Petrology and geochemistry of the Saga and Sangsang ophiolitic massifs, Yarlung Zangbo Suture Zone,...

Lithos 113 (2009) 48–67

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Petrology and geochemistry of the Saga and Sangsang ophiolitic massifs, YarlungZangbo Suture Zone, Southern Tibet: Evidence for an arc–back-arc origin

É. Bédard a,⁎, R. Hébert a, C. Guilmette a, G. Lesage a, C.S. Wang b, J. Dostal c

a Département de Géologie et de Génie Géologique, Université Laval, Québec, Qc., Canada G1K 7P4b Research Center for Tibetan Plateau Geology, China University of Geosciences, 29 Xueyuan Road, Haidian District, 100083 Beijing, Chinac Department of Geology, Saint Mary's University, Halifax, NS, Canada B3H 3C3

⁎ Corresponding author. Tel.: +1 418 656 2131 # 491E-mail addresses: [email protected] (É. Béda

[email protected] (R. Hébert), [email protected] (J. Dostal).

0024-4937/$ – see front matter © 2009 Elsevier B.V. Adoi:10.1016/j.lithos.2009.01.011

a b s t r a c t
a r t i c l e i n f o
Article history:Received 3 June 2008Accepted 26 January 2009Available online 8 February 2009

Keywords:OphiolitesYarlung Zangbo Suture ZoneGeochemistryMantle refertilization

The Saga and Sangsang ophiolites are located about 600 and 450 km west of Lhasa and represent a westernextention of the central portion of the Yarlung Zangbo Suture Zone (YZSZ) ophiolite belt. The Saga massifcomprises fresh mantle lherzolite and cpx-harzburgite, an ophiolite mélange (±amphibolite), metamor-phosed mafic crustal rocks (meta-gabbro, meta-basalts and amphibolites) and a sequence of uppermostcrustal rocks (chert, basaltic lavas, diabase sills and dikes). The Sangsang ophiolite consists of an ophiolitemélange (harzburgite) and upper mantle harzburgite with minor lavas and gabbro.Peridotites from both massifs show variable degrees of serpentinization. Their Mg# varies between 0.89 and0.91. All peridotites show distinct flat REE (rare earth elements) patterns with La/YbN ratios close to 1,probably indicative of a refertilized mantle. The Olivine-Spinel equilibrium and the spinel chemistry for theSaga (Cr#~0.10–0.22) and Sangsang (Cr#~0.30–0.55) peridotites suggest that the Saga peridotites have adeeper mantle provenance (N20 kbar) and have undergone lower degrees of partial melting (5–12%) thanthe Sangsang peridotites (b15 kbar; 17–30%). The composition of the Saga peridotites is similar to thecomposition of pre-oceanic peridotites while peridotites from the Sangsang massif resemble abyssal andsubduction-related peridotites. Mafic rocks from both ophiolites have basalt and basaltic andesitecompositions. They are slightly depleted in light REE with respect to the heavy REE, with La/YbN between0.5 and 0.8 and with a small negative Ta-Nb anomaly suggesting the presence of a subduction component.The abundances of incompatible elements in these mafic rocks are similar to N-MORB (mid-ocean ridgebasalt) or back-arc-basin basalts (BABB).Our data suggest that the Saga and Sangsang ophiolites belong to an intraoceanic suprasubduction zone segmentas postulated for other ophioliticmassifs in the eastern portion of the YZSZ. However, the geochemistry ofmantlerocks from these two massifs is different compared to each other (fertile vs. refractory) and compared to otherYZSZ ophiolites suggesting different petrogenetic histories. Field relationships and geochemical data furthermoresuggest that the Saga and Sangsang ophiolites were formed in a complex arc–back-arc setting where at least twosubducting slabs must have been active. This study places onemore piece in the YZSZ puzzle and lead to a betterunderstanding of the morphology of the convergence zone before the final stage of collision which led to thepresent configuration of the suture zone.

© 2009 Elsevier B.V. All rights reserved.

1. Introduction

The E–W-trending Yarlung Zangbo Suture Zone (YZSZ) ophiolitebelt is a thin and discontinuous section of oceanic lithosphereassociated with deep sea sediments and mélanges (Wu and Deng,1980; Nicolas et al., 1981; Tapponnier et al., 1981a,b; Allègre et al.,1984; Girardeau et al., 1985a,b). This Late Jurassic to Early Cretaceousophiolite represents a remnant of the Tethys ocean floor. The Tethyan

7; fax: +1 418 656 7339.rd),@ulaval.ca (C. Guilmette),

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oceanic lithosphere was probably destroyed along two differentsubduction zones: one andean-type (Allègre et al., 1984; Aitchisonet al., 2000; McDermid et al., 2002) and the other intraoceanic(Hébert et al., 1999, 2000, 2001, 2003; Aitchison et al., 2000; Huot etal., 2002; Varfalvy et al., 2002; Dubois-Coté et al., 2005; Dupuis et al.,2005a,b; 2006; Guilmette, 2005) leading to the Eocene collisionbetween the Indian terrane to the south and the Eurasian plate to thenorth (Molnar and Tapponnier, 1975). The YZSZ ophiolites areheterogeneous relicts of various sections of the complex Neo-Tethyanoceanic domain and most have intraoceanic suprasubduction zoneaffinities. These remnants were obducted towards the south over theIndian passive margin during the earliest stage of collision, around theCretaceous–Tertiary time boundary (Tapponnier et al., 1981a,b;Allègre et al., 1984; Burg et al., 1987).

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Fig. 1. Tectonic division of the Tibetan plateau KSZ, Kokoxili Suture Zone; BNSZ, Bangong Nujiang Suture Zone; YZSZ, Yarlung Zangbo Suture Zone; NB, Namche Barwa; EHS, EasternHimalayan Syntaxis; ISZ, Indus Suture Zone.

49É. Bédard et al. / Lithos 113 (2009) 48–67

Since 1998, our team has been studying the mineralogy, geochem-istry andgeochronologyof a numberof ophioliticmassifs along theYZSZin order to reassess the geodynamic and petrogenetic significance ofthese Tethyan oceanic crust remnants. Seven massifs were studied,covering a more than 300 km long portion of the YZSZ, between Buma(Angren) to thewest and Zedong to the east (Fig.1). Themost importantresult of these studies is a confirmation that most ophiolites wereformed in an oceanic marginal basin overlying a suprasubduction zoneas first proposed by Miyashiro in 1973 (Hébert et al., 1999, 2000, 2001,2003; Aitchison et al., 2000; Huot et al., 2002; Varfalvy et al., 2002;Dubois-Coté et al., 2005; Dupuis et al., 2005a,b, 2006). Petrographic and

Fig. 2. Schematic geologic map of south central Tibet modified from Ding et al. (2005). GCTYZMT, Yarlung Zangbo Mantle thrust; ZGT, Zhongba-Gyangze thrust.

geochemical data show that the easternmost massifs were generated inan intraoceanic arc setting (a fore-arc setting for Luobusa ophiolite)while the westernmost massifs have back-arc basin affinity (Dubois-Coté et al., 2005). Unfortunately, there are few available data for theophiolite complexes east or west of the central portion of the YZSZ.

The ophiolitic massifs under study are located near the villages ofSaga and Sangsang, respectively about 350 and 200 km west of theXigaze area ophiolites (Fig. 2). Except for the publication of Ding et al.(2005) which contains a geological map of the Saga area, there was upto now no known study in western literature on the Saga andSangsang ophiolites. Therefore, this study will allow to fill the gap

, Great Counter thrust; GT, Gangdese thrust; STDS, south Tibetan detachment system;

50 É. Bédard et al. / Lithos 113 (2009) 48–67

between the well-known Xigaze ophiolites to the east and the Ladakharea ophiolites to the west. The aims of this work are therefore toidentify the geological, petrographic and geochemical characteristicsof these two ophiolitic massifs and their associated rocks and tocompare these characteristics to those of the massifs to the east inorder to ascertain whether they are all related petrogenetically andgeochronologically. Also, this work brings new ideas to the study ofophiolite petrogenesis in general, and especially to the study of mantleprocesses, since relatively rare geochemical features are observed forthe Saga massif mantle rocks.

2. Geological setting

2.1. Regional geology

The YZSZ is the youngest and southernmost suture across theTibetan Plateau (Fig.1). This suture is related to the development of theHimalaya–Tibet orogen as well as to the uplift of the Tibetan Plateau,during the accretion of India to the Lhasa terrane in the Late Cretaceousand Early Tertiary (~70–40Ma; Gansser 1974; Molnar and Tapponnier,1975). This collision was accompanied by the progressive destructionof the Tethyan oceanic floor while the Neo-Tethys marginal basin wastrapped between India and Eurasia through a complex configuration ofsubduction zone systems (Aitchison et al., 2000; Wang et al., 2000;Dubois-Coté et al., 2005; Mahéo et al., 2004). The Tethys Ocean wasformed by the rifting along the southernmargin of the Lhasa terrane inthe Early Triassic marking the initial separation of Gondwanaland(including India) from Laurasia (and Lhasa terrane) (Gaetani andGarzanti, 1991). The creation of the Neo-Tethyan marginal basin isrelated to the existence of Late Jurassic–Cretaceous intraoceanicsubduction zones within the Tethys Ocean which led to the formationand opening of suprasubduction intraoceanic arcs and back-arc basinsclose to themargin of the Lhasa terrane (Zhou et al., 1996; Aitchison etal., 2000; Hébert et al., 2000, 2001, 2003; McDermid et al., 2000, 2001,2002; Huot et al., 2002; Dubois-Coté et al., 2005; Dupuis et al., 2005a,b,2006). During the India-Asia collision, Neo-Tethyan ophiolites wereobducted toward the south onto the Indian basement and then back-thrusted northward (Tapponnier et al., 1981a; Allègre et al., 1984; Burget al., 1987). The ophiolites were also affected by late strike–slip andeast–west extension. These deformational events led to the partialdismemberment of the ophiolitic massifs. Three main tectonicdomains outcrop along the YZSZ (Burg et al., 1987; Hodges, 2000;Dupuis et al., 2005a). These domains are delineated by major thrustfaults and are, from north to south, the active paleomargin, the oceanicdomain and the passive Indian paleomargin. In order to compare thegeology of the Saga and Sangsang massifs to other YZ ophiolites, themain characteristics of these three domains are briefly summarizedbelow. A more detailed description is given by Burg et al. (1987),Hodges (2000) and Dupuis et al. (2005a).

North of the YZSZ, the active paleomargin (i.e. the Lhasa terrane) iscomposed of calc-alkaline igneous rocks and sedimentary sequences(Nicolas et al., 1981; Allègre et al., 1984). Most of the southernmostpart of Lhasa terrane (Allègre et al., 1984) is made up of the Gangdeseor Transhimalaya batholithwhile diverse volcanic belts (i.e. Linzizong;110–80 Ma and 60–40 Ma) are found north and south of the terrane(Coulon et al., 1986). This magmatism is related to the subduction ofNeo-Tethys oceanic lithosphere beneath the Lhasa terrane. New dataindicate that two major stages of calc-alkaline magmatism areassociated with the growth of the Gangdese continental arc; thefirst stage spreading took place between 103 and 80 Ma (probably asearly as 188.1±1.4 Ma (Chu et al., 2006)) and the second between 65and 46 Ma (Chung et al., 2007). The Late Cretaceous Xigaze Groupsiliciclastic turbidites, interpreted to be forearc basin sediments(Einsele et al., 1994; Dürr, 1996; Wang et al., 2000), accumulatedalong the southern margin of the Gangdese and were incorporated tothe YZSZ during the collision (Nicolas et al., 1981). The Xigaze

sediments overlie the ophiolitic sequence with either a stratigraphic(Girardeau et al., 1984; Girardeau and Mercier, 1988) or faultedcontact (Aitchison et al., 2000). Since the Cenozoic, several events ofeast–west extension associated with deformational and partialmelting episodes affected the Lhasa terrane (Tapponnier et al.,1981a; Mo et al., 2006). Numerous intrusions resulted from post-collisonal magmatic activity (45 Ma–present) which occurred withinthe whole Tibetan Plateau (Mo et al., 2006). Some Miocene intrusionswere found to be cutting the ophiolite at Saga and Sangsang (Hébert etal., 2007a,b).

Typically the oceanic domain is composed of a disrupted ophiolitebelt resting on a dynamothermal sole and/or an ophiolitic mélange(Nicolas et al., 1981). These ophiolitic sequences are often underlainby sedimentary and volcanic oceanic-floor rocks of Tethys oceanorigin. Diverse ages were obtained for the YZSZ ophiolitic massifsdepending on their location along the suture zone. Plagiogranitesfrom the crustal section of the Xigaze area ophiolite gave an U/Pbformation age of 120±10Ma (Göpel et al., 1984) while a zircon from apegmatitic gabbro yielded an U/Pb age of 132.0±2.9 Ma (Chan et al.,2007). Zyabrev et al. (1999) also reported similar ages (125–110 Ma)for radiolarites overlaying the volcanic rocks. Hornblende from abasaltic dike crosscutting the Yungbwa ophiolite, located 1000 kmto the west, yielded an 40Ar/39Ar age of 152±33 and 123 Ma (Milleret al., 2003). The 123 Ma age for this ophiolite was confirmed byChan et al. (2007; U/Pb zircon age; 123.4±0.9 Ma and 123.9±0.9 Ma). West of the Jinlu area, zircon and amphibole from crustalrocks from the Zedong terrane gave U/Pb and 40Ar/39Ar ages of about152–156 Ma (McDermid et al., 2002). Chan et al. (2007) also obtainednew U/Pb ages on zircon for Luobusa (diabase; 149.7±3.4 Ma and150.0±5.0 Ma), Dangxiong (gabbro; 126.7±0.4 Ma and 123.4±0.8 Ma) and Kiogar ophiolites (gabbro; 159.7±0.5 Ma). A Pb modelage of 404 Ma obtained on clinopyroxene (cpx) from harzburgites ofthe Xigaze ophiolite suggested that the contact between the crustalsection and the mantle is tectonic so that the crust and the mantle arenot genetically related (Göpel et al., 1984). However, this age hasnever been reassessed.

Southward, the ophiolitic massifs are underlain by an ophioliticmélange which is typically composed of cm- to km-sized blockssurrounded by a highly sheared serpentine matrix (Nicolas et al.,1981). Blocks within the mélange are mainly serpentine, diabase andgabbro fragments though sedimentary blocks have also been identified.In addition, highly foliated (±garnet) amphibolite blockswere found indifferent areas within the suture zone (Guilmette and Hébert, 2003;Guilmette, 2005; Guilmette et al., 2005, 2008). 40Ar/39Ar ages of 123–128 Ma were obtained from these blocks by Guilmette et al. (2005,2008) indicating that they were produced contemporaneously to theformation of the overlying ophiolitic crust during the birth of a newsubduction zone in a Neo-Tethyan back-arc basin.

To the south, the ophiolitic mélange rests on a tectonic mélangeknown as the Yamdrock mélange (Searle et al., 1987; Dupuis et al.,2005b) or the Bainang terrane (Aitchison et al., 2000). This mélange isan imbricate thrust sheet zone containing several distinct thrust slicesin which an ocean floor stratigraphy is preserved (Aitchison et al.,2000). The northern part of the Yamdrockmélange is composed of redsiliceous shales, radiolarian cherts and minor alkaline basalts whilethe southern part is made up of fine-grained, thinly bedded deepmarine shales (Chang, 1984; Aitchison et al., 2000; Dupuis et al.,2005a,b). Dupuis et al. (2005b) suggested that the source for thesesediments was the Indian passive margin. Zhu et al. (2008) reportedoccurrences of intraplate mafic volcanics dated at 144.7±2.4 Ma fromthe Indian margin, which are similar to the alkaline basalt blocks ofthe Yamdrock mélange. The structural style of the Yamdrock mélangeresembles rock sequences typically found in subduction complexesassociated with intraoceanic arc systems (Aitchison et al., 2000)so that the imbricate thrust slices were probably off-scrapedfrom a subducting oceanic slab (Chang 1984; Ziabrev et al., 2001).

Fig. 3. Simplified geologic map of the Saga Area.


South of the oceanic domain, the Indian passive paleomargindomain includes three major tectonic units (from north to south): acontinental margin turbidite sequence, carbonate flysch and a plat-form sequence. These units are Permian–Cretaceous deposits ofprogressively deeper water thrusted southward over India duringEarly Paleocene times (Burg and Chen 1984; Burg et al., 1987; Liu andEinsele, 1996, 1999). Based on geochemical affinities the source forthese sediments is believed to be a passive margin (Dupuis et al.,2006).

Fig. 4. Simplified geologic ma

2.2. Saga ophiolitic massif

The Saga massif is a 25 km-long body of an incomplete ophioliticsequence within the YZSZ (Fig. 3). From north to south, the massifcomprises fresh mantle tectonite, an ophiolite mélange, metamor-phosedmafic upper crustal rocks and a sequence of uppermost crustalrocks. The mantle section only outcrops in the western part of themassif (~1.5 km thick) and is mostly composed of lherzolite withminor cpx-harzburgite. To the south of the mantle section the

p of the Sangsang Area.

Fig. 5. Microphotographs of Saga (A–B–C) and Sangsang (D–E–F) massif samples. A) Lherzolite with brownish-green spinel, B) Brecciated and altered lava from the uppermostcrustal unit, C) Amphibolite from the metamorphosed upper crustal unit, D) Harzburgite with red spinel, E) Mafic lavas, F) Diabase with an ophitic texture.


ophiolite mélange crops out throughout the whole massif with amaximum thickness of 2 km and less than 1 km in the western andeastern parts, respectively. Lherzolite, dunite as well as localamphibolite blocks where identified within the mélange. Underlyingthe ophiolite mélange, the metamorphosed upper crust (~2–2.5 kmthick) is composed of meta-gabbro and meta-basalts in addition toamphibolites as the more metamorphosed end-member. The upper-most crustal unit is 2–2.5 km thick and is composed of ocean floorrocks such as green and red chert, basaltic pillowed, massive orfragmented lavas as well as diabase sills and dikes. It is not certainwhether the uppermost crustal unit and to some extent, themetamorphosed mafic upper crust, could be compared to theYamdrock mélange of other YZSZ ophiolites. Middle Mioceneshoshonitic intrusions (trachyandesite and trachydacite) crosscuttingeither the mantle tectonite or the ophiolite mélange have also beenidentified (not shown on Fig. 3; Hébert et al., 2007a,b).

The upper section of the ophiolite is overlain by an ultramaficconglomerate (i.e. rounded peridotite clasts with a carbonate matrix)and chromite-bearing sandstone. Considering that the conglomerate

only includes peridotite clasts and is deformed along with theophiolite, it does not appear to belong to the Xigaze Group which isfound along the northern contact of the Saga massif. Spinel-bearingsandstones are also found in the upper section of the ophiolite. To thesouth, the Saga massif is overlying passive margin Indian sedimentsand igneous rocks. Ar/Ar ages obtained on hornblende separates frommeta-gabbro/diabase and amphibolite from the metamorphosedoceanic upper crust unit and the ophiolite mélange (Guilmette et al.,2008) indicate that a major metamorphic event occurred within theNeo-Tethys basin between 123.3±1.1 Ma and 128.8±1.4 Ma. Similarages from amphibolite within an ophiolitic mélange have beenreported from the easternmost ophiolitic massifs (Guilmette andHébert, 2003; Guilmette et al., 2005, 2007; Chan et al., 2007).

2.3. Sangsang ophiolitic massif

The Sangsang massif is 100 km long and seems to be the easternextension of the Saga massif and/or the western extension of theBuma ophiolite. The Sangsang massif consists mainly of an ophiolite

Fig. 6. Mineralogical and textural evidence of melt-rock interactions. A) Orthopyroxene (Opx) partly replaced by secondary clinopyroxene (Cpx), B) Embayment into Opx crystalfilled with secondary olivine showing a symplectite-like texture with secondary Opx, C) Intergranular extensions of spinel between adjacent olivine, D) Opx and spinel symplectite.


mélange and mantle tectonite (Fig. 4). Like the Saga massif, theophiolite mélange crops out throughout the whole massif with amaximum thickness of about 5 km in the westernmost part. It ismostly composed of harzburgite blocks. It is also typically intruded bygabbro and diabase plugs which are rodingitized. To the north, mantletectonite is only found in the central part of the massif where an up to4 km thick section of harzburgitic rocks outcrops. In the centralsegment of the Sangsang massif, a thin slice (b1 km) of gabbro/diabase and lavas is adjoining themantle tectonite. In thewestern partof the Sangsang massif, chert beds occur in the ophiolite mélange.These chert layers are more than 15 km long and appear to becontinuous within the mélange. Occurrences of sandstones were alsoobserved in the westernmost and easternmost extremities of themassif. Several Miocene xenolith-bearing trachyandesite and trachy-dacite intrusions were also identified within the Sangsang ophiolite(Hébert et al., 2007a,b). Unlike the Saga massif or other YZSZophiolites, no MORB-type ocean floor rocks are found to the southof the ophiolite mélange of the Sangsang massif. To the north, theSangsang massif is in faulted contact with the Xigaze Group while tothe south the massif is underlain by the Indian margin Triassic flysch.

3. Petrography

3.1. Saga ophiolite

Sagamassif lherzolites and clinopyroxeneharzburgites are freshwithusually less than 15% secondary serpentine (Fig. 5A), which occurs asveinlets or is locally pervasive. Clinopyroxene crystals (5–30%) are eitherprismatic phenocrysts (up to 3 mm in size) or occur as irregularinterstitial grains (~0.5 mm). Some clinopyroxene crystals are alsoassociated with orthopyroxene in orthopyroxenitic aggregates. Theyshow undulatory extinction and can contain fine orthopyroxeneexsolution lamellae. Orthopyroxene (10–75% in websterite) is gener-ally ~2–5 mm in size, but it can reach up to 8 mm. Orthopyroxene

phenocrysts aredominantamong theSagaophiolite peridotites althoughsome grains form orthopyroxenitic aggregates (~10 mm in size). Theymayalso contain exsolutions of clinopyroxene and are typically in spatialassociation with spinel. Some clinopyroxenes and orthopyroxenes areslightly folded or kinked and can also include small grains of olivine.Peridotites contain up to 70% of fresh olivine crystals (0.5–4 mm). Somegrains showa slight undulatory extinctionwhile othersmight be smallerrecrystallized crystals. Spinel, the content ofwhich ranges from0.5 to 2%,is usually brownish-green, although some spinels tend to be redder andmore opaque. Spinel grain size varies from 0.5 mm to 2 mm and thelarger spinel seems to bemore euhedral than the smaller ones,which areslightly vermicular. Minor secondary chlorite has also been observed asan alteration mineral after pyroxene.

Numerous mineralogical and textural characteristics indicative ofreplacement and/or re-equilibration have been identifiedwithin severalsamples. These textures include (1) orthopyroxene partly replaced bysecondary clinopyroxene (Fig. 6A), (2) embayments of orthopyroxenecrystals by secondary olivine which locally forms a symplectite-liketexture with secondary orthopyroxene (Fig. 6B), (3) intergranularextensions of spinel between adjacent olivine (Fig. 6C), (4) pseudo-morphs of altered pyroxene surrounded by relatively fresh olivine and(5) orthopyroxene and spinel symplectite (Fig. 6D). Thesemineralogicaland textural characterisitics have been identified as evidence of melt-rock interactions (Hellebrand et al., 2002; Seyler et al., 2007; Tamuraet al., 2008).

Lava samples from the uppermost crustal unit are typicallybrecciated and strongly altered (Fig. 5B). It is difficult to determineif these rocks are either altered lavas or mafic crystal tuff. They arecomposed of angular to subangular clasts (20–30%) of 0.5 to 1 mm insize, which are mainly pyroxene, quartz and plagioclase crystals.However, most of the pyroxene clasts are partially or completelyaltered to a secondary green amphibole (~15%). Up to 15% ofinterstitial plagioclase can be found in these samples, which areremarkably clear and thus seem to be recrystallized. They are

Table 1ARepresentative microprobe analyses of Saga massif UM minerals (FeO(T): total Fe as FeO; FeT: total Fe).

Olivine Lherz. Lherz. Cpx-Har. Spinel Lherz. Cpx-Har. Harz. Cpx Cpx-Har. Lherz. Harz. Opx Cpx-Har. Lherz. Ortho.

06SA68CR 5

06SA84CV5

06SA42B1

06SA68CR 4

06SA50AV2

07SA23AR1

06SA42V4

06SA52R2

07SA23AN3

06SA53BR3

06SA87N3

06SA71R1

SiO2 40.90 40.79 40.72 SiO2 0.01 0.03 0.03 SiO2 52.70 52.46 51.79 SiO2 54.55 54.29 56.75TiO2 0.00 0.00 0.00 TiO2 0.02 0.09 0.01 TiO2 0.21 0.21 0.03 TiO2 0.09 0.05 0.05Al2O3 0.01 0.01 0.00 Al2O3 55.09 31.21 41.98 Al2O3 3.55 3.93 4.05 Al2O3 4.89 5.30 1.56Cr2O3 0.06 0.03 0.00 Cr2O3 12.43 33.57 24.95 Cr2O3 0.60 0.42 1.20 Cr2O3 0.70 0.55 0.55MgO 50.04 49.73 50.14 Fe2O3 1.76 3.95 2.57 FeO(T) 2.36 2.64 2.40 FeO(T) 6.18 6.29 5.52CaO 0.01 0.02 0.00 MgO 19.41 12.61 15.61 MnO 0.04 0.10 0.06 MnO 0.11 0.12 0.13MnO 0.09 0.18 0.16 CaO 0.00 0.01 0.02 MgO 17.19 17.68 16.66 MgO 33.24 32.77 34.98FeO 9.18 9.48 9.23 MnO 0.00 0.00 0.00 CaO 22.41 21.43 23.29 CaO 0.62 0.53 0.59CoO 0.06 0.00 0.12 FeO 10.41 16.90 14.16 Na2O 0.50 0.63 0.07 Na2O 0.02 0.03 0.00NiO 0.41 0.31 0.28 CoO 0.05 0.05 0.07 K2O 0.00 0.00 0.01 K2O 0.00 0.00 0.00Total 100.75 100.53 100.66 NiO 0.32 0.08 0.15 NiO 0.09 0.02 0.05 NiO 0.11 0.10 0.18Si 0.99 0.99 0.99 ZnO 0.15 0.21 0.36 Total 99.63 99.51 99.58 Total 100.52 100.03 100.32Ti 0.00 0.00 0.00 Na2O 0.00 0.00 0.00 Si 1.92 1.91 1.89 Si 1.88 1.88 1.95Al 0.00 0.00 0.00 Total 99.65 98.69 99.94 Aliv 0.08 0.09 0.11 Aliv 0.12 0.12 0.05Cr 0.00 0.00 0.00 Si 0.00 0.01 0.01 Alvi 0.07 0.08 0.07 Alvi 0.08 0.09 0.01Mg 1.81 1.81 1.82 Ti 0.00 0.02 0.00 Fe3+ 0.03 0.05 0.01 Fe3+ 0.03 0.02 0.03Ca 0.00 0.00 0.00 Al 13.65 8.86 11.12 Cr 0.02 0.01 0.03 Cr 0.02 0.01 0.01Mn 0.00 0.00 0.00 Cr 2.07 6.40 4.43 Ti 0.01 0.01 0.00 Ti 0.00 0.00 0.00Fe 0.19 0.19 0.19 Fe3+ 0.28 0.72 0.44 Fe2+ 0.05 0.03 0.06 Fe2+ 0.14 0.16 0.13Co 0.00 0.00 0.00 Mg 6.08 4.53 5.23 Mn 0.00 0.00 0.00 Mn 0.00 0.00 0.00Ni 0.01 0.01 0.01 Ca 0.00 0.00 0.01 Mg 0.93 0.96 0.91 Mg 1.71 1.69 1.79Mg# 0.91 0.90 0.91 Mn 0.00 0.00 0.00 Ca 0.87 0.84 0.91 Ca 0.02 0.02 0.02Fo 90.69 90.35 90.63 Fe2+ 1.83 3.40 2.66 Na 0.04 0.04 0.00 Na 0.00 0.00 0.00Fa 9.31 9.65 9.37 Co 0.01 0.01 0.01 K 0.00 0.00 0.00 K 0.00 0.00 0.00

Ni 0.05 0.02 0.03 Wo 45.64 43.47 48.01 Wo 1.19 1.03 1.10Zn 0.02 0.04 0.06 En 48.72 49.90 47.79 En 89.27 89.09 90.69Na 0.00 0.00 0.00 Fs 3.81 4.32 3.95 Fs 9.45 9.76 8.21Mg# (Fe2+) 0.77 0.57 0.66 Ac 1.83 2.30 0.25 Ac 0.08 0.12 0.00Mg# (FeT) 0.74 0.52 0.63 Mg# (Fe2+) 0.95 0.97 0.94 Mg# (Fe2+) 0.92 0.91 0.93Cr# 0.13 0.42 0.29 Mg# (FeT) 0.93 0.92 0.93 Mg# (FeT) 0.91 0.90 0.92


associated with a greenish-brown (silicified?) matrix (30%) withminor chlorite (10%). Quartz, carbonate, chlorite as well as prehniteveinlets (~5–10%) were also identified in several samples. The lavasamples from the passive margin Indian sediments and igneous rocksare identical to the lava samples from the uppermost crust. In fact,these two units are only distinguishable using the whole rockchemistry data.

Diabase from the uppermost crustal unit is composed of up to 35%of prismatic plagioclase (0.1–0.2 mm) associated with up to 6% ofclinopyroxene and orthopyroxene relics (0.1–0.8 mm). Most of thepyroxenes were completely chloritized (30%) and chlorite is nowinterstitial to plagioclase crystals. Up to 10% of actinolite needles (withminor epidote) have also been identified. A significant proportion(15%) of carbonate veinlets (±quartz) crosscut the diabase sample.

Amphibolites from the metamorphosed mafic upper crustal unitare very similar to one another (Fig. 5C). They typically containbetween 20 and 40% of cloudy plagioclase (recrystallized?) partially tocompletely altered to an albite–prehnite assemblage. The average sizeof plagioclase crystals is usually between 0.05 and 0.1 mm but grainsup to 1 mm have been observed. Plagioclase is associated withmetamorphic green hornblende (30–50%) which forms a distinctivefoliation in many samples. Hornblende seems to be interstitial to theplagioclase and occurs as crystals of 0.1 to 0.2 mm in size. Somecrystals are also larger than 1 mm or form large aggregates of smallerhornblende grains. In some amphibolite samples, hornblende crystalsare not completely recrystallized and vestiges of igneous maficminerals are distinguishable. Prehnite, quartz and chlorite±epidoteveinlets (5–15%) are also common in the amphibolite samples. Minoroxides and titanite were also observed.

The composition of sandstones from the Saga ophiolite is diverse.Typically, they contain subangular to subrounded clasts up to 1 cm indiameter. Clasts are mainly quartz (10–40%), sedimentary rocks (10–60%), actinolitized or chloritized pyroxene or amphibole pseudo-morphs and altered mafic to intermediate volcanics (5–30%). Clasts of

radiolarian chert (up to 50%) were also identified in two samples.Spinel fragments (b1%) have been identified in all sandstone samples.In general, the quartz clasts are deformed and show slightlyundulatory extinction. Clasts within the sandstones are either floatingor self-supporting in a siliceous matrix with minor chlorite/carbonate.

3.2. Sangsang ophiolite

Compared to Saga peridotites, the Sangsang ophiolite harzburgites(and clinopyroxene harzburgites) aremore affected by serpentinizationwith an average serpentine content of 25 to 30% (Fig. 5D). Serpentiniza-tion also appears to bemore pervasive close to faults or shear zones. Therocks contain no more than 2 to 5% of clinopyroxene crystals which areb2 mm in size and are interstitial. Orthopyroxene (15–30%) occurs aslarger (5–10 mm) phenocrysts or smaller (1–2 mm) interstitial grains.Some also form orthopyroxenitic aggregates. Most of the orthopyroxeneis slightly or completely altered to serpentine, chlorite or actinolite.Modal proportions of fresh olivine vary from 5 to 75%. Olivine grainshave an average size of 2mmbut can be up to 10mm. Spinel (1–2.5%) inthese rocks is dark red-brown to opaque, and the more opaque spinelsare moderately to completely altered into silicate mineral assemblages.Once again, larger crystals (2 mm) are euhedral while smaller grains(0.5 mm) are vermicular. Magnesian aragonite veinlets and prehniteveins have also been observed in some samples.

Mafic to intermediate lavas from the Sangsang ophiolitic massifare typically very fine-grained and show an interstitial microlitictexture characterized by clinopyroxene (±actinolitized or chlori-tized; 15–30%), microlites of plagioclase (0.1–0.2 mm; 40–60%)slightly to moderately replaced by an albite-prehnite assemblageand secondary interstitial chlorite (5–15%) as well as minor oxides(~2–10%; Fig. 5E). Most of these lavas contain 10–25% of subidio-morphic plagioclase glomerocrysts (up to 2 cm) which are partiallyto completely altered to albite, prehnite and/or sericite. Some ofthem also contain partially to completely chloritized pyroxene

Table 1BRepresentative microprobe analyses of Sangsang massif UM minerals (FeO(T): total Fe as FeO; FeT: total Fe).

Olivine Harz. Harz. Cpx-Har. Spinel Cpx-Har. Harz. Harz. Cpx Webst. Cpx-Har. Harz. Opx Cpx-Har. Harz. Webst.

06SG10ACR 5

07SG27CB2

07SG47N3

07SG24CV1

07SG26V1

07SG64R2

07SG41AV1

07SG47B3

06SG04ACN 4

06SG12ACR 6

07SG20BV1

07SG41AR1

SiO2 40.83 40.95 41.12 SiO2 0.01 0.06 0.03 SiO2 52.81 52.49 53.67 SiO2 55.47 55.78 54.67TiO2 0.01 0.00 0.00 TiO2 0.04 0.03 0.07 TiO2 0.24 0.03 0.05 TiO2 0.03 0.00 0.11Al2O3 0.05 0.00 0.01 Al2O3 26.91 28.25 40.63 Al2O3 1.44 3.07 2.45 Al2O3 4.11 3.00 1.44Cr2O3 0.00 0.05 0.00 Cr2O3 41.37 41.30 26.53 Cr2O3 0.20 1.24 0.74 Cr2O3 0.69 0.90 0.15MgO 49.89 50.43 49.60 Fe2O3 2.26 1.58 3.08 FeO(T) 4.75 2.79 2.09 FeO(T) 6.70 6.13 11.69CaO 0.02 0.02 0.05 MgO 12.95 14.73 17.51 MnO 0.13 0.05 0.06 MnO 0.13 0.09 0.19MnO 0.08 0.06 0.13 CaO 0.00 0.01 0.00 MgO 16.49 17.87 17.44 MgO 33.19 34.41 28.95FeO 8.95 8.23 9.66 MnO 0.00 0.00 0.00 CaO 22.89 22.17 23.91 CaO 1.04 0.91 2.19CoO 0.01 0.00 0.11 FeO 16.30 13.65 10.93 Na2O 0.20 0.14 0.04 Na2O 0.02 0.00 0.02NiO 0.45 0.31 0.32 CoO 0.03 0.00 0.08 K2O 0.00 0.01 0.00 K2O 0.00 0.01 0.00Total 100.28 100.05 101.00 NiO 0.04 0.10 0.26 NiO 0.03 0.05 0.04 NiO 0.11 0.07 0.02Si 1.00 1.00 1.00 ZnO 0.13 0.18 0.15 Total 99.18 99.89 100.49 Total 101.49 101.31 99.42Ti 0.00 0.00 0.00 Na2O 0.00 0.00 0.00 Si 1.95 1.91 1.94 Si 1.90 1.91 1.96Al 0.00 0.00 0.00 Total 100.04 99.88 99.27 Aliv 0.05 0.09 0.06 Aliv 0.10 0.09 0.04Cr 0.00 0.00 0.00 Si 0.00 0.01 0.01 Alvi 0.02 0.04 0.04 Alvi 0.06 0.03 0.02Mg 1.81 1.83 1.80 Ti 0.01 0.01 0.01 Fe3+ 0.04 0.03 0.00 Fe3+ 0.03 0.06 0.03Ca 0.00 0.00 0.00 Al 7.67 7.92 10.75 Cr 0.01 0.04 0.02 Cr 0.02 0.02 0.00Mn 0.00 0.00 0.00 Cr 7.91 7.77 4.71 Ti 0.01 0.00 0.00 Ti 0.00 0.00 0.00Fe 0.18 0.17 0.20 Fe3+ 0.41 0.28 0.52 Fe2+ 0.11 0.06 0.06 Fe2+ 0.16 0.12 0.32Co 0.00 0.00 0.00 Mg 4.67 5.23 5.86 Mn 0.00 0.00 0.00 Mn 0.00 0.00 0.01Ni 0.01 0.01 0.01 Ca 0.00 0.00 0.00 Mg 0.91 0.97 0.94 Mg 1.69 1.75 1.54Mg# 0.91 0.92 0.90 Mn 0.00 0.00 0.00 Ca 0.91 0.86 0.93 Ca 0.04 0.03 0.08Fo 90.83 91.64 90.16 Fe2+ 3.30 2.72 2.05 Na 0.01 0.01 0.00 Na 0.00 0.00 0.00Fa 9.17 8.36 9.84 Co 0.01 0.00 0.02 K 0.00 0.00 0.00 K 0.00 0.00 0.00

Ni 0.01 0.02 0.05 Wo 45.77 44.80 47.89 Wo 1.98 1.70 4.23Zn 0.02 0.03 0.03 En 45.90 50.23 48.60 En 87.84 89.29 77.82Na 0.00 0.00 0.00 Fs 7.60 4.46 3.37 Fs 10.12 9.02 17.87Mg# (Fe2+) 0.59 0.66 0.74 Ac 0.73 0.51 0.14 Ac 0.05 0.00 0.07Mg# (FeT) 0.56 0.64 0.69 Mg# (Fe2+) 0.89 0.95 0.94 Mg# (Fe2+) 0.91 0.94 0.83Cr# 0.51 0.50 0.30 Mg# (FeT) 0.86 0.92 0.94 Mg# (FeT) 0.90 0.91 0.82

Harz.=Harzburgite.Cpx-Har.=Cpx-Harzburgite.Webst.=Websterite.


phenocrysts (~5–10%). One lava sample is also characterized byamygdules (15%) filled with chlorite, quartz and carbonate. Carbo-nate and chlorite veinlets were also observed.

The gabbroic rocks in the Sangsangmassif are variable, ranging fromcoarse- to fine-grained quartz leuco-gabbro to melano-gabbro. Gabbrosare composed of subidiomorphic plagioclase (25–75%) moderately orcompletely altered into the secondary mineral assemblage albite-prehnite. Plagioclase crystals are usually about 0.5 to 4 mm in size but

Fig. 7. Variations of NiO vs. Mg# in olivines from peridotites. Fields outline olivine compositiomafic plutonic rocks from the oceanic domain. The arrow is compatible with the fractional

can reach 1 cm. Varying percentages of fresh subidiomorphic toallotriomorphic clinopyroxene 0.2 mm–2 cm in size are observed (5–25%). These crystals are generally replaced by metamorphic greenamphibole (15–35%) or minor chlorite. Amphibole is either massive oroccurs as neoblastic smaller grains. Large poikilitic crystals containingsmaller amphiboles have also been identified. In a few samples, theremight be small relics of a more brownish amphibole suggesting amagmatic origin. Unfortunately, these relics are very scarce and almost

ns in forearc peridotites (Ishii et al., 1992) and various mantle peridotites, ultramafic andcrystallization trend (Constantin 1999).


completely replaced by the green amphibole. Only some gabbroscontain orthopyroxene crystals (up to 20%). Up to 10% of secondary Ti-rich phases replace primary titanite in several gabbroic rocks. Prehniteveinlets (3–4%) were also observed. Most of the gabbros show asubophitic texture although one or two rocks have a distinctivecumulate texture.

The mineralogy of the diabase samples is about the same as for thegabbros. They are also typically fresher than the gabbros and show anophitic to subophitic texture (Fig. 5F). They are composed of 30–40% ofinterstitial subidiomorphic clinopyroxene of 0.5 to 3mm in size. Severalcrystals are poikilitic and include small prismatic plagioclase grains.Plagioclase (30–40%) is idiomorphic to subidiomorphic crystals (0.4–0.8 mm) and usually not altered. Secondary chlorite (5–15%) occurs asan alteration of clinopyroxenewhile asmuch as 15% of secondary Ti-richphases completely replace primary titanite.

Like the Saga ophiolite sandstones, the composition of theSangsang ophiolite sandstones is very diverse. They are composedof subangular to subrounded clasts up to 1 cm in diameter which aremainly quartz (15–25%), sedimentary rocks (15–25%), altered maficto intermediate volcanics (0–25%), feldspars (~10%) and felsicplutonic rocks (0–10%) as well as actinolitized or chloritizedpyroxene or amphibole pseudomorphs (1–4%). One sample is verydistinctive as it is composed of about 70% of carbonate oolites and10% of sedimentary rocks clasts cemented by carbonate (~20%). Thissandstone is also the only one containing minor spinel clasts. Most ofthe quartz clasts are more or less deformed showing an undulatoryextinction or are even polycrystalline. Less than 35% of siliceousmatrix as well as local carbonate cement is found in the sandstonesso that the clasts are self-supporting.

4. Mineral chemistry

4.1. Analytical conditions

Mineral analyses on more than 70 samples from both the Saga andSangsang ophiolites were done on a Cameca SX-100 five-spectrometerelectron microprobe at Université Laval. Analytical conditions were15 kV, 20 nAwith a counting time of 20 s on the peaks and 10 s on thebackground. Calibration standards used were generally oxides (GEOStandard Block of P and H developments), or minerals where needed

Fig. 8. Spinel composition in peridotites and sandstones. Data for Dazhuqu, Luobusa and Lhaset al. (2003). Fields for spinel in abyssal peridotites are taken from Dick and Bullen (1984)represents the experimental percentage of wet melting of the host peridotites (Hirose and

[Mineral StandardMountMINM25-53 of Astimex Scientific; referencesamples from Jarosewich et al. (1980)]. Datawere reduced using a PAPmodel. The main mineral phases analysed for the ultramafic sampleswere spinel, clinopyroxene, orthopyroxene and olivine while clin-opyroxene, orthopyroxene, plagioclase, amphibole and chlorite wereanalyzed for the mafic samples. Only spinel was analysed for thesandstones. Spinel analyses with a SiO2 content of more than 1% wererejected, assuming that these grains were too altered to be safely used.A large number of analyses were done (i.e.N240 spinels, 360pyroxenes and 130 olivines were analyzed just for the ultramaficrocks) but only representative data for the most important mineralphases for the ultramafic samples (olivine, spinel, clinopyroxene,orthopyroxene) are presented in Table 1A (Saga) and B (Sangsang). Acomplete set of analytical data is given in Bédard (2009).

4.2. Olivine

The composition of olivine in the Saga and Sangsang peridotites isvery similar. Olivine from the Saga massif peridotites has Fo valuesranging from 89.9 to 91.3 with NiO contents from 0.27 to 0.52 wt.%(Fig. 7).Olivine fromthe Sangsangperidotites are however slightlymoreforsteritic (Fo90.1–91.9) with an NiO content between 0.29 and 0.45.

4.3. Spinel

The majority of spinels from the Saga massif peridotites have atypical Mg# between 0.70 and 0.79 (for some samples it is as low as0.50) with a corresponding Cr# varying from ~0.10 to 0.22, although itwas as high as 0.64 for a few spinels. TiO2 content for these spinelsvaries from 0 to 0.34%. Spinels from the Sangsang massif peridotiteshave a lower Mg# (~0.50–0.65), a higher corresponding Cr# (~0.30–0.58) along with a lower TiO2 content (0–0.08%). Using calculationssimilar to those by Hirose and Kawamoto (1995; Fig. 8), the Sagaspinels indicate only low degrees of partial melting (5–12%) while thecomposition of the Sangsang spinels suggests higher degrees of partialmelting (17–30%).

Fresh spinels found in Saga massif sandstones have either a highMg# (~0.65–0.75) with a low corresponding Cr# (~0.17–0.27) or alarge spectrum of Mg# values (~0.40–0.70) along with Cr# valueshigher than 0.45 (Fig. 8).

e ophiolites are fromHébert et al. (2003) and data for Yungbwa ophiolite are fromMiller. Data for spinel in forearc peridotites are from Ishii et al. (1992). The curve with ticksKawamoto 1995).

Fig. 9. Diagrams of TiO2 (wt.%) vs. Mg# of clinopyroxenes from Saga and Sangsang massif peridotites. Fields outline clinopyroxene compositions in abyssal peridotites (Johnson et al.,1990) and forearc peridotites (Ishii et al., 1992).


4.4. Clinopyroxene

Clinopyroxenes from the Saga massif peridotites show high Mg#values (0.93–0.99) along with Al2O3, Cr2O3, TiO2 contents varying from0.44 to 6.98%, 0.15 to 1.65% and 0.04 to 0.37%, respectively (Fig. 9).Clinopyroxenes fromthe Sangsangophiolite peridotites have lowerMg#(0.89–1.00) as well as lower Al2O3 (0.25–5.11), Cr2O3 (0.04–1.60) andTiO2 (0–0.28) percentages. Clinopyroxenes from both massifs havediopsidic compositions. The wide range of the clinopyroxene composi-tion is related to the type of rocks inwhich they occur. For example, TiO2

and Al2O3 contents are lower in clinopyroxene from cpx harburgite butare higher in lherzolite. This is particularly evident for Saga peridotiteswhich show more varied clinopyroxene abundances.

Clinopyroxenes in lavas, meta-gabbros and amphibolites from theSaga massif uppermost crust and metamorphosed upper crust haveMg# values varying from 0.63 to 0.86 (Fig. 10). Their Al2O3, Cr2O3, TiO2

contents range from 0.56 to 6.00%, 0 to 1.20% and 0.06 to 1.70%. Mg# inthe clinopyroxene from Sangsang massif gabbros and lavas varies from0.72 to 0.84, Al2O3 from 1.54 to 5.06, Cr2O3 from 0 to 0.47 and TiO2 from

Fig. 10. Cr2O3 (wt.%) vs. Mg# in clinopyroxene from peridotites and crustal mafic rocks (baforearc peridotite field from Ishii et al. (1992), and Back-arc-basin basalt (BABB) field from

0.31 to 2.32. All clinopyroxene analyses also follow the main oceanictholeiitic fractional crystallisation trend of Constantin (1999).

4.5. Orthopyroxene

Orthopyroxene in the Saga massif peridotites has a reasonablyuniform composition (Mg# between 0.90 and 0.91) with Al2O3 andCr2O3 contents varying from 1.25 to 6.16 and 0.16 to 0.94, respectively(Fig. 11). The composition of orthopyroxene from the Sangsangperidotites seems to be less uniform. Some are enriched in iron whileothers are even more magnesian than those from the Saga peridotites(Mg# ~0.82–0.94) with Al2O3 and Cr2O3 values of 1.41–5.25 and 0.09–0.97. Orthopyroxene from both massifs are enstatite. Larger (porphyr-oclasts) orthopyroxenes seem to have higher Cr2O3 values than thesmaller (interstitial) orthopyroxenes. Generally the composition of theorthopyroxene varies with that of clinopyroxene.

The few orthopyroxenes identified in Saga massif mafic rocks show aMg# between 0.72 and 0.74 along with Al2O3, Cr2O3, TiO2 percentagesbetween1.81–3.52, 0.01–0.08 and0.11–0.31, respectively. Orthopyroxenes

salts, diabase and gabbros). Abyssal peridotite field taken from Johnson et al. (1990),Hawkins and Allan (1994).

Fig. 11. Diagrams of Al2O3 (wt.%) vs. Mg# of orthopyroxenes from Saga and Sangsang massif peridotites. Fields outline orthopyroxene compositions in abyssal peridotites (Johnsonet al., 1990), forearc peridotites (Ishii et al., 1992), boninites (Van der Laan et al., 1992), and ultramafic and mafic plutonic rocks from Pito, Terevaka and Garrett (Constantin 1999).


in gabbros and lavas from Sangsang ophiolite have slightly lower Mg#(0.58–0.75) as well as lower Al2O3, Cr2O3, TiO2 contents (0.96–1.65; 0–0.08; 0.08–0.18).

4.6. Feldspar

Feldspar has been analyzed in amphibolites and meta-gabbros fromthe Saga massif and in gabbros/diabases and lavas from the Sangsangmassif. Feldspars from both massifs are mostly secondary sodicplagioclase. Plagioclase composition varies from very sodic (Ab97.4) topure albite (Ab100) for the Saga massif while for the Sangsang massif,plagioclase is slightly more calcic (Ab85.8–99.3). The predominance ofalbitic plagioclase is an indication of hydrothermal metamorphism.

4.7. Amphiboles

Only metamorphic amphiboles were analyzed in amphibolites ormeta-gabbros from the Saga massif and in gabbros/diabases andlavas from the Sangsang massif. The Saga massif metamorphicamphiboles are mainly magnesio-hornblende and have low TiO2

contents (0.47–1.04 wt.%). Sangsang amphiboles are actinolitichornblende along with magnesio-hornblende and have highlyvariable TiO2 contents (0.05–2.00 wt.%).

5. Whole rock chemistry

5.1. Analytical conditions

More than 120 samples (2006 and 2007 campaign)were selected forwhole-rock geochemical analyses. The concentrations of major ele-ments and some trace elements (V, Cr, Co, Ni, Zn, Ga, Rb) weredetermined by X-ray fluorescence at the Regional Geochemical Centre,Saint Mary's University (Halifax, Nova Scotia). Analytical errors, asdetermined from replicate USGS standard rock analyses, are±2% for themajor oxides and between 5% and 10% for minor and trace elements(Dostal et al.,1986). Additionnal trace elements (Y, Zr, Nb, Ba, REE, Hf, Ta,Th, U)were analyzed by inductively coupled plasma-mass spectrometry(ICP-MS) on 2006 samples using a Na2O2-sintering technique at theDepartment of Earth Sciences of theMemorial University of Newfound-land. Themethod is described by Longerich et al. (1990). Theprecision isbetween 2 and 4%. Samples collected in 2007 were analyzed for majorand some trace elements by ICP-AES and for additional trace elementsby ICP-MS at Activation Laboratories Ltd in Ancaster, Ontario. A lithium

metaborate/tetraborate fusion method was used. Since the abundancesof several trace elementswere close to detection limits, selected sampleswere re-analyzed using larger samples. The data for the ultramafic andthemafic rocks obtained from theMemorial are very similar to the datafor the Sangsang massif samples analysed by Activation Laboratories.Data for major and trace elements are presented in Table 2A and B.

5.2. Mantle rocks

Peridotites from bothmassifs show variable degrees of serpentiniza-tion (LOI: 0.7–13.6%) but most of them are fresh. Their Mg# variesbetween 0.89 and 0.92. Saga peridotites have relatively high Al2O3

values (2–3.5%) while Sangsang peridotites show more refractorycompositions with Al2O3 contents between 0.5 and 2%. The Sagaperidotites are enriched in iron (Fe2O3T content averaging 9% withvalues ashighas11%). Someof the Sangsangperidotites alsohave Fe2O3T

values close to 9%. On a chondrite-normalized REE diagram (Fig. 12A),Saga massif peridotites show distinct flat patterns with (La/Yb)Nratios close to 1. REE patterns of most samples are parallel to an N-MORB pattern characterized by a flat heavy REE (HREE) segment anda slight depletion of light REE (LREE) compared to HREE. Most ofSangsang massif peridotites have U-shaped profiles (Fig. 12C). Twosamples show relatively flat patterns with slight LREE positive slopessimilar to Saga massif flat profiles.

5.3. Mafic rocks

The uppermost crustal and metamorphosed mafic crustal rockcompositions correspond to basalt and basaltic andesite. When plottedon a chondrite-normalized multielement diagram (Fig.12B), they showa slight depletion in highly incompatible (LREE) elements with respectto the less incompatible elements (HREE) with La/YbN between 0.5 and0.8. Their patterns are relatively flat from middle REE (MREE) to HREEand also display a slight negative Ta anomaly suggesting the presence ofa subduction component. Some sample even show a small distinctnegative Ti anomaly. However, three samples are enriched in incompa-tible elements (La to Zr). On the discrimination diagram of Pearce andNorry (1979), crustal rocks fall in the N-MORB and volcanic arc basaltfields (Fig. 13A). By contrast, the Indian margin rocks have tephrite–basanite compositions and are strongly enriched in LREE (La/YbN~6.6–7.8). Their patterns are also characterized by slight positive Ta–Tianomalies. Furthermore, when plotted on the Pearce and Norry (1979)diagram, they clearly fall in the within-plate basalt field.

Table 2AElemental content for Saga massif UM and mafic rocks (D.L.: Detection limit; L.O.I: lost on ignition; Fe2O3(T): total Fe as Fe2O3).

UM rocks 06-SA-38A 06-SA-40B 06-SA-42 06-SA-50A 06-SA-53B 06-SA-61 06-SA-63 06-SA-71 06-SA-72 06-SA-84B

Wehrlite Lherzolite Cpx-harzburgite Cpx-harzburgite Cpx-harzburgite Lherzolite Lherzolite Orthopyroxenite Lherzolite Lherzolite

SiO2 39.18 41.37 41.50 42.22 43.63 43.26 42.46 52.78 40.89 44.22TiO2 0.34 0.03 0.06 0.04 0.07 0.05 0.03 0.07 0.07 0.06Al2O3 4.68 0.78 1.82 0.98 2.54 2.32 1.76 1.63 2.58 2.25Fe2O3(T) 15.23 8.46 9.09 8.83 8.80 9.26 8.06 6.14 7.48 8.28MnO 0.18 0.12 0.12 0.13 0.13 0.13 0.10 0.13 0.11 0.15MgO 28.84 39.74 40.78 40.34 39.85 40.65 35.78 32.64 36.30 44.19CaO 2.80 0.46 1.34 1.66 2.91 2.42 1.41 3.00 2.34 0.57Na2O 0.15 D.L. D.L. D.L. D.L. D.L. D.L. D.L. D.L. 0.00K2O 0.09 0.06 0.06 0.00 0.00 0.00 0.01 0.01 0.00 0.00P2O5 0.06 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01L.O.I. nd 9.4 5.2 5.4 0.7 0.7 10.5 2.4 11.4 14Total 91.73 91.30 95.00 94.48 98.35 98.36 89.99 96.75 90.10 87.67Mg# 0.81 0.91 0.91 0.91 0.91 0.91 0.91 0.92 0.91 0.92V 71 32 38 54 59 68 60 45 62 38Cr 211 2535 2052 2892 3294 2606 3421 3006 2492 2638Co 121 106 108 103 104 111 92 64 96 102Ni 1153 2119 2287 2172 1948 2180 2061 1326 1943 2226Zn 93 45 50 50 51 52 46 38 55 61Ga 4 0 D.L. D.L. 2 D.L. 2 2 2 0Rb 6 9 5 6 6 7 6 6 8 7Sr 23 5 4 4 13 6 9 14 11 7Y (ICP-MS) 6.18 0.26 0.76 0.46 1.87 1.26 0.70 0.82 1.59 1.28Zr 27.80 2.86 2.69 1.50 3.72 2.00 1.67 3.99 3.13 2.48Nb 0.94 0.51 0.26 0.26 0.35 0.18 0.30 0.34 0.38 0.30Ba 12.9 19.0 17.8 4.0 5.5 5.5 7.2 7.6 6.7 4.6La 1.00 0.10 0.09 0.06 0.32 0.08 0.16 0.16 0.11 0.03Ce 3.00 0.24 0.18 0.09 0.65 0.18 0.35 0.24 0.21 0.18Pr 0.49 0.03 0.01 0.02 0.09 0.02 0.04 0.04 0.04 0.03Nd 2.33 0 0.01 0.23 0.46 0.16 0.24 0.57 0.27 0.0Sm 0.74 0 0 0.07 0.15 0.07 0.04 0.12 0.10 0.04Eu 0.28 0.02 0.01 0.02 0.05 0.03 0.03 0.08 0.06 0.0Gd 0.93 0.01 0.08 0.08 0.22 0.10 0.11 0.10 0.22 0.05Tb 0.17 0 0.01 0.02 0.04 0.03 0.02 0.03 0.05 0.03Dy 1.11 0.04 0.13 0.09 0.30 0.21 0.06 0.23 0.28 0.20Ho 0.22 0 0.02 0.03 0.07 0.05 0.04 0.04 0.08 0.06Er 0.76 0.02 0.06 0.05 0.25 0.19 0.07 0.17 0.18 0.22Tm 0.11 0 0.01 0.02 0.03 0.03 0.01 0.04 0.03 0.04Yb 0.73 0.04 0.09 0.17 0.27 0.19 0.12 0.24 0.25 0.16Lu 0.10 0.01 0.01 0.03 0.04 0.03 0.03 0.05 0.04 0.01Hf 0.57 0.02 0.04 0.12 0.11 0.17 0.09 0.14 0.12 0.0Ta 0.05 0.02 0 0.01 0.01 0.02 0.02 0.03 0.00 0.0Th 0.05 0.05 0.02 0.01 0.05 0.04 0.05 0.04 0.00 0.0U 4 5 2 4 8 6 4 4 8 6

Mafic rocks 06-SA-02A 06-SA-08 06-SA-10C 06-SA-16 06-SA-18 06-SA-22A 06-SA-23 06-SA-26A 06-SA-34

Brecciated tuff Mafic tuff Altered diabase Brecciated basalt Gabbro Gabbro Amphibolite Altered basalt Amphibolite

SiO2 47.31 48.05 42.15 43.38 46.42 47.62 48.36 49.31 49.42TiO2 1.68 1.30 5.75 4.07 1.52 1.46 1.01 1.23 0.86Al2O3 15.96 14.76 12.23 13.86 15.60 13.89 15.21 16.76 10.32Fe2O3(T) 14.73 11.37 18.86 16.18 10.89 11.27 9.14 9.54 9.91MnO 0.23 0.17 0.26 0.44 0.18 0.17 0.15 0.17 0.18MgO 7.07 7.66 5.22 7.47 8.00 8.08 7.60 6.06 11.57CaO 8.20 8.46 6.25 5.22 8.05 10.70 11.36 7.44 12.49Na2O 2.50 2.86 2.42 3.34 2.43 2.84 2.46 3.86 1.66K2O 1.97 0.52 0.23 0.04 1.84 0.23 0.47 0.42 0.12P2O5 0.21 0.11 0.63 0.42 0.17 0.14 0.09 0.13 0.07L.O.I. 3.6 3.5 4.9 5 3.5 2.5 3.2 4.7 2.2Total 95.73 95.36 94.21 94.58 95.22 96.50 95.96 94.99 96.74Mg# 0.51 0.60 0.38 0.50 0.62 0.61 0.65 0.58 0.72V 319 262 520 489 309 286 203 247 242Cr 235 215 D.L. 71 326 191 306 46 602Co 48 38 47 51 45 40 40 33 42Ni 92 88 24 158 129 156 150 113 192Zn 134 84 155 130 84 51 67 64 64Ga 17 14 19 21 14 14 13 14 11Rb 51 19 12 5 40 12 13 17 7Sr 113 50 350 390 111 119 63 39 35Y (ICP-MS) 32.07 27.38 48.30 26.91 27.30 26.78 22.57 23.18 17.13Zr 80.23 103.69 328.71 236.96 94.92 82.70 88.56 87.68 53.56Nb 1.43 2.34 70.15 45.69 5.85 1.76 2.07 1.99 1.13Ba 150.7 58.1 712.3 37.4 108.9 30.7 35.5 35.4 11.8La 2.32 3.19 37.99 26.07 3.70 2.34 2.45 2.09 1.56Ce 7.09 9.74 80.79 58.55 10.17 7.93 7.44 7.10 4.73Pr 1.25 1.66 10.65 7.75 1.67 1.46 1.25 1.30 0.86(continued on next page)


Table 2A (continued)

Mafic rocks 06-SA-02A 06-SA-08 06-SA-10C 06-SA-16 06-SA-18 06-SA-22A 06-SA-23 06-SA-26A 06-SA-34

Brecciated tuff Mafic tuff Altered diabase Brecciated basalt Gabbro Gabbro Amphibolite Altered basalt Amphibolite

Pr 1.25 1.66 10.65 7.75 1.67 1.46 1.25 1.30 0.86Nd 7.17 9.34 46.23 34.54 8.81 8.23 7.45 7.35 4.94Sm 2.50 3.20 9.76 7.60 3.03 2.95 2.60 2.65 1.87Eu 0.84 1.25 2.84 2.44 0.99 1.10 1.12 0.88 0.66Gd 4.87 4.39 10.53 7.82 4.40 4.41 3.60 3.75 2.79Tb 0.89 0.75 1.53 1.17 0.81 0.85 0.61 0.70 0.51Dy 6.65 5.10 8.85 6.59 5.54 5.54 4.17 4.88 3.40Ho 1.27 1.15 1.79 1.14 1.12 1.11 0.94 0.96 0.70Er 3.68 3.37 4.85 2.99 3.30 3.25 2.78 2.92 2.06Tm 0.52 0.48 0.66 0.40 0.48 0.48 0.43 0.43 0.30Yb 3.46 3.36 4.11 2.42 3.20 3.22 2.89 2.85 1.99Lu 0.56 0.45 0.57 0.34 0.46 0.45 0.40 0.41 0.28Hf 1.85 2.36 7.47 5.04 2.05 1.99 2.36 2.16 1.50Ta 0 0.05 3.20 2.13 0.23 0.07 0.07 0.09 0.06Th 0 0.09 3.70 2.65 0.34 0.07 0.28 0.16 0.10U 1 6 11 3 3 6 3 4 2

Mafic rocks 06-SA-37 06-SA-39 06-SA-64A 06-SA-65A 06-SA-74B 06-SA-74C 06-SA-75

Amphibolite Hematised tuff Amphibolite Amphibolite Andesitic basalt Andesitic basalt Brecciated basalt

SiO2 48.93 56.96 43.42 39.69 52.78 50.29 45.19TiO2 1.38 0.66 1.33 2.40 0.97 1.06 1.40Al2O3 15.74 15.11 15.61 13.87 15.63 14.93 14.16Fe2O3(T) 11.12 9.25 11.32 19.00 10.53 9.60 12.09MnO 0.18 0.13 0.19 0.29 0.16 0.16 0.19MgO 6.03 10.06 10.03 9.34 4.53 6.88 9.12CaO 8.00 3.53 16.91 14.22 7.11 8.48 10.07Na2O 3.50 4.02 0.86 0.73 3.55 3.77 1.69K2O 0.84 0.07 0.09 0.05 0.67 0.15 0.89P2O5 0.13 0.05 0.12 0.27 0.10 0.10 0.13L.O.I. 2.6 7.4 4 4 2.7 3 4.4Total 95.93 92.24 96.08 95.95 96.11 95.53 95.06Mg# 0.54 0.71 0.66 0.52 0.49 0.61 0.62V 271 153 244 390 320 219 283Cr 88 482 368 198 25 236 302Co 34 45 38 53 31 32 45Ni 43 310 139 105 61 98 164Zn 84 59 69 178 67 68 89Ga 14 11 11 10 15 12 14Rb 22 8 7 5 23 11 24Sr 83 181 70 37 68 82 45Y (ICP-MS) 29.51 12.80 26.33 63.77 21.45 22.80 27.29Zr 111.78 59.16 73.32 128.49 59.65 73.49 102.18Nb 2.44 2.34 8.94 21.47 1.52 1.77 2.33Ba 65.7 38.1 13.1 11.5 118.2 53.6 156.7La 2.98 3.58 3.09 9.83 2.42 2.15 2.70Ce 9.29 8.90 9.07 25.60 6.76 6.77 8.68Pr 1.67 1.03 1.44 4.04 1.16 1.18 1.58Nd 8.98 3.94 6.99 20.81 5.99 6.71 8.84Sm 3.00 0.69 2.17 6.32 2.02 2.58 3.42Eu 1.06 0.28 0.89 2.02 0.76 0.86 1.17Gd 4.33 1.50 3.73 9.74 3.15 3.61 4.53Tb 0.78 0.26 0.66 1.70 0.57 0.64 0.82Dy 5.25 1.96 4.81 11.72 4.06 4.43 5.54Ho 1.21 0.39 1.03 2.53 0.83 0.93 1.11Er 3.63 1.10 2.95 7.66 2.50 2.74 3.39Tm 0.53 0.13 0.39 1.07 0.36 0.38 0.51Yb 3.70 0.93 2.59 7.30 2.53 2.65 3.33Lu 0.50 0.12 0.39 1.08 0.39 0.41 0.52Hf 2.75 0.83 1.47 3.46 1.31 1.53 2.43Ta 0.07 0.00 0.24 0.52 0.06 0.08 0.09Th 0.07 0.83 0.08 0.07 0.21 0.10 0.12U 2 6 4 0 2 8 4


Sangsang massif mafic rocks (lavas and gabbros) have basalt tobasaltic andesite compositions. Surprisingly, most of them have highLa/YbN values (4.6–8.0). They also fall in the within-plate basalt fieldof the Pearce and Norry (1979) diagram (Fig. 13B). Some samples alsoshow lower La/YbN ratios (0.6–0.8). Their patterns are characterizedby a slight depletion in LREE as well as a flat MREE-LREE segment.These samples also display a slight negative Ta anomaly. One sample isenriched in incompatible elements (La to Zr) and is therefore similarto the three enriched samples of the Saga massif.

6. Discussion

6.1. Affinities and petrogenesis

The chemistry of the main minerals from the ultramafic and maficrocks reflects the different affinities and petrogenesis of the Saga andSangsang ophiolitic massifs. Olivine from both massifs is similar toeach other and also to olivine from forearc and upper mantle abyssalperidotites (Fig. 7). According to the Mg# and Cr# of spinel (Fig. 8),

Table 2BElemental content for Sangsang massif UM and mafic rocks (D.L.: Detection limit; L.O.I: lost on ignition; Fe2O3(T): total Fe as Fe2O3).

UM rocks 07-SG-22 07-SG-24C 07-SG-25 07-SG-27A 07-SG-27B 07-SG-41A 07-SG-44 07-SG-46

Cpx-harzburgite Cpx-harzburgite Harzburgite Harzburgite Harzburgite Websterite Serp. Harzburgite Dunite

SiO2 41.21 38.81 40.99 41.74 39.61 49.58 41.40 41.76TiO2 0.02 0.02 0.02 0.02 0.03 1.19 0.02 0.02Al2O3 1.07 0.27 0.67 0.64 0.44 15.77 0.80 0.81Fe2O3(T) 8.16 8.99 8.50 8.49 8.15 9.90 8.55 8.41MnO 0.12 0.12 0.12 0.12 0.11 0.17 0.06 0.12MgO 38.87 45.25 41.56 42.91 44.47 7.31 38.46 41.25CaO 1.42 0.74 0.99 0.65 0.41 9.52 0.16 1.09Na2O 0.00 0.00 0.00 0.00 0.00 3.02 0.00 0.00K2O 0.01 0.00 0.00 0.01 0.00 0.72 0.00 0.00P2O5 0.00 0.01 0.01 0.01 0.01 0.10 0.01 0.01L.O.I. 9 6 7.3 5.6 7 2.3 10.8 6.6Total 91.30 94.40 93.15 94.93 93.61 97.38 89.80 93.80Mg# 0.91 0.92 0.92 0.92 0.92 0.62 0.91 0.92V (ICP-MS) 41 13 39 35 21 127 37 158Cr 2350 1090 2210 2940 2640 680 2290 1110Co 48 82 97 94 54 39 99 64Ni 880 1550 1800 1820 980 340 1920 430Cu 0 0 20 0 0 1080 0 30Zn 0 0 40 40 0 0 40 130Rb 2 0 0 0 0 0 0 2Sr 0 0 0 0 0 9 5 187Y 0 0 0 0 0 1.9 0 3.3Zr 2 0 0 0 0 5 0 3Nb 0 0 0 0 0 0 0 0Cs 0 0 0 0 0 0.8 3.5 2.4Ba 0 0 0 0 0 0 0 6La 0.32 0.18 0 0.13 0.07 0.38 0.2 0.25Ce 0.41 0.21 0.08 0.32 0.17 0.56 0.21 0.47Pr 0.02 0 0 0.03 0.02 0.04 0 0.05Nd 0.09 0 0 0.14 0.09 0.24 0 0.37Sm 0.03 0 0 0.04 0.03 0.09 0.04 0.16Eu 0.01 0 0 0.01 0.01 0.04 0.02 0.09Gd 0.02 0 0 0.01 0.02 0.14 0.03 0.26Tb 0 0 0 0 0 0.03 0 0.06Dy 0.03 0.02 0 0.03 0.05 0.26 0.04 0.5Ho 0 0 0 0 0.01 0.07 0.01 0.12Er 0.04 0.01 0 0.02 0.04 0.27 0.04 0.39Tm 0.01 0 0 0 0.01 0.05 0.01 0.06Yb 0.06 0.02 0.03 0.04 0.04 0.31 0.04 0.44Lu 0.02 0.00 0.00 0.01 0.01 0.06 0.01 0.07Hf 0 0 0 0 0 0.2 0 0Ta 0 0 0 0 0 0.01 0 0Pb 0 0 0 0 0 0 0 0Th 0.06 0 0 0.06 0 0 0 0U 0 0 0 0.01 0 0 0 0

Mafic rocks 07-SG-14 07-SG-17A 07-SG-20A 07-SG-28A 07-SG-32B 07-SG-41C 07-SG-41B 07-SG-42B 07-SG-53

Hematised basalt Hematised basalt Arkosic sandstone Basalt Gabbro Gabbro Coarse gabbro Diabase Basalt

SiO2 43.44 47.52 66.20 50.29 45.24 48.92 49.72 51.29 43.64TiO2 2.08 3.20 0.54 0.90 0.94 1.11 0.12 0.32 3.89Al2O3 16.60 13.80 15.68 14.91 18.08 12.53 9.51 13.86 12.71Fe2O3(T) 10.33 17.34 3.05 7.94 11.55 11.60 8.76 5.32 14.76MnO 0.18 0.18 0.03 0.08 0.11 0.19 0.15 0.06 0.34MgO 4.95 4.04 1.10 9.74 4.50 10.77 18.18 12.59 6.67CaO 12.08 5.02 2.10 8.64 13.84 8.95 10.09 10.86 8.55Na2O 1.80 4.85 2.81 2.16 2.16 2.39 0.56 1.78 3.63K2O 1.65 0.13 4.41 0.02 0.23 0.36 0.12 0.55 0.02P2O5 0.37 0.36 0.21 0.09 0.03 0.10 0.01 0.03 0.65L.O.I. 6.3 3.1 3.2 4.8 2.6 2.8 2 2.2 4.9Total 93.67 96.59 96.35 94.86 96.76 97.05 97.43 96.84 95.05Mg# 0.51 0.34 0.44 0.73 0.46 0.67 0.82 0.84 0.50V (ICP-MS) 185 331 55 216 292 218 141 205 316Cr 280 20 30 260 0 390 990 830 60Co 35 34 6 36 42 26 56 37 20Ni 90 0 0 110 0 90 370 290 0Cu 30 120 50 60 0 20 20 0 20Zn 100 110 70 80 50 0 40 0 80Rb 31 3 177 0 5 3 2 7 0Sr 473 180 452 43 201 100 166 130 220Y 26.5 36.5 10.6 19.9 13.4 29.4 3.1 12.5 31.1Zr 192 256 192 68 27 75 5 15 135Nb 29.9 35.1 7.1 1.1 0.7 0.8 0 0 20.7Cs 3.6 0.3 3.7 1.1 16.8 1.1 2.3 8.8 0(continued on next page)


Table 2B (continued)

Mafic rocks 07-SG-14 07-SG-17A 07-SG-20A 07-SG-28A 07-SG-32B 07-SG-41C 07-SG-41B 07-SG-42B 07-SG-53

Hematised basalt Hematised basalt Arkosic sandstone Basalt Gabbro Gabbro Coarse gabbro Diabase Basalt

Cs 3.6 0.3 3.7 1.1 16.8 1.1 2.3 8.8 0Ba 225 103 941 3 94 13 4 13 549La 25.6 33.8 68 2.35 1.72 2.51 0.16 0.79 22Ce 51.4 73.6 128 6.92 3.51 8.26 0.38 1.71 51.2Pr 5.43 8.29 12.4 1.04 0.41 1.29 0.06 0.19 6.26Nd 22.1 34.4 37 5.51 2.14 7.32 0.43 1.24 27.8Sm 5.38 8.15 6.74 2.18 0.71 2.77 0.2 0.48 7.1Eu 1.99 2.83 1.47 0.84 0.34 1 0.10 0.24 2.8Gd 5.24 7.83 3.9 2.86 1.16 3.95 0.31 1.04 7.24Tb 0.83 1.21 0.45 0.53 0.25 0.73 0.07 0.24 1.07Dy 4.84 6.93 2.05 3.46 1.87 4.77 0.5 1.89 5.81Ho 0.94 1.35 0.35 0.77 0.46 1.03 0.12 0.46 1.11Er 2.62 3.94 0.9 2.36 1.58 3.22 0.4 1.52 3.09Tm 0.38 0.55 0.12 0.36 0.26 0.47 0.06 0.25 0.41Yb 2.16 3.3 0.76 2.33 1.76 2.94 0.42 1.62 2.34Lu 0.31 0.48 0.12 0.35 0.28 0.44 0.08 0.25 0.33Hf 4.6 6.6 5.3 1.9 0.9 2.2 0.2 0.5 3.6Ta 2.14 2.64 0.68 0.06 0.05 0.06 0 0.01 1.64Pb 18 0 55 8 0 0 6 0 0Th 3.23 4.42 39.3 0.1 0.16 0.09 0 0.11 1.57U 0.62 0.87 6.37 0.82 0.07 0.02 0 0.02 0.43

Mafic rocks 07-SG-61 07-SG-63A 07-SG-68 07-SG-70A 07-SG-71

Andesite Gabbro Altered basalt Felsic lava Diabase

SiO2 48.19 48.77 50.42 65.88 49.98TiO2 1.66 3.95 1.14 0.64 1.44Al2O3 16.59 12.56 14.76 14.39 14.75Fe2O3(T) 10.29 14.77 10.49 6.50 12.57MnO 0.21 0.24 0.15 0.04 0.20MgO 6.66 4.36 5.33 1.43 6.87CaO 6.89 7.07 10.36 1.66 6.30Na2O 4.26 4.33 3.40 6.89 3.94K2O 0.45 0.95 0.15 0.03 0.23P2O5 0.19 0.50 0.11 0.13 0.12L.O.I. 4.2 2.3 3 2 3.5Total 95.63 97.66 96.37 97.66 96.50Mg# 0.59 0.39 0.53 0.33 0.55V (ICP-MS) 249 295 276 59 242Cr 140 0 80 40 170Co 36 35 28 14 23Ni 50 0 30 0 40Cu 130 0 60 10 20Zn 100 200 100 40 40Rb 7 15 3 0 3Sr 1240 222 69 116 118Y 27.3 50.5 29.3 62 30.2Zr 147 344 80 301 87Nb 21.2 37.5 1.4 3 1.1Cs 0.4 0.4 0 0 0.3Ba 264 271 14 8 12La 17 37.7 2.95 7.86 2.98Ce 37.5 83.5 9.02 24.4 9.52Pr 4.23 9.63 1.37 3.53 1.44Nd 17.7 40.4 8.24 18.5 8.3Sm 4.6 10.2 2.89 5.95 2.92Eu 1.58 3.4 1.13 1.39 1.19Gd 4.98 10.6 3.89 7.5 3.98Tb 0.82 1.65 0.72 1.35 0.75Dy 4.87 9.23 4.74 9.19 4.89Ho 0.98 1.81 1.03 2.12 1.09Er 2.85 5.16 3.2 6.68 3.31Tm 0.41 0.72 0.47 1.03 0.48Yb 2.47 4.26 2.9 6.61 2.96Lu 0.36 0.61 0.45 1.03 0.44Hf 3.9 8.7 2.3 7.8 2.4Ta 1.47 2.86 0.06 0.21 0.07Pb 0 8 0 0 0Th 1.97 4.49 0.12 0.46 0.13U 0.6 1.18 0.04 0.19 0.04


the Saga massif peridotites fall in the field of abyssal peridotite (andfore-arc peridotites for four samples) while the Sangsang peridotitesplot in the field of abyssal peridotite and fore-arc peridotites. Inaddition, when the Cr# of spinel is plotted against the Mg# of olivine

(Fig. 14), the Saga and Sangsang peridotites have different composi-tions. In this Cr# (spinel)–Mg# (olivine) diagram, the Saga peridotiteshave a deeper mantle provenance (N20 kbar) than the Sangsangperidotites (b15 kbar). The composition of the Saga peridotites

Fig. 12. A) REE diagram for Saga peridotites, nomalized to C1 chondrites. Data are from Bodinier and Godard (2003) and references therein for the unmetasomatized fertileperidotites from the Pyrenees, and from Nìu and Hékinian (1997) and Bodinier and Godard (2003) for the refertilized refractory peridotites from the East Pacific Rise. B)Multielement diagram for the Saga passive margin Indian rocks, uppermost crustal and metamorphosed upper crustal rock units, nomalized to C1 chondrites. N-MORB values arefrom Sun and McDonough (1989). C) REE diagram for Sangsang peridotites, nomalized to C1 chondrites. D) Multielement diagram for the Sangsang lavas and diabase and gabbro,nomalized to C1 chondrites. N-MORB values are from Sun and McDonough (1989). (For interpretation of the references to colour in this figure legend, the reader is referred to theweb version of this article.)


resembles the composition of pre-oceanic peridotites while perido-tites from the Sangsang massif are similar to abyssal and subductionmargin peridotites. Fresh spinels from the Saga sandstones can besubdivided into two groups. Spinels with a high Mg# (group 1) aresimilar to most of those from the Saga massif peridotites while thespinels with a high Cr# (group 2) come from another source havingfore-arc peridotites affinities (e.g. the Sangsang massif). Fig. 9 showsthat the composition of clinopyroxene in peridotites from Saga is closeto the composition of clinopyroxene from modern abyssal peridotiteswhile clinopyroxene in peridotites from the Sangsang massifresembles those of fore-arc peridotites. The compositional range ofclinopyroxene from the mafic crustal rocks is wide and is similar tothat of back-arc-basin basalts (BABB) (Fig. 10). Compared toorthopyroxene from modern settings, the composition of orthopyr-oxene in peridotites from both the Saga and Sangsang massifs issimilar to that of abyssal peridotites and fore-arc, respectively (Fig.11). Thus it appears that mantle rocks from the Saga ophiolitic massifare comparable to abyssal peridotites while the Sangsang massifperidotites can be correlated with fore-arc peridotites.

Major elements chemistry (e.g. the high Al2O3 and Fe2O3T values)for the Saga massif peridotites suggests relatively fertile compositions(Bodinier and Godard, 2003) while peridotites from the Sangsangmassif show more refractory compositions. However, the observediron enrichment is often associated with textural and mineralogicalevidence for refertilization ormetasomatism and is attributed tomelt-rock interactions (Bodinier and Godard, 2003). The REE contents inthe Saga peridotites (and the two samples from the Sangsang massif)

suggest that their composition is similar to a chondritic mantle(Fig. 12). The REE abundance in these rocks correlates inversely withthe modal abundance of olivine. The U-shaped profiles for most of theSangsang peridotites are interpreted to be characteristic of interactionbetween LREE-enriched melt and REE-depleted mantle residues(Dupuis et al., 2005b) and are reminiscent of the process producingboninites (Crawford, 1991).

The abundances of incompatible elements in most rocks of theSaga massif uppermost crustal and metamorphosed mafic uppercrustal units are similar to N-MORB. However, the slight negative Taand Ti anomalies indicative of a subduction component suggest thatthey are more likely BABB. The three samples enriched in incompa-tible elements are comparable to volcanics from mature intraoceanicisland arcs. In comparison, the geochemical signatures of the Indianmargin mafic rocks are typical of ocean-island basalt (OIB) with theirhigh La/YbN values. Most of the Sangsang massif mafic rocks are alsoenriched in incompatible elements and thus show geochemicalsignatures typical of OIB. (Fig. 12D). These rocks are similar to thepassive margin Indian mafic rocks of the Saga ophiolite. Thegeochemical characteritics of the mafic rocks from the Sangsangmassif with lower La/YbN ratios and a slight negative Ta anomalyresemble the BABB mafic rocks of the Saga ophiolite (metamorphosedupper crustal and uppermost crustal units). Also, the only incompa-tible element-enriched sample from Sangsang can be correlated withthe three IAT rocks from the Saga massif. The whole rock chemistrythus is indicative of the suprasubduction zone affinities of the Sagaand Sangsang ophiolitic massifs.

Fig. 13. Discrimination diagram from Pearce and Norry (1979) for the Saga mafic rocks.

Fig. 14. Polybaric origin of Saga and Sangsang massif peridotites as deduced from theCr# in spinels and coexisting Mg# in olivines. The olivine-spinel mantle array (OSMA;dotted field) and compositional range in abyssal peridotites are from Arai (1994). Thecompositional ranges in pre-oceanic and subduction margin peridotites are fromBonatti and Michael (1989). The dashed line represents the mean compositionalvariations in the Troodos peridotites (Sobolev and Batanova 1995). The 5 and 10 kbarcurves are from Sobolev and Batanova (1995), and the 15 kbar curve is from Jaques andGreen (1980).


6.2. Mantle refertilization

The Saga massif peridotites differ from the Sangsang peridotitesand also from most of the “typical” refractory peridotites observed inseveral ophiolitic massifs around the globe. Their flat REE patterns aresimilar to those of chondrites or primitive mantle and suggest thatthese rocks are not refractory. The distinct iron enrichment combinedwith the mineralogical and textural characteristics indicative of melt/fluid-rock interactions (e.g. orthopyroxene partly replaced by sec-ondary clinopyroxene, embayments into orthopyroxene crystals filledwith secondary olivine) suggests that these peridotites are refertilizedrefractory peridotites. The Saga peridotites might represent refractoryperidotites which have undergone variable degrees of partial meltingprior to enrichment events of percolation/entrapment by a fluid/meltof N-MORB/BABB affinities (Fig. 12A, blue lines). Alternatively, theseperidotites could be unmetasomatized fertile peridotites (Fig. 12A,grey field) representing pieces of oceanic mantle which has under-gone a very small partial melting event (~1–2%).

Similar flat REE patterns of peridotites have been observedelsewhere (e.g., the Pyrenees and the East Pacific Rise; Bodinier andGodard, 2003 and references therein). The REE patterns for the Sagaperidotites are consistent with the model calculations of partialmelting with melt entrapment of Godard et al. (2000). Therefore, werefine the model using the equations of Hellebrand et al. (2002) in

order to determine if the chemical effect of a melt entrapment on aresidual peridotite is consistent with the Saga REE patterns. Wemodelled the enrichment of a refractory harzburgite by a BABB melt(Fig. 15A). The results show that the most rocks can be accounted forby an entrapment of 2–5% of melt fraction. However, the calculatedmodel fails to explain the upper REE pattern, especially the gapbetween this pattern and the REE patterns with the absoluteabundances. An entrapment of a larger fraction (5–20%) of a primitiveBABBmelt (represented by our upper REE pattern) can account for theREE patterns of most peridotites. This result is in agreement withGodard's model (Godard et al., 2000) where flat REE patterns ofperidotites are the result of an enrichment of a refractory peridotite bypercolation or entrapment of more than 10% of an N-MORB melt.

6.3. Comparison with other YZSZ ophiolites

There are several similarities aswell as differences between the Sagaand Sangsang ophiolites and other YZSZ ophiolites (see Hébert et al.,2003; Dubois-Coté et al., 2005). From a lithological point of view, theSaga and Sangsang massifs share several features with western andeastern ophiolites. The Saga massif resembles the Yungbwa ophiolite tothe west which is also characterized by the absence of a typical crustalsection overlying themantle rockswhile the Sangsangmassif resemblesthe Xigaze area ophiolites. The occurrence of ocean-floor rocks (i.e.Saga's uppermost crustal and metamorphosed mafic upper crustalunits) underneath the ophiolitic mélange has also been recognized inmassifs to the east where it is known as the Yamdrock mélange. Thereare also somemassifs which directly overlie Indian affinity rocks such asthe Triassic flysch of the Sangsang ophiolite. An important feature is thehigh proportion of lherzolitic peridotites in the Sagamassif compared tothe harzburgitic rocks of Sangsang ophiolite and other massifs to theeast. A similar proportion of lherzolite has also been described forthe westernmost Yungbwa ophiolite and easternmost Dazhuquophiolite. The chemical composition of the main mineral phases of

Fig. 15. Chemical effect of a melt entrapment on an initially refractory harzburgite aftera linear mixing with a BABB melt (A) and a melt corresponding to our upper REE curve(B). Model calculations after Hellebrand et al. (2002).

Fig. 16. Simplified geodynamic model for the evolution of the Saga and Sangsangophiolites.


the peridotites and mafic rocks from both the Saga and Sangsangophiolites is similar to the forearc and abyssal compositions of otherYZSZ ophiolites as shown by Hébert et al. (2003; Fig. 9). However,according to the mineral chemistry, Saga peridotites tend to be moreprimitive or enriched than peridotites from other massifs, includingthe Sangsang massif. The whole rock geochemistry of Saga maficrocks can be correlated to the geochemistry of mafic rocks from thecrustal section of the seven ophiolitic massifs to the east describedby Dubois-Coté et al. (2005). Most of the rocks from the uppermostcrustal and metamorphosed mafic upper crustal units are compar-able to Group 1 rocks of BABB affinities of Dubois-Coté et al. (2005).Only three other samples are similar to their Group 2 arc-like rocks.The most outstanding attribute of the Saga ophiolite is the traceelement contents of the peridotites. No such flat chondritic REEpatterns have been reported for any YZSZ ophiolites.

6.4. Geodynamic implications

Numerous geodynamic models have already been proposed for thegenesis of the YZSZ ophiolites in a suprasubduction zone environment(Aitchison et al., 2000; Dubois-Côté et al., 2005; Dupuis et al., 2005a,b;Guilmette, 2005). The main differences among these models are thenumber of subducting slabs involved in the genesis of the ophiolites aswell as their spatial configuration. According to ourdata for the Saga andSangsang ophiolites, it is probable that more than one intraoceanic

subduction zone is required to explain the formation of the YZSZophiolites as first proposed by Guilmette (2005).We are thus proposinga complementary geodynamic model for the genesis of the YZSZophiolites.

According to the along-strike lithologic correlation and the agerelationships (~123–128Mametamorphic event), the Saga and Sangsangmassifs belong to the sameophiolite segment asotherophioliticmassifs tothe east (Xigaze ophiolites) and the west (Yungbwa ophiolite). Mineralchemistry data show that both the Saga and Sangsang ophiolites wereformed in a back-arc–arc environment. The Saga massif represents theback-arc end-memberwith abyssal peridotiteswhile the Sangsangmassifrocks are closer to the arc end-memberwith compositions similar to thoseof forearc peridotites like Luobusa ophiolite. The presence of two distinctenvironments (back-arc/arc) is also consistent with the whole rockgeochemistry data for the mafic rocks accreted to the Saga ophiolitemantle section and themafic rocks of the Sangsangmassif. Trace elementsshow that the Saga's rocks of the uppermost crustal and the metamor-phosed mafic upper crustal units as well as some mafic rocks from theSangsangmassif havebeen formed inaback-arcbasinenvironmentwhereminor mature arc volcanic material had access to the basin. The traceelement contents of the Saga massif peridotites suggest that these rockswere generated in a zonewheremagmatic fluids ormelts have interactedwith mantle rocks. Sangsang massif peridotites also show evidence ofinteraction of a melt with the mantle rocks.

Since Saga suprasubduction zone (SSZ)mafic rocks do not form aproper ophiolite crustal section above the mantle crustal sectionand are instead « accreted » to the ophiolite, theywere likely formedwithin a first suprasubduction basin and subsequently metamor-phosed and buried with the development of a second subductionzone. Thus, at least two subduction slabs must have been active in acomplex arc–back-arc configuration (Fig. 16). The first stage wouldhave produced an BABB–island arc tholeiite affinity oceanic crust(e.g. Saga and Sangsang ophiolitic massifs) before 130 Ma whilethe second subduction event around 127 Ma would have triggeredthe off-scraping of the uppermost crust rocks of this BABB–IATcrust. Saga massif mantle rocks have probably been refertilized byenriched magmatic melts and/or fluids within the first suprasub-duction zone basin before 130 Ma or a little after the inception ofthe second subduction event. Dilek et al. (2008) reported analogousevidence of mantle source enrichment at or prior to the time of meltextraction.

The occurrence and spatial association of MORB- and SSZ-relatedophiolites have been recognized in many well-known ophioliticsmassifs (e.g. Dilek et al., 2008). The associations have been usually


explained by a process where an initial spreading phase that producedthe MORB crust was followed by the inception of an intraoceanicsubduction zone that resulted in the development of the SSZ crust.However, Dilek et al. (2008) showed that the observed MORB and SSZaffinities of the Albian ophiolites do not require separate tectonicsettings and significantly different times of formation. They showed thatan evolution from protoarc to forearc magmatism in response to slab-roll back combined to changes in mantle sources could produce thesevaried affinities. In fact, their MORB-like rocks are also suprasubductionproducts but only display a very small subduction component. Such ageochemical signature is similar to the signature of BABB-affinitiesrocks, which are characterized by a MORB signature showing a weak tomoderate contributionof subduction. Therefore, the BABBmafic rocks ofthe Saga massif are also the products of an early stage of protoarcmagmatism followedbya short stage of arcmagmatismbefore orduringthe inception of the second subduction event at 127 Ma.

7. Conclusions

The Saga and Sangsang ophiolites belong to the same intraoceanicsuprasubduction zone segment as other ophiolitic massifs of the E–W-trending Early Cretaceous ophiolite belt of the Yarlung Zangbo SutureZone. Both the Saga and Sangsang massifs are incomplete ophioliticsequences and comprise a prominent ophiolite mélange overlain by athin mantle section. The Saga peridotites are mostly lherzolitic withminor cpx-harzburgite while the Sangsang peridotites are harzburgi-tic rocks. Crustal rocks have been identified within both massifs butform a rather complete Penrose-type ophiolite stratigraphy only in theSangsang area (lavas and gabbros). Crustal mafic rocks of the Sagamassif are represented by a belt of metamorphosed upper crustalrocks and a sequence of uppermost crustal rocks « accreted » beneaththe ophiolite mélange. Peridotites from the Saga massif resembleabyssal peridotites and have undergone only low degrees of partialmelting (5–12%). According to their trace elements contents, theserocks are either unmetasomatized fertile peridotites or refertilizedrefractory peridotites (La/YbN~1). The Sangsang peridotites compriseboth abyssal and subduction-related peridotites and have undergonehigher degrees of partial melting (17–30%). The geochemicalcharacterisitics of the mantle rocks from these two massifs aredifferent compared to each other suggesting different petrogenetichistories. The uppermost crustal and metamorphosed upper crustalrocks of the Saga ophiolite contain all three N-MORB, BABB and IATcomponets. Our geodynamical model suggests that the Saga andSangsang ophiolitic massifs were formed in a complex arc–back-arcconfiguration where at least two intraoceanic subducting slabs musthave been active. This arrangement of subducting slabs is required toexplain field relationships such as in the Saga ophiolite, where a firstsubduction event would have produced an IAT–BABB affinity oceaniccrust which is then forced to plunge into themantle, triggering the off-scraping of the uppermost crust rocks of this young IAT–BABB crust.

Acknowledgements

The authors thank the Natural Sciences and Engineering ResearchCouncil (NSERC Grant No. 1253 to RH and a NSERC graduatescholarship to EB). We also wish to thank M. Choquette for microp-robe analyses at the Université Laval as well as anonymous reviewersfor their constructive comments.

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Petrology and geochemistry of the Saga and Sangsang ophiolitic massifs, Yarlung Zangbo Suture Zone,...

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