Two stages of zircon and monazite growth in anatectic leucogneiss: SHRIMP constraints on the duration and intensity of Pan-African metamorphism in Prydz Bay, East Antarctica

SHRIMP U–Pb data from two samples of foliated anatectic leucogneiss bracket the high-grade metamorphic history of metasediments from southern Prydz Bay, East Antarctica. Magmatic zircons in these two samples have mean crystallization ages of 535 ± 13 and 536 ± 35 Myr, and provide a good estimate for the timing of peak metamorphism and partial melting. Low-Th monazite grains in the same samples have mean ages of 528 ± 4 and 527 ± 11 Myr, which are within error of the zircon ages but may reflect a lower blocking temperature. High-Th monazite rims with younger mean ages of 518 ± 3 and 512 ± 13 Myr occur around low-Th cores in both samples, and were precipitated from hydrous fluids released during the crystallization of residual melts. These magmatic fluids were channelled along the retrograde foliation present in both samples. All mineral assemblages and fabrics preserved in the metasediments reflect intense Pan-African metamorphism and deformation, associated with high degrees of partial melting and significant vertical displacements of the crust. Continental reconstructions assuming limited Pan-African tectonism in East Antarctica should be reassessed in the light of these new data.     

Analysis of noble gases in water using a quadrupole mass spectrometer in static mode

A quadrupole-based mass spectrometer used in static mode has been employed to determine noble gas concentrations of water samples by isotope dilution. Water samples are degassed and spiked with minor isotopes of each noble gas. After separation of the various gas components, the isotopic ratios are measured in the mass spectrometer. Because a spike is added to each sample, the recovery of each gas component does not have to be quantitative. Equilibration temperatures are calculated from the gas concentrations, based on noble gas solubility data. An accuracy of ±1°C, from the air equilibration temperature, is attainable for air-saturated water samples prepared under controlled conditions in the laboratory. The method has been applied to groundwaters from the London Basin Chalk aquifer. The noble-gas-derived temperatures are consistent with climatic conditions prevailing at the periods of recharge.     

The mafic-ultramafic complex near Finero (Ivrea-Verbano Zone), I. Chemistry of MORB-like magmas

Geochemical data are presented for the meta-igneous, mafic-ultramafic complex near Finero. This complex is in contact with a phlogopite-bearing mantle peridotite and is subdivided into the Internal Gabbro unit, the Amphibole Peridotite unit, and the External Gabbro unit. The Internal Gabbro and the Amphibole Peridotite units consist of coarse-grained, chemically heterogeneous cumulates, whereas the External Gabbro unit is generally massive, chemically more uniform and approximately representative of the residual melt with MgO contents between 6.6 and 9.1% and Mg numbers between 38 and 58. Both whole-rock and mineral contents of Ni and Cr are significantly higher (at similar Mg numbers) in the Amphibole Peridotite unit than in the Internal Gabbro unit. The most straightforward interpretation of this is that the Amphibole Peridotite unit accumulated after the influx of fresh mafic (or ultramafic) magma into the magma chamber. Major-element chemical trends are continuous from the Amphibole Peridotite unit to the External Gabbro unit and are consistent with closed-system fractionation with no further addition of magma or contamination by wall or roof rock assimilation. In the External Gabbro unit, total FeO and TiO₂ contents are strongly correlated with each other (and with P₂O₅ and Zr) and reach values as high as 19 and 4%, respectively, indicating an advanced degree of crystal fractionation along a tholeftic trend. The External Gabbro samples have generally smooth normalized trace element patterns, which are consistent with being representative of a liquid composition. The residual nature of the External Gabbro magma is also indicated by negative Eu and Sr anomalies, clear evidence for prior feldspar fractionation. REE patterns are otherwise indistinguishable from N-type MORB, but Th and U are significantly more depleted than in MORB. This Th and U depletion is similar to that found in olivine basalts and picrites on Iceland and Hawaii; its origin is not well understood. No evidence is seen for any assimilation of crystal material, in sharp contrast with the situation of the igneous complex in Val Sesia near Balmuccia, where the magma composition is dominated by assimilation of crust. We suggest that the heat provided by at most two injections of magma near Finero was insufficient to induce crystal anatexis, in contrast with the excess heat supplied by multiple magma injections at Balmuccia.     

Temporal and tectonic evolution of the granulite-eclogite association from the Bergen Arcs, western Norway

The U-Pb and Sm-Nd dating of deep crustal rocks from the Bergen Arcs system helps resolve enigmatic aspects of the tectonic evolution of the Caledonian Orogen in western Norway and yields insights into the arrested stages of eclogite development within the granulites of the area. The U-Pb dating of zircon from one of the eclogite facies shear zones yields an upper intercept age of 945 ± 5 Ma [all errors two standard deviations (2σ)], which is similar to other zircon ages from the granulite facies protolith. The age is interpreted to represent the time of late Proterozoic (Sveconorwegian) granulite metamorphism. The U-Pb ages of sphene and epidote show that the eclogites formed early in the evolution of the Caledonian Orogen (pre-Scandian phase) at about 460 Ma. An eclogite facies quartz vein yields a Sm-Nd whole rock-garnet isochron of 440 ± 12 Ma that may reflect the onset of cooling immediately after peak eclogite facies conditions, although the Sm-Nd systematics reveal some isotopic disequilibrium within the sample. In tandem with previous ⁴⁰Ar/³⁹Ar age determinations from, an adjacent eclogite of 450 Ma for hornblende and 430 Ma for muscovite, these data indicate that < 30 Ma elapsed between formation of the eclogites and the initial stages of cooling and exhumation to at least mid-crustal levels. This corresponds to minimum cooling rates of 14 °C/m.y. The timing relations suggest that the formation and exhumation of these eclogites from the overlying Caledonian Nappe wedge in western Norway are related to an early phase of crustal subduction during or somewhat before the major phase of continent-continent collision.

The short period of time between the formation of the eclogites and the initial stages of exhumation and rapid cooling is consistent with the only partial and localized transformation of the granulite to eclogite. Isolated occurrences of eclogite within the granulite, the formation of eclogite along metasomatic fronts and the formation of hydrous eclogite facies minerals within the “dry” granulite all point to the importance of fluids in the transformation and re-equilibration of the granulite to eclogite. Together, field and isotopic data demonstrate that both the localized and limited access of fluids and the rapid cycling of continental crust through the deepest portions of the orogen to upper crustal levels resulted in the preservation of the arrested stages of eclogite formation and survival of the granulites metastably through eclogite facies conditions.     

Organic carbon isotope ratios in Quaternary cores from the Gulf of Mexico

All of the major deep-water sedimentary provinces of the Gulf of Mexico were sampled with 48 piston cores, representative of the late Quaternary. The amount (per cent) and δ5C¹³ of the organic carbon in the sediment was measured at intervals within each core.Graphs of δC¹³ versus depth for each core give an indication of the sedimentological history of the Gulf. They show the extent of terrestrial influence on the Gulf during the late Pleistocene.Changes in δC¹³ of up to 6.0%.(from ~ ?19%.to ?25%. vs NBS-20) were measured across the Pleistocene-Holocene boundary in cores from the abyssal plain. These changes are consistent with a model wherein varying amounts of land-derived organic carbon were transported to the Gulf basin during glacial periods.By comparing graphs for cores from different areas, it was concluded that the major parameter affecting the δC¹³ values of organic carbon from marine sediments is the relative amount of terrestrial material present in the sediment. The maximum possible effect of the Pleistocene-Holocene temperature change in the Gulf was determined to be ~1.0%, if such an effect occurs at all.     

Beitrag zur Kenntnis des K/Rb-Verhältnisses In Magmatischen Gesteinen

Zusammenfassung Die röntgenfluoreszenzanalytische Bestimmung von Kalium und Rubidium in 210 basaltoiden und granitoiden Gesteinen der Böhmischen Masse (Waldviertel), der Ost- und Westalpen und Dinariden zeigt, daß im Anschluß an neuere Arbeiten der Durchschnittswert an K/Rb für Basalte vermutlich höher, in Graniten tiefer anzusetzen ist, als allgemein angenommen wird.In der Abfolge von Effusiv- und Intrusivgesteinen ist in mehreren Fällen eine Abhängigkeit des K/Rb-Wertes von der zeitlichen Abfolge (z. B. varistische Granite des Waldviertels, oststeirisches Vulkangebiet) festzustellen, bei der der K/Rb-Wert von basischeren zu sauren Gliedern abnimmt.Oberflächennahe Intrusionen und autometasomatisch veränderte Granitoide zeigen besonders niedrige Werte, die in ein eigenes Konzentrationsfeld fallen. Konkordante Granitoide haben K/Rb-Werte > 150, diskordante < 150.

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Summary X-ray fluorescence analyses of potassium and rubidium of 210 basaltic and granitic rocks of the Bohemian Mass (Waldviertel), of the Eastern and Western Alps, and Dinarides show higher average values of K/Rb-ratios for basalts, lower ones for granites, than generally supposed.The sequency of volcanic and plutonic rocks is controlled by the K/Rb-values, as shown in some cases (such as Hercynian granites of the Waldviertel, Tertiary volcanic rocks of the Alpes etc.). The K/Rb-ratio increases from basic to acid members.Intrusions near the surface and autometasomatically changed granitic rocks show particularly low values, which fall into a special field of concentration. Concordant granites tend to K/Rb-values > 150, discordant ones to < 150.

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Résumé L'analyse par fluorescence X du potassium et du rubidium dans des roches basaltiques et granitiques du Massif Bohêmien (Waldviertel), des Alpes de l'Est et des Dinarides montre que la valeur moyenne de K/Rb est plus grande pour les basaltes que pour les granites, comme prévu normalement.A la suite de l'étude des roches effusives et intrusives on a pu constater dans plusieurs cas une dependance de la valeur K/Rb par la suite temporaire (p. e. les granites varisques du Waldviertel, les roches volcaniques tertiaires des Alpes), par laquelle le rapport K/Rb diminue des valeurs basiques à acides.Les intrusions près de la surface et les granitoïdes changés par autométasomatose présentent des valeurs particulièrement basses.Les granites concordants un rapport K/Rb > 150, les discordants < 150.

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On the correlation of certain lower Dalradian successions in northwest Donegal

Within the Dalradian Series of Donegal, the Creeslough, Maas and Fintown successions, which have hitherto been regarded as stratigraphically distinct, are correlated. A standard terminology for this sequence is proposed and a detailed correlation is made with the Ballachulish Succession, which thus forms all the country around the Donegal Granites, northwest of the faults and thrusts which separate it from the Islay Succession.     

The Origin of Associated Basic and Acid Rocks in the Lebombo--Nuanetsi Igneous Province, Southern Africa, as implied by Strontium Isotopes

Whole rock Rb–Sr measurements were made on associatedbasic and acid intrusives and extrusives from the northern andsouthern ends of the Lebombo monocline with a view to determineif the acid rocks came from the mantle or from the crust. Inthe south, in Swaziland (now called Ngwame) and Zululand, rhyolitesand granophyres (age 202 14 m.y.) have an initial Si⁸⁷–Sr⁸⁶ratio of 0.7042 0.0005; in the north, in the Nuanetsi syncline,rhyolites (age 206 13 m.y.) and granites from ring complexes(age 177 7 m.y.) have initial ratios of 0.7081 0.0008 and0.7085 0.0007, respectively. The initial ratios of the basicrocks, basalts and gabbros, vary in both areas and have a rangefrom 0.7042 to 0.7125 which can be explained by crustal contamination.The low initial ratios and isotopic homogeneity of the acidmagmas over long distances (160 km) in Swaziland and Zululandare interpreted as implying that they were derived from themantle.     

Carbonatite in the tuffs of the West Eifel,Germany

Newly-discovered carbonatitic fragments from the West Eifel are described in terms of their petrography and trace-element content.