Isotopic U-Pb ages of monazites and zircons from the crust-mantle transition and adjacent units of the ivrea and ceneri zones (Southern Alps,Italy)

The Ivrea zone forms a part of the Southern Alps and is composed of basic rocks interfingered with granulite facies acidic rocks. According to geophysical evidence, this zone represents the transition between crust and uplifted and overthrusted mantle. Towards the Ceneri zone the metamorphic grade changes to amphibolite facies. Paragneisses, migmatites and anatectic gneisses dominate, within which postmetamorphic granites occur. Concordant monazite U-Pb ages of 275+2 m.y. were obtained from paragneisses of the Ivrea zone. The apparent zircon ages are discordant indicating a minimum age of 1900 m.y. for the oldest population and an apparent lead loss of 99 to 85 % about 285–300 m.y. ago. The zircons show features such as rounded habitus, low trace element contents and well ordered crystal lattices characteristic for detrital, recrystallised populations. Monazite from the neighbouring Ceneri zone migmatite yielded concordant U-Pb ages at 295±5 m.y. The discordant zircon age pattern indicates a time of formation of 450 m.y., similar to other newly formed zircons in anatectic rocks of the Ceneri zone, and an episodic or continuous lead loss at, or until 300 m.y. ago. The majority of the zircons are euhedral and have elevated trace element contents, features typical for zircons formed in the present-day host rocks. Concordant, 295±5 m.y. old monazite dates the formation of the postmetamorphic Mont' Orfano granite. Again zircon fractions yielded discordant ages, pointing in contrast to the above discordancies to a recent or continuous lead loss. The concordant ages of the monazites demonstrate the usefulness of this mineral for dating purposes in metamorphic and granitic rocks and contrast with the discordant age patterns of all zircon suites. From the general agreement between the monazite ages and the time of lead loss inferred from the zircon age patterns as well as from the geological relationships of the rocks and their metamorphic grade it is concluded that 295±5 m.y. is the minimum age for the regional granulite to upper amphibolite facies metamorphism of the Ivrea zone and that the uplift and overthrust of the upper mantle started prior to 295 m.y. ago, and that the basic rocks of the Ivrea zone are synmetamorphic intrusions. The decrease from 310–320 m.y. to 170–200 m.y. of the K-Ar and Rb-Sr mineral ages from the Ceneri towards the Ivrea zone is accompanied by decreases from 450 m.y. to 295 m.y. and on to 275 m.y. in the U-Pb ages of monazites. The zircon age pattern also shows a decrease from 450 m.y. to approximately 300 m.y. The main lowering of the ages occurs approximately at the petrographic boundary between the two zones and is related to the Hercynian uplift and overthrust of the mantle which may have started as early as 450 m.y. ago. The Insubric line which terminates the Ivrea zone towards the North must therefore be of pre-Alpine age, or a precursor of the Insubric line must have existed at the time of the mantle uplift.     

Age and origin of detrital zircons from the pre-Permian basements of the Bohemian Massif and the Alps

U-Pb isotopic analyses were made on detrital zircon populations from sandstones and quartzites of the pre-Permian basement in an attempt to shed light on the presedimentary history of the zircons and the age of their primary source rocks. Eight rock samples were collected from the Saxothuringian and Moldanubian parts of the Bohemian Massif, the western part of the Upper Austroalpine Nappes, and the Southern Alps. The heterogeneous populations were separated into fractions of different size, magnetic susceptibility, color, and shape. Because of their typically pitted surface all zircon grains from the sandstones and quartzites appear to be detrital. Only in three samples from the Alps—one from a contact metamorphic aureole—the zircons show surface recrystallization and minor new growth. With the exception of some euhedral crystals in the Saxothuringian quartzites all zircon fractions have highly discordant U-Pb ages. On a concordia diagram their data points scatter slightly around best-fit lines with upper intersections between 2000 and 2300 m.y. From this pattern the following conclusions are reached:

1. A large proportion of the material of the metasedimentary basement rocks in the Bohemian Massif as well as in the Alps derives from one or more sources, about 2000 to 2300 m.y. old.
2. The estimated proportion of detrital zircons with primary ages of 700 to 1500 m.y. is less than 10%.
3. The existence of a regional high-grade metamorphism in the Bohemian Massif as well as in the Alps during 700 to 1500 m.y. can be excluded. From Rb-Sr isotopic data, a metamorphism for the time prior to 1500 m.y. is very unlikely.

The lower intersections of the best-fit lines with the concordia curve cannot be clearly correlated with an episodic disturbance of the U-Pb systems during weathering and sedimentation and/or during regional metamorphism. For the zircons of the Bohemian Massif a disturbing event, about 550 to 600 m.y. ago, is likely. Clear, euhedral, but nevertheless detrital zircons found among the zircon populations of two Saxothuringian quartzites (“Plattenquarzit” of the pre-Ordovician “Arzberger Serie” and Lower Ordovician “FrauenbachQuarzit”) crystallized most probably during the Upper Proterozoic and/or the Assyntian petrogenesis. The highly discordant age pattern of the detrital zircons from the Alps is likely to be the result of the Caledonian and/or Hercynian (=Variscan) metamorphism. Differences in concentration levels of common lead in detrital zircons and the problem of red zircons as indicators of Precambrian origin are discussed.     

Importance of Hercynian tectonics within the framework of the Southern Alps

The Hercynian remnants present within the Alpidic structural zones of the Southern Alps are reviewed. The pattern of Hercynian metamorphism is zonal from granulites in the western area to anchimetamorphic facies in the eastern one. In the folded zone at the eastern margin a severe Hercynian folding phase took place during the Westphalian. Thrust sheets comprising sequences of different facies ranging in age from Caradocian to early Westphalian are sutured by late Westphalian molasse deposits. The assumption that the Southern Alps and the external Dinarides remained outside the Hercynian folding front is contradicted by field evidence.     

Dating synmagmatic folds: a case study of Schlingen structures in the Strona-Ceneri Zone (Southern Alps, northern Italy)

The Strona-Ceneri Zone (Southern Alps) contains folds with moderately to steeply inclined axial planes and fold axes, and amplitudes of up to several kilometres (so-called 'Schlingen'). These amphibolite facies folds deform the main schistosity of Late Ordovician metagranitoids and are discordantly overlain by unmetamorphic Permian sedimentary rocks. Mutually cross-cutting relationships between these folds and garnet-bearing leucotonalitic dykes indicate that these dykes were emplaced during folding. Sm–Nd systematics and the strongly peraluminous composition of these dykes point to an anatectic origin. Pb step leaching of magmatic garnet from a leucotonalitic dyke yielded a 321.3±2.3  Ma intrusive age. Rb–Sr ages on muscovites from leucotonalitic dykes range from 307 to 298  Ma, interpreted as cooling ages during retrograde amphibolite facies metamorphism. Conventional U–Pb data of zircons from an older granodioritic dyke that pre-dates the Schlingen folds yielded discordant U–Pb ages ranging from 371 to 294  Ma. These ages reflect a more complicated multi-episodic growth history which is consistent with the observed polyphase structural overprint of this dyke. Schlingen folding was accompanied by prograde amphibolite facies metamorphism, during the thermal peak of which the leucotonalitic dyke material was generated by partial melting in a deeper source region from where these S-type magmas intruded the presently exposed level. Because partial melting may occur in a relatively late stage of a clockwise P–T–t path, or even during decompression on the retrograde path, we do not exclude the possibility that Schlingen folding had already started in Early Carboniferous time. Schlingen folds also occur in Penninic and Austroalpine basement units with a very similar pre-Alpine history, indicating that Variscan folding affected large segments of the future Alpine realm.     

Alpine and pre-Alpine magmatism in the root-zone of the western Central Alps

 The highest grade of metamorphism and associated structural elements in orogenic belts may be inherited from earlier orogenic events. We illustrate this point using magmatic and metamorphic rocks from the southern steep belt of the Lepontine Gneiss Dome (Central Alps). The U-Pb zircon ages from an anatectic granite at Verampio and migmatites at Corcapolo and Lavertezzo yield 280–290 Ma, i.e., Hercynian ages. These ages indicate that the highest grade of metamorphism in several crystalline nappes of the Lepontine Gneiss Dome is pre-Alpine. Alpine metamorphism reached sufficiently high grade to reset the Rb-Sr and K-Ar systematics of mica and amphibole, but generally did not result in crustal melting, except in the steep belt to the north of the Insubric Line, where numerous 29 to 26 Ma old pegmatites and aplites had intruded syn- and post-kinematically into gneisses of the ductile Simplon Shear Zone. The emplacement age of these pegmatites gives a minimum estimate for the age of the Alpine metamorphic peak in the Monte Rosa nappe. The U-Pb titanite ages of 33 to 31 Ma from felsic porphyritic veins represent a minimum-age estimate for Alpine metamorphism in the Sesia Zone. A porphyric vein emplaced at 448±5 Ma (U-Pb monazite) demonstrates that there existed a consolidated Caledonian basement in the Sesia Zone. Received: 23 May 1995/Accepted: 12 October 1995     

Palaeobotanical and geochronological evidence for the Alpine age of the metamorphism in the Sesia-Zone

For a long time the age of the last metamorphism of the Sesia-zone was considered to be Hercynian or older. Basement inclusions in basic volcanics were the main argument for this interpretation. The Trachyandesites — Andesites of the Sesia-zone were regarded as Permian, analogoues to the widespread Permian volcanics of the Southern Alps. Recently, plant remnants have been found in tuffitic interlayers of the Sesia volcanics. These fossils have been described as palaeozoic plants, in contradiction to structual observations and to the numerous radiometric data acquired in the region. To check the age of the fossils a detailed palaeobotanical study was carried out. Our samples contained a very modern flora of definitely Tertiary age. Not one palaeozoic fossil was detected. The Tertiary age found on palaeobotanical evidence proved to be between 29 and 33 m. y. on the basis of total rock K-Ar ages. The last high pressure metamorphism of the Sesia-zone occurred between 90 and 60 m. y. as has been shown by radiometric ages on micas. The external part of the zone was overprinted by the Lepontine phase of metamorphism in greenschist facies 38 m. y. ago.     

Thermal history of the Sonnblick Dome,south-east Tauern Window,Austria: Implications for heterogeneous uplift within the Pennine basement

The Sonnblick Dome is one of several domal structures affecting the interface between basement and cover within the Pennine Zone of the Tauern Window in the eastern Alps. Rb-Sr isotopic data, comprising 19 biotite and 22 white mica ages from variably deformed granitic gneisses, provide new evidence of the thermal and tectonic history of the dome and its relationships with other parts of the south-east Tauern Window. White mica ages generally cluster between 26 and 30 Ma although there are values up to 82 Ma, which appear to reflect incomplete equilibration during Tertiary metamorphism under low amphibolite facies conditions; six closely spaced samples from an intensely sheared gneiss lamella are more tightly grouped between 26 and 27.6 Ma and provide the best estimate of the age of syntectonic crystallization. Biotite ages are systematically younger, ranging from 19 to 23.5 Ma, reflecting closure during post-metamorphic cooling. Sonnblick Dome and the Hochalm Dome approximately 20 km further east, where closure of Rb-Sr in biotite did not occur until 16.5 Ma; the metamorphic peak here is also probably younger, possibly as late as 22 Ma. The Sonnblick Dome was formed before 27 Ma and the deformation style had changed to extension before biotite closure by 19 Ma. In contrast, rapid updoming in the Hochalm Dome was previously dated at 16.5 Ma and the differences in thermal history can be linked to differences in deformation history. Overall the geochronological data from the south-east Tauern Window demonstrate the heterogeneity of thermal history on a geographical scale of 10 km and emphasize the importance of tectonic displacements in controlling temperature within orogenic belts.     

Timing of metamorphism in the Tauern Window, Eastern Alps: Rb-Sr ages and fabric formation

The Peripheral Schieferhülle of the Tauern Window of the Eastern Alps represents post-Hercynian Penninic cover sequences and preserves a record of metamorphism in the Alpine orogeny, without the inherited remnants of Hercynian events that are retained in basement rocks. The temperature-time-deformation history of rocks at the lower levels of these cover sequences have been investigated by geochronological and petrographic study of units whose P-T evolution and structural setting are already well understood. The Eclogite Zone of the central Tauern formed from protoliths with Penninic cover affinities, and suffered early Alpine eclogite facies metamorphism before tectonic interposition between basement and cover. It then shared a common metamorphic history with these units, experiencing blueschist facies and subsequent greenschist facies conditions in the Alpine orogeny. The greenschist facies phase, associated with penetrative deformation in the cover and the influx of aqueous fluids, reset Sr isotopes in metasediments throughout the eclogite zone and cover schists, recording deformation and peak metamorphism at 28-30 Ma. The Peripheral Schieferhülle of the south-east Tauern Window yields Rb-Sr white mica ages which can be tied to the structural evolution of the metamorphic pile. Early prograde fabrics pre-date 31 Ma, and were reworked by the formation of the large north-east vergent Sonnblick fold structure at 28 Ma. Peak metamorphism post-dated this deformation, but by contrast to the equivalent levels in the central Tauern, peak metamorphic conditions did not lead to widespread homogenization of the Sr isotopes. Localized deformation continued into the cooling path until at least 23 Ma, partially or wholly resetting Sr white mica ages in some samples. These isotopic ages may be integrated with structural data in regional tectonic models, and may constrain changes in the style of crustal deformation and plate interaction. However, such interpretations must accommodate the demonstrable variation in thermal histories over small distances.     

Early Tertiary eclogite facies metamorphism in the Monviso Ophiolite

Although crucial for the construction of tectonic models of the Alps the timing of high pressure metamorphism is still poorly determined and controversial. It is likely to vary from one tectonic unit to another depending on when each became involved in subduction. This in turn relates to palaeogeographic position with respect to the active ocean basin. Well defined, reliable geochronological data are too few to test this hypothesis. This paper extends the database by determining Sm–Nd mineral isochrons on two samples from the Monviso Ophiolite in the Piemonte Zone, carefully selected to minimize the problems of Sm–Nd dating of eclogites encountered elsewhere in the Alps. The dated samples have eclogite facies mineral assemblages typical of the Lago Superiore unit of the ophiolite; mineral compositions are similar to previously reported samples and indicate pressures of around 2  GPa and temperatures of 400–500  °C. Sm–Nd isochron ages of 60±12 and 62±9  Ma are defined by garnet and clinopyroxene, while the Rb–Sr age on phengite which is part of the high- P assemblage is 40±1  Ma. The new data fit an emerging pattern of ages in which high- P metamorphism in the oceanic realm is Early Tertiary, with slightly older ages in the overlying Sesia Zone and younger, Oligocene ages in the underlying internal basement massifs which only became involved in subduction when closure of the Piemont ocean was complete.     

U-Pb and Rb-Sr radiometric dates and their correlation with metamorphic events in the granulite-facies basement of the serre,Southern Calabria (Italy)

An approximately 7 km thick, continuous sequence of granulite-facies rocks from the lower crust, which contains a lower granulite-pyriclasite unit and an upper metapelite unit, occurs in the NW Serre of the Calabrian massif. The lower crustal section is overlain by a succession of plutonic rocks consisting of blastomylonitic quartz diorite, tonalite, and granite, and is underlain by phyllonitic schists and gneisses.Discordant apparent zircon ages, obtained from granulites and aluminous paragneisses, indicate a minimum age of about 1,900 m.y. for the oldest zircon populations. The lower intersection point of the discordia with the concordia at 296±2 m.y. is also marked by concordant monazites. Therefore, the age of 296±2 m.y. is interpreted as the minimum age of granulite-facies metamorphism.Concordant zircon ages were obtained from a metamorphic quartz monzogabbronorite sill (298±5 m.y.) and an unmetamorphosed tonalite (295±2 m.y.); they are interpreted as the intrusion ages.Discordant zircon ages from a blastomylonitic quartz diorite gneiss, situated between the lower crustal unit and the non-metamorphosed tonalite, reveal recent or geologically young lead loss by diffusion. The ²⁰⁷Pb/²⁰⁶Pb ages of the two analysed size-fractions point to an intrusion age similar to that of the overlying tonalite.Rb-Sr mineral ages are younger in the granulite-pyriclasite unit than in the overlying metapelite unit. Feldspars from the granulite-pyriclasite unit yield ages of about 145 m.y. and those from the metapelite unit 176±5 m.y. In the same way, the biotite cooling ages range between 108 and 114 m.y. in the granulitepyriclasite and between 132 and 135 m.y. in the metapelite unit and the tonalite. Some still younger biotite ages are explained by the influence of tectonic shearing on the Rb-Sr systems. A muscovite from a postmetamorphic aplite in the metapelite unit yields a cooling age of 203±4 m.y.The Rb-Sr isotopic analyses from migmatite bands do not lie on an isochron, perhaps due to limited isotopic exchange between the small scale layers during the long cooling period after the peak of metamorphism.In the phyllonitic gneisses and schists a Hercynian metamorphism is indicated by a muscovite age of 268±4 m.y., whereas the biotite age of 43±1 m.y. from the same sample can be correlated with an Alpine greenschist-facies metamorphism.On the basis of the radiometric dates and of the P-T path of the lower crustal section deduced petrologically, the following model is presented: the end of the Hercynian granulite-facies metamorphism was accompanied by an uplift of the lower crustal rocks into intermediate crustal levels and by synchronous plutonic intrusions into the lower crust and higher crustal levels, but essentially into the latter. Substantial further uplift did not occur until after cooling from the temperature of the granulite-facies metamorphism to the biotite closing temperature. This cooling lasted for about 185 m.y. in the lower part and for about 160 m.y. in the upper part of the lower crust section.A comparison between the geologic evolutions of the NW Serre of Calabria and the Ivrea Zone of the Alps demonstrates striking similarities. The activity of deep seated faults in both areas at least since late Hercynian time raises the possibility that a fault precursor of the boundary of the Adriatic microplate already existed at this time.     

Evidence of late oligocene/early miocene backthrusting in the central alpine “root zone”

Abstract

Two groups of stretching lineations can be distinguished in the Central Alpine " root zone " between Ticino and Mera :

1) Steeply plunging lineations formed during retrograde metamor-Phism under amphibolite/greenschist facies conditions indicate an uplift movement of the Central Alps. The lineations can be related to an important back-thrusting event of late Oligocene/early Miocene age.

2) Gently plunging lineations formed under lower greenschist facies conditions display a pattern typical of a dextral strike-slip system. These lineations are of early Miocene age.

This cpmbined movement, achieved by ductile deformation along the lnsubric line was followed by a stage of brittle deformation in a dextral strike-slip system (= Tonale line).

The signification of this interpretation is shown in a new crustal cross section through the Central Alpine/Southern Alpine border zone in the Iicino area.     

Isotopic age and metamorphic history of the banded gneiss at Danmarkshavn,East Greenland

Along the northern part of the East Greenland coast the Caledonian structures are superimposed on an older fold system called the Carolinidian. Traces of this Carolinidian belt are preserved in a few isolated fragments within the Caledonian fold belt. According to Haller (1970) one of these fragments exhibiting the typical Carolinidian NNW to NW-trending infrastructural folds is the peninsula of Germania Land which is accessible near the Danish weather station Danmarkshavn. The rock sampled there is a banded gneiss of granodioritic composition with steeply inclined, NNW-trending layers. Isotopic age determinations yielded essentially two groups of ages: 1) 3,000±150 m.y. (zircon suite and Rb/Sr whole rock analyses of layers) and 2) 320–380 m.y. (Rb/Sr mineral isochrons, U-Th-Pb on sphene, K/Ar on hornblende and biotite). The egg-shaped zircons support a sedimentary origin of the banded gneiss and in conjunction with the Rb/Sr whole rock ages determine the age of formation of the banded gneiss (or its last high grade metamorphism) some 3,000 m.y. ago. No other Precambrian metamorphism or orogeny is recorded in the rock. The ages between 320–380 m.y. date a thermal event of lower amphibolite facies grade related to a late Caledonian spasm.The new isotopic data reveal the existence of very old rocks in the hinterland — away from the direction of thrusting—of the East Greenland Caledonian belt. With respect to the age of the Carolinidian fold system three geological interpretations are compatible with the results of this study:

Alpine metamorphism of Hercynian hornblende granodiorite beneath the blueschist facies schistes lustrés nappe of NE Corsica

Abstract The Hercynian granitic basement which forms the Tenda Massif in NE Corsica represents part of the leading edge of the European Plate during middle-to-late Cretaceous (Eoalpine) high P metamorphism. The metamorphism of this basement, induced by the overthrusting of a blueschist facies (schistes lustrés) nappe, was confined to a major ductile shear zone (c. 1000m thick) within which deformation increases upwards towards the overlying nappe. Metamorphism within the basement mostly records lower blueschist facies conditions (crossite + epidote) except near the base of the shear zone where the greenschist facies assemblage albite + actinolitic amphibole has developed instead of crossite. Study of the primary mafic phase breakdown reactions within hornblende granodiorite reveals the following metamorphic zonation. Zone 1: biotite to chlorite. Towards zone 2: biotite to phengite. Zone 2: Hornblende to actinolitic Ca-amphibole + albite + sphene, and biotite to actinolitic Ca-amphibole + albite + phengite + Ti-ore + epidote. Zone 3: Hornblende to crossite + low Ti-biotite + phengite + sphene, and biotite to crossite + low Ti-biotite + phengite + Ti-ore + sphene ± epidote. P-T conditions at the base of the shear zone are estimated to have been 390-490°C at 600-900 M Pa (6-9kbar) and the Corsican basement is therefore deduced to have been buried to 20-30 km during metamorphism. This relatively shallow metamorphism contrasts with some other areas in the Western Alps where the Eoalpine event apparently buried the European continental crust to depths of 80 km or more. As there is no evidence for a long history of blueschist facies metamorphism prior to the involvement of the European continent, it is deduced that the Eoalpine blueschists were produced during the collision of the Insubric plate with Europe, rather than during Tethyan intraoceanic subduction. Coherent blueschist terrains such as the schistes lustres probably record buovant feature collision and obduction tectonics rather than any preceding oceanic subduction.     

A lead isotope study of the northeastern Ivrea zone and the adjoining Ceneri zone (N-Italy): evidence for a contaminated subcontinental mantle

Lead isotope data on whole rocks, feldspars and sulfides from the Ivrea zone indicate that the magmas which formed the Hercynian (?) mafic complex derived from an isotopically anomalous mantle with a lead isotope composition that resembled average continental crustal leads. Contamination during magma emplacement played a minor role and probably accounts for second order variations of the μ values. Similar leads were observed in Caledonian orthogneisses, late-Hercynian granites and volcanics of the near-by Ceneri zone. Metasediments of the kinzigitic series of the Ivrea zone as well as metasediments of the Ceneri zone contain lead with high μ values which indicates of prolonged crustal history in agreement with the mean ages of 1900 to 2500 my of their detrital zircon populations. Amphibolites of the kinzigitic series still contain MORB-type leads, and contamination by metasediments played only a minor role in spite of the regional metamorphism in upper amphibolite facies. The trace leads of stratiform sulfide ores are variable mixtures of mantle-derived and crustal leads depending on the host rock lithologies which are either amphibolites, amphibolites plus metasediments or metasediments.     

On the Precambrian tectonics of Bulgaria

The Precambrian basement of Bulgaria consists of two sharply defined units: the Upper Precambrian—Lower Paleozoic (?) Diabase-phyllitoid (Vlasina) complex, characterized by greenschist facies metamorphism, and the high-grade (amphibolite facies) basement (Thracian massif), subdivided into Rhodopian and pre-Rhodopian complexes. According to the style and dominant fold trend five Precambrian structural zones are defined within the Thracian massif: Ograzhden Anticlinorium; Sredna Gora Anticlinorium; Rhodope Anticlinorium; Pirin structural zone; Upper Thracian structural zone.The pre-Rhodopian development (4 major fold phases in amphibolite facie metamorphic conditions) resulted in formation of a grate-shape structure with centrifugal vergence, and smooth curvature of the dominant trend. The consecutively formed Rhodopian complex was folded following a simple “dome-and-basin” interference pattern, in amphibolite facies conditions, and uplift tendencies for the peripheral anticlinoria. The structure was complicated by torsion in the Pirin zone, and by setting up of the first (“ancestral”) fault zones along the junction lines between the separate Precambrian structural zones. After erosion the Diabase-phyllitoid complex covered the periphery of the Thracian massif and incorporated fragments of it as cores of positive structures. The Precambrian structural plan has been largely inherited by later (Hercynian and Alpine) tectonics.     

Some thoughts on the pre-Alpine metamorphic history of the austridic basement of the Eastern Alps

Summary In several places of the old crystalline basement of the Eastern Alps a classification of the pre-Alpine metamorphic effects into an older, high-to intermediate-pressure metamorphism (eclogites, kyanite) and a younger, lower-pressure one (and±ky±cord) is recognizable. Some local geological situations allow a sharp chronological distinction to be made between these two events; and the available radiometric age values demonstrate the Caledonian age (500 m.y.) of the older metamorphism and the Hercynian age (320 m.y.) of the younger one. Elements exist showing that the Caledonian metamorphism, belongs to a complex cycle of geological processes which took place substantially during the Ordivician age (?) and has all the ingredients of the orogenic cycles. This evolutional picture represents a possible model for the whole of the Eastern Alps.

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Überlegungen zur Entwicklung der voralpidischen Metamorphose im Ostalpin

Zusammenfassung An mehreren Stellen im altkristallinen Grundebirge der Ostalpen ist eine Gliederung der voralpidischen metamorphen Ereignisse in eine ältere Metamorphose, die einer hoch-bis mitteldruckbetonten Faziesserie (Eklogite, Disthen) angehört und eine jüngere weniger druckbetonte Metamorphose (Andalusit±Disthen±Cordierit) erkennbar. Einzelne lokale geologische Situationen erlauben eine scharfe chronologische Trennung dieser zwei Ereignisse; das Caledonische Alter ( 500m.y.) der älteren und das Hercynische Alter ( 320 m.y.) der jüngeren Metamorphose wird durch radiometrische Altersdaten demonstriert. Es gibt Hinweise, daß die Caledonische Metamorphose zu einem komplexen geologischen Ereignis mit allen Kennzeichen eines orogenen Zyklusses gehört, das im wesentlichen während des Ordoviziums stattgefunden hat. Diese genetischen Vorstellungen scheinen als mögliches Modell für die gesamten Ostalpen annehmbar.

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With 3 Figures     

Geochronological constraints on the timing of granitoid magmatism, metamorphism and post-metamorphic cooling in the Hercynian crustal cross-section of Calabria

Exposed cross‐sections of the continental crust are a unique geological situation for crustal evolution studies, providing the possibility of deciphering the time relationships between magmatic and metamorphic events at all levels of the crust. In the cross‐section of southern and northern Calabria, U–Pb, Rb–Sr and K–Ar mineral ages of granulite facies metapelitic migmatites, peraluminous granites and amphibolite facies upper crustal gneisses provide constraints on the late‐Hercynian peak metamorphism and granitoid magmatism as well as on the post‐metamorphic cooling. Monazite from upper crustal amphibolite facies paragneisses from southern Calabria yields similar U–Pb ages (295–293±4 Ma) to those of granulite facies metamorphism in the lower crust and of intrusions of calcalkaline and metaluminous granitoids in the middle crust (300±10 Ma). Monazite and xenotime from peraluminous granites in the middle to upper crust of the same crustal section provide slightly older intrusion ages of 303–302±0.6 Ma. Zircon from a mafic to intermediate sill in the lower crust yields a lower concordia intercept age of 290±2 Ma, which may be interpreted as the minimum age for metamorphism or intrusion. U–Pb monazite ages from granulite facies migmatites and peraluminous granites of the lower and middle crust from northern Calabria (Sila) also point to a near‐synchronism of peak metamorphism and intrusion at 304–300±0.4 Ma. At the end of the granulite facies metamorphism, the lower crustal rocks were uplifted into mid‐crustal levels (10–15 km) followed by nearly isobaric slow cooling (c. 3 °C Ma^?1) as indicated by muscovite and biotite K–Ar and Rb–Sr data between 210±4 and 123±1 Ma. The thermal history is therefore similar to that of the lower crust of southern Calabria. In combination with previous petrological studies addressing metamorphic textures and P–T conditions of rocks from all crustal levels, the new geochronological results are used to suggest that the thermal evolution and heat distribution in the Calabrian crust were mainly controlled by advective heat input through magmatic intrusions into all crustal levels during the late‐Hercynian orogeny.     

U–Pb zircon dating of the Gruf Complex: disclosing the late Variscan granulitic lower crust of Europe stranded in the Central Alps

Permian granulites associated with noritic intrusions and websterites are a common feature of the post-Variscan European crust. Such granulites are common in the Southern Alps (e.g. Ivrea Zone), but occur only in the Gruf Complex in the Central Alps. To understand the geotectonic significance of these granulites, in particular in the context of Alpine migmatisation, zircons from 15 high-grade samples have been U–Pb dated by SHRIMP II analysis. Oscillatory zoned zircons from charnockite sheets, interpreted as melts generated through granulite facies fluid-absent biotite melting at 920–940°C, yield ages of 282–260 Ma. Some of these zircons contain inclusions of opx, unequivocally attributable to the granulite facies, thus confirming a Permian age for the charnockites and associated granulites. Two samples from an enclave-rich orthogneiss sheet yield Cambrian and Ordovician zircon cores. Two deformed leucogranites and six ortho- and augengneisses, which compose two-thirds of the Gruf Complex, give zircon ages of 290–260 Ma. Most zircons have milky rims with ages of 34–29 Ma. These rims date the Alpine amphibolite facies migmatisation, an interpretation confirmed by directly dating a leucosome pocket from upper amphibolite facies metapelites. The Gruf charnockites associated with metre-scale schlieren and boudins of opx–sapphirine–garnet–granulites, websterites and gabbronorites can thus be identified as part of the post-Variscan European lower crust. A geotectonic reconstruction reveals that this piece of lower crust stranded in the (European) North upon rifting of the Neotethys, such contrasting the widespread granulite units in the Southern Alps. Emplacement of the Gruf lower crust into its present-day position occurred during migmatisation and formation of the Bergell Pluton in the aftermath of the breakoff of the European slab.     

Long-term thermo-tectonic evolution of the Montes de Toledo area (Central Hercynian Belt, Spain): constraints from apatite fission-track analysis

In the Montes de Toledo area, located in the axial part of the Central Hercynian zone, a long-term thermo-tectonic evolution can be deduced from apatite fission-track (AFT) data in conjunction with tight geological constraints derived from the knowledge of regional geology and other independent chronometers. The area is composed of two different blocks separated by the Toledo Shear Zone (TSZ). The northern block is a granulite facies anatectic terrane. The southern block is composed of greenschist facies Paleozoic sediments intruded by a late Hercynian granitic pluton. A total of 13 samples have been recovered for AFT analysis. AFT ages in both blocks cluster around 189–221 Ma, with mean confined track lengths between 11.4 m and 12.4 m. Modeling of the AFT data indicates that the thermal history is broadly similar in both blocks, which constrains the main movement of the TSZ, as essentially before the Upper Permian. AFT ages in the TSZ cluster around 124–164 Ma, and the track lengths vary between 11.4 m and 12.4 m. These data reveal that the fault must have been affected by a later thermal overprint as AFT ages are significantly younger than those of the footwall and hangingwall blocks. This differential thermal resetting is likely related to the advection of localized hydrothermal fluids that are responsible for the widespread Pb–Zn mineralization along the TSZ. These results give an example of resetting AFT data by hydrothermal events. The long-term evolution suggests a lack of important Alpine tectonism in the Montes de Toledo block, in clear contrast to other nearby Hercynian areas such as the Sierra de Guadarrama, where the important effect of Alpine tectonism has almost totally erased the previous thermal signal in the AFT system.     

Response of detrital zircon and monazite,and their U–Pb isotopic systems,to regional metamorphism and host‐rock partial melting,Cooma Complex,southeastern Australia

Progressive Early Silurian low‐pressure greenschist to granulite facies regional metamorphism of Ordovician flysch at Cooma, southeastern Australia, had different effects on detrital zircon and monazite and their U–Pb isotopic systems. Monazite began to dissolve at lower amphibolite facies, virtually disappearing by upper amphibolite facies, above which it began to regrow, becoming most coarsely grained in migmatite leucosome and the anatectic Cooma Granodiorite. Detrital monazite U–Pb ages survived through mid‐amphibolite facies, but not to higher grade. Monazite in the migmatite and granodiorite records only metamorphism and granite genesis at 432.8 ± 3.5 Ma. Detrital zircon was unaffected by metamorphism until the inception of partial melting, when platelets of new zircon precipitated in preferred orientations on the surface of the grains. These amalgamated to wholly enclose the grains in new growth, characterised by the development of {211} crystal faces, in the migmatite and granodiorite. New growth, although maximum in the leucosome, was best dated in the granodiorite at 435.2 ± 6.3 Ma. The combined best estimate for the age of metamorphism and granite genesis is 433.4 ± 3.1 Ma. Detrital zircon U–Pb ages were preserved unmodified throughout metamorphism and magma genesis and indicate derivation of the Cooma Granodiorite from Lower Palaeozoic source rocks with the same protolith as the Ordovician sediments, not Precambrian basement. Cooling of the metamorphic complex was relatively slow (average ～12°C/10⁶y from ～730 to ～170°C), more consistent with the unroofing of a regional thermal high than cooling of an igneous intrusion. The ages of detrital zircon and monazite from the Ordovician flysch (dominantly composite populations 600–500 Ma and 1.2–0.9 Ga old) indicate its derivation from a source remote from the Australian craton.