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.