Full text of DOI:10.1007/BF02906879

Vol. 44 No. 1
SCIENCE IN CHINA (Series D)
January 2001
Isotopic characteristics of shoshonitic rocks in eastern
Qinghai-Tibet Plateau: Petrogenesis and its tectonic
implication
ZHANG Yuquan (张玉泉)1, XIE Yingwen (谢应雯)1, LI Xianhua (李献华)1,
QIU Huaning (邱华宁)1, LIANG Huaying (梁华英)1, LI Jianping (李建平)1,
ZHAO Zhenhua (赵振华)1, DENG Wanming (邓万明)2
& CHUNG Sunlin (钟孙霖)3
1. Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, China;
2. Institute of Geology, Chinese Academy of Sciences, Beijing 100029, China;
3. Department of Geology, Taiwan University, Taibei 106-17
Correspondence should be addressed to Zhang Yuquan
Received July 31, 1999
Abstract The Cenozoic magmatic rocks of shoshonitic series in the eastern Qinghai-Tibet Pla-
teau include potassic alkaline plutonic rocks, volcanic rocks, lamprophyres and acidic porphyries.
Analytical results show that these different lithological rocks are extremely similar in Sr, Nd and Pb
isotopic compositions with the range of 0.705 187—0.707 254 for 87Sr/86Sr, 0.512 305—0.512 630
for 143Nd/144Nd, 18.53—18.97 for 206Pb/204Pb, 15.51—15.72 for 207Pb/204Pb and 38.38—39.24 for
208Pb/204Pb. They are isotopically similar to the EMII end-member. This indicates that mantle me-
tasomatism must have taken place in their source region. The formation of these particular rocks is
related to crustal thinning and mantle upwelling in a large-scale strike-slip and pull-apart fault zone
at about 40 Ma in northern and eastern Qinghai-Tibet Plateau.
Keywords: isotopic characteristics, magmatic rocks, shoshonitic series, eastern Qinghai-Tibet Plateau.
The Cenozoic shoshonitic rocks in the eastern Tibet are distributed along the Ailao Moun-
tain-Jinsha River fault zone and its two sides. The rocks consist of both intrusive and eruptive se-
ries, and display wide compositions ranging from ultrabasic, basic, intermediate to acidic. The
samples investigated in this study include potassic alkaline plutonic rocks, volcanic rocks, lam-
—
prophyres and acidic porphyries, for which some Sr, Nd and Pb[1 4] isotope data have previously
been reported. Due to their uniformity in temporal-spatial distribution, these rocks are regarded as
the same series in this study. On the other hand, these rocks are closely associated with the large
and super-large deposits of Cu, Mo and Au. Therefore, it is important to undertake isotopic studies
on them in order to characterize the nature of their source materials, and to understand its petro
genesis as well as the mechanism of associated mineralization.

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SCIENCE IN CHINA (Series D)
Vol. 44
1
Sr-Nd-Pb isotopic characteristics
1.1 Samples and sample locations
The rocks studied include potassic alkaline plutonic rocks (malignite, shonkinite and diopside
syenite), volcanic rocks (tephrite, shoshonite, latite and trachyte), lamprophyres (alkali-minette
and minette) and acidic porphyries (quartz monzonitic porphyry, monzonitic granoporphyry,
syenite-granoporphyry and alkali-feldspar granoporphyry). They were collected from north to
south: Nangqen, Yulong, Zalagar, Mangzong, Doxarsumdo, Mamupu, Zongguo, Xiaoqiaotou,
Gaoxingcun, Shitoncun, Wozhong, Xiaogu’ancun, 91#, Midu and Laowangzhai (fig. 1).
1.2 Analytical method
The samples have been analyzed for
Sr, Nd and Pb isotopic compositions in the
Isotopic Geochemistry Laboratory of
Guangzhou Institute of Geochemistry,
Chinese Academy of Sciences. Spikes of
84Sr and 146Nd were added to the precisely
weighted samples. The samples were then
dissolved in HF+HNO3 in Teflon. Sr and
REE are separated by cation resin. The
separation of Nd and Pb was made by
HDEHP and HBr-anion resin, respectively.
Sr, Nd and Pb isotope ratios were measured
on a mass spectrometer VG-354. 87Sr/86Sr
and 143Nd/144Nd ratios were normalized to
0.119 4 and 0.348 417, respectively. The
analyses of standards during this period give
0.710 340 ± 20 for 86Sr/87Sr of NBS-987
and 0.511 860 ±10 for 143Nd/144Nd of La
Fig. 1. A map showing the sampling localities. 1, Nangqen; 2,
Jolla. Lead isotopic ratios were corrected for
mass fractionation using NBS-981 as a cali-
bration standard. Analytical precision is
Yulong; 3, Zalagar; 4, Mangzong; 5, Doxarsumdo; 6, Mamupu; 7,
Zongguo; 8, Xiaoqiaotou; 9, Gaoxingcun; 10, Shitoucun; 11,.
Wozhong; 12, Xiaoguancun; 13, 91#; 14, Midu; 15, Laowangzhai.
about 0.02%. The analytical results are given in table 1. Since the samples are very young (30 Ma[2]),
the isotopic ratios are not corrected for the time effect and there is no significant difference between
modern and initial Sr-Nd isotopic ratios.
1.3 Sr, Nd and Pb isotopic characteristics
The different rocks of the shoshonitic series have extremely similar isotopic compositions to
143Nd/144Nd ranging from 0.512 305 to 0.512 630 and 87Sr/86Sr ratio varying from 0.705 187 to

No. 1
ISOTOPIC CHARACTERISTICS OF SHOSHONITIC ROCKS IN E. QINGHAI-TIBET PLATEAU
3
0.707 254. The variation in Pb isotopic ratios is very small (18.53—18.97 for 206Pb/204Pb, 15.51—
15.72 for 207Pb/204Pb and 18.53—18.97 for 206Pb/204Pb). These results show that the different
rocks of the shoshonitic series from eastern Tibet are nearly homogeneous in Sr, Nd and Pb iso-
topic compositions, suggesting that they may be cogenetic in origin. Similar isotopic compositions
have been found for the Cenozoic volcanic and plutonic rocks of Kerguelen Island [5,6]. It is clear
from fig. 2 that the different rock types of the shoshonitic series from eastern Tibet are all plotted
in a small area, overlapping those of Cenozoic magmatic rocks in the East Africa rift[7] and
Kerguelen Island. They are significantly different from MORB[8]. High 87Sr/86Sr and low
143Nd/144Nd ratios suggest that their source region must have been metasomatized in the past.
Table 1 Sr,Nd and Pb isotopic data of shoshonitic rocks in eastern Tibet and its neighboring region
Nd
143 Nd
144 Nd
Sr
87Sr
86Sr
Pb
206 Pb
204 Pb
207 Pb
204 Pb
208Pb
204 Pb
No.a) Sample
ε Nd
εSr
(×10−6)
(×10−6)
(×10−6)
1
2
3
4
5
6
7
8
9
96-2
96-8
84.88
1 332.3
974.6
23.3 69.48
17.6 11.34
9.8 39.97
32.2 22.28
18.7 21.32
21.2 22.46
26.0 35.32
13.5 30.52
20.6 22.85
24.0 41.40
9.8 56.75
28.7 53.96
26.4 29.28
16.7 6.96
20.0 26.14
37.2 45.57
25.9 47.67
28.3 28.40
32.1 16.36
39.1 25.61
0.512 528 ± 9 −2.18
0.512 597 ± 11 −0.84
0.512 507 ± 13 −2.59
0.512 427 ± 19 −4.15
0.512 509 ± 9 −2.56
0.512 485 ± 8 −3.02
0.512 522 ± 9 −2.30
0.512 532 ± 11 −2.11
0.512 496 ± 11 −2.81
0.512 496 ± 10 −2.81
0.512 552 ± 16 −1.72
0.512 517 ± 8 −2.40
0.512 544 ± 39 −1.87
0.512 536 ± 13 −2.03
0.512 544 ± 9 −1.87
0.512 305 ± 23 −4.89
0.512 596 ± 7 −0.86
0.512 403 ± 25 −4.62
0.512 630± 10 −0.20
0.706 139 ± 14
0.705 739 ±
18.716 ± 24 15.642 ± 26 38.872 ± 65
18.908 ± 17 15.608 ± 14 38.996 ± 35
18.783 ± 17 15.613 ± 19 38.729 ± 53
18.971 ± 18 15.669 ± 20 38.996 ± 21
18.867 ± 26 15.661 ± 24 38.963 ± 28
18.870 ± 19 15.648 ± 29 38.955 ± 33
18.883 ± 13 15.629 ± 10 38.921 ± 18
18.852 ± 14 15.634 ± 15 38.915 ± 18
18.960 ± 23 15.721 ± 30 39.236± 25
18.825 ± 19 15.659 ± 19 38.958 ± 19
18.861 ± 9 15.659 ± 11 38.965 ± 17
18.678 ± 24 15.601 ± 25 38.664 ± 28
18.576 ± 11 15.514 ± 9 38.382 ± 23
18.865 ± 68 15.562 ± 68 39.120 ± 79
18.656 ± 15 15.610 ± 13 38.644 ± 21
18.532 ± 3 15.523 ± 14 38.492 ± 18
18.813 ± 13 15.675 ± 16 39.082 ± 35
18.564 ± 11 15.552 ± 13 38.408 ± 16
18.730 ± 6 15.599 ± 5 38.776 ± 14
18.811 ± 15 15.613 ± 20 38.541 ± 21
126.42
96-21 296.89
83-284 72.41
82-136 64.93
82-141 39.99
83-404 47.39
83-375 43.31
83-99 130.89
6 600.1
989.5
0.705 192 ± 17
0.706 765 ± 8
0.705 817 ± 21
0.705 997 ± 9
0.706 329 ± 14
0.705 453 ± 2
0.705 962 ± 8
0.706 191 ± 6
0.705 187 ± 9
0.706 519 ± 11
0.706 360± 14
0.705 677 ± 7
0.705 911 ± 9
0.707 119 ± 6
0.706 324 ± 5
0.706 497 ± 5
0.706 759 ± 6
0.707 254 ± 16
1 002.2
750.6
627.8
276.3
1 585.4
1 545.3
2 281.1
1 603.7
1 003.5
615.4
10 83-110 70.68
11 83-85 95.31
12 83-778 49.50
13 86-192 25.97
14 86-152 49.53
15 86-4
16 86-99
17 86-76
48.07
37.75
92.45
1 527.7
1 221.0
2 588.6
1 106.0
986.3
18 86-177 33.20
19 86-33
20 95-3
33.57
86.36
1 526.2
−0.84
0.512 597± 9
a) Numbers 4—8 from ref. [4]; 1—3, Nangqen; 4, Yulong; 5, 6, Zalagar; 7, Mangzong; 8, Doxarsumdo; 9, 10, Mamupu;
11, Zongguo; 12, Xiaoqiaotou; 13, Wozhong; 14, 15, Gaoxingcun; 16, Shitoucun; 17, 91#; 18, Xiaogu’ancun; 19, Midu; 20,
Laowangzhai; 1, diopside syenite-porphyry; 2, 18, 19, latite; 3, 9, 16, 20, minette; 4, quartz monzonitic porphyry; 5, 8, monzo-
nitic granoporphyry; 6, syenite-granoporphyry; 7, alkali-feldspar granoporphyry; 10, 12, diopside alkali-feldspar syenite; 11,
trachyte; 13, shonkinite; 14, malignite; 15, tephrite; 17, pyrope-bearing alkali-minette.
2
Discussion
2.1 Attribute of acidic porphyries
The acidic porphyries investigated in this paper include ore-bearing porphyries and their co-
existing typical potassic alkaline plutonic and volcanic rocks. They are characterized by high al-
kali content, high-K and high K2O/Na2O ratio (>1)[10] and thus belong to the potassic alkaline se-

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SCIENCE IN CHINA (Series D)
Vol. 44
Fig. 2. Sr-Nd-Pb covariation diagrams. (a) Sr-Nd, (b) Pb-Sr and (c) Pb-Nd diagram. MORB, Oceanic basalts; Ea, East Africa
Rift; Ker, Kerguelen Island; DMM, EMI, EMII, HIMU end-members of mantle[9]; QT, Eastern Qinghai-Tibet plateau and their
neighboring region; 1, plutonic rocks; 2, volcanic rocks; 3, lamprophyres; 4, acidic porphyries.
ries. Moreover, there are no alkaline ferromagnesian minerals in syenites and granites. This makes
them different from sodic alkaline series which generally contain ferromagnesian minerals. These
acidic porphyries thus can be classified as a part of potassic alkaline plutonic rocks. It is worth
indicating that the nomenclature of the acidic porphyry in this paper is specially designated and is
distinct from the common syenitic and quartz syenitic porphyry typical of alkaline plutonic rocks.
2.2 Tectonic setting of shoshonitic rocks
On the basis of geochemical characteristics, the shoshonitic rocks have been divided into five
subgroups which are believed to occur in five different tectonic settings[11]: i.e. within-plate, con-
tinental arc, post-collisional arc, oceanic (island) arc, initial and late oceanic arc. In the following,
the possible tectonic settings at which the shoshonitic rocks occur are discussed on the basis of
geological and geophysical data.
1) Since the late Eocene, a large thick red molasse, gypsum and saline-bearing formations of
Gonjo, Baofengsi, Meile and Jinsichang groups, the coal-bearing subformations, the Lawula and

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ISOTOPIC CHARACTERISTICS OF SHOSHONITIC ROCKS IN E. QINGHAI-TIBET PLATEAU
5
Jianchuan formation of alkaline basalts and trachytes were accumulated in a series of faulted basin
along the Jinsha River-Red River. The total thickness of these formations is greater than 8 200 m. 2)
In the Tertiary faulted basins between Red River and Jinsha River, a series of lakes developed. From
south to north, the Erhai Lake, Zibai Lake, Jianhu Lake and the Manghu Lake in eastern Tibet form a
bead-like distribution pattern which is similar to the morphology [12] in the East Africa Rift. 3) The
remote sensing data[13] on the Gonjo group of the Eogene System show that an expand area of sedi-
mentation was formed in the Tertiary inherited faults around the center of the Manghu Lake. The fact
that the biggest tectonic lake ( Manghu Lake ) in eastern Tibet is located in the center of the Tertiary
sedimentary system strongly suggests that this sedimentary center continues to subside during the
Quaternary period. 4) The drilling data[14] indicate that the sediments in the Gonjo faulted basin be-
come thinner from west to east, pointing to an asymmetric distribution feature. There are three-level
faults on the west side for the Tertiary Dali basin from the foot of Cangshan Mountain to the Erhai
Lake. The electric data[15] show a total offset of the fault movement of about 500 m. Its asymmetric
feature also suggests a pull-apart characteristics. Positive Bouguer gravity anomalies are found at
Jianchuan, Heqing and the side of west Lijiang, and trend northwardly. This contrasts with the re-
gional background of negative gravity anomaly. Although these geophysical features may be related
to the presence of high density basalt layer, they more likely reflect crustal thinning and the gravity
features of the rift valley. 5) On the basis of the geophysical data[16], the faulted basin along the Red
River-Jinsha River is coincident with a zone of mantle upwelling. The depth to the Moho beneath
this zone is shallower by 2—3 km than those of the adjacent regions. 6) There is a marked, nearly
south-north trend linear magnetic anomaly along the joint zone of the Jinsha River[17]. The aero-
magnetic intensity increases discontinuously from east to west with a variation range of 30—80 γ.
The eastern side of the joint zone is characterized by negative magnetic values close to zero. In con-
trast, the Qamdo block to the west side displays positive values. The anomaly distribution pattern
forms a NS trend short-axis shape to the east of the joint zone. But it becomes NNW-trend linear
shape to the west. The difference in the magnetic anomaly between the two sides of the joint zone
suggests that there is a large-scale intra-plate fault system along the Jinsha River. Moreover, the
electrical and magnetic sounding data suggest a discontinuity of the upper mantle beneath the joint
zone of the Jinsha River, which subsides in the northeastern side but uplifts in the southwestern side.
It is possible that the upper mantle in this region is stretched.
The above-mentioned geological, geomorphic and geophysical data all suggest that the pos-
sible tectonic setting is an intra-plate strike-slip and pull-apart fault zone or initial rift at which the
shoshonitic rocks formed along the Jinsha-Red River zone.
2.3 Petrogenetic mechanism
The ages of Cenozoic shoshonitic rocks in this region vary between 40 Ma and 30 Ma[2].
These ages are roughly coincident with or slightly later than the time of collision between the In-
dian and Eurasian continents at the Yarlung Zangbo River suture zone. It can therefore be con-

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Vol. 44
cluded that initiation of the large-scale strike slip-pull apart fault zone along the Red River-Jinsha
River-Kekexili is related to continuous northward movement of the Indian Plate, which is in-turn
blocked by ancient basements of Tarim and Qaidam. The place where basins formed is also char-
acterized by crustal thinning and mantle upwelling. This provides favorable conditions to melt
metasomatized mantle resulting in the formation of shoshonitic rocks. This is also the reason why
the Cenozoic shoshonitic rocks in this region commonly occur as groups and are closely associ-
ated with the Tertiary faulted basins.
Acknowledgements The authors wish to thank Dr. Xu Yigang for improving English. This study was supported by “Na-
tional and CAS Tibet Research Project” (G1999043203, G1998040800) and CAS (kz952-S1-414).
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