Proofreading Schweizerische Mineralogische und Petrographische Mitteilungen 83, X1–X24, 2003
Geochemistry of metabasalts from ophiolitic and adjacent distal continental margin units: Evidence from the Monte Rosa region (Swiss and Italian Alps)
Julia Kramer1, Rainer Abart1, Othmar Müntener2, Stefan M. Schmid1 and Willem-B. Stern1
Abstract
1 In this paper we present new whole rock analyses of amphibolites from the ophiolitic and adjacent continental tectonic units in the Monte Rosa region. Mg numbers and Ni contents indicate that these amphibolites were derived from fractionated magmas with compositions ranging from E- to N-MORB. Based on their Ni, Ti, REE and Nb systematics, the metabasalts from the ophiolitic Zermatt-Saas and Antrona units and from the continental units of 5 the Furgg zone and the Portjengrat unit are ascribed to a common origin. They represent a coherent suite ranging from T- to N-MORB. In contrast, amphibolites from the continental Siviez-Mischabel and Monte Rosa nappes were derived from enriched MORB and/or gabbroic precursors, which are not related to the metabasalts from the ophiolites, the Furgg zone or the Portjengrat unit. The geochemical differences between the basalts of the ophiolitic Zermatt-Saas and Antrona units and the adjacent 10 continental Furgg zone and the Portjengrat unit are very subtle. Most mafic rocks were derived from low to moderate degrees of melting of an N-MORB type mantle source. Some compositional parameters such as (Ce/Sm)n, Zr* and (Nb/Zr)n indicate a transition from T-MORB compositions in the continental units towards less enriched composi- tions in the ophiolitic units. Y, Ti, V and Zr concentrations are highly correlated in the metabasalts from the Furgg zone, whereas such inter-element correlations are less well defined in the metabasalts from the ophiolitic units. This renders? 15 renders the previously proposed interpretation of the Furgg zone amphibolites as tectonically incorporated ophiolitic fragments unlikely. Our data rather suggest that the distal continental units (Portjengrat unit and Furgg zone) and the nearby ophiolitic units were intruded by similar magmas. Portjengrat unit and Furgg zone are inter- preted as a formerly continuous tectonic unit which, based on structural grounds, represents the ocean-continent transition zone of the Briançonnais to the immediately adjacent oceanic Antrona unit. However, the ambiguity in the 20 paleogeographic provenance of the Antrona unit (Valais vs. Piemont-Liguria ocean) cannot be resolved with the existing geochemical data.
Keywords: Metabasalt, geochemistry, continental crust, Furgg zone, Monte Rosa, Central-Western Alps.
1. Introduction The paleogeographic relations were obscured 1 in the course of polyphase Tertiary deformation. The Western Alpine Arc is characterized by the NW-directed nappe stacking started in Eocene occurrence of remnants of different Mesozoic times under eclogite facies conditions (deforma- oceanic basins (e.g. Stampfli and Marchant, 1997 tion phases D1 and D2). This was immediately 5 and references therein; Schmid and Kissling, followed by exhumation during ongoing crustal 2000). In the Penninic nappe edifice of the Mon- shortening. Post-collisional SE-directed folding te Rosa region, the Zermatt-Saas and the Antro- and oblique thrusting of the entire nappe pile (de- na ophiolitic units represent the remnants of formation phases D3 and D4) finally resulted in oceanic crust. They are sandwiched between sev- the complex present-day geometry (Figs. 1 and 2). 10 eral slivers of continental crust, which pertain to The paleotectonic reconstruction of the Mon- the Monte Rosa and Siviez-Mischabel nappes, te Rosa region is still under discussion (Escher et and to the Portjengrat and Stockhorn units al., 1997; Froitzheim, 2001; Keller and Schmid, (Escher et al., 1997). The situation is illustrated 2001; Kramer, 2002). Controversies partially arise in a schematic geological sketch map of the from the interpretation of mafic dykes and bou- 15 Monte Rosa region (Fig. 1) and in a simplified dins, which occur within continental units and N–S cross-section (Fig. 2). which possibly represent derivatives from former
1 Department of Earth Sciences, University of Basel, Bernoullistr. 32, CH-4056 Basel.
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Sample locations in a simplified geological sketch map of the Monte Rosa region,
Fig. 1 Fig. (2000), Weber (2001), Bacher (2002), Bearth (1953a, 1954a,b), Steck et al. (1999), and mapping by Kramer (2002). (1999), Steck et al. 1954a,b), Bearth (1953a, Bacher (2002), (2001), Weber (2000),
55 Proofreading Geochemistry of metabasalts from the Monte Rosa region 3
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Fig. 2 Schematic S–N cross-section of the Monte Rosa region (Kramer, 2002), modified after Milnes et al. (1981). 20 oceanic domains. In the Monte Rosa region, mafic dome. It is comprised of large masses of Permian dykes and boudins are abundant in both the con- granitoids and orthogneisses, which were derived tinental and ophiolitic tectonic units, and they oc- from granitoid intrusions (Frey et al., 1976). These cur in Paleozoic continental crystalline crust, as granitoids were emplaced into pre-Permian, pri- 25 well as in Mesozoic cover sequences. Mafic bou- marily meta-pelitic paragneisses with subordinate dins are particularly abundant in two mylonitic mafic rocks (Hunziker, 1970). The present-day shear zones that overprint the nappe contacts be- geometry of the Monte Rosa nappe is largely con- tween continental and ophiolitic tectonic units: trolled by a large-scale SSE-facing D4 fold, re- (1) in the so-called “Furgg zone” and (2) in the ferred to as “Vanzone backfold” by Milnes et al. 30 “intensely strained Monte Rosa basement and (1981) and Escher et al. (1988, 1997). The ophio- cover” (Figs. 1 and 2; Kramer, 2002). Due to the litic Zermatt-Saas and Antrona units form an al- abundant mafic boudins, Froitzheim et al. (2001) most complete envelope around the continental interpreted the Furgg zone as a tectonic mélange Monte Rosa nappe, except for the area between with ophiolitic fragments. Other authors, howev- Gornergrat and Monte della Preja, where only 35 er, considered the mafic boudins as dyke intru- isolated slivers of ophiolites are present (Fig. 1). sions into continental lithosphere (e.g. Jaboyedoff The Furgg zone (FZ in Figs. 1 and 2) and the et al., 1996), and the Furgg zone as an intraconti- intensely strained Monte Rosa cover and base- nental shear zone (Escher et al., 1997; Keller and ment (ISMR in Figs. 1 and 2) represent up to one Schmid, 2001; Kramer, 2002). km thick mylonitic shear zones found north and 40 In this paper we present new whole rock major south of the Monte Rosa main crest, respectively. and trace element data on mafic rocks collected in They overprinted basement-cover contacts of the (1) the Furgg zone, (2) the adjacent continental continental tectonic units, as well as nappe con- Portjengrat unit and in the Monte Rosa and tacts between ophiolitic and continental tectonic Siviez-Mischabel nappes as well as (3) in the units during WNW-directed nappe stacking (de- 45 ophiolitic Zermatt-Saas and Antrona units. Our formation phases D1 and D2; Kramer, 2002). My- own and previously published data (Beccaluva et lonitic shearing resulted in the superposition of al., 1984; Pfeifer et al., 1989) shed light on magma two phases of isoclinal and/or sheath folding af- sources and evolution, and on the paleogeograph- fecting the nappe contacts between Zermatt-Saas ic relations among the tectonic units in the Monte unit and Monte Rosa nappe in case of the ISMR, 50 Rosa region. and between Portjengrat and Stockhorn unit, Monte Rosa nappe and Antrona unit in case of the FZ, respectively (see Fig. 2; Kramer, 2002). 2. Geological setting Subsequent reactivation of the Furgg zone during syn-D3 and syn-D4 shearing partly led to 55 The Monte Rosa nappe forms the “backbone” of the imbrication of isoclinal D1 and D2 folds, now the Penninic nappe edifice west of the Lepontine forming isolated fold cores that consist of a varie- Proofreading 4 J. Kramer, R. Abart, O. Müntener, S.M. Schmid and W.-B. Stern
1 ty of rocks of different provenance which are em- Siviez-Mischabel nappe forms the central part of bedded within a highly strained matrix. D3 and the Grand St. Bernhard nappe system. Whereas D4 deformation also accounted for large-scale its cover comprises only Permo-Triassic rocks in SE-directed folding and dextral oblique thrusting most parts of the region, a complete sedimentary 5 (e.g. Keller and Schmid, 2001; Kramer, 2002), in sequence of up to Eocene age is preserved in the the course of which both the Furgg zone and the “Barrhorn series” in Mattertal (Sartori, 1987, base- intensely strained Monte Rosa and cover were 1990). Since the mafic rocks that were analyzed in ment? tilted into their present-day structural position this study were sampled in the Furgg zone, the enveloping the Monte Rosa nappe. ophiolitic Zermatt-Saas and Antrona units, the 10 Several larger coherent imbricates of ultrama- continental Monte Rosa and Siviez-Mischabel fic and mafic rocks represent mappable units and nappes, as well as in the continental Portjengrat were therefore not regarded as part of the Furgg unit, these units will be described here in more zone, but assigned to the Zermatt-Saas (at detail. Gornergrat) and the Antrona units (at Monte del- 15 la Preja and in Furggtal), respectively. On struc- 2.1. Furgg zone tural grounds the numerous smaller mafic bou- dins found in both Furgg zone and intensely The Furgg zone comprises the following rock strained Monte Rosa basement and cover, howev- types derived from the neighbouring tectonic er, are considered as intrusions of former dykes units (Monte Rosa nappe and Portjengrat-Stock- 20 and sills into continental lithosphere, that were horn unit): para- and orthogneisses of Paleozoic overprinted by mylontitic shearing later. Hence, age, along with the adjoining meta-sedimentary these mafic boudins are regarded as part of both cover consisting of garnet micaschists, meta-arko- shear zones (FZ and ISMR) overprinting conti- ses, quartzites, and marbles of inferred Permo-Tri- nental tectonic units. It is one of the aims of this assic age (Bearth, 1954a,b, 1957; Klein, 1978; Ja- 25 publication to test this structural interpretation boyedoff et al., 1996; Rössler, 2000; Keller and by adding geochemical data. Note that our struc- Schmid, 2001; Bacher, 2002; Kramer, 2002). The tural interpretation differs from the view of abundant mafic intercalations, which typically Froitzheim (2001), who considers both the large take the form of disrupted layers and boudins are imbricates of ultramafics and the small mafic bou- a remarkable feature of the Furgg zone. In some 30 dins as ophiolitic imbricates and defines the Furgg places these mafics represent more than 50% of zone as an ophiolitic subduction mélange the rock volume on outcrop scale. In some of the (Froitzheim, 2001). mafic boudins, eclogitic assemblages are pre- The intensely strained Monte Rosa basement served (Wetzel, 1972; Liati et al., 2001). As de- and cover on the southern side of the Monte Rosa scribed in the previous section, several larger co- 35 massif (ISMR) was, however, not reactivated dur- herent slivers comprising ultramafics and mafics ing D3 and D4 shearing. There, syn-D1- and D2- were mapped as ophiolitic imbricates derived shearing mainly resulted in a complex superposi- from the Zermatt-Saas or the Antrona ophiolitic tion of two mylonitic folding phases on basement unit, and not as part of the Furgg zone (Keller and and cover of the Monte Rosa nappe. Because of Schmid, 2001; Bacher, 2002; Kramer, 2002). Be- 40 the abundant occurrence of mafic boudins in a tween Gornergrat and Monte della Preja (Fig. 1), highly strained matrix, the ISMR strongly resem- the Furgg zone overprints the nappe contacts of bles the Furgg zone in the field. However, the two the Monte Rosa nappe with the Stockhorn and shear zones differ in respect to their tectonic sig- Portjengrat units, respectively (Bearth, 1953a,b, nificance, and, as will be shown, also geochemical- 1954b, 1957; Escher and Sartori, 1991; Escher et 45 ly. Therefore, we distinguish between these two al., 1997). shear zones, although some authors consider the Within less deformed portions of the Furgg ISMR as part of the FZ (Dal Piaz, 1964; Froitz- zone rock types with a stratigraphy similar to that heim, 2001). In the following we will treat the of the Permo-Triassic meta-sediments in the adja- ISMR as part of the Monte Rosa nappe. cent Portjengrat-Stockhorn unit and Monte Rosa 50 The Portjengrat unit, as well as its southern nappe were mapped (Jaboyedoff et al., 1996; Kel- equivalent, the Stockhorn unit, consists of conti- ler and Schmid, 2001; Bacher, 2002; Kramer, nental basement and a cover series interpreted to 2002). Such a stratigraphic sequence is particular- represent Permo-Triassic sediments. The Portjen- ly well preserved in the Passo della Preja area grat unit is either interpreted as part of the Grand (Fig. 1), where Triassic dolomite and calcite mar- 55 St. Bernhard nappe system (e.g. Bearth, 1956) or bles are associated with rauhwackes, quartzites, rauhwackes? as a derivative of the Monte Rosa nappe (e.g. and meta-arkoses. These Triassic marbles were in- Bearth, 1957; Keller and Schmid, 2001). The truded by well preserved mafic dykes (see Fig. 5 in Proofreading Geochemistry of metabasalts from the Monte Rosa region 5
Jaboyedoff et al., 1996), which were collected for cate rocks (Bearth, 1952, 1954 a, b, 1957; Dal Piaz, 1 chemical analysis (“F4: Furgg zone marbles” in 1966, 1971; Escher et al., 1997), which were intrud- Table 1). ed by late Carboniferous to Permian granites (310–260 Ma; Hunziker, 1970; Frey and Hunziker, 2.2. Zermatt-Saas ophiolitic unit 1976; Engi et al., 2001). Peak metamorphic condi- 5 tions of 2.3 GPa and 520 °C were inferred from The Zermatt-Saas unit largely consists of meta- white schists within the Monte Rosa basement morphosed ultramafic and mafic rocks (Bearth (Le Bayon et al., 2001) and dated at 35–40 Ma and Stern, 1971, 1979; Pfeifer et al., 1989) with a (Hunziker, 1970; Gebauer, 1999). late Jurassic formation age (166–160 Ma, Gebau- Meta-sedimentary strata of inferred Permo- 10 er, 1999). The associated meta-sediments are Triassic age are rare, but partly preserved in upper manganese-rich quartzites (meta-radiolarites), Valle d’Ayas, Valle di Gressoney and Valle di Lo- marbles and calcareous micaschists (“Bündner- ranco (Keller, 2000; Kramer, 2002). These meta- schiefer” or “schistes lustrés”; Bearth, 1976; sediments consist of garnet micaschists, meta- Bearth and Schwander, 1981) of probably late arkoses and quartzites as well as calcite and dolo- 15 Jurassic to late Cretaceous age (Marthaler, 1981, mite marbles. They contain numerous mafic lay- 1984). The calcareous micaschists may contain ers and boudins. These meta-sediments are simi- meta-basalt boudins. High-pressure peak meta- lar to the Permo-Triassic cover of the Portjengrat morphic conditions of 2.6–2.8 GPa and 590–630 °C unit. Yet, they were interpreted as Paleozoic in are recorded by meta-radiolarites (Reinecke, 1991, age by Dal Piaz (1966, 2001). With respect to their 20 1998; van der Klauw et al., 1997) while conditions spectrum of rock types, boudin-rich parts of the of 1.75–2.0 GPa and 550–600 °C are reported from Monte Rosa basement and its meta-sedimentary the ophiolites (Barnicoat and Fry, 1986). Peak cover are rather similar to the Furgg zone and re- metamorphic conditions were reached during the ferred to as “intensely strained Monte Rosa cover Middle Eocene, i.e. at 44 Ma (Amato et al., 1999; and basement” (ISMR) in Fig. 1. 25 Rubatto and Gebauer, 1999; Gebauer, 1999) while the retrograde greenschist facies overprint oc- 2.5. Siviez-Mischabel nappe curred during the early Oligocene (Müller, 1989; Barnicoat et al., 1995; Gebauer, 1999). The Siviez-Mischabel nappe consists of a Paleo- zoic polymetamorphic basement (Bearth, 1945; 30 2.3. Antrona ophiolitic unit Thélin et al., 1993; Escher et al., 1997) and its rela- tively well-preserved Permo-Mesozoic cover (i.e. The Antrona unit consists of metamorphosed ul- Barrhorn series; Marthaler, 1984; Sartori, 1990). In tramafic and mafic rocks and associated marbles contrast to all the other tectonic units described in and calcareous micaschists (Carrupt and Schlup, this contribution, no eclogite facies event is docu- 35 1998; Colombi, 1989; Pfeifer et al., 1989). So far, no mented. Nappe emplacement occurred under radiometric formation ages have been reported greenschist facies conditions, estimated at 350 °C for the meta-basic rocks. Recently discovered ec- to 450 °C and 0.4–0.6 GPa by Sartori (1990) in late logites (Keller, pers. comm.) probably represent Eocene times (Markley et al., 1998). The basement the remnants of a high-pressure metamorphic rocks contain numerous mafic layers and boudins 40 event. Engi et al. (2001) related the peak meta- (see Thélin et al., 1990; Eisele et al., 1997). morphic conditions found within the eastern Monte Rosa nappe to the emplacement of this 2.6. Portjengrat and Stockhorn units nappe into the surrounding units at 35–40 Ma ago, hence the high pressure metamorphic peak within The Portjengrat unit comprises basement of 45 the adjacent Antrona unit might fall into the same Paleozoic age, consisting of calc-silicates, para- time span. The Oligocene retrograde greenschist gneisses and orthogneisses (Huang, 1935a,b; facies overprint as a result of the exhumation of Bearth, 1945; Escher et al., 1997) and a cover of the nappe pile, and a prograde amphibolite facies presumed Permo-Mesozoic age (Dubach, 1998). metamorphic overprint in the area of the Lepon- Both basement and Permo-Triassic cover contain 50 tine dome were described by Pfeifer et al. (1989) numerous mafic layers and boudins. In Valle di and by Colombi (1989). Loranco, the abundance of mafic layers and bou- dins, as well as strain intensity, gradually increase 2.4. Monte Rosa nappe towards the Furgg zone (Keller and Schmid, 2001). This is one of the reasons, why we regard 55 The basement of the Monte Rosa nappe consists the Furgg zone as a shear zone that overprints of pre-Carboniferous paragneisses and calc-sili- continental crust, in this case the Portjengrat unit Proofreading 6 J. Kramer, R. Abart, O. Müntener, S.M. Schmid and W.-B. Stern
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(see Kramer, 2002). Note, however, that further di Loranco (sample 99-105). Here the Antrona 1 west (in Furggtal and Saastal), the basement of ophiolites occur within the core of an isoclinal an- the Portjengrat unit contains far fewer mafic tiform (Kramer, 2002), which refolds a mappable dykes and boudins (Rössler, 2000; Weber, 2001; tectonic contact between Antrona unit and Furgg Bacher, 2002). zone (Keller, 2000; Bacher, 2002). 5 The cover of the Portjengrat unit, which is well preserved e.g. at the Mittaghorn in Saastal and at 3.2. Metabasalts from the Zermatt-Saas the Zwischbergen pass in Almagellertal-Zwisch- ophiolitic unit (O2) bergental (Huang, 1935a,b; Bearth, 1954b, 1957; Dubach, 1998) consists of a sequence of garnet The Zermatt-Saas unit was sampled close to the 10 micaschists, meta-conglomerates, meta-arkoses, contact with the Portjengrat-Stockhorn unit rauhw quartzites, marbles, and rauhwackes. Boudinaged (sample 99-53 from Mittaghorn in Saastal) and meta-basalt layers are oriented parallel to the with the Monte Rosa nappe (sample 99-31 from main foliation. As the metasediments of pre- Rifugio Mezzalama in Valle d’Ayas), respectively. sumed Permo-Triassic age have been continuous- Sample 99-53 is from the northern limb of the D3 15 ly traced all the way to Gornergrat in Mattertal, Mittaghorn synform, 50 m south of the contact the Portjengrat and Stockhorn units are correlat- with the calcareous micaschists. Sample 99-31 was ed (Kramer, 2002). Near the Stockhorn at collected ten meters west of the contact with the Gornergrat, a prograde eclogite facies mineral as- highly strained sedimentary cover of the Monte semblage of early Oligocene age (34.9 ± 1.4 Ma) is Rosa nappe. 20 preserved in Permo-Mesozoic quartzites (“Gor- nergrat series”) for which conditions of P < 1.4– 3.3. Meta-basalt sills from the continental base- 1.5 GPa, T < 500–550 °C were estimated by Ru- ment of the Siviez-Mischabel nappe (C1) batto and Gebauer (1999). Samples were taken at localities in Mattertal and 25 Saastal. The sampling sites are structurally locat- 3. Sample descriptions ed in the core of the D3 “Mischabel backfold”, a south-facing kilometer-scale antiform. Samples Metabasalts sampled for geochemical analysis 99-14 and 99-15 (near Zermatt in Mattertal) were were taken from the Furgg zone and all adjacent collected approximately 60 m north of the base- 30 units described above. Sample localities are ment-cover contact. Samples 99-40 and 99-41 shown in Fig. 1 and brief sample descriptions are (near Zen Eisten in Saastal) were collected within listed in Table 1. Based on their lithologic-tectonic basement far off any tectonic or sedimentary con- setting, the metabasalts are divided into 9 classes. tact. All samples are from up to 1.5 m thick, partly These classes may be grouped into three broad boudinaged and/or isoclinally folded foliation- 35 categories: (1) metabasalts from ophiolitic units parallel layers. They are interpreted as early (pre- (Antrona and Zermatt-Saas units); (2) mafic sills, Alpine?) sills, which intruded the paragneisses of dykes and boudins, found within continental the Siviez-Mischabel nappe and were boudinaged basement and associated meta-sedimentary cover and folded during subsequent deformation. series (Siviez-Mischabel nappe, Portjengrat- 40 Stockhorn unit and Monte Rosa nappe); (3) me- 3.4. Isolated meta-basalt boudins from tabasalts from the Furgg zone. basement and cover of the southwestern Monte Rosa nappe (C2) 3.1. Metabasalts from the Antrona ophiolitic unit (O1) This group comprises samples from the intensely 45 strained Monte Rosa cover and basement in the Three samples were taken from the main body of southwestern part of the Monte Rosa massif the Antrona unit (sample 99-98 near Alpe Varallo (ISMR), which was considered as part of the in Valle di Bognanco; sample 99-102 at Cavalli Furgg zone by some authors (i.e. Dal Piaz, 1964; lake in Valle d’Antrona), one of them close to the Froitzheim, 2001). Meta-basalt boudins frequent- 50 contact with the Moncucco-Camughera unit ly occur in paragneisses whereas they are scarce (sample 98-53; Antronapiana in Valle d’Antrona). in meta-granites. Since one of the metabasalt bou- A large coherent imbricate of the Antrona unit, dins was found to be intruded by a granite of sup- situated near the Simplon shear zone, was sam- posed late Paleozoic age (Kramer, 2002) near the pled at San Marco in Valle d’Antrona (sample 98- Mezzalama refuge in upper Valle d’Ayas, at least 55 07). The “Loranco-Lappen” (Bearth, 1954 b, 1957) some of these boudins are probably Paleozoic in was sampled close to Alpe Campolamana in Valle age. Furthermore, paragneisses occasionally con- Proofreading 10 J. Kramer, R. Abart, O. Müntener, S.M. Schmid and W.-B. Stern
1 tain pre-Alpine sillimanite, oriented parallel to trix of isoclinally folded orthogneisses, garnet mi- the foliation and the principal extension direction caschists and meta-arkoses. As the sample locali- of the embedded mafic boudins (Dal Piaz, 1966, ties are structurally situated in the flat hinge re- 1971). This also suggests a Paleozoic age for these gion of the D4 Vanzone antiform, the distance to 5 metabasalts. Since mafic boudins are abundant the nappe contact of the overlying Zermatt-Saas within paragneisses of the Monte Rosa nappe, unit, as inferred from cross-section (Kramer, and particularly so near the tectonic contact with 2002), is probably less than 100 meters. Hence the the Zermatt-Saas unit, field evidence does not al- boudins could also represent tectonically incor- low us to exclude the possibility that they may porated ophiolitic imbricates. 10 represent ophiolitic imbricates incorporated dur- ing nappe stacking. 3.5. Meta-basalt sills from the Permo-Triassic Metabasalts from this group were sampled in cover of the Portjengrat-Stockhorn unit and Valle di Gressoney at the Alpe Bettolina (sample northern Monte Rosa nappe (C3) 99-20) and at the Plateau del Lys (samples 99-66, 15 99-69, 99-70, 99-71, 99-72). The Bettolina sample This group of samples comprises meta-basalt oc- (99-20) stems from a 0.3 m thick boudin, whereas currences, which represent intrusions into Permo- stems sample 99-69 stems from a large imbricate of ap- Triassic country rocks, for which mélange forma- proximately 25 by 12 meters in size. The other tion can be excluded. Metabasalts taken from the samples were collected from 3 m thick boudins. cover of the Portjengrat-Stockhorn unit were 20 Except for sample 99-66, all sampled boudins oc- sampled in tectonically relatively undisturbed cur in a matrix of garnet micaschists, which are meta-arkoses and marbles below the Mittaghorn believed to represent Permo-Mesozoic cover. in Saastal (samples 98-75, 99-50, 99-54, 99-55). The Sample 99-66 occurs in an intensely strained ma- samples were collected from 0.6 to 1 m thick, part-
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O+K 5 2 Na 3 Basaltic 40 Picrobasalt andesite 1 Basalt
41 45 49 53 57 SiO2 (wt%) 45 F1: Boudins from the Furgg zone at Gornergrat F2: Boudins from the Furgg zone in Saastal F3: Boudins from the Furgg zone in Furggtal and Valle d'Antrona F4: Dykes and sills in marbles of the Furgg zone
50 O1: Meta-basalts from the Antrona unit O2: Meta-basalts from the Zermatt-Saas unit
C1: Sills in basement of the Siviez-Mischabel nappe C2: Boudins from the intensely strained Monte Rosa cover & basement 55 C3: Sills in marbles of the Monte Rosa and Portjengrat cover series Fig. 3 Total alkali versus silica diagram after Le Bas et al. (1986). The symbols are generally larger than the 3 sigma error. Proofreading Geochemistry of metabasalts from the Monte Rosa region 11 ly boudinaged foliation-parallel layers, interpret- Metabasalts from Furggtal were collected 1 ed as sills or parallelized dykes. from up to 1.5 m thick boudins embedded in a One amphibolite was collected from the Mon- matrix of meta-arkoses (sample 99-57), garnet mi- te Rosa unit in Valle di Loranco, i.e. from an ap- caschists (sample 98-102) and orthogneisses proximately 3 m thick, boudinaged foliation-par- (sample 99-61). The meta-arkoses are intercalated 5 allel layer, found in carbonate-bearing meta- with garnet micaschists, at least partly due to iso- arkoses of inferred Permo-Triassic age (sample clinal folding. One meta-basalt sample collected 99-13 at Bottarello glacier). from Valle d’Antrona (sample 98-77 from west of Lake Cavalli) stems from a 1 m thick boudin in a stems 3.6. Metabasalts from the Furgg zone at matrix of garnet micaschists. It was collected 100 10 Gornergrat (F1) m east of and structurally below the contact to the basement of the Monte Rosa nappe. Here the Furgg zone consists of an association of paragneisses and calc-silicates of supposed Paleo- 3.9. Dykes and sills in marbles of the zoic age and a cover sequence, including garnet mi- Furgg zone (F4) 15 caschists, meta-arkoses, quartzites and calcite-dol- omite marbles of inferred Permo-Triassic age. The This group of Furgg zone samples comprises me- meta-basalt samples from this part of the Furgg tabasalts found in marbles of the Furgg zone in zone were collected in the Stockchnubel area the Monte della Preja area. Samples stem from (samples 99-78, 99-86, 99-87) and east of P. 3223 Alpe Preja (sample 98-82) and from the core of a stem 20 (sample 99-85; see Table 1 for exact location). Sam- synform at the Passo della Preja between Valle di ple 99-78 was collected from an approximately 30 Loranco and Valle di Bognanco (samples 98-68, by 10 m large boudin, while the other samples are 99-103, 99-104). Here the metabasalts may readily from smaller boudins up to 1 m in thickness. The be interpreted as representing sills or parallelized boudins in the vicinity of the Stockchnubel are em- dykes, which intruded Triassic marbles. 25 bedded in a matrix of garnet micaschists and The post-Triassic intrusion age is particularly quartzites; the sample east of P. 3223 (sample 99- evident for sample 99-104, which was collected 85) is surrounded by a matrix of dolomite marble. from a 0.3 m thick discordant dyke described by Jaboyedoff et al. (1996, see there Fig. 5), obviously 3.7. Metabasalts from the Furgg zone post-dating the Triassic marbles. Samples 98-82 30 in Saastal (F2) and 98-68 were collected from 0.8–1.5 m thick fo- liation-parallel boudins, sample 99-103 from a 0.6 Here the Furgg zone consists of an assemblage of m thick foliation-parallel layer. highly strained, fine-grained orthogneisses and the presumed Permo-Triassic cover of garnet mi- 35 caschists and subordinate meta-arkoses (see 4. Mineralogical composition of metabasalts Bearth, 1954b, 1957; Rössler, 2000; Weber, 2001). It contains metabasalts, which were sampled near All samples are fine-grained, foliated amphibo- the Allalin glacier (samples 98-73, 98-100, 99-95) lites, with a grain size of generally less than 2 mm. and at the river Saaser Vispa (sample 97-15). Sam- Mineral parageneses (see Table 1) include yellow 40 ples from the Allalin glacier area were collected and green actinolitic hornblende (act), albitic pla- from two up to 0.5 m thick boudins (samples 98- gioclase (ab), clinozoisite (cz), zoisite (zo), epi- 73 and 98-100) and one 3 m thick boudin (sample dote (ep), chlorite (chl), white mica (wm) and ti- 99-95), all embedded in garnet micaschists and tanite (ttn) +/– biotite (bt). This assemblage is in- meta-arkoses. Sample 97-15 (river Saaser Vispa) terpreted to have formed during retrograde 45 was collected from a 0.8 m thick boudinaged layer greenschist facies overprint. Accessory minerals embedded in meta-arkoses. are rutile (rt), ilmenite (ilm), +/– hematite (hem), +/– pyrite (py), +/– zircon (zr), +/– quartz (qtz), +/ 3.8. Metabasalts from the Furgg zone in – calcite (cc). Furggtal and in Valle d’Antrona (F3) Relics of an earlier high-pressure metamor- 50 phic event could only be identified in amphibo- Here the Furgg zone mostly comprises the same lites collected from the Permo-Mesozoic sedi- lithologies as described for Saastal. However, in mentary cover of the Portjengrat-Stockhorn unit Valle d’Antrona quartzites occur less and marbles and the Monte Rosa nappe, where abundant gar- more frequently in the Furgg zone. Mafic boudins net is found. This garnet is partly retrogressed to 55 are particularly abundant, and some of them are blue-green hornblende and / or chlorite. Wetzel several tens of meters long. (1972) and Liati et al. (2001) also describe relics of Proofreading 12 J. Kramer, R. Abart, O. Müntener, S.M. Schmid and W.-B. Stern
1
5
10
15
ratio, the Ce and Yb values are the Ce and ratio,
n 20
/Yb
n
25
ratio. For the Ce For ratio.
n
/Yb
n
30
35
40
chondrite-normalized; normalization values were taken from Boynton (1984).
45 Y ratio and Ce Zr/ Mg# number, Analytical data,
Table 2
50
55
1 8/8
5
10
15
20
25
30
35
40 Proofreading Geochemistry of metabasalts from the Monte Rosa region 13
1
5
10
15
20
25
Continued. 30
Table 2
35
40
45
50
55
1 8/8
5
10
15
20
25
30
35
40 Proofreading 14 J. Kramer, R. Abart, O. Müntener, S.M. Schmid and W.-B. Stern
1 high-pressure assemblages within amphibolites of glass discs. The XRF analyses (SRS-3400 Bruker- the Furgg zone (see below). Two generations of AXS) were done at the geochemical laboratory of albitic plagioclase and actinolitic hornblende are the University of Basel. The statistical uncertainty common. A first generation of plagioclase and at the 3 sigma-level is less than 2 relative % for all 5 hornblende (ab 1, act 1) forms symplectites which major elements, except for K2O (5–10 relative %) partly replace blue-green hornblende and, proba- and TiO2 (2–20 relative %). Trace elements were bly more commonly, former omphacite (see analyzed by ICP-MS at the CNRS in Nancy, Kramer, 2002). These symplectites are replaced France. For this purpose, 300 mg of sample powder by a second generation of synkinematically grown were fused with LiBO2 and dissolved in HNO3. De- 10 albite and actinolitic hornblende (ab 2, act 2), or tection limits range from 0.01 to 6 ppm (Table 2). by post-kinematic clinozoisite and zoisite. The ret- Uncertainties are usually less than 10 relative % rograde breakdown of garnet produced chlorite, except for values close to the detection limit. zoisite/ clinozoisite and more rarely blue-green or yellow-green hornblende. Titanite is commonly 5.2. Major and trace element composition 15 the breakdown product of ilmenite and rutile, but also occurs as xenoblastic grains in the matrix. The major and trace element compositions of the amphibolites are given in Table 2. Additional analyses from the Zermatt-Saas and Antrona 5. Whole rock chemistry ophiolitic units may be found in Pfeifer et al. 20 (1989). The SiO2 concentrations generally fall into 5.1. Analytical methods a range between 46 and 52 wt%. In the total alkali versus silica (TAS) diagram after Le Bas et al. Samples were carefully selected to avoid cracks or (1986), the analyses plot into the basalt and basal- veins, and weathered crusts were removed. The tic andesite fields (Fig. 3). However, all samples 25 samples were crushed in a jawbreaker and ground are amphibolites and it is therefore likely that al- in an agate mill to grain sizes < 60 µm. For major kali metals were affected to some degree by ei- element analyses 300 mg of ignited powder were ther ocean floor (hydrothermal) and/or regional mixed with 4700 mg of dried Li2B4O7 and fused to metamorphism (Pearce, 1976; MacGeehan and
30 Furgg zone Ophiolites Continental basement & cover 2.5
2.0
1.5
35 (wt%) 2 1.0
TiO 0.5
40 50
40
30
20 45 (ppm) Y
150
50 Zr (ppm) 100
50 40 45 50 55 60 65 45 50 55 60 65 45 50 55 60 65 55 Mg# Mg# Mg# Fig. 4 Concentrations of selected major and trace elements versus Mg# (MgO/(MgO+FeOtot) in moles); symbols as in Fig. 3. Proofreading Geochemistry of metabasalts from the Monte Rosa region 15
MacLean, 1980; Gelinas et al., 1982; Mottl, 1983; The large range of MgO (4.06–8.62 wt%) cou- 1 Rollinson, 1983; Saunders and Tearney, 1984). The pled with relatively low Ni contents (< 140 ppm, same is true for other large ion lithophile ele- Table 2, see also Fig. 8) indicates that all amphibo- ments (LILE; Cs, Rb, Ba, Pb and Sr), which is re- lites are derived from differentiated magmas and flected by large concentration variations in the do not represent unmodified liquids that were in 5 different samples (see Table 2 and Fig. 7). This equilibrium with the mantle. This is illustrated in makes it difficult to use major elements and espe- Fig. 4. Mg# (molar Mg/(Mg+Fetot)) are less than cially the alkali metals to decipher the igneous 65 and decrease with increasing incompatible ele- history of the amphibolites. Therefore we mainly ment contents. rely on trace elements in the following discussion, Harker diagrams of selected major and trace 10 which are considered to be less mobile during al- elements (Fig. 5) show that incompatible ele- teration and metamorphism (Dostal and Capedri, ments (Ti, Y, V) are positively correlated with Zr. 1979; Grauch, 1989; Gelinas et al., 1982; Hum- These correlations are well defined for the sam- phris, 1984). ples from the Furgg zone, but less clear for the 15 Furgg zone Ophiolites Continental basement & cover 2.50
2.00
1.50 20 (wt%) 2 1.00 TiO 0.50
25 50
40
30 Y (ppm) Y 30 20
12.50 10.00 35 7.50
5.00 Nb (ppm) 2.50
40 300
250
200 V (ppm) 150 45
50 40
30 50
20
Zr/ Nb (ppm) 10 0 25 75 125 175 75 125 175 75 125 175 55 Zr (ppm) Zr (ppm) Zr (ppm) Fig. 5 Concentrations of selected major and trace elements versus Zr concentrations; symbols as in Fig. 3. Proofreading 16 J. Kramer, R. Abart, O. Müntener, S.M. Schmid and W.-B. Stern
1
5
10
15
20
25
30
35
Fig. 6 Chondrite-normalized rare earth element con- centrations, normalization values from Boynton (1984): La: 0.31; Ce: 0.808; Pr: 0.122; Nd: 0.60; Sm: 0.195; Eu: 40 0.0735; Gd: 0.259; Tb: 0.0474; Dy: 0.322; Ho: 0.0718; Er: 0.21; Tm: 0.0324; Yb: 0.209; Lu: 0.0322.
45 ophiolitic units and the C1 and C2 units. This sug- In MORB the REE concentrations may vary gests that the Furgg zone amphibolites were de- from less than 10 times chondritic in primitive ba- rived form a single cogenetic magma suite, where- salts to about 50 times chondritic in more evolved as a more heterogeneous magma source and/or a basalts (Schilling et al., 1983; Venturelli et al., more heterogeneous magma evolution was asso- 1981; Pearce, 1982; Cullers and Graf, 1984; Hen- 50 ciated with mafic magmatism in the ophiolitic derson, 1984; Saunders, 1984; Rollinson, 1993; units. Desmurs et al., 2002). Low-pressure fractional The Zr/Nb ratio in metabasalts from the crystallization leads to an overall enrichment of ophiolitic units is about 30 (Fig. 5). Samples from the incompatible elements, but does not change the Furgg zone generally plot at somewhat lower the slope of the REE pattern (Schilling, 1983; 55 Zr/Nb ratios indicating a higher T-type MORB af- Frey et al., 1976). This is why the shape of REE finity than the classical N-type MORB of the patterns observed in evolved basalts largely re- ophiolitic units (Wood et al., 1979). flects the REE chemistry of the parent liquid, and Proofreading Geochemistry of metabasalts from the Monte Rosa region 17
JK 98-77 1 100 100 JK 98-102 JK 98-53 Furgg B JK 99-57 Serpentinte JK 99-98 50 50 JK 99-61 JK 99-102 JK 98-82 5 JK 99-105 JK 98-68 JK 98-07 JK 99-103 JK 99-31 JK 99-104 10 JK 99-53 10 10 5 5
15
100 100 JK 99-78 JK 99-14 JK 99-20 Furgg A JK 99-85 JK 99-15 JK 99-69 50 JK 99-86 50 JK 99-40 JK 99-70 Primitive mantle normalized JK 99-87 JK 99-41 JK 99-71 20 JK 99-95 JK 99-72 JK 98-100 JK 99-66 JK 97-15 10 JK 98-73 10 25 5 5 Primitive mantle normalized
30 Nb La Pr Nd Sm Gd Dy Er Yb Ta Ce Sr Zr Eu Tb Ho Tm Lu 100 JK 99-50 Fig. 7 Primitive mantle-normalized diagrams of se- 50 JK 98-75 lected trace elements of amphibolites from the Monte JK 99-55 Rosa region and surrounding areas (normalization after JK 99-13 Gabbro of Liati et al. 35 Sun and McDonough 1989). JK 99-54
10
REE are particularly useful for discrimination 5 40 between different MORB magma types. With re- spect to the relative abundance of light, interme- diate and heavy REE, the investigated samples fall into two broad categories (Fig. 6). A first group shows relatively flat REE pat- Nb La Pr Nd Sm Gd Dy Er Yb 45 terns with 10 to 20 times chondritic concentra- Ta Ce Sr Zr Eu Tb Ho Tm Lu tions, typical for T-type MORB, with (Ce/Yb) be- tween 1.00 and 1.76 (Table 2). This group includes all samples from the two ophiolitic units (O1 and teristic feature of low-degree melts generated in O2), all samples from the Furgg zone (F1–F4) and the spinel peridotite field (e.g. Casey, 1997). Sr 50 the basalts of group C3 (Fig. 6). Primitive mantle- (and Rb, which is not shown) is highly variable normalized trace element patterns (Fig. 7; after and may reflect alteration processes related to the Sun and MacDonough, 1989) indicate a consist- breakdown of plagioclase. ent positive Zr anomaly, with the exception of two A second group of samples comprises dykes samples, which probably represent former gabbro and sills from the Siviez-Mischabel basement unit 55 cumulates (see below). Such a trace element pat- (C1) and isolated boudins found within the south- tern is not uncommon in basalts and is a charac- western part of the Monte Rosa basement and Proofreading 18 J. Kramer, R. Abart, O. Müntener, S.M. Schmid and W.-B. Stern
1 50 14
(a)Furgg zone (b) Saas Zermatt & Antrona ophiolite 12 40 Portjengrat & Monte Rosa cover 5 Monte Rosa Basement & cover Siviez-Mischabel basement 10
30 8 Ce (ppm) 10 Nb (ppm) 6 20
4
10 15 2
0 0 0 20 40 60 80 100 120 140 0 20 40 60 80 100 120 140 Ni (ppm) Ni (ppm)
20 Fig. 8 (a) and (b). Compatible (Ni) versus incompatible element (Nb, Ce) plots. All samples (filled symbols) from the Furgg zone (except one, open circle), the Zermatt-Saas and Antrona ophiolitic units and the Portjengrat unit (except one, open diamond) indicate a coherent chemical trend with decreasing Ni and increasing incompatible elements (Ce, Nb). Open symbols show either low incompatible elements at low Ni content or high incompatible element content at variable Ni content (all samples from the Monte Rosa basement, and the Siviez-Mischabel nappe). These samples are probably not metabasalts but gabbro cumulates or gabbros with high proportions of 25 trapped interstitial liquid or (in the case of the Siviez Mischabel nappe) metabasalts with a source enriched in incom- patible elements.
cover (C2). These samples have highly variable from the ophiolites, the Furgg zone (with one ex- REE patterns and slopes (Fig. 6). The compatible/ ception) and the Portjengrat unit (with one ex- 30 incompatible element ratios (Fig. 8) are different ception) show a smooth increase of incompatible from those of the first group. As can be seen from Ce and Nb with decreasing Ni content. The sam- Fig. 8, the C1 and C2 samples show different Ce/ ples falling off the trend most probably represent Ni ratios and highly variable Nb contents. These cumulates. These samples are not further consid- features indicate that the C1 and C2 samples have ered in the discussion1 . 35 another origin than all the other samples. Compared to the samples from the ophiolitic units, the Furgg zone and the Portjengrat unit, the samples from the Siviez-Mischabel basement 6. Discussion and conclusions (C1) and the southwestern Monte Rosa nappe (C2) exhibit generally higher LREE contents at 40 6.1 Gabbro or basaltic precursors of comparable Ni (Fig. 8) and REE spectra with the amphibolites? higher LREE/ HREE ratios (Fig. 6). In addition, in the C2 group Nb is highly variable, which prob- It is often difficult to decide from whole rock ably reflects different proportions of Fe–Ti oxides analyses of amphibolites whether they were de- 45 rived from basaltic or gabbroic precursor rocks. 1 This problem is particularly difficult to tackle if U–Pb dating of zircons from sample 99-13 (Monte Rosa cover) yielded a Paleozoic age (Liati et al., 2001), which the metamorphosed mafic rocks are differentiat- was interpreted to represent the time of crystallization of ed, because Fe–Ti-oxide gabbros are often fine- the igneous (gabbroic?) protolith by these authors. How- grained and resemble metamorphosed basalts. ever, the chemistry of this sample (Figs. 6, 8) is indicative 50 Compatible element (Ni) versus incompatible el- of a cumulate precursor, and hence the sample is not rep- ements (Ce, Nb) covariation trends may provide a resentative of the suite of amphibolites in Furgg zone chemical criterion for the distinction between dif- (group F1–F4), Portjentgrat unit and northern Monte ferent protoliths (see Fig. 8). If samples represent Rosa nappe (group C3). Since we do not regard the sam- ple as part of the same magmatic suite, its Paleozoic age is a single suite of crystallized liquids (e.g. basaltic not considered as representative of the other samples. 55 rocks), smooth negative correlations between Furthermore, we doubt that the dated zircons necessarily compatible and incompatible elements are ex- represent the age of the igneous protolith of sample 99- pected. As can be seen from Fig. 8, all samples 13, as the zircons might be inherited. Proofreading Geochemistry of metabasalts from the Monte Rosa region 19
1 Furgg zone 1.8 (a) Saas Zermatt & Antrona ophiolite Portjengrat & Monte Rosa cover 1.4 Platta T-Morb 5
n Platta N-Morb Gets T-Morb 1.0 IL N-Morb EL T-Morb (Nb/Zr) Corsica N-Morb Fig. 9 Incompatible element ratio plots for metabasalts from the Furgg zone and adjacent 0.6 Corsica T-Morb 10 areas, and comparison with basalts from other Piemont-Ligurian ophiolites. Only samples with filled symbols from Fig. 8 are plotted here. (a) 0.2 (Nb/Zr)n vs. (Ce/Sm)n plot. High values for T- MORBs from the Gets and Platta nappe indi- 0.6 0.8 1.0 1.2 1.4 1.6 cate an enriched mantle source, whereas the 15 (Ce/Sm)n lower values for the N-MORBs from the Platta nappe, Ligurides and Corsica are similar to those (b) of present-day N-MORBs. The samples from the 1.8 Furgg zone are mostly T-MORB, while some samples from the Zermatt-Saas and Antrona 1.6 ophiolitic units are approaching N-MORB. (b) 20 Zr* (Zrn/((Smn+Ndn)/2)) vs. (Nb/Zr)n plot. Zr* is sensitive to changes in the degree of partial melt- 1.4 ing producing the basalts, whereas (Nb/Zr)n is characteristic of the source of melting. All data Zr* 1.2 from the Furgg zone and adjacent areas indicate 25 relatively low degrees of partial melting. Data 1.0 sources (averages, thin lines indicate 2 standard deviation of the mean): Gets nappe: Bill et al. 0.8 (2000); Platta nappe: Desmurs et al. 2002; Inter- nal and External Ligurides: Rampone et al. (1998); Corsica: Saccani et al. (2000). 30 0.2 0.6 1.0 1.4 1.8 (Nb/Zr)n in these samples. The smooth REE spectra from salts from Piemont-Ligurian ophiolites from out- 35 the Siviez-Mischabel basement (C1) may indicate side the study area. The Cen/Smn and the Nbn/Zrn enriched MORB (e.g. Bill et al., 2000) derived ratios of the Antrona and Zermatt-Saas ophio- from a source with initially high incompatible ele- lites cluster in two different groups, one with Cen/ ment content. The samples from the southwestern Smn < 1 and Nbn/Zrn < 0.4, and one with Cen/Smn Monte Rosa nappe (C2) do not show any system- > 1 and Nbn/Zrn > 0.4, respectively (Fig. 9 a). The 40 atics with respect to incompatible elements (Fig. latter are very similar to the Furgg zone metaba- 8). In addition, field relations indicate a possible salts and are intermediate between T-MORB Paleozoic origin for some of the amphibolites from the Gets and upper Platta nappe, and T- from the southwestern Monte Rosa nappe. This is MORB from the external Ligurides and Corsica why we think that the C1 and C2 amphibolites (Fig. 9b). This suggests that the Furgg zone meta- 45 cannot be directly compared with the amphibo- basalts are T-MORB from a single homogenous lites from the ophiolites, the Furgg zone and the magma source while in the metabasalts from the Portjengrat unit. adjacent ophiolitic units the source was more het- erogeneous, producing basalts of T- to N-MORB 6.2. Possible sources of the MORB magmatism composition. All metabasalts from the Furgg zone 50 in the Furgg zone, the Portjengrat unit and and from the Antrona and Zermatt-Saas ophio- adjacent ophiolitic units lites show a positive anomaly in Zr (Zr*). This probably indicates a low degree of melting in the In Figure 9a, the Nbn/Zrn and the Cen/Smn ratios spinel field (Casey, 1997). The Zr* decreases from of samples from the Furgg zone, the Portjengrat T- to N-MORB in Corsica and the Ligurides. This 55 unit, the Antrona ophiolites and the Zermatt- indicates that they are the products of variable Saas ophiolites are plotted and compared to ba- degrees of partial melting (higher for N- than T- Proofreading 20 J. Kramer, R. Abart, O. Müntener, S.M. Schmid and W.-B. Stern
1 MORB). Taken together, these data show that crust, which separates the Briançonnais micro- partial melting of a slightly enriched (lithospher- continent (= Portjengrat-Stockhorn unit) from ic?) mantle source led to the formation of the me- the European plate (= Monte Rosa nappe). How- tabasalts from the Furgg zone (group F1–F4) and ever, the geochemical data presented above do 5 Portjengrat unit (group C3), whereas partial melt- not exclude the possibility that the Antrona and ing of a more depleted mantle source accounted Zermatt-Saas units were an originally continuous for the formation of the metabasalts of the ophio- unit forming part of the Piemont-Ligurian ocean, litic units (group O 1 and O2). as was proposed by Escher et al. (1997). The different geochemical signatures of the 10 6.3. Tectonic implications of the metabasaltic boudins from the intensely strained geochemical data Monte Rosa cover and basement of the south- western Monte Rosa nappe (ISMR, group C2) The geochemical data discussed above suggest preclude a direct correlation with the Furgg zone, that the abundant small metabasaltic boudins of as was proposed by Dal Piaz (1966). Furthermore, 15 the Furgg zone (group F1–F4) are T-MORB type their derivation from the Zermatt-Saas ophiolitic intrusives that were derived from a slightly en- unit can also be excluded on geochemical argu- riched (lithospheric?) and single cogenetic mag- ments. Hence, the chemical data presented in this ma suite. In this respect they differ from the study do not support the idea that the ISMR rep- ophiolitic units that exhibit a tendency towards a resents the prolongation of the Furgg zone in 20 more depleted, T- to N-MORB type and towards terms of an ophiolitic mélange (Froitzheim, 2001). a more heterogeneous magma source. If the On the other hand, larger mappable ophiolitic Furgg zone metabasalts were derived from ophio- slivers containing ultramafic rocks, which are sur- litic units, one would expect a complete overlap of rounded by the Furgg zone, do exist. However, the chemistry of the Furgg zone metabasalts with they demonstrably are either isoclinal D3 fold 25 that of the metabasalts from the ophiolitic units. cores of the Zermatt-Saas unit (at Gornergrat) or This is, however, not supported by our data (Figs. fold cores of the Antrona unit (in the Monte della 8 and 9). Furthermore, the Zr, Y, V, and Ti inter- Preja region) that were fragmented during D3 element correlations are significantly higher in overprint (Kramer, 2002) and that are not regard- the Furgg zone metabasalts than in the mafic ed as part of the Furgg zone (Keller and Schmid, 30 rocks from the ophiolitic units. This excludes the 2001; Kramer, 2002). possibility that the Furgg zone metabasalts were derived through tectonic incorporation of adja- 6.4. Paleogeographic implications of the cent ophiolitic components. In summary, these geochemical data data indicate that the Furgg zone cannot be inter- 35 preted as an ophiolitic mélange, as proposed by Based on the geochemistry of the metabasalts, Froitzheim (2001). two stages of mafic magmatism are inferred. An Given the geochemical similarities of the me- early stage is recorded by the samples of groups tabasaltic boudins of the Furgg zone with those of C1 and C2. The amphibolite boudins of group C2 the Portjengrat unit (group C3), the main differ- are embedded in the basement of the southwest- 40 ence between the two units seems to be the con- ern Monte Rosa nappe (ISMR), as well as in gar- tinuous increase in strain intensity towards the net micaschists of its supposedly Permo-Triassic Furgg zone. Since the Portjengrat unit is undoubt- meta-sedimentary cover. The boudins are parallel edly a fragment of continental upper crust, the to the foliation of the basement rocks, which is, at Furgg zone may conclusively be interpreted as an least in part, defined by pre-Alpine sillimanite 45 intracontinental shear zone that overprints both (Dal Piaz, 1966). Hence, Dal Piaz concluded, that Portjentgrat unit and adjacent Monte Rosa foliation and embedded boudins are Paleozoic in nappe. This is consistent with structural evidence age. This view is corroborated by the observation indicating that the Monte Rosa nappe and the of amphibolite boudins, which are intruded by Portjengrat-Stockhorn units were originally con- granites of probably Permian (or Late Carbonif- 50 nected to form a single Portjengrat – Stockhorn – erous) age (Kramer, 2002). Late Carboniferous to Monte Rosa continental fragment, separating the Permian igneous activity, producing MORB-type Antrona ophiolites from the Zermatt-Saas ophio- and also more differentiated magmas has been lites (Keller and Schmid, 2001; Kramer, 2002). described from several regions in the Alps (e.g. This is again in marked contrast to the model pro- von Raumer et al., 1990). A pre-Triassic age is also 55 posed by Froitzheim (2001), who considers the likely for the amphibolites of group C1 from the Furgg zone, together with the Antrona ophiolitic Siviez-Mischabel nappe, which were collected unit, as representing North Penninic oceanic within the basement only. Proofreading Geochemistry of metabasalts from the Monte Rosa region 21
A later, presumably post-Triassic age is pro- MORB dykes have neither been reported from 1 posed for all T-type MOR basalts. The main rea- the sedimentary cover of the Siviez-Mischabel soning is that some of the metabasalts of groups nappe (internal Briançonnais), nor from the C3 (Portjengrat and Stockhorn units) and F4 southwestern part of the Monte Rosa nappe (parts of the Furgg zone) intruded Triassic meta- (most internal Briançonnais situated directly ad- 5 arkoses and/or marbles (Jaboyedoff et al., 1996; jacent to undisputed Piemont-Liguran ophiolites, Kramer, 2002). Our geochemical data and field i.e. the Zermatt-Saas unit; see Figs. 1 and 2). To- relations suggest that this post-Triassic MORB- gether with the fact that ophiolitic imbricates de- type magmatism affected pericontinental as well rived from the Antrona unit (Valaisan according as oceanic units. This has recently been described to Keller and Schmid, 2001) can be found in the 10 from other ocean-continent transition zones in Furgg zone, these structural and field arguments the Eastern Central Alps (e.g. Puschnig, 2000; may indicate that the basaltic intrusions into dis- Desmurs et al., 2002) and in the Gets nappe (Bill tal continental margin units occurred in the con- et al., 2000). However, in none of these regions text of Late Jurassic to Early Cretaceous opening were comparable volume proportions of mafic of the Valais ocean. 15 rocks found within continental upper crustal Notwithstanding the different paleotectonic basement, including its Triassic cover. interpretations, an important finding of this study The question whether the Furgg zone magma- is that T-type MOR basalts are not only found in tism was linked in space and time with the mag- ophiolitic units but also in (thinned) distal conti- matism in the ophiolitic Antrona or with that of nental margin units. The volume of mafic rocks 20 the Zermatt-Saas unit cannot be answered une- intruding the Briançonnais distal continental quivocally with the existing geochemical data margin adjacent to the Valais ocean or, alterna- alone. In case of the existence of only one single tively, to the Piemont-Ligurian ocean, is distinctly oceanic basin in the working area, as proposed by higher than the volume of mafic rocks found in Escher et al. (1997), all ophiolitic units would the Adriatic distal continental margin (e.g. 25 have belonged to the Piemont-Ligurian ocean. Puschnig, 2000; Desmurs et al., 2002). This ‘asym- According to this tectonic model, this single oce- metry’ in the distribution of mafic rocks along anic suture derived from South Penninic oceanic continental margins is consistent with asymmetric crust would separate Austroalpine units from the low angle detachment faulting during the latest Briançonnais microcontinent, the southern mar- stages of rifting. The finding of T-MOR basalts 30 gin of which would be represented in our working intruding distal parts of the continental crust is area. similar to the evolution of the Red Sea rifting sys- In case of the existence of two oceanic basins tem (e.g. Voggenreiter et al., 1988). in the working area, the Antrona and Zermatt- Saas units would have been part of the Valais and Acknowledgements 35 Piemont-Ligurian oceans, respectively (e.g. Keller and Schmid, 2001). Those metabasalts that per- The authors gratefully acknowledge the contributions tained to the Valaisan ocean, would be Late of all the members of the Basel, Bonn and Mainz “Mon- te Rosa group” (Nikolaus Froitzheim, Ronan Le Bayon, Jurassic or younger in age. The fact that the adja- Katharina Dubach, Christiane Rössler, Lukas Keller, cent Portjengrat unit and Furgg zone were intrud- Andreas Weber, Corinne Bacher, Lukas Baumgartner, 40 ed by dykes and sills with T- to N-MORB compo- Sabine Pawlig). Furthermore, we benefitted from dis- sitions similar to the ophiolites suggests that both cussions with and/or reviews from Hans-Rudolf Pfeifer, Philippe Monjoie, Dieter Gebauer, and Martin Engi. units may have been derived from the same distal J. Kramer acknowledges support from the Werenfels- continental margin within the Briançonnais, that Fonds, Freiwillige Akademische Gesellschaft, Basel. S. was stacked between the Valaisan and Piemont- Schmid acknowledges support from NF-projects Nr. 20- 45 61814.00 and 2000-068020/1, initiated by the late Martin Ligurian oceanic sutures. According to this sce- Frey. The authors are grateful for Martin’s initiative and nario the Portjengrat-Stockhorn-Monte Rosa support during the initial stages of this study. continental fragment (including the Furgg zone) represents the eclogitic part of the Briançonnais References microcontinent, as proposed by Keller and 50 Schmid (2001) and Kramer (2002). Amato, J.M., Johnson, C.M., Baumgartner, L.P. and Structural studies indicate that the distal mar- Beard, B.L. (1999): Rapid exhumation of the Zer- gin of the Briançonnais, represented by the matt-Saas ophiolite deduced from high-precision Sm–Nd and Rb–Sr geochronology. Earth Planet. Sci. Portjengrat unit and the Furgg zone, may have Lett. 171, 425–438. been located adjacent to the more external Valai- Bacher, C. (2002): Strukturen und Metamorphose in der 55 san oceanic basin (Keller and Schmid, 2001). In Furggzone (Furggtal bei Saas Almagell, Wallis). Un- this context it is interesting to note that T-type publ. diploma thesis, University of Basel, 83 pp. Proofreading 22 J. Kramer, R. Abart, O. Müntener, S.M. Schmid and W.-B. Stern
1 Barnicoat, A.C. and Fry, N. (1986): High-pressure meta- nantly basic and ultrabasic rocks. In: Henderson, P. 8/8 morphism of the Zermatt-Saas ophiolite zone, Swit- (ed.): Developments in Geochemistry 2 – Rare zerland. J. Geol. Soc. London 143, 607–618. Earth Element Geochemistry. Elsevier, Amsterdam, Barnicoat, A.C., Rex, D.C., Guise, P.G. and Cliff, R.A. 237–274. 5 (1995): The timing of and nature of greenschist Dal Piaz, G.V. (1964): Il cristallino antico del versante 5 facies deformation and metamorphism in the upper meridionale del Monte Rosa paraderivati a preva- Pennine Alps. Tectonics 14, 279–293. lente metamorphismo alpino. Rendiconti Soc. Min. Bearth, P. (1945): Ueber das Verhältnis von Kristallisa- Italiana Anno XX, 101–135; 4 Tav. tion und Bewegung in der Monte Rosa-Bernhard- Dal Piaz, G.V. (1966): Gneiss ghiandoni, marmi ed anfi- 10 (Mischabel)-Decke. Schweiz. Mineral. Petrog. Mitt. boliti antiche del ricoprimento Monte Rosa nell’alta 25, 537–538. Valle d’Ayas. Boll. Soc. Geol. Italiana 85, 103–132. Bearth, P. (1952): Geologie und Petrographie des Monte Dal Piaz, G.V. (1971): Nuovi ritrovamenti di cianite alpi- 10 Rosa. Beitr. Geol. Karte Schweiz (Neue Folge) 96, 1– na nel Cristallino antico del Monte Rosa. Rend. Soc. 94. Italiana Min. Petrol. 27, 437–477. 15 Bearth, P. (1953a): Blatt Zermatt, Geologischer Atlas Dal Piaz, G.V. (2001): Geology of the Monte Rosa mas- der Schweiz, Nr. 29. 1:25’000. Basel, Schweiz. Geol. sif: historical review and personal comments. Kommission. Schweiz. Mineral. Petrogr. Mitt. 81, 275–303. Bearth, P. (1953b): Erläuterungen Blatt Zermatt, Geolo- Desmurs, L., Müntener, O. and Manatschal, G. (2002): 15 gischer Atlas der Schweiz. Nr. 29. 1:25’000. Basel, Onset of magmatic accretion within a magma-poor 20 Schweiz. Geol. Kommission. rifted margin: A case study from the Platta ocean- Bearth, P. (1954a): Blatt Monte Moro, Geologischer At- continent transition, Eastern Switzerland. Contrib. las der Schweiz, Nr. 30. 1:25’000. Basel, Schweiz. Mineral. Petrol. 144, 365–382. Geol. Kommission. Dostal, J. and Capedri, S. (1979): Rare earth elements in Bearth, P. (1954b): Blatt Saas, Geologischer Atlas der high-grade metamorphic rocks from the Western 2025 Schweiz, Nr. 31. 1:25’000. Basel, Schweiz. Geol. Alps. Lithos 12, 41–49. Kommission. Dubach, K. (1998): Die Grenzzone von Portjengrat- Bearth, P. (1956): Geologische Beobachtungen im Einheit und Siviez-Mischabel-Decke im Almagel- Grenzgebiet der lepontinischen und penninischen lertal, Wallis. Unpubl. diploma thesis, University of Alpen. Eclogae geol. Helv. 49, 279–290. Basel, 61 pp. 30 Bearth, P. (1957): Erläuterungen Blatt Saas und Monte Eisele, J., Geiger, S. and Rahn, M. (1997): Chemical char- Moro, Geologischer Atlas der Schweiz. Nr. 30, 31. acterization of metabasites from the Turtmann Val- 25 Basel, Schweiz. Geol. Kommission. ley (Valais, Switzerland): implications for their pro- Bearth, P. (1976): Zur Gliederung der Bündnerschiefer toliths and geotectonic origin. Schweiz. Mineral. in der Region von Zermatt. Eclogae geol. Helv. 69, Petrogr. Mitt. 77, 403–417. 35 149–161. Engi, M., Scherrer, N.C. and Burri, T. (2001): Metamor- Bearth, P. and Schwander, H. (1981): The post-Triassic phic evolution of pelitic rocks of the Monte Rosa sediments of the ophiolite zone Zermatt-Saas Fee nappe: Constraints from petrology and single grain 30 and the associated manganese mineralizations. Ec- monazite age data. Schweiz. Mineral. Petrogr. Mitt. logae geol. Helv. 74, 189–205. 81, 305–328. 40 Bearth, P. and Stern, W. (1971): Zum Chemismus der Escher, A., Hunziker, J.-C., Marthaler, M., Masson, H., Eklogite und Glaukophanite von Zermatt. Schweiz. Sartori, M. and Steck, A. (1997): Geologic frame- Mineral. Petrogr. Mitt. 51, 349–359. work and structural evolution of the western Swiss- Bearth, P. and Stern, W. (1979): Zur Geochemie von Italian Alps. In: Pfiffner, O.A., Lehner, P., Heitz- Metapillows der Region Zermatt-Saas. Schweiz. mann, P., Müller, S. and Steck, A. (eds.): Deep struc- 4535 Mineral. Petrogr. Mitt. 59, 349–373. ture of the Swiss Alps: Results from NRP 20. Beccaluva, L., Dal Piaz, G.V. and Macciotta, G. (1984): Birkhäuser, Basel, 205–221. Transitional to normal MORB affinities in ophiolitic Escher, A., Masson, H. and Steck, A. (1988): Coupes metabasites from Zermatt-Saas, Combin and Antro- géologiques des Alpes occidentales suisses. Mém. na units, Western Alps. Geol. Mijnbouw 63, 165–177 Géol. Lausanne, Rapp. géol. serv. hydrol. géol. natl. 2, 50 Bill, M., Nägler, T.F. and Masson, H. (2000): Geochemis- 1–11, incl. 1 map, 2 tables, 1 plate. 40 try, Sm–Nd and Sr isotopes of mafic rocks from the Escher, A. and Sartori, M. (1991): The geology of the earliest oceanic crust of Alpine Tethys. Schweiz. Zermatt-Gornergrat area. NFP 20 Bull. 9, 5–11. Mineral. Petrogr. Mitt. 80, 131–145. Frey, M., Hunziker, J.C., R, O.N.J. and Schwander, H.W. RONI? Boynton, W.V. (1984): Cosmochemistry of the rare earth (1976): Equilibrium-disequilibrium relations in the 55 elements: meteorite studies. In: Henderson, P. (ed.): Monte Rosa Granite, Western Alps; petrological, Developments in Geochemistry 2 – Rare Earth Ele- Rb–Sr and stable isotope data. Contrib. Mineral. Pet- 45 ment Geochemistry. Elsevier, Amsterdam, 63–114. rol. 55, 147–179. Carrupt, E. and Schlup, M. (1998): Métamorphisme et Froitzheim, N. (2001): Origin of the Monte Rosa nappe tectonique du versant sud du Val Bognanco (Pen- in the Pennine Alps – A new working hypothesis. 60 nique, Alpes italiennes). Bull. Soc. Vaud. Sci. Nat. 86, Geol. Soc. Am. Bull. 113, 604–614. 29–59. Gebauer, D. (1999): Alpine geochronology of the Cen- Casey, J.F. (1997): Comparison of major- and trace ele- tral and Western Alps: new constraints for a complex ment geochemistry of abyssal peridotites and mafic geodynamic evolution. Schweiz. Mineral. Petrogr. 50 plutonic rocks with basalts from the MARK region Mitt. 79, 191–208. 65 of the Mid-Atlantic Ridge. Proc. Ocean. Drill. Prog. Gelinas, L., Mellinger, M. and Trudel, P. (1982): Archae- Sci. Res. 153,181–241. an mafic metavolcanics from the Rouyn-Noranda Colombi, A. (1989): Métamorphisme et géochimie des district, Abitibi greenstone belt, Quebec. 1. Mobility roches mafiques des Alpes ouest-centrales (géopro- of the major elements. Canad. J. Earth Sci. 19, 2258– fil Viège-Domodossola-Locarno). Mém. Géol. 4. Im- 2275. 5570 privite S.A., Lausanne, 216 pp. Grauch, R.I. (1989): Rare earth elements in metamor- Cullers, R.L. and Graf, J.L. (1984): Rare earth elements phic rocks. In: Lipin, B.R. and McKay, G.A. (eds.): in igneous rocks of the continental crust: predomi- Geochemistry and mineralogy of rare earth ele- Proofreading Geochemistry of metabasalts from the Monte Rosa region 23
ments. 21. Mineralogical Society of America, Wash- Mattirolo, E., Novarese, V., Franchi, S. and Stella, A. 1 1 ington D.C., 147–167. (1912): Foglio Monte Rosa, Carta Geologica d’ Ita- 8/8 Henderson, P. (1984): General geochemical properties lia. No. 29. 1:100 000. Novara, Serv. Geol. Ital. and abundances of the rare earth elements. In: Milnes, A.G., Greller, M. and Müller, R. (1981): Se- Henderson, P. (ed.): Developments in Geochemistry quence and style of major post-nappe structures, 5 2 – Rare Earth Element Geochemistry. Elsevier, Simplon-Pennine Alps. J. Struct. Geol. 3/4, 411–420. 5 Amsterdam Oxford New York Tokyo, 1–32. Mottl, M.J. (1983): Metabasalts, axial hot springs, and the Huang, T.-K. (1935a): Carte géologique de la région structure of hydrothermal systems at mid-ocean Weissmies-Portjengrat (Valais). 1:25’000. Basel, ridges. Geol. Soc. Am. Bull. 94, 161–180. Schweiz. Geol. Kommission. Müller, C. (1989): Albitisation of the Zermatt area, 10 Huang, T.-K. (1935b): Etude géologique de la région Western Alps. PhD thesis, University of Basel, 285 Weissmies-Portjengrat (Valais). Bull. Soc. Neuchat. pp. Sci. Nat. 60, 3–76. Pearce, J.A. (1976): Statistical analysis of major element 10 Humphris, S.E. (1984): The mobility of the rare earth el- patterns in basalts. J. Petrol. 17, 15–43. ements in the crust. In: Henderson, P. (ed.): Develop- Pearce, J.A. (1982): Trace element characteristics of la- 15 ments in Geochemistry 2 – Rare Earth Element Ge- vas from destructive plate boundaries. In: Thorpe, ochemistry. Elsevier, Amsterdam, 317–342. R.S. (ed.): Andesites – Orogenic andesites and relat- Hunziker, J.C. (1970): Polymetamorphism in the Monte ed rocks. John Wiley and Sons, Chichester, 525–548. Rosa, Western Alps. Eclogae geol. Helv. 63, 151–161. Pfeifer, H.R., Colombi, A. and Ganguin, J. (1989): Zer- 15 Jaboyedoff, M., Béglé, P. and Lobrinus, S. (1996): Stratig- matt-Saas and Antrona Zone: A petrographic and 20 raphie et évolution structurale de la zone de Furgg, geochemical comparison of polyphase metamorphic au front de la nappe du Mont-Rose. Bull. Soc. Vaud. ophiolites of the West-Central Alps. Schweiz. Miner- Sci. Nat. 84, 191–210. al. Petrogr. Mitt. 69, 217–236. Keller, L. (2000): Kinematik der duktilen Scherung an Puschnig, A.R. (2000): The oceanic Forno Unit (Rhetic der Front der Monte Rosa Decke (Val Loranco, Ital- Alps). Eclogae Geol. Helv. 93, 103–124 25 20 ien). Unpubl. diploma thesis, University of Basel, 90 Rampone, E., Hofmann, A.W. and Raczek, I. (1998: Iso- pp. topic contrasts within the Internal Liguride Ophio- Keller, L. and Schmid, S.M. (2001): On the kinematics of lite (N. Italy): the lack of a genetic mantle-crust link. shearing near the top of the Monte Rosa nappe and Earth Planet. Sci. Lett. 163, 175–189. the nature of the Furgg zone in the Val Loranco Reinecke, T. (1991): Very-high-pressure metamorphism 30 (Antrona valley, N. Italy): tectono-metamorphic and and uplift of coesite-bearing metasediments from paleogeographical consequences. Schweiz. Mineral. the Zermatt-Saas zone, Western Alps. Eur. J. Miner- 25 Petrogr. Mitt. 81, 347–367. al. 3, 7–17. Klein, J.A. (1978): Post-nappe folding southeast of the Reinecke, T. (1998): Prograde high-to ultrahigh-pres- Mischabelrückfalte (Pennine Alps) and some as- sure metamorphism and exhumation of oceanic sed- 35 pects of the associated metamorphism. Leidse geol. iments at Lago di Cignana, Zermatt-Saas Zone, Medel. 51, 233–312. western Alps. Lithos 42, 147–189. Kramer, J. (2002): Structural evolution of the Penninic Rollinson, H.R. (1993): Using geochemical data: evalua- 30 units in the Monte Rosa region (Swiss and Italian tion, presentation, interpretation. Longman, Singa- Alps). PhD thesis No. 23, University of Basel, 140 pp, pore, 352 pp. 40 2 maps, 2 encl. Rössler, C. (2000): Transport und Faltung in der Furgg- Le Bas, M.J., Le Maître, R.W., Streckeisen, A. and Zan- zone und in den angrenzenden Einheiten (südliches ettin, B. (1986): A chemical classification of volcanic Saastal, Wallis, Schweiz). Unpubl. diploma thesis, rocks based on the total alkali-silica diagram. J. Pet- University of Basel, 74 pp. rol. 27, 745–750. Rubatto, D. and Gebauer, D. (1999): Eo/Oligocene (35 45 35 Le Bayon, R., Schmid, S.M. and De Capitani, C. (2001): Ma) high-pressure metamorphism in the Gorner- The metamorphic evolution of the Monte Rosa grat Zone (Monte Rosa, Western Alps): implications nappe and its relation to exhumation by fore- and for paleogeography. Schweiz. Mineral. Petrogr. Mitt. back-thrusting in the Western Alps. Geol. Paläont. 79, 353–362. Mitt. Innsbruck 25, 132–133. Saccani, E., Padoa, E. and Tassinari, R. (2000): Prelimi- 50 Liati, A., Gebauer, D., Froitzheim, N. and Fanning, M. nary data on the Pineto gabbroic massif and Nebbio 40 (2001): U–Pb SHRIMP geochronology of an amphi- basalts: Progress toward the geochemical characteri- bolitized eclogite and an orthogneiss from the Furgg zation of Alpine Corsica ophiolites. Ofioliti 25, 75– zone (Western Alps) and implications for its geody- 85. namic evolution. Schweiz. Mineral. Petrogr. Mitt. 81, Sartori, M. (1987): Structure de la zone du Combin entre 55 379–393. les Diablons et Zermatt (Valais). Eclogae geol. Helv. MacGeehan, P.J. and MacLean, W.H. (1980): An Ar- 80, 789–814. 45 chaean sub-seafloor geothermal system, ‘calc-alkali’ Sartori, M. (1990): L’unité du Barrhorn (Zone pennique, trends, and massive sulphide genesis. Nature 286, Valais, Suisse). PhD thesis, University of Lausanne, 767–771. 156 pp. 60 Markley, M.J., Teyssier, C., Cosca, M.A., Caby, R., Hun- Saunders, A.D. (1984): The rare earth element charac- ziker, J.C. and Sartori, M. (1998): Alpine deforma- teristics of igneous rocks from the ocean basins. In: tion and 40Ar/39Ar geochronology of synkinematic Henderson, P. (ed.): Developments in Geochemistry white mica in the Siviez-Mischabel nappe, Western 2 – Rare Earth Element Geochemistry. Elsevier, 50 Pennine Alps, Switzerland. Tectonics 17, 407–425. Amsterdam Oxford New York Tokyo, 205–236. 65 Marthaler, M. (1981): Découverte de foraminifères Saunders, A.D. and Tearney, J. (1984): Geochemical planctoniques dans les “schistes lustrés” de la pointe characteristics of basaltic volcanism within back-arc de Tourtemagne (Valais). Bull. Soc. Vaud. Sci. Nat. basins. In: Kohelaar, B.P. and Howells, M.F. (eds.): 75, 171–178. Marginal basin geology. Spec. Publ. 16. Geol. Soc. Marthaler, M. (1984): Géologie des unités penniniques London, 59–76. 70 55 entre le Val d’Anniviers et le Val de Tourtemagne Schilling, J.-G., Zajac, M., Evans, R., Johnston, T., White, (Valais, Suisse). Eclogae geol. Helv. 77, 395–448. W., Devine, J.D. and Kingsley, R. (1983): Petrologic Proofreading 24 J. Kramer, R. Abart, O. Müntener, S.M. Schmid and W.-B. Stern
1 and geochemical variations along the Mid-Atlantic Venturelli G., Thorpe R.S. and Potts P.J. (1981): Rare 8/8 Ridge from 27°N to 73°N. Am. J. Sci. 283, 510–586. earth and trace element characteristics of ophiolitic Schmid, S.M. and Kissling, E. (2000): The arc of the west- metabasalts from the Alpine-Apennine belt. Earth ern Alps in the light of geophysical data on deep Planet. Sci. Lett. 53, 109–123. 5 crustal structure. Tectonics 19, 62–85. von Raumer, J.F., Galetti, G., Pfeifer, H.R. and Ober- 5 Stampfli, G.M. and Marchant, R.H. (1997): Geodynamic hänsli, R. (1990): Amphibolites from Lake Emosson/ evolution of the Tethyan margins of the Western Aiguilles Rouges, Switzerland: tholeiitic basalts of a Alps. In: Pfiffner, O.A.e.a. (eds.): Deep structure of Paleozoic continental rift zone. Schweiz. Mineral. the Swiss Alps: results of NRP 20. Birkhäuser, Basel, Petrogr. Mitt. 70, 419–435. 10 223–239. Weber, A. (2001): Zur strukturellen und metamorphen Steck, A., Epard, J.-L., Escher, A., Gouffon, Y. and Mas- Entwicklung der Furgg-Zone und angrenzender son, H. (1999): Carte téctonique des Alpes de Suisse Einheiten im südlichen Saastal, Wallis, Schweiz. Un- 10 occidentale et des régiones avoisinantes. Feuille publ. diploma thesis, University of Bonn, 85 pp. Monte Rosa, No. 47. 1:100000. Bern, Serv. hydrol. Wetzel, R. (1972): Zur Petrographie und Mineralogie 15 géol. nat. der Furgg-Zone (Monte Rosa-Decke). Schweiz. Sun, S.S. and McDonough, W.F. (1989): Chemical and Mineral. Petrogr. Mitt. 52, 161–236. isotopic systematics of ocenaic basalts: implications Wood, D.A., Joron, J.-L. and Treuil, M. (1979): A re-ap- for mantle composition and processes. In: Saunders, praisal of the use of trace elements to classify and 15 A.D. and Norry, M.J. (eds.): Magmatism in ocean ba- discriminate between magma series erupted in dif- 20 sins. Spec. Pub. 42. Geol. Soc. London, 313–345. ferent tectonic settings. Earth Planet. Sci. Lett. 45, Thélin, P., Sartori, M., Burri, M., Gouffon, Y. and Ches- 326–336. sex, R. (1993): The Pre-Alpine Basement of the Bri- ançonnais (Wallis, Switzerland). In: von Raumer, J.F. and Neubauer, F. (eds.): Pre-Mesozoic Geology in 2520 the Alps. Springer, Berlin, 297–315. Thélin, P., Sartori, M., Lengeler, R. and Schaerer, J.P. (1990): Eclogites of Paleozoic or early Alpine age in the basement of the Penninic Siviez-Mischabel Nappe, Wallis, Switzerland. Lithos 25, 71–88. 30 van der Klauw, S.N.G.C., Reinecke, T. and Stöckhert, B. (1997): Exhumation of ultrahigh-pressure metamor- 25 phic oceanic crust from Lago di Cignana, Piemon- Received tese zone, western Alps: the structural record in me- Accepted in revised form I will tabasites. Lithos 41, 79–102. Editorial handling: M. Engi fill in 35
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