Fig. 5f). Reactions (2) and (3) are considered to represent large-scale melt-producing reactions in the Mashan complex. The typical minerals, e.g., staurolites and andalusites, representing for lower metamorphic grade were not found in this study. But the final stage of retrograde metamorphism was widely developed: the partial alteration of the peak minerals to muscovite and chlorite, which were demonstrated by the following reactions. Pl+Sil+H 2 O=Ms+Qtz (7) Grt+H 2 O=Chl+Qtz (8) The mineral paragenesis of the metepelites of the Mashan complex is shown in the Table P-T Estimations and the P-T Path The mineral assemblages in Al-rich metapelites from the Mashan complex provide limited potential geothermobarometers to estimate the temperature and pressure conditions during the peak and subsequent retrograde metamorphic evolution. We summarize them in Table 12. The calculated results are briefly discussed below. Figure 6. (a) Back-scattered electron image photograph shows the perthite texture of sample MS3-3; lamellae concentrated core is labeled (C) and whole feldspar grain containing lamellae-bearing core and lamellae-free rim is labeled (W). (b) Ternary plot of feldspar compositions along with re-integrated recovered one-phase feldspar compositions Solvus curves are calculated at 0.8 GPa using the model of Fuhrman and Lindsley (1988). Open squares indicate the chemical composition of the host and the lamellae. Solid black circles represent the re-integrated pre-exsolution one phase feldspar composition for the whole grain.

13 Zn-Rich Spinel in Association with Quartz in the Al-Rich Metapelites from the Mashan Khondalite Series, NE China 219 Table 11 The mineral paragenesis of the metepelites from the Mashan complex Ternary feldspar geothermometer The Spl-bearing Grt-Sil schist samples of the metapelites show a typical mesoperthite texture (the host: potassium feldspar and the lamella: plagioclase). This gerthermometer is referred to the method of Fuhrman and Lindsley (1988). Area scanning electron microprobe analysis of fine-scale exsolution textures is usually applied to obtain pre-exsolutuion original compositions of feldspars (e.g., Harley, 1985; Sandiford, 1985). Chemical composition of the lamella with the highest Ca was treated as that of plagioclase domain of the exsolution. Chemical composition of the host with the highest K was regarded as that of K-feldspar domain of the exsolution. Area proportions of the host and the lamella were estimated by IMAGE TOOL V3.0. The calculated pre-exsolution chemical compositions of the feldspar core and the whole grain chemical composition of the feldspar are Or : Ab : An=80.8 : 16.0 : 2.9 and Or : Ab : An=81.9 : 15.2 : 2.6, respectively, yielding a temperature of 820 (Fig. 6b). This thermometer was applied to the Spl-free Grt-Sil gneiss (Lg25-3) and Spl-bearing Grt-Sil schist (MS3-3 and SL06-4). Garnet-biotite theomometry using the calibration of Bhattacharya et al. (1992) yields up to 756. However, the computation using Hodges and Spear (1982) shows temperatures up to 671. The estimated temperatures are markedly lower than that of ternary feldspar geothermometer. This might caused by the retrograde Fe-Mg exchange re-equilibration, but there are no effect on ternary feldspar geothermometer (Frost and Chacko, 1989) Ti-in-biotite geothermometry Ti-in-biotite geothermometry (Henry et al., 2005) was also used to check the metamorphic temperatures of the Al-rich metapelites (Fig. 7), which contained ilmenite or rutile and had equilibrated at roughly 4 6 kbar. The maximum temperature estimated by this method is about 747 and the minimum temperature is about 690. Based on the above computations of different geothermometries, 820 is regarded as the peak temperature for the metapelites from the Mashan complex. This peak temperature is consistent with Jiang s (1992) result GASP and Grt-Crd-Sil geobarometer The GASP (garnet-sillimanite-quartz-plagioclase) (Holland and Powell, 1998) and Grt-Crd-Sil (Harris and Holland, 1984) geobarometries were used to estimate the pressure of the Mashan complex. The maximum pressure is calculated

14 220 Xinzhuan Guo, Akira Takasu, Yongjiang Liu and Weimin Li Table 12 Summary of pressure-temperature data of Al-rich metapelites from the Mashan complex Sample No. MS3-3 SL06-4 LG25-3 SL06-7 SL06-1 Geothermometry (5 kbar) Grt-Bt Bhattacharya et al. (1992) Temperature ( ) Temperature ( ) Temperature ( ) Temperature ( ) Hodges and Spear (1982) Temperature ( ) Temperature ( ) Temperature ( ) Temperature ( ) Ti-in-biotite Temperature ( ) Temperature ( ) Temperature ( ) Temperature ( ) Henry (2005) Ternary feldspar Temperature ( ) Fuhrman and Lindsley (1988) 820 Geobarometry Grt-Crd-Sil (690 ) Pressure (kbar) Harris and Holland (1984) 5.7 Grt-Sil-Pl-Qtz Pressure (kbar) Pressure (kbar) Holland and Powell (1998) Ti (a.p.f.u.) P= 4-6 kbar SL06-1 SL06-4 SL06-2 LG25-3 Sl Mg/ ( Mg+ Fe 2+ ) Figure 7.Ti-in-biotite geothermometer (Henry et al., 2005). The dashed curves represent the intermediate 50 interval isotherms. Symbols in the diagram show the Ti (a.p.f.u.) vs. Mg/(Mg+Fe) of biotites from different samples in this study. based on the assemblage in the sample Lg25-3, yielding a pressure of 8.0 kbar (T=756, calculated from Grt-Bt thermometer, Bhattacharya et al., 1992). This pressure is regarded as the peak pressure of the metapelites from the Mashan complex Petrogenetic grids Pattison et al. s petrogenetic grid (Pattison et al., 2003) with the equilibrium curves delimiting the stability fields of specific minerals and mineral assemblages was used in this study to clarify the P-T path of the Mashan complex (Fig. 8). Based on the petrography study and the chemical composition of the minerals, the metamorphism of the Al-rich metapelites in the Mashan complex is divided into three metamorphic stages: pre-peak (lower to upper amphibolites facies), peak (granulite facies) and retrograde stages. The first occurrence of assemblages, sillimanite+ K-feldspar, indicates the transition from the lower amphibolite to the upper amphibolites facies. With temperature increasing, association of cordierite and garnet occurs, which marks the onset of the granulite facies. The widespread anatectic veins are believed to occur at the peak metamorphic conditions. The exsolutions of the feldspar occur in the highest granulite facies metamorphic stage. Orthopyroxene is not found in the Al-rich metapelites, so the upper temperature limit can be controlled in the lower temperature site by the reaction line Bt+Qtz+Grt+Pl=Opx+Crd+Liq±K-feldspar (Fig. 8). As discussed before, the decompression texture and the exsolution textures in the rim of the plagioclase suggest that the Mashan complex underwent an isothermal decompression process. The last stage can be controlled by the reaction: Grt+H 2 O=Chl+Qtz. Finally, a clockwise orogen style P-T path was constructed. 5.3 Spinel-Quartz Assemblage Spinel+quartz assemblage has been documented from a large number of localities. In most cases, the other diagnostic UHT assemblages, such as sapphirine+quartz, orthopyroxene+ sillimanite±quartz and osumilite, also occur (summarized by Kelsey, 2008 and Harley, 2008). Reaction microstructures for spinel-quartz assemblage include: 1) a corona of cordierite (Barbosa et al., 2006); 2) a spatially organised corona with sillimanite adjacent to spinel and garnet adjacent to quartz (Dasgupta et al., 1995); 3) garnet or sillimanite coronas (e.g., Perchuk et al., 1989); 4) a spatially organised corona of sillimanite and orthopyroxene (Bose et al., 2000); 5) a mantle of sapphirine±sillimanite (Tsunogae and van Reenen, 2006); 6) a multiple of corona of garnet, sapphirine and sillimanite (Ellis et al., 1980). All the textures above show close relationships between spinel and other diagnostic high temperature minerals (e.g., cordierite, orthopyroxene and sapphirine). But the assemblage spinel+quartz lonely cannot be used as a diagnostic assemblage for UHT. The chemical impurity in spinel will change

15 Zn-Rich Spinel in Association with Quartz in the Al-Rich Metapelites from the Mashan Khondalite Series, NE China 221 Figure 8. Experimental and thermodynamically predicted positions of reactions that limit the stability of granulite-facies mineral associations. Continuous lines, from Pattison et al. (2003); dotted lines, high-grade FMASH equilibria from Harly (1998). The inserted images represent different textures that occurred in different metamorphic stages. Dashed line means the P/T conditions are not confirmed. its stability. Nichols et al. (1992) demonstrated that high Zn content in spinel with quartz lowered the stability temperature. Santosh et al. (2006) described both low Zn and high Zn spinels in the same samples of granulites from the North China Craton and showed that the low Zn spinels in equilibrium with quartz formed at ultrahigh-temperature conditions, whereas the high Zn spinels formed during the retrograde stage. Numerical studies (Powell and Sandiford, 1988; Ackermand et al., 1987; Hensen, 1986) have shown that the stability of spinel+quartz will move to lower temperature field under oxidizing conditions through incorporation of Fe 3+ in to spinel. Zn and Fe 3+ contents of spinel in our samples are high. Therefore, the spinel+quartz assemblage in our samples cannot indicate UHT metamorphism. The Zn-rich spinel has been attributed to the decomposition of either sphalerite (Spry and Scott, 1986), or biotite (Dietforst, 1980), or, most frequently, to the breakdown of Zn-rich staurolite in assemblages both bearing (Schumacher, 1985) and devoid of muscovite+quartz (Tuccillo et al., 1992; Atkin, 1978). The first genetic possibility is easily to be excluded based on the petrographic research. In the Mashan complex, spinel and quartz only occur as inclusions in the peak minerals, and in most cases they coexist with biotite. The reaction microstructure reported by Dietforst (1980) is very similar to that in our study (Figs. 3a, 3d). Assuming that the formation of gahnite is due to the breakdown of Zn-rich biotite, then spinel should show higher Zn value in the rim closer to the boundary between spinel and biotite and the biotite should contain certain amount of Zn. In fact, almost all the spinels show Zn decreasing from core to rim (Fig. 4), which is completely opposite to Dietforst s (1980) result. Furthermore, almost no Zn component is found in the neighboring biotite. Therefore, it can be concluded that the gahnite-rich spinels are not the resultant of the breakdown of biotites. Albee (1972) showed that Zn strongly partitioned into staurolite and substitute into a Fe 2+, IV site. Staurolite is the only common metamorphic mineral with such a cation site (Griffen and Ribbe, 1973). Zn 2+ also prefers the tetrahedral sites in the (normal) spinel structure. As a result, staurolite may be a direct precursor of Zn-rich spinel. Stoddard (1979) reported the formation of zinc-rich hercynite of Adirondack specimens (the minerals assemblages are similar to that of the spinel bearing metapelites in the Mashan complex) was due to the dehydration of staurolite via a uni-