The mafic–ultramafic rock association of Loderio–Biasca

(lower Pennine nappes, Ticino, Switzerland): Cambrian oceanic

magmatism and its bearing on early Paleozoic paleogeography

Urs Schaltegger *, Dieter Gebauer , Albrecht von Quadt

Department of Earth Sciences, Institute for Isotope Geology and Mineral Resources, Federal Institute of Technology ETH,

Sonneggstrasse 5, 8092 Zurich, Switzerland

Accepted 7 December 2001

Abstract

Dismembered relics of mafic and ultramafic rock in high-grade basement rocks often record pre-collisional stages of deep

lithospheric and/or oceanic mafic magmatism in an orogen. A lens of amphibolites and serpentinites, intercalated between the

crystalline Simano and Leventina nappes (lower Penninic nappes, Central Alps, Switzerland) was investigated for the protolith

age and chemical and isotopic composition. Despite the polyphase high-grade metamorphic overprinting, primary isotopic and

chemical relationships are still preserved and are indicative of an origin in an active margin situation. Trace element

geochemistry and Nd isotopes of amphibolites argue for the fractionated gabbros as protoliths, which were formed by the partial

melting of the upper mantle. The rocks show a variation of eNd values from + 7.3 to + 4.2 and Nd model ages up to 2 Ga, taken

as evidence for the contamination by a geochemically enriched, old lithospheric source. Sr isotopes are heavily disturbed

through the percolating crustal fluids after the emplacement into the continental crust. U–Pb age determination of zircon from

two amphibolites using both in situ (SHRIMP) and conventional methods converge at an age of 518F 11 Ma for the

crystallization of the protoliths. The rock association of Loderio–Biasca is a further example from the Alpine basement

recording an oceanic domain between Gondwana and the different Gondwana-derived microcontinents in the Cambrian–

Ordovician times. Remnants of this ocean were incorporated into the accretionary wedges that formed during the subduction/

collision events from Ordovician to Carboniferous, commonly summarized as the Caledonian and Variscan orogeny, and are

today spread over the so-called Saxothurigian and Moldanubian domain of the Variscan orogen. These domains consist, thus to

large extents, of heterochronous accretionary wedge sequences. D 2002 Elsevier Science B.V. All rights reserved.

Keywords: Cambrian; Ordovician; Island-arc; MORB; U–Pb dating; Ionprobe; SHRIMP

1. Introduction

High-grade basement units in the orogenic domains

often carry isolated lenses of mafic–ultramafic rock

associations, which are hard to root and interpret.

Despite their polyphase and high-grade metamorphic

reworking, geochemical and isotopic compositions

0009-2541/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved.

PII: S0009 -2541 (02 )00005 -0

* Corresponding author. Present address: Departement de

Mineralogie, Universite de Geneve, rue des Maraıchers 13, 1205

Geneve, Switzerland. Tel.: +41-22-7026638; fax: +41-22-3205732.

E-mail addresses: urs.schaltegger@terre.unige.ch

(U. Schaltegger), gebauer@erdw.ethz.ch (D. Gebauer),

quadt@erdw.ethz.ch (A. von Quadt).

www.elsevier.com/locate/chemgeo

Chemical Geology 186 (2002) 265–279

are often astonishingly well preserved and offer the

only tool for a tentative geodynamic reconstruction of

their formation environment. The polyorogenic crys-

talline basement of the Alpine belt carries ubiquitous

examples of such mafic–ultramafic associations (e.g.

Pfeiffer et al., 1993; see Fig. 1a). They either occur

as ocean-floor sequences of mainly Mesozoic age or as

relics of the Paleozoic and older oceanic stages

within the pre-Mesozoic basement. Very often, dis-

membered amphibolites and ultramafic rocks carry

Fig. 1. (a) Variscan units of the Alpine domain; mafic associations with protolith ages between 460 and 530 Ma are indicated with grey dots (for

references, see text). Modified after von Raumer et al. (2002). MC=Massif Central; BM=Bohemian Massif. (b) Geological setting of the

Loderio–Biasca mafic–ultramafic complex within the high-grade Lepontine area of the Central Alps, intercalated between the Simano and

Leventina nappes. The occurrence of the coeval rocks at Cima di Gagnone (Adula nappe) is indicated by a grey dot.

U. Schaltegger et al. / Chemical Geology 186 (2002) 265–279266

important pieces of information about a deep litho-

spheric stage at the onset of the continental breakup or

a subsequent oceanic stage in the evolution of an

orogenic terrain.

This study investigates the mafic and ultramafic

rocks at Loderio near Biasca (Val Blenio, southern

Switzerland), which form a lens-shaped body with

tectonic contacts between the overlying Simano and

the underlying Leventina nappes, respectively (Fig.

1b). It is impossible to assign the body to either

tectonic units. The rocks were emplaced between the

Simano and Leventina nappes during the Alpine nappe

thrusting. They do not show an HP overprint as other

similar units in the Penninic nappe stack (e.g. the

Adula–Cima–Lunga nappe) and are overprinted

under the amphibolite facies conditions at temper-

atures up to 700 jC during the Alpine metamorphism

(Trommsdorff and Evans, 1970; Vance and O’Nions,

1992; Todd and Engi, 1997). Both Mesozoic and

Paleozoic–Proterozoic oceanic lithosphere are possi-

ble origins for the Loderio association. In the first case,

the rocks would be coeval to the Jurassic gabbros in the

basaltic units of the Piemont–Ligurian oceanic basin,

as they are dated in the Zermatt–Saas ophiolites

(Rubatto et al., 1998) or the Platta (Desmurs et al.,

1999) and Gets nappes (Bill et al., 1997) in the Central

and Western Alpine realms, or to the Cretaceous

basalts of the Valais ocean basin as suggested for the

Cima–Lunga unit by Pfiffner (1999).

An alternative origin as a sheared relic from the

basement series of either nappes also has to be con-

sidered and will be substantiated by the dating results

in this paper. The pre-Mesozoic evolution of the

crystalline basement of both the Simano and Leventina

nappes is recorded by the structural and mineralogical

inheritance from the Variscan orogenic cycle. Numer-

ous mafic to ultramafic inliers in the Alpine basement

gneissses have, however, been dated at the Lower

Paleozoic protolith ages in the Penninic and Austro-

alpine nappes (Stille and Tatsumoto, 1985; Gebauer,

1996; Schenk-Wenger, 1993; Schaltegger et al., 1997),

and in the external crystalline massifs (Menot et al.,

1988; Paquette et al., 1989; Gebauer, 1993; Oberli et

al., 1994; Biino et al., 1999). These different occur-

rences were assigned to a variety of geodynamic

environments, including MOR-, island-arc- and active

continental margin settings and mostly dated at ages

between 530 and 460 Ma (see compilation in Fig. 1a).

These data have been taken as an argument for the

existence of an oceanic domain extending during the

Cambrian and early Ordovician times off the Gond-

wana continental margin.

The present study provides a geochemical, isotopic

and geochronological database of the mafic rocks at

Loderio near Biasca to suggest a possible paleogeo-

graphic–geodynamic environment during the em-

placement of the Lower Paleozoic mafic–ultramafic

rocks of the Alpine realm. The Biasca–Loderio rocks

herewith enjoy the first modern scientific investiga-

tion almost 100 years after their initial description by

Hezner (1909), who in fact, was the first woman who

studied natural sciences at ETH Zurich.

2. Regional geology and petrography of sampled

material

The mafic–ultramafic rocks of Loderio are bor-

dered by migmatitic gneisses of the Simano nappe at

the top and by the Leventina gneiss at their base (see

Fig. 1b for a geological overview). Both granitoid

lithologies are interpreted as Variscan granite intru-

sions that underwent high-grade metamorphism dur-

ing the Alpine metamorphism (Frey et al., 1980). The

mafic–ultramafic complex is some 50 to 100 m thick,

of which the ultramafic part makes up 30 m. The

whole association is poorly exposed in a post-glacial

landslide; an overview on the lithological content may

be gained along the small road at the bottom of the

landslide, where the studied samples were also col-

lected.

The petrography of the rocks has been described by

Hezner (1909). The ultramafic body is considered as a

former peridotite with primary olivine and pyroxene.

Alpine new formations are serpentine, talc, carbonate

(magnesite), chlorite and hyperstene, as well as the

tremolite – actinolite forming radial aggregates

(Trommsdorff and Evans, 1970). The contact between

the peridotite and underlying amphibolites is formed

by an (Alpine?) the blackwall containing green horn-

blende, actinolite and chlorite. Among the amphibo-

lites, Hezner (1909) only describes an epidote

amphibolite. There are no further systematic studies

of petrography or mineral chemistry available. The

petrographic reconnaissance in the landslide mass,

however, revealed the existence of a variety of amphib-

U. Schaltegger et al. / Chemical Geology 186 (2002) 265–279 267

Table 1

Concentrations of major, trace, and rare-earth elements

Element Bi-1 Bi-2 SU-93-2 SU-93-3 SU-93-4 SU-93-5 SU-93-6

(%)

SiO2 47.80 48.50 50.40 49.50 54.40 48.20 45.70

Al2O3 15.40 14.40 15.90 14.80 14.70 14.70 13.50

Fe2O3 11.80 13.80 10.60 13.50 10.00 12.60 16.10

TiO2 1.42 2.17 1.07 1.94 1.49 1.47 2.88

MnO 0.23 0.66 0.26 0.33 0.26 0.72 0.33

MgO 6.44 5.24 8.15 6.63 5.90 7.75 6.88

CaO 10.30 10.00 9.60 10.70 9.90 11.40 11.40

K2O 0.53 0.40 0.38 0.40 0.35 0.52 0.67

Na2O 3.54 2.58 3.30 2.33 3.01 3.02 2.00

P2O5 0.37 0.49 0.33 0.33 0.35 0.33 0.34

LOI 0.58 0.15 0.63 0.52 0.44 0.84 0.41

Sum 98.41 98.39 100.62 100.98 100.80 101.55 100.21

(ppm)

Ba 104.0 46.7 30.5 62.5 85.3 58.3 54.8

Rb 7.5 11.1 8.8 7.8 7.1 14.4 15.8

Sr 207 167 307 126 285 221 77.6

V 280 213 203 372 228 307 694

Ni 71.5 70.6 125.0 50.5 50.3 84.6 122.0

Co 51.4 41.3 82.2 80.8 77.1 68.4 86.3

Cr 110 164 348 42.5 56.1 113 33.0

Zn 112 101 112 72.6 69.6 121 133

Cu 24.6 27.8 6.1 188 16.6 24.9 17.0

Sc 34.1 36.9 31.1 41.4 31.6 39.8 51.3

Zr 256 256 123 141 176 129 70.3

Hf 4.61 4.70 2.62 3.12 3.71 2.70 1.63

Mn 1709 4972 1896 2367 1802 1168 2330

Nb 4.08 8.68 3.56 4.19 4.55 3.19 1.79

Ta 0.24 0.59 0.38 0.43 0.58 0.33 0.22

Th 0.93 1.29 1.44 0.96 3.07 0.88 0.97

U 0.39 0.62 0.60 0.31 0.88 0.39 0.48

Y 33.8 47.0 23.5 32.1 34.5 31.0 23.5

La 8.68 11.71 7.97 6.83 11.33 8.96 4.12

Ce 22.24 30.38 19.06 17.95 27.29 22.94 10.32

Pr 3.18 4.48 2.59 2.61 3.67 3.27 1.52

Nd 15.75 23.35 12.58 13.75 17.70 16.45 8.04

Sm 4.02 6.08 3.20 3.90 4.56 4.49 2.48

Eu 1.35 2.31 1.12 1.45 1.48 1.69 1.11

Gd 4.20 6.38 3.22 4.11 4.52 4.71 2.79

Tb 0.82 1.22 0.60 0.80 0.89 0.85 0.56

Dy 5.38 8.05 3.91 5.40 5.65 5.34 3.92

Ho 1.17 1.71 0.82 1.16 1.22 1.09 0.82

Er 3.29 4.45 2.18 3.10 3.32 2.92 2.25

Tm 0.61 0.79 0.39 0.54 0.57 0.50 0.42

Yb 3.29 4.15 2.18 2.86 3.11 2.69 2.15

Lu 0.51 0.65 0.35 0.46 0.50 0.40 0.34

REE 74.49 105.71 60.17 64.92 85.81 76.30 40.84

U. Schaltegger et al. / Chemical Geology 186 (2002) 265–279268

olite types, from fine-grained to pseudo-gabbroic tex-

tures, with and without garnet (see short character-

ization in Appendix A). Garnet is present in some of

the amphibolites but often consumed to various extents

by kelyphites containing plagioclase and hornblende.

3. Analytical methods

3.1. Geochemistry

Analyses were done at Centre de Geochimie de la

Surface in Strasbourg/F. The samples were digested

using the Li– tetraborate fusion; all concentration

determinations of one sample were carried out from

the same fusion. Major elements were measured with

an ARL 14,000 arc discharge spectrometer, achieving a

precision of F 1.5%, except for Na and K, which were

analyzed by the optic emission spectroscopy. Trace

elements (Sr, Ba, V, Ni, Co, Cr, Zn, Cu, Sc, Y, Zr, Mn,

P) were determined on a ARL 35,000C ICP-OES,

achieving F 10% precision and 1–5 ppm limits of

detection according to the element. Rb, Nb, Hf, Ta, Th,

U and REE were analyzed on a PQ II + ICP-MS with

5% precision and 0.1 ppm limit of detection. Precision

and accuracy of the methods were routinely controlled

by replicate analyses of the international rock stand-

ards. Results are summarized in Table 1.

3.2. Sr, Nd isotope determinations

Data are presented in Table 2. The details for the

chemical procedure of the Sm–Nd and Rb–Sr analy-

ses were described by von Quadt (1997). The whole-

rock samples (50–250 mg) were dissolved in Teflon

capsules with a mixture of HF–HNO3 (4:1) for 2 h

using a microwave system. The heating procedure of

the microwave system comprised several temperature

settings of variable duration. Prior to dissolution,

highly enriched 150Nd/149Sm and 84Sr/87Rb tracer

solutions were added. Mass spectrometry was done

on a MAT 262 thermal ionisation mass spectrometer in

the static multicollection mode. The Nd isotopic ratios

were normalized to a 146Nd/144Nd ratio of 0.7219. The

Table 2

Rb–Sr and Sm–Nd isotopic results

Sample

number

Rb

(ppm)

Sr

(ppm)

87Rb/86Sr

87Sr/86Sr

sample (0)

87Sr/86Sr

sample (T)

Sm

(ppm)

Nd

(ppm)

147Sm/144Nd

sample (0)

143Nd/144Nd

sample (0)

143Nd/144Nd

sample (T)

eNd(0)

eNd(T)

TDM(Ga)

SU-93-3 7.78 126 0.174 0.709665F 8 0.709536 4.600 15.454 0.1797 0.512956F 7 0.512344 6.16 7.31 0.87

SU-93-4 7.10 285 0.070 0.703644F 8 0.703592 2.861 11.230 0.1538 0.512818F 8 0.512294 3.47 6.34 0.85

SU-93-5 14.38 221 0.183 0.705954F 9 0.705819 2.719 8.682 0.1891 0.512828F 6 0.512184 3.67 4.18 2.00

SU-93-6 15.78 77.6 0.572 0.703871F 8 0.703448 0.423 1.292 0.1979 0.512973F 7 0.512299 6.50 6.43 1.71

Table 3

U–Pb isotopic data from the ion microprobe spot dating (SHRIMP)

Spot Concentrations (ppm) Th/U f 206Pb (%)a Isotopic ratiosb Apparent ages,

U Th Pb rad. 206Pb/238U (1r) 207Pb/235U (1r)206Pb/238U (1r)

Garnet–amphibolite, Biasca 1

1.113 1295 1384 118 1.07 0.44 0.07705F 202 0.5995F 188 478F 12

1.108c 944 776 91 0.82 0.14 0.08463F 222 0.6543F 208 524F 13

1.110 459 338 38 0.74 0.48 0.07915F 208 0.6171F 246 491F12

1.109c 2174 3501 243 1.61 0.14 0.08410F 221 0.6721F191 521F13

1.112 347 303 27 0.87 0.73 0.07071F187 0.5312F 291 440F 11

Garnet–amphibolite, Biasca 2

2.80c 439 421 43 0.96 0.25 0.08405F 221 0.6769F 286 520F 13

2.77c 61 20 5 0.33 1.73 0.08203F 219 0.6414F 456 508F 13

a Percentage of common 206Pb relative to the total measured 206Pb.b Standard deviations (1r level) refer to the last significant digits of the corresponding ratios.c Planar-oscillatory, magmatic domains. All other domains consist of or contain irregular, rounded domain boundaries.

U. Schaltegger et al. / Chemical Geology 186 (2002) 265–279 269

Table 4

Results of the conventional U–Pb dating

Number Descriptiona Weight Number Concentrations Atomic ratios Apparent ages Error(mg) of grains

U Pb rad.

(ppm)

Pb nonrad.

(pg)

Th/Ub 206/204c 206/238d Error

2s (%)

207/235d Error

2s (%)

207/206d Error

2s (%)

206/238 207/235 207/206corr.

SU-93-3

1 transp clrls 0.0137 38 308 27.60 21.8 1.23 874 0.07147 0.35 0.5580 0.79 0.05662 0.66 444.9 450.9 481.9 0.56

2 euh turbid

G-type

0.0130 35 576 48.20 18.1 1.07 1806 0.06923 0.34 0.5393 0.54 0.05649 0.39 431.4 437.9 471.9 0.69

3 bulk 0.0120 c 40 254 22.25 16.6 1.13 835 0.07153 0.34 0.5595 0.97 0.05673 0.85 445.3 451.2 481.0 0.50

4 transp euh 0.0074 12 364 30.98 9.3 1.06 1297 0.07051 0.33 0.5516 0.34 0.05673 0.38 439.2 446.0 481.2 0.36

5 larger transp

euh

0.0120 8 582 45.87 13.2 0.98 2206 0.06655 0.34 0.5163 0.43 0.05627 0.22 415.2 422.7 462.9 0.86

6 turbid 0.0061 10 314 27.14 8.9 1.10 970 0.07099 0.33 0.5553 0.65 0.05673 0.51 442.1 448.4 480.9 0.63

SU-93-4

7 spr to subh 0.0068 5 500 43.65 1.3 0.69 12086 0.07941 0.33 0.6269 0.38 0.05726 0.14 492.6 494.2 501.4 0.93

8 prism 0.0075 4 281 24.27 12.7 0.67 850 0.07902 0.48 0.6228 0.71 0.05717 0.57 490.2 491.6 498.1 0.60

9 prism 0.0088 3 425 36.07 1.4 0.58 13145 0.07940 0.34 0.6262 0.38 0.05720 0.15 492.5 493.7 499.1 0.92

10 round, turbid 0.0058 6 478 39.37 2.4 0.74 5388 0.07399 0.34 0.5784 0.40 0.05670 0.16 460.2 463.5 479.8 0.92

a euh = euhedral, clrls = colourless, incl = inclusions, prism = prismatic, spr = short-prismatic, subh = subhedral, transp = transparent, G-type zircons according to Pupin (1980);

analyses 1 to 6 unabraded, 7 to 10 abraded.b Calculated on the basis of radiogenic 208Pb/206Pb ratios, assuming concordancy.c Corrected for fractionation and spike.d Corrected for fractionation (0.10%), spike, blank, and common lead (Stacey and Kramers, 1975).

U.Schalteg

ger

etal./Chem

icalGeology186(2002)265–279

270

reproducibility of the data was estimated by measuring

the La Jolla Standard, the mean value of 15 runs during

this work was 143Nd/144Nd = 0.511842F 6 (2r mean).

The Nd model age TDM was calculated using147Sm/144Nd = 0.213 and 143Nd/144Nd = 0.513150. Sr

isotopic ratios were normalized to 88Sr/86Sr = 8.37521.

The reproducibility of the Sr data was estimated by

measuring the NBS 987 standard. The mean of 12 runs

during this work was 87Sr/86Sr = 0.710245F 8 (2rmean).

3.3. Ion microprobe (SHRIMP) dating:

The individual zircon crystals were analyzed for

cathodoluminescence (CL) previous to the ion

microprobe analyses, which were performed in 1990

using the SHRIMP I at ANU in Canberra. Detailed

descriptions for the CL- and SHRIMP-techniques are

given by Gebauer (1996) and Compston et al. (1992).

The SHRIMP-data are plotted on a conventional

concordia diagram to better present the discordant

data points. Correction for the common lead via204Pb, 208Pb or 207Pb concentrations yielded identical

results for the 206Pb/238U ages; the 204Pb-corrected

results are presented in Table 3.

3.4. Conventional U–Pb dating

The procedures follow very closely those described

in Schaltegger et al. (1999); procedural blank levels

were 10–20 pg Pb for analyses 1–6 (carried out in

1996), about 2 pg for analyses 7–10 (carried out in

1998). The zircon aggregates of sample SU-93-3 were

not air-abraded because they turned out to be too

fragile and instantly disaggregated. The performance

of the ion counting system mounted on the MAT 262

mass spectrometer at ETH was controlled by contin-

uous monitoring of the NBS 982 and 983 standards.

The data are presented in Table 4.

4. Presentation and discussion of the results

4.1. Major, trace and rare-earth element geochemistry

The geochemical composition of the studied rocks

is presented in the MORB-normalized variation dia-

grams (Fig. 2; Pearce et al., 1984). The patterns are

characterized by an enrichment of compatible LIL

elements (K, Rb, Ba, Th), as well as the depletion of

Nb and Cr. The latter may be explained by the

fractionation of a Ti-phase and spinel. The enrichment

of LIL elements, slightly decreased Mg concentrations

and the Cr and Nb point to the fractionated protoliths.

The Biasca amphibolites are characterized by the low

HFSE concentrations scattering around the MORB

line in Fig. 2a, and suggesting an oceanic origin for

these rocks. Low Nb is, however, considered to be

characteristic for stable Ti-bearing phases in the

mantle wedge and therefore may suggest an island-

arc environment for these gabbros (e.g. Saunders et

al., 1980); on the other hand, it may reflect the low-

temperature fractionation of ilmenite or sphene during

the crystallization of the gabbro, not indicative for an

island-arc environment. The REE patterns exhibit a

slight enrichment in the light REE, no or slightly

positive Eu anomalies and flat heavy REE (Fig. 2b),

indicating that the melting was shallow and did not

leave residual garnet in the source. The suspicious

systematic decrease in Gd may be an analytical

artifact, which unfortunately could not be resolved

with the data of the standards measured with this suite

of samples. Sample SU-93-6 shows the highest degree

of fractionation, indicated by low Nb, Cr and a

slightly positive Eu anomaly, but a flat REE pattern.

The high phosphorous concentrations, as well as the

strongly variable Rb and Ba concentrations point to

the post-emplacement element mobility during the

Alpine, Variscan and/or Caledonian metamorphic

overprints. Trace and rare-earth element distribution

patterns would agree with a hypothetical geodynamic

scenario of shallow melting at an intraoceanic active

plate margin or of an enriched source in an MOR

setting (T-MORB). Basalts derived from an OIB-type

source would show much higher La/Yb ratios due to

the equilibration in the garnet stability field.

In the Zr/Y vs. Zr diagram (Fig. 2c), which tries to

differentiate the island-arc, mid-ocean ridge and

within-plate settings, the data mostly lie in the WPB

field, which may be an effect of the melt fractionation;

the Zr vs. xMg plot (Fig. 2c, inset) reveals the high

degree of fractionation/cumulation but does not show a

clear fractionation trend among the studied samples.

Despite their metamorphic overprint, they, however,

exhibit much less scatter than the coeval gabbroic and

tonalitic rocks with an inferred island-arc setting from

U. Schaltegger et al. / Chemical Geology 186 (2002) 265–279 271

the Silvretta nappe (Schaltegger et al., 1997), as well as

a suite of basalts with inferred MORB characteristics

from the southern Penninic units (Bellinzona–Bosco–

Berisal; Schenk-Wenger, 1993). The latter mostly plot

outside the fields for fresh basaltic rocks, indicating a

high degree of Zr, Y mobility during the Alpine

Fig. 2. (a) MORB-normalized variation diagram (after Pearce et al., 1984); enrichment of LIL elements, as well as the fractionation of Nb and Cr

demonstrates the evolved nature of the melt. (b) Chondrite-normalized REE patterns (Sun and McDonough, 1989). (c) Zr/Y vs. Zr plot after

Pearce and Norry (1979); the Biasca–Loderio data are mainly situated within the WPP field. The Zr vs. xMg diagram (inset) demonstrates the

high degree of fractionation of the studied meta-basalts and -gabbros. WPB=within-plate basalt; MORB=mid-ocean ridge basalt; IAT= island-

arc tholeiite.

U. Schaltegger et al. / Chemical Geology 186 (2002) 265–279272

alteration. Fractionation and cumulation effects and

post-emplacement element mobility demonstrate that

the application of the geochemical classification

schemes is limited in cases as the present one, where

primary lithologies within a polyorogenic basement

are unknown and most likely did not represent liquidus

compositions.

The overall geochemical characteristics are consid-

ered to be magmatic; some disturbance of Rb, Sr, Th

concentrations visible in the MORB-normalized varia-

tion diagram (Fig. 2a) are likely to be caused by the

high-grade metamorphic overprint(s) and/or by the

oceanic alteration. The present position of the Biasca–

Loderio rock association as a sliver between basement

nappes mainly consisting of felsic gneisses strongly

favours flushing with crustal fluids.

4.2. Isotope geochemistry

Measured 87Sr/86Sr values of the four samples show

a wide range from 0.7036 to 0.7097 (see Fig. 3 and

Table 2). The large variations in Rb/Sr ratios indicate

that one or both elements were mobile during the

polymetamorphic evolution of these rocks. Open-sys-

tem behaviour of the Sr isotopic system may be

envisaged during the seafloor alteration but much more

likely due to flushing with crustal fluids during the

Alpine metamorphism, since some of the data points

have higher 87Sr/86Sr values than the seawater 518 Ma

ago (Burke et al., 1982; Keto and Jacobsen, 1987).

Fig. 2 (continued ).

Fig. 3. Nd–Sr evolution diagram, time-corrected for an age of 518 Ma. Insets: DM= depleted mantle; SW= seawater (Burke et al., 1982; Keto

and Jacobsen, 1987). Same symbols as in Fig. 2c. Own data from amphibolites of Biasca are compared to the amphibolites with inferred MORB

character from the Bellinzona zone, selected from Schenk-Wenger (1993).

U. Schaltegger et al. / Chemical Geology 186 (2002) 265–279 273

Sample SU-93-4 and -6 are situated virtually on the

mantle array (Fig. 3), indicating that they may repre-

sent the least altered rocks. The Nd isotopes, on the

other hand, seem to retain a more original geochemical

signature: they point to a rather homogeneous source

for samples SU-93-3, -4 and -6 with eNd of + 7.3 to 6.3at an age of 518 Ma, whereas SU-93-5 displays a

deviation to a lower (eNd= + 4.2) value. A differential

mobility of Sm and Nd is not substantiated by the REE

patterns, therefore the scatter of eNd values between 4.2and 7.3 is regarded as of primary nature. They point to

the participation of a geochemical component with a

time-integrated low Sm/Nd ratio in the genesis of these

rocks; an average age of this component may be

approximated by the Nd model ages between 0.85

and 2 Ga.

4.3. Ionprobe U–Pb age determinations of zircon

(SHRIMP)

Samples Bi-1 and Bi-2 contained little zircon of

prismatic morphology, which displays magmatic

oscillatory zoning in CL (Fig. 4a). A total of seven

ion microprobe analyses from these two samples

yielded slightly scattering data along the concordia

curve (Fig. 5a). Four well-defined concordant points

Fig. 4. CL and SEM pictures of analyzed zircon populations.

U. Schaltegger et al. / Chemical Geology 186 (2002) 265–279274

showing magmatic-type CL-patterns, however, define

a mean age of 518F 11 Ma (95% c.l.), interpreted to

reflect the protolith age. Three data points, giving

apparent 206Pb/238U ages between 440 and 491 Ma,

are considered to be the result of partial lead loss.

These data are derived from domains with irregular,

rounded boundaries and that show recrystallization

phenomena resulting in the fading of the oscillatory

zoning to various degrees accompanied by the en-

hancement of luminescence (Fig. 4b). Interestingly

and in contrast to the usually observed loss of U and

the reduction of the Th/U ratios during recrystalliza-

tion, this ‘‘bleaching’’ seems to be only accompanied

by the loss of U. Thus, the corresponding data points

are excluded from the calculation of the mean proto-

lith age. On the other hand, no age of recrystallization

can be given with the limited data set available for

these domains.

4.4. Conventional U–Pb age determinations

Sample SU-93-3 contained very small zircon

grains and 50 Am small raspberry-like aggregates.

The zircons show signs of resorption (Fig. 4c); the

raspberry structures (Fig. 4d) could have formed by

welding together the pre-existing grains during new

growth or recrystallization processes. The aggregates

are fragile and could not be air-abraded; the material

analyzed by the conventional U–Pb techniques (anal-

yses 1–6) therefore contains zircon components or

domains of variable age and/or lead loss, yielding

strongly discordant data (Fig. 5b). The data show a

little scatter along the discordia and thus yield only ill-

defined upper and lower intercept ages of 519F 39

and 241F 77 Ma. The lower intercept age is consid-

ered to be a combination of Variscan and Alpine lead

loss, an additional lead loss of Ordovician age cannot

be excluded either. The second sample analyzed by

the conventional methods (SU-93-4) fortunately con-

tained prismatic zircons, which could be treated by

air-abrasion and yielded near-concordant results

between 206Pb/238U ages of 490 and 493 Ma. They

define a discordia line together with a fraction of six

turbid and round zircons (analysis 10), which inter-

sects the concordia at 511F 8 and 298F 49 Ma. The

age of episodic lead loss and/or the formation of

recrystallized concordant domains seems to be of

uniquely Variscan age. However, an Alpine and/or

Caledonian influence cannot be excluded either. The

two conventional ages agree within their 2r errors

margin with the SHRIMP age of 518F 11 Ma. The

Fig. 5. (a) U–Pb concordia diagramswith ionmicroprobe (SHRIMP)

analyses of samples Bi-1 and -2; shaded ellipses refer to the

recrystallized high-CL zones and were rejected for the calculation of

the protolith age. (b) Conventional U–Pb dating results of samples

SU-93-3 and -4: the data indicate the multiepisodic lead loss and/or

growth possibly during the Ordovician, Carboniferous (Variscan) and

Tertiary (Alpine) metamorphism.

U. Schaltegger et al. / Chemical Geology 186 (2002) 265–279 275

SHRIMP-age is considered to be closest to the em-

placement age of the protoliths. The hypothesis of a

Mesozoic age (Piemont–Ligurian or Valais ocean) is

herewith clearly disproved.

5. Geodynamic setting of the Loderio–Biasca

protolith

The protoliths of the Biasca amphibolites are

interpreted to be fractionated gabbros. The partial

melting occurred in the upper mantle in the stability

field of spinel, indicated by the fractionation of Cr,

and lacking HREE depletion. Basaltic rocks would

not crystallize magmatic zircon and the interpretation

of all zircon being inherited is a very speculative and

unlikely one, considering the presence of exclusively

Cambrian magmatic zircons. Lack of magmatic

resorption is a further argument against the exclu-

sively inherited zircons. The Nd isotopic character-

istics with the eNd values between + 7.3 and + 4.2

argue against a mid-ocean rift setting but are rather

indicative for an oceanic scenario involving litho-

spheric or subducted components. The Nd isotopic

data of the amphibolites, however, indicate that they

cannot be derived from an N-MORB source; primary

lithospheric contamination would rather favour the

active margin or island-arc alternative, as it has been

suggested for the coeval gabbros of the Silvretta

nappe (Schaltegger et al., 1997) or a T-MORB origin.

We argue that the Biasca–Loderio amphibolites rep-

resent geochemically the evolved island-arc or T-

MORB gabbros, the source of which has been con-

taminated by the old enriched lithosphere.

6. Cambrian–Ordovician associations in the

Alpine realm

The Biasca occurrence is not unique but adds to a

list of the Late Cambrian to Ordovician mafic–ultra-

mafic units of different geodynamic settings within the

pre-Alpine basement (see Fig. 1a). Three different

types of settings have been recognized so far: (1)

MOR basalt origin was proposed for amphibolites in

the zone Bellinzona–Bosco–Berisal (Schenk-Wenger,

1993) in the Cima–Lunga unit (Adula nappe; Pfiffner,

1999) and the Lac Cornu area (Aiguilles Rouges

massif; Paquette et al., 1989). The first defined a re-

gional Sm–Nd whole-rock reference line with an

apparent age of approximately 0.9 Ga (Schenk-Wenger

and Stille, 1990). Some island-arc or E-type MORB

gabbros of about 870 Ma were found in the Gotthard

Massif (e.g. Gebauer, 1993). (2) Ophiolitic associa-

tions of the ultramafic and mafic rocks—meta-horn-

blendites, pyroxenites, basalts and serpentinites—were

described from Berisal, Simplon area, with an age of

approximately 475 Ma (Stille and Tatsumoto, 1985),

from Chamrousse (Belledonne massif), 496F 6 Ma

(Menot et al., 1988), from the Oetztal unit, 481F 9 Ma

(Gebauer and Sollner, 1993), and also from the nearby

Cima di Gagnone locality (Adula nappe; Fig. 1b), with

an age of 528F 6 Ma (Gebauer, 1996). A possible

plagiogranite from the Silvretta nappe was reported by

Muller et al. (1996) with an age of 532F 30 Ma. (3)

Gabbroic intrusions in active continental or island-arc

settings, with subsequent high-pressure overprints

were described from the Alpine External Massifs, with

ages of 479F 5 Ma (Biino et al., 1999, Aar massif),

467 + 5/� 4 and 471 + 6/� 7 Ma (Oberli et al., 1994,

Gotthard and Tavetsch massifs), 462F 6 Ma (Rubatto

et al., 2001; Argentera) and from the Austroalpine

Silvretta nappe with an age of 525F 5 Ma (Muller et

al., 1995; Schaltegger et al., 1997).

The geodynamic setting of the above-discussed

occurrences mainly relies on the geochemical and

isotopic characteristics, which is mostly uncertain

and disputable. Groups (2) and (3) were therefore

combined in Fig. 1a. They scatter across a large

number of different Alpine tectonic units, which were

assigned to different terranes of contrasting origin by

von Raumer et al. (2002).

7. Lower Paleozoic paleogeography of the Variscan

domain

The large-scale Ordovician–Silurian geodynamic

scenario in Europe comprises continental bimodal

volcanic associations, e.g. in the Massif Central (e.g.

Pin and Duthou, 1990, Gebauer et al., 1981), the

Iberian Massif (Gebauer, 1994) or continental crust-

derived magmatism in various places (Silvretta,

Poller, 1997; Poller et al., 1997; Lusitania, Kroner et

al., 1994; Ossa Morena, Ordonez Casado, 1998).

Cambrian and Ordovician oceanic magmatism for

U. Schaltegger et al. / Chemical Geology 186 (2002) 265–279276

example is documented in the Saxothuringian Zone of

NE-Bavaria (e.g. Gebauer and Grunenfelder, 1979;

von Quadt, 1997), the Moldanubian Zone (e.g. Geba-

uer and Grunenfelder, 1982) or at Cabo Ortegal in

Spain (Ordonez Casado et al., 2001).

The data from the Alpine realm suggest the exis-

tence of at least one oceanic domain at the end of the

Cambrian, extending between the continental mass of

Gondwana, which experienced the Pan-African orog-

eny prior to the breakup, and at least two Gondwana-

derived terranes, one on the west, Avalon, and one on

the north. The latter is termed ‘‘Armorica’’, clearly

aware of the fact that there must be several disrupted

fragments attributed to both continental slices (Iberia,

Armorica, parts of Mid-German Crystalline Rise,

etc.). The period of the oceanic magmatism postdated

the fragmentation of the Pan-African Gondwana con-

tinent. The presence of the different tectonic and

magmatic events within a diversity of collision- and

subduction-cycles was explained in terms of the so-

called ‘‘Hun Superterrane’’ by Stampfli (1996, 2000).

This implies a common origin of active margin, back-

arc and oceanic domains, which were active during

the Ordovician.

The oldest HP rocks reached their metamorphic

peak conditions at around 468 Ma (Gotthard Massif,

Gebauer, 1990) or between 479 and 456 Ma (Aar

massif, Biino et al., 1999). In the Moldanubian/Sax-

othuringian Zone, Silurian eclogite-facies metamor-

phism around 424 Ma has been found in the Upper

Palatine (von Quadt and Gebauer, 1993) or in northern

Bohemia (Gebauer, 1991). Acadian subduction around

380–390 Ma is known from the Saxothuringian Zone

(e.g. Gebauer and Grunenfelder, 1979; von Quadt

(1990) or in NE-Iberia (Ordonez Casado et al.,

2001). The youngest subduction/collision cycle in the

European Variscides is the Lower Carboniferous,

which occurred after the closure of all intervening

oceanic basins during the collision of Laurasia and

Gondwana, respectively.

In conclusion, the rocks of the Variscan Moldanu-

bian terrain sensu lato, i.e. the areas of the Bohemian,

French, Armorican and Iberian massifs including the

pre-Alpine basement, have to be regarded as contain-

ing sequences of fore-arc sediments with the dispersed

and dismembered remnants of arc and back-arc mag-

matism. There are Ordovician, Silurian, Devonian and

Carboniferous subduction cycles. The Moldanubian

s.l. is therefore considered to contain large and multi-

episodic fore-arc/accretionary wedge sequences.

Acknowledgements

The study was supported by many people at

different stages. J. Samuel, R. Rouault (Strasbourg)

are thanked for the help with the geochemical analyses,

W. Wittwer, R. Aubert, I. Ivanov, M.T. Bar, M. Meier

and M. Ovtcharova (Zurich) for the help with the

mineral separation, chemistry and mass spectrometry.

DG acknowledges W. Compston and I.S. Williams for

their help in the collection and evaluation of the

SHRIMP data. Valuable comments by O. Muntener,

and journal reviews by F. Bussy and an anonymous

referee improved the manuscript. The study benefited

from the financial support to the US and DG by the

Schweizerischer National fonds, which is gratefully

acknowledged.

Studied samples

Bi-1: Very fresh, fine-grained amphibolite, no

foliation, garnet, hornblende, plagioclase.

Bi-2: Foliated garnet amphibolite.

SU-93-3: Medium-grained amphibolite with weak

foliation, relic garnet with plagioclase rims.

SU-93-4: Medium-to-coarse-grained amphibolite

with relic magmatic texture, weak foliation, horn-

blende, plagioclase, large titanites, no garnet.

SU-93-5: Coarse-grained amphibolite without foli-

ation, hornblende, plagioclase, no garnet.

SU-93-6: Strongly foliated, fine-grained amphibo-

lite, metamorphic segregation in nearly monomineralic

layers, garnet relics with plagioclase haloes, porphyro-

blastic hornblende and titanite with sieve structures.

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