An eclogite-bearing continental tectonic slice in the Zermatt–Saas high-pressure ophiolites at Trockener Steg (Zermatt, Swiss Western Alps)

Weber, Sebastian; Bucher, Kurt

doi:10.1016/j.lithos.2015.07.010

Cited by 25 publications

(41 citation statements)

References 87 publications

(151 reference statements)

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“…Lenses and blocks of eclogite from some centimetres to many metres in size are common within the Grt‐Ph schists (all abbreviations for mineral names after Whitney & Evans, ). The composition of the TGU eclogites is distinctly different from the eclogites of the ZSU (Weber & Bucher, ). From TGU eclogite assemblages, Weber and Bucher () obtained 2.1 ± 0.1 GPa and 580 ± 65°C for the P–T conditions at maximum depth in the subduction zone.…”

Section: Geological Backgroundmentioning

confidence: 97%

“…The composition of the TGU eclogites is distinctly different from the eclogites of the ZSU (Weber & Bucher, ). From TGU eclogite assemblages, Weber and Bucher () obtained 2.1 ± 0.1 GPa and 580 ± 65°C for the P–T conditions at maximum depth in the subduction zone. Lu‐Hf garnet whole‐rock ages of 56.5 ± 2.7 and 58.2 ± 1.4 Ma for two TGU eclogite samples indicate that eclogite facies metamorphism occurred in the Palaeocene (Weber et al, ).…”

Section: Geological Backgroundmentioning

confidence: 97%

“…The garnet porphyroblasts presented in this paper occur in garnet–phengite schists (GPS) of the Theodul Glacier Unit (TGU) (Weber & Bucher, ), a 2 km long and 100 m thick slab of eclogite facies continental basement rocks within the Zermatt–Saas meta‐ophiolite unit (ZSU) of the Western Alps (Figure ). GPS, eclogite, and biotite gneiss are the tectonically interleaved rock types of the TGU.…”

Section: Geological Backgroundmentioning

confidence: 99%

“…The Mischabel and Monte Rosa nappe are Penninic crystalline basement (European plate), the Dent Blanche nappe represents Austroalpine continental basement (Apulian plate). TGU = Theodul Glacier Unit (Weber & Bucher, ), LdC = Lago di Cignana coesite locality (Reinecke, , ), eclogitic pillow lavas at Pfulwe locality (Bucher et al, ), eclogite facies metagabbro at Allalinhorn (Bucher & Grapes, ). The centre of the TGU outcrops is located at N 46°58′6.24″ and E 7°42′53.28″…”

Section: Geological Backgroundmentioning

confidence: 99%

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Decoding the complex internal chemical structure of garnet porphyroblasts from the Zermatt area, Western Alps

Bucher

Weisenberger

Klemm³

et al. 2019

Journal Metamorphic Geology

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Garnet is a prototypical mineral in metamorphic rocks because it commonly preserves chemical and textural features that can be used for untangling its metamorphic development. Large garnet porphyroblasts may show extremely complex internal structures as a result of a polycyclic growth history, deformation, and modification of growth structures by intra‐ and intercrystalline diffusion. The complex internal structure of garnet porphyroblasts from garnet–phengite schists (GPS) of the Zermatt area (Western Alps) has been successfully decoded. The centimetre‐sized garnet porphyroblasts are composed of granulite facies garnet fragments overgrown by a younger generation of grossular‐rich eclogite facies garnet. The early granulite facies garnet (G‐Grt) formed from low‐P, high‐T metamorphism during a pre‐Alpine orogenic event. The late garnet (E‐Grt) is typical of high‐pressure, low‐temperature (HPLT) metamorphism and can be related to Alpine subduction of the schists. Thus, the garnet of the GPS are polycyclic (polymetamorphic). G‐Grt formation occurred at ~670 MPa and 780°C, E‐Grt formed at ~1.7 GPa and 530°C. The G‐Grt is relatively rich in Prp and poor in Grs, while E‐Grt is rich in Grs and poor in Prp. The Alm content (mol.%) of G‐Grt is 68 of E‐Grt 55. After formation of E‐Grt between and around fragmented G‐Grt at 530°C, the GPS have been further subducted and reached a maximum temperature of 580°C before exhumation started. Garnet composition profiles indicate that the initially very sharp contacts between the granulite facies fragments of G‐Grt and fracture seals of HPLT garnet (E‐Grt) have been modified by cation diffusion. The profiles suggest that Ca did not exchange at the scale of 1 µm, whereas Fe and Mg did efficiently diffuse at the derived maximum temperature of 580°C for the GPS at the scale of 7–8 µm. The Grt–Grt diffusion profiles resulted from spending c. 10 Ma at 530–580°C along the P–T–t path. The measured Grt composition profiles are consistent with diffusivities of log DMgFe = −25.8 m2/s from modelled diffusion profiles. Mg loss by diffusion from G‐Grt is compensated by Fe gain by diffusion from E‐Grt to maintain charge balance. This leads to a distinctive Fe concentration profile typical of uphill diffusion.

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Section: Geological Backgroundmentioning

confidence: 97%

Section: Geological Backgroundmentioning

confidence: 97%

Section: Geological Backgroundmentioning

confidence: 99%

Section: Geological Backgroundmentioning

confidence: 99%

See 2 more Smart Citations

Decoding the complex internal chemical structure of garnet porphyroblasts from the Zermatt area, Western Alps

Bucher

Weisenberger

Klemm³

et al. 2019

Journal Metamorphic Geology

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“…Lago di Cignana unit: Reinecke, ; Groppo et al ., ; Etirol‐Levaz Unit (ELU): Fassmer et al ., and references therein]. A few slices of continental affinity have been reported along this shear zone [ELU, Theodul Glacier Unit (TGU): Weber & Bucher, ; ChU, Chatillon Unit (ChU): Rolfo et al ., ; see also Ballèvre et al ., ; Fig. ].…”

Section: Geological Settingmentioning

confidence: 98%

Fluid pathways and high‐P metasomatism in a subducted continental slice (Mt. Emilius klippe, W. Alps)

Angiboust

Yamato

Hertgen

et al. 2017

Journal Metamorphic Geology

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The Mt. Emilius klippe (Western Alps, Italy) corresponds to a segment of the stretched Adriatic continental margin metamorphosed at granulite facies during Permian. This slice was subducted during the early Cenozoic Alpine subduction with the underlying eclogite facies remnants of the Tethyan seafloor (Zermatt‐Saas zone). Near the base of the Mt. Emilius massif, there is a shear zone with eclogite facies hydrofracture systems associated with deformation‐induced re‐equilibration of granulites during high‐P metamorphism. In the basal part of the massif, a pluri‐hectometre domain of sheared mafic boudins is hosted in the granulitic paragneiss. In these mafic boudins, there are garnetites, garnet veins and clinopyroxenites, as well as clinozoisite and calcite veins. These features record multiple events of fracture opening, brecciation, boudinage and parallelization of structures coevally with fluid–rock interaction, metasomatism and volume change. This integrated petrological, micro‐textural and geochemical investigation illustrates the multiplicity and the chemical variability of fluid sources during prograde to peak metamorphic evolution in the lawsonite–eclogite‐facies field (at ~2.15–2.4 GPa, 500–550 °C) during subduction of the Mt. Emilius slice. The calcite veins crosscutting the garnetites have relatively low δ18OVSMOW values (∼+6.5‰) near those for marble layers (and nearby calcsilicates) embedded within the metasomatized granulites (+8 to +10‰). It is proposed that infiltration of externally‐derived H2O‐rich fluids derived from the plate interface flushed the marbles, promoting decarbonation followed by short‐distance transport and re‐precipitation along garnetite fractures. This study highlights the importance of inherited structural heterogeneities (such as mafic bodies or sills) in localizing deformation, draining fluids from the downgoing plate and creating long‐lasting mechanical instabilities during subduction zone deformation.

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Nappe stack of Piemonte–Ligurian units south of Aosta Valley: New evidence from Urtier Valley (Western Alps)

Ellero

Loprieno²

2017

Geological Journal

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New results of structural and lithostratigraphic investigations performed in the Urtier Valley, located south of Aosta Valley at the northern flank of the Gran Paradiso Massif, shed new light on the nappe stack architecture of the Western Alps. Two distinct Piemonte–Ligurian units were recognized in the area: (a) the blueschist facies Broillot unit, with a well‐preserved lithostratigraphic succession that includes ophiolites and their metasedimentary cover and (2) the eclogitic Bardonney unit with clasts and/or slices of ophiolites embedded in a carbonate/quarzitic and/or metabasic matrix. Three major deformation stages have been recognized (D1, D2, and D3). D1 phase occurred during the peak metamorphic conditions (i.e., eclogite or blueschist facies). D2 and D3 phases are related to multistage exhumation processes, from early stacking to late refolding of former nappe contacts. Despite the intensity of deformation and metamorphism, the original lithostratigraphic succession of the Broillot unit allowed us to use a lithostratigraphic approach to reconstruct the structures in the study area. For the first time, differences in the nappe stack exposed south and north of the Aosta Valley are reported. In fact, contrary to what is observed in the north, the eclogitic units exposed in the Urtier Valley lay above and not below the blueschist units. A new model for the tectonic evolution, which ranges from stages of subduction to large‐scale nappe refolding, is proposed. It is able to explain the difference in the architecture of the north and south of the Aosta Valley.

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An eclogite-bearing continental tectonic slice in the Zermatt–Saas high-pressure ophiolites at Trockener Steg (Zermatt, Swiss Western Alps)

Cited by 25 publications

References 87 publications

Decoding the complex internal chemical structure of garnet porphyroblasts from the Zermatt area, Western Alps

Decoding the complex internal chemical structure of garnet porphyroblasts from the Zermatt area, Western Alps

Fluid pathways and high‐P metasomatism in a subducted continental slice (Mt. Emilius klippe, W. Alps)

Nappe stack of Piemonte–Ligurian units south of Aosta Valley: New evidence from Urtier Valley (Western Alps)

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