Converging P-T paths of Mesozoic HP-LT metamorphic units (Diego de Almagro Island, Southern Chile): evidence for juxtaposition during late shortening of an active continental margin

Summary On Diego de Almagro Island in Chilean Patagonia (51°30S), a convergent strike slip zone, the Seno Arcabuz shear zone, separates the Diego de Almagro Metamorphic Complex from very low grade metagreywackes in the east, which were intruded by Jurassic granitoids. The Diego de Almagro Metamorphic Complex is composed of a metapsammopelitic sequence containing blueschist intercalations in the west and (garnet) amphibolite lenses in the east. Peak metamorphic conditions (stage I) at 9.5–13.5kbar, 380–450°C in the blueschist and at 11.2–13.2, 460–565°C in the amphibolite indicate subduction and accretion at different positions within the deepest part of the accretionary wedge. A K–Ar age of 117±28Ma of amphibole approximately dates the peak of metamorphism in the amphibolite. The early retrograde stage of metamorphism occurred under static conditions and resulted in localized equilibration (stage II) at 6.3–9.6kbar, 320–385°C in the blueschist and 6.1–8.4kbar, 310–504°C in the amphibolite. Both P-T paths converge within a midcrustal level.In contrast, an orthogneiss of trondhjemitic composition occurring within the Seno Arcabuz shear zone is associated with a garnet mica-schist containing a high temperature/intermediate pressure assemblage formed at 4.9–6.5kbar, 580–690°C. A muscovite K–Ar age of 122.2±4.6Ma dates cooling after this event which is related to a concomitant magmatic arc. These rocks were overprinted by a mylonitic deformation, which is caused by convergent strike slip shearing and ends during formation of a retrograde phengite-chlorite-stilpnomelane assemblage at a minimum pressure of approximately 5.7kbar (at 300°C).Zircon fission track ages from rocks of the Seno Arcabuz shear zone are 64.9±2.7 and 64.9±2.7Ma; they record the end of shearing in the Seno Arcabuz shear zone that juxtaposed all rocks in the middle crust. Zircon fission track ages ranging from 78 to 105Ma in the South Patagonian batholith to the east indicate earlier cooling through 280°C. The rocks of the Diego de Almagro Metamorphic Complex were initially slowly exhumed and resided at a midcrustal level before being emplaced via shearing in the Seno Arcabuz shear zone. Apatite fission track ages (54±8Ma) from the Seno Arcabuz shear zone show that exhumation and cooling rates increased after this event. The incorporation of continental crust within the subduction system was a late process, which modified the Cretaceous accretionary wedge, resulting in considerable shortening of the convergent margin.     

Formation of Ni–Cu–Platinum Group Element sulfide mineralization in the Sudbury Impact Melt Sheet

Summary The Ni–Cu–Platinum Group Element (PGE) sulfide deposits of the Sudbury Structure have provided a major portion of the worlds total nickel production and their host rocks have been the subject of numerous research studies, yet a number of perplexing problems remain to be solved. On the one hand, studies seeking to explain the formation of the Sudbury Structure have now converged on a genetic model which proposes that the Main Mass and Offset Dykes of the Sudbury Igneous Complex (SIC) were produced by crystallization of an impact-generated melt sheet. On the other hand, these models have yet to be fully reconciled with the production of the very large volume of magmatic Ni, Cu, Co, and PGE-rich sulfide mineralization and the associated mafic rock types. This paper explores this problem using new precious metal data from the Main Mass and Offset Dykes. These data are used to understand the relationships between these rocks, and to provide constraints on how the Ni–Cu–PGE sulfide ore deposits fit into the geological evolution of the Sudbury Structure.In the two drill cores selected for study in this project, the Mafic Norite has 1–5 modal percent pyrrhotite plus chalcopyrite, and elevated Ni (40–1000ppm), Cu (40–1140ppm), and PGE (1.9–7.8ppb Pd, 1.8–7.3ppb Pt); this is overlain by Felsic Norite that contains pyrrhotite, and has a wide range in concentration of Ni (13–257ppm), Cu (7–328ppm), and PGE (<0.01–6.4ppb Pd, <0.01–5ppb Pt). For a similar range of MgO, the upper portion of the Felsic Norite unit has 5–10 times lower Ni and Cu abundances than within-plate basalts and local crustal rocks, and PGE abundance levels are mostly below analytical determination limits. Stratigraphic studies of other compositional profiles around the SIC demonstrate that this depletion signature of Ni, Cu, and PGE is widespread and developed not only above mineralized embayments and offsets, but also above barren sections of the lower contact of the SIC.The depletion of the upper part of the Felsic Norite in Ni, Cu and PGE is presumably due to equilibration of the magma with magmatic sulfide, and accumulation of this dense sulfide liquid. Results of modeling indicate that the parental magma giving rise to the Mafic and Felsic Norites had initial Ni and Cu contents of 210 and 110ppm, respectively. In addition, Ni, Cu and PGE tenors calculated in 100% sulfide from the Copper Cliff Offset average 13% Cu, 6% Ni, 18ppm Pd, and 19ppm Pt indicating that these sulfides had formed by fractionation from magmas that contained 310ppm Ni, 310ppm Cu, 18ppb Pd and 19ppb Pt. These values are factors of 3 to 5 higher than the Ni, Cu, Pd, and Pt contents of the Onaping Formation with average values of 55ppm Ni, 48ppm Cu, and 4.9ppb Pd as well as the marginal sulfide-poor phase of the Worthington Offset quartz diorite, which has average values of 61ppm Ni, 59ppm Cu, 2.8ppb Pd and 4.0ppb Pt. Both the Onaping Formation and the marginal quartz diorite are believed to represent the initial composition of a large component of the melt sheet. There is therefore a fundamental problem in reconciling the initial metal contents of the SIC magma as indicated by the marginal phases of the Offset dykes and that of the Onaping Formation with the composition of the SIC magma at the times of formation of the sulfides as indicated by their Ni, Cu and PGE tenors.It is proposed that because the SIC melt sheet was initially superheated with a temperature of 1700°C, it was able to dissolve 5 times as much S as it could at its liquidus temperature of 1200°C. It was also initially composed of an emulsion of mafic and felsic melts (Marsh and Zieg, 1999), which may have formed discrete magma cells. As the temperature of the melt sheet decreased, some of these magma cells became S-saturated and the resultant Ni–Cu–PGE sulfides settled downwards and on reaching magma cells lower in the melt sheet were re-dissolved thereby raising the Ni, Cu and PGE contents of the lower magma cells. It was from these enriched magma cells that precipitation of the ore-forming Ni–Cu–PGE sulfide melts eventually took place.The mineral potential of Offset and embayment structures appears to be empirically linked to the thickness of the overlying noritic rocks; for example, the most heavily mineralized embayments and Offset Dykes are located in areas where the Felsic Norite is thickest. It appears unlikely that the entire 1–3km-thick melt sheet was convectively mixing throughout its lateral extent, and so the heterogeneity in sulfide distribution was retained after crystallization and cooling.     

Phosphorus-rich topaz from fractionated granites (Podlesí, Czech Republic)

Summary A detailed electron microprobe study of P, F, Ge and Ga-contents in rock-forming topaz was performed on a suite of Variscan granites at Podlesí in the western Kruné Hory Mts., Czech Republic. Topaz crystals from the relatively less evolved biotite- and protolithionite granites display homogeneous cathodo-luminescence (CL) intensities, whereas topaz from the marginal pegmatite, highly fractionated zinnwaldite granite and greisens show intense oscillatory zoning. Phosphorus contents reach 1.15wt% P₂O₅ in topaz from the zinnwaldite granite. Many topaz crystals are distinctly zoned with a maximum P content in the transition zone between core and rim. Phosphorus is incorporated into the topaz lattice by berlinite substitution: Al³⁺+P⁵⁺=Si⁴⁺+Si⁴⁺. The majority of analysed topazes are highly saturated in F, reaching 90–97% of the theoretical maximum saturation. Topaz from the marginal pegmatite only reaches 87–90% of F-saturation. There is a positive correlation between P_topaz and P_{whole rock}, but no correlation between F_topaz and F_{whole rock}. No difference has been found in P and F contents between magmatic and the hydrothermal (=greisen stage) topaz. Contents of Ge and Ga vary from around the detection limit (50ppm) up to 200ppm Ge and 100ppm Ga, respectively.     

Nickel tenor variations between Archaean komatiite-associated nickel sulphide deposits, Kambalda ore field, Western Australia: the metamorphic modification model revisited

Summary Geochemical data for 870 ore samples of 14 nickel sulphide (NiS) deposits from throughout the major Archaean Kambalda ore field, Western Australia, reveal highly heterogeneous Ni tenor (wt% Ni in 100wt% sulphide) variation that is difficult to explain solely by magmatic processes. The Ni tenor values for the deposits range from 6.2wt% Ni (Helmut deposit) to 19.7wt% Ni (Carnilya Hill deposit), close to the range for within single deposits (9.7–19.3wt% Ni). Contents of Ni and platinum-group elements (PGE) broadly increase with decreasing Fe and with increasing abundance of metamorphic pyrite+magnetite±silicates. In turn, the abundance of the metamorphic phases appears to be complexly related to structural setting, metamorphic grade, alteration type, and proximity to felsic intrusion. Chondrite-normalised multi-element plots of deposit compositions reveal relative depletions in Au, As, Bi, and Te.The relationship of increasing Ni content with secondary phase abundance indicates a strong role for metamorphic modification in the tenor variation. Replacement of pyrrhotite by pyrite+magnetite±chlorite during oxidation reduced the abundance of Fe sulphide relative to Ni sulphide and increased the Ni tenor of the residual sulphide. The extent of the oxidation reflects the extent of alteration fluid ingress along deformation structures and fabrics during talc-carbonate alteration, regional metamorphism, and felsic intrusion related to D3. The relative depletions of Au, As, Bi and Te combined with relative enrichments of these metals in nearby orogenic gold deposits mean that NiS deposits could represent metal reservoirs for Archaean gold hydrothermal systems.     

Metagranitoids from the eastern part of the Central Rhodopean Dome (Bulgaria): U–Pb, Rb–Sr and 40Ar/39Ar timing of emplacement and exhumation and isotope-geochemical features

Summary Geochronological data (U–Pb, Rb–Sr and ⁴⁰Ar/³⁹Ar) are used to unravel the Late Alpine high-grade metamorphism, migmatisation and exhumation of Variscan granitoids within the core of the Central Rhodopean dome, Bulgaria. The age of the granitoid protolith is 300±11Ma, as determined by U–Pb analyses on single zircons selected from the core of the dome structure.Rb–Sr whole rock data define an errorchron with a large scatter of the data points due to the Late Alpine metamorphic overprint. The slope of the reference line indicates a Variscan magmatic event. Strontium characteristics are used to discriminate the samples most influenced by metamorphism from those, which reflect possible differences in the protolith age of the granitoids.Petrological-geochemical data, the initial strontium ratio of 0.708±0.001, and _Hf zircon values ranging from –2.58 to –3.82 point to a mixed, but crust-dominated origin of the Variscan magmas; young crustal material and mantle fragments were sources for the I-type metagranitoids.The exhumation of the granitoids from depths greater than 20–25km to about 5km below the surface was a rapid geological process. It started with the formation of granitic eutectic minimum melts at the temperature peak of metamorphism. Monazite crystallisation at about 650°C continued during isothermal decompression to possible depths of about 10–12km. An age of 35.83±0.40Ma was determined using conventional U–Pb isotope methods on four multigrain monazite fractions. A maximum average age of 36.6–37.5Ma (assuming same error uncertainties) for crystallisation of the metamorphic monazites was calculated assuming 10 to 20% monazite resetting during the subsequent Oligocene volcanism and hydrothermal activity in the region of the Central Rhodopean Dome. The rocks were then cooled to about 350–300°C at 35.35±0.22Ma according to ⁴⁰Ar/³⁹Ar ages of biotites and below 300°C at 35.31±0.25Ma (Rb–Sr data), as indicated by crystallisation of adularia in an open vein subsequent to pegmatite intrusion. A minimum exhumation rate of 3–5km per 1 million years can therefore be calculated for the exhumation of the metagranitoids during the period from 38–35Ma.     

Sedimentary provenance of Mid-Devonian clastic sediments in the Teplá-Barrandian Unit (Bohemian Massif): U–Pb and Pb–Pb geochronology of detrital zircons by laser ablation ICP-MS

Summary The provenance of the Mid-Devonian clastic sediments in the Teplá-Barrandian Unit (TBU) of the Bohemian Massif was investigated by laser ablation ICP-MS U–Pb zircon dating, bulk sediment geochemistry and mineralogical study of the heavy mineral fraction. In contrast to the island arc provenance of the TBU Neoproterozoic sediments, the Early Palaeozoic sediments contain significant amounts of differentiated crustal material. The detrital zircon populations in the Barrandian Mid-Devonian siltstones and sandstones show ages ranging from Archaean (3.0Ga) to Early Palaeozoic (0.39Ga). Major age maxima are at 2.6Ga, 2.0–2.25Ga, 0.62 and 0.51Ga. The youngest identified zircons so far correspond to Lower and Mid-Devonian ages. The extensive mechanical abrasion of zircons having Archaean (3.0, 2.8 and 2.6Ga) to Paleoproterozoic ages (2.25–2.0Ga) suggest their provenance from recycled old sedimentary sequences. The relatively large number of zircons with ages between 2.0 and 3.0Ga may indicate the presence of relicts of the Archaean/Paleoproterozoic crust in the source areas of the studied Mid-Devonian sediments. The absence of detrital zircon ages between 0.9 and 1.2Ga and the presence of zircon ages of 2.0–2.25 and 0.5–0.8Ga correspond to the zircon age pattern from the Gondwana-related North African, rather than Gondwana-related South American and Baltic terranes. The material was entering the basin predominantly from the west and consisted primarily of detrital material of Cambrian granitoids and recycled material of Neoproterozoic meta-sedimentary sequences.     

Mineralogy and composition of the chromitites and their platinum-group minerals from Ortaca (Muğla-SW Turkey): evidence for ophiolitic chromitite genesis

Summary A large number of podiform chromitite bodies of massive, disseminated and nodular type have been located in ultramafic units, composed of depleted mantle harzburgite and dunite of the Marmaris Peridotite from Ortaca (Mula, SW Turkey). The chromite ore bodies are surrounded by dunite envelopes of variable thickness, exhibiting transitional boundaries to harzburgite host rocks. Chromitites, containing a large number of inclusions, i.e. silicates, base metal sulphides and alloys, and platinum-group minerals (PGM) have a wide range of chemical composition. The Cr# [Cr/(Cr+Al)] values of most chromitites are high (0.61–0.81) and Mg# [Mg/(Mg+Fe²⁺)] values range between 0.65 and 0.71 with TiO₂ content lower than 0.24wt.%, which may reflect the crystallization of chromites from boninitic magmas in supra-subduction setting environment.Platinum-group minerals (PGM) such as laurite, erlichmanite and Os–Ir alloys, silicates such as olivine, clinopyroxene and amphibole, and base metal sulphides (BM-S), alloys (BM-A) and arsenides (BM-As) are found as inclusions in chromite or in the serpentine matrix. Platinum-group element (PGE) concentrations of the Ortaca chromitites (OC) are low in all samples. Total PGE (Ir+Ru+Rh+Pt+Pd) ranges from 63ng/g to 266ng/g and Pd/Ir ratios range between 0.23 and 4.75. PGE content is higher and the Pd/Ir ratio lower in Cr-rich chromitites compared to Al-rich ones. There is a strong negative correlation between the Cr# and Pd/Ir ratios (r=–0.930). The PGE patterns show a negative slope from Ru to Pt and a positive slope from Pt to Pd. The low PGE content in the majority of the OC may reflect a lack of sulphur saturation during an early stage of their crystallization. The laurite compositions show a wide range of Ru–Os substitution caused by relatively low temperature and increasing f(S₂) during the chromite crystallization. The high Cr# of and hydrous silicate mineral inclusions in chromite imply that chromite crystallized in a supra-subduction setting.     

Sr-rich minerals in a carbonatite skarn, Fuerteventura, Canary Islands (Spain)

Summary Metamorphosed carbonatites and related skarn deposits, located in Fuerteventura Basal Complex, contain unusual Sr-rich minerals. Maximum SrO concentration in the following minerals are: calcite, 7.23wt%; apatite, 5.22wt%; epidote, 11.64wt%; clinozoisite, 1.25wt%; allanite, 5.63wt%; britholite, 4.11wt% and a Sr–Na aluminosilicate (probably stronalsite), 16.44wt% SrO. Calcite and apatite are chemically similar to those found in carbonatites and are therefore considered to be of igneous origin. Textural evidence indicates that the first skarn stage garnet+diopside+Sr–Na aluminosilicate formed as the result of chemical interaction between carbonatites and adjacent silicate rocks. The formation of Sr-bearing epidote/clinozoisite, allanite and britholite appears to be related to the release of Sr into the fluid phase from the breakdown of high temperature assemblages during the retrograde skarn stage. During the final evolution stages, further alteration of britholite by bastnäsite and törnebohmite took place. The occurrence of REE minerals shows that the fluids responsible for this metasomatism must also have transported significant quantities of REE.     

Pervasive rehydration of granulites during exhumation – an example from the Pulur complex, Eastern Pontides, Turkey

Summary High-grade gneisses from the Pulur complex in NE Turkey bear evidence for biotite-dehydration melting at 820°C and 0.7–0.8GPa, melt segregation and near-isothermal decompression to 0.4–0.5GPa. During further exhumation, the rocks underwent secondary pervasive rehydration at temperatures between 400 and 230°C and fluid pressures between 0.3 and 0.1GPa. Metamorphic peak conditions are dated at 331–327Ma, while hydrothermal retrogression occurred significantly later at 315–310Ma under static conditions. During the rehydration event, primary high-grade mineral assemblages including garnet, cordierite, sillimanite, spinel, biotite, plagioclase and ilmenite were extensively replaced by muscovite, paragonite, margarite, corundum, diaspore, chlorite, kaolinite, pumpellyite, prehnite, epidote, titanite, anatase, pyrite and chalcopyrite. Secondary mineral assemblages indicate that the infiltrating fluids were characterized by low fO₂, very low X_CO2 (<0.002), variable activities of Ca²⁺, K⁺, Na⁺ and H⁺ and relatively high activities of H₂S and CH₄. Quartz veins that might have acted as pathways for the fluids are rare. Ubiquitous veinlets consisting of (i) albite, (ii) chlorite+calcite+quartz or (iii) K-feldspar+calcite+quartz were formed after the pervasive rehydraton event by precipitation from aqueous solutions that were somewhat richer in CO₂.     

Solid solution of rare earth elements in synthetic titanite: a reconnaissance study

Summary Titanite varieties doped with rare earth elements (REE) have been prepared by ceramic synthesis and quenching in air. Their crystal structure was determined by Rietveld analysis of the powder X-ray diffraction patterns. Two different substitution schemes, Ca_1–xNa_x/2Sm_x/2TiSiO₅ and Ca_1–xDy_xTi_1–x SiO₅, are studied at x=0.2. Both synthetic varieties of titanite adopt space group A2/a. This implies that both single-site and complex multivalent substitutional schemes destroy the coherence of the off-centering of octahedral chains in the titanite structure resulting in a P2₁/aA2/a phase transition. Unit cell dimensions obtained for the REE-bearing titanite varieties are as follows: a=7.0541(1)Å; b=8.7247(1)Å; c=6.5664(1)Å; =113.732(1)° for Ca_0.8Na_0.1Sm_0.1TiSiO₅; and a=7.0021(1)Å; b=8.7256(1)Å; c=6.5427(1)Å; =113.294(1)° for . Both REE-doped titanite samples and a control sample of the pure titanite end member have similar unit cell parameters and consist of polyhedra distorted to a similar extent with the exception of more-distorted SiO₄ tetrahedron in CaSiTiO₅. The structural data suggest that the Ca_1–xNa_x/2Sm_x/2TiSiO₅ and Ca_1–xDy_xTi_1–xFe_xSiO₅ solid solutions adopting the titanite structure might extend to x sufficiently greater than 0.2 and involve both heavier and lighter trivalent rare earth elements.Permanent address: Geological Institute KSC RAS, 14 Fersmana St., Apatity, 184200 Russia     

Strontiumgehalte in Süddeutschen Malmkalken

Zusammenfassung Kurven der Strontiumgehalte an Profilen süddeutscher Malmkalke zeigen einen bemerkenswert gleichsinnigen Verlauf über große regionale Verbreitung. Dies hat seine Ursache darin, daß die Strontiumgehalte, wie gezeigt werden kann, an die Gehalte an nichtkarbonatischen Rückständen gebunden sind. Zur Deutung dieses Befundes wird eine Hypothese entwickelt, die besagt, daß das Strontium bei der Umwandlung von Aragonit in Calcit an Tonminerale adsorbiert wurde. In einer noch unbewiesenen Verallgemeinerung gestattet dies die Verteilung von Strontium in Sedimenten allgemein zu skizzieren.

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Curves of strontium contents from profiles of Upper Jurassic limestones in Southern Germany are remarkably similar over great distances. The reason for it is that the strontium is mostly adsorbed by the clay minerals. It is assumed that this strontium was set free during the early transformation of aragonite to calcite. On the basis of this hypothesis and a further extrapolation an attempt is made to outline the principles of the distribution of strontium in sediments.

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Résumé Les courbes de teneur en strontium dans des séries du Jurassique supérieur de l'Allemagne du Sud présentent des grandes analogies d'une coupe à l'autre. On observe en outre que le strontium est lié aux résidus non carbonatés de la roche. Pour expliquer ce résultat, on suppose que le strontium a été adsorbé sur les minéraux argileux au moment de la transformation de l'aragonite en calcite. Cela permet d'esquisser la distribution générale du strontium dans les sédiments.

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Mineralogy and geochemistry of Al-phosphate and Al-borosilicate-bearing metaquartzites of the northern Serra do Espinhaço (State of Bahia, Brazil)

Summary The investigated Al-phosphate- and Al-borosilicate-bearing metaquartzite horizon belongs to the Middle Proterozoic Serra de Vereda member of the São Marcos Formation of the northern Espinhaço fold belt. Outcrops are located in the NNW–SSE trending hills of the Serra de Canas/Cana Brava (Northern Serra do Espinhaço, State of Bahia, NE Brazil) about 10km west of the town of Boquira. Sky blue lazulite and deep blue to lilac dumortierite are enriched in layers which sometimes show relic cross bedding. The occurrence of hematite patches and layers is typical. In rocks of type A, phosphorus contents range from 1.89wt.% to 10.73wt.% P₂O₅ and boron contents are mostly below 10ppm whereas in the rocks of type B phosphorus contents are below 1% and boron contents reach 2600ppm with a mean value of 1139ppm. REE distribution patterns of rocks are predominantly controlled by the amount of xenotime, monazite and zircon. Oxygen thermometry on quartz and hematite and the stability of kyanite, augelite, trolleite and berlinite allow to estimate the minimum metamorphic peak P-T conditions at 475°C and 3.8kbar i.e. within the stability field of scorzalite–lazulite and dumortierite. The association of Al-phosphates with hematite layers provides evidence for the deposition of the metaquartzite protolith in a seawater-fed sabkha-like sand flat fringing the Espinhaço rift. It is proposed that apatite, Al-phosphates and Al-borosilicates were chemically precipitated together with iron hydroxides from pore fluids. The antithetic behaviour of phosphorus and boron contents in the metaquartzites is the consequence of an influx of toxic boron-rich water that drastically affected algae populations and consequently reduced phosphorus precipitation.     

Molybdenum mineralization at Alpeiner Scharte, Tyrol (Austria): results of in-situ U–Pb zircon and Re–Os molybdenite dating

Summary Vein-type Mo mineralization at Alpeiner Scharte occurs in the Penninic units of the western Tauern Window in the Eastern Alps. Three types of previously undated metagranitoids (central gneisses) are distinguished and preserve intrusive contacts with pre-Alpine metamorphosed supracrustal rocks. The granitic protoliths represent fractionated late to post-orogenic, calc-alkaline, I-type magmas with minor S-type components. The Mo veins are restricted to a biotite and alkali feldspar-rich gneiss variety and occur in E–W trending normally sub-vertical quartz veins with adjacent thin discontinuous garnet- and biotite-rich zones; the latter are interpreted as metamorphosed vein selvages. Prior to this work the age of the intrusive host rocks as well as the age of Mo mineralization were unknown.The pre-Alpine Mo deposit and its host rocks were affected by four Alpine deformation events (D₁–D₄) and Young-Alpine regional metamorphism. The P-T conditions of this metamorphic event were 550°C and 8kbar and are in agreement with results of previous regional studies.Zircon grains from two orthogneiss samples were dated with the U–Pb method using ion probe techniques. Zircons from the metagranitic host rock of the Mo-veins yielded an emplacement age of 306.8±3.8Ma (2). A second sample from a more leucocratic gneiss lacking Mo-veins gave 305.0±6.6Ma (2). Re–Os dating of molybdenite from the veins yielded an age of 306.8±3.1Ma, in good agreement with the U–Pb zircon ages.This study confirms one of two alternative hypotheses discussed in the literature. It supports the idea that vein-type Mo-mineralization in the western Tauern Window is genetically related to Late Carboniferous (Westphalian) granitoids that were emplaced during the late to post-orogenic stage of the Variscan orogeny. They do not constitute an Alpine metamorphic-hydrothermal deposit. This study further confirms the strength of the Re–Os molybdenite chronometer, in that it was unaffected by subsequent Alpine medium grade regional metamorphism.Present address: Kremstalstraße 32, A-4501 Neuhofen an der Krems, Austria     

K–Ar and Ar–Ar dating of the Palaeozoic metamorphic complex from the Mid-Bosnian Schist Mts., Central Dinarides, Bosnia and Hercegovina

Summary K–Ar and Ar–Ar whole rock and mineral ages are presented for 25 samples of metamorphic rocks from the Mid-Bosnian Schist Mts., representing one of the largest allochthonous Palaeozoic terranes incorporated within the Internal Dinarides. Four main age groups can be distinguished: 1) Variscan (343Ma), 2) post-Variscan (288–238Ma), 3) Early Cretaceous (mainly 121–92Ma), and 4) Eocene (59–35Ma) ages. Apart from this, an Oligocene (31Ma) age was obtained on Alpine vein hyalophane. The radiometric dating indicates a polyphase metamorphic evolution of the Palaeozoic formations and suggests a pre-Carboniferous age of the volcano-sedimentary protoliths, an Early Carboniferous age of Variscan metamorphism and deformation, post-Variscan volcanism, an Early Cretaceous metamorphic overprint related to out-of-sequence thrusting of the Palaeozoic complex, and an Eocene and Oligocene metamorphic overprint related to the main Alpine compressional deformation and subsequent strike-slip faulting, and uplift of the metamorphic core. Accordingly, the Mid-Bosnian Schist Mts. can be correlated in its multistage geodynamic evolution with some Palaeozoic tectonostratigraphic units from the Austroalpine domain in the Eastern Alps.Deceased     

SiO2-Diagenese in Tiefseesedimenten

Zusammenfassung Im SiO₂-Kreislauf des Weltozeans ist Kieselsäure biogener Herkunft (Skelettopal) der wichtigste SiO₂-Lieferant, während vulkanogene und andere Quellen zurücktreten. Die Erhaltung von biogenem Opal und seine spätere Umbildung in authigenen Opal-CT und Klinoptilolith wird durch hohe Kieselplankton-Produktivität und Sedimentationsraten (z. B. in Auftriebsgebieten) begünstigt. Vulkanglas wird langsamer gelöst und führt zur Ausfällung von authigenem Smektit und Phillipsit, aber nicht zur Hornsteinbildung.Im allgemeinen verläuft die SiO₂-Diagense als ein diskontinuierlicher zeit-, teufen-, temperatur- und faziesabhängiger Reifungsprozeß von instabilem biogenem Opal (Opal-A) über metastabilen Opal-CT (fehlgeordneter Tief-Cristobalit/Tridymit) zu stabilem Quarz. Opal-CT ist also immer die erste (10–65 Ma nach der Ablagerung gefällte) SiO₂-Phase, aus der dann erst später (50–140 Ma nach Ablagerung der ursprünglichen Kieselsedimente) echte Quarzhornsteine entstehen. Unabhängig davon wird akzessorischer Quarz schon frühdiagenetisch in Porzellaniten ausgefällt, wo er Hohlräume füllt oder den Calcit von Fossilien verdrängt.Während die Opal-AOpal-CT-Umwandlung meist über einen Lösungsschritt geht, können Kieselskelette auchin-situ in Opal-CT umgewandelt werden. Das Opal-CT-Gitter erfährt einen teufen- und temperaturabhängige, röntgenographisch nachweisbare strukturelle Reifung. Diese führt später zur Opal-CTQuarz-Umwandlung, die möglicherweise ohne generelle Lösung und Wiederausfällung abläuft.Obwohl Alter/Versenkungstiefen-Diagramme ein weit überlappendes Vorkommen von Opal-A, Opal-CT und Quarz zeigen, läßt sich im allgemeinen eine positive Korrelation der Reife der SiO₂-Phasen mit diesen Parametern feststellen. Die Umwandlung von biogenem Opal in authigenen Opal-CT verläuft allerdings etwas rascher in karbonatischem als in tonigem Milieu, während tonige Fazies die Opal-CTQuarz-Umbildung beträchtlich verlangsamt. Die SiO₂-Transformationen werden allerdings nicht nur durch die Faktoren Zeit, Teufe (Temperatur) und Gastsedimentfazies gesteuert. Weitere, noch weitgehend unbekannte Parameter spielen vermutlich eine bedeutende Rolle.

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Biogenic silica is the most important source for the global oceanic SiO₂ cycle, whereas volcanogenic and other sources are less significant. High silica plankton fertility and sedimentation rates (e. g. in upwelling areas) favour the preservation of skeletal opal and its later transformation into authigenic opal-CT and clinoptilolite. Volcanic glass is less easily dissolved and leads to the precipitation of authigenic smectite and phillipsite, but not to the formation of porcellanites and cherts.In general, silica diagenesis proceeds as a discontinuous age-, burial- and facies-dependant maturation from instable biogenic opal (opal-A) via metastable opal-CT (disordered low-temperature cristobalite/tridymite) to stable quartz. Opal-CT is always the precursor silica phase (precipitated 10–65 m.y. after deposition), followed by the formation of genuine quartz cherts (about 50–140 m.y. after deposition of the original siliceous oozes). Already during early diagenesis, accessory quartz is directly precipitated in porcellanites and fills voids or replaces calcitic fossils.Whereas the opal-Aopal-CT transformation usually involves a solution step, opaline skeletons can also be transformedin situ into opal-CT. A maturation of the opal-CT structure takes place with increasing burial depth, which later leads to an opal-CT quartz transformation, possibly without any major silica mobilization.Altough age/burial depth diagrams show a wide overlap for the distribution of opal-A, opal-CT, and quartz, a general positive correlation of the maturity of the silica phases with these parameters is evident. On the other hand, the rate of the opal-Aopal-CT transformation is slightly faster in calcareous than in clayey sediments, whereas clayey facies retards the opal-CTquartz transformation considerably. However, the silica transformations are not only controlled by the factors time, burial depth (temperature) and host rock facies, but also by additional, largely unknown parameters.

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Résumé Dans le cycle SiO₂ des océans les organismes siliceux sont les fournisseurs principaux de SiO₂. Les autres sources de silice, volcanogénes ou autres, sont de moindre importance. Une haute productivité de plancton siliceux alliée à un taux d'accumulation élevé (par exemple, dans les zones des upwellings) favorisent la conservation de l'opale biogène et sa transformation subséquente en opale-CT et en clinoptilolite. Les verres volcaniques se dissolvent plus lentement et donnent des smectites authigènes et des phillipsites, mais pas de cherts.En général, la diagénèse de la silice se poursuit comme un processus des maturation discontinu, dépendant des facteurs temps, profondeur, température et faciès: au cours de ce cycle, l'opale biogène instable (»opale-A«) se transforme en opale-CT métastable (-cristobalite/tridymite avec désordre unidimensionnel), puis en quartz stable. L'opale-CT est donc toujours la première phase SiO₂ (précipité 10–65 Ma après le dépÔt du sédiment), suivi plus tard (50–140 Ma après le dépÔt des sédiments biosiliceux originaux) par des cherts à quartz. Indépendamment, du quartz accessoire peut Être précipité au cours d'un stade précoce de la diagénèse dans les porcellanites, ou il remplit les vacoules ou remplace la calcite des fossiles.Alors que la transformation de l'opale-A en opale-CT s'opère, de manière générale, par l'intermédiaire d'une phase de dissolution, les restes des organismes siliceux peuvent Être transformésin situ en opale-CT. La grille de l'opale-CT subit un processus de maturation structurale en relation avec la profondeur et la température, visible par l'analyse aux rayons X. C'est cette maturation de la grille qui conduira plus tard à la transformation de l'opale-CT en quartz, cette étape pouvant avoir lieu sans dissolution totale ni reprécipitation.Bien que les diagrammes âges-profondeurs montrent des recouvrements importants de l'opale-A, de l'opale-CT et du quartz, on peut remarquer souvent une corrélation positive entre la maturité des phases de silice et ces paramètres. La transformation de l'opale biogène en opale-CT authigène se poursuit toutefois un peu plus rapidement en milieu carbonatique qu'en milieu argileux, alors que les faciès argileux freinent considérablement la transformation d'opale-CT en quartz. Cependant les transformations de SiO₂ ne sont pas uniquement dirigées par les facteurs temps, profondeur (température) et faciès du sédiment-hÔte. D'autres paramètres encore mal connus semblent jouer un rÔle important au cours de ces phénomènes.

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Geringfügig erweiterte Fassung eines auf der 69. Jahrestagung der Geologischen Vereinigung in Heidelberg gehaltenen übersichtsreferates.     

Tectonic controls on the genesis of ignimbrites from the Campanian Volcanic Zone, southern Italy

Summary ¶The Campanian Plain is an 80×30km region of southern Italy, bordered by the Apennine Chain, that has experienced subsidence during the Quaternary. This region, volcanologically active in the last 600ka, has been identified as the Campanian Volcanic Zone (CVZ). The products of three periods of trachytic ignimbrite volcanism (289–246ka, 157ka and 106ka) have been identified in the Apennine area in the last 300ka. These deposits probably represent distal ash flow units of ignimbrite eruptions which occurred throughout the CVZ. The resulting deposits are interstratified with marine sediments indicating that periods of repeated volcano-tectonic emergence and subsidence may have occurred in the past. The eruption, defined as the Campanian Ignimbrite (CI), with the largest volume (310km³), occurred in the CVZ 39ka ago. The products of the CI eruption consist of two units (unit-1 and unit-2) formed from a single compositionally zoned magma body. Slightly different in composition, three trachytic melts constitute the two units. Unit-1 type A is an acid trachyte, type B is a trachyte and type C of unit-2 is a mafic trachyte.The CI, vented from pre-existing neotectonic faults, formed during the Apennine uplift. Initially the venting of volatile-rich type A magma deposited the products to the N–NE of the CVZ. During the eruption, the Acerra graben already affected by a NE–SW fault system, was transected by E–W faults, forming a cross-graben that extended to the gulf of Naples. E–W faults were then further dislocated by NE–SW transcurrent movements. This additional collapse significantly influenced the deposition of the B-type magma of unit-1, and the C-type magma of unit-2 toward the E–SE and S, in the Bay of Naples. The pumice fall deposit underlying the CI deposits, until now thought to be associated with the CI eruption, is not a strict transition from plinian to CI-forming activity. It is derived instead from an independent source probably located near the Naples area. This initial volcanic activity is assumed to be a precursor to the CI trachytic eruptions, which vented along regional faults.Received October 23, 2002; revised version accepted July 29, 2003     

Platinum-group element alloy inclusions in chromites from Archaean mafic-ultramafic units: evidence from the Abitibi and the Agnew-Wiluna Greenstone Belts

Summary The paper investigates the role of primary magmatic phases in the fractionation and concentration of PGE in Archaean mafic and ultramafic systems. The composition of chromites and olivines in sulphur-poor (S<0.6wt%) komatiites from the Agnew-Wiluna Belt (Western Australia), and of chromite concentrated from komatiitic basalt, ferropicritic basalt and tholeiitic basalt from the Abitibi Belt (Canada) were analysed. The results of laser ablation ICP-MS analyses show that PGE-bearing alloys are not stable in crystallising komatiite and that ruthenium is soluble in chromite during crystallisation. Conversely, analyses of chromites separated from Theos Flow tholeiitic basalt indicate that Ir–Os–(±Pt) enrichments (>200ppb) reflect the presence of PGM. Chromites from Freds Flow komatiitic basalt contain Ir-rich clusters, whereas Pt enrichments (>370ppb) in Boston Creek ferropicritic basalt reflect the presence of Pt-rich compounds. The presence of PGE-bearing alloys in Theos Flow and Freds Flow is due to late S-supersaturation, whereas the presence of Pt-rich compounds in Boston Creek Flow reflects high state of melt oxidisation. The lack of PGE-bearing alloys in the olivines and chromites of komatiites can be explained by thermal instability of PGM, depletion in PGE at the mantle source, early S-supersaturation, the oxidisation conditions of the melt, or a combination of these factors.     

Comparative organic petrology of interlayered sandstones,siltstones, mudstones and coals in the Upper Carboniferous Ruhr basin,Northwest Germany,and their thermal history and methane generation

In the coal-mining Ruhr-area, Upper Carboniferous rocks (ca. 4000 m) consist of interlayered sandstones, siltstones, mudstones and coals. They were deposited in a tropical, paralic environment where alternating fluvial sedimentation, occasional marine ingressions, and swamp growth resulted in an irregular cyclic succession. The total sedimentary package contains on an average 6 Vol.-% of organic matter. About 70 Vol.-% of the organic matter occurs in coal seams, the rest as dispersed organic matter in clastic rocks. The organic matter is autochthonous in the coals and allochthonous in associated sandstones and siltstones. It consists of about 70% vitrinite, 20% inertinite, and 10% liptinite. The overall maceral group composition is the same for coals and dispersed organic matter. This surprising similarity is caused by a nearly exclusive input of land-plant derived organic matter to swamps and fluvial systems and a similar degree of preservation. Highest average liptinite contents (% of total macerals) were found in unrooted mudstones, highest average inertinite contents in coarse-grained siltstones and highest average vitrinite percentages in sandstones.Maturities of the sediments studied are well within the hydrocarbon generation window, e. g. vitrinite reflectivities range from 0.6% to 1.6%. Reflectivities measured on dispersed particles in clastic rocks are similar to those measured in coal seams. Calculations of the amount of methane generated indicate that coal seams contributed more to the total hydrocarbon generation than dispersed organic matter.

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Zusammenfassung Das Oberkarbon des Ruhrgebietes besteht aus ca. 4000 Metern wechsellagernder Sandsteine, Siltsteine und Kohleflöze. Der Ablagerungsraum der Sedimente war eine Region mit tropischem Klima, genauer ein paralisches Environment mit unregelmäßig alternierenden Folgen von fluviatilen Sedimenten, marinen Transgressionen und verbreiteten Sumpfablagerungen. Der Anteil organischer Substanzen an der gesamten Sedimentmenge beträgt durchschnittlich 6%. 70% dieser organischen Masse ist in Kohleflözen angereichert, der Rest liegt verteilt in den klastischen Gesteinen vor. Die Herkunft der organischen Substanz in den Kohleflözen ist autochthon, die in den benachbarten Sand- und Siltsteinen allochton. Die Zusammensetzung des organischen Materials ist 70% Vitrinit, 20% Inertinit und 10% Liptinit. Die allgemeine Zusammensetzung nach Mazeral-Gruppen ist für die Kohleflöze und das verteilte Material in den Nachbargesteinen identisch. Diese Ähnlichkeit beruht auf der gemeinsamen Herkunft der organischen Substanz in Sümpfen und Flußsystemen, die fast ausschließlich von Landpflanzen bestimmt wird, und einer sich entsprechenden Konservierung.Der Reifegrad der analysierten Sedimente liegt innerhalb des Kohlenwasserstoff-Bildungsbereiches (die Vitrinit-Reflexionen reichen von 0,6–1,6%). Dabei entsprechen die in den klastischen Gesteinen beobachteten Reflexionswerte weitgehend denen der Kohleflöze. Anhand der Menge des erzeugten Methans läßt sich erkennen, daß der Anteil der Kohleflöze an der Kohlenwasserstoff-Produktion höher ist als der Anteil, den disperse organische Substanzen der Klastika beisteuern.

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Résumé Dans la région minière de la Ruhr, le Carbonifère supérieur constitue une succession, épaisse de 4.000 m environ, de grès, de siltites et de charbon. Leur dépôt, dans un milieu paralique tropical, a été marqué par des alternances de sédimentation fluiviale, de transgressions marines occasionnelles, et d'épisodes marécageux; il en résulte une disposition cyclique irrégulière. La série sédimentaire contient dans l'ensemble 6% de matière organique en moyenne. Environ 70% de cette matière organique se trouve dans les couches de charbon où elle est autochtone; le reste est allochtone et dispersé dans les grès et siltites. La matière organique se répartit approximativement en 70% de vitrinite, 20% d'inertinite et 10% de liptinite. La composition moyenne du groupe «macéral» est la même pour les charbons et la matière organique dispersée. Cette identité surprenante est l'expression d'une alimentation provenant exclusivement de plantes terrestres et d'un même degré de conservation.La maturité des sédiments étudiés se situe à l'intérieur du domaine de genèse des hydrocarbures: le pouvoir réflecteur de la vitrinite s'échelonne entre 0,6% et 1,6%. Les gradients du pouvoir réflecteur observés dans les roches détritiques et dans les couches de charbon sont analogues. Le calcul de la quantité de méthane engendré indique que la contribution des couches de charbon à la production totale d'hydrocarbure est supérieur à celle de la matière organique dispersée.

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Crustal rifting,and related thermo-mechanical processes in the lithosphere beneath Iceland

Recent observations of crustal rifting in the NE-Iceland axial zone are summarized and discussed. The thermal state of the crust in the axial zone is discussed on the basis of thermal gradient measurements, magnetotelluric soundings, and modelling of the accretion mechanism. The three independent methods are in good general agreement. Thermal modelling indicates that a zone of partial melting generally exists below approximately 6 km depth in the axial zone. It is suggested, that thermal stresses induced by the cooling of the lithosphere as it moves away from the axis are largely responsible for the off-axis volcanism, and that the thermal stresses also contribute to enhancing the vertical permeability for geothermal waters at intermediate crustal depths on the flanks of the axial zone.

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Zusammenfassung Neue Beobachtungen von Driftbewegungen der Erdkruste in dem zentralen Bereich der aktiven Riftzone in Nordost-Island werden zusammengefaßt und diskutiert. Der thermische Zustand der Zentralzone wird diskutiert an Hand von Temperaturgradientmessungen, magnetotellurischen Messungen und Modelrechnungen, die die Entstehung der Kontinentalplatten beschreiben. Die Ergebnisse der drei unabhängigen Methoden sind in guter Übereinstimmung miteinander. Thermische Modelrechnungen weisen darauf hin, daß in einer Tiefe von etwa 6 km unterhalb der zentralen Zone, eine Schicht von teilweise geschmolzenen Gesteinsmassen allgemein vorhanden ist. Es wird vorgeschlagen, daß thermische Spannungen, die durch die Abkühlung der Lithosphäre auf seiner Bewegung weg von der zentralen Riftzone erzeugt werden, hauptsächlich für die vulkanische Aktivität außerhalb der Zentralzone verantwortlich sind. Die thermischen Spannungen erhöhen vermutlich auch eine vertikale Permeabilität für geothermales Wasser in der mittleren Kruste an den Flanken der aktiven Zone.

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Résumé L'auteur résume et discute des observations récentes de formation de rift crustal dans la zone axiale du NE de l'Islande. Il discute l'état thermique de la croûte dans la zone axiale sur la base de mesures du gradient thermique, de sondages magnéto-telluriques et d'un modèle du mécanisme d'accroissement. Ces trois méthodes indépendantes sont en général en bon accord. Le modèle thermique indique qi'il existe généralement une zone de fusion partielle en-dessous d'une profondeur approximative de 6 km dans la zone axiale. Il est suggéré que des tensions thermales induites par le refroidissement de la lithosphere au fur et à mesure qu'il s'écarte de l'axe sont largement responsables du volcanisme à l'écart de l'axe, et que les tensions thermiques contribuent aussi à activer la perméabilité, suivant la verticale, pour les eaux chaudes aux profondeurs crustales moyennes sur les flancs de la zone axiale.

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The Geochemistry of Lake Joyce, McMurdo Dry Valleys, Antarctica

Lake Joyce is one of the least studied lakes of the McMurdo Dry Valleys. Similar to other lakes in this region, Lake Joyce is a closed-basin, permanently ice-covered, meromictic lake. We present here a detailed investigation of major ions, nutrients, and dissolved trace elements for Lake Joyce. Specifically, we investigate the role of iron and manganese oxides and hydrous oxides in trace metal cycling.Lake Joyce is characterized by fresh, oxic waters overlying an anoxic brine, primarily Na–Cl. Surface waters have a maximum nitrate concentration of 26M with a molar dissolved inorganic nitrogen to phosphorus ratio of 477. The supply of nitrogen is attributed to atmospheric deposition, possibly from polar stratospheric clouds. Dissolved phosphorus is scavenged by hydrous iron oxides. The pH is highest (10.15) just beneath the 7-m thick ice cover and decreases to a minimum of 7.29 in the redox transition zone. Dissolved Al exceeds 8M in surface waters, and appears to be controlled by equilibrium with gibbsite. In contrast, concentrations of other trace elements in surface waters are quite low (e.g., 5.4nM Cu, 0.19nM Co, <20pM La). Dissolved Fe, Mn, Ni and Cd were below our detection limits of 13 nM, 1. 8 nM, 4.7 nM and 15pM (respectively) in surface waters. There was a 6-m vertical separation in the onset of Mn and Fe reduction, with dissolved Mn appearing higher in the water column than Fe. Based on thermodynamic calculations, dissolved Mn appears to be controlled by equilibrium with hausmannite (Mn3O4). Co tracks the Mn profile closely, suggesting Co(III) is bound in the lattice of Mn oxides, whereas the Ce profile is similar, yet the Ce anomaly suggests oxidative scavenging of Ce. Release of Cu, Ni, Cd and trivalent REE appears to be controlled by pH-induced desorption from Fe and Mn oxides, although Cu (and perhaps Ni) may be scavenged by organic matter in surface waters.