Enrichment In Incompatible Elements Research Articles

Geochemical constraints on the petrogenesis of Triassic alkaline basalts of Sierra de Valle Fértil, Western Sierras Pampeanas, Argentina: implications for their origin, evolution and tectonic setting

The first geochemical and Sr-Nd isotope data of a series of Triassic basaltic rocks from Sierra de Valle Fértil, Western Sierras Pampeanas, Argentina, are herein presented. The mafic rocks are the result of continental intraplate magmatism related to an extensional tectonic setting after the climax of the Gondwanide orogeny (Upper Carboniferous-Permian). Major minerals phases of these rocks are olivine, clinopyroxene, plagioclase and Fe-Ti oxides. Olivine and clinopyroxene occur both as phenocrysts and as groundmass, while feldspar is mostly restricted to the groundmass. The rocks belong to a typical alkaline series displaying SiO2 contents of 43.9–51.6 wt%, Na2O between 3.1 and 5.4 wt%, K2O between 0.9 and 2.9 wt% and high TiO2 2.2–3.6 wt% and P2O5 0.5–1.2 wt% contents. Nepheline on its CIPW normative composition indicates that it crystallized from silica-undersaturated magmas. Samples have moderate to low Mg# (58.1–15.9) values and Cr and Co contents that indicate they are not primary magmas but have undergone fractional crystallization with earlier fractionation of olivine and clinopyroxene. Geochemical patterns are similar to those found in the ocean island basalts (OIBs), with enrichment of alkalis and incompatible elements: LILE, HFSE and LREE, strong fractionation of HREE relative to LREE [(La/Yb)N = 9.6–22.4], no depletion of Nb and Ta and absence of negative Eu anomaly. Observed LILE-enrichment is indicated by the alkaline nature of the basaltic rocks and suggests melting of an enriched mantle. The Eu and Sr positive anomalies, not related to plagioclase accumulation, argue against significant plagioclase fractionation and are original geochemical characteristics of the primary magma. The chemical features, together with high incompatible-element ratios, are consistent with low degrees of partial melting of a dominantly garnet-pyroxenite mantle source. The trace-element patterns and isotopic data with 143Nd/144Nd(i) (0.51257–0.51267), positive εNd(i) and low 87Sr/86Sr(i) (0.70292–0.70339) values, suggest a mantle source influenced by an enriched-component. Active subduction-related mechanisms promoted the conditions for decompression melting in the asthenospheric mantle. Primary basaltic magma experienced subsequent compositional modifications by fractional crystallization in shallow-level low-pressure magma storage reservoirs. There is no evidence of crustal contamination during magma ascent.

Rifting of the northern margin of the Indian craton in the Early Cretaceous: Insight from the Aulis Trachyte of the Lesser Himalaya (Nepal)

Abstract To reconstruct the early tectonic history of the Himalayan orogen before final India-Asia collision, we carried out geochemical and geochronological studies on the Early Cretaceous Aulis Trachyte of the Lesser Himalaya. The trace-element geochemistry of the trachytic lava flows suggests formation in a rift setting, and zircon U-Pb ages indicate that volcanism occurred in Early Cretaceous time. The felsic volcanics show enrichment of more incompatible elements and rare earth elements, a pattern that is identical to the trachyte from the East African Rift (Kenya rift), with conspicuous negative anomalies of Nb, P, and Ti. Although much of the zircon age data are discordant, they strongly suggest an Early Cretaceous eruption age, which is in agreement with the fossil age of intravolcanic siltstones. The Aulis Trachyte provides the first corroboration of Cretaceous rifting in the Lesser Himalaya as suggested by paleomagnetic data associated with the concept that the northern margin of India separated as a microcontinent and drifted north in the Neo-Tethys before terminal collision of India with Asia.

Pleistocene basaltic volcanism in the Krông Nô area and vicinity, Dac Nong Province (Vietnam)

Basaltic and related- pyroclastic products aged 0.67, 0.46 and 0.2 Ma produced by eruption of Chư B'luk (Buon Choah), Phu Son (Pass 52, Quang Phu) volcanoes in the Krông Nô district, and Ea T’ling and Nam Dong volcanoes in the Cu Jut district covers an area of 120 km2 with a thickness of a few meters to about 80 m. The volcanoes are aligned along a sub-meridian oriented extensional fault, from Phu Son in the south via Chư B'luk, Ea T’ling in the middle to Nam Dong in the north. The basalts are divided into geochemically alkaline and tholeiitic groups. Ne-normative alkaline basalts include samples from the upper layer of Phu Son volcano, those in the Trinh Nu waterfall area (Ea T’ling volcano) and cored samples conducted near the Dray Sap waterfall. Q-normative tholeiitic basalts include all the samples from Chư B'luk and Nam Dong volcanoes, and massive basalts in the lower layer of Phu Son volcano. Geochemically the tholeiitic basalts are vastly different from the alkaline basalts in that they have higher SiO2 (50–55 wt% compared to <47 wt% in alkaline basalt), Al2O3 (>14–16 wt% compared to <14 wt%), lower K2O (<1.5 wt% compared to >1.5–2.5 wt%) and lower TiO2 (<1.8 wt% as compared to >2 wt% in alkaline basalt). Melting temperature and pressure calculations based on computing primitive melt compositions showed that the tholeiitic basalts may be produced in a narrow temperature range of 1365–1385 °C, corresponding to pressure of 1.2–1.6 GPa. The alkaline basalts may be produced in a temperature range between 1400 and 1440 °C and pressure varying from 3.1 to 3.35 GPa. Both the tholeiitic and alkaline basalts are free of crustal contamination. They show geochemically similar characteristics to those of continental intraplate basalts although the alkaline basalt show much more enrichment in incompatible elements. The 87Sr/86Sr and 143Nd/144Nd ratios of the two basaltic groups are quite similar, varying within narrow ranges of, respectively, 0.7038–0.7041, 0.5129–0.51286 but their 206Pb/204Pb isotopic ratios vary widely relative to 207Pb/204Pb and 208Pb/204Pb suggesting their interaction with different isotopic components and/or isotopic evolution history. Isotopic modeling relative to major mantle isotopic components suggests that the alkaline basalts may be generated by interaction between an N-MORB-like and an enriched mantle type 1 (EM1) at deep levels. The isotopic component is usually known as DUPAL-like regional mantle isotopic anomaly believed to be widespread in SE Asian mantle. The isotopic compositions of tholeiitic basalt may be generated by mixing between the DUPAL-like component with an enriched mantle type 2 (EM2)-like at higher levels in the lithospheric mantle.

Assessment of the ore-forming process of the Gejiu tin district (South China)

The Gejiu tin district of southern China is the largest tin–polymetallic ore district in the world. Even though the mining operation has lasted more than 2000 years in this area, the metallogenic mechanism of this gigantic tin mineralization is still controversial. Based on petrographic, geochemical, and zirconological information from this study, we present a particular model of the magma evolution and tin mineralization for the Gejiu tin district. The ore-forming processes involved both a batholithic biotite monzogranite (BMG) emplaced at ∼94 Ma and a satellitic muscovite alkali-feldspar granite (MAG) emplaced at ∼72 Ma. The BMG and the MAG were produced from a long-lived magma chamber by two stages of magmatic activity during intracontinental orogeny in the Youjiang block. The hiatus between the two magmatic stages indicates that the deep-seated magma chamber persisted during the dominant phase of the orogeny. Progressive fractional crystallization of the magma caused the extreme enrichment of incompatible elements, which include ore-forming components such as tin, aqueous fluids, and other volatiles, in the residual magma. When the regional tectonic environment evolved into the extensional regime at ∼72 Ma, the highly-fractionated residual magma ascended and once emplaced it formed the highly-evolved MAG with a mean zircon crystallization age of 71.8 ± 1.2 Ma. The MAG is temporally and spatially associated with cassiterite-bearing quartz vein (CQV) with a mean zircon crystallization age of 72.6 ± 1.6 Ma. This metallogenic model of the BMG–MAG–CQV association is generally applicable to other granite-related metallic deposits in South China, Southeast Asia and elsewhere (Australia, Bolivia, Brazil, USA, etc.). The recognition of this BMG–MAG–CQV association in regional geological survey may facilitate the future exploration for this type of deposit.

The closure time of the easternmost segment of the Solonker-Xar Moron-Changchun-Yanji Suture: determined by the Yangjin’gou Granite Porphyry in the Hunchun Region, northeast China

The timing of closure of the paleo-Asian Ocean in northeastern China remains contentious. To investigate this problem and better constrain the final closure time of the paleo-Asian Ocean, we report the geochemical characteristics, zircon U-Pb chronology, and Hf isotopes of the Yangjin’gou granite porphyry in the Hunchun region in the easternmost segment of the Solonker- Xar Moron-Changchun-Yanji Suture (SXCYS). The zircon U-Pb age of the Yangjin’gou granite porphyry is 247.1 ± 1.5 Ma (mean square weighted deviation = 0.7). Initial eHf(t) values are high and positive, ranging from +11.2 to +13.3 with an average of +12.7, and the two-stage Hf isotope model age TDM2 is 520–350 Ma. The geochemical characteristics of the granite porphyry indicate that the rocks belong to the calc-alkaline series and are rich in Si, alkali, and Al, and poor in Ca. These rocks also have a rare-earth element distribution pattern that is smooth and inclined down to the right in spider diagrams, weak negative to positive Eu anomalies (Eu/ Eu* = 0.83–1.01), moderate enrichment of large-ion lithophile elements (e.g., Cs, Ba, Rb, and K) and radioactive incompatible elements (e.g., Th and U), and depletion in high field strength elements (e.g., Ti, Nb, and Ta). The geochemical and isotope data of the rocks indicate that the Yangjin’gou granite porphyry is a high-Ba-Sr granite and that the primitive magma of the Yangjin’gou rocks originated from the partial melting of juvenile crust. Given the location of the Yangjin’gou granite prophyry in the Hunchun region in the easternmost segment of the SXCYS, these rocks are inferred to have formed in a tectonic setting that changed from continental collision to post-collision extension at ~247 Ma, clearly later than the middle Permian closure time of the paleo-Asian Ocean along the middle segment of the North China Craton. The age of ~247 Ma, or a little earlier, represents the closure time of the paleo-Asian Ocean in the easternmost segment of the SXCYS.

Chemical Disequilibria, Lithospheric Thickness, and the Source of Ocean Island Basalts

AbstractWe examine REE (Rare-Earth Element) and isotopic (Sr–Hf–Nd–Pb) signatures in OIB (Ocean Island Basalts) as a function of lithospheric thickness and show that the data can be divided into thin- (&amp;lt;12 Ma) and thick-plate (&amp;gt;12 Ma) sub-sets. Comparison to geophysically constrained thermal plate models indicates that the demarcation age (∼12 Ma) corresponds to a lithospheric thickness of about 50 km. Thick-plate OIB show incompatible element and isotopic enrichments, whereas thin-plate lavas show MORB-like or slightly enriched values. We argue that enriched signatures in thick-plate OIB originate from low-degree melting at depths below the dry solidus, while depleted signatures in MORB and thin-plate OIB are indicative of higher-degree melting. We tested quantitative explanations of REE systematics using melting models for homogeneous fertile peridotite. Using experimental partition coefficients for major upper mantle minerals, our equilibrium melting models are not able to explain the data. However, using a new grain-scale disequilibrium melting model for the same homogeneous lithology the data can be explained. Disequilibrium models are able to explain the data by reducing the amount of incompatible element partitioning into low degree melts. To explore new levels of detail in disequilibrium phenomena, we employ the Monte-Carlo Potts model to characterize the textural evolution of a microstructure undergoing coarsening and phase transformation processes simultaneous with the diffusive partitioning of trace elements among solid phases and melt in decompressing mantle. We further employ inverse methods to study the thermochemical properties required for models to explain the OIB data. Both data and theory show that OIB erupted on spreading ridges contain signatures close to MORB values, although E-MORB provides the best fit. This indicates that MORB and OIB are produced by compositionally indistinguishable sources, although the isotopic data indicate that the source is heterogeneous. Also, a posteriori distributions are found for the temperature of the thermomechanical lithosphere-asthenosphere boundary (TLAB), the temperature in the source of OIB (Tp, oib) and the extent of equilibrium during melting (i.e. grain size). TLAB has been constrained to 1200–1300°C and Tp, oib is constrained to be &amp;lt;1400°C. However, we consider the constraints on Tp, oib as a description of all OIB to be provisional, because it is a statistical inference from the global dataset. Exceptional islands or island groups may exist, such as the classical ‘hotspots’ (Hawaii, Reunion, etc) and these islands may originate from hot sources. On the other hand, by the same statistical arguments their origins may be anomalously hydrated or enriched instead. Mean grain size in the source of OIB is about 1–5 mm, although this is also provisional due to a strong dependence on knowledge of partition coefficients, ascent rate and the melting function. We also perform an inversion in which partition coefficients were allowed to vary from their experimental values. In these inversions TLAB and Tp, oib are unchanged, but realizations close to equilibrium can be found when partition coefficients differ substantially from their experimental values. We also investigated bulk compositions in the source of OIB constrained by our inverse models. Corrections for crystallization effects provided ambiguous confirmations of previously proposed mantle compositions, with depleted mantle providing the poorest fits. We did not include isotopes in our models, but we briefly evaluate the lithospheric thickness effect on isotopes. Although REE data do not require a lithologically heterogeneous source, isotopes indicate that a minor enriched component disproportionately contributes to thick-plate OIB, but is diluted by high-degree melting in the generation of thin-plate OIB and MORB.

Terrestrial-like zircon in a clast from an Apollo 14 breccia

A felsite clast in lunar breccia Apollo sample 14321, which has been interpreted as Imbrium ejecta, has petrographic and chemical features that are consistent with formation conditions commonly assigned to both lunar and terrestrial environments. A simple model of Imbrium impact ejecta presented here indicates a pre-impact depth of 30–70 km, i.e. near the base of the lunar crust. Results from Secondary Ion Mass Spectrometry trace element analyses indicate that zircon grains recovered from this clast have positive Ce/Ce⁎ anomalies corresponding to an oxygen fugacity +2 to +4 log units higher than that of the lunar mantle, with crystallization temperatures of 771±88 to 810 ± 37°C (2σ) that are unusually low for lunar magmas. Additionally, Ti-in-quartz and zircon calculations indicate a pressure of crystallization of 6.9±1.2 kbar, corresponding to a depth of crystallization of 167±27 km on the Moon, contradicting ejecta modelling results. Such low-T, high-fO2, and high-P have not been observed for any other lunar clasts, are not known to exist on the Moon, and are broadly similar to those found in terrestrial magmas.The terrestrial-like redox conditions inferred for the parental magma of these zircon grains and other accessory minerals in the felsite contrasts with the presence of Fe-metal, bulk clast geochemistry, and the Pb isotope composition of K-feldspar grains within the clast, all of which are consistent with a lunar origin. The dichotomy between redox conditions and the depth of origin inferred from the zircon compositions compared to the ejecta modelling necessitates a multi-stage petrogenesis. Two, currently unresolvable hypotheses for the origin and history of the clast are allowed by these data. The first postulates that the relatively oxidizing conditions were developed in a lunar magma, possibly by fractional crystallization and enrichment of incompatible elements in a fluid-rich, phosphate-saturated magma, at the base of the lunar crust to form the zircon grains and their host felsite. Subsequent excavation by the Imbrium impact introduced more typical lunar features to the clast but preserved primary chemical characteristics in zircon and some other accessory minerals. However, this hypothesis fails to explain the high P of crystallization. Alternatively, the felsite and its zircon crystallized on Earth at a modest depth of 19±3 km in the continental crust where oxidizing, low-T, fluid-rich conditions are common. Subsequently, the clast was ejected from the Earth during a large impact, entrained in the lunar regolith as a terrestrial meteorite with the evidence of reducing conditions introduced during its incorporation into the Imbrium ejecta and host breccia.

Second-stage Caribbean Large Igneous Province volcanism: The depleted Icing on the enriched Cake

A long-standing question has been whether the Lower Nicaraguan Rise in the western Caribbean Sea represents thinned continental crust or is part of the Caribbean Large Igneous Province (CLIP). During the R/V Meteor cruise M81/2, the northeastern part of the Lower Nicaraguan Rise including the Hess Escarpment was extensively sampled. Only volcanic and carbonate rocks were recovered and no metamorphic rocks or rocks with continental crustal affinities, indicating that at least the uppermost basement is volcanic and belongs to a volcanic province, probably the CLIP. We provide a comprehensive geochemical dataset, including major and trace elements and Sr-Nd-Hf-Pb double spike isotope ratios, for the volcanic rocks. The basalts are geochemically subdivided into two groups: 1) depleted and 2) enriched. The majority belong to the depleted group with 6–9 wt% MgO and highly depleted incompatible element (e.g. (La/Yb)N = 0.15–0.76, (La/Sm)N = 0.21–0.65) compositions and depleted radiogenic isotope (e.g. εNdi = 8.9–11.0, εHfi = 13.6–16.5, 206Pb/204Pbi = 18.28–18.98) ratios. The subordinate enriched group basalts have 5–8 wt% MgO and were solely recovered from the lower part of the central Hess Escarpment. These rocks possess enriched incompatible element ((La/Sm)N = 1.15–1.60, (La/Yb)N = 1.40–2.57) and isotopic (εNdi = 7.52–8.34, εHfi = 12.6–12.9, 206Pb/204Pbi = 18.87–19.07) ratios resembling E-MORB. The depleted group is distinct from Pacific and Atlantic MORB based on combined 206Pb/204Pb, 143Nd/144Nd and 176Hf/177Hf data. It is similar, but not identical to the highly depleted Gorgona komatiites, thought to be generated during the CLIP event, and depleted Galápagos hotspot track lavas accreted to the Pacific margin of Central America. Ages of ~81 Ma from rocks drilled on the Hess Escarpment postdate the main CLIP phase (95-83 Ma), and indicate that at least the northeastern part of the Nicaraguan Rise formed during a later volcanic event. Lithospheric thinning after the main CLIP phase is suggested to have triggered upwelling of still anomalously hot, partially-melted or unmelted CLIP mantle material to form the younger depleted group of lavas. Our investigations imply that second-stage melting to generate depleted compositions during formation of oceanic large igneous provinces, as has been recently proposed for the Manihiki Plateau, is a more common process than originally thought.

Influence of water on granite generation: Modeling and perspective

Water influences the physics and chemistry of magmatic differentiation because it reduces the melting point, decreases melt viscosity, modifies phase equilibria, and controls how latent heat is released. Here, we explore how compositional trends of fractionating magmas can be used to evaluate the amount of water in the crystallizing system. Of interest is the role of water in generating silicic magmas such as granites and rhyolites. Water expands the crystallization window over which silicic melts are stable, such that granites are limited to small residual melt fractions (F < 0.2) under dry conditions but stable over a wider range of residual melt fractions (F < 0.6) at high water contents. Variations of SiO2 versus F are relatively insensitive to oxygen fugacity and can be used as a hygrometer for granitoids, where F is estimated through relative enrichments in incompatible elements, such as Th and K. We also show that generation of granitoids in arc environments follow distinctly different Mg#-SiO2 trends compared with anorogenic or intra-plate granites. We confirm that arc magmas differentiate at higher water contents and possibly higher oxygen fugacities than anorogenic/intra-plate magmas. Finally, Archean TTGs (tonalite, trondhjemite, and granodiorite) show similar Mg#-SiO2 systematics as Phanerozoic arc-related granitoids, suggesting similar petrogenetic physical and chemical conditions.

Origin of Late Triassic mafic–ultramafic intrusions in the Hongqiling Ni–Cu sulfide deposit, Northeast China: evidence from trace element and Sr–Nd isotope geochemistry

There are many Late Triassic mafic–ultramafic intrusions in the Hongqiling magmatic Ni–Cu sulfide deposit, Northeast China. Research on magma evolution leading to formation of these mafic–ultramafic intrusions is of great significance for understanding the mantle beneath Northeast China and associated Ni–Cu mineralization. A trace element study of the No. 1, 3, and 7 intrusions in the Hongqiling deposit reveals that these mafic–ultramafic intrusions are characterized by enrichment of incompatible elements, which however cannot be interpreted by subduction modification. Furthermore, model of batch partial melting of depleted mantle accompanied by upper crustal contamination can simulate the trace element patterns of these mafic–ultramafic intrusions, but partial melting of depleted mantle accompanied by lower crustal contamination model cannot work. In addition, Sr–Nd isotopic compositions of the Hongqiling No. 1, 3, and 7 intrusions also indicate that crustal contamination could have occurred mainly during the magma ascent. Consequently, a possible scenario for the magma evolution is that the primary mafic–ultramafic magma was derived from batch partial melting of a depleted mantle, and then contaminated by Cambrian–Ordovician metamorphic rocks of the Hulan Group during ascent. We conclude that the mantle source contained no significant crustal component in the Late Triassic and was also independent of substantial contribution from subducted material, and therefore the Mesozoic large-scale lithospheric delamination beneath eastern China may happen after a period of time of the Late Triassic.

Multiple mantle metasomatism beneath the Leizhou Peninsula, South China: evidence from elemental and Sr-Nd-Pb-Hf isotope geochemistry of the late Cenozoic volcanic rocks

ABSTRACTWe analysed whole-rock major and trace elements and Sr-Nd-Pb-Hf isotopes of the late Cenozoic volcanic rocks in the Leizhou Peninsula, South China to investigate their mantle source characteristics. These volcanic rocks, collected from Jiujiang, Tianyang and Huoju areas of the Leizhou Peninsula, are characterized by incompatible element enrichment but variable isotopic depletion. The volcanic rocks from Jiujiang and Tianyang show prominent primitive-mantle-normalized positive Nb, Ta and Sr anomalies and depleted Sr-Nd-Pb-Hf isotope compositions, whereas those from Huoju show slight positive to negative Nb and Ta anomalies, a prominent positive Pb anomaly, and more enriched Sr-Nd-Pb-Hf isotope compositions. Two types of mantle metasomatism are required to explain the geochemical characteristics of these rocks. The Jiujiang and Tianyang samples were largely derived from a mantle source metasomatized recently by a low-F melt. Such low-F melt is generated within the asthenospheric mantle, which is enriched in volatiles and incompatible elements with positive Sr anomaly and depleted Sr-Nd-Pb-Hf isotope compositions. The Huoju samples were largely derived from a mantle source metasomatized by recycled upper continental crust material. These two types of mantle metasomatism beneath the Leizhou Peninsula are consistent with trace element characteristics of mantle mineralogy (e.g. clinopyroxene vs. amphibole), which reflects source evolution in space and time (e.g. tectonic setting change).

Mapping a hidden terrane boundary in the mantle lithosphere with lamprophyres

Lamprophyres represent hydrous alkaline mantle melts that are a unique source of information about the composition of continental lithosphere. Throughout southwest Britain, post-Variscan lamprophyres are (ultra)potassic with strong incompatible element enrichments. Here we show that they form two distinct groups in terms of their Sr and Nd isotopic compositions, occurring on either side of a postulated, hitherto unrecognized terrane boundary. Lamprophyres emplaced north of the boundary fall on the mantle array with εNd −1 to +1.6. Those south of the boundary are enriched in radiogenic Sr, have initial εNd values of −0.3 to −3.5, and are isotopically indistinguishable from similar-aged lamprophyres in Armorican massifs in Europe. We conclude that an Armorican terrane was juxtaposed against Avalonia well before the closure of the Variscan oceans and the formation of Pangea. The giant Cornubian Tin-Tungsten Ore Province and associated batholith can be accounted for by the fertility of Armorican lower crust and mantle lithosphere.

New geochemical constraints on the origins of MARID and PIC rocks: Implications for mantle metasomatism and mantle-derived potassic magmatism

MARID (Mica-Amphibole-Rutile-Ilmenite-Diopside) and PIC (Phlogopite-Ilmenite-Clinopyroxene) rocks are unusual mantle samples entrained by kimberlites and other alkaline volcanic rocks. The formation of MARID rocks remains hotly debated. Although the incompatible element (for example, large ion lithophile element) enrichment in these rocks suggests that they formed by mantle metasomatism, the layered textures of some MARID samples (and MARID veins in composite xenoliths) are more indicative of formation by magmatic processes. MARID lithologies have also been implicated as an important source component in the genesis of intraplate ultramafic potassic magmas (e.g., lamproites, orangeites, ultramafic lamprophyres), due to similarities in their geochemical and isotopic signatures. To determine the origins of MARID and PIC xenoliths and to understand how they relate to alkaline magmatism, this study presents new mineral major and trace element data and bulk-rock reconstructions for 26 MARID and PIC samples from the Kimberley-Barkly West area in South Africa. Similarities between compositions of PIC minerals and corresponding phases in metasomatised mantle peridotites are indicative of PIC formation by pervasive metasomatic alteration of peridotites. MARID genesis remains a complicated issue, with no definitive evidence precluding either the magmatic or metasomatic model. MARID minerals exhibit broad ranges in Mg# (e.g., clinopyroxene Mg# from 82 to 91), which may be indicative of fractionation processes occurring in the MARID-forming fluid/melt. Finally, two quantitative modelling approaches were used to determine the compositions of theoretical melts in equilibrium with MARID rocks. Both models indicate that MARID-derived melts have trace element patterns resembling mantle-derived potassic magma compositions (e.g., lamproites, orangeites, ultramafic lamprophyres), supporting inferences that these magmas may originate from MARID-rich mantle sources.

Variolites of the Paleoproterozoic Hekpoort Formation (Transvaal sub-basin, Kaapvaal craton): Multistage undercooling textures?

Globular structures in volcanic and hypabyssal rocks called varioles (or spherulites) have been described from a wide variety of places and rock types including komatiites, basalts, dacites, rhyolites as well as alkaline dykes, but the processes leading to their formation are still debated. Here, we present a field, petrological and geochemical study of variole-bearing lavas (variolites) in the Paleoproterozoic volcanic Hekpoort Formation in the southern part of the Transvaal sub-basin of South Africa. Variolites were only observed in a restricted area, corresponding to a ≥3.5 km long basin or depression. The variolites are locally up to 50 m thick and correspond to a succession of one-meter-thick lava flows. Most of the flows consist of two distinct layers, with an upper layer rich in spherical varioles (1–32 cm in diameter), overlying a layer generally devoid in varioles but often showing vertical jointing. Both layers show randomly oriented acicular textures of centimetre-sized skeletal clinopyroxene crystals (Al-rich augite) that precede the formation of varioles. The varioles themselves are mainly composed of radiating acicular augite (1 μm in diameter), the crystallization of which caused diffusion of incompatible elements, which became enriched in the groundmass around the varioles. The whole rock geochemistry of the Hekpoort variolites is indistinguishable from that of non-variolitic Hekpoort lavas with the same MgO content. This excludes liquid immiscibility as the origin of the varioles. The acicular texture (of Al-rich augite) in combination with the incompatible element enrichment around the varioles suggests that both skeletal and acicular augite are related to undercooling rather than devitrification. Sub-aqueous deposition is thought to be the cause of undercooling.

Elemental and Sr-Nd isotopic geochemistry of Mesoproterozoic sedimentary successions from NE Lesser Himalaya, northern India: Implications for Proterozoic climate and tectonics

The sedimentation history of northeastern Himalayan Proterozoic basins is poorly known. In the current study, we examine major, trace, rare earth elements (REEs) and Sr-Nd isotope geochemistry of pelites and quartzites from Mesoproterozoic metasedimenatary sequences of Bomdila and Rupa Groups, NE Himalaya in order to understand provenance, tectonic setting and paleoweathering conditions of deposition. The salient geochemical parameters, such as, high SiO2 and Al2O3 concentrations, relatively low MgO values, typical enrichment of immobile incompatible elements (Al2O3/TiO2 = 32.2, Th/Sc = 2.1), fractionated REE patterns (La/Ybn = 28) and a strong negative europium anomaly (Eu/Eu∗ = 0.3–0.7) indicate dominant felsic source for Bomdila sediments. Bomdila sediments show significantly radiogenic εNd0.95Ga values (−10.25 to −17.90, avg. −15.24) compared to Bundelkhand craton (avg. −26) and Singhbhum craton (avg. −30) and yield much younger TDM ages (2.3–2.7 Ga) compared to Archean Bundelkhand (3.2–3.5 Ga) and Singhbhum cratons (3.4–3.9 Ga). These results clearly rule out any significant detritus contribution from old cratonic sources. εNd and TDM data of Mesoprterozoic Bomdila sediments envelop the signature of Lower Proterozoic Inner Lesser Himalayan (ILH) terrane. Prominent U-Pb age peaks at 1702, 1651, ∼1400, 1248 and 1118 Ma in detrital zircons indicate major contribution from Late Paleoroterozoic to Late Mesoproterozic sources. However, initial Nd isotope ratios and TDM do not favour any juvenile inputs. We argue that ILH source terrain may have been extensively reworked during Late Paleoroterozoic to Late Mesoproterozic time and could have been part of a long lived orogenic belt.. Severe depletion of Ca, Na and Sr in Bomdila sediments indicate intense chemical weathering of the source region, and can be attributed to warm and humid climatic conditions, a conclusion which is consistent with the CO2-rich atmosphere during Mesoproterozoic period.

Assembly and breakup of the Nuna supercontinent: Geodynamic constraints from 1800 to 1600 Ma sedimentary basins and basaltic magmatism in northern Australia

Basin formation from 1800 to 1640 Ma in northern Australia took place in a backarc extensional upper plate setting accompanied by lithospheric thinning and voluminous but episodic basaltic magmatism. West-dipping subduction combined with oceanward retreat of an associated magmatic arc drove these processes and commenced after the separation of Laurentia, precluding any connection with eastern Australia from 1800 Ma until basin closure and re-accretion of the arc ca. 1650–1640 Ma. Early rift-related basaltic magmatism generated km-thick piles of interstratified 1785–1775 Ma shallow-water sediment and tholeiitic lava whose low- and high-Ti compositions bear some resemblance to basalts erupted during the earlier stages of Gondwana breakup. Later basalts evolved in a western Pacific-style marginal marine basin and exhibit compositions consistent with melting of a subcontinental mantle source that had either been metasomatically altered during flat-slab subduction or contaminated with crustal material during an earlier subduction-related event. From 1700 to 1670 Ma, the crust and underlying lithospheric mantle rapidly thinned, and basaltic magmatism increasingly occurred in a turbidite-dominated deep marine environment with concomitant changes towards more subdued subduction-related signatures in the east and less enrichment in incompatible elements. Coincidently, granites emplaced from 1780 to 1670 Ma at deeper crustal levels were extensionally unroofed along with their country rocks. Backarc extension concluded no later than 1655 Ma by which time MORB-like magmas were being sourced directly from the asthenosphere and northern Australia had transitioned from an active to passive continental margin lying along the inboard side of the marginal sea. With ongoing subduction and consumption of oceanic crust to the east, Laurentia began to encroach upon both the arc and Australian margin, resulting initially in collapse of the backarc basin and then continent–continent collision by ca. 1620 Ma. These events represent an important milestone in assembly of the Nuna supercontinent and mark the end of one complete Wilson cycle from the time that the two continents first separated until their re-amalgamation 150 Myr later. Timeframes for the opening and closure of ocean basins in southern Laurentia are similar, raising the possibility that the same convergent margin and backarc extensional processes may once have extended along strike into ancestral North America.

Incompatible elements in bottom sediments of the Itaipú Dam Reservoir by EDXRF

Sediments play an important role in the distribution of trace elements in the aquatic system and act as a sink for metals. Incompatible elements (IEs) such as rare earth elements as well as other refractory are considered useful tracers of geochemical processes. In this work using XRF analytical techniques, the refractory Ti,Y, Zr, Nb, Ba, La, Ce, and Nd as well as Fe and Mn contents in bottom sediments along the Paraguayan side of the reservoir were investigated and compared with those found in the river before impoundment. Spidergrams results show an enrichment of IE as well as a strong contribution of sedimentary material into bottom sediments.

Geochemical Constrains on MORB Composition and Magma Sources at East Pacific Rise Between 1°S and 2°S

The East Pacific Rise (EPR) is a typical fast spreading ridge. To gain a better understanding of the magmatism under ridges, Mid Ocean Ridge Basalts (MORBs) with remarkably heterogeneous compositions are obtained from (EPR) 1°–2°S and multielement geochemical and radioisotope analyses are conducted. Results show that these MORBs have wide variation ranges in trace element concentrations and isotopic ratios. Sample 07 has low concentrations of incompatible elements, and very low 87Sr/86Sr, and high 143Nd/144Nd from 0.70213 to 0.702289 and 0.513234 to 0.513289, respectively. However, other samples show enrichment in incompatible elements to varying degrees, and medium values of 87Sr/86Sr and 143Nd/144Nd from 0.702440 to 0.702680 and 0.513086 to 0.513200, respectively. This study proposes that one depleted source and two enriched sources contribute to the formation of MORBs from EPR 1°–2°S. Samples 02 and 10 are formed by mixing between one enriched source and one depleted source, while sample 07 is crystallized from the depleted source with no mixing process involved. However, the formation of samples 06 and 11 are different, and thus further research is required to determine genesis.

Pleistocene alkaline rocks of Martin Vaz volcano, South Atlantic: low-degree partial melts of a CO2-metasomatized mantle plume

ABSTRACTSeveral alkaline massifs on inland southeastern Brazil extend offshore, roughly parallel to ~20° S, through a seamount chain of the Vitoria-Trindade ridge. This paper presents the first extensive work on the Martin Vaz volcano through whole-rock and Sr and Nd isotopic composition of volcanic and subvolcanic lithotypes from the Martin Vaz Island, located at the easternmost of this volcanic chain. These alkaline rocks were generated during the Plio-Pleistocene (~0.47 My, 40Ar/39Ar dating in whole-rock) and represent the crystallization of sodic magmas of nephelinitic composition that evolved through fractional crystallization towards phonolites. Calculations from P–TLiquidus using PELE software show temperatures of 1045°C and 818°C, viscosity of 2.47 log Poise and 5.02 log Poise, and densities of 2.57 g/cm3 and 2.26 g/cm3 for nephelinite and phonolite, respectively. Like in Trindade Island, the nephelinitic volcanism in Martin Vaz may represent a Strombolian and/or Hawaii-type eruption due to low viscosity magma according to its physical properties whereas phonolitic intrusions present higher viscosity characteristics forming lava domes. The 87Sr/86Sr (~ 0.703800) and 143Nd/144Nd (~ 0.512750) ratios of lavas from the seamounts and Martin Vaz do not vary significantly, pointing to partial melting process from a homogeneous mantle source showing isotope signature close to HIMU. Beside the restrict variation on these isotopic ratios, a conspicuous enrichment in incompatible trace elements, mainly LREE, indicates that metasomatism is a recent process and not a long-term source characteristic. Non-modal partial melting models (fractional melting and batch melting) suggest that the source of the Martin Vaz magmatism is consistent with the garnet-lherzolite mantle stability field (>90 km depth; Tb/Yb >0.7), generated about 3.0 GPa by very small degree of partial melting of an enriched wet mantle source (F = 0.03–0.04) with 2.5 wt. % of CO2.

Late Cretaceous tectono–magmatic event in Songliao Basin, NE China: New insights from mafic dyke geochronology and geochemistry analysis

The ages and source of Late Cretaceous mafic dolerite dykes of the Songliao Basin were determined using geochronological, geochemical, zircon Hf, whole‐rock Sr–Nd isotopic, and zircon fission tract. The results show that (a) petrographic data reveal doleritic, poikiloblastic textures, and amygdale structures of the dolerite dykes. (b) LA‐MC‐ICP‐MS U–Pb analysis of zircons yielded an age of 70.0 ± 3.0 Ma of one of these mafic dykes in agreement with the zircon fission track age (69–68 Ma) of host rocks within uncertainty, placing them within the Late Cretaceous. (c) Geochemical researches indicate that the dykes are classified as the alkaline and tholeiitic series according to their major elements and characterized by high Na2O (2.79–4.89 wt.%), TiO2 (1.63–2.71 wt.%), and low K2O (0.46–2.45 wt.%). And they have enrichment of incompatible elements (e.g., K, Ti, Rb, Ba, Th, U, Nb, Ta, and REEs), high Rb/Sr (0.026–0.066), LaN/YbN (3.83–15.93), near‐chondritic HREEs ([Gd/Yb]N = 1.78–3.37), low Sm/Nd (0.202–0.282) ratios, and with negligible Eu anomalies (Eu/Eu* = 0.98–1.06). The dykes contain relatively low amounts of radiogenic Sr, as shown by (87Sr/86Sr)i values (0.7037–0.7045), and have positive values of εNd(t) (3.66–5.40) and εHf (t) (2.66–7.45), suggesting that the mafic dykes have been derived from partial melting of the depleted lithospheric mantle and contaminated by crust in the source region and formed in back‐arc rift settings caused by the subduction of the Pacific Oceanic Plate. All the above indicate large‐scale extensional event in Songliao Basin NE China at about 70 Ma.