Dating eclogite-facies metamorphism in the Eastern Alps – approaches, results, interpretations: a review (original) (raw)
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Tectonophysics, 2004
New 40 Ar/ 39 Ar geochronology places time constraints on several stages of the evolution of the Penninic realm in the Eastern Alps. A 186F2 Ma age for seafloor hydrothermal metamorphic biotite from the Reckner Ophiolite Complex of the Pennine-Austroalpine transition suggests that Penninic ocean spreading occurred in the Eastern Alps as early as the Toarcian (late Early Jurassic). A 57F3 Ma amphibole from the Penninic subduction-accretion Rechnitz Complex dates high-pressure metamorphism and records a snapshot in the evolution of the Penninic accretionary wedge. High-pressure amphibole, phengite, and phengite+paragonite mixtures from the Penninic Eclogite Zone of the Tauern Window document exhumation through V15 kbar and N500 8C at~42 Ma to~10 kbar and~400 8C at~39 Ma. The Tauern Eclogite Zone pressure-temperature path shows isothermal decompression at mantle depths and rapid cooling in the crust, suggesting rapid exhumation. Assuming exhumation rates slower or equal to high-pressure-ultrahigh-pressure terrains in the Western Alps, Tauern Eclogite Zone peak pressures were reached not long before our high-pressure amphibole age, probably at V45 Ma, in accordance with dates from the Western Alps. A late-stage thermal overprint, common to the entire Penninic thrust system, occurred within the Tauern Eclogite Zone rocks at~35 Ma. The high-pressure peak and switch from burial to exhumation of the Tauern Eclogite Zone is likely to date slab breakoff in the Alpine orogen. This is in contrast to the long-lasting and foreland-propagating Franciscanstyle subduction-accretion processes that are recorded in the Rechnitz Complex. D
Mineralogy and Petrology, 2003
A petrological study has been undertaken in the Austroalpine basement units, along a SSE-NNW pro®le across the northern parts of the Saualpe Complex in the Seetal Alps, the Micaschist-Marble Complex (Rappold and W olz Units) in the W olz Tauern and the Speik Complex with Permian cover sequences at Hochgr oûen Massif in the Rottenmann Tauern. With exception of the Permian cover rocks all units experienced Pre-Alpine as well as Eo-Alpine metamorphism. Relicts of early Variscan eclogite facies metamorphism (397 Ma) are preserved in the Speik Complex in the Hochgr oûen Massif. Different compositional zoning and inclusion patterns in garnet porphyroblasts from the Rappold and W olz Units allow to distinguish two Pre-Alpine metamorphic events: a Permian ($ 250 Ma) low-P=high-T and an Upper Carboniferous (330 Ma?) medium-pressure metamorphism of amphibolite facies conditions. Eo-Alpine metamorphism ($ 100 Ma) is characterized by a southward grading from upper greenschist facies (510 AE 10 C and 0.8 GPa) in the Permian cover sequences in the north, through amphibolite facies (600±650 C and 1.0±1.1 GPa) in the W olz and Rappold Units to eclogite facies (690 AE 20 C and 1.8 AE 0.1 GPa) in the Saualpe Complex in the south. The P±T conditions of Eo-Alpine metamorphism estimated for the different Austroalpine units indicate a subduction type geothermal gradient of 10±16 C=km. The rocks that formed at different P±T conditions are now exposed within south dipping thrust units in the Eastern Alps.
Mafic rocks containing eclogite relics are fairly widespread in the crystalline nappe Stack of lhe Swiss Central Alps. This study addresses the spatial distribution of eclogite relics in the Central Alps, their field relations. structural and petrological characteristics. and their PTt-history. Implications for the assembly of the nappe Stack are explored. The majority of eclogite-facies relics is confined lo a single super-unit of tectonic melange. interpreted as a tec¬ tonic accretion Channel (TAC). Numerous mafic high-pressure (HP) lenses have been discovered through systematic fieldwork in the TAC units of the Central Alps, an up-lo-date inventory of which is presented. Systematic documentation of select samples with HP imprint yields clockwise PT-paths. Prograde phase relations are seldom preserved. except in the chemical zoning of garnet porphyroblasts. However. when present. relic assemblages indicate HP-LT conditions indicative of a subduction setting. Maximum recorded pressures are substantially different from one loca¬ tion to the next (1.9 to 3.3 GPa). Depending on the degree of rehydration. reaction sequences are derived from observed relics. local replacement relations and assemblages. Quantitative constraints on the detailed PT-path are extracted by combining isochemical phase diagrams and 7Wß-thermobarometry with petrographie information. HP lenses from different locations display substantially different paths. both within and between different melange zones of the TAC. PT-conditions reflecting the late-Alpine Barrovian overprint of mafic HP lenses are in agreement with the coherent regional pattern derived from metasediments. i.e.. maximum temperatures (-600°C in the central Lepontine belt. 700-750°C in the southern parts) were reached at pressures between 0.75 and 0.55 GPa Four samples have been dated by Lu-Hf isotopic analysis of garnet. clinopyroxene. malrix phases and wholerock powders.The age span Covers a ränge from >70 to-.36 Ma.much larger than previously documented for Alpine HP rocks from the Central Alps. Petrological data of the samples and their Lu-Hf isotopic system indicate a protracted HP historv for at least some of the sub-units of the TAC. with garnet growth under eclogite-facies conditions starting before 70 Ma in some parts of the TAC. and continuing as late as 36 Ma in others. Iliese data have implications for the dynamics of melange formation within the TAC with internal fragmentation and mixing. and pronounced mobility of lhe tectonic zones, probably during the early. subduclional stages and again during the post-collisional extrusion along the plate boundary. After 32 Ma. when the Barrovian overprint reached its maximum temperature. the TAC appears to have been exhumed as part of the then-coherenl crystalline nappe Stack.
Evidence for deep subduction of Austroalpine crust (Texel Complex, NE Italy)
Rendiconti Lincei, 2013
We report here the discovery of UHP eclogites in the Ulfas Valley within the Texel Complex (Meran, NE Italy), which has important implications on the evolution of the Austroalpine nappe stack in the Eastern Alps. Exsolved K-feldspar and phengite lamellae within omphacite relics, together with an unusually high K-content of the host clinopyroxene point to equilibrium pressures at least close to the quartz/coesite transition. The age of the high pressure metamorphism is constrained by U-Pb dating of metamorphic zircon at 85 ± 4 Ma. These new data suggest pressure conditions for the Cretaceous eclogitic metamorphism of the Texel Complex higher than hitherto reported data, possibly indicating that several sectors of the Africa-Adria derived Austroalpine margin were subducted to mantle depths during the early stages of the Alpine orogeny.
Geologische Rundschau, 1990
With 8 figures and 3 tables Zusammenfassung Basanitische Ganggesteine (Lokalbezeichnung Ehrwaldite) treten in einer schmalen, hber fiinfzig Kilometer langen, Ost-West verlaufenden Zone der Lechtal-Decke in den N6rdlichen Kalkalpen auf. Die Ganggesteine durchschlagen mesozoische Sedimente bis zur Unterkreide, welche in unmittelhater Umgebung der G~nge kontakt-metamorphisiert wurden. Die basanitischen Gesteine enthalten neben zum Tell noch frischem Olivin (fo 91-87) wechsetnde Anteile yon Titanaugit, Kaersutit, Titanbiotit und Zeolithen. Nebengemengteile sind Titanomagnetit, Picotit, Apatit, Ilmenit, Karbonat und Sulfide. Mantelxenolithe und Mantelxenokristalle k~Snnen his zu fihnf Prozent des Gesteins ausmachen. Die Gesamtgesteinschemie der Ehrwaldite belegt deren Charakter als primitive Mantelschmelzen und ist typisch fiir Nephelinbasanite (ne = 11-17 Gew.%, mg-Zahl = 74-78). Alle Spurenelementgehalte der Ehrwaldite stimmen ausgezeichnet mit denjenigen yon Basaniten und Olivinnepheliniten terti~irer und quart~rer Vulkanprovinzen in Europa iiberein. Dasselbe gilt fiir die Isotopencharakteristik der G~nge (mittleres end yon + 4.7, mittleres eSr yon-8.2). Die Daten belegen ~hnliche Mantelausgangsgesteine und Schmelzcharakteristika wie fi~r andere europ~iische Alkaligesteine und damit Ahnlichkeiten des europ~iischen subkontinentalen Mantels an verschiedenen Orten fiir den Zeitraum der letzten 100 Millionen Jahre. Die Isotopendaten stehen im eindeutigen Gegensatz zur Isotopencharakteristik des penninischen subozeanischen Mantels. Geochemische Daten und laufende Piston Cylinder Experimente ergeben eine Ursprungstiefe der Ehrwaldite als Mantelteilschmelzen yon > 80 km bei Temperaturen yon mindestens 1250 ~ K-At Gesamtgesteinsalter der Ehrwaldite an petrographisch verschiedenen Proben aus verschiedenen Aufschliissen ergeben sehr konsistente Alter von ca. 100 Millionen Jahren (oberes Alb). Nach ihrer Platznahme erlitten die Ehrwaldite Bedingungen der ,,Diagenesezone-mit Temperaturen um 120 ~ wobei die prim;ir gebildeten Zeolithe hberlebten.
Contributions to Mineralogy and Petrology, 1994
The 4~ 39Ar degassing spectra of white micas and amphiboles from three tectonic units of the central Tauern Window (Pennine basement and cover in the Eastern Alps) have been measured. White micas are classified as (1) pre-Alpine low-Si relic micas with an age value of 292 Ma, variously disturbed by the Alpine metamorphism; (2) Alpine phengitic micas of variable composition with an age between 32 and 36 Ma; (3) Alpine low-Si micas with a maximum age of 27 Ma. We attribute the higher Alpine ages to a blueschist facies event, whereas the lower age reflects the late cooling of the nappe pile. Blueschist facies phengites from the basement (Lower Schieferhfille) and the tectonic cover (Upper Schieferh~lle) crystallized at a temperature below the closure temperature (To) for argon diffusion in white mica and record ages of 32 to 36 Ma. At the same time a thin, eclogite facies unit (Eclogite Zone) was thrust between the Lower and the Upper Schieferhfille and cooled from eclogite facies conditions at about 600~ at 20kbar to blueschist facies conditions at 450~ or even 300~ at >10kbar. Eclogite facies phengites closed for argon diffusion and record cooling ages, coinciding with the crystallization ages in the hanging and the footwall unit. Amphibole age spectra (actinolite, glaucophane, barroisite) are not interpretable in terms of geologically meaningful ages because of excess argon.
Pre-Alpine contrasting tectono-metamorphic evolutions within the Southern Steep Belt, Central Alps
Lithos, 2018
In the Southern Steep Belt, Italian Central Alps, relicts of the pre-Alpine continental crust are preserved. Between Valtellina and Val Camonica, a poly-metamorphic rock association occurs, which belongs to the Austroalpine units and includes two classically subdivided units: the Languard-Campo nappe (LCN) and the Tonale Series (TS). The outcropping rocks are low to medium grade muscovite, biotite and minor staurolite-bearing gneisses and micaschists, which include interlayered garnet- and biotite-bearing am- phibolites, marbles, quartzites and pegmatites, as well as sillimanite-bearing gneisses and micaschists. Permian intrusives (granitoids, diorites and mi- nor gabbros) emplaced in the metamorphic rocks. We performed a detailed structural, petrological and geochronological analysis focusing on the two main lithotypes, namely, staurolite-bearing micaschists and sillimanite-bearing paragneisses, to reconstruct the Variscan and Permian-Triassic history of this crustal section. The reconstruction of the tectono-metamorphic evolution allows for the distinction between two different tectono-metamorphic units during the early pre-Alpine evolution (D1) and predates the Permian intrusives, which comprise rocks from both TS and LCN. In the staurolite-bearing micaschists, D1 developed under amphibolite facies conditions (P=0.7- 1.1 GPa, T=580-660◦C), while in the sillimanite-bearing paragneisses formed under granulite facies conditions (P=0.6-1.0 GPa, T>780◦C). The two tectono- metamorphic units coupled together during the second pre-Alpine stage (D2) under granulite-amphibolite facies conditions at a lower pressure (P=0.4-0.6 GPa, T=620-750◦C) forming a single tectono-metamorphic unit (Languard-Tonale Tectono-Metamorphic Unit), which comprised the previously distin- guished LCN and TS. Geochronological analyses on zircon rims indicate ages ranging between 250 and 275 Ma for D2, contemporaneous with the emplacement of Permian intrusives. This event developed under a high thermal state, which is compatible with an extensional tectonic setting that occurred during the exhumation of the Languard-Tonale Tectono-Metamorphic Unit. The extensional regime is interpreted as being responsible for the thinning of the Adriatic continental lithosphere during the Permian, which may be related to an early rifting phase of Pangea.
Contrasting Eoalpine P-T evolutions in the southern Koralpe, Eastern Alps
Mineralogy and Petrology, 1997
The Koralpe crystalline complex and the Plankogel unit represent two lithologically distinct units within the Koralpe region of the southeasternmost Austroalpine crystalline basement. The Eoalpine P-T evolution of these two units is derived from new petrographical data. The Plankogel unit and the Koralpe crystalline complex show markedly different P-T evolutions during the early stages of the Eoalpine event. The rocks of the Koralpe crystalline complex experienced eclogite facies conditions with minimum pressures in the range of 15-16 kbar and temperatures in excess of 700 °C. At the same time the Plankogel unit resided in a shallower environment at pressures of 10-11 kbar and temperatures of less than 600 °C. The tectonic emplacement of the Plankogel unit into its present position on top of the Koralpe crystalline complex took place after the eclogite facies event in a relatively shallow crustal level. After their juxtaposition the Koralpe crystalline complex and the Plankogel unit were affected by a common amphibolite facies metamorphic overprint. The distinctly different P-T evolution during the early stages of the Eoalpine event and a common history at later stages imply that major tectonic processes were operative in this part of the Austroalpine crystalline basement during the Cretaceous. Such processes may have involved subduction of oceanic and continental lithosphere which may have lead to significant crustal shortening within the Austroalpine basement. Zusammenfassung Unterschiedliche Eoalpine P-T Entwicklungen in der Siidlichen Koralpe, Ostalpen Das Koralpenkristallin und die Plankogelserie stellen zwei unterschiedliche lithologische Einheiten in der stidlichen Koralpe des ostalpinen Kfistallins dar. Die P-T Entwicklung dieser beiden Einheiten w~hrend der Eoalpinen Metamorphose wurde anhand neuer petrographischer Daten abgeleitet. Das Koralpenkristallin und die Plankogelserie zeigen deutlich unterschiedliche P-T Entwicklungen in einem frtihen 62 D. Gregurek et al. Stadium der Eoalpinen Metamorphose. Die Gesteine des Koralpenkristallins waren eklogitfaziellen Bedingungen mit Mindestdrucken im Bereich yon 15 bis 16 kbar und Temperaturen von fiber 700 °C ausgesetzt. Die Plankogelserie verweilte zur gleichen Zeit in einem relativ seichten Niveau bei Drucken yon 10 bis 11 kbar und Temperaturen unterhalb 600 °C. Die Platznahme der Plankogelserie in ihrer heutigen Position im tektonisch Hangenden des Koralpenkristallins erfolgte nach dem eklogitfaziellen Ereignis in einem relativ seichten Krustenniveau. Nach ihrer Vereinigung erfuhren die beiden Einheiten eine gemeinsame amphibolitfazielle 1Jberpr/igung. Die markant unterschiedlichen P-T Entwicklungen in einem friihen Stadium der eoalpinen Orogenese und die gemeinsame Entwicklung in einem sp~teren Stadium k6nnen als Hinweis auf eine umfangreiche tektonische Aktivitiit in diesem Teil des ostalpinen Grundgebirges in kretazischer Zeit gewertet werden. Diese Tektonik bestand eventuell in einer Subduktion yon ozeanischer und kontinentaler Lithosph/ire, und kann zu einer signifikanten Krustenverk'tirzung im Ostalpinen Kristallin gefiihrt haben.
Journal of Metamorphic Geology, 2021
The hinterland of the Cenozoic Northern Apennines fold-and-thrust belt exposes the metamorphic roots of the chain, vestiges of the subduction-related tectono-metamorphic evolution that led to the buildup of the Alpine orogeny in the Mediterranean region. Like in other peri-Mediterranean belts, the tectono-metamorphic evolution of the Paleozoic continental basement in the Apennines is still poorly constrained, hampering the full understanding of their Alpine orogenic evolution. We report the first comprehensive tectonometamorphic study of the low-grade metasedimentary (metapsammite/metapelite) succession of the Monti Romani Complex (MRC) that formed after Paleozoic protoliths and constitutes the southernmost exposure of the metamorphic domain of the Northern Apennines. By integrating fieldwork with microstructural studies, Raman spectroscopy on carbonaceous material and thermodynamic modelling, we show that the MRC preserves a D1/M1 Alpine tectono-metamorphic evolution developed under HP-LT conditions (~ 1.0-1.1 GPa at T ~400 °C) during a non-coaxial, top-to-the-NE, crustal shortening regime. Evidence for HP-LT metamorphism is generally cryptic within the MRC, dominated by graphite-bearing assemblages with the infrequent blastesis of muscovite ± chlorite ± chloritoid ± paragonite parageneses, equilibrated under cold paleo-geothermal conditions (~ 10 °C/km). Results of this study allow extending to the MRC the signature of subduction zone metamorphism already documented in the hinterland of the Apennine orogen, providing further evidence of the syn-orogenic ductile exhumation of the HP units in the Apennine belt. Finally, we discuss the possible role of fluid-mediated changes in the reactive bulk rock composition on mineral blastesis during progress of regional deformation and metamorphism at low-grade conditions.