Assessing the Valais ocean, Western Alps: U-Pb ...

Eur. J. Mineral. 2006, 18, 299–308

Assessing the Valais ocean, Western Alps: U-Pb SHRIMP zircon geochronology of eclogite in the Balma unit, on top of the Monte Rosa nappe ANTHI LIATI1, * and NIKOLAUS FROITZHEIM2 1Institute

of Isotope Geology and Mineral Resources, Swiss Federal Institute of Technology (ETH), Sonneggstrasse 5, 8092 Zurich, Switzerland *Corresponding author, e-mail: [email protected] 2Geological Institute, University of Bonn, Nussallee 8, 53115 Bonn, Germany

Abstract: Recent tectonic studies suggest that the Balma unit, a nappe sliver of eclogite and serpentinite directly overlying the Monte Rosa nappe in the Western Alps, represents a remnant of the Cretaceous Valais ocean. In order to test this hypothesis, U-Pb SHRIMP dating was carried out on zircons from an eclogite of the Balma unit. For the eclogite studied, PT conditions of 500–590 °C, 13– 14.5 kbar (minimum P) are inferred by geothermobarometry. Oscillatory zoned (co-magmatic) zircon domains yielded a 206Pb/238U weighted mean age of 93.4 ± 1.7 Ma, interpreted as the time of magmatic crystallization of the gabbroic protolith. Metamorphic zircon domains yielded a 206Pb/238U weighted mean age of 40.4 ± 0.7 Ma. These domains contain inclusions of rutile and garnet displaying a composition similar to the rim of the matrix garnet. ‘In situ’ LA-ICPMS trace/REE analyses revealed low Th, Nb and Ta contents of the metamorphic zircon domains indicating concurrent growth with the HP phases clinozoisite (a sink for Th) and rutile (a sink for Nb, Ta). The lack of a negative Eu anomaly in the chondrite-normalized REE pattern of metamorphic zircon points to the absence of plagioclase at the time of metamorphic zircon formation. Thus, formation of metamorphic zircon took place probably at the peak of metamorphism or the early retrograde stage at HP. The 40.4 ± 0.7 Ma age is therefore ascribed to HP metamorphism. The 93.4 ± 1.7 Ma protolith age of the eclogite dated is identical to protolith ages reported for metabasic rocks of the Chiavenna ophiolites (Central Alps), attributed to the Valais ocean. The 40.4 ± 0.7 Ma HP metamorphism is marginally older than the HP metamorphism in the Valais-derived Antrona ophiolites, Western Alps, and in the Chiavenna ophiolites, Central Alps. The data provide a correlation between these units and indicate that the Cenomanian-Turonian ocean floor formation in the Valais ocean is more widespread than previously assessed. Our geochronological data (both the 93.4 ± 1.7 Ma time of crystallization and the 40.4 ± 0.7 Ma time of metamorphism) independently arrive at the conclusion of the tectonic studies that the Balma unit is part of the Valais ocean. The 40.4 ± 0.7 Ma age of metamorphism of the Balma unit fits well the model suggesting that the Valais rocks were metamorphosed earlier than those of the European margin (ca. 35 Ma) and later than the Piemont-Ligurian oceanic rocks (ca. 44 Ma). The Balma unit very likely represents part of the youngest oceanic crust in the Valais ocean. Key-words: zircon, U-Pb, Valais ocean, Alps, Monte Rosa, eclogite, ophiolite.

1. Introduction Mafic/ultramafic rock associations in the Western and Central Alps provide evidence of the existence of at least two ocean basins located between Europe and Africa before the onset of Alpine convergence: the Piemont-Ligurian ocean representing the Mesozoic Tethys, which opened from the Middle Jurassic onward (e.g. De Wever et al., 1995) and the Valais ocean, located NW of the Brian¸connais peninsula, probably opening from the Early Cretaceous onward (Trümpy, 1980; Florineth & Froitzheim, 1994; Stampfli et al., 1998). In the course of Alpine convergence between Europe and Africa, subduction of these ocean basins and subsequent continental collision resulted in the formation of a DOI: 10.1127/0935-1221/2006/0018-0299

nappe stack in the western and central Alpine chain. Geochronological data can contribute to a better understanding of the pre-orogenic paleogeography and the subsequent geodynamic evolution of the different nappes. In the present study, an eclogite associated with serpentinites has been dated, which is located directly on top of the Monte Rosa nappe in the Western Alps (see below; Fig. 1). The aim of this work was to clarify the paleogeographic position of this eclogite/serpentinite nappe, by determining the time of crystallization of the protolith of the metabasic rocks and the time of metamorphism. We applied the U-Pb ion microprobe dating (SHRIMP) technique assisted by cathodoluminescence (CL) imaging, trace-element and REE analysis of the zircon crystals. 0935-1221/06/0018-0299 $ 4.50

ˇ 2006 E. Schweizerbart’sche Verlagsbuchhandlung, D-70176 Stuttgart

300

A. Liati, N. Froitzheim

sions affected by Alpine eclogite-facies metamorphism (Lange et al., 2000; Engi et al., 2001), followed by greenschist- to amphibolite-facies metamorphic overprinting. At the top of the Monte Rosa nappe, mostly in its northern part, a strongly deformed, lithologically heterogeneous zone occurs called the Furgg zone (see summary by Dal Piaz, 2001), from which Cambro-Ordovician and Permian protolith ages

2. Geological setting In the broad area of study, the nappe stack comprises (from bottom to top) the Monte Rosa nappe, the Zermatt-Saas zone, the Combin zone, and the Dent Blanche nappe (Fig. 1). The Monte Rosa nappe consists of Hercynian basement with Carboniferous (?) and Permian granitoid intru-

D

ZURICH

Antrona

Gornergrat

MONTE ROSA

BERN

Balma unit PIS1

F

A IT

asin se b s a l Mo

Zermatt

Furgg zone

CH

l He

*

ve

tic

zo

Alagna

ne

Misox

GENEVA DB

MR

Chiavenna

Furgg

Insubric line

Se sia

N

Balma Po basin

Profile (B)

GP

100 km Brian onnais microcontinent Valais basin

DM

(A)

Margna-Sesia continental fragment

European margin

Piemont-Ligurian ocean

Adriatic margin

Bergell intrusion

NW

SE

Combin fault

Co m

Dent Blanche

bin

Sesia

Balma St. Bernard

r Ze

m

at

t

Monte Rosa Furgg

Simplon fault Southern Alps

na tro An Camughera Moncucco

10 0

(B)

20km

Fig. 1. (A): Tectonic map of the Swiss-Italian Alps. The Balma unit, as well as the Furgg zone are best distinguished on the inset map, due to their small size. Their location is marked by the arrows. The asterisk in the inset map shows the sample locality (PIS1). GP: Gran Paradiso; DB: Dent Blanche; MR: Monte Rosa; DM: Dora-Maira. Inset map from Liati et al. (2001), based on Steck et al. (1999). (B): Tectonic profile through the Western Alps showing also the Balma unit. Modified after Escher et al. (1997) and Froitzheim (2001). Balma unit after Pleuger et al. (2005). Balma unit and Furgg zone not to scale.

Assessing the Valais ocean, Western Alps

have so far been reported (Liati et al., 2001). This zone is overlain by the continental, Brian¸connais-derived St. Bernard nappe system, wedging out towards the south. The overlying Zermatt-Saas zone is formed by Mesozoic ophiolites of the Piemont-Ligurian ocean. Protolith ages of metagabbros in the Zermatt-Saas zone are Middle Jurassic (ca. 164 Ma) and (eclogite-facies, locally UHP) metamorphic ages are ca. 44 Ma (e.g., Rubatto et al., 1998; Dal Piaz et al., 2001; see below). The Zermatt-Saas ophiolites are overlain by the Combin zone, which consists of an upper blueschistto blueschist-eclogite transitional facies (Bousquet et al., 2004) ophiolite-bearing mélange interleaved with thin layers of continental-derived Mesozoic sedimentary rocks. The Combin zone is overlain by the Dent Blanche nappe, a Hercynian continental basement derived either from the Adriatic margin (Dal Piaz et al., 1972) or from the Margna-Sesia continental fragment located close to the Adriatic continental margin (Froitzheim & Manatschal, 1996). According to the tectonic interpretation of the Monte Rosa area by Froitzheim (2001), the structurally deepest layer of the Zermatt-Saas zone, directly on top of the Monte Rosa nappe represents the suture of the Valais ocean. On the southern side of Monte Rosa, the Valais and Zermatt-Saas ophiolites are in direct contact because of the southeastward wedging-out of the St. Bernard nappe (Brian¸connais). The ophiolite nappe directly on top of the Monte Rosa nappe, at its southern end, has been recently assigned to a distinct unit, named ‘Balma unit’ newly introduced by Pleuger et al. (2005). It is exactly from this unit that the eclogite dated in the present paper was collected. The Balma unit comprises metabasalts, metagabbros, and serpentinites. Its thickness varies between a few metres and up to 500 m in thickened fold hinges. It is locally separated from the main mass of the Zermatt-Saas ophiolites by thin slivers of paragneiss representing continental crust of possible Brian¸connais origin (Pleuger et al., 2005). 0.7

garnet endmembers (mol%)

0.6

0.5

0.4

Almandine Pyrope Grossular Spessartine

0.3

0.2

0.1

301

3. Description and petrological data of the dated rock The eclogite dated here (sample PIS1) was collected halfway between Alpe la Balma and Bocchetta delle Pisse, NW of Alagna Valsesia (Fig. 1A, inset), in a small gully under an old cable railway (coordinates: E 7°53’58’’/N 45°52’4’’; Swiss coordinates: E 635’780/N 80’820). Serpentinite is exposed on the southwestern side of the gully and eclogite on the northeastern side. Both rock types can be followed several hundred metres along strike. The sample was taken only a few metres from the serpentinite/eclogite boundary and has a well-developed foliation that is shallowly dipping to the SW. The rock studied is characterised by the following mineral assemblage: garnet + omphacite + green/bluish amphibole + zoisite/clinozoisite + white mica + rutile + quartz + opaques. In some cases, very fine-grained albite/oligoclase + diopside symplectites were observed around omphacite. Omphacite and green-bluish amphibole occur also as inclusions in garnet, sometimes as coexisting phases. Zircon was not observed in the thin sections studied. Electron-microprobe analyses of minerals revealed the following compositions of the main mineral constituents: Garnet is prograde zoned showing an increase in pyrope and decrease in grossular and spessartine components from core to rim: Py10Alm62Gr26Sp2 (core), Py19-22Alm55-56 Gr20-24 Sp1 (rim) (Table 1). Almandine content shows also a general decrease from core to rim. A garnet profile is shown in Fig. 2. Omphacite in the matrix has a jadeite content ranging between ca. 38 and 43 mol % (Table 1). Inclusions of omphacite in garnet have similar compositions (maximum jadeite content of omphacite inclusions: 43 mol %). Amphibole is of the sodic-calcic group (according to the classification scheme of IMA, 1978). Both matrix amphiboles and inclusions in garnet are barroisites (Table 1). White mica is paragonite. Fe2+-Mg partitioning between coexisting garnet-omphacite pairs (Ellis & Green, 1979; Powell, 1985; Krogh-Ravna, 2000) yields T between 500 °C and 590 °C (for an assumed P = 14 kbar), depending on the garnet-clinopyroxene geothermometer used, the Ellis & Green (1979) calibration giving the highest and the Krogh-Ravna (2000) the lowest temperatures. Based on the maximum jadeite content (Jd43) of the analyzed omphacite, pressures of 13–14.5 kbar (for T = 500–600 °C) are derived (Gasparik & Lindsley, 1981). These are minimum pressure estimates, since the three phases necessary for the application of the geobarometer calibrated for the reaction albite = jadeite + quartz were not found to coexist in equilibrium.

0 1 (core)

2

3

4

5 (rim)

inclusion in zircon

Fig. 2. Garnet profile showing an increase in pyrope and decrease in grossular and spessartine component from core to rim, typical for prograde zoning. The composition of a garnet inclusion in metamorphic zircon is also plotted for comparison. Note the similarity in composition of the garnet enclosed in zircon with that of the matrix garnet rim.

4. Analytical techniques and data evaluation Zircon separation: 15 zircon grains and grain fragments were separated from a ca. 2.5 kg eclogite sample. The mineral separation was carried out at the laboratories of the ETH in Zurich. After crushing, milling and sieving, the rock powder was passed over a magnetic drum separator. Zircon was

2.979 0.009 1.959 0.069 1.839 0.056 0.300 0.786 – –

2.978 0.006 1.978 0.065 1.765 0.054 0.402 0.746 – –

22

2.982 0.008 1.982 0.047 1.785 0.047 0.439 0.704 – –

38.4 0.10 21.8 0.79 27.5 0.71 3.80 8.47 – – 101.53 2.955 0.001 2.004 0.088 1.661 0.039 0.675 0.573 – –

12 oxygens 2.980 2.989 0.008 0.008 1.981 1.983 0.053 0.030 1.715 1.655 0.038 0.033 0.506 0.571 0.714 0.724 – – – –

40 38.4 0.02 22.1 1.52 25.8 0.60 5.88 6.95 – – 101.197

24 38.4 0.12 21.8 0.51 25.5 0.51 4.93 8.70 – – 100.38

23 38.4 0.08 21.9 0.91 26.5 0.58 4.38 8.60 – – 101.21

rim

2.950 0.001 1.983 0.119 1.656 0.042 0.666 0.579 – –

37.9 0.02 21.6 2.03 25.4 0.63 5.74 6.94 – – 100.303

44

rim

3.001 0.001 2.012 0.000 1.794 0.039 0.618 0.533 – –

38.6 0.01 22.0 0.00 27.6 0.60 5.33 6.40 – – 100.49

inclusion in zircon 01/2-9

1.994 0.002 0.383 0.057 0.117 0.000 0.480 0.535 0.432 0.000

55.2 0.08 8.99 2.09 3.88 0.01 8.92 13.8 6.17 0.00 99.18

35

matrix

4 cations 1.987 0.001 0.448 0.024 0.097 0.002 0.474 0.518 0.449 0.000

55.5 0.04 10.60 0.91 3.24 0.06 8.87 13.5 6.46 0.00 99.15

43

omphacite matrix

1.985 0.001 0.445 0.013 0.112 0.000 0.486 0.532 0.428 0.000

55.4 0.04 10.50 0.48 3.74 0.00 9.11 13.9 6.17 0.00 99.30

inclusion in garnet 27

22 oxygens 6.228 0.009 5.745 – 0.046* 0.001 0.035 0.035 1.449 0.134

49.3 0.09 38.6 – 0.44* 0.01 0.18 0.26 5.92 0.83 95.23

29

paragonite matrix

Alvi Ti Fe3+ Fe2+ Mn Mg

Si Aliv

7.348 0.652 8.000 0.537 0.010 0.746 0.201 0.008 3.497 5.000 1.273 0.727 2.000

SiO2 52.6 TiO2 0.10 Al2O3 7.21 Fe2O3 7.09 FeO 1.72 MnO 0.07 MgO 16.8 CaO 8.50 Na2O 2.87 K2O 0.13 97.0

12

core

Ca Jd 37.7 43.5 42.8 Na(B) Alm 61.7 59.5 60.0 57.7 55.5 56.3 56.3 60.1 Ac 5.5 1.4 0.2 Py 10.1 13.6 14.8 17.0 19.1 22.9 22.6 20.7 En 24.0 23.7 24.3 Gros 26.4 25.1 23.7 24.0 24.3 19.5 19.7 17.9 Fe 5.9 4.9 5.6 K 0.023 Spes 2.0 1.8 1.6 1.3 1.1 1.3 1.4 1.3 Wo 25.3 25.3 25.9 * total Fe as FeO. Alm: almandine; Py: pyrope; Gros: grossular; Spes: spessartine; Jd: jadeite; Ac: acmite; En: enstatite; Fe: ferossilite; Wo: wollastonite Analyses grt 40-omph 35 and grt 44-omph 43 were used for geothermometry; Fe2+/Fe3+ calculation for garnets and pyroxenes based on charge balance; for amphiboles after Tindle & Webb (1994), based on Rock & Leake (1984).

Si Ti Al Fe3+ Fe2+ Mn Mg Ca Na K

21

38.0 0.07 21.6 1.11 27.0 0.82 3.45 8.90 – – 100.90

20

SiO2 37.7 0.15 TiO2 21.2 Al2O3 1.13 Fe2O3 FeO 27.9 MnO 0.81 MgO 2.55 CaO 9.28 – Na2O – K2O TOTAL 100.70

garnet garnet profile from core (20) to rim (24)

Table 1. Representative electron microprobe analyses of minerals from the eclogite of Balma unit, Western Alps.

7.063 0.937 8.000 0.557 0.017 0.853 0.236 0.009 3.327 5.000 1.309 0.691 2.000 0.023

0.038

50.8 0.16 9.12 8.16 2.03 0.08 16.1 8.79 3.16 0.13 98.5 23 oxygens 7.220 0.780 8.000 0.874 0.015 0.390 0.585 0.005 3.130 5.000 1.270 0.730 2.000

51.7 0.15 10.1 3.71 5.01 0.04 15.0 8.49 3.35 0.21 97.8

amphibole rim inclusion in garnet 14 48

302 A. Liati, N. Froitzheim

303


further enriched using heavy liquids and finally, after repeated magnetic separation (Frantz), handpicked. Dating technique: The age data were obtained on a SHRIMP II ion microprobe at the Geological Survey of Canada, Ottawa. The spot size used for analysis was about 25 µm. For data collection, seven scans through the critical mass range were made. U/Pb ratios were calibrated relative to standard BR266, which is a piece of a Sri Lanka gemquality zircon. For a detailed description on the SHRIMP technique, data processing including the reasons for using mainly 206Pb/238U-ages for Phanerozoic zircons, the reader is referred to Compston et al. (1992), Williams (1998) or Stern (1997). CL-images were collected from a split screen on a CamScan CS 4 scanning electron microscope (SEM) at ETH in Zürich operating at 13 kV (Gebauer, 1996 for detailed technical description). In general, dark CL contrast reflects high amounts of minor and trace elements while bright CL contrast corresponds to low amounts of minor and trace elements, including U (e.g., Sommerauer, 1974). Thus, the U contents can be qualitatively predicted via CL. For the calculation of the 238U/206Pb ratios and ages, the data were corrected for common Pb using the 207Pb correction method, following the standard procedures of Compston et al. (1992) and Williams et al. (1996). The data are graphically presented on Tera-Wasserburg (TW) diagrams (Tera & Wasserburg, 1972), where total 207Pb/206Pb vs. the calibrated, total 238U/206Pb is plotted. This diagram allows fast estimation of the amount of common Pb of the projected analyses. The amount of common Pb was calculated using the isotope composition at the time of zircon formation obtained from the model of Cumming & Richards (1975). The

ellipses on the TW diagrams are plotted with a 2 c error. For the individual analyses listed in Table 2 and shown on the zircon CL-images, 1 c errors are given. The error on the weighted mean ages is provided at the 95 % confidence level (c.l.). For all diagrams and weighted mean age calculations, the ISOPLOT program of Ludwig (2000) was used. Electron-microprobe analyses: The EMP analyses were carried out with a JEOL Superprobe (JXA-8200) at the Institute of Mineralogy, ETH, Zurich (15 kV, 20 nA). Natural and synthetic oxides and silicates were used as standards. Trace and REE analyses: ‘In situ’ trace and REE analyses of zircon were obtained by LA-ICPMS (Excimer 193nm ArF Laser, coupled with an ELAN6100 quadrupole MS) at the Institute of Isotope Geology and Mineral Resources, ETH Zurich. The SRM 610 glass from NIST was used as external standard. Repetition rate was 10 Hz and the size of the spot 30 µm, in diameter. Oxide production rate was tuned to < 0.5 % ThO. A detailed compilation of instrument and data acquisition parameters for the LA-ICPMS is given in Pettke et al. (2004).

5. CL characteristics of zircon and SHRIMP results Six out of the 15 zircons (crystals and crystal fragments) separated from the eclogite sample are entirely of magmatic origin showing, in general, well developed oscillatory zoning (e.g. Fig. 3A). The magmatic zircon crystals are prismatic, more or less euhedral, ranging in size between ca. 300– 180 µm in length and ca. 150–80 µm in width. The relatively large size of the zircons suggests that the protolith of this ec-

Table 2. U, Th, Pb SHRIMP data for co-magmatic and metamorphic zircons from an amphibolitized eclogite of the Balma unit, Western Alps. Sample

U (ppm)

Th (ppm)

Th/U

rad.Pb (ppm)

f206 %

U/206Pb (1 c ) (uncorrected)

207Pb/206Pb

238

(1 c ) (uncorrected)

Age (Ma) 206Pb/238U

1. 2. 3. 4. 5. 6. 7. 8. 9.

co-magmatic domains PIS1-4.1 502 PIS1-1.1 1199 PIS1-4.2 1018 PIS1-1.2 600 PIS1-S6.1 1007 PIS1-S11.1 914 PIS1-S11.2 953 PIS1-3.1 367 PIS1-S1.1 353

141 461 442 157 335 326 336 294 348

0.290 0.397 0.449 0.271 0.343 0.369 0.364 0.828 1.018

7 18 15 8 15 14 14 5 5

0.010 0.000 0.003 0.006 0.005 0.001 0.005 0.007 0.019

68.82 ± 0.98 68.67 ± 0.95 68.47 ± 0.97 70.29 ± 1.08 65.59 ± 1.62 66.52 ± 1.38 66.61 ± 1.40 76.45 ± 2.43 78.38 ± 2.24

0.0480 ± 0.0004 0.0495 ± 0.0003 0.0494 ± 0.0004 0.0488 ± 0.0006 0.0516 ± 0.0009 0.0491 ± 0.0011 0.0518 ± 0.0009 0.0532 ± 0.0010 0.0628 ± 0.0021

93.0 ± 1.3 93.0 ± 1.3 93.3 ± 1.3 91.0 ± 1.4 97.1 ± 2.4 96.1 ± 2.0 95.6 ± 2.0 83.2 ± 2.6 80.2 ± 2.3 WM: 93.4 ± 1.7 Ma

10. 11. 12. 13. 14. 15. 16. 17.

metamorphic domains PIS1-2.1 146 PIS1-2.2 1082 PIS1-2.3 1103 PIS1-2.4 118 PIS1-2.5 995 PIS1-S3.1 703 PIS1-S4.1 519 PIS1-S7.1 615

0.19 2.80 2.70 0.31 2.62 4.44 1.21 2.35

0.001 0.003 0.002 0.003 0.003 0.006 0.002 0.004

1 6 6 1 6 4 3 3

0.031 0.001 0.006 0.084 0.008 0.017 0.015 0.023

159.48 ± 2.94 161.10 ± 2.47 159.46 ± 2.17 144.06 ± 3.64 151.95 ± 2.55 153.98 ± 3.43 152.43 ± 3.66 157.98 ± 3.13

0.0640 ± 0.0023 0.0486 ± 0.0008 0.0497 ± 0.0006 0.1161 ± 0.0041 0.0594 ± 0.0007 0.0602 ± 0.0022 0.0590 ± 0.0023 0.0653 ± 0.0018

39.4 ± 0.7 39.8 ± 0.6 40.2 ± 0.6 40.7 ± 1.1 41.6 ± 0.7 41.0 ± 0.9 41.5 ± 1.0 39.7 ± 0.8 WM: 40.4 ± 0.7 Ma

WM: weighted mean; error on WM is given at the 95 % confidence level 1. Uncertainties are given at the 1 c level. 2. f206 % denotes the percentage of 206Pb that is common Pb. 3. Analyses in italics (Nr. 8 and 9) are not considered for the WM calculation (see text).

304


A

C 41.5 ± 1.0

B

D

93.3 ± 1.3 93.0 ± 1.3 garnet

rutile

30µm 30µm PIS1-4 0.070

207Pb/206Pb (uncorrected)

0.066 0.062

PIS1-2

30µm 20µm PIS1-S9

PIS1-S4

age of magmatic protolith: WM: 93.4 ± 1.7 Ma (95% c.l.) 3.1

age of metamorphism: WM: 40.4 – 0.7 Ma (95% c.l.)

0.12

0.10

0.058 0.054

0.08

s1.1

0.050 106 102 98

0.046

94

90

86

0.06

82

78

78

82

0.042 58

62

66

70

74

86

0.04 130

48

46

140

44

42

150

40

160

238U/206Pb (uncorrected)

238U/206Pb (uncorrected)

(E)

(F)

38

170

Fig. 3. (A–D): Cathodoluminescence pictures of zircon crystals from the eclogite of Balma unit, Western Alps. Crystal (A) consists to a large extent of a co-magmatic, oscillatory zoned domain and probably an inherited core (not analyzed). Crystals (B–D) show an entirely metamorphic pattern (patchy zoning). Crystal C contains a rutile inclusion and crystal D a garnet inclusion (see text). Circles correspond to SHRIMP spots. (E), (F): Tera Wasserburg diagrams with data of zircons from the eclogite of the Balma unit (errors on the ellipses are 2 c ). In (E) analyses from the co-magmatic, oscillatory zoned zircon domains are plotted: two analyses plot to the right side of the mixing line, due to Pb-loss caused by a later metamorphic event. In (F), analyses from metamorphic zircon domains are plotted (see text).

logite was a gabbro rather than basalt, because such large crystals cannot grow from rapidly cooling basaltic melts. The rest of the zircons consist entirely of one domain, which in CL shows ‘patchy zoning’. These zircons are prismatic and range in size between ca. 200–90 µm in length and ca. 100–45 µm in width. Zircons of this type are interpreted to be of metamorphic origin (e.g. Fig. 3B–D). Metamorphic zircon crystals may contain rutile and garnet inclusions (Fig. 3C, D; see below). Seven analyses of the oscillatory zoned, co-magmatic domains plot, on a TW diagram, on a mixing line with common Pb and calibrated total 238U/206Pb as end members. Two analyses plot on the right side of the mixing line (Fig. 3E), probably due to Pb loss caused by subsequent metamorphism. The 206Pb/238U weighted mean age of the seven concordant analyses is 93.4 ± 1.7 Ma (error at 95 % c.l.), which is interpreted as the time of crystallization of the magmatic (gabbroic) protolith of this eclogite (Table 2). Eight analyses from the metamorphic domains plot, on a TW diagram, on a mixing line with common Pb and cali-

brated total 238U/206Pb as end members (Fig. 3F). The 206Pb/ weighted mean age of these analyses is 40.4 ± 0.7 Ma (Table 2). Interpretation of this metamorphic age is discussed below in the light of mineral inclusions and trace/ REE data of the metamorphic zircon domains. 238U

6. Interpretation of the SHRIMP metamorphic age Inclusions: As shown by EMP analyses, metamorphic zircons have inclusions of garnet and rutile, which is, as a rule, the high-pressure Ti-phase (e.g., Helmann & Green, 1979; Xiong et al., 2005). One garnet and 3 rutile inclusions were found in 3 (out of 9) different metamorphic zircon crystals. The composition of one garnet inclusion in metamorphic zircon is: Py21Alm60Gr18Sp1, comparable to that of the rim of the matrix garnet (Table 1; Fig. 2). This feature excludes the possibility of metamorphic zircon formation during prograde metamorphism and rather indicates that it formed ei-

305


100

Fig. 4. Chondrite-normalized REE patterns of two metamorphic zircon crystals (shown in CL) from the eclogite of Balma unit. Concentrations of La, Pr, Nd and Sm for both crystals analyzed are not indicated on the diagram, because they are below detection limit. For Sm, the value plotted on the diagram is the maximum possible value (corresponding to the d.l. value) and is indicated with a bar. The lack of a negative Eu anomaly for crystal 2 and a weak negative Eu anomaly for crystal S4 are indicative of zircon formation under high pressures (see also text). Normalization values after Sun & McDonough (1989).

zircon / chondrite

PIS1-S4

10 PIS1-2 1

0.1 PIS1-2 PIS1-S4 0.01 La

Table 3. Trace and REE data of metamorphic zircon from the eclogite of Balma unit, Western Alps. (ppm)

PIS1-S4

PIS1-2

La < d.l. < d.l. Ce 0.094 0.050 Pr < d.l. < d.l. Nd < d.l. < d.l. Sm < d.l. < d.l. Eu 0.061 0.089 Gd 1.092 1.403 Tb 0.669 0.952 Dy 7.629 11.65 Ho 2.507 3.759 Er 9.137 16.41 Tm 1.794 3.159 Yb 18.04 32.71 Lu 3.608 7.030 Y 80.85 124.5 Hf 9738 9929 Nb 0.284 0.267 Ta 0.025 < d.l. 1: < d.l.: below detection limit; 2: Detection limit for Sm (plotted in Fig. 4) is 0.064 for analysis PIS1-S4 and 0.147 for analysis PIS1-2.

ther when the rock reached highest PT conditions or afterwards. This inference is in line with the presence of rutile inclusions in metamorphic zircon domains. Trace and REE: The metamorphic zircon crystals have very low Th contents and Th/U ratios, compared to the magmatic zircons (Table 2). The very low Th contents of metamorphic zircons can be explained by coeval crystallization with clinozoisite (Bingen et al., 2004) during eclogite-facies metamorphism. Two metamorphic zircon crystals were analysed ‘in situ’ by LA-ICPMS for their trace and REE composition. Their very low Nb and Ta contents (Table 3) may be assigned to concurrent growth with rutile, which is a sink for Nb and Ta (Li et al., 2005). La, Pr, Nd and Sm are below detection limit (d.l.)

Ce

Pr

Nd

Sm

Eu

Gd

Tb

Dy

Ho

Er

Tm

Yb

Lu

for LA-ICPMS, while HREE are enriched with respect to MREE (Table 3). The chondrite-normalized REE patterns of the analyzed zircons are shown in Fig. 4. For Sm, the d.l. value has been plotted on the diagram as a maximum possible value (indicated by a bar), because the concentration of Sm is critical for the identification of any Eu anomaly. The absence of a negative Eu anomaly can be ascertained for one of the two zircon analyses (crystal 2 of Fig. 4). It is very likely that this applies also to the second analysis (crystal S4) for which only a weak negative Eu anomaly results, even when the maximum possible Sm content equal to the detection limit is assumed. The significance of the presence or absence of a negative Eu anomaly in the REE pattern of zircon relies on the fact that Eu is preferentially accommodated in plagioclase and K-feldspar. For rocks potentially containing plagioclase at low pressures, like the eclogite studied here, the lack of a negative Eu anomaly in metamorphic zircon implies that zircon formed above the plagioclase stability field, that is under at least HP conditions (see summary by Hoskin & Schaltegger, 2003 and references therein). Based on the above arguments, we suggest that the 40.4 ± 0.7 Ma age obtained from the metamorphic domains of zircon reflects the time when the rocks were either at the PTpeak or on the early retrograde part of the PT-path, still at HP. The generally high rates of exhumation (> 1–2 cm/y) in high-pressure rocks from numerous orogens worldwide (e.g., Gebauer et al., 1997; Amato et al., 1999; Cartwright & Barnicoat, 2002 and references therein) compensate for any possible deviation of the zircon age from the precise PTpeak. The uncertainties on the age (referring to the 95 % c.l. errors on the weighted mean ages) are probably larger than the time difference between HP-peak and HP-retrograde metamorphism.

7. Conclusions and geodynamic implications Magmatism: The Piemont-Ligurian ocean, one of the two main ocean basins in the Western and Central Alps, opened

306


from the Middle Jurassic onward (e.g., De Wever & Baumgartner, 1995). Radiometric data on the time of crystallization of central and western Alpine ophiolitic rocks of this oceanic basin range between 142 ± 5 Ma and 166 ± 1 Ma (see summary by Liati et al., 2003 and references therein; Stucki et al., 2003). The second ocean basin, the Valais ocean, opened from the Early Cretaceous onward, based on paleontological data (Frisch, 1979; Trümpy, 1980; Florineth & Froitzheim, 1994; Stampfli et al., 1998). Geochronological data of 93.0 ± 2.0 Ma and 93.9 ± 1.8 are known for the time of crystallization of oceanic rocks in the Chiavenna unit of the Central Alps (Fig. 1; Liati et al., 2003). Based on its structural position, the Chiavenna unit is considered to be part of the Valais ocean (e.g., Schmid et al., 1996), which is confirmed by geochronological data (Liati et al., 2003). The Late Cretaceous (93.4 ± 1.7 Ma) age obtained for the time of crystallization of the magmatic protolith of our eclogite sample from the Balma unit is identical to the 93.0 ± 2.0 Ma and 93.9 ± 1.8 SHRIMP ages of metabasites from the Chiavenna ophiolites. The Balma unit, on top of the Monte Rosa nappe, therefore, represents a piece of the youngest oceanic crust in the Valais ocean. Metagabbros with Jurassic protolith ages of ca. 155 to 163 Ma found in other Valaisan ophiolite units (Misox zone and Antrona ophiolite, Liati et al., 2005) are interpreted as remnants of Piemont-Ligurian ocean floor inside the Valais geotectonic domain. In that case, Valaisan oceanic crust formed by spreading and re-rifting of existing Piemont-Ligurian oceanic crust during the Cretaceous (see Liati et al., 2005 and references therein, for details). The ca. 93 Ma eclogite protolith age from the Balma unit, taken together with the data from Chiavenna, has important implications for the paleogeography of the Penninic zone: (1) Cenomanian-Turonian ocean floor, so far only found in the Chiavenna unit (Liati et al., 2003), is not a singularity but may have been widespread. (2) At 93 Ma, the Iberia-Brian¸connais plate was not yet approaching Europe but was still moving away from it, probably in a sinistrally transtensive manner. This yields an important constraint for Cretaceous plate motions, which are otherwise poorly defined because of the lack of paleomagnetic reversals. (3) A ca. 50 Ma long time gap exists between 142 ± 5 Ma (the youngest Piemont-Ligurian ophiolites) and the ca. 93 Ma ophiolites of the Balma and Chiavenna units. Either there was no spreading in the Penninic zone during this time interval, or the Early Cretaceous ocean floor has been completely subducted. It is also possible that Early Cretaceous ophiolites exist but have simply not been dated yet. Therefore, further geochronological work is necessary to constrain if oceanic spreading in the Penninic zone was continuous or episodic. Metamorphism: Regarding the time of metamorphism, subduction of the Piemont-Ligurian oceanic crust to (U)HP conditions occurred at ca. 44–45 Ma (zircon SHRIMP-data by Rubatto et al., 1998; Rubatto & Hermann, 2003). A longer time bracket extending from 48.8 ± 2.1 to 40.6 ± 2.6 Ma (duration of the prograde PT-path) has been proposed by Lapen et al. (2003), based on Lu-Hf data in combination

with Sm-Nd data of Amato et al. (1999). The large time span between the Sm-Nd age of 40.6 ± 2.6 Ma (Amato et al., 1999) and a Lu-Hf age of 48.8 ± 2.1 Ma (Lapen et al., 2003) is very likely the result of the lack of isotopic homogenization and/or mixing of real and apparent ages of different mineral components (see Liati et al., 2003 and Rubatto & Hermann, 2003 for details on this topic). Thus, we prefer here the SHRIMP data of ca. 44–45 Ma as most reliable, as there is also very good evidence (e.g. high-pressure inclusions) that they reflect (U)HP metamorphism (Rubatto et al., 1998; Rubatto & Hermann, 2003). Subduction of the Valais geotectonic domain to eclogitefacies conditions in the Western Alps has been dated at 38.5 ± 0.9 Ma (U-Pb SHRIMP data on zircon of an amphibolitized eclogite of the Antrona ophiolites, Passo del Mottone; Liati et al., 2005). In the Central Alps, the Valais geotectonic domain (represented by the Chiavenna ophiolites) was metamorphosed at 37.1 ± 0.9 Ma (U-Pb SHRIMP data on zircon of an amphibolite) but it is uncertain whether it reached HP (Liati et al., 2003). The 40.4 ± 0.7 Ma metamorphic age obtained for the eclogite at Bocchetta delle Pisse, Balma unit, is in the same range as earlier data for the metamorphism of Valais ocean rocks: It is marginally older than the 38.5 ± 0.7 Ma metamorphic age of the Antrona ophiolites and the 37.1 ± 0.9 Ma metamorphic age of the Chiavenna amphibolites by 0.5 Ma and by 1.7 Ma, respectively. This deviation can be interpreted by assuming that the eclogite of the Balma unit was subducted slightly earlier than both the Antrona amphibolitized eclogite (at least by 0.5 Ma) and the Chiavenna amphibolites (at least by 1.7 Ma). The Chiavenna amphibolites may have reached lower depths during subduction, which can also account for the difference in the time of metamorphism. On the other hand, if the ca. 40 Ma Sm-Nd age reported by Amato et al. (1999) is really corresponding to HP metamorphism of the Piemont-Ligurian ophiolites and not compromised by problems of Sm-Nd garnet dating (disequilibrium; see above), this would suggest that subduction of the two oceans overlapped in time, although subduction of the Piemont-Ligurian ocean started earlier. This is in line with a model assuming the partly simultaneous activity of two subduction zones consuming the Valais and Piemont-Liguria oceans (Froitzheim et al., 2003). Regional geologic implications: Pleuger et al. (2005), based on detailed tectonic studies, suggested the existence of a separate serpentinite/eclogite rock association at the southern border of the Monte Rosa nappe, as a distinct tectonic unit, named Balma unit. These authors proposed that the Balma unit is part of the Valais ocean. The main argument in favour of this hypothesis is the parallelization with the situation at the northwestern border of the Monte Rosa nappe (locality Stockknubel). In that area, a layer of serpentinite with eclogite boudins, lithologically identical with the Balma unit occurs between the Monte Rosa nappe below (considered as part of the European margin; e.g., Froitzheim, 2001) and various gneisses above, which belong to the Brian¸connais-derived St. Bernard nappe (Fig. 1B). Parallelizing the Balma unit with this ophiolite layer, Pleuger et al. (2005) arrived at the conclusion that the Balma units is


structurally deeper than the Brian¸connais and therefore most likely derived from the Valais ocean, given the general rule of south-over-north stacking during early thrusting in this part of the Alps. Taken together, the Balma unit, the ophiolites at Stockknubel, and the Antrona ophiolites which occur in the same structural position further northeast, form a discontinuous ophiolite layer over the Monte Rosa nappe and represent the suture of the Valais ocean. The SHRIMP results of the present study independently arrive at the same conclusion as the tectonic studies, since both the crystallization, as well as the metamorphic age of the eclogite from this unit are well within the range of ages suggested for Valaisan oceanic crust. The finding of identical ca. 93 Ma (Late Cretaceous) crystallization ages and similar metamorphic ages in the Western (Balma unit) and Central Alps (Chiavenna unit) further argues for a coherent evolution of the Monte Rosa nappe and the Adula-Gruf nappe (including Alpe Arami, Cima di Gagnone and Gruf Complex; Gebauer, 1996; Liati & Gebauer, 2003). Both nappes were interpreted to represent the distal European continental margin originally north of the Valais ocean (e.g., Gebauer, 1996; Froitzheim, 2001; Rubatto & Gebauer, 1999). The 40.4 ± 0.7 Ma metamorphic age in the Balma unit fits the model of northwestwards propagating subduction episodes of continental and oceanic crust (from the Adriatic plate in the SSE to the European plate in the NNW; see e.g., Gebauer, 1999; Liati et al., 2005 and references therein). The Valais rocks were metamorphosed earlier than those of the European margin (ca. 35 Ma) lying to the north and generally later than the Piemont-Ligurian oceanic rocks (ca. 44 Ma) lying to the south of the Valais geotectonic domain. Some overlap in time between the Piemont-Ligurian and Valais subduction episodes cannot be excluded. Acknowledgements: We very much appreciate the help of M. Hamilton and R.A. Stern, GSC Ottawa, during various stages of the SHRIMP work. Thanks are due to D. Gebauer, ETH Zurich, and J. Pleuger, Bonn University, for constructive discussions. The help of T. Pettke during LA-ICPMS analyses and E. Reusser, ETH Zurich, during electron-microprobe analyses, is greatly appreciated. The paper benefited from constructive reviews by B. Bingen, Norwegian Geological Survey, and T. Brewer, University of Leicester. This study was supported by a grant of the Swiss National Science Foundation (20-52662.99). The work of N.F. was supported by the ‘Deutsche Forschungsgemeinschaft’, grant Nr. FR700/6.

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Received 6 June 2005 Modified version received 18 December 2005 Accepted 12 January 2006