Deqing-Benzilan and two traverses across the Ailaoshan Suture Zone Yakou-Laowangzai and Mojiang-Yuanjiang
as well as biostratigraphical, tectonic, chemostratigraphical and isotope geochronological investigations of other areas
including Xumai and Shuanggou.
2. Tectonostratigraphic subdivision
The metamorphic rocks exposed in the Jinshajiang– Ailaoshan Suture Zone represent a mixed assemblage
resulting from various plate tectonic processes, polyphase metamorphism and deformation. Each huge mixed body
comprises many secondary rock blocks of different size and origin. For example, the original Gajinxueshan and
Zhongxinrong groups, established by the Sichuan Team of Regional Geology STRG in 1997, are an associa-
tion of several huge mixed rock bodies, widely exposed in the Jinshajiang Suture Zone. In a sense, they might
be considered as a set of “strata” formed at different times and in different tectonic settings. Based on studies
of the tectonic settings of these rock assemblages, their tectonic boundaries, metamorphism and deformation, it
is suggested that four tectono-stratigraphic units be recognized, i.e. the Jinshajiang Ophiolitic Melange, rede-
fined Gajinxueshan “Group”, Zhongxinrong “Group” and the Eaqing Complex in the Jinshajiang Suture Zone
Fig. 4.
X. Wang et al. Journal of Asian Earth Sciences 18 2000 675–690 676
Fig. 1. Principal continental lithospheric blocks terranes and sutures of East and Southeast Asia. WB West Burma, SWB South West Borneo, S Semitau Terrane, HT Hainan Island Terranes, L Lhasa Terrane, QT Qiangtang Terrane, QS Changdu-Simao Terrane, SG Songpan Ganzi accre-
tionary complex, KL Kunlun Terrane, QD Qaidam Terrane, AL Ala Shan Terrane, KT Kurosegawa Terrane. After Metcalfe 1998.
2.1. The Jinshajiang Ophiolitic Melange The Jiangshajiang Ophiolitic Melange, as a special
component of the Jinshajiang Suture Zone, is well-exposed in the Xiaruo-Tuoding, Shusong-Gongka and Xumai-
Xuedui areas to the south of Zhongzan Figs. 3 and 4 and shows characteristics of a typical variably disrupted ophio-
litic suite. It is easily distinguished from the redefined Gajinxueshan “Group” by its different rock assemblages
and is usually distributed on both sides of the Gajinxueshan “Group”. In the Jinshajiang Ophiolitic Melange the clasts
consist mainly of basic and ultrabasic rocks and limestones,
X. Wang et al. Journal of Asian Earth Sciences 18 2000 675–690 677
Fig. 2. Continental blocks and suture zones of SW China and adjacent regions showing the general location of the Jinshajiang and Ailaoshan Suture Zones and traverses E-E Yakou-Laowangzai and F-F Muojiang-Yuanjiang. After Metcalfe 1996b.
while the matrix comprises scaly serpentinites, siltstones, pelitic and siliceous rocks Fig. 5. The Jinshajiang Ophio-
litic Melange
was formed
by subduction–accretion
processes. Early Carboniferous radiolarians and conodonts are reported from chert and siliceous limestone interbeds
within pillow basalts in the Xiaruo area, Deqing Wu, 1993; Feng et al., 1997. The main elements include the
radiolarians, Albaillella indensis indensis Won, Entactinia
X. Wang et al. Journal of Asian Earth Sciences 18 2000 675–690 678
Fig. 3. Geological sketch map of the Jinshajiang Suture Zone modified from SBGMR, 1991; YBGMR, 1982 showing localities of studied traverses: A-A Changbo-Zhongzan, B-B Ganzhangniuchang-Benjinlong, C-C Deqing-Benzilan, and D-D Xiquhe. Pt
1–2
Er Eaqing Complex Early–Middle Proterozoic, D
3
- Pji Jiashajiang Ophiolitic Melange Late Devonian–Permian, C-Pg Gajinxueshan “Group” Carboniferous–Permian, T
1-2
zh Zhongxinrong “Group” Early– Middle Triassic. For general location see inset map A and also Fig. 2.
vugaris vugaris Won, E. parva Won, E. tortispina Ormis- ton and Lane and the conodonts Gnathodus typicus Cooper,
G. girtyi rhodesi Higgins and Spathognathodus sp. Some Early Permian radiolarians, including Albaillella sp., Pseu-
doalbaillella sakmarensis Kozur?, Follicucullus ventrico- sus Ormiston and Babcock? and Follicucullus sp. are
found in purple chert intercalated within pillow basalts at Gongka, Benzila. Two zircon U–Pb ages of 340 3 and
294 3 Ma are separately obtained from the Shusong and Xuitui plagiogranites, respectively, within the ophiolitic
assemblages see below. These two plagiogranite U–Pb ages probably indicate that the Jinshajiang Ophiolitic
rocks was formed during the latest Devonianearliest Carbo- niferous or Carboniferous times in association with opening
and spreading of the Jiangshajiang ocean basin. The Jinsha- jiang Ophiolitic Melange is interpreted to have formed
X. Wang et al. Journal of Asian Earth Sciences 18 2000 675–690 679
Fig. 4. Distribution of tectono-stratigraphic units in the Jinshajiang Suture Zone. For general location see Fig. 3.
mainly in the Carboniferous, but ranging up to Permian on the basis of fossils and isotopic data. The ophiolitic melange
of the Ailaoshan Suture Zone was included in the Madeng Lithogroup by the Yunnan Bureau of Geology and Mineral
Resources YBGMR, 1996 Fig. 3. It is quite similar to the Jinshajiang Ophiolitic Melange in terms of rock assem-
blages, and metamorphic and deformation characteristics, and is well exposed in the Shuanggou-Mojiang area. A
plagiogranite from the Shuanggou Ophiolitic assemblage yields a U–Pb age of 362 41 Ma Jian et al., 1998a.
These are the first isotopic ages to indicate that the Jinsha- jiang and Ailaoshan ophiolitic melanges are contiguous and
equivalent to each other. It is suggested here that the ophio- litic melange of the Ailaoshan Suture Zone should be sepa-
rated from the Madeng Lithogroup and named the Shuanggou Ophiolitic Melange.
2.2. The Gajinxueshan “Group” The Gajinxueshan “Group”, characterized by turbiditic
clastic sediments, is redefined here as including the major part of the original Gajinxueshan Group and the lower part
of the former Zhongxinrong Group. It is the most widely distributed tectonostratigraphic unit of the Jinshajiang
Suture Zone and is widely exposed on both sides of the Jinshajiang river. Investigations of the three traverses,
Dongda-Nilu, Ganzhangniuchang-Benjinlong and Changbo- Zhongza Figs. 5b, 6a and b indicate that the original
X. Wang et al. Journal of Asian Earth Sciences 18 2000 675–690 680
Fig. 5. a Cross-section of the Jinshajiang Ophiolitic Melange exposed between Susong and Gongka see Fig. 3 for location. b Cross-section of the Jiaren “Formation” Gajinxueshan “Group” between Dongda and Nilu on the western bank of the Jinshajiang river modified from Zhan et al., 2000.
X. Wang
et al.
Journal of
Asian Earth
Sciences 1
8 2000
675 –
690
681 Fig. 6. a Cross-section of the Gajinxueshan “Group” Jiaren and a part of Yangla “formations” between Ganzhangniuchang and Benjinnong on the eastern bank of the Jinshajiang river. b Cross-section of the
Jiaren “Formation” between Changbo and Zhongzai.
Gajinxueshan Group includes four rock assemblges formed in different tectono-palaeogeographic environments:
1. ophiolitic melange produced by subductionobduction, i.e the Jinshajiang Ophiolitic Melange;
2. turbiditic greywackes of hemi-deep water on the continen- tal marginal slope or island arc, sometimes together with
basic volcanic rocks and allochthonous or sea-mount massive limestone blocks;
3. carbonates with volcanic interbeds on the shallow to deeper shelf;
4. turbidites of the residual Jinshajiang–Ailaoshan back-arc ocean basin, i.e. the redefined Zhongxinrong “Group”.
These rock assemblages with different deformation and metamorphic structures are separated from each other by
faults. The redefined Gajinxueshan “Group” mainly comprises
the above assemblage 2., i.e. clastic flysch sediments, and assemblage 3., i.e. carbonates with volcanic interbeds. The
former is called the Jiaren “Formation” Figs. 5a, 6a and b and the latter the Yangla “Formation” Fig. 7. The type
sections are situated in the Gajinxueshan area Fig. 6a. The massive limestone blocks exposed in the upper part
of the Jiaren “Formation” yield the corals, Wentzelella sp., Yuanophyllum sp., Kueichowpora sp., Diphyphyllum sp.,
Lophophyllum sp., Syringopora sp. and Palaeosmilia sp. in the Gajinxueshan area and the conodonts, Delinognatho-
dus lateralis Higgins and Bouckaert, Delinognathodus sp. and Neogondollella sp. and the fusulinids, Pseudofusulina
sp. in the Benzinan area Chengdu Institute of Geology and Mineral Resources, i.e. CIGMR and YBGMR, 1992.
Among these fossils the corals represent the Lower Carbo- niferous, except Wentzelella. which is usually found in the
Permian. The conodonts and fusulinids are common Carbo- niferous forms. The Jiaren “Formation” is therefore inter-
preted to be of Carboniferous age, but possibly extending into the Permian. It is not yet clear whether the massive
limestone represents an allochthonous block or blocks, which should then be referred to the Jinshajiang Ophiolitic
Melange, or to an oceanic island or sea mount deposit He and Chen, 1998.
The Yangla “Formation” comprises carbonate interca- lated with basic volcanics, which are usually distributed
on both sides of the Jinshajiang river, together with the redefined Jiaren “Formation”. The type section is located
in Yangla, Deqing Fig. 6a. Basalt collected from the Yangla “Formation” of Yangla area has provided U–Pb
ages of 362 9–296 7 Ma
; which indicates a Carbonifer-
ous age for the Yangla “Formation” see below. 2.3. The Zhongxinrong “Group”
Recent work has shown that two volcanic rock units, separately included in the original Zhongxinrong and Gajin-
xueshan groups, are the same unit, and are equivalent to the Yangla “Formation” of the Gajinxueshan “Group”. This
necessitates a redefinition of the Zhongxinrong “Group”. The redefined Zhongxinrong “Group” is characterized by
flysch sediments, consisting of low grade metamorphosed sandstone and siltstone and carbonaceous slate with inter-
beds of thin-bedded limestone and rare basic volcanic rock. It is in fault contact with volcanics of the Yangla “Forma-
tion” in the Zhongmu and Zhongxinrong areas. Study of sedimentary
facies indicates
that the
Zhongxinrong “Group” formed as bathyal–abyssal turbidite deposits in
the narrowing Jinshajiang–Ailaoshan ocean basin. No fossils have been found so far, but the isotopic geochrono-
logical dating of gabbro and granite, intruded into the Zhongxinrong “Group” of the Zhongmu area, gives Rb–Sr
ages of 227 5 and 255 8 Ma
; respectively see below.
Hence the Zhongxinrong “Group” would probably have been formed in the latest Permian to early Middle Triassic
time and hence be younger than the Gajinxueshan “Group”.
2.4. The Eaqing Complex The Eaqing Complex, exposed in the Batang-Suwalong
area and on both banks of the Jinshajiang River, west of Xumai, is composed of high-grade metamorphic rocks.
The metamorphic grade attains epidote–amphibolite facies to amphibolite facies together with common migmatization.
The name “Eaqing Complex” is derived from the Eaqing Group Sichuan Team of Regional Geology, 1980, which
was proposed to replace the Gajinxueshan and Zhongxin- rong groups by CIGMR and SBGMR 1992. SBGMR
1997 redefined the Eaqing Group as a formation including four members, and interpreted it to be of Permian age. This
stratigraphic subdivision of an amphibolite unit is ques- tioned here. Isotopic geochronological studies indicate
X. Wang et al. Journal of Asian Earth Sciences 18 2000 675–690 682
Fig. 7. TiO
2
–MnO–P
2
O
5
plot for basalt from the Jinshajiang Ophiolite from Han et al., 1996. Circles show the basalt from Bamaxueshan and
Gongka.
that the metamorphic age of these high-grade metamorphic rocks, exposed in the Suwalong area, are pre-Devonian, and
possibly as old as Early–Middle Proterozoic. The redefined Eaqing Complex might represent the metamorphic base-
ment remnant micro-continental fragment of the Jinsha- jiang area or high-grade metamorphic accretionary complex
rocks.
2.5. Correlation In the Ailaoshan Suture Zone two sets of metamorphic
rocks with characteristic rock assemblages, metamorphism and deformational structure were established by YBGMR
1990, 1996: the Madeng Lithogroup and another, unnamed, lithogroup. The former is characterized by meta-
morphic rocks of lower greenschist facies with strong defor- mation. The latter is further subdivided into the Meiziqing,
Kudumu, Shuoshan, Nazhuang and Maoheshan lithoforma- tions. We deduce that these two sets of metamorphic rocks
were formed in different tectonicpalaeogegraphic positions. Integrated studies on the mixed metamorphic rocks of the
Jiangshajiang and Ailaoshan suture zones suggests that the Eaqing Complex is equivalent to the Ailaoshan Complex
and that the redefined Jiaren “Formation” equates to the Madeng “Group”, excluding the Shuanggou Ophiolitic
Melange. The Yangla “Formation” equates to the Maohe- shan, Nazhuang and Suoshan Lithoformations, and the
Zhongxinrong “Group” is similar to the Ganbatang Lithogroup in lithological association. The Jinshajiang and
Shuanggou ophiolitic melanges appear to have been formed at the same time and to be contiguous with each other see
below. A correlation of the tectono-stratigraphy of the Jinshajiang and Ailaoshan suture zones is shown in Table 1.
3. Geochemical implications
