ORDOVICIAN Ar/ Ar PHENGITE AGES FROM THE BLUESCHIST-FACIES ONDOR SUM SUBDUCTION-ACCRETION COMPLEX (INNER MONGOLIA) AND IMPLICATIONS FOR THE EARLY
PALEOZOIC HISTORY OF CONTINENTAL BLOCKS IN CHINA AND ADJACENT AREAS
KOEN DE JONG*,§, WENJIAO XIAO**, BRIAN F. WINDLEY***, HIDEKI MASAGO‡, and CHING-HUA LO§
ABSTRACT. We obtained 453.2ⴞ 1.8 Ma and 449.4 ⴞ 1.8 Ma (2) laser step-heating
40
Ar/39Ar plateau ages for phengite from quartzite mylonites from the
blueschist-facies Ondor Sum subduction-accretion complex in Inner Mongolia (northern China). These ages are within error of the inverse isochron ages calculated using the plateau steps and the weighted mean ages of total fusion of single grains. The compositional change from glaucophane in the cores to crossite in the rims of blue amphiboles, as revealed by electron microprobe analysis, points to decompression, probably caused by progressive exhumation of the subducted material. The Late Ordovician ages were not affected by excess argon incorporation because in all likelihood the oceanic sediments were wet on arrival at the trench and free of older detrital mica. The ca. 450 Ma ages are, hence, interpreted as the time of crystallization during mylonitization under high fluid activity at fairly low temperatures. This means that accretion of the quartzite mylonite unit occured about 200 Ma before final closure of the Paleo-Asian Ocean, amalgamation of the Siberian, Tarim and North China cratons, and formation of the end-Permian Solonker suture zone.
We argue that the Early Paleozoic evolution of the Ondor Sum complex occurred along the northeastern Cimmerian margin of Gondwana, which was composed of micro-continents fringed by subduction-accretion complexes and island arcs. The later evolution took place during the building of the Eurasian continent following middle Devonian and younger rifting along the East Gondwanan margin and northward drift of the detached North China craton. An extensive review shows that this type of two-stage scenario probably also applies to the geodynamic evolution of other micro-continents like, South China, Tarim, a number of Kazakh terranes, Alashan, Qaidam and Kunlun, as well as South Kitakami and correlatives in Japan, and probably Indochina. Like the North China craton, these were bordered by Early Paleozoic subduction-accretion complexes, island arcs or contained calc-alkaline volcanic mar-gins, like for example, the central Tienshan, North Qinling, North Qaidam-Altun, North Qilian and Kunlun belts in China, as well as the Oeyama and Miyamori ophiolites and Matsugadaira-Motai blueschist belt of Japan and the dismembered Sergeevka ophiolite of the southern part of the Russian Far East. This implies that a vast orogenic system, comprising an archipelago of micro-continents, seems to have existed along the Cimmerian margin of East Gondwana in Early Paleozoic time in which the ultrahigh-pressure metamorphism that characterizes the early evolution of many of the Asian micro-continents occurred.
introduction and aim
The Central Asian Orogenic Belt (CAOB) is a complex collage of island arcs, micro-continental fragments and remnants of oceanic crust, including small forearc *Institut des Sciences de la Terre d’Orle´ans, UMR 6113Universite´ d’Orle´ans, 45067 Orle´ans 7, Cedex 2, France; Koen.deJong@univ-orleans.fr
**State Key Laboratory of Lithosphere Tectonic Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China
***Department of Geology, University of Leicester, Leicester LE1 7RH, United Kingdom
‡Japan Agency for Marine-Earth Science and Technology (JAMSTEC), Center for Deep Earth
Explora-tion, 3173-25 Showa-machi, Kanazawa-ku, Yokohama, Kanagawa 236-0001, Japan
§Argon Geochronology Laboratory, Department of Geosciences, National Taiwan University, Taipei
10699, Republic of China
and backarc basins, situated between the Siberian craton to the north and the Kazakhstan, North China (or Sino-Korean) and Tarim cratons in the south (Zonen-shain, 1973; Zonenshain and others, 1990; Mossakovsky and others, 1994; Jahn and others, 2000; Filippova and others, 2001; Badarch and others, 2002; Kheraskova and others, 2003; Xiao and others, 2003; Jahn, 2004; fig. 1). The CAOB likely continues eastward as far as the Hida belt in Japan (fig. 1, loc. 11; Arakawa and others, 2000; Jahn and others, 2000) that was part of the Asian mainland before the Miocene (Maruyama and others, 1997). Although the southern margin of Siberia was the site of subduction-accretion tectonics from the Neoproterozoic on (Dobretsov, 2003; Dobretsov and others, 2003; Khain and others, 2003), the CAOB formed mainly during the Paleozoic by accretion of oceanic plate sediments and magmatic arcs, as well as by amalgamation of terranes of different type and derivation (Coleman, 1994; Didenko and others, 1994; Mossakovsky and others, 1994; Dobretsov and others, 1995; Buslov and others, 2001; Filippova and others, 2001; Badarch and others, 2002; Windley and others, 2002; Dobretsov, 2003; Kheraskova and others, 2003; Xiao and others, 2003, 2004a). This process was accompanied by emplacement of immense volumes of mafic and granitic magmas (Chen and others, 2000; Jahn and others, 2000; Wu and others, 2002; Jahn, 2004). The CAOB resulted from the closure of an oceanic basin (Mongolian seaway: Maruyama and others, 1997; Paleo-Asian Ocean: Xiao and others, 2003) by double subduction below the southern active margin of the Siberian craton and the northern margins of North China and Tarim (Wang and Liu, 1986). Li and Wu (1996) argued that intermixing of floras typical for the Siberian craton on the one hand and the North China and Tarim cratons on the other constrains their incipient collision as early Late Permian. Cope and others (2005) envisaged that convergence and ultimate collision resulted in a paleo-current reversal in North China due to the onset of uplift and creation of topography on the northern margin of the craton, from mid-Permian time on. The collision resulted in the formation of the Solonker suture zone, a major structure developed just north of the North China craton (Badarch and others, 2002) and Tarim craton (Xiao and others, 2004b), and that can be followed all along the CAOB from Kyrgyzstan in the west to the coastal area of the Sea of Japan in northernmost North Korea in the east (fig. 1). Different lines of evidence point to a suturing that occurred progressively later from west to east (Dobretsov, 2003; Xiao and others, 2004b; Cope and others, 2005). During the final stages of the collision crustal shortening was partitioned into vertical, ductile, strike-slip shear zones between the micro-continents that often reactivated older sutures, during the late Carboniferous to early Permian in the western part (Buslov and others, 2001, 2004) and the late Permian in the central part (Laurent-Charvet and others, 2003) of the orogen. Early Permian A-type granites, occurring in a narrow belt along the suture zone, formed as result of post-collisional slab break-off, and Late Triassic to Early Jurassic A-type granites from subsequent lithospheric delamination (Wu and others, 2002). Triassic molassic sedi-ments that overly weakly foliated to undeformed about 230-Ma old granite plutons (Wang and Liu, 1986; Chen and others, 2000) mark their cooling and exhumation, and herald the end of the collision and uplift of the orogen. A compilation by Wang and Liu (1986) and data obtained by the Bureau of Geology and Mineral Resources of Inner Mongolia (Nei Mongol) (BGMRIM, 1991) indicate that terrestrial sediments were deposited across the suture zone in early Jurassic time.
S¸engo¨r and others (1993) favored continuous, forearc subduction-accretion and arc collision along the southern margin of the Siberian craton. However, ophiolite belts in the CAOB have been interpreted alternatively as discrete suture zones between terranes, rather pointing to punctuated closure of multiple oceanic basins (Hsu¨ and others, 1991; Mossakovsky and others, 1994; Buchan and others, 2002). Xiao and others (2003) proposed a model for the evolution of the CAOB with three main stages
Fig. 1. Tectonic map of Asia with the main terranes, the Central Asian Organic Belt, and the Solonker suture zone [string of black elongated dots, modified after Badarch and others (2002) and Xiao and others (2004b)]. White star indicates Ondor Sum region; numbered stars indicate locations discussed in the text: 1). Sergeevka ophiolite, 2). Jiamusi and Songliao-Zhangguangcai blocks, 3). North Qinling belt, 4). Central Tianshan, 5). Western Kunlun range, 6). North Qilian Mountains, 7). Zhuguangshan batholith, 8). Song Chay complex, 9). Dai in Loc and Kontum massifs, 10). South Kitakami terrane, 11). Hida belt, with small stars: main occurrences of the Oeyama ophiolite and correlatives. K: Kyushu; H: Honshu. The boundary between South China and Indochina follows the Song Ma zone, allowing early Paleozoic sedimentary series (Findlay, 1998) and Permian Emeishan flood basalts (Chung and others, 1997) to be on the South China craton for the time slice considered. AT: Altyn Tagh fault; RR: Red River segment of the Ailao Shan-Red River shear zone. Modified after: Chang, 1996; Chung and others, 1997; Arakawa and others, 2000; Jahn and others, 2002; Ota and others, 2002; Wilde and others, 2000; Khain and others, 2003; Song and others, 2003; Jahn, 2004; Jia and others, 2004; Oh and others, 2005). Azimuthal equal-area projection.
that are related to progressive two-way subduction of the Paleo-Asian Ocean: (1) early to mid-Paleozoic Japanese-type subduction-accretion, (2) a Permian Andean-stage when the two opposing margins became sufficiently consolidated, and (3) continent-continent collision, leading to the formation of the Solonker suture zone at the end of the Permian during the final closure of the Paleo-Asian Ocean due to its coeval southward and northward subduction beneath the Tarim and North China cratons and Siberia, respectively.
The 60-km wide Erdaojing complex forms part of the Solonker suture zone in Inner Mongolia (fig. 2) and consists of tectonic me´langes with blocks of meta-chert, marble, and ultramafic-mafic rocks of blueschist-facies metamorphic grade (Tang, 1990; Xu and others, 2001). The latter authors obtained a40Ar/39Ar age of 383⫾ 13 Ma (mid-Devonian) on a sodic-amphibole from a blueschist block (fig. 2, for location). Recently, late Guadalupian (middle Permian) radiolarians have been found (Shang, 2004). The subduction-accretion complex can be compared to the Jilin complex that occurs northeast of the Sino-Korean craton in northeastern China (Li, 2006). The suture has a multiple character, because it occurs within two opposing subduction-accretion complexes formed by two-way subduction (Xiao and others, 2003). Although the former controversy about the timing of the formation of the Solonker suture zone has been resolved as end-Permian (Chen and others, 2000; Wu and others, 2002; Xiao and others, 2003; Cope and others, 2005), the age and significance of the early accretionary stages are poorly known and understood. The Ondor Sum accretion-subduction complex offers a key opportunity to constrain the timing of the early accretion along the northern margin of the North China craton. Therefore, we applied the 40Ar/39Ar dating method to well-selected samples of blueschist-facies, phengite-bearing quartzitic mylonite in the Ulan Valley in the Ondor Sum region (figs. 2 and 3), where the best section through the subduction-accretion complex is exposed. The ca. 450 Ma40Ar/39Ar ages that we obtained on phengites imply that ductile deformation in the Ondor Sum subduction-accretion complex took place some 200 Ma before the formation of the Late Permian Solonker suture zone. We argue that the Late Ordovician deformation and metamorphism took place along the margin of East Gondwana, parts of which, such as Tarim and the North and South China cratons, were rifted off in Late Paleozoic time and entered the Paleo-Asian Ocean as micro-plates, in part fringed by Early Paleozoic subduction-accretion complexes and island arcs.
the northern margin of the north china craton
The basement of the North China craton consists of Archean predominantly trondhjemitic-tonalitic-granodioritic gneisses and minor mafic igneous rocks, as well as Paleoproterozoic metasedimentary and igneous rocks, which are locally overlain by Neoproterozoic to Early Paleozoic passive margin sediments (Hsu¨ and others, 1991; Zhao, 2001; Kusky and Li, 2003; Zhai and others, 2003, 2005; Jia and others, 2004). Cambrian to middle Ordovician platform carbonates are unconformably covered by Carboniferous-Permian strata (BGMRIM, 1991). Based on40Ar/39Ar plateau ages of amphiboles from the Bayan Obo area (fig. 2), Chao and others (1997) postulated a regional metamorphic event that started at about 425 Ma and continued until at least 395 Ma. By the end of the Carboniferous the northern side of the North China craton was an active continental margin into which Andean-type granites and granodiorites were intruded, onto which andesites, basalts, dacites, rhyolites were extruded, and tuffs were deposited (Xiao and others, 2003). The Late Carboniferous to Permian series contain detritus (polycrystalline quartz, feldspar and lithic fragments) from both plutonic and metamorphic sources that supplied detrital zircons with Archean-Paleoproterozoic and Late Paleozoic Sensitive High-Resolution Ion MicroProbe (SHRIMP) U-Pb ages (Cope and others, 2005). The latter, 425 to 275 Ma, age group increases in importance stratigraphically upward and reveals that the northern margin
Fig. 2. Tectonic sketch map of central Inner Mongolia from which late Mesozoic-Cenozoic strata are omitted for clarity (modified after Xiao and others, 2003). Arrow indicates the sampling area in the Ulan Valley (fig. 3).
of North China was a continental margin arc already in the Silurian and which activity would last into Permian time (Cope and others, 2005). Late Paleozoic granite and granodiorite intrusions in the Precambrian basement of the northernmost North China craton (BGMRIM, 1991; Chao and others, 1997; figs. 2 and 10 of Xiao and others, 2003) probably form the source that is being progressively eroded in the terminal Paleozoic (Cope and others, 2005). The Carboniferous-Permian strata ex-posed near the most northern limit of the Archean basement are deformed into a series of tight north-vergent overturned to isoclinal folds, which are unconformably overlain by Early Jurassic fluvial-lacustrine sediments (Cope and others, 2005).
The Chifeng-Bayan Obo fault (fig. 2) is widely regarded as the northern boundary of the North China craton (Wang and Liu, 1986; Shao, 1989; Tang and Yan, 1993; Bai Fig. 3. Geological map of the Ondor Sum subduction-accretion complex in the Ulan Valley based on Xiao and others (2003), showing its main litho-tectonic units and structures. Samples for radiometric dating (phengite-bearing quartzite mylonites IM-2 and IM-5) and microprobe analysis (glaucophane-bearing foliated margin of meta-gabbro/leucogabbro IM-19) are indicated. Position indicated in figure 2. The position of the blow-up map of the contact between units 1 and 2 (fig. 5) is outlined.
and others, 1993a, 1993b). A number of Early Paleozoic island arcs occur to the north of the fault. The Ulan arc formed within the Ondor Sum subduction-accretion complex during this period, as part of a north-dipping, oceanward-directed subduc-tion system along the northern margin of the craton, and subsequently, the Bain-aimiao arc formed as a result of southward subduction (fig. 4; Xiao and others, 2003). The latter arc consisted of calc-alkaline tholeiitic basalts to minor felsic lavas, alkaline basalts, agglomerates, volcanic breccias and tuffs, as well as gabbros, granodiorites and granites (Hu and others, 1990). Zircon from a granodiorite porphyry has a U-Pb age of about 466 Ma and a muscovite from a granite yielded a K-Ar age of ca. 430 Ma (Zhang and Tang, 1989). Xiao and others (2003) argued that the high initial strontium isotope ratio (87Sr/86Sri⫽ 0.7146) of granites (Shao, 1989) and theεNd value of 2.4⫾ 1.7 of granodiorite (Nie and Bjørlykke, 1999) imply that the Bianaimiao arc was formed by mixing between mantle-derived and crustal rocks in an active continental Cordilleran-type margin, rather than in an island arc, as commonly thought. A 20 to 30 km wide and 120 km long east-trending belt occurring about 50 km south of Bayan Obo (fig. 2) contains ca. 455 Ma-old granitic plutonic rocks that intrude Proterozoic metasedi-ments (Chao and others, 1997). The ca. 430 Ma K-Ar muscovite from a granite probably reflects the cooling of the Bainaimiao arc. The occurrence of shallow-marine clastic sediments and carbonates (with late Silurian fossil corals) on top of early Paleozoic granites in the western Ondor Sum region (Wang and Liu, 1986) constrains the exhumation of the Bainaimiao-type magmatic rocks, which may point to the extinction of the arc. In the Silurian, Devonian and Carboniferous no island arcs were generated and accreted to the northern margin of the North China craton (Xiao and others, 2003).
Fig. 4. Cartoon-like profiles demonstrating the tectonic evolution of the northern margin of the North China craton in Inner Mongolia (A) Ordovician; (B) Ordovician-Silurian) in the present-day geographic reference frame, modified after Xiao and others (2003).
ondor sum subduction-accretion complex
Isolated outcrops of ophiolites occur around Ondor Sum, in an area of about 70 km along strike associated with high-pressure metamorphic rocks, and further east-ward in the area of Kedenshan and along the northern banks of the Xar Moron River (fig. 2). These rocks form part of the Ondor Sum subduction-accretion complex (fig. 2), which incorporates the Ondor Sum Group (Zhang and Wu, 1998), Ondor Sum Ophiolite Belt (Wang and Liu, 1986), Wentermiao (Tang and others, 1983) or Wendurmiao (Hu and others, 1990) Group or the Ondor Sum magmatic-metamor-phic complex (Zhai and others, 2003). The subduction-accretion complex comprises Early Paleozoic rocks (Xiao and others, 2003) that have been thrust onto the continental margin of the North China craton. The Mesoproterozoic amphibolite-facies orthogneisses and supracrustal rocks as well as Neoproterozoic greenstones described by Zhai and others (2003) probably belong to this basement.
We have structurally subdivided the Ondor Sum complex at its type locality in the Ulan Valley (also known as: Ulangou, Wulan valley, or Wulangou) northeast of Ondor Sum into three units. These are, from south to north (fig. 3), in order of increasing intensity of deformation and structural position: Unit 1 (undeformed pillow basalt with a cover of chert and clastics), Unit 2 (a series of volcanic rocks) and Unit 3 (mylonitic meta-chert with lenses of ocean-floor derived lithologies). The three units of the subduction-accretion complex are described below in more detail.
Unit 1
Unit 1 comprises about 600 meters of undeformed pillow basalt, in which lensoid bodies of gabbro and limestone occur imbricated (fig. 3). Pillows are up to 30 to 50 cm long, have vesicular chilled margins and well-preserved amygdaloids filled wth carbon-ate, chlorite and occasionally albite. Pillow breccias and hyaloclastites, with centimeter-size lava fragments, are found near the bottom of the lavas. Many flows have a shear foliation parallel to the bedding of the pillows. The basalts are considered to be of MORB-type, on the basis of their major and trace elements compositions (Tang, 1992; Wu and others, 1998). The basalts are characterized by the metamorphic assemblage: lawsonite⫹ albite ⫹ pumpellyite ⫹ chlorite ⫹ aragonite ⫹ calcite ⫹ quartz ⫹ epidote; lawsonite occurs mainly in the cores of pillows and decreases in abundance outward (Yan and others, 1989).
The series of pillow basalt form the upper part of a locally preserved ophiolite sequence of, from bottom to top; serpentinized harzburgite and dunite, cumulate gabbro, and diabase (Wang and Liu, 1986), developed to the south of the area displayed in figure 3. The ultramafic rocks have the metamorphic mineral assemblage: talc⫹ calcite ⫹ magnesite ⫹ chlorite ⫹ magnetite (Yan and others, 1989). Gabbros contain: omphacite (Jd% 35-25) ⫹ chlorite ⫹ epidote ⫹ pumpellyite ⫹ albite ⫹ titanite⫹ quartz as high-pressure metamorphic assemblage (Yan and others, 1989).
The basalts in the uppermost part of unit 1 are covered by a thrust-imbricated, but stratigraphically intact succession of red to purple-colored chert and overlying sand-stone (figs. 3 and 5). Chert may contain m-size lenses of serpentinite and magnetite-pyrolusite-quartz rock. Chert has yielded fossil sponge spicules, Hyolithes, Monoplaco-phora, radiolaria, acritarchs and spores, indicative of a Late Precambrian to Cambrian age (Peng, 1984; Wang and Liu, 1986). Xiao and others (2003) interpreted the thrust duplex on the ophiolitic rocks as representing the ridge-trench transition in ocean plate stratigraphy.
The north-dipping contact zone with the overlying lava series of unit 2 is made up of a 200 to 250 m thick zone in which pillow basalts, gabbro and more or less serpentinized harzburgite occur intimately imbricated (fig. 5). At the uppermost thrust contact ultramafic rocks are transformed into talc schists or magnesite-chlorite schists, pointing to pervasive retrogressive hydration.
Unit 2
Unit 2 contains calc-alkaline basalts, andesites, dacites, rhyolites and bedded pyroclastic tuffs. The unit is separated from the structurally underlying unit 1 and overlying unit 3 by north-dipping thrust faults (fig. 3). Xiao and others (2003) regarded this series of volcanic rocks as a relic of a dismembered island arc that was incorporated into the subduction-accretion complex.
Unit 3
Unit 3 occupies the structurally highest position and is the principal target of our study. It consists of a ca. 400 m-thick series of phengite-bearing quartzitic mylonites that are typically reddened due to the presence of hematite. The mylonites contain a well-developed, moderately northward plunging mineral and stretching lineation (fig. 3). The regional foliation is deformed into open and upright folds with wavelengths in the order of 50 m and axes that shallowly plunge in approximately east- and westward directions. The mylonites contain lenses, which are up to 200 meters across, of several different lithologies such as: meta-basaltic greenschist and blueschist (fig. 3), chlorite schist and chlorite-magnetite-epidote schist, meta-chert, magnetite-pyrolusite-quartz rock, quartz-pyrite-hematite rock, quartz-sericite schist, K-white mica-rich quartzitic schist and marble. Some quartzites contain deerite, glaucophane, hematite, magnetite, minnesotaite, piemontite and stilpnomelane (Yan and others, 1989). Phase equilibria point to temperatures of 250 to 350°C and pressures in the order of 0.60 to 0.75 GPa (Tang and Yan, 1993).
Xiao and others (2003) considered the quartzites of unit 3 to be derived from ocean-floor chert enriched in Fe and Mn and imbricated with slices of oceanic basalt and gabbro during subduction.
Fig. 5. Detailed geological map of the contact zone between the pillow basalts of unit 1 (footwall) and the arc volcanic rocks of unit 2 (hanging wall) of the Ondor Sum subduction-accretion complex in the Ulan Valley. Position indicated in figure 3.
Age Constraints
A number of small bodies of different leucocratic rocks, like fine-grained pla-giogranite and granophyric trondhjemite, occur associated with the gabbroic rocks and pillow lavas in the Ondor Sum region between Tulinkai and Deyenqimiao to the east of the Ulan Valley (Zhao and Li, 1987; Tang, 1990; Jian and others, 2006). The latter authors obtained U-Pb SHRIMP ages on zircon from these rocks: 477⫾ 10 Ma (ophiolite), 477⫾ 6 Ma (low-K rhyolites), 467 ⫾ 12 Ma (trondhjemite associated with island arc tholeiites) and 457⫾ 10 Ma (adakite). These ages are much younger, but within error of the Rb-Sr whole-rock age of 509 ⫾ 40 Ma that is often quoted for a meta-basalt of unit 1 (for example, Yan and others, 1989; Tang, 1990). However, analytical data for this particular sample are not given and it is not stated where it is taken, rendering its value somewhat uncertain.
40
Ar/39Ar isochron ages of 446⫾ 15 Ma and 426 ⫾ 15 Ma are often quoted for glaucophane (for example, Zhang and Liou, 1987; Yan and others, 1989; Tang and Yan, 1993). In spite of the fact that these dates are the only available isotopic constraints on the timing of the high-pressure metamorphism of the Ondor Sum complex their meaning is uncertain, because analytical data, descriptions of the samples and of their provenance are lacking,
The late Silurian shallow-marine clastic sediments and carbonates that unconform-ably overlie the ophiolites, blueschists and cherts in the central part of the Ondor Sum region (Tang and others, 1983) do not place tight age constraints on the timing of exposure at the surface of the subduction-accretion complex.
sample description, petrography and mineral chemistry
The dated mica-bearing quartzites of unit 3 (fig. 3) are intensely foliated and lineated, fine-grained mylonites. The pronounced mineral and stretching lineation plunges moderately to the north. IM-2 has a brick red color, whereas IM-5 is more brownish due to some secondary limonitization. The main minerals in both mylonites are quartz and (minor) phengite, whereas hematite is the major accessory phase. The thoroughly dynamically recrystallized quartz has a grain size of ⬍ 200 m. The mylonite foliation is formed by an alternation of red, hematite-rich and milky white, hematite-free quartz layers at the scale of less than a millimeter. The hematite-free layers are interpreted as quartz veins that were rotated parallel to the mylonite foliation as a result of the intense elongation of the rocks during their ductile deformation. Less deformed folded and boudinaged quartz veins that are discordant to the mylonite foliation also occur. Foliation boudinage resulted in an anastomosing foliation. Cross cutting, mm-thick quartz veins occur sub-perpendicular to both foliation and linea-tion, and contain fibrous crystals with their c-axis parallel to the stretching linealinea-tion, pointing to a crack-seal mechanism during brittle extension. The occurrence of different generations of progressively stronger deformed quartz veins points to alternat-ing phases of brittle and ductile deformation. These are probably associated with variations in strain rate and/or fluid pressure. Yellowish green phengite occurs in lozenge-shaped aggregates of a few millimeters long between anastomosing quartz layers, but may also be extremely elongated forming several centimeter-long and highly attenuated lenses. These structures are interpreted as micro-boudins.
Blueschist sample IM-19 is taken from the foliated and lineated margin of a 70 m-diameter meta-gabbro/leucogabbro lens imbricated in the quartzite mylonite (fig. 3). It is a fine-grained rock (fig. 6) with a weakly developed planar and linear fabric, containing the following mineral paragenesis: Na-amphibole ⫹ stilpnomelane ⫹ chlorite ⫹ plagioclase ⫹ phengite ⫹ calcite ⫹ dolomite ⫹ quartz, with accessory titanite and magnetite.
The sodic-amphiboles occur as⬃0.8 mm-long, rectangular or flaky porphyroblasts with a distinctive deep blue to pale violet pleochroism. They are optically and compositionally zoned, usually with deep bluish cores, which seems to be common for the sodic-amphiboles in the area (Yan and others, 1989). Occasionally, the amphiboles have more irregular and complicated spotted and/or oscillatory zoning patterns. A small number of sodic-amphiboles have rectangular mineral inclusions, presumed to be relict igneous augite, some of which are boudinaged (fig. 6). Electron microprobe analyses show that Na-amphibole varies in composition from glaucophane in the cores to crossite in the rims (fig. 7). The cores generally have a slightly higher Fe2⫹/Mg ratio, higher M4 site Na (⬎1.8) and Al(VI) contents, but a lower Fe3⫹content than the rims (table 1, fig. 7). The blue amphibole that seals the pulled-apart augite (fig. 6) has a crossitic composition.
Plagioclase is almost pure albite (Ab⬎ 99; table 2) and forms irregular porphyro-blasts. The main matrix minerals are: pale-green stilpnomelane that occurs abundantly as aggregates of tiny flakes or laths, relatively rare chlorite, and occasional K-white mica that forms tiny laths. Electron microprobe analysis demonstrates that the K-white mica in this sample is a phengite with 1.67 Al pfu and 3.64 Si pfu and a negligible paragonite component (table 2). Magnetite is the major accessory phase, whereas titanite, although rare, occurs as relatively large grains up to 1.2 mm long.
Calcite and dolomite are abundant in the matrix and occur as anhedral crystals that cement cracks and grain boundaries of other matrix minerals, suggesting second-ary infiltration of a CO2-rich fluid. Both carbonates have dolomitic rims of higher Mg Fig. 6. Photomicrograph of boudinaged augitic (Aug) relics within a sodic-amphibole (Na-Amp) crystal in IM-19, the foliated margin of a meta-gabbro/leucogabbro. Note the filling of pull-aparts by amphibole with a crossite composition.
composition (table 2). The Ca/(Ca⫹Mg) ratio in the cores of calcite and dolomite is ca. 0.01 and 0.31, respectively, whereas the ratio in the rim of both is about 0.35 to 0.38. Both minerals have a small siderite component (XSd⬍ 0.1), which shows a concomi-tant increase with magnesite in the rim.
The mineral paragenesis of IM-19 points to high-pressure metamorphism related to subduction, but it does not permit a quantitative estimate of its pressure-temperature condition. At first sight the maximum pressure seems constrained by the stability of albite instead of sodic-pyroxene and the lack of lawsonite. Yan and others (1989) noted that lawsonite and glaucophane never occur in the same assemblage in the high-pressure metamorphic rocks in the Ulan Valley. Absence of lawsonite might also be partly due to the presence of an elevated CO2component in the metamorphic fluid in this particular sample, which strongly reduces the stability field of the mineral (Nitsch, 1972). Glaucophane cores of sodic-amphiboles imply relatively high-pressure conditions during the prograde stage. The increase of Fe3⫹, in combination with the decrease in the alumina and Na(M4) contents and the Fe2⫹/Mg ratio, toward grain rims points to decreasing pressure and increasing temperature (Brown, 1977; Ma-ruyama and others, 1986) during advanced metamorphic stages. The decompression was most likely due to progressive exhumation of the subducted material. The sealing of pulled-apart augite (fig. 6) by blue amphibole of crossitic composition instead of glaucophane indicates that exhumation occurred during deformation.
40ar/39ar dating results
40
Ar/39Ar dating of phengite occurred by incremental heating of 64 to 100m diameter grains with a defocused, continuous ultraviolet laser beam. Fusion was achieved in the final step by beam focusing. All grains of IM-5 were step-heated, whereas splits of sample IM-2 were step-heated separately in two experiments (IM-2a and IM-2b). A number of grains of each sample were dated by total fusion too. The 40
Ar/39Ar analytical data are listed in tables 3 and 4; the ages are summarized in table 5 and portrayed as age spectra in figures 8 and 9, respectively. The inverse of the Fig. 7. Composition diagrams for sodic-amphiboles in meta-gabbro/leucogabbro IM-19 from the Ondor Sum subduction-accretion complex in the Ulan Valley. (A) Na(M4) versus Al/(Al⫹Fe3⫹) for cores and rims of zoned amphibole crystals, (B) Fe2⫹/(Fe2⫹⫹Mg) versus Al/(Al⫹Fe3⫹) ratios for cores and rims of zoned amphiboles. For EPMA data, see table 1; for analytical details, see footnote to table 1.
39
Ar/40Ar intercept yields an inverse isochron age, whereas the inverse of the36Ar/ 40
Ar intercept [(40/36)i in table 5] indicates the composition of the trapped non-radiogenic argon component (Roddick and others, 1980; Heizler and Harrison, 1988). With the use of IsoPlot (Ludwig, 2000) plateau ages were calculated if 55 percent or more of the39Ar was released in three or more contiguous steps with a probability-of-fit of the weighted mean of more than 5 percent. All errors are quoted at the 2 level. For analytical details the reader is referred to Appendix A.
Step-heating yielded somewhat irregular age spectra. The first apparent ages are as young as about 395 Ma (IM-2a; fig. 8) and 385 Ma (IM-5; fig. 9) and rise to constant values over the following 20 to 25 percent of39Ar release. Both samples yielded almost concordant plateau ages of 453.2⫾ 1.9 Ma (IM-2a) and 449.4 ⫾ 1.8 Ma (IM-5) for 79 percent and 57.3 percent of the released 39Ar, respectively. Sample IM-2b yielded a weighted mean age of 453.2⫾ 2.4 Ma (46.6% of39Ar release) for the flat upper part of
Table 1
Representative mineral analyses
*Iron calculated as FeO cation (O⫽23)
Representative electron probe microanalyses of sodic-amphibole (Na-Amp) in meta-gabbro/ leucogabbro IM-19 from the Ondor Sum subduction-accretion complex in the Ulan Valley. The analyses were made with a JEOL-8800 electron probe X-ray micro-analyzer (Department of Earth and Planetary Science, Tokyo Institute of Technology), operated at 15 kV with a beam current of 1.20⫻ 10⫺8Å and a⬍ 4 m beam diameter.
the dome that is concordant to the plateau age of sample split IM-2a (fig. 8). The decreasing apparent ages for part of the spectrum following the flat part of the dome are probably due to the observed turning of some bigger grains during the final part of the experiment, exposing parts that had apparently not homogeneously degassed earlier, given the younger apparent ages of the last four steps. The relatively strong age variation of contiguous steps in IM-5 (fig. 9) is probably due to the same phenomenon. The plateau ages are within error of the inverse isochronal ages calculated using the plateau steps (table 5). The weighted mean ages of total fusion of single grains are concordant to the plateau and inverse isochronal ages of each sample, but they are a couple of Ma’s older than the integrated ages of all heating steps (table 5).
interpretation and discussion
Although research during the last decade of the past millennium made it clear that phengite in high-pressure metamorphic rocks have often taken up excess argon, the mineral has also commonly yielded meaningful40Ar/39Ar ages (Okay and Monie´, 1997; Bosse and others, 2000; Gao and Klemd, 2003; Rodriguez and others, 2003). Incorporation of excess argon by phengite has been explained by its partial
recrystalli-Table 2
Representative mineral analyses
Fe-Ox* Iron calculated as FeO; Fe-Ox⫹ Iron calculated as Fe2O3; Phengite cation (O⫽11); Chlorite
cation (O⫽28); Plagioclase cation (O⫽8)
Representative electron probe microanalyses of various minerals in meta-gabbro/leucogabbro IM-19 from the Ondor Sum subduction-accretion complex in the Ulan Valley. Abbreviations according to Kretz (1983); Phn: phengite. Analytical details, see footnote to table 1.
zation during subsequent metamorphism at lower pressure (Hammerschmidt and Franz, 1992; Hannula and McWilliams, 1995; de Jong and others, 2001), or by strongly restricted fluid mobility during the high-pressure metamorphism (Scaillet, 1996; Boundy and others, 1997; de Jong, 2003). The fluid activity in rocks depends on their pre-metamorphic history (Scaillet, 1996; Okay and Monie´, 1997). In our case, the late Precambrian to Cambrian pelagic siliceous sediments that cover the ocean-floor pillow basalts (unit 1) and the Fe- and Mn-rich ocean-floor chert protoliths of the dated
Table 3
40Ar/39Ar analytical data of phengite from IM-2
40Ar/39 analytical data of phengite from quartzite mylonite IM-2 from the Ondor Sum
subduction-accretion complex in the Ulan Valley, a. Step-heating experiment on about 10 (IM-2a) and 20 (IM-2b) grains. b. Total fusion of 10 single grains.40Ar* is radiogenic argon from natural K-decay;37Ar,38Ar, and39Ar
phengite-bearing quartzite mylonite (unit 3) were in all probability still relatively water-rich at their arrival at the trench and during early accretion. The occurrence of different generations of progressively stronger deformed quartz veins points to the presence of a fluid phase throughout the deformation history. In addition, the formation of glaucophane at the cost of magmatic minerals in the basaltic rocks may have liberated water, depending on the exact reaction by which it formed. The dated quartzite mylonites were therefore not metamorphosed under fluid-deficient condi-tions and were not recrystallized at a later stage. Under these metamorphic condicondi-tions, the still relatively young, pelagic sediments had, hence, not cumulated significant, if any, radiogenic argon. It is thus highly unlikely that the obtained ages of ca. 450 Ma were affected by excess argon. The concordant step-heating and single grain total fusion ages of the two samples provide additional supportive evidence against the latter
Table 4
40Ar/39Ar analytical data of phengite from IM-5
40Ar/39Ar analytical data of phengite from quartzite mylonite IM-5 from the Ondor Sum
subduction-accretion complex in the Ulan Valley. a. Step-heating experiment on about 90 grains. b. Total fusion of 4 single grains.
phenomenon, because phengite with excess argon may show highly different plateau ages for neighboring samples or different grains from a given sample (Ruffet and others, 1995; de Jong and others, 2001).
The amount of older detrital mica in these pelagic sediments was probably negligible, because microstructures indicative of such components, like porphyroclasts and strain shadows are lacking. In addition, straightforwardly recognizable detrital feldspar is absent. Moreover, we can argue on geochemical grounds that it is highly unlikely that the ages of about 450 Ma are due to this phenomenon. In the very first place, the presence of older detrital K-white mica would have resulted in a much greater spread in single grain total fusion ages. Secondly, white mica being a hydrous mineral; it releases Ar during step-heating under vacuum as a consequence of chemical and structural changes within the crystals, rather than by volume diffusion (see references in: de Jong and others, 2001). A major consequence of this is that39ArKand 40
Ar* are released simultaneously from cores and rims of crystals, leading to homogeni-zation of ages. In contrast, the presence of different generations of chemically distinct white micas in grain separates often results in complicated or dome-shaped40Ar/39Ar age spectra due to their differential degassing over a temperature interval (Wijbrans and McDougall, 1986; de Jong, 2003). In such cases37ArCa/39ArKratios - proxies for the Ca/K ratios of the degassing material - are also irregular. The finding of age plateaux and constant37ArCa/39ArKratios can thus be taken as an argument in favor of degassing of chemically homogeneous white mica, further underscoring the absence of detrital components. Finally, the phengite ages are comparable to the 460 to 410 Ma 40
Ar/39Ar ages of glaucophane in high-pressure metamorphic rocks from the Ulan Valley referred to by Chinese authors.
Because the metamorphic temperature of the main tectono-metamorphic recrys-tallization during which the quartzite mylonites were formed was well below the blocking temperature of K-white mica [see Villa (1998) for a listing], the ca. 450 Ma age is interpreted as the crystallization age of phengite during mylonitization. White mica ages in the accretionary complexes of Japan, represented by the Sambagawa and Mikabu belts, have been interpreted in a similar way (Takasu and Dallmeyer 1990; de Jong and others, 2000 and references therein). Our ca. 450 Ma phengite ages are
Table 5
Summary of the40Ar/39Ar age results of phengite IM-2 and IM-5. MSDW⫽ SUMS/(n-2), with SUMS ⫽
minimum weighted sum of residuals; n⫽ number of points fitted. (40/36)iinverse of the
36Ar/40Ar
probably a better and in any case a tighter constraint on the age of accretion-related tectono-metamorphic recrystallization than the range of 460 to 410 Ma40Ar/39Ar ages of glaucophane quoted by Chinese authors. Glaucophane is K-poor and submicro-scopic inclusions of K-rich minerals like mica, which may have formed during later recrystallization or alteration, may critically influence its age (Sisson and Onstott, 1986). The meta-gabbro/leucogabbro lens that occurs in the quartzite mylonite of unit 3 (fig. 3) has a foliated and lineated margin from which blueschist IM-19 was sampled. The micro-boudinage of magmatic augite in IM-19 occurred in stability field of crossite that was formed at the expense of glaucophane during exhumation. This implies that the final stages of the penetrative ductile deformation during which the dated quartzite mylonites IM-2 and IM-5 were formed also occurred during exhumation. Our ca. 450 Ma phengite dates provide, hence, an age constraint on the final phase of the formation of the Ondor Sum subduction-accretion complex.
Xiao and others (2003) pointed out that the right-way-up ocean plate stratigraphy and the upward-facing pillows of unit 1 of the Ondor Sum accretion-subduction complex, together with the consistent northward dip of the entire section, indicate that the rocks have not been overturned. Therefore, they envisaged that southward accretion took place on a north-dipping subduction zone that generated the Ulan arc. Our ca. 450 Ma plateau ages would thus constrain the dynamic recrystallization of the quartzite mylonites formed during this phase of southward accretion (fig. 4A). Accordingly, this event was of Late Ordovician age according to the time-scale of Harland and others (1990) or earliest Ashgill using the recent chronostratigraphic scale of Webby and others (2004).
Fig. 8. 40Ar/39Ar age spectra of phengite from quartzite mylonite IM-2 obtained by step-heating of
The supra-subduction zone ophiolite sequence and a variety of subduction-related leucocratic rocks in the Tulinkai area that yielded zircons with U-Pb SHRIMP ages of 477 to 457 Ma, as indicated above, have highly variableεNd(t)(⫹11.9 – -0.75) and high initial Sr ratios (87Sr/86Sri⫽ 0.7043 – 0.7062) (Jian and others, 2006). On the basis of these data the authors favor a near-trench setting for these magmatic rocks and generation by ridge-trench interaction. This interpretation supports our model of an early Paleozoic intra-oceanic evolution of the Ondor Sum subduction-accretion com-plex and the Ulan arc (fig. 4A). Jian and others (2006) also pointed out that younger ages of ca. 437 Ma for metamorphic zircons from some of these leucocratic rocks may reflect intra-oceanic overthrusting and crustal thickening under amphibolite-facies conditions. A ca. 420 Ma age of an albitite dike in a serpentinite places an upper time constraint on this event (Jian and others, 2006). These ages strengthen our interpreta-tion that the ca. 450 Ma age of phengite from the Ulan Valley better constrains the timing of the high-pressure metamorphism in the Ondor Sum subduction-accretion complex than the 40Ar/39Ar ages of glaucophane referred to by Chinese authors. Indeed, the youngest glaucophane age of ca. 426 Ma agrees with the timing of the post-accretion metamorphism and may be related to recrystallization caused by it.
early paleozoic belts and terranes in china, kazakhstan, mongolia, russia, vietnam, japan
Our radiometric dating implies that a 200 Ma gap existed between the early accretion in the Ondor Sum subduction-accretion complex along the northern margin of the North China craton and the formation of the Solonker suture zone, which marks Fig. 9. 40Ar/39Ar age spectrum of phengite from quartzite mylonite IM-5 obtained by step-heating of
the final closure of the Paleo-Asian Ocean. As shown below, other margins of the North China craton and of many other continental fragments in the Paleo-Asian Ocean were bordered by early Paleozoic subduction-accretion complexes, island arcs or contained calc-alkaline volcanic margins, which were formed well before the final closure of this oceanic basin.
Kazakhstan and Junggar
A number of early Paleozoic ophiolites occur along the northern margin of the Junggar terrane (Xinjiang, China; fig. 1), namely, the Tangbale-Mayile-Hongguleleng-Aermantai complexes (Wang and others, 2003). Radiometric and micro fossil data show that the Tangbale ophiolite was formed in late Cambrian-Ordovician time, whereas, the age of the other complexes is less well constrained, but probably pre-middle Ordovician (Wang and others, 2003). The ophiolites are associated with volcanic and volcanoclastic sequences, as well as radiolarian cherts, turbidites and limestones of Ordovician and/or Silurian age (Wang and others, 2003).40Ar/39Ar ages of sodic-amphibole from blueschists in the Tangbale ophiolite me´lange (table 6) point to accretion and high-pressure metamorphism in middle Ordovician time along the northern margin of Junggar.
The Altai-Sayan fold belt (fig. 1) comprises terranes of different age and deriva-tion that are separated by subducderiva-tion-accrederiva-tion complexes that contain ophiolites, and are cut by strike-slip faults of different age (Zonenshain and others, 1990; Berzin and Dobretsov, 1994; Buslov and others, 2001; Windley and others, 2002). The Chara ophiolite belt is located along the northeastern margin of the Kazakhstan composite terrane and was formed in Late Carboniferous-Permian time as a strike-slip zone during the collision with Siberia, but also contains older serpentine me´langes and accreted terranes (Buslov and others, 2001). The oldest subduction-accretion complex is of Early Paleozoic age and contains amongst others: cherts, eclogites, amphibolites and blueschists, which yielded K-white mica with Late Ordovician to Early Silurian K-Ar ages (table 6).
Mongolia, Russian Far East and Northeastern China
Mongolia and bordering areas in Russia contain a number of crustal blocks situated along the southern margin of Siberia (Badarch and others, 2002; Dobretsov and others, 2003; Khain and others, 2003). A compilation by these authors revealed that these amphibolite- and locally granulite-facies metamorphic rocks yielded many radiometric ages in the 535 to 450 Ma range. They explained these data by high-grade metamorphism and anatexis related to Cambro-Ordovician ridge subduction and/or terrane collision.
Northeastern China and adjacent areas of the Russian Far East, contain a complex suite of continent-related terranes composed of rocks of in part early and middle Paleozoic age of diverse tectonic settings (Khanchuk and others, 1996; S¸engo¨r and Natal’in, 1996; Kojima and others, 2000; Nokleberg and others, 2001, 2004; Jia and others, 2004). A number of these terranes that are situated just north of the North China craton have been grouped as the Khanka superterrane (Kojima and others, 2000; Nokleberg and others, 2001, 2004).
The Khanka superterrane in the southern part of the Russian Far East (fig. 1, loc. 1) comprises four terranes that amalgamated in the Silurian (Kojima and others, 2000) and were covered by similar Devonian and Mississippian continental-rift-related volca-nic and sedimentary series (Nokleberg and others, 2001). The superterrane consists of a series of related terranes of early and middle Paleozoic age, representing a continental-margin, an island arc and subduction-accretion complexes (amongst others the Voznesenka terrane) with Cambrian ophiolite, chert, clastic rocks and limestone (Khanchuk and others, 1996; Kojima and others, 2000; Nokleberg and others, 2001).
Table 6 Compilation of published isotopic ages of early Paleozoic tectonic and magmatic events in Central and East Asia
Table
6
Table 6 (continued) TIMS ⫽ Thermal Ionization Mass Spectrometry; SHRIMP ⫽ Sensitive High-Resolution Ion MicroProbe.
The Sergeevka terrane is located along the southeastern margin of Khanka and comprises a gneissose meta-dioritic complex, granite and associated metamorphic rocks that may have been formed in an island arc (Khanchuk and others, 1996; Kojima and others, 2000; Nokleberg and others, 2001). Rocks from this long-lived and complex terrane yielded isotopic ages in the 490 to 530 Ma range and the dismem-bered Sergeevka ophiolite contains metagabbros with 430 to 470 Ma-old hornblendes (table 6) that are as old as hornblendes of some ophiolites in Japan (see below).
Jia and others (2004) argued, on the basis of whole-rock40Ar/39Ar plateau ages of blueschists and syn-tectonic granites, that two terranes that are not part of Khanka, namely the Jiamusi and Songliao-Zhangguangcai blocks (fig. 1, loc. 2) collided along the Heilongjiang belt (also known as Mudanjiang belt) at about 450 to 410 Ma. This belt is regarded as a subduction-accretion complex and contains, amongst others, serpentinized ultramafic rocks and gabbro, rare plagiogranite, pillow basalt, marble, a variety of siliceous-argillaceous metasediments and blueschists (Yan and others, 1989). The amphibolite- to granulite-facies metamorphic Mashan complex, which forms part of the Jiamusi block, has yielded ca. 500 Ma SHRIMP zircon ages (table 6).
North and South Qinling Belts
The Qinling belt of eastern central China, developed to the south of the North China craton (fig. 1, loc. 3), is divisible into northern and southern belts that are separated by metamorphic rocks with Late Carboniferous40Ar/39Ar hornblende ages (Zhai and others, 1998).
The northernmost part of the North Qinling belt contains sediments accumulated on the southern passive margin of the North China craton formed by latest Mesopro-terozoic metamorphic rocks (Zhai and others, 2003). The youngest sediments are mid-Ordovician that were deformed and metamorphosed under greenschist-facies conditions before deposition of mid-Carboniferous strata (Xue and others, 1996a, 1996b). The central Qinling island-arc, farther south, contains deformed, medium pressure/temperature (P/T) meta-igneous rocks with 487 to 470 Ma single-zircon 207
Pb/206Pb evaporation ages, and relatively undeformed calc-alkaline granitoid plu-tons with 422 to 383 Ma isotopic ages (Erlangping unit: Ratschbacher and others, 2003 and references therein) that truncate older structures (Xue and others, 1996a, 1996b). 434 to 404 Ma40Ar/39Ar hornblende ages constrain the time of metamorphism related to these intrusions (Sun and others, 2002a) or their cooling (Zhai and others, 1998). The lower Qinling unit developed farther south comprises gneisses (local granulites), amphibolites and marbles, with Paleoproterozoic and Neoproterozoic metamorphic ages, as well as early Paleozoic K-Ar mineral ages [that is 370 – 480 Ma see Ratschbacher and others (2003) for a compilation]. The northern zone of the unit contains lenses and blocks of eclogite and gneiss that have yielded garnets with inclusions of coesite and zircons with included micro-diamonds. The ultrahigh-pressure metamorphism occurred at around 500 Ma (table 6; Yang and others, 2003).
These data have been interpreted in terms of the formation of an intra-oceanic island arc (Erlangping unit) caused by southward (Xue and others, 1996a, 1996b) or northward (Ratschbacher and others, 2003) subduction of the oceanic basin border-ing the North China craton in the early Ordovician. Followborder-ing the closure of the North Qinling back-arc basin (Sun and others, 2002a), the island arc, micro-continents (like the lower Qinling unit) and subduction-accretion complexes collided with the south-ern margin of North China before the intrusion of latest Silurian to Early Devonian post-collisional calc-alkaline granitoids (Xue and others, 1996a, 1996b; Ratschbacher and others, 2003). Intrusion of these stitching plutons is part of the building of a ca. 400 Ma Andean-type continental margin arc along the amalgamated southern margin of the North China craton due to northward subduction of oceanic lithosphere (Ratschbacher and others, 2003).
In contrast in the South Qinling belt, most of the isotopic dates that refer to the age of the (ultra) high-pressure metamorphism that formed the local coesite- and diamond-bearing eclogites span the late Carboniferous to late Triassic (references in: Gao and others, 1995; Chang, 1996; Zhai and others, 1998; Sun and others, 2002a, 2002b; Roger and others, 2003; Liou and others, 2004). Recently, however, Qiu and Wijbrans (2006) obtained Late Ordovician to Silurian40Ar/39Ar ages from eclogites in the Dabieshan by stepwise crushing of garnet (table 6) and they suggest that these ultrahigh-pressure metamorphic rocks were first formed in the early Paleozoic. Also eclogite in the northwest Dabieshan yielded early Paleozoic isotopic ages that were interpreted by Yang and others (2003) as dating the amphibolite-facies recrystalliza-tion of the (ultra) high-pressure metamorphic rocks (table 6).
Tianshan Belt
The ca. 400 km wide and 2500 km long Tianshan belt similarly records about 200 Ma of tectonic history of collision of the Tarim craton and intervening smaller crustal fragments with the southern Angaran active margin of the Siberian continent (Shu and others, 2002, 2004; Laurent-Charvet and others, 2002, 2003; Xiao and others, 2004b). A central belt is separated from the northern and southern Tianshan belts by suture zones (Windley and others, 1990) that contain me´langes with a schistose meta-pelitic matrix that surrounds blocks of (ultra) mafic magmatic rocks, tholeiitic pillow lavas, siliceous rocks and limestones of Ordovician to Late Silurian age, as well as blueschists, eclogites and (mafic) granulites (Gao and others, 1998; Shu and others, 2002, 2004; Xiao and others, 2004b).
The Central Tianshan belt (fig. 1, loc. 4) is considered as an Early Ordovician to Early Silurian magmatic arc (table 6) formed on Proterozoic basement and overlain by Late Paleozoic platform sediments (Windley and others, 1990; Laurent-Charvet and others, 2002; Shu and others, 2002, 2004; Xiao and others, 2004b). The immature, intra-oceanic Harlik-Dananshu arc, situated in the central Asian archipelago to the north of the central Tianshan arc, was also initiated in the Early Ordovician (Xiao and others, 2004b, and references there-in).
Despite the formation of an Ordovician-Silurian magmatic arc due to subduction of oceanic lithosphere isotopic ages for blueschists and eclogites from the me´langes along the Central Tianshan’s northern limit in western China cluster closely around 350 Ma (Gao and others, 1998; Gao and Klemd, 2003). Geochronologic indications for early Paleozoic high-pressure metamorphism in this area and correlative belts farther west in Kyrgyzstan (Atbashy eclogite belt) and in Tajikistan (Fan-Karategin blueschist belt) are not conclusive. However, the latest Neoproterozoic to earliest Early Cambrian Weiya granulite from the northern margin of the eastern Central Tianshan belt experienced an early Paleozoic retrograde thermo-tectonic event (Shu and others, 2004; table 6).
The Makbal coesite-grade eclogite in the northern Tianshan belt in Kyrgyzstan may have been formed around Cambrian Ordovician boundary times (table 6).
Kunlun Belt
The Kunlun belt occurs to the south of the Tarim craton and the Qaidam block and is divided into a western and an eastern range that are offset along the sinistral Altyn Tagh strike-slip system (fig. 1). The Kunlun belt contains the remnants of superimposed early Paleozoic and late Paleozoic-Triassic arcs. In both the western and the eastern Kunlun belt the older suite of batholiths shows a pronounced concentra-tion of 450 to 490 Ma radiometric ages, pointing to the presence of an Early to Late Ordovician magmatic arc (Yuan and others, 2002; Cowgill and others, 2003; Schwab and others, 2004). The youngest isotopic ages obtained by Cowgill and others (2003) on batholiths in both belts are identical (384 ⫾ 2 and 389 ⫾ 5 Ma, that is Middle
Devonian). Ca. 405 Ma-old lamprophyres and a 384 ⫾ 2 Ma-old pluton are post-tectonic (Schwab and others, 2004).
Based on data from the Kudi area (fig. 1, loc. 5) Xiao and others (2002) proposed a model of an early Paleozoic arc-accretionary prism for the western Kunlun belt that has resemblance to the model for the early Paleozoic tectonic evolution of the Ondor Sum subduction-accretion complex, outlined earlier in this paper. In agreement with Mattern and Schneider (2000), they envisaged that in the Late Cambrian to earliest Ordovician an oceanward-dipping intra-oceanic subduction zone was generated to the south of Tarim. This led to the formation of the earliest Ordovician intra-oceanic Yixieke arc that was emplaced northward onto the margin of Tarim (Xiao and others, 2002). This process gave rise to flip in the subduction polarity, and consequent northward subduction beneath the southern margin of the accreted Yixieke arc led to the formation of an Early to Mid-Ordovician Andean-type continental margin on the southern side of Tarim (Xiao and others, 2005). The end of northward subduction may be related to the docking of the Kudi terrane, a Precambrian continental fragment with gneisses with Late Ordovician to Silurian isotopic ages (table 6) that probably record the collision (Matte and others, 1996; Zhou and others, 1999). Yuan and others (2002) argued that the end of the collision is constrained by the 405⫾ 2 Ma single-grain zircon U-Pb age of the post-dynamic, A-type North Kudi pluton and coeval lamprophyre dikes, indicating the beginning of extensional deformation that would last until the Early Permian. Schwab and others (2004) underlined the similarities in Paleozoic tectonic evolution between the Kunlun belt and the Qinling belt, 2500 to 3000 km farther to the east. They compared the formation and accretion of the intra-oceanic Yixieke arc, with that of the Erlangping unit, and the docked Kudi terrane with the Qinling micro-continent.
Liu (ms, 2000) obtained 440 to 360 Ma hornblende and mica40Ar/39Ar ages from Proterozoic gneisses from both sides of the central Kunlun fault in the eastern Kunlun range. He interpreted these ages to reflect the age of amphibolite-facies metamor-phism during collision of the South and North Kunlun blocks that followed the closure of the small oceanic or marginal basin between them.
North Qilian Belt
The North Qilian Belt (fig. 1, loc. 6) comprises (1) a northern terrane with middle Cambrian sediments (Wu and others, 1993), (2) a ca. 1000 km long, discontinuous belt composed mainly of greenschist-facies metamorphic felsic and mafic volcanic rocks interpreted as an Ordovician volcanic (island) arc (Yin and Harrison, 2000; Wang and others, 2005), and (3) a southern terrane accretion-subduction complex with abyssal and bathyal sedimentary rocks, an imbricated ophiolite (Qian and Zhang, 2001), with blueschists, blocks of eclogite, gabbro, and serpentinized and carbonated ultramafic rocks (Liu, ms, 2000; Wang and others, 2005). Phengite and glaucophane from the southern terrane yielded 460 to 440 Ma K-Ar and 40Ar/39Ar ages (Xiao and others, 1986; Song and Wu, 1992; Wu and others, 1993; Zhang and others, 1997; Liu, ms, 2000). Granodiorite and associated skarn formed in Proterozoic continental fragments present in the southern terrane yielded Late Ordovician isotopic ages (table 6). Ordovician to Early Silurian (table 6) rhyolites and basalts (Wang and others, 2005) and intrusive rocks were emplaced in an island-arc setting (Zhang and others, 2006).
Wang and others (2005) interpreted the geochemistry of the volcanic rocks as pointing to an Ordovician volcanic arc formed on the southern margin of the North China craton above a northward subduction zone, possibly evolving into an island arc separated from the continent by a back-arc basin. The northward drifting of the Central Qilian microcontinental fragment, with Archean basement rocks (Liu, ms, 2000), eventually resulted in the amalgamation of both terranes and their collision with the North China craton in Silurian times (Wang and others, 2005).
North Qaidam-Altun Eclogite Belt
The North Qaidam-Altun eclogite belt occurs over almost the entire length of the northern margin of the Qaidam block (constituted by Precambrian metamorphic rocks with a Paleozoic sedimentary cover), and is displaced by the Altyn Tagh fault (fig. 1). The belt comprises quartzo-feldspathic and pelitic gneisses with local garnet peridotites and eclogite lenses and layers, which may contain ultrahigh-pressure minerals (Song and others, 2003; Zhang and others, 2004; Liou and others, 2004).
Metamorphic zircon from eclogites and garnet peridotites yielded U-Pb TIMS (Thermal Ionization Mass Spectrometry) and SHRIMP ages that span 436 to 504 Ma; eclogites have whole-rock-garnet-omphacite Sm-Nd isochrons in the 500 to 435 Ma range (Zhang and others, 2001, 2004, 2005; Yang and others, 2002; Song and others, 2003), which according to these authors date the (ultra) high-pressure metamorphic conditions. Exhumation to lower-crustal depths is constrained by40Ar/39Ar ages of 477 to 407 Ma (Zhang and others, 2005; table 6). I-type granites in the region yielded SHRIMP ages of 496 to 445 Ma (Yang and others, 2002). The North Qaidam-Altun eclogite belt probably constituted a tectonic collage of multiple (ultra) high-pressure metamorphic units formed as a result of early Paleozoic (that is 500 – 445 Ma) subduction and subsequent collision between the Qilian and Qaidam blocks, which were probably both Precambrian micro-continents (Song and others, 2003; Zhang and others, 2005).
Bian and others (2004) regarded the entire Kunlun-Qilian-Qinling system during the Early Ordovician to latest Silurian as constituted by a number of colliding micro-continents fringed by subduction-accretion complexes and island arcs devel-oped as a result of northward subduction and closure of oceanic basins.
South China and Indochina
Although the tectonic significance and geodynamic setting are unclear, scarce information implies that the Indochina block (or Annamia) and the southern part of the Cathaysia block (that is, the southern South China craton) were affected by Silurian magmatism, regional sub-greenschist facies metamorphism and deformation. This is indicated by U-Pb zircon ages (table 6) and by a lack of Silurian sediments, a regional angular unconformity between unmetamorphosed Late Devonian and younger deposits and folded Neoproterozoic to early Paleozoic metasediments of the South China craton with isotopic ages of about 440 to 415 Ma (Zhao and Cawood, 1999; Roger and others, 2000, and references therein). The Dai Loc and Kontum massifs in the Indochina craton of central Vietnam (fig. 1, loc. 9) contain early Paleozoic, in part granulite-facies gneisses (Carter and others, 2001; Lan and others, 2003) and granodi-orite (Nagy and others, 2001) (table 6).
Japanese Belts
The Japanese terranes oceanward of the Hida belt (fig. 1, loc. 11), the geotectonic element the closest to the Asian continent and that can be regarded as the eastward continuation of the CAOB, are generally interpreted as subduction-accretion com-plexes (Isozaki, 1997a, 1997b). Only a small number of terranes have been considered as derived from a mature island arc or a micro-continent (for example, Faure and Charvet, 1987; Aitchison and others, 1991; Maruyama and others, 1997; Hada and others, 2001; Takagi and Arai, 2003), and occur isolated in the Japanese islands due to late Triassic and younger tectonism. These are the Kurosegawa terrane (western Honshu and Kyushu; indicated in fig. 1 as H and K, respectively), the Hida Gaien (or Marginal) terrane (central Honshu), the South Kitakami terrane (central and north-eastern Honshu; fig. 1, loc. 10) and parts of the Paleo-Ryoke terrane (in restricted areas from Kyushu to the Kanto Mountains of central Honshu). These four terranes can be correlated on the basis of similarities in litho- and bio-stratigraphy of Late Silurian to
early Middle Devonian and Late Palaeozoic sedimentary series, isotopic ages and petrochemistry of late Ordovician and Permian granitoids (Umeda, 1998; Ehiro, 2000; Hada and others, 2000; Takagi and Arai, 2003; Kurihara, 2004; Kawajiri, 2005). The Hida Gaien terrane as used in this paper refers to the recently redefined Hida Gaien belt by Tsukada and others (2004) that crops out along the southern margin of the Hida belt (fig. 1, loc. 11), which it structurally underlies. These authors pointed out that part of the tectonic elements of the classic belt can be regarded as belonging to the Paleozoic Renge, Suo, Akiyoshi and Maizuru subduction-accretion complexes.
The Oeyama suite is interpreted as an early Paleozoic ophiolite nappe that occupies the highest structural position in the stack of superimposed subduction-accretion complexes of southwest Japan (Ishiwatari and Tsujimori, 2003), and is here regarded as belonging to the Renge belt, as defined by Nishimura (1998). A ca. 560 Ma Sm-Nd age of a gabbro dyke with magmatic clinopyroxene, plagioclase and (?) hornblende, and MORB-like affinities, suggests that the Oeyama ophiolite formed during the Cambrian (Hayasaka, 1995, in: Tsujimori and Itaya, 1999). The peridotite of the Oeyama suite may have been formed in a supra-subduction zone mantle beneath an intra-oceanic arc (Tsujimori and Itaya, 1999; Ishiwatari and Tsujimori, 2003). The Osayama serpentinite me´lange, which is associated with the Oeyama belt, contains tectonic blocks of kyanite- and staurolite-bearing high-pressure metamorphic gabbros and a rare blueschist with an eclogite-facies mineral assemblage that yielded early Paleozoic isotopic ages (table 6), which match those from the Sergeevka ophiolite of the southern part of the Russian Far East (fig. 1, loc. 1; table 6).
Ishiwatari and Tsujimori (2003) and Sakashima and others (2003) have consid-ered the South Kitakami terrane as an early Paleozoic active continental margin or mature island arc, on the basis of late Cambrian to Ordovician isotopic ages from ophiolite, blueschists, as well as calc-alkaline and granitic rocks (table 6). Metamorphic rocks with hornblendes that have 400 to 445 Ma K-Ar ages furthermore occur in a serpentinite me´lange in the Kurosegawa terrane (Tsujimori and Itaya, 1999; Ishiwatari and Tsujimori, 2003).
geodynamics
S¸engo¨r and others (1993) considered all Precambrian crustal blocks present in the CAOB as fragments derived from the margin of the Laurentian part of the former Meso-Neoproterozoic supercontinent Rodinia (that is from Siberia and, probably, North China). These were emplaced as a result of the separation of a single magmatic arc from this margin in latest Neoproterozoic-Early Cambrian time (namely, the Kipchak arc), rather than as micro-continents. In contrast, based on paleomagnetic data that point to predominant migration of such continental fragments from Gond-wana toward Siberia, Mossakovsky and others (1994) and Didenko and others (1994) regarded these fragments as Gondwana-derived micro-continents. Other authors like for example, Dobretsov and others (1995) and Buslov and others (2001, 2004), however, underlined the heterogeneity of these fragments and inferred that they were composite micro-continents with Gondwanan and “Laurentian” fragments. The evolu-tion of micro-continental fragments can be quite complicated as illustrated by the Altai-Sayan terrane, for which Fortey and Cocks (2003) proposed that it rifted from Siberia around Cambrian-Ordovician boundary times and subsequently drifted to obtain a peri-Gondwanan position in the Caradoc.
In the following section we will point out that micro-continents that currently form part of Asia as a result of latest Paleozoic to earliest Mesozoic collision were at least in part peri-Gondwanan terranes. These terranes were situated close to the northeastern Cimmerian margin of the Gondwana supercontinent in the early Paleo-zoic, and fringed by subduction zones, accretionary complexes and calc-alkaline volcanic margins, before they separated from this margin after the middle Devonian
and subsequently drifted northward. The early Paleozoic tectonic cycle was preceded by, and a consequence of, the break-up of Rodinia. This event is the rationale behind the accumulation of late Neoproterozoic to earliest Paleozoic passive margin sedi-ments of the eastern margin of Gondwana, the North China craton and many of the micro-continents of mainland Asia.
The Early Paleozoic Gondwanan Margin
Paleogeographical reconstructions by Li and Powell (2001) imply that during the Early Cambrian, the North and South China cratons (the latter forming part of a continental ribbon with northwest Tasmania in its southern tip) were located adjacent to the Australia-New Zealand-Antarctica continental margin of eastern Gondwana, close to or in the Paleo-Pacific oceanic basin. The magmatic and tectonic evolution of the eastern Gondwanan margin is related to different subduction stages of the Paleo-Pacific Ocean and associated marginal basins (Li and Powell, 2001; Veevers, 2004; Foster and others, 2005), as a result of which the Terra Australis Orogen was formed that continued into South America (Cawood, 2005). Oblique subduction was initiated in the East Gondwana segment in late Neoproterozoic time (580 – 560 Ma; Cawood, 2005), or in the earliest Phanerozoic (ca. 550 Ma; Veevers, 2004). The main pulse of convergence started at about 530 to 520 Ma (Cawood, 2005) and enduring westward subduction occurred from 490 Ma onward (Veevers, 2004). Ultrahigh-pressure metamorphic conditions were attained during the Ross orogeny in Antarctica that are dated by Sm-Nd and 238U-206Pb methods at about 500 Ma, whereas the amphibolite-facies overprinting is constrained by40Ar/39Ar ages of 490 to 486 Ma for Ca-amphibole (Liou and others, 2004).
Paleomagnetic data suggest that the northwestern Tasmania part of the ribbon continent that also contained South China, may have accreted to the Pacific margin of Gondwana by the Late Cambrian (Li and Powell, 2001). Due to the oblique subduction of Paleo-Pacific oceanic lithosphere below the eastern Gondwanan margin in Early Cambrian time, the South and North China micro-continents traveled along the active margin, reaching their position near the Cimmerian re-entrant by the Early Ordovi-cian (Li and Powell, 2001; fig. 10). The Arabia-Iran-Himalayan-part-of-India segment of this re-entrant may have been an active margin from the terminal Neoproterozoic to at least the Early Cambrian, characterized by magmatic arc complexes and possibly a back-arc zone of attenuated continental crust, developed above a continent-ward dipping subduction zone (Ramezani and Tucker, 2003). Research by many workers has revealed the Himalayan part of the Gondwanan margin experienced widespread early Paleozoic tectonism. Its manifestations in various regions of the Himalayas include Late Cambrian to Early Ordovician large-scale thrusting, crustal thickening and ductile deformation; medium- to high-grade regional metamorphism and the generation of granitic crustal melts; uplift and erosion of metamorphic rocks, as well as accumulation of thick sequences of synorogenic sediments (DeCelles and others, 2000; Miller and others, 2001; Gehrels and others, 2003, 2006a, 2006b; Schwab and others, 2004, and references therein). Rolland and others (2002) regarded the Masherbrum Greenstone Complex of the southern Karakoram (NE Pakistan) as a dismembered ophiolite comprising relics of an Early Ordovician subduction-accretion complex, a volcanic arc and a back-arc system formed along the southern margin of a Northern Karakoram micro-continent. The Karakoram micro-continent, together with the Hel-mand block of central Afghanistan and the Lhasa block of Tibet, belonged to the peri-Gondwanan Cimmerian domain during the Early Ordovician (Le Fort and others, 1994). The Cimmerian re-entrant is regarded as composed of a number of semi-independent blocks during the Cambro-Silurian and not as a single entity (Fortey and Cocks, 2003), although for clarity it is depicted as such in figure 10.
Fig. 10. Geodynamic reconstructions of the major lithospheric plates for the Late Ordovician (modified after Li and Powell, 2001). Positions of micro -continents (those discussed in the text are dark shaded) and subduction zones modified mainly after: Pickering and Smith (1995), Li and Powell (2001), Fortey and C ocks (2003), Torsvik and Cocks (2004). Ar ⫽ Armorica; Av ⫽ Avalonia; I ⫽ Indochina; NC ⫽ North China Craton; Q ⫽ Qaidam-Qilian block; SC ⫽ South China Craton; T ⫽ Tarim.
![Fig. 1. Tectonic map of Asia with the main terranes, the Central Asian Organic Belt, and the Solonker suture zone [string of black elongated dots, modified after Badarch and others (2002) and Xiao and others (2004b)]](https://thumb-ap.123doks.com/thumbv2/9libinfo/8679098.196954/3.729.95.640.104.787/tectonic-terranes-central-organic-solonker-elongated-modified-badarch.webp)
