Zermatt-Saas Ophiolite Research Articles

The Molybdenum isotope subduction recycling conundrum: A case study from the Tongan subduction zone, Western Alps and Alpine Corsica

Molybdenum isotopes have emerged as novel tracers for high-temperature igneous and metamorphic processes. The debate remains to what extent different subducted slab lithologies, such as oceanic crust and marine sediments, contribute to the Mo isotope signature of arc magmas and, hence, exert different controls on the terrestrial Mo cycle. Here we investigate Mo isotope systematics from input to output at the Tongan subduction zone: Arc lavas from different Tongan islands, pelagic sediments and altered oceanic crust (AOC) samples from DSDP site 595/596 on the subducting Pacific plate. For complementary insights into the fate of Mo and its isotopic signatures during prograde subduction metamorphism, we also present data of metasediments and variably altered AOC-type eclogites from the Zermatt-Saas ophiolite, Switzerland and Italy, and the Schistes Lustrés Complex in Alpine Corsica.Manganese oxide-rich pelagic sediments from DSDP site 595/596 show variable, depth-dependent Mo/Mn ratios and Mo isotope compositions controlled by diagenetic reactions. As subducted equivalents, Mn-rich eclogitic metapelites display lower Mo contents and δ98/95Mo ratios compared to their non-subducted protolith. This indicates prominent loss of Mo along with isotope fractionation during early subduction metamorphism. In comparison to unaltered MORB, low temperature seafloor alteration has shifted Mo/Ce and δ98/95Mo in studied AOC samples towards lower ratios, in the range of most mafic eclogites published so far. However, some mafic eclogites show even lower Mo/Ce and δ98/95Mo ratios compared to AOC, likely due to fluid-related Mo loss upon early subduction and preferential incorporation of light Mo into residual rutile.Our data document a prominent loss of isotopically heavy Mo before and upon early subduction metamorphism at shallow depths in the forearc region. Moreover, when prograde rutile crystallizes at ~30 km depth, it fixes the largest fraction of Mo in the subducting material. This creates an “arc Mo-conundrum” as devolatization of the slab at subarc depths is not able to account for the fluid-mobile Mo source responsible for observed higher Mo/Ce and δ98/95Mo in Tongan arc lavas compared to the mantle. As an alternative scenario, Mo mobilization by slab-derived aqueous fluids during the early stages of subduction into the forearc mantle produces serpentinites enriched in Mo with a possible heavy Mo isotopic signature. Mechanical transport and devolatization of forearc serpentinites at subarc regions is a plausible alternative recycling process accounting for the observed Mo systematics in Tongan arc lavas. This is supported by positive covariations of Mo/Ce and δ98/95Mo with elements such as As, Sb, and Cs, which are thought to be mostly released from the subducting material during early stages of subduction. We propose that a multi-stage recycling of metasomatized forearc mantle can be an important process in recycling of Mo and possibly other elements.

HP tectono‐metamorphic evolution of the Internal Piedmont Zone in Susa Valley (Western Alps): New petrologic insight from garnet+chloritoid‐bearing micaschists and Fe–Ti metagabbro

AbstractNew petrologic data from meta‐ophiolites exposed in a relatively poorly known sector of the Western Alps (i.e. Susa Valley, Internal Piedmont Zone) are reported. Garnet+chloritoid‐bearing phengitic micaschist and Fe–Ti metagabbro preserving eclogite facies assemblages have been investigated in detail to constrain the HP tectono‐metamorphic evolution of meta‐ophiolites in the study area. Microstructural investigations allowed to identify different assemblages related to different tectono‐metamorphic stages, whose pressure–temperature (P–T) conditions of the HP stages were constrained using the pseudosection modelling approach combined with multi‐equilibrium thermobarometry. Four main metamorphic stages have been recognized: (a) an Alpine peak‐P stage (M1a), which reached the HP–UHP eclogite facies boundary (P = 25–29 kbar, T = 460–510°C); (b) a slightly prograde decompression stage (M1b), still under eclogite facies conditions (P = 21–25 kbar, T = 500–530°C); (c) a greenschist facies re‐equilibration stage (M2); (d) a nearly isobaric late heating stage (M3), developed at the boundary between greenschist‐ and amphibolite facies conditions. The new P–T path inferred for the HP phases recorded by the studied meta‐ophiolites is compatible with that reported from other meta‐ophiolite units in the Western Alps, that is, the Zermatt–Saas ophiolite to the north and the Monviso ophiolite to the south, confirming that subduction of the Tethys oceanic lithosphere occurred at similar P–T regimes all along its length. The comparison with the tectonic evolution of the adjacent Dora Maira continental crustal unit suggest that the early exhumation of eclogitized oceanic crust likely occurred along the plate interface in the subduction channel, without the buoyancy contribution of continental crust.

Petrochronological close-up on the thermal structure of a paleo-subduction zone (W. Alps)

We present the results from a study combining zoned geochronology, mineral trace element geochemistry and phase equilibria modeling of a mafic lithology from the Zermatt Saas Ophiolite in order to elucidate the tectonic rates and thermal structure of the Western Alps paleo-subduction interface. The Zermatt-Saas Ophiolite represents dismembered slices derived from an eclogitized, 60-km wide coherent fragment of Tethyan oceanic lithosphere. Two cm-sized garnet crystals from a pyrite-rich chlorite-talcschist (representative of sub-seafloor hydrothermally-altered basalts) from the Servette mine locality (St. Marcel Valley, Italy) were microdrilled to separate core and rim growth generations and dated using Sm-Nd geochronology to determine the overall duration of garnet growth. Garnet cores were dated to 46.9±1.6Ma, signifying the timing of the initiation of garnet growth. Garnet rims were dated to 43.5±1.3Ma, signifying the timing of burial to peak depths (constrained to ∼75 km). Major and trace element zoning in garnet suggest two distinct generations of garnet growth. Garnet crystal cores display evidence for rapid nucleation and growth, while garnet crystal rims suggest growth at relatively slow (tectonic) rates. The duration of garnet growth (3.4±2.1Ma), when coupled to constraints from phase equilibria modeling and Zr-in-rutile thermometry, provides estimates on the burial and heating rate of 4.7 (+7.9−2.1) km/Myr and 13.2 (+17.8−6.1)°C/Myr, respectively. Using the depth and temperature conditions recorded by laterally equivalent meta-ophiolites from across the Western Alps, a ‘snapshot’ of the geothermal gradient at the Alpine subduction interface is presented. This reveals a pressure-temperature trajectory highlighted by a significant decrease of the thermal gradient from ∼10°C/km down to ∼3°C/km at ∼45 Ma. We further present geometric calculations to identify the tectonic parameters that best fit this dual-segmented thermal gradient. Using convergence rates from paleogeographic reconstructions during Eocene times (on the order of 1.5-2 cm/yr), our calculations require a paleo-slab angle of 15-20°, in line with dip estimates from most modern subduction zones. This study highlights how a detailed petrochronology-based approach enables assessing the thermal structure and geometry of a paleo-subduction zone, with implications on seismicity, stress distribution, and devolatilization potential of deeper portions of the subduction interface.

Superposed Sedimentary and Tectonic Block-In-Matrix Fabrics in a Subducted Serpentinite Mélange (High-Pressure Zermatt Saas Ophiolite, Western Alps)

The primary stratigraphic fabric of a chaotic rock unit in the Zermatt Saas ophiolite of the Western Alps was reworked by a polyphase Alpine tectonic deformation. Multiscalar structural criteria demonstrate that this unit was deformed by two ductile subduction-related phases followed by brittle-ductile then brittle deformation. Deformation partitioning operated at various scales, leaving relatively unstrained rock domains preserving internal texture, organization, and composition. During subduction, ductile deformation involved stretching, boudinage, and simultaneous folding of the primary stratigraphic succession. This deformation is particularly well-documented in alternating layers showing contrasting deformation style, such as carbonate-rich rocks and turbiditic serpentinite metasandstones. During collision and exhumation, deformation enhanced the boudinaged horizons and blocks, giving rise to spherical to lozenge-shaped blocks embedded in a carbonate-rich matrix. Structural criteria allow the recognition of two main domains within the chaotic rock unit, one attributable to original broken formations reflecting turbiditic sedimentation, the other ascribable to an original sedimentary mélange. The envisaged geodynamic setting for the formation of the protoliths is the Jurassic Ligurian-Piedmont ocean basin floored by mostly serpentinized peridotites, intensely tectonized by extensional faults that triggered mass transport processes and turbiditic sedimentation.

Dating of ultramafic rocks from the Western Alps ophiolites discloses Late Cretaceous subduction ages in the Zermatt-Saas Zone

AbstractThe Zermatt-Saas Zone was part of the Middle to Late Jurassic Tethyan lithosphere that underwent oceanic metamorphism during Mesozoic time and subduction during Eocene time (HP to UHP metamorphism). In upper Valtournanche, serpentinite, metarodingite and eclogite record a dominant S2 foliation that developed under 2.5±0.3 GPa and 600±20°C during Alpine subduction. Serpentinites contain clinopyroxene and rare zircon porphyroclasts. Clinopyroxene porphyroclasts show fringes within S2 with similar compositions to that of grains defining S2. Zircon cores show zoning typical of magmatic growth and thin fringes parallel to the S2 foliation. These features indicate crystallization of clinopyroxene and zircon fringes during HP syn-D2 metamorphism, related to the Alpine subduction. The U–Pb zircon dates for cores and fringes reveal crystallization at 165±3.2 Ma and 65.5±5.6 Ma, respectively. The Middle Jurassic dates are in agreement with the known ages for the oceanic accretion of the Tethyan lithosphere. The Late Cretaceaous - Paleocene dates suggest that the Zermatt-Saas Zone experienced high-pressure to ultra-high-pressure (HP–UHP) metamorphism at c. 16 Ma earlier than previously reported. This result is in agreement with the evidence that in the Western Alps the continental Sesia-Lanzo Zone reached the subduction climax at least from 70 Ma and was exhumed during ongoing oceanic subduction. Our results are further evidence that the Zermatt-Saas ophiolites diachronically recorded heterogeneous HP–UHP metamorphism.

Record of Jurassic mass transport processes through the orogenic cycle: Understanding chaotic rock units in the high-pressure Zermatt-Saas ophiolite (Western Alps)

The eclogite facies Zermatt-Saas ophiolite in the Western Alps includes a composite chaotic unit exposed in the Lake Miserin area, in the southern Aosta Valley region. The chaotic unit is characterized by a block-in-matrix texture consisting of ultramafic clasts and blocks embedded within a carbonate matrix. This unit overlies massive serpentinite and ophicarbonate rocks and is unconformably overlain by layered calcschist. Despite the effects of subduction and collision-related deformation and metamorphism, the internal stratigraphy and architecture of the chaotic unit are recognizable and are attributed to different types of mass transport processes in the Jurassic Ligurian-Piedmont Ocean. This finding represents an exceptional record of the preorogenic history of the Alpine ophiolites, marked by different pulses of extensional tectonics responsible for the rough seafloor topography characterized by structural highs exposed to submarine erosion. The Jurassic tectonostratigraphic setting envisioned is comparable to that observed in present-day magma-poor slow- and ultraslow-spreading ridges, characterized by mantle exposure along fault scarps that trigger mass transport deposits and turbiditic sedimentation. Our preorogenic reconstruction is significant in an eclogitized collisional orogenic belt in which chaotic rock units may be confused with the exclusive product of subduction-related tectonics, thus obscuring the record of an important preorogenic history.

MANTLE ORIGIN OF THE ANTRONA SERPENTINITES (ANTRONA OPHIOLITE, PENNINE ALPS) AS INFERRED FROM MICROSTRUCTURAL, MICROCHEMICAL, AND NEUTRON DIFFRACTION QUANTITATIVE TEXTURE ANALYSIS

The Antrona ophiolite is located in the Italian side of Western Central Alps. In the Alpine nappe stack, it lies at low structural levels, being sandwiched between the overlying continental Monte Rosa Nappe (upper Penninic) and the underlying Camughera-Moncucco continental Unit (middle Penninic). The ophiolite sequence includes serpentinized ultramafites, metagabbros and mafic rocks covered by calcschists. The ultramafic portion of the Antrona ophiolite consists of serpentinized peridotites, with interbedded layers of various mafic/ultramafic rocks, and underlies the mafic rocks and metasediments. In spite of the Alpine tectonic and metamorphic evolution of the Antrona ophiolite and its heavy serpentinization, the ultramafic rocks preserve relict texture and mineralogy that allow discussing the nature of their protoliths. Olivine-clinopyroxene-spinel-bearing serpentinites still retain relict porphyroclastic texture, commonly attributed to mantle peridotites. This inference is supported by quantitative textural analysis of Lattice Preferred Orientation by neutron diffraction, performed for the first time in olivine crystals of Alpine ophiolites, suggesting T conditions > 800°C for the activation of slip systems. Mineral chemistry of fresh olivine, clinopyroxene and spinel also contribute to support the mantle nature of the serpentinite protolith. The Antrona ophiolite may thus be regarded as a fossilized fragment of oceanic lithosphere including mantle rocks, volcanics and deep-see sediments. Although their extention is smaller, the Antrona ophiolite seems to be comparable with a coherent ophiolitic slice, such as Zermatt Saas ophiolite more than with a serpentinite melange.

Initial water budget: The key to detaching large volumes of eclogitized oceanic crust along the subduction channel?

We herein investigate the extent to which extensive hydration of the oceanic lithosphere influences the preservation and exhumation of large-scale ophiolite bodies from subduction zones. The Zermatt–Saas ophiolite (ZS, W. Alps), which was subducted during the late stages of oceanic subduction, preserves a complete section of Mesozoic Tethys oceanic lithosphere and particularly fresh eclogites, and represents, so far, the largest and deepest known portion of exhumed oceanic lithosphere. Pervasive hydrothermal processes and seafloor alteration led to the incorporation of large amounts of fluid bound in the hydrated upper layers of the oceanic crust (now as lawsonite eclogites, glaucophanites, and chloritoschists) and in associated ultramafic rocks. Internally, the ZS ophiolite is made up of a series of tectonic slices of oceanic crust (150–300 m thick) which are systematically separated by a 5 to 100 m thick layer of serpentinite. This stack of slices is separated from the underlying eclogitized continental crust (e.g., Monte Rosa) by a thick (~ 500 m) serpentinite sole. Field observations, textural relationships and pseudosection modelling reveal that lawsonite was abundant and widespread in mafic eclogites when the ophiolite detached from the slab at around 550 °C and 24 kbar. Comparison between fresh eclogitic samples and pseudosection modelling shows that (i) water remained in excess from burial to eclogitic peak conditions, (ii) the lightest eclogitized metabasalts correspond to the portions of oceanic crust where metasomatism was the strongest, (iii) crystallization of widespread hydrated parageneses (such as lawsonite, glaucophane and phengite) instead of garnet and omphacite decreased by 5 to 10% the rock density and subsequently enhanced its buoyancy. We propose that this density decrease acted as a ‘float’ which prevented the slices from an irreversible sinking in the mantle. These slices were subsequently detached from the downgoing slab and stacked in the serpentinized subduction channel at pressures between 15 and 20 kbar, in the epidote blueschist facies. Exhumation of the underlying, positively buoyant continental crust dragged this “frozen” nappe-stack from the subduction channel towards the surface.

The Eclogite-facies Allalin Gabbro of the Zermatt-Saas Ophiolite, Western Alps: a Record of Subduction Zone Hydration

The Allalin gabbro is a 2 km × 0·5 km block of layered olivine-gabbro and troctolite included in the Zermatt–Saas ophiolite nappe of the Western Alps. Comprehensive texture, mineral and rock composition data together with a thermodynamic analysis of the complex phase associations permit a detailed reconstruction of the igneous and metamorphic reaction history recorded by the gabbroic rocks. Based on rock and mineral composition data, the Allalin gabbro represents part of a Middle Jurassic underplate of mafic magma at the base of the continental crust of the Apulian plate (Dent Blanche–Sesia Lanzo system). Granulite-facies recrystallization during cooling at ∼825°C and 1·0 GPa, involving the formation of Opx–Grt coronas between Ol–Pl, can be related to crustal thickening. Eocene subduction of the Tethys oceanic lithosphere under the Apulian plate detached the gabbro from the base of the continental crust and incorporated it into the ophiolite. Increasing pressure in the descending slab had little effect on parts of the gabbro, which still locally contain unaltered igneous Ol, Aug and Pl and well-preserved magmatic textures. With increasing subduction depth an increasing amount of aqueous fluid accessed the gabbro and transformed Pl to Zo–Jd–Ky–Qtz. At about 2·5 GPa (93 km) at c. 610°C, a dramatic hydration process converted most of the rocks (>90 vol. %) into a fully hydrated eclogite-facies assemblage of Omp + Zo + Tlc + Cld ± Grt ± Ky + Rt. The full hydration under water-present conditions occurred at the greatest depth reached by the gabbro. After detachment from the downgoing slab (i.e. along the ascent path), Gln, Pg and Mrg formed as additional hydrates. This last phase of hydration desiccated the metagabbro at a depth of c. 78 km and from then on the rocks were essentially devoid of a free fluid phase. The Allalin gabbro provides strong evidence that the fundamental high-pressure transformation of mafic rocks involves the hydration of gabbro to form eclogite.

Petrogenesis and structure of the Buck Creek mafic-ultramafic suite, southern Appalachians: Constraints on ophiolite evolution and emplacement in collisional orogens

Integration of detailed field, petrological, structural, and geochemical observations of the Buck Creek mafic-ultramafic suite, which is among the larger southern Appalachian ultramafic exposures, points to an igneous origin as a mid-ocean-ridge cumulate massif, with subsequent emplacement in a deep subduction-zone setting. These results help to clarify the unresolved role of “alpine-type” ultramafic bodies in the southern Appalachians, a problem complicated by their generally small and fragmented nature and by significant tectonic overprinting. As an ophiolite fragment, the Buck Creek rocks are unusual in their preservation of relatively high-pressure, anhydrous conditions, and thus they provide constraints on mechanisms of ophiolite emplacement. Field support for a cumulate origin for the Buck Creek suite includes centimeter- to meter-scale interlayering of dunite, troctolite, anorthosite, and spinel-rich layers, and local gradation between Buck Creek troctolitic rocks and surrounding amphibolites. Relict anorthitic plagioclase, forsteritic olivine, and augitic clinopyroxene define a “cumulate triangle” on an MgO versus Al 2 O 3 diagram: meta-troctolite and meta-dunite compositions define a linear array between olivine and plagioclase compositions, and many amphibolites fall within this triangle, reflecting gabbroic protoliths. The relatively abundant troctolite cumulates, scarce pyroxene in the ultramafic rocks, and the high Al 2 O 3 /TiO 2 and low Cr numbers in spinel best match LOT- or L-type (lherzolite) ophiolites, consistent with crystallization in a slow-spreading mid-ocean-ridge setting. Sapphirine-bearing, spinel symplectites in the metatroctolites suggest that Buck Creek rocks remained anhydrous to ~800 °C and ~0.9–1.1 GPa, representing conditions atypical of ophiolite emplacement. Comparisons to the Zermatt-Saas ophiolite and Bergen eclogite suggest rapid subduction of Buck Creek rocks to depths of ~30 km, where partial hydration, perhaps facilitated by pro-grade dehydration reactions in surrounding rocks, caused strain softening that permitted ductile disaggregation and aided in their emplacement into the overlying accretionary complex. Alternatively, emplacement might have resulted from a switch in subduction polarity, with hydration following emplacement. Uplift, shortening, and hydration within the accretionary sequences are responsible for the dominant structural features.

Rhenium–osmium isotope and elemental behaviour during subduction of oceanic crust and the implications for mantle recycling

This study presents major-, trace-element, and rhenium–osmium (Re–Os) isotope and elemental data for basalts and gabbros from the Zermatt-Saas ophiolite, metamorphosed to eclogite-facies conditions during the Alpine orogeny. Igneous crystallisation of the gabbros occurred at 163.5 ± 1.8 Ma and both gabbro and basalt were subject to ‘peak’ pressure–temperature ( P– T) conditions of > 2.0 GPa and ~ 600 °C at about 40.6 ± 2.6 Ma. Despite such extreme P– T conditions, Re–Os isotope and abundance data for gabbroic rocks suggest that there has been no significant loss of either of these elements during eclogite-facies metamorphism. Indeed, 187Re– 187Os isotope data for both unaltered gabbros and gabbroic eclogites lie on the same best-fit line corresponding to an errorchron age of 160 ± 6 Ma, indistinguishable from the age of igneous crystallisation. In contrast, metamorphosed basalts do not yield age information; rather most possess 187Re/ 188Os ratios that cannot account for the measured 187Os/ 188Os ratios, given the time since igneous crystallisation. Taken with their low Re contents these data indicate that the basalts have experienced significant Re loss (∼ 50–60%), probably during high-pressure metamorphism. Barium, Rb and K are depleted in both gabbroic and basaltic eclogites. In contrast, there is no evident depletion of U in either lithology. Many ocean-island basalts (OIB) possess radiogenic Os and Pb isotope compositions that have been attributed to the presence of recycled oceanic crust in the mantle source. Published Re–Os data for high- P metabasaltic rocks alone (consistent with this study) have been taken to suggest that excessive amounts of oceanic crust are required to generate such signatures. However, this study shows that gabbro may exert a strong influence on the composition of recycled oceanic crust. Using both gabbro and basalt (i.e. a complete section of oceanic crust) calculations suggest that the presence of ≥ 40% of 2 Ga oceanic crust can generate the radiogenic Os compositions seen in some OIB. Furthermore, lower U/Pb ratios in gabbro (compared to basalt) serve to limit the 206Pb/ 204Pb ratios generated, while having a minimal effect on Os ratios. These results suggest that the incorporation of gabbro into recycling models provides a means of producing a range of OIB compositions having lower (and variable) 206Pb/ 204Pb ratios, but still preserving 187Os/ 188Os compositions comparable to HIMU-type OIB.

Coupling of oceanic and continental crust during Eocene eclogite-facies metamorphism: evidence from the Monte Rosa nappe, western Alps

High precision U–Pb geochronology of rutile from quartz–carbonate–white mica–rutile veins that are hosted within eclogite and schist of the Monte Rosa nappe, western Alps, Italy, indicate that the Monte Rosa nappe was at eclogite-facies metamorphic conditions at 42.6 ± 0.6 Ma. The sample area [Indren glacier, Furgg zone; Dal Piaz (2001) Geology of the Monte Rosa massif: historical review and personal comments. SMPM] consists of eclogite boudins that are exposed inside a south-plunging overturned synform within micaceous schist. Associated with the eclogite and schist are quartz–carbonate–white mica–rutile veins that formed in tension cracks in the eclogite and along the contact between eclogite and surrounding schist. Intrusion of the veins at about 42.6 Ma occurred at eclogite-facies metamorphic conditions (480–570°C, >1.3–1.4 GPa) based on textural relations, oxygen isotope thermometry, and geothermobarometry. The timing of eclogite-facies metamorphism in the Monte Rosa nappe determined in this study is identical to that of the Gran Paradiso nappe [Meffan-Main et al. (2004) J Metamorphic Geol 22:261–281], confirming that these two units have shared the same Alpine metamorphic history. Furthermore, the Gran Paradiso and Monte Rosa nappes underwent eclogite-facies metamorphism within the same time interval as the structurally overlying Zermatt-Saas ophiolite [∼50–40 Ma; e.g., Amato et al. (1999) Earth Planet Sci Lett 171:425–438; Mayer et al. (1999) Eur Union Geosci 10:809 (abstract); Lapen et al. (2003) Earth Planet Sci Lett 215:57–72]. The nearly identical P–T–t histories of the Gran Paradiso, Monte Rosa, and Zermatt-Saas units suggest that these units shared a common Alpine tectonic and metamorphic history. The close spatial and temporal associations between high pressure (HP) ophiolite and continental crust during Alpine orogeny indicates that the HP internal basement nappes in the western Alps may have played a key role in exhumation and preservation of the ophiolitic rocks through buoyancy-driven uplift. Coupling of oceanic and continental crust may therefore be critical in preventing permanent loss of oceanic crust to the mantle.

Extraction faults

We propose the term “extraction fault” for a planar structure that forms at the trailing edge of a discrete block when it is forced or extracted out of the surrounding material. This process results in the merging of two block-bounding faults with opposite senses of displacement. An extraction fault differs fundamentally from other faults in that its two sides have approached each other substantially in the direction perpendicular to the fault. The fault-parallel displacement may be either zero (pure extraction faults) or not (mixed extraction faults). Pure small-scale extraction faults can result from boudinage. A large-scale example may be the S-reflector of the Galicia passive continental margin which is related to rifting and continental breakup. When the strong portion of the lithosphere, i.e. the upper mantle and the lower crust, underwent necking, thermally weak mantle from below and upper crust from above collapsed into the opening gap in the rift centre and an extraction fault formed at the trailing edge of the strong lithosphere. Extraction faults are also potentially important in the exhumation of high-pressure metamorphic rocks in collisional orogens. We propose that the Combin fault on top of the eclogite-facies Zermatt-Saas ophiolites in the Penninic Alps, earlier interpreted either as a normal fault or as a thrust, is in fact an extraction fault.

THE ANTRONA NAPPE: LITHOSTRATIGRAPHY AND METAMORPHIC EVOLUTION OF OPHIOLITES IN THE ANTRONA VALLEY (PENNINE ALPS)

The Antrona ophiolite (western Central Alps) represents a tectonic fragment of the oceanic lithosphere of the Upper Jurassic - Lower Cretaceous Ligurian- Piedmont basin, a section of the Western Alpine Tethyan Ocean. The Antrona ophiolite occurs at lower structural levels in the Alpine nappe stack and is sandwiched between the overlying continental Monte Rosa Nappe (upper Penninic) and the underlying Camughera-Moncucco continental Unit (middle Penninic). The Monte Rosa Nappe is overlain by the Zermatt Saas ophiolitic Unit. Despite the tectono-metamorphic reworking, the Antrona ophiolite exhibits all typical lithologies of oceanic lithosphere: ultramafic and mafic plutonic rocks, mafic volcanic rocks and deep-sea sediments can be recognized. Several rock types were distinguished among mafic rocks. New findings and inferences are: i) the occurrence of probable relics of magmatic structures in some amphibolites, inferred to be pillow lavas or pillow breccia; ii) the occurrence of lawsonite pseudomorphs-bearing amphibolites, not described so far in the study area as precursor of the origin of epidote-amphibolites the Antrona Valley. A qualitative P-T diagram deduced from the stability conditions of mineral parageneses is presented and compared with published P-T paths for the Antrona and Zermatt-Saas ophiolites. The metamorphic evolution of the studied rocks is characterized by blueschist prograde path followed by high pressure (eclogitic) metamorphic peak. P-T estimates for the metamorphic peak were calculated by the Na-clinopyroxene garnet equilibria and the jadeite content in omphacite. T = 372°C for a nominal pressure of P = 1 GPa and T = 386°C for a nominal pressure of P = 1.5 Gpa were obtained. Jd30 as maximum Jadeite content suggests P > 1 Gpa. Retrograde path, although not well constrained, is dominated by epidote-amphibolite/ amphibolite facies conditions, in accord with published data, differing from those inferred so far for the overlying Zermatt-Saas ophiolite.

Blueschists, eclogites, and decompression assemblages of the Zermatt-Saas ophiolite: High-pressure metamorphism of subducted Tethys lithosphere

The Zermatt-Saas ophiolite of the Swiss Alps represents a complete sequence of Mesozoic Tethys oceanic lithosphere. The ophiolite was subducted during early phases of the Alpine orogeny and the mafic rocks were transformed to eclogites and blueschists. Metabasalts locally preserve pillow structures in which glaucophanite forms rims on eclogitic pillow cores. Omphacite-garnet-glaucophane-epidote-ferroan dolomite-Mg-chloritoid-talc-paragonite-chlorite–rutile form characteristic coeval blueschist- and eclogite-facies assemblages. Omphacite + garnet + glaucophane + epidote + rutile represents an equilibrium assemblage that formed during deformation and in the period when the rocks reached the greatest depth of subduction. In rocks containing this assemblage, an additional significant mineral pair is Mg-chloritoid + talc. Coarse chloritoid ( X Mg ~ 0.45) and talc formed in dispersed clusters after the last penetrative deformation. The assemblage may require > 2.7 GPa pressure to form. It developed at maximum pressure conditions corresponding to the return-point of the ophiolite in the subduction zone. Coarse paragonite and chlorite replaced parts of the earlier formed assemblages and removed free H 2 O from the rocks. Exhumation of the HP to UHP ophiolite rocks was accompanied by development of symplectite rims and other replacement products along grain boundaries of the eclogite minerals by decompression reactions in a fluid-deficient regime. Particularly noteworthy is the formation of margarite, paragonite, chlorite, albite, barroisite, and preiswerkite. The latter mineral, a very rare Na-biotite, formed as a result of the decomposition of chloritoid + paragonite and is associated with magnetite and hercynite. Omphacite breakdown produced diopside-albite-barroisite symplectites. Calculated equilibrium assemblage phase diagrams for metabasite compositions indicate P-T conditions of ~2.5–3.0 GPa and ~550–600 °C. The conditions of the subduction-related metamorphism denote P and T at the return-point, which coincide with the upper P-T limit of antigorite. Antigorite-serpentinites constitute the largest volume of rocks within the ophiolite. We suggest that the P-T conditions recorded by the exhumed mafic rocks are coupled to those of antigorite breakdown in the serpentinite that released large amounts of dehydration water in the subducted serpentinite slab facilitating exhumation of the Zermatt-Saas eclogites and blueschists.

Reply to Comment by W. Kurz on “Tectonic map and overall architecture of the Alpine orogen”

99We very much welcome the comment by Kurz (2005), whichraises some important questions and allows us to further clari-fy some concepts behind our correlations between Westernand Eastern Alps (Schmid et al. 2004). As a matter of fact, theprincipal question raised by Kurz (2005), namely the possiblecontinuation of the Northpenninic (=Valais) ophiolites andBundnerschiefer, originally defined in the Western Alps(Trumpy 1955, 1960), towards the east and into the area of theTauern window (see Fig. 1 and plate 1 in Schmid et al. 2004)does not concern “details” (Kurz 2005), but represents a firstorder problem in any attempt to propose possible correlationsof tectonic units and paleogeographical domains along theAlpine chain between Nice and Vienna. As correctly stated by Kurz (2005), the Glockner nappe(referred to as “Upper Schieferhulle Unit” by Schmid et al.2004), comprises remnants of an oceanic basement in the senseof a partly incomplete ophiolitic sequence, of course besidesthe volumetrically dominating calcschists referred to as “Bund-nerschiefer” or “schistes lustres” in the Western Alps. Indeedcalcschists and ophiolitic remnants are not diagnostic for theLower Penninic nappes derived from the Valaisan paleogeo-graphical domain, and it is true that they also occur in UpperPenninic ophiolitic units of the Western Alps, derived from thePiedmont-Liguria ocean (such as in the Zermatt-Saas ophio-lites of Western Switzerland and the Avers Bundnerschieferof Eastern Switzerland). However, we do not understand whythe fact that the Matrei zone at the southern rim of the Tauernwindow is occasionally missing, the Glockner nappe oftenbeing in direct contact with the Austroalpine lid, “wouldfavour a Southpenninic (= Piedmont-Liguria) origin of theGlockner nappe” (Kurz 2005). The Matrei zone, which ofcourse also contains calcschists (besides metapelites), is actu-ally defined by the presence of tectonic slivers (or olistolithsaccording to the interpretation of Frisch et al. 1987) of Aus-troalpine derivation (Kurz et al. 1998). This clearly makes theMatrei zone an analogue of the Upper Penninic Arosa zone ofEastern Switzerland (Manatschal et al. 2003), also character-ized by material derived from the Austroalpine domain (asparts of a tectonic melange according to Ludin 1987). In Eastern Switzerland, as well as in the Tauern Window,these Upper Penninic units are considered as an integral partof the Cretaceous-age top-WNW nappe edifice by all workers.However, we differ from many Alpine geologists in that wefollow Froitzheim et al. (1994, 1996) and consider top-WNWthrusting during Cretaceous orogeny as being very distinctfrom top-N thrusting during Tertiary orogeny, the two oroge-nies being separated by a Late Cretaceous (syn-Gosau) exten-sional event. It is Tertiary orogeny that led to the nappe stack-ing of the Lower Penninic (e.g. Glockner nappe), and the Sub-penninic units (e.g. the “Zentralgneise” nappes, see Kurz et al.1998), which constitute the deeper and major part of theTauern window. Hence, contrary to the “classical interpreta-tion” of most Austrian geologists (e.g. Frisch et al. 1987, Kurzet al. 1998), that envisage collision between the Zentralgneiseand the Austroalpine margin to have occurred during Creta-ceous orogeny, we interpret the contact between Glocknernappe and Matrei zone to mark a tectonic contact between twonappe edifices that formed during two distinct orogenies (Cretaceous vs. Tertiary). Thereby we are led by the stronganalogies between the architecture of the Engadine window,which demonstrably was not closed before Tertiary times(Froitzheim et al. 1994), and the Tauern window. This stronganalogy is additionally supported by the results of along-strikereflection seismic data (Pfiffner and Hitz 1997, their line E2)which demonstrate that the Lower Penninic Bundnerschieferof the Engadine window are directly underlain by basement

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

Rubidium-Sr and Sm-Nd isotope data and rare earth element (REE) concentrations of the metasedimentary rocks within the Zermatt-Saas (ZS) 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 8 7 Sr/ 8 6 Sr variations for the ZS 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 8 7 Sr/ 8 6 Sr isotope systematics were nearly completely homogenized for most of the ZS metasediments during early Tertiary metamorphism, probably because they were relatively wet prior to metamorphism. In contrast to the Sr isotope data, REE data and Nd isotope compositions of the ZS metasediments overlap those of average upper continental crust, average shale, and the local nappes, and Nd model ages of the ZS metasediments overlap with those of Variscan-age rocks. These relations suggest that the REEs of the ZS metasediments were not disturbed during high- to ultra-high pressure Alpine metamorphism. Based on REE data, Nd isotope compositions, and mixing models, the ZS metasediments comprise two groups that require distinct source 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.

Metamorphic Processes in Rodingites of the Zermatt-Saas Ophiolites

Three types of rodingites can be distinguished in the serpentinite complex of the Zermatt-Saas ophiolites based on mineral assemblages, texture, and chemical characteristics of bulk-rock and mineral compositions. These three types of rodingites show a genetic relationship on the basis of their mineral evolution. The mineral assemblage Czo-Hgrs-Chl-Di ± Uvr (rodingite I) was developed during ocean-floor metamorphism and represents the earliest rodingitic rock. A second phase of oceanic alteration is characterized by the formation of andradite-rich hydrogarnet. Subsequent progressive metamorphism resulted in formation of vesuvianite and continued formation of hydroandradite, producing the assemblage Hgrs-Vsv-Hadr-Chl-Di (rodingite II). The low-variance mineral assemblage Hadr-Vsv-Chl (rodingite III) represents equilibrium conditions, and the most completely rodingitized rock. Complex variation in chemical composition and mineral assemblages (i.e., distribution of rodingite) is the result of variation in protoliths and degrees of rodingitization.

Burial rates during prograde metamorphism of an ultra-high-pressure terrane: an example from Lago di Cignana, western Alps, Italy

Estimation of burial rates and duration of prograde metamorphism of ultra-high-pressure (UHP) rocks (T=590–630°C, P=2.7–2.9 GPa) of the Zermatt–Saas ophiolite from Lago di Cignana, Italy, may be made through combined Lu–Hf and Sm–Nd garnet geochronology in conjunction with petrologic estimates of the prograde P–T path. We report a Lu–Hf garnet–omphacite–whole-rock isochron age of 48.8±2.1 Ma from the UHP locality at Lago di Cignana, which stands in contrast to the Sm–Nd age of 40.6±2.6 Ma [Amato et al., Earth Planet. Sci. Lett. 171 (1999) 425–438] obtained from the same sample and mineral material. The Sm–Nd and Lu–Hf ages, as well as other ages determined on metamorphic garnet, zircon and white mica [Amato et al., Earth Planet. Sci. Lett. 171 (1999) 425–438; Mayer et al., Eur. Union Geosci. 10 (1999) Abstr. 809; Rubatto et al., Contrib. Mineral. Petrol. 132 (1998) 269–287; Dal Piaz et al., Int. J. Earth Sci. 90 (2001) 668–684] from Lago di Cignana and elsewhere in the Zermatt–Saas ophiolite, lie within a range of ∼50–38 Ma, which we interpret to encompass the duration of prograde metamorphism, and possibly the duration of garnet growth. The difference in measured Sm–Nd and Lu–Hf ages from Cignana can be accounted for by expected core to rim variations in Lu, Hf, Sm, and Nd. The measured yttrium content in garnet, which may be a proxy for Lu, is highest in garnet core and lowest in the mineral rim, generally following a profile that is predicted by Rayleigh fractionation. Preferential enrichment of Lu in the core produces a Lu–Hf age that is weighted toward the older garnet core. Sm–Nd ages, as predicted by Rayleigh fractionation of Sm and Nd during garnet growth, however, reflect later grown garnet as compared to Lu–Hf ages. The difference in Sm–Nd and Lu–Hf ages from a single sample should therefore be a minimum estimate for the duration of garnet growth and prograde metamorphism so long as Sm–Nd and Lu–Hf blocking temperatures were not exceeded for a long period of time. Based on the 12 Myr duration of prograde garnet growth estimated in this study, burial rates for rocks at Lago di Cignana were on the order of 0.23–0.47 cm/yr. These values correlate with continuous shortening rates of 0.4–1.4 cm/yr between the European plate and the African–Adriatic promontory between 50 and 38 Ma, which is on the order of that calculated for plate velocities from plate reconstructions, suggesting that the Zermatt–Saas ophiolite may have remained well-coupled to the down-going slab to UHP conditions.

Tertiary age and paleostructural inferences of the eclogitic imprint in the Austroalpine outliers and Zermatt–Saas ophiolite, western Alps

The Austroalpine Sesia–Lanzo inlier and upper Austroalpine Dent Blanche, Mt. Mary and Pillonet outliers occur on top of the western-Alpine orogenic wedge and, as a whole, override the structurally composite ophiolitic Piemonte zone. Instead, the Mt. Emilius, Glacier–Rafray, Etirol–Levaz and other lower Austroalpine eclogitic outliers are inserted within the Piemonte zone, between its upper (Combin) and lower (Zermatt–Saas) tectonic elements, or within the latter. Rb–Sr dating on phengitic micas show that the eclogitic imprint in the lower Austroalpine outliers, conventionally regarded as Late Cretaceous by comparison with the Sesia–Lanzo inlier, is of Eocene age (49–40 Ma), like the underlying Zermatt–Saas ophiolite (45–42 Ma) between the Aosta valley and Gran Paradiso massif. 40Ar–39Ar plateau ages on the same mica concentrates of the ophiolitic Zermatt–Saas nappe (46–43 Ma) are consistent with Rb–Sr dating, whereas that on the Austroalpine Glacier–Rafray klippe (92 Ma) is influenced by argon excess. The lower Austroalpine outliers underwent the subduction metamorphism concurrently with the Zermatt–Saas nappe, 20–25 Ma later than the eclogitic Sesia–Lanzo inlier and blueschist Pillonet klippe. The temporal gap and present intra-ophiolitic position mean that the lower Austroalpine outliers were probably derived from an intraoceanic extensional allochthon (Mt. Emilius domain) stranded inside the Piemonte–Ligurian ocean far from the Dent Blanche–Sesia domain and Adriatic margin.