The Jinshajiang–Ailaoshan Suture Zone, China: tectonostratigraphy, age and evolution Xiaofeng Wang a , I. Metcalfe b, * , Ping Jian a , Longqing He a , Chuanshan Wang a a Yichang Institute of Geology and Mineral Resources, CAGS, Yichong, People’s Republic of China b Asia Centre, University of New England, Armidale, Australia Received 12 March 2000; accepted 20 July 2000 Abstract The Jinshajiang Suture Zone is important for enhancing our understanding of the evolution of the Paleo-Tethys and its age, tectonic setting and relationship to the Ailaoshan Suture Zone have long been controversial. Based on integrated tectonic, biostratigraphic, chemostrati- graphic and isotope geochronological studies, four tectono-stratigraphic units can be recognized in the Jinshajiang Suture Zone: the Eaqing Complex, the Jinshajiang Ophiolitic Melange, the Gajinxueshan “Group” and the Zhongxinrong “Group”. Isotope geochronology indicates that the redefined Eaqing Complex, composed of high-grade-metamorphic rocks, might represent the metamorphic basement of the Jinsha- jiang area or a remnant micro-continental fragment. Eaqing Complex protolith rocks are pre-Devonian and probably of Early–Middle Proterozoic age and are correlated with those of the Ailaoshan Complex. Two zircon U–Pb ages of 340 ^ 3 and 294 ^ 3 Ma; separately dated from the Shusong and Xuitui plagiogranites within the ophiolitic assemblage, indicate that the Jinshajiang oceanic lithosphere formed in latest Devonian to earliest Carboniferous times. The oceanic lithosphere was formed in association with the opening and spreading of the Jinshajiang oceanic basin, and was contiguous and equivalent to the Ailaoshan oceanic lithosphere preserved in the Shuanggou Ophiolitic Melange in the Ailaoshan Suture Zone; the latter yielded a U–Pb age of 362 ^ 41 Ma from plagiogranite. The re-defined Gajinxueshan and Zhongxinrong “groups” are dated as Carboniferous to Permian, and latest Permian to Middle Triassic respectively, on the basis of fossils and U–Pb dating of basic volcanic interbeds. The Gajinxueshan “Group” formed in bathyal slope to neritic shelf environments, and the Zhongxinrong “Group” as bathyal to abyssal turbidites in the Jinshajiang–Ailaoshan back-arc basin. Latest Permian–earliest Middle Triassic synorogenic granitoids, with ages of 238 ^ 18 and 227 ^ 5–255 ^ 8 Ma; respectively, and an Upper Triassic overlap molasse sequence, indicate a Middle Triassic age for the Jinshajiang–Ailaoshan Suture, formed by collision of the Changdu-Simao Block with South China. q 2000 Elsevier Science Ltd. All rights reserved. Keywords: Jinshajiang; Ailaoshan; Suture zone; Tectonostratigraphy 1. Introduction Eastern Asia is an amalgamation of allochthonous conti- nental terranes (Fig. 1) which had their origin on the margin of Gondwanaland (Metcalfe, 1988; Metcalfe, 1996a,b, 1998). The remnants of oceanic basins which once sepa- rated these continental terranes are now preserved along narrow suture zones that form the boundaries between the various allochthonous blocks. Detailed studies of the various suture zones of the region provide much valuable information on the history of the ocean basins they represent and on the tectonic history of the continental blocks they bound. This paper presents new information on the Jinsha- jiang and Ailaoshan suture zones in south-western China (Fig. 2), here interpreted to be contiguous, which necessi- tates a reinterpretation of the tectonic history of this region. Metamorphic rocks are widespread in the Jinshajiang– Ailaoshan Suture Zone and represent several huge mixed or accumulated bodies. They comprise mixed and overlap- ping rock bodies or rock blocks of different age and origin, which underwent several episodes of metamorphism and deformation. These huge mixed bodies stretch along both sides of the ophiolitic melange situated in the central part of the suture zone from north to south (Fig. 3). Traditional stratigraphic principles are not applicable to these dismem- bered metamorphic rocks. The origin, age and tectonic environment of these rocks, and their relationship to Paleo-Tethys evolution, have long been controversial. This paper presents results of our ongoing studies on the basis of three traverses across the Jinshajiang Suture Zone (Ganzhangniuchang-Benjinnong, Zhongza-Changbo and Journal of Asian Earth Sciences 18 (2000) 675–690 1367-9120/00/$ - see front matter q 2000 Elsevier Science Ltd. All rights reserved. PII: S1367-9120(00)00039-0 www.elsevier.nl/locate/jseaes * Corresponding author. Fax: 161-2-67733596. E-mail address: [email protected] (I. Metcalfe).
Transcript
The Jinshajiang–Ailaoshan Suture Zone, China: tectonostratigraphy,age and evolution
aYichang Institute of Geology and Mineral Resources, CAGS, Yichong, People’s Republic of ChinabAsia Centre, University of New England, Armidale, Australia
Received 12 March 2000; accepted 20 July 2000
Abstract
The Jinshajiang Suture Zone is important for enhancing our understanding of the evolution of the Paleo-Tethys and its age, tectonic settingand relationship to the Ailaoshan Suture Zone have long been controversial. Based on integrated tectonic, biostratigraphic, chemostrati-graphic and isotope geochronological studies, four tectono-stratigraphic units can be recognized in the Jinshajiang Suture Zone: the EaqingComplex, the Jinshajiang Ophiolitic Melange, the Gajinxueshan “Group” and the Zhongxinrong “Group”. Isotope geochronology indicatesthat the redefined Eaqing Complex, composed of high-grade-metamorphic rocks, might represent the metamorphic basement of the Jinsha-jiang area or a remnant micro-continental fragment. Eaqing Complex protolith rocks are pre-Devonian and probably of Early–MiddleProterozoic age and are correlated with those of the Ailaoshan Complex. Two zircon U–Pb ages of 340^ 3 and 294̂ 3 Ma; separatelydated from the Shusong and Xuitui plagiogranites within the ophiolitic assemblage, indicate that the Jinshajiang oceanic lithosphere formedin latest Devonian to earliest Carboniferous times. The oceanic lithosphere was formed in association with the opening and spreading of theJinshajiang oceanic basin, and was contiguous and equivalent to the Ailaoshan oceanic lithosphere preserved in the Shuanggou OphioliticMelange in the Ailaoshan Suture Zone; the latter yielded a U–Pb age of 362^ 41 Ma from plagiogranite. The re-defined Gajinxueshan andZhongxinrong “groups” are dated as Carboniferous to Permian, and latest Permian to Middle Triassic respectively, on the basis of fossils andU–Pb dating of basic volcanic interbeds. The Gajinxueshan “Group” formed in bathyal slope to neritic shelf environments, and theZhongxinrong “Group” as bathyal to abyssal turbidites in the Jinshajiang–Ailaoshan back-arc basin. Latest Permian–earliest Middle Triassicsynorogenic granitoids, with ages of 238̂18 and 227̂ 5–255^ 8 Ma; respectively, and an Upper Triassic overlap molasse sequence,indicate a Middle Triassic age for the Jinshajiang–Ailaoshan Suture, formed by collision of the Changdu-Simao Block with South China.q 2000 Elsevier Science Ltd. All rights reserved.
Eastern Asia is an amalgamation of allochthonous conti-nental terranes (Fig. 1) which had their origin on the marginof Gondwanaland (Metcalfe, 1988; Metcalfe, 1996a,b,1998). The remnants of oceanic basins which once sepa-rated these continental terranes are now preserved alongnarrow suture zones that form the boundaries between thevarious allochthonous blocks. Detailed studies of thevarious suture zones of the region provide much valuableinformation on the history of the ocean basins they representand on the tectonic history of the continental blocks theybound. This paper presents new information on the Jinsha-jiang and Ailaoshan suture zones in south-western China
(Fig. 2), here interpreted to be contiguous, which necessi-tates a reinterpretation of the tectonic history of this region.
Metamorphic rocks are widespread in the Jinshajiang–Ailaoshan Suture Zone and represent several huge mixedor accumulated bodies. They comprise mixed and overlap-ping rock bodies or rock blocks of different age and origin,which underwent several episodes of metamorphism anddeformation. These huge mixed bodies stretch along bothsides of the ophiolitic melange situated in the central part ofthe suture zone from north to south (Fig. 3). Traditionalstratigraphic principles are not applicable to these dismem-bered metamorphic rocks. The origin, age and tectonicenvironment of these rocks, and their relationship toPaleo-Tethys evolution, have long been controversial.This paper presents results of our ongoing studies on thebasis of three traverses across the Jinshajiang Suture Zone(Ganzhangniuchang-Benjinnong, Zhongza-Changbo and
Journal of Asian Earth Sciences 18 (2000) 675–690
1367-9120/00/$ - see front matterq 2000 Elsevier Science Ltd. All rights reserved.PII: S1367-9120(00)00039-0
Deqing-Benzilan) and two traverses across the AilaoshanSuture Zone (Yakou-Laowangzai and Mojiang-Yuanjiang)as well as biostratigraphical, tectonic, chemostratigraphicaland isotope geochronological investigations of other areasincluding Xumai and Shuanggou.
2. Tectonostratigraphic subdivision
The metamorphic rocks exposed in the Jinshajiang–Ailaoshan Suture Zone represent a mixed assemblageresulting from various plate tectonic processes, polyphasemetamorphism and deformation. Each huge mixed bodycomprises many secondary rock blocks of different size
and origin. For example, the original Gajinxueshan andZhongxinrong groups, established by the Sichuan Teamof Regional Geology (STRG) in 1997, are an associa-tion of several huge mixed rock bodies, widely exposedin the Jinshajiang Suture Zone. In a sense, they mightbe considered as a set of “strata” formed at differenttimes and in different tectonic settings. Based on studiesof the tectonic settings of these rock assemblages, theirtectonic boundaries, metamorphism and deformation, itis suggested that four tectono-stratigraphic units berecognized, i.e. the Jinshajiang Ophiolitic Melange, rede-fined Gajinxueshan “Group”, Zhongxinrong “Group” andthe Eaqing Complex in the Jinshajiang Suture Zone(Fig. 4).
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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 specialcomponent of the Jinshajiang Suture Zone, is well-exposedin 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 redefinedGajinxueshan “Group” by its different rock assemblagesand is usually distributed on both sides of the Gajinxueshan“Group”. In the Jinshajiang Ophiolitic Melange the clastsconsist mainly of basic and ultrabasic rocks and limestones,
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Fig. 2. Continental blocks and suture zones of SW China and adjacent regions showing the general location of the Jinshajiang and Ailaoshan Suture Zones andtraverses 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–accretionprocesses. Early Carboniferous radiolarians and conodonts
are reported from chert and siliceous limestone interbedswithin pillow basalts in the Xiaruo area, Deqing (Wu,1993; Feng et al., 1997). The main elements include theradiolarians,Albaillella indensis indensisWon, Entactinia
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Fig. 3. Geological sketch map of the Jinshajiang Suture Zone (modified from SBGMR, 1991; YBGMR, 1982) showing localities of studied traverses: A-AChangbo-Zhongzan, B-B Ganzhangniuchang-Benjinlong, C-C Deqing-Benzilan, and D-D Xiquhe. Pt1–2Er Eaqing Complex (Early–Middle Proterozoic), D3-Pji Jiashajiang Ophiolitic Melange (Late Devonian–Permian), C-Pg Gajinxueshan “Group” (Carboniferous–Permian), T1-2zh Zhongxinrong “Group” (Early–Middle Triassic). For general location see inset map A and also Fig. 2.
vugaris vugarisWon, E. parvaWon, E. tortispina(Ormis-ton and Lane) and the conodontsGnathodus typicusCooper,G. girtyi rhodesiHiggins andSpathognathodussp. SomeEarly Permian radiolarians, includingAlbaillella sp.,Pseu-doalbaillella sakmarensis(Kozur)?,Follicucullus ventrico-sus (Ormiston and Babcock)? andFollicucullus sp. arefound in purple chert intercalated within pillow basalts atGongka, Benzila. Two zircon U–Pb ages of 340^ 3 and
294^ 3 Ma are separately obtained from the Shusong andXuitui plagiogranites, respectively, within the ophioliticassemblages (see below). These two plagiogranite U–Pbages probably indicate that the Jinshajiang Ophioliticrocks was formed during the latest Devonian/earliest Carbo-niferous or Carboniferous times in association with openingand spreading of the Jiangshajiang ocean basin. The Jinsha-jiang Ophiolitic Melange is interpreted to have formed
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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 onthe basis of fossils and isotopic data. The ophiolitic melangeof the Ailaoshan Suture Zone was included in the MadengLithogroup by the Yunnan Bureau of Geology and MineralResources (YBGMR, 1996) (Fig. 3). It is quite similar to theJinshajiang Ophiolitic Melange in terms of rock assem-blages, and metamorphic and deformation characteristics,and is well exposed in the Shuanggou-Mojiang area. Aplagiogranite from the Shuanggou Ophiolitic assemblageyields a U–Pb age of 362̂ 41 Ma (Jian et al., 1998a).These are the first isotopic ages to indicate that the Jinsha-jiang and Ailaoshan ophiolitic melanges are contiguous andequivalent 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 theShuanggou Ophiolitic Melange.
2.2. The Gajinxueshan “Group”
The Gajinxueshan “Group”, characterized by turbiditicclastic sediments, is redefined here as including the majorpart of the original Gajinxueshan Group and the lower partof the former Zhongxinrong Group. It is the most widelydistributed tectonostratigraphic unit of the JinshajiangSuture Zone and is widely exposed on both sides of theJinshajiang river. Investigations of the three traverses,Dongda-Nilu, Ganzhangniuchang-Benjinlong and Changbo-Zhongza (Figs. 5b, 6a and b) indicate that the original
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Fig. 5. (a) Cross-section of the Jinshajiang Ophiolitic Melange exposed between Susong and Gongka (see Fig. 3 for location). (b) Cross-section of theJiaren“Formation” (Gajinxueshan “Group”) between Dongda and Nilu on the western bank of the Jinshajiang river (modified from Zhan et al., 2000).
X.
Wa
ng
et
al.
/Jo
urn
alo
fA
sian
Ea
rthS
cien
ces
18
(20
00
)6
75
–6
90
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 theJiaren “Formation” between Changbo and Zhongzai.
Gajinxueshan Group includes four rock assemblges formedin different tectono-palaeogeographic environments:
1. ophiolitic melange produced by subduction/obduction, i.ethe Jinshajiang Ophiolitic Melange;
2. turbiditic greywackes of hemi-deep water on the continen-tal marginal slope or island arc, sometimes together withbasic volcanic rocks and allochthonous or sea-mountmassive limestone blocks;
3. carbonates with volcanic interbeds on the shallow todeeper shelf;
4. turbidites of the residual Jinshajiang–Ailaoshan back-arcocean basin, i.e. the redefined Zhongxinrong “Group”.
These rock assemblages with different deformation andmetamorphic structures are separated from each other byfaults.
The redefined Gajinxueshan “Group” mainly comprisesthe above assemblage 2., i.e. clastic flysch sediments, andassemblage 3., i.e. carbonates with volcanic interbeds. Theformer is called the Jiaren “Formation” (Figs. 5a, 6a and b)and the latter the Yangla “Formation” (Fig. 7). The typesections are situated in the Gajinxueshan area (Fig. 6a).The massive limestone blocks exposed in the upper partof the Jiaren “Formation” yield the corals,Wentzelellasp.,Yuanophyllumsp., Kueichowporasp., Diphyphyllum sp.,Lophophyllumsp., Syringoporasp. andPalaeosmiliasp.in the Gajinxueshan area and the conodonts,Delinognatho-dus lateralis(Higgins and Bouckaert),Delinognathodussp.and Neogondollellasp. and the fusulinids,Pseudofusulinasp. in the Benzinan area (Chengdu Institute of Geology andMineral Resources, i.e. CIGMR and YBGMR, 1992).Among these fossils the corals represent the Lower Carbo-
niferous, exceptWentzelella. which is usually found in thePermian. The conodonts and fusulinids are common Carbo-niferous forms. The Jiaren “Formation” is therefore inter-preted to be of Carboniferous age, but possibly extendinginto the Permian. It is not yet clear whether the massivelimestone represents an allochthonous block or blocks,which should then be referred to the Jinshajiang OphioliticMelange, or to an oceanic island or sea mount deposit (Heand Chen, 1998).
The Yangla “Formation” comprises carbonate interca-lated with basic volcanics, which are usually distributedon both sides of the Jinshajiang river, together with theredefined Jiaren “Formation”. The type section is locatedin Yangla, Deqing (Fig. 6a). Basalt collected from theYangla “Formation” of Yangla area has provided U–Pbages of 362̂ 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 theYangla “Formation” of the Gajinxueshan “Group”. Thisnecessitates a redefinition of the Zhongxinrong “Group”.The redefined Zhongxinrong “Group” is characterized byflysch sediments, consisting of low grade metamorphosedsandstone 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 ofsedimentary facies indicates that the Zhongxinrong“Group” formed as bathyal–abyssal turbidite deposits inthe narrowing Jinshajiang–Ailaoshan ocean basin. Nofossils have been found so far, but the isotopic geochrono-logical dating of gabbro and granite, intruded into theZhongxinrong “Group” of the Zhongmu area, gives Rb–Srages of 227̂ 5 and 255̂ 8 Ma; respectively (see below).Hence the Zhongxinrong “Group” would probably havebeen formed in the latest Permian to early Middle Triassictime and hence be younger than the Gajinxueshan “Group”.
2.4. The Eaqing Complex
The Eaqing Complex, exposed in the Batang-Suwalongarea and on both banks of the Jinshajiang River, west ofXumai, is composed of high-grade metamorphic rocks.The metamorphic grade attains epidote–amphibolite faciesto amphibolite facies together with common migmatization.The name “Eaqing Complex” is derived from the EaqingGroup (Sichuan Team of Regional Geology, 1980), whichwas proposed to replace the Gajinxueshan and Zhongxin-rong groups by CIGMR and SBGMR (1992). SBGMR(1997) redefined the Eaqing Group as a formation includingfour members, and interpreted it to be of Permian age. Thisstratigraphic subdivision of an amphibolite unit is ques-tioned here. Isotopic geochronological studies indicate
X. Wang et al. / Journal of Asian Earth Sciences 18 (2000) 675–690682
Fig. 7. TiO2–MnO–P2O5 plot for basalt from the Jinshajiang Ophiolite(from Han et al., 1996). Circles show the basalt from Bamaxueshan andGongka.
that the metamorphic age of these high-grade metamorphicrocks, exposed in the Suwalong area, are pre-Devonian, andpossibly as old as Early–Middle Proterozoic. The redefinedEaqing Complex might represent the metamorphic base-ment (remnant micro-continental fragment) of the Jinsha-jiang area or high-grade metamorphic accretionary complexrocks.
2.5. Correlation
In the Ailaoshan Suture Zone two sets of metamorphicrocks with characteristic rock assemblages, metamorphismand 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 rockswere formed in different tectonic/palaeogegraphic positions.Integrated studies on the mixed metamorphic rocks of theJiangshajiang and Ailaoshan suture zones suggests that theEaqing Complex is equivalent to the Ailaoshan Complexand that the redefined Jiaren “Formation” equates to theMadeng “Group”, excluding the Shuanggou OphioliticMelange. The Yangla “Formation” equates to the Maohe-shan, Nazhuang and Suoshan Lithoformations, and theZhongxinrong “Group” is similar to the GanbatangLithogroup in lithological association. The Jinshajiang andShuanggou ophiolitic melanges appear to have been formedat the same time and to be contiguous with each other (see
below). A correlation of the tectono-stratigraphy of theJinshajiang and Ailaoshan suture zones is shown in Table 1.
3. Geochemical implications
A detailed study on the basalts exposed in the JinshajiangOphiolitic Melange from the Baimaxueshan, Shusong andGongka areas was made by Han et al. (1996). The resultsindicated that the basalt composition falls mainly into theMORB on the TiO2–MnO–P2O5 diagram, only a few datapoints falling in the oceanic island tholeiite (OIT) field (Fig.7). This suggests that at least a part of the remnant oceaniccrust in the Jinshajiang Suture might be derived from ocea-nic ridge spreading (ORS). On the other hand, elementdistribution from these basalts also shows a transitionalcharacter from MORB to OIB. A similar result is alsoobtained from basalts of the Ailaoshan Ophiolitic melange.
4. Isotopic geochronology
4.1. Jinshajiang Ophiolitic Melange
Plagiogranite is considered a typical oceanic granitoid,being the product of magmatic differentiation, and is morewidely found in the Jinshajiang Ophiolitic Melange.Samples for isotopic study were collected from separatedplagiogranites, occurring in the metamorphic peridotites ofShusong, Xuedui and Rongjiaoxishan. Preliminary resultsare here presented from the Shusong plagiogranite andXuedui plagiogranite. The former is exposed in a road
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Table 1Tectono-stratigraphic subdivision and correlation of the Jinshajiang–Ailaoshan Suture Zone
Age
T
T
P
C
D
Pre D(Pt?)
3
1-2
neritic shelf
Tectonostratigraphic unitJinshajiang Zone
continentalslope
continentalslope
oceanbasin
oceanbasin
residualocean basin
residualocean basin
neritic shelf
Jiapila Formation Yiwanshui Formation
Zhonxing-rong "Gr"
Yangla"Fm"
(Gajinxue-shan "Gr")
Jiaren"Fm"
(Gajinxue-shan "Gr")
JinshajiangOphioliticMelange
Maohesan"Fm"
Nazhuang"Fm"
Saoshan"Fm"
Maden"Gr"
ShuanggouOphioliticMelange
Ganbatang"Gr"
Eaqing Complex Ailaoshan Complex
Ailaoshan ZoneTectonic event
Molasseformation
Basin closure
Syn-collisionalgranite
emplacement
Subduction
Formation ofocean lithosphereBasin spreading
Intracontinentalrift
cutting between Deqing and Banzilan (Fig. 3). The latter isfound in the Xuedui Ophiolitic assemblage. Both plagiogra-nites occur in tectonic contact with metamorphic peridotite,which is a part of the Jinshajiang Ophiolitic belt.
Zircons were extracted from a fresh 15 kg sample of theShusong plagiogranite (SA 9738). U–Pb dating wasperformed in the isotopic laboratory of the Tianjin Instituteof Geology and Mineral Resources, China. Isotopic analyseswere undertaken on a VG 354 Mass Spectrometer. Themixed spike of205Pb1 235U was used. Total blanks werePb (0.05 ng) and U (0.02 ng). The ages were calculatedusing thepbdat and isoplot programs (Ludwig, 1987).All the ages obtained are at the 95% confidence level.
Zircons from the Shusong plagiogranite are a very homo-geneous population with short prismatic crystals. Threeanalyses give206Pb/238U ages ranging from 339–347 Maand 207Pb/235U ages of 323–346 Ma. These results show aslight discordance. The weighted mean206Pb/238U age is340^ 2:5 Ma (Fig. 8, Table 2). This age probably repre-
sents the emplacement age of the Shusong plagiogranite,indicating that the Jinshajiang Ocean was already activelyspreading during the latest Devonian to earliestCarboniferous.
Zircons extracted from the Xuedui plagiogranite (SA9722) are elongated and prismatic in appearance. Threeanalyses give206Pb/238U ages ranging from 289–296 Maand 207Pb/235U ages from 289–294 Ma (Table 2). Theanalyses show basic concordance. The 294^ 4 Ma averageage of206Pb/238U would represent the formation age of theXuedui plagiogranite, indicating that the Jinshajiang ocea-nic spreading occurred in the Carboniferous.
4.2. Gajinxueshan Group
So far, no isotopic ages nor any fossils have beenobtained from the clastic flysch deposits in the redefinedJiaren “Formation”, which equates to most of the Gajinxue-shan “ Group”. Some new isotopic results, however, wereobtained from the redefined Yangla “Formation”, whichincludes the original Yangla and Linong lithoformations(Zhan et al., 2000). Two zircon-bearing samples (Zr 1 andZr 2) were collected from the bedded metamorphic basalt inthe Linong mining area. A sample of 60 kg provided only30–50 zircons with rounded shapes, probably caused bystrong solution. Three analysis give206Pb/238U ages of523–1354 Ma and207Pb/238U ages of 757–1786 Ma, show-ing obvious discordance. These zircons have obviousmagmatic corrosion, and it is suggested that they werederived from the basement rocks, and are probably crustalrelics, caught up in the basalt magma. In association withother inherited zircons of 911̂ 31 and 2119̂ 78 Ma (Jianet al, 1998b), this data further supports the possibility of aProterozoic metamorphic basement existing in the Jinsha-jiang Suture Zone.
4.3. Eaqing Complex
A fresh sample (SA 9730) sillimanite-bearing two micaschist was collected from a 20 m exposure at Gangda,Batang, West Sichuan (Fig. 3). Rb–Sr analysis was under-taken in the Isotopic laboratory of Yichang Institute ofGeology and Mineral Resources (YIGMR). Six analyses
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Fig. 8. U–Pb concordia diagram of zircons from the Shusong plagiogranite.
Table 2U–Pb analyses of zircons from the Jinshajiang plagiogranites
give a slightly scattered isochron. The Rb–Sr age is 423^
40 Ma (Fig. 9a, Table 3). This age is interpreted as the ageof the high-grade metamorphic event which produced themetamorphic minerals mica, sillimanite, kyanite, etc. Thedispersed data points are likely related to modifications thatdid not affect all minerals in the primary rocks. The EaqingComplex is derived from the Eaqing Group, which is corre-lated with the original Gajinxueshan Group by CIGMR andSBGMR (1991) and was interpreted as Permian in age bythe SBGMR (1997).
However, a new study of the Eaqing Complex shows that
its rock assemblage and metamorphic grade are similar tothat of the Ailaoshan Complex exposed in the Ailaoshanarea. The metamorphic age of the Eaqing Complex, more-over, is dated at 423̂ 40 Ma: Its protolith age should betherefore older than the Devonian. A zircon sample (SA9731) from the Gadi plagioclase–amphibolite (Figs. 3 and4) is dated by the U–Pb method, and shows a more compli-cated pattern (Table 4). It gives an upper intercept age of1627^ 192 Ma; providing a minimum age for the volcanicprotolith. This indicates the presence of a Meso- to Neo-Proterozoic remnant metamorphic basement, or micro-continental fragment, in the Jinshajiang area, with an ageolder than 1627̂ 192 Ma: If the time of opening of theJinshajiang Ocean corresponds to that of the AilaoshanOcean, the Eaqing Complex is likely to be equivalent tothe Ailaoshan Complex. An analogous high-grademetamorphic series to the Eaqing Complex, exposed inthe Batang area, is also found in both sides of the JinshajiangRiver in the Xumei area, Deqing. Based on the rock assem-blage and high metamorphic grade, this series in Xumeishould be referred to the Eaqing Complex.
X. Wang et al. / Journal of Asian Earth Sciences 18 (2000) 675–690686
Table 5Rr-Sr data of granite and gabbro from Jinshajiang Suture Zone
Sample no. Rock W(Sr)/2610 W(Sr)/2610 87Rb/86Sr 87Sr/86Sr
4.4.1. The Xumai foliated granite (SA 9725)Samples were collected from foliated granites, intrud-
ing the Eaqing Complex, about 1 km to the east ofXumai village (Figs. 3 and 4). The metamorphiccomplex there attains epidote amphibolite facies toamphibolite facies and underwent several episodes of
migmatization. The intrusive relationship between theXumai granite and its country rocks can be seen in thefield. The foliation of the granite is concordant with thatof the metamorphic complex. Six whole rock analysesdefine an isochron age of 238̂18 Ma: The age prob-ably reflects the time of a collisional orogenic event. The(87Sr/86Sr) initial value is 0:70421^ 0:00012; which issignificantly lower than the average value of the crust,
X. Wang et al. / Journal of Asian Earth Sciences 18 (2000) 675–690 687
Fig. 10. Schematic diagram illustrating the tectonic evolution of the Jinshajiang–Ailaoshan Suture Zone.
implying that the granite probably originated from themantle (Fig. 9b, Table 5).
4.4.2. The Zhongmu granite (SA 9717)The Zhongmu granite, intrudes the flysch formation of
the Zhongxinrong “Group ” as a stock. The sample wascollected in the river bank of the Jinshajiang near Zhongmuvillage (Figs. 3 and 4). The isochron age defined by sixwhole rock analyses is 255̂ 8 Ma; with (87Sr/86Sr) initialvalue of 0:709079̂ 0:00008 (Fig. 9c, Table 5). The higher(87Sr/86Sr) initial value indicates that the studied granite wasrelated to crustal melting.
The Rb–Sr ages of the Jiaren and Linong granites, indi-cate that they separately intruded into the redefined Jiarenand Yangla “formations”. These Rb–Sr ages are 208^ 6and 227̂ 2 Ma with (87Sr/86 Sr) initial values of 0.71027–0.70640, respectively, (Zhan et al., 2000). These data showthat these granites, occuring along the Jinshajiang Suture,formed during Late Triassic to earliest Jurassic times, andthat the Jinshajiang ocean was already closed at this time.
4.4.3. Geochronology of high-level gabbro (SA 9715)Diabase and gabbro are widespread in the Zhongmu area
and are in tectonic contact with the Zhongxinrong “Group”.However no associated ultramafic rocks (metamorphic peri-dotite, etc.) and pillow basalt are found in this area. Theserocks, therefore, appear not to be part of the ophioliticassemblage. At present limited petrological evidence isavailable to determine the petrogenesis of these rocks.Gabbro samples were collected on the eastern bank of theJinshajiang near Zhongmu village. The isochron age of fivewhole rock Rb–Sr analyses is 227̂5 Ma; with (87Sr/86Sr)initial value of 0:70459^ 0:00006 (Fig. 9c, Table 5),further indicating that the Jinshajiang ocean closed duringthe Indosinian orogenic episode.
5. Tectonic evolution of Jinshajiang–Ailaoshan SutureZone
The Jinshajiang–Ailaoshan Suture, palaeogeographi-cally, is situated in the eastern part of the Paleo-TethysOcean. From the above discussion, we suggest that theJinshajiang and Ailaoshan suture zones are contiguous andrepresent the same ocean basin. This has important implica-tions for a greater understanding of the origin and evolutionof the Paleo-Tethys. This conclusion is based on: (a) the twosuture zones having comparable lithological assemblages,tectonic deformation, similar metamorphic history and ages;(b) the Ophiolitic Melange of the two suture zones beingconsistent in their tectonic setting, and ages, furthersupported by new geochemical and isotopic geochronologi-cal data; and (c) the discovery of several Palaeozoic–Meso-zoic synorogenic granites, such as the Xumai and Zhongmugranites, and a Late Triassic molasse overlap sequence,unconformably overlying the metamorphic rocks of both
suture zones, further indicate identical suturing ages.Comparison of the basement compositions, pre-Devoniansedimentary features as well as the Palaeozoic palaeobio-geographic faunal and floral affinites of the Changdu-Simaoand South China blocks (Fig. 1) suggests that the Changdu-Simao micro-continental block probably rifted from theSouth China Block by back-arc extension and subsequentocean floor spreading. In this case, the Changdu-SimaoBlock must be regarded as a separate continental terrane,as previously suggested by Wu et al. (1995), and not anintegral part of the Indochina Terrane, as previouslysuggested by Metcalfe (1996a, 1998). The general lack ofopen ocean sediments (e.g. ribbon-bedded cherts) presentlysupports this view. Further work is required in Burma andNorthern Thailand to delineate the southern termination ofthe Changdu-Simao Terrane and elucidate its tectonic rela-tionships with the Sibumasu Terrane. The evolution of theJinshajiang–Ailaoshan Suture Zone is here interpreted tohave occurred in four evolutionary stages as discussedbelow (Fig. 10).
5.1. Intracontinental rift stage
The Palaeo-Tethys Ocean basin opened when NorthChina, South China, Indochina and Tarim rifted and sepa-rated from the northern margin of Gondwanaland in theDevonian. The main branch of Palaeo-Tethys is representedby the Lancangjiang and Changning–Menglian sutures inSW China, by the Nan–Uttaradit–Sra Kaeo Suture in Thai-land and the Bentong–Raub Suture in Peninsular Malaysia(Metcalfe, 1996a,b, 1998; Metcalfe and Spiller, 1999).Following initial sea-floor spreading of the Lancangjiangand Changning–Menglian segments of the Palaeo-Tethysin the Early Devonian, deposition of Middle–Late Devonianoceanic ribbon-bedded cherts (Liu et al., 1991, 1993)occurred in the ocean, and subduction of the Palaeo-Tethysbeneath South China and Indochina in the Middle–LateDevonian to earliest Carboniferous led to back-arc exten-sion and rifting of the Changdu-Simao Massif from SouthChina. The initial Jinshajiang–Ailaoshan back-arc intracon-tinental basin contains graptolite-bearing deep water clas-tics deposited in this basin along the western margin of theSouth China Block from Luchun-Mojiang to Batang.
5.2. Opening and spreading stage
During the Devonian–Carboniferous transition period theJinshajiang–Ailaoshan back-arc basin reached its oceanicspreading stage due to continued extension and subductionof the Lancangjiang–Changning–Menglian ocean. Oceanlithosphere protolith rocks of the Jinshajiang and Shuang-gou ophiolitic melanges began to be formed at this time andcontinued up to Early Permian. These protolith rocksinclude harzburgites, cumulus crystal gabbros and pillowbasalts, indicating oceanic lithospheric spreading. SomeEarly Carboniferous and Early Permian radiolarian-bearingcherts were present in the abyssal, starved basin in the
X. Wang et al. / Journal of Asian Earth Sciences 18 (2000) 675–690688
medial part of the Jinshajiang–Ailaoshan basin. The flyschand turbidite deposits of bathyal facies and carbonate depos-its of neritic facies, intercalated with volcanic rocks (Jiarenand Yangla “formations”), were developed on both passivecontinental margins of the new oceanic back-arc basin.Massive limestones of limited extent, with Early Carboni-ferous corals, are interpreted as sea-mount deposits. Anumber of volcanic rocks occurring in the carbonate suiteof the Yangla “Formation” are likely related to the subduc-tion of the Lancangjiang Paleo-Tethys Ocean toward theChangdu-Simao Massif.
5.3. Subduction stage
From about the Early–Late Permian boundary theJinshajiang–Ailaoshan back-arc basin ceased spreadingand began its subduction stage. Subsequently, the easternmargin of the Changdu-Simao Massif was changed from apassive continental margin to an active one. This changefrom spreading to subduction is indicated by four lines ofevidence: (a) the occurrence of the Late Permian–earlyMiddle Triassic Zhongxinrong “Group” flysch formed inthe narrowing residual Jinshajiang–Ailaoshan basin; (b)the increase of latest Permian to Early Triassic basic volca-nic deposits, unconformably overlying the Yangla “Forma-tion”, in the Jinshajiang Suture Zone; (c) the development ofthe Jiangda (Jomda)-Weixi volcanic arc in the MiddlePermian–Late Triassic on the eastern margin of theChangdu-Simao Block by westwards subduction of theJinshajiang–Ailaoshan (Mo et al., 1993); and (d) the exis-tence of Late Permian radiolarian cherts (Sun et al., 1997)and passive continental marginal deposits, belonging to theophiolitic melange in the eastern slope of the Jinshajiang–Ailaoshan oceanic basin. These data additionally indicatethat the Jinshajiang–Ailaoshan oceanic basin could not haveclosed before the Middle Triassic.
5.4. Collision stage
The closure of the Jinshajiang–Ailaoshan back-arc basinis considered an important event in the evolution of not onlythe Jinshajiang–Ailaoshan Suture Zone, but also Palaeo-Tethys in general. In the Jinshajiang–Ailaoshan SutureZone the collision, occurring along with the closure of theoceanic basin, led to structural deformation, regional meta-morphism of varying degrees, disruption of the Jinshajiangand Shuanggou ophiolitic melanges, the flysch suites of theGajinxueshan and Madeng “groups” and the turbidites ofbathysal-abyssal facies of the Zhongxinrong and Ganbatang“groups”, formed in the different tectono-palaeogeographicsettings of the Jinshajiang–Ailaoshan back-arc basin duringthe Early Carboniferous to early Middle Triassic. LatestPermian–Middle Triassic synorogenic granitoids, such asthe Xumai �238^ 18 Ma� and Zhongmu�255^ 8–227^2 Ma� granites are distributed along the orogenic belt. TheLate Triassic molasse overlap sequences of the Jiapila andYiwanshui formations, unconformably overlie the meta-
morphic rocks in the Jinshajiang–Ailaoshan Suture Zoneand mark the end of evolutionary history of this suture.The age of the suture is thus constrained as Middle Triassic.
Acknowledgements
We would like to thank the Sichuan and Yunnan Bureausof Geology and Mineral Resources for their assistance andcooperation during fieldwork. We gratefully acknowledgethe Geological Survey of China and an Australian ResearchCouncil Grant to I.M. for financial support. Constructivereviews of the paper by Mark Allen and Dietrich Helmckeare gratefully acknowledged.
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