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

Transcription

1 Provenance of Jurassic Tethyan sediments in the HP/UHP Zermatt-aas ophiolite, western Alps Nancy J. Mahlen Clark M. Johnson Department of Geology and Geophysics, University of Wisconsin, 1215 West Dayton treet, Madison, Wisconsin 53706, UA Lukas P. Baumgartner Institute of Mineralogy and Petrology, BFH2, CH-1015 Lausanne, witzerland Brian L. Beard # Department of Geology and Geophysics, University of Wisconsin, 1215 West Dayton treet, Madison, Wisconsin 53706, UA ABTRACT Rubidium-r and m-nd isotope data and rare earth element (REE) concentrations of the metasedimentary rocks within the Zermatt-aas (Z) ophiolite complex of the western Alps are used to investigate element mobility and to determine the provenance of the metasediments in order to place constraints on the precollisional paleogeography of the Piemont-Ligurian portion of the neo-tethys ocean. Present-day 87 r/ 86 r variations for the Z metasediments scatter about an early Tertiary Alpine metamorphic age, whereas local nappes that have been interpreted to reflect African/Apulian and European basements scatter about a Variscan-like age; this suggests 87 r/ 86 r isotope systematics were nearly completely homogenized for most of the Z metasediments during early Tertiary metamorphism, probably because they were relatively wet prior to metamorphism. In contrast to the r isotope data, REE data and Nd isotope compositions of the Z metasediments overlap those of average upper continental crust, average shale, and the local nappes, and Nd model ages of the Z metasediments overlap with those of Variscan-age rocks. These relations suggest that the REEs of the Z metasediments were not disturbed during high- to ultra-high pressure Alpine metamorphism. Based on REE data, Nd isotope compositions, and mixing models, the Z metasediments comprise two groups that require distinct source mahlen@geology.wisc.edu. clarkj@geology.wisc.edu. lukas.baumgartner@img.unil.ch. # beardb@geology.wisc.edu. terranes: one group (Group I) seems to be mixing of an old, crustal component, such as the paragneissic basement nappe samples, with metasediments similar to the second group (Group II). The source material for Group II is dominated by homogenization of the Variscan-like orthogneissic basement nappe samples. The provenance of Group I samples is interpreted to be local, where source nappes must have been proximal to the Piemont-Ligurian basin prior to Alpine convergence. The similarity in dispersion of Nd isotope compositions of the metasediments and likely source terranes suggests that the metasediments reflect deposition in small, isolated basins early in the formation of the Piemont-Ligurian ocean. Keywords: Western Alps, Zermatt, metasediments, m/nd, Rb-r, rare earth elements, provenance. INTRODUCTION Trace-element compositions, particularly the rare earth elements (REEs) and Rb-r and m- Nd isotope data, are often used for determining provenance and tectonic setting of sedimentary rocks. Numerous studies have noted that REE contents are useful indicators of source terranes in oceanic basins (e.g., McLennan et al., 1993; Gleason et al., 1995; Ugidos et al., 1997). Enrichment or depletion of light REEs relative to heavy REEs and the nature of Eu anomalies can provide additional clues to relate sediments to the bulk compositions of source regions. Because m/nd ratios do not readily experience fractionation during processes such as diagenesis, chemical weathering, erosion, or sedimentary sorting (Taylor and McLennan, 1985; McLennan, 1989), m-nd isotope variations in sedimentary rocks should also faithfully record their source regions. Further, m and Nd are relatively immobile during metamorphic events (Green et al., 1969; Jahn, 2000), which suggests that m-nd isotope systematics may be useful in interpretation of more geologically complex provenance problems. Jurassic rifting of the African/Apulian plates from the European plate followed by Late Jurassic spreading formed the Piemont-Ligurian basin of the neo-tethys ocean (e.g., Hunziker, 1974). Fine-grained Piemont-Ligurian ocean floor and basin sediments, caught in Eocene Alpine collisional tectonics, were eventually metamorphosed to high-pressure (HP) and ultra-high-pressure (UHP) conditions, and the Zermatt-aas ophiolite complex represents one such unit. Determining the provenance of the Piemont-Ligurian basin sediments can provide restrictions on proposed models of restoration of European and African/Apulian units prior to the Alpine orogeny. In addition, understanding the variability of isotopic compositions across a basin may provide information as to basin size and physiography. Here REE and Rb-r and m-nd isotope data are reported for metasedimentary rocks in the Zermatt-aas ophiolite, and these results are compared with new data from various nappe units of the western Alps as probable oceanic and continental source regions for the sediments. Despite hydrothermal alteration during Mesozoic divergence and/or metasomatic alteration during subsequent subduction and metamorphism, the REEs in the rocks studied do not appear to have been mobilized. m- Nd isotope systematics of the sedimentary rocks in the ophiolite were subjected to HP/UHP conditions, but at temperatures generally less than 600 C, suggesting the measured Nd isotope GA Bulletin; March/April 2005; v. 117; no. 3/4; p ; doi: /B ; 9 figures; 3 tables. 530 For permission to copy, contact editing@geosociety.org 2005 Geological ociety of America

2 PROVENANCE OF JURAIC TETHYAN EDIMENT Inset a NW 97JA-18 97JA-24 Breuil 96JA-16, 17 96JA-1, 26 96JA-30a, b 01NM-44, 45, C 47b Champoluc Matterhorn Z GB 97JA-1 DB Täsch 97JA JA-3 97JA-2 Dent Blanche Allalinhorn C a 96JA- 21 C Unter Gabelhorn Matterhorn C Lago di Cignana Valtournenche DB MR Z Zermatt Gornergrat 97JA-14 97JA-16 Mezzalama t. Jacques C Z 97JA-12 97JA-13 a' E Randa 96JA-35 96JA-38 96JA witzerland Italy 3 4 Pfülwe Z metasediment sample location a thrust sheet MR 5 aas Fee a aas Almagell Mattmark ee a' 5 km N Figure 1. implified tectonic map of the western Alps depicting major tectonic units: 1. Continental outer Penninic zone including Grand t. Bernhard (GB) and Combin (C) units. 2. Continental inner Penninic zone including Monte Rosa (MR) and Gran Paradiso (not shown on map). 3. Piemont-Ligurian remnants including Zermatt-aas ophiolite (Z). 4. Low- to mediumpressure African/Apulian Dent Blanche (DB) including Arolla series and Valpelline series (4a). 5. High- to ultra-high-pressure African/Apulian esia zone () including esia equivalent (5a) to Valpelline series. Inset is cross section a to a. Modified from Dal Piaz, compositions may have been preserved through the sedimentary and metamorphic cycles. Although the Piemont-Ligurian basin sediments were subducted to HP/UHP conditions, which reset r isotopes, REE and m-nd isotope variations appear to be useful indicators of provenance for the Zermatt-aas metasediments. These sediments appear to have been derived entirely from local sources, i.e., the present-day nappes, which must have been exposed adjacent to the Piemont-Ligurian basin in the Jurassic. GEOLOGIC BACKGROUND The Alps are a classic continent-continent convergent setting, which formed through closure of the Piemont-Ligurian ocean basin during latest Cretaceous to early Tertiary collision of the European and African/Apulian plates (e.g., Hunziker, 1974; Dal Piaz and Ernst, 1978). lices of the southeastern margin of the European plate and the northern margin of the African/Apulian plates were diachronously subducted with the Piemont-Ligurian ocean basin to variable temperatures and pressures. The major tectonic units of the western Alps are, from northwest to southeast (Fig. 1; Dal Piaz and Ernst, 1978; Escher et al., 1993): the Helvetic zone, which represents the European margin; the outer Penninic zone, which represents rifted European fragments that were metamorphosed at low to medium pressures (e.g., Grand t. Bernhard nappe); the inner Penninic zone, which includes remnants of the Piemont-Ligurian ocean basin (e.g., the Zermatt-aas ophiolite) and fragments of the European margin that were metamorphosed at high to ultra-high pressures (e.g., Monte Rosa and Gran Paradiso nappes); and the Austroalpine zone, which represents the African/Apulian margin (e.g., Dent Blanche and esia zone basement nappes). Geology of the Zermatt-aas Ophiolite The Zermatt-aas ophiolite (Fig. 1) comprises peridotites, serpentinites, eclogitized metagabbros and metabasalts that contain local examples of deformed sheeted dikes and clear pillow structures, and a cover series of calcareous and siliceous metasediments from the Piemont-Ligurian basin (Bearth, 1967; Dal Piaz and Ernst, 1978; Barnicoat and Fry, 1986). The ocean floor of the Piemont-Ligurian basin was largely gabbro, Geological ociety of America Bulletin, March/April

3 MAHLEN et al. followed by tholeiitic basalt flows and pillow lavas (Lemoine et al., 1987). The metasedimentary cover series of the Zermatt-aas ophiolite comprises manganese-rich quartzites and an overlying locally variable sequence of marbles, metapelites, calc-schists, and micaceous quartzites that capped the ophiolite sequence prior to tectonic dismemberment (Bearth and chwander, 1981; Reinecke, 1991). The earliest deposition age of the Zermatt-aas metasediments is late Jurassic based on U-Pb ages of magmatic zircons (166 ± 1 Ma) and a 40 Ar- 39 Ar age of magmatic hornblende (165.9 ± 2.2 Ma) both from gabbros in the Gets nappe, French Alps (Bill et al., 1997), and U-Pb ages of detrital zircons (161 ± 11 Ma) from metasediments and magmatic zircons (ca. 164 Ma) from metagabbros in the Zermatt- aas ophiolite (Rubatto et al., 1998). The Zermatt-aas ophiolite locally underwent ocean-floor hydrothermal alteration, as suggested by the presence of Mn-quartzites (radiolarites) believed to have been deposited directly on basalts of the ophiolite (Barnicoat and Cartwright, 1995). Cartwright and Barnicoat (1999), however, suggested that only minor amounts of fluid could have been involved based on the similarities in oxygen isotope compositions of whole rock, quartz, and white micas from Zermatt-aas metasediments with metasediments of similar compositions elsewhere in the Alps. The mineral assemblages of the Zermatt-aas metasediments from this study are typical of HP metapelites, and include quartz + white mica (typically phengite and/or talc, which form a well-defined foliation) + garnet ± epidote/ clinozoisite ± chlorite ± chloritoid ± carbonate ± rutile ± titanite ± tourmaline (Table 1; Bearth and chwander, 1981; Reinecke, 1991, 1998). The Zermatt-aas ophiolite experienced blueschist- to eclogite-facies metamorphism with peak pressure and temperature estimates of GPa and C in the Täschalp region (Oberhänsli, 1980; Barnicoat and Fry, 1986), and GPa and C at Lago di Cignana (Reinecke, 1991, 1998; van der Klauw et al., 1997). Constraints on the age of HP/UHP metamorphism include early m-nd studies (52 ± 18 Ma; Bowtell et al., 1994), U-Pb zircon ages (averaging 44.1 Ma; Rubatto et al., 1998), m-nd geochronology on acid-leached minerals (40.6 ± 2.6 Ma; Amato et al., 1999), and Lu-Hf geochronology (48.9 ± 2.1 Ma; Lapen et al., 2003). In aggregate, this span of ages is taken to reflect initial garnet growth in mafic rocks due to subduction beginning at ca. 52 Ma and attainment of peak metamorphic conditions around 40 Ma (Lapen et al., 2003). Upon exhumation, a greenschist-facies overprint at ca. 38 Ma (Reinecke, 1998; Amato et al., 1999) affected much of the Zermatt-aas ophiolite complex, although metastable UHP relics are preserved in portions of the ophiolite (Dal Piaz and Ernst, 1978). These age relations indicate that the Zermatt-aas ophiolite was rapidly exhumed from peak pressure conditions. Potential ource Terranes everal nappe units that represent European and African crust were chosen to characterize potential source terranes for the Zermatt-aas metasediments and to test the possibility that current local units may have provided sediment to the Zermatt-aas unit (Fig. 1). uch a model is consistent with paleogeographic reconstructions that suggest the nappe units could have been proximal to the Piemont-Ligurian basin (Escher et al., 1997; tampfli et al., 1998; Froitzheim, 2001). The outer Penninic Grand t. Bernhard nappe system comprises Variscan basement, Triassic volcaniclastic, and Briançonnais cover sequence rocks (Bearth, 1967; Dal Piaz, 1999). The Grand t. Bernhard system is partially overlain by the low-pressure metasediments of the Combin zone. The inner Penninic Monte Rosa and Gran Paradiso nappes include pre-variscan basement comprising paragneisses and augen gneisses that are intruded by a Variscan-age plutonic complex (Bearth and chwander, 1981; Dal Piaz, 2001). The Gran Paradiso nappe was metamorphosed to eclogite-facies conditions at ca. 45 Ma (Meffan-Main, 1998; Brouwer et al., 2002) and reached maximum temperatures and pressures of ~500 C and 1.5 GPa, respectively (Rubatto and Gebauer, 1999; Brouwer et al., 2002). The Monte Rosa nappe reached peak conditions ca Ma (Meffan-Main, 1998; Engi et al., 2001) at temperatures of ~ C and pressures that varied from 1.2 to 2.5 GPa (Chopin and Monié, 1984; Borghi et al., 1996; Le Bayon et al., 2000; Engi et al., 2001). The inner Penninic rocks were overprinted by greenschistto amphibolite-facies metamorphism at ca Ma (Hurford and Hunziker, 1989; Hurford et al., 1991; Freeman et al., 1997; Brouwer et al., 2002), suggesting rapid exhumation. Units that represent the African/Apulian margin include the low-pressure Dent Blanche nappe and the eclogitic esia zone. The Dent Blanche nappe (Fig. 1) comprises pre-alpine basement paragneisses (Valpelline series) and local granitoids, late-paleozoic orthogneisses (Arolla series), and a Mesozoic metasedimentary cover series (Venturini, 1995; Dal Piaz, 1999). Maximum metamorphic temperatures and pressures of ~ C and 1.0 GPa occurred over a poorly constrained time interval of Ma (Pfeifer et al., 1991; Cortiana et al., 1998; Dal Piaz, 1999; Gebauer, 1999). The esia zone comprises a pre-alpine high-temperature basement complex that is likely of Variscan age but was affected by Alpine metamorphism, including paragneisses (comparable to the Valpelline series of the Dent Blanche) and Variscanage gabbros and granites that are overlain by a late Permian to early Triassic mono-metamorphic cover series (Compagnoni et al., 1977; Venturini et al., 1996). Maximum metamorphic temperatures and pressures are estimated to have been ~550 C and GPa, where peak conditions were reached at ca. 65 Ma (Inger et al., 1996; Duchêne et al., 1997; Ruffet et al., 1997; Rubatto et al., 1999; Dal Piaz et al., 2001). AMPLE AND METHOD amples of the Zermatt-aas metasediments were collected from geographically dispersed locations that include the aas-fee, Täsch, and Zermatt areas from the wiss portion of the ophiolite and the Lago di Cignana and Valtournanche areas in Italy (Fig. 1). amples were also collected from selected African/Apulian and European basement nappes for controls on provenance, including the esia zone in Italy and the Dent Blanche nappe in Italy and witzerland, as well as portions of the European plate as represented by the Monte Rosa, Gran Paradiso, and Grand t. Bernhard nappes from outcrops in Italy and witzerland (Fig. 1). Professor J.C. Hunziker, from the Institute of Mineralogy and Petrology in Lausanne, witzerland, provided additional esia zone and Dent Blanche nappe samples (see Venturini, 1995). Whole-rock samples (~2 5 kg) were crushed and powdered and 50 mg sample-size portions were spiked with Rb-r and REE tracers for concentration and isotopic analyses. ample dissolution, chemical, and mass analysis procedures follow those of Johnson and Thompson (1991); all chemical separations and mass analyses were done in the Radiogenic Isotope Laboratory at the University of Wisconsin Madison. trontium isotope compositions were measured using a GV Instruments ector 54 thermal ionization mass spectrometer (TIM) using a three-jump dynamic multicollector analysis; 87 r/ 86 r isotope ratios were exponentially normalized to an 86 r/ 88 r = Using this analysis method, the measured 87 r/ 86 r of NIT RM-987 was ± 13 (2-D, n = 22) during the course of this study. Laboratory blanks were typically ~450 pg for r and less than 200 pg for Rb, which are negligible. Neodymium was analyzed as NdO + using single Re filaments and silica gel and phosphoric acid as the oxygen source, and 18 O/ 16 O and 17 O/ 16 O ratios of and , respectively, were used to correct the data. Mass analysis was done using a ector 54 TIM via a three-jump multicollector dynamic analysis and a power-law 532 Geological ociety of America Bulletin, March/April 2005

4 PROVENANCE OF JURAIC TETHYAN EDIMENT TABLE 1. PERCENT MINERAL MODE FOR METAEDIMENT AND BAEMENT NAPPE AMPLE ample no. Location Description Qtz WM Fsp Gt Bi Cht Carb Ep Am Other GROUP I, Zermatt-aas Metasediments 96JA-30b Lago di Cignana, Italy Gt-WM-Qtz schist rutile, titanite, tourmaline, opaque 97JA-1a aasfee-plattjen, witz. Gt-WM-Qtz schist rutile, titanite 97JA-24 Breuil-Cervinia, Italy Gt-Carb-WM-Qtz schist JA-16a Lago di Cignana, Italy Gt-WM-Qtz schist rutile, titanite, tourmaline 96JA-35 Täsch, witzerland Gt-WM-Qtz schist JA-4 aasfee-allalin, witz. Gt-Carb-WM-Qtz schist rutile 97JA-1b aasfee-allalin, witz. Gt-WM-Qtz schist rutile, chloritoid, opaque 97JA-15b Gornergletcher, witz. Gt-WM-Carb-Qtz schist rutile 01NM-47a Lago di Cignana, Italy Gt-WM-Qtz schist rutile, tourmaline, opaque 96JA-46 Pfulwe, witzerland Gt-WM-Qtz schist titanite, tourmaline GROUP II, Zermatt-Metasediments 96JA-30a Lago di Cignana, Italy WM-piemontite quartzite piemontite, opaque 97JA-2 aasfee-allalin, witz. Gt-Carb-WM-Qtz schist biotite 97JA-12 Gornergletcher, witz. WM-Qtz schist rutile 97JA-18 Breuil-Cervinia, Italy Gt-WM-Carb-Qtz schist JA-21 Lago di Cignana, Italy Qtz carbonate titanite 97JA-16 Gornergletcher, witz. WM-Gt quartzite JA-26 Lago di Cignana, Italy Gt-Carb-WM-Qtz schist NM-45 Lago di Cignana, Italy Gt-WM-Qtz schist rutile 01NM-44 Lago di Cignana, Italy Gt-WM-Qtz schist blue amphibole, rutile Mafi c Zermatt aas ample 97JA-3c aasfee-allalin, witz. Metagabbro chloritoid, titanite Europe Gran Paradiso 01NM-50 Valsavarenche Valley, Italy Paragneiss NM-51 Valsavarenche Valley, Italy Augen orthogneiss pyroxene Europe Grand t. Bernhard 01NM-11 Tennje, witzerland Orthogneiss NM-12 Mattsand, witzerland Orthogneiss NM-17 Randa, witzerland Paragneiss Europe Monte Rosa 01NM-31 Allalin dam, witzerland Paragneiss NM-32 Allalin dam, witzerland Paragneiss NM-33 Brusson, Italy Orthogneiss chloritoid 01NM-34 Mezzalama, Italy Orthogneiss NM-36 Mezzalama, Italy Orthogneiss NM-39 Gressonay, Italy Paragneiss pyroxene Africa/Apulia Dent Blanche 01NM-24 Unter Gabelhorn, witz. Amphibolite NM-25 Unter Gabelhorn, witz. Amphibolite NM-26 Unter Gabelhorn, witz. Amphibolite NM-27 Unter Gabelhorn, witz. Amphibolite NM-61 Valpelline Valley, Italy Amphibolite 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 NM-60 Valpelline Valley, Italy Paragneiss NM-62 Valpelline Valley, Italy Carbonate paragneiss pyroxene 02NM-52 Arolla Valley, witz. Orthogneiss NM-56 Valpelline Valley, Italy Leucocratic orthogneiss NM-57 Valpelline Valley, Italy Orthogneiss NM-58 Valpelline Valley, Italy Orthogneiss pyroxene Africa/Apulia Lanzo 01NM-49 Near Pont t Martin, Italy Paragneiss pyroxene, titanite V-911a* Arnad, Italy Albitic paragneiss x x x x pyroxene, titanite, rutile mb 1k/91* avenca, Italy Mesocratic gneiss x x x x x x sillimanite, ilmenite V-9110b* Bonze, Italy Massive ortho-leucogneiss x x x x x x rutile V-9128c* Cavalcurt, Italy Leucocratic orthogneiss x x x x x rutile 01NM-48 Near Pont t Martin, Italy Orthogneiss chloritoid, titanite V-913e* Elvo Valley, Italy Metagranodiorite x x x x x x rutile, zircon V-914e* Elvo Valley, Italy Metagranite x x x x x x rutile, zircon V-925a* Arnad, Italy Massive orthogneiss x x x x x x x V92 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. *amples from J.C. Hunziker, Lausanne, witzerland (see Venturini, 1995). Geological ociety of America Bulletin, March/April

5 MAHLEN et al. normalization to 146 Nd/ 144 Nd = During the course of this study, the measured 143 Nd/ 144 Nd of the BCR-1 UG rock standard was ± 9 (2-D, n = 2), and this is taken to be equal to present-day CHUR (e.g., Wasserburg et al., 1981). Laboratory blanks were ~180 pg for Nd and <50 pg for m, which are negligible. All isotopic compositions are calculated to an initial age of 160 Ma for comparison, which is a reasonable estimate of the depositional age of the sediments (Rubatto et al., 1998). REULT Twenty-three samples of Zermatt-aas metasediments and 36 samples from the African/ Apulian and European source nappes were analyzed for Rb-r and m-nd isotope compositions and REE contents. Rare-Earth-Element Data Two distinct groups of metasediments from the Zermatt-aas ophiolite are resolved from the chondrite-normalized REE patterns (Fig. 2A), and both groups resemble a continental pattern. Group I samples have ΣREE contents (where ΣREE is the sum in ppm of Ce, Nd, m, Eu, Gd, Dy, Er, and Yb analyzed by isotope dilution in this study) ranging from ppm to ppm, with an average ΣREE concentration of ppm, moderate negative Eu anomalies (average Eu/Eu* = 0.66), and an average (m/nd) n ratio of 0.60 (Table 2, where the subscript n refers to chondrite-normalized values; Fig. 2A, dashed lines). The overall REE pattern indicates light-ree enrichment relative to the heavy REEs with an average (Ce/Yb) n ratio of 8.1. The Group I samples are primarily strongly foliated garnet-mica-quartz schists ± carbonate with nearly equal modal abundances of quartz and white mica (between 20% 35%; Table 1). Group II samples have ΣREE contents ranging from 90.2 ppm to 55.8 ppm, with an average ΣREE concentration of 71.1 ppm, moderate negative Eu anomalies (average Eu/Eu* = 0.68), and an average (m/nd) n ratio of 0.64 (Table 2; Fig. 2A, solid lines). The REE pattern displays less extreme light-ree enrichment relative to the heavy REEs and has a lower average (Ce/Yb) n of 6.6 than the Group I samples (Table 1). Group II samples include strongly foliated garnet-mica-quartz schists ± carbonate, mica-quartz schists, quartzcarbonates, a garnet-rich micaceous quartzite, and a micaceous piemontite-bearing quartzite (Table 1). Although Group II samples contain higher modal abundances of quartz (30 40%) than white mica (10 30%) compared to Group I samples, this cannot explain the factor of two difference in ΣREE contents between the two groups through, for example, quartz dilution. One sample, 97JA-3c, has a REE pattern that is unique relative to the Group I and Group II metasediments (Fig. 2A, short dashed line). This sample has low ΣREE abundance of 34.8 ppm and no distinctive Eu anomaly (Eu/Eu* = 0.94). ample 97JA-3c has a (m/nd) n ratio of 1.0 and is depleted in light REEs relative to heavy REEs (Table 1). In contrast to the more continental-like patterns of Groups I and II, this sample more closely resembles a depleted oceanic REE pattern. This metagabbro-like sample contains epidote + white mica + biotite + feldspar + chloritoid + titanite (Table 1). Eleven samples from the European nappes were analyzed to characterize European crust that may have been proximal to the Ligurian basin. The ΣREE contents of the European samples range from ppm to 54.9 ppm with an average of ppm (Table 2; Fig. 2B). These samples display a light-ree enrichment relative to the heavy REEs and have an average (Ce/Yb) n of 8.7. Europium anomalies range 100 A Zermatt-aas metasediments Group I-dashed, n = 12 high ΣREE B European basement paragneiss-dashed, n = 5 orthogneiss-solid, n = 6 Chondrite normalized mafic, n = 2 C Group II-solid n = 11, low ΣREE esia zone paragneiss-dashed, n = 3 orthogneiss-solid, n = 7 D mafic, n = 5 Dent Blanche nappe paragneiss-dashed, n = 6 orthogneiss-solid, n = 4 mafic-short dashed, n = 5 Figure 2. Chondrite-normalized rare-earth-element patterns normalized to Anders and Grevesse (1989) chondritic compositions. Lines represent individual samples. (A) Zermatt-aas metasediments (Z). Z shaded areas plotted with (B) European basement samples including Monte Rosa, Grand t. Bernhard, and Gran Paradiso. (C) African/ Apulian esia zone samples. (D) African/Apulian Dent Blanche nappe samples. Nappe samples in B, C, and D are subdivided by lithology. 1 dark grey shading = Z Group I light grey shading = Z Group II Ce Nd m Eu Gd Dy Er Yb Ce Nd m Eu Gd Dy Er Yb 534 Geological ociety of America Bulletin, March/April 2005

6 PROVENANCE OF JURAIC TETHYAN EDIMENT ample Description wiss Grid coordinates TABLE 2. RARE EARTH ELEMENT DATA FOR METAEDIMENT AND BAEMENT NAPPE AMPLE Ce Nd m GROUP I, Zermatt-aas Metasediments 96JA-30b Gt-WM-Qtz schist N.D N.D JA-1a Gt-WM-Qtz schist JA-24 Gt-Carb-WM-Qtz schist JA-16a Gt-WM-Qtz schist JA-35 Gt-WM-Qtz schist JA-4 Gt-Carb-WM-Qtz schist JA-1b Gt-WM-Qtz schist JA-15b Gt-WM-Carb-Qtz schist NM-47a Gt-WM-Qtz schist N.D N.D. N.D JA-46 Gt-WM-Qtz schist JA-46 Gt-WM-Qtz schist JA-46 Gt-WM-Qtz schist GROUP II, Zermatt-aas Metasediments 96JA-30a WM-piemontite quartzite JA-30a WM-piemontite quartzite JA-2 Gt-Carb-WM-Qtz schist JA-12 WM-Qtz schist JA-18 Gt-WM-Carb-Qtz schist JA-21 Qtz carbonate JA-16 WM-Gt quartzite JA-26 Gt-Carb-WM-Qtz schist JA-21 Qtz carbonate NM-45 Gt-WM-Qtz schist NM-44 Gt-WM-Qtz schist Mafi c Zermatt-aas ample 97JA-3c Metagabbro JA-3c Metagabbro Europe Gran Paradiso 01NM-50 Paragneiss NM-51 augen orthogneiss Europe Grand t. Bernhard 01NM-11 Orthogneiss NM-12 Orthogneiss NM-17 Paragneiss Europe Monte Rosa 01NM-31 Paragneiss NM-32 Paragneiss N.D N.D. N.D NM-33 Orthogneiss NM-34 Orthogneiss NM-36 Orthogneiss NM-39 Paragneiss Africa/Apulia Dent Blanche 01NM-24 Amphibolite NM-25 Amphibolite NM-26 Amphibolite N.D N.D. N.D NM-27 Amphibolite N.D N.D N.D NM-61 Amphibolite KAW-558 Paragneiss N.D. N.D KAW-560 Paragneiss N.D. N.D KAW-682 Paragneiss N.D. N.D NM-59 Paragneiss NM-60 Paragneiss NM-62 Carbonate paragneiss NM-52 Orthogneiss NM-56 Leucocratic orthogneiss NM-57 Orthogneiss NM-58 Orthogneiss Africa/Apulia esia Lanzo 01NM-49 Paragneiss V-911a Albitic paragneiss N.D. N.D mb 1k/91 Mesocratic gneiss N.D. N.D N.D N.D. N.D V-9110b Massive ortho-leucogneiss N.D. N.D V-9128c Leucocratic orthogneiss N.D. N.D NM-48 Orthogneiss V-913e Metagranodiorite N.D. N.D V-914e Metagranite N.D. N.D V-925a Massive orthogneiss N.D. N.D V92-1mt Metagranite N.D. N.D Note: wiss grid coordinates in italics are approximate. ample description abbreviations as in Table 1. Eu/Eu* = Eu n /(m n * Gd n ) 1/2 ; n chondrite normalized values, from Anders and Grevesse (1989); N.D. no data. Replicate analyses. amples from J.C. Hunziker, Lausanne, witzerland (see Venturini, 1995). Eu Gd Dy Er Yb Eu/ Eu* (m/ Nd) n (Gd/ Yb) n (Ce/ Yb) n ΣREE Geological ociety of America Bulletin, March/April

7 MAHLEN et al. from Eu/Eu* = and (m/nd) n ratios range from 0.55 to There appears to be no apparent distinction between the three European nappes, although when the nappes are denoted by parent lithology, paragneissic samples tend to have higher overall REE contents than the orthogneissic samples. Twenty-five samples were analyzed to characterize African/Apulian crust compositions. The ten esia Lanzo samples have variable REE patterns and abundances, where ΣREE contents range from to 30.3 ppm (Table 2; Fig. 2C). These samples have Eu anomalies ranging from positive (Eu/Eu* = 1.3) to extremely negative (Eu/Eu* = 0.01), and an average (m/nd) n of The REE patterns generally indicate light-ree enrichment, where 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 to 39.6 ppm, moderate Eu anomalies (average Eu/Eu* = 0.71), and an average (m/nd) n ratio of 0.58 (Table 2). The overall REE pattern also indicates light-ree enrichment relative to the heavy REEs, with average (Ce/Yb) n = 9.1 (Fig. 2D). The distinction between orthogneissic versus paragneissic parent lithology in the African/Apulian samples is not as clear as in the European samples, but the samples that have higher overall REE concentrations tend to be paragneissic samples (Figs. 2C and 2D). Five additional Dent Blanche samples have REE patterns that are fairly flat, (Ce/Yb) n ratios that range from 0.7 to 4.1, ΣREE abundances between 95 and 14 ppm, average (m/nd) n = 0.94, and average Eu/Eu* of ~1.2 (Table 2; Fig. 2D, light-grey short-dashed lines). These samples contain amphibole + epidote/clinozoisite + white mica + feldspar + chlorite + quartz (Table 1). These five amphibolite samples, collected near Unter Gabelhorn, have REE data that are similar to depleted, oceanic REE patterns rather than continental patterns, and they are not likely to be representative of the Dent Blanche nappe. Rb-r Isotope Data Present-day 87 Rb/ 86 r 87 r/ 86 r variations for the Zermatt-aas metasediments are distinct from those of the European and African/Apulian nappes (Fig. 3). The majority of the metasediments lie along a common trend that falls along a Ma isochron, which may be interpreted to reflect a metamorphic, Alpine-related age (Fig. 3A and 3C). Three metasediment samples, however, lie along a steeper array, similar to the basement nappes. Group I metasediments have a relatively restricted range in 87 Rb/ 86 r ratios, varying from 1.21 to 6.97, whereas 87 Rb/ 86 r ratios for Group II metasediments are more variable, ranging from 0.04 to (Table 3). The majority of the African/Apulian and European nappe samples scatter about a Variscan-age array (Fig. 3C) consistent with their most common basement age. There is little correlation of 87 Rb/ 86 r ratios and REE patterns for the basement nappes, with the exception that the five Dent Blanche samples that have low (Ce/Yb) n ratios (Table 1) have very low 87 Rb/ 86 r ratios, consistent with the inferred mafic composition (Table 3; Fig. 3C). m-nd Isotope Data In contrast to the 87 Rb/ 86 r 87 r/ 86 r relations, present-day 147 m/ 144 Nd 143 Nd/ 144 Nd variations 87 r/ 86 r(0) 87 r/ 86 r(0) This study: Z Group I Z Group II Z mafic Other studies: Z, Amato et al., 1999 Z, Dal Piaz et al., , 0.9 t» 360 Ma t» 260 Ma for the Zermatt-aas metasediments do not define a correlation along a m-nd isochron, but instead have a significant range in Nd isotope compositions relative to a restricted range in m/ Nd ratios (Fig. 4, where f m/nd = 147 m/ 144 Nd measured / ). The lack of 147 m/ 144 Nd 143 Nd/ 144 Nd variations about an Alpine or Variscan-age isochron supports the premise that the REEs in the Zermatt-aas sediments reflect neither derivation from a single Variscan-age source nor significant mobilization during Eocene Alpine metamorphism. An important observation from the isotopic data is that, commensurate with their distinctive REE patterns and 147 m/ 144 Nd ratios, the Group I and II samples have different ranges in Nd isotope compositions, where ε Nd (0) ranges from 7.9 to 14.9 for Group I samples t» 100 Ma t» 360 Ma B t» 60 Ma t» 40 Ma Zermatt-aas metasediments t» 260 Ma 0.74 This study: European African/Apulian Other studies: 0.72 esia, Dal Piaz et al., 2001 Monte Rosa, Basement Pawlig, 2000 nappe samples C t» 60 Ma t» 40 Ma Rb/ r Figure Rb/ 86 r 87 r/ 86 r isotope plot. (A) Zermatt-aas metasediments (Z). (B) Exaggeration of lower left-hand corner of A. (C) European and African/Apulian basement nappes. This study: European basement samples include Monte Rosa, Grand t. Bernhard, and Gran Paradiso; African/Apulian samples include Dent Blanche and esia zone. A 536 Geological ociety of America Bulletin, March/April 2005

8 PROVENANCE OF JURAIC TETHYAN EDIMENT ample TABLE 3. m-nd AND Rb-r IOTOPIC DATA FOR METAEDIMENT AND BAEMENT NAPPE AMPLE m Nd m/ Nd/ 2-E ε Nd (0) ε Nd (160) ƒ m/nd T DM r Rb Rb/ r/ Nd Nd m (Ga) r GROUP I, Zermatt-aas Metasediments 96JA-30b ± JA-1a ± JA ± JA-16a ± JA ± JA-35 N.D N.D ± N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. 97JA ± JA-1b ± JA-1b N.D N.D ± N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. 97JA-15b ± NM-47a ± JA ± JA ± JA ± JA-46 N.A. N.A. N.A ±8000 N.A. N.A. N.A. N.A. N.A. N.A. N.A N.A. GROUP II, Zermatt-aas Metasediments 97JA ± JA ± JA ± JA ± JA ± N.D. N.D. N.D. N.D. N.D. 96JA ± JA ± JA-21 N.D N.D ± N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. 01NM ± NM ± JA-30a ± JA-30a ± JA-30a N.A. N.A. N.A ±8000 N.A. N.A. N.A. N.A. N.A. N.A. N.A N.A. Mafi c Zermatt-aas ample 97JA-3c ± N.A JA-3c ± N.A Europe Gran Paradiso 01NM ± NM ± N.D. N.D N.D. Europe Grand t. Bernhard 01NM ± NM ± NM ± Europe Monte Rosa 01NM ± NM ± NM ± NM ± NM ± NM ± Africa/Apulia Dent Blanche 01NM ± N.A NM ± N.A NM ± N.A NM ± N.A NM ± KAW ± KAW ± KAW ± NM ± NM ± NM ± NM ± NM ± NM ± NM ± Africa/Apulia esia Lanzo 01NM ± N.D. N.D N.D. V-911a ± mb 1k/ ± V-9110b ± V-9128c ± N.D NM ± V-913e ± V-914e ± V-925a ± V92-1mt ± N.D Note: Nd normalized to 146 Nd/ 144 Nd = Present-day CHUR 143 Nd/ 144 Nd = and 147 m/ 144 Nd = r normalized to 86 r/ 87 r = Initial age for all samples is 160 Ma. m measured ratio; N.D. no data; N.A. not applicable. Replicate analyses. Unspiked samples. 86 r m 87 r/ 86 r 160 Geological ociety of America Bulletin, March/April

9 MAHLEN et al. and 7.8 to 10.1 for Group II samples (Fig. 4). Neodymium isotope compositions, however, are best compared at the time of sediment deposition, and ε Nd (160) values range from 6.5 to 13.2 for Group I samples, whereas ε Nd (160) values for Group II samples vary from 6.3 to 8.7 (Table 3). Present-day Nd isotope compositions of the African/Apulian and European basement nappes overlap those of the Zermatt-aas metasediments. When the nappes are distinguished by parent lithology, orthogneissic versus paragneissic groupings are apparent (Fig. 4). The Group I metasediments overlap ε Nd (0) 147 m/ 144 Nd variations observed for the paragneissic nappe samples, and Group II samples overlap the range defined by the orthogneissic nappe samples (Fig. 4). The esia zone ε Nd (160) values range from 2.2 to 12.8, whereas ε Nd (160) values for the Dent Blanche samples vary from 5.4 to 12.0 (Table 3). ε Nd (160) values for the European samples, which include the Monte Rosa, Gran Paradiso, and Grand t. Bernhard nappes, range from 5.3 to 13.8 (Table 3). The wide range in 143 Nd/ 144 Nd ratios is not correlated with 147 m/ 144 Nd ratios (Fig. 4), consistent with data from Variscan rocks elsewhere that have not been subjected to Alpine metamorphism (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. INTERPRETATION OF RARE EARTH ELEMENT AND IOTOPE DATA Our data support the conclusion of previous studies that have interpreted the REEs to be relatively immobile during sedimentary processes such as diagenesis, weathering, erosion, and sorting, as well as during metamorphism (e.g., Green et al., 1969; McCulloch and Wasserburg, 1978; Goldstein et al., 1984; Taylor and McLennan, 1985; McLennan, 1989; Taylor and McLennan, 1995; Jahn, 2000), and our results indicate that the REEs are immobile even to UHP conditions where metamorphic temperatures were 600 C. amarium-nd isotope and REE data indicate a close relation between the Zermatt-aas metasediments and the local nappe samples with regard to provenance, suggesting local derivation. The Rb-r system, however, has long been known to be relatively easily reset during metamorphism (e.g., Goldstein and Jacobsen, 1988; Lee and Chang, 1997), and our results suggest that r isotope variations were nearly completely reset for the majority of the Zermatt-aas metasediment samples during Eocene Alpine metamorphism. 87 Rb- 87 r Mobilization The contrasting 87 r/ 86 r variations for the African/Apulian and European basement nappes, which scatter about a Variscan-like ( Ma) array (Fig. 3C) relative to those of the Zermatt-aas metasediments, which cluster around a 60 Ma isochron (Fig. 3A), are interpreted to reflect contrasting mobility of the Rb-r isotopic system in wet (open-system) versus dry (closed-system) metamorphic conditions, given the similar P-T-t history of the majority of the samples. That the Zermatt-aas metasediments generally cluster along a Ma isochron is interpreted to reflect nearly complete homogenization of 87 r/ 86 r ratios during Alpine metamorphism. The homogenization of the r isotope system is particularly striking given the wide range in Nd isotope compositions of the Zermatt-aas metasediments (Fig. 4), which suggests that prior to Alpine metamorphism the 87 r/ 86 r ratios were also likely to have been quite variable. It remains unclear why three Zermatt- aas metasediment samples (97JA-1a, 97JA-1b, and 97JA-12; 87 r/ 86 r >0.74) did not become homogenized during Alpine metamorphism, as these samples do not appear to be anomalous in mineralogy or Rb and r contents. ε Nd (0) Primitive source LREE enriched Z Group I Z Group II Z mafic Eur samples AA samples The Rb-r isotope data reported here for the basement nappes generally match those of Pawlig (2001), where it was suggested that the Monte Rosa nappe of the European plate, for example, was dry as it underwent Alpine metamorphism. The Monte Rosa metagranites from Pawlig (2001) primarily plot along a 360 Ma isochron (Fig. 3C), indicating they have generally preserved their Variscan Rb-r ages, despite the fact that they were metamorphosed to HP/UHP conditions similar to those of the Zermatt-aas metasediments. Conversely, the Ligurian basaltic crust and overlying sediments could be considered to have been wet during subduction and Alpine metamorphism, due to prior seawater and hydrothermal fluid interaction with the oceanic crust and sediment cover, and/or dewatering of the slab, which would have provided sufficient fluids to mobilize r (Tatsumi et al., 1986). upport for locally high fluid pressures in the Zermatt-aas rocks comes from the Lago di Cignana, Italy, locality (van der Klauw et al., 1997), where quartz and albite vein shapes and orientations are distinct from other rock structures. Unfortunately, it cannot be distinguished with certainty if r isotope homogenization in the Zermatt-aas metasediments occurred during MORB source LREE depleted 360 Ma 160 Ma -12 Old 50 Ma -14 UCC source ƒ m/nd Nappes-paragneiss Nappes-orthogneiss Nappes-DB mafic Figure 4. f m/nd ε Nd (0) isotope plot. Zermatt-aas metasediments (Z); European nappe samples (Eur) include Monte Rosa, Grand t. Bernhard, and Gran Paradiso; African/ Apulian nappe samples (AA) include Dent Blanche and esia. Nappes primarily cluster into two groups based on lithology. MORB, primitive, and old upper continental crust (UCC) source regions from Taylor and McLennan (1985), Hofmann (1988), Taylor and McLennan (1995), respectively. 538 Geological ociety of America Bulletin, March/April 2005