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.     

U-Pb zircon and Sm—Nd geochronology of mafic and ultramafic rocks from the central part of the Tauern Window (eastern Alps)

Geochronological and geochemical analyses were carried out in order to identify the pre-Variscan basement of the Tauern Window (eastern Alps). Maficultramafic rocks from the central part of the Tauern window have been studied by REE-analysis and U-Pb and Sm-Nd isotopic analyses on whole rock, zircons, garnets and sphene. U-Pb and Sm—Nd zircon dating define both magmatic Pan-African and Cambro-Ordovician events from 650 Ma to 486 Ma within the Alpine fold belt. This indicates a time span of 150 Ma for magmatic activities in the Tauern Window of the eastern Alps. The ages of 657 Ma (U-Pb zircon) and 644 (Sm—Nd zireon) obtained from an amphibolite are the oldest dates of the Eastern Alps; they may be related to the Pan-African orogeny, and imply an early cycle of magmatic intrusion before major activity started at around 500 Ma. Sm-Nd whole rock analyses of the Precambrian rocks do not define an isochron, reflecting heterogenities within the mantle source. The initial _Nd values (+1.2 to +4.7) are very low, implying an enrichment of the magma source. The second main phase of magmatic activity (539 486 Ma) is characterized by the emplacement of mafic/ultramafic rock sequences. As no ophiolitic relies are observed in these domains, the Early Paleozoic magmatism was likely associated with extensional tectonics. Obtained ages of 301±3 and 314+4/-3 Ma point to a Variscan metamorphism. The first combined U-Pb zircon/Sm—Nd zircon data for an amphibolite from the Basal Amphibolite Formation (BAF) favoured the Sm-Nd zircon isochron age as a magmatic age, whereas the low initial _Nd value point to an enriched magma source as well as to heterogenities within the magma source. The obtained ages suggest that parts of the pre-Variscan basement within the Alpine fold belt were formed during the Pan-Africa cycle. The detection of Pre-Variscan ages within the Alpine basement must reffect a complex history involving significant pre-Variscan activity.     

Dating of granulites involved in the hercynian fold-belt of Europe: An example taken from the granulite-facies orthogneisses at La Picherais,Southern Armorican Massif,France

Situated within the crystalline metamorphic complex of Champtoceaux NE of Nantes, the orthogneiss of La Picherais (near St Mars-du-Désert, Loire Atlantique, France) show relicts of a granulite facies paragenesis. Comparison with other granulitic rocks in the Hercynian fold-belt suggest possible ages ranging from Lower Proterozoic to Phanerozoic. The Rb-Sr whole rock method yields an errorchron of 570±110 m.y. for the Picherais orthogneiss, whereas the U-Pb zircon method indicates an upper intersection on Concordia at 1,880±120 m.y. and a lower intersection at 423±10 m.y. Several interpretations are possible for these data: the granite emplacement age was (1) 1,900 m.y. ago. (2) more likely Upper Proterozoic — Lower Palaeozoic. The zircons concordant at 1,900 m.y. were either present in the granitic magma at its time of origin or were introduced into the magma during emplacement. These zircons could be derived from sedimentary horizons such as found in the Lower Ordovician sandstones of the Armorican massif whose zircon age data are presented here.     

A U-Pb zircon,and Rb-Sr and U-Th-Pb whole-rock study of a polymetamorphic terrane in the central Alps,Switzerland

U-Pb analyses of zircons from the southern paragneiss zone of the Gotthard massif in the central Alps indicate these rocks were derived from one or more source areas ≧ 1400 m.y. old and were strongly affected by both the Caledonian and Hercynian orogenies. Rb-Sr whole-rock analyses also appear to reflect the Hercynian event while Rb-Sr analyses of a metamorphic inclusion and a boudin indicate that these small-scale samples were affected by the Alpine orogeny. U-Pb whole rock data appear to reflect only the Hercynian event; these data, when corrected for primordial lead, furthermore yield an upper concordia intercept of 4500 m.y., possibly resulting from a U-Pb fractionation very early in the history of the earth. A more refined three-stage U-Pb evolution model yields an age of about 4480 m.y. rather than 4500 m.y. for this hypothesized early terrestrial differentiation. Geologically these data emphasize that: 1) the southern paragneiss zone of the Gotthard massif contains metasediments which have experienced the Caledonian orogeny and are at least 400 m.y. old; 2) the Hercynian episode in this region was pervasive indeed; and 3) the Alpine orogeny affected the Rb-Sr and U-Pb whole rock systems to a far lesser degree than the preceding orogenic episodes and apparently did not affect the U-Pb zircon systems investigated at all.     

Zircon U-Pb ages of the Franzfontein granitic suite,northern South West Africa

U-Pb isotopic data are presented for composite and size fractions of zircons from 15 samples of the Franzfontein granitic suite. These data reveal two distinctly different discordance trends that yield concordia intercept ages of 1730±30 m.y. and 1870±30 m.y. that are believed to encompass the age of emplacement of the suite. The previously published Rb-Sr isochron age of 1580±20 m.y. is now interpreted as recording a time of Rb and/or Sr migration through the system. The stratigraphic implications of the new zircon data are discussed.     

Comparative geochronology in the reversely zoned plutons of the Bottle Lake Complex,Maine: U-Pb on zircons and Rb-Sr on whole rocks

The Bottle Lake Complex is a composite granitic batholith emplaced into Cambrian to Lower Devonian metasedimentary rocks. Both plutons (Whitney Cove and Passadumkeag River) are very coarse grained hornblende and biotite-bearing granites showing petrographic and geochemical reverse zonation. Two linear whole rock Rb/Sr isochrons on xenolith-free Whitney Cove and Passadumkeag River samples indicate ages of 379±5 m.y. and 381±4 m.y., respectively, in close agreement with published K-Ar ages for biotite from Whitney Cove of 377 m.y. and 379 m.y., and for hornblende ⁴⁰Ar/³⁹Ar determinations from Passadumkeag River which indicate an age of 378±4 m.y. The initial Sr isotopic ratio for Whitney Cove is 0.70553 and for Passadumkeag River is 0.70414. A whole-rock isochron on a suite of xenoliths from the Passadumkeag River granite indicates a whole rock Rb-Sr age of 496±14 m.y., with an initial Sr isotopic ratio of 0.70262.Two types of zircon exhibiting wide petrographic diversity are evident in variable proportions throughout the batholith. One of these types is preferentially found in a mafic xenolith and it is widely dispersed in the host granites forming discrete grains and probably as inclusions in the other type of zircon. U-Pb analyses of zircons give concordia intercept ages of 399±8 m.y. for Whitney Cove, 388±6 m.y. for Passadumkeag River, 415 m.y. for a mafic xenolith in Passadumkeag River, and 396±32 for combined Whitney Cove and Passadumkeag River granite. The zircons show a spread of up to 20 m.y. in the ²⁰⁷Pb/²⁰⁶Pb ages. Omitting the finest zircon fraction in the Passadumkeag River results in a concordia intercept age of 381±3 m.y., in better agreement with the whole-rock Rb-Sr and mineral K-Ar ages. For the Whitney Cove pluton, exclusion of the finest fraction does not bring the zircon age into agreement with the Rb-Sr data.Age estimates by the whole rock Rb-Sr, mineral K-Ar and Ar-Ar methods suggest that the crystallization age of the plutons is about 380 m.y., slightly younger than the U-Pb zircon intercept ages. A possible reason for this discrepancy is that the zircons contain inherited lead. Thus, zircon U-Pb ages might represent a mixture of newly developed zircon and older inherited zircon, whereas the Rb-Sr whole rock age (380 m.y.) reflects the time of crystallization, and the argon ages result from rapid cooling after emplacement.     

Time calibration of a PT-path from the Western Tauern Window,Eastern Alps: the problem of closure temperatures

New Hornblende K-Ar and ³⁹Ar-⁴⁰Ar and mica Rb-Sr and K-Ar ages are used to place specific timemarks on a well-constrained pressure-temperature path for the late Alpine metamorphism in the Western Tauern Window. After identification of excess ⁴⁰Ar, the closure behavior of Ar in hornblende is compared with that of Sr and Ar in phengite and biotite. Samples were collected in three locations, whose maximum temperatures were 570° C (Zemmgrund), 550° C (Pfitscher Joch), and 500–540° C (Landshuter Hütte).The average undisturbed age sequence found is: Phengite Rb-Sr (20 Ma)>hornblende K-Ar (18 Ma)>phengite K-Ar (15 Ma)>biotite Rb-Sr, K-Ar (13.3 Ma)>apatite FT (7 Ma). Except for the phengite Rb-Sr age, the significance of which is debatable, all ages are cooling ages. No compositional effects are seen for closure in biotite. Additionally, Rb-Sr phengite ages from shearzones possibly indicate continuous shearing from 20 to 15 Ma, with reservations regarding the validity of the initial Sr correction and possible variations of the closure temperatures. The obviously lower closure temperature (T _c) for Ar in these hornblendes than for Sr in the unsheared phengites indicates that the T _c sequence in the Western Tauern Window is different from those observed in other terrains. In spite of this discrepancy, valuable geological conclusions can be drawn if the application of closure temperatures is limited to this restricted area with similar T, P and : (1) All ages of samples located on equal metamorphic isotherms decrease from east to west by about 1 Ma which is the result of a westward tilting of the Tauern Window during uplift. (2) In a PT-path, the undisturbed cooling ages yield constantly decreasing uplift rates from 3.6 mm/a to 0.1 mm/a. (3) Use of recently published diffusion data for Ar in hornblende (T _c=520° C) and biotite (T _c=320° C) suggests an extrapolated phengite closure temperature for Sr at 550° C. This suggests that the prograde thermal metamorphism at this tectonic level of the Tauern Window lasted until some 20 Ma ago.     

Paleozoic migmatites affected by high-grade tertiary metamorphism in the central Alps (Valle Bodengo,Italy)

The Lepontine Gneiss Complex of southern Switzerland and northern Italy is characterized by high-grade metamorphism and intensive deformation of Alpine age with migmatites prevalent in the area with the highest metamorphic grade. Petrological and structural observations are generally inconclusive but indicate in some places an Alpine age for the migmatite formation. To determine the time of migmatite formation a geochronologic study was undertaken in one of the best exposed areas, the Valle Bodengo, Italy. Rb-Sr whole-rock errorchrons of intrusive migmatite phases and of two rather homogeneous granitoid gneiss bodies yield apparent ages between 280 and 350 m.y. They suggest a Hercynian or older igneous history for these rocks. The U-Pb ages of the euhedral zircons are highly discordant, but they do point to the presence of zircon components more than 450 m.y. old. The concordia-intercept ages are incompatible with the Rb-Sr data and the low initial ⁸⁷Sr/⁸⁶Sr ratios of about 0.706. These low initial ratios suggest that either the bulk of the granitoid material is not much older than Hercynian, or older crustal material was isotopically homogenized on a regional scale with rocks that had low Rb/Sr and ⁸⁷Sr/⁸⁶Sr ratios (e.g. the lower crust or upper mantle) during a Hercynian metamorphism. Rb-Sr small-scale whole-rock isochrons and tie lines of adjacent, lithologically different rock phases give Alpine ages, the best isochron yielding 22 m.y. This coincides with concordant U-Pb ages of monazites of 23 to 24 m.y. Rb-Sr mineral isoohrons (muscovite, biotite, feldspars, apatite) give ages of 18–21 m.y. Our interpretation is that this age pattern resulted due to rapid cooling after the climax of the last phase of the Alpine metamorphism and we conclude that high-grade metamorphic conditions existed during the upper Oligocene or early Miocene. Other investigators have suggested that the Alpine metamorphism had a climax 35–40 m.y. ago and that the younger mineral ages are a result of simple continuous cooling due to uplift. Based on this study and other recent geochronological studies in the Lepotine Gneiss Complex we suggest that there had to be a thermal maximum at about 20–25 m.y. The example of Valle Bodengo demonstrates that the areal coincidence of the zone of highest-grade metamorphism with the occurrence of migmatites does not necessarily mean that metamorphism and migmatite formation were coeval and related to each other.     

Geochronology of Archaean gneisses and tonalites from north of the Frederikshåbs isblink,S.W. Greenland

The main rock types in the area north of the Frederikshåbs isblink are streaky gneisses, massive tonalites and ‘supracrustals’. The gneisses are thought to be the parent rocks of the tonalite and can be seen to merge into tonalite across a narrow zone of nebulite. Rb-Sr whole rock points from samples of gneiss and tonalite fall on a common isochron with an age of 2662 ± 116 m.y. (2σ) and initial ratio of 0.7032 ± 0.0008 (2σ) (half-life of ⁸⁷Rb = 50 b.y.). The uncertainties in the isochron could mask small age and initial ratio differences between the gneiss and tonalite. However, our present interpretation is that the isochron reflects a homogenization of Sr isotopes within and between the two rock types. The presence of two out of four K-feldspar points on the whole rock isochron is interpreted as evidence that the K-feldspar became closed to Sr isotope migration at the same time as the whole rocks. Subsequent local isotopic disturbance has resulted in a minor loss of radiogenic strontium from two of the samples. The interpretation of the K-feldspar as a product of the epidoteamphibolite facies metamorphism allows the conclusion that the whole rock-K-feldspar isochron is recording a Sr isotopic homogenization during this event and is not related to the formation of the gneiss or the tonalite. Rb-Sr closure ages of ca. 2515 m.y. for muscovite and ca. 1950 m.y. for biotite could be recording separate isotopic disturbances or the cessation of strontium isotope migration as the minerals cooled through their characteristic blocking temperatures. Zircons from both the gneiss and the tonalite have igneous morphological features. Their U-Pb systems are complex, however, and suggest a multistage history of isotopic disturbance. Whereas the zircon U-Pb and whole rock Rb-Sr results suggest a maximum age of approximately 3000 m.y. for the parent rocks of the gneiss and tonalite they do not entirely exclude the possibility that the rocks represent older crust in which the isotopic systems have been almost completely reset ca. 2700 m.y. ago.     

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.     

K-Ar and Rb-Sr Dating of blue amphiboles,micas, and associated minerals from the Western Alps

The results of 63 new radiometric K-Ar and Rb-Sr measurements on metamorphic minerals from the internal units of the Western Alps show Hercynian, Permian, as well as three Alpine age groups. The first of the Alpine ages cover the period between 78 and 100 m.y. and refer to high pressure parageneses. The second group comprises K-Ar 39 to 50 m.y. ages; these values are affected by some inherited argon, as indicated by Rb-Sr measurements which point to 35–36±4–5 m.y., i.e. similar to the culmination of the Lepontine crystallization. The final group includes 15 to 30 m.y. ages. It is not yet clear which geologic processes have led to this isotope re-equilibration. Large amounts of inherited argon have been found in Alpine metamorphic minerals of the basement rocks.     

Geochronology of igneous rocks at and near to the Nezhdaninka gold deposit, Yakutia, Russia: U-Pb, Rb-Sr, and Sm-Nd isotopic data

The intrusive rocks associated with the large Nezhdaninka gold deposit (Au > 470 t) hosted in the Permian carbonaceous terrigenous sequence have been dated on zircon and rock-forming minerals with precision U-Pb (ID-TIMS) and Rb-Sr methods. The lamprophyre of the dike complex that occurs in the ore field and spatially is related to gold mineralization has concordant U-Pb zircon age (121 ± 1 Ma) and the same isochron Rb-Sr age (121.0 ± 2.8 Ma). The concordant U-Pb zircon age of granodiorite that dominates in the Kurum pluton is 94 ± 1 Ma, whereas the Rb-Sr isochron age of various intrusive rocks from this pluton is 1–4 Ma younger. This difference is caused by long-term cooling of the Kurum pluton and later closure of Rb-Sr isotopic system of biotite (300–350°C) and other rock-forming minerals as compared with U-Pb isotopic system of zircon (～ 900°C). The Rb-Sr age of quartz diorite from the Gel’dy group of stocks (92.6 ± 0.8 Ma) coincides within uncertainty limits with the age of the Kurum pluton. Thus, the rocks pertaining to two epochs of magmatic activity, which developed in the South Verkhoyansk Foldbelt and divided by a time span of 25–28 Ma, are documented in the Nezhdaninka ore field. Taking into account that the age of gold mineralization is no less than 120 Ma, the data obtained allow us to specify the previously proposed formation model of the Nezhdaninka deposit. These data give grounds to rule out the Late Cretaceous Kurum pluton and the Gel’dy group of stocks from constituents of the ore-magmatic system, and to suggest that an Early Cretaceous deep-seated magma source existed beneath the deposit. Along with host terrigenous rocks, this magma source participated in the supply of matter to the hydrothermal system. The Nd, Sr, and Pb isotopic systematics of igneous rocks and ore mineralization in the Nezhdaninka ore field show that the Early and Late Cretaceous magma sources were formed in the Precambrian crust dated at ～1.8 Ga.     

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.     

Alpine metamorphism in the south-east Tauern Window, Austria: 1. P–T variations in space and time

Abstract The Pennine rocks exposed in the south-east Tauern Window, Austria, contain mineral assemblages which crystallized in the mid-Tertiary ‘late Alpine’regional metamorphism. The pressure and temperature conditions at the thermal peak of this event have been estimated for rocks at four different structural levels using a variety of published and thermochemically derived geobarometers and geothermometers. The results are: (a) In the garnet+chlorite zone, 2–5 km structurally above the staurolite+biotite isograd: T= 490.50°C, P= 7° 1 kbar; (b) Within 0.5 km of the staurolite+biotite isograd: T= 560±300C, P=7.1 kbar; (c) In the staurolite+biotite zone, c. 2.5 km structurally below the staurolite+biotite isograd: T= 610±30°C, P=7.6±1.2 kbar; (d) In the staurolite+biotite zone, 3–4 km structurally below the staurolite+biotite isograd: T= 630±40°C, P= 6.6±1.2 kbar. The pressure estimates imply that the total thickness of overburden above the basement-cover interface in the mid-Tertiary was c. 26.4 km. This overburden can only be accounted for by the Austro-Alpine units currently exposed in the vicinity of the Tauern Window, if the Altkristallin (the ‘Middle Austro-Alpine’nappe) was itself buried beneath an ‘Upper Austro-Alpine’nappe or nappe-pile which was 7.4 km thick at that time. The occurrence of epidote + margarite + quartz pseudomorphs after lawsonite in garnet, indicates that part of the Mesozoic Pennine cover sequence in the south-east Tauern experienced blueschist-facies conditions (T<450°C, P<12 kbar) in early Alpine times. Evidence from the central Tauern is used to argue that the blueschist-facies imprint post-dated the main phase of tectonic thickening (D¹_A) and was thus a direct consequence of continental collision. Combined oxygen-isotope and fluid-inclusion studies on late-stage veins, thought to have been at lithostatic pressure and in thermal equilibrium with their host rocks during formation, suggest that they crystallized from aqueous fluids at 1.1±0.4 kbar and 420.20°C. Early Alpine, late Alpine and vein-formation P–T constraints have been used to construct a P–T path for the base of the Mesozoic cover sequence in the south-east Tauern Window. The prograde part of the P–T path, between early and late Alpine metamorphic imprints, is unlikely to have been a smooth curve and may well have had a low dP/dT overall; the decompression (presumably due to erosion) which occurred immediately before the thermal peak and possibly also earlier in the Tertiary, was probably partly or completely cancelled by the effects of early- to mid-Tertiary (D²_A) tectonic thickening. The thermal peak of metamorphism was followed by a phase of almost isothermal decompression, which implies a period of rapid uplift in the middle Tertiary. The peak metamorphic P–T estimates are compared with the solutions of England's (1978) one-dimensional conductive thermal model of the Eastern Alps, and are shown to be consistent with the idea that the late Alpine metamorphism was caused by tectonic burial of the Pennine Zone beneath the Austro-Alpine nappes in the absence of extraneous heat sources, such as large intrusions, at depth.     

Isotopic geochronology of mesozoic acid volcanic rocks in Zhejiang Province,China

Volcano-sedimentary series of the Upper Jurassic to the Lower Cretaceous are extensively developed in Zhejiang Province. But ages and stratigraphic correlation concerning these rocks have long been a controversial problem. Systematic sampling was made of volcanic rocks of the Laocun, Huangjian, Shouchang and Moshishan Formations in western Zhejiang considered thus far as the Late Jurassic. Isotopic age determinations show that U-Th-Pb zircon ages are approximately concordant with Rb-Sr isochron ages, whereas K-Ar biotite ages and K-Ar isochron ages are all slightly lower. It can therefore be established that the ages of volcanic rocks mentioned above range from 134±6 to 122±2 m.y., corresponding to the “transitional period” from Jurassic to Cretaceous. It can also be concluded that the rocks have not undergone apparent epigenetic metamorphism. The initial⁸⁷Sr/⁸⁶Sr ratio is about 0.7089–0.7121, on the basis of which it may be postulated that the volcanic magma seems to have originated from the upper mantle with contamination by sialic materials subsequent to differentiation. For age determinations of such acid volcanic rocks Rb-Sr isochron method is considered more suitable in view of its following advantages: the high reliability of results; wide applicability to different samples; smaller sample requirement and the possibility for further studies involving petrogenesis by use of initial⁸⁷Sr/⁸⁶Sr ratio.     

Syn‐metamorphic folding in the Tauern Window,Austria dated by Th–Pb ages from individual allanite porphyroblasts

High‐precision ²³²Th–²⁰⁸Pb dates have been obtained from allanite porphyroblasts that show unambiguous microstructural relationships to fabrics in a major syn‐metamorphic fold in the SE Tauern Window, Austria. Three porphyroblasts were analysed from a single garnet mica schist from the Peripheral Schieferhülle in the core of the Ankogel Synform, one of a series of folds which developed shortly before the thermal peak of Alpine epidote–amphibolite facies metamorphism: allanite grain 1 provided two analyses with a combined age of 27.7 ± 0.7 Ma; grain 2, which was slightly bent and fractured during crenulation, provided two analyses with a combined age of 27.7 ± 0.4 Ma; a single analysis from grain 3, which overgrew an already crenulated fabric, gave an age of 28.0 ± 1.4 Ma. The five ²³²Th–²⁰⁸Pb ages agree within error and define an isochron with an age of 27.71 ± 0.36 Ma (95% confidence level; MSWD = 0.46). The results imply that the crenulation event was in progress in a short interval (<1 Ma) c. 28 Ma, and that the Ankogel Synform was forming at this time. The thermal peak of regional metamorphism in the SE Tauern Window was probably attained shortly after 28 Ma, only c. 5 Ma after eclogite facies metamorphism in the central Tauern Window. Metasediment may contain allanite porphyroblasts with clear‐cut microstructural relationships to fabric development and metamorphic crystallization; for such rocks, ²³²Th–²⁰⁸Pb dating on microsamples offers a powerful geochronological tool.     

Rb/Sr geochronology in the Eastern Alps

New Rb/Sr data on mineral and whole rock samples from in and around the south-east corner of the Tauern Window are presented. Pennine orthogeneisses yield an Rb/Sr whole rock age of 279±9 m.y., while orthogneiss samples from the Altkristallin Sheet near Innerkrems, Carinthia, yield an age of 381±30 m.y. by the same technique. The apparent mineral age break across the margins of the Tauern Window is investigated in an area of good structural and petrofabric control. The post-Palaeozoic history of the Eastern Alps is then discussed in the context of the available Rb/Sr data. It is argued that the bulk of the Katschberg Phyllites are of pre-Mesozoic age; that the major overthrusting movements of the Austroalpine Units were completed by 60–65 m.y.; and that the Peri-Adriatic intrusives can be little older than middle Tertiary.     

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.     

Age and isotopic studies of some Pan-African granites from North-Central Nigeria

Twenty-nine Rb-Sr whole-rock isotopic analyses and three U-Pb zircon analyses on foliated granites and largely unfoliated charnockitic rocks indicate that the central part of the Pan-African belt in west Africa was characterised by intense orogenic plutonism. These data and Rb-Sr analyses on muscovite books from late cross-cutting pegmatites indicate that the peak of magmatic activity occurred 610 ± 10 m.y. ago.Initial ⁸⁷Sr/⁸⁶Sr ratios for the granitic and charnockitic rocks are in the range 0.7065–0.7125, and indicate a significantly older crustal component in the magmas.     

Rb-Sr age of Godhra and related granites,Gujarat, India

Rubidium and strontium determinations are reported for Godhra and geographically related granites from central Gujarat. The whole rock data define a Rb-Sr isochron corresponding to a common age of 955±20 m.y. and initial Sr ratio of 0·7130±0·001. This age is distinctly older than the age of 735 m.y. reported for the Erinpura suite of rocks from Mount Abu in western Rajasthan and from Idar in northern Gujarat. There are at least two generations of post-Delhi intrusive rocks in the Gujarat precambrian. Biotites associated with these granites have the same age as the whole-rocks within experimental error indicating the absence of significant metamorphic heating since the time of emplacement. It is significant that rocks of similar age occur in the Rajasthan Precambrian mainly in the axial zone of the Aravalli Mountains.