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Dating of igneous rocks from paleo-island arc complexes of orogenic belts makes it possible to determine the precise timing of subduction processes that occurred during the formation of the orogen. Within the central (East Transbaikal) part of the Mongol–Okhotsk orogenic belt, the Kamenka island-arc terrane is an example of such a complex. Its intrusive part is included into the Bereya complex, represented by a gabbro-diorite-tonalite-plagiogranite series of rocks demonstrating subduction geochemical characteristics. The dating of zircons from acidic rocks of this complex by the U–Pb classical method showed that they were formed in a narrow time interval of 203 ± 1–205 ± 1 Ma, which corresponds to the Norian–Rhaetian boundary of the Late Triassic. Taking into account the previously obtained age of the diorites (254 ± 5 Ma), the timing of formation of the entire series of the intrusive rocks is about 50 Ma, thus indicating the probable subduction along the northern (in modern coordinates) margin of the Mongol–Okhotsk Paleocean under the Siberian paleocontinent in the Late Permian – Late Triassic. The diorites of the first phase have positive values of εND(254 Ma) = 3.2–3.6 (TNd(DM) = 879–994 Ma), and the plagiogranites have ɛND(205 Ma) = 2.3–3.5 (TNd(DM) = 859–1028 Ma), which points to the relationship between these rocks and the substance of the depleted mantle source and is consistent with the Sm–Nd isotope characteristics of the juvenile crust of the Central Asian orogenic belt. [ABSTRACT FROM AUTHOR]

The origin of the recently discovered Flatreef remains debated due to the pronounced interaction of the magmatic rocks with sedimentary floor rocks, resulting in a complex intrusive stratigraphy. In this study, we report new Nd isotopic compositions of Flatreef lithologies intersected by borehole UMT-393 on the farm Macalacaskop in order to improve our understanding of the magmatic history of the deposit and to further test the putative correlation between the Flatreef/Platreef and the Upper Critical Zone of the remainder of the Bushveld Complex. The initial epsilon Nd (εNdi) values for the Flatreef range between −5.2 and −7.6, overlapping with εNdi values of the Upper Critical Zone from the eastern (ranging between − 4.8 and − 8.5) and the Upper Critical Zone and Main Zone from the western limb (−6.3 and −7.6, and −6.3 and −7.4 respectively) of the Bushveld Complex. The Flatreef εNdi values also overlap with those of the Platreef; however, due to the varying footwall lithologies of the Platreef along strike, Platreef rocks display a wider variation in isotopic composition. Our findings support the correlation of the Flatreef with the Upper Critical Zone — Main Zone transition interval in the remainder of the Bushveld Complex, which includes the Merensky and Bastard reefs. Due to significant overlap between the εNdi values of the Flatreef and local potential contaminants occurring at the base of the Northern Limb, we propose that the Sr–Nd isotopic composition of the magmas that gave rise to the Flatreef are most likely attributable to the interaction of mantle-derived magma with upper and lower crustal rocks of the Kaapvaal Craton within a sub-Bushveld staging chamber, with possible syn- to post-emplacement modification as a result of interaction with dolomitic footwall rocks. [ABSTRACT FROM AUTHOR]

The Pauzhetka Caldera (27 × 18 km) was formed in the South Kamchatka during the Golygin Ignimbrite eruption (420–440 ka), the largest known eruption in the region in the past 1 Myr. The eruption was preceded by the 3 Ma-old mafic and intermediate volcanism. After the caldera-forming eruption, a variety of products, from basalt to rhyolite, were ejected within the caldera. For understanding the origin of voluminous silicic magmatism in thin mafic South Kamchatka crust, we used geochemical and isotope data. Our research has characterized the major and trace element composition of Golygin ignimbrite, intra-caldera hydrothermally altered deposits, pre-caldera (Mt. Orlinoe Krylo, Mt. Klyuchevskaya) and post-caldera (Kambalny Ridge, Chernye Skaly) eruptive centers. The Sr–Nd isotope composition of the Golygin ignimbrite and some eruptive post-caldera products was investigated. The isotope variations indicate that parental magmas for all rocks of the Pauzhetka area were obtained from a weakly evolved source derived through fluid-assisted melting of a subducted slab. Geochemical data support that the formation of most magmas of the Pauzhetka caldera was mainly controlled by fractional crystallization in the lower to middle crust. MELTS-modelling agrees with geochemical data. The fractional crystallization of Kambalny basalt with 2 wt % H2O at 6 kbar provides the best fit to the observed composition of the Golygin dacite. [ABSTRACT FROM AUTHOR]

The symmetry and mixed-symmetry states of 144 - 154 Nd isotopes are studied via the framework of the proton–neutron interacting boson model. The F-spin values of theoretical energy states are also investigated. The calculated electromagnetic transition probabilities B(E2), B(M1) and the mixing ratio δ (E 2 / M 1) are compared with available experimental data. The results obtained show that the 2 3 + and 2 4 + states have an F = F max - 1 value in the 144 Nd and 146 , 148 Nd isotopes, respectively. In the 150 - 154 Nd isotopes, the 1 1 + state is the lowest mixed-symmetry state. [ABSTRACT FROM AUTHOR]

We reconstructed the past deep-water character of the equatorial Indian Ocean using the isotope ratio of neodymium (εNd) in the Fe–Mn coating of mixed-species foraminifera. When compared with previous εNd records at the same site (ODP 758) and at another site to the west (SK 129), the three datasets were consistent and showed glacial-interglacial variations, even though the other two records were extracted from different media (cleaned foraminifera and bulk sediment leach). This confirms that while the foraminiferal coating is the preferred medium for reconstructing past bottom water εNd records, for carbonate-dominated lithologies, weak acid extraction of bulk sediment is also a viable option offering high-resolution capabilities. When the lithology includes volcanic particles or high organics, the extraction protocol may need to be adjusted to guard against detrital contamination or a slight correction may need to be applied. During glacials, the deep waters bathing the equatorial Indian Ocean had a larger AABW component and during interglacials a larger NADW component. Our HI1808-GPC04 record supplements the ODP 758 record in the interval with prominent AABW signal (MIS 6/5 transition and MIS 7) and reveals regional effects in some non-radiogenic intervals. The smaller differences between the HI1808-GPC04/ODP 758 and SK 129 records seem to reflect regional Nd input from river systems and non-radiogenic Nd from the boundaries. [ABSTRACT FROM AUTHOR]

The Nové Město Unit and the northern part of the Zábřeh Unit comprise back-arc basin mafic rocks metamorphosed during Variscan times. In both units, nearly identical metabasites comprise variously enriched tholeiites (from N-MORB to transitional E-MORB-OIB), high-Ca, low-silica boninites and low-Ti tholeiites. The tholeiites (Ti/V: 22–58, εNd540: + 7.6 to − 4.7) represent 10–15% melting at ca. 30–60 km and temperatures of 1380–1230 °C of a depleted MORB mantle-type (DMM) wedge heterogeneously modified shortly before fusion by OIB-like melts (enriched mantle possibly of EM1–EM2 type) presumably derived from decompression melting of upwelling asthenosphere. Much less common meta-boninites (Ti/V: 6–23, εNd540: + 6.7 to − 2.9) formed by 15–25% re-melting of residual mantle (DMM after ~ 15% melt extraction) at depths of 40–65 km and temperatures of ~ 1420–1300 °C. Scarce low-Ti meta-tholeiites (Ti/V: 18–19, εNd540: + 7.1) resulted from < 10% melting of an unenriched DMM-type source or re-melting of residual mantle (after < 15% of former melting). Trace elements and Nd isotope compositions imply random fluxing of tholeiitic- and boninitic magma sources by components released from a subducted slab. The metasomatic enrichment (Th, LREE–MREE) was induced not only by sediment-derived melts but also by fluids supplied by subducted sediments or juvenile crust. The Nové Město–Zábřeh association of metabasites points to an easterly prolongation (in present-day coordinates) of the Cadomian subduction system of the Teplá–Barrandian. Boninite-type magmatism and OIB-like input into mantle beneath an extensional back-arc basin suggest an upflow of hot asthenosphere through subducted ridge (slab window) followed by a cessation of subduction zone activity. [ABSTRACT FROM AUTHOR]

The Silurian volcanic rocks of the Prague Basin represent within-plate, transitional alkali to tholeiitic basalts, which erupted in a continental rift setting through the thick Cadomian crust of the Teplá-Barrandian Unit (Bohemian Massif). Despite the variable, often intense alteration resulting in post-magmatic replacement of the basalt mass due to carbonatization, the geochemical signatures of Silurian basalts are still sufficiently preserved to constrain primary magmatic processes and geotectonic setting. The studied interval of Silurian volcanic activity ranges from Wenlock (Homerian, ~431 Ma) to late Ludlow (Gorstian, ~425 Ma) with a distinct peak at the Wenlock/Ludlow boundary (~428 Ma). Trace-element characteristics unambiguously indicate partial melting of a garnet peridotite mantle source. Wenlock basalts are similar to alkaline OIB with depleted radiogenic Nd signature compared to Ludlow basalts, which are rather tholeiitic, EMORB-like with enriched radiogenic Nd signature. The correlation of petrogenetically significant trace-element ratios with Nd isotopic compositions points to a mixing of partial melts of an isotopically heterogeneous, possibly two-component mantle source during the Wenlock-Ludlow melting. Lava eruptions were accompanied by intrusions of doleritic basalt and meimechite sills. The latter represent olivine-rich cumulates of basaltic magmas of probably predominantly Ludlow age. Meimechites with dolerites and, to a lesser extent, some lavas were subject to alteration due to wall-rock-fluid interaction. The trigger for the Wenlock-to-Ludlow (431-425 Ma) extension and related volcanism in the Prague Basin is related to far-field forces, namely slab-pull regime due to progressive closure of the Iapetus Ocean. The main stage of the Baltica-Laurentia collision then caused the Prague Basin rift failure at ca. 425 Ma that has never reached an oceanic stage. [ABSTRACT FROM AUTHOR]

It is well established that A-type granites enriched in high field strength elements, such as Zr, Nb and the REE, form in anorogenic tectonic settings. The sources of these elements and the processes controlling their unusual enrichment, however, are still debated. They are addressed here using neodymium and oxygen isotope analyses of samples from the 1.24 Ga Strange Lake pluton in the Paleoproterozoic Core Zone of Québec-Labrador, an A-type granitic body characterized by hyper-enrichment in the REE, Zr, and Nb. Age-corrected εNd values for bulk rock samples and sodic amphiboles (mainly arfvedsonite) from the pluton range from −0.6 to −5.7, and −0.3 to −5.3, respectively. The εNd values for the Napeu Kainiut quartz monzonite, which hosts the pluton, range from −4.8 to −8.1. The Sm/Nd ratios of the suite and the host quartz monzonite range from 0.0967 to 0.1659, large variations that can be explained by in situ fractionation of early LREE-minerals (Strange Lake), and late hydrothermal HREE remobilization. Oxygen isotope analyses of quartz of both Strange Lake and the host yielded δO values between +8.2 and +9.1, which are considerably higher than the mantle value of 5.7 ± 0.2‰. Bulk rock oxygen isotope analyses of biotite-gneisses in the vicinity of the Strange Lake pluton yielded δO values of 6.3, 8.6 and 9.6‰. The negative εNd values and positive δO values of the Strange Lake and Napeu Kainiut samples indicate that both magmas experienced considerable crustal contamination. The extent of this contamination was estimated, assuming that the contaminants were sedimentary-derived rocks from the underlying Archean Mistinibi (para-) gneiss complex, which is characterized by low εNd and high δO values. Mixing of 5-15% of a gneiss, having an εNd value of −15 and a δO value of +11, with a moderately enriched mantle source (εNd = +0.9, δO = +6.3) would produce values similar to those obtained for the Strange Lake granites. Based on analogies between the Nain Plutonic Suite and the Gardar alkaline igneous province (SW-Greenland), we conclude that the Strange Lake pluton and associated REE-mineralized anorogenic bodies formed from a combination of subduction-induced fertilization of the sublithospheric mantle, crustal extension and in situ magma evolution. [ABSTRACT FROM AUTHOR]

Significant PGE and Cr mineralization occurs in a number of 2.44-2.50-Ga mafic layered intrusions located across the Karelian and Kola cratons. The intrusions have been interpreted to be related to mantle plume activity. Most of the intrusions have negative ε values of about −1 to −2 and slightly radiogenic initial Sr isotope compositions of about 0.702 to 0.703. One potential explanation is crustal contamination of a magma derived from a mantle plume, but another possibility is that the magma was derived from metasomatized sub-continental lithospheric mantle. Samples from the upper chromitite layers of the Kemi intrusion and most samples from the previously studied Koitelainen and Akanvaara intrusions have supra-chondritic γ values indicating some crustal contamination, which may have contributed to the formation of chromitites in these intrusions. Chromite separates from the main ore zone of the Kemi and Monchepluton intrusions show nearly chondritic γ, similar to the coeval Vetreny belt komatiites. We suggest that the Os isotope composition of the primitive magma was not significantly changed by crustal contamination due to a high Os content of the magma and a low Os content of the contaminant. Modeling suggests that the Os and Nd isotope compositions of the Monchepluton and Kemi intrusions cannot be explained by assuming a magma source in the sub-continental lithospheric mantle with sub-chondritic γ. A better match for the isotope data would be a plume mantle source with chondritic Re/Os and Os isotope composition, followed by crustal contamination. [ABSTRACT FROM AUTHOR]

Kaersutite to pargasite phenocrysts from Tertiary alkali basalts (Rhön, Central European Province, Germany) yield new high-precision Ar/Ar ages of 24.0-24.1 Ma. Major and trace element compositions demonstrate that these high-TiO (4-7 wt%) amphiboles are in equilibrium with their host rock. Chemically, these amphibole phenocrysts resemble amphibole from magmatic veins in upper mantle rocks but differ from disseminated amphibole from peridotite. Most amphiboles have similar isotope characteristics to their alkaline basaltic host rocks (Sr/Sr = 0.7035-0.736, εNd = +3.8-+4.0, Pb/Pb = 19.21-19.37, Pb/Pb = 15.58-15.62, Pb/Pb = 38.95-39.16), but two samples show contrasting isotopic compositions (εNd = −4.0 and −2.9; Pb/Pb = 17.08 and 18.11; Pb/Pb = 15.51 and 15.58; Pb/Pb = 37.41 and 37.99), indicating involvement of an ancient crust-derived component during melting. The O isotopic composition of the amphibole phenocrysts ranges from 5.4 to 7.5 ‰, reflecting O isotope heterogeneity of the upper mantle sources. The contrasting isotopic composition of amphibole and host rock pairs furthermore indicates that phenocrysts record the early stages of the volcanic history of the Rhön volcanic field on a regional scale and at a different depth within the lithospheric mantle. Temperature and pressure estimates range from 1,010 and 1,080 °C and 0.7 and 1.0 GPa and are compatible with the experimental results on the stability of amphibole in alkaline rocks derived from the upper mantle. Geochemistry of major and trace elements and isotopic compositions shows that igneous amphibole from alkali basalts may preserve isotope heterogeneities suggesting that they sample heterogeneous upper mantle lithologies on a small scale. [ABSTRACT FROM AUTHOR]

There is a broad correlation between the ε values for rivers (including both the water and the particulate material it carries) and the age of the source terrain. This paper presents Nd isotope distribution data for soil, soil water, groundwater, and stream water samples gathered in a small catchment in northern Sweden. The results show that the release of Nd and Sm from boreal forests into streams and, eventually, into the oceans is more complicated than previously realized. The weathering of till causes changes in both the Nd isotopic composition and Sm/Nd ratios. Both the Sm/Nd ratio and ε were higher in strongly weathered soils horizons than in less weathered till, since minerals with high Sm/Nd ratios were, on average, more resistant to weathering than those with low Sm/Nd ratios. In contrast to the situation for the main minerals and the major elements, the weathering of rare earth elements (REE) was not restricted to the E-horizon: the measured REE concentrations continued to increase with depth in the C-horizon. In addition, REE released by weathering in the upper parts of the soil profile were partly secondarily retained at deeper levels. Therefore, the dissolved Nd released by weathering in the upper soil horizons was trapped and did not enter the groundwater directly. Rather, the Nd in the groundwater largely originated from weathering within the groundwater zone. However, this was not the only source of Nd in the stream water. The Nd isotope composition and Sm/Nd ratio were determined by the mixing between of Nd and Sm in the groundwater and REE-carrying organic material washed out of the soil profile. The groundwater close to the stream reaches the upper soil horizons during high discharge events such as snowmelts, and organic matter carrying Nd and Sm is washed out of the soils and thus released into the stream. Therefore, the Nd exported from catchment is derived from both the weathering within the groundwater zone, and the organic matter washed out from the soil. If longer timescales with more advanced weathering stages in the groundwater zone are considered, it cannot be ruled out that there will be a shift towards more radiogenic values in the exported Nd. Recorded shifts in the Nd isotopic composition in the ocean may thus not only reflect changed source regions, but also the weathering history of the same source region. [ABSTRACT FROM AUTHOR]

Despite its importance in regulating the global climate, the past deep ocean circulation in the western North Pacific has still been poorly understood. Nd isotopes of ferromanganese crusts have been proven to be a good proxy to monitor paleoceanic circulation changes. In this study, late Cenozoic Nd isotopic records are recovered from two ferromanganese crusts located near Mariana arc but at different water depths (MKD13: 1530 m, MDD53: 2700 m), and their implications for paleocirculation change in this area are explored. From the early to late Miocene, Nd isotopic compositions of MDD53 remained stable, and they were also characterized by the least radiogenic signatures (ɛ −4.0 to −5.0) compared to crusts of similar water depths in the Miocene North Pacific. Afterward, an abrupt increase in its ɛ value occurred in the Pliocene. In contrast, Nd isotopes of MKD13 became more radiogenic with time in the Miocene and were almost invariable thereafter. The continual increase in ɛ of shallower crust MKD13 is interpreted as reflecting progressive closure of Indonesian Seaway in the Miocene, while the deep western boundary current sourced from the Southern Pacific may have dominated Nd isotopes of deeper crust MDD53 during the same time interval. The lack of Nd isotopic variation of MKD13 in the Pliocene indicates that there were no changes in Nd sources in shallower waters and the final restriction between the Indian and Pacific may have only occurred since then. Therefore the observed large shift to more radiogenic Nd isotopes of MDD53 in the Pliocene should not be resulted from changes in vertical input from shallower to deeper water. Instead, a decreased ventilation of deep southern component current along the studied water depth range (∼2700 m) may have continued in the Pliocene. [ABSTRACT FROM AUTHOR]

The Humr Akarim and Humrat Mukbid plutons, in the central Eastern Desert of Egypt, are late Neoproterozoic post-collisional alkaline A-type granites. Humr Akarim and Humrat Mukbid plutonic rocks consist of subsolvus alkali granites and a subordinate roof facies of albite granite, which hosts greisen and Sn-Mo-mineralized quartz veins; textural and field evidence strongly suggest the presence of late magmatic F-rich fluids. The granites are Si-alkali rich, Mg-Ca-Ti poor with high Rb/Sr (20-123), and low K/Rb (27-65). They are enriched in high field strength elements (e.g., Nb, Ta, Zr, Y, U, Th) and heavy rare earth elements (La/Yb = 0.27-0.95) and exhibit significant tetrad effects in REE patterns. These geochemical attributes indicate that granite trace element distribution was controlled by crystal fractionation as well as interaction with fluorine-rich magmatic fluids. U-Pb SHRIMP zircon dating indicates an age of ~630-620 Ma but with abundant evidence that zircons were affected by late corrosive fluids (e.g., discordance, high common Pb). εNd at 620 Ma ranges from +3.4 to +6.8 (mean = +5.0) for Humr Akarim granitic rocks and from +4.8 to +7.5 (mean = +5.8) for Humrat Mukbid granitic rocks. Some slightly older zircons (~740 Ma, 703 Ma) may have been inherited from older granites in the region. Our U-Pb zircon data and Nd isotope results indicate a juvenile magma source of Neoproterozoic age like that responsible for forming most other ANS crust and refute previous conclusions that pre-Neoproterozoic continental crust was involved in the generation of the studied granites. [ABSTRACT FROM AUTHOR]

The Xing-Meng Orogenic Belt is the eastern extension of the Central Asian Orogenic Belt, which marks the boundary between the North China and Siberian blocks. Key information on the collision between these two blocks and the evolution of this orogenic belt at the end of the Paleozoic are preserved in the widely distributed Paleozoic sedimentary sequences. Petrographic studies of the clastic sedimentary rocks from the Ordovician to the Permian in this region have shown that the rocks are mainly greywackes and arkosic sandstones characterized by low maturity, poor sorting and a large number of lithic fragments. Provenances of Paleozoic sandstones are mainly newly accreted crustal materials. Nd model ages range from the Neoproterozoic to the Late Mesoproterozoic. These ages are very different from those of the North China Block, but the change of ɛ( t) (from the initial ɛ value) is similar to that of igneous rocks in the Xing-Meng Orogen. This result indicates that provenances of these Paleozoic sedimentary rocks mainly arise from the Xing-Meng Orogen itself. These clastic sedimentary rocks mainly consist of inputs of juvenile accreted crustal materials and limited recycled old crusts. Their provenance is controlled by the following three end members: a felsic basement, a partially melted mafic, and a mafic end member. From the point view of lithology, these three end members must have mixed in a ratio of 53:41:5 to form the average composition of the clastic sedimentary rocks. The depositional environment began to change from an arc to an orogen during the Permian, and therefore the collision was complete before the Triassic. This interpretation is consistent with the argument that the final collision time was at the end of the Permian or beginning of the Triassic. [ABSTRACT FROM AUTHOR]

Radiogenic Neodymium (Nd) isotopes in the central North Pacific deep water have been generally interpreted as reflecting Pacific arcs input according to previous studies, while little is known about the specific path and mode of radiogenic Nd from these arcs to deep water. In this study, Nd isotopic data from surface scrapings of 17 Fe-Mn crusts from the central North Pacific deep water (around 20°N) have been reported. Based on these data together with recently published data on dissolved Nd isotopes in seawater for this region, we propose that radiogenic Nd isotopes in North Pacific Intermediate Water (NPIW 300-800 m) from the western Pacific margin is an important source for the central North Pacific deep water, while the surface water of this area has little influence on dominating deep water Nd isotopes. Such a view emphasizes the role of vertical mixing and advection of the marginal ocean currents in balancing Nd isotopes in central North Pacific deep water, and helps to understand oceanic Nd cycling. Moreover, the values of ε from surface scrapings of Fe-Mn crusts are relatively homogenous, with an average value of about −3.4, which is higher than that of modern seawater (∼−3.9). This difference over short time scales reflects the quick evolution of Nd isotopes in central North Pacific deep water. [ABSTRACT FROM AUTHOR]

The development of a regional stratigraphy in Palaeoproterozoic basins within the Tanami region, Northern Australia has been hindered by the difficulty of discriminating sedimentary units and facies across this isolated and poorly exposed basin. A regional stratigraphy is important as it provides constraints on sedimentary basin evolution and assists in gold exploration. Gold is known to be more concentrated in certain rock formations. Based on Nd isotopes and whole rock geochemistry, five main sedimentary events have been identified in the Tanami region. Some sedimentary units were derived from homogeneous local sources, whereas others contain evidence of a well-mixed fine-grained remote provenance. Within the basins, major gold-bearing lithologies are characterised by mafic source indicators: (1) high Cr/Th ratios; (2) low Th/Sc ratios; (3) low (La/Yb)PAAS ratios relative to Post-Archaean Average Shale (); (4) Eu anomalies equal to ∼1; and (5) distinctive ranges in initial εNd values. Potential future exploration target areas have been identified in the Tanami region at the Cashel and Sunline prospects using these geochemical parameters. [ABSTRACT FROM AUTHOR]

New geochemical, isotopic, and geochronological data and interpretations are presented for late Neoproterozoic intrusive carbonates and related rocks of southern Sinai, Egypt (northernmost Arabian–Nubian Shield). The Tarr carbonates are coarsely crystalline and related to explosive emplacement of hypabyssal and volcanic albitite at 605 ± 13 Ma. The carbonates associated with the albitites are divisible into two types: primary dolomitite and secondary breunneritite (Fe-rich magnesite). The dolomitite was clearly intrusive but differs from classic igneous carbonatites, containing much lower abundances of incompatible elements, such as REE, U, Th, Rb, Nb, Y, P, Sr, Zr, Ba, and total alkalies. The breunneritite is a secondary replacement of dolomitite, probably marking the roots of a vigorous hydrothermal system. Albitites show pristine abundances of major and trace elements and were not subjected to a major metamorphic overprint. They are relatively more fractionated, alkaline and related to within-plate A-type magmas, were emplaced in an extensional or non-compressive tectonic regime in the cupola of high-level A-type granite. Tarr albitites may represent residual magma remaining after near-total crystallization of an A-type granite pluton at depth, forcibly emplaced into the roof above the cooling pluton. The intrusive dolomitite exsolved from highly differentiated albitite melt, in the apical regions of a still-buried alkaline “A-type” granite pluton that was rich in CO2; these volatiles migrated upwards and towards the cooler margins of the magma body. Late NNE-SSW extension allowed a shallow-level cupola to form, into which albitite melts and carbonate fluids migrated, culminating in explosive emplacement of albitite breccia and intrusive carbonate. Isotopic compositions of Tarr dolomitite and albitite indicate these are consanguineous and ultimately of mantle origin. Magmatic volatiles fenitized the wall rock, while submarine hydrothermal activity transformed some of the dolomitite into breunneritite. Recognition of Tarr-type should encourage similar hypabyssal complex intrusions to be sought for in association with A-type granitic plutons elsewhere. [ABSTRACT FROM AUTHOR]

The Eoarchaean (>3,600 Ma) Itsaq Gneiss Complex of southern West Greenland is dominated by polyphase orthogneisses with a complex Archaean tectonothermal history. Some of the orthogneisses have c. 3,850 Ma zircons, and they vary from rare single phase metatonalites to more common complexly banded migmatites. This is due to heterogeneous strain, in situ anatexis and granitic veining superimposed during younger tectonothermal events. In the single-phase tonalites with c. 3,850 Ma zircon, oscillatory-zoned prismatic zircon is all 3,850 Ma old, but shows patchy ancient loss of radiogenic Pb. SHRIMP spot analyses and laser ablation ICP-MS depth profiling show that thin (usually < 10 μm) younger (3,660–3,590 Ma and Neoarchaean) shells of lower Th/U metamorphic zircon are present on these 3,850 Ma zircons. Several samples with this simple zircon population occur on islands near Akilia. In contrast, migmatites usually contain more complex zircon populations, with often more than one generation of igneous zircon present. Additional zircon dating of banded gneisses across the Complex shows that samples with c. 3,850 Ma igneous zircon are not just a phenomenon restricted to Akilia and adjacent islands. For example, migmatites from Itilleq (c. 65 km from Akilia) contain variable amounts of oscillatory-zoned 3,850 Ma and 3,650 Ma zircon, interpreted, respectively, as the rock age and the time of crustal melting under Eoarchaean metamorphism. With only 110–140 ppm Zr in the tonalites and likely magmatic temperatures of >850°C, zircon solubility–melt composition relationships show that they were only one-third saturated in zircon. Any zircon entrained in the precursor magmas would thus have been highly soluble. Combined with the cathodoluminesence imaging, this demonstrates that the c. 3,850 Ma oscillatory zoned zircon crystallised out of the melt and hence gives a magmatic age. Thus the rare well-preserved tonalites and palaeosome in migmatites testify that c. 3,850 Ma quartzo–feldspathic rocks are a widespread (but probably minor) component in the Itsaq Gneiss Complex. C. 3,850 Ma zircon with negative Eu anomalies (showing growth in felsic systems) also occurs as detrital grains in rare c. 3,800 Ma metaquartzites and as inherited grains in some 3,660 Ma granites ( sensu stricto). These demonstrate that still more c. 3,850 Ma rocks were present, but were recycled into Eoarchaean sediments and crustally derived granites. The major and trace element characteristics (e.g. LREE enrichment, HREE depletion, low MgO) of the best-preserved c. 3,850 Ma rocks are typical of Archaean TTG suites, and thus argue for crust formation processes involving important contributions from melting of hydrated mafic crust to the earliest Archaean. Five c. 3,850 Ma tonalites were selected as the best preserved on the basis of field criteria and zircon petrology. Four of these samples have overlapping initial ɛNd (3,850 Ma) values from +2.9 to +3.6± 0.5, with the fourth having a slightly lower value of +0.6. These data provide additional evidence for a markedly LREE-depleted early terrestrial mantle reservoir. The role of c. 3,850 Ma crust should be considered in interpreting isotope signatures of the younger (3,800–3,600 Ma) rocks of the Itsaq Gneiss Complex. [ABSTRACT FROM AUTHOR]

The Izera Block in the West Sudetes, which is composed of granites, gneisses (and transitional granite-gneisses) and minor mica schists, is one of the largest outcrops of Early Palaeozoic (ca. 500 Ma) metagranitoid rocks in the basement units of the Variscides of Central Europe. The Izera granites show S-type features: magmatic cordierite, relict garnet and sillimanite, lack of mafic enclaves, and absence of coexisting tonalites and diorites. The paucity of pegmatites indicates that the granitic magma was relatively dry. The S-type character of these granites is further supported by their peraluminous character (A/CNK 1.0–1.63), high content of normative corundum (up to 3.5%) and relatively high 87Sr /86Sr initial ratio. The chemical variation of these rocks was controlled by the fractional crystallization of plagioclase (CaO, Sr, Eu/Eu*), biotite and cordierite (Al2O3, MgO, FeO), zircon (Zr, Hf) and monazite (REE). Initial εNd values range from −5.2 to −6.9 (mean: −5.9, SD=0.6). These largely negative εNd values imply that the granitic magmas emplaced ca. 500 Ma were extracted from a source reservoir that was strongly enriched in LREE (i.e., with low Sm/Nd ratio) on a time-integrated basis. The relatively consistent depleted mantle model ages (1,730–2,175 Ma; mean: 1,890 Ma) is in agreement with the earlier reported presence of ca. 2.1 Ga old inherited Pb component in zircon from the closely related Rumburk granite. This points to an old (Early Proterozoic) crustal residence age of the inferred metasedimentary protoliths of the Izera granitoids, with only minor contribution to their protoliths of juvenile components of Late Proterozoic/Early Palaeozoic age. Although the Izera granites show some trace element features reminiscent of syn-collisional or post-collisional granitoids, they more likely belong to the broad anorogenic class. Our data corroborate some previous interpretations that granite generation was connected with the Early Palaeozoic rifting of the passive margin of the Saxothuringian block, well documented in the region by bimodal volcanic suites of similar age (Kaczawa Unit, eastern and southern envelope of the Karkonosze–Izera Block). In this scenario, granite magmatism and bimodal volcanism would represent two broadly concomitant effects of a single major event of lithospheric break-up at the northern edge of Gondwana. [ABSTRACT FROM AUTHOR]

U-Pb SHRIMP dating of zircons of metamagmatites from the Bayerischer Wald (Germany) reveals a complex evolution of this section of the Moldanubian Zone exposed in the western Bohemian Massif of the central European Variscan belt. In the south-western part of the Bayerischer Wald Upper Vendian magmatism is constrained by pooled 206Pb/238U mean ages of 555±12, 549±7 and 549±6 Ma from metarhyolites and a metabasite. Inherited zircon cores were not observed. Zircon overgrowths, yielding pooled 206Pb/238U ages of 316±10 and 319±5 Ma, provide evidence for Variscan metamorphic zircon growth; cathodoluminescence imaging reveals a two-stage metamorphic overprint. In contrast, Lower Ordovician magmatism and anatexis are documented in the north-eastern parts of the Bayerischer Wald by metagranitoids (480±6, 486±7 Ma), an eclogitic metabasite (481±8 Ma) and a leucosome (491 to 457 Ma). Inherited zircon cores are found in Lower Ordovician metagranitoids and the leucosome, indicating a Palaeoproterozoic-Archaean (ca. 2.7, 2.0 Ga) source region, presumably of Gondwana affinity (West African craton), and documenting Cadomian magmatism (ca. 640 Ma). Post-Cadomian metamorphism is inferred from concordant ages of 433±4 and 431±7 Ma. Upper Vendian magmatism is assumed at an active continental margin with ensialic back-arc development (εNd(t) –3.01 to +1.22); the lack of inherited zircon is due to either derivation from juvenile (?volcanic arc) material or complete isotopic resetting of pre-existing zircon. An active continental margin setting, possibly with some lateral variation (accretion/collision) is envisaged for the Lower Ordovician, producing granitoids, rhyolites and leucosomes (εNd(t) -0.5 to -6.27); MORB-type metabasites may be related to ZEV or Mariánské Lázně Complex metabasites. A tentative palaeogeographic reconstruction puts the “Bayerischer Wald” in close relationship with the Habach terrane (proto-Alps), as the “eastern” extension of terranes of the northern Gondwana margin. [ABSTRACT FROM AUTHOR]

High grade granitoid orthogneisses occur in several metamorphic units of the Erzgebirge in the Saxothuringian Zone of the Variscan Belt. The determination of protolith ages and the geochemical characterization of these rocks permit a reconstruction of the Neoproterozoic to early Palaeozoic magmatic and geodynamic history of the Erzgebirge. Single zircon Pb-Pb evaporation and SHRIMP ages combined with major and trace element data and Sm-Nd isotope systematics indicate at least two discrete magmatic events concealed in the so-called red gneisses, one at ~550 Ma in rocks of the medium pressure—medium temperature (MP-MT) unit and the other at ~500–480 Ma in rocks of the high pressure units. The transition zones comprise both Neoproterozoic granitoids and early Palaeozoic metarhyolites. The granitoid gneisses represent Neoproterozoic calc-alkaline granitoids with REE patterns similar to those produced in Andean-type continental margins. The early Palaeozoic muscovite gneisses are geochemically distinct from the older granitoids and may be derived from melts generated in a back-arc setting. Initial εNd values in all samples overlap and range from −4.1 to −9.2, corresponding to crustal sources with average residence times of 1.5 to 1.9 Ga. Zircon xenocryst ages as old as 2992 Ma provide evidence for Grenvillian, Svecofennian-Birimian-Aazonian and older age components and suggest an association of the Erzgebirge with Avalonia. [ABSTRACT FROM AUTHOR]

Previous detailed studies of large rhyolite bodies propose that their elemental and isotopic characteristics were largely acquired in shallow crustal magma chambers. This model explains the common chemical and isotopic zonations of large volumes of rhyolites as well as the less common chemical and isotopic homogeneity of such bodies. We report an intermediate situation (the Estérel massif, southeast France) in which chemical variations contrast with Nd-isotope homogeneity. We thus infer that, in this case, large volumes of rhyolite resided for enough time in shallow magma chambers to develop chemical zonations through differentiation, but this process was not accompanied by crustal assimilation. The subordinate amount of mafic rocks cropping out in the Estérel probably evolved from basalt to trachyte through assimilation and fractional crystallization. The relatively radiogenic Nd-isotope signatures of the rhyolite compared with the Hercynian crust show that it cannot have been generated by partial melting of exposed basement rocks. Several geological similarities with large rhyolitic provinces could suggest that the rhyolite was purely mantle derived or, alternatively, generated by partial melting of an ad hoc crustal component. However, mineralogical, geochemical, and geodynamic connections between the Estérel rhyolite and the hypersolvus anorogenic granites of Corsica, as well as the extreme Nd-isotope homogeneity of the rhyolite, lead us to propose that the rhyolite was generated by mixing between mantle-derived magmas and a mafic lower crust. This scenario accounts for the relatively radiogenic Nd-isotope signatures of the rhyolite compared with the Hercynian crust. The good Nd-isotope homogeneity observed in the rhyolite implies that the mixing process, which occurred in the deep crust, was complete and provided a shallow magma chamber with isotopically and probably chemically homogeneous magmas. [ABSTRACT FROM AUTHOR]

The REE and Pb, Sr, Nd isotopes in three xenoliths from limburgite and scoria-breccias, including spinel-lherzolite, spinel-garnet-lherzolite and phlogopite-gamet-lherzolite, were analysed. The REE contents of the xenoliths are 1.3 to 3.3 times those of the chondrites with their REE patterns characterized by weak LREE depletion. TheNd/Nd values of whole rocks and minerals range from 0.51306 to 0.51345 with εNd=+ 8.2− +15.8,Pb/ Pb < 18.673, andPb/Pb < 15.574. All this goes to show that the upper mantle in Mingxi at the depth of 67-82 km is a depleted mantle of MORB type, withSr/ Sr ratios 0.70237-0.70390. In Nd-Sr diagram the data points of whole rocks are all out of the mantle array, implying that the xenoliths from Mingxi have more radiogenic Sr isotopes than those of the mantle array. [ABSTRACT FROM AUTHOR]