CORE Metadata, citation and similar papers at core.ac.uk Provided by RERO DOC Digital Library Published in Terra Nova 20, issue 2, 118-125, 2008 1 which should be used for any reference to this work High-pressure metamorphism in Taiwan: from oceanic subduction to arc-continent collision? O. Beyssac,1 F. Negro,2 M. Simoes,3 Y. C. Chan4 and Y. G. Chen5 1Laboratoire de Ge´ologie, ENS–CNRS, Paris, France; 2Institut de Ge´ologie, Universite´ de Neuchaˆtel, Suisse; 3Institut de Physique du Globe, Universite´ Paris 7–CNRS, Paris, France; 4Institute of Earth Sciences, Academia Sinica, Taipei, Taiwan; 5Department of Geosciences Neuch^atel, National Taiwan University, Taipei, Taiwan ABSTRACT The Taiwan orogen has been the focus of a number of models of followed by a rapid isothermal decompression, with exhuma- mountain building processes, but little attention has been paid tion possibly as rapid as burial. These units have subsequently to high-pressure (HP) metamorphic rocks that are found as been stored at a pressure of 3 kbar for 4–5 Myr, before exotic blocks intermingled within the deepest units of the their final exhumation, probably facilitated by the accretion of mountain belt. In this study, we re-appraise from updated passive margin sequences during the Late Cenozoic collision. petrological and thermodynamic databases the physical condi- Therefore, HP units in Taiwan maintain a record of processes at tions of HP metamorphism in Taiwan, and we combine our depth from the early stages of oceanic subduction to the findings with available geochronological data to estimate the present arc-continent collision. thermal history of these rocks. Our results indicate that peak metamorphic conditions of 550 °C and 10–12 kbar have been Complex, TC) exhumed to the east amphiboles from the Juisui blueschists Introduction (Ho, 1986) (Fig. 1). This young oro- in the range 8–14 Ma. Ages of Petrological and geochronological gen is characterized by high rates of 10–12 Ma were obtained by K–Ar investigations of high-pressure (HP) deformation and erosion (e.g. Simoes (Juang and Bellon, 1986) and metamorphic rocks have provided and Avouac, 2006) and has been 40Ar–39Ar (Lo and Yui, 1996) on valuable insights into the kinematics intensively studied in terms of moun- phengites from these blueschists. Lo of burial and exhumation of these tain building processes (e.g. Suppe, and Yui (1996) retrieved 40Ar–39Ar materials (e.g. Chopin, 2003; Jolivet 1981; Dahlen and Barr, 1989; Fuller ages of 4.5 Ma on phlogopites from et al., 2003; Ernst, 2005) and hence et al., 2006; Simoes et al., 2007). How- the Wanjung rocks. It is to be noted have been key in better understanding ever, in these models, little attention that these HP units have geochrono- the mechanical and rheological has been paid to the HP metamorphic logical ages only slightly older than the aspects of the crust and lithosphere rocks reported within the TC along onset of the late Cenozoic collision. (e.g. Burov et al., 2001; Gerya and the suture zone (Fig. 1). These rocks The purpose of this study was to Stockhert, 2006). This study focuses are found in ÔexoticÕ blocks that are re-appraise the physical conditions of on HP rocks that have been reported characterized by totally different lith- HP metamorphism in Taiwan. We in Taiwan (Liou et al., 1975; Yui and ological and petrological properties re-investigated the mineralogy and Lo, 1989) (Fig. 1). The Taiwan moun- from those observed in the surround- petrology of these units on the basis tain belt results from the collision ing TC schists (Liou, 1981). Two HP of recent thermodynamic databases, between the Luzon volcanic arc and blocks have been described: the Juisui which have undergone considerable the Chinese continental margin, dated block with glaucophane schists and improvement over the last years. at 6.5 Ma (Lin et al., 2003). This amphibolites (Liou et al., 1975) and When combined with geochronologi- collision followed the subduction of the Wanjung block with omphacite- cal ages, the P–T history of HP rocks the South China Sea oceanic crust bearing ultramafic rocks within a ser- provides possible insights into the below the Philippine Sea plate, which pentinite melange (Yui and Lo, 1989). kinematics of burial and exhumation initiated 15 Ma ago (e.g. Huang Because of extremely bad surface during the transition from oceanic et al., 2006) and which is still active exposure, the tectonic relationship of subduction to arc-continent collision. to the south of Taiwan along the these blocks to the bulk of the TC remains largely unclear. Petrological Manila trench (e.g. Teng, 1990; Mal- Sampling and methods avieille et al., 2002). This young oro- investigations (Liou et al., 1975; Yui gen consists mostly of metasediments, and Lo, 1989) indicate peak metamor- In the Juisui area, no rocks were including remnants of the basement of phic conditions of 400–450 °C and found in place because of extremely the Chinese passive margin (Tananao 6–8 kbar, which certainly need to be bad exposure conditions. All the re-evaluated in light of more recent samples were collected in two small Correspondence: Dr O. Beyssac, Labora- thermodynamic databases. Various rivers draining exclusively the HP toire de Ge´ologie, Ecole Normale geochronological techniques were units according to the map of Liou Supe´rieure, CNRS UMR 8538, 24 rue applied to these HP rocks and provide et al. (1975). A large diversity of rocks Lhomond, Paris F-75005, France. Tel.: constraints on the timing of the was found in the Juisui area: glauco- +33144322294; fax: +33144322000; exhumation of these units. Jahn et al. phane schists (glnS – samples Y0403 e-mail: [email protected] (1981) obtained Rb–Sr ages on a and Juisui02-1), garnet-epidote 2 using standard conditions (15 kV, 10 nA) and the chemical standards described in Table 1. The garnet blackschists from the Juisui exotic block were analysed by the Raman spectroscopy of carbonaceous mate- rial (RSCM) technique, which pro- vides a sensitive geothermometer for peak metamorphic temperature in such rocks (Beyssac et al., 2002, 2004). The samples were analysed using the experimental conditions described by Beyssac et al. (2007) and following the procedure de- scribed by Beyssac et al. (2003). Results are summarized in Table 2. Petrology and P–T history Figure 2 illustrates representative mineral assemblages observed in the rocks from Juisui and Wanjung. All mineral abbreviations are after Kretz (1983) except for amphibole (am) and water (w). The glnS contains zoned amphiboles (Ca-am core and gln rim, and vice-versa), grt + phg as an early assemblage, and chl, ep, ab as a retrograde assemblage. The grtS contains grt locally retrograded to chl, phg and ab. More details regard- ing the petrology of these rocks may be found in Liou et al. (1975) or Liou (1981). Representative analyses and structural formulae of px, gln, grt, chl and phg for Juisui are given in Table 1. Ternary plots for phengite Fig. 1 Geological map of Taiwan and geodynamic context. Taiwan is composed of and garnets, and an amphibole com- the Taiwan mountain belt to the west and the Coastal Range (CoR – remnant of the positional diagram, are given in Luzon Arc) to the east. They are separated by the Longitudinal valley (LV), a deep Fig. 3. Blue amphiboles from the elongated sedimentary basin that forms the suture between the two plates. The Juisui glnS are essentially sodic, with Taiwan mountain belt results from the collision between the Chinese continental margin and the Luzon volcanic Arc. From west to east, the Taiwan mountain belt is glaucophane compositions with aver- age XMg values of 0.46 and Fe3+ ⁄ composed of the western foothills (WF – fold and thrust belt), the Slate belt, which 3+ VI includes the Cenozoic metasediments of the Hsuehshan Range units (HR) and the (Fe +Al ) ranging between 0.31 Backbone Slates (BS), and the Tananao complex (TC) consisting of pre-tertiary and 0.45 (Fig. 3a). Phengites show basement of the margin [Tailuko belt (TB) and Yuli belt (YB)]. Metamorphic grade variable composition in garnet associated with the late Cenozoic collision generally increases from west to east, blackschists and glnS (Fig. 3). High reaching temperatures of up to 500 °C in the eastern TC, with some complexities in Si-content phengites with significant the western slate belt (e.g. Beyssac et al., 2007). celadonite content (up to 45%) are observed within the glnS, whereas phengites from the grtS have inter- amphibolites, epidote amphibolites serpentinite me´lange. All these rocks mediate compositions up to 35% (not described here, as no HP evidence may have an oceanic origin from deep (Fig. 3b). Garnets generally show fer- was detected in these rocks) and sea sediments (glnS) to basalt (am- rous compositions with XAlm > 60% garnet-bearing blackschists (grtS – phibolites) or gabbro (ompR). in both the garnet blackschists and samples Y0404 a, b, c and f). In For all the rocks, mineralogy was glnS from Juisui (Fig. 3c). In the Wanjung, the omphacite-bearing determined through optical micros- Wanjung ompR, mineralogy is re- rocks (ompR – samples Y0402 a and copy, completed by X-ray diffrac- stricted to the presence of px, ep, b), possibly metagabbro, were col- tion. The chemical analysis of minor ab and qtz for the peak assem- lected from the abandoned serpentine minerals was performed using a blage, and late ab, chl and phl for quarry described by Yui and Lo Cameca SX100 electron microprobe the retrograde assemblage. Pyroxenes (1989) as an inclusion within a (Camparis, Universite´ Paris VI) are omphacitic with variable jadeite 3 Table 1 Representative electron microprobe analysis of px, gln, am, grt, phg and chl in the glaucophane schists (glnS) and garnet blackschists (grtS) from Juisui and the omphacite-bearing rock (am, ompR) from Wanjung.