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

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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. For permission to copy, contact [email protected] 530 © 2005 Geological Society of America PROVENANCE OF JURASSIC TETHYAN SEDIMENTS 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).
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