Provenance of Jurassic Tethyan Sediments in the HP/UHP Zermatt-Saas Ophiolite, Western Alps

Provenance of Jurassic Tethyan sediments in the HP/UHP Zermatt-Saas ophiolite, western Alps

Nancy J. Mahlen† Clark M. Johnson‡ Department of Geology and Geophysics, University of Wisconsin, 1215 West Dayton Street, Madison, Wisconsin 53706, USA Lukas P. Baumgartner§ Institute of Mineralogy and Petrology, BFSH2, CH-1015 Lausanne, Switzerland Brian L. Beard# Department of Geology and Geophysics, University of Wisconsin, 1215 West Dayton Street, Madison, Wisconsin 53706, USA

ABSTRACT terranes: one group (Group I) seems to be 1985; McLennan, 1989), Sm-Nd isotope varia- mixing of an old, crustal component, such tions in sedimentary rocks should also faithfully Rubidium-Sr and Sm-Nd isotope data and as the paragneissic basement nappe samples, record their source regions. Further, Sm and Nd rare earth element (REE) concentrations of with metasediments similar to the second are relatively immobile during metamorphic the metasedimentary rocks within the Zer- group (Group II). The source material for events (Green et al., 1969; Jahn, 2000), which matt-Saas (ZS) ophiolite complex of the west- Group II is dominated by homogenization suggests that Sm-Nd isotope systematics may ern Alps are used to investigate element mobil- of the Variscan-like orthogneissic basement be useful in interpretation of more geologically ity and to determine the provenance of the nappe samples. The provenance of Group complex provenance problems. metasediments in order to place constraints I samples is interpreted to be local, where Jurassic rifting of the African/Apulian plates on the precollisional paleogeography of the source nappes must have been proximal to from the European plate followed by Late Juras- Piemont-Ligurian portion of the neo-Tethys the Piemont-Ligurian basin prior to Alpine sic spreading formed the Piemont-Ligurian ocean. Present-day 87Sr/86Sr variations for convergence. The similarity in dispersion basin of the neo-Tethys ocean (e.g., Hunziker, the ZS metasediments scatter about an early of Nd isotope compositions of the metasedi- 1974). Fine-grained Piemont-Ligurian ocean Tertiary Alpine metamorphic age, whereas ments and likely source terranes suggests fl oor and basin sediments, caught in Eocene local nappes that have been interpreted to that the metasediments refl ect deposition in Alpine collisional tectonics, were eventually refl ect African/Apulian and European base- small, isolated basins early in the formation metamorphosed to high-pressure (HP) and ments scatter about a Variscan-like age; this of the Piemont-Ligurian ocean. ultra-high-pressure (UHP) conditions, and the suggests 87Sr/86Sr isotope systematics were Zermatt-Saas ophiolite complex represents one nearly completely homogenized for most of Keywords: Western Alps, Zermatt, metasedi- such unit. Determining the provenance of the the ZS metasediments during early Tertiary ments, Sm/Nd, Rb-Sr, rare earth elements, Piemont-Ligurian basin sediments can provide metamorphism, probably because they were provenance. restrictions on proposed models of restoration relatively wet prior to metamorphism. of European and African/Apulian units prior to In contrast to the Sr isotope data, REE INTRODUCTION the Alpine orogeny. In addition, understanding data and Nd isotope compositions of the the variability of isotopic compositions across a ZS metasediments overlap those of aver- Trace-element compositions, particularly the basin may provide information as to basin size age upper continental crust, average shale, rare earth elements (REEs) and Rb-Sr and Sm- and physiography. Here REE and Rb-Sr and and the local nappes, and Nd model ages of Nd isotope data, are often used for determining Sm-Nd isotope data are reported for metasedi- the ZS metasediments overlap with those provenance and tectonic setting of sedimentary mentary rocks in the Zermatt-Saas ophiolite, of Variscan-age rocks. These relations sug- rocks. Numerous studies have noted that REE and these results are compared with new data gest that the REEs of the ZS metasediments contents are useful indicators of source ter- from various nappe units of the western Alps as were not disturbed during high- to ultra-high ranes in oceanic basins (e.g., McLennan et probable oceanic and continental source regions pressure Alpine metamorphism. Based on al., 1993; Gleason et al., 1995; Ugidos et al., for the sediments. Despite hydrothermal altera- REE data, Nd isotope compositions, and 1997). Enrichment or depletion of light REEs tion during Mesozoic divergence and/or metaso- mixing models, the ZS metasediments com- relative to heavy REEs and the nature of Eu matic alteration during subsequent subduction prise two groups that require distinct source anomalies can provide additional clues to relate and metamorphism, the REEs in the rocks stud- sediments to the bulk compositions of source ied do not appear to have been mobilized. Sm- regions. Because Sm/Nd ratios do not readily Nd isotope systematics of the sedimentary rocks †E-mail: [email protected]. ‡E-mail: [email protected]. experience fractionation during processes such in the ophiolite were subjected to HP/UHP con- §E-mail: [email protected]. as diagenesis, chemical weathering, erosion, ditions, but at temperatures generally less than #E-mail: [email protected]. or sedimentary sorting (Taylor and McLennan, 600 °C, suggesting the measured Nd isotope

GSA Bulletin; March/April 2005; v. 117; no. 3/4; p. 530–544; doi: 10.1130/B25545.1; 9 ﬁ gures; 3 tables.

620000 640000 Saas C Randa Fee Saas Almagell GB 97JA-1 Täsch DB 97JA-4 000 97JA-3 100000 100 97JA-2 Dent Blanche Allalinhorn 96JA-35 Unter a Gabelhorn 96JA-38 Figure 1. Simpliﬁ ed tectonic Zermatt 96JA-46 Mattmark map of the western Alps Pfülwe See depicting major tectonic units: C ZS 1. Continental outer Pen- ninic zone including Grand Gornergrat St. Bernhard (GB) and Com- Matterhorn 97JA-14 bin (C) units. 2. Continental 97JA-16 97JA-12 97JA-13 inner Penninic zone including Monte Rosa (MR) and Gran 97JA-18 Paradiso (not shown on map). 97JA-24 MR Switzerland 3. Piemont-Ligurian remnants Breuil including Zermatt-Saas ophio- Italy Mezzalama lite (ZS). 4. Low- to medium- pressure African/Apulian Dent Blanche (DB) including Arolla 96JA- Lago di Cignana series and Valpelline series (4a). 000 000 80 21 Valtournenche 80 5. High- to ultra-high-pressure 96JA-16, 17 96JA-1, 26 St. Jacques African/Apulian Sesia zone (S) 96JA-30a, b including Sesia equivalent (5a) 01NM-44, 45, C to Valpelline series. Inset is 47b a' cross section a to a′. Modiﬁ ed Champoluc S from Dal Piaz, 1999.

Inset Matterhorn 1 3 5 a a' a N DB 2 4 a ZS C 5 km C MR S ZS ZS metasediment sample location thrust sheet NW SE compositions may have been preserved through during latest Cretaceous to early Tertiary col- and fragments of the European margin that were the sedimentary and metamorphic cycles. lision of the European and African/Apulian metamorphosed at high to ultra-high pressures Although the Piemont-Ligurian basin sedi- plates (e.g., Hunziker, 1974; Dal Piaz and Ernst, (e.g., Monte Rosa and Gran Paradiso nappes); ments were subducted to HP/UHP conditions, 1978). Slices of the southeastern margin of the and the Austroalpine zone, which represents the which reset Sr isotopes, REE and Sm-Nd iso- European plate and the northern margin of the African/Apulian margin (e.g., Dent Blanche and tope variations appear to be useful indicators African/Apulian plates were diachronously sub- Sesia zone basement nappes). of provenance for the Zermatt-Saas metasedi- ducted with the Piemont-Ligurian ocean basin to ments. These sediments appear to have been variable temperatures and pressures. The major Geology of the Zermatt-Saas Ophiolite derived entirely from local sources, i.e., the tectonic units of the western Alps are, from present-day nappes, which must have been northwest to southeast (Fig. 1; Dal Piaz and The Zermatt-Saas ophiolite (Fig. 1) comprises exposed adjacent to the Piemont-Ligurian basin Ernst, 1978; Escher et al., 1993): the Helvetic peridotites, serpentinites, eclogitized metagab- in the Jurassic. zone, which represents the European margin; bros and metabasalts that contain local examples the outer Penninic zone, which represents rifted of deformed sheeted dikes and clear pillow struc- GEOLOGIC BACKGROUND European fragments that were metamorphosed tures, and a cover series of calcareous and sili- at low to medium pressures (e.g., Grand St. Ber- ceous metasediments from the Piemont-Ligurian The Alps are a classic continent-continent nhard nappe); the inner Penninic zone, which basin (Bearth, 1967; Dal Piaz and Ernst, 1978; convergent setting, which formed through includes remnants of the Piemont-Ligurian Barnicoat and Fry, 1986). The ocean ﬂ oor of closure of the Piemont-Ligurian ocean basin ocean basin (e.g., the Zermatt-Saas ophiolite) the Piemont-Ligurian basin was largely gabbro,

Geological Society of America Bulletin, March/April 2005 531 MAHLEN et al. followed by tholeiitic basalt fl ows and pillow ophiolite complex, although metastable UHP basement complex that is likely of Variscan lavas (Lemoine et al., 1987). The metasedimen- relics are preserved in portions of the ophiolite age but was affected by Alpine metamorphism, tary cover series of the Zermatt-Saas ophiolite (Dal Piaz and Ernst, 1978). These age relations including paragneisses (comparable to the Val- comprises manganese-rich quartzites and an indicate that the Zermatt-Saas ophiolite was rap- pelline series of the Dent Blanche) and Variscan- overlying locally variable sequence of marbles, idly exhumed from peak pressure conditions. age gabbros and granites that are overlain by a metapelites, calc-schists, and micaceous quartz- late Permian to early Triassic mono-metamor- ites that capped the ophiolite sequence prior to Potential Source Terranes phic cover series (Compagnoni et al., 1977; tectonic dismemberment (Bearth and Schwander, Venturini et al., 1996). Maximum metamorphic 1981; Reinecke, 1991). The earliest deposition Several nappe units that represent European temperatures and pressures are estimated to have age of the Zermatt-Saas metasediments is late and African crust were chosen to characterize been ~550 °C and 1.4–2.0 GPa, where peak con- Jurassic based on U-Pb ages of magmatic zircons potential source terranes for the Zermatt-Saas ditions were reached at ca. 65 Ma (Inger et al., (166 ± 1 Ma) and a 40Ar-39Ar age of magmatic metasediments and to test the possibility that 1996; Duchêne et al., 1997; Ruffet et al., 1997; hornblende (165.9 ± 2.2 Ma) both from gabbros current local units may have provided sediment Rubatto et al., 1999; Dal Piaz et al., 2001). in the Gets nappe, French Alps (Bill et al., 1997), to the Zermatt-Saas unit (Fig. 1). Such a model is and U-Pb ages of detrital zircons (161 ± 11 Ma) consistent with paleogeographic reconstructions SAMPLES AND METHODS from metasediments and magmatic zircons that suggest the nappe units could have been (ca. 164 Ma) from metagabbros in the Zermatt- proximal to the Piemont-Ligurian basin (Escher Samples of the Zermatt-Saas metasediments Saas ophiolite (Rubatto et al., 1998). et al., 1997; Stampfl i et al., 1998; Froitzheim, were collected from geographically dispersed The Zermatt-Saas ophiolite locally underwent 2001). The outer Penninic Grand St. Bernhard locations that include the Saas-Fee, Täsch, and ocean-fl oor hydrothermal alteration, as sug- nappe system comprises Variscan basement, Zermatt areas from the Swiss portion of the gested by the presence of Mn-quartzites (radio- Triassic volcaniclastic, and Briançonnais cover ophiolite and the Lago di Cignana and Valtour- larites) believed to have been deposited directly sequence rocks (Bearth, 1967; Dal Piaz, 1999). nanche areas in Italy (Fig. 1). Samples were also on basalts of the ophiolite (Barnicoat and Cart- The Grand St. Bernhard system is partially over- collected from selected African/Apulian and wright, 1995). Cartwright and Barnicoat (1999), lain by the low-pressure metasediments of the European basement nappes for controls on prov- however, suggested that only minor amounts Combin zone. The inner Penninic Monte Rosa enance, including the Sesia zone in Italy and the of fl uid could have been involved based on the and Gran Paradiso nappes include pre-Variscan Dent Blanche nappe in Italy and Switzerland, similarities in oxygen isotope compositions of basement comprising paragneisses and augen as well as portions of the European plate as whole rock, quartz, and white micas from Zer- gneisses that are intruded by a Variscan-age represented by the Monte Rosa, Gran Paradiso, matt-Saas metasediments with metasediments plutonic complex (Bearth and Schwander, 1981; and Grand St. Bernhard nappes from outcrops of similar compositions elsewhere in the Alps. Dal Piaz, 2001). The Gran Paradiso nappe was in Italy and Switzerland (Fig. 1). Professor J.C. The mineral assemblages of the Zermatt-Saas metamorphosed to eclogite-facies conditions at Hunziker, from the Institute of Mineralogy and metasediments from this study are typical of HP ca. 45 Ma (Meffan-Main, 1998; Brouwer et al., Petrology in Lausanne, Switzerland, provided metapelites, and include quartz + white mica 2002) and reached maximum temperatures and additional Sesia zone and Dent Blanche nappe (typically phengite and/or talc, which form pressures of ~500 °C and 1.5 GPa, respectively samples (see Venturini, 1995). a well-defi ned foliation) + garnet ± epidote/ (Rubatto and Gebauer, 1999; Brouwer et al., Whole-rock samples (~2–5 kg) were crushed clinozoisite ± chlorite ± chloritoid ± carbonate 2002). The Monte Rosa nappe reached peak con- and powdered and 50 mg sample-size portions ± rutile ± titanite ± tourmaline (Table 1; Bearth ditions ca. 45–32 Ma (Meffan-Main, 1998; Engi were spiked with Rb-Sr and REE tracers for and Schwander, 1981; Reinecke, 1991, 1998). et al., 2001) at temperatures of ~515–650 °C and concentration and isotopic analyses. Sample dis- The Zermatt-Saas ophiolite experienced pressures that varied from 1.2 to 2.5 GPa (Cho- solution, chemical, and mass analysis procedures blueschist- to eclogite-facies metamorphism pin and Monié, 1984; Borghi et al., 1996; Le follow those of Johnson and Thompson (1991); with peak pressure and temperature estimates Bayon et al., 2000; Engi et al., 2001). The inner all chemical separations and mass analyses were of 1.4–2.0 GPa and 500–600 °C in the Täschalp Penninic rocks were overprinted by greenschist- done in the Radiogenic Isotope Laboratory at the region (Oberhänsli, 1980; Barnicoat and Fry, to amphibolite-facies metamorphism at ca. 30– University of Wisconsin–Madison. Strontium 1986), and 2.6–3.0 GPa and 590–630 °C at 35 Ma (Hurford and Hunziker, 1989; Hurford et isotope compositions were measured using a GV Lago di Cignana (Reinecke, 1991, 1998; van al., 1991; Freeman et al., 1997; Brouwer et al., Instruments Sector 54 thermal ionization mass der Klauw et al., 1997). Constraints on the 2002), suggesting rapid exhumation. spectrometer (TIMS) using a three-jump dynamic age of HP/UHP metamorphism include early Units that represent the African/Apulian multicollector analysis; 87Sr/86Sr isotope ratios Sm-Nd studies (52 ± 18 Ma; Bowtell et al., margin include the low-pressure Dent Blanche were exponentially normalized to an 86Sr/88Sr = 1994), U-Pb zircon ages (averaging 44.1 Ma; nappe and the eclogitic Sesia zone. The Dent 0.1194. Using this analysis method, the measured Rubatto et al., 1998), Sm-Nd geochronology on Blanche nappe (Fig. 1) comprises pre-Alpine 87Sr/86Sr of NIST SRM-987 was 0.710263 ± 13 acid-leached minerals (40.6 ± 2.6 Ma; Amato et basement paragneisses (Valpelline series) and (2-SD, n = 22) during the course of this study. al., 1999), and Lu-Hf geochronology (48.9 ± 2.1 local granitoids, late-Paleozoic orthogneisses Laboratory blanks were typically ~450 pg for Sr Ma; Lapen et al., 2003). In aggregate, this span (Arolla series), and a Mesozoic metasedimen- and less than 200 pg for Rb, which are negligible. of ages is taken to refl ect initial garnet growth tary cover series (Venturini, 1995; Dal Piaz, Neodymium was analyzed as NdO+ using single in mafi c rocks due to subduction beginning at 1999). Maximum metamorphic temperatures Re fi laments and silica gel and phosphoric acid ca. 52 Ma and attainment of peak metamorphic and pressures of ~400–500 °C and 1.0 GPa as the oxygen source, and 18O/16O and 17O/16O conditions around 40 Ma (Lapen et al., 2003). occurred over a poorly constrained time interval ratios of 0.002110 and 0.000387, respectively, Upon exhumation, a greenschist-facies over- of 75–40 Ma (Pfeifer et al., 1991; Cortiana et al., were used to correct the data. Mass analysis was print at ca. 38 Ma (Reinecke, 1998; Amato et 1998; Dal Piaz, 1999; Gebauer, 1999). The Sesia done using a Sector 54 TIMS via a three-jump al., 1999) affected much of the Zermatt-Saas zone comprises a pre-Alpine high-temperature multicollector dynamic analysis and a power-law

532 Geological Society of America Bulletin, March/April 2005 PROVENANCE OF JURASSIC TETHYAN SEDIMENTS

TABLE 1. PERCENT MINERAL MODES FOR METASEDIMENTS AND BASEMENT NAPPE SAMPLES Sample no. Location Description Qtz WM Fsp Gt Bi Cht Carb Ep Am Other GROUP I, Zermatt-Saas Metasediments 96JA-30b Lago di Cignana, Italy Gt-WM-Qtz schist 35 20 15 10 5 10 rutile, titanite, tourmaline, opaque 97JA-1a SaasFee-Plattjen, Switz. Gt-WM-Qtz schist 35 35 15 5 5 rutile, titanite 97JA-24 Breuil-Cervinia, Italy Gt-Carb-WM-Qtz schist 30 20 10 15 10 15 96JA-16a Lago di Cignana, Italy Gt-WM-Qtz schist 30 30 15 10 5 rutile, titanite, tourmaline 96JA-35 Täsch, Switzerland Gt-WM-Qtz schist 35 35 10 10 5 97JA-4 SaasFee-Allalin, Switz. Gt-Carb-WM-Qtz schist 35 25 15 5 10 5 rutile 97JA-1b SaasFee-Allalin, Switz. Gt-WM-Qtz schist 30 35 15 10 rutile, chloritoid, opaque 97JA-15b Gornergletcher, Switz. Gt-WM-Carb-Qtz schist 35 15 15 30 rutile 01NM-47a Lago di Cignana, Italy Gt-WM-Qtz schist 30 30 15 10 5 5 rutile, tourmaline, opaque 96JA-46 Pfulwe, Switzerland Gt-WM-Qtz schist 25 30 10 10 10 10 titanite, tourmaline GROUP II, Zermatt-Metasediments 96JA-30a Lago di Cignana, Italy WM-piemontite quartzite 65 15 5 5 piemontite, opaque 97JA-2 SaasFee-Allalin, Switz. Gt-Carb-WM-Qtz schist 50 15 10 5 15 biotite 97JA-12 Gornergletcher, Switz. WM-Qtz schist 40 40 5 10 rutile 97JA-18 Breuil-Cervinia, Italy Gt-WM-Carb-Qtz schist 30 15 10 10 20 10 96JA-21 Lago di Cignana, Italy Qtz carbonate 35 5 10 50 5 titanite 97JA-16 Gornergletcher, Switz. WM-Gt quartzite 65 15 10 5 96JA-26 Lago di Cignana, Italy Gt-Carb-WM-Qtz schist 35 15 5 5 25 15 01NM-45 Lago di Cignana, Italy Gt-WM-Qtz schist 40 25 10 5 15 rutile 01NM-44 Lago di Cignana, Italy Gt-WM-Qtz schist 40 20 10 5 10 blue amphibole, rutile “Maﬁ c” Zermatt–Saas Sample 97JA-3c SaasFee-Allalin, Switz. Metagabbro 10 10 10 5 5 40 chloritoid, titanite Europe–Gran Paradiso 01NM-50 Valsavarenche Valley, Italy Paragneiss 50 25 10 10 2 2 01NM-51 Valsavarenche Valley, Italy Augen orthogneiss 50 20 20 1 2 2 3 pyroxene Europe–Grand St. Bernhard 01NM-11 Tennje, Switzerland Orthogneiss 60 25 10 01NM-12 Mattsand, Switzerland Orthogneiss 62 15 18 4 01NM-17 Randa, Switzerland Paragneiss 58 31 1 9 Europe–Monte Rosa 01NM-31 Allalin dam, Switzerland Paragneiss 45 20 10 15 5 01NM-32 Allalin dam, Switzerland Paragneiss 40 30 15 01NM-33 Brusson, Italy Orthogneiss 40 15 30 5 3 3 2 chloritoid 01NM-34 Mezzalama, Italy Orthogneiss 40 25 15 15 01NM-36 Mezzalama, Italy Orthogneiss 40 25 20 15 01NM-39 Gressonay, Italy Paragneiss 45 10 15 10 10 5 pyroxene Africa/Apulia–Dent Blanche 01NM-24 Unter Gabelhorn, Switz. Amphibolite 20 10 20 40 01NM-25 Unter Gabelhorn, Switz. Amphibolite 20 15 60 01NM-26 Unter Gabelhorn, Switz. Amphibolite 20 5 35 40 01NM-27 Unter Gabelhorn, Switz. Amphibolite 15 10 20 50 02NM-61 Valpelline Valley, Italy Amphibolite 10 80 KAW-558* Matterhorn, Italy Paragneiss x x x x x x KAW-560* Matterhorn, Italy Paragneiss x x x x x sillimanite KAW-682* Matterhorn, Italy Paragneiss x x x x x opaque 02NM-59 Valpelline Valley, Italy Paragneiss 40 30 5 10 5 5 02NM-60 Valpelline Valley, Italy Paragneiss 40 10 20 5 15 5 2 02NM-62 Valpelline Valley, Italy Carbonate paragneiss 5 10 60 15 pyroxene 02NM-52 Arolla Valley, Switz. Orthogneiss 50 20 25 3 02NM-56 Valpelline Valley, Italy Leucocratic orthogneiss 70 5 10 5 5 02NM-57 Valpelline Valley, Italy Orthogneiss 40 30 20 2 5 02NM-58 Valpelline Valley, Italy Orthogneiss 45 20 15 2 10 pyroxene Africa/Apulia–Lanzo 01NM-49 Near Pont St Martin, Italy Paragneiss 73 21 3 pyroxene, titanite SV-911a* Arnad, Italy Albitic paragneiss x x x x pyroxene, titanite, rutile mb 1k/91* Savenca, Italy Mesocratic gneiss x x x x x x sillimanite, ilmenite SV-9110b* Bonze, Italy Massive ortho-leucogneiss x x x x x x rutile SV-9128c* Cavalcurt, Italy Leucocratic orthogneiss x x x x x rutile 01NM-48 Near Pont St Martin, Italy Orthogneiss 64 11 3 3 8 5 chloritoid, titanite SV-913e* Elvo Valley, Italy Metagranodiorite x x x x x x rutile, zircon SV-914e* Elvo Valley, Italy Metagranite x x x x x x rutile, zircon SV-925a* Arnad, Italy Massive orthogneiss x x x x x x x SV92–1mt* Montestrutto, Italy Metagranite x x x x rutile Note: Modes determined from visual inspection of thin sections. Abbreviations: Qtz—quartz; WM—white mica; Fsp—feldspar; Gt—garnet; Bi—biotite; Cht—chlorite; Carb—carbonate; Ep—epidote/zoisite/clinozoisite group; Am—amphibole; x—percent mode not available for this mineral. *Samples from J.C. Hunziker, Lausanne, Switzerland (see Venturini, 1995).

Geological Society of America Bulletin, March/April 2005 533 MAHLEN et al. normalization to 146Nd/144Nd = 0.7219. During Ce, Nd, Sm, Eu, Gd, Dy, Er, and Yb analyzed (Table 1). Although Group II samples contain the course of this study, the measured 143Nd/144Nd by isotope dilution in this study) ranging from higher modal abundances of quartz (30–40%) of the BCR-1 USGS rock standard was 0.512643 169.2 ppm to 106.9 ppm, with an average ΣREE than white mica (10–30%) compared to Group ± 9 (2-SD, n = 2), and this is taken to be equal concentration of 140.0 ppm, moderate negative I samples, this cannot explain the factor of two to present-day CHUR (e.g., Wasserburg et al., Eu anomalies (average Eu/Eu* = 0.66), and an difference in ΣREE contents between the two

1981). Laboratory blanks were ~180 pg for Nd average (Sm/Nd)n ratio of 0.60 (Table 2, where groups through, for example, quartz dilution. and <50 pg for Sm, which are negligible. All the subscript n refers to chondrite-normalized One sample, 97JA-3c, has a REE pattern that isotopic compositions are calculated to an “ini- values; Fig. 2A, dashed lines). The overall REE is unique relative to the Group I and Group II tial” age of 160 Ma for comparison, which is a pattern indicates light-REE enrichment relative metasediments (Fig. 2A, short dashed line). reasonable estimate of the depositional age of the to the heavy REEs with an average (Ce/Yb)n This sample has low ΣREE abundance of 34.8 sediments (Rubatto et al., 1998). ratio of 8.1. The Group I samples are primarily ppm and no distinctive Eu anomaly (Eu/Eu*

strongly foliated garnet-mica-quartz schists ± = 0.94). Sample 97JA-3c has a (Sm/Nd)n ratio RESULTS carbonate with nearly equal modal abundances of 1.0 and is depleted in light REEs relative to of quartz and white mica (between 20%–35%; heavy REEs (Table 1). In contrast to the more Twenty-three samples of Zermatt-Saas meta- Table 1). Group II samples have ΣREE con- “continental-like” patterns of Groups I and II, sediments and 36 samples from the African/ tents ranging from 90.2 ppm to 55.8 ppm, this sample more closely resembles a depleted Apulian and European source nappes were ana- with an average ΣREE concentration of 71.1 oceanic REE pattern. This metagabbro-like lyzed for Rb-Sr and Sm-Nd isotope compositions ppm, moderate negative Eu anomalies (aver- sample contains epidote + white mica + biotite and REE contents. age Eu/Eu* = 0.68), and an average (Sm/Nd)n + feldspar + chloritoid + titanite (Table 1). ratio of 0.64 (Table 2; Fig. 2A, solid lines). The Eleven samples from the European nappes Rare-Earth-Element Data REE pattern displays less extreme light-REE were analyzed to characterize European crust enrichment relative to the heavy REEs and that may have been proximal to the Ligurian

Two distinct groups of metasediments has a lower average (Ce/Yb)n of 6.6 than the basin. The ΣREE contents of the European sam- from the Zermatt-Saas ophiolite are resolved Group I samples (Table 1). Group II samples ples range from 219.6 ppm to 54.9 ppm with from the chondrite-normalized REE patterns include strongly foliated garnet-mica-quartz an average of 117.7 ppm (Table 2; Fig. 2B). (Fig. 2A), and both groups resemble a “conti- schists ± carbonate, mica-quartz schists, quartz- These samples display a light-REE enrichment nental” pattern. Group I samples have ΣREE carbonates, a garnet-rich micaceous quartzite, relative to the heavy REEs and have an average contents (where ΣREE is the sum in ppm of and a micaceous piemontite-bearing quartzite (Ce/Yb)n of 8.7. Europium anomalies range

A Zermatt-Saas metasediments B European basement paragneiss-dashed, n = 5 100 orthogneiss-solid, n = 6 Group I-dashed, n = 12 high ΣREE

Figure 2. Chondrite-normalized mafic, rare-earth-element patterns nor- 10 n = 2 Group II-solid malized to Anders and Grevesse n = 11, low ΣREE (1989) chondritic compositions. Lines represent individual sam- C Sesia zone Dent Blanche nappe D ples. (A) Zermatt-Saas metasedi- paragneiss-dashed, n = 3 paragneiss-dashed, n = 6 ments (ZS). ZS shaded areas 100 orthogneiss-solid, n = 7 orthogneiss-solid, n = 4 plotted with (B) European base-

Chondrite normalized mafic-short dashed, n = 5 ment samples including Monte Rosa, Grand St. Bernhard, and Gran Paradiso. (C) African/ Apulian Sesia zone samples. (D) African/Apulian Dent Blanche nappe samples. Nappe samples 10 in B, C, and D are subdivided by mafic, lithology. n = 5

dark grey shading = ZS Group I light grey shading = ZS Group II 1 Ce Nd Sm Eu Gd Dy Er Yb Ce Nd Sm Eu Gd Dy Er Yb

534 Geological Society of America Bulletin, March/April 2005 PROVENANCE OF JURASSIC TETHYAN SEDIMENTS

TABLE 2. RARE EARTH ELEMENT DATA FOR METASEDIMENTS AND BASEMENT NAPPE SAMPLES Sample Description Swiss Grid Ce Nd Sm Eu Gd Dy Er Yb Eu/ (Sm/ (Gd/ (Ce/ ΣREE

coordinates (ppm) (ppm) (ppm) (ppm) (ppm) (ppm) (ppm) (ppm) Eu* Nd) n Yb)n Yb) n (ppm) GROUP I, Zermatt-Saas Metasediments 96JA-30b Gt-WM-Qtz schist 612100 80600 87.13 33.69 6.90 N.D. 6.19 5.24 2.97 2.77 N.D. 0.63 1.85 8.48 144.89 97JA-1a Gt-WM-Qtz schist 639200 103570 82.26 34.37 6.40 1.27 5.53 5.84 3.32 3.09 0.65 0.57 1.48 7.17 142.09 97JA-24 Gt-Carb-WM-Qtz schist 615875 87100 66.41 28.98 6.25 1.38 5.98 5.03 2.77 2.48 0.68 0.66 1.99 7.22 119.27 96JA-16a Gt-WM-Qtz schist 612700 80500 93.00 41.09 7.81 1.54 6.46 5.11 2.98 2.86 0.66 0.58 1.87 8.77 160.83 96JA-35 Gt-WM-Qtz schist 629100 99050 67.11 29.34 5.67 1.19 5.01 4.30 2.32 2.13 0.68 0.59 1.95 8.50 117.06 97JA-4 Gt-Carb-WM-Qtz schist 637850 101420 63.39 26.78 5.05 1.01 4.23 3.83 1.96 1.71 0.66 0.58 2.05 9.98 107.96 97JA-1b Gt-WM-Qtz schist 639200 103570 98.17 40.71 7.56 1.45 6.51 6.89 4.17 3.74 0.63 0.57 1.44 7.06 169.21 97JA-15b Gt-WM-Carb-Qtz schist 624700 90500 59.83 26.78 5.47 1.15 5.47 4.10 2.17 1.88 0.64 0.63 2.40 8.55 106.86 01NM-47a Gt-WM-Qtz schist 612100 80600 94.47 33.87 6.85 1.40 5.99 4.38 2.20 N.D. 0.66 0.62 N.D. N.D. 149.16 96JA-46 Gt-WM-Qtz schist 629750 96825 92.73 39.56 7.33 1.43 6.13 5.71 3.25 3.17 0.65 0.57 1.60 7.88 159.31 96JA-46† Gt-WM-Qtz schist 629750 96825 84.99 37.49 6.99 1.37 5.82 5.42 3.09 2.94 0.65 0.57 1.64 7.79 148.11 96JA-46† Gt-WM-Qtz schist 629750 96825 89.51 39.42 7.34 1.43 6.04 5.64 3.21 3.11 0.65 0.57 1.61 7.77 155.68 GROUP II, Zermatt-Saas Metasediments 96JA-30a WM-piemontite quartzite 612100 80600 43.37 18.74 3.86 0.85 3.56 3.23 1.83 1.66 0.70 0.63 1.77 7.04 77.09 96JA-30a† WM-piemontite quartzite 612100 80600 41.81 18.06 3.72 0.83 3.45 3.14 1.77 1.54 0.70 0.63 1.85 7.30 74.31 97JA-2 Gt-Carb-WM-Qtz schist 638050 101300 36.71 17.05 3.50 0.78 3.34 3.19 1.82 1.64 0.69 0.63 1.69 6.04 68.03 97JA-12 WM-Qtz schist 624725 90125 31.81 14.44 3.10 0.56 2.86 2.81 1.44 1.20 0.57 0.66 1.98 7.17 58.21 97JA-18 Gt-WM-Carb-Qtz schist 617300 88500 39.63 19.00 3.99 0.94 3.84 3.43 1.87 1.62 0.73 0.65 1.96 6.60 74.32 96JA-21 Qtz carbonate 611050 81125 44.80 19.02 3.96 0.89 3.71 3.40 1.92 1.63 0.70 0.64 1.88 7.40 79.33 97JA-16 WM-Gt quartzite 624600 90750 34.90 14.72 2.92 0.62 2.69 2.93 1.76 1.62 0.67 0.61 1.38 5.80 62.17 96JA-26 Gt-Carb-WM-Qtz schist 611925 80500 37.80 18.10 3.74 0.82 3.52 2.87 1.55 1.44 0.69 0.64 2.03 7.09 69.85 96JA-21 Qtz carbonate 611050 81125 39.99 17.68 3.76 0.85 3.63 3.31 1.90 1.62 0.70 0.65 1.85 6.64 72.73 01NM-45 Gt-WM-Qtz schist 612150 80500 53.34 20.12 4.17 0.92 3.99 3.71 2.08 1.83 0.68 0.64 1.80 7.83 90.16 01NM-44 Gt-WM-Qtz schist 611925 80500 30.11 11.96 2.52 0.58 2.74 3.56 2.31 2.05 0.67 0.65 1.10 3.95 55.84 “Maﬁ c” Zermatt-Saas Sample 97JA-3c Metagabbro 637945 101375 9.35 8.59 2.79 0.98 3.49 4.37 2.64 2.33 0.95 1.00 1.24 1.08 34.53 97JA-3c† Metagabbro 637945 101375 9.74 8.56 2.77 0.97 3.56 4.37 2.68 2.35 0.94 1.00 1.25 1.12 35.00 Europe–Gran Paradiso 01NM-50 Paragneiss 582271 43974 45.95 17.45 3.43 0.70 2.88 2.47 1.49 1.38 0.67 0.60 1.73 8.99 75.75 01NM-51 augen orthogneiss 581974 42699 54.56 24.89 5.45 0.72 5.63 4.94 2.73 2.50 0.40 0.67 1.86 5.88 101.43 Europe–Grand St. Bernhard 01NM-11 Orthogneiss 628275 113325 32.47 14.28 2.55 0.53 1.82 1.50 0.87 0.90 0.74 0.55 1.67 9.73 54.92 01NM-12 Orthogneiss 627404 110563 44.44 20.57 4.81 0.61 4.78 4.77 2.53 2.13 0.38 0.72 1.86 5.63 84.63 01NM-17 Paragneiss 626791 107263 102.96 39.07 7.19 1.34 5.87 4.70 2.38 2.12 0.63 0.57 2.29 13.07 165.64 Europe–Monte Rosa 01NM-31 Paragneiss 639925 99650 131.02 53.53 10.03 1.42 8.32 7.61 3.98 3.66 0.47 0.58 1.88 9.64 219.57 01NM-32 Paragneiss 639925 99650 86.27 36.90 7.36 1.15 6.63 5.81 2.96 N.D. 0.50 0.61 N.D. N.D. 147.07 01NM-33 Orthogneiss 624192 65782 35.57 15.69 3.49 0.43 3.06 2.66 1.31 1.18 0.40 0.68 2.15 8.14 63.39 01NM-34 Orthogneiss 624943 85049 71.52 31.65 6.67 0.82 5.23 3.94 1.96 1.78 0.42 0.65 2.43 10.82 123.57 01NM-36 Orthogneiss 624922 85055 57.52 25.23 5.11 1.20 4.61 4.55 2.64 2.52 0.75 0.62 1.51 6.15 103.37 01NM-39 Paragneiss 631803 82641 94.33 35.36 6.72 1.10 6.09 5.97 3.22 2.82 0.52 0.58 1.78 9.00 155.61 Africa/Apulia–Dent Blanche 01NM-24 Amphibolite 621221 96609 27.53 14.52 4.22 1.67 5.50 6.32 3.92 3.66 1.05 0.89 1.24 2.03 67.35 01NM-25 Amphibolite 621221 96609 7.45 4.92 1.36 0.72 1.74 1.81 1.06 0.89 1.42 0.85 1.61 2.24 19.96 01NM-26 Amphibolite 620927 96552 3.46 3.47 1.23 0.62 1.81 1.97 1.12 N.D. 1.27 1.09 N.D. N.D. 13.67 01NM-27 Amphibolite 620796 96567 3.81 3.95 1.38 0.93 N.D. 2.52 1.57 1.46 N.D. 1.08 N.D. 0.70 15.62 02NM-61 Amphibolite 604485 83623 39.86 26.12 6.82 2.44 7.16 6.45 3.20 2.59 1.06 0.80 2.28 4.14 94.64 KAW-558‡ Paragneiss N.D. N.D. 86.71 37.46 7.27 1.62 6.49 6.19 3.57 3.36 0.71 0.60 1.60 6.95 152.67 KAW-560‡ Paragneiss N.D. N.D. 113.79 48.76 9.09 1.89 7.86 7.27 4.16 3.98 0.68 0.57 1.63 7.70 196.82 KAW-682‡ Paragneiss N.D. N.D. 84.21 36.55 6.96 1.49 5.91 5.67 3.68 3.71 0.71 0.59 1.32 6.11 148.18 02NM-59 Paragneiss 604244 83344 93.72 41.22 8.00 1.49 6.81 6.18 3.67 4.10 0.61 0.60 1.37 6.15 165.20 02NM-60 Paragneiss 604264 83433 128.85 57.05 10.90 2.04 8.67 7.19 4.04 3.80 0.64 0.59 1.89 9.13 222.55 02NM-62 Carbonate paragneiss 603977 83241 33.89 15.01 2.89 0.61 2.54 2.42 1.44 1.40 0.69 0.59 1.49 6.50 60.20 02NM-52 Orthogneiss 602682 95692 43.61 15.46 2.54 0.51 1.83 1.35 0.75 0.80 0.72 0.50 1.88 14.67 66.84 02NM-56 Leucocratic orthogneiss 590818 75528 21.00 9.24 2.27 0.69 2.29 2.18 1.12 0.85 0.92 0.76 2.23 6.67 39.63 02NM-57 Orthogneiss 590818 75528 74.49 27.77 4.97 0.88 4.05 3.63 1.96 1.75 0.60 0.55 1.91 11.47 119.51 02NM-58 Orthogneiss 590797 75587 69.39 22.18 3.42 0.85 2.67 2.22 1.24 1.22 0.85 0.47 1.81 15.33 103.19 Africa/Apulia–Sesia Lanzo 01NM-49 Paragneiss 623876 51279 79.37 33.96 6.35 1.14 4.53 2.07 0.58 0.36 0.65 0.58 10.45 59.73 128.36 SV-911a‡ Albitic paragneiss N.D. N.D. 130.85 64.25 10.35 0.45 6.82 4.70 2.19 1.82 0.16 0.50 3.09 19.32 221.43 mb 1k/91‡ Mesocratic gneiss N.D. N.D. 42.66 22.92 4.57 1.46 4.67 6.38 4.15 N.D. 0.96 0.61 N.D. N.D. 86.80 SV-9110b‡ Massive ortho-leucogneiss N.D. N.D. 27.76 26.64 10.90 0.05 12.56 13.38 7.00 6.01 0.01 1.26 1.73 1.24 104.30 SV-9128c‡ Leucocratic orthogneiss N.D. N.D. 70.41 34.79 6.25 2.25 4.31 1.91 0.55 0.31 1.32 0.55 11.53 61.42 120.77 01NM-48 Orthogneiss 623864 50498 59.85 25.32 5.14 1.11 4.73 4.28 2.28 1.89 0.69 0.62 2.07 8.52 104.61 SV-913e‡ Metagranodiorite N.D. N.D. 92.51 41.82 6.96 0.94 5.07 3.81 2.03 1.70 0.48 0.51 2.47 14.69 154.84 SV-914e‡ Metagranite N.D. N.D. 32.90 17.17 3.32 0.57 2.58 2.13 1.10 0.91 0.59 0.59 2.34 9.74 60.66 SV-925a‡ Massive orthogneiss N.D. N.D. 87.12 44.13 8.31 1.41 7.18 6.60 3.56 2.77 0.55 0.58 2.14 8.46 161.09 SV92-1mt‡ Metagranite N.D. N.D. 12.53 6.48 2.06 0.13 2.42 3.79 1.65 1.20 0.18 0.98 1.67 2.82 30.27

1/2 Note: Swiss grid coordinates in italics are approximate. Sample description abbreviations as in Table 1. Eu/Eu* = Eun/(Smn * Gdn) ; n—chondrite normalized values, from Anders and Grevesse (1989); N.D.—no data. †Replicate analyses. ‡Samples from J.C. Hunziker, Lausanne, Switzerland (see Venturini, 1995).

Geological Society of America Bulletin, March/April 2005 535 MAHLEN et al.

86 from Eu/Eu* = 0.38–0.75 and (Sm/Nd)n ratios Sr ratios for Group II metasediments are more for the Zermatt-Saas metasediments do not range from 0.55 to 0.72. There appears to be no variable, ranging from 0.04 to 32.66 (Table 3). defi ne a correlation along a Sm-Nd “isochron,” apparent distinction between the three European The majority of the African/Apulian and Euro- but instead have a signifi cant range in Nd isotope nappes, although when the nappes are denoted pean nappe samples scatter about a Variscan-age compositions relative to a restricted range in Sm/ 147 144 by parent lithology, paragneissic samples tend array (Fig. 3C) consistent with their most com- Nd ratios (Fig. 4, where fSm/Nd = Sm/ Ndmeasured/ to have higher overall REE contents than the mon basement age. There is little correlation of 0.1967–1). The lack of 147Sm/144Nd–143Nd/144Nd orthogneissic samples. 87Rb/86Sr ratios and REE patterns for the base- variations about an Alpine or Variscan-age Twenty-fi ve samples were analyzed to ment nappes, with the exception that the fi ve isochron supports the premise that the REEs characterize African/Apulian crust composi- Dent Blanche samples that have low (Ce/Yb)n in the Zermatt-Saas sediments refl ect neither tions. The ten Sesia Lanzo samples have ratios (Table 1) have very low 87Rb/86Sr ratios, derivation from a single Variscan-age source nor variable REE patterns and abundances, where consistent with the inferred mafi c composition signifi cant mobilization during Eocene Alpine ΣREE contents range from 221.4 to 30.3 ppm (Table 3; Fig. 3C). metamorphism. An important observation from (Table 2; Fig. 2C). These samples have Eu the isotopic data is that, commensurate with anomalies ranging from positive (Eu/Eu* = 1.3) Sm-Nd Isotope Data their distinctive REE patterns and 147Sm/144Nd to extremely negative (Eu/Eu* = 0.01), and an ratios, the Group I and II samples have different 87 86 87 86 average (Sm/Nd)n of 0.68. The REE patterns In contrast to the Rb/ Sr– Sr/ Sr relations, ranges in Nd isotope compositions, where εNd(0) generally indicate light-REE enrichment, where present-day 147Sm/144Nd–143Nd/144Nd variations ranges from –7.9 to –14.9 for Group I samples most samples have (Ce/Yb)n >8.5, although this varies considerably from 61.4 to as low as 1.2. Ten Dent Blanche samples have ΣREE concentrations ranging from 222.5 to 39.6 ppm, moder- This study: 0.80 ZS Group I 0.72 ate Eu anomalies (average Eu/Eu* = 0.71), and ZS Group II an average (Sm/Nd) ratio of 0.58 (Table 2). The ZS mafic n Other studies: overall REE pattern also indicates light-REE ZS, Amato enrichment relative to the heavy REEs, with 0.78 et al., 1999 0.71 ZS, Dal Piaz B average (Ce/Yb)n = 9.1 (Fig. 2D). The distinc- et al., 2001 0 1 2 3 4 5 6 7

a tion between orthogneissic versus paragneissic (0) M

Sr 0.76 parent lithology in the African/Apulian samples 360 86 » t is not as clear as in the European samples, Sr/ » 260 Ma t but the samples that have higher overall REE 87 0.74 Ma concentrations tend to be paragneissic samples t » 100 t » 60 Ma (Figs. 2C and 2D). Five additional Dent t » 40 Ma Blanche samples have REE patterns that are 0.72 fairly ﬂ at, (Ce/Yb)n ratios that range from 0.7 to 4.1, ΣREE abundances between 95 and 14 Zermatt-Saas metasediments A ppm, average (Sm/Nd)n = 0.94, and average 0.70 Eu/Eu* of ~1.2 (Table 2; Fig. 2D, light-grey short-dashed lines). These samples contain 4.3, 0.9 a amphibole + epidote/clinozoisite + white mica 0.80 M Ma 360 » » 260 + feldspar + chlorite + quartz (Table 1). These t t ﬁ ve amphibolite samples, collected near Unter Gabelhorn, have REE data that are similar to 0.78 depleted, oceanic REE patterns rather than continental patterns, and they are not likely to be (0) representative of the Dent Blanche nappe. Sr 0.76 86 Sr/

Rb-Sr Isotope Data 87 0.74 This study: » 60 Ma Present-day 87Rb/86Sr–87Sr/86Sr variations for European t African/Apulian t » 40 Ma the Zermatt-Saas metasediments are distinct Other studies: 0.72 Sesia, Dal Piaz from those of the European and African/Apulian et al., 2001 nappes (Fig. 3). The majority of the metasedi- Monte Rosa, Basement ments lie along a common trend that falls along Pawlig, 2000 nappe samples C 0.70 a 60–40 Ma “isochron,” which may be inter- 0 10 20 30 preted to reﬂ ect a metamorphic, Alpine-related 87 86 age (Fig. 3A and 3C). Three metasediment sam- Rb/ Sr ples, however, lie along a steeper array, similar Figure 3. 87Rb/86Sr–87Sr/86Sr isotope plot. (A) Zermatt-Saas metasediments (ZS). (B) Ex- to the basement nappes. Group I metasediments aggeration of lower left-hand corner of A. (C) European and African/Apulian basement have a relatively restricted range in 87Rb/86Sr nappes. This study: European basement samples include Monte Rosa, Grand St. Bernhard, ratios, varying from 1.21 to 6.97, whereas 87Rb/ and Gran Paradiso; African/Apulian samples include Dent Blanche and Sesia zone.

536 Geological Society of America Bulletin, March/April 2005 PROVENANCE OF JURASSIC TETHYAN SEDIMENTS

TABLE 3. Sm-Nd AND Rb-Sr ISOTOPIC DATA FOR METASEDIMENTS AND BASEMENT NAPPE SAMPLES

147 143 87 87 87 Sample Sm Nd Sm/ Nd/ 2-SE εNd(0) εNd(160) ƒSm/Nd TDM Sr Rb Rb/ Sr/ Sr/ 144 144 86 86 86 (ppm) (ppm) Nd Ndm (Ga) (ppm) (ppm) Sr Srm Sr160 GROUP I, Zermatt-Saas Metasediments 96JA-30b 6.90 33.69 0.12447 0.512232 ±75 –7.92 –6.45 –0.37 1.56 185.65 77.87 1.214 0.70729 0.70453 97JA-1a 6.40 34.37 0.11325 0.511879 ±79 –14.80 –13.10 –0.42 1.91 80.20 146.44 5.309 0.75767 0.74559 97JA-24 6.25 28.98 0.13097 0.512203 ±79 –8.50 –7.16 –0.33 1.73 123.25 92.90 2.181 0.71022 0.70526 96JA-16a 7.81 41.09 0.11556 0.512043 ±88 –11.62 –9.96 –0.41 1.71 136.17 138.85 2.952 0.71286 0.70615 96JA-35 5.67 29.34 0.11744 0.512029 ±69 –11.89 –10.27 –0.40 1.76 196.36 135.74 2.001 0.71272 0.70810 96JA-35† N.D. 29.35 N.D. 0.512023 ±91 –12.00 N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. 97JA-4 5.05 26.78 0.11463 0.512113 ±74 –10.24 –8.56 –0.42 1.59 129.41 112.55 2.517 0.71213 0.70620 97JA-1b 7.56 40.71 0.11283 0.511875 ±352 –14.89 –13.18 –0.43 1.91 77.64 186.54 6.974 0.74086 0.72500 97JA-1b† N.D. 40.69 N.D. 0.511905 ±80 –14.31 N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. 97JA-15b 5.47 26.78 0.12418 0.512210 ±84 –8.35 –6.87 –0.37 1.59 42.95 97.24 6.455 0.71346 0.69873 01NM-47a 6.85 33.87 0.12301 0.512141 ±89 –9.70 –8.20 –0.37 1.69 131.82 146.56 3.218 0.71159 0.70427 96JA-46 7.33 39.56 0.11265 0.511996 ±74 –12.53 –10.81 –0.43 1.73 111.97 155.13 4.011 0.71539 0.70626 96JA-46† 6.99 37.49 0.11335 0.511942 ±78 –13.57 –11.87 –0.42 1.82 118.19 178.25 4.367 0.71546 0.70553 96JA-46† 7.34 39.42 0.11311 0.511963 ±81 –13.17 –11.47 –0.42 1.79 118.54 175.52 4.287 0.71542 0.70567 96JA-46‡ N.A. N.A. N.A. 0.512006 ±8000 N.A. N.A. N.A. N.A. N.A. N.A. N.A. 0.71544 N.A. GROUP II, Zermatt-Saas Metasediments 97JA-2 3.50 17.05 0.12471 0.512208 ±93 –8.39 –6.92 –0.37 1.60 59.99 59.49 2.870 0.71300 0.70647 97JA-12 3.10 14.44 0.13066 0.512199 ±78 –8.56 –7.21 –0.34 1.73 15.80 176.75 32.657 0.80231 0.72803 97JA-18 3.99 19.00 0.12761 0.512227 ±82 –8.02 –6.61 –0.35 1.62 218.83 35.63 0.471 0.71020 0.70913 96JA-21 3.96 19.02 0.12655 0.512118 ±78 –10.14 –8.71 –0.36 1.79 605.92 8.58 0.041 0.70878 0.70869 97JA-16 2.92 14.72 0.12069 0.512227 ±78 –8.02 –6.47 –0.39 1.51 N.D. N.D. N.D. N.D. N.D. 96JA-26 3.74 18.10 0.12564 0.512206 ±84 –8.42 –6.97 –0.36 1.62 216.10 50.24 0.673 0.71179 0.71026 96JA-21 3.76 17.68 0.12911 0.512120 ±94 –10.11 –8.73 –0.34 1.84 603.39 8.42 0.040 0.70879 0.70870 96JA-21† N.D. 17.68 N.D. 0.512123 ±74 –10.05 N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. 01NM-45 4.17 20.12 0.12596 0.512178 ±101 –8.98 –7.53 –0.36 1.68 50.93 85.46 4.858 0.71515 0.70410 01NM-44 2.52 11.96 0.12778 0.512174 ±94 –9.06 –7.65 –0.35 1.72 20.05 92.34 13.339 0.71892 0.68858 96JA-30a 3.86 18.74 0.12518 0.512238 ±73 –7.80 –6.34 –0.36 1.56 177.00 70.90 1.159 0.70937 0.70674 96JA-30a† 3.72 18.06 0.12530 0.512241 ±65 –7.75 –6.29 –0.36 1.56 182.38 80.25 1.273 0.70936 0.70646 96JA-30a‡ N.A. N.A. N.A. 0.512242 ±8000 N.A. N.A. N.A. N.A. N.A. N.A. N.A. 0.70938 N.A. “Maﬁ c” Zermatt-Saas Sample 97JA-3c 2.79 8.59 0.19742 0.513053 ±84 8.10 8.09 0.00 N.A. 47.90 77.90 4.708 0.71186 0.70116 97JA-3c† 2.77 8.56 0.19674 0.513063 ±79 8.30 8.30 0.00 N.A. 52.28 90.73 5.023 0.71188 0.70045 Europe–Gran Paradiso 01NM-50 3.43 17.45 0.11950 0.511888 ±84 –14.63 –13.06 –0.39 2.02 75.74 104.98 4.023 0.74033 0.73117 01NM-51 5.45 24.89 0.13295 0.512284 ±82 –6.91 –5.61 –0.32 1.62 74.38 N.D. N.D. 0.73451 N.D. Europe–Grand St. Bernhard 01NM-11 2.55 14.28 0.10850 0.512275 ±82 –7.09 –5.29 –0.45 1.27 73.73 108.53 4.261 0.71341 0.70371 01NM-12 4.81 20.57 0.14213 0.512258 ±84 –7.42 –6.31 –0.28 1.88 67.73 210.73 9.039 0.74961 0.72905 01NM-17 7.19 39.07 0.11182 0.511843 ±81 –15.50 –13.78 –0.43 1.94 161.94 100.77 1.803 0.72467 0.72056 Europe–Monte Rosa 01NM-31 10.03 53.53 0.11390 0.511946 ±81 –13.49 –11.81 –0.42 1.82 121.25 29.74 0.712 0.74155 0.74004 01NM-32 7.36 36.90 0.12125 0.511948 ±77 –13.47 –11.93 –0.38 1.96 94.90 157.66 4.823 0.74269 0.73172 01NM-33 3.49 15.69 0.13535 0.512151 ±73 –9.50 –8.25 –0.31 1.92 50.51 289.59 16.695 0.77328 0.73531 01NM-34 6.67 31.65 0.12802 0.512203 ±73 –8.49 –7.09 –0.35 1.67 10.86 28.95 7.779 0.79340 0.77682 01NM-36 5.11 25.23 0.12314 0.512172 ±82 –9.08 –7.58 –0.37 1.64 67.80 16.04 0.686 0.73082 0.72935 01NM-39 6.72 35.36 0.11543 0.511963 ±85 –13.17 –11.52 –0.41 1.83 116.25 130.73 3.260 0.72841 0.72100 Africa/Apulia–Dent Blanche 01NM-24 4.22 14.52 0.17666 0.513057 ±91 8.17 8.58 –0.10 N.A. 447.60 0.76 0.005 0.70343 0.70341 01NM-25 1.36 4.92 0.16766 0.513037 ±84 7.79 8.38 –0.15 N.A. 390.67 1.47 0.011 0.70316 0.70314 01NM-26 1.23 3.47 0.21567 0.513090 ±105 8.82 8.44 0.10 N.A. 425.05 0.41 0.003 0.70322 0.70321 01NM-27 1.38 3.95 0.21287 0.513081 ±124 8.64 8.32 0.08 N.A. 220.42 0.65 0.009 0.70401 0.70399 02NM-61 6.82 26.12 0.15866 0.512758 ±74 2.34 3.12 –0.19 1.09 363.04 6.86 0.055 0.70500 0.70489 KAW-558 7.27 37.46 0.11793 0.512051 ±79 –11.44 –9.84 –0.40 1.74 207.95 9.55 0.133 0.71662 0.71633 KAW-560 9.09 48.76 0.11332 0.511937 ±83 –13.68 –11.98 –0.42 1.83 107.59 18.37 0.495 0.73583 0.73477 KAW-682 6.96 36.55 0.11570 0.511995 ±78 –12.54 –10.89 –0.41 1.78 175.77 17.45 0.288 0.72500 0.72438 02NM-59 8.00 41.22 0.11800 0.512079 ±84 –10.91 –9.31 –0.40 1.70 348.49 102.15 0.849 0.71438 0.71257 02NM-60 10.90 57.05 0.11610 0.512113 ±82 –10.24 –8.59 –0.41 1.61 312.92 81.44 0.753 0.71411 0.71250 02NM-62 2.89 15.01 0.11702 0.512036 ±72 –11.75 –10.13 –0.41 1.74 4737.40 47.15 0.029 0.70835 0.70829 02NM-52 2.54 15.46 0.09977 0.512258 ±68 –7.41 –5.43 –0.49 1.19 99.50 5.68 0.165 0.71751 0.71716 02NM-56 2.27 9.24 0.14959 0.512250 ±87 –7.57 –6.61 –0.24 2.11 108.59 6.31 0.168 0.71997 0.71961 02NM-57 4.97 27.77 0.10881 0.512198 ±80 –8.58 –6.78 –0.45 1.38 105.65 24.48 0.671 0.72143 0.72000 02NM-58 3.42 22.18 0.09381 0.512251 ±65 –7.56 –5.46 –0.52 1.14 198.82 10.18 0.148 0.71348 0.71316 Africa/Apulia–Sesia Lanzo 01NM-49 6.35 33.96 0.11367 0.512119 ±79 –10.13 –8.44 –0.42 0.96 149.82 N.D. N.D. 0.71687 N.D. SV-911a 10.35 64.25 0.09793 0.512422 ±82 –4.21 –2.20 –0.50 1.56 15.96 156.52 28.522 0.75933 0.69445 mb 1k/91 4.57 22.92 0.12106 0.511901 ±86 –14.38 –12.84 –0.38 2.04 80.23 146.87 5.317 0.74827 0.73618 SV-9110b 10.90 26.64 0.24860 0.512501 ±88 –2.68 –3.74 0.26 1.67 21.60 119.71 16.128 0.76602 0.72933 SV-9128c 6.25 34.79 0.10910 0.511997 ±88 –12.50 –10.71 –0.45 N.D. 729.88 40.49 0.161 0.71457 0.71420 01NM-48 5.14 25.32 0.12328 0.512187 ±98 –8.80 –7.30 –0.37 1.61 187.61 122.33 1.888 0.71651 0.71221 SV-913e 6.96 41.82 0.10113 0.512185 ±86 –8.84 –6.90 –0.49 1.30 146.55 145.78 2.882 0.72165 0.71509 SV-914e 3.32 17.17 0.11738 0.512218 ±74 –8.20 –6.59 –0.40 1.47 83.43 142.18 4.942 0.73212 0.72088 SV-925a 8.31 44.13 0.11444 0.512267 ±75 –7.23 –5.55 –0.42 1.35 239.51 100.59 1.216 0.71226 0.70950 SV92-1mt 2.06 6.48 0.19336 0.512369 ±70 –5.25 –5.19 –0.02 N.D. 27.15 39.93 4.340 0.91293 0.90368 Note: Nd normalized to 146Nd/144Nd = 0.7219. Present-day CHUR 143Nd/144Nd = 0.512638 and 147Sm/144Nd = 0.1967. Sr normalized to 86Sr/87Sr = 0.1194. Initial age for all samples is 160 Ma. m—measured ratio; N.D.—no data; N.A.—not applicable. †Replicate analyses. ‡Unspiked samples.

Geological Society of America Bulletin, March/April 2005 537 MAHLEN et al. and –7.8 to –10.1 for Group II samples (Fig. 4). 87Rb-87Sr Mobilization The Rb-Sr isotope data reported here for Neodymium isotope compositions, however, are the basement nappes generally match those best compared at the time of sediment deposi- The contrasting 87Sr/86Sr variations for the of Pawlig (2001), where it was suggested that tion, and εNd(160) values range from –6.5 to African/Apulian and European basement the Monte Rosa nappe of the European plate,

–13.2 for Group I samples, whereas εNd(160) nappes, which scatter about a Variscan-like for example, was “dry” as it underwent Alpine values for Group II samples vary from –6.3 to (360–260 Ma) array (Fig. 3C) relative to those of metamorphism. The Monte Rosa metagranites –8.7 (Table 3). the Zermatt-Saas metasediments, which cluster from Pawlig (2001) primarily plot along a Present-day Nd isotope compositions of the around a 60 Ma “isochron” (Fig. 3A), are inter- 360 Ma “isochron” (Fig. 3C), indicating they African/Apulian and European basement nappes preted to refl ect contrasting mobility of the Rb-Sr have generally preserved their Variscan Rb-Sr overlap those of the Zermatt-Saas metasedi- isotopic system in “wet” (open-system) versus ages, despite the fact that they were metamor- ments. When the nappes are distinguished by “dry” (closed-system) metamorphic conditions, phosed to HP/UHP conditions similar to those parent lithology, orthogneissic versus para- given the similar P-T-t history of the majority of the Zermatt-Saas metasediments. Conversely, gneissic groupings are apparent (Fig. 4). The of the samples. That the Zermatt-Saas metasedi- the Ligurian basaltic crust and overlying sedi- 147 Group I metasediments overlap εNd(0)– Sm/ ments generally cluster along a 60–40 Ma iso- ments could be considered to have been “wet” 144Nd variations observed for the paragneissic chron is interpreted to refl ect nearly complete during subduction and Alpine metamorphism, nappe samples, and Group II samples overlap homogenization of 87Sr/86Sr ratios during Alpine due to prior seawater and hydrothermal fl uid the range defi ned by the orthogneissic nappe metamorphism. The homogenization of the Sr interaction with the oceanic crust and sediment samples (Fig. 4). The Sesia zone εNd(160) val- isotope system is particularly striking given the cover, and/or dewatering of the slab, which ues range from –2.2 to –12.8, whereas εNd(160) wide range in Nd isotope compositions of the would have provided suffi cient fl uids to mobi- values for the Dent Blanche samples vary from Zermatt-Saas metasediments (Fig. 4), which lize Sr (Tatsumi et al., 1986). Support for locally

–5.4 to –12.0 (Table 3). εNd(160) values for the suggests that prior to Alpine metamorphism the high ﬂ uid pressures in the Zermatt-Saas rocks European samples, which include the Monte 87Sr/86Sr ratios were also likely to have been quite comes from the Lago di Cignana, Italy, locality Rosa, Gran Paradiso, and Grand St. Bernhard variable. It remains unclear why three Zermatt- (van der Klauw et al., 1997), where quartz and nappes, range from –5.3 to –13.8 (Table 3). The Saas metasediment samples (97JA-1a, 97JA-1b, albite vein shapes and orientations are distinct wide range in 143Nd/144Nd ratios is not correlated and 97JA-12; 87Sr/86Sr >0.74) did not become from other rock structures. with 147Sm/144Nd ratios (Fig. 4), consistent with homogenized during Alpine metamorphism, as Unfortunately, it cannot be distinguished with data from Variscan rocks elsewhere that have these samples do not appear to be anomalous in certainty if Sr isotope homogenization in the not been subjected to Alpine metamorphism mineralogy or Rb and Sr contents. Zermatt-Saas metasediments occurred during (e.g., Peucat et al., 1988; Gerdes et al., 2000), providing support for the interpretation that the REEs were not mobilized in the basement nappes during Alpine metamorphism. 10 8 MORB INTERPRETATION OF RARE EARTH Primitive source 6 source ELEMENT AND ISOTOPE DATA 4 Our data support the conclusion of previous 2 LREE LREE studies that have interpreted the REEs to be 0 enriched depleted relatively immobile during sedimentary pro- (0) -2 Nd

cesses such as diagenesis, weathering, erosion, ε and sorting, as well as during metamorphism -4 360 Ma (e.g., Green et al., 1969; McCulloch and Was- -6 serburg, 1978; Goldstein et al., 1984; Taylor -8 and McLennan, 1985; McLennan, 1989; Tay- 160 Ma lor and McLennan, 1995; Jahn, 2000), and our -10 results indicate that the REEs are immobile -12 Old 50 Ma even to UHP conditions where metamorphic -14 UCC temperatures were ≤600 °C. Samarium-Nd source -16 isotope and REE data indicate a close relation -0.6 -0.5 -0.4 -0.3 -0.2 -0.1 0.0 0.1 0.2 0.3 between the Zermatt-Saas metasediments and ƒ the local nappe samples with regard to prov- Sm/Nd enance, suggesting local derivation. The Rb-Sr ZS Group I Eur samples Nappes-paragneiss system, however, has long been known to be ZS Group II AA samples Nappes-orthogneiss relatively easily reset during metamorphism ZS mafic Nappes-DB mafic

(e.g., Goldstein and Jacobsen, 1988; Lee and Figure 4. fSm/Nd–εNd(0) isotope plot. Zermatt-Saas metasediments (ZS); European nappe Chang, 1997), and our results suggest that samples (Eur) include Monte Rosa, Grand St. Bernhard, and Gran Paradiso; African/ Sr isotope variations were nearly completely Apulian nappe samples (AA) include Dent Blanche and Sesia. Nappes primarily cluster into reset for the majority of the Zermatt-Saas two groups based on lithology. MORB, primitive, and old upper continental crust (UCC) metasediment samples during Eocene Alpine source regions from Taylor and McLennan (1985), Hofmann (1988), Taylor and McLennan metamorphism. (1995), respectively.

538 Geological Society of America Bulletin, March/April 2005 PROVENANCE OF JURASSIC TETHYAN SEDIMENTS prograde metamorphism or during retrograde fl uid-present conditions, spurring homogeniza- components of upper continental crust mixed greenschist-facies overprinting without further tion, and have similar blocking temperatures for with a younger component or a more radiogenic mineralogical data, although the timing can be white micas (<500 °C for Sr, Butler and Free- source, such as arc material. The lower overall generally confi ned to the Alpine metamorphic man, 1999; ~350 °C for Ar, Jäger, 1979), which REE concentrations of the Group II Zermatt- event. For example, Amato et al. (1999) analyzed would have been exceeded during the Alpine Saas metasediments may refl ect mixing of source two quartzites from the Zermatt-Saas metasedi- orogenic event in the Zermatt-Saas ophiol- materials, such as old crustal material with young ments and calculated a whole rock–phengite Rb- ite (~550–600 °C; Barnicoat and Fry, 1986; recycled crust or mafi c source components (e.g., Sr isochron at 38 ± 2 Ma, and these data overlap Reinecke, 1991; van der Klauw et al., 1997). McLennan, 1989; McLennan et al., 1993). with the Zermatt-Saas metasediments from this Alternatively, the lower ΣREEs may refl ect study (Fig. 3A and 3B). Amato et al. (1999) 147Sm–143Nd–REE Immobility preferential addition of quartz during transport interpret this date as the age of greenschist- and deposition, either due to quartz mobility facies overprint on the HP/UHP metamorphic There is no evidence for homogenization of from fl uid fl ow during metamorphism or a more assemblage. Phengitic micas in mafi c samples Nd isotope compositions, or the REEs in gen- coarse-grained, quartz-rich protolith than Group of the African/Apulian lower outliers and the eral, in the Zermatt-Saas metasediments during I samples (Condie, 1991; Taylor and McLennan, Sesia zone and phengitic micas from quartzites Alpine metamorphism. The REE profi les of the 1995), although, as noted above, modal quartz of the Zermatt-Saas ophiolite are interpreted to metasediments are consistent with the REE pro- contents of the Group II samples are only mod- have grown during Alpine metamorphism at fi les that have been proposed for shale compos- erately greater than those of the Group I samples. ca. 45 Ma (Dal Piaz et al., 2001), and their Rb-Sr ites (e.g., Haskin and Haskin, 1966; Taylor and Either scenario would result in a general lowering isotope compositions overlap those of the Zer- McLennan, 1985; Gleason et al., 1995; Ugidos of the REE concentrations, although it is impor- matt-Saas metasediments (Figs. 3A and 3B). et al., 1997) as well as average upper continental tant to note that “quartz dilution” would not Rubidium-strontium concentrations are similar crust (Taylor and McLennan, 1995). Specifi cally, explain the less extreme Eu anomalies of Group to those of upper continental crust and compos- the ΣREE abundances (>100 ppm), (Ce/Yb)n II samples or the different light- and heavy-REE ite shales (Taylor and McLennan, 1985; Taylor ratios (6.7–9.9), (Sm/Nd)n ratios (0.55–0.66), fractionations. We therefore suggest that the pri- and McLennan, 1995). There appears to be no and negative Eu anomalies (Eu/Eu* = 0.6–0.7) mary cause of the differences between the REE evidence for extensive Rb or Sr addition or deple- for the Group I samples overlap those of the abundances of Group I and Group II samples is tion, such as might be observed by a metasomatic upper continental crust and composite shale related to distinct contrasts in source materials. event. Homogenization of Sr isotope composi- profi les (Haskin and Haskin, 1966; Taylor and Present-day 147Sm/144Nd–143Nd/144Nd variations tions therefore seems most likely to refl ect inter- McLennan, 1985; Gleason et al., 1995; Taylor of the Group II samples closely overlap those of nal mobilization of Sr. and McLennan, 1995; Ugidos et al., 1997). The the orthogneissic samples from both European REE patterns of the Group II metasediments are and African/Apulian nappes (Fig. 4), which is

Implications for Alpine Geochronology also similar to those of continental crust and also apparent when the frequency of εNd(160) val- shale composites, although there are differences ues of the metasediments and nappe samples are Our results suggest that the Zermatt-Saas in the specifi c REE data (described below) that compared (Fig. 5). Orthogneisses of the Euro- metasediments may hold promise for detailed suggest the Group II metasediments may have a pean basement refl ect Variscan-age intrusions, geochronologic studies of Alpine metamorphism different provenance than the Group I samples. whereas African/Apulian orthogneissic samples using the Rb-Sr isotope system. Whole-rock Rb- The restricted range in 147Sm/144Nd ratios for may be associated with Variscan-age magmatism Sr data may provide ages related to early Alpine the Zermatt-Saas metasediments (0.113–0.131) or Permian rifting and/or faulting (Baud et al., events, whereas mineral isochrons are likely to overlaps that of average shale (0.115–0.120; 1993). That the orthogneisses cluster about the record late Alpine metamorphic events (e.g., Goldstein et al., 1984; Gleason et al., 1995; 360–260 Ma “isochron” array (Fig. 4) suggests Amato et al., 1999). Detailed mineralogic and Ugidos et al., 1997), average upper continental that the Group II metasediments were derived petrologic studies of the white micas in the crust (0.118–0.119; Jahn and Condie, 1995; from sources that were typical of the Variscan metasediments are required before speculating Goldstein et al., 1984), and average marine orogeny (Liew and Hofmann, 1988; Gerdes et on what segment of the Alpine P-T path will sediments (~0.131; Patchett et al., 1984). These al., 2000). The wide range in 147Sm/144Nd ratios be recorded by Rb-Sr geochronology. In addi- observations support the interpretation that the of orthogneissic nappe samples (Fig. 4) likely tion, more geochemical data on these samples Zermatt-Saas metasediments have not expe- refl ects variability of individual intrusive units, (whole-rock chemistry, oxygen isotopes, etc.) rienced signifi cant mobilization of the REEs which were later homogenized through erosion, are necessary to provide interpretable isochrons. during the Alpine metamorphism. transport, mixing, and deposition of the Group II K-Ar and 40Ar/39Ar geochronology has been Zermatt-Saas sediments (Figs. 4 and 5).

particularly problematic in the Zermatt-Saas Provenance of the Zermatt-Saas The relatively restricted Sm/Nd–εNd(160) ophiolite and Monte Rosa nappe due to strong Metasediments range for the Zermatt-Saas Group II metasedi- inherited components (Bocquet et al., 1974; ments may be qualitatively explained through Arnaud and Kelley, 1995; Ruffet et al., 1995), The dominant source terrane of the Group I homogenization of European and African/ but the near-complete homogenization of Sr Zermatt-Saas metasediments appears to contain Apulian orthogneisses (Fig. 6A). First-cycle isotopes in the Zermatt-Saas metasediments a recycled, old continental crustal component. mixing was simulated through mixing of ran- suggests that Rb-Sr studies may complement Although the REE data of the Group I Zer- dom proportions of endmembers that are repre- 40Ar/39Ar geochronology. It is likely that homog- matt-Saas metasediments are consistent with sentative of the range in Sm/Nd ratios seen in the enization of Sr isotopes may have also been those of upper continental crust, the REE data orthogneissic nappe samples. The range in Sm/ accompanied by complete resetting of the K-Ar of the Group II metasediments are less similar. Nd ratios produced by one mixing stage is fur- system during the Alpine metamorphic event. The dominant source terrane for the Group II ther restricted following a second cycle of mix- Rb-Sr and K-Ar are potentially mobile under Zermatt-Saas metasediments appears to have ing (Fig. 6A). These calculations illustrate that

Geological Society of America Bulletin, March/April 2005 539 MAHLEN et al.

-5 European Nappes Group II A ZS II Orthogneissic -6 M source M P G P M G G 1st cycle -7 2nd cycle African/Apulian Nappes D -8 D -9

D S (160) S D S DDS S S S S Nd -10

ε Breuil-Cervinia All Nappes, Sed. vs. Ign. Sed. Gornergletcher

Frequency -11 Ign. Lago di Cignana -12 Pfülwe Group I Saas-Fee -13 Zermatt-Saas Täsch Frequency: 1 unit = sample metasediments, ZS I -14 Group I vs. Group II 0.110 0.115 0.120 0.125 0.130 0.135 ZS II 147Sm/144Nd -4 B ZS I -5 Group II Figure 7. 147Sm/144Nd– (160) isotope dia- metasediment ZS II εNd -14 -12 -10 -8 -6 -4 -2 -6 source, gram. The isotope data for the Zermatt-Saas ε -7 -0.36, -6 Nd(160) metasediments are subdivided to illustrate -8

(160) geographic distribution based on sample site. Figure 5. Frequency distribution and his-

Nd -9 ε togram of εNd(160) values showing overlap -10 of Zermatt-Saas (ZS) Group I and Group -11 Old crustal II samples with paragneissic (Sed.) and -12 paragneissic locality at Lago di Cignana seems to be domi- source, orthogneissic (Ign.) nappe samples, respec- -13 nated by Group II compositions (Fig. 7). -0.55, -20 tively. There is no correlation with specifi c -14 Neodymium model ages (TDM) of sedimen- basement nappe units (Gran Paradiso—P; -0.55 -0.50 -0.45 -0.40 -0.35 -0.30 -0.25 -0.20 tary rocks provide a useful expression for the ƒ Grand St. Bernhard—G; Monte Rosa—M; Sm/Nd possible average age of the source terrane(s) Dent Blanche—D; Sesia—S) with ZS Group or their precursors. Model ages of the Zermatt- Figure 6. (A) Mixing diagram based on I or Group II samples. Saas metasediments range from ca. 1.5 Ga to Johnson and Winter (1999) for Group II 1.9 Ga (Table 3), and this is comparable to the Zermatt-Saas metasediments (ZS II), shown average model ages for Variscan-age terranes 20× actual frequency in black. First-cycle (e.g., Liew and Hofmann, 1988; Peucat et al., mixing of 1000 random points of orthog- the relatively restricted range of Sm/Nd ratios 1988; Pacquette et al., 1989; Gerdes et al., neissic source in medium grey. Second-cycle in the Group II Zermatt-Saas metasediments 2000). A dramatic exception is sample 97JA- mixing of 100 10-point averages of fi rst cycle may be produced through mixing and homog- 3c whose light-REE-depleted REE pattern mixing in light grey. (B) Mixing diagram for enization of the local basement nappes. We and high 143Nd/144Nd ratios indicate that it had Group I ZS metasediments (ZS I) shows mix- therefore infer that the Nd isotope compositions a different source as compared to most Group ing between an average ZS II component and of the Zermatt-Saas Group II metasediments I and II metasediments (Figs. 2A and 4). The an average old crustal component, which refl ect signifi cant mixing and homogenization most likely source for this sample was mafi c in includes European and African/Apulian processes, which most likely occurred on the composition, such as an oceanic debris fl ow in paragneisses. See text for discussion. continental shelves along plate margins during a forearc setting (McLennan, 1989). Model ages Jurassic and Cretaceous seafl oor spreading. of paragneissic nappe samples are fairly consis-

In contrast, the Group I Zermatt-Saas metased- tent (TDM,ave = 1.8 Ga) and are similar to those iments plot along a more vertical Sm/Nd–Nd iso- Saas Group II metasediments, which themselves of the Group I Zermatt-Saas metasediments tope trend (Fig. 4), suggesting mixing of sources refl ect homogenization of the orthogneissic (TDM,ave = 1.73). Model ages of orthogneissic 143 144 with highly variable Nd/ Nd ratios, but with nappe samples and represent the high εNd(160) nappe samples are younger and more variable a restricted range in Sm/Nd ratios. Paragneissic end member (Fig. 6B). The Group I metasedi- than the paragneissic samples (TDM,ave = 1.55), samples of both the European and African/ ments, therefore, would seem to refl ect a higher consistent with mixing old detritus and young Apulian nappes seem to follow the vertical mix- degree of sedimentary cycling than the Group II or mafi c materials, similar to the Group II ing trend of the Group I samples (Figs. 4 and 5), metasediments; this extensive cycling may have metasediments (TDM,ave = 1.66). which refl ects mixing of pre-Variscan-age source occurred during deposition on the continental The REE patterns (Fig. 2) and Nd isotope materials prior to deposition of the protoliths to rise or in the deep-ocean basin. compositions (Figs. 4 and 5) of the European and the paragneisses. The spread in εNd values of the The provinciality of Groups I and II may be African/Apulian nappes generally overlap those paragneissic nappe samples, however, is not as refl ected geographically as well, as there is some of the Zermatt-Saas metasediments indicating large as that of the Zermatt-Saas metasediments, distinction between geographic sampling loca- that the nappes that are locally exposed may be where εNd values of the metasediments extend to tion and metasediment group (Fig. 7). Group I reasonable source materials for the metasedi- less negative εNd values than the majority of the samples, which have the lowest εNd(160) values, ments, followed by homogenization during paragneissic samples (Figs. 4 and 5). The range tend to occur in northern areas of Saas-Fee, transport and deposition. In detail, there are in Nd isotope compositions of the Zermatt-Saas Täsch, and Pfülwe. In contrast, Group II sam- some distinctions between specifi c samples of Group I metasediments are well explained ples appear to be more common in the southern the basement nappes, particularly the basement through mixing of the European and African/ localities at Breuil, Gornergletcher, and Lago di orthogneisses and some Sesia zone samples, and Apulian paragneissic samples and the Zermatt- Cignana. It is notable that the well-studied UHP the Zermatt-Saas metasediments. These unusual

540 Geological Society of America Bulletin, March/April 2005 PROVENANCE OF JURASSIC TETHYAN SEDIMENTS

B area = 1,500 km2 area = 40,000 km2 stands in contrast to the relatively homogeneous ZS Group I compositions that are measured for several recent ZS Group II Bay of Bengal sediments [4] to modern marine basins, when compared to their source regions. Figure 8A illustrates that Mediterranean surface seds. [8] basins that acquire large sediment loads from Mediterranean sediments [8] extensive river systems draining highly variable 0 1 2 3 4 5 0 1 2 3 4 5 source terranes, such as the Amazon fan, Bay of relative 2-SD ε Nd Bengal, and the Indus River basin, become rela- 25 A tively homogenized in at the terminus of the 7 εNd 4 6 river system (including the main channel, shelf, 2 3 20 and fan sediments), as reﬂ ected by the fact that their 2-σ values plot far above the 1:1 line. Some variability laterally across such basins is expected 15 1 (e.g., Colin et al., 1999; Pierson-Wickmann et al., 8 2001), but this variability is still smaller than the of averaged source total variability in the source terranes and likely 10 5 (0) reﬂ ects different average Nd isotope composi-

Nd 1:1 line

ε tions for different drainage basins. For example, 5 "not possible" east-west variability in Nd isotope compositions 2-SD in modern Mediterranean sediments (εNd = –6 to –13; Freydier et al., 2001) and surface sediments 0 (ε = –2 to –12; Weldeab et al., 2002) reﬂ ects 0 264 8 10 12 Nd contributions from diverse sources, including 2-SD ε (0) of averaged sediment Nd major contributions of Nile River sediment in

ZS all, this study the east (average εNd ≈–3; Goldstein et al., 1984) ZS Group II ("orthogneissic") 4 Bay of Bengal fan to dominance of Saharan sand input further west

ZS Group I ("paragneissic") 5 Himalayan foreland basin (εNd = –3 to –27, average −13; Grousset et al., 1 Alpine Molasse basin 6 Indus Molasse basin 1998). Detailed studies of the Bay of Bengal and 2 Amazon fan 7 Indus River basin the Mediterranean basins demonstrate that over 3 Amazon foreland basin 8 Mediterranean sediments relatively small areas, on the order of 1500 km2,

the range in εNd values is relatively restricted, 2 Figure 8. (A) Comparison of 2σsediment (reﬂ ecting variability of εNd of basin sediments) versus whereas over large areas, such as 40,000 km ,

2σsource (reﬂ ecting variability of εNd of all potential sources of the respective basin). (B) Com- isotopic diversity is greater (Fig. 8B, and refer- parison of 2σsediment (εNd) for small versus large basin areas. Zermatt-Saas metasediments ences therein). In each case, however, studies of (ZS). Other data sources: 1—Alpine molasse basin, Spiegel et al. (2002). 2—Amazon fan, Nd isotope composition of modern marine basins McDaniel et al. (1997). 3—Amazon foreland basin, Basu et al. (1990); McDaniel et al. (1997). adjacent to orogenic systems highlight the fact

4—Bay of Bengal fan, Galy et al. (1996); Colin et al. (1999); Clift et al. (2002); Robinson et that spatial variations in εNd values for modern al. (2001). 5—Himalayan foreland basin, Robinson et al. (2001). 6—Indus molasse basin, sediments vary smoothly regardless of basin size Clift et al. (2001). 7—Indus River basin, Clift et al. (2002). 8—Mediterranean sediments, (e.g., Freydier et al., 2001; Weldeab et al., 2002).

Gülec (1991); Grousset et al. (1998); Freydier et al. (2001); Weldeab et al. (2002). The correlation of εNd values of the Zer- matt-Saas metasediments with local basement nappes, where the Group I metasediments appear to refl ect mixing of Group II sediments patterns and isotopic data may indicate that these qualitative basis for the extent of homogeniza- with the paragneissic nappe samples and the particular nappe samples are not representative tion during sedimentation. For example, if the Group II metasediments seems likely to refl ect of the dominant source material. spread in isotope compositions of the source cycling and homogenization of the ortho- To assess the degree of homogenization of Nd terranes matches that of the sediments, it would gneissic nappe samples (Figs. 5 and 6), suggests isotope compositions of source terranes during plot on a 1:1 line in Figure 8, suggesting minimal that the wide range in Nd isotope compositions transport and deposition, we compare the stan- homogenization of isotopic compositions during was unlikely to have been present in a single, dard deviation (2σ) of the average Nd isotope transport and deposition. This may refl ect an end- small marine basin (Fig. 8B). When viewed in compositions of specifi c source terranes with member case where sediment is transported very comparison to modern sedimentary systems in

the 2σ of sediments derived from those source short distances and deposited in local, proximal, orogenic belts, the wide range in εNd values for terranes (Fig. 8). Such an approach assumes short-lived basins, where little homogenization the Zermatt-Saas sequence most likely refl ects that the data provide an adequate representation has occurred through mixing. In general, how- subduction-related collapse and stacking of of the source materials, which may be diffi cult ever, mixing processes are expected to produce a geographically extensive basin (≥105 km2) to establish. Clearly this approach is most valid compositions that lie above a 1:1 line on a plot of or subduction-related collapse and stacking for large databases. These issues not withstand- 2-σsediment versus 2-σsource (Fig. 8). of multiple, isolated basins that retained their ing, it is illustrative to compare the dispersion of It is striking that the spread in εNd values for the isotopic provinciality, preserving an overall isotopic data for source materials and derivative Zermatt-Saas metasediments and their respec- range in isotopic compositions that is slightly sediments, which, at the least, may provide a tive source nappes lies near the 1:1 line, which more refi ned than the possible source terranes.

Geological Society of America Bulletin, March/April 2005 541 MAHLEN et al.

There is no evidence that extensive river sys- NW SE tems drained a large and extensive orogen, as European African/Apulian in the Himalaya, based on the close correla- margin at 130 Ma margin tion between 2σ deviations for Zermatt-Saas metasediments and local nappes. A GP VB GB PL? MR? PL? AA Implications for Paleogeography B VB GB PL Crustal extension affecting the Apulian and MR/GP AA European margins accommodated by en echelon ? ? normal faulting was likely occurring as a result C of mid-Jurassic rifting and magmatism. This may MR/GP VB GB PL AA? PL? PL? AA have led to fragmented blocks of continental crust amid oceanic crust resulting in horst and Figure 9. Simplifi ed cartoons of precollisional paleogeography showing relationship of basin topography along the continental margin Piemont-Ligurian basin (PL) with African/Apulian and European margins: (A) small basins and stretching into the newly forming ocean based on Platt (1986), (B) a single large basin, and (C) proposed in this paper; Gran Paradiso basin. There is little argument that a small basin, (GP), Grand St. Bernhard (GB), Monte Rosa (MR), African/Apulian undifferentiated (AA), i.e., the Valais basin, formed along the European Valais basin (VB). Based on Rubatto et al. (1998); Froitzheim (2001); and references therein. continental margin, where denuded continental basement likely mixed with oceanic basement. This basin was separated from the Piemont-Ligu- rian basin proper by a rifted and uplifted terrane ophiolite refl ects randomly obducted ophiolite ples with the second group (Group II) metasedi- believed by most to be of European affi nity (e.g., fragments from the entire Piemont-Ligurian ments. Group II is explained through mixing

Dal Piaz, 1999). Evidence for both the continental ocean basin that has been collapsed into a single of high εNd Variscan-like African/Apulian and terrane and the basin, including ophiolitic mate- unit that has limited current geographic extent. European orthogneissic protoliths, and this mix- rial, exists in the Grand St. Bern hard nappe today. Collapse of a single, very large (≥105 km2) basin ing relationship agrees well with Nd isotope data. In addition to the formation of the Valais basin, is certainly possible, but our preferred interpreta- Group I metasediments, therefore, seem likely to Dal Piaz et al. (2001) suggest a paleoreconstruc- tion is that the close correspondence in the range refl ect a greater degree of cycling as compared tion with blocks rifted from the African/Apulian of εNd values relative to the range for local base- to the Group II metasediments. The contrasting margin existing as microcontinents separating ment nappes refl ects multiple small basins that mixing relations of the metasediments may indi- portions of the Piemont-Ligurian basin, based were adjacent to source regions at the time of cate provinciality of the sediments in the basin. on geochronologic data of attainment of peak deposition (Fig. 9C). Collapse of these initially It appears Group II sediments may have been conditions of the different units. Other pro- isolated basins would have then occurred during deposited at the continental shelves, whereas posed models for restoration of the European Alpine orogenesis. Support for our preferred Group I sediments, if they refl ect greater mix- and African/Apulian units prior to the Alpine model comes from the diachronous attainment ing, may have been deposited along continental orogeny consider various locations of different of peak conditions for the different nappe slices slopes or in marine basins. Greater mixing and units and variable dimensions of the Piemont- during closure of the Piemont-Ligurian ocean chemical maturity for Group II samples is indi- Ligurian basin (400–700 km wide) based on basin (Dal Piaz et al., 2001). cated by their relatively quartz-rich nature. Group temporal, structural, or lithologic relationships I samples appear to be more common in the of specifi c units (e.g., Platt, 1986; Stampfl i et al., CONCLUSIONS northern regions of the ophiolite, whereas Group 1998; Froitzheim, 2001). Figure 9 illustrates very II samples are dominant in the southern regions general and simplifi ed cartoons based on these Rare earth element (REE) contents and Sm- of the ophiolite for the localities studied. Both models. Figure 9A is based on a model sug- Nd isotope compositions of the metasedimen- groups, however, can be found within hundreds gested by Platt (1986), for example, that assumes tary rocks within the Zermatt-Saas ophiolite of meters of each other in the fi eld. multiple interoceanic fragments from European complex were not disturbed during Alpine high/ The probable source regions of the Zermatt- continental margins that segregate portions of ultra-high-pressure metamorphism. The REE Saas metasediments appear to be the basement the Piemont-Ligurian basin. The segregation of patterns overlap those that characterize upper nappes that are currently in close proximity to the small basins implies that sediments within indi- continental crust and average shale composites, Zermatt-Saas ophiolite. Because of the overlap vidual basins will refl ect local source derivation. as well as local nappes that have been inter- in REE contents and Nd isotope compositions Collapsing multiple basins one on top of the other preted to refl ect the African/Apulian and Euro- of the African/Apulian- and European-derived during Alpine subduction and exhumation would pean basements. Moreover, Nd isotope data for nappes, our data cannot distinguish these sources explain the diversity in Nd isotope compositions the Zermatt-Saas metasediments overlap those in specifi c metasediment samples. Because of the of the metasediments in such a model. of Variscan basement nappes. similar dispersion of Nd isotope compositions of Alternatively, Figure 9B illustrates an exam- Source terranes of the Zermatt-Saas metasedi- the metasediments relative to the local basement ple of a large, single basin, which, as discussed ments appear to fall into two distinct sets based nappes, the metasediments are likely to have been previously, would likely refl ect relatively gradu- on REE patterns, Nd isotope compositions, and deposited in multiple small basins (<1500 km2), ally varying Nd isotope compositions across the mixing models: the fi rst group (Group I), recog- consistent with the inference that the metasedi- basin, drawing upon the analogy with modern nized by variable εNd values, is best explained by ments refl ect a local source derivation. This inter- marine basins. Assuming such analogies are mixing of pre-Variscan polycyclic paragneisses pretation implies that the metasediments largely valid, this model implies that the Zermatt-Saas of the African/Apulian and European nappe sam- refl ect deposition during the early rifting stages

542 Geological Society of America Bulletin, March/April 2005 PROVENANCE OF JURASSIC TETHYAN SEDIMENTS

Basu, A.R., Sharma, M., and DeCelles, P.G., 1990, Nd and Pillonet klippe and Sesia Lanzo basal slice in the Ayas of the Piemont-Ligurian basin. An alternative Sr isotopic provenance and trace element geochemistry Valley and evolution of the Austroalpine-Piedmont explanation is that the isotopic diversity of the of Amazonian foreland basin fl uvial sands, Bolivia and nappe stack: Memorie di Scienze Geologiche, Padova, metasediments refl ects tectonic collapse of a very Peru; Implications for ensialic Andean Orogeny: Earth v. 50, p. 177–194. and Planetary Science Letters, v. 100, p. 1–17, doi: Dal Piaz, G.V., 1999, The Austroalpine-Piedmont nappe 5 2 large (≥10 km ) basin into the currently exposed 10.1016/0012-821X(90)90172-T. stack and the puzzle of the Alpine Tethys: Memorie di section of the Zermatt-Saas ophiolite. Tests of Baud, A., Marcoux, J., Guiraud, R., Ricou, L.E., and Gaetani, Scienze Geologiche, Padova, v. 51, p. 155–176. 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Bearth, P., 1967, The ophiolites of the Zermatt-Saas Fee gen, v. 81, p. 275–303. zone: Beitraege zur Geologischen Karte der Schweiz, Dal Piaz, G.V., and Ernst, W.G., 1978, Areal geology and In contrast to the relative immobility of the v. 132, p. 130. petrology of eclogites and associated metabasites of REEs, Sr isotopes of the Zermatt-Saas metasedi- Bearth, P., and Schwander, H., 1981, The post-Triassic the Piemonte ophiolite nappe, Breuil–St. Jacques area, ments have been nearly completely homogenized sediments of the ophiolite zone Zermatt-Saas Fee and Italian western Alps: Tectonophysics, v. 51, p. 99–126, the associated manganese mineralizations: Eclogae doi: 10.1016/0040-1951(78)90053-7. during Alpine metamorphism in most samples, Geologicae Helvetiae, v. 74, p. 198–205. Dal Piaz, G.V., Cortiana, G., Del Moro, A., Martin, S., Pen- probably due to the high fl uid contents of the oce- Bill, M., Bussy, F., Cosca, M.A., Masson, H., and Hunziker, nacchioni, G., and Tartarotti, P., 2001, Tertiary age and J.C., 1997, High-precision U-Pb and 40Ar/39Ar dating paleostructural inferences of the eclogitic imprint in anic crust and sediments during subduction and of an Alpine ophiolite (Gets nappe, French Alps): Eclo- the Austroalpine outliers and Zermatt-Saas ophiolite, metamorphism. Present-day 87Sr/86Sr variations gae Geologicae Helvetiae, v. 90, p. 43–54. western Alps: International Journal of Earth Sciences, for the Zermatt-Saas metasediments scatter about Bocquet, J., Delaloye, M., Hunziker, J.C., and Krum- v. 90, p. 668–684, doi: 10.1007/S005310000177. menacher, D., 1974, K-Ar and Rb-Sr dating of blue Duchêne, S., Blichert-Toft, J., Luais, B., Télouk, P., Laradeaux, an Alpine (ca. 50 Ma) metamorphic age; such amphiboles, micas, and associated minerals from the J.M., and Albarède, F., 1997, The Lu-Hf dating of garnets homogenization is striking in light of the likeli- western Alps: Contributions to Mineralogy and Petrol- and the ages of the Alpine high-pressure metamorphism: hood that 87Sr/86Sr ratios were highly variable ogy, v. 47, p. 7–26. Nature, v. 387, p. 586–589, doi: 10.1038/42446. Borghi, A., Compagnoni, R., and Sandrone, R., 1996, Com- Engi, M., Scherrer, N.C., and Burri, T., 2001, Metamorphic prior to metamorphism, given the wide range posite P-T paths in the internal Penninic massifs of the evolution of pelitic rocks of the Monte Rosa nappe: in Nd isotope compositions. These observations western Alps: Petrological constraints to their thermo- Constraints from petrology and single grain monazite mechanical evolution: Eclogae Geologicae Helvetiae, age data: Schweizerische Mineralogische und Petro- suggest that Rb-Sr geochronology may success- v. 89, p. 345–367. graphische Mitteilungen, v. 81, p. 305–328. fully determine the ages of peak metamorphism Bowtell, S.A., Cliff, R.A., and Barnicoat, A.C., 1994, Sm- Escher, A., Masson, H., and Steck, A., 1993, Nappe geometry in rocks that were wet during metamorphism, in Nd isotopic evidence on the age of eclogitization in in the western Swiss Alps: Journal of Structural Geology, the Zermatt-Saas ophiolite: Journal of Metamorphic v. 15, p. 501–509, doi: 10.1016/0191-8141(93)90144-Y. contrast to the apparent failure of Rb-Sr geochro- Geology, v. 12, p. 187–196. Escher, A., Hunziker, J.C., Marthaler, M., Masson, H., nology on dry Variscan basement units. Brouwer, F.M., Vissers, R.L.M., and Lamb, W.M., 2002, Sartori, M., and Steck, A., 1997, Geologic framework Structure and metamorphism of the Gran Paradiso and structural evolution of the western Swiss–Italian massif, western Alps, Italy: Contributions to Mineral- Alps, in Pfi ffner, O.A., et al., eds., Deep structure of the ACKNOWLEDGMENTS ogy and Petrology, v. 143, p. 450–470. Swiss Alps—Results from NRP 20: Basel, Birkhäuser, Butler, R.W.H., and Freeman, S.R., 1999, Dating Alpine shear p. 205–221. The authors would like to thank Professor J.C. zones, using Rb-Sr geochronology on white mica, to Freeman, S.R., Inger, S., Butler, R.W.H., and Cliff, R.A., Hunziker, from the Institute of Mineralogy and Petrol- gain greenschist deformation ages in the western Alps: 1997, Dating deformation using Rb-Sr in white mica: ogy in Lausanne, Switzerland, for providing whole- Memorie di Scienze Geologiche, v. 51, p. 177–189. Greenschist facies deformation ages from the Entrelor rock powders from the Dent Blanche nappe and the Cartwright, I., and Barnicoat, A.C., 1999, Stable isotope shear zone, Italian Alps: Tectonics, v. 16, p. 57–76, Sesia zone for analysis. Some samples jointly col- geochemistry of Alpine ophiolites: A window to doi: 10.1029/96TC02477. ocean-fl oor hydrothermal alteration and constraints on Freydier, R., Michard, A., De Lange, G., and Thomson, J., lected with Jeff Amato. This paper benefi ted greatly fl uid-rock interaction during high-pressure metamor- 2001, Nd isotopic compositions of eastern Mediter- from the thorough and thought-provoking reviews of phism: International Journal of Earth Sciences, v. 88, ranean sediments: Tracers of the Nile infl uence dur- Associate Editor John Bartley and reviewers François p. 219–235, doi: 10.1007/S005310050261. ing sapropel S1 formation: Marine Geology, v. 177, Bussy and John Shervais. 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REVISED MANUSCRIPT RECEIVED 26 JULY 2004 and gabbroic ocean fl oor of the Ligurian Tethys (Alps, Rubatto, D., and Gebauer, D., 1999, Eo/Oligocene (35 Ma) MANUSCRIPT ACCEPTED 7 SEPTEMBER 2004 Corsica, Apennines): In search of a genetic model: high-pressure metamorphism in the Gornergrat zone Geology, v. 15, no. 7, p. 622–625. (Monte Rosa, western Alps): Implications for paleo- Printed in the USA

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