B. Wang 1,2, M. Faure 1, D. Cluzel 1, L. Shu 2, J. Charvet 1

Transcription

1 TECTONIC EVOLUTION OF THE NORTHERN PART OF THE YILI BLOCK (WESTERN CHINESE TIANSHAN) B. Wang 1,2, M. Faure 1, D. Cluzel 1, L. Shu 2, J. Charvet 1 1 Institut des Sciences de la Terre d Orléans, Université d Orléans, F45067, Cedex 2, France 2 Department of Earth Sciences, Nanjing University, Nanjing, , China, bo.wang@univ-orleans.fr The Tianshan range, that extends E-W over more than 3000 km from NW China to Kazakhstan and Kirgizstan, separates the Tarim Basin to the south from the Junggar Basin to the north. It is a key region for the understanding of the Late Paleozoic geodynamic evolution of Central Asia. The Tianshan Belt is considered to result from the Paleozoic amalgamation of several continental blocks, arcs and accretionary complexes [Coleman, 1989; Windley et al., 1990; Sengör et al., 1993]. In the Xinjiang Province of NW China, the geodynamic development of eastern Tianshan is clarified as a two-stage north-verging deformation [Charvet el al., 2001; Laurent-Charvet, 2002]. The ealier closure of the South Tianshan Paleo-ocean that led to accretion of Central Tianshan to Tarim continued until the Devonian and generated large thrusts and nappes of ophiolitic mélange that formed the present South Tianshan Unit. It also resulted in the southward subduction of the North Tianshan Paleo-ocean under the Ordovician Central Tianshan volcanic arc and subsequent accretion of the North Tianshan unit. The Late Devonian to Late Carboniferous southward subduction of the North Tianshan Paleo-ocean was responsible for the development of widespread volcanic-arc rocks in the North Tianshan continental block. At the end of the Carboniferous, the collision of the Junggar Block with the volcanic arc terminated the "oceanic" part of the evolution of this area. In the western Tianshan evolution models have been already proposed [Xiao et al., 1992; Gao et al., 1998], but no agreement has been reached yet and the correlation with East Tianshan is still problematical. Moreover, the Yili Block, located between North Tianshan and the Central-South Tianshan, played an important role during the mountainbuilding of the West Tianshan belt, but its tectonic features are poorly constrained. In order to clarify the Late Paleozoic geodynamic evolution of the northern part of West Tianshan Belt, we focused our work on volcanic rocks that crop out in the Yili Block, ophiolitic mélange and the North Tianshan shear zone (Fig. 1). 200 volcanic rock samples were collected for geochemistry (XRF & ICP-MS) and oriented samples were taken for kinematic of the shear zone. The volcanic rocks collected from the Yili Block are dated as Early to Middle Carboniferous. The petrological and geochemical features of these rocks infer that: (1) they 274

2 belong to the calc-alkaline series showing a linear differentiation trend from andesite (including basaltic andesite), rhyodacite to trachyandesite; (2) they have relatively high total REE concentrations ranging from 66 to 219 ppm. The Chondrite-normalized REE distribution patterns are characterized by a negative slope with (La/Yb) N > 2.66 showing the Fig. 1. Structural map of the western Chinese Tianshan with location of the study areas fractionation of LREE relative to HREE, which is consistent with their calc-alkaline character; and most volcanic rocks show a weak negative Eu anomaly ( Eu=0.6113~0.9135) possibly related to plagioclase fractionnation, while some of them have no Eu anomaly ( Eu=0.9975~1.0417) with a more primitive character (0.22< Mg# <0.37). (3) On the expanded REE and trace elements spiderdiagram normalised to the MORB, they display a prominent enrichment of LILE relative to HFSE which have MORB concentrations, they also display a moderate negative anomaly of Nb and Ta relative to Th and Ce. These characters are consistent with subduction-related magmas. Classical petrotectonic discrimination diagrams [Wood et al., 1979; Pearce and Peate, 1995] indicate that the Carboniferous calc-alkaline volcanic rocks of Yili are likely to have erupted in a continental active margin setting. Carboniferous arc-type granites with age of 350~310 Ma (Zircon U-Pb) are widespread within the Yili Block. Moreover, the sedimentary evolution of the platform series shows evidence for Late Carboniferous filling-up of the basin instead of rifting as previously proposed. This tectonic setting is also consistent with that of Bogda- 275

3 Harlike arc in northern East Tianshan, which is considered as a Carboniferous continental volcanic arc formed by the subduction of Junggar Paleo-ocean [Charvet et al., 2002; Shu et al., 2002]. In the study area, a prominent subduction complex is exposed along the northern boundary of the Yili Block (Fig. 1), it is composed of thick mid-carboniferous turbidite (flysch) and an ophiolitic mélange. The melange is formed of decametre to kilometre-size blocks of serpentinized peridotite, diabase, massive and pillowed basalt, abyssal chert, and flysch included in a coarse turbidite. The published geochemical features of the mafic rocks allow three magma types to be distinguished: N-MORB, OIB and IAT [Xiao et al., 1992; Li, Du, 1994], depicting a marginal basin. The cherts yield Late Devonian to Early Carboniferous radiolarian and conodonts [Xiao et al., 1992; Li, Du, 1994]. These lines of evidence infer that during the Late Carboniferous, the northern boundary of the Yili Block was an active continental margin. The south-directed subduction of a Late Devonian-Early Carboniferous "oceanic" plate accounts well for the formation of both accretionary complex and magmatic arc. To the south of the Carboniferous turbidite belt, an East-West trending ductile shear zone, 5 km wide and extending laterally over more than 100 km separates the turbidite and ophiolitic mélange to the north from the Yili volcanic-arc to the south (Fig. 1). Steeply dipping foliation with an average N100E strike bears a sub-horizontal lineation. Outcrop scale kinematic criteria such as asymmetric pressure shadows around feldspar or quartz porphyroclasts indicate a dextral strike-slip shearing. Under the microscope, kinematic criteria, including -type feldspar and calcite porphycroclasts with asymmetric mica and recrystallized quartz "tails", Fig. 2. Asymmetric quartz pressure shadows around fedspar porphyroclast and mica-fish in slate showing a ductile dextral shear along the Northern Tianshan strike-slip fault. In the field, the subvertical E-W foliation bears a subhorizontal lineation. asymmetric pressure shadows around rigid feldspar grains (Fig. 2), mica-fish structure, and quartz dynamic recrystallization texture confirm the dextral sense of shearing. These kinematic features are consistent with the large-scale dextral strike-slip deformation described in Central and Eastern Tianshan [Shu et al., 1999; Laurent-Charvet et al., 2002, 2003]. Therefore we suggest that a southward subduction regime, that closed the northern Tianshan oceanic basin and generated the ophiolitic mélange and active continental mar- 276

4 ginal magmatism, was followed by collision of the Yili Block with the Junggar continent. During the Late Permian, this suture zone was reactivated by dextral wrench faulting. The present architecture of the North Tianshan Belt is mainly due to the Himalayan collision. As a result of intracontinental deformation, the Paleozoic units are thrust northward over the Junggar basin and therefore hide the bulk of the Carboniferous suture (Fig. 3). Fig. 3. Interpretative crustal-scale cross section of the North Tianshan Belt west of Urumqi showing Late Carboniferous polyphase top-to-the-north thrusting, Permian dextral wrench faulting, and Cenozoic top-to-the-north reworking. A geodynamic model involving oceanic subduction, continental collision and strain partitioning between frontal subduction and lateral escape is discussed in order to better understand the Paleozoic evolution of the West Tianshan Belt during the Late Paleozoic. References Charvet J., Laurent-Charvet S., Shu L.S., Ma R.S. Paleozoic continental accretions in Central Asia around Junngar Block: new structural and geochronological data, Gondwana Research 4(4), , Coleman R.G. Continental growth of Northwest China, Tectonics, 8, , Gao J., Li M.S., Xiao X.C., Tang Y.Q., He G.Q. Paleozoic tectonic evolution of the Tianshan Orogen, northern China, Tectonophysics, 287, , Laurent-Charvet S., Charvet J., Monie P., Shu L.S. Late Paleozoic strike-slip shear zones in eastern Central Asia (NW China): new structural and geochronological data, Tectonics 22(2), , Laurent-Charvet S., Charvet J., Shu L.S., Ma R.S., Lu H.F. Palaeozoic late collisional strikeslip deformations in Tianshan and Altay, eastern Xinjiang, NW China, Terra Nova, 14(4), , Li S.H., Du Q. The ophiolites in Motogou-Gurt of Wusu County, Xinjiang Geology 12(3), , (In Chinese with English abstract). 277

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