Eocene high pressure metamorphism in the Penninic units of the Tauern Window (Eastern Alps): evidence from40Ar?39Ar dating and petrological investigations (original) (raw)
Related papers
Chemical Geology, 1995
Bulk samples and single grains of high-pressure white micas and a biotite from two restricted areas (Mucrone and Marine) 10 km apart in the Sesia Zone (Western Alps, Italy) were analysed by the conventional (step-heating) and continuous laser probe (step-heating and spot fusion) 4°Ar/39Ar techniques, respectively. The analysed minerals crystallized during the Eoalpine, eclogitic facies metamorphism. The phengites from the Mucrone area display ages which scatter between ~ 104 and ~ 180 Ma. In the Marine area (a 50-m-wide outcrop) the phengites from four samples with different bulk-rock compositions display precise plateau ages, which scatter between 69.4___ 0.7 and 76.9 + 0.6 Ma, and homogeneous age maps, whereas two minerals (phengite and biotite) from the same rock sample provide two very "discordant" plateau ages, 69.4 + 0.7 and 140.5 __+ 0.6 Ma, respectively. In the light of the present knowledge of the history of the Alps, ages as old as 180 Ma are unrealistic and are related to excess argon, whereas ages in the range 70-80 Ma, perhaps even 140 Ma, could be significant. We suggest that excess argon, probably conveyed by a fluid phase and trapped in defects within the structure of the phengite grains at the time of their isotopic closure, is the best explanation for the observed spread in apparent ages on a regional to single grain scale, in spite of a nearly universal existence of plateau ages and homogeneous age maps. We propose that the age of closure of the phengites to argon loss is ~<69.4+ 0.7 Ma.
Mineralogy and Petrology, 2006
In the Eastern Alps, different tectonic units contain eclogites of Carboniferous, Cretaceous (eo-Alpine), and Cenozoic age: (i) In the structurally lower Penninic unit, which is exposed in the Tauern window, eclogites formed at peak P-T conditions of 2.4 GPa and 630 C as a result of subduction of the former European continental margin at the base of an imbricate stack of Mesozoic oceanic rocks below the Austroalpine domain. Amphibole and phengite 40 Ar-39 Ar and Rb-Sr multimineral isochron ages suggest an Eocene-Early Oligocene (<45-31 Ma) age for the stage of deepest subduction and rapid exhumation=cooling. (ii) Variscan MORB-type eclogites are present in the central Ö tztal basement (2.7 GPa=730 C; mean Sm-Nd age: 347 AE 9 Ma), the eastern Silvretta (ca. 350 Ma), and the Ulten high-grade crystalline (336 AE 4 Ma). These domains pertain to the N Apulian (Austroalpine) crust. (iii) In the southern Austroalpine units, eclogites and high-P metapelites are known from the Texel complex (1.3 GPa=520-600 C), the Schober area (1.8 GPa= 690 C), and the Saualpe-Koralpe-Pohorje domain (2.2-2.7 GPa= 750 C). Their tectonic position and near-identical Cretaceous ages suggest a common subduction-exhumation history. This ''eo-Alpine high-pressure belt'' (EHB) resulted from burial of the strongly thinned, distal passive Neotethyan margin of the Meliata back-arc basin and, further west, pre-Alpine Austroalpine crust along an intracontinental subduction zone during the Late Mesozoic convergence of Apulia and Europe. Peak metamorphism and initial decompression=exhumation is dated by the Sm-Nd, Lu-Hf, U-Pb and Rb-Sr systems as close to 90 Ma, with mean exhumation rates in the range of 5-10 km=Ma for the time between 90 and 85 Ma B.P.
39 Ar− 40 Ar dating of multiply zoned amphibole generations (Malenco, Italian Alps)
Contributions to Mineralogy and Petrology, 2000
Igor M. Villa á JoÈ rg Hermann á Othmar MuÈ ntener Volkmar Trommsdor 39 Ar) 40 Ar dating of multiply zoned amphibole generations (Malenco, Italian Alps) Abstract Ma®c rocks of a Permian crust to mantle section in Val Malenco (Italy) display a multi-stage evolution: pre-Alpine exhumation to the ocean¯oor, followed by burial and re-exhumation during Alpine convergence. Four prominent generations of amphiboles were formed during these stages. On the basis of microstructural investigations combined with electron microprobe analyses two amphibole generations can be assigned to the pre-Alpine decompression and two to the Alpine metamorphic P±T evolution. The dierent amphiboles have distinct Na M4 , Ca, K and Cl contents according to dierent P±T conditions and¯uid chemistry. Analysing these mixed amphiboles by the 39 Ar) 40 Ar stepwise heating technique yielded very complex age spectra. However, by correlating amphibole compositions directly obtained from the electron microprobe with the components deduced from the release of Ar isotopes during stepwise heating, obtained ages were consistent with the geological history deduced from ®eld and petrological studies. The two generations of pre-Alpine amphiboles gave distinguishable Triassic to Late Jurassic/Early Cretaceous ages (225 and 130±140 Ma respectively). High-Na M4 amphiboles have higher isotopic ages than low-Na M4 ones, in agreement with their decompressional evolution. The exhumation of the Permian crust to mantle section is represented by the former age. The latter age concerns Cl-dominated amphibole related to an Early Cretaceous oceanic stage. For the early Alpine, pressure-dominated metamorphism we obtained a Late Cretaceous age (83±91 Ma). The later, temperature-dominated overprint is signi®-cantly younger, as indicated by 39 Ar) 40 Ar ages of 67± 73 Ma. These Late Cretaceous ages favour an Adriatic origin for the Malenco unit. Our data show that 39 Ar) 40 Ar dating combined with detailed microprobe analysis can exploit the potential to relate conditions of amphibole formation to their respective ages.
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.
39Ar−40Ar dating of multiply zoned amphibole generations (Malenco, Italian Alps)
Contributions to Mineralogy and Petrology, 2000
Ma®c rocks of a Permian crust to mantle section in Val Malenco (Italy) display a multi-stage evolution: pre-Alpine exhumation to the ocean¯oor, followed by burial and re-exhumation during Alpine convergence. Four prominent generations of amphiboles were formed during these stages. On the basis of microstructural investigations combined with electron microprobe analyses two amphibole generations can be assigned to the pre-Alpine decompression and two to the Alpine metamorphic P±T evolution. The dierent amphiboles have distinct Na M4 , Ca, K and Cl contents according to dierent P±T conditions and¯uid chemistry. Analysing these mixed amphiboles by the 39 Ar) 40 Ar stepwise heating technique yielded very complex age spectra. However, by correlating amphibole compositions directly obtained from the electron microprobe with the components deduced from the release of Ar isotopes during stepwise heating, obtained ages were consistent with the geological history deduced from ®eld and petrological studies. The two generations of pre-Alpine amphiboles gave distinguishable Triassic to Late Jurassic/Early Cretaceous ages (225 and 130±140 Ma respectively). High-Na M4 amphiboles have higher isotopic ages than low-Na M4 ones, in agreement with their decompressional evolution. The exhumation of the Permian crust to mantle section is represented by the former age. The latter age concerns Cl-dominated amphibole related to an Early Cretaceous oceanic stage. For the early Alpine, pressure-dominated metamorphism we obtained a Late Cretaceous age (83±91 Ma). The later, temperature-dominated overprint is signi®-cantly younger, as indicated by 39 Ar) 40 Ar ages of 67± 73 Ma. These Late Cretaceous ages favour an Adriatic origin for the Malenco unit. Our data show that 39 Ar) 40 Ar dating combined with detailed microprobe analysis can exploit the potential to relate conditions of amphibole formation to their respective ages.
Radiometric ages of Alpine metamorphic rocks in the western and central Alps
The Island Arc, 1993
The chronological characteristics of Alpine metamorphic rocks are described and Alpine metamorphic events are reinterpreted on the basis of chronological data for the western and central Alps from 1960 to 1992. Metamorphic rocks of the Lepontine, Gran San Bernardo, Piemonte, Internal Crystalline Massifs and Sesia-Lanzo mostly date Alpine metamorphic events, but some (along with granitoids and gneisses from the Helvetic and Southern Alps) result from the Variscan, Caledonian or older events and thus predate the Alpine events. Radiometric age data from the Lepontine area show systematic age relations: U-Pb monazite (23-29 Ma), Rb-Sr muscovite (15-40 Ma) and biotite (15-30 Ma), K-Ar biotite (10-30 Ma), muscovite (15-25 Ma) and hornblende (25-35 Ma), and FT zircon (10-20 Ma) and apatite (5-15 Ma), which can be explained by the different closure temperatures of the isotopic systems. A 121 Ma U-Pb zircon age for a coesite-bearing whiteschist (metaquartzite) from the Dora-Maira represents the peak of ultra-high pressure metamorphism. Coesite-free eclogites and blueschists related to ultra-high pressure rocks in the Penninic crystalline massifs yield an 40Ar-39Ar plateau age of about 100 Ma for phengites, interpreted as the cooling age. From about 50 Ma, eclogites and glaucophane schists have also been reported from the Piemonte ophiolites and calcschists, suggesting the existence of a second high PIT metamorphic event. Alpine rocks therefore record three major metamorphic events: (i) ultra-high and related high P/T metamorphism in the early Cretaceous, which is well preserved in continental material such as the Sesia-Lanzo and the Penninic Internal Crystalline Massifs; (ii) a second high P/T metamorphic event in the Eocene, which is recognized in the ophiolites and calcschists of the Mesozoic Tethys; and (iii) medium PIT metamorphism, in which both types of high P/T metamorphic rocks were variably reset by Oligocene thermal events. Due to the mixture of minerals formed in the three metamorphic events, there is a possibility that almost all geochronological data reported from the Alpine metamorphic belt show mixed ages. Early Cretaceous subduction of a Tethyan mid-ocean ridge and Eocene continental collision triggered off the exhumation of the high pressure rocks.
Contributions to Mineralogy and Petrology, 2001
Ma®c eclogites sampled from a restricted area in the Lanterman Range (Antarctica) retrogressed variably under amphibolite facies metamorphism. Assemblages range from well-preserved eclogite, with minor growth of Na±Ca amphibole, to strongly retrogressed ones with extensive development of Ca amphibole. 40 Ar± 39 Ar furnace step-heating experiments on the different amphiboles yield results varying from plateau ages of 498Matoanear−plateauageof498 Ma to a near-plateau age of 498Matoanear−plateauageof490 Ma, and the greater the amphibolite retrogression, the younger the age. 40 Ar± 39 Ar infrared laser-probe analyses on rock chips from a well-preserved eclogite and a slightly retrogressed one reveal the presence of an excess argon component. Whereas excess argon is invariably present in garnet and clinopyroxene developed under highpressure metamorphism, it is heterogeneously distributed in amphibole on a millimetre scale. Results indicate that excess argon was incorporated during high-pressure metamorphism; this component was then lost during retrogression, while a change in composition of ambient argon to atmospheric argon occurred. New 40 Ar± 39 Ar data and previously published Sm±Nd garnet and U±Pb rutile ages obtained from the same well-preserved eclogite sample suggest that the oldest Na±Ca amphibole age is reliable and not an artefact due to the incorporation of excess argon. The variably retrogressed eclogites are thought to derive from dierent parts of the enclosing metasedimentary rocks that were variably invaded by¯uids during amphibolite facies metamorphism. Thus the circulation of¯uids promoting (re)crystallisation, and not temperature, was the main process controlling the rate of argon transport in the studied eclogites. The dierent 40 Ar± 39 Ar ages are interpreted to record diachronous amphibole growth at dierent crustal levels during exhumation. Data indicate that there was about a 10-Ma interval between the eclogite facies stage (at ³1.5 GPa) and the Ca amphibole-hydration forming reaction (at 0.3±0.5 GPa); this translates into an average exhumation rate of 3±4 km/ Ma.
Mineralogy and Petrology, 2005
The crystalline basement rocks of the Sopron Mountains are the easternmost and most isolated outcrops of the Austroalpine basement of the Eastern Alps. Ar=Ar and K=Ar dating of phengitic mica indicates that the Eoalpine high-pressure metamorphism of the area occurred between 76 and 71 Ma. Short-lived metamorphism is characterised by fluid-poor conditions. Fluid circulation was mostly restricted to shear zones, thus the degree of Alpine overprint has an extreme spatial variation. In several metamorphic slices Variscan mineral assemblages have been preserved and biotite yielded Variscan and Permo-Triassic Ar ages. Different mineral and isotope thermometers (literature data) yielded temperatures of 500-600 C for the peak of Alpine metamorphism in the Sopron Mountains, but muscovite and biotite do not show complete argon resetting. Thus, we consider this crystalline area as a well constrained natural test site, which either indicates considerably high closure temperatures (around 550 C) for Ar in muscovite and biotite in a dry metamorphic environment, or which is suitable for testing the widely applied methods of temperature estimations under disequilibrium conditions.
Mineralogical Magazine, 1995
Amphibolites in the Mesozoic part of the parautochthonous Lower Schieferhülle (LSH), the allochthonous Upper Schieferhülle (USH) and the overlying Austroalpine basement (AA) in and around the western Tauern Window (Eastern Alps) suffered a progressive Alpine deformation. Lineations and foliations L1-S1, L2-S2 defined by preferentially oriented (Na-Ca) amphiboles as well as F3 folds and further foliations Smyl and S4 in the metabasites are structures of successive deformational stages with a constant W-E main extension axis of strain. The (Na-Ca) amphiboles in assemblages with epidote, chlorite, albite/oligoclase and quartz are zoned with similar continuous zonation trends from early actinolite in the cores to magnesio-hornblende and tschermakitic hornblende, and from magnesio-hornblende to late actinolite in the rims in the three lithostratigraphic units. Geothermobarometry involving tremolite-edenite and (pargasite-hastingsite)-tremolite end-member equilibria in amphiboles allowed ...