Silvretta Nappe Research Articles

Rheology Dependent on the Distance to the Propagating Thrust Tip—(Ultra‐)Mylonites and Pseudotachylytes of the Silvretta Basal Thrust

AbstractTo evaluate how the presence of pseudotachylytes affects the strength of crustal rocks, deformed pseudotachylytes and their relationship with pristine pseudotachylytes at the base of the Silvretta nappe are analyzed. Pseudotachylytes formed associated with high‐stress crystal plasticity (σd > 400 MPa), as indicated by twinned amphiboles in gneisses. Mylonitic quartz clasts enclosed within deformed pseudotachylytes and mylonitic vein‐quartz, hosting folded pseudotachylyte injection veins, reflect creep at lower stresses (ca. 100 MPa) after seismic rupturing. Deformed pseudotachylytes can be crosscut by pristine pseudotachylytes, indicating a second, independent stage of coseismic rupturing after creep. The evidence of dynamic dislocation creep of quartz and the presence of stilpnomelane and epidote associated with all fault rocks indicate similar ambient greenschist facies conditions during all deformation stages. Whereas the intermediate stage of creep is interpreted to represent deformation at large distance to the propagating thrust tip, the pristine pseudotachylytes represent the last stage of rupturing eventually leading to nappe decoupling from its basement. Gneiss clasts in an ultramylonitic matrix (i.e., deformed pseudotachylyte) reveal that pseudotachylytes have a lower strength during creep in relation to the hosting gneisses. In contrast, during coseismic high‐stress crystal plasticity, the coarse gneisses accumulate a higher amount of strain. This strength‐relationship explains that only those rocks rupture, which have not been previously deformed before. The study demonstrates the importance of different strengths of crustal rocks at specific stress‐ and strain‐rate conditions in dependence on the distance to the propagating fault tip.

Cadomian to Cenerian accretionary orogenic processes in the Alpine basement: the detrital zircon archive

New whole-rock geochemical and detrital zircon U–Pb and Lu–Hf data of metasedimentary sequences of the Silvretta Nappe, Orobic Alps, Strona-Ceneri Zone, Gotthard Massif and Venediger Nappe are presented. These units seem to share a common early to middle Paleozoic geological record, which has alternatively been interpreted as the result of intraplate or orogenic processes. Detrital zircon data mainly indicate late Ediacaran to early Ordovician maximum sedimentation ages for the studied sequences, suggesting that they were intimately related to Cadomian and Cenerian orogenic processes along the northwestern Gondwana margin. The common presence of late Ediacaran to Cambrian Cadomian ages associated with variable subchondritic to suprachondritic Lu–Hf compositions points to recycling processes of Cadomian sequences, further supported by geochemical data indicating a relatively low to moderate maturity of sedimentary protoliths. The occurrence of Cenerian arc-related intrusions in Austroalpine and South Alpine basement unit points to an arc/back-arc position in the early Paleozoic Cenerian orogen, except for the Strona-Ceneri Zone, which was likely located closer to the forearc region, as indicated by the presence of high-pressure metamorphism. Younger sequences, such as the Landeck Quartz-phyllite, document post-Cenerian sedimentation, whereas those of the Venediger Nappe more likely record the early stages of Variscan subduction, as indicated by Devonian maximum deposition ages.

The pseudotachylytes at the base of the Silvretta Nappe: A newly discovered recent generation and the tectonomometamophic evolution of the Nappe

The base of the Silvretta Nappe (Austroalpine Unit, Eastern Alps) has localized extensive deformation, including the occurrence of multiple generations of pseudotachylytes, alternating with (ultra)mylonites, in the host amphibolite and gneiss. Previous works attributed the formation of pseudotachylytes and associated ultramylonites to the Eoalpine deformation phase (mid. Cretaceous). In this work, we report the presence of a younger generation of pseudotachylytes, which overprints the previously characterized pseudotachylyte-ultramylonite association. A detailed petrographic study of selected samples from the Jamtal, Tyrol (Austria), provides further constraints on the possible tectonic evolution of the Silvretta Nappe.Contrary to the Eoalpine pseudotachylytes, the younger pseudotachylytes are not completely recrystallized, foliated, and epidotized, but rather preserve the typical features of frictional melts (e.g., microlites, glassy groundmass, evidence of melt immiscibility, etc.). This suggests formation in relatively shallow conditions, rather than under greenschist facies, as assumed for the Eoalpine pseudotachylytes. The “young” pseudotachylytes, commonly discordant to the foliation, thus likely formed during the final subduction of the Penninic Unit in the Paleogene. This occurrence lends further support to the localization of deformation at the base of the upper plate, in this case represented by the Silvretta Nappe, as observed in other portions of the Alps.

Stresses during pseudotachylyte formation - Evidence from deformed amphibole and quartz in fault rocks from the Silvretta basal thrust (Austria)

Pseudotachylyte-bearing, amphibole-rich gneisses with quartz-rich layers concordant to the foliation from the base of the Silvretta nappe were analyzed by polarized light microscopy and scanning electron microscopy to constrain the deformation history. Amphibole shows microfractures, kink bands and mechanical twins associated with pseudotachylytes. (−101)[101] twins document high differential stresses of ≥ 400 MPa. Quartz grains in contact with twinned amphibole in gneisses show subbasal deformation lamellae, short-wavelength undulatory extinction and new grains along cracks with diameters of < 15 μm and crystallographic preferred orientation (CPO) controlled by the orientation of host grains. This quartz microstructure indicates dislocation glide-controlled deformation coeval with high-stress amphibole deformation and pseudotachylyte formation.Quartz-rich layers concordant to the gneiss' foliation commonly show evidence of dislocation creep by homogenous recrystallized grain aggregates with CPO indicating <a> dislocation glide. Systematic differences in distribution of the recrystallized grains and grain sizes, indicate deformation at differential stresses from a few tens to hundred MPa. Yet, no evidence of increasing strain towards the contact to amphibole-rich gneisses is observed. In contrast, a decreasing number of recrystallized grains towards the contact occurs locally. In relation to pseudotachylytes, the recrystallized quartz is affected by shear fractures offsetting the recrystallized microstructure. This, and the missing or decreasing strain gradient of dislocation creep towards the contact to amphibole-rich gneisses, indicate that 1) dislocation creep in quartz-rich layers is preceding and 2) not directly related to the high-stress and high strain-rate deformation, ruling out a major influence of thermal runaway for the formation of the pseudotachylytes. The differences in the deformation behavior is interpreted to be controlled by the distance to the propagating fault tip. This study demonstrates the potential of microstructures to constrain the deformation history controlling seismic faulting and creep at hypocentral depth not directly accessible for in-situ stress measurements.

The structural evolution of the Albula Pass region, Graubünden, eastern Switzerland: the origin of the various vergences in the structure of the Alps

The Albula Pass region lies between the Lower Austroalpine Err Nappe and the Middle Austroalpine Silvretta Nappe. They will be treated here as the frame of the non- to gently metamorphic sedimentary units between the two during the Alpide times. Sedimentation started on a metamorphic Hercynian basement during the latest Carboniferous(?) and continued into the Permian. Then a sequence from the Alpine Buntsandstein to the medial Jurassic to early Cretaceous Aptychenkalk (=Maiolica) and radiolarites were deposited in an environment of rifting and subsidence. The succeeding Palombini clastics were laid down after the Aptychenkalk and mark the onset of shortening in the Alpine realm. The initial structures that formed were at least two north-dipping normal faults which formed before the deposition of the Jurassic sedimentary rocks. When shortening set in, the first structure that came into being was the south-vergent Elalbula Nappe, bending the normal faults into close antiforms. It became further dismembered into two pieces creating parts of the future Ela and Albula nappes in the Albula region. This motion was later reversed, when the entire ensemble became bulldozed by the immense body of the Silvretta Nappe along numerous, closely spaced thrust faults, some of which only very locally followed horizontal bits of the old normal faults, but in principle they determined their own course. No evidence for westerly motion could be identified, although microstructures in the structural fabric were not studied. The reason for this may be the pre-orogenic fabric in the bounding tectonic units.

The Schwarzhorn Amphibolite (Eastern Rätikon, Austria): an Early Cambrian intrusion in the Lower Austroalpine basement

Abstract The Alpine nappe stack in the Penninic-Austroalpine boundary zone in the Rätikon (Austria) contains a 4×1 km tectonic sliver of meta-diorite, known as the Schwarzhorn Amphibolite. It was deformed and metamorphosed in the amphibolite facies and is unconformably overlain by unmetamorphic Lower Triassic sandstone, indicating pre-Triassic metamorphism. Cataclastic deformation and brecciation of the amphibolite is related to normal faulting and block tilting during Jurassic rifting. Zircon dating of the Schwarzhorn Amphibolite using LA-ICP-MS gave a U-Pb age of 529+9/-8 Ma, interpreted as the crystallization age of the protolith. Geochemical characteristics indicate formation of the magmatic protolith in a supra-subduction zone setting. The Cambrian protolith age identifies the Schwarzhorn Amphibolite as a pre-Variscan element within the Austroalpine basement. Similar calc-alkaline igneous rocks of Late Neoproterozoic to Early Cambrian age are found in the Upper Austroalpine Silvretta Nappe nearby and in several other Variscan basement units of the Alps, interpreted to have formed in a peri-Gondwanan active-margin or island-arc setting.

The Arosa zone in Eastern Switzerland: oceanic, sedimentary burial, accretional and orogenic very low- to low grade patterns in a tectono-metamorphic mélange

In the area of Arosa–Davos–Klosters (Eastern Switzerland) the different tectonic elements of the Arosa zone mélange e.g. the Austroalpine fragments, the sedimentary cover of South Penninic ophiolite fragments, as well as the matrix (oceanic sediments and flysch rocks) show distinctively different metamorphic histories and also different climaxes (“peaks”) of Alpine metamorphism. This is shown by a wealth of Kübler-Index, vitrinite and bituminite reflectance measurements, and K-white mica b cell dimension data. At least six main metamorphic events can be recognized in the area of Arosa–Davos–Klosters: (1) A pre-orogenic event, typical for the Upper Austroalpine and for instance found in the sediments at the base of the Silvretta nappe but also in some tectonic fragments of the Arosa zone (Arosa zone mélange). (2) An epizonal oceanic metamorphism observed in the close vicinity of oceanic basement rocks units of the Arosa zone (South Penninic) is another pre-orogenic process. (3) A metamorphic overprint of the adjacent Lower Austroalpine nappes and structural fragments of the Lower Austroalpine in the Arosa zone. This metamorphic overprint is attributed to the orogenic metamorphic processes during the Late Cretaceous. (4) A thermal climax observed in the South Penninic sediments of the Arosa zone can be bracketed by the Austroalpine Late Cretaceous event (3) and the middle Tertiary event (5) in the Middle Penninic units and predates Oligocene extension of the “Turba phase”. (6) North of Klosters, in the northern part of our study area, the entire tectonic pile from the North Penninic flysches to the Upper Austroalpine is strongly influenced by a late Tertiary high-grade diagenetic to low-anchizone event. In the Arosa zone mélange an individual orogenic metamorphic event is evidenced and gives a chance to resolve diagenetic–metamorphic relations versus deformation. Six heating episodes in sedimentary rocks and seven deformation cycles can be distinguished. This is well explained by the propagation of the Alpine deformation front onto the foreland units. Flysches at the hanging wall of the mélange zone in the north of the study area (Walsertal zone) show data typical for low-grade diagenetic thermal conditions and are therefore sandwiched between higher metamorphic rock units and separated from theses units by a disconformity. The Arosa zone s.s., as defined in this paper, is characterised by metamorphic inversions in the hanging wall and at the footwall thrust, thus shows differences to the Walsertal zone in the north and to the Platta nappe in the south.

Timing of Palaeozoic magmatism in the Maggia and Sambuco nappes and paleogeographic implications (Central Lepontine Alps)

Magmatic rocks from the pre-Mesozoic basements of the Sambuco and Maggia nappes have been dated by U–Pb zircon ages with the LA-ICPMS technique. Several magmatic events have been identified in the Sambuco nappe. The mafic banded calc-alkaline suite of Scheggia is dated at 540 Ma, an age comparable to that of mafic rocks in the Austroalpine Silvretta nappe. The Sasso Nero peraluminous augengneiss has an age of 480–470 Ma, like many other “older orthogneisses” in Alpine basement units. It hosts a large proportion of inherited zircons, which were dated around 630 Ma, a Panafrican age indicating the Gondwanan affiliation of the Sambuco basement. The calc-alkaline Matorello pluton yielded ages around 300 Ma, similar to numerous Late Carboniferous intrusions in other basement units of the Lower Penninic (Monte Leone, Antigorio, Verampio) and Helvetic domains (Gotthard and other External Crystalline Massifs). Associated lamprophyric dykes are slightly younger (300–290 Ma), like similar dykes sampled in gneiss blocks included in the sedimentary cover of the underlying Antigorio nappe (290–285 Ma). The Cocco granodiorite and Ruscada leucogranite, both intruding the basement of the neighbouring Maggia nappe, yielded ages of ca. 300–310 Ma, identical within errors to the age of the Matorello pluton. They are significantly older than former age determinations. This age coincidence, coupled with remarkable petrologic similarities between the Cocco and Matorello granodiorites, strongly suggests paleogeographic proximity of the Sambuco and Maggia nappes in Late Carboniferous times. In recent publications these two nappes have been interpreted as belonging to distinct Mesozoic paleogeographic domains: “European” for Sambuco and “Brianconnais” for Maggia, separated by the “Valais” oceanic basin. In this case, the similarity of the Matorello and Cocco intrusions would demonstrate the absence of any significant transcurrent movement between these two continental domains. Alternatively, according to a more traditional view, Sambuco and Maggia might belong to a single large Alpine tectonic unit.

U-Pb zircon age of volcaniclastic layers in Middle Triassic platform carbonates of the Austroalpine Silvretta nappe (Switzerland)

We present precise U-Pb age determinations from two volcaniclastic layers within Middle Triassic carbonates in the Upper Austroalpine Silvretta nappe near Davos (Switzerland). The two volcaniclastic layers were dated using annealing-leaching techniques and yielded ages of 240.91 ± 0.26 Ma (Prosanto Formation) and 239.89 ± 0.21 Ma (Altein Formation), respectively. The high resolution ages allow comparison of the Upper Austroalpine record of the Ducan with sections in the Southern Alps. The upper Prosanto Formation is, thus, equivalent to the middle part of the Buchenstein Formation (Middle Pietra Verde, Earliest Ladinian), and the Altein Formation is equivalent to the upper part of the Buchenstein Formation in the section with the Global boundary Stratotype Section and Point (GSSP) for the base of the Ladinian (Bagolino, northern Italy). This study demonstrates that we can use precise, accurate and carefully intercalibrated U-Pb zircon ages from volcaniclastic layers to infer the stratigraphic position of their host sediments on zone level. The older volcaniclastic layer (240.91 ± 0.26 Ma) allows a precise age determination (earliest Ladinian) for the marine vertebrate beds in the upper Prosanto Formation.

Structure of Austroalpine and Penninic units in the Tilisuna area (Eastern Rätikon, Austria): Implications for the paleogeographic position of the Allgäu and Lechtal nappes

This study presents new data related to the architecture and evolution of the Penninic-Austroalpine plate boundary in the vicinity of the Prattigau halfwindow (eastern Switzerland). The Tilisuna area in the northeastern corner of the Prattigau halfwindow provides a structural section from the Middle Penninic Sulzfluh nappe through various South Penninic and Lower Austroalpine units up to the Upper Austroalpine Phyllitgneis zone. The Tertiary age basal thrust of the units derived from the Apulian margin over the Penninic domain is located at the top of the lowermost subunit of the Arosa zone, which represents the remnants of the South Penninc ocean. The hanging wall of this basal thrust is formed by a nappe pile of higher South Penninic, Lower Austroalpine and Upper Austroalpine units that was already assembled in Late Cretaceous times. It is shown that the Cretaceous nappe stack beneath the Phyllitgneis zone in the study area is identical to that found below the Silvretta nappe southeast of the Prattigau halfwindow. In this scheme the Arosa Dolomites nappe represents the southern prolongation of the Allgau nappe of the Northern Calcareous Alps. The results confirm earlier petrographic studies proposing that the Phyllitgneis zone and the Silvretta nappe are not separated by an important Alpine fault. Instead, the Phyllitgneis zone and the Silvretta nappe represent one and the same unit. Accordingly, the deposition area of the western Calcareous Alps (Lechtal- and Allgau nappes) was located north of the Silvretta nappe.

Petrogenesis and geotectonic setting of ultramafic rocks in the Eastern Alps: constraints from geochemistry

In the Eastern Alps, a heterogeneous pile of Pre-Mesozoic and Mesozoic nappes was emplaced during several distinct orogenic events. Ultramafic rocks are preserved in: (1) Pre-Mesozoic basement units of the Penninic Tauern Window and in Austroalpine basement complexes, and (2) in Mesozoic units within Penninic windows and in overlying Austroalpine segments. Geochemical data (major, trace and rare earth elements, REE, of bulk rock samples) of ultramafic rocks from 18 regions covering all the major tectonic units are presented. Metamorphic olivine-rich and clinopyroxene-rich ultramafic rocks and serpentinites from the Cambro-Ordovician Lower Schist cover units of the Tauern Window (Stubach Group, “Greiner Series”) are grouped into peridotites (#Mg values=molecular 100×Mg/(Mg+Fe tot), 87–92; <2 wt.% Al 2O 3) and clinopyroxene-rich pyroxenites (#Mg=80–92; 1–3 wt.% Al 2O 3). Major and trace element characteristics indicate a residual origin of most peridotites as restites after 10–20% partial melting. Most probably, the pyroxenites are crystal segregates from a basaltic melt in a mantle transition zone. Small lenses of completely serpentinised ultramafic rocks in the Cambrian Habach Group (#Mg=90–92) also have residual character, although they are strongly metasomatised. The Stubach and Habach Groups developed in a back arc and volcanic arc setting, respectively, along the northern margin of Gondwana. Metamorphic harzburgite and dunite (#Mg=89–90; 0.5–2.4 wt.% Al 2O 3) veined by metagabbroic dikes were sampled in the Austroalpine Silvretta nappe (Hochnörderer). The harzburgites represent restites after 12% partial melting in suboceanic mantle, most probably during the Cambrian. In the Austroalpine basement east of the Tauern Window, metamorphosed ultramafic rocks occur in association with amphibolite and eclogite in the Proterozoic (about 750 Ma) Speik Complex. Harzburgite and dunite (#Mg=89–92; 0.1–0.8 wt.% Al 2O 3) are highly depleted in incompatible elements (2.6–35 ppb Yb) and are interlayered with small amounts of more fertile peridotite (amphibole-rich harzburgite and lherzolite, #Mg=89; up to 3.0 wt.% Al 2O 3), metaclinopyroxenite (#Mg∼85) and hornblendite. Coarse-grained orthopyroxenite (#Mg=89–92; 0.5–0.7 wt.% Al 2O 3; 19–36 ppb Yb) is present as veins and stocks in the harzburgite–dunite sequence. Chondrite-normalised rare earth element patterns of harzburgite–dunite and orthopyroxenite are characteristically U-shaped. The geochemical data indicate that harzburgites and some dunites are products of multiple melting events. They can be modelled as restites after 20–30% partial melting from already depleted mantle in a suprasubduction zone geotectonic setting. Orthopyroxenites crystallised from high-(Si, Mg) melts with a significant crustal component, probably during a Cambrian subduction event. In Mesozoic units of the Penninic windows (Lower Engadine, Tauern and Rechnitz windows), serpentinised ultramafic rocks of harzburgitic composition (#Mg=90–92; 1.1–2.0 wt.% Al 2O 3) are associated with metagabbro, metabasalt, radiolarite and ophicarbonate. Furthermore, serpentinised lherzolites (#Mg=88–90; 2.9–4.6 wt.% Al 2O 3) are exposed in the Matrei Zone (a tectonic mélange zone between Penninic and Austroalpine nappes), in the Lower Austroalpine Reckner Complex and in a body of unclear tectonic affiliation at the southern margin of the Lower Engadine Window at Nauders. The harzburgites are residual mantle left behind after 10–15% partial melting of a fertile source, and form part of the Mesozoic (Jurassic) incomplete ophiolite sequences of the Ligurian–Piemontais ocean. The lherzolites, having experienced less than 10% partial melting, are either attributable to an early (Permian) rifting episode during break-up of Pangaea, or represent subcontinental mantle from the Adria plate tectonically incorporated into the Neo-Tethys.

Combination of Single-Grain Fission-Track Chronology and Morphological Analysis of Detrital Zircon Crystals in Provenance Studies: Sources of the Macigno Formation (Apennines, Italy)

Fission track (FT) analyses on unannealed detrital min- erals provide a powerful tool both for refining provenance models de- rived from traditional methods and for collecting information about erosion rates of the source area. Their power is increased if they are coupled with the study of zircon morphology. This combination of methods is applied to the Chattian-Aquitanian (25-23 Ma) Macigno turbidite complex. Basin-fill patterns and petrographical studies con- sistently identify the uplifting western Central Alps as the main source region for the Macigno Formation. Most zircon grains fall into a young age cluster ( ; 40-30 Ma), de- rived from a rapidly exhuming crystalline source region with a high cooling rate. Within this cluster, two age subgroups can be distin- guished at 30 and 40 Ma. In the younger subgroup, the zircon mor- phology supports the presence of two main populations: (1) from ig- neous rocks (S-type euhedral zircons), which appear to be partly de- rived from airborne tuffs; and (2) from metasedimentary units. In huge volumes of these metamorphic rocks, mica Ar-Ar and zircon fission- track thermochronometers have been reset, because of high geothermal gradients in the vicinity of the Periadriatic intrusives in mid-Oligocene times. At the present surface of the Alps, zircon FT ages around and slightly less than 30 Ma are reported in the Sesia-Lanzo zone, the Gran Paradiso Massif, the Upper Pennine nappes, the Monte Rosa Massif, and the Dent Blanche complex. The older subgroup of the Tertiary zircons (40 Ma) may have been supplied by metamorphic and mig- matitic rocks affected by an Eocene high-temperature phase. A Late Cretaceous age cluster ( ; 70-60 Ma) is related to cooling after the main Austroalpine metamorphic event at 110-100 Ma. Most of the recently exposed Austroalpine nappe complex displays mica cooling ages and zircon FT ages between 95-70 Ma and 99-55 Ma, respectively. Finally, an ill-defined Jurassic age cluster, with a mean in Late Ju- rassic times, is related to rift-shoulder heating of the Austroalpine/ South-Alpine crystalline basement due to rifting of the Pennine oceanic domain. Presently, the Silvretta nappe complex, situated at the western termination of the Austroalpine realm, and the South-Alpine basement west of the Canavese Line, display similar zircon FT ages. Therefore, a westward continuation of the Silvretta complex prior to deep Neo- gene erosion is suggested.

UPb single-zircon dating under cathodoluminescence control (CLC-method) : application to polymetamorphic orthogneisses

The dating of polymetamorphic rocks with the U-Pb zircon method often results in discordant data. Thus a new technique combining cathodoluminescence imaging (CL) and thermal ion mass spectrometry (TIMS) measurements was developed. Cathodoluntinescence-controlled UPb dating ( CLC-method) works with bisected zircons and was successfully applied to the multistage S-type granitoids of the Swiss Silvretta nappe. CLC-method enables the resolution of the different anatectic events and later major overprints (Cambrian, Ordovician, Silurian) and gives information about the protolith (Archaean) of the investigated gneisses. This is achieved by conventional UPb dating of selected grains, recovered from the SEM (scanning electron microscope) mounts after cathodoluminescence (CL) imaging. Applying this new combination of two established techniques yields geologically meaningful UPb zircon ages even for complex polymetamorphic rocks.

Early Cambrian oceanic plagiogranite in the Silvretta Nappe, eastern Alps: geochemical, zircon U-Pb and Rb-Sr data from garnet-hornblende-plagioclase gneisses

Garnet-hornblende-plagioclase gneisses rich in incompatible elements occur in the crystalline basement of the Austro-Alpine Silvretta nappe and are associated with clinopyroxene norites and harzburgite cumulates. It is proposed here that the gneisses were formerly oceanic plagiogranites. An eNd(530) value of +5.6 for the gneisses as well as initial87Sr/86Sr values of 0.7036–0.7037 for the gabbroic rocks and 0.7026–0.7027 for the ultramafic rocks suggest a mantle source for this rock association. The geochemical characteristics of the garnet-hornblende-plagioclase gneisses indicate that their precursors were derived by fractional crystallization from a basaltic parent magma, by the same process which produced the adjacent clinopyroxene norites and ultramafic cumulates as well. The combined U-Pb upper intercept ages of zircons from two gneiss samples yield an igneous crystallization age of 532 ± 30 Ma, similar to previously dated (mostly calcalkaline) orthogneisses in the same area. High-quality transparent zircons showed the least degree of discordance, but contain extremely low U and Pb levels. The rock suite, including this plagiogranite, was emplaced within oceanic crust which formed in the latest Precambrian-early Palaeozoic off the northern continental margin of Gondwana.

U-Pb and Pb-Pb zircon dating of the older orthogneiss suite in the Silvretta nappe, eastern Alps: Cadomian magmatism in the upper Austro-Alpine realm

Two orthogneiss suites dominate the Silvretta nappe. Primary crystallization of the larger suite (‘younger orthogneisses’) is assumed to be Ordovician in age. The second, adjacent magmatic suite consists of older, alkaline to calc-alkaline, ultrabasic, basic to intermediate and granitic rocks known as ‘older orthogneisses’. U-Pb data of multigrain zircon fractions, as well as single zircon stepwise evaporation 207Pb/206Pb results suggest a latest Proterozoic to early Cambrian intrusion age for the protoliths of the ‘older orthogneisses’ as both dating methods yield early Cambrian crystallization ages of 526±7 and 519±7 Ma for an alkaline granite gneiss; similar results were obtained for two neighbouring calc-alkaline orthogneisses (207Pb/206Pb ages of 533 ± 4 and 568 ± 6 Ma, respectively). The crystal habitus corresponds to P5, S19 and S9 zircons of magmatic origin. Whole-rock initial Sr isotope ratios indicate a primitive source. The igneous protoliths of these ‘older orthogneisses’ represent a fragment of a Cadomian (Pan-African) crust found in places within the basement of the European Hercynides.

U–Pb and Pb–Pb zircon dating of the older orthogneiss suite in the Silvretta nappe, eastern Alps: Cadomian magmatism in the upper Austro-Alpine realm

U–Pb and Pb–Pb zircon dating of the older orthogneiss suite in the Silvretta nappe, eastern Alps: Cadomian magmatism in the upper Austro-Alpine realm

Formation of Alpine mylonites and pseudotachylytes at the base of the Silvretta nappe, Eastern Alps

Optical microscopy and transmission electron microscopy (TEM) have been used to investigate deformation-induced microstructures in Alpine pseudotachylytes, mylonites and ultramylonites from the Upper Austro-Alpine Silvretta nappe at the southwestern margin of the Lower Engadine Window in the Eastern Alps of Switzerland and Austria. The earliest Alpine stage of deformation (D1) produced mylonitic gneisses, with dislocation glide being the dominant deformation mechanism in quartz. In the second stage (D2a), mylonites were formed as a result of strain softening in quartz (manifest by dynamic recrystallization and a lattice preferred orientation). TEM observations are used to characterize the initial stages of migration and rotation recrystallization. Pseudotachylytes were generated during D 2b when strain hardening due to dislocation tangling was not outweighed by strain softening. Several generations of mylonites and pseudotachylytes are observed. A model for the propagation of seismic fractures, initiated in comparatively rigid amphibolites, into an otherwise ductile environment is proposed for the pseudotachylytes. As such they are formed as transient discontinuities in the mylonites. D 1 and D 2a/b are related to the Eo-Alpine detachment of the Silvretta nappe. Subsequent Eo-Alpine metamorphism in the investigated area reached the stilpnomelane zone, and this coincided with the development of ultramylonites from both the mylonites and pseudotachylytes during D 3 and peak metamorphism. The increased temperature combined with fluid infiltration from the overridden Penninic strata favoured grain-boundary sliding (superplasticity) as the dominant deformation mechanism during D 3.

Eclogites from the Silvretta nappe (Switzerland): Geochemical constraints on the nature and geotectonic setting of their protoliths

In the Austroalpine Silvretta nappe of central Europe eclogitic rocks of presumed Paleozoic metamorphic age have recently been found. Their chemical composition suggests that they were derived from evolved tholeiitic magmas which have undergone variable degrees of fractionation at shallow depths involving crystallization of plagioclase plus olivine and/or pyroxene. The igneous protoliths probably originated at a constructive plate margin. Although some chemical features closely resemble those of mid-ocean ridge basalts the composition of the metasedimentary country rocks suggests a tectonic setting close to a continental block, perhaps in a back-arc basin situation or incipient oceanic ridge.

Deutung diskordanter Zirkonalter der Silvrettadecke und des Gotthardmassivs (Schweizer Alpen)

U-Pb isotopic analyses are reported for zircons of nine igneous rocks and high grade metamorphic paragneisses from the Silvretta nappe and the Gotthard massif, Switzerland.

Rb-Sr age determinations on orthogneisses of the Silvretta (Switzerland)

Eleven Rb–Sr measurements on total rock samples from granitic and aplitic orthogneisses of the Upper East-Alpine Silvretta nappe give two isochron ages of 428 ± 17 m.y. and 351 ± 14 m.y. The apparent ages are ascribed to intrusions of the Caledonian and Variscan orogeny. Mineral isochrons for individual gneiss samples yield a metamorphic cooling age of 295 ± 12 m.y. Sr evolution diagrams for other samples give mixed mica ages between Variscan and younger Alpine metamorphism. The influence of Alpine metamorphism on micas, the deficit of radiogenic Sr in feldspars, and their deviation from the isochron are discussed.