1 Introduction

La zone de suture d'Anyemaqen s'étend sur une distance de 1200 km le long de la bordure nord-est du Tibet. Elle sépare les terrains triasiques de la chaı̂ne de Songpan–Garze située au sud, du bloc du Qaidam au nord (Fig. 1a) [22,24,33]. La question cruciale pour la géodynamique de cette région est de savoir si cette suture représente réellement la trace de l'océan téthysien paléozoı̈que.

La suture présente une association péridotites–gabbros–ophicalcites–radiolarites, qui est typique et comparable à des séries ophiolitiques connues dans les Alpes et le long de la marge Ibérique [1–3,19–21,23]. Elle contient des blocs de taille et de composition variées, qui représentent des fragments de lithosphère dont l'origine océanique paléo-téthysienne est contestable en raison de leur transformation et altération postérieures.

La discussion sur la nature (composition chimique) et l'origine même des ophiolites de la suture d'Anyemaqen provient aussi du fait qu'elles sont essentiellement connues à partir des données des roches volcaniques [27,33]. C'est pourquoi l'analyse des roches qui représentent les parties les plus profondes de la croûte inférieure et du manteau supérieur est indispensable.

Nous présentons, dans cette étude, les premières données obtenues pour cette région sur la composition chimique des minéraux résiduels contenus dans les péridotites d'Anyemaqen.

2 Contexte géologique

3 La péridotite de la suture Anyemaqen

Un échantillon de péridotite exceptionnellement bien préservé des déformations et rétromorphoses secondaires a été découvert et sélectionné pour des analyses détaillées. Cette roche contient seulement 5 % de serpentine. Sa composition modale est de ⩾5 % de clinopyroxène (Cpx) résiduel, 25 % d'orthopyroxène (Opx), ⩽68 % d'olivine (Ol) et 2 % de spinelle (Sp), ce qui correspond à une transition entre une harzburgite riche en clinopyroxène et une lherzolite à spinelle pauvre en clinopyroxène. La foliation mylonitique est bien marquée. Les lamelles d'exsolution de Cpx dans les grands (jusqu'à 1 cm) porphyroclastes d'Opx étirés sont parallèles aux plans de cisaillement, et obliques à l'allongement de l'Opx qui donne la foliation (Fig. 2A–C). Les porphyroclastes de Cpx sont petits (190–380 μm, rarement jusqu'à 600 μm) et fortement résorbés par une néo-blastèse en mosaı̈que de grains de Ol et de Sp (Fig. 2D et E). La texture mylonitique de la roche est aussi soulignée par des bandes de Ol recristallisées, étendues parallèlement à l'alignement de porphyroclastes de Opx et de grains de Sp allongés (Fig. 2F).

4 Discussion et conclusion

Cette étude suggère que les péridotites de la suture d'Anyemaqen caractérisent le matériel du manteau supérieur, qui a subi moins de 10 % de fusion partielle avant sa mise en place (Fig. 3c et d) [14]. La teneur en éléments tels que Al, Ti, et Fe est très élevée dans les minéraux résiduels de la péridotite étudiée, ce qui la rend très semblable aux péridotites résiduelles de dorsales océaniques à vitesse lente (Fig. 3a–d, Tableau 1) [9,15,16,28,29] et aux péridotites de la marge de Galice [10,13] et de l'ı̂le de Zabargad [5]. La composition chimique des minéraux et l'analyse texturale des péridotites d'Anyemaqen (texture de l'Ol et obliquité des Opx) montrent que ces roches ont subi une déformation plastique en cisaillement ductile non coaxiale dans des conditions subsolidus de basse température (800–1000 °C) et une forte contrainte déviatorique [25], suivie d'un stade de re-équilibration en base de lithosphère. Une déformation fragile tardive affecte les lherzolites et les gabbros. On distingue dans les roches des fractures multiples remplies par de la serpentine et des carbonates, ainsi que des veines d'ophicalcite et des brèches carboniques.

Table 1

Microprobe analyses of Ol, Cpx, Opx and Sp from the Anyemaqen peridotite (AM-16) (Cameca microprobe, ‘Université Montpellier-2’, Montpellier, France), and a whole rock analysis (wet chemistry, VIMS, Moscow)

Analyses microsonde de Ol, Cpx, Opx et Sp de la péridotite d'Anyemaqen (AM 16) (microsonde Cameca, université Montpellier-2), et l'analyse de roche totale (VIMS, Moscou)

Sample	AM-16	AM-16	AM-16	AM-16	AM-16	AM-16	Sample	AM-16	Sample	AM-16
mineral	Ol-1	Ol-2	Cpx-1	Cpx-1	Opx-1	Opx-2	mineral	Sp	rock	peridotite
	relic	neoblast	core	rim	with	free of		core
					exsolution	exsolution
SiO2	41.28	41.27	53.41	54.27	54.86	55.78	SiO2	0.146	SiO2	42.60
TiO2	n.d.	n.d.	0.22	0.24	0.03	0.06	TiO2	0.100	TiO2	0.05
Al2O3	0.03	n.d.	3.86	1.96	5.50	4.41	Al2O3	50.032	Al2O3	1.79
Cr2O3	n.d.	n.d.	0.94	0.29	0.51	0.59	Cr2O3	17.614	FeO	5.46
Fe2O3	–	–	–	–	–	–	Fe2O3	1.787	Fe2O3	3.03
${\mathrm{FeO}}^{*}$	9.71	9.93	2.15	2.28	6.71	6.36	FeO	13.166	MnO	0.16
MnO	0.23	0.30	0.10	0.13	0.00	0.17	MnO	0.237	MgO	40.16
MgO	50.01	49.37	16.34	17.53	32.22	32.50	MgO	17.538	CaO	2.25
CaO	0.04	0.01	24.32	23.98	0.66	1.03	NiO	–	Na2O	0.06
Na2O	–	–	–	–	0.26	0.06			K2O	0.07
Total	101.28	100.87	101.34	100.67	100.75	100.96	Total	100.441	P2O5	0.01
							${\mathrm{FeO}}^{*}$	14.774	H2O	0.04
									LOI	4.68
	O=4		O=6		O=6			O=32
Si	1.032	1.038	1.929	1.971	1.926	1.950	Si	0.031	Total	100.36
Ti	–	–	0.006	0.007	0.001	0.002	Ti	0.016	CO2	0.15
Al	0.001	–	0.164	0.084	0.227	0.182	Al	12.653
Cr	–	–	0.027	0.008	0.014	0.016	Cr	2.988	Rb	5.000
Fe3+	–	–	–	–	–	–	Fe3+	0.288	Ba	46.000
${\mathrm{Fe}}^{2{+}_{*}}$	0.203	0.209	0.065	0.069	0.197	0.186	Fe2+	2.362	Sr	360.000
Mn	0.005	0.006	0.003	0.004	0.000	0.005	Mn	0.043	Cr	2216.200
Mg	1.862	1.849	0.879	0.948	1.685	1.692	Mg	5.607	Ni	175.000
Ca	0.001	0.000	0.940	0.933	0.025	0.039	Ni	–	Zn	8.000
Na	0.000	0.000	0.000	0.000	0.018	0.004
Z	1.032	1.038	2.099	2.061	2.154	2.133	A	8.012
XY	2.071	2.064	1.914	1.962	1.939	1.942	B	15.976
Wo			0.499	0.478	0.013	0.020	Cr	0.188
En			0.466	0.486	0.884	0.883	Al	0.794
Fs			0.034	0.036	0.103	0.097	Fe3+	0.018
Mg#	0.902	0.899	0.931	0.932	0.895	0.901	Mg#	0.704
							Cr#	0.191
							Fe#	0.018

Ophiolites in the Northeast Tibet are found in association with the Triassic Yushu–Jinshajiang River suture between the Qiangtang and the Songpan–Garze terranes, and with the Early Palaeozoic and Permo-Triassic Eastern Kunlun sutures, which separate the Songpan–Garze and the Kunlun (Qaidam) terranes (Fig. 1a). A crucial question for the geology of this region is: do sutures represent the remnants of the Palaeo-Tethyan Ocean?

Early studies in southern Tibet demonstrated that the Yarlung Tsangbo ophiolite belt could represent the suture zone between Eurasia and Gondwana [12]. However, palaeontological and stratigraphical evidence strongly suggest now that this boundary lies much further north. According to more recent studies, Gondwana-type sedimentary rocks with cold-water fossils such as Eurydesma and Lytrolasma occur much farther north of Tibet [31,32], even north of the Yushu–Jinshajiang River Suture, close to the south margin of Kunlun [30]. The same type of rocks also occurs in the easternmost part of the Tibetan Plateau [11,22]. The distribution of these sedimentary rocks suggests that most of the Tibetan Plateau was part of Gondwanaland and that the northern boundary between the Eurasian and Gondwana plates lies near the Kunlun Mountains.

Two sutures – the Early Palaeozoic Middle Kunlun suture and the Permo-Triassic South Kunlun (Anyemaqen) suture – are distinguished at the northern margin of the Tibetan Plateau in the eastern Kunlun area (Fig. 1a). The Middle Kunlun suture represents the oldest belt, and contains ophiolites of Cambrian age. These rocks crop out in the central part of eastern Kunlun, extend for at least a few hundred kilometres and are believed to reflect a small ocean basin [33]. South of it, the Anyemaqen suture extends nearly 1200 km and contains numerous ophiolitic blocks of Early Permian to Middle Triassic age and exotic Early Permian bioclastic limestone blocks in a matrix of the Triassic flysch [33].

The origin of the Anyemaqen ophiolites is debatable. Previously, the lavas from this suture were determined to be similar to within-plate tholeiites or plume-related mid-oceanic ridge basalts originated at a passive margin during Permian rifting [27]. The origin during a rifting event for Anyemaqen ophiolites has also been proposed by [24]. However, the later study has shown [33] that ophiolites from the Anyemaqen suture consist of volcanic rocks with geochemical affinities typical for the rocks originated at mid-oceanic ridge, oceanic islands and volcanic arcs. The belt was interpreted as the suture zone between Gondwana and Eurasia [33]. Obviously, the data concerning only volcanic rocks do not allow us to determine the nature of the ophiolites from the Anyemaqen suture and new data on the deeper parts of the ophiolitic section are needed.

We describe for the first time the chemical composition of the primary phases of peridotites sampled in the Anyemaqen suture.

The Anyemaqen suture trends to the east-southeast at 35°N latitude west of Xining, North Tibet (Fig. 1a). It is reactivated by the active Kunlun Fault, which separates the Kunlun Range from the Tibetan Plateau. The suture is characterized by the occurrence of radiolarites, pillow lava basalts, granodiorites, flaser gabbros, amphibolites and peridotites, which form individual bodies from less than 1 km² to 10 km². This association is typical of ophiolitic series found in the Alps or along the Iberia margin [1–3,19–21,23]. All these rocks, components of a dismembered ophiolitic sequence, crop out as imbricated tectonic lenses within the Kunlun fault zone. Blocks of neritic bioclastic Permian limestones, which represent marginal basin deposits, are also reported [6].

The studied geological section is located near the West Huashexia–South Tong Xi Tso Lake area (Fig. 1b). The series fall steeply to the north. The ophiolites are separated from Neogene sedimentary rocks to the south and from massive limestones to the north by two segments of the recent brittle Kunlun fault. The limestones contain benthic Foraminifera fossil remnants indicating internal platform environment, from which D. Vachard determined a Lower Permian age (Middle Asselian) Sphaeroshwagerina shamovi (1949) and Rugosofusulina pandae (1962), which have Angara affinity. The ophiolite sequence is composed mostly of peridotites and flaser gabbro-amphibolites. Lenses of ophicalcites, reddish argillites and radiolarites, locally recrystallized in quartzites, also occurred (Fig. 1b). Peridotites and gabbro-amphibolites are deformed into mylonites along narrow zones, which bound more preserved domains. The ophicalcites found among peridotites and gabbro-amphibolites are composed of magnesite tectonic breccias or magnesite-magnetite layered aggregates, which contain isolated crushed and euhedral Cr-spinel crystal relics and zones of Fe–Ni mineralisation.

A well-preserved sample (AM-16) of peridotite from the ophiolitic section was discovered and selected for a detailed study (Fig. 1b). The peridotite is exceptionally weakly altered and contains about 5% serpentine, as determined from the optical microscope. It is consistent with loss on ignition (LOI) value in the whole-rock analysis (Table 1). The modal composition of the peridotite is ⩽68% olivine (Ol), 25% orthopyroxene (Opx), ⩾5% residual clinopyroxene (Cpx) and 2% CrAl spinel (Sp), corresponding to transition between a clinopyroxene-rich harzburgite and a clinopyroxene-poor spinel lherzolite.

The texture of the peridotite AM-16 is mylonitic. The large elongated Opx porphyroclasts have a preferred alignment, which defines the foliation of the rock. The matrix of the peridotite is composed of fine-grained, dynamically recrystallized Ol with rare grains of Cpx and amoeboid interstitial grains of Cr–Al Sp, elongated parallel to the rock foliation.

Orthopyroxene porphyroclasts (up to 1 cm long) are characterized by a maximum elongation ratio about 1:3–1:5. They are abraded by deformation and are plastically deformed by (100) glide, displaying undulose extinction, kink structures and exsolution lamellae of Cpx (Fig. 2A). The shear planes in porphyroclasts are usually oblique to the foliation in the rock that indicates non-coaxial deformation (Fig. 2B). The Opx porphyroclasts are often flanked by a mantle of tapering, recrystallized grain aggregates, of both Opx and Ol, developed by high stress recrystallization (Fig. 2B). In other cases, the Opx porphyroclasts display traces of resorption as a result of interaction with a low-silica content melt or fluid: most of the porphyroclasts are irregularly shaped and clearly replaced by neoblasts of Ol (up to 80–130 μm) and black interstitial Sp, occurring at the edges of deformed porphyroclasts (Fig. 2C). The composition of Opx slightly differs in porphyroclasts from Wo_2.01En_88.29Fs_9.7 to Wo_1.31En_88.36Fs_10.3 (Table 1).

Clinopyroxene occurs as sub-euhedral or irregular residual porphyroclasts (190–380 μm and very rarely up to 600 μm). Most of them are internally deformed, displaying undulose extinction and exsolution lamellae. Like the Opx porphyroclasts, the Cpx grains are transformed at their periphery in neoblasts of Ol and Sp (Fig. 2D and E). The composition of Cpx in cores of the residual host grains differs slightly from that in its rims (Fig. 3d). Cpx has a composition Wo_49.92En_46.64Fs_3.4 in the cores and is characterized by higher contents of Al₂O₃, Cr₂O₃, CaO and slightly lower TiO₂ compared to Cpx in the rims (Wo_47.83En_48.62Fs_3.6) (Table 1). The decrease of Cr and Al contents in the rim compared to the core suggests subsolidus re-equilibration at lower temperature with formation of additional Sp in a closed system.

Olivine forms fine-grained groundmass of the peridotite and occurs as elongated grains (about 80 μm) or polycrystalline ribbons (170 μm, up to 600 μm) and as small dynamically recrystallized olivine neoblasts (about 20–30 μm). The Ol ribbons are elongated with aspect ratio varying from 3:1 to 7:1 and aligned parallel to the foliation defined by Opx and Sp. The relic Ol grains display subgrain boundaries and undulose extinction and replaced at the edges by the Ol neoblasts. The Ol composition varies from Fo₉₀ in relic grains to Fo₈₉ in neoblasts (Table 1).

Spinel occurs in two morphologies. One phase is represented by green–brown irregular amoeboid grains (up to 1 mm long) elongated parallel to the Opx and Ol ribbon foliations (Fig. 2F). The other phase of Sp is observed as smaller black interstitial grains (about 10 μm) dispersed in the olivine matrix (Fig. 2C, E and F). The composition of spinel elongated amoeboid grains in lherzolite is characterized by high Al₂O₃ contents (Table 1) and plots on a Cr# vs. Mg# diagram close to those from oceanic residual peridotites, determined by [9] as being of a ‘high-alumina type’ (Fig. 3b).

Secondary phases: two late systems of serpentine veins occur in the peridotites, parallel to or crosscutting the mylonitic foliation at 35–40° or 90°. Serpentine minerals are optically determined as chrysotile and lizardite, which are laced and streaked with dusty magnetite. The Opx and Cpx porphyroclasts crossed by serpentine veins are partially replaced by bastite lizardite (Fig. 2D). Sp grains are surrounded by inner rim of magnetite and outer rim of lizardite, which is usually spatially connected to the main phase of serpentine veins (Fig. 3F).

The textural, mineralogical and modal characteristics of studied peridotites from the Anyemaqen suture indicate their affinity with ultramafic tectonites, as defined by [26], whereas the typical features of magmatic cumulates in the peridotites are lacking. The peridotite belongs most likely to the upper mantle.

Observed mineral chemistry of the Anyemaqen spinel lherzolite shows relatively high abundances of the magmaphile elements Fe, Al, Ti in the main primary mineral phases. The exhumed mantle rock displays a moderate percentage of melting, which corresponds to less than 10% by comparison with the experimental studies on the Tinaquillo peridotite (Fig. 3c and d) [14]. Its composition mostly resembles that of the less depleted oceanic residual peridotites, collected in fracture zones or rift valley of slow spreading ridge segments (South-West Indian Ridge, Atlantic-Antarctic Ridge, Equatorial Mid-Atlantic Ridge) [7,15,16,28] and in the Galicia passive continental margin [10,13] (Fig. 3).

Mylonite texture, marked by plastically deformed Opx porphyroclasts, elongated green-brown Cr–Al Sp grains and Ol ribbons, indicates that this rock underwent a plastic shear deformation event. The obliquity of the shear plane in Opx porhyroclasts with respect to the foliation suggests that the deformation was achieved by non-coaxial shear flow. The significant difference in size of large Opx porphyroclasts and small olivine neoblasts in the matrix suggests that the deformation developed by plastic flow at low temperatures 800–1000 °C [25].

The Anyemaqen peridotite underwent a subsolidus reequilibration, which probably occurred during the ductile shear deformation and appears to be active after it, because both kinked Opx and Cpx porphyroclasts are resorbed at their edges by secondary Ol and small black Sp neoblasts. The mineral assemblage in the rock suggests that the reequilibration occurred under conditions of temperature and pressure appropriate to spinel peridotite facies. We therefore propose that a mineralogical reaction took place, consuming orthopyroxene and clinopyroxene, and formation of olivine + spinel ± melt [17,18].

Textural and chemical characteristics of residual peridotites from the Anyemaqen suture are consistent with two possible hypotheses for their origin. The mantle material could have been emplaced at a shallow depth during incipient rifting on the passive Asian margin, similar to peridotites reported from the Galicia passive margin [10,13] Alternatively, we suggest that the residual lherzolites represent the mantle material uplifted at slow spreading ridge setting in Palaeo-Tethyan Ocean, similar to peridotites occurring presently in the mid axial rift valley of the Equatorial Mid-Atlantic Ridge, South-West Indian Ridge, Atlantic-Antarctic Ridge [7,15,16,28]. Further researches are needed to make a definite choice between these two alternatives.