Paleozoic geodynamic evolution of Yili Block
in West Chinese Tianshan

Abstract : The Yili Block is a triangular area bordered by the northern and southern fault zones of the
Western Chinese Tianshan Belt, it is considered as a micro-continental block with Proterozoic
basement extending westward into Kazakhstan with unclear boundaries. The Yili Block is
important for understanding the Paleozoic geodynamic evolution of the Tianshan Belt, which
resulted from polyphase subduction-collision orogenies. The Tianshan Belt is classically divided
into three subunits: North Tianshan, Central Tianshan and South Tianshan. But our structural,
geochemical and paleomagnetic studies suggest that these subunits and their boundaries should be
The Yili Block, previously considered as a part of the Central Tianshan, is in fact a Late
Paleozoic magmatic arc situated upon a Precambrian basement and an Early Paleozoic platform.
Carboniferous volcanic rocks are widespread along the margins of Yili Basin. Petrological and
geochemical features of Carboniferous volcanic rocks and granitoids show that: (1) volcanic rocks
are mainly composed of andesite, rhyolite and minor basaltic rocks, granitoids consist of gabbro,
diorite, granodiorite and granite, both volcanic and granitic rocks are belong to the calc-alkaline
series, (2) magmatic rocks display prominent Nb-Ta negative anomalies consistent with
subduction - related magmas, and (3) HFSE-based discriminations place volcanic rocks in the
field of continental arcs, and granitoids in the field of syn-collisional granites or arc-type granites.
Rb-Sr and Sm-Nd isotopic studies indicate that these magmatic rocks derived from a depleted
magma reservoir of mantle source, and suffered an important contamination of upper crust or
terrigenous sediments. Taking into account the shallow water sedimentation associated with the
magmatism, the Carboniferous magmatic rocks are suggested to form in an active continental
margin setting. Systematic zircon U-Pb ICPMS dating on the volcanic and granitic rocks from the
whole Yili Block indicates that their ages range from 389~301 Ma, i.e. Late Middle Devonian to
Carboniferous time, but the peak magmatism took place during 360~301 Ma.
The northern boundary of the Yili Block is represented by the North Tianshan Late
Carboniferous turbidite and Bayingou-Motuogou ophiolitic mélange. Stratigraphical and
petrological studies suggest that the turbidite and ophiolitic mélange form a subduction complex.
The ophiolitic mélange was a result of intra-oceanic tectonism and seafloor redeposition and
subsequent deformation during the subduction of the North Tianshan oceanic basin, which existed
at least during Late Devonian to Early Carboniferous on the basis of the microfossils found in
chert of ophiolite. Structural, kinematic, chronological and geochemical evidences from ophiolitic
mélange and turbidite argue that a southward subduction of the North Tianshan oceanic basin is
responsible for the formation of the Late Paleozoic Yili magmatic arc. Since this subduction
complex was redeformed and thrust northward upon the Junggar Cenozoic basin, the real North
Tianshan Suture is probably hidden by the Cenozoic thrust. New geochemical studies from
Houxia area indicate that the dolerite, andesite and dacitic rhyolite show similar features with
those of Yili arc, and therefore correspond to the North Tianshan arc. Temporal consistency and
spatial correlation between the North Tianshan subduction complex and the North Tianshan Arc suggest that the North Tianshan Suture extends probably eastwards until to the north of Bogdashan.
More detailed studies are needed to test this conclusion.
The southern boundary of the Yili Block is a complicated deformation zone including
high-pressure (HP) metamorphic complex, ophiolitic mélange, Proterozoic basement and platform
sedimentary rocks, affected by strike-slip faulting. Detail geological survey along the Kekesu
River revealed a top-to-the-North ductile shearing that is developed both in ocean-derived HP
rocks and in the southern margin of the Yili Block. Evidences for an extensional event, formed in
retrograde greenschist facies conditions, are recognized in the HP metapelites. Ar-Ar laser probe
dating of white mica in retrogressed blueschist facies metapelites and in greenschist facies
quartzite provides 330-315 Ma ages that are interpreted as the date of the exhumation of the HP
rocks. Taking into account the previous results, HP/UHP metamorphism should have taken place
earlier than 350Ma. Since the final magmatism of the Yili Arc is significantly younger than both
peak metamorphism and retrograde metamorphism, and the top-to-the-North kinematics of the HP
rocks, our results do not support the previous interpretation of the north directed subduction of the
“South Tianshan Ocean” producing the Yili magmatic arc. Near to the HP metamorphic belt,
Ophiolitic mélange of Changawuzi-Qiongkushitai areas extends eastward joining with Gangou -
Mishigou ophiolitic mélange, mafic rocks yield ages from Ordovician to Silurian, and the
kinematics of Gangou ophiolitic mélange is consistent with that of Kekesu HP metamorphic rocks.
To the south of Kekesu HP metamorphic complex, Qiongkushitai and Gangou mélange zone,
Ordovician-Silurian arc-type volcano-sedimentary rocks were recognized to be unconformably
covered by Carboniferous sandstone and carbonate. Proterozoic amphibolite facies metamorphic
rocks are distinguished as the basement of this volcanic arc, which is defined as the Central
Tianshan block. Therefore, the metamorphic and mélange zone is interpreted as a southward
subduction of Tianshan Ocean beneath the Central Tianshan block generating the
Ordovician-Silurian volcanic arc.
In Aheqi, Wushi and Heiyingshan areas, along the southern slope of the Central Tianshan, a
Late Devonian-Early Carboniferous ophiolitic mélange includes ca 390 Ma gabbro blocks with a
back-arc-basin geochemical affinity. Moreover, the evolution from platform carbonate
sedimentation to banded cherts deposition argues for a progressive deepening of the southern
margin of the Central Tianshan Block that is responsible for the opening of a marginal oceanic
basin between the Central Tianshan and Tarim during the Early to Middle Devonian. Field
observation and kinematics suggest that the mélange has been thrust to the North above the
deformed Silurian-Devonian volcaniclastic rocks and marble series. This ductile shearing occurred
before the deposition of the Lower Carboniferous sandstone and limestone that unconformably
cover the mélange and its tectonic substratum. These observations are quite consistent with those
from Kulehu and Kumux-Yushugou areas. Therefore, this ophiolitic mélange zone that stretches
from Aheqi to Kumux is defined as the South Tianshan Mélange Belt. This mélange has
previously often been considered as south-directed tectonic klippes, but our study indicates that
they are actually thrust from South to North, and rooted in the southernmost suture of the Chinese
Tianshan Belt.
The two boundaries of the Yili Block have been reworked by the Permian strike-slip faulting.
North Tianshan Fault redeformed the southern part of North Tianshan Carboniferous turbidite (the
northern boundary of the Yili Block). First-hand kinematic results suggest that the North Tianshan
Fault is a dextral ductile strike-slip fault, new Ar-Ar whole rock dating constraint this deformation from 285 to 245 Ma that is consistent with the age of Main Tianshan Shear Zone (280~250Ma).
The Nalati Fault that extends eastwards joining with the Qingbulak Fault cut the southern
boundary of the Yili Block, kinematic observations from Kekesu, Laerdun Pass and
Sangshuyuanzi coherently indicate a dextral shearing, previous chronological study also provide
~280 Ma age. The Permian right-lateral faulting is associated with alkaline magmatic rocks, such
as continental tholeiite, felsic volcanic rocks and alkaline granite. Late Permian redbeds and
conglomerate are widespread in whole Tianshan Belt and unconformably cover the pre-Permian
rocks. Therefore, Permian tectonics appears to be geodynamically distinct from the convergent
orogeny of the Tianshan Belt, and plays an important role on the final formation of its present
In order to better constrain Permian transcurrent tectonics, primary paleomagnetic results on
Ordovician, Carboniferous and Permian rocks are obtained from the Yili Block and its adjacent
areas. More than 500 sedimentary and volcanic samples were collected from 61 sites. Laboratory
measurements on rock magnetism and magnetic remanence indicate that the magnetite and
hematite are principal remanent carriers for characteristic remanent magnetization. After careful
analysis of reliable characteristic remanences from Zhaosu, Xinyuan and Gongliu areas and
detailed discussion on remanence ages, two primary poles of Late Carboniferous and Late
Permian are calculated for the Yili Block. Comparison of these paleomagnetic poles of the Yili
Block with coeval poles of Junggar, Tarim and Siberia indicates (1) no significant relative motion
between the Yili and Junggar blocks since Late Carboniferous, (2) no significant or weak
latitudinal relative motion occurred since Late Carboniferous among these blocks, but (3) the
46.2°±15.1° and the 31.6°±15.1° counterclockwise rotations of the Yili-Junggar blocks with
respect to Tarim and Siberia took place during Late Carboniferous to Permian. These rotations are
accommodated by the Permian dextral strike-slip faults along the northern and southern sides of
the Tianshan Belt and sinistral strike-slip faulting along the Erqishi Fault of the Altay Belt,
resulting in about 1000 km and 600 km lateral displacements in the Tianshan and Altay belts,
Finally, on the basis of the removal of Permian lateral displacement and Cenozoic
reactivation during the Indo-Asian collision, a simplified evolutionary model involving the Yili
Block and its adjacent areas is proposed. During the Ordovician-Early Silurian, an oceanic basin
called the Tianshan Ocean existed between the Yili Block and the Central Tianshan Block. This
ocean began to close by southward subduction beneath the Central Tianshan during Late
Ordovician and Silurian generating the Central Tianshan volcanic arc. From Middle Silurian to
Middle Devonian time, the Tianshan Ocean was progressively closing. The oceanic subduction
was followed by the continental subduction of the southern part of the Yili Block and resulted in
the development of the HP metamorphic complex. Coevally with the closure of the Tianshan
Ocean, a marginal sea opened to the South of the Central Tianshan Block, as suggested by the
ophiolitic mélange and the progressive deepening of the southern margin of the Central Tianshan
Block during the Early to Middle Devonian. During the Early Paleozoic, the limestone and
sandstone deposits suggest that the northern margin of the Yili Block was a passive margin. From
the Late Devonian to Early Carboniferous, due to the closure of the Tianshan Ocean, the Yili and
Central Tianshan Blocks were welded together to form a single continental mass. At that time, the
HP metamorphic rocks were exhumed. The closure of the back arc basin was accommodated by a
south-directed subduction below the Tarim Block as suggested by the northward thrusting of the South Tianshan ophiolitic mélange. During Late Mid-Devonian to Early Carboniferous, southward
subduction of an oceanic basin, called the North Tianshan Ocean was responsible for the Yili
magmatic arc and the North Tianshan accretionary prism. The oceanic subduction ended in Late
Carboniferous when the continental collision occurred between Junggar Block and the Yili Block.
At the end of the Carboniferous, the N-S convergence finished when all the continental blocks
were amalgamated. Permian lateral large-scale displacement dragged the main blocks of Central
Asia close to their present position, and probably also disturbed original continuity (e.g. Yili and
Bogda magmatic arcs).
Document type :
Tectonics. Université d'Orléans, 2006. English
Contributor : Nathalie POTHIER <>
Submitted on : Wednesday, April 11, 2007 - 8:12:44 AM
Last modification on : Wednesday, April 11, 2007 - 12:41:31 PM


  • HAL Id : tel-00140948, version 1


Bo Wang. Paleozoic geodynamic evolution of Yili Block
in West Chinese Tianshan. Tectonics. Université d'Orléans, 2006. English. <tel-00140948>




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