The age of the Mont-Blanc granite (Western Alps) : a heterogeneous isotopic system dated by Rb-Sr whole rock determinations on its microgranular enclaves

New Rb-Sr whole rock age data have been obtained for the Mont-Blanc calc-alkaline Hercynian granite. Different facies of the granite have been sampled, as well as locally abundant microgranular enclaves of magmatic origin. The analytical data for the granite and for some of the enclaves are scattered within the Sr evolution diagram, which points to isotopic disequilibrium. Other enclaves form a homogeneous system which gives an age of 316.1 ± 19.5 Ma and an initial 87Sr/86Sr ratio of 0.7058 ± 0.0005. The following evolution is plausible: - individualization of microgranular enclaves due to the dismembering of mafic synplutonic dikes within the granitic magma; - complete isotopic equilibrium between granite and enclaves during a late-magmatic stage; - remobilization ofthe granite and certain enclaves in a post-magmatic stage and during Alpine orogenesis. Certain enclaves are not affected and retain the original characteristics of the system. The 316 Ma age of these "relics" and their initial ratio of 0.7058 are therefore thought to be valid for the granite as well. These results are similar to those obtained for other calc-alkaline Hercynian plutons in Western Europe. They indicate an origin for the granitic magma in the deep crust with involvement of mantle-derived material in unknown proportions, represented by the microgranular enclaves

Stability and isotopic dating of monazite and allanite in partially molten rocks: examples from the Central Alps

We investigated the stability of monazite and allanite as a function of bulk rock composition within several types of Tertiary Alpine anatexites, characterized by different compositions and melting reactions, but similar P-T conditions of melting. The investigated rocks consist of: (1) orthogneisses in which the melting reaction was triggered by water infiltration from the Bergell pluton; (2) anatectic tonalites, which were affected by water-assisted melting; and (3) metapelitic migmatites, which underwent muscovite dehydration melting. The studied anatexites cover a large range of Ca contents and water activities during partial melting, and allow an assessment of how much these parameters affect the stability of accessory phases. The different melting reactions that affected these rocks generated different water activities during the melt-present stage; they were highest in the water-saturated, contact metamorphic anatexites, and lowest in the metapelitic anatexites that underwent dehydration melting. These differences go together with different accessory phases within the migmatites. Whereas metapelitic anatexites only contain monazite, anatexites derived from tonalitic and granodioritic protoliths mainly contain allanite. This is consistent with observations made on Tertiary Alpine anatexites, suggesting that the growth of specific accessory phases is determined by the water activity and Ca content during melting. We measured single-grain monazite U/Pb isotope ages. One grain has relics of old cores, which have also been detected in Y-zonation patterns of the monazite. The data of unzoned monazites indicate partial melting in the Southern Steep Belt between 30.78 ± 0.14 and 28.10 ± 0.28M

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

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

Formation of intra-arc volcanosedimentary basins in the western flank of the central Peruvian Andes during Late Cretaceous oblique subduction: field evidence and constraints from U-Pb ages and Hf isotopes

During late Early to Late Cretaceous, the Peruvian coastal margin underwent fast and oblique subduction and was characterized by important arc plutonism (the Peruvian Coastal Batholith) and formation of volcanosedimentary basins known as the Western Peruvian Trough (WPT). We present high-precision U-Pb ages and initial Hf isotopic compositions of zircon from conformable volcanic and crosscutting intrusive rocks within submarine volcanosedimentary strata of the WPT hosting the Perubar massive sulfide deposit. Zircons extracted from both the volcanic and intrusive rocks yield concordant U-Pb ages ranging from 67.89±0.18Ma to 69.71±0.18Ma, indicating that basin subsidence, submarine volcanism and plutonic activity occurred in close spatial and temporal relationship within the Andean magmatic arc during the Late Cretaceous. Field observations, satellite image interpretation, and plate reconstructions, suggest that dextral wrenching movements along crustal lineaments were related to oblique subduction. Wrench tectonics is therefore considered to be the trigger for the formation of the WPT as a series of pull-apart basins and for the emplacement of the Coastal Batholith. The zircon initial εHf values of the dated magmatic rocks fall between 5.5 and 7.4, and indicate only very subordinate influence of a sedimentary or continental component. The absence of inherited cores in the zircons suggest a complete lack of old basement below the WPT, in agreement with previous U-Pb and Sr isotopic data for batholithic rocks emplaced in the WPT area. This is supported by the presence of a most likely continuous block of dense (~3.0g/cm3) material observed beneath the WPT area on gravimetric crustal cross sections. We suggest that this gravimetric anomaly may correspond to a piece of lithospheric mantle and/or oceanic crust inherited from a possible Late Permian-Triassic rifting. Such young and mafic crust was the most probable source for arc magmatism in the WPT are

Volcanic ash layers in the Upper Cretaceous of the Central Apennines and a numerical age for the early Campanian

At Montagna della Maiella and at Gola del Furlo (central Apennines) two discrete layers of bentonic clay are intercalated within the pelagic (Furlo) and turbiditic/pelagic limestones (Maiella) of the Upper Cretaceous basinal succession of the Umbrian basin (Scaglia facies). The bentonite layers are dated by planktonic foraminifera to the Globotruncanita elevata zone, early Campanian, and by calcareous nannofossils to the Aspidolithus parcus zone (CC 18); they fall into the reversed interval of chron 33. Detailed correlation shows the layers to be of exactly the same age. The upper layer is dated by U/Pb on magmatic zircons to 81.67±0.21Ma, an age compatible with the Cretaceous time-scale of Obradovich. The mineralogy of the bentonitic clays is almost pure montmorillonite and contrasts sharply with the clay mineral assemblage of the enclosing pelagic and turbiditic limestones, which is dominated by soil-derived smectite and illite in different proportions. The bentonite seams are interpreted as the submarine alteration products of wind-borne volcanic ashes. They can be followed with only minor changes in thickness over 200km and must be derived from distant volcanic sources and related to extreme volcanic events. A possible source area is present in the Dinarides where Upper Cretaceous subduction-related magmatic rocks are widesprea

The temporal evolution of the Mitu group, south-east Peru – first U-Pb age data

The Eastern Cordillera of southern Peru formed along a crustal zone that has been active as part of the western Gondwana margin since the middle Paleozoic. The present study investigates the Mitu Group of south-east Peru in the area of Abancay-Cusco-Sicuani-Titicaca. This unit comprises continental clastic sediments deposited in syn-sedimentary basins during an extensional period in Permo-Triassic times and has not benefitted from a thorough geochemical-geochronological investigation so far. One of the main reasons for this lack of data is a complex structure of the graben system, tectonically complicated by compressional inversion of the extensional basins during Andean orogeny. Due to dominating coarse-grained clastics, the Mitu Group is devoid of fossils and its age is only poorly bracketed to be Permo -Triassic based on its stratigraphic relation to the underlying Copacabana and overlying Pucara groups. The upper levels of the Copacabana have been constrained by palynology to the Artinskian (Doubinger and Marocco, 1981). However, a hiatus may be observed between the Copacabana and the Mitu groups in most places, rendering the age estimate of the basal Mitu imprecise. The Pucara Group, regarded by Rosas et al. (2007) as thermal sag after Mitu extension, is attributed to the late Triassic - early Jurassic on the basis of ammonite fossils and U-Pb zircon ages from ash beds (Schaltegger et al., 2008). The aim of this study is to provide more accurate and precise age constraints for the age and duration of the Mitu Group by using U-Pb geochronology of volcanic zircon in rhyolitic lavas, and of detrital zircon in clastic sediments. For andesitic volcanic lithologies, age approximations will be obtained by Ar-Ar techniques applied to amphibole and groundmass samples. Field data were obtained from a long and apparently complete section through the Mitu, situated 120km SE of Cusco near the city of Sicuani. This section consists of typical Mitu deposits; continental red beds, breccias and andesitic lavas. However, a zircon-bearing rhyolitic lava at the bottom gives us the opportunity to date the start of Mitu sedimentation by U-Pb ID-TIMS; this analysis will provide a precise age for the base of the Mitu group for the first time. In the Sicuani area the Mitu unconformably overlies the Ambo group, suggesting that the entire Copacabana is missing. Laser-ablation ICP-MS U-Pb data of detrial zircons from a sandstone just below the unconformity indicate a maximum age of latest Carboniferous (303Ma) for the underlying Ambo group. This maximum age overlaps with the palynological age of the lower Copacabana (Azcuy et al., 2002), raising the question whether the Ambo and Copacabana are truly diachronous or just coeval units of different sedimentary facies associations. In another section, 100km W of Cusco, near the city of Abancay, we found Mitu sediments overlying the Copacabana Group. Here the Copacabana contains well preserved plant fossils of the lycopsids family also found elsewhere in Peru and Bolivia. Lack of acidic volcanism during Mitu extension in this region prevents from dating of lavas using the U-Pb method. The detrital zircon population in a sandstone in the lowermost part of the Mitu was analysed for U-Pb ages, using LA-ICP-MS techniques. The youngest zircons in the population are around 235 Ma hence providing a maximum age for the onset of Mitu group sedimentation. The Artinskian age for the upper Copacabana from Doubinger and Marocco (1981) has also been obtained from the Abancay region, establishing a hiatus of some 50 Myrs between the two units. The Mitu Group is intruded by a 220 Ma granite body (Lipa and Saraiva, 2008) indicating significant burial of the sediments at this time. 500km SE of Cusco, on the Bolivian shores of lake Titicaca, the Ambo Group features plant fossils of the Lycopsids family like those found in the Copacabana near Abancay. Our detrital zircon LA-ICPMS study on a quartz arenite just below the fossils indicates a maximum U-Pb age of 343Ma. However a zircon-bearing ash bed will allow for more precise calibration of the fossil age by ID-TIMS techniques. The zircon U-Pb data will provide a test whether the Copacabana and the Ambo group are indeed diachronous or just lateral variations of a sedimentary system

Contrasting magma types and timing of intrusion in the Permian layered mafic complex of Mont Collon (Western Alps, Valais, Switzerland): evidence from U/Pb zircon and 40Ar/39Ar amphibole dating

Abstract.: We have selected and dated three contrasting rock-types representative of the magmatic activity within the Permian layered mafic complex of Mont Collon, Austroalpine Dent Blanche nappe, Western Alps. A pegmatitic gabbro associated to the main cumulus sequence yields a concordant U/Pb zircon age of 284.2 ±0.6Ma, whereas a pegmatitic granite dike crosscutting the latter yields a concordant age of 282.9 ±0.6Ma. A Fe-Ti-rich ultrabasic lamprophyre, crosscutting all other lithologies of the complex, yields an 40Ar/39Ar plateau age of 260.2 ±0.7Ma on a kaersutite concentrate. All ages are interpreted as magmatic. Sub-contemporaneous felsic dikes within the Mont Collon complex are ascribed to anatectic back-veining from the country-rock, related to the emplacement of the main gabbroic body in the continental crust, which is in accordance with new isotopic data. The lamprophyres have isotopic compositions typical of a depleted mantle, in contrast to those of the cumulate gabbros, close to values of the Bulk Silicate Earth. This indicates either contrasting sources for the two magma pulses - the subcontinental lithospheric mantle for the gabbros and the underlying asthenosphere for the lamprophyres - or a single depleted lithospheric source with variable degrees of crustal contamination of the gabbroic melts during their emplacement in the continental crust. The Mont Collon complex belongs to a series of Early Permian mafic massifs, which emplaced in a short time span about 285-280Ma ago, in a limited sector of the post-Variscan continental crust now corresponding to the Austroalpine/Southern Alpine domains and Corsica. This magmatic activity was controlled in space and time by crustal-scale transtensional shear zone

Cenozoic granitoids in the Dinarides of southern Serbia: age of intrusion, isotope geochemistry, exhumation history and significance for the geodynamic evolution of the Balkan Peninsula

Two age groups were determined for the Cenozoic granitoids in the Dinarides of southern Serbia by high-precision single grain U-Pb dating of thermally annealed and chemically abraded zircons: (1) Oligocene ages (Kopaonik, Drenje, Željin) ranging from 31.7 to 30.6Ma (2) Miocene ages (Golija and Polumir) at 20.58-20.17 and 18.06-17.74Ma, respectively. Apatite fission-track central ages, modelling combined with zircon central ages and additionally, local structural observations constrain the subsequent exhumation history of the magmatic rocks. They indicate rapid cooling from above 300°C to ca. 80°C between 16 and 10Ma for both age groups, induced by extensional exhumation of the plutons located in the footwall of core complexes. Hence, Miocene magmatism and core-complex formation not only affected the Pannonian basin but also a part of the mountainous areas of the internal Dinarides. Based on an extensive set of existing age data combined with our own analyses, we propose a geodynamical model for the Balkan Peninsula: The Late Eocene to Oligocene magmatism, which affects the Adria-derived lower plate units of the internal Dinarides, was caused by delamination of the Adriatic mantle from the overlying crust, associated with post-collisional convergence that propagated outward into the external Dinarides. Miocene magmatism, on the other hand, is associated with core-complex formation along the southern margin of the Pannonian basin, probably associated with the W-directed subduction of the European lithosphere beneath the Carpathians and interfering with ongoing Dinaridic-Hellenic back-arc extensio