Fritz Finger

Do U-Pb zircon ages from granulites reflect peak metamorphic conditions?

Granulite facies metamorphic events are constrained commonly through application of U-Pb zircon geochronometry. Zircon growth related to high-grade metamorphism is interpreted as reflecting the age of peak pressure-temperature ( P-T ) conditions. However, these ages obtained from granulites need to be interpreted with considerable care. Under conditions of high-grade metamorphism, it is important that the possible presence of melt is considered. Our modeling of partial melting and its impact on zircon stability implies that zircon crystallization in hot, isothermally uplifted granulites could postdate the pressure peak of the P-T path. In a case study of felsic granulites from the Bohemian massif of Variscan central Europe, it appears likely that most zircons in the rocks would have grown after they were exhumed to medium pressure levels. Thus, zircon growth related to high-grade metamorphism should not be automatically assumed as reflecting the age of peak P-T conditions.

Replacement of primary monazite by apatite-allanite-epidote coronas in an amphibolite facies granite gneiss from the eastern Alps

Abstract Accessory monazite crystals in granites are commonly unstable during amphibolite facies regional metamorphism and typically become mantled by newly formed apatite-allanite- epidote coronas. This distinct textural feature of altered monazite and its growth mechanism were studied in detail using backscattered electron imaging in a sample of metagranite from the Tauern Window in the eastern Alps. It appears that the outer rims of the former monazites were replaced directly by an apatite ring with tiny thorite intergrowths in connection with Ca supply through metamorphic fluid. Around the apatite zone, a proximal allanite ring and a distal epidote ring developed. This concentric corona structure, with the monazite core regularly preserved in the center, shows that the reaction kinetics were diffusion controlled and relatively slow. Quantitative electron microprobe analyses suggest that the elements released from monazite breakdown (P, REE, Y, Th, U), were diluted and redistributed in the newly formed apatite, allanite, and epidote overgrowth rings and were unable to leave the corona. This supports the common hypothesis that these trace elements are highly immobile during metamorphism. Furthermore, microprobe data suggest that the preserved monazite cores lost little, possibly none of their radiogenic lead during metamorphism. Thus, metastable monazite grains from orthogneisses appear to be very useful for constraining U-Th-Pb protolith ages. On the basis of these findings and a review of literature data, it seems that monazite stability in amphibolite facies metamorphic rocks depends strongly on lithologic composition. While breaking down in granitoids, monazite may grow during prograde metamorphism in other rocks such as metapelites.

Variscan granitoids of central Europe: their typology, potential sources and tectonothermal relations

SummaryDuring the Variscan orogenic cycle, central Europe was intruded by numerous granitoid plutons. Typological and age relationships show that the characteristics of the granitoid magmatism changed during the course of the Variscan orogeny. Five genetic groups of granitoids may be distinguished:1.Late Devonian to early Carboniferous “Cordilleras” I-type granitoids (ca. 370-340 Ma): These early Variscan granitoids are mainly tonalites and granodiorites. They often have hornblende and occur in association with diorites and gabbros. They form plutonic massifs in the Saxothuringian unit, in Central Bohemia and the intra-Alpine Variscides. In terms of existing models, they can be interpreted as volcanic arc granites, being related to the subduction of early Variscan oceans. Models involving mantle sources and AFC may be feasible.2.Early Carboniferous, deformed S-type granite/migmatite associations (ca. 340 Ma): These occur in the footwall of a thick thrust in Southern Bohemia (Gföhl nappe) and seem to represent a phase of water-present, syn-collisional crustal melting related to nappe stacking.3.Late Visean and early Namurian S-type and high-K, I-type granitoids (ca. 340-310 Ma): These granitoids are mainly granitic in composition and particularly abundant along the central axis of the orogen (Moldanubian unit). This zone experienced a high heat flow at this time, probably as a consequence of post-collisional extension and magmatic underplating. Most of group 3 granitoids formed through high-T fluid-absent melting in the lower crust. Enriched mantle melts interacted with some crustal magmas on a local scale to form durbachites. Partial melting events in the middle crust produced a number of high-T/low-P, S- and I-type diatexites and some S-type granite magmas.4.Post-collisional, epizonal I-type granodiorites and tonalites (ca. 310-290 Ma): These plutons can be found throughout the Central European Variscides. However, most of them occur in the Alps (near the southern flank of the orogen). Such late I-type plutons could be related to renewed subduction along the southern fold belt flank, and/or to extensional decompression melting near the crust/mantle boundary. Post-collisional mantle or slab melting may have occurred in connection with remnant subduction zones below the orogen undergoing thermal relaxation and dehydration.5.Late Carboniferous to Permian leucogranites (ca. 300-250 Ma): Many of these rocks are similar to sub-alkaline A-type granites. Potential sources for this final stage of plutonism could have been melt-depleted lower crust or lithospheric mantle.ZusammenfassungIm Verlauf der variszischen Orogenese intrudierten im mitteleuropäischen Raum große Massen von Granitoiden. Eine Bewertung geochronologischer and granittypologischer Daten zeigt, daß sich die Magmencharakteristik mit der Zeit verändert hat. Fünf Hauptgruppen von Granitoiden können unterschieden werden:1.I-Typ Granitoide des sädten Devon and fruhen Karbon (ca. 370-340 Ma): Es handelt sich dabei durchwegs um I-Typ Tonalite and Granodiorite, welche häufig Hornblende fühen. Typisch für these Plutone ist die Präsenz gabbroischer oder dioritischer Endglieder. Eine Magmenentstehung aus Mantelquellen mit Modifikation durch AFC und eine genetische Verbindung zu frühvariszischen Subduktionszonen ist denkbar.2.Syntektonische S-Typ Granite and Migmatite (ca. 340 Ma): Große Massen solcher Granitoide treten im Deckenstapel der südlichen Böhmischen Masse auf. Sie repräsentieren wassergesättigte, syn-kollisionale Krustenschmelzen, die sich in der Nähe von tektonischen Überschiebungsbahnen gebildet haben.3.S-Typ and kalireiche L-Typ Granitoide des spdten Vise and fruhen Namur (ca. 340-310 Ma): Diese Plutone haben in der Regel granitische Zusammensetzung und intrudierten vornehmlich in der moldanubischen Zentralzone des Orogens. Die dortige kontinentale Kruste war zu dieser Zeit einem extrem hohen Wärmefluß ausgesetzt, der vermutlich durch postkollisionale Extension mit rascher Krustenhebung und magmatischem „underplating” verursacht wurde. Die meisten dieser Granite bildeten sich durch Dehydratationsschmelzen der Unterkruste aus Paragneisen und eventuell auch intermediären kaliumreichen Orthogneisen. Einige wenige Plutone zeigen Interaktionen mit mafischen Magmen, die aus einem angereicherten Lithosphärenmantel stammen (Durbachite). Schmelzprozesse in der mittleren Kruste führten weiträumig zur Bildung von Migmatiten mit grßgen Anteilen an S-Typ and I-Typ Diatexiten.4.Postkollisionale, epizonale I-Typ Granodiorite and Tonalite (ca. 310-290 Ma): Die Hauptverbreitung dieser Plutone liegt in den Alpen. Eine genetische Verbindung zu einer spätvariszischen Subduktionszone am Variszikums-Siidrand erscheint möglich. Andererseits könnte auch die bloße Reaktivierung and Dehydratation von alten (frühvariszischen) Subduktionszonen unter dem Orogen die Produktion entsprechender I-Typ Magmen bewirkt haben, ebenso wie ein postkollisionales Druckentlastungsschmelzen von I-Typ Quellen im Bereich der Krusten-Mantel Grenze ohne Subduktionzusammenhang.5.Leukogranite des sädten Karbon and Perm (ca. 300-250 Ma): Viele dieser Plutone zeigen Eigenschaften von A-Typ Graniten. Die entsprechenden Magmen sind vermutlich durch Schmelzprozesse in einer restitischen Unterkruste oder im lithosphärischen Mantel entstanden.

Deducing the ancestry of terranes: SHRIMP evidence for South America–derived Gondwana fragments in central Europe

We present here an example of how the sensitive high-resolution ion microprobe (SHRIMP) zircon dating method can provide a terrane-specific geochronological fingerprint for a rock and thus help to reveal major tectonic boundaries within orogens. This method, applied to inherited zircons in a ca. 580 Ma metagranitoid rock from the eastern Bohemian Massif, has provided, for the first time in the central European Variscan basement, unequivocal evidence for Mesoproterozoic and late Paleoproterozoic geologic events ca. 1.2 Ga, 1.5 Ga, and 1.65–1.8 Ga. The recognition of such zircon ages has important consequences because it implies that parts of the Precambrian section of Variscan central Europe were originally derived from a Grenvillian cratonic province, as opposed to the common assumption of an African connection. A comparison with previously published SHRIMP data suggests, however, that these Mesoproterozoic and late Paleoproterozoic zircon ages may be restricted to the Moravo-Silesian unit in the eastern Variscides, whereas the Saxothuringian and Moldanubian zones appear to contain a typical north African (i.e., Neoproterozoic plus Eburnian) inherited-zircon age spectrum. This finding supports new tectonic concepts, according to which Variscan Europe is composed of a number of completely unrelated terranes with extremely different paleogeographic origins. The Moravo-Silesian unit can be best interpreted as a peri-Gondwana terrane, which was situated in the realm of the Amazonian cratonic province by the late Precambrian, comparable to the Avalonian terranes of North America and the United Kingdom.

Deciphering the petrogenesis of deeply buried granites: whole-rock geochemical constraints on the origin of largely undepleted felsic granulites from the Moldanubian Zone of the Bohemian Massif

The prominent felsic granulites in the southern part of the Bohemian Massif (Gfohl Unit, Moldanubian Zone), with the Variscan (∼340 Ma) high-pressure and high-temperature assemblage garnet+quartz+hypersolvus feldspar ± kyanite, correspond geochemically to slightly peraluminous, fractionated granitic rocks. Compared to the average upper crust and most granites, the U, Th and Cs concentrations are strongly depleted, probably because of the fluid and/or slight melt loss during the high-grade metamorphism (900–1050°C, 1·5–2·0 GPa). However, the rest of the trace-element contents and variation trends, such as decreasing Sr, Ba, Eu, LREE and Zr with increasing SiO 2 and Rb, can be explained by fractional crystallisation of a granitic magma. Low Zr and LREE contents yield ∼750°C zircon and monazite saturation temperatures and suggest relatively low-temperature crystallisation. The granulites contain radiogenic Sr ( 87 Sr/ 86 Sr 340 = 0·7106–0·7706) and unradiogenic Nd ( = − 4·2 to − 7·5), indicating derivation from an old crustal source. The whole-rock Rb–Sr isotopic system preserves the memory of an earlier, probably Ordovician, isotopic equilibrium. Contrary to previous studies, the bulk of felsic Moldanubian granulites do not appear to represent separated, syn-metamorphic Variscan HP–HT melts. Instead, they are interpreted as metamorphosed (partly anatectic) equivalents of older, probably high-level granites subducted to continental roots during the Variscan collision. Protolith formation may have occurred within an Early Palaeozoic rift setting, which is documented throughout the Variscan Zone in Europe.

The Brunovistulian: Avalonian Precambrian sequence at the eastern end of the Central European Variscides?

Abstract An outline is presented of the present state of research on the Precambrian evolution history of the Brunovistulian, a large (30 000 km2), mainly sediment covered Peri-Gondwana basement block at the eastern end of the Central European Variscides. On the basis of recent chemical, isotopic and geochronological data it is argued that the eastern half of the Brunovistulian (Slavkov Terrane) originated in an island-arc environment, documenting the rare case of Neoproterozoic crustal growth in central Europe. The western half of the Brunovistulian, the Thaya Terrane, includes more mature, recycled cratonic material and is considered to have been originally part of the Neoproterozoic Gondwana continent margin. A phase of regional metamorphism at c. 600 Ma, followed by extensive granitoid plutonism, probably marks the stage when the Slavkov Terrane was accreted to the Thaya Terrane by arc-continent collision. A belt of metabasites, which is intercalated between the two terranes, may represent relics of the incipient arc or a back-arc basin. A comparison of geochronological data shows that the timing of geological events recorded in the Brunovistulian does not correlate with the evolution history of the Cadomian crust in the Teplá-Barrandian zone and the Saxo-Thuringian belt. This supports the theory that the Brunovistulian is not part of Armorica but derived from a different sector of the Neoproterozoic Gondwana margin. A correlation with the Avalonian superterrane appears feasible.

Late Variscan Magmatic Evolution of the Alpine Basement

After having experienced the Variscan orogenic episodes, the pre-Mesozoic Alpine basement was subjected to large-scale shearing effects accompanying lithosphere distensional thinning, “Basin and Range”-like tectonics and high geothermal regimes. As a result of intrusion of mantle-derived magmas and induced crustal anatexis, almost all pieces of basement within the Alpine chain display contrasting magma associations.

Migmatization and “secondary” granitic magmas: effects of emplacement and crystallization of “primary” granitoids in Southern Bohemia, Austria

The Sauwald and Mühl zones of the prebatholithic, Moldanubian, middle crust in northern Austria contain metapelites and metaluminous to weakly peraluminous metagreywackes, respectively. Both zones were affected by low-pressure, high-temperature metamorphism and anatexis. The metapelites of the Sauwald zone became in-situ diatexites, probably by fluid-absent reactions involving the breakdown of muscovite and the partial breakdown of biotite. The biotite-plagioclase-quartz gneisses of the Mühl zone experienced only slight melting. Following this event, and while the mid crust was still hot, additional heat was locally advected into the Mühl zone by the intrusion of the Weinsberg granite. This brought about fluid-present partial melting of the biotite-plagioclasequartz gneisses, producing relatively large volumes of metaluminous to weakly peraluminous, I-type Schlieren granite. This cool, wet, restite-rich magma remained close to its site of generation. Thus, infracrustal I-type granitoids may be formed anywhere in the crust, and not always at high T. Under special circumstances the heat and fluids from granitic magmas can spawn “secondary” granites. Also, relatively low initial 87Sr/86Sr values (of around 0.707) in I-type rocks do not necessarily indicate either lower crustal magma sources or mixing with mantle-derived magma. The Weinsberg granite magma came from the lower crust (P probably <700 MPa), where widespread fluid-absent breakdown of biotite-plagioclase-quartz assemblages occurred. The necessary high heat flow was probably provided by newly underplated mafic magmas. However, these seem not to have mixed or mingled with the crustally derived Weinsberg magmas. Deep equivalents of the Mühl-zone metagreywackes may have formed the Weinsberg protolith. Fluid-absent experiments show that the melting temperature probably exceeded 850°C and that a garnet-bearing, orthopyroxene-rich residue should be present in the lower crust. Fluid-present experiments demonstrate that the availability of free H2O can radically alter the characteristics of the partial melts, from apparent S-type mineralogy (with fluid-absent melting) to I-type mineralogy (with wet melting).

Hybrids, magma mixing and enriched mantle melts in post-collisional Variscan granitoids: the Rastenberg Pluton, Austria

Abstract The composite Rastenberg Pluton in the South Bohemian Massif preserves an example of generation of relatively homogeneous granitoid hybrids by mixing of mafic and felsic magmas. Metaluminous melagranites and quartz monzonites, the main lithologies of the pluton, are interpreted to be hybrids. In contrast, a slightly peraluminous biotite granodiorite is considered to be a lower-crustal melt. In addition, abundant mafic ultrapotassic enclaves, country-rock lamprophyres and quartz monzodiorite bodies represent distinct lithospheric mantle-derived magmas, which were only slightly modified by fractionation and/ or magma mixing. Almost continuous linear chemical and isotope correlations joining the enclaves, quartz monzonites, melagranites and granodiorites indicate the importance of a mixing process in the generation of these rocks. Incompatible elements decrease with increasing silica and moderate negative Eu anomalies disappear towards the granodioritic endmember. Pb, Sr and Nd isotopes show typical crustal values in all granitoids but are more radiogenic in the ultrapotassic endmember. In the Rastenberg Pluton, the interaction of mantle- and crustal-derived magmas has produced unequivocal petrographic, chemical and isotopic evidence for mixing and mingling. Because similar features are lacking in most other Variscan plutons, we suggest that mantle magmas were not substantially involved in their genesis. Nevertheless, small-volumes of hybrids, involving variably enriched mantle-derived melts, do crop out locally throughout the central Variscides. In view of the generally strongly enriched nature of these small volume hybrid magmas, we suggest that voluminous mantle melting and large-scale magmatic underplating are unlikely to have occurred.

The Saxo-Danubian Granite Belt: magmatic response to post-collisional delamination of mantle lithosphere below the southwestern sector of the Bohemian Massif (Variscan orogen)

The Saxo-Danubian Granite Belt: magmatic response to post-collisional delamination of mantle lithosphere below the southwestern sector of the Bohemian Massif (Variscan orogen) On the basis of the synchronicity of geochronological data and the similarity of granite types, it is proposed that the mid-Carboniferous Fichtelgebirge/Erzgebirge Batholith in the Saxothuringian Zone of the central European Variscan Fold Belt and the South Bohemian Batholith in the Moldanubian Zone (including the intervening Oberpfalz and Bavarian Forest granite areas) belong to one coherent and cogenetic, ca. 400 km long plutonic megastructure. Unlike older (syn-collisional) plutonic structures in the Bohemian Massif, this Saxo-Danubian Granite Belt (nov. nom.) has developed discordant to the Devonian/Early Carboniferous collision-related tectonic architecture of the Bohemian Massif. It is argued that the Saxo-Danubian Granite Belt formed in response to a post-collisional detachment of lithospheric mantle below the south-western sector of the Bohemian Massif.