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 Oce´aniques’, IUEM-UBO, Plouzane´ 29280, France bDe´partement de Ge´ologie et de Ge´nie Ge´ologique, 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 1367-9120/02/$ - see front matter q 2002 Elsevier Science Ltd. All rights reserved. PII: S1367-9120(02)00053-6 308 F. Huot et al. / Journal of Asian Earth Sciences 21 (2002) 307–322 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.