Paleo-Tethys Evolution Recorded in the Changning-Menglian Belt, Western Yunnan, China

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0 Academic des sciences / Elsevier, Paris Geodynamical / Ghodynamique

Paleo-Tethys evolution recorded in the Changning-Menglian Belt, western Yunnan, China

Evolution de ia Pa/&o-Tbthys reconstitu6e dans ia ceinture Changning-Mengiian, Ouest de I’Yunnam, Chine

Nianqiao FANG, Qinglai FENG, Shihong ZHANG and Xunlian WANG

Xueguan road 29. Beijing 100083. China

ABSTRACT As a trunk part of the Paleo-Tethyan archipelago ocean, the Changning-Menglian belt of western Yunnan, China, presents varied volcano-sedimentary records ranging in time from Early Devonian through Middle-Late Triassic. The typical pelagic sediments represented by radiolarites can not only signify a continuously evolved ocean but also constitute a complicated basin comprising different tectono-sedimentary units. It is likely that oceanic plateaus of diverse origins played a key role in constructing the pattern of eastern Tethys. The opening and closing of the Yunnan Tethys are discussed in the paper as well. 0 AcadCmie des sciences 1 Elsevier. Paris.

Keywords: Paleo-Tethys, Western Vunnan, Oceanic plateau, Pelagic sedimentation, Magmatism

La zone de Changning-Menglian permet de reconstituer un rjaste secteur de la Pal&-Tt?thys. Des bassins et des arch@els volcaniques ant 6tk reconstituks d partir de 1 kznalyse des skies uolcano- Gdimentaires de couvertures s’ktalant du Dhonien infkrieur au Trias moyen-sup&eur. Les skdiments p6lagiques (radiolarites) conduisent ci reconnal^tre la permanence d’un o&an aujourd’hui dkmantelt? en plusieurs unit& tectono-skdimentaires On montre que cet oct?an est composite : des plateaux ockaniques d’origines et de natures diueses rendent compte de l’&olution de la Pa&-T& tbys de son ouverture d sa fermeture. 0 Acadkmie des sciewes /Else~ier, Paris.

Mots cl& : Pal&-TEtthys. Vunnan occidental, Plateau ocitanique, Sedimentation pblagique, Magmatisme

Version abr6gCe (voir p. 281)

1. Introduction each of them some evidence of remnant oceanic crust can be found (Chen et al., 1987). These ancient ocean basins, together with numerous small- and medium-sized terranes Western Yunnan is a key area for understanding the east- separated by them, constitute an archipelago configura- ern Paleo-Tethys evolution in China (Huang and Chen, tion which features the Paleo-Tethyan ocean. 1987). Generally acknowledged, it includes tectonically four Hercynian-lndosinian folding belts: Ailaoshan- The scale of the Tethyan ocean in Yunnan and the Honghe (Jinshajiang) belt, Lancangjiang (Mekong) belt, position of its main suture have long been the subject of Changning-Menglian belt and Nujiang (Salween) belt. In intense debate (e.g. Huang and Chen, 1987; Liu et al.,

Note prCsenr& par Jean Uercourt. Note remix le 24 octobrc 1997, accept& apres r&Ion le 6 janvier 1998

C. R. Acad. Sci. Paris, Sciences de la terre et des planetes / Earth & Planetary Sciences 275 1998. 326,275-202

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1991;Zhongand Wang, 1991;Moetal., 1991;Fangetal., paleontology, lithologic association and gochemistry, we 1994, 1996a; Metcalfe, 1995; Wu et al., 1995; Zhang et are successful in recognizing and organizing the following al., 1996). Our study has come to the conclusion that the records: ocean might have existed for more than 200 million years - huge flysch accumulation of Early Devonian to Late in Yunnan and the Changning-Menglian belt was a trunk Triassic being representative of passive continental mar- part of the ocean. The reasons we hold can be summarized gin; as follows. - unmixed radiolarian silica-argillaceous sequence of - The Lincang-Simao massifs with typical Cathaysia flora Early Devonian to mid-Permian accompanied occasion- and fauna (Wang et al., 1996) and Gengma-Baoshan ally with MORB (mid oceanic ridge basalt) aged 386 Ma massifs blanketed by Permo-Carboniferous moraine de- or manganese deposits of Devonian, which indicates an posits (Wang, 1983; Wopfner, 1996) and yielding biologi- abyssal oceanic floor environment; cally the diverse Gondwanaphile elements (Yang and Liu, - oceanic island sequence, ranging in time from Early 1996) occur just in the two flanks of the Changning- Carboniferous to Late Permian, composed of ultramafic Menglian belt. lavas, alkaline basalts, radiolarites, basaltic graywackes, - The most complicated framework mirrored materially phosphorites and shallow-water carbonates; by different volcano-depositional associations can be sat- - Mariannes-type oceanic island-arc silica-tuffaceous se- isfactorily established in the Changning-Menglian belt, quence of mid-Permian to Ladinian and Japan-type conti- which varies more than all other belts in western Yunnan nental island-arc sequences of Late Carboniferous to and must stand proxy for a grand space (Feng, 1992; Fang Middle-Late Triassic; and chert-elastic and pelitic deposits et al., 1996a, 199613). dated Late Permian to Late Triassic and representing an - In addition to the relict oceanic crust, the longest pe- oceanic plateau of tectonic origin. lagic record has been found in the Changning-Menglian Such long, continuous and variant oceanic and mar- belt (Fang et al., 1994). ginal records preserved in the Changning-Menglian belt - Magnetic data show the Changning-Menglian belt constitute an ‘omnipositional’ pattern for reconstructing could be a broad basin. To be specific, the paleolatitude of the history of the ocean basin. the Baoshan massif occurring to the west of the belt is 38.0”-43.0”s during Middle-Late Devonian (Zhang et al., 1996) and 30.4”-34.1 “S during Carboniferous (Huang and 3. Some sensitive subjects in the Chen, 1987; Zhang et al., 1996), while the Simao massif evolutionary process of the eastern and western part of Yangtze land to the east of the belt is Tethys recorded in the O”-4.5”s (Fang et al., 1989; Zhang et al., 1996). On this ac:count, the Changning-Menglian belt is now Changning-Menglian belt the most conspicuous part of the eastern Tethys separating the boreal land (Laurasia) from the austral land (Gond- 3.1. Opening of the ocean wanaland) during most of the Late Paleozoic time. Making The Tethys defined originally is ‘a broad zone of marine a dissection of the newest materials from the belt, we deposits of Mesozoic age’ (Suess, 1893). Sengdr (1979) approach some important things to re-examine the evolu- put forward a conception of Paleo-Tethys differing from tionary history of eastern Paleo-Tethys. the classical Tethys. Temporally, this Paleo-Tethys is a synonym of Permo-Triassic Tethys. Huang and Chen 2. A survey of the Changning-Menglian belt (1987) proposed that the term ‘Paleo-Tethys’ should be applied to describe the Tethys occurring during the Paleo- The Changning-Menglian belt is currently limited within a zoic. In spite of that, they placed the birthday of the narrow and long region, about 400 km in the N-S direc- Paleo-Tethys in the Early Permian or Late Carboniferous. tion and only 50-70 km in the E-W direction (figure 1). Metcalfe (1995) put the opening time of the Paleo-Tethys Its northward elongation submerges into the waspwaist in the Late Devonian. His postulate is likely, however, to part of the !janjiang tectonic zone (‘Sanjiang’, meaning be based on the data from those peripheral regions. In his ‘Three-Rivers’ in Chinese namely Jinshajiang River, Lan- diagram, the Devonian Tethys was only a narrow ocean cangjiang River and Nujiang River) because of the Ceno- trough. zoic compressions while its southward extension into The Paleo-Tethys in western Yunnan is not constrained Burma has not been documented. According to the data by the Permo-Triassic periods. Actually, the record of the available (Barr et al., 1990; Sashida et al., 1993; Metcalfe, Changning-Menglian ocean can be traced to the lowest 1995), however, we believe that the Chiang Rai-Chiang Devonian. According to the data available, we believe Mai belt of Thailand and the Raub-Bentong belt of Malay- that its prime period lies in the Middle Devonian to Early sia are probably a prolongation of the Changning- Carboniferous times. In respect of the beginning of the Menglian belt (Caridroit et al., 1992, 1993; Caridroit, 1993). ocean, it might be at a certain stage of Early Paleozoic. Like most erogenic belts, the Changning-Menglian belt Some of our colleagues suggest that there is a Proto- affords a great quantity of varied and broken volcano- Tethys living during the Early Paleozoic which disappears depositional sequences. Depending on the study of micro- with the process of the Caledonian orogeny (Liu et al.,

276 C. R Acad. Sci. Paris, Sciences de la terre et des plan&es / Earth & Planetary Sciences 1998.326.275-282 中国科技论文在线 http://www.paper.edu.cn Paleo-Tethys in Western Yunnan (China)

Figure 1. Sketch map showing the distribution of the volcano-sedimentary records (D,-T,) in the Changning-Menglian belt, western Yunnan, whose tectonic surroundings are described in the small-scale attached chart. 1. Continental marginal records. Terrigenous deep- and shallow-water deposits (D,-T3). 2. Oceanic basinal records. Radiolarian silica-argillo-manganese deposits (D,-P,). 3. Oceanic plateau records. I) Basalt-radiolarite-phosphorite-carbonate association (C,-P,). II) Chert-elastic rock and radiolarian shale association (P,-T,). III) Terrig- enous arenite and radiolarian shale association (C,?-P,). IV) Pyroclastic arenite and radiolarite association (P,-T,). 4. Island-arc records (C,-P,). Basalt-andesite-radiolarite-tuffaceous clayrock association. 5. Island-arc records (P,-T,). Basalt-andesite-pyroclastic rock-clayrock association. 6. Ultramafic lavas. 7. MORBs. 8. Pre-Cambrian basement. 9. Tectonic zone. 0 Jinshajiang-Ailaoshan Belt, Q Lancangjiang (Mekong Belt, 0 Nujiang (Salween) Belt, @ Changning-Menglian Belt, 0 Chiang Rai-Chiang Mai Belt, @ Raub-Benton Belt. 10. late Paleozoic glacio-marine deposits. .=~ *, Carte sch&natique montrant la r6partition des formations volcano-sCdimentaires CD,-T,) dans /a ceinture de Changning-Menglian, Ouest- Yunman. Les &ments structuraux sont d&fits dans /a carte 2 petite &he//e. 1. formations continentales de mange : d&p&s terrigPnes d’eau profonde etpeu profonde (D,-T,). 2. Formations de bassin ocCanique : d@p&s silica-argileux mangan&if&es d radiolaires (D,-P,). 3. Formations de plateau o&anique : I) association basalte-radiolarite-phosphorite-carbonate (C,-fz) ; /I) association roche clastique d chert-shale 2 radio/ai- res (P2-7;) ; ///) association arhnite terrigene-shale 2 radiolaires CC, ?--P2) ; /V) association argnite pyroc/astique-rddio/arite (P,-TJ. 4. formations d’art insulaire (C,-P,I : association basa/te-and&ite-radiolarite-roche argileuse t&a&e. 5. Formations d’art insulaire (P,-T,) : association basalte-and&site-roche pyroclastique-roche argileuse. 6. Laves ultramafiques. 7. MORBs. 8. Socle pr&ambrien. 9. Zone tectonique. 0 Cein- ture jinshajiang-Ailaoshan, 0 Ceinture Lancangjiand (Mekong, @ Ceinfure Nujiang (Salween)), @ Ceinture Changning-Menglian, 0 Ceinture Chiang Rai-Chiang Mai, @ Ceinture RauU3enton. 10. Dhp&s glacio-marins de /a fin du PalGozoi’que.

C. R. Acad. Sci. Paris, Sciences de la terre et des plan&es / Earth & flonetory Sciences 277 1998. 326,275282 中国科技论文在线 http://www.paper.edu.cn N. Fang et al.

1993; Zhong and Ding, 1993). Though there is not any Gengma shows an extremely low rate of sedimentation significant evidence, there is somehow a vague opinion (2-5 mm/l OOOa). Applying Bhatia’s REE diagnostic crite- that the Proto-Tethys closed just before the Devonian and ria (1985) for the thin-bedded plagio-arkosic turbidites afterwards. intercalating to the sequence, we have gained a sequence Zhang and Wang (1996) give an opposite conclusion by reflecting a typically Mariana-type basin associated to an analysis of geometric deformation to the closure of the oceanic island-arc (Fang et al., 1994). Changning-Menglian ocean at the end of Early Paleozoic. In the sequence numerous radiolarian skeletons have Wang et al. (1996) consider also by comparing the rugose been seen and four assemblage zones established. The genera from the Baoshan massif with those from the Yantse zones, named Wangia, Shengia yini, Triassocampe coro- land (Cathaysia) that there must be an ocean wide enough nata and Triassocampe deweveri, cover successively a to obstruct the exchange of fauna between the north and period from the top of the Permian to the Ladinian (Feng south domains at the beginning of the Devonian. We have and Ye, 1996). drawn the same point from a few Early Devonian se- The Anisian and Ladinian radiolarian faunas display quences in the Changning-Menglian belt (Fang et al., surprisingly a distant echo with western Tethys. The radi- 1996c). olarian faunas from the above sequence exhibit a high Conformably underlying the Middle-Late Devonian pe- similarity to those of the southern Alps reported by Dumi- lagic sequences, the record of the Early Devonian in the trica et al. (1980) and Gorican and Buser (1990). This Changning-Menglian belt is commonly characterized by should signify the relict water was westwards free from a dominance of thin-bedded argillo-silica-manganese de- any obstruction even if the Changning-Menglian ocean posits occasionally intercalated by quartz fine-grained had much shrunk at that epoch (Fang et al., 1996b). sandstone with distal turbidite facies. The lowermost horizon of the sedimentary association is black radiolarian Ranging temporally from late Late Permian to Anisian, shale about 0.5 m thick yielding fossil graptolite another deep water sequence, which outcrops at Monograptus uniformis, M. aegualibis and M. microdon Cangyuan and Gengma, shows a tectono-sedimentary and fossil radiolaria Eaolbaillella lilaensis, Entactinia? facies differing completely from the above one. It is com- strena, Entactinosphaera echinata and so on (Feng and posed of chert, shale and chert-elastic rock. The chert and Liu, 1993). \we have not gained any layer underlying the shale resulted from typical pelagic sedimentation while black shale. The lithofacies of the Lower Devonian ex- the elastic rock displays many structural features of gravity hibit, however, a mature sea rather than a newly opening current and contour current sedimentations (Fang et al., one at the beginning of the Devonian. 1994, 1996c). A contrast of colour exists between the Permian pelagic 3.2. Closure of the ocean sediments and the Triassic ones. A few black shale layers It was largely accepted that the ocean declined during the dominate over the pelagic facies under the P-T border. Early Permian and closed at the end of the Late Permian Conversely, all of the Triassic mudrock and chert show red (e.g. Huang ,and Chen, 1987; Liu et al., 1991, 1993). Two or yellow colours and therefore imply a great and abrupt deep water sedimentary sequences we found lately in the change of oceanographic redox background at the begin- Changning-Menglian belt shake the base of the above ning of the Mesozoic. Compared with the island-arc se- point. They are representative of an oceanic island-arc quence, this one shows two specific features. basin and lifted oceanic plateau respectively. Interestingly, (1) The chert-elastic beds with typical structures of both the two sequences are continuously developed from gravity current constitute a substantial part of the se- Late Permian to Middle-Late Triassic and their lithologic quence. All detritus constructing the beds are dramatically compositions completely differ each from other (Fang et from the cherts yielding abundantly fossil radiolarias of the al., 1994, 1996~). Middle Devonian to the Late Permian. We argue that only Consisting mainly of thin-bedded radiolarian cherts, the an environment of oceanic plateau transformed tectoni- sequence forming anciently in the oceanic island-arc set- cally from the ocean floor could unmixedly provide the ting and outcroping presently at Lancang, Menglian and fragmentated cherts.

Figure 2. Columnar sections constituting a complete pattern of the ancient Changning-Menglian ocean. Note in each section the silicolites play a key role in dating the strata, establishing the spatial and temporal relations between them and revealing their paleogeographic positions and primary tectono-sedimentary settings. The 22 radiolarian zones, developing continuously from Early Devonian to Middle Triassic, are marked in the figure and the paleolatitudinal data are given by measuring the bedded radiolarites with a superconductive magnetometer. 1. Siliceous rock; 2. clayrock; 3. sandrock; 4. manganese deposit; 5. limestone; 6. mud; 7. andesitic tuff; 8. basalt; 9. ultramafic lava; 10. horizon yielding a great number af radiolaria. :yi Sections verficales repkentant la configuration ghx?rale de l’ancien o&an Changning-Menglian. Noter que, dans chaque section, /es roches siliceuses jouent un rele cl.6 dans /a datation des strates, ktablissant leurs relations spatio-temporelles et indiquant leurs positions pa/Cogkographi- ques ei /es dkpBts tectono-sgdimentaires primaires. Les 22 zones 9 radiolaires, qui se d&eloppent continClment en&e le d&but du Dhonien et le milieu du Trias, sent indiqukes sur /a figure, et /es don&es paleolatitudinales sont fournies en mesurant /es radiolarites /@es au magr&omPtre superconducteur. 1. Roche siliceuse. 2. Roche argileuse. 3. Roche sableuse. 4. Dkp& de manganke. 5. Calcaire. 6. Limon. 7. Tufand&itique. 8. Basalte. 9. 1 ave ultramafique. 70. Horizon riche en radiolaires.

270 C. R. Acad. Sc. Paris, Sciences de la terre et des plan&tes / Earth & Planetary Sciences 1998. 326.275-282 中国科技论文在线 Passive slope Continental arc Oceanic arc Abyssal floor Oceanic plateau Oceanic plateau Oceanic plateau ( 1 : mantle plume) (11:micromassif) (MI: elevated floor) M.A. 1

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(2) The Triassic radiolarian fauna, whose representative the different volcano-sedimentary associations, four types members are Spongopalliurn sp. cf. S. koppi (Lahm), Be- of oceanic plateaus: clusters of eruptive pile of the mantle turiella robusta Dumitrica, Kozur et Mostler, displays a plume, inactive island-arc, detached or submerged conti- strong local colour, which fully differs from the faunas of nental fragment and tectono-thermal uplifted ocean (Fang the oceanic island-arc sequence. It shows that the sea floor et al., 1996b), which coincide with the models summa- elevation accompanied with the ocean diminution results rized from the present western Pacific (Ben-Avraham et al., in impeding water mass and isolating planktonic species 1981). in the interior of the Changning-Menglian ocean. A great amont of the oceanic plateaus of different origins Being equally a part of the Changning-Menglian ocean constitutes the basic configuration of the Carboniferous to but presenting such a great difference in lithologic asso- Triassic Changning-Menglian ocean. The successive ac- ciation and radiolarian community, the two sequences cretion of a lot of oceanic plateaus (allochthonous ter- together mirror a complicated configuration of the relict ranes) to the Laurasia margin may be a major mechanism ocean. Lacking an obviously terrigenous component, both of the Paleo-Tethys perishment. the two sequences have the deep-sea facies and show no sign of the final closure of the Paleo-Tethys. In reality, a 3.4. Radiolarites and their tectono-paleogeographic series of the newest discovery of the records, such as implication Middle-Late Triassic turbidite and Late Triassic radiolarite Numerous and various radiolarian siliceous rocks impres- (Ding and Zhong, 1995; Zhong Dalai, personal commu- sively develop in the Changning-Menglian belt. It is on the nication), enables us to believe that the Changning- basis of establishing 22 radiolarian zones of Early Devo- Menglian relict sea remains to be sizeable until the nian to Middle Triassic that we have rearranged the strati- Middle-Late Triassic and even opened a thorough-fare graphic records and reconstructed the evolution of the linking up the western Tethys. The termination of the ocean (Liu et al., 1991; Feng and Ye, 1996) (figure 2). Changning-Menglian ocean should be later than those Figure 2 is a summary of the data for the radiolarian times. siliceous rocks, in which the different facies associations, fossil assemblage zones, paleolatitudinal positions and 3.3. Mantle event and oceanic plateau inferred tectono-depositional settings are included. With The epoch of the Early Carboniferous seems to be a great reference to the figure we can draw the evolution for the turn in the history of the Changning-Menglian Tethys. The ocean as follows. most important event in this epoch is the massive emer- A quite large ocean with a simple frame existed in the gence of ultramafic lavas (Fang et al., 1996a). Tethyan domain during the Devonian. The active mantle The komatiitic rocks largely outcroping around plume resulted in a rapid growth of oceanic ridge or Menglian have a high MgO content (27-31 wt%), low plateau from the Early Carboniferous. Promoted probably SiO, content (40 wt%), spinifex texture and other chilling by the equatorial upwelling circulating the plateaus and features. Additionally, the existence of xenocrystalline the free silicium released by volcanic exhalation, the olivine, titanomagnetite, picotite, talc and other hypogene radiolarias in the cherts with plateau facies show a greater minerals in the komatiite is likely to further reveal the abundance, higher diversity and quicker succession than plutonic origin of the magmatic melt. Generally the ko- those in the cherts with other facies. Late Carboniferous matiitic rocks are overlaid by the pillow alkali basalt and and early Early Permian were a glacial episode. We find different oceanic sediments including bedded radiolarite, little radiolarian fossils in the Upper Carboniferous to phosphorous deposit, manganese nodule, basalt-elastic bottom Early Permian strata and their recovery and flour- graywacke and shallow water carbonate. According to the ishing at mid-Early Permian (Late Wolfcampian). The con- radiolarian assemblage from the sedimentary intercala- siderable reduction of silicolites from mid-Permian to tions, all ultramafic lavas are just of Early Carboniferous in Triassic may indicate a great diminution of the oceanic age. basin. During the last stage the pelagic biosiliceous sedi- Storey et al. (1991) attribute the generation of the Cre- mentation was still very important. According to the taceous komatiite from Corgona Island, Colombia, to an record available, the relict ocean was of considerable oceanic plateau setting with mantle plume. Our discovery scale until Late Triassic time. provides another example for illustrating the same origin of Phanerozoic komatiitic rocks. As a trunk part of the Paleo-Tethys, the Changning-Menglian ocean seems to 4. Conclusion be a simple and quite large basin during Devonian. But at the epoch of Early Carboniferous the ancient Changning- So far the volcano-sedimentary record preserved in the Menglian plate passed over hot spot(s) and the formation Changning-Menglian belt of western Yunnan is the best of the oceanic ridges or plateaus complicated the configu- one to understand the eastern Paleo-Tethyan ocean. There ration of the oceanic basin from then on. is not only remnant oceanic crust, but also an unparalleled The hot spot may be the most important kinetic origin and very long and continuous pelagic record and varied but is not only one for generating the oceanic plateaus. We facies of oceanic plateau in the belt, which mirrors a have found in the Changning-Menglian belt, by scanning long-lived and complicated oceanic basin. We have not 280 C. R. Acad. Sci. Paris, Sciences de la terre et des plan&es / Earth & Planetary Sciences 1990. 326,275282 中国科技论文在线 http://www.paper.edu.cn Paleo-Tethys in Western Yunnan (China)

currently evidence enough to determine precisely the within the Tethys domain during the whole Late Paleozoic beginning and end of the ocean but an animated ocean and Early Mesozoic can be identified.

VERSION ABRI~G~E

Le Yunnan occidental permet de comprendre l’&olution de la oceaniques de nature ocitanique (laves komatiitiques, basaltes Pal6eT6thys. La zone de Changning-Menglian, comprise en- alcalins), (3) les cort+ges sedimentaires (arenites basaltiques, tre les cratons precambriens et associ+e P des structures radiolarites, phosphorites, carbonates, couverture de bassins complexes, affleure, et permet de reconstituer les &apes de ou de plateaux oceaniques). La complexit est accrue par la 1’6volution de ce paleo-o&an i partir : presence de plateaux d’%ges varies. L‘ensemble constitue un - d’informations palComagr&iques sur les zones voisines, qui c’seuil lithospherique 1)(sensu Vrielvnck et Dercourt, in Nairn et conduisent ri reconstruire un o&an tres large ; al., 1996), comme on en connait d’autres dans la Tkthys. Le - d’informations tectoniques tr6s nombreuses, d’autant plus seuil de Changning-Menglian contrBle la circulation ocCani- que la zone Changning-Menglian s’etend au sud, jusque dans que, puis, lors de la fermeture, la tectonique dans la zone. la mer de Java (elle prend le nom de zone de Chang Rai-Chiang- Le d&tail de la reconstitution historique de l’ocean est 6tabli Mai en Thai’lande et de zone de Bentong-Raub en Malaisie). gdce aux radiolaires. Nous avons etabli une biozonation L’aire oceanique qui affleure dans la zone Changning-Men- (22 assemblages) concernant la pCriode du DCvonien au Trias. glian sirparait les continents boreal et austral, et participe de la Nous montrons : Pal&--Tethys ; des Faunes de radiolaires (et d’autres microfos- - qu’au Devonien : l’ocean, large, presentait une morphologie siles) ont et6 d&rites ; elles nous conduisent P modifier trPs simple et homogkne ; significativement l’histoire de cet ocean. - qu’au Carbonif&e infirieur, un (des) panache(s) mantel- L’ouverture de la crofite Ioc@anique de la zone Changning- lique(s) c+ai(en)t dans l’ockan des rides morphologiques et Menglian s’est produite avant le Devonien, comme cela est surtout des plateaux intra-ocCaniques magmatiques ; ce sect- 6tabli par l’lge des ophiolites (386Ma) (Metcalfe, 1995) et par eur etait le siege de courants ascendants (upweZlin@ qui celui des fossiles dans les passages, les depcits de manganese et favorisaient le d6veloppement de la faune de radiolaire ; sur les les turbidites distales silica-clastiques. plateaux intra-oc6aniques, les radiolaires incluses dans les La fermeture de cet o&an est posterieure au Pal6ozolque. couches permettent d’etablir une stratigraphie. done une re- contrairement aux hypotheses actuelles (e.g. Huang et Chen, constitution historique precise ; 1987 : Liu et al.. 1993), car les pClagites sont tr6s developpees du Permien supkrieur au Trias moyen. Les assemblages de - qu’au Carbonifitre supkrieur et au Permien infirieur (base), radiolaires de I’Anisien et du Ladinien dans les Gdiments la rare&Z des radiolaires observCe dans les skdiments serait due volcano-clastiques (arc insulaire) sont semblables 1 ceux des aux glaciations globales ; Alpes meridionales, ce qui appuie l’hypothese d’un domaine - qu’au Permien inferieur (partie mediane = Wolfcampien), au ocCanique qui s’etendrait de la Tethys occidentale (Alpes) 1 Trias moyen-sup&rieur, l’abondance des radiolaires dans les l’horizontale (Zone Changning-Menglian) (Marcoux et al.? sediments jaspeux noirs reprend ; ce type de d6pBt cesse de 1993). En outre, des genres locaux sont canton&s sur les s’accumuler au tours du Permien (ce qui indiquerait une plateaux intra-oceaniques reconstitu&. La fermeture post-date reduction de l’aire oceanique), alors que les radiolaires con- ces sediments et serait done du Trias supCrieur (ou m@me du tinuent P prolifirer dans les sediments volcano-clastiques as- Jurassique infirieur). soci& aux arcs insulaires, ainsi que dans les sediments qui se La complexit de la crocte oceanique est etablie par (1) la deposent sur les plateaux intra-oceaniques. presence de la crofite classique existant dans les ophiolites La Gdimentation pitlagique fossilifire se d&eloppe jusqu’au largement affleurantes, (2) l’existence de plateaux intra- Trias moyen-supCrieur.

Acknowledgements This work is supported financially by the National Science Foundation of China (Nos 9487002,49172 102) and the Ministry of Geology and Mineral Resources of China (No. 96-30-25).

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