Origin and Source Evolution of the Leucite Hills Lamproites: Evidence from Sr-Nd-Pb-O Isotopic Compositions

Whole-rock major and trace element and O, Sr, Nd and Pb isotopicdata are reported for 3·0–0·89 Ma lamproitesfrom the Leucite Hills, Wyoming, USA. The two main groups oflamproites, madupitic lamproites and phlogopite lamproites,are geochemically distinct and cannot be related to one anotherby either fractional crystallization or crustal contamination.It seems likely that the geochemical differences between thesetwo rock types are related to variations in source mineralogyand depth of partial melting. The high Mg-number and large ionlithophile element abundances and negative Nd values of thelamproites indicate a mantle source that has experienced stagesof both depletion and enrichment. The negative Nb, Ta and Tianomalies in mantle-normalized trace element diagrams and lowtime-integrated U/Pb, Rb/Sr and Sm/Nd ratios of both lamproitegroups and other Cenozoic igneous rocks from the Wyoming ArcheanProvince indicate an ancient metasomatic enrichment (>1·0Ga) of the mantle source associated with the subduction of carbonate-bearingsediments. Other chemical characteristics of the Leucite Hillslamproites, especially their high K₂O and volatile contents,are attributed to more recent metasomatism (<100 Ma) involvinginflux from upwelling mantle during back-arc extension or plumeactivity. KEY WORDS: isotopes; lamproites; metasomatism; Leucite Hills; Wyoming     

The Central Scandinavian Dolerite Group—Protracted hotspot activity or back-arc magmatism?: Constraints from U–Pb baddeleyite geochronology and Hf isotopic data

The Central Scandinavian Dolerite Group (CSDG) occurs in five separate complexes in central Sweden and SW Finland. U–Pb baddeleyite ages of dolerite dikes and sills fall into three age intervals: 1264–1271 (the Dalarna complex), 1256–1259 (the Västerbotten-Ulvö-Satakunta complexes) and 1247 Ma (the Jämtland complex). Timing and spatial distribution of CSDG are unlike expressions of the voluminous and short-lived magmatism which characterises plume-associated large igneous provinces (LIPs). Protracted mafic magmatism in association with mantle plume tail (hotspot) activity beneath the Fennoscandian lithosphere or discrete events of extension behind an active margin (subduction) are considered more plausible tectonic settings. Both settings are consistent with timing, relative magma volumes between complexes and vertical ascent of individual magma pulses through the crust, as inferred from seismic sections [Korja, A., Heikkinen, P., Aaro, S., 2001. Crustal structure of the northern Baltic Sea palaeorift. Teconophysics 331, 341–358]. In the hotspot model, the lack of a linear track of intrusions can be explained by an almost stationary position of Fennoscandia relative to the hotspot, in agreement with palaeomagnetic data [Elming, S.-Å., Mattsson, H., 2001. Post Jotnian basic intrusion in the Fennoscandian Shield, and the break up of Baltica from Laurentia: a palaeomagnetic and AMS study. Precambrian Res. 108, 215–236]). Together with geological evidence, dolerite sill complexes and dike swarms in Labrador (Canada), S Greenland and central Scandinavia in the range 1234–1284 Ma are best explained by long-lived subduction along a continuous Laurentia-Baltica margin preceding Rodinia formation. There is no support for the hypothesis that CSDG was fed by magma derived from a distal mantle plume located between Baltica and Greenland and, hence, for rifting between the cratons at 1.26 Ga.The epsilon-Hf in various members of the CSDG varies between 4.7 and 10.3, which are overall higher than both older and younger Mesoproterozoic mafic intrusions in central Fennoscandia. Magma generated from a hotspot mantle source that was mixed to highly variable degrees with an enriched subcontinental lithospheric mantle could account for the wide range in Hf isotope composition. In the course of Hf isotope development work during this project we have analysed four fragments of the Geostandard 91500 reference zircon and after evaluating the existing ICPMS and TIMS data we calculate a mean ¹⁷⁶Hf/¹⁷⁷Hf value of 0.282303 ± 0.000003 (2σ).     

Recharge areas and geochemical evolution of groundwater in an alluvial aquifer system in the Sultanate of Oman

A regional hydrogeochemical model was developed to evaluate the geochemical evolution of different groundwaters in an alluvial aquifer system in the Interior of Oman. In combination with environmental isotopes the model is able to extract qualitative and quantitative information about recharge, groundwater flow paths and hydraulic connections between different aquifers. The main source of water to the alluvial aquifer along the flow paths of Wadi Abyadh and Wadi Muaydin in the piedmont is groundwater from the high-altitude areas of the Jabal Akhdar and local infiltration along the wadi channels. In contrast, the piedmont alluvial aquifer along Wadi Halfayn is primarily replenished by lateral recharge from the ophiolite foothills to the east besides smaller contributions from the Jabal Akhdar and local infiltration. Further down gradient in the Southern Alluvial Plain aquifer a significant source of recharge is direct infiltration of rain and surface runoff, originating from a moisture source that approaches Oman from the south. The model shows that the main geochemical evolution of the alluvial groundwaters occurs along the flow path from the piedmont to the Southern Alluvial Plain, where dedolomitization is responsible for the observed changes in the chemical and carbon isotope composition in these waters.

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Resumen Un modelo hidrogeoquímico regional fue desarrollado, para evaluar la evolución geoquímica de diferentes aguas subterráneas en un sistema acuífero aluvial ubicado en el interior de Omán. El modelo, en combinación con isótopos ambientales, es capaz de obtener información cualitativa y cuantitativa sobre recarga, direcciones de flujo de agua subterránea y sobre las conexiones hidráulicas entre diferentes acuíferos. La fuente principal de agua para el acuífero aluvial en el piedemonte son las aguas subterráneas, a lo largo de las direcciones de flujo del Wadi Abyadh y del Wadi Muaydin, desde las áreas de gran altura de Jabal Akhdar y como infiltración local a lo largo de los canales del wadi. En contraste, el acuífero aluvial del piedemonte a lo largo del Wadi Halfayn, esta alimentado principalmente por recarga lateral desde las partes bajas de las montañas de ofiolitas hacia el Este, además por pequeñas contribuciones desde el Jabal Akhdar e infiltración local. Siguiendo gradiente abajo en el acuífero de la Llanura Aluvial del Sur, allí tanto la infiltración directa de lluvia, como la escorrentía superficial constituyen fuentes significativas de recarga, las cuales se originan en una fuente de humedad que llega a Omán proveniente del Sur. El modelo muestra, que la evolución geoquímica principal de las aguas subterráneas en el aluvión, sucede a lo largo de la dirección de flujo, que va desde el piedemonte hacia la Llanura Aluvial Sur, en donde el proceso de des-dolomitización es el responsable de los cambios observados en la composición química y también del contenido de isótopos de carbono en estas aguas.

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Résumé On a développé un modèle régional hydrogéochimique pour évaluer lévolution géochimique des différentes eaux souterraines dans le système aquifère de lintérieur dOman. Avec la contribution des isotopes denvironnement, le modèle est capable de fournir des informations qualitatives et quantitatives sur la recharge, les directions découlement et sur les connections hydrauliques entre les différents aquifères du système. La source principale pour laquifère situé au long de la direction découlement des oueds de Abdyah et de Muaydin dans le piémont est leau souterraine provenant des zones de haute altitude de Jabal Akhbar ainsi que linfiltration locale au long des canaux de loued. Par contre, à part dune petite contribution provenant des infiltrations locaux et de Jabal Akhbar, l aquifère alluvial de Wadi Halfayan est en principal alimenté par la recharge latérale provenant des ophiolites situées dans la partie est. Les plus importantes sources de recharge dans laval de laquifère de la Plaine Alluviale sont linfiltration directe et le ruissellement provenant dune source dhumidité qui sapproche dOman par le sud. Le modèle montre que la principale évolution géochimique se produise au long de la direction découlement qui part de piémont de la Plaine Alluviale du Sud, où le procès de dédolomisation est responsable pour les changements observés dans la composition géochimique et en carbone des eaux.

     

South Equatorial Current (SEC) driven changes at DSDP Site 237, Central Indian Ocean, during the Plio-Pleistocene: Evidence from Benthic Foraminifera and Stable Isotopes

This study attempts to analyse paleoceanographic changes in the Central Indian Ocean (Deep Sea Drilling Project Site 237), linked to monsoon variability as well as deep-sea circulation during the Plio-Pleistocene. We used factor and cluster analyses of census data of the 34 most dominant species of benthic foraminifera that enabled us to identify five biofacies: Astrononion umbilicatulum–Uvigerina proboscidea (Au–Up), Pullenia bulloides–Bulimina striata (Pb–Bs), Globocassidulina tumida–Nuttallides umbonifera (Gt–Nu), Gyroidinoides nitidula–Cibicides wuellerstorfi (Gn–Cw) and Cassidulina carinata–Cassidulina laevigata (Cc–Cl) biofacies. Knowledge of the environmental preferences of modern deep-sea benthic foraminifera helped to interpret the results of factor and cluster analyses in combination with oxygen and carbon isotope values. The biofacies indicative of high surface productivity, resulting from a stronger South Equatorial Current (Au–Up and Pb–Bs biofacies), dominate the early Pliocene interval (5.6–4.5 Ma) of global warmth. An intense Indo-Pacific ‘biogenic bloom’ and strong Oxygen Minimum Zone extended to intermediate depths (1000–2000 m) over large parts of the Indian Ocean in the early Pliocene. Since 4.5 Ma, the food supply in the Central Indian Ocean dropped and fluctuated while deep waters were corrosive (biofacies Gt–Nu, Gn–Cw). The Pleistocene interval is characterized by an intermediate flux of organic matter (Cc–Cl biofacies).     

Rhinocerotid tooth enamel 18O/16O variability between 23 and 12 Ma in southwestern France

The relationship between the oxygen isotope ratio of mammal tooth enamel and that of drinking water was used to reconstruct changes in the Miocene oxygen isotope ratio of rainfall (meteoric water ${\delta }^{18}$O_MW). These, in turn, are related to climatic parameters (temperature, precipitation and evaporation rate). ${\delta }^{18}$O values of rhinocerotid teeth from the Aquitaine Basin (southwestern France) suggest a significant climatic change between 17 and 12 Ma, characterized by cooling together with precipitation increase, in agreement with other terrestrial and oceanic records. To cite this article: I. Bentaleb et al., C. R. Geoscience 338 (2006).     

云南降水中稳定同位素变化的模拟和比较

利用MUGCM的模拟,云南地区日、月、年时间尺度下降水中稳定同位素的变化、降水量效应以及δD/δ18O之间的关系被分析。无论是在日时间尺度下还是在月、年时间尺度下,降水同位素均存在显著的降水量效应。与实测结果相比,模拟的降水中δ18O与降水量之间具有更强的相关性。对于单站而言,蒙自站和腾冲站的大气水线被较好地模拟。但在思茅站和昆明站,模拟结果未能准确再现实际降水中δD与δ18O的关系,模拟的大气水线斜率比实测结果偏高。这意味着,在云南这个特殊的地区,模式可能高估了HDO的贫化。     

Shoshonite and sub-alkaline magmas from an ultrapotassic volcano: Sr–Nd–Pb isotope data on the Roccamonfina volcanic rocks,Roman Magmatic Province,Southern Italy

The Roccamonfina volcano is characterised by two stages of volcanic activity that are separated by volcano-tectonic caldera collapses. Ultrapotassic leucite-bearing rocks are confined to the pre-caldera stage and display geochemical characteristics similar to those of other volcanoes in the Roman Province. After the major sector collapse of the volcano, occurred at ca. 400 ka, shoshonitic rocks erupted from cinder cones and domes both within the caldera and on the external flanks of the pre-caldera Roccamonfina volcano. On the basis of new trace element and Sr–Nd–Pb isotope data, we show that the Roccamonfina shoshonitic rocks are distinct from shoshonites of the Northern Roman Province, but are very similar to those of the Neapolitan volcanoes. The last phases of volcanic activity erupted sub-alkaline magmas as enclaves in trachytic domes, and as lavas within the Monte Santa Croce dome. Ultrapotassic rocks of the pre-caldera composite volcano are plagioclase-bearing leucitites characterised by high levels of incompatible trace elements with an orogenic signature having troughs at Ba, Ta, Nb, and Ti, and peaks at Cs, K, Th, U, and Pb. Initial values of ⁸⁷Sr/⁸⁶Sr range from 0.70926 to 0.70999, ¹⁴³Nd/¹⁴⁴Nd ranges from 0.51213 to 0.51217, while the lead isotope rations vary between 18.788–18.851 for ²⁰⁶Pb/²⁰⁴Pb, 15.685–15.701 for ²⁰⁷Pb/²⁰⁴Pb, and 39.048–39.076 for ²⁰⁸Pb/²⁰⁴Pb. Shoshonites show a similar pattern of trace element depletions and enrichments to the earlier ultrapotassic leucite-bearing rocks but have a larger degree of differentiation and lower concentrations of incompatible trace elements. On the other hand, shoshonitic rocks have Sr, Nd, and Pb isotopes consistently different than pre-caldera ultrapotassic leucite-bearing rocks. ⁸⁷Sr/⁸⁶Sr ranges from 0.70665 to 0.70745, ¹⁴³Nd/¹⁴⁴Nd ranges from 0.51234 to 0.51238, ²⁰⁶Pb/²⁰⁴Pb ranges from 18.924 to 19.153, ²⁰⁷Pb/²⁰⁴Pb ranges from 15.661 to 15.694, and ²⁰⁸Pb/²⁰⁴Pb ranges from 39.084 to 39.212. High-K calc-alkaline samples have intermediate isotopic values between ultrapotassic plagioclase leucitites and shoshonites, but the lowest levels of incompatible trace element contents. It is argued that ultrapotassic magmas were generated in a modified lithospheric mantle after crustal-derived metasomatism. Interaction between the metasomatic agent and lithospheric upper mantle produced a low-melting point metasomatised veined network. The partial melting of the veins alone produced pre-caldera leucite-bearing ultrapotassic magmas. It was possibly triggered by either post-collisional isotherms relaxation or increasing T°C due increasing heat flow through slab tears. Shoshonitic magmas were generated by further melting, at higher temperature, of the same metasomatic assemblage with addition 10–20% of OIB-like astenospheric mantle material. We suggest that addition of astenospheric upper mantle material from foreland mantle, flowing through slab tearing after collision was achieved. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

河南桐柏围山城地区主要侵入岩年代学与地球化学特征

河南省桐柏围山城地区位于华北板块与扬子板块之间的秦岭造山带与大别造山带的衔接部位,本文对产于该区的主要侵入岩体开展了锆石SHRIMP测年和岩石地球化学特征研究。总共对8件样品开展了SHRIMP测年,其中产于破山银矿区的云斜煌岩获得的锆石SHRIMP测年结果为461.8 ± 9.7 Ma,MSWD＝1.10,表明其形成于加里东中期。在桃园岩体及其南部混染带内产出的变辉长岩、石英闪长岩、花岗闪长岩和黑云母花岗岩共获得了6组年龄数据,分别是431.7 ± 8.3 Ma （MSWD＝0.76）、432.3 ± 7.5Ma （MSWD＝0.45）、470.3 ± 8.3Ma（MSWD＝1.3）、431.5 ± 8.2 Ma（MSWD＝1.6）、433 ± 11 Ma（MSWD＝1.8）和426.7 ± 9.1 Ma（MSWD＝1.6）,说明桃园岩体是加里东晚期岩浆侵位产物。梁湾岩体花岗闪长岩和二长花岗岩的锆石SHRIMP测年结果分别为132.5 ± 2.3Ma （MSWD＝1.5）和137.0 ± 3.4Ma （MSWD＝1.9）,为燕山中期。锆石SHRIMP测年结果表明,围山城地区主要侵入岩的岩浆活动集中在三个时期,分别是460-470Ma,430Ma和135Ma。侵入岩体的岩石地球化学研究结果表明,与加里东晚期花岗岩相比,燕山中期花岗岩明显富K、Nb和Rb贫Na和Y。Pb-Sr-Nd同位素示踪研究表明,加里东晚期花岗岩明显富含放射性成因Pb,其206Pb/204Pb值为19.342～19.712,平均值19.512,（87Sr/86Sr）i值较低,变化于0.70211~0.704899,εNd (t)值均为正值,变化范围为+3.55～+7.53,显示成岩物质来源较深,可能来自亏损地幔;而燕山中期花岗岩的放射性成因Pb含量低,206Pb/204Pb值为17.278～17.492,平均值17.370, （87Sr/86Sr）i值相对较高,为0.707004~0.716968,εNd (t)值均为较大负值,变化范围为-16.95～-9.76,显示成岩物质来源较浅,可能来自下地壳。围山城地区加里东晚期花岗岩与燕山中期花岗岩岩石地球化学特征上的差别与它们形成时所处的大地构造环境有关,加里东晚期花岗岩形成于弧后环境,而燕山中期花岗岩则形成于华北与扬子两大板块碰撞后的岩石圈减薄与伸展构造环境。     

Geochemical, Sr–Nd and zircon U–Pb–Hf isotopic studies of Late Carboniferous magmatism in the West Junggar, Xinjiang: Implications for ridge subduction?

Voluminous granitic intrusions are distributed in the West Junggar, NW China, and they can be classified as the dioritic rocks, charnockite and alkali-feldspar granite groups. The dioritic rocks (SiO₂ = 50.4–63.8 wt.%) are calc-alkaline and Mg enriched (average MgO = 4.54 wt.%, Mg^# = 0.39–0.64), with high Sr/Y ratios (average = 21.2), weak negative Eu (average Eu/Eu = 0.80) and pronounced negative Nb–Ta anomalies. Their Sr–Nd and zircon Hf isotopic compositions ((⁸⁷Sr/⁸⁶Sr)_i = 0.7035–0.7042, ε_Nd(t) = 4.5–7.9, ε_Hf(t) = 14.1–14.5) show a depleted mantle-like signature. These features are compatible with adakites derived from partial melting of subducted oceanic crust that interacted with mantle materials. The charnockites (SiO₂ = 60.0–65.3 wt.%) show transitional geochemical characteristics from calc-alkaline to alkaline, with weak negative Eu (average Eu/Eu = 0.75) but pronounced negative Nb–Ta anomalies. Sr–Nd and zircon Hf isotopic compositions ((⁸⁷Sr/⁸⁶Sr)_i = 0.7037–0.7039, ε_Nd(t) = 5.2–8.0, ε_Hf(t) = 13.9–14.7) also indicate a depleted source, suggesting melts from a hot, juvenile lower crust. Alkali-feldspar granites (SiO₂ = 70.0–78.4 wt.%) are alkali and Fe-enriched, and have distinct negative Eu and Nb–Ta anomalies (average Eu/Eu = 0.26), low Sr/Y ratios (average = 2.11), and depleted Sr–Nd and zircon Hf isotopic compositions ((⁸⁷Sr/⁸⁶Sr)_i = 0.7024–0.7045, ε_Nd(t) = 5.1–8.9, ε_Hf(t) = 13.7–14.2). These characteristics are also comparable with those of rocks derived from juvenile lower crust. Despite of the differences in petrology, geochemistry and possibly different origins, zircon ages indicate that these three groups of rocks were coevally emplaced at ~ 305 Ma.A ridge subduction model can account for the geochemical characteristics of these granitoids and coeval mafic rocks. As the “slab window” opened, upwelling asthenosphere provided enhanced heat flux and triggered voluminous magmatisms: partial melting of the subducting slab formed the dioritic rocks; partial melting of the hot juvenile lower crust produced charnockite and alkali-feldspar granite, and partial melting in the mantle wedge generated mafic rocks in the region. These results suggest that subduction was ongoing in the Late Carboniferous and, thus support that the accretion and collision in the Central Asian Orogenic Belt took place in North Xinjiang after 305 Ma, and possibly in the Permian.     

Nonsulfide zinc deposits in the Silesia–Cracow district,Southern Poland

The Silesia–Cracow district in Poland has been one of the world’s principal sources of zinc from nonsulfide zinc ore (Polish: galman). The still remaining nonsulfide ore resources can be estimated at 57 Mt at 5.6% Zn and 1.4% Pb. Nonsulfide mineralization is mainly hosted by Lower Muschelkalk (Triassic) limestone and is associated with different generations of the hydrothermal ore-bearing dolomite (OBD I, II, III). A fundamental ore control is believed to have been exerted by the basement faults, which were repeatedly reactivated during the Alpine tectonic cycle, leading to the formation of horst-and-graben structures: these dislocations may have caused short periods of emersion and the circulation of meteoric waters during the Cenozoic. Nonsulfide ores show a wide range of morphological characteristics and textures. They occur as earthy masses, crystalline aggregates, and concretions in cavities. Breccia and replacement textures are also very common. The most important mineral phases are: smithsonite, Fe–smithsonite, Zn–dolomite, goethite, and Fe–Mn(hydr)oxides. Minor hemimorphite and hydrozincite have also been detected. Two distinct nonsulfide ore types occur: the predominant red galman and the rare white galman. In the white galman, Fe–smithsonite and Zn–dolomite are particularly abundant. This ore type is commonly considered as a peripheral hydrothermal alteration product related to the same fluids that precipitated both the OBD II–III and the sulfides. In contrast, a supergene origin is commonly assumed for the red galman. Evidence of the petrographic and mineralogical difference between white and red galman is also found in stable isotope data. Smithsonite from red galman shows a limited range of δ ¹³C_VPDB values (−10.1 to −11.4‰), and δ ¹⁸O_VSMOW values (25.3‰ to 28.5‰, mean 26.8 ± 0.3‰). The uniform and low carbon isotope values of red galman smithsonite are unusual for supergene carbonate-hosted deposits and indicate the predominance of a single organic carbon source. Smithsonite from white galman has a more variable, slightly more positive carbon isotope (−2.9‰ to −7.4‰), but broadly similar oxygen isotope composition (26.8‰ to 28.9‰). The relationship of the white galman ore with the hydrothermal system responsible for OBD II and sulfide generation is still uncertain. The most important paleoweathering events took place in both Lower and Upper Silesia during Late Cretaceous up to Paleogene and early Neogene time. During this period, several short-lasting emersions and intense weathering episodes facilitated the formation of sinkholes in the Triassic carbonate rocks and the oxidation of sulfide orebodies through percolating meteoric waters. These phenomena may have lasted until the Middle Miocene.