The Beimarang Mélange (Southern Tibet) Brings Additional Constraints in Assessing the Origin, Metamorphic Evolution and Obducti

Journal of Asian Earth Sciences 21 (2002) 307–322 www.elsevier.com/locate/jseaes

The Beimarang me´lange (southern Tibet) brings additional constraints in assessing the origin, metamorphic evolution and obduction processes of the Yarlung Zangbo ophiolite

Franc¸ois Huota,*,Re´jean He´bertb,Ve´ronika Varfalvya, Georges Beaudoinb, Chengshan Wangc, Zhifei Liud, Jo Cottena, Jaroslav Dostale

aUMR 6538 ‘Domaines Oceániques’, IUEM-UBO, Plouzane´ 29280, France bDe´partement de Geólogie et de Geńie Geólogique, Universite´ Laval, Sainte-Foy, Que., Canada G1K 7P4 cInstitute of Sedimentary Geology, Chengdu University of Technology, Chengdu, Sichuan 610059, People’s Republic of China dDepartment of Marine Geology and Geophysics, Tonji University, 1239 Siping Road, Shanghai 200012, People’s Republic of China eGeology Department, St Mary’s University, Halifax, NS, Canada B3H 3C3

Received 30 July 2001; revised 20 February 2002; accepted 25 April 2002

Abstract The Beimarang massif is one of many ophiolitic remnants which crop out discontinuously along the Yarlung Zangbo suture zone in southern Tibet. The southern contact of these remnants is marked by a highly sheared serpentinite me´lange made up of blocks of serpentinites, diabases, gabbros and composite blocks of serpentinites and mafic injections. The Beimarang me´lange has been investigated in order to bring additional constraints on the origin, metamorphic evolution and obduction processes of the Yarlung Zangbo ophiolite. Petrography and geochemical data suggest that the ultramafic components are similar to moderately depleted upper mantle peridotites. They may represent the already cooled and serpentinized Tethyan upper mantle which was trapped in a mantle wedge at the onset of a north- dipping Early Cretaceous intra-oceanic subduction zone located south of the Gangdese arc. These peridotites were then intruded by back-arc- like mafic magmas whose moderately depleted mantle source was affected by a subduction component. Ultramafic and mafic secondary mineral assemblages suggest that early low-(P/T ) metamorphic intra-oceanic conditions reached the amphibolite facies (.550 8C) before being retrograded down to the pumpellyite–prehnite facies (,280 8C). The Beimarang me´lange, interpreted as an obduction me´lange formed near a spreading ridge, was subjected to metamorphic conditions in the pumpellyite–prehnite facies which favored re-serpentinization of the peridotites and partial rodingitization of the mafic rocks. Unlike subduction-related me´langes and their associated lithological units found in the Ladakh area, we found no evidence of high-(P/T ) conditions in lithologies from the Beimarang me´lange. q 2002 Elsevier Science Ltd. All rights reserved.

Keywords: Ophiolites; Supra-subduction; Me´langes; Obduction; Tibet

1. Introduction ophiolitic massifs are the most common lithological assemblages. Although late backthrusting (Tapponnier The convergence between India and Eurasia, already et al., 1981a), strike-slip (Molnar and Tapponnier, 1975; active during the Middle Cretaceous, consumed the tethyan Alle`gre et al., 1984) and active east–west extensional oceanic domains beneath the Lhasa Block along one or (Tapponnier et al., 1981b) features are widespread along the many north-dipping subduction zones (Alle`gre et al., 1984; suture, it is believed that the Yarlung Zangbo ophiolites Aitchison et al., 2000). During Eocene times (Molnar and were ﬁrst obducted towards the south over India-related ` Tapponnier, 1975), the continental collision between these terranes (Gansser, 1974; Tapponnier et al., 1981a; Allegre et al., 1984). mega-plates developed the more than 1000-km long steeply Previous studies have shown the heterogeneity of the dipping Yarlung Zangbo suture zone (Fig. 1(a)) into which lithological units found along the suture zone. For instance, * Corresponding author. Tel.: þ33-1-418-656-2193; fax: þ33-1-418- we recall that the arc-related volcaniclastic assemblages 656-7339. (Honegger et al., 1982) and the subduction-related blue- E-mail address: [email protected] (F. Huot). schists (Honegger et al., 1989), spatially associated with the

Fig. 1. (a) Schematic tectonic map of the Himalayas, the Tibetan Plateau and surrounding areas showing the different crustal blocks separated by suture zones (modified after Coulon et al. (1986)). MBT, main boundary thrust; MCT, main central thrust; YZSZ, Yarlung Zangbo suture zone; (b) Geological map of the central portion of the Yarlung Zangbo suture zone (modified after Wang et al. (2000)) showing the location of the ophiolitic massifs discussed in the text. ophiolites, are present in Ladakh but have not been found in 2. Characteristics of the ophiolitic massifs of the Yarlung the central portion of the suture zone except for the Zedang Zangbo suture zone (central part) terrane interpreted by Aitchison et al. (2000) as an arc assemblage. Moreover, ages of the ophiolitic remnants are The Yarlung Zangbo ophiolite is located south of Xigaze quite different, being as old as 177 ^ 1 Ma for the Spontang and Lhasa along the middle part of the Yarlung Zangbo ophiolite in Ladakh (Pedersen et al., 2001) and as young as suture zone. This ophiolite is divided into spatially, 120 ^ 10 Ma for the Xigaze massif (Go¨pel et al., 1984)in lithologically and chemically distinct massifs named after the central part of the suture zone. the village located nearby. These massifs are, from west to The present work is part of a Sino-Canadian project east, Liuqu, Jiding, Beimarang, Qunrang, Baigang, Dazhuqu, initiated in 1998 and devoted to the Tibetan ophiolitic Zedang and Luobusa (Fig. 1(b)). In this paper we avoid the massifs located in the central part of the Yarlung Zangbo use of the term ‘Xigaze massif’, used previously in various suture zone between Liuqu and Luobusa (Fig. 1(b)). papers, and prefer its subdivision into the Beimarang and Previous works have already shown a large compositional Qunrang massifs. The reasons are that the Xigaze massif is diversity among the ophiolitic massifs in the study area heterogeneous and that it is located far from the town of (Girardeau et al., 1984, 1985a; He´bert et al., 2000). Here we Xigaze. The Liuqu, Beimarang, Qunrang and Dazhuqu present new observations and geochemical data on the massifs are considered to represent individual complete blocks and matrix of the Beimarang me´lange. Our aim is to ophiolitic sequences (Nicolas et al., 1981; Girardeau et al., better constrain the magmatic and metamorphic evolution of 1984) although they have a relatively thin crust (,3.5 km). the lithologies found in the me´lange in order to improve our All massifs have been vertically tilted by orogenic understanding on the geodynamic history of this portion of processes. From north to south, or top to bottom, these the Yarlung Zangbo suture zone. This evolution might be massifs are composed of a volcano-sedimentary sequence, a drastically different from that described from other parts of diabasic sill complex, and an upper mantle section. In the the suture zone (e.g. Ladakh area). area of the Beimarang and Qunrang massifs, interbedded F. Huot et al. / Journal of Asian Earth Sciences 21 (2002) 307–322 309 radiolarian cherts and lavas of the volcano-sedimentary truncated architecture caused by orogenic dismemberment sequence of the ophiolite are apparently stratigraphically that would have removed the gabbro section. overlain by the Upper Aptian–Lower Cenomanian rhyth- Early interpretations on the paleo-setting of the Yarlung mic flyschoid formation of the Xigaze Group (Tapponnier Zangbo ophiolite pointed towards a Tethys intra-oceanic et al., 1981a; Marcoux et al., 1982). However, more recent origin (Nicolas et al., 1981; Girardeau et al., 1985a,b). works suggest a faulted contact with the sedimentary rocks These authors based their arguments on (1) the petrofacies of the Xigaze Group (Alle`gre et al., 1984; Aitchison et al., of the mantle sequences such as the abundance of Cr 2000), a detrital sequence accumulated in a fore-arc basin diopside-rich harzburgites and lherzolites, and (2) the located south of the Gangdese arc (Alle`gre et al., 1984; abyssal tholeiitic nature of the mafic rocks. The preferred Girardeau et al., 1984). The volcanic rocks of the ophiolite site of formation of the ophiolite was probably related to a include almost exclusively massive flows and pillow lavas pull-apart basin located immediately south of Eurasia which whereas volcaniclastic products are nearly absent. At its was unrelated to a subduction zone (Girardeau et al., base, the volcanic unit is intruded by diabasic sills and 1985b). However, more complete chemical data presented progressively grades downwards into a sill complex here and in other works (He´bert et al., 2000, in preparation) (Girardeau et al., 1985a). Diabasic sills, and more rarely reveal that all massifs bear supra-subduction zone rather crosscutting dykes, form the main intrusive feature, whereas than typical MORB signatures. A subduction-related origin plutonic lithologies are rather scarce. Locally, isotropic was first set forth by Wu and Deng (1980) on the basis of gabbros and trondhjemites are observed as screens in major element chemistry. These oceanic remnants could between diabases. Mafic and ultramafic crustal plutonics are have formed in a neo-tethyan supra-subduction zone basin only present in small volume at Dazhuqu, Jiding and located south of the newly formed Gangdese arc, and into Baigang (He´bert et al., 2000). Isolated pockets of gabbros which the fore-arc sediments of the Xigaze Group would occur in the contact zone between the crustal and the upper have been later deposited. Furthermore, a lead isotopic mantle sections (Girardeau and Mercier, 1988). The upper study (Go¨pel et al., 1984) supports two separate origins for mantle sequence is well-exposed in the central part of the the geodynamic setting of the ‘Xigaze’ massif. These Yarlung Zangbo suture zone. The sequence is characterized authors concluded that the ophiolite was the product of a by the abundance of diabase and gabbro injections, and by propagating ridge (crustal sequence and diabases in the the relatively large volume of Cr diopside-rich harzburgites mantle) overprinting an interarc basin remnant (mantle and lherzolites compared to sensu stricto harzburgites and sequence). dunites more typical of supra-subduction zone ophiolites (Pearce et al., 1984). Girardeau et al. (1985a,b) published that serpentinization is common in the upper part of the 3. Field observations on the Beimarang me´lange mantle section but decreases downwards. However, recent investigation shows that the serpentinization overprint is not The Beimarang massif exposes, on its southern side, a so systematic and might be intense at the apparent base. nearly continuous serpentinite me´lange averaging 1 km in Geological and isotopic evidences suggest that the upper- thickness (Fig. 2). This me´lange stretches roughly east– level serpentinites formed during an oceanic hydration and west on both sides of the Beimarang valley but pinches out were later affected by meteoric fluids after the obduction on its western side. It is fault-bounded to the late Triassic– (Agrinier et al., 1988). According to Nicolas et al. (1981), Early Cretaceous red radiolarites and to the ophiolitic upper diabase injections are only rarely rodingitized as opposed to mantle. The latest movements on these two sub-vertical gabbro dykes. These observations suggest that gabbro dykes fault zones are related to backthrusting and dextral strike- were emplaced before the serpentinization and were slip. Backthrusting explains why some early north-dipping affected by the concomitant rodingitization. Then diabases low-angle thrust faults are now south-dipping giving an intruded the already serpentinized peridotites escaping the abnormal me´lange-over-mantle structural relationship rodingite alteration (Nicolas et al., 1981; Girardeau et al., (Fig. 2). 1985a). The lowermost part of the upper mantle is The matrix of the me´lange is exclusively composed of a juxtaposed to a highly sheared serpentinite me´lange (e.g. sheared and flaky serpentinite with frequent dextral and Beimarang me´lange) which is faulted against late Triassic to sinistral S-C fabrics. No sedimentary matrix of terrigenous Early Cretaceous red radiolarites and, locally, against the or calcareous nature has been observed. Shear zones are Late Cretaceous–Early Tertiary Liuqu conglomerate nearly vertical and are oriented in an east–west direction (Aitchison et al., 2000). The ophiolite obduction towards (Fig. 3(a)). We also observed, locally, that the schistosity of the south over India-related terranes is responsible for the the serpentinite matrix is folded along sub-horizontal axial development of tectonic me´langes at the base of some planes oriented at N1158/258SSW. The me´lange is zoned in ophiolitic massifs (Gansser, 1974; Girardeau et al., 1984). terms of the nature of blocks. Some areas are exclusively According to Nicolas et al. (1981) the actual lithological made up of ultramafic blocks while others also include sequence of their so-called Xigaze massif represents a mafic ones. The gradient of deformation also varies across preserved continuous ophiolitic section rather than a the me´lange exposure. The structural lowest part, i.e. that 310 F. Huot et al. / Journal of Asian Earth Sciences 21 (2002) 307–322

Fig. 2. (a) Geological map of the Yarlung Zangbo ophiolite in the area of Xigaze (modified after Girardeau et al. (1984)); (b) detailed geological map of the Beimarang massif showing the location of the sampled outcrops; (c) schematic cross-section showing the lithological units and the main structural features across the Beimarang massif. adjacent to the red radiolarites, is extremely deformed and orthopyroxene and spinel are crosscut by serpentinite shear contains abundant shearing and faulting structures with a zones. Dunites seem to be relatively common near the chaotic block-in-matrix aspect. Towards the upper mantle contact with diabase injections. Intrusive contacts with the section the me´lange shows decametric undeformed zones serpentinized peridotites are sharp and regular suggesting with preserved magmatic relationships. These zones are the peridotites had significantly cooled at the time of mafic made up of massive serpentinites and crosscutting diabases. injections. These dykes may be as wide as 5 m. The largest They are thought to represent large blocks of unsheared ones are generally coarser-grained and occasionally foliated upper mantle bounded by serpentinite shear zones. The towards their center. Preserved magmatic textures vary origin of the me´lange is purely tectonic and contrasts with between the aphanitic, porphyritic, diabasic and gabbroic that of me´langes such as the Kiogar nappe (Gansser, 1974). types, both in diabase injections and mafic clasts. When However, the Beimarang me´lange is quite similar to the present, phenocrysts are plagioclase and amphibolitized sheared serpentinite units reported in Robertson (2000) from clinopyroxene. Rodingitized dykes and blocks, recognized the Ladakh area into which arc-like diabases are found. by their whitish weathered surface, are observed throughout Massive serpentinized peridotites in both large slabs and the whole me´lange exposure. Some diabase injections are small blocks are generally rich in pyroxene pseudomorphs rooted either into large coarse-grained gabbroic pods as (15–25 vol%). The extent of serpentinization (,100%) wide as 150 m or into north–south trending dykes as prevents us from being certain that clinopyroxene was described by Nicolas et al. (1981) for diabases from the present in the majority of the samples but we believe that upper mantle section. In a few exposures, we observed that clinopyroxene-harzburgites account for a large proportion plagioclase is aligned parallel to the elongated gabbroic of peridotites found in the me´lange. Harzburgites and pods and reoriented in the direction of their branching dunites are also commonly found while lherzolites are rather dykes, which indicates a liquid-supported magmatic defor- rare. High-temperature foliations marked by stretching of mation. Metamorphic foliations defined by amphibole have F. Huot et al. / Journal of Asian Earth Sciences 21 (2002) 307–322 311

Fig. 3. Lower hemisphere projections showing poles to (a) serpentinite shear zones and (b) diabase contacts with the serpentinized peridotites. The average strike and dip of the shear zones is N0948/838S. N stands for the number of data. been observed as well. Measurements of the diabase spinel and pyroxene pseudomorphs. In most cases spinel has orientations are scattered, though a population tends to be undergone complex alterations transforming the limpid parallel to the me´lange shear zones (Fig. 3(b)). The phase into a porous and heterogeneous amalgamation of proposed average orientation (N0508/528NW) for injections silicate-bearing ferritchromite with SiO2 values reaching in the serpentinized peridotites of the Beimarang upper 28%. Anhedral spinel is the most common variety but mantle (Girardeau et al., 1985c) does not apply for diabases partially resorbed crystals are occasionally present in in the me´lange. Truncations of diabases by serpentinite clinopyroxene-harzburgites. The latter are often spatially shear zones are common and the offsets display right-lateral associated with pseudomorphs of orthopyroxene. When displacements. fresh, dark brown spinels are relatively Al-rich (Cr# 35–60) in clinopyroxene-harzburgite samples (Fig. 4(c)). Spinels from a dunitic sample yield more Cr-rich values (Cr# 70– 4. Mineral chemistry 74). By comparison with spinels from other Yarlung Zangbo massifs, those from the Beimarang me´lange show a 4.1. Analytical method chemical variation following the Luobusa trend defined by He´bert et al. (in preparation), and overlap the two The mineral chemistry was investigated using a compositional fields of spinels from the upper mantle of CAMECA SX-100 five-spectrometer electron microprobe the Beimarang massif. The Luobusa trend is mainly (Universite´ Laval, Sainte-Foy, Canada). Analytical con- ascribed to chemical variations caused by different degrees ditions were 15 kV, 20 nA with a counting time of 20 s on of apparent partial melting of the host peridotites. Spinels peaks and 10 s on background. Spinel, clinopyroxene, from He´bert et al. (in preparation), associated with the orthopyroxene, serpentine, plagioclase, amphibole and Luobusa trend, are characterized by their low-TiO2 contents prehnite compositions can be electronically transmitted (Fig. 4(d)) when compared with spinels from the Dazhuqu upon request to the first author. trend. Spinel compositions from this study, although plotting along the Luobusa trend, have both low and 4.2. Serpentinites relatively high-TiO2 contents. The latter, with TiO2 between 0.2 and 0.5%, have compositions superposed on the high- Both the me´lange matrix and the ultramafic blocks are TiO2 fields from Cannat et al. (1997) and He´bert et al. (in totally serpentinized independently from the extent of preparation). High-TiO2 spinels in peridotites have been deformation. Only one sampled ultramafic block still interpreted to be the result of magma–mantle interactions contains accessory clinopyroxenes whose compositions, (Cannat et al., 1997). typical of diopside, indicate re-equilibration temperatures Two types of pyroxene pseudomorphs transformed into less than 500 8C(Fig. 4(a)). Al2O3 contents in clinopyrox- bastite, a serpentine polymorph, are discriminated based on enes (Fig. 4(b)) are similar to the lowest values found in petrographic observations. Orthopyroxene pseudomorphs clinopyroxenes from moderately depleted abyssal perido- are larger, have higher refringence and generally finer tites (He´bert et al., 1990; Johnson et al., 1990) but are higher exsolution lamellae than those thought to be derived from than those from highly depleted fore-arc peridotites (Ishii clinopyroxenes (Fig. 5(a)). Moreover, orthopyroxene et al., 1992). Other primary phases such as olivine and pseudomorphs, with occasional kink-bands, tend to be orthopyroxene are transformed into serpentine polymorphs. aligned parallel to spinels whereas clinopyroxene pseudo- Magnetite is an ubiquitous phase associated with serpentine, morphs are rather xenomorphic crystals randomly oriented. 312 F. Huot et al. / Journal of Asian Earth Sciences 21 (2002) 307–322

Fig. 4. Mineral chemistry of ultramaﬁc rocks in the Beimarang me´lange. (a) Clinopyroxene compositions from a serpentinized clinopyroxene-harzburgite projected in the Di–En–Hd–Fs quadrilateral. Temperature curves (at 1 atm) are taken from Lindsley (1983) and the nomenclature is from Morimoto et al.

(1989); (b) TiO2 (wt%) vs Al2O3 (wt%) for clinopyroxenes from a serpentinized clinopyroxene-harzburgite. Comparative ﬁelds are for clinopyroxenes from abyssal (dotted, He´bert et al., 1990; stippled, Johnson et al., 1990) and fore-arc peridotites (continuous, Ishii et al., 1992). The large arrow shows compositional variations in clinopyroxenes with a progressive increase in the partial melting of a hypothetical host peridotite. The small arrow shows variations induced by the percolation of a tholeiitic magma in a peridotite; (c) 100 £ Cr/(Cr þ Al) vs 100 £ Mg/(Mg þ Fe2þ) for spinels from serpentinized peridotites and a diabase. Arrows indicate variations in spinel compositions following an apparent increasing partial melting (Luobusa Trend) and magma–mantle reactions

(Dazhuqu Trend) according to He´bert et al. (in preparation); (d) TiO2 vs 100 £ Cr/(Cr þ Al) for spinels from serpentinized peridotites and a diabase. The continuous ﬁeld outlines spinel compositions following the Luobusa Trend while the stippled ﬁeld is for those of the Dazhuqu Trend (He´bert et al., in preparation). The stippled arrow shows the compositional variations in spinels with increasing partial melting of a hypothetical host peridotite.

Bastite compositions are ambiguous in discriminating 4.3. Mafic blocks between the two pyroxenes because of similar values in major oxides. However, olivine-derived serpentine is easily All mafic lithologies experienced alteration processes chemically recognizable by its low Al2O3 (,0.6%), Cr2O3 severely overprinting primary mineral compositions. Rare (,0.15%) and high NiO (.0.28%) contents. relicts of igneous phases in the groundmass are clinopyr- From petrographic observations serpentine polymorphs oxene, plagioclase and ilmenite. A brownish type of include mesh-texture lizardite, fibrous chrysotile and platy amphibole might also be included among the primary antigorite. This latter polymorph is the most common. phases. Clinopyroxene and/or plagioclase are present as Antigorite mainly replaces olivine but is also associated phenocrysts in many samples. Metamorphic phases are with a late phase of veining in massive serpentinites. This mainly amphibole and prehnite but magnetite, sphene, serpentine polymorph is abundant in the sheared matrix of leucoxene, chlorite, epidote, calcite, quartz, and pumpellyite the me´lange too. Lizardite, after olivine, is uncommon and also occur as accessory phases. is generally replaced by antigorite. Lizardite is also present Plagioclase is generally randomly oriented and, together in the matrix of the me´lange. Chrysotile is found in thin and with clinopyroxene, displays sub-ophitic to intersertal early veins which are crosscut by larger antigorite veins. On magmatic textures. More rarely, laths of plagioclase are a chemical basis all serpentine phases have compositions aligned along a magmatic foliation. Fresh surfaces are which cluster around that of antigorite. Tremolite, talc, scarce because of abundant saussuritization. Plagioclases clinochlore, Al-bearing chrysotile and brucite seem to be have compositions ranging from almost pure albite to An68 absent from the secondary assemblage. for the whole mafic group but large variations also exist in F. Huot et al. / Journal of Asian Earth Sciences 21 (2002) 307–322 313

Fig. 5. (a) Bastitized orthopyroxene (high refringence) and clinopyroxene (low refringence) in a clinopyroxene-harzburgite (16A); (b) a brownish and limpid magnesiohornblende partially rims a clinopyroxene in an interstitial position with respect to plagioclase. The contact between clinopyroxene and amphibole is iv marked by a vermicular or spongy texture (06A); (c) a (Al ,TiO2)-rich brown magnesiohornblende, partially replaced by chlorite and epidote, is rimmed by a iv (Al ,TiO2)-poor green magnesiohornblende (14A); (d) radial prehnite has partially replaced foliated and recrystallized magnesiohornblende (06B). Cpx, clinopyroxene; Opx, orthopyroxene; Pl, plagioclase; Prh, prehnite. individual samples. Interstitial clinopyroxenes are colorless (Fig. 6(d)). Its composition approaches that of TiO2-rich and have compositions ranging from augite to diopside. magmatic pargasites reported by Girardeau et al. (1985a) Their compositional variations reflect sub-solidus reequili- from the Beimarang isotropic gabbros. bration (Fig. 6(a)). TiO2 (,0.8%), Cr2O3 (,1.7%) and Secondary amphiboles display several aspects and can be Al2O3 (2–6%) abundances in clinopyroxenes are in general grouped into three types. They replace other amphiboles, more similar to those of clinopyroxenes from back-arc basin primary clinopyroxene, volcanic glass and more rarely basalts although a group of data plots into the fields for plagioclase. The most abundant type is a fibrous and iv clinopyroxenes from island arc tholeiites, boninites and N- elongate actinolite (Al , 0.5; TiO2 , 0.5%) with greenish MORB (Fig. 6(b)). All fields are empirically drawn from colors. The two others are magnesiohornblendes which are data obtained on basalts collected in diverse modern-day optically distinguishable by their green and brown colors geodynamic settings. Despite the fact that overlapping do (Fig. 5(c)). The greenish one, developed from other exist between clinopyroxene compositions of the magmatic amphiboles, clinopyroxene or volcanic glass, is Aliv-poor series, such a diagram is useful in discriminating between (0.5–1.0) and TiO2-poor (,1.0%). The crystals are found end-member compositions such as those from island arc either surrounding brownish magnesiohornblende or form- tholeiites and N-MORB. Scattering of our data is reminis- ing individual recrystallized and variably foliated neoblasts. cent of the large array of magma compositions found in each Green magnesiohornblende, developed after plagioclase, is iv supra-subduction zone setting, and in particular that known rather Al -rich and TiO2-poor and define a different trend from back-arc regions (Hawkins and Melchior, 1985; (Fig. 6(c)). The brownish magnesiohornblende is Aliv-rich Hawkins and Allan, 1994). Only one crystal of spinels has (.1.0) and TiO2-rich (.1.0%). Clinozoisite and chlorite been observed in a diabase. It is a Fe–Cr-rich variety such commonly partially replace this brown amphibole. Pum- as a ferritchromite (Fig. 4(c)) with a low-TiO2 content (Fig. pellyite is observed on the fringe of actinolite and represents 4(d)). Among the four optically identified types of a late secondary phase. Veins of radiating Fe-poor prehnite amphiboles, one is thought to have a magmatic origin. crosscut all metamorphic fabrics. Accessory prehnite also This brownish and clear amphibole is sporadically found at replaces actinolite, pumpellyite, clinopyroxene and the rim of clinopyroxene and in an interstitial position with plagioclase. respect to plagioclase. The contact between clinopyroxene Five out of 12 analyzed mafic samples contain more than and amphibole is commonly marked by a vermicular or 50 modal% of prehnite and high amounts of associated spongy texture (Fig. 5(b)). This feature might be interpreted secondary amphiboles. Prehnite, a Ca-bearing mineral, is as evidence for late hydrous magmatic conditions of the often related to the specific metasomatism of metabasites crystallizing magma. This type of amphiboles is a TiO2-rich called rodingitization (Rice, 1983). Commonly associated (.1.0%) and Al2O3-rich (.1.0%) magnesiohornblende minerals such as diopside, grossular, tremolite and carbon- (Fig. 6(c)) which plots along the pargasitic substitution line ates are absent. Prehnite displays either a radial crystallizing 314 F. Huot et al. / Journal of Asian Earth Sciences 21 (2002) 307–322

Fig. 6. Mineral chemistry of maﬁc rocks in the Beimarang me´lange. (a) Clinopyroxene compositions projected in the Di–En–Hd–Fs quadrilateral.

Temperature curves (at 1 atm) are taken from Lindsley (1983) and the nomenclature is from Morimoto et al. (1989); (b) Al2O3 (wt%) vs TiO2 þ Cr2O3 (wt%) for clinopyroxenes. Fields are outlined from clinopyroxene compositions in boninites (van der Laan et al., 1992); island arc tholeiites and back-arc basin basalts (Hawkins and Allan, 1994); and N-MORB (Stakes and Franklin, 1994). The number on each contour line refers to the percentage of data outside the ﬁeld; (c) iv TiO2 (wt%) vs Al for amphiboles. The continuous arrow represents the temperature-dependant compositional variation of amphiboles replacing other amphiboles, clinopyroxene and volcanic glass while the stippled arrow indicates the trend for compositional variation of amphiboles replacing plagioclase and possibly garnet too. Subdivisions in terms of Aliv are from Leake et al. (1997); (d) Na þ K (c.f.u.) vs Aliv for amphiboles. Edenitic (Ed), pargasitic (Prg), and tschermakitic (Ts) substitutions are indicated along with an idealized ‘hornblende’ (Hbl) substitution (Hietanen, 1974). The ﬁeld is for Ti-rich magmatic pargasites from Girardeau et al. (1985a). growth (Fig. 5(d)) or an equant and allotriomorphic shape. It trace element contents. Detection limits for each element is not affected by deformation and always post-dates the are written in Table 2. For detailed technical procedures, the metamorphic foliation shown by the amphiboles. Garnet readers are referred to Cotten et al. (1995). Compositions was possibly present in one of the rodingitized sample (06B) used in the graphs have been normalized on an anhydrous but is now replaced by chlorite and minor amphiboles. The basis. The calculated Fe2O3 is assumed to represent 10% garnet composition is unknown but based on its replacement total iron analyzed as Fe2O3. Because of the plausible phases, it might have been a Fe-rich variety such as almandine. For clarity of the discussion, samples with Table 1 abundant prehnite (,50 modal%) will be referred to as Major oxides of the massive serpentinites from the Beimarang me´lange rodingites whereas the others represent the spilites. 05Aa 16A 16C 18B 18C

SiO2 38.30 38.35 39.00 39.35 39.00 5. Whole-rock chemistry TiO2 0.02 0.02 0.01 0.02 0.00 Al2O3 1.05 0.79 1.27 0.73 0.09 b Fe2O3 8.45 8.35 7.95 7.70 8.10 5.1. Analytical methods MnO 0.11 0.12 0.11 0.11 0.06 MgO 37.70 38.00 37.50 37.80 39.50 CaO 0.07 0.12 0.07 0.10 0.03 Whole-rock data for ultramaﬁc and maﬁc samples Na2O 0.01 0.04 0.01 0.02 0.00 reported in Tables 1 and 2, respectively, have been analyzed K2O 0.00 0.00 0.01 0.00 0.00 by ICP-AES (Universite´ de Bretagne Occidentale, Brest, P2O5 0.01 0.01 0.01 0.01 0.01 France). For major elements, the accuracy is plus or minus LOIc 13.88 13.86 14.06 13.81 13.10 0.001% for concentrations lower than 0.5% and the standard Total 99.60 99.66 100.00 99.65 99.89 deviations close to 2% for the others. For trace elements, the Notes: Oxides are in weight percent. relative standard deviations are less than or equal to 5% for a Sample number refers to that shown in Fig. 2. b concentrations higher than eight times the detection limits. Total Fe reported as Fe2O3. The latter is calculated on the basis of three sigma for low c Loss on ignition. F. Huot et al. / Journal of Asian Earth Sciences 21 (2002) 307–322 315

Table 2 Major and trace element analyses of the maﬁc rocks from the Beimarang me´lange

03Aa 03D 04A 06A 06B 14A 15A 16B 18A 20A 21A 21B DLb

SiO2 43.70 44.30 50.30 47.00 45.80 50.50 49.20 45.20 52.00 50.00 42.00 51.20 – TiO2 1.18 0.98 1.15 0.72 0.62 1.41 1.06 0.59 1.34 0.98 1.35 0.96 – Al2O3 15.25 16.35 15.85 15.25 15.20 15.85 15.75 15.38 15.85 15.40 17.60 16.80 – c Fe2O3 10.18 8.50 9.75 7.95 7.55 10.35 9.20 7.00 10.15 9.72 10.30 8.50 – MnO 0.17 0.14 0.16 0.13 0.13 0.17 0.15 0.12 0.17 0.16 0.17 0.14 – MgO 7.20 7.24 7.00 8.05 8.10 5.75 6.85 8.60 5.70 6.65 6.00 6.36 – CaO 18.00 18.40 9.40 16.90 19.00 8.95 12.44 19.00 8.40 12.40 18.00 9.50 –

Na2O 0.21 0.53 3.65 1.30 0.37 4.28 2.75 0.26 4.24 2.74 0.38 3.97 – K2O 0.04 0.10 0.35 0.17 0.04 0.17 0.19 0.03 0.28 0.18 0.04 0.40 – P2O5 0.11 0.10 0.12 0.07 0.07 0.11 0.11 0.07 0.12 0.09 0.13 0.09 – L.O.I.d 3.99 3.63 1.76 2.63 3.42 2.12 2.29 3.99 1.63 1.80 4.50 2.34 –

Total 100.03 100.27 99.49 100.17 100.30 99.66 99.99 100.24 99.88 100.12 100.47 100.26 Rb 0.4 1.1 2.3 1.4 0.7 1.5 1.6 0.3 2.5 1.3 0.6 2.9 – Sr 68 284 342 182 16 308 520 24 155 236 58 310 0.5 Ba 12 12 34 18 1 7 14 2 6 10 4 11 1.0 Sc 37 34 36 37 32 34 33 32 34 37 35 34 0.1 V 280 230 250 215 190 295 240 185 260 255 275 250 1.0 Cr 71 255 190 270 500 132 120 320 90 125 54 93 1.0 Co 36 36 38 37 33 32 34 35 28 35 32 30 1.0 Ni 39 82 76 90 130 50 60 120 38 55 148 58 1.0 Y 26.5 22.0 28.5 19.5 17.5 27.5 26.5 16.0 31.0 25.0 31.0 22.5 0.3 Zr 61 58 53 32 21 25 37 34 33 18 45 27 1.0 Nb 1.00 0.90 1.50 0.60 0.35 1.20 1.00 0.80 1.50 1.00 1.40 1.40 0.5 La 2.70 2.40 3.20 1.60 1.60 2.55 2.50 1.60 3.00 1.70 2.10 2.30 0.5 Ce 7.50 7.00 9.00 4.50 5.00 7.50 7.50 4.50 9.00 5.80 9.00 6.50 1.5 Nd 7.30 6.60 8.60 4.20 3.80 7.60 7.50 3.70 9.40 6.10 6.40 6.40 0.6 Sm 2.50 2.00 2.75 1.65 1.35 2.35 2.50 1.30 3.20 2.10 2.90 2.25 0.6 Eu 0.91 0.94 1.10 0.75 0.69 1.01 0.94 0.55 1.10 0.87 1.02 0.90 0.1 Gd 3.60 3.15 3.95 2.40 2.30 3.60 3.75 2.25 4.50 3.30 4.90 3.25 1.0 Dy 4.45 3.60 4.55 3.10 2.75 4.20 4.20 2.45 5.15 4.05 5.20 3.80 0.3 Er 2.70 2.20 2.80 2.00 1.80 2.75 2.60 1.60 3.10 2.50 3.10 2.25 0.6 Yb 2.61 2.13 2.66 1.92 1.70 2.70 2.54 1.57 2.91 2.36 3.00 2.17 0.1 Th 0.2 0.2 0.1 0.1 0.1 0.1 0.1 0.2 0.0 0.1 0.0 0.1 0.3

Notes: Oxides are in weight percent. Trace elements are in parts per million (ppm). a Sample number refers to that shown in Fig. 2. b Detection limit. c Total Fe reported as Fe 2O3. d Loss on ignition. existence of non-solubilized zirconium minerals prior to protoliths of harzburgite with probable clinopyroxene ICP-AES analytical determinations (J. Cotten, personal pseudomorphs and one dunite. This latter sample, totally communication), we decided to select a few samples to be devoid of pyroxene, was collected less than 1 m away from analyzed by the ICP-MS method (St Mary’s University, a diabase. Major oxides such as Al2O3 and CaO are useful in Halifax, Canada; Table 3). These samples were prepared via evaluating degrees of depletion of peridotites (Ishii et al., acid dissolution. To achieve a total digestion, a closed 1992; Pearce et al., 1992) and effects of metasomatism (Fig. beaker was used to allow the acid to reﬂux, thereby allowing 7). The relatively high percentages of pyroxene pseudo- a continuous exposure of the sample to the acid. Precision morphs in the serpentinites from the Beimarang me´lange, and accuracy values were generally within 5% (at 2 standard except for the dunite, explain the Al2O3 high values (0.85– deviations) for values greater than 10 times the detection 1.48 wt%). These values are as high as those in abyssal limits. These limits are reported in Table 3. Note that the peridotites and a lot higher than values in peridotites from sample numbers in the tables refer to the outcrop numbers in Mariana and Izu-Bonin fore-arcs. The abundance of Fig. 2(b). clinopyroxene pseudomorphs should also result in high CaO values. However, CaO is very low (,0.14 wt%) in our 5.2. Massive serpentinites samples. Because of high mobility of Ca we propose that this element was heavily leached from the serpentinites The ﬁve analyzed massive serpentinites include four whereas Al2O3 remained unchanged or only slightly 316 F. Huot et al. / Journal of Asian Earth Sciences 21 (2002) 307–322

Table 3 relatively immobile rare earth elements and other incompa- Trace element analyses of selected maﬁc rocks from the Beimarang tible trace elements measured by ICP-AES prevent us from me´lange (ICP-MS) discussing further the whole-rock chemistry of these 06Aa 14A 16B 18A 20A 21A DLb lithologies.

La 0.95 2.08 1.24 2.78 1.55 2.72 0.2 5.3. Mafic blocks Ce 3.59 7.04 4.02 9.62 5.65 9.52 0.2 Pr 0.70 1.26 0.72 1.71 1.06 1.74 0.2 Samples were selected from mafic blocks in the Nd 4.37 6.97 3.81 9.58 6.17 9.37 0.5 Sm 1.70 2.46 1.40 3.26 2.32 3.36 0.5 sheared serpentinite matrix and from dykes crosscutting Eu 0.70 0.97 0.55 1.13 0.87 1.13 0.2 the massive serpentinites found as large fragments in Gd 2.57 3.58 2.13 4.38 3.44 4.52 0.3 the me´lange. Graphs involving the mobile oxides, SiO2, Tb 0.46 0.64 0.37 0.76 0.61 0.80 0.5 CaO, and Na2O(Fig. 8(a)–(c)), clearly show that the Dy 3.14 4.23 2.48 5.21 4.10 5.18 0.3 composition of rodingites (empty circles) diverges from Er 2.05 2.84 1.60 3.32 2.71 3.30 0.05 Ho 0.68 0.94 0.54 1.11 0.89 1.14 0.2 that of spilites (other symbols). Moreover, H2Oisalso Tm 0.29 0.40 0.23 0.48 0.38 0.48 0.3 higher and K2O lower in rodingites when compared Yb 1.83 2.64 1.54 3.02 2.39 2.98 0.5 with spilites (not shown). Enrichments in CaO and H2O Lu 0.275 0.394 0.226 0.448 0.382 0.463 0.01 and depletions in SiO2,Na2OandK2O have already Y 18.23 25.18 14.79 30.41 24.35 30.33 0.05 been ascribed to rodingitization (Honorez and Kirst, Th ND 0.06 ND 0.07 ND 0.05 0.5 U ND 0.02 0.02 0.02 0.01 0.02 0.03 1975; Rice, 1983). Spilites show their usual increase in Zr 33.15 33.55 34.60 58.69 42.96 60.40 1.0 Na2O and decrease in CaO relative to unaltered oceanic Hf 1.09 1.32 0.98 1.83 1.35 2.01 0.05 basalts. On the other hand, the relatively immobile Nb 0.26 0.83 0.47 0.99 0.56 0.80 0.5 TiO2 displays only one trend for both types of Rb 1.10 1.15 ND 2.51 1.01 ND – alteration (Fig. 8(d)). Increasing values of TiO along Sr 191.45 324.50 23.29 162.22 309.25 58.23 – 2 Cs 0.02 0.06 ND 0.16 0.10 0.01 – the liquid line of descent plot in a range intermediate between compositions for island arc tholeiites and back- Notes: All analyses are in parts per million (ppm). arc basin basalts. These values are significantly lower a Sample number refers to that shown in Fig. 2. b Detection limit. than those for the N-MORB low-pressure fractionation trend. For comparisons with the mafic lithologies of the lowered. This interpretation is in agreement with the Beimarang ophiolitic massif we also plotted analyses of conclusion of He´bert et al. (1990) who showed that Al2O3 diabases and basalts from Girardeau et al. (1985a) and is only slightly lowered in pyroxene-derived serpentine. He´bert (unpublished data). The majority of these CaO, on the other hand, is virtually absent in serpentine analyses plots along the trend defined by the samples derived from olivine, clinopyroxene and orthopyroxene. of this study. Extremely low CaO content may explain the absence of Trace elements plotted in a multi-element diagram secondary tremolite and carbonates in our samples. normalized to primitive mantle (Fig. 9) show abun- Compositions of relatively fresh peridotites from the dances lower than or equal to N-MORB values from Beimarang upper mantle (He´bert, unpublished data) plotted Sun and McDonough (1989). All samples show the in Fig. 7 might reflect pre-serpentinization approximate same general depleted pattern in terms of light rare- compositions of our samples. Extremely low abundances in earth elements with a flat shape from the middle to heavy rare-earth elements. The relative degree of fractionation deduced from Mg#s (Fig. 8)isalso coherent with respect to elemental abundances, i.e. the lower the Mg#, the higher the elemental abundances, suggesting derivation from a single mantle source. By comparison with neighboring elements, strong anomalies in large ion lithophile elements (Rb, Ba, K), Sr and Zr are striking. Smaller negative anomalies in Nb and Ti also occur. Spilitized diabases are typically considerably enriched in large ion lithophile elements and Sr.

Fig. 7. Al2O3 (wt%) vs CaO (wt%) for serpentinized peridotites from the Rodingites are, however, usually strongly depleted in Beimarang me´lange and upper mantle. Samples from this study are these elements. Nb, Zr and Ti form negative anomalies clinopyroxene-harzburgites (ﬁlled circles) and a dunite (open circle). Data in almost all samples no matter the type of alteration. from the Beimarang upper mantle peridotites (ﬁlled diamonds) are from Basalt and diabase samples from the Beimarang He´bert (unpublished data). Fields for abyssal and fore-arc peridotites ´ together with the trend for increasing partial melting are taken from Pearce massif (Hebert, unpublished data) display similar et al. (1992). The stippled arrow represents the suggested trend for elemental abundances and anomalies when compared metasomatism related to serpentinization. with our data. Though negative Zr anomalies do exist in F. Huot et al. / Journal of Asian Earth Sciences 21 (2002) 307–322 317

Fig. 8. (a) CaO, (b) SiO2, (c) Na2O and (d) TiO2 (wt%) vs Mg# for mafic rocks from the Beimarang me´lange. These graphs also include data for mafic rocks from the Beimarang ophiolitic massif taken from Girardeau et al. (1985a) and He´bert (unpublished data). Gray fields show the composition of rodingites. In graph (d) the arrows represent the chemical evolution of different magmatic series taken from Huot et al. (submitted for publication). mafic lithologies from the Beimarang massif, we are sample, we noticed that ratios from the former method concerned with the very large anomalies presented in were either lower or equal to the ratios of the latter. Fig. 9 for samples collected in the me´lange, and Despite these differences negative anomalies do exist analyzed by ICP-AES. By comparing Zr/Zrp ratios since Zr/Zrp ratios are almost systematically lower than p (where Zr ¼ðNd þ SmÞPMnorm=2)obtainedbyboth one. We conclude that Zr did precipitate into the form ICP-AES and ICP-MS methods for each selected of ZrO prior to ICP-AES analytical determinations

Fig. 9. Multi-element patterns normalized to the primitive mantle for mafic rocks from the Beimarang me´lange. The gray field outlines the compositional range for mafic rocks from the Beimarang ophiolitic massif (He´bert, unpublished data). The pattern with squares refers to N-MORB (Sun and McDonough, 1989). Numbers indicate the range in Mg#s for the mafic rocks from the me´lange. Normalizing values are from Sun and McDonough (1989). Symbols as in Fig. 8. 318 F. Huot et al. / Journal of Asian Earth Sciences 21 (2002) 307–322 whichcausedthelargenegativeanomaliesshownin and Frey (1994), diopside is the Ca-bearing phase (CMSH Fig. 9. system) stable below 520 and 500 8C along the kyanite and sillimanite geotherms, respectively, in the absence of CO2 (Curve #3). Moreover, Burkhard (1993) proposed that 6. Discussion porous heterogeneous spinels, similar to those in our samples, are related to serpentinization within the pumpel- 6.1. Mineral assemblages and P–T history lyite–prehnite facies. Late chrysotile locally observed in the massive serpentinites and in the sheared matrix may be Metamorphic overprinting is widespread but is incom- coeval with this low-temperature alteration (curve #1). plete both in ultramafic and mafic lithologies. We will first Attributing a range of P–T conditions from the studied discuss the ultramafic-related mineral parageneses and then ultramafic rocks of the me´lange is highly hazardous. proceed with those from the mafic rocks. All curve numbers Parageneses indicate that conditions might have evolved mentioned in the following text refer to Fig. 10 which from the greenschists down to the pumpellyite–prehnite represents a composite P–T diagram mixing reactions for facies. However, the lack of geobarometers in these facies both ultramafic and mafic rocks. precludes any interpretation of the prevailing surrounding We previously mentioned that the different serpentine pressures at the time of serpentinization. polymorphs all had an antigorite composition. Antigorite is From petrographical observations on spilitized mafic the most stable phase under P–T conditions accompanying components of the me´lange, we deduce that the TiO2-rich serpentinization of ultramafic rocks. Its stability domain brownish magnesiohornblende (Aliv . 1.0) was the first extends from 250 to 550 8C at relatively low pressure in the metamorphic phase to crystallize. Indeed, this type of MSH system (Bucher and Frey, 1994). Above 550 8C amphibole is both partially replaced by clinozoisite and antigorite is unstable and is replaced by forsterite and talc chlorite, and rimmed by a TiO2-poor green magnesiohorn- (Curve #4). Below 250 8C antigorite is transformed into blende (Aliv , 1.0). This paragenesis suggests a water- chrysotile (Curve #1). Partial re-equilibration of diopside controlled retrograde alteration. Coexisting Ca-plagioclase has been influenced by metamorphic temperatures lower and albite occur in these samples together with clinozoisite than 500 8C as is shown in Fig. 4(a). According to Bucher and chlorite which suggest a decrease in temperature from

Fig. 10. Composite P–T diagram mixing reactions for both ultramafic and mafic rocks from the Beimarang me´lange. Stippled lines with numbers in circles ¼ define stability fields for mineral assemblages of ultramafic rocks in a CMASH system at PH2O Ptotal: These stable assemblages are written in between parentheses. Continuous lines with numbers in diamonds are for mineral assemblages in mafic rocks. Metamorphic facies are separated by thick gray lines; BS, blueschist facies; Pmp–Prh, pumpellyite–prehnite facies; GS, greenschist facies; Amp, amphibolite facies. Geotherms and curve reactions are all from Bucher and Frey (1994) with the following exceptions. The curve for maximum temperature plagioclase solvus is drawn from data given by Maruyama et al. (1982). The dashed area is the greenschist–amphibolite transition zone from Maruyama et al. (1983). The ‘chlorite-in/out’ curve is from Liou et al. (1974). Curves for the wet and dry solidus are from Yoder and Tilley (1962). The black arrow indicates the evolution of the low-(P/T ) intra-oceanic retrograde metamorphism recorded in the upper mantle peridotites and mafic rocks before the tectonic dismemberment. P–T conditions during the me´lange formation are represented by the empty star. Ab, albite; Act, actinolite; Atg, antigorite; Ath, anthophyllite; Brc, brucite; Chl, chlorite; Ctl, chrysotile; Di, diopside; En, enstatite; Ep, epidote; Fo, forsterite; Gln, glaucophane; Jd, jadeite; Lmt, laumontite; Lws, lawsonite; Pg, paragonite; Pl, plagioclase; Pmp, pumpellyite; Prh, prehnite; Qtz, quartz; Tlc, talc; Tr, tremolite; Zo, zoisite. F. Huot et al. / Journal of Asian Earth Sciences 21 (2002) 307–322 319 the amphibolite facies down to the greenschist–amphibolite during this late-stage metamorphism. Overall, the succes- transition zone (dashed zone in Fig. 10) defined by Liou et al. sive replacement of secondary phases in the spilites suggests (1974) and Maruyama et al. (1983). The upper stability limit a retrograde low-(P/T ) oceanic-type metamorphism. In of this transition zone is about 450–550 8C for the low- and other words, these rocks probably recorded the waning medium-(P/T ) conditions, respectively. According to Liou thermal regime of an oceanic ridge. et al. (1974) and Liou and Ernst (1979), the presence of Rodingitized diabases and gabbros include abundant transitional greenschist–amphibolite assemblages devel- prehnite found both in veins and as a post-deformational oped in rocks of basaltic composition is indicative of very secondary phase partially to completely replacing the low-(P/T ) metamorphism. The absence of high-pressure mineralogy of spilites. An equant green magnesio- minerals (Curves 13, 14 and 15) such as glaucophane, hornblende is often associated with prehnite though a few lawsonite and garnet or that of their pseudomorphs samples still preserve the metamorphic TiO2-rich brownish precludes any P–T conditions along the jadeite þ quartz- magnesiohornblende. The absence of common Ca-bearing and the kyanite-geotherms. It rather constrains the overall silicates typically found in rodingites such as grossular, me´lange setting to a relatively low-(P/T ) environment. diopside, tremolite and carbonates is a consequence of the These conditions contrast with those deduced from the once prevailing temperatures together with the low CO2 blueschist assemblages existing along the suture zone in the activity of the fluid. Surrounding conditions below 280 8C Ladakh region (9–11 kbar; Honegger et al., 1989). More- and 3 kbar (Curve #11) are necessary for the formation of over, secondary amphiboles analyzed in our samples are all prehnite at the expense of actinolite and zoisite (Bucher and Na-poor (Nac.f.u. , 0.6) whereas those from the blueschists Frey, 1994). in the Ladakh area are Na-rich (1.3 , Nac.f.u. , 2.0; Honegger et al., 1989). One diabase sample, displaying a 6.2. A scenario of magmatic, metamorphic and possible garnet pseudomorph, might be the only indication deformational events of medium-(P/T ) conditions. However, this hypothetical occurrence of garnet is dependant on the whole-rock Pervasive deformation and metamorphism of the lithol- composition of mafic rocks. If garnet ever formed the ogies in the me´lange strongly obliterate primary magmatic condition of its appearance are constrained at about 500 8C features. Nevertheless, a few preserved areas allow to and 5 kbar following a kyanite-type geothermal gradient retrace its intra-oceanic pre-deformational magmatic his- (Bucher and Frey, 1994). All other samples are thought to be tory. As an oceanic upper mantle lithosphere, peridotites of the result of low-(P/T ) metamorphism; that is below the the Beimarang massif were intruded by mafic magmas (Fig. kyanite-geotherm. Early overall pressure is estimated to 11). The peridotites were already cooled (400–500 8C) at have been below 4 kbar with temperatures exceeding the initiation of diabase injections and, according to Nicolas 550 8C over which chlorite is unstable (Liou et al., 1974; et al. (1981) and Girardeau et al. (1985a,b), they had already Bucher and Frey, 1994). The transitional state was reached undergone serpentinization processes. The high proportion when temperatures were lowered to 450 8C at 3 kbar of clinopyroxene-harzburgites over harzburgites and (Maruyama et al., 1983; Bucher and Frey, 1994). Under dunites, the abundance of pyroxenes (15–25%), and the these P–T conditions, the solvus of the plagioclase presence of accessory clinopyroxene (,2%) are all (Maruyama et al., 1982) was reached which caused the petrographic characteristics consistent with moderately coexistence of Ca-plagioclase and albite. Subsequent depleted abyssal peridotites. Moreover, clinopyroxene and parageneses include the partial replacement of equant spinel chemistries and Al2O3 contents in the serpentinites green TiO2-poor magnesiohornblende and Ca-plagioclase indicate apparent degrees of depletion intermediate between by fibrous actinolite, albite, sphene, chlorite and rare those of abyssal and fore-arc peridotites. Dunite might epidote. Such mineral products are typical of the greenschist represent the result of extreme mantle depletions based on facies under low-(P/T ) conditions found in the oceanic the chemistry of its spinels. Dissolution of pyroxene in the metamorphism (Liou and Ernst, 1979). The rarity of wall-rock peridotites caused by ascending migration of carbonates suggests that hydrothermal fluids were CO2- tholeiitic melts (Varfalvy et al., 1996) is an alternative poor. The last non-veining metamorphic replacement is explanation for the origin of that dunite which is in contact shown by fibrous actinolite locally overprinted by amor- with a diabase. On the other hand, high values of TiO2 in phous pumpellyite (Curve #12). Abundant prehnite-bearing clinopyroxenes and spinels together with the presence of veins in spilites, into which brucite fibers have been undeformed clinopyroxene pseudormorphs in clinopyroxene- described, crosscut all metamorphic assemblages including harzburgites are probably the results of melt impregnation actinolite and pumpellyite. These veins were formed during (Varfalvy et al., 1996; Cannat et al., 1997) related to the the latest metamorphic event in the spilites which occurred mafic magmatic event. So, both phase dissolution and melt at a very low-grade metamorphism with temperatures and impregnation phenomena could be preserved in the pressures below 280 8C and 3 kbar (Curve #11). Once more serpentinized peridotites of the me´lange. the presence of pumpellyite and prehnite, and the absence of This study also supports a single event for the injections high-pressure minerals preclude any high-(P/T ) conditions of mafic magmas. These melts were probably derived from a 320 F. Huot et al. / Journal of Asian Earth Sciences 21 (2002) 307–322

Fig. 11. Simplified sketch of the geodynamic setting south of the Gangdese arc during Early Cretaceous. The thick black line with arrows shows the tectonic dismemberment of the upper mantle near the spreading center. The rectangle represents the preferred original site of the lithologies preserved as the Beimarang me´lange. single mantle source based on TiO2 variations and the (Nicolas et al., 1981; Girardeau et al., 1985a). However, smooth increase of rare-earth element abundances along according to Aitchison et al. (2000), vestiges of an Early with the low-pressure fractional crystallization. Such a Cretaceous intra-oceanic arc, composed of lava flows and conclusion is in disagreement with observations made by volcaniclastics with island arc tholeiite affinities, exist in the Nicolas et al. (1981) which stated that the rodingitization Zedang area. event temporally separates gabbro from diabase injections. Once solidified, the mineralogy of the diabases was Clinopyroxene compositions together with significant affected by sub-solidus re-equilibration and metamorphism. negative anomalies in Nb and Zr, small negative anomalies The succession of secondary mineral assemblages is typical in Ti and variably enriched large ion lithophile elements for of phase replacements under progressively decreasing spilitized mafic rocks are suitable with tholeiitic magmas temperatures. Pressure-indicating minerals are absent whose moderately depleted mantle source was affected by a except for a possible pseudomorph of garnet for which the subduction component (Pearce et al., 1984, 1992; Hawkins initial composition is unknown. We suggest that the and Allan, 1994). When compared with modern-day prograde metamorphic overprint was the result of a high oceanic settings, diabases from the me´lange more closely geothermal gradient related to the spreading ridge. Early resemble back-arc basin basalts with strong arc-related metamorphic temperatures, in excess of 550 8C, certainly characteristics. The occurrence of TiO2-rich magnesiohorn- caused partial dehydration of the host-rock serpentinized blende which probably has a magmatic origin is also in peridotites in which serpentine breakdown usually begins at agreement with a supra-subduction zone origin for the 350 8C(Raleigh and Paterson, 1965). Retrograde reactions diabases, a setting known to be richer in volatiles than mid- are related to metamorphism in an intra-oceanic domain oceanic ridges (Hawkins and Melchior, 1985). under relatively constant low pressures in the supra- The proposed model for the magmatic evolution of the subduction zone setting. Yarlung Zangbo ophiolite (Fig. 11) as deduced from the We believe that initiation of the decoupling occurred in Beimarang me´lange involves at least two distinct stages the vicinity of the supra-subduction zone spreading ridge, separated in time by the onset of a north-dipping Early and probably as proposed by Girardeau et al. (1985c) along Cretaceous intra-oceanic subduction. This subduction led to thrust faults initiated on previously existing transform the trapping of the already cooled, perhaps serpentinized, faults. Such a decoupling might be associated with the moderately depleted abyssal peridotites in the mantle development of a dynamothermal amphibolite sole wedge. These ultramafic rocks, dated at 404 Ma (Go¨pel (Nicolas, 1989). Blocks of garnet-bearing amphibolites et al., 1984), might be the vestiges of a Tethyan lithosphere found in the me´lange near Bainang might represent such a which was later intruded by mafic magmas with back-arc sole (Nicolas et al., 1981). Further thrusting of the ophiolite affinities. In the Ladakh region, highly sheared serpentinites onto the India-related units evolved into an obduction crosscut by arc-related diabases and gabbros are known. me´lange, herein referred to as the Beimarang me´lange, in According to Robertson (2000) such lithologies represent which serpentinized peridotites accommodated compressive the tectonically disrupted oceanic substratum of the stresses in developing a mechanically produced matrix. structurally juxtaposed Cretaceous oceanic Dras arc com- Metamorphic overprint related to the tectonic dismember- plex. In the central portion of the suture zone, such arc ment of the Beimarang upper mantle was probably limited assemblages are not well developed. Crustal sections are to extensive re-serpentinization of peridotites and con- more similar to those associated with slow-spreading ridges comitant partial rodingitization of spilitized mafic rocks in F. Huot et al. / Journal of Asian Earth Sciences 21 (2002) 307–322 321 the pumpellyite–prehnite facies. These conditions contrast (NSERC/Grant no. 1253) for financial support of the Tibet with those deduced from the blueschist assemblages found in project. The first author also thanks the NSERC (scholarship subduction-related me´langes and associated lithological units PGS B) and FCAR for financial support. We are also from the Ladakh area. Such a late serpentinization over- thankful to M. Choquette for microprobe analyses. Critical printing, occurring after the initial decoupling, has been reviews by K. Burke, K. Stu¨we, J.C. Vannay, L. Webb and proposed by Girardeau and Mercier (1988) and Agrinier et al. R.C. Maury were appreciated. (1988). Later collisional rotational movements, concomitant with late strike-slip faulting, are thought to be the cause of the scattered diabase orientations in the me´lange. References

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