Full text of DOI:10.1007/BF02882257

Vol. 44 No. 3
SCIENCE IN CHINA (Series D)
March 2001
Tectonics of Precambrian basement of the Tarim craton
GUO Zhaojie (郭召杰)1, ZHANG Zhicheng (张志诚)1, JIA Chengzao (贾承造)2
& WEI Guoqi (魏国齐)2
1. Department of Geology, Peking University, Beijing 100871, China;
2. Tarim Oilfield Company, CNPC, Korla 841000, China
Correspondence should be addressed to Guo Zhaojie (email: zguo@geoms.geo.pku.edu.cn)
Received June 2, 2000
Abstract The Altyn Tagh Mountain is the main area where the Precambrian basements of Tarim
craton are exposed. There are two ophiolitic belts in Altyn Tagh: one belt is exposed in the northern
margin of Altyn Tagh whose formation age is about (829±60) Ma, the other is situated along the
southern margin of Altyn Tagh and has a formation age of about (1449±270) Ma. This paper
proposes a simple tectonic model for the Precambrian basement of Tarim craton established from
ophiolites in Altyn Tagh area. The south Tarim block had amalgamated with Qaidam block during
about 1400—1500 Ma along the present Altyn fault, while the south Tarim-Qaidam united block
was still separated from the north Tarim block by an ocean. The united block of south Tarim and
Qaidam collided with north Tarim block along the zone of high positive anomaly of central Tarim,
Hongliugou and Lapeiquan in about 800 Ma. So since the Sinian (beginning at 800 Ma) there has
been an integrated basement for Tarim craton.
Keywords: Tarim, Altyn Tagh, craton basement, ophiolite, block collision.
Tarim Basin is the largest sedimentary basin in China and one of the largest unexplored
basins in the world. Recent study indicates that the basement of the Tarim Basin is Precambrian
continental crust and it underwent a long period and complex tectonic evolution process[1]. A few
rises circle the Tarim Basin, which are commonly considered to represent the basin basement. The
Altyn Tagh is one of these rises and also the main study target in this paper.
1
Altyn Tagh ophiolite
Altyn Tagh is the main area where the Precambrian basement of Tarim craton is exposed. The
tectonic evolution of Altyn Tagh is till a debatable question: many geologists believe that Altyn
—
5]
Tagh is an Early Paleozoic orogenic belt, while some think it is a Late Paleozoic orogenic belt[2
.
Recently some geologists proposed new ideas of Altyn Tagh tectonics based on the new
discoveries of high-pressure metamorphic rocks, ophiolites and chronological data in Altyn Tagh
—
8]
area[6
.
Along the north front of Altyn Tagh Mountain, the ophiolite occurs within a linear belt from
Hongliugou and Lapeiquan in Xinjiang to Aksay and Subei areas in Gansu Province (fig. 1). In
Hongliugou and Lapeiquan area, the ophiolite crops to the south of the granulite complex (Miran
Group), and ophiolite sequence mainly contains pillow lava, spilite, silicalite and many

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ultramafic-mafic fragments. The basalts and associated sedimentary sequences in this zone have
been categorized as the Jixian System (1400—1000 Ma) by Xinjiang Bureau of Geology and
Mineral Resources (BGMR), but they do not have strong evidence[9]. In Aksay and Subei area, the
ophiolite sequence is categorized as the Sinian system, named Duoruonur Group by Gansu BGM,
because microplant fossils have been found in the sequence[10]
.
Fig. 1. Simplified geological map and ophiolite distribution in Altyn Tagh area. 1, Cenozoic-
Mesozoic; 2, Paleozoic; 3, Proterozoic; 4, Archean; 6, fault; 7, ophiolite; 8, granite.
We have reported the simplified petrologic features of north Altyn Tagh ophiolite[7], and we
made a geochemistry study on the main members of ophiolite. The results of the major element
analyses are given in table 1 and rare earth elements results in table 2. We collected samples of
north Altyn Tagh ophiolite from Ban Obotu gully east to Aksay. Table 1 shows that the average
value of major elements from basalts is as follows: SiO2, 49.57%; TiO2, 2.37%; Al2O3, 13.76%;
Fe2O3, 4.88%; FeO, 7.21%; MnO, 0.18%; MgO, 4.82%; CaO, 7.41%; Na2O, 3.40%; K2O, 1.15%.
The average value of major elements from gabbros is as follows: SiO2, 49.73%; TiO2, 0.52%;
Al2O3, 16.0%; Fe2O3, 1.32%; FeO, 4.91%; MnO, 0.13%; MgO, 7.44%; CaO, 12.52%; Na2O,
2.87%; K2O, 0.11%.
The total REE average value of basalts from north Altyn Tagh ophiolite is 131.47×10−6,
their La/Yb = 10.27, so they have a medium LREE-enriched pattern in the chondrite-normalized
REE curves (fig. 2). The chondrite-normalized REE patterns for gabbros from north Altyn Tagh
ophiolite are given in fig. 3. We can see that they have a horizontal slightly LREE-enriched
pattern.
In general, the geochemistry characteristics of ophiolite in north Altyn Tagh show that they
are typical ophiolite but not formed in normal mid-ocean ridge.
There is another ophiolite belt along the south front of Altyn Tagh Mountain. A highly
dismembered block of ophiolite is mainly distributed from Tura to Mangya Asbestos Mine. Near
Munablark between Qiemo and Tura, there is an ultramafic block which is the biggest one in

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Fig. 2. Chondrite-normalized REE patterns for basalts of north Altyn Tagh ophiolite.
Fig. 3. Chondrite-normalized REE patterns for gabbros of north Altyn Tagh ophiolite.
Xinjiang[9,12]. In Qingshuiquan area between Tura and Mangya Mine, we also see serpentinized
ultramafic and meta-gabbros blocks. From metagabbros we still can see its proto-cumulative
textures but pyroxenes have changed to hornblendes. The ophiolitic blocks are mainly distributed
within Proterozoic metamorphic complex, so they underwent a long and complex deformation and
metamorphic history. The geochemical and REE results of metagabbros are given in table 1 and
table 2. The chondrite-normalized REE patterns for meta-gabbros from south Altyn Tagh ophiolite
are given in fig. 4, we can see they have a horizontal LREE-enriched pattern, but sample MN-21
has a very high LREE-enriched pattern because the new mineral allanite was formed during later
metamorphism.
2
Age of the Altyn Tagh ophiolite
The ophiolite in Altyn Tagh has been dated in two places, one near Aksay along the north
front of Altyn Tagh Mountain and one in Qingshuiquan area along the south front of the mountain.
In Aksay, Sm-Nd isochronal age for cumulative gabbros is (829±60) Ma(2σ )[7]. The Sm-Nd
isotope analysis results for gabbros from Qingshuiquan are given in table 3 and its isochron is

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Fig. 4. Chondrite-normalized REE patterns for gabbros of south Altyn Tagh ophiolite.
given in fig. 5. From the results we can get an isochron age of south Altyn Tagh ophiolite as
(1449±270) Ma. This implies that two ophiolite belts in Altyn Tagh have very different ages: the
south Altun Tagh ophiolite formed much earlier than the north Altyn Tagh ophiolite.
Table 3 Sm-Nd isotope analysis results of the Altyn Tagh ophiolitesa)
147Sm
144 Nd
143 Nd
Sm/μg·g−1
Nd/μg·g−1
±2σ
No.
Sample site
Field No.
Rock name
basalt
144 Nd
1
2
Ban Obotu
Ban Obotu
Bg-7
Bg-8
6.253
7.166
6.804
3.148
0.965
1.600
1.257
0.648
0.620
27.067
31.921
30.611
9.726
2.566
4.634
3.572
1.425
1.509
0.139 7
0.135 8
0.134 4
0.195 8
0.227 5
0.208 8
0.212 9
0.275 1
0.248 6
0.512 578
0.512 533
0.512 553
0.512 947
0.513 144
0.513 049
0.513 064
0.513 389
0.513 256
12
8
basalt
3
4
Ban Obotu
Ban Obotu
Bg-10
Bg-13
basalt
7
bojite pegmatite
gabbro
17
14
11
10
14
12
5
6
7
8
9
Ban Obotu
Ban Obotu
Ban Obotu
Ban Obotu
Ban Obotu
Bg-14
Bg-18
Bg-19
Bg-20
Bg-21
gabbro
gabbro
coarse gabbro
gabbro
pegematite
meta-gabbro
meta-gabbro
meta-gabbro
meta-gabbro
meta-gabbro
meta-gabbro
10 Qingshuiquan Qs-3
11 Qingshuiquan Qs-4
12 Qingshuiquan Qs-7
1.925
0.679
0.242
0.829
1.200
24.076
6.123
2.026
0.189 9
0.202 8
0.237 2
0.151 5
0.151 7
0.096 3
0.513 033
0.513 121
0.513 573
0.512 781
0.512 742
0.512 108
11
9
0.617
13
7
13 Munablark
14 Munablark
15 Munablark
Mn20-1
303.8
Mn20-2
Mn-21
4.784
20
7
151.203
a) Analyzed by Geochronology Lab at Institute of Geology, the Chinese Academy of Sciences. The results of La Jolla
standard sample are 0.511855—0.511864.
It is worth pointing out that Liu et al.[8] have reported that they had got a Sm-Nd isochronal
age of 481.3 Ma from basalts near Mangya Mine, and they interpreted it as the time of south Altyn
Tagh ophiolite formation. In fact, the basalts in Mangya area just like basalts in Lapeiquan area,
are members of bimodal volcanic rocks, so they are formed in a paleo-rift in Ordovician period[13]
.
An Yin[4] pointed out that the ophiolitic fragments along the Altyn Tagh strike-slip fault may have
been dragged into the fault zone from Qilian Mountains or Kunlun Mountains and then displaced
for a long distance. There are Precambian ophiolites in Qilian Mountains[14], but because there are

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235
no detailed studies to correlate these ophiolites in
Qilian Mountains and in Altyn Tagh, we do not
know whether they are the same ophiolitic belt.
3
Discussion on the amalgamation of the
Tarim craton
The present Tarim Basin has a late Cenozoic
feature, as Neogene sediments cover it. It is a
long debated question as to when and how the
basement of the Tarim craton formed. Recent
drilling, seismic-reflection profiles, and field
Fig. 5. Sm-Nd isochron of gabbro from south Altun Tagh
ophiolite.
studies in the basin suggest that it has had a protracted history of deformation since the late
Precambrian[1]. Most geologists believe that there is a middle Paleozoic suture system between the
North Tarim and South Tarim block which is the western extension of the Qilian suture, and this
—
6]
suture has been offset by the late Cenozoic Altyn Tagh fault for 300—500 km left-laterally[4
.
Our new dating result of Sm-Nd isochron proves that the north Altyn Tagh ophiolite formed in
Qingbaikouan, so it cannot trace the continuation of that in the Qilian Mountains orogen whose
age is Early Paleozoic.
Our conclusions are consistent with new drilling, seismic reflection profiles and field studies
around Tarim Basin. We have proposed that the Central Tarim high aeromagnetic anomaly may
represent the western trend of the north Altyn Tagh ophiolitic zone[7], so it may be the collision
suture between North and South Tarim block. Both the North and South Tarim blocks contain
similar carbonates and sandstone of Cambrian and Ordovician age, so the central Tarim suture
should have formed at least prior to the Cambrian. The Precambrian strata and metamorphic rocks
in the North and South Tarim block are very different from each other. For example, the Sinian
system only lies conformably above the north Tarim block and is absent on the south Tarim
block1). In Kuruk Tagh area which is the outcrop of the North Tarim block there are Archean
metamorphic rocks[11], but the oldest rocks in Tieklik (southeastern Tarim uplift) and Altyn Tagh
to the south of Hongliugou-Lapeiquan suture zone are only lower Proterozoic system[9]. So the
north Tarim block contains the Archean basement but the south Tarim block just has a Proterozoic
basement.
The aeromagnetic anomaly map[1] shows that the north and south Tarim block has different
features. In the north Tarim block, the magnetic field is in east-west direction, with flat and gentle
negative anomalies. In the south Tarim block, the magnetic field presents NE-trending alternation
of positive and negative anomalies. This direction is parallel to the trend of south Altyn Tagh
ophiolitic zone and it cannot pass through the latitudinal high positive anomaly of central Tarim.
1) Jia Chengzao, Wei Guoqi et al., The structural feature and hydrocarbon accumulation of the Tarim Basin, 96-111-01-01
project report, 1998.

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We propose that NE-trending anomalies in south Tarim may represent the textures that resulted
from the collision between south Tarim block and Qaidam block, while central Tarim positive
anomaly represents the collision suture between the north Tarim block and south Tarim block
(amalgamation with Qaidam block).
The following summarizes the tectonic evolution of Tarim craton’s basement. A collision
between the south Tarim block and the Qaidam block occurred during the Late Changchengian
along the south Altyn Tagh suture, while this amalgamated south Tarim-Qaidam block was still
separated by an ocean from the north Tarim block. The late Qingbaikouan-Early Sinian collision
between the north Tarim and South Tarim-Qaidam block along the Central Tarim-Hongliugou-
Lapeiquan-Aksay suture caused the formation of united Precambrian Tarim craton. The Cambrian
system is the first cover above the united Tarim craton.
Acknowledgements Our research in Altyn Tagh and in Tianshan was supported by the National Natural Science
Foundation of China (Grant Nos. 49972080 and 49832040) to Z. Guo. Professor Qian Xianglin, Lu Huafu, An Yin and Han
Baofu are particularly thanked for their discussion and help.
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