Journal of Earth Science, Vol. 21, No. 5, p. 581–597, October 2010 ISSN 1674-487X Printed in China DOI: 10.1007/s12583-010-0128-7

Progress and Controversy in the Study of HP-UHP Metamorphic Terranes in the West and Middle Central China Orogen

Liu Liang* (刘良) State Key Laboratory of Continental Dynamics, Department of Geology, Northwest University, Xi’an 710069, China Yang Jiaxi (杨家喜) College of Earth Sciences and Land Resources, Chang’an University, Xi’an 710054, China Chen Danling (陈丹玲) State Key Laboratory of Continental Dynamics, Department of Geology, Northwest University, Xi’an 710069, China Wang Chao (王超) State Key Laboratory of Continental Dynamics, Department of Geology, Northwest University, Xi’an 710069, China; Xi’an Institute of Geology and Mineral Resource, Xi’an 710054, China Zhang Chengli (张成立), Yang Wenqiang (杨文强), Cao Yuting (曹玉亭) State Key Laboratory of Continental Dynamics, Department of Geology, Northwest University, Xi’an 710069, China

ABSTRACT: During the past ten years, various types of HP-UHP metamorphic rocks have been dis- covered in the South Altyn Tagh, the North Qaidam and the North Qinling (秦岭) in the West and Middle Central China orogen. The UHP rocks, as lentoid bodies in regional gneisses, include eclogite (garnet-bearing pyroxenite), garnet peridotite and various pelitic or felsic gneisses. There are many re- cords of minerals and microstructures of exsolution indicate the UHP metamorphism, such as coesite (or its pseudomorph), diamond, exsolution of clinopyroxene/amphibole/+rutile or rutile+quartz+apatite in garnet, exsolution of quartz in omphacite and exsolution of kyanite+spinel in precursor stishovite. The discovery of microstructure evidence for the This study was supported by the National Basic Research Pro- presence of precursor stishovite in typical Al- gram of China (No. 2009CB825003), the National Natural rich gneiss from the South Altyn Tagh reveals Science Foundation of China (Nos. 40972128, 40572111), and continental subduction and exhumation to and the Ministry of Science and Technology of China for the State from a depth of more than 350 km. It is the Key Laboratory of Continental Dynamics of Northwest Uni- petrological record of the deepest subduction versity. and exhumation of continental rock in the world. *Corresponding author: [email protected] The in situ zircon U-Pb dating using LA-ICP- © China University of Geosciences and Springer-Verlag Berlin MS or SHRIMP methods shows that the meta- Heidelberg 2010 morphic ages of the HP-UHP rocks in the South Altyn Tagh, the North Qaidam and the North Manuscript received May 20, 2010. Qinling are 475–509, 420–457, and 485–514 Ma, Manuscript accepted July 31, 2010. respectively. The metamorphic ages of HP-UHP 582 Liu Liang, Yang Jiaxi, Chen Danling, Wang Chao, Zhang Chengli, Yang Wenqiang and Cao Yuting

rocks in the North Qaidam are 20–80 Ma younger than those in the South Altyn Tagh and the North Qinling, and the metamorphic ages do not systematically increase or decrease from the South Altyn Tagh through the North Qaidam to the North Qinling. The absence of time transgressive variety of the metamorphism in the three regions does not support the hypothesis that the HP-UHP rocks in these re- gions form the same HP-UHP metamorphic zone. And the HP-UHP rocks in these regions can not be simply correlated to the collision between the North China plate and the South China plate. At present, the study of the HP-UHP rocks in the West and Middle Central China orogen faces several key issues or challenges, such as: (1) the continental subduction to the mantle depth of stishovite stability field (>9 GPa) is occasional or universal; (2) the mechanism of exhumation for the continental rocks subducted to the depth of stishovite stability field (>300 km); (3) the tectonic setting and geodynamical mechanism of producing the HP-UHP metamorphic zones in the South Altyn Tagh, the North Qaidam and the North Qinling. Further studies aiming at these problems will make important progress not only in me- tamorphism of the HP-UHP rocks in the West and Middle Central China orogen, but also in continen- tal deep subduction and exhumation in solid earth science. It will also contribute to the establishment of the theory of continental deep subduction. KEY WORDS: HP-UHP metamorphic zone, continental deep subduction, zircon LA-ICP-MS and SHRIMP dating, South Altyn Tagh, North Qaidam, North Qinling.

INTRODUCTION standing. In this article, we present an overview of re- Since the discoveries of coesite in metamor- cent progress in the study of these regions as well as phosed sedimentary rocks from the Dora Maira massif the challenges for future studies. of the weatern Alps by Chopin (1984) and the West- ern Gneiss region of Norway by Smith (1984), and IMPORTANT PROGRESS microdot diamond in metamorphosed sedimentary During the past ten years, various HP-UHP me- rocks from the Kokchetav of Kazakstan by Sobolev tamorphic rocks have been discovered in the South and Shatsky (1990) and in eclogite from the Dabie Altyn Tagh, the North Qaidam and the North Qinling Mountain of China by Xu et al. (1992), ultrahigh (Yang and Powell, 2008; Liu et al., 2007a, 2005, pressure (UHP) metamorhism, as well as explicated 2004a, b, 2003a, b, 2002, 2001, 1997, 1996a; Song et deep subduction of continental crust, has become one al., 2006, 2005a, b, 2004, 2003a, b; Yang J S et al., of the hottest and most competitive fields in earth sci- 2003, 2002a, b, 2001, 2000, 1998; Zhang J X et al., ence. So far, UHP metamorphic rocks have been dis- 2005a, 2002, 2001, 1999; Song and Yang, 2001; Liu, covered in at least 22 orogenic belts around the world 1999; Liu and Zhou, 1995; Yang J J et al., 1994;). (Liou et al., 2009, 2007; Carswell and Compagnoni, They were believed to be the products of deep sub- 2003), including 6 regions of China as the Dabie-Sulu duction of continental crusts in the Early Paleozoic. (Cong, 1996), the North Qinling (Liu et al., 2003; Some rocks show a very deep origin of depth of >200 Yang et al., 2002b), the North Qaidam (Song et al., km (>6–7 GPa) (Liu et al., 2005, 2004b, 2003a; Song 2003a; Song and Yang, 2001; Yang et al., 2001), the et al, 2005b, 2004) or even >350 km (>12–14 GPa) South Altyn Tagh (Liu et al., 2002, 2001) and the (Liu et al., 2007a). Southwest Tianshan (Zhang L F et al., 2002). In recent years many important progresses in the study of high South Altyn Tagh and ultrahigh pressure (HP-UHP) metamorphic rocks Detailed field survey and petrological work show have been made in the South Altyn Tagh, the North that the HP-UHP metamorphic rocks in the South Al- Qaidam and the North Qinling which belong to the tyn Tagh orogenic belt mostly occur in three areas, west and middle parts of the Central China orogen. Jianggalesayi, Qingshuiquan and Yinggelisayi (Fig. 1). However, some problems about HP-UHP meta- The UHP rocks are eclogite, kyanite- and garnet- morhism in these regions are in need of better under- bearing pelitic gneiss, magnesite-bearing garnet Progress and Controversy in the Study of HP-UHP Metamorphic Terranes 583

Figure 1. Geological and tectonic map of the Altyn Tagh orogen (modified after Liu et al., 2002). TRB. Ta- rim basin; QL. Qilian Mountains; QDB. Qaidam basin; HMLY. Himalaya Mountains; INP. Indian plate; WKL. western Kunlun Mountains; EKL. eastern Kunlun Mountains; I. Tarim block; II. Qaidam block; III.

Altyn Tagh orogenic belt; III1. North Altyn Tagh Archean complex; III2. North Altyn Tagh subduction- collision complex; III3. Milanhe-Jinyanshan block; III4. South Altyn Tagh subduction-collision complex. lherzolite, Kfs-bearing garnet pyroxenite and garnet- assemblage of Kfs-bearing garnet clinopyroxenite is bearing granitic gneiss. All of them are lenticular bo- Grt+Cpx (clinopyroxene)+Kfs (K-feldspar)+SiO2 dies occurring in widespread middle- or high-graded (maybe coesite), and the pressure is estimated to be >7 granitic gneiss, paragneiss and marble. GPa by using the geobarometer for volume percentage In the eclogites from Jianggalesayi, UHP micro- of exsolved pyroxene in garnet and the Si-(Al+Cr) structures include pseudomorphs of coesite in garnets geobarometer for majoritic garnet (Liu et al., 2005). (Fig. 2a), exsolved rods of quartz in omphacites (Fig. P-T estimation for the peak-stage mineral assemblage, 2b) (Liu et al., 2009a; Zhang J X et al., 2002) and the Grt+Per (perthite) (before exsolution)+Ttn (titanite) exsolutions of ilmenite in rutile (Liu et al., 2004b). (before exsolution)+Ky (kyanite)+Zoi (zoisite)+Qz/ The peak-stage metamorphic mineral assemblage of Coe (coesite)±Cpx, produces the result of 3.7–4.3 GPa the eclogite is Grt (garnet)+Omp (Omphacite)+Ph for granitic gneiss (Liu et al., 2004a). Grt+Ol (oli- (phengite)+Ru (rutile)+Qz (quartz), Zhang J X et al. vine)+Opx+Cpx±Mgs (magnisite) is the peak-stage (2002) estimated P-T conditions at 820–850 ℃ and metamorphic mineral assemblage of garnet lherzolite, 2.8–3.2 GPa by using garnet-omphacite-phengite ba- and peak P-T conditions are estimated at 3.8–5.1 GPa rometry and garnet-omphacite and garnet-phengite and 880–970 ℃ using garnet-orthopyroxene barome- thermometry for the eclogite (Fig. 3a). Based on the ter and garnet-clinopyroxene Fe-Mg exchange ther- microstructures and P-T estimation, the depth of sub- mometer (Liu et al., 2002). Subsequently, Zhang et al. duction is estimated to be >200 km (Liu et al., 2004b). (2005a) thought that the garnet lherzolite, Grt clino- In Yinggelisayi, clinopyroxene+rutile exsolutions in pyroxenite and granitic gneiss in Yinggelisayi under- garnets as UHP microstructures are found in the gar- went HP/UHT metamorphism owing to the discovery net lherzolite and Kfs-bearing garnet clinopyroxenite, of sapphirine-bearing metabasites which occur as ma- the exsolutions of rod-like plagioclase+amphibole in fic layers in the garnet peridotite. Wang et al. (in re- titanite are found in granitic gneiss (Liu et al., 2005, view) reported that the Yinggelisayi garnet peridotites 2004, 2002). The peak-stage metamorphic mineral have the records of an early transformation from 584 Liu Liang, Yang Jiaxi, Chen Danling, Wang Chao, Zhang Chengli, Yang Wenqiang and Cao Yuting spinel peridotite into garnet peridotite (peak stage at decompression products of Al-Fe-bearing stishovite. 4.2–6.0 GPa and 920–990 ℃), a latter transformation This discovery suggests that the minimum metamor- into amphibole garnet peridotite and a final transfor- phic pressure was estimated to exceed 12 GPa, and the mation into spinel peridotite (Fig. 3a). Moreover, ori- depth of subduction/exhumation of continental crust is ented rods of kyanite+spinel are observed in quartz possibly >350 km (Fig. 3b). from kyanite- and garnet-bearing pelitic gneisses from LA-ICP-MS or SHRIMP zircon in situ U-Pb Jianggalesayi (Fig. 2c ) (Liu et al., 2007a). Based on dating (Table 1) shows that the metamorphic ages of microstructure observations, EBSD (electron back- eclogite from the Jianggelesayi and its country rock, scattering diffraction) and universal stage analyses, garnet biotite gneiss, are 493±4.3 and 499±27 Ma, re- petrological experiments under high temperature and spectively. The protolith of the eclogite formed at the pressure conditions, the rods were interpreted to be the age of 754±9 Ma (Liu et al., 2007b). The metamorphic

Figure 2. Microstructures of representative UHP rocks from South Altyn Tagh and North Qaidam. Amp. Amphibole; Sp. Spinel. Progress and Controversy in the Study of HP-UHP Metamorphic Terranes 585

Figure 3. P-T path of HP-UHP rocks from South Altyn Tagh, North Qaidam and North Qinling. And. An- dalusite; Sill. sillimanite; Ky. kyanite; Hey. hercynite; Grt. garnet; Sp. spinel; Pl. plagioclase; Qz. quartz.

Table 1 Zircon U-Pb geochronological data for HP-UHP rocks from South Altyn Tagh

Sample locality/ Protolith age Metamorphic age (Ma) Method Data source rock type (Ma) Jianggalesayi Eclogite 495±4 750 LA-ICP-MS Liu et al., 2007b 504±5 TIMS Zhang et al., 1999 Grt-biotite gneiss 499±27 700–940 LA-ICP-MS Liu et al., 2007b Yinggelisayi Grt-bearing gran- 487±10 842±18 SHRIMP Zhang A D et al., 2004 itic gneiss 495±7 846±17 LA-ICP-MS Liu et al., 2009a 493±7 SHRIMP Zhang et al., 2005a Kfs-bearing Grt 488.7±8.3 821±27 LA-ICP-MS Liu et al., 2009a clinopyroxenite 497±11 SHRIMP Zhang et al., 2005a Grt lherzolite 509±12 844±9 LA-ICP-MS Liu et al., 2009a 501±16 SHRIMP Zhang et al., 2005a Qingshuiquan Grt-bearing gran- 499±4 LA-ICP-MS Unpublished data itic gneiss

Grt-Ky-Ms gneiss 486±5 719±14 LA-ICP-MS Cao et al., 2009 586 Liu Liang, Yang Jiaxi, Chen Danling, Wang Chao, Zhang Chengli, Yang Wenqiang and Cao Yuting ages of magnesite-bearing garnet lherzolite, potash the garnet peridotites are discovered only at the feldspar-bearing garnet pyroxenite and garnet-bearing Shenglikou (Yang and Powell, 2008; Song et al., 2004; granitic gneiss from Yinggelisayi are 509±12, 489±8, Yang et al., 1994). This UHP terrane was first estab- and 487±10 Ma, respectively, and their protoliths are lished because of identification of coesite inclusions in 844±9, 821±27, and 842±18 Ma (Liu et al., 2009a; zircon from a paragneiss, the wall rocks of Yematan Zhang et al., 2005, 2004). The metamorphic ages of eclogite (Song et al., 2003a; Yang et al., 2001). Sub- Grt-bearing granitic gneiss and kyanite- and garnet- sequently, diamond inclusions are found in zircons, bearing biotite gneiss from the Qingshuiquan are exsolved Ti-Fe oxides are found in olivines and the 499±4 and 486±5 Ma, respectively (Cao et al., 2009; exsolution of pyroxene+soda amphibole is in garnet Liu et al., 2009a). These metamorphic ages from dif- from garnet peridotite at Shenglikou (Figs. 2d, 2e) ferent areas are consistent within the analytical errors, (Song et al., 2004). Relict coesites also have been confirming that the HP-UHP rocks of the three areas discovered in garnets of eclogites at Shaliuhe and formed in the same time (Early Paleozoic age of Yukahe (Fig. 2f) (Zhang G B et al., 2009a; Zhang J X 475–509 Ma in the South Altyn Tagh tectonic belt). et al., 2009). A peak eclogite facies stage with the as- semblage Grt+Omp+Ky+Rt+Phn is at 2.9–3.3 GPa North Qaidam and 729–746 ℃, and a high-pressure granulite facies The HP-UHP metamorphic rocks in the North stage with Grt+Cpx (Jd<30)+Pl (An=24–29)+Scp Qaidam region mainly occur at Yukahe, Xitieshan and (scapolite) at 1.9–2.0 GPa and 873–948 ℃ for Shenglikou (also known as the Lüliangshan) areas of Shaliuhe eclogite (Fig. 3a) (Song et al., 2003a). In the Da Qaidam Town as well as Yematan and Yukahe, using Grt-Phn and Grt-Cpx Fe-Mg geother- Shaliuhe areas of the Dulan County, lying northwest mometer and Grt-Omp-Phn geothermometer, P-T es- to southeast for over 700 km (Fig. 4). Recognized timates yield 3.0–3.4 GPa and 620–680 ℃ for the UHP rocks are eclogite, garnet peridotites and various eclogite with the peak-stage mineral assemblage of gneisses. Eclogites occur as lentoid bodies in gneisses Grt+Omp+Phn+Rt, and 2.3–2.4 GPa and 700–720 ℃ at Xitieshan, Yukahe, Yematan and Shaliuhe areas, for the retrograde stage with Grt+Omp+Phn+Amp

Figure 4. Geological map of the North Qaidam showing distribution of the HP-UHP terrane (modified after Yang et al., 2001). Progress and Controversy in the Study of HP-UHP Metamorphic Terranes 587

Table 2 Geochronological data of zircons for HP-UHP rocks in different areas from the North Qaidam

Sample locality/ Metamorphic Protolith age Method Data source rock type age (Ma) (Ma) Yukahe Eclogite 436±3; 431±4 783–793 LA-ICP-MS Chen et al., 2009 Plagiogneiss 431±3 LA-ICP-MS Chen et al., 2009 Pelitic gneiss 432±19 LA-ICP-MS Chen et al., 2007a Xitieshan Eclogite 421–458 699–710 LA-ICP-MS Chen et al., 2007b 440–460 SHRIMP Zhang G B et al., 2009 437±6 SHRIMP Meng et al., 2005 Paragneiss 437±16 Zhang J X et al., 2008 435±7 890±14 Zhang J X et al., 2008 Shenglikou Garnet olivine 423±5; 420±5 484–444 SHRIMP Song et al., 2005a Paragneiss 447±3; 421±5 SHRIMP Zhang J X et al., 2008 454±6 1 005–1 844 SHRIMP Zhang J X et al., 2008 443±4 SHRIMP Zhang J X et al., 2008 Granitic or- 448±13 891±31 SHRIMP Zhang J X et al., 2008 thogneiss

Yematan Eclogite 422±4; 449±2 SHRIMP Mattinson et al., 2007 442±4; 433±5 457±7 SHRIMP Song et al., 2006 Coesite-gneiss 440±17 SIMS Yang et al., 2003 Coesite-gneiss 423±6 909–2 192 SHRIMP Song et al., 2006 Shaliuhe Eclogite 425 730 LA-ICP-MS Chen et al., 2007b 445±7 SHRIMP Zhang G B et al., 2008 assemblage (Fig. 3a) (Chen et al., 2005). Zhang et al. were formerly mantle peridotites emplaced into the (2009) also obtained P-T conditions of peak eclogite oceanic crust and then were subjected to serpentiniza- stage at 2.9–3.2 GPa and 566–613 ℃ , and retrograde tion by seawater-derived fluids near the sea floor prior stage at 1.8–2.5 GPa and 670–700 ℃ for Yuka ec- to subduction to 3.0–3.5 GPa and 700 ℃ (Yang and logites (Fig. 3a). Mineral-pairs P-T estimates using Powell, 2008) (Fig. 3a). It is mentionable that the P-T Al-in-Opx geothermobarometer and Grt-Ol geother- estimates of eclogites from Shaliuhe and Yukahe sug- mometer yield re-equilibrium conditions of 960–1 040 gest that the eclogites were overprinted by HP ℃ and 5.0–6.5 GPa, together with the microstructures granulite-facies metamorphism during exhumation of the peridotite from Shenglikou, Song et al. (2004) (Zhang G B et al., 2009; Chen et al., 2005; Zhang et inferred that the garnet peridotites were crystalized at al., 2005b; Song et al., 2004, 2003a) (Fig. 3a). a depth of >200 km (Fig. 3a). However, recent petro- Quite a number of the metamorphic ages of logic study and P-T estimates based on several pseu- HP-UHP rocks obtained by different methods in vari- dosections suggest that these UHP ultramafic rocks ous areas of the North Qaidam have been reported 588 Liu Liang, Yang Jiaxi, Chen Danling, Wang Chao, Zhang Chengli, Yang Wenqiang and Cao Yuting

(Table 4 in Liu et al., 2009a). It is now clear that the in North Qinling situ zircon U-Pb dating by SHRIMP and LA-ICP-MS The HP-UHP rocks in the North Qinling mostly methods is more meaningful than those achieved by occur in the areas along Guanpo and Shiziping of TIMS method (Liu et al., 2009a). The ages achieved Lushi County in Henan Province, and in the Song- by TIMS method at earlier years are always 40–50 Ma shugou area of the Shangnan County in Shaanxi older than those achieved by SHRIMP and Province (Fig. 5). The eclogites mostly occur in LA-ICP-MS methods. The possible reason is that zir- Guanpo and Shiziping areas, and the typical mineral cons of various rocks in the region have inherited assemblage is Grt+Omp±Ru (Hu et al., 1995a). Coe- cores formed in those protoliths. The TIMS dating site pseudomorph is first reported in the eclogite by ages might be the mixing age of the metamorphic age Hu et al. (1995b), together with P-T estimate, and in- and the forming age of the protoliths and hence have dicates the eclogite having a clockwise P-T path (Fig. no actual significance (Liu et al., 2009a). In addition, 3a) (Hu et al., 1995a). However, there is no sufficient the metamorphic age of a Xitieshan eclogite obtained evidence for the coesite to be approved. Subsequently, by Zhang et al. (2005b) using SHRIMP method is diamond inclusions have been found in zircons from 480±16 Ma (Zhang et al., 2005b). After a careful re- eclogites and gneisses provide additional evidence for view of the initial data in that paper (Table 7 in Zhang the UHP metamorphism (Yang et al., 2002b). The HP et al., 2005b), it can be found that the 206Pb/238U ages basic granulite, garnet pyroxenite and felsic granulite of seven data used for calculating the average age vary (Liu et al., 1996b, 1995a), as well as the garnet- significantly from 450 to 502 Ma. The errors of five amphibole rocks (Yang Y et al., 1994; Chen et al., age data are up to 39–59 Ma. Therefore, the age of 1993) mostly occur in Songshugou area, and the gar- 480±16 Ma is unbelievable. Table 2 just lists high net pyroxenite can be traced discontinuously from precision ages achieved by SHRIMP and LA-ICP-MS eastward to the north of Zhaigen area of Xixia County methods for HP-UHP rocks in different areas from the in Henan Province. The peak metamorphic mineral North Qaidam. Thus, reasonable metamorphic age of assemblage of HP basic granulite in Songshugou area the HP-UHP rocks in the North Qaidam is 420–457 is Grt+Cpx+Q+Pl, and peak condition is estimated at Ma. 1.4–1.5 GPa and 800–900 ℃ (Liu et al., 1995a, b). A peak granulite facies stage of HP felsic granulite with

Figure 5. Geological map of the North Qinling showing distribution of the HP-UHP terrane. Progress and Controversy in the Study of HP-UHP Metamorphic Terranes 589 the assemblage of Grt+Ky+Per+Rt+Qz is at 1.3–1.6 the Guanpo eclogite is 507±37 (Yang et al., 2002b) or GPa and 800–900 ℃ (Liu et al., 1996b). Later, Liu et 505±12 Ma (Liu et al., 2009b), and the protolith age is al. (2003) found that garnets in the felsic granulite >750 Ma. In the Songshugou area, the metamorphic from Songshugou area contain abundant exsolved rods age of the basic granulite is 485±3.3 Ma (Chen et al., of rutile+quartz+apatite, which indicate the rock may 2004), that of the amphibole eclogite is 514±9 undergo UHP metamorphism, and the forming pres- (SHRIMP) or 500±10 Ma (LA-ICP-MS) (Liu et al., sure is deduced to be >7 GPa (Liu et al., 2003). Pre- 2009b), and the protolith age is >720 Ma. The meta- vious studies show that the Songshugou HP granulite morphic age of the Zhaigen garnet pyroxenite is also have the clockwise P-T paths (Liu et al., 1996b, 500±6 Ma (Liu et al., 2009b). Accordingly, the meta- 1995b). morphic ages of the various HP-UHP rocks in various LA-ICP-MS or SHRIMP in situ zircon U-Pb areas of the North Qinling are 485–514 Ma. dating (Table 3) shows that the metamorphic age of

Table 3 Geochronological data of zircons for HP/UHP rocks in different areas from the North Qinling

Sample locality/ Metamorphic age Protolith age Method Data source rock type (Ma) (Ma)

Guanpo Eclogite 507±37 SHRIMP Yang et al., 2002b Eclogite 505±12 >750 LA-ICP-MS Liu et al., 2009b Songshugou Grt amphibolite 514± 9 <720 SHRIMP Liu et al., 2009b 500±10 LA-ICP-MS Liu et al., 2009b 501±10 SHRIMP Su et al., 2004 518±19 LA-ICP-MS Liu and Sun, 2005 HP balsic granulite 485±3 LA-ICP-MS Chen et al., 2004 Felsic HP granulite 486–511 1 180–1 453 LA-ICP-MS Liu et al., 2009b Zhaigen Grt clinopyroxenite 500±6 LA-ICP-MS Liu et al., 2009b

DISCUSSION—PERSPECTIVES AND CHAL- know whether or not similar rocks exist in other LENGE HP-UHP metamorphic zones and whether the conti- Ultra-deep Subduction of Continental Crust to a nental subduction >350 km deep is occasional or not. Mantle Depth of Stishovite Stability Field (>9 In addition, the common rock types in the HP-UHP GPa)—Occasional or Universal metamorphic zones of the continental subduction type It is well known that stishovite is one of a poly- are various felsic gneisses (including orthogneiss and morphic variety of silica stabilized at the very HP of paragneiss), pelitic schists or gneisses as well as mar- >9 GPa (at the temperature of about 1 000 ℃), and bles, in which eclogites, garnet peridotites and garnet was found only in meteorite pits before (El-Goresy et pyroxenites occur as lentoid bodies with a total vol- al., 2000; Sharp et al., 1999; Chao et al., 1962). The ume proportion of <5% in the area. It is obvious that microstructural evidence for the precursor stishovite in the study to find out the evidence for continental sub- the kyanite- and garnet-bearing pelitic gneiss from the duction of mantle depth of stishovite stability field (>9 South Altyn Tagh HP-UHP zone reveals that some GPa) is very important for complete understanding the continental rocks can subducted to the mantle depth of type of HP-UHP metamorphic zones of the continen- at least >350 km and from which exhume to the earth tal subduction. surface (Liu et al., 2007a). However, it is important to 590 Liu Liang, Yang Jiaxi, Chen Danling, Wang Chao, Zhang Chengli, Yang Wenqiang and Cao Yuting

Mechanism of Exhumation for Ultra-deep Sub- exsolution of pyroxenes in garnet from garnet peri- ducted Felsic Gneiss dotite (Sautter et al., 1991; Haggerty and Sautter, The mechanism of the exhumation from deep 1990), the exsolution of Ti-Fe oxide in olivine mantle to earth surface for the subducted continental (Dobrzhinetskaya et al., 1996), as well as rocks is an important problem in the study of UHP MgSi-perovskite, CaSi-perovskite, majoritic garnet, metamorphism and continental geodynamics, and has TAPP (tetragonal almandine pyrope phase) and Egg been paid a lot attention for near twenty years (Warren phase (Wirth et al., 2007; Hayman et al., 2005; Wirth et al., 2008; Ernst et al., 2007, 1994; Liu Y C et al., and Matsyuk 2005; Brenker et al., 2002; Harte et al., 2007; Massonne et al., 2007; Ernst, 2006; Xu et al., 1999; Scott-Smith et al., 1984) as inclusions in dia- 2006a, b; Walsh et al., 2004; Froitzheim et al., 2003; monds from kimberlites. The following issues still Zheng et al., 2003; Li et al., 2002; Hacker et al., 2000; remain poorly understood: (1) whether or not the fel- Chemenda et al. 1995; Davies and Von-Blanckenburg, sic continental rocks can subduct to such deep mantle 1995; Nie et al., 1994; Michard et al., 1993; Platt, as indicated by these ultrahigh pressure mineral 1993; Hsü, 1991). Slab dynamics is controlled by a phases or microstructures; (2) how deep the continen- balance between driving and resisting forces, which tal felsic rocks can be subducted into the mantle; (3) can vary with depth, subduction rate, and the what is the fate of deep subducted felsic continental long-term evolution of the slab (Billen, 2008). At pre- crusts in the mantle; (4) what are the diagnostic indi- sent, most researchers agree to that the buoyancy of cators for the deep subducted continental felsic rocks. lighter continental rocks broken off from subduction It is well known that some diagnostic minerals continental plate is the cause of exhumation. However, (such as coesite and diamond) and microstructures of Yamato et al. (2008) provided a thermodynamically exsolution (such as exsolution of pyroxene in garnet) and thermomechanically numerical model explaining are indicative of UHP metamorphism and depth of that the syn-convergent exhumation is a transient continental subduction. Isomorphism and exsolution process largely controlled by buoyancy forces in the are very common for many rock-forming minerals. depth interval of 100–350 km, without significant im- For recognition of microstructures of exsolution as in- pact from slab break-off. Additionally, it has been dicators of UHP conditions and pressure estimation, it demonstrated in the laboratory that continental crust is crucial to understand the crystallochemical behavior will lose its buoyancy at depth of about 300 km be- of the host minerals under high temperature and pres- cause its density becomes larger than surrounding sure condition (Liu et al., 2009b). However, there are mantle below such depths (Wu et al., 2009; Irifune et only a few minerals (such as garnet, pyroxene and oli- al., 1994). Hence, the “break off” mode is not appro- vine) that have been investigated with the solubility of priate for the exhumation of the pelitic gneisses of the some isomorphic components or chemical composi- South Altyn Tagh region where the UHP rocks might tions under high temperature and pressure conditions. have been subducted to a very deep mantle depth The deficiency of the knowledge about crystal- (>350 km). Therefore, attention will be paid to the lochemistry under UHP condition for UHP rocks re- mechanism of exhumation for such very deeply sub- sults in controversy for interpretation of microstruc- ducted rocks as the Altyn Tagh felsic gneisses. The tures of exsolution observed in those rocks. For exam- prevailing model of exhumation for UHP rock faces ple, the exsolution of gedrite in kyanite has been new challenges. found in UHP rocks from the South Altyn Tagh region. It may be a new indicator of subduction/exhumation Petrological Evidence for Continental Ultra-deep of continental rocks that needs to be testified by ex- Subduction and Other Diagnostic Indicators perimental studies. On the other hand, exsolution may Studies of microminerals and exsolution struc- be diversity for one specific host mineral. For example, tures from mantle rocks have revealed that the host the exsolved minerals from garnet may be clinopy- rocks came from the mantle of depth >300 km or even roxene and/or orthopyroxene (Spengler et al., 2006; from the transitional zone (410–660 km), such as the Van-Roermund et al., 1998), clinopyroxene+rutile+ Progress and Controversy in the Study of HP-UHP Metamorphic Terranes 591 apatite (Ye et al., 2000) and rutile+quartz+ apatite sections of the Central China orogen are important (Mposkos and Kostopoulos, 2001). Similarly, the ex- parts of the north margin of the proto-Tethys and solved minerals from clinopyroxene may be paleo-Tethys during the Late Neoproterozoic to Pa- quartz/coesite (Zhang et al., 2005, 2002; Dobrzhinet- leozoic. The paleotectonics were characterized by skaya et al., 2002; Katayama et al., 2000; Tsai and multiple microcontinents and islands (Zhang et al., Liou, 2000), garnet (Smith,1988), clinoenstatite (Liu 2001; Yin and Zhang, 1998). They formed the com- X W et al., 2007; Bozhilov et al., 1999), potash feld- plex tectonic background for convergence, collision spar (Becker and Altherr, 1992), and phlogopite+ and subduction between microcontinents, islands and coesite/quartz (Zhu and Ogasawara, 2002). Obviously, continent, resulting in the HP-UHP metamorphic ter- the essential of producing this diversity is the differ- ranes. However, the mechanism remains unknown for ence in composition of the host minerals under pres- the close of the paleo-ocean and the collision between sure and temperature conditions. Therefore, experi- continent and continent or between continent and is- mental investigations on crystallochemical behavior land. Further studies in the subduction and exhuma- for the minerals in UHP rocks are very important for tion processes will help establishing the ancient tec- understanding the microstructures of exsolution. tonics of the HP-UHP metamorphic rocks in the west- ern China. The expected progress in divergence and Tectonic Background of HP-UHP Metamorphic convergence of a lot of microcontinents and islands, Zones in the Western Central China Orogen provide important information for the conditions, The two HP-UHP metamorphic zones in the west scale and tectonic framework of the continental deep Central China orogen, the South Altyn Tagh and the subduction, and contribute to the buildup of continen- North Qaidam, have different HP-UHP metamorphic tal geodynamics. ages. The metamorphic age of the North Qaidam (420–457 Ma) is 20–80 Ma younger than that of the EPILOGUE South Altyn Tagh (475–509 Ma). It is in contrast to The discoveries of UHP (>7 GPa) rocks in the the eastern Central China orogen, where the HP-UHP South Altyn Tagh, the North Qaidam and the North rocks from the Dabie region and the Sulu region have Qinling in the western China, especially the discovery similar ages of peak metamorphism and are consid- of the rock resulting from the deepest continental ered from the same HP-UHP metamorphism zone. subduction (>350 km) in world at present, the western Thus, the difference in metamorphic ages between the China has become an important natural laboratory of South Altyn Tagh and the North Qaidam preclude UHP metamorphism and continental deep subduction them from the same HP-UHP metamorphic zone. The after the Dabie-Sulu region in the eastern China metamorphic age of the HP-UHP rocks in the North worldly. Further study of the microstructures of exso- Qinling (485–514 Ma) is also different from that of lution discovered in the UHP metamorphic rocks from the North Qaidam, the metamorphic ages of the the western China, combined with HT-HP experi- HP-UHP rocks do not spatially increase or decrease ments at ultra-deep condition, and to further find pe- from the South Altyn Tagh through the North Qaidam trological evidence of melt/fluid during the continental to the North Qinling, i.e., no timetransgressive. deep subduction and exhumation, as well as the study Therefore, the three metamorphic terranes in the of geochemistry and chronology for related rocks in western Central China orogen can not be regarded as focus regions, will make progress in metamorphism of the same HP/UHP metamorphic terrain. These ter- continental deep subduction and exhumation. Forward ranes are unlikely the products of the collision be- progress will not only promote understanding of the tween the North China and the South China during the UHP metamorphism of the continental deep suduction Early Paleozoic. It is therefore important to know their in the western China, but also provide new informa- tectonic background as well as geodynamic implica- tion about solid earth science and contributions to the tions. establishment of theory of continental deep subduc- Previous studies have shown that the western tion. 592 Liu Liang, Yang Jiaxi, Chen Danling, Wang Chao, Zhang Chengli, Yang Wenqiang and Cao Yuting

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