A Introduction
Nous avons vu, dans le chapitre précédent que le Junggar Occidental résultait d’une histoire polycyclique, marquée par le recyclage de la croûte juvénile cambro-ordovicienne pendant les épisodes magmatiques dévono-carbonifères. L’évolution au Paléozoïque supérieur, présentée dans le chapitre 6, a été largement étudiée, et de nombreux modèles d’évolution sont actuellement proposés et débattus. Inversement, il existe très peu d’informations sur la géologie du Paléozoïque inférieur et seules des données géochimiques et géochronologiques sur les mélanges ophiolitiques sont disponibles (Zhang et al., 1993 ; Jian et al., 2005). Le modèle proposé par Wang et al (2003), suggère une continuité des phénomènes d’accrétion durant tout le Paléozoïque, pas en accord avec les données géologiques présentées dans le chapitre 4. Xiao et al. (2008) ont suggéré un collage d’arcs intraocéaniques. Cependant, la polarité de la subduction est mal contrainte, et la géométrie proposée des complexes d’accrétion et des arcs magmatique est très discutable (cf. chapitre 3). L’accrétion d’arcs intra-océaniques et de microcontinents est aussi avancée par Buckman & Aitchison (2004), mais il existe de nombreuses incertitudes sur les terranes de Mayila et de Laba. De plus, ce modèle s’intéresse uniquement à l’évolution du domaine sud et les régions de Tarbagatay et de Sharburt ne sont pas considérées. L’absence d’études géologiques détaillées sur les complexes de roches de Paléozoïque inférieur est le problème majeur des scénarii géodynamiques existants. Le premier objectif de ce chapitre est donc de documenter précisément la géologie des massifs ordoviciens à siluriens du Junggar Occidental, en apportant de nouvelles contraintes structurales, géochronologiques et géochimiques.
Dans de nombreuses études, le Junggar Occidental est considéré comme un système insulaire isolé au sein de l’Océan paléo-asiatique (Xiao et al., 2008 ; 2010), bien que des corrélations avec le Junggar Oriental ont aussi été envisagées (Wang et al., 2003). Cependant, les nouvelles données géochronologiques (Chen et al., 2010) et paléontologiques (He et al., 2011) obtenues dans les régions de Tarbagatay et de Tangbale suggèrent des similitudes entre les unités du Junggar Occidental et celles du Kazakhstan. Le second objectif de ce chapitre est d’explorer ces possibles connections entre le Junggar Occidental et le Kazakhstan Oriental, en s’appuyant sur les synthèses régionales récentes proposées par Windley et al. (2007) et Degtyarev (2011). Cette étude comparative permet ainsi de replacer l’évolution géodynamique du Junggar Occidental dans le cadre général des Altaïdes Occidentales, marquée notamment par la formation du microcontinent kazakh (Kheraskova et al., 2003).
B Article à soumettre : Structure and evolution of the Early Palaeozoic accretionary complexes in West Junggar and their place within the Altaids collage.
Abstract
Understanding the development and the evolution of accretionary orogens is crucial for characterizing continental crust growth in time and space. In the Altaids tectonic collage (Central Asia), divergent geodynamical models have been suggested to spell out the origin of the Palaeozoic orogenic belts, which have extensively contributed to the formation of a mostly juvenile continental crust. But, chronology of this continental crust growth is not well documented, as it is highly dependant on the geodynamic evolutionary scenario. This study focuses on West Junggar (NW China), a segment of the Altaids. Multidisciplinary approach, including detrital zircon provenance studies, geochemistry, and field structural analysis, deals with three Early Palaeozoic units in West Junggar. Subduction seems to have been predominant in Early Palaeozoic time, as indicated by ophiolitic mélanges, volcaniclastic turbidite series and magmatic arc suites. However, olistostrome, molasses deposits and magmatic events chronology reveal interruptions of subduction. Discrete collisions are inferred by the structure of the belt, which displays allochtonous units rooted in the suture zone. Magmatic and sedimentary successions in West Junggar bear some resemblance to Eastern Kazakhstan rocks, suggesting a lateral continuation of units. On the basis of this new results, we propose the following evolution: 1) Ordovician subductions below intra-oceanic island arcs, 2) Late Ordovician amalgamation of island arcs against Kazakhstan margin, 3) Early to Middle Silurian resumption of oceanic subduction and arc magmatism and 4) Late Silurian-Early Devonian collision with a microcontinent. This scenario, consistent with the formation of the Kazakhstan microcontinent supports a model of multiple accretions for the Altaids tectonic collage. These results also imply that continental crust growth in Central Asia might result from alternation of vertical and lateral growth episodes throughout Palaeozoic.
B.1 Introduction
Accretionary orogens are major sites for the production of juvenile magmas, and therefore they significantly contribute to the growth of the continents (Rudnick, 1995; Condie, 2005; Cawood & Buchan, 2007). A fundamental issue is the timing of the crustal growth process. Since accretionary orogen formation is related to long-lived subduction zones, the architecture and evolution of these orogens are usually complex (Cawood et al., 2009). Accretionary orogens were extensively studied throughout the world for instance in the Western America cordilleras (Coney et al., 1980; Clowes et al., 2005; Ring, 2008), Alaska (Byrne, 1984; Sisson et al., 2003), Japan (Maruyama, 1997), or Australia (Foster & Gray, 2000; Glen, 2005). However, when the age of formations is not well established by palaeontological evidence, or when stratigraphic relationships between units are overprinted by late tectonics, it becomes complicated to decipher the construction steps leading to the formation of an accretionary orogen. Using an indirect method, like the provenance of sediments is a powerful method for reconstructing the reasonable pattern of an accretionary orogeny (e.g. Ledent et al., 1964; Gaudette et al., 1981; Cawood et al. 1999). Such an approach has been efficiently used in numerous worldwide orogens (e.g. Gehrels et al., 2002; Griffin et al., 2004; Willner et al., 2008; Bahlburg et al., 2009). Despite multiple sedimentary cycles, detrital zircons can preserve the initial isotopic ratios of their host magmas and disclose to their origin (Fedo et al., 2003). Zircon U-Pb geochronology and Hf isotopes provide the age of the source rock, and direct information on the juvenile or contaminated character of the source rock, respectively (Stevenson & Patchett, 1990; Griffin et al., 2004; Wu et al., 2010). Therefore, the study of detrital zircons provenance may help for tracking the formation and evolution of the continental crust (Condie et al., 2005).
In Central Asia, the Altaids collage (Sengör et al., 1993; Sengör and Natal’in, 1996a), also called the Central Asian Orogenic Belt (CAOB) (Mossakovsky et al., 1993; Windley et al., 2007; Xiao et al., 2010), consists of numerous ribbon-like units of Precambrian microcontinents, magmatic arcs, and accretionary wedges with ophiolitic remnants amalgamated during the Palaeozoic (Fig. 5.B.1a). The timing of continental crust growth during the Palaeozoic is uncertain, as the steps of the collage formation are not clearly established yet. The Kipchak arc model (Sengör et al., 1993) assumes a long-lived subduction zone of several thousands of kilometres, active from Late Neoproterozoic to Early Permian. During Permian, large transcurrent motions disrupted the belt (Laurent-Charvet et al., 2003) and gave birth to the puzzle structure of the Altaids collage. The Kipchak arc also
model implies a continuous accretion during the Palaeozoic. Alternatively, archipelago models (Mossakovsky et al., 1993; Buslov et al., 2001; Filippova et al., 2001; Badarch et al., 2002; Xiao et al., 2008) propose the existence of several independent and diachronous subduction zones, by analogy with the present-day Western Pacific setting. The resulting magmatic arcs and accretionary wedges were subsequently amalgamated (see reviews in Windley et al., 2007; Xiao et al., 2010), but timing of the final collision is largely debated (Charvet et al., 2011).
Figure 5.B.1: a: Tectonic map of the Western Altaids illustrating the relationships between northern China, Kazakhstan and Kirghizstan (modified after Windley et al., 2007; Charvet et al., 2011; Degtyarev, 2011). b: Tectonic map of West Junggar Mountains (adapted after Feng et al., 1989, BGMRXUAR, 1993, and Buckman
& Aitchison, 2004) illustrating the location of Early Palaeozoic accretionary complexes, mélanges and
magmatic rocks. Numbers in open circle refer to the ophiolitic mélanges mentioned in the text. 1: Late Palaeozoic Dalabute mélange; 2: Late Palaeozoic Baijantan mélange; 3: Undated Barliek mélange; 4: Late Palaeozoic Kokeshentan mélange; 5: Early Palaeozoic Mayila mélange ; 6: Early Palaeozoic Tangbale mélange 7: Early Palaeozoic Hongguleleng mélange sensu stricto, 8: Early Palaeozoic Hobuksar mélange, 9: Early Palaeozoic Bayanhe mélange. These three mélange are refered to a single Hongguleleng mélange sensu lato.
According to Sengör et al. (1993), the resulting Altaid accretionary orogens can account to almost 50% of the Earth’s total, newly added, Palaeozoic continental crust. The juvenile character of the crust was largely documented by positive εNd values of Late Palaeozoic granites (Heinhorst et al., 2000; Jahn et al., 2000a and 2000b; Wu et al., 2000; Kovalenko et al., 2004; Jahn, 2004; Kroner et al., 2008). However, some geochemical studies (Heinhorst et al., 2000; Chen & Jahn, 2004; Kröner et al., 2008) point out a recycling of juvenile material and/or Precambrian crust during arc and post-collisional magmatic events. Therefore, this partial recycling of the substratum during orogenic processes shows that continental crust growth is a complex process and it cannot be only limited to a simple transfer of mantle-derived magma into the crust. Moreover, recent field investigations in Western Altaids have revealed a polycyclic character for several diachronous accretionary orogens, e.g. the Tian Shan (Charvet et al., 2007; 2011) or the Chingiz belt (Degtyarev, 2011) (Fig. 5.B.1a). This feature is not in agreement with the single long-lived subduction model or the subduction complex tectonic collage model, but polycyclic orogens may explain how a Precambrian or Palaeozoic basement can be reworked during late magmatic events. These considerations are particularly important for the formation and evolution of the Early Palaeozoic Kazakhstan microcontinent (see a review in Degtyarev, 2011), where unit contacts are often hindered by a late tectonic overprint (Burtman, 1964; Samygin, 1974; Abdullin et al., 1980; Laurent-Charvet et al., 2003; Yakubchuk, 2004; van der Voo et al., 2006).
In West Junggar, northwest China, like in many places in the Altaids, Early and Late Palaeozoic ophiolitic mélanges are juxtaposed with diversely aged turbidites and volcanic rocks (Feng et al., 1989; Zhang et al., 1993; Wang et al., 2003; Buckman & Aitchison, 2004). Several models based on geochemistry and geochronology (Wang et al., 2003), or structure (Buckman & Aitchison, 2004; Xiao et al., 2008; 2010) have been proposed. Lateral duplication of a single long-lived subduction complex (Wang et al., 2003) and diachronous subductions of multiple oceanic basins (Buckman & Aitchison, 2004) have been alternatively proposed to explain the Palaeozoic geodynamic evolution of West Junggar. Numerous geochemical (Jian et al., 2005; Gu et al., 2009; Lei et al., 2008; Geng et al., 2009; Liu et al., 2009), and structural data (Zhang et al., 2011a; Choulet et al., 2011) have been recently published, but they mainly deal with the Late Palaeozoic events. Geological constraints on the Pre-Devonian evolution only consist of a stratigraphic inventory (BGMRXUAR, 1993), and geochemical and geochronological data from boulders in the
mélange (Zhang et al., 1993; Wang et al., 2003; Buckman & Aitchison, 2001; 2004; Jian et al., 2005). The architecture of Early Palaeozoic rock complexes is thus not well constrained. However, a recent study of zircon U-Pb geochronology and Hf isotopes on detrital zircons from Palaeozoic accretionary complex rocks (Choulet et al., submitted) has shown the coexistence of juvenile magmatic input with recycled juvenile magma, as inferred by the geochemical signature of some late to post-orogenic plutons (Chen & Jahn, 2004; Chen & Arakawa, 2005; Su et al., 2006a). An arc source with a likely Kazakhstan provenance for these zircons is involved, (Choulet et al., submitted). These results imply that the juvenile basement is recycled during the Palaeozoic and it rules out the idea of a diachronous collage of several magmatic arcs (Xiao et al., 2008). Another consequence is the genetic link between West Junggar and the Kazakhstan microcontinent that has been often disregarded (Wang et al., 2003; Buckman & Aitchison, 2004; Xiao et al., 2008).
The purpose of this paper is to provide new data demonstrating that West Junggar is a polycyclic accretionary orogen, with cycles of accretion separated by intervals of erosion and deposition of a molasses and olistostromes. Therefore we investigated the Chingiz-Tarbagatay, Mayila and Tangbale Early Palaeozoic accretionary complexes of West Junggar (Fig. 5.B.1b). We report here zircon U-Pb and Hf isotope data from a set of detrital samples collected in clastic deposits. In addition, we present new geochemical and geochronological data from magmatic rocks. These new results, combined with data from the literature, provide time constraints on the emplacement of the magmatic arcs, the activity of the subduction zones, and the subsequent erosion of the accretionary orogen. We also use these data to explore the possible connection of West Junggar, with neighbouring Kazakhstan during the Early Palaeozoic geological evolution.
B.2 An outline of the Early Palaeozoic units of West Junggar
The West Junggar accretionary orogen is a mountainous massif located in northwestern China, near the Kazakh border (Fig. 5.B.1b). The primary relationships between the tectonic units have been by Permian strike-slip faulting (Allen & Vincent, 1997; Choulet et al., 2010) and by emplacement of numerous Late Carboniferous-Early Permian plutons (Kwon et al., 1989; Chen & Jahn, 2004; Han et al., 2006; Zhou et al., 2008b; Geng et al., 2009). The West Junggar accretionary orogeny results of a polyclic evolution with Devonian-Carboniferous subduction complexes superimposed on an Ordovician-Silurian basement itself edificated through out accretionary process (Feng et al., 1989). The Late Palaeozoic evolution is marked by Devonian Carboniferous events, which have significantly affected the
primary archicture of the Ordovician-Silurian basement. The Late Palaeozoic West Junggar was a part of the Kazakh orocline (Abrajevitch et al., 2008). The subduction of the Junggar Balksh Ocean during Devonian and Carboniferous (Buckman & Aitchison, 2004; Choulet et al., 2011) gave birth to the Barliek arc and the West Karamay accretionary complex (Fig.
5.B.1b), which display Carboniferous turbidites (Jin & Li, 1999) and the Karamay and
Dalabute ophiolitic mélanges (Zhang et al., 2011a). The northern part of West Junggar Mountains exposes a second Devonian-Carboniferous subduction belt formed by the Sawuer arc and the Erquis accretionary complex, including the Kokeshentan ophiolitic mélange (Buslov et al., 2001; Zhou et al., 2008b; Chen et al., 2010a; 2010b) (Fig. 5.B.1b).
Three Early Palaeozoic massifs are exposed in the West Junggar Mountains (Fig.
5.B.1b). The Chingiz-Tarbagatay Unit that may extend to the eastern Kazakhstan, crops out in
Xiemisitai-Sharburt Mountains and Tarbagatay Mountains (Feng et al., 1989; Chen et al., 2010a). Because of the difficulty of access, the Ordovician series of the Tarbagatay Mountains were not investigated in detail, and only a brief outline is given here. Conversely, the Mayila and Tangbale units that are well exposed in the southern part of West Junggar are analysed in detail here (Feng et al., 1989; Buckman & Aitchison, 2004; Chen et al., 2010a) (Fig. 5.B.1b). The three areas will be discussed further and especially their correlations with Kazakhstan.
B.2.i The Chingiz-Tarbagatay Unit
Pre-Devonian rocks crop out in the Tarbagatay, Sharburt, Xiemisitai, North Wuerkashier and Saier Mountains (Fig. 5.B.2). There, serpentinite mélanges are recognized from west to east in the Bayanhe, Hobuksar and Hongguleleng areas. Although no radiometric data support a contemporaneous origin of these three mélanges, they display similar lithology and can be grouped in a single mélange unit, which will be called the Hongguleleng mélange. Plagiogranite, troctolite, diabase, and gabbro from the Hongguleleng mélange were dated at 625±25 Ma by Sm-Nd isochron method (Huang et al., 1995; Jin et al., 1999). However, this age is conspicious since damouritization of the gabbro plagioclase is reported and since 5 out the 7 analyzed rocks display Nd and Sm content lower than 1 ppm (Huang et al., 1995). The Late Ordovician Nd-Sm isochron age at 444±27 Ma (Zhang & Huang, 1992), obtained from four gabbroic cumulates and one basalt, is more acceptable and is compatible with the 475 Ma age, obtained by SHRIMP on 24 zircons from an anorthosite (Jian et al., 2005). The serpentinite matrix of the mélange includes various-sized boulders of
websterite, gabbroic pegmatite, dolerite and basalts (Zhang et al., 1993; Jin et al., 1999). According to Huang et al. (1995), these ophiolites formed in an oceanic setting, but a back-arc setting cannot be ruled out (Zhang et al., 1993). In the Honguleleng mélange (Fig. 5.B.2), slices of tuffs, graywackes, and andesitic lavas from the Bulukqi Formation are also exposed in the Sharburt Mountains and limestone interlayers yield Middle to Late Ordovician trilobites, brachiopods and gasteropods (BGMRXUAR, 1993). Further to the west, in the Tarbagatay Mountains (Fig. 5.B.1b), Zhu and Xu (2006) dated a gabbro from the Kujibai ophiolite at 478±3 Ma, reworked into an Early Carboniferous conglomerate. The Hongguleleng, Hobuksar and Bayanhe mélanges experienced intense ductile shearing, documented by highly schistone serpentinite matrix and sigmoidal blocks of pillow-basalt, (Fig. 5.B.3a). The mélange is usually in fault contact with the surrounding younger rocks, except to the east of Hongguleleng, where unconformable shallow dipping Late Silurian rocks cover the sub-vertical mélange rocks (Fig. 5.B.2).
Middle Ordovician volcanoclastic rocks of the Kekesayi Formation are exposed in the Tarbagatay Mountains (Fig. 1; BGMRXUAR, 1993). Upper Ordovician ashes and tuffaceous sandstones pass upward to an olistostrome, which consists of blocks of conglomerate and limestone included into a shaly matrix. According to Feng et al. (1989), the conglomerate contains boulders of chert, calk-alkaline lavas and ophiolitic rocks. In the Saier Mountains (Fig. 5.B.2), the fossiliferous limestone has yielded Late Ordovician corals, gasteropods and trilobites (BGMRXUAR, 1993). Early Silurian tuffaceous siltstones and graptolite-rich shales (Bulong Formation) conformably overlie the Late Ordovician olistostrome (Feng et al., 1989; Mu et al., 1986). In the Sharburt Mountains (Fig. 5.B.2), a thick pile of volcanic
Figure 5.B.2 Geological map of Sharburt-Xiemisitai Mountains (modified after Feng et al., 1989; Chen et al.,
rocks (Sharburt Formation) is composed of andesitic tuff (Fig. 5.B.3b), porphyrytic andesite lava and andesitic (Fig. 5.B.3c) (Feng et al., 1989). The base of the series is not observed (BGMRXUAR, 1993). Rare fossils of Middle Silurian age are reported from interlayered sandstone and limestone (Mu et al., 1986). The geochemistry of these volcanic rocks is not available. The uppermost part of the formation is composed of a dark greenish to reddish conglomerate, including boulders of the underlying volcanic rocks (Fig. 5.B.3d), but also fragments of the Hongguleleng ophiolites (Jin et al., 1999). Limestone, containing Ordovician fossils (Jin et al., 1999) and Middle Silurian (Wenlock) corals (Mu et al., 1986), were initially described as synsedimentary lenses, but our investigations indicate that these blocks are rather olistoliths enclosed in a terrigenous conglomeratic matrix. The size of the blocks can reach several tens of metres (Fig. 5.B.3e), and there is no concordance between the bedding of the matrix and the folded bedding within the limestone olistoliths (Fig. 5.B.3f). Various-scaled breccias are exposed at the base of the blocks and support a syn-tectonic redeposition setting for the limestone (Fig. 5.B.3g and 5.B.3h). This block-in-matrix formation must not be confused with the serpentinite mélange described above. Instead, it represents a Middle to Late Silurian olistostrome (Wenlock-Pridoli), formed by a gravitational unstability.
The Kekexiongkuduke Formation, which overlies the olistostrome, consists of tuffaceous variegated sandstone, and limestone lenses containg Middle Silurian-Late Silurian age (Wenlock-Pridoli) corals and brachiopods (Mu et al., 1986; BGMRXUAR, 1993). The volcaniclastic series grades into a thick sequence of pyroclastic-rich turbidites (Feng et al., 1989) and is conformably overlain by the lower Devonian clastic sandstones (Hobuksar formation). In the Sharburt Mountains, the base of the Lower Devonian (Wutabulak sub-formation) is similar to the Kekexiongkuduke Formation, with turbidites containing bivalves, graptolites and trilobites (Mu et al., 1986; Feng et al., 1989). As shown by ichnofacies variations (Gong, 1993), the sedimentary deposits became shallower in the middle Lower Devonian Mangeer sub-Formation, with abundant immature debris (Wei et al., 2009). The presence of conglomerate suggests molasse-like deposits. At the top of the Lower Devonian series, calcareous siltstone and limestone contain abundant plant debris (BGMRXUAR, 1993). In the Wuerkashier Mountains (Fig. 5.B.2), the Lower Devonian rocks of the Malasu and Mengbulak formations are composed of terrigenous clastic rocks interlayered with lavas and tuffs (BGMRXUAR, 1993). The base of the middle Devonian (Eifelian) is lacking (Soto & Lin, 2000; Wei et al., 2009). Sedimentation starts again at the end of the Middle Devonian
with continental sandstone of the Hujiersite Formation overlain by the variegated tuffaceous sandstone and conglomerate of the Zulumute Formation (BGMRXUAR, 1993). Fossil plant assemblage (Cai & Wang, 1995; Xu & Wang, 2008), and detrital zircon geochronology (Choulet al., submitted) provide a Givetian age for the Hujiersite Formation, whereas the Frasnian age of the Zulumute formation is based on rare plant debris (BGMRXUAR, 1993).
The overlying Hongguleleng formation, which should not be confused with the Honggulelleng mélange is composed of calcareous siltstone, calcarenite and limestone. Famennian faunae of conodonts, rugose corals, brachiopods and echinoderms were reported (Xia, 1996; Lane et al., 1997; Soto & Lin, 2000; Waters et al., 2003). To the north, the Middle to Late Devonian rocks of the Sawuer Mountains expose thick sections of andesite and tuff, related to the Sawuer arc (BGMRXUAR, 1993; Shen et al., 2008) (Fig. 5.B.2). The Carboniferous sedimentary deposits are chiefly volcanocastic turbidites associated with lavas (BGMRXUAR, 1993). Continental molasses deposits usually characterize the Permian deposits (Feng et al., 1989).
A ten of granitic intrusions (A-type granite, diorite, and K-feldspar granite), initially assigned to Permian yield zircon SHRIMP and LA-ICPMS ages ranging from 422 Ma to 405 Ma (Chen et al., 2010a) (Fig. 5.B.2). These Late Silurian-Early Devonian ages imply that