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Abstract: Recalculated whole rock major-, trace-element and isotope compositions from mineral analyses of Finsch mantle peridotites indicate a complex history of the South African sub-continental lithospheric mantle (SA-SCLM). According to the Lu–Hf whole rock isotopic data last depletion coupled with the final cratonization of the Kaapvaal craton underneath Finsch occurred at ~2.6Ga. The data of this study indicate several depletion and enrichment processes during that early history of Finsch SCLM. Calculated degrees of partial melting using major and rare earth element (REE) compositions show that all depletion episodes together have removed more than 40% of melt from the SA-SCLM. As indicated by elevated Hf concentrations and subchondritic Lu/Hf, many Finsch peridotites were metasomatically overprinted between the last and earlier depletion stages. Silica enrichment, expressed in high modal abundance of orthopyroxene in the majority of the samples likely took place at that early stage. The last coupled melting and re-enrichment event (at ~2.6Ga) resulted in complete re-equilibration of trace element and Hf isotope signatures at the whole rock scale and thus, erased most signs of previous events. After the final depletion and before the final kimberlite eruption, the Finsch mantle suffered from further metasomatic overprints. Metasomatism is expressed in the enrichment of light REE (LREE) and some large ion lithophile elements (e.g. Sr, Rb, Th). Additionally, the Nd isotope signatures of all samples as well as the Hf isotope compositions of some samples were modified. Late enrichment is documented by a modally metasomatized sample that contains rutile and sulfide. This sample is characterized by strong enrichment of high-field-strength elements and LREE and was probably overprinted with a sulfide bearing silico-carbonatitic melt with high contents of Ti, Fe and Ni. This melt likely originated from an enriched mantle that was isolated for a long time during which it developed a very unradiogenic Hf isotope signature, characteristic for this type of metasomatism. [Copyright &y& Elsevier]

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]

Abstract: The warm and saline North Atlantic inflow to the Arctic Ocean is a major component of high northern latitude circulation and the main mechanism of deep water renewal in the Arctic Ocean. Knowledge of its past variability is critical for understanding the high latitude feedback mechanisms of the climate system. Here we present the first combined seawater Hf and Nd isotope compositions of past Arctic Intermediate Water extracted from the authigenic Fe–Mn oxyhydroxide fraction of two sediment cores recovered near the North Pole, to reconstruct changes in contributions from glacial brines of the Eurasian shelf and past inflow of Atlantic waters. The Hf and Nd isotopic compositions obtained from leachates of the authigenic fraction show closely coupled and environmentally controlled variations over the past 14 million years. An observed offset of these data from seawater ε Hf and ε Nd compositions from other ocean basins (seawater array) is interpreted as the result of continuously prevailing glacial weathering conditions on the high latitude Eurasian continent. In the late Quaternary, large amplitude Hf and Nd isotopic variations of Arctic Intermediate Water (AIW) was characterized by more radiogenic isotope signatures generally prevailing under glacial conditions and less radiogenic values during interglacial periods. On the basis of the close coupling between Nd and Hf isotopes, we suggest that the evolution of Hf isotope compositions of central Arctic AIW has primarily been controlled by changes in ocean circulation and provenance of weathering inputs, rather than changes in weathering regime. [Copyright &y& Elsevier]

Abstract: The Nuvvuagittuq Greenstone Belt (NGB) in Northern Quebec, Canada, is dominated by mafic and ultramafic rocks metamorphosed to at least upper amphibolite facies. Rare felsic intrusive rocks provide zircon ages of up to ∼3.8Ga (David et al., 2009; Cates and Mojzsis, 2007) establishing the minimum formation age of the NGB as Eoarchean. Primary U-rich minerals that may provide reliable formation ages for the dominant mafic lithology, called the Ujaraaluk unit, have yet to be found. Metamorphic zircons, rutiles and monazites are present in the unit and give variably discordant results with 207Pb/206Pb ages ranging from 2.8Ga to 2.5Ga. The younger ages overlap 2686±4Ma zircon ages for intruding pegmatites (David et al., 2009) and Sm–Nd ages for garnet formation in the Ujaraaluk rocks suggesting this era as the time of peak metamorphism and metasomatism in the NGB, coeval with regional metamorphism of the Superior craton. Sm–Nd data for Ujaraaluk whole rocks scatter about a Sm/Nd vs. 143Nd/144Nd correlation (MSWD=134) whose slope would correspond to 3.6±0.2Ga if interpreted as an isochron. This “isochron” is seen to consist of a series of younger ∼3.2–2.5Ga slopes for the different geochemical groups within the Ujaraaluk, emanating from a baseline distribution older than 4Ga. The 146Sm–142Nd chronometer is less affected by metamorphism at 2.7Ga because of 146Sm extinction prior to ∼4Ga. Expansion of the 142Nd dataset for the Ujaraaluk and associated ultramafic rocks continues to show a good correlation between Sm/Nd ratio and 142Nd/144Nd that corresponds to an age of 4388 +15 −17 Ma. The dataset now includes samples with superchondritic Sm/Nd ratios that extend the correlation to 142Nd excesses of up to 8ppm compared to the terrestrial standard with a total range in 142Nd/144Nd of 26ppm. The upper Sm/Nd ratio end of the Ujaraaluk correlation is defined by rocks that are interpreted to be cumulates to compositionally related extrusive rocks indicating that this crystal fractionation had to occur while 146Sm decay was active, i.e. well before 4Ga. Intruding gabbros give 143Nd and 142Nd isochron ages of respectively 4115±100Ma and 4313 +41 −69 Ma, also supporting an Hadean age for the gabbros and providing a minimum age for the intruded Ujaraaluk unit. 3.6Ga tonalites surrounding the NGB, 3.8Ga trondhjemitic intrusive veins, and a 2.7Ga pegmatite show a deficit in 142Nd compared to the terrestrial standard. These felsic rocks plot to the low Sm/Nd ratio side of the Ujaraaluk isochron and do not show a correlation between their Sm/Nd and 142Nd/144Nd ratios, which can be explained if they are melts of ancient LREE-enriched mafic rocks, such as the Ujaraaluk, with the melting occurring after 146Sm was extinct. A subset of least disturbed Ujaraaluk samples has coherent isotopic compositions for both short-lived and long-lived Nd isotopic systems giving 143Nd and 142Nd isochron ages overlapping within error of 4321±160Ma (MSWD=6.3) and 4406 +14 −17 Ma (MSWD=1.0), respectively. This age represents our best age estimate for the Ujaraaluk unit. The NGB thus preserves over 1.6 billion years of early Earth history including an expanse of mafic crust formed in the Hadean. [Copyright &y& Elsevier]

Abstract: A study of sediment provenance and tectonic setting was carried out for the first time on metasedimentary rocks of the Rio Itapicuru greenstone belt, Serrinha Block, São Francisco craton, Brazil, using a combination of LA-ICP-MS zircon U–Pb geochronology, whole-rock geochemistry and Sm–Nd isotope data. The protoliths of the studied phyllites, micaschists and biotite gneisses were classified mostly as mudstones and impure sandstones, i.e. shales and greywackes. The chemical index of alteration (CIA) ranges from 39 to 70 indicating moderate chemical weathering in source areas. Intermediate and felsic rocks (granite, granodiorite, rhyolite and andesite) are the likely main source rocks, and less often mafic rocks (basalt and gabbro). The depleted-mantle Nd model ages (T DM), most in the range 2.0–2.2Ga, and the positive ɛ Nd values for the proposed deposition time (ca. 2115Ma) suggest source rocks mainly in the greenstone belt. The U–Pb data on detrital zircon grains of four samples yielded 207Pb/206Pb age mostly in the timespan 2125–2237Ma, with age populations clustering at 2164 ± 4Ma and 2209 ± 4Ma, and minor outliers at ca. 2112Ma, 2270Ma, and 2415Ma. The results indicated that the metasedimentary rocks of the Rio Itapicuru greenstone belt have source areas mainly in Palaeoproteroic terranes such as the Rio Itapicuru and Rio Capim greenstone belts. The zircon populations between 2.17Ga and 2.24Ga are not found in the Serrinha block, thus requiring sources that are not known in the region. Diagrams of tectonic setting suggest that the sediments may have been deposited on continental island arc. Our data support a model in which basalts and arc granites of the Rio Itapicuru greenstone belt were accreted onto an unknown continental or arc margin and the entire pile collided with, and were thrust onto a microcontinent represented by the adjacent Archaean basement complex. [Copyright &y& Elsevier]

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]

Abstract: Paleoproterozoic granites and metamorphic rocks of the Badu complex, the oldest rocks found in Cathaysia Block, provide a window for investigating the crustal evolution of the Cathaysia basement. SHRIMP and LA-ICP-MS zircon U–Pb ages of five representative granite samples from Jinluohou (Lizhuang) and Jingju granitic complexes indicate that the magmatism took place during Paleoproterozoic, and overprinted by early Mesozoic thermal event. The Jinluohou garnet-bearing biotite granite (1878±28Ma) has high A/CNK (1.28–1.42), low contents of P2O5 and low FeOT/(FeOT +MgO). It contains a large amount of biotites and garnets with minor inherited zircons but no primary muscovites, showing the S-type granite affinity. It was formed at low water fugacity and high Zr saturation temperature (792–809°C). The Jingju medium-coarse grained K-feldspar granite (1861±35Ma), Jingju medium-fine grained K-feldspar granite (1849±30Ma) and Jinluohou gneissic granodiorite (1877±10Ma) can reach strongly peraluminous (A/CNK=0.97–1.26), are enriched in alkalis and HFSE (Zr, Nb, Ce, Y) and have high Ga/Al and FeOT/(FeOT +MgO) ratios. Their high Zr saturation temperatures (840–854°C, most of them are higher than 830°C) together with high Rb/Nb and Y/Nb ratios suggest that they belong to the A2 subgroup of A-type granites. The ages of inherited zircon cores from Jinluohou garnet-bearing biotite granite (2088–2929Ma) reveal the existence of the Archean basement, and the Hf and Nd isotopes and chemical compositions suggest that these S- and A-type granites were originated from the Paleoproterozoic and Archean crustal sources with different proportions of input from juvenile crust materials. The Paleoproterozoic S- and A-type granites intruded simultaneously, and formed under intraplate extensional geodynamic setting, which is an important event of episodic crustal growth and reworking. The age of Jingju porphyritic quartz monzonite (226.2±1.4Ma) and the lower intercept age of Jinluohou gneissic granodiorite, Jingju K-feldspar granite (224–231Ma) may indicate another important thermal overprinting event in this region. [Copyright &y& Elsevier]

Abstract: Sm–Nd isotopic systematics has been proven to be a powerful tool for investigating the source provenance and tectonic significance of sedimentary rocks. Here, I report the Nd isotopic compositions of Mesozoic to Paleogene siliciclastic sediments from southern Tibet in an attempt to determine the temporal variations in their provenance. The data reveal a shift in ε Nd(0) values of Mesozoic strata in the Tethyan Himalaya, varying from −16.3 to −12.7 in the Jurassic to −8.1 to −3.7 in the Cretaceous. The less negative ε Nd(0) values of the Cretaceous siliciclastic rocks indicate increasing input of a juvenile mantle component from the source region. Such a shift can be attributed to the Early Cretaceous mantle-derived magmatism that was predominant in northern Greater India. This study indicates that there is a significant overlap in the ε Nd(0) values of rocks from the Tethyan Himalaya and Lhasa block, implying that Nd isotopes may not be a good provenance proxy to constrain the initiation of the India–Asia collision. Such an inference is also evidenced by similar Nd isotopic compositions between rocks from the Bhainskati Formation in the Nepalese foreland basin, sourced from the Tethyan Himalaya, and the Enba and Zhaguo formations in southern Tibet, sourced from the Lhasa block. [Copyright &y& Elsevier]

Abstract: Late Paleoproterozoic sedimentary basins across eastern Australia record a significant change in their neodymium isotopic composition at ca. 1655Ma. Prior to ca. 1655Ma, detritus was derived from comparatively evolved sources generating bulk ɛ Nd(1650Ma) values of generally −8 to −6. Subsequent sedimentary successions, which accumulated between ca. 1655Ma and 1600Ma, have bulk ɛ Nd(1650Ma) values of −2 to −1. This change is interpreted to reflect the input of sediments from a new, probable felsic volcanic source, corresponding to a fundamental change in tectonism and/or tectonic drivers for basin evolution in northern Australia. One possible driver for the change in sedimentary source is the initiation of renewed rifting from 1655Ma along the eastern margin of Paleoproterozoic Australia. As the 1655–1600Ma sedimentary successions that record the isotopic change are voluminous, a large, juvenile source of volcanic detritus must have been present, either within Paleoproterozoic Australia or in the cratonic block immediately to the east of Australia in the NUNA supercontinent. Although voluminous juvenile felsic magmatic sources are known in eastern and central Australia (e.g. 1639–1631Ma volcanics in the Warumpi Province; 1620–1610Ma granites of the St. Peter Suite in the Gawler Province), these sources are too young to have acted as a source for the juvenile detritus. Felsic intrusions of ∼1650Ma age are present in the Mount Isa Province, but the known exposed volume is very small. Of these possible sources, we favour either a buried or eroded eastern Proterozoic Australian source or an outboard Laurentian source. [Copyright &y& Elsevier]

Abstract: Circulation in the North Atlantic during the Late Cretaceous has implications for poleward heat transport and nutrient distribution during an extreme greenhouse interval with episodes of ocean anoxia. Nd isotopes of fossil fish teeth and debris represent one of the few water mass tracers that can be used to reconstruct deep ocean circulation. We present Nd isotopic data interpreted as bottom water values for 290 samples from three Ocean Drilling Program sites on Demerara Rise (Sites 1258, 1260, and 1261) along with 102 analyses from four other North Atlantic sites (Cape Verde, Goban Spur, Bermuda Rise and Blake Nose) that provide additional geographic and bathymetric control. Our results confirm the presence of a water mass with low εNd values (−14 to −17) that are believed to be influenced by continental material during local water mass formation at low latitudes. This Demerara Bottom Water (DBW) is the primary water mass in the region from the Cenomanian to Coniacian and from the late Campanian through early Maastrichtian following a hiatus of ~10my. A positive 8 εNd unit excursion occurs during Ocean Anoxic Event 2 (OAE2) that cannot be explained by changes in weathering inputs, diagenesis or magmatic sources of Nd; instead, it appears to represent an influx of bottom waters sourced from the Tethys or North Atlantic. This replacement of DBW during OAE2 argues for decreased production of DBW or enhanced production of Tethys/North Atlantic waters during peak greenhouse conditions. From the late Campanian through early Maastrichtian, DBW becomes the only water mass recorded at Demerara Rise and it appears to expand to abyssal depths at Cape Verde in the Campanian. This water mass is ultimately replaced by waters that appear to be sourced from the North Atlantic starting in the late Maastrichtian. Observed variations in circulation in the tropical North Atlantic during the Late Cretaceous can account for extensive deposition of black shales in the region prior to the hiatus, as well as local warming during a global cooling event in the Maastrichtian. [Copyright &y& Elsevier]

Abstract: We have analyzed the Nd isotopic composition of both ancient seawater and detrital material from long sequences of carbonated oozes of the South Indian Ocean which are ODP Site 756 (Ninety East Ridge (−30°S), 1518m water depth) and ODP Site 762 (Northwest Australian margin, 1360m water depth). The measurements indicate that the εNd changes in Indian seawater over the last 35Ma result from changes in the oceanic circulation, large volcanic and continental weathering Nd inputs. This highlights the diverse nature of those controls and their interconnections in a small area of the ocean. These new records combined with those previously obtained at the equatorial ODP Sites 757 and 707 in the Indian Ocean (Gourlan et al., 2008) established that the distribution of intermediate seawater εNd was uniform over most of the Indian Ocean from 35Ma to 10Ma within a geographical area extending from 40°S to the equator and from −60°E to 120°E. However, the εNd value of Indian Ocean seawater which kept an almost constant value (at about −7 to −8) from 35 to 15Ma rose by 3 εNd units from 15 to 10Ma. This sharp increase has been caused by a radiogenic Nd enrichment of the water mass originating from the Pacific flowing through the Indonesian Passage. Using a two end-members model we calculated that the Nd transported to the Indian Ocean through the Indonesian Pathway was 1.7 times larger at 10Ma than at 15Ma. The Nd isotopic composition of ancient seawater and that of the sediment detrital component appear to be strongly correlated for some specific events. A first evidence occurs between 20 and 15Ma with two positive spikes recorded in both εNd signals that are clearly induced by a volcanic crisis of, most likely, the St. Paul hot-spot. A second evidence is the very large εNd decrease recorded at ODP Sites 756 and 762 during the past 10Ma which has never been previously observed. The synchronism between the εNd decrease in seawater from 10 to 5Ma and evidences of desertification in the western part of the nearly Australian continent suggests enhanced weathering inputs in this ocean from this continent as a result of climatic changes. [Copyright &y& Elsevier]

New detrital zircon and isotopic (Nd and Sr) analyses from the eastern Pamir provide information on the depositional age and tectonic terrane affiliation of regional metamorphic terranes. Our results show the following. First, detrital zircon analyses from metasedimentary units along the Kongur Shan extensional system dominantly yield Triassic maximum depositional ages, with a similar age distribution to the Tibetan Songpan-Ganzi terrane. Further, zircon analyses from quartzofeldspathic gneisses in the core of the Muztaghata massif show the protoliths are Triassic granites. These units are interpreted to be part of the Permian-Triassic Karakul-Mazar arc-accretionary complex terrane. Second, εNd(0) compositions of Triassic granites overlap with other metasedimentary units not analyzed for detrital zircons and are also interpreted to be part of the Karakul-Mazar terrane. Third, schists in the Sares-Muztaghata gneiss dome structurally above Triassic orthogneisses yield an Ordovician maximum depositional age with a distinct detrital age distribution similar to the Tibetan Qiangtang terrane and are interpreted to be part of the Central Pamir terrane. Finally, Triassic and Ordovician schists along the Muztaghata massif record an Early Jurassic metamorphic event interpreted to date south-directed subduction of the Karakul-Mazar terrane beneath the Central Pamir during final closure of the Paleo-Tethys. These results, integrated with previously published results and field relations, reveal a complex Mesozoic to Cenozoic interleaving of tectonic terranes in the eastern Pamir with emplacement of the Karakul Mazar terrane both above and below the Kunlun and Central Pamir terranes to the north and south, respectively. [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]

Abstract: The Paleoproterozoic is considered the main period of crustal growth in the South American continent, with voluminous production of granitoid suites emplaced during the onset of the ca. 2.2–2.1Ga Trans-Amazonian orogeny. This work documents and interprets mineral chemistry, whole rock geochemistry, Nd isotopes and U–Pb geochronology data for the high-aluminium Lagoa Dourada TTG suite. This suite constitutes a hitherto unknown episode of Paleoproterozoic (ca. 2350Ma) juvenile felsic plutonism in the southern São Francisco craton, Minas Gerais, Brazil. It is also a rare example of juvenile continental crust production in the global crustal age-gap between 2.45 and 2.2Ga. The suite is composed of metaluminous to slightly peraluminous, low-Mg#, low-K and high-Ca, biotite-hornblende to hornblende-biotite (± garnet-magnetite) tonalites and biotite trondhjemites, varying from ∼62 to 73wt.% SiO2. The trace element geochemistry of the suite is characterized by mildly depleted Nd isotope compositions [ɛ Nd(t)=+1.0 to +2.1], low large-ion-lithophile element (LILE, i.e., Rb, Ba, and including the highly incompatible Th) and heavy Rare Earth element (REE, Yb<1.00ppm) contents. The suite is also characterized by high Sr/Y ratios (≥41 up to 81), high (La/Yb)N ratios (≥12 up to 46), and positive Eu/Eu* anomalies. These data are consistent with the derivation of the suite from a tholeiitic metabasaltic source, which had a short crustal residence time prior to melting. The chemical diversity of the suite is attributed mainly to the fractional crystallization of hornblende and accessory minerals, coupled with plagioclase accumulation in the tonalitic parental melt. Emplacement of new batches of magma could explain samples which diverge from the proposed liquid line of descent of the suite. The composition and age of the Lagoa Dourada suite suggest a link with the accretion of greenstone belt packages in an intra-oceanic tectonic setting. [Copyright &y& Elsevier]

Abstract: We present the first combined 146,147Sm–142,143Nd and 176Lu–176Hf study of mafic rocks (amphibolites) from the western part of the Isua Supracrustal Belt (ISB, SW Greenland). Sm–Nd and Lu–Hf whole-rock isochrons yield identical ages within error: 3.72±0.08 and 3.67±0.07Ga, respectively. 142Nd measurements confirm and extend the database of 142Nd excesses in Isua samples (μ142Nd=7–16ppm relative to the terrestrial Nd standard), indicating that early-differentiated reservoirs escaped complete homogenization by mantle convection until the Archean. Combined 146,147Sm–142,143Nd systematics suggest differentiation of the Isua amphibolite reservoir at maximum ages of ~4.53, ~4.47, and ~4.32Ga. These ages are relative to, respectively, ordinary-chondritic, enstatite-chondritic, and higher-than-chondritic mantle compositions. The intercept of the Sm–Nd whole-rock isochron is in accordance with 142Nd results and consistent with a superchondritic initial 143Nd/144Nd ratio (ε143Nd3.7Ga =+1.41±0.98). In contrast, the corresponding initial ε176Hf3.7Ga =−1.41±0.57 is subchondritic. Since Lu/Hf and Sm/Nd fractionate similarly during mantle processes, the Sm–Nd and Lu–Hf isotope systems display incongruent relative parent–daughter behavior in the source of Isua amphibolites. Based on high-pressure and -temperature phase partition coefficients, we propose a model that satisfies 147Sm–143Nd, 176Lu–176Hf, and 142Nd results, as well as trace element characteristics. A deep-seated source composed largely of magnesium perovskite (98% MgPv) and containing 2% calcium perovskite satisfactorily explains the Nd and Hf isotopic discordance observed for Isua amphibolites. The negative HFSE anomalies characterizing Isua basalts likewise could have been inherited from such an early (4.53–4.32Ga) deep mantle cumulate. Since the ~4Ga old Acasta Gneisses (Northwest Territories, Canada) have radiogenic 176Hf/177Hf (Scherer et al., 2010), dissociation of the Lu–Hf and Sm–Nd isotope systems may not be a ubiquitous feature of the Hadean mantle, but rather reflects that a deep-seated source was involved in the formation of ISB lavas. [Copyright &y& Elsevier]

Abstract: Major-element and trace-element compositions, as well as Nd isotope ratios and protolith model ages of Early to Middle Paleozoic granitoids from the Russian Gorny Altai are summarized and discussed in terms of the compositions and tectonic settings. The observed geographic variations of compositions in the granitoid assemblages correlate with three groups of late Neoproterozoic to Middle Paleozoic terranes corresponding to oceanic, island arc, and turbidite basin settings. Plutonic rocks in the oceanic and island arc terranes are commonly low-alkali metaluminous tonalite and trondhjemite that formed by melting of oceanic crust. The island arc terranes also contain tonalite–trondjemite–granodioritic and tonalite–granodiorite–granitic rocks, with higher alkali and potassium contents, which result from anatectic melting of volcanics and sediments. Granitoids in the turbidite basins are potassic peraluminous monzogranite produced by anatectic melting of upper crustal turbidites, as well as subalkaline tonalite, granodiorite, and granite derived from upper crustal metasediments and metamorphosed mafic oceanic crust underlying the turbidite basins. Alkaline granitoids emplaced during two major tectonomogamatic events (Early and Late Devonian) and are present in terranes of all three settings. Their source is of crustal origin, like in other groups of granitoids, but it was a mixture of crustal and mantle material. Thus the compositional trends of Early and Middle Paleozoic granitoids in the Gorny Altai record the history of continental crust. [Copyright &y& Elsevier]

Abstract: The Precambrian terranes of north east Africa are still poorly known. It is generally believed that the oldest rocks in the region are the charnockitic gneisses of the Uweinat massif, located at the triple junction between Libya–Egypt–Sudan, because they have yielded Rb–Sr ages around 2.6Ga and Nd TDM model ages around 3.0–3.2Ga. Here we confirm that these rocks are indeed Archean with SHRIMP U–Pb zircon ages as old as 3.0Ga, but we also report older rocks to the east, in the neighboring Gebel Kamil region. This area contains a metaigneous complex formed of tonalite–trondhjemite–granite (TTG) and gabbro-diorite (GbD) gneisses with a whole-rock Sm–Nd isochron age of 3.16±0.16Ga, average Nd T CR of 3.17±0.04Ga, and average ɛNd(3.2Ga) of 3.4±0.3. The oldest TTG gneisses, which are also the oldest rocks found to date in north east Africa, contain large magmatic zircons with SHRIMP U–Pb crystallization ages peaking at 3.22±0.04Ga, closely matching the Sm–Nd and the Nd T CR model ages. These ages are interpreted to represent arc-magmas produced between 3.1Ga and 3.3Ga. Other TTGs have younger Archean zircon ages that form a continuum between 3.1Ga and 2.55Ga, with three peaks at around 2.97Ga, 2.85Ga, and 2.6–2.7Ga; these rocks were apparently generated from the older TTGs during repeated events of crustal recycling. The crust was stable from 2.55Ga to 2.0Ga, when an intense thermal event generated rims of variable thickness over the Archean zircons. Neither the reworking from 3.1Ga to 2.55Ga, nor the metamorphism at 2.0Ga involved the addition of juvenile material to the crust of this area, which behaved as an almost closed system from 3.1Ga to 0.75Ga, when the intrusion of Pan-African I-type granitoids with a juvenile component began. [Copyright &y& Elsevier]

Abstract: Mount Narryer and Jack Hills metasedimentary rocks in the Narryer Gneiss Complex of the Yilgarn Craton, Western Australia, contain zircons with ages up to 4.4Ga, the oldest known crustal materials on Earth, and monazites up to 3.6Ga. In this study, we have investigated 147Sm–143Nd systematics of detrital and metamorphic monazites from these metasedimentary rocks using laser ablation-multicollector-inductively coupled plasma mass spectrometry (LA-MC-ICPMS). All detrital monazites have negative initial εNd(t) values, indicating that their parental magmas formed by remelting of older crustal materials. A comparison between the initial εNd(t) values of the detrital monazites and granitoids in the Narryer Gneiss Complex indicates that the Mt. Narryer and Jack Hills sediments were partly derived from the most isotopically enriched surrounding granitoids with ages of ca. 3.6 and 3.3Ga. The metamorphic monazites generally have lower initial εNd(t) values when compared to the detrital monazites. However, the detrital and metamorphic monazites show similar distributions of εNd(t) at the ages of sediment deposition (3.28Ga for Mt. Narryer and 3.05Ga for Jack Hills). In addition, multiple analyses on single monazite grains having core-rim structures reveal that the cores and the recrystallized rims had identical Nd isotopic compositions at the time of recrystallization. These findings indicate that older monazites are source of light rare earth elements for younger metamorphic monazite formation and, therefore, that monazite can inherit its primary Sm–Nd isotopic signature during the recrystallization processes. We calculated the Nd model ages for all analyzed monazites to estimate crustal residence time of their source materials. We find that no igneous monazites older than 4.0Ga were recrystallized to form the monazites. This implies that the lack of Hadean monazites is not due to recrystallization of ancient monazites during later metamorphism, but due to high-Ca compositions of the parental magmas of Hadean detritus, which prevent growth of magmatic monazite. [Copyright &y& Elsevier]

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]

This paper reports detailed stratigraphic analysis, whole rock geochemistry and Nd isotopic ratios of basalts from the northern Maine inliers. These data place constraints on first-order controversies about the tectonic conditions leading up to and during the early stages of the Acadian Orogeny in northern Maine. Late Silurian and early Devonian stratigraphic sequences indicate a progressive change from shallow water and subaerial exposure (Ripogenns and The Forks formations, East Branch Group) to a rapidly subsiding basin prior to the onset of Acadian deformation Subsidence was accompanied by mafic to intermediate volcanism of the West Branch, Spider Lake, and Fish Pond volcanics, and that continued with intrusion of the syn-deformational Greenville Plutonic Belt, including the Flagstaff Lake Igneous Complex, whose properties we report in this paper. Trace element geochemistry indicates that magmatism is transdiscriminant, showing aspects of within-plate, volcanic arc, and back-arc affinities and Nd isotopic ratios are moderately positive (+2.3 to + 3.8) indicating either an uncontaminated and moderately enriched mantle source, or a depleted mantle source contaminated by continental crust. Four possible tectonic models of the Acadian Orogeny and the Siluro-Devonian sedimentary-volcanic sequences of northern Maine are evaluated in the context of a subsiding basin associated with this magmatism developed on the amalgamated Laurentian plate. These include 1) slab detachment during southeast-directed subduction of the Laurentian continental margin; 2) "Laramide-style" thrust basins above a shallow, northwest-dipping subduction zone; 3) back-arc extension followed by thin skinned shortening above a northwest-dipping subduction zone; and 4) "Moluccan style" dual-dipping subduction zones. [ABSTRACT FROM AUTHOR]

Abstract: Attempts to decipher the petrogenesis of Archean Trondhjemite–Tonalite–Granodiorite (TTG) and establish trends in their compositions over time typically do not assess the possible role of assimilation–crystal fractionation processes. Studies of TTG suites worldwide, however, demonstrate that reprocessing of older crust is common. Felsic volcanic and intrusive rocks from the North Caribou Lake area, located in the nucleus of the Superior Province, exhibit a range of compositions that are not accounted for by variable degrees of amphibolite melting alone. Characteristic of Archean TTG, the rocks mainly fall in the lower right hand quadrant of a Nb/Ta vs Zr/Sm plot but they also show clear trends between εNdT and ratios such as Al2O3/TiO2, Th/Nb, [Tb/Yb]N and Eu/Eu*. The lowest Nb/Ta values in the TTG likely result from crystal fractionation including titanite, which strongly partitions and fractionates these elements. [Copyright &y& Elsevier]

Abstract: This work provides five new U–Pb zircon dating and the corresponding Nd isotope data for felsic granulites from the south Itabuna-Salvador-Curaçá Block (ISCB), in the São Francisco Craton, Brazil. Three major sets of felsic granulites can be recognised. The oldest set is tonalitic in composition and of TTG affinity. It is Archaean in age with magmatic zircon cores dated at 2675 ± 11 Ma by LA-ICPMS and up to ca 2.7–2.9 Ga by SHRIMP on an other sample. It exhibits epsilon Nd values between −8 and −11 at 2.1 Ga. This Nd signature is similar to that of granulites found in the western Archaean Jequié Block. Cartographically, this set of Archaean terrains represents at least 50% of the granulites in the studied area. The second set corresponds to a Palaeoproterozoic calc-alkaline tonalitic suite with zircon ages from 2019 ± 19 Ma to 2191 ± 10 Ma and epsilon Nd values between −3 and −4 at 2.1 Ga, corresponding partially to a newly formed crust. The third set of granulites is also Palaeoproterozoic. It is shoshonitic to monzonitic in composition and synchronous with the high grade metamorphism dated by metamorphic zircons at 2086 ± 7 Ma (average of five samples). The Nd isotope signature for this alkaline set is similar to that of the Palaeoproterozoic calc-alkaline one. Nd isotopes appear to be a very efficient tool to distinguish Archaean from Palaeoproterozoic felsic protoliths in granulitic suites of the Itabuna-Salvador-Curaçá Block (ISCB). Finally, the southern part of the ISCB is composed of a mixture of Archaean and Palaeoproterozoic protoliths, in similar amounts, suggesting that it was probably an active margin between 2.1 and 2.2 Ga located on the eastern border of the Archaean Jequié Block. A major crustal thickening process occurred at ca 2.09 Ga in the ISCB and seems significantly younger towards the west, in the Jequié granulites, where an average of 2056 ± 9 Ma is determined for the high grade event. [Copyright &y& Elsevier]

Abstract: The 618Ma Curral de Cima tonalite and 577Ma Lourenço monzodiorite, northeastern Brazil, are magmatic epidote-bearing plutons that carry ferrohornblende, biotite, titanite, and epidote. Major, trace, and isotope chemistry suggests that the major magmas of the two plutons followed similar differentiation trends but derived from source rocks that differed in age and isotopic composition. The mineral phases of the Curral de Cima tonalite, the presence of amphibole-rich clots, and juvenile component (average εNd=−3.55) point to an I-type source for these rocks. These data and high calculated δ18O(w.r.) (10.0‰) for the tonalite and high δ18O value for a clot (9.3‰) argue that the clots are fragments of a metabasaltic source rock that has been hydrothermally altered at a low temperature. In contrast, average calculated δ18O(w.r.) for the Lourenço monzodiorite =7.8‰, 87Sr/86Sr=0.7083, εNd=−14.6, and TDM =1.92Ga. These data are compatible with a long crustal residence time of lower crust amphibolites source. Epidote in the Curral de Cima pluton crystallized close to the NNO buffer, and hornblende chemistry, due to Al reequilibration, yielded sub-solidus temperature and pressure. In contrast, in the Lourenço pluton epidote crystallized close to the HM buffer and Al-in-hornblende points to near-solidus solidification (685°C) around 4.4Kbar. This study confirms that magmatic epidote in granitic plutons can crystallize at pressures lower than 5.5Kbar under higher fO2 as experimentally foreseen. Rapid magma transportation through hot continental crust during the peak of metamorphism in early stages of an orogenic cycle prevents epidote dissolution. [Copyright &y& Elsevier]

Abstract: Geochronological (sensitive high-resolution ion microprobe, SHRIMP II, and 40Ar/39Ar), major and trace element, and Nd isotopic data are presented for newly discovered early Mesozoic volcanic rocks in and around the central Qiangtang metamorphic belt on the northern Tibetan Plateau. The data indicate that volcanism/magmatism was active during the Triassic and Jurassic and show a bimodal composition. Mafic lavas are generally similar to oceanic island basalt, characterized by enrichments in Nb and Ta relative to similarly incompatible elements, high rare earth element (REE) concentrations, enrichment in light REEs, absence of Eu anomalies, and positive εNd values. Felsic lavas are characterized by enrichment in large ion lithophile elements and light REEs, depletion in high field strength elements (especially for Nb and P), and very low εNd values (<−10). The trace element and isotopic data suggest that the felsic lavas were derived from partial melting of the continental crust. The volcanism was caused by post-orogenic collapse of the central Qiangtang metamorphic belt due to break-off of the subducting western Qiangtang slab, which may have induced decompressional melting that resulted in the eruption of small volumes of basaltic rocks and the relatively rapid exhumation of high-pressure to ultrahigh-pressure rocks. [Copyright &y& Elsevier]

Abstract: The Gawler Craton forms the bulk of the South Australian Craton and occupies a pivotal location that links rock systems in Antarctica to those in northern Australia. The western Gawler Craton is a virtually unexposed region where the timing of basin development and metamorphism is largely unknown, making the region ambiguous in the context of models seeking to reconstruct the Australian Proterozoic. Detrital zircon data from metasedimentary rocks in the central Fowler Domain in the western Gawler Craton provide maximum depositional ages between 1760 and 1700Ma, with rare older detrital components ranging in age up to 3130Ma. In the bulk of samples, ɛ Nd(1700Ma) values range between −4.3 and −3.8. The combination of these data suggest on average, comparatively evolved but age-restricted source regions. Lu–Hf isotopic data from the ca 1700Ma aged zircons provide a wide range of values (ɛ Hf(1700Ma) +6 to −6). Monazite U–Pb data from granulite-grade metasedimentary rocks yield metamorphic ages of 1690–1670Ma. This range overlaps with and extends the timing of the widespread Kimban Orogeny in the Gawler Craton, and provides minimum depositional age constraints, indicating that basin development immediately preceded medium to high grade metamorphism. The timing of Paleoproterozoic basin development and metamorphism in the western Gawler Craton coincides with that in the northern and eastern Gawler Craton, and also in the adjacent Curnamona Province, suggesting protoliths to the rocks within the Fowler Domain may have originally formed part of a large ca 1760–1700Ma basin system in the southern Australian Proterozoic. Provenance characteristics between these basins are remarkably similar and point to the Arunta Region in the North Australian Craton as a potential source. In this context there is little support for tectonic reconstruction models that: (1) suggest components of the Gawler Craton accreted together at different stages in the interval ca 1760–1680Ma; and (2) that the North Australian Craton and the southern Australian Proterozoic were separate continental fragments between 1760 and 1700Ma. [Copyright &y& Elsevier]

Abstract: New geochemical, Nd-isotope and U–Pb zircon data from Neoproterozoic volcanic rocks from Wadi Sawawin in northwestern Saudi Arabia provide important constraints on the evolution of the crust in this part of the Arabian–Nubian Shield (ANS). The Ghawjah volcanic rocks range from tholeiitic to calc-alkaline and are metamorphosed to greenschist facies. U–Pb zircon analyses for Ghawjah andesite yield a weighted mean 206Pb/238U age of 763±25Ma, indicating that these are some of the oldest rocks of the Midyan terrane. Ghawjah volcanic rocks are mostly moderately fractionated, as indicated by Mg-numbers between 28 and 67, Cr between 5 to 537ppm and Ni from 4 to 175ppm, REE patterns are slightly fractionated [(La/Yb)N=1.2 to 4.0], and multi-element diagrams show Ba, Sr, Rb and K enrichments and Nb and Ta depletions, typical of modern convergent-margin igneous rocks. Ghawjah volcanic rocks have positive εNd (+5.4 to +8.2) and a mean model age of 0.71Ga. Ghawjah volcanic rocks are similar to the “Younger Volcanic” rocks from the Central Eastern Desert (CED) of Egypt, in terms of stratigraphic relations, chemical compositions, Nd-isotopic compositions, and U–Pb zircon ages (~750Ma), indicating that both were generated by partial melting of subduction-modified depleted mantle. The Ghawjah volcanic rocks are interpreted to have formed at ~750Ma in an arc setting during an important episode of crust formation. [ABSTRACT FROM AUTHOR]

Devonian-Carboniferous granites are widespread in Tasmania. In eastern Tasmania, Devonian granites intrude Ordovician-Early Devonian quartz-rich turbidites of the Mathinna Supergroup. The earliest (∼400 Ma) I-type granodiorites may be arc-related. Following the Tabberabberan Orogeny (∼389 Ma), more felsic and, finally, strongly fractionated I- and S-type granites were emplaced until ∼373 Ma. In contrast, western Tasmania granites intrude a more diverse terrane of predominantly marine shelf successions, with depositional ages as old as Late Mesoproterozoic. They are mostly felsic and fractionated I- and S-types emplaced from ∼374-351 Ma, possibly in response to post-collisional crustal extension following juxtaposition of the eastern and western Tasmanian terranes. Granites from the two terranes are readily distinguishable by the age spectra of their inherited zircon, which are noticeably similar to those of the detrital zircon from sedimentary successions in their respective terranes. Furthermore, within each terrane, both I and S-types yield similar inheritance patterns. This suggests a pivotal role for the sedimentary successions in the petrogenesis of both types. Western Tasmanian granites are also enriched in ∼1600 Ma zircon, which is essentially unrepresented in the exposed supracrustal succession. Subtle differences between the inheritance and detrital age spectra in eastern Tasmania probably relate to unrepresentative sampling of the supracrustal rocks. Nd, Sr and Pb isotopic characteristics of the granites are consistent with their derivation by mixing of magmas derived from the mantle, possibly the lower crust, and from supracrustal rocks. Systematic isotopic trends in some eastern Tasmanian I-types, particularly in the Scottsdale Batholith, correlate well with major and trace element geochemistry and age. The isotopes are inconsistent with simple restite unmixing or crystal fractionation in a closed magma chamber, and indicate progressive contamination by the Mathinna Supergroup, or similar rocks. The isotopic characteristics of late, strongly fractionated granites, although sometimes obscured by hydrothermal alteration, are also consistent with concurrent assimilation-fractional crystallisation processes. Together with the close association of some strongly fractionated I- and S-types, this suggests that such granites were generated directly in the lower crust, and were not derived from unfractionated parental granite magmas. [ABSTRACT FROM AUTHOR]

The Tamazert Eocene alkaline complex of the Central High Atlas Range of Morocco hosts the largest outcropping occurrences of carbonatites in northern Africa. The complex consists of carbonatites and undersaturated ultramafic to syenitic alkaline to peralkaline silicate rocks. Mineralogically and geochemically the Tamazert carbonatites are classified as calciocarbonatites, magnesiocarbonatites and silicocarbonatites. They are enriched in light rare earth elements and large ion lithophile elements (Cs, Rb, Ba, U, Th), but depleted in high field strength elements (particularly, Ti, Nb and Ta). Stable and radiogenic isotope ratios vary in the range of δ13CPDB = –5·8 to 1·8‰, δ18OSMOW = 6·9–23·5‰, initial 87Sr/86Sr = 0·7031–0·7076, 143Nd/144Nd = 0·5125–0·5129 and 206Pb/204Pb = 18·29–19·89. Calciocarbonatites intruding Jurassic limestones have the highest δ13C and δ18O values and the most radiogenic initial 87Sr/86Sr, but least radiogenic 143Nd/144Nd, 206Pb/204Pb and 208Pb/204Pb isotope ratios, and are interpreted to have interacted with the limestones (crustal components). The magnesio- and silicocarbonatites have Sr, Nd and Pb isotope ratios that are nearly identical to those of low-87Sr/86Sr calciocarbonatites. The isotope signature of the high-Sr, low-87Sr/86Sr calciocarbonatites with mantle-type O and C isotopic compositions indicates the presence of HIMU- and EMI-type components in the mantle source of the Tamazert carbonatites, similar to what has been proposed for the Cape Verde and Canary Islands. The close similarity in carbonatite composition between the Cape Verde and Canary Islands and Tamazert suggests a common sublithospheric source for these carbonatites. We therefore propose that the Tamazert carbonatites originated through melting of Canary plume material that may have flowed through a sub-lithospheric corridor extending from the Atlantic near the Canary Islands to the Middle Atlas, formed by the delamination of the subcontinental lithosphere in response to Africa–Europe collision at c. 42 Ma. Seismic tomography data suggest that the common source may be within the lower mantle at depths > 1000 km. [ABSTRACT FROM PUBLISHER]

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]

Abstract: Determination of the emplacement ages and initial isotopic composition of kimberlite by conventional isotopic methods using bulk rock samples is unreliable as these rocks usually contain diverse clasts of crustal- and mantle-derived materials and can be subject to post-intrusion sub-aerial alteration. In this study, 8 samples from 5 kimberlites in southern Africa and twelve samples from 7 kimberlites from Somerset Island, Canada have been selected for in situ perovskite U–Pb isotopic age determination and Nd isotopic analysis by laser ablation using thin sections and mineral separates. These fresh perovskites occur as primary groundmass minerals with grain-sizes of 10–100μm. They were formed during the early stage of magmatic crystallization, and record data for the least contaminated or contamination-free kimberlitic magma. U–Pb isotopic data indicate that the majority of the southern Africa kimberlites investigated were emplaced during the Cretaceous with ages of 88±3 to 97±6Ma, although one sample yielded an Early Paleozoic age of 515±6Ma. Twelve samples from Somerset Island yielded ages ranging from 93±4Ma to 108±5Ma and are contemporaneous with other Cretaceous kimberlite magmatism in central Canada (103–94Ma). Although whole-rock compositions of the kimberlites from southern Africa have a large range of ε Nd(t) values (−0.5 to +5.1), the analysed perovskites show a more limited range of +1.2 to +3.1. Perovskites from Somerset Island have ε Nd(t) values of −0.2 to +1.4. These values are lower than that of depleted asthenospheric mantle, suggesting that kimberlites might be derived from the lower mantle. This study shows that in situ U–Pb and Nd isotopic analysis of perovskite by laser ablation is both rapid and economic, and serves as a powerful tool for the determination of the emplacement age and potential source of kimberlite magmas. [Copyright &y& Elsevier]

Abstract: We have measured 182W/184W for Eoarchean rocks from the Itsaq Gneiss Complex (3.8–3.7Ga pillow meta-basalts, a meta-tonalite, and meta-sediments) and Acasta Gneiss Complex (4.0–3.6Ga felsic orthogneisses) to assess possible W isotopic heterogeneity within the silicate Earth and to constrain W isotopic evolution of the mantle. The data reveal that 182W/184W values in the Eoarchean samples are uniform within the analytical error and indistinguishable from the modern accessible mantle signature, suggesting that the W isotopic composition of the upper mantle has not changed significantly since the Eoarchean era. The results imply either that chemical communication between the mantle and core has been insignificant in post-Hadean times, or that a lowermost mantle with a distinctive W isotope signature has been isolated from mantle convective cycling. Most terrestrial rock samples have a 0.2ε 142Nd/144Nd higher than the chondrite average. This requires either the presence of a hidden enriched reservoir formed within the first 30Ma of the Solar System, or the bulk Earth having a ∼5% higher Sm/Nd than the chondrite average. We explored the relevance of the 182Hf–182W isotope system to the 146Sm–142Nd isotope system during early silicate differentiation events on Earth. In this context, we demonstrate that the lack of resolvable 182W excesses in the Itsaq rocks, despite 142Nd excesses compared to the modern accessible mantle, is more consistent with the view that the bulk Earth has a non-chondritic Sm/Nd. In the non-chondritic Sm/Nd Earth model, the 182W–142Nd chronometry constrains the age of the source mantle depletion for the Itsaq samples to more than ∼40Ma after the Solar System origin. Our results cannot confirm the previous report of 182W anomalies in the Eoarchean Itsaq meta-sediments, which were interpreted as reflecting an impact-derived meteoritic component. [Copyright &y& Elsevier]

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]

Abstract: The 1690–1670Ma felsic Tunkillia Suite was generated through the central Gawler Craton, southern Australia, during or after the waning stages of the 1730–1690Ma Kimban Orogeny. Previous models for the generation of the Tunkillia Suite used trace element tectonic discrimination diagrams to suggest a subduction-related petrogenesis. Subsequently, Tunkillia Suite magmatism was used to infer an active plate margin in paleotectonic reconstruction models. In part, this led to the suggestion of a long-lived active margin on the southern edge of paleo-Australia that was correlated with the long-lived active margin of south-western Laurentia. The more detailed geochemical and isotopic analyses used in this study highlight the shortcomings of analysis based upon simple application of trace-element tectonic discrimination diagrams. The combination of detailed geochemical assessment and new geochronology demonstrates that some intrusions previously used to characterise the tectonic setting of the Tunkillia Suite are in fact unrelated. Tunkillia Suite intrusions are typically high-K, alkali-calcic and magnesian and are dominantly felsic (>70wt% SiO2), moderately peraluminous granitoids. Trace and rare earth element abundances display large variations. Sr and Y concentrations range from 40 to 550ppm and from 4.7 to 41ppm, respectively. Eu/Eu* values range from 0.2 to 1.6 and combined with REE patterns demonstrate the varying composition of restite associated with melt generation of the Tunkillia Suite. The majority of the Tunkillia Suite display chemical characteristics typical of a plagioclase- or hornblende-dominated restite with only three samples demonstrating garnet-rich restite composition. Nd-isotope characteristics of Tunkillia Suite intrusions suggest a crustal column of variable age exists in the Gawler Craton, as variation in ɛ Nd(T) values (−6.3 to +2.6) cannot be accounted for simply through assimilation and fractional crystallisation processes. The limited SiO2 compositional range of the Tunkillia Suite inhibits unequivocal tectonic classification due to the inability to determine mantle melt composition and infer mantle melting conditions leading to the generation of the Tunkillia Suite. Geochemical, isotopic and mineralogical characteristics of the Tunkillia Suite are most consistent with a ‘late- to post-tectonic’ setting for petrogenesis. The term ‘late- to post-tectonic’ is used in this instance as a genetic descriptor based upon the comparison with Phanerozoic granitoid suites formed in such a setting after collisional orogenesis. Contrary to the suggestion of previous studies, a subduction-related arc setting is not readily reconcilable with the chemical and mineralogical characteristics of the Tunkillia Suite. [Copyright &y& Elsevier]

Abstract: Carolinia comprises a collection of Neoproterozoic–Early Paleozoic magmatic arc and sedimentary terranes that were amalgamated and accreted to Laurentia in the early to middle Paleozoic. In central North Carolina, mafic rocks of the Stony Mountain gabbro intrude sub-aqueous volcanic and sedimentary rocks and submarine epiclastic sedimentary rocks of the Albemarle Group. The age of the Stony Mountain gabbro is constrained to the Early Cambrian–Middle Ordovician. Field relations indicate that the gabbro represents the final phase of magmatism following the eruption and deposition of the Neoproterozoic–earliest Cambrian Albemarle Group, yet the gabbro pre-dates regional metamorphism and tectonism related to the Late Ordovician accretion of Carolinia to Laurentia. The Stony Mountain gabbro has a sub-alkaline basaltic composition, variable TiO2, MgO and Ni/Cr values. The rocks have a geochemical signature typical of island-arcs; the degree of LREE enrichment, prominent negative Nb anomalies and Nb/Th ratios are all features of low-K to medium-K tholeiitic basalts in modern island-arc, subduction-related lavas. Isotope data are dominated by juvenile compositions that are consistent with derivation from lithospheric and asthenospheric sources during decompression melting of the mantle. The Stony Mountain gabbro records subduction zone magmatism in a rifted island arc setting and can be modeled as the product of ~10–15% hydrous partial melting of variable mixtures of MORB- and OIB-like mantle sources overprinted by a minor subducted-slab derived hydrous fluid component. By analogy with modern settings the rocks of the Stony Mountain gabbro are comparable to MORB-like to OIB-type enriched rocks from the Lau Island and Sumisu Rift and are interpreted to have formed within an evolving Early Paleozoic island arc–back arc rift–basin system. The presence of an Early Cambrian arc–back arc rift system in Carolinia is broadly coeval with arc–back arc volcanism in other peri-Gondwanan blocks of the Appalachians and may be related to the Early Paleozoic opening of the Rheic Ocean. [Copyright &y& Elsevier]

Abstract: Nd isotopes preserved in fossil fish teeth and ferromanganese crusts have become a common tool for tracking variations in water mass composition and circulation through time. Studies of Nd isotopes extracted from Pleistocene to Holocene bulk sediments using hydroxylamine hydrochloride (HH) solution yield high resolution records of Nd isotopes that can be interpreted in terms of deep water circulation, but concerns about diagenesis and potential contamination of the seawater signal limit application of this technique to geologically young samples. In this study we demonstrate that Nd extracted from the >63μm, decarbonated fraction of older Ocean Drilling Program (ODP) sediments using a 0.02M HH solution produces Nd isotopic ratios that are within error of values from cleaned fossil fish teeth collected from the same samples, indicating that the HH-extractions are robust recorders of deep sea Nd isotopes. This excellent correlation was achieved for 94 paired fish teeth and HH-extraction samples ranging in age from the Miocene to Cretaceous, distributed throughout the north, tropical and south Atlantic, and composed of a range of lithologies including carbonate-rich oozes/chalks and black shales. The strong Nd signal recovered from Cretaceous anoxic black shale sequences is unlikely to be associated with ferromanganese oxide coatings, but may be derived from abundant phosphatic fish teeth and debris or organic matter in these samples. In contrast to the deep water Nd isotopic signal, Sr isotopes from HH-extractions are often offset from seawater values, suggesting that evaluation of Sr isotopes is a conservative test for the integrity of Nd isotopes in the HH fraction. However, rare earth elements (REE) from the HH-extractions and fish teeth produce distinctive middle REE bulge patterns that may prove useful for evaluating whether the Nd isotopic signal represents uncontaminated seawater. Alternatively, a few paired HH-extraction and cleaned fish teeth samples from each site of interest can be used to verify the seawater composition of the HH-extractions. The similarity between isotopic values for the HH-extraction and fish teeth illustrates that the extensive cleaning protocol applied to fish teeth samples is not necessary in typical, carbonate-rich, deep sea sediments. [Copyright &y& Elsevier]

Abstract: The Yunkai Terrane is one of the most important pre-Devonian areas of metamorphosed supracrustal and granitic basement rocks in the Cathaysia Block of South China. The supracrustal rocks are mainly schist, slate and phyllite, with local paragneiss, granulite, amphibolite and marble, with metamorphic grades ranging from greenschist to granulite facies. Largely on the basis of metamorphic grade, they were previously divided into the Palaeo- to Mesoproterozoic Gaozhou Complex, the early Neoproterozoic Yunkai ‘Group’ and early Palaeozoic sediments. Granitic rocks were considered to be Meso- and Neoproterozoic, or early Palaeozoic in age. In this study, four meta-sedimentary rock samples, two each from the Yunkai ‘Group’ and Gaozhou Complex, together with three granite samples, record metamorphic and magmatic zircon ages of 443–430Ma (Silurian), with many inherited and detrital zircons with the ages mainly ranging from 1.1 to 0.8Ga, although zircons with Archaean and Palaeoproterozoic ages have also been identified in several of the samples. A high-grade sillimanite–garnet–cordierite gneiss contains ∼242Ma metamorphic zircons, as well as ∼440Ma ones. Three of the meta-sedimentary rocks show large variations in major element compositions, but have similar REE patterns, and have t DM model ages of 2.17–1.91Ga and ε Nd (440Ma) values of −13.4 to −10.0. Granites range in composition from monzogranite to syenogranite and record t DM model ages of 2.13–1.42Ga and ε Nd (440Ma) values of −8.4 to −1.2. It is concluded that the Yunkai ‘Group’ and Gaozhou Complex formed coevally in the late Neoproterozoic to early Palaeozoic, probably at the same time as weakly to un-metamorphosed early Palaeozoic sediments in the area. Based on the detrital zircon population, the source area contained Meso- to Neoproterozoic rocks, with some Archaean material. Palaeozoic tectonothermal events and zircon growth in the Yunkai Terrane can be correlated with events of similar age and character known throughout the Cathaysia Block. The lack of evidence for Palaeo- and Mesoproterozoic rocks at Yunkai, as stated in earlier publications, means that revision of the basement geology of Cathaysia is necessary. [Copyright &y& Elsevier]

Abstract: Systematic analysis of Pb, Sr and Nd isotopes of 32 fluvial clay samples (<2µm fraction) from many of the major tributaries of the vast (1.106 km2) Murray Darling Basin (MDB), located in semiarid southeastern Australia, displays similar isotopic values between some MDB clays and dust from several ice core samples from the EPICA Dome C in Antarctica. Close scrutiny of several ratios of the four Pb isotopes, and in particular 208Pb/207Pb versus 206Pb/207Pb, shows that several samples from the Darling-sub-basin of the MDB display similar values for the same isotopes for Dome C samples from different ages, and more particularly during wet phases in Australia [Marine Isotopic Stages 5e, 3 and 1]. The combination of Nd and Sr isotopic ratios from the same MDB fluvial clays clearly eliminates the Murray sub-basin, and supports the Darling sub-basin as a potential source of aeolian material to Antarctica. Overall, the Australian dust supply to Antarctica predominantly occurred during interglacial periods. The work presented here shows that aerosols generated in southeastern Australia can travel to parts of West Antarctica and this is supported by atmospheric observations and models. In addition, evidence of Australian dust in Antarctic ice cores further implies dust deposition in the Southern Ocean would have occurred in the past. Current meteorological observations also imply that the western Pacific and Indian Ocean sector of the Southern Ocean would frequently receive aeolian dust components originating from southeastern Australia. [Copyright &y& Elsevier]

Abstract: Mafic granulites and amphibolites in the Masang Kang area of NW Bhutan Himalaya have been investigated for their geochemical and isotopic characteristics in order to determine their protolith history. Bulk-rock major and trace element geochemistry indicate that the rocks were originally tholeiitic and alkali basalts with minor ultramafics. U–Pb zircon SIMS data suggest an age of 1742±39Ma for mafic magmatism. The age-corrected ε Nd (1742) values of the rocks are highly variable, ranging from high positive (+8.4) to negative (−3.3). The positive value suggests a primitive magma source, similar to that of rift-related tholeiites. We suggest that the rocks of the Masang Kang suite were produced during a major late Paleoproterozoic thermal event that caused the mobilization and enrichment of the sub-continental lithospheric mantle beneath the north Indian margin. The geochemical signature of these rift-related metabasic rocks may have been produced during an earlier episode of oceanic underplating or subduction from which the fluid required to mobilize and enrich the overlying sub-lithospheric mantle may have been derived. Though their occurrence is rare, Paleoproterozoic igneous rocks within the Greater Himalayan Sequence (GHS), in addition to sources identified throughout the LHS, may have contributed to the detrital zircon population that form the 1.7–1.9Ga peak in the age spectra of the Lesser Himalayan Sequence (LHS). In addition, the coeval Paleoproterozoic magmatism in both LHS and GHS suggests that the two lithotectonic units may have belonged to the same continental plate at that time period. [Copyright &y& Elsevier]

The composite intrusions of Drumadoon and An Cumhann crop out on the SE coast of the Isle of Arran, Scotland and form part of the larger British and Irish Palaeogene Igneous Province, a subset of the North Atlantic Igneous Province. The intrusions (shallow-level dykes and sills) comprise a central quartz–feldspar-phyric rhyolite flanked by xenocryst-bearing basaltic andesite, with an intermediate zone of dark quartz–feldspar-phyric dacite. New geochemical data provide information on the evolution of the component magmas and their relationships with each other, as well as their interaction with the crust through which they travelled. During shallow-crustal emplacement, the end-member magmas mixed. Isotopic evidence shows that both magmas were contaminated by the crust prior to mixing; the basaltic andesite magma preserves some evidence of contamination within the lower crust, whereas the rhyolite mainly records upper-crustal contamination. The Highland Boundary Fault divides Arran into two distinct terranes, the Neoproterozoic to Early Palaeozoic Grampian Terrane to the north and the Palaeozoic Midland Valley Terrane to the south. The Drumadoon Complex lies within the Midland Valley Terrane but its isotopic signatures indicate almost exclusive involvement of Grampian Terrane crust. Therefore, although the magmas originated at depth on the northern side of the Highland Boundary Fault, they have crossed this boundary during their evolution, probably just prior to emplacement. [ABSTRACT FROM PUBLISHER]

Abstract: Geochronology of detrital zircons and their overgrowths combined with whole-rock geochemical and Sm–Nd isotopic data can be used to distinguish different stages of sediment recycling and metamorphism during multiphase orogenic evolution. This approach is applied to the Paleoproterozoic sedimentary rocks of the Tampere and Pirkanmaa belts (southern Finland) in the center of the composite Svecofennian orogen. The lower part of the Tampere belt succession and bulk of the Pirkanmaa belt are characterized by turbidites whereas the upper part of the Tampere belt succession is dominated by 1.90–1.89Ga mature arc-type volcanic rocks. Detrital zircon geochronology indicates that the Tampere and Pirkanmaa belts have a coeval 1.92–1.89Ga depositional and tectonic history. Ages of pre-depositional zircon overgrowths vary from 1.91Ga to 2.0Ga with clusters at 1.92Ga and 1.98Ga. Within the Pirkanmaa belt, post-depositional zircon overgrowths indicate metamorphic culmination at c. 1885Ma in the Vammala Ni-zone and at c. 1875Ma in the northern part. The lower conglomerates and graywackes in the Tampere belt and their equivalents in the Pirkanmaa belt are rich both in Neoarchean and Paleoproterozoic grains, the latter ranging in age from 1.9 to 2.1Ga. Compared to these, a sample from the Vammala Ni-zone has an exotic provenance with at least c. 1.90Ga, 2.04–2.15Ga, 2.38–2.48Ga and 2.57–2.63Ga components. A sedimentary recycling and tectonic model for the central Fennoscandia is proposed, in which the Paleoproterozoic Keitele+Bergslagen continent was formed during an unnamed orogeny at 1.98–1.97Ga. The Archean Norrbotten microcontinent was attached to the continent at 1.97–1.93Ga. Upper Kaleva turbidites, derived from the Lapland-Kola orogen in the north, were deposited before 1.92Ga on a passive margin of the Archean Karelia craton. The Karelia craton collided with the Keitele+Bergslagen+Norrbotten continent at c. 1.92Ga forming the Lapland-Savo orogen. Subsequent evolution led to rifting and break-up of the latter continent into two microcontinents in the hinterland. At 1.92–1.91Ga the rift was developed into a subsiding passive margin of the Keitele microcontinent with voluminous turbidite deposition, now seen as graywackes in the Tampere, Pirkanmaa and Pohjanmaa (western Finland) belts. The turbidite material was derived from the rising Lapland-Savo orogen and included recycled Upper Kaleva, recycled (sandstones) and first-cycle 2.03–1.97Ga detritus from Keitele, 1.93–1.92Ga Savo arc material, as well as detritus from the Archean craton and its cover deposits. The collision between Karelia and Keitele caused a subduction reversal and the onset of Tampere arc volcanism at 1.90Ga. Arc-derived materials started to deposit and were mixed with older sedimentary rocks, and trench-parallel distal turbidites from exotic source were being deposited in the accretionary wedge. [Copyright &y& Elsevier]

Abstract: Mafic dyke swarms represent short-lived magmatic events that carry important temporal and chemical constraints for the evolution of the lithospheric mantle. Five baddeleyite U–Pb isotopic analyses of Paleoproterozoic mafic dyke swarms intruding the Ungava Peninsula yielded ages of 2508±6, 2220±1, 2212±3, 2199±5 and 2149±3Ma. These ages fall within the 2.5–2.0Ga period previously defined by five other swarms, which together comprise a magmatic record of more than 500Ma. The similar age, composition and surface area of two 2.51Ga swarms, along with the lack of coeval supracrustal rocks, suggest they correspond to failed rifts. An important magmatic hiatus of 280Ma follows, but ends with the intrusion of numerous dykes between 2.23 and 2.20Ga in the thinnest lithosphere of the northern Ungava Peninsula. These dykes exhibit chemical signatures that vary from enriched older 2229–2212Ma dykes (ɛNd(t) <+0.4, La/Yb>4) to relatively depleted 2209–2199Ma voluminous younger dykes (ɛNd(t) >+1.4, La/Yb<4). The depleted character, great volume, and occurrence of these younger dykes in a thin lithosphere may represent decompression melting of the asthenosphere, following delamination of the Archean lithospheric keel ca. 2.21–2.20Ga. Later dykes emplaced ca. 2.17Ga within a rejuvenated Paleoproterozoic lithosphere also have depleted chemical compositions, and map the extent of rifting coeval with the emplacement of early basalts in the Labrador Trough. Dykes emplaced in the 2.15–2.00Ga period further to the South are proximal to coeval continental basalts, with which they share enriched isotopic compositions (ɛNd(t) =−1.5 to −7) and a positive correlation between La/Yb and Zr/Nb (La/Yb=4–10; Zr/Nb=7–20) that suggest assimilation of an Archean crustal component. A progressive decrease in the ɛNd(t) values of these magmas with decreasing age further implies increasing contamination, until the Archean lithosphere failed ca. 2.0Ga and permitted the eruption of isotopically depleted basalts. The ca. 2.2Ga depleted northern dykes and the 2.1–1.9Ga Povungnituk basalts of the Cape Smith foldbelt share a negative correlation between Zr/Nb and La/Yb that is expected for the incorporation of an enriched lithospheric component. This component is represented by the alkaline rocks comprised in the Circum-Ungava supracrustal belts (carbonatites, lamprophyric lavas, alkaline basalts) and by the Lac Aigneau lamprophyric dykes (1941±3Ma) intruding the Ungava Peninsula. These alkaline magmas collectively have 2.2–2.1Ga depleted-mantle Nd model ages (T DM) similar to a 2.15Ga Sm–Nd isochron defined by the basaltic rocks of the supracrustal belts. Although these Nd T DM ages and isochron are 100–300Ma older than magmatic ages, they are identical with the ca. 2.2Ga mafic dykes of the Ungava Peninsula. This time correlation, along with the similar chemical signature of the northern dykes and younger Povungnituk basalts, suggest that the composition of younger lavas and alkaline magmas was influenced by an enriched component associated with the pervasive metasomatism of the lithosphere, coeval with the emplacement of the many ca. 2.2Ga dykes. [Copyright &y& Elsevier]

It has been shown before that the combined use of Sr and Nd isotopes allows the geographical origin of Roman primary glass production to be traced. Nd isotopes are characteristic of the mineral fraction other than quartz in the silica sand used for the glass, while Sr isotopes are in most cases characteristic of the lime component, either attributed to the sand or as a separate constituent in the form of shell. In this study, the relationship between a sand raw material and the resulting ancient glass in terres of their trace element and Sr-Nd isotopic compositions is investigated. Especially the relevance of REE versus the use of isotopic techniques in the provenance determination of glass production is evaluated. The REE content of Roman glass is shown to be dependent on the clay fraction of the sand, and is identical for most possible provenance areas in the Mediterranean. Conversely, elements such as Zr, Hf, Ti, Sn are controlled by heavy mineral concentrations in sand and offer great potential as indicators of glass provenance. [ABSTRACT FROM AUTHOR]

Abstract: Olmoti Crater is part of the Plio-Pleistocene Ngorongoro Volcanic Highland (NVH) in northern Tanzania to the south of Gregory Rift. The Gregory Rift is part of the eastern branch of the East African Rift System (EARS) that stretches some 4000km from the Read Sea and Gulf of Aden in the north to the Zambezi River in Mozambique. Here, we (1) characterize the chemistry and mineral compositions of lavas from Olmoti Crater, (2) determine the age and duration of Olmoti volcanic activity through 40Ar/39Ar dating of Olmoti Crater wall lavas and (3) determine the genesis of Olmoti lavas and the relationship to other NVH and EARS volcanics and (4) their correlation with volcanics in the Olduvai and Laetoli stratigraphic sequences. Olmoti lavas collected from the lower part of the exposed crater wall section (OLS) range from basalt to trachyandesite whereas the upper part of the section (OUS) is trachytic. Petrography and major and trace element data reflect a very low degree partial melt origin for the Olmoti lavas, presumably of peridotite, followed by extensive fractionation. The 87Sr/86Sr data overlap whereas Nd and Pb isotope data are distinct between OLS and OUS samples. Interpretation of the isotope data suggests mixing of enriched mantle (EM I) with high-μ-like reservoirs, consistent with the model of Bell and Blenkinsop [Bell, K., Blenkinsop, J., 1987. Nd and Sr isotopic compositions of East African carbonatites: implications for mantle heterogeneity. Geology 5, 99–102] for East African carbonatite lavas. The isotope ratios are within the range of values defined by Oceanic Island Basalt (OIB) globally and moderate normalized Tb/Yb ratios (2.3–1.6) in these lavas suggest melting in the lithospheric mantle consistent with other studies in the region. 40Ar/39Ar incremental-heating analyses of matrix and anorthoclase separates from Olmoti OLS and OUS lavas indicate that volcanic activity was short in duration, lasting ∼200kyr from 2.01±0.03Ma to 1.80±0.01Ma. The age of Olmoti activity overlaps with ages reported for Ngorongoro Caldera, implying contemporaneous activity of multiple NVH volcanic centers during part of the eruption interval. Olmoti is considered the source for the bulk of interbedded volcanics and volcaniclastic deposits that comprise much of the upper Bed I section of nearby Olduvai Gorge, and part of the Laetoli sequence, both known for their well preserved fossils and archaeological remains. Age and chemical data reported here are compatible with those derived from tephra and lava interbedded in Bed I at Olduvai Gorge and from the Olpiro Beds at Laetoli. [Copyright &y& Elsevier]

Abstract: The trace element signature of Earth''s continental crust resembles that of arc lavas, but the continents may have formed during ancient igneous pulses that are hard to reconcile with supra-subduction zone magmatism. We explore the role of coupled arc–back-arc accretionary processes in crust generation by considering the tectonic context of whole rock Nd isotope and zircon Hf–O isotope data from igneous rocks of the Australian Tasmanides (515–230 Ma), which is thought to have evolved by the repeated opening and closure of sediment-filled back-arc basins behind a long-lived subduction zone. The significance of this process for continental crust formation has yet to be evaluated from an isotopic perspective. Granitic rocks in this area define striking secular ε Nd–ε Hf–δ18O trends that correlate with the pattern of deformational events and register changes in magma source during tectonic activity. These trends reveal that juvenile magmatic input was enhanced during extensional, back-arc rifting episodes that immediately followed crustal thickening, suggesting a relationship between slab rollback and continental growth. Interaction between juvenile magma and sedimentary units deposited during a preceding back-arc rifting cycle was integral to the formation of stable continental material. This highlights the importance of back-arc environments for both the generation and differentiation of continental crust. The juvenile component within the Tasmanide igneous rocks increased from the Cambrian to the Triassic, consistent with a diminished input from craton-derived metasedimentary material as the subduction zone migrated outboard of the Gondwana margin. Subduction zone retreat formed large tracts of new crust in eastern Australia at comparable rates to crust generation at modern island arcs, providing a mechanism for rapid continental growth at convergent margins. Using isotopic tracers to link tectonic evolution and crust generation in modern and ancient orogens can lead to a better understanding of the geodynamic controls on planetary differentiation. [Copyright &y& Elsevier]

Abstract: Intrusive rocks are an important heat source for the maturation of organic matter contained in sediments from the Paraná Basin, South America. In this study, we have investigated the geochemical effects induced by the thermal effect of a 13-m thick basic sill intruded on carbonates and black shales of the Permian Irati Formation, which constitutes one of the most important oil source horizons in the Paraná Basin. Detailed sampling was performed on the sedimentary host rocks located in the upper and lower contacts. XRD mineralogical examination of the host rocks reveals major mineralogical changes induced by the basic intrusion that led to the formation of talc, serpentine and pyroxene. Trace elements and neodymium (Nd) isotopes across the contact indicate that geochemical interaction between the basic magma and the sedimentary rocks was not significant, suggesting that heat was transferred predominantly by diffusion. The data also reveal that: (i) the sedimentary rocks located below the sill are more enriched in hydrous minerals than those in the upper part, suggesting that the sill behaved as a hydraulic barrier to fluid percolation; and (ii) carbonate samples show a systematic decrease in carbon (C) and oxygen (O) isotope values towards the contact, indicating release of CO2 from carbonate consumption during metamorphic reactions. Thermal effects are also observed on spores present in black shales. They show coloration index (SCI) values up to 10 for distances below 1.65m from the lower contact with the basic sill. Beyond this point, the spore coloration index drops to 4–4.5 at 12m from the contact, which is the deepest point from which samples could be collected. Assuming that heat was transported by diffusion, we modeled the variation of temperature near the contact using a one-dimensional transfer model of a one-dimensional infinite slab. Modeled temperature across the contact is, however, not compatible with the SCI values, which indicate a narrow thermally affected zone. We argue that heat was absorbed by mineral dehydration, pore water volatilization, organic matter maturation and decarbonization reactions. [Copyright &y& Elsevier]

Abstract: The Chinese Altai orogen is a typical domain of the Central Asian Orogenic Belt and in previous syntheses was divided into six tectonic units (or terranes). In this paper, 41 new and 36 published whole-rock Nd and Sr isotopic data for granitoid intrusions and mafic dykes within units 2 and 3 of the central Altai and units 4 to 6 of the southern Altai were used for isotopic mapping. The age of the samples used is known from U–Pb zircon dating. ε Nd(t) values range from −4 to +2 in the central Altai units 2 and 3, and from +1.4 to +6 in the southern Altai units 4 and 5 and up to +8.4 in unit 6. Correspondingly, Nd model ages (T DM) decrease from 1.6–1.1 Ga in the central Altai to 1.0–0.5 Ga in the southern Altai, except for a continental fragment in unit 4. I Sr is elevated in the central units, decreasing to <0.705 in the southern units 5 and 6. These results demonstrate that the central Altai contains widespread old crustal components in its basement, whereas the southern Altai comprises a higher proportion of juvenile crust. When integrated with published U–Pb zircon ages, our data show that new crust grew southwards in the Chinese Altai. In addition, our data suggest that units 2 and 3 are a single tectonic unit, whereas the southern Altai (unit 4), which previously was regarded as a single young accretionary terrane, should be subdivided into two units: a younger arc and an older continental fragment. The granitoids in the Chinese Altai can be divided into syn- and post-/non-orogenic groups. The syn-orogenic granitoids have ca. 50–80% mantle-derived component, whereas the post/non-orogenic granitoids contain ca. 70–90%. The generation of syn- and post-/non-orogenic granitoids represents horizontal (lateral) and vertical crustal growth, which contributed ca. 18–28% and 7–8% respectively to the area of the Chinese Altai. This work shows that isotopic mapping is an effective approach to elucidate terrane composition in the Central Asian Orogenic Belt and thereby better constrain Palaeozoic crustal growth. [Copyright &y& Elsevier]

Abstract: Submarine tholeiitic and calc-alkaline basalt and andesite with subordinate dacite, metamorphosed to greenschist facies, are exposed at Wadi Kareim and Wadi El Dabbah, Central Eastern Desert of Egypt. These metavolcanic sequences are representative of early stages of Neoproterozoic crust formation in the northern part of the Arabian-Nubian Shield. The magmas were primitive to moderately fractionated, as indicated by average Mg# (54), Cr (191ppm) and Ni (69ppm). Their REE patterns are slightly fractionated [(La/Yb)N =1.3–3.9], and multi-element diagrams show Ba, Sr, K and Rb enrichments and Nb and Ta depletions, typical of modern convergent margin igneous rocks. Tectonic discrimination diagrams also show affinities with modern convergent margin magmas, suggesting that Wadi El Dabbah lavas formed in a volcanic arc that experienced cryptic crustal contamination, whereas Wadi Kareim lavas formed in a back-arc basin. Positive initial ɛNd (+5.1 to +8.9) and Nd model ages (0.64–0.79Ga) indicate that the crust is juvenile and was extracted from a depleted mantle source. Wadis Kareim and El Dabbah metavolcanic rocks are dated at ∼750Ma (U–Pb SHRIMP) but contain abundant pre-Neoproterozoic zircons which might be derived from underlying sediments or perhaps older continental crust. These results indicate that a major ∼750Ma crust-forming event involving ophiolite generation and eruption of juvenile melts was important for the formation of the crust. [Copyright &y& Elsevier]

Abstract: The bulk of sedimentary provenance studies use similarities in the detrital zircon age patterns or “barcodes” in sedimentary systems and potential source regions to make interpretations about palaeogeographic settings. While this “age-only” approach is generally considered to be effective, it is limiting because the timing of zircon growth events may not be unique to specific terrains and important rock forming events may be associated with little zircon growth. To a significant extent these limitations can be overcome by employing additional isotopic data sets such as Sm–Nd and Lu–Hf that provide information on the crustal evolution of the source region, and allow comparisons with sedimentary packages. As an example, the age spectra of detrital zircons in Palaeoproterozoic metasedimentary rocks in the eastern Gawler Craton in southern Australia are virtually identical to the dominant zircon growth timelines in adjacent older domains of the Gawler Craton, presenting a prima facie case that it was the source region. However, whole rock Nd isotopic data indicate that the pre-existing proto Gawler Craton was isotopically too crustally evolved (ε Nd (1850 Ma ) −10) to have supplied the bulk of the sediment to the relatively more juvenile metasedimentary units (ε Nd (1850 Ma) −1 to −5). In addition, zircon Hf isotopic compositions from ca 2000 Ma detrital zircons in the metasedimentary rocks (ε Hf (2000 Ma) +2 to +5) are significantly more juvenile than 2000 Ma rocks in the adjacent Gawler Craton (ε Hf (2000 Ma) −2 to −5). The combination of bulk rock Nd and Hf zircon data suggest that the Gawler Craton is not a viable source region for the metasedimentary packages, despite the striking similarity between detrital zircon ages and zircon crystallisation events within the craton. [Copyright &y& Elsevier]

Abstract: The Ikkattoq gneisses of the Archaean gneiss complex in the Nuuk region, southern West Greenland, are the orthogneiss component within the amphibolite facies Tre Brødre terrane. They have mostly granodioritic compositions, with a small amount of quartz diorite. Sm–Nd isotopic data for a quartz diorite and five granodiorite Ikkattoq gneiss samples from within 5 km of the Ikkattoq (fjord) type locality yielded a regression with a slope equivalent to 2005±52 Ma (MSWD=0.72). Regardless of the low MSWD, this cannot be the true age of the Ikkattoq gneisses, because all Ikkattoq gneisses yield U–Pb zircon dates of c. 2825 Ma and they are cut by the undeformed 2560 Ma Qôrqut granite complex. This anomalously young regression ‘age’ resulted instead from mixing of different Nd components, indicating that the Ikkattoq gneisses are derived from mixed source materials. Taking the true age of the Ikkattoq gneisses as 2825 Ma from U–Pb zircon dating, the range of initial ε Nd in the Ikkattoq gneisses is −7.1 to −1.8. The negative initial ε Nd values mean that older, light rare earth enriched, sialic crust contributed to the igneous precursors of the Ikkattoq gneisses. This Nd evidence for contribution of older sialic crust is supported by positive ε Sr values for the Ikkattoq gneisses. With ε Nd values as low as −7.1 this older crustal component has to be Eoarchaean. The presence of scarce quartz diorites (low SiO2, high MgO) suggests that ultramafic rocks (upper mantle?), metasomatised by the passage of fluids or silicic melts, were another contributing source. The Ikkattoq gneisses are proposed as a complex suite incorporating material derived from melting of much older sialic crust and probably upper mantle. The intercalation of tectonostratigraphic terranes during collisional orogeny at c. 2720 Ma destroyed the architecture of this 2825 Ma magmatic system, and the Ikkattoq gneisses now form a slice tectonically isolated from their source region. In terms of trace element parameters, the Ikkattoq gneisses resemble Phanerozoic volcanic arc granites. Thus an Andean-style arc setting for the generation of the Ikkattoq gneiss precursors is possible. Other Archaean TTG suites of the Nuuk region are generally thought to represent predominantly juvenile additions to the crust. In the broadest sense they do, because isotopic work over the past 30 years has demonstrated that they do not represent wholesale recycling of considerably older crust. However in detail, within these broadly juvenile suites, a contribution from older crust can be detected. Thus, c. 3000 Ma type-Nûk gneisses from around Nuuk town show a spread in ε Nd values down to −1.7. In this case, the likely older crustal component was 3230 Ma quartz diorite that occurs as enclaves in the c. 3000 Ma suite. Thus to a lesser or greater degree, some Meso- to Neoarchaean TTG suites in the Nuuk region display the same internal complexities and evidence for mixed sources as modern arc suites developed near the margins of older crust. [Copyright &y& Elsevier]

Abstract: The Trans-Amazonian cycle was an important rock-forming event in South America, generating voluminous juvenile and reworked fractions of continental crust. The Bacajá domain, in the southern sector of the Maroni-Itacaiúnas Province in the Amazonian craton, is an example of the Trans-Amazonian terranes adjacent to the Archean Carajás block. Zircon Pb-evaporation and whole-rock Sm–Nd analyses were carried out on representative samples of six lithological units, and allowed the proposal of a comprehensive tectonic-magmatic evolutionary sequence for the central and eastern parts of this domain, from the Neoarchean to the Rhyacian. Gneisses with ages of ca. 2.67 and 2.44Ga are the oldest rocks recorded in the region, and probably represent remnants of island and continental arcs. The Três Palmeiras succession, emplaced between 2.36 and 2.34Ga, hosts gold deposits and represents the first record of Siderian supracrustal rocks in the Amazonian craton. It was probably part of an island arc/ocean floor accreted to a craton margin. Rhyacian granitogenesis lasted for ca. 140My (2.22–2.08Ga), marking different stages of the Trans-Amazonian cycle. The first stage is represented by continental arc granitoids formed by melting of Archean crust at 2.22–2.18Ga. The second is characterized by the production of juvenile material between 2.16 and 2.13Ga. The third and final stage at ca. 2.08Ga is represented by a large volume of granitoids originated from either juvenile material or reworked crust during compressive stresses. Nd isotopes reveal that juvenile rocks dominated in the northern part of the domain, whereas those formed from reworked crust predominate in the south. The present-day configuration of the Bacajá domain results from collision against the Archean Carajás block at the end of the Trans-Amazonian cycle. [Copyright &y& Elsevier]