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1

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Magma pulses in the Central Variscan Belt: episodic melt generation and emplacement during lithospheric thinning (1997)

Schaltegger, Urs

Oxford, UK : Blackwell Publishing Ltd

Terra nova 9 (1997), S. 0

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ISSN: 1365-3121

Source: Blackwell Publishing Journal Backfiles 1879-2005

Topics: Geosciences

Notes: Magmatism in the central part of the Variscan orogenic belt in Europe has occurred in several distinct episodes. Each episode spanned a few million years and was characterized by a distinct geochemical signature and geodynamic setting: (i) between 340 and 330 Ma (Visean), high-K diorites and granitoids within a zone extending parallel the orogen axis, possibly tracing a palaeosuture; (ii) at 310–307 Ma (Westphalian), calc-alkaline granitoids in the Intra-Alpine domain; (iii) at 304–295 Ma (Stephanian), subalkaline granitoids in Intra-Alpine and Pennine domains; and finally (iv) at 280–270 Ma (Permian) granitoids in Pennine, Austroalpine and South-Alpine domains. The generation and emplacement of granitoid melts in the Variscan crust are suggested to be short-term pulses that are controlled by extensional processes in the lithosphere such as thermal erosion and relaxation of the lithospheric mantle, and decompressional melting.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1111/j.1365-3121.1997.tb00021.x

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2

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Multiple mantle sources during island arc magmatism: U–Pb and Hf isotopic evidence from the Kohistan arc complex, Pakistan (2002)

Schaltegger, Urs ; Zeilinger, Gerold ; Frank, Martin ; [et al.]

Oxford, UK : Blackwell Science Ltd

Terra nova 14 (2002), S. 0

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ISSN: 1365-3121

Source: Blackwell Publishing Journal Backfiles 1879-2005

Topics: Geosciences

Notes: This paper presents high-precision U–Pb ages and initial Hf isotopic compositions of zircon from mafic to felsic rocks of the Kohistan Arc Complex, Pakistan. Three magmatic pulses tapping geochemically different reservoirs are distinguished. Partial melting of mantle with MORB-type isotopic characteristics generated 99–92-Ma-old magmas. Plutonism around 85 Ma tapped a more fertile mantle source, most likely consisting of a 〉600-Ma-old metasomatically enriched mantle, or of mantle contaminated by an old sedimentary component; 82-Ma-old felsic peraluminous dykes have MORB-type isotopic compositions considered to be inherited from remelting earlier magmas in the deep base of the arc. The isotopic results demonstrate several and rather rapid changes in melt source region during arc development. They also show that there was subordinate continental influence and negligible importance of slab components for the Hf budget during the generation of the Kohistan Arc Complex.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1046/j.1365-3121.2002.00432.x

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3

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The transition from rifting to sea-floor spreading within a magma-poor rifted margin: field and isotopic constraints (2002)

Schaltegger, Urs ; Desmurs, Laurent ; Manatschal, Gianreto ; [et al.]

Oxford UK : Blackwell Science Ltd

Terra nova 14 (2002), S. 0

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ISSN: 1365-3121

Source: Blackwell Publishing Journal Backfiles 1879-2005

Topics: Geosciences

Notes: We provide new geological and isotope geochemical constraints on the evolution from continental rifting to sea-floor spreading along a segment of the Jurassic Tethyan margin exposed in the Platta and Err nappes (eastern Central Alps). Field observations show that the ocean–continent transition zone is characterized by oceanward-dipping detachment faults leading to the exhumation of subcontinental mantle rocks subsequently intruded by gabbro bodies and dolerite dikes, and covered by pillow basalts and radiolarites. Zircons extracted from gabbros and albitite yield concordant U–Pb ages of 161 ± 1 Ma; their initial ɛHf (+ 14.4 to + 14.9) as well as bulk rock ɛNd values of from gabbros and basalts (+ 7.3 to + 9.5) point to a MORB-type depleted mantle source. These data suggest that the onset of magmatic activity coincides with the latest phase of mantle exhumation along low-angle detachment faults and may be controlled by upwelling asthenosphere beneath a zone of exhumed continental mantle.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1046/j.1365-3121.2002.00406.x

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4

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Early Cambrian oceanic plagiogranite in the Silvretta Nappe, eastern Alps: geochemical, zircon U-Pb and Rb-Sr data from garnet-hornblende-plagioclase gneisses (1996)

Müller, Bernhard ; Schaltegger, Urs ; Klötzli, Urs ; [et al.]

Springer

International journal of earth sciences 85 (1996), S. 822-831

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ISSN: 1437-3262

Keywords: Austro-Alpine ; Eastern Alps ; Oceanic rocks ; Rb-Sr ; Zircon ; U-Pb ; Magmatism

Source: Springer Online Journal Archives 1860-2000

Topics: Geosciences

Notes: Abstract Garnet-hornblende-plagioclase gneisses rich in incompatible elements occur in the crystalline basement of the Austro-Alpine Silvretta nappe and are associated with clinopyroxene norites and harzburgite cumulates. It is proposed here that the gneisses were formerly oceanic plagiogranites. An εNd(530) value of +5.6 for the gneisses as well as initial87Sr/86Sr values of 0.7036–0.7037 for the gabbroic rocks and 0.7026–0.7027 for the ultramafic rocks suggest a mantle source for this rock association. The geochemical characteristics of the garnet-hornblende-plagioclase gneisses indicate that their precursors were derived by fractional crystallization from a basaltic parent magma, by the same process which produced the adjacent clinopyroxene norites and ultramafic cumulates as well. The combined U-Pb upper intercept ages of zircons from two gneiss samples yield an igneous crystallization age of 532 ± 30 Ma, similar to previously dated (mostly calcalkaline) orthogneisses in the same area. High-quality transparent zircons showed the least degree of discordance, but contain extremely low U and Pb levels. The rock suite, including this plagiogranite, was emplaced within oceanic crust which formed in the latest Precambrian-early Palaeozoic off the northern continental margin of Gondwana.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1007/BF02440113

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5

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The evolution of the polymetamorphic basement in the Central Alps unravelled by precise U−Pb zircon dating (1993)

Schaltegger, Urs

Springer

Contributions to mineralogy and petrology 113 (1993), S. 466-478

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ISSN: 1432-0967

Source: Springer Online Journal Archives 1860-2000

Topics: Geosciences

Notes: Abstract The U−Pb age determinations of zircon and rutile from the Aar massif reveal a complex evolution of the Central Alpine basement. The oldest components are found in zircons of metasediments, which bear cores of Archean age; the U−Pb age of discordant prismatic zircons of the same rocks ranges between 580 and 680 Ma, an age that is typical for Pan-African metamorphism. The zircons are interpreted as Pan-African detritus with Archean inheritance. The provenance region of the Pan-African zircons is assumed to be a terrane of Gondwana-affinity, i.e. the W. African craton or the Pentevrian microplate. The Caledonian metamorphism left a pervasive structural imprint in amphibolite facies on the rocks of the Aar massif; it is dated at 456±2 and 445 Ma by zircons of a layered migmatitic gneiss and a migmatitic leucosome, respectively, both occurring in the northernmost zones of the massif. Hercynian metamorphism never exceeded greenschist-facies conditions and is recorded by zircon in a garnet-amphibolite and by rutile in a meta-psammite that yield an age of 330 Ma. Both zircon and rutile are considered to be products of retrograde mineral reactions and therefore do not date the peak conditions of Hercynian metamorphism. The Gastern granite at the western end of the Aar massif is a contaminated granite that intruded at 303±4 Ma, contemporaneously with the wide-spread late Hercynian post-collisional I-type magmatism. The study demonstrates the potential of isotope dilution U−Pb dating of single grains and microfractions in deciphering complex evolutionary histories of polymetamorphic terrains.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1007/BF00698316

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6

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The age and source of late Hercynian magmatism in the central Alps: evidence from precise U−Pb ages and initial Hf isotopes (1992)

Schaltegger, Urs ; Corfu, Fernando

Springer

Contributions to mineralogy and petrology 111 (1992), S. 329-344

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ISSN: 1432-0967

Source: Springer Online Journal Archives 1860-2000

Topics: Geosciences

Notes: Abstract This study presents U−Pb ages for zircon, titanite, allanite and epidote, and initial Hf isotopic compositions for zircon of Upper Carboniferous granites, diorites and syenites from the Aar massif, central Alps. The rocks were emplaced during three magmatic pulses after Hercynian collisional tectonics: (A) a shoshonitic-ultrapotassic series at 334±2.5 Ma; (B) scattered diorites and granites at 308–310 Ma; and (C) a high-K cale-alkaline granite batholith at 298±2 Ma. Inheritance of old zircons is negligible among all three groups. The Southern Aar granite, in contrast, is a syn-tectonic, probably ca. 350 Ma old granite that contains large amounts of inherited Precambrian zircons. Alpine metamorphism caused weak lead loss in many analyzed zircon fractions, but left the titanite U−Pb system undisturbed: thorites were almost completely reset by Alpine and recent lead loss. Mineral isochrons defined by titanite, allanite, epidote and apatite yield initial Pb isotopic compositions that are in agreement with the model values of Stacey and Kramers. Initial Hf isotopic compositions range from ɛHf=−8 to +3.5. The data follow a trend of increasing ɛHf with decreasing age. The ɛHf versus element concentration relationships suggest mixing between a mantle and a crustal component. These relationships can be explained in terms of generation of the melts from a subcontinental mantle that had been enriched during subduction events at about 1 Ga and by 300 Ma had developed an isotopic signature distinct from that of MORB-type mantle. Further contamination of the melts occurred during ascent and differentiation in the crust. This late Hercynian magmatism can be related to post-collisional strike-slip tectonics.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1007/BF00311195

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7

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Post-granulite facies monazite growth and rejuvenation during Permian to Lower Jurassic thermal and fluid events in the Ivrea Zone (Southern Alps) (1999)

Vavra, Gerhard ; Schaltegger, Urs

Springer

Contributions to mineralogy and petrology 134 (1999), S. 405-414

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ISSN: 1432-0967

Source: Springer Online Journal Archives 1860-2000

Topics: Geosciences

Notes: Abstract U-Pb analyses of single monazite grains from two granulite facies metapelites in the Ivrea Zone (Southern Alps) reveal the presence, in both samples, of at least three different ages and prove that earlier interpretations of supposedly concordant monazite data as cooling ages are unwarranted. One group of monazite data defines a subconcordant discordia line with an upper intercept age of 293.4 ± 5.8 Ma and a lower intercept age of 210 ± 14 Ma. The upper intercept is interpreted as the real cooling age of the monazites. The lower intercept is interpreted as an episode of fluid-driven Pb-loss, indicated by the presence of internal and external corrosion structures not only of the monazites but also of the zircons in the same samples that are also rejuvenated at 210 ± 12 Ma. Another group of monazite data lies above the concordia. The presence of excess 206Pb indicates that these crystals have grown below the monazite blocking temperature, thus after the granulite facies metamorphism. The age of growth of the new monazite crystals is approached by their 207Pb/235U ages that range between 273 and 244 Ma. The two groups of post-cooling age (post-293.4 ± 5.8 Ma) monazite data correspond to two distinct late- and post-Variscan geotectonic regimes that affected the Southern Alps, (1) Permian transtension with decompression and anatectic melting; (2) Upper Triassic to Lower Jurassic rifting with geographically dispersed hydrothermal activity and alkaline magmatism.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1007/s004100050493

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8

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Early Cambrian oceanic plagiogranite in the Silvretta Nappe, eastern Alps: geochemical, zircon U–Pb and Rb–Sr data from garnet–hornblende–plagioclase gneisses (1996)

Müller, Bernhard ; Klötzli, Urs ; Schaltegger, Urs ; [et al.]

Springer

Geologische Rundschau 85 (1996), S. 822-831

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ISSN: 0016-7835

Keywords: Key words Austro-Alpine ; Eastern Alps ; Oceanic rocks ; Rb ; Sr ; Zircon ; U ; Pb ; Magmatism

Source: Springer Online Journal Archives 1860-2000

Topics: Geosciences

Notes: Abstract Garnet–hornblende–plagioclase gneisses rich in incompatible elements occur in the crystalline basement of the Austro-Alpine Silvretta nappe and are associated with clinopyroxene norites and harzburgite cumulates. It is proposed here that the gneisses were formerly oceanic plagiogranites. An εNd( 530 ) value of +5.6 for the gneisses as well as initial 87Sr/86Sr values of 0.7036–0.7037 for the gabbroic rocks and 0.7026–0.7027 for the ultramafic rocks suggest a mantle source for this rock association. The geochemical characteristics of the garnet–hornblende–plagioclase gneisses indicate that their precursors were derived by fractional crystallization from a basaltic parent magma, by the same process which produced the adjacent clinopyroxene norites and ultramafic cumulates as well. The combined U–Pb upper intercept ages of zircons from two gneiss samples yield an igneous crystallization age of 532±30 Ma, similar to previously dated (mostly calc-alkaline) orthogneisses in the same area. High-quality transparent zircons showed the least degree of discordance, but contain extremely low U and Pb levels. The rock suite, including this plagiogranite, was emplaced within oceanic crust which formed in the latest Precambrian–early Palaeozoic off the northern continental margin of Gondwana.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1007/s005310050114

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