Early Paleozoic Tectonic Evolution of the South Tianshan Collisional Belt: Evidence from
Geochemistry and Zircon U-Pb Geochronology of the Tie’reke Monzonite Pluton, Northwest
China
Author(s): He Huang, Zhaochong Zhang, M. Santosh, Dongyang Zhang, Zhidan Zhao, and Junlai
Liu
Source: The Journal of Geology, Vol. 121, No. 4 (July 2013), pp. 401-424
Published by: The University of Chicago Press
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Early Paleozoic Tectonic Evolution of the South Tianshan Collisional
Belt: Evidence from Geochemistry and Zircon U-Pb Geochronology
of the Tie’reke Monzonite Pluton, Northwest China
He Huang, Zhaochong Zhang,* M. Santosh, Dongyang Zhang,
Zhidan Zhao, and Junlai Liu
State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences,
Beijing 100083, China
ABSTRACT
We report an Early Paleozoic hornblende quartz monzonitic pluton from the Tie’reke region in the central South
Tianshan Collisional Belt (STCB). Laser ablation ICP-MS U-Pb zircon dating reveals that the pluton was emplaced
during the Late Silurian at ∼419.9 Ⳳ 2.1 Ma. Our data, together with those from coeval intrusive rocks in the eastern
STCB and the eastern Northern Margin of the Tarim Block (NMTB), indicate a major late Early Paleozoic magmatic
event in the region. This magmatic event is supported by a detrital zircon U-Pb age population of 462–395 Ma obtained
from a Cenozoic sandstone sample from the Kangsu region and an Early Paleozoic metasandstone sample from the
Jigen region. Geochemically, the Tie’reke pluton is intermediate in composition, with SiO2 contents ranging from
60.73 to 64.73 wt%, and belongs to the alkali-calcic and shoshonitic series. The pluton displays relative depletion
of Nb, Ta, P, and Ti and enrichment of large-ion lithophile elements (Ba, K, and Rb), typical of continental arc–related
igneous rocks. Whole-rock Sr-Nd and zircon Hf isotopic data reveal that the magma was derived dominantly from
partial melting of the Paleoproterozoic continental crust, with input from juvenile materials from a depleted-mantle
wedge. In general, geochronological, geochemical, and isotopic features of the Late Silurian igneous rocks in the
present STCB and NMTB, coupled with detrital zircon U-Pb geochronological data from the two sedimentary rocks,
suggest that the northern margin of the Paleozoic Tarim Block was an Andean-type active continental margin during
Middle Ordovician to Middle Devonian time. Given the coeval magmatism in the Central Tianshan Block, which
necessitates a northward subduction of the Paleozoic South Tianshan Ocean, we propose a double-subduction model
for the evolution of the Paleozoic South Tianshan Ocean during the Late Ordovician to Middle Devonian period.
During the Late Devonian to Middle Carboniferous, the northern margin of the Paleozoic Tarim Block was likely
characterized by tectonomagmatic quiescence, whereas the Central Tianshan Block was still extensively affected by
arc-type magmatism, furthering the northward subduction of the Paleozoic South Tianshan Ocean.
Online enhancements: supplementary tables.
Introduction
gens on the planet and contains numerous tectonic
units, including microcontinents, island arcs, oceanic plateaus, seamounts, ophiolites, and accretionary complexes (Windley et al. 1990, 2007; Jahn
et al. 2004; Rojas-Agramonte et al. 2011; Kröner et
al. 2013). In general, the Neoproterozoic and Paleozoic tectonics of the CAOB were closely related
to the evolution of the Paleo-Asian Ocean and the
amalgamation of different terranes (Qian et al.
2007; Han et al. 2011; Xiao et al. 2013; Xu et al.
The Central Asia Orogenic Belt (CAOB), also
known as the Altaid Tectonic Collage or Altaids,
is sandwiched between the Siberian and European
Cratons to the north and the Karakum, Tarim, and
North China Cratons to the south (fig. 1a; Şengör
et al. 1993; Cawood et al. 2009; Xiao et al. 2013).
The CAOB is one of the largest accretionary oroManuscript received June 25, 2012; accepted February 3,
2013.
* Author for correspondence; e-mail: [email protected].
[The Journal of Geology, 2013, volume 121, p. 401–424] 䉷 2013 by The University of Chicago.
All rights reserved. 0022-1376/2013/12104-0005$15.00. DOI: 10.1086/670653
401
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402
H. HUANG ET AL.
Figure 1. a, Tectonic sketch map of the Central Asian Orogenic Belt. b, Geological map of the Central Tianshan
Block, the South Tianshan Collisional Belt, and the Northern Margin of the Tarim Block (modified from Gao et al.
2011; Huang et al. 2012b). c, Geological map of the Tie’reke hornblende quartz monzonitic pluton.
2013). However, the nature of individual terranes
in the CAOB and their complete geological record
remain to be investigated.
The South Tianshan Collisional Belt (STCB),
along the southern margin of the CAOB, was formed
by the final amalgamation of the Tarim Block to the
south and the Central Tianshan Block to the north
(present coordinates). Before this collisional event,
the two blocks were separated by the Paleozoic
South Tianshan Ocean, the southernmost branch of
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Journal of Geology
S O U T H T I A N S H A N C O L L I S I O N A L B E LT
the gigantic Paleo-Asian Ocean. Notably, almost all
paleogeographic studies regard the Tarim Block as
the last terrane to dock with the CAOB; thus, the
formation of the STCB may represent the termination of the prolonged Paleozoic Central Asian accretionary orogeny (Xiao et al. 2010, 2013 and references therein). Consequently, the geological
evolution of the STCB is critical for understanding
the reconstruction of the Paleozoic tectonic framework of the CAOB. Previous studies have mainly
focused on the Late Paleozoic tectonic evolution,
including the time of closure of the Paleozoic South
Tianshan Ocean and the subsequent collision between the Tarim and Central Tianshan Blocks.
The Early Paleozoic tectonic evolution of the region is not well constrained, however. One of the
disputed aspects of this evolution involves the Paleozoic South Tianshan Ocean. Some authors argue
that the opening of the Paleozoic South Tianshan
Ocean was triggered by back-arc extension during
southward subduction of the Paleozoic Terskey
Ocean (PTO, or Paleo-Tianshan Ocean in some literature; Long et al. 2011a; Wang et al. 2011), which
existed between the Paleozoic Central Tianshan
Block and the Kazakhstan-Yili Block (KYB). In contrast, other workers suggest that the opening of the
Paleozoic South Tianshan Ocean was nearly simultaneous with or even earlier than that of the
PTO (see Ge et al. 2012 and references therein). The
subduction polarity of the Paleozoic South Tianshan Ocean is also in dispute. Although the northward-subduction (present coordinates) model has
been widely advocated (see Han et al. 2011; Xiao
et al. 2013 and references therein), a southwardsubduction model, predominantly based on the
north-verging ductile deformation in the accretionary complexes, has recently been proposed (e.g.,
Charvet et al. 2007, 2011; Wang et al. 2011).
Intermediate to felsic igneous rocks, widely distributed in the STCB, can be used to constrain the
STCB’s geodynamic and tectonic settings. However, most of the rocks examined in previous studies display Late Carboniferous to Early Permian
ages and were emplaced in nonsubduction (postcollisional or intraplate) settings. In comparison
with those in other terranes of the CAOB, Early
Paleozoic igneous rocks are generally rare in the
STCB. We have recently discovered a Late Silurian
hornblende quartz monzonitic pluton intruding the
Precambrian basement strata in the Tie’reke region
in the central STCB. This is the first report of detailed investigations of this pluton that includes
laser ablation (LA)-ICP-MS U-Pb and Lu-Hf isotopic
data and bulk-rock geochemical and Sr-Nd isotopic
compositions. In addition, detrital zircon U-Pb ages
403
from a Cenozoic sandstone sample from the Kangsu
region and another Early Paleozoic metasandstone
sample from the Jigen region are also presented to
constrain the geological history of the region. In
combination with previously published data, a new
tectonic model is proposed for the Paleozoic evolution of the southern margin of the CAOB.
Geological Setting
The Tianshan (Tien Shan, or Tian Shan) range, extending more than 2500 km from Uzbekistan to
southwestern Mongolia, is on the southern margin
of the CAOB. Roughly separated by the TuokexunKumishi High Road, the Chinese Tianshan is generally subdivided into eastern and western domains
(Gao et al. 2009). Tectonically, the western Tianshan
is composed of, from north to south, the North Tianshan Collisional Belt, the KYB, the Central Tianshan
Block, the STCB, and the Northern Margin of the
Tarim Block (NMTB; fig. 1b). Among these tectonic
domains, the North Tianshan Collisional Belt and
the STCB are Paleozoic accretionary/collisional
belts, whereas the KYB, the Central Tianshan Block
and the NMTB are microcontinental blocks or terranes (Li et al. 2006). Detailed descriptions of the
geological evolution of the western Tianshan are
given in Han et al. (2011), Xiao et al. (2010, 2013),
and Long et al. (2011a). Our study focuses mainly
on the Central Tianshan Block, the STCB, and the
NMTB, and a brief summary on the geological settings of these three terranes is given below.
Although the Central Tianshan Block was once
considered the southern extension of the so-called
Central Tianshan–Yili-Kazakhstan Block, recent
geological studies suggest that it is an independent
tectonic terrane separated from the KYB by the
“Nikolaev Line” in Kyrgyzstan and Kazakhstan
and the “North Nalati Fault” in Xinjiang in northwestern China (fig. 1b). Ophiolites are occasionally
exposed along the Nikolaev Line–North Nalati
Fault. These ophiolites are interpreted as remnants
of a paleo-ocean, the PTO, which existed from the
end Neoproterozoic until the Early Ordovician
(Qian et al. 2007; Gao et al. 2009). To the south,
the Central Tianshan Block is separated from the
STCB by the South-Central Tianshan Suture
(SCTS), also known as the Atbashy-Inyl’chek Fault
in Kyrgyzstan and the South Nalati–Qawabulak
Fault in northwestern China. The Central Tianshan Block is underlain by Meso- to Neoproterozoic
basement, is composed of gneisses, schists, migmatites, and marbles, and is covered by Ediacaran
carbonates and tillites. Recent detrital zircon U-Pb
geochronology and Hf isotope data (Ma et al. 2012)
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H. HUANG ET AL.
reveal that the Central Tianshan Block was likely
a component of the Proterozoic “Tarim Craton” (or
Xinjiang Craton); therefore, the basement rocks of
the Central Tianshan Block show some similarities
in age to the STCB and the NMTB. CambrianOrdovician carbonate and flysch are well exposed
in Kyrgyzstan (Qian et al. 2007) but are absent in
the Chinese segment of the Central Tianshan
Block. Late Silurian and Carboniferous island-arc
volcanic and volcaniclastic rocks were locally
thrust over the basement rocks (Gao et al. 1998;
Zhu et al. 2005, 2009). Early Ordovician to Early
Permian dioritic and granitic intrusions are extensively exposed in this terrane.
The STCB might represent the northernmost
margin of the Paleozoic Tarim Block before the final amalgamation, given that the Central Tianshan
and Tarim Blocks collided along the present SCTS
(Qian et al. 2007; Gao et al. 2009; Han et al. 2011).
The basement rocks of this belt are represented by
the Paleoproterozoic Xingditagh Formation and the
Mesoproterozoic Akesu Formation, both exposed
in the central part of the STCB (Yang and Zhou
2009). An accretionary complex composed of Paleozoic ophiolites/ophiolitic mélanges, representing remnants of the Paleozoic South Tianshan
Ocean, and two typical high-pressure/low-temperature metamorphic belts, is well exposed south of
the SCTS. Ediacaran to Lower Cambrian sedimentary rocks and Cambrian to Carboniferous carbonates, clastic rocks, cherts, and interlayered volcanics are widely exposed in the STCB. Permian
strata are sparse and are occasionally composed of
terrestrial volcanics unconformably overlying the
strongly folded upper Carboniferous marine carbonate rocks (Huang et al. 2012b). Compared to
those of other Tianshan terranes, the STCB intrusive rocks comprise only a small proportion, ca.
5%, of the total area and were emplaced mostly
during Late Carboniferous to Early Permian time.
Some Late Silurian granitoids in the Precambrian
basement have been recently identified in the Kuluketage area, the easternmost part of the STCB
(Han et al. 2004; Zhang et al. 2007a).
The NMTB is the area between the North Tarim
Suture to the north and Tarim Desert to the south.
In general, the NMTB basement is represented by
Archean and Paleoproterozoic metamorphic rocks
in the block’s eastern part (Zhang et al. 2010a).
Lower to Middle Paleozoic marine chert, limestone, and flysch occur in the central part. Upper
Paleozoic limestone, sandstone, and shale with minor volcanic rocks are also widely exposed. In contrast to those of the STCB, the NMTB Permian
sedimentary strata are better exposed and conform-
ably overlie the upper Carboniferous marine carbonate rocks. Except for one recently identified
Late Silurian porphyritic granodioritic pluton in the
Korla region (Ge et al. 2012), most NMTB Paleozoic
igneous rocks are of latest Early Permian (∼282–
275 Ma) age, and their genesis has been attributed
to a mantle plume (Zhang et al. 2008, 2010a; Zhou
et al. 2009).
Geology and Petrography of the Tie’reke Pluton
The Tie’reke region is in the central part of the
STCB. The Mesoproterozoic Akesu Formation,
composed predominately of granulite facies metamorphic rocks, is well exposed in the southern part
of the study area. To the north, the Middle Devonian Alatage Formation, consisting of metasedimentary rocks, tectonically overlies the Akesu Formation. The Middle Carboniferous Kangkelin
Formation, in the eastern part of the area, is dominated by shallow marine carbonate and clastic sequences and discordantly overlies the earlier Akesu
and Alatage Formations.
At least two discrete pulses of magmatism have
been recognized in the study area. The earlier is
represented by a Neoproterozoic pluton with an
emplacement age of ∼657 Ma (Xinjiang Bureau of
Geology and Mineral Resources, unpublished data).
The hornblende quartz monzonitic pluton of
Tie’reke reported in our study is exposed immediately north of the Neoproterozoic pluton, representing the younger magmatic pulse. The pluton
outcrops occupy over an area of ∼3.5 km2 and exhibit an intrusive contact relationship with the Mesoproterozoic Akesu Formation. Round to oblong
mafic microgranular enclaves (MMEs) commonly
occur in the pluton (fig. 2, top).
The Tie’reke pluton is mainly composed of hornblende quartz monzonite. The rocks exhibit
medium-grained textures and are predominately
composed of hornblende (∼20 vol%), K-feldspar
(∼35 vol%), plagioclase (∼25 vol%), biotite (∼10
vol%), and quartz (∼5 vol%), with accessory Fe-Ti
oxides, titanite, zircon, and apatite (fig. 2, middle).
Some feldspar grains are partly altered to kaolinite
or muscovite. Poikilitic texture characterized by
several anhedral hornblende, biotite, and quartz
grains enclosed by subhedral K-feldspar grains is
also observed in some thin sections (fig. 2, bottom).
Description of Sedimentary and
Metasedimentary Samples
Since no Early Paleozoic rocks have been found in
the western STCB or in the NMTB, we employ
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Journal of Geology
S O U T H T I A N S H A N C O L L I S I O N A L B E LT
405
detrital zircon U-Pb data to constrain the geological
history of the region. As shown in figure 1b, one
Cenozoic sandstone sample (DKX-10) was collected
from the Kangsu region and one Early Paleozoic
metasandstone sample (WGST-1) from the Jigen
region.
Sample DKX-10 is coarse grained, with a modal
content of 170% quartz, 20% K-feldspar, and 5%
biotite. Chlorite is the main alteration product.
Sample WGST-1 is medium grained and consists
mainly of quartz (75 vol%), plagioclase (15 vol%),
and biotite (5 vol%), with minor amounts of muscovite. Recrystallized biotite and quartz grains are
commonly observed in this sample.
Analytical Methods
Figure 2. Top, mafic microgranular enclave (MME) in
the Tie’reke pluton. Middle, the pluton is predominately
composed of euhedral hornblende and biotite, subhedral
K-feldspar and plagioclase, and anhedral quartz. Bottom,
several anhedral hornblende, biotite, and quartz grains
are enclosed by a subhedral K-feldspar grain, suggesting
poikilitic texture of the pluton. Bi p biotite; Hb p hornblende; Kf p K-feldspar; Pl p plagioclase; Qz p quartz.
A color version of this figure is available in the online
edition or from the Journal of Geology office.
Zircon U-Pb and Lu-Hf Isotopic Analyses. A representative sample from the monzonite pluton
(TLK-2) was selected for zircon separation and UPb age and Lu-Hf isotope analyses. Zircons from
the two sandstone/metasandstone samples (DKX10 and WGST-1) were also analyzed. The zircons
were separated from crushed rocks with conventional magnetic and density techniques and by
handpicking under a binocular microscope. Before
isotopic analysis, the morphology and internal
structure of the zircon grains were investigated
through backscattered electron and cathodoluminescence (CL) imaging that used a LEO1450VP
scanning electron microscope with a MiniCL detector at the Institute of Geology, Chinese Academy of Geological Sciences. The CL images were
used to select the positions for LA-ICP-MS U-Pb
and Lu-Hf analyses (fig. 3a).
Zircon U-Pb dating of zircons from sample TLK2 was carried out at the Key Laboratory of Continental Collision and Plateau Uplift, Chinese Academy of Sciences, Beijing, with an Elan 6100 DRC
ICP-MS equipped with 193-nm Excimer lasers. UTh-Pb ratios were determined relative to the Plešovice standard zircon, and the absolute abundances of U, Th, and rare earth elements (REEs)
were determined by using the NIST 612 standard
glass. A mean age of 338.2 Ⳳ 1.5 Ma was obtained
for the Plešovice zircon standard. Most analyses
used a beam with a 40-mm diameter. Corrections
for common Pb were made with the method of Andersen (2002). Data were processed via the Glitter
and Isoplot (Ludwig 2003) Excel programs. Errors
on individual analyses by LA-ICP-MS are quoted at
the 94.5% (1j) confidence level. Details of the analytical procedures can be found in Yuan et al.
(2004).
U-Pb dating of detrital zircon grains from the
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406
H. HUANG ET AL.
Figure 3. a, Representative cathodoluminescence images showing the internal structure of zircons of sample TLK2 from the Tie’reke pluton. The U-Pb (solid circles) and Hf (dashed circles) analytical sites are marked, and the
numbers refer to the analytical data presented in tables S1 and S3. b, U-Pb concordia diagrams of zircons. c, Weightedaverage diagram of zircons. Data-pt errs p data-point errors.
sandstone/metasandstone samples used LA-ICPMS at the State Key Laboratory of Geological Processes and Mineral Resources, China University of
Geosciences, Wuhan. A pulse Geolas of 193-nm
ArF Excimer laser with 50-mJ/pulse energy at a repetition ratio of 10 Hz, coupled to an Agilent 7500a
quadrupole ICP-MS, was used for ablation. Zircon
91500 was used as an external standard to normalize isotopic fractionation during analysis. A
mean age of 1062 Ⳳ 8 Ma was obtained for the
91500 zircon standard. Zircon standard GJ-1 was
analyzed as a controlled standard and was measured
four times during the analysis of each sample. Processing of lead isotopic data, U-Pb ages, and traceelement contents used the ICPMSDataCal software
(Liu et al. 2010). The external errors of the standard
91500 were propagated to the ultimate results of
each analytical spot.
In situ zircon Hf isotope analysis of zircon grains
from sample TLK-2 was conducted at Institute of
Mineral Resources, Chinese Academy of Geological Sciences, Beijing, and used a New Wave UP213
LA microprobe attached to a Neptune multicollector ICP-MS. With the CL images as guides, Lu-Hf
spots were selected to be partly overlapping with
or close to those of U-Pb dating. Instrumental conditions and data acquisition were comprehensively
described by Hou et al. (2007) and Wu et al. (2006).
A stationary spot was used for the analyses, with
a beam diameter of either 40 or 55 mm, depending
on the size of ablated domains. Helium was used
as the carrier gas to transport the ablated sample,
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S O U T H T I A N S H A N C O L L I S I O N A L B E LT
mixed with argon, from the LA cell to the ICP-MS
torch via a mixing chamber. In order to correct the
isobaric interferences of 176Lu and 176Yb on 176Hf, a
176
Lu/175Lu of 0.02658 and a 176Yb/173Yb of 0.796218
were determined (Chu et al. 2002). For instrumental mass-bias correction, Yb isotope ratios were normalized to a 172Yb/173Yb of 1.35274 (Chu et al. 2002)
and Hf isotope ratios to a 179Hf/177Hf of 0.7325 with
an exponential law. The mass-bias behavior of Lu
was assumed to follow that of Yb. Zircon GJ-1 was
used as the reference standard, with a weighted
mean 176Hf/177Hf of 0.282013 Ⳳ 0.00008 (2j, n p
10) or 0.282013 Ⳳ 0.000024 (2j, n p 10) during our
routine analyses. These values are not distinguishable from the weighted mean 176Hf/177Hf of
0.282013 Ⳳ 19 (2j) from in situ analysis by Elhlou
et al. (2006).
Major and Trace Elements. After careful petrographic examination, a representative suite of six
fresh samples from the pluton was crushed and
powdered in an agate mill for geochemical analysis.
Major and trace (including rare earth) elements
were determined in the Key Laboratory of Orogenic
Belts and Crustal Evolution, Ministry of Education,
School of Earth and Space Sciences, Peking University. Major-element compositions were measured by a scanning wavelength-dispersion X-ray
fluorescence spectrometer (Thermo ARL Advant
XP⫹) on fused glass disks. The analytical precision,
as determined on the Chinese National standards
GSR-1 and GSR-3, was 11%.
Trace-element compositions were determined as
solute by Agilent 7500ce ICP-MS. About 50 mg of
powder of each sample was dissolved for about 7
days at ca. 100⬚C with HF-HNO3 (10 : 1) mixtures
in screw-top Teflon beakers, followed by heating to
70⬚C for 12 h (overnight). Subsequently, the material was dissolved in 7N HNO3, taken to incipient
dryness again, and then redissolved in 2% HNO3
to a sample-solution weight ratio of 1 : 1000. According to the analyses of Chinese national standards GSR-1 and GSR-3, the accuracy for most elements was estimated to be better than 5%–15%
(relative), depending on their concentrations.
Sr and Nd Isotope Analysis. Two samples from the
Tie’reke pluton were analyzed for Sr-Nd isotopic
compositions at the National Research Center for
Geoanalysis, Chinese Academy of Geological Sciences, Beijing. The Sr isotopic compositions were
measured by isotope dilution on a Finnigan MAT262 mass spectrometer. Nd isotopic compositions
were acquired by a Nu Plasma HR MC-ICP-MS (Nu
Instruments). The procedures followed those described by He et al. (2007). Total procedural blanks
407
were !100 pg for Sr and !50 pg for Nd, and the
estimated analytical uncertainties of 147Sm/144Nd
and 87Rb/86Sr were !0.5%. The 143Nd/144Nd and
87
Sr/86Sr were corrected for mass fractionation
by normalization to 146 Nd/142 Nd p 0.7219 and
86
Sr/ 88Sr p 0.1194, respectively.
Results
Zircon U-Pb Geochronology. Zircon grains separated from sample TLK-2 are prismatic, transparent, and colorless or pale yellow, with lengths varying from 100 to 250 mm and width-length ratios
generally ranging from 1 : 1.5 to 1 : 4 (fig. 3a).
Thirty-six grains were analyzed, and the results are
listed in table S1 (tables S1–S5 are available in the
online edition or from the Journal of Geology office). Among these, six spots are discordant, probably reflecting postmagmatic lead loss, and the
other 30 fall along a concordia (fig. 3b). The 27
youngest ones are clustered around 206Pb/238Pb ages
of 410–430 Ma, with a weighted mean age of
419.9 Ⳳ 2.1 Ma (n p 27, MSWD p 2.8; fig. 3c).
The clear oscillatory zoning and high Th/U (0.4–
2.0, with an average of 1.0) of the youngest zircon
population indicate a magmatic origin; thus, the
weighted mean age can be interpreted to be the
crystallization age of the Tie’reke pluton. The second group of three zircons, with 206Pb/238Pb ages of
614 Ⳳ 5, 521 Ⳳ 5, and 661 Ⳳ 6 Ma, also shows oscillatory zoning and high Th/U. They are likely to
be inherited zircons captured during magma ascent
from the surrounding, slightly older (Neoproterozoic to Cambrian) magmatic rocks.
A total of 85 detrital zircons, of which 41 grains
were separated from sample DKX-10 and 44 from
sample WGST-1, were analyzed. The U-Pb results
are listed in table S2. Sixty-six detrital zircon grains
have Th/U 1 0.4 (0.41–2.04), and the other 19 yield
Th/U from 0.10 to 0.37. Most of the analyses from
this sample are concordant or nearly concordant
(fig. 4a, 4c). Detrital zircons from sample DKX-10
yield predominantly Paleozoic ages, except for 11
grains with Proterozoic 206Pb/238U ages and one
grain with a Triassic 206Pb/238U age (fig. 4b). The
Paleozoic grains yield a 206Pb/238U age range of 462–
253 Ma and show two age populations of 462–420
and 302–276 Ma, with two major peak ages at 440
and 293 Ma. In addition, among the sample DKX10 Paleozoic zircons, two grains show 206Pb/238U
ages of 396 Ma and one has a 206Pb/238U age of 253
Ma. Sample WGST-1 detrital zircons exhibit a wide
range of ages, from latest Mesoarchean to latest
Late Ordovician (2814 to 443 Ma, fig. 4d). Among
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408
H. HUANG ET AL.
Figure 4. Combined concordia plots (a, c) and relative probability density-histogram plots (b, d) of detrital zircon
ages from samples DKX-10 (a, b) and WGST-1 (c, d).
the zircons with ages !1.0 Ga, 14 give 206Pb/238U
ages between 904 and 786 Ma and additional seven
yield Early Paleozoic 206Pb/238U ages, defining two
major age populations.
Zircon Lu-Hf Isotopes. Thirty zircons from the
grains analyzed for U-Pb that showed a well-defined
concordia in the 207Pb/235Pb-versus-206Pb/238Pb diagram were selected for in situ Lu-Hf isotopic analysis (fig. 3b). The decay constant of 176Lu,
1.867 # 10⫺11 per year (Scherer et al. 2001), was
adopted in the calculation. Single-stage model
ages (TDM) were calculated on the basis of the chondrite model with 176 Hf/177 Hf p 0.282772 and
176
Lu/177 Hf p 0.0332 (Blichert-Toft and Albarède
1997) and the depleted-mantle model with presentday 176 Hf/177 Hf p 0.28325 and 176 Lu/177 Hf p
0.0384 (Griffin et al. 2000). The two-stage continental model age (TDM2) was calculated on the assumption that the initial 176Hf/177Hf of zircon refers
back to the depleted-mantle growth curve and using 176 Lu/177 Hf p 0.015 for the average continental
crust (Griffin et al. 2000).
Twenty-seven spots on the 27 youngest zircon
grains (∼410–430 Ma) give 176Lu/177Hf varying from
0.001103 to 0.004662, with an average of 0.002066,
indicating very low radiogenic Hf (see table S3).
These grains yield variable 176Hf/177Hf between
0.282224 and 0.282505 and corresponding ␧Hf(t) values of ⫺11.4 to ⫺1.4 (calculated at respective 206Pb/
238
Pb ages; fig. 5a, 5c). The corresponding two-stage
model ages (TDM2) were calculated at ca. 1.5–2.1 Ga
(fig. 5b). The xenocrystic zircons with ages of 521,
614, and 661 Ma show 176Hf/177Hf of 0.282350,
0.282346, and 0.282446, respectively. Computations based on the 206Pb/238Pb ages yielded ␧Hf(t) values of ⫺4.0, ⫺3.0, and ⫹2.1, respectively (fig. 5).
Major and Trace Elements. The major- and traceelement data for six representative samples are pre-
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S O U T H T I A N S H A N C O L L I S I O N A L B E LT
409
sented in table S4. The loss on ignition ranges from
0.94 to 1.94 wt%. In accordance with the petrography, the rocks are intermediate in composition,
with SiO2 contents from 60.73 to 64.73 wt%. The
other major-element ranges are as follows: 3.96–
5.61 wt% for Fe2O3T, 2.64–3.96 wt% for MgO, 2.54–
3.97 wt% for CaO, 0.19–0.43 wt% for P2O5, and
0.52–0.75 wt% for TiO2. The rocks show high K2O
(3.87–5.38 wt%), Na2O (3.84–3.33 wt%), and
K2O⫹Na2O (7.20–8.22 wt%) contents. Except for
one sample that plots in the alkalic series field, the
majority plot in the alkalic-calcic field in the SiO2versus–K 2 O ⫹ Na 2 O ⫺ CaO diagram (fig. 6a). In the
SiO2-versus-K2O diagram (fig. 6b), they classify as
a shoshonitic series. The Al2O3 contents of the pluton are high, ranging from 14.88 to 16.03 wt%, with
A/CNK (molar Al 2 O 3 /(CaO ⫹ Na 2 O ⫹ K 2 O)) values ranging from 0.91 to 1.02, exhibiting metaluminous and slightly peraluminous affinities on A/
CNK-versus-A/NK (molar Al 2 O 3 /(Na 2 O ⫹ K 2 O)
plots (fig. 6c).
The primitive mantle–normalized spider diagram shows that the Tie’reke pluton is relatively
depleted in high-field-strength elements (e.g., Nb,
Ta, and Ti, and REEs) but enriched in large-ion lithophile elements (e.g., Rb, Ba, and Th; fig. 7a). The
rocks show moderate total REE concentrations,
from 181.1 to 337.2 ppm. On the chondrite-normalized REE diagram, light REEs are moderately
enriched relative to heavy REEs (HREEs), with (La/
Yb)N varying from 10.3 to 19.4. HREEs show a relatively flat distribution pattern (fig. 7b). The rocks
lack any prominent Eu anomaly, with Eu/Eu∗ ranging from 0.77 to 0.91. These geochemical features
resemble those of modern arc volcanics and arcrelated plutons (e.g., Pearce et al. 1984; Pearce and
Peate 1995) and are comparable to the ∼420-Ma
granodiorites in the Korla region in the eastern
NMTB (see fig. 7; Ge et al. 2012). The latter has
been considered to be emplaced in a continentalarc setting.
Sr-Nd Isotopic Compositions. Sr and Nd isotopic
data for the Tie’reke samples are listed in table S5,
and initial Sr and Nd isotopic ratios are calculated
at the age of 420 Ma. The results are shown in the
plots of (87Sr/86Sr)t versus ␧Nd(t) in figure 8a and intrusive age versus ␧Nd(t) in figure 8b. Some of the
published isotopic data for Paleozoic igneous rocks
Figure 5. Histograms showing the distribution of ␧Hf(t)
values (a) and two-stage model ages (TDM2; b) and ageversus-␧Hf(t) diagram (c) for all analyzed zircons from
sample TLK-2 and those from the Korla granodioritic pluton (Ge et al. 2012); additional data for the Tarim Block
Neoarchean basement from Long et al. (2010, 2011b).
CHUR p chondrite uniform reservoir.
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410
H. HUANG ET AL.
from the STCB and the Central Tianshan Block are
also shown for comparison.
The two Tie’reke pluton samples have low 87Rb/
86
Sr (0.4052 and 0.6382) and demonstrate relatively
high age-corrected (87Sr/86Sr)t (0.71161 and 0.70792).
The two samples exhibit 147Sm/144Nd of 0.09317
and 0.10563 and 143Nd/144Nd of 0.51202 and
0.51204. The age-corrected ␧Nd(t) values for the
samples are ⫺6.8 and ⫺6.6.
Discussion
Tectonic Setting and Sources of the Tie’reke Pluton.
The petrographic data on the Tie’reke pluton document the ubiquitous presence of hornblende, and
the geochemical data show an intermediate, alkalicalcic, shoshonitic, and metaluminous to weakly
peraluminous character, broadly comparable with
an I-type magmatic source. Furthermore, our samples show relatively low 10000Ga/Al as well as a
negative correlation between SiO2 and P2O5 (fig. 9),
further indicating an I-type affinity. The intermediate, alkali-calcic and I-type features of the
Tie’reke pluton, in combination with the relative
depletion of Nb, Ta, P, and Ti and enrichment of
large-ion lithophile elements (Ba, K, and Rb), are
typical features of arc-related magmatism and are
suggestive of an ocean-derived fluid metasomatized
source, as in the cases of most modern subductionrelated volcanic and plutonic rocks (e.g., Wood et
al. 1979; Briqueu et al. 1984; Pearce and Peate
1995). Furthermore, our samples exhibit moderately fractionated REE patterns, low HREE abundances, and ancient Hf and Sr-Nd isotopic compositions, all of which resemble those of
Andean-type continental-arc granitoids (Högdahl et
al. 2008; Ge et al. 2012). In the SiO2-versusFeOT/(FeOT ⫹ MgO) and M-versus-F diagrams (Maniar and Piccoli 1989), all of the Tie’reke samples
plot in the IAG⫹CAG⫹CCG field (fig. 10a, 10b).
In the Y-versus-Nb and Y⫹Nb-versus-Rb discrimination diagrams (Pearce et al. 1984), the rocks fall
within the volcanic-arc granite area (VAG; fig. 10c,
10d). We therefore infer that the Tie’reke pluton
probably formed in an Andean-type active continental margin.
Alternately, the above features could be an inFigure 6. Chemical classification diagrams for the
Tie’reke and Huangjianshishan plutons in the South
Tianshan Collisional Belt, the Korla pluton in the
Northern Margin of the Tarim Block, and some granitoids in the Central Tianshan Block. a, SiO2-versusK2O ⫹ Na2O ⫺ CaO diagram (after Frost et al. 2001). b,
SiO2-versus-K2O diagram (after Peccerillo and Taylor
1976). c, A/CNK (Al2O3/(CaO ⫹ Na2O ⫹ K2O))-versus-
A/NK (Al2O3/(Na2O ⫹ K2O) diagram (after Maniar and
Piccoli 1989). Data sources: Korla pluton: Ge et al. (2012);
Huangjianshishan pluton: Zhang et al. (2007a); granitoids
exposed in the Central Tianshan Block: Yang et al. (2006);
Qian et al. (2007); Gao et al. (2009); Long et al. (2011a);
Gou et al. (2012).
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411
members of magma sources may include (1) aqueous fluids and/or partial melts released by dehydration of altered oceanic crust and/or subducted
sediments, (2) subcontinental mantle wedge, and
(3) overriding continental crust (Hawkesworth et
al. 1997; Bedard 1999; Macdonald et al. 2000; Elburg
et al. 2002; Zhu et al. 2009; Zhang et al. 2010b). In
Figure 7. a, Primitive mantle–normalized spidergram
for the Tie’reke hornblende quartz monzonitic pluton. b,
Chondrite-normalized rare earth element (REE) pattern
of the Tie’reke hornblende quartz monzonitic pluton.
Trace-element and REE patterns for samples from the
Korla granodioritic pluton are shown for comparison,
based on the data set published by Ge et al. (2012). Normalized values are from Sun and McDonough (1989).
heritance from the source rocks that were ultimately generated in subduction-related tectonic
settings. In this case, the arc-like nature cannot be
directly utilized to constrain the possible tectonic
setting. However, many coeval intermediate to felsic rocks intruding the Precambrian basement
rocks have been recently identified in the STCB
and the NMTB (see fig. 1b; Han et al. 2004; Zhang
et al. 2007a; Ge et al. 2012), and they mark an important tectonothermal event along the NMTB in
the Early Paleozoic. Interestingly, despite their variable SiO2 contents, the rocks examined in this
study are uniformly characterized by continental
arc–like geochemical features (figs. 9, 10). It is unlikely that all of these arc-like geochemical characteristics were inherited from source rocks, in
which case a highly variable geochemical and isotopic signature should result. We therefore infer
that the Silurian magmatic rocks in the STCB and
the NMTB were formed in an Andean-type continental-arc setting.
In a continental-arc setting, the possible end
Figure 8. Plots of (87Sr/86Sr)t versus ␧Nd(t) (a) and intrusive age versus ␧Nd(t) (b) for the Tie’reke and other intrusive rocks in the South Tianshan Collisional Belt (STCB),
the Northern Margin of the Tarim Block, and the Central
Tianshan Block. Data sources: Korla pluton: Ge et al.
(2012); Huangjianshishan pluton: Zhang et al. (2007a);
Jingbulake mafic-ultramafic intrusion: Yang and Zhou
(2009); granitoids exposed in the Central Tianshan Block:
Zhu et al. (2006); Qian et al. (2007); Long et al. (2011a);
Gou et al. (2012). The Sr-Nd data for West Junggar, East
Junggar, and the Chinese Altai are obtained from Long
et al. (2011a). CHUR p chondrite uniform reservoir.
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412
H. HUANG ET AL.
Figure 9. K2O⫹Na2O–versus–10000Ga/Al (a) and Nbversus–10000Ga/Al (b) diagrams of Whalen et al. (1987)
and an SiO2-versus-P2O5 diagram (c), showing the I-type
nature of the Tie’reke pluton. Symbols and data sources
are as in figure 6.
the case of the Tie’reke pluton, our samples generally display high K2O contents (3.87–5.38 wt%)
as well as “ancient” whole-rock Sr-Nd isotopic
signatures ((87Sr/ 86Sr)t p 0.71161 and 0.70792,
␧ Nd(t) p ⫺6.8 and ⫺6.6), and zircon Hf (␧Hf(t) values
for zircon grains with ages of 430–410 Ma are from
⫺11.0 to ⫺1.4, all of which suggests that continental crust was the major source for the magma.
The two-stage Hf model ages for ∼410- to ∼430-Ma
zircon grains vary from 1.5 to 2.2 Ga, which implies
that the source region was mainly composed of Paleoproterozoic basement rocks.
The pluton exhibits moderate Sr (305–614 ppm)
and high Y (21.3–27.6 ppm) and Yb (2.2–2.8 ppm)
concentrations, resulting in low Sr/Y (13.9–26.5) and
Sr/Yb (125–284). In addition to the flat HREE patterns (fig. 7), these features preclude garnet as a residual phase (Defant and Drummond 1990; Drummond and Defant 1990; Gou et al. 2012). Besides,
the insignificant Eu anomalies (Eu/Eu∗ p 0.77–
0.91) imply a lack of any significant plagioclase fractionation. All these features point to melt generation
in the lower part of a normal to slightly thickened
crust (!50 km; Ge et al. 2012; Gou et al. 2012). According to previous experimental studies (e.g., Beard
and Lofgren 1991; Rapp and Watson 1995), the K2O
content of crustal melts is predominately controlled
by source rocks and pressure. Furthermore, dehydration melting involving the breakdown of hydrous
minerals, such as hornblende or zoisite, more readily
gives rise to K-rich melts. At low to normal pressures, high-K crustal magma can be generated by
partial melting of moderately hydrous medium- to
high-K basaltic rocks (Sisson et al. 2005). All of our
samples have low Al 2 O 3 /(FeOT ⫹ MgO ⫹ TiO2 )
(1.67–2.27) and fall in the partial-melt field of
amphibolites in a Al 2 O 3 ⫹ FeOT ⫹ MgO ⫹ TiO2–
versus–Al 2 O 3 /(FeOT ⫹ MgO ⫹ TiO2 ) diagram (not
shown; Patiño Douce 1999), further indicating that
the crustal magma was mainly derived from partial
melting of K-rich, metabasaltic rocks.
However, the presence of xenocrystic zircons,
captured by the magma, with Neoproterozoic to
Cambrian 206Pb/238Pb ages implies that in addition
to the Paleoproterozoic basement rocks, some comparatively younger magmatic rocks were probably
assimilated during magma ascent. Indeed, their
presence also testifies to some earlier Neoproterozoic to Cambrian magmatic pulses in the study
area, although the tectonic settings are still unclear. More importantly, the large variation in
∼410- to ∼430-Ma zircon Hf isotopic compositions
(up to 10 ␧ units for zircons from a single sample;
see fig. 5) precludes the possibility of magma generation in a closed system but instead necessitates
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413
Figure 10. Tectonic-setting discrimination diagrams for the granitoids exposed in the present-day South Tianshan
Collisional Belt, the Northern Margin of the Tarim Block, and the Central Tianshan Block: SiO2-versus–
FeOT/(FeOT ⫹ MgO) (a) and M-versus-F (b) diagrams after Maniar and Piccoli (1989); Y-versus-Nb (c) and Y⫹Nb-versusRb (d) diagrams after Pearce et al. (1984). Symbols and data sources are as in figure 6. a, b, CAG p continental-arc
granitoids; CCG p continental-collision granitoids; CEUG p continental epeirogenic-uplift granitoids; IAG p islandarc granitoids; POG p postorogenic granitoids; RRG p rift-related granitoids. c, d, ORG p ocean ridge granites; synCOLG p syn-collisional granites; VAG p volcanic-arc granites; WPG p within-plate granites.
a complex process that juxtaposes magmas with
variable Hf isotopic compositions. This interpretation is also supported by the occurrence of MMEs
and poikilitic texture, both suggesting that a
magma-mixing process may have been involved in
the genesis of the pluton. Experimental work by
Rapp and Watson (1995) showed that melts with
Mg# (molar MgO/(MgO ⫹ FeOT)) greater than 40
can be obtained only with a mantle component.
Thus, the high Mg# values (61–64) for our samples
indicate that the mafic magma involved in the genesis was likely derived from a depleted-mantle
wedge. If this is the case, then the absence of zircons with positive ␧Hf(t) values can be explained by
possibilities including (1) use of an insufficient
number of zircons in the study and/or (2) the fact
that zircons are fairly difficult to crystallize in mantle-derived magmas, particularly when they are
mafic or ultramafic in composition, until these
magmas have sufficiently interacted with crustal
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414
H. HUANG ET AL.
melts and cooled down to the zircon saturation
temperature (Watson and Harrison 1983; Ge et al.
2012).
In summary, the Tie’reke continental-arc rocks
were dominantly produced by partial melting of a
Paleoproterozoic crustal source, probably triggered
by underplating of mantle wedge-derived magmas,
followed by the assimilation of Neoproterozoic to
Cambrian igneous rocks during magma ascent. In
conjunction with previously published Hf isotopic
data for the Korla granodiorites (Ge et al. 2012), we
conclude that the involvement of mantle materials
is a common feature for the Silurian continental
arc–type magmatism in the STCB and the NMTB.
Timescale of the Paleozoic South Tianshan Ocean.
Some of the fundamental issues related to the tectonic evolution of the Paleozoic South Tianshan
Ocean, including its nature and subduction polar-
ity, are still under contention. The timescale of the
paleo-ocean is an important factor in this context,
and we show in figure 11a a compilation of the ages
of ophiolitic rocks, including mafic-ultramafic igneous rocks and the sedimentary strata.
Two Late Neoproterozoic ages for gabbro and basalt within the Dalubayi ophiolite (see figs. 1b, 11)
have recently been reported by Yang et al. (2005).
Basalts and gabbros from this ophiolite were interpreted as ocean island igneous rocks (Yang et al.
2005). This interpretation implies that the opening
of the Paleozoic South Tianshan Ocean and, consequently, the separation of the Central Tianshan
Block from the Proterozoic Tarim Craton took place
as early as the Late Neoproterozoic. Note that recent
studies have shown that the PTO was likely to have
opened in the late Neoproterozoic to early Cambrian
and to have closed in the Ordovician (Lomize et al.
Figure 11. Simplified schematic diagram showing ages of mafic-ultramafic rocks and sedimentary strata of the
representative ophiolites of the STCB. Data sources: Yang et al. (2005, 2011); Liu and Hao (2006); Long et al. (2006);
Alekseev et al. (2007); Li et al. (2007); Wang et al. (2007a, 2011, 2012); Kang et al. (2010, 2011); Han et al. (2011).
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S O U T H T I A N S H A N C O L L I S I O N A L B E LT
1997; Bazhenov et al. 2003; Qian et al. 2007; Gao et
al. 2009). We infer that the opening of the Paleozoic
South Tianshan Ocean was nearly simultaneous
with or even earlier than that of the PTO.
According to published data sets, the majority of
mafic-ultramafic rocks within ophiolites exposed
in the STCB have Late Ordovician to Middle Devonian emplacement ages (fig. 11). In fact, the geochemical data on some of the mafic-ultramafic
rocks in the ophiolite suites indicate a suprasubduction zone (SSZ) origin. For instance, Wang et al.
(2011) reported that mafic rocks—including oceanic island basalt–like alkali basalt and andesite,
normal mid-ocean ridge basalt (MORB)–like tholeiitic basalt, sheeted diabase dikes, cumulate gabbro, and peridotite—of the Heiyingshan ophiolite
display suprasubduction signatures and were emplaced in a continental-margin setting. However,
many researchers have argued that some of the
ophiolites exposed on the southern side of the
SCTS are tectonic klippen (e.g., Windley et al. 1990;
Gao et al. 1998; Han et al. 2011; Xiao et al. 2013
and references therein), and as a result, whether
those SSZ-type rocks were initially emplaced in the
northern margin of the Early Paleozoic Tarim Block
or the southern margin of the Early Paleozoic Central Tianshan Block is debatable. Thus, the SSZ features barely contribute to clarification of the subduction polarity of the Paleozoic South Tianshan
Ocean (see below). However, in spite of such uncertainties, the SSZ signatures still suggest that
subduction of the Paleozoic South Tianshan Ocean
should have begun as early as the Ordovician. In
this case, the onset of the subduction of the Paleozoic South Tianshan Ocean was evidently earlier
than the emplacement of the Silurian to Carboniferous igneous rocks exposed in the present
NMTB, STCB, and Central Tianshan Block, including the Tie’reke pluton.
The time of closure of the Paleozoic South Tianshan Ocean and that of the collisional event are
also debated. Within the sedimentary rocks of the
ophiolites, Middle Devonian to Early Carboniferous microfossils (i.e., radiolarians and conodonts)
are well preserved (see fig. 11). This suggests that
the final closure of the Paleozoic South Tianshan
Ocean, as well as the subsequent collisional event,
occurred after the Early Carboniferous. Considering
the widespread occurrence of Late Carboniferous
to Early Permian collision-related granitoids in the
STCB and the Central Tianshan Block (see Gao et
al. 2011 and references therein; Han et al. 2011;
Huang et al. 2012b) and the ages of high-/ultrahighpressure metamorphic rocks exposed along the
SCTS (see Gao et al. 2009, 2011 and references
415
therein; Han et al. 2011), we tentatively propose
that the Paleozoic South Tianshan Ocean was
closed in the Late Carboniferous, although some
workers prefer a Permian- or Triassic-closure model
(Xiao et al. 2013 and references therein; also Zhang
et al. 2007b).
Subduction Polarity of the Paleozoic South Tianshan
Ocean. It has been widely accepted that the Early
Paleozoic accretionary processes of the Tianshan
tectonic collage were closely associated with the
evolution of several oceanic basins, all of which were
branches of the Paleo-Asian Ocean (e.g., Coleman
1989; Charvet et al. 2007, 2011; Kröner et al. 2007;
Xiao et al. 2008, 2013). However, in the case of the
Paleozoic South Tianshan Ocean (the southernmost
constituent of the Paleo-Asian Ocean), the subduction direction is still a controversial issue. Previous
studies were mainly based on kinematic analyses.
For instance, the Kyrgyzstan geological data from
areas near Atbashi revealed top-to-the-south thrusting on north-dipping thrusts as well as southward
propagation of thrusts during the Middle Carboniferous to Early Permian (Hegner et al. 2010; Xiao et
al. 2013), supporting a northward-subduction model.
In contrast, a southward-subduction model was recently proposed for the Paleozoic South Tianshan
Ocean, mainly based on deformation structures in
which north-vergent (D1) thrust sheets are documented in the Chinese STCB and NMTB (Charvet
et al. 2007, 2011; Lin et al. 2009; Wang et al. 2011).
In addition, the SSZ features of some mafic-ultramafic rocks within ophiolites or ophiolitic mélanges, which are exposed in the present STCB, are considered strong evidence for southward subduction
(Wang et al. 2011).
Some authors (e.g., Coutand et al. 2002; Makarov
et al. 2010) emphasize that Tianshan Paleozoic deep
structures may have been strongly overprinted and
obliterated by Permian-Mesozoic transcurrent and
extensional tectonics and thereafter by Cenozoic intracontinental shortening. Furthermore, it is still
unknown whether the ophiolites and ophiolitic mélanges are tectonic klippen. As a result, models that
rely on kinematic analyses and SSZ features are
equivocal. Evaluation of the Paleozoic arc-type igneous rocks in the NMTB, the STCB, and the Central Tianshan Block is a more reliable tool in reconstructing the polarity of oceanic subduction.
Because the closure of the PTO occurred in the
early Late Ordovician, the Central Tianshan Block
should have been amalgamated with the KYB before the Silurian, forming the united Paleozoic Central Tianshan–Yili-Kazakhstan Block. In other
words, during Early Silurian to Middle Carboniferous time, the present narrow Central Tianshan
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416
H. HUANG ET AL.
Block constituted the southernmost margin of the
united Central Tianshan–Yili-Kazakhstan Block. In
addition, previous studies have suggested that only
the igneous rocks in the northern margin of the
present KYB were related to the southward subduction of the Paleozoic Junggar Ocean (North
Tianshan Ocean in some literature; e.g., Long et al.
2011a). Taking into account the above geological
features and the timescale of the Paleozoic South
Tianshan Ocean, we postulate that the majority, if
not all, of the Early Silurian to Middle Carboniferous igneous rocks located south of the Nikolaev
Line–North Nalati Fault (in the present Central
Tianshan Block, STCB, and NMTB) should be genetically related to the subduction of the Paleozoic
South Tianshan Ocean. We plot in figure 12 the upto-date information on the ages of intrusive rocks
from the three terranes.
During the subduction of oceanic crust beneath
a continental block, subduction-related magmatism takes place only along the active margin of
the overriding plate (Han et al. 2011). According to
the northward-subduction model, the NMTB has
been interpreted as a long-lived passive continental
margin.
However, some previously studied calc-alkalic to
alkali-calcic felsic rocks in the eastern part of the
STCB and the NMTB (fig. 12a; Han et al. 2004;
Zhang et al. 2007a; Ge et al. 2012) consistently
show Silurian ages (430–420 Ma). Our identification of the ∼420-Ma arc-like signature for the
Tie’reke monzonitic pluton within the middle segment of the Chinese STCB strongly favors an active
continental margin during the Late Silurian. Notably, the Tie’reke pluton intruded into the Mesoproterozoic Akesu Formation, believed to be a
constituent of the basement of the Tarim Block
(Zhang et al. 2011). Furthermore, when the timescale of the Paleozoic South Tianshan Ocean is
taken into account, the Akesu Formation was unlikely to have belonged to the accretionary complex
that was produced by the consumption of the Paleozoic South Tianshan Ocean. In addition, considering the established model that the Central Tianshan and Tarim Blocks collided along the present
SCTS, it is reasonable to infer that the pluton was
initially emplaced in the northern margin of the
Early Paleozoic Tarim Block.
In the western part of the STCB and the NMTB,
no Early Paleozoic rocks have yet been found. Detrital zircon U-Pb data can constrain the geological
history of the region. As illustrated in figure 13a,
51 Paleozoic to Early Mesozoic detrital zircon
grains exhibit two distinct age populations, 462–
395 and 302–276 Ma. In contrast to the Central
Figure 12. Statistics of the zircon ages obtained for the
intrusive rocks intruding into the South Tianshan Collisional Belt (STCB) and the Northern Margin of the Tarim Block (NMTB, a) and the Central Tianshan Block (b).
Data sources for a: this study; Han et al. (2004); Konopelko et al. (2007, 2009); Wang et al. (2007a, 2007b); Zhu
et al. (2008a, 2008b); Zhang et al. (2010a); Huang et al.
(2011, 2012a, 2012b); Long et al. (2011a); De Grave et al.
(2012); Ge et al. (2012). Data sources for b: Qian et al.
(2007); Gao et al. (2009) and references therein; Dong et
al. (2011); Long et al. (2011a); Gou et al. (2012).
Tianshan Block samples (fig. 13b; details discussed
below), the Cenozoic sandstone sample from the
Kangsu region lacks detrital zircon grains with ages
ranging from 395 to 302 Ma (Middle Devonian to
Late Carboniferous), precluding the possibility that
the detrital zircon grains were partly derived from
the igneous rocks initially emplaced in the Central
Tianshan Block. In summary, the abundance of
462–395-Ma detrital zircons in the sandstone samples indicates that the northwestern margin of the
Paleozoic Tarim Block was extensively affected by
a Middle Ordovician to Middle Devonian mag-
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S O U T H T I A N S H A N C O L L I S I O N A L B E LT
Figure 13. Combined relative probability density and
histogram plots of the detrital zircon ages from one Cenozoic sandstone and one Early Paleozoic sandstone sample collected from the western part of the South Tianshan
Collisional Belt (STCB; our unpublished data; a) and
those from modern river sands in the Central Tianshan
Block (from Ren et al. 2011; b). The data for detrital zircon grains from the STCB are listed in table S2. A total
of 85 detrital zircon grains were dated, and only the 51
grains that exhibit Paleozoic to Early Mesozoic ages are
shown here.
matic event, likely associated with the subduction
of the Paleozoic South Tianshan Ocean.
The foregoing discussion suggests that the northern margin of the Paleozoic Tarim Block transformed into an active continental margin with arcrelated magmatism no later than the Late Silurian,
probably as early as the Middle Ordovician. This
conclusion is also sustained by sedimentary evidence (Ge et al. 2012). A widespread unconformity
at the Ordovician-Silurian boundary is well documented in the northern margin and the inner part
of the Tarim Block and has been interpreted as representing the transition of the tectonic setting from
extension to compression (e.g., Carroll et al. 2001;
Shi et al. 2007; Ge et al. 2012). However, even
though the northern margin of the Early Paleozoic
Tarim Block has been confirmed to have been an
active continental margin, the present data still do
not support a southward-subduction model. As
417
shown in figure 12b, a large number of continental
arc–like igneous rocks with ages ranging from Early
Ordovician to Middle Devonian are documented in
the present Central Tianshan Block (see references
in fig. 12). Even though those generated earlier than
the Early Silurian can be regarded as products of
PTO subduction, the majority, if not all, of the
Early Silurian to Middle Devonian arc-type magmatic records should be interpreted by the subduction of the Paleozoic South Tianshan Ocean. This
indicates that, during this time span, the southern
margin of the united Paleozoic Central Tianshan–
Yili-Kazakhstan Block was an active continental
margin as well; accordingly, a double-subduction
model is reasonable for the Middle Ordovician to
Middle Devonian subduction of the Paleozoic
South Tianshan Ocean.
The age data synthesized in figure 12a do not
show any Middle Devonian to Middle Carboniferous magmatism in the present STCB and NMTB,
and the sandstone samples do not contain any
∼395- to ∼302-Ma detrital zircon grains (fig. 13a).
Furthermore, the Middle Devonian to Carboniferous shallow marine sedimentary strata, which were
interpreted as passive-margin sediments, generally
occur in the southern part of the STCB and unconformably overlie the Silurian sequence (Zhou and
Chen 1990; Han et al. 2011 and references therein).
In contrast to the tectonomagmatic quiescence of
the northern margin of the Paleozoic Tarim Block,
both geochronological data of intrusive rocks and
the detrital zircon U-Pb ages of modern river sands
from the present Central Tianshan Block (figs. 12b,
13b) exhibit numerous Late Devonian to Carboniferous ages, confirming long-lived Late Paleozoic
magmatism in the southern margin of the united
Central Tianshan–Yili-Kazakhstan Block. Thus, it
is reasonable to infer that the northern margin of
the Paleozoic Tarim Block must have changed into
a passive continental margin at least since the Middle Devonian, whereas the southern margin of the
united Paleozoic Central Tianshan–Yili-Kazakhstan Block was still an active continental margin
during this period.
A New Tectonic Evolution Model. Considering
these various points, we propose a new tentative
model for the tectonic evolution of the southern
margin of the CAOB during Late Neoproterozoic
and Paleozoic time, as illustrated in figure 14. Our
model is derived in part from the ideas developed
by previous studies (e.g., Gao et al. 1998, 2009,
2011; Chen et al. 1999; Qian et al. 2007; Xiao et al.
2008, 2009, 2013).
In this model, the opening of the PTO and the
Paleozoic South Tianshan Ocean and, conse-
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Figure 14. Tectonic-evolution model for the southern margin of the Central Asia Orogenic Belt.
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Journal of Geology
S O U T H T I A N S H A N C O L L I S I O N A L B E LT
quently, the separation of Central Tianshan Block
and the KYB from the Proterozoic Tarim Craton (or
Xinjiang Craton) are presumed to have occurred in
the Late Neoproterozoic (fig. 14a), as also recorded
by the Xiate transitional MORB (Yang et al. 2005)
and the Dalubayi ophiolite mélanges (Qian et al.
2007), respectively. The southward or double subduction of the PTO took place during the Ordovician (Qian et al. 2007; Gao et al. 2009; Long et
al. 2011a), leading to the final closure of the oceanic
basin and the amalgamation of the Central Tianshan Block in the south and the KYB in the north
(fig. 14b) before the Silurian. The double subduction of the Paleozoic South Tianshan Ocean is assumed to have occurred during the Late Ordovician
to Middle Devonian, when both the northern margin of the Paleozoic Tarim Block and the southern
margin of the united Central Tianshan–YiliKazakhstan Block were active continental margins
(fig. 14c). Subsequently, the subduction polarity of
the Paleozoic South Tianshan Ocean changed to
just northward until the final closure (fig. 14d).
Conclusions
1. LA-ICP-MS U-Pb zircon dating indicates that
the Tie’reke hornblende quartz monzonitic pluton
was emplaced at ∼419.9 Ⳳ 2.1 Ma, in the Late
Silurian.
2. The pluton was produced by partial melting of
a Paleoproterozoic crustal source, probably through
the underplating of mantle wedge-derived magmas,
followed by assimilation of Neoproterozoic to
Cambrian igneous rocks during magma ascent.
3. The Tie’reke pluton, as well as the other Late
419
Silurian magmatic rocks exposed in the present
STCB and NMTB, likely formed in an Andean-type
continental arc. Given the coeval arc magmatism
in the Central Tianshan Block, a double-subduction
model seems applicable to the Late Ordovician to
Middle Devonian evolution of the Paleozoic South
Tianshan Ocean.
4. Several lines of evidence suggest that the
northern margin of the Paleozoic Tarim Block
transformed into a passive continental margin at
least since the Middle Devonian, whereas the
southern margin of the united Central Tianshan–
Yili-Kazakhstan Block was still an active continental margin during the period.
ACKNOWLEDGMENTS
We thank editor D. Rowley and referees W. J. Xiao
and A. Robinson for their constructive and helpful
comments, which helped improve our manuscript.
This study was financially supported by National
305 Project (2011BAB06B02-04) and a Natural Science Foundation of China grant (40925006). A Talent Award to M. Santosh from the Chinese Government also helped support this work. We are
grateful to Z. Chen of the Institute of Geology, Chinese Academy of Geological Sciences; L. Ding of
the Key Laboratory of Continental Collision and
Plateau Uplift, Chinese Academy of Sciences; S. H.
Tang of National Research Center for Geoanalysis,
Chinese Academy of Geological Sciences; and
other workers for their assistance in geochemical,
isotopic, and LA-ICP-MS determinations. Special
thanks are to Y. Z. Zheng and C. R. Feng for their
kind help during the fieldwork.
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