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Notes: [Auszug] The timescales and mechanisms for the formation and chemical differentiation of the planets can be quantified using the radioactive decay of short-lived isotopes. Of these, the 182Hf-to-182W decay is ideally suited for dating ...

Notes: In the Orlica-Śnieżnik complex at the NE margin of the Bohemian Massif, high-pressure granulites occur as isolated lenses within partially migmatized orthogneisses. Sm–Nd (different grain-size fractions of garnet, clinopyroxene and/or whole rock) and U–Pb [isotope dilution-thermal ionization mass spectrometry (ID-TIMS) single grain and sensitive high-resolution ion microprobe (SHRIMP)] ages for granulites, collected in the surroundings of Červený Důl (Czech Republic) and at Stary Gierałtów (Poland), constrain the temporal evolution of these rocks during the Variscan orogeny. Most of the new ages cluster at c. 350–340 Ma and are consistent with results previously reported for similar occurrences throughout the Bohemian Massif. This interval is generally interpreted to constrain the time of high-pressure metamorphism. A more complex evolution is recorded for a mafic granulite from Stary Gierałtów and concerns the unknown duration of metamorphism (single, short-lived metamorphic cycle or different episodes that are significantly separated in time?). The central grain parts of zircon from this sample yielded a large spread in apparent 206Pb/238U SHRIMP ages (c. 462–322 Ma) with a distinct cluster at c. 365 Ma. This spread is interpreted to be indicative for variable Pb-loss that affected magmatic protolith zircon during high-grade metamorphism. The initiating mechanism and the time of Pb-loss has yet to be resolved. A connection to high-pressure metamorphism at c. 350–340 Ma is a reasonable explanation, but this relationship is far from straightforward. An alternative interpretation suggests that resetting is related to a high-temperature event (not necessarily in the granulite facies and/or at high pressures) around 370–360 Ma, that has previously gone unnoticed. This study indicates that caution is warranted in interpreting U–Pb zircon data of HT rocks, because isotopic rejuvenation may lead to erroneous conclusions.

Notes: Intrusion-related migmatites comprise a substantial part of the high-grade part of the southern Damara orogen, Namibia which is dominated by Al-rich metasedimentary rocks and various granites. Migmatites consist of melanosomes with biotite+sillimanite+garnet+cordierite+hercynite and leucosomes are garnet- and cordierite-bearing. Metamorphic grade throughout the area is in the upper amphibolite to lower granulite facies (5–6 kbar at 730–750 °C). Field evidence, petrographic observations, chemical data and mass balance calculations suggest that intrusion of granitic magmas and concomitant partial melting of metasedimentary units were the main processes for the generation of the migmatites. The intruding melts were significantly modified by magma mixing with in situ partial melts, accumulation of mainly feldspar and contamination with garnet from the wall rocks. However, it is suggested that these melts originally represented disequilibrium melts from a metasedimentary protolith. The occurrence of LILE-, HFSE- and LREE-enriched and -depleted residues within the leucosomes implies that both quartzo-feldspathic and pelitic rocks were subjected to partial melting. Isotope ratios of the leucosomes are rather constant (143Nd/144Nd (500 Ma): 0.511718–0.511754, ε Nd (500 Ma): −3.54 to −5.11) and Sr (87Sr/86Sr (500 Ma): 0.714119–0.714686), the metasedimentary units have rather constant Nd isotope ratios (143Nd/144Nd (500 Ma): 0.511622–0.511789, ε Nd (500 Ma): −3.70 to −6.93) but variable Sr isotope ratios Sr (87Sr/86Sr (500 Ma): 0.713527–0.722268). The most restitic melanosome MEL 4 has a Sr isotopic composition of 87Sr/86Sr (500 Ma): 0.729380. Oxygen isotopes do not mirror the proposed contamination process, due to the equally high δ18O contents of metasediments and crustal melts. However, the most LILE-depleted residue MEL 4 shows the lowest δ18O value (〈10). Mass balance calculations suggest high degrees of partial melting (20–40%). It is concluded that partial melting was promoted by heat transfer and release of a fluid phase from the intruding granites. High degrees of partial melting can be reached as long as the available H2O, derived from the crystallization of the intruding granites, is efficiently recycled within the rock volume. Due to the limited amounts of in situ melting, it seems likely that such regional migmatite terranes are not the sources for large intrusive granite bodies. The high geothermal gradient inferred from the metamorphic conditions was probably caused by exhumation of deep crustal rocks and contemporaneous intrusion of huge masses of granitoid magmas. The Davetsaub area represents an example of migmatites formed at moderate pressures and high temperatures, and illustrates some of the reactions that may modify leucosome compositions. The area provides constraints on melting processes operating in high-grade metasedimentary rocks.

Notes: Abstract The Erzgebirge Crystalline Complex (ECC) is a rare example where both‘crustal’eclogites and mantle-derived garnet-bearing ultramafic rocks (GBUs) occur in the same tectonic unit. Thus, the ECC represents a key complex for studying tectonic processes such as crustal thickening or incorporation of mantle-derived material into the continental crust. This study provides the first evidence that high-pressure metamorphism in the ECC is of Variscan age. Sm-Nd isochrons define ages of 333 ± 6 (Grt-WR), 337± 5 (Grt-WR), 360± 7 (Grt-Cpx-WR) (eclogites) and 353 ± 7 Ma (Grt-WR) (garnet-pyroxenite). 40Ar/39Ar spectra of phengite from two eclogite samples give plateau ages of 348 ± 2 and 355 ± 2 Ma. The overlap of ages from isotopic systems with blocking temperatures that differ by about 300 ° C indicates extremely fast tectonic uplift rates. Minimum cooling rates were about 50° C Myr-1. As a consequence, the closure temperature of the specific isotopic system is of minor importance, and the ages correspond to the time of high-pressure metamorphism. Despite textural equilibrium and metamorphic temperatures in excess of 800° C, clinopyroxene, garnet and whole rock do not define a three-point isochron in three of four samples. The metamorphic clinopyroxenes seem to have inherited their isotopic signature from magmatic precursors. Rapid tectonic burial and uplift within only a few million years might be the reason for the observed Sm-Nd disequilibrium. The εNd values of the eclogites (+4.4 to +6.9) suggest the protoliths were derived from a long-term depleted mantle, probably a MORB source, whereas the isotopically enriched garnet-pyroxenite (εNd–2.9) might represent subcontinental mantle material, emplaced into the crust prior to or during collision. The similarity of ages of the two different rock types suggests a shared metamorphic history.

Notes: Eclogite facies carbonate rocks have been discovered associated with the granulite–eclogite transitional rocks within Bergen Arc system, Caledonian Orogen of western Norway. The local occurrences of marbles and calc-silicates are found subparallel to the mafic eclogite facies shear zones on Holsnøy Island. Marbles contain the assemblage calcite (Ca0.99Sr0.01CO3), calcian strontianite (Ca0.18−0.44Sr0.53−0.84CO3), clinopyroxene (Jd7−32), epidote/allanite (Ps0−33), titanite, garnet (Alm52−56Grs28−33Pyp11−16), barite (Ba0.90−0.99Sr0.01−0.10SO4), celestine (Sr0.67−0.98Ba0.01−0.23Ca0.01−0.11SO4), and one apparently homogeneous grain of intermediate composition (Ba0.49Ca0.01Sr0.50SO4). Adjacent eclogites have clinopyroxene with similar jadeite contents (Jd14−34) and similar garnet (Alm51−60Grs26−36Pyp8−14) compositions. The marbles have high contents of Sr (9500–11000 p.p.m) and Y (115–130 p.p.m). However, low concentrations of some key trace elements (110–160 p.p.m. Ba and 〈5 p.p.m. Nb) appear to indicate that the marble is not a metamorphosed carbonatite. The 87Sr/86Sr ratios range from 0.7051 to 0.7059. Field and petrological relationships suggest that metasomatic reactions and fluids played a significant role in producing and/or modifying the marbles. The breakdown of scapolite in the granulite into carbonates and sulphates during eclogite facies metamorphism may have contributed to the metasomatic formation of the marbles along shear zones.Fluids involved during subduction are an important catalyst for metamorphism and are recognized to have played a critical role in the localized transformation from granulite to eclogite in the Holsnøy Island area. Thermobarometry indicates 640–690 °C and 18–20 kbar for adjacent eclogites and temperatures of 580–650 °C for the calc-silicates. The marble assemblages are consistent with fluid that is dominantly comprised of H2O (XCO2 〈 0.03) under high-pressure conditions. Phase equilibria of the marbles constrain the fO2 of the fluids and imply oxidizing conditions of the deep crustal fluids. At present the source of the fluids remains unresolved. The results provide additional insights into the variable and evolving nature of fluids related to subduction and high-pressure metamorphism.

Notes: 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.

Notes: Sm–Nd (garnet), U–Pb (monazite) and Rb–Sr (biotite) ages from a composite migmatite sample (Damara orogen, Namibia) constrain the time of high-grade regional metamorphism and the duration of regional metamorphic events. Sm–Nd garnet whole-rock ages for a strongly restitic melanosome and an adjacent intrusive leucosome yield ages of 534±5, 528±11 and 539±8 Ma. These results provide substantial evidence for pre-500 Ma Pan-African regional metamorphism and melting for this segment of the orogen. Other parts of the migmatite yield younger Sm–Nd ages of 488±9 Ma for melanosome and 496±10, 492±5 and 511±16 Ma for the corresponding leucosomes. Garnet from one xenolith from the leucosomes yields an age of 497±2 Ma. Major element compostions of garnet are different in terms of absolute abundances of pyrope and spessartine components, but the flat shape of the elemental patterns suggests late-stage retrograde equilibration. Rare earth element compositions of the garnet from the different layers are similar except for garnet from the intrusive leucosome suggesting that they grew in different environments. Monazite from the leucosomes is reversely discordant and records 207Pb/235U ages between 536 and 529 Ma, indicating that this monazite represents incorporated residual material from the first melting event. Monazite from the mesosome MES 2 and the melanosome MEL 3 gives 207Pb/235U ages of 523 and 526 Ma, and 529 and 531 Ma, respectively, which probably indicates another thermal event. Previously published 207Pb/235U monazite data give ages between 525 and 521 Ma for composite migmatites, and 521 and 518 Ma for monazite from neosomes. Monazite from granitic to granodioritic veins indicates another thermal event at 507–505 Ma. These ages are also recorded in 207Pb/235U monazite data of 508 Ma from the metasediment MET 1 from the migmatite and also in the Sm–Nd garnet ages obtained in this study. Taken together, these ages indicate that high-grade metamorphism started at c. 535 Ma (or earlier) and was followed by thermal events at c. 520 Ma and c. 505 Ma. The latter event is probably connected with the intrusion of a large igneous body (Donkerhoek granite) for which so far only imprecise Rb–Sr whole-rock data of 520±15 Ma are available. Rb–Sr biotite ages from the different layers of the migmatite are 488, 469 and 473 Ma. These different ages indicate late-stage disturbance of the Rb–Sr isotopic system on the sub-sample scale. Nevertheless, these ages are close to the youngest Sm–Nd garnet ages, indicating rapid cooling rates between 13 and 20°C Ma−1 and fast uplift of this segment of the crust. Similar Sm–Nd garnet and U–Pb monazite ages suggest that the closure temperatures for both isotopic systems are not very different in this case and are probably similar or higher than the previously estimated peak metamorphic temperatures of 730±30°C. The preservation of restitic monazite in leucosomes indicates that dissolution of monazite in felsic water-undersaturated peraluminous melts can be sluggish. This study shows that geochronological data from migmatites can record polymetamorphic episodes in high-grade terranes that often contain cryptic evidence for the nature and timing of early metamorphic events.

Notes: The most widely used technique for the determination of high precision mineral growth ages in igneous and metamorphic rocks is dating of zircons with the U-Pb method. The interpretation of these ages, particularly in metamorphic settings, is hampered by an incomplete understanding of the common phenomenon of partial Pb-loss in zircon. In principle, this Pb-loss may occur in four very different ways: diffusion in metamict zircon, diffusion in pristine zircon, leaching from metamict zircon and recrystallization of metamict zircon. Here it is argued that, under conditions common in the continental crust, Pb-loss is only possible in partially to strongly metamict zircons. Pb-diffusion in the pristine zircon lattice is insignificant up to temperatures of at least 1000 °C. Pb-loss is only possible if the zircons experienced a time interval below their annealing temperature of about 600–650 °C, because only below this temperature can the lattice damage through α-decay and spontaneous fission accumulate. Zircons that remain above this temperature do not lose Pb by diffusion and will stay closed systems. Complete resetting of the U-Pb system in zircon under crustal conditions is only possible through dissolution and reprecipitation of zircon. Partial resetting results from recrystallization, leaching or diffusion in metamict zircon. As a consequence, special care has to be taken to interpret lower intercepts on concordia diagrams defined by discordant U-Pb data. Lower intercept ages may be significant only if they are defined by zircons with low U-content (〈100 p.p.m.) or if confirmed by other geochronological methods. In addition, the accuracy of the lower intercept should be confirmed by abrading the zircon fractions that define the discordia.