Re–Os and <Superscript>40</Superscript>Ar/<Superscript>39</Superscript>Ar ages of the Jiguanshan porphyry Mo deposit,Xilamulun metallogenic belt,NE China,and constraints on mineralization events

The large Jiguanshan porphyry Mo deposit, with more than 100 Mt of ore and grades ranging from 0.08% to 0.11%, is located in the newly identified Xilamulun metallogenic belt along the northern margin of the North China Craton. The Mo mineralization is predominantly disseminated in the host granite porphyry, but locally occurs as stockworks in lithic tuff and rhyolitic rocks. ⁴⁰Ar/³⁹Ar dates of samples from groundmass material in the host granite porphyry, post-ore diabase, and quartz porphyry dikes show plateau ages of 155.1 ± 1.9, 149.4 ± 0.9, and 147.6 ± 0.9 Ma, with inverse isochron ages of 156.0 ± 1.8, 149.3 ± 1.3, and 148.3 ± 1.2 Ma, respectively. Seven samples of disseminated molybdenite yielded a weighted average ¹⁸⁷Re-¹⁸⁷Os age of 155.3 ± 0.9 Ma, whereas six veinlet-type molybdenite samples yielded a weighted average ¹⁸⁷Re-¹⁸⁷Os age of 153.0 ± 0.9 Ma, providing direct timing constraints for the Mo mineralization at 153–155 Ma. The regional geological setting together with the emplacement of post-ore diabase and quartz porphyry dikes in the Jiguanshan deposit, are indicative of an extensional regime in Late Jurassic, which was probably linked to lithospheric extension in northeast China.     

The Dorn gold deposit in northern Victoria Land,Antarctica: Structure,hydrothermal alteration,and implications for the Gondwana Pacific margin

The Dorn gold deposit in northern Victoria Land, Antarctica is a fault related gold-only deposit and it represents the first described occurrence of gold mineralization in Paleozoic terranes of the Antarctic continent. The deposit is hosted in lower greenschist facies Middle Cambrian metavolcanic and metasedimentary rocks of the Bowers terrane. Gold-bearing veins are located in a brittle–ductile reverse high-strain zone, which was produced by transpressional deformation that overprints the structures of the Cambrian–Ordovician Ross Orogeny. The vein system is surrounded by a hydrothermal alteration zone that is as much as 300-m-wide, where the host rocks are partially to completely transformed into Fe–Mg carbonate-rich rocks with different degrees of replacement of the original mineralogy and texture.The type of host rock, the temperature estimates for mineralization from 290–320 °C, the quartz dominant vein system with sulfides and Fe-rich carbonates, and the controlling structures linked to a convergent margin tectonic setting together suggest that this mineralized vein system can be classified as an orogenic gold deposit. Close analogies are found with deposits of the Stawell zone in western Victoria, which is consistent with the correlation between units and hydrothermal events in northern Victoria Land and southeastern Australia.     

Shifts in an epibenthic trophic web across a marine frontal area in the Southwestern Atlantic (Argentina)

Marine benthic trophic relationships and food web structures may be influenced by benthic–pelagic coupling processes, which could also be intensified by the physical dynamics of marine fronts. In this work, we employed stable isotope (δ¹³C and δ¹⁵N) analysis to investigate the influence of the Southwest (SW) Atlantic shelf-break front (SBF; 38–39°S, 55–56°W; Argentina) on an epibenthic trophic web. Epibenthic organisms were sampled, at depths of ~ 100 m, with a non-selective dredge from a sandy bottom community located in frontal (F) and marginal (M) areas. The SBF position and the chlorophyll-a (chl-a) concentrations were inferred using satellite data of the sea surface temperature (SST) and satellite chl-a concentration, respectively. The most noticeable shifts in stable isotopes between the sampled areas were those of the Patagonian scallop, Zygochlamys patagonica (δ¹³C), and those of the sea urchin, Sterechinus agassizi (δ¹⁵N). Diet analyses inferred from stable isotopes and mixing models demonstrated that the dominant component of this community, Z. patagonica, had variable contributions to higher trophic levels between areas. More importantly, the epibenthic assemblage in F areas showed δ¹³C-enriched and δ¹⁵N-depleted isotopic signatures with respect to the M areas. Collectively, this evidence suggests that frontal dynamics promotes the accumulation of δ¹³C-enriched phytoplankton in the seabed in F areas, while in M areas the more degraded organic matter becomes more important in the trophic web, decreasing the δ¹⁵N isotopic signature of the assemblage. Therefore, the trophic web was sustained by fresher food in F areas than in M areas, demonstrating the role of frontal dynamics in the shaping of these communities.     

Computation of analytical partial derivatives of phase and group velocities for Rayleigh waves with respect to structural parameters

Summary The analytical expressions used to compute the partial derivatives of phase and group velocity of Rayleigh waves with respect to the P- and S-wave velocity and the density are derived and the related computer code is developed. The results of the analytical computations were satisfactorily tested against numerically determined values. Several examples of partial derivatives for a given structural model are presented.     

1.1 Ga K-rich alkaline plutonism in the SW Grenville Province

U–Pb zircon and baddeleyite dating of six syenitic stocks establishes that the ultrapotassic, potassic alkaline and shoshonitic magmatism with island-arc affinities in the Central Metasedimentary Belt (CMB) of the southwestern Grenville Province, Canada took place between 1089 and 1076 Ma, along a 400-km-long, northeast-trending plutonic belt. These ages indicate that ultrapotassic rocks with arc affinities are not unique to the Phanerozoic. West to east emplacement ages along a northern and southern cross-section of this belt range from 1083±2 Ma (Kensington), through 1081±2 Ma (Lac Rouge) to 1076 _–1 ⁺³ Ma (Loranger) in the north, and from 1089 _–3 ⁺⁴ Ma (loon Lake) and 1088±2 Ma (Calabogie), to 1076±2 Ma (Westport) in the south. Although closely spaced in time, in detail these ages suggest a slight younging of this magmatic activity to the southeast. Integration of the geochronological data with the spatial extent and potassic character of the plutons shows that the K-rich alkaline suite is distinct from the nepheline-syenite belt of the Bancroft terrane and from the syenite-monzonite suite of the Frontenac terrane of the CMB, and it is considered to be a magmatic episode unique to the Elzevir terrane and its Gatineau segment. The timing and the postmetamorphic emplacement of these plutons indicate that the regional greenschist to granulite-facies metamorphism of the country rock (precise age unknown) is older than 1089 Ma throughout the entire Elzevir terrane. The potassic magmatism is interpreted as the initiation of the 1090–1050 Ma Ottawan Orogeny in the Elzevir terrane; thus, the regional metamorphism in this terrane, previously assigned to the Ottawan Orogeny, is an earlier event. The contemporaneous emplacement of this postmetamorphic plutonic belt with Keweenawan volcanism is at variance with current tectonic models which consider the Keweenawan rift to be formed at the same time as regional metamorphism in the CMB.     

Metamorphic CO2 production from calc-silicate rocks via garnet-forming reactions in the CFAS–H2O–CO2 system

The type and kinetics of metamorphic CO₂-producing processes in metacarbonate rocks is of importance to understand the nature and magnitude of orogenic CO₂ cycle. This paper focuses on CO₂ production by garnet-forming reactions occurring in calc-silicate rocks. Phase equilibria in the CaO–FeO–Al₂O₃–SiO₂–CO₂–H₂O (CFAS–CO₂–H₂O) system are investigated using P–T phase diagrams at fixed fluid composition, isobaric T–X(CO₂) phase diagram sections and phase diagram projections in which fluid composition is unconstrained. The relevance of the CFAS–CO₂–H₂O garnet-bearing equilibria during metamorphic evolution of calc-silicate rocks is discussed in the light of the observed microstructures and measured mineral compositions in two representative samples of calc-silicate rocks from eastern Nepal Himalaya. The results of this study demonstrate that calc-silicate rocks may act as a significant CO₂ source during prograde heating and/or early decompression. However, if the system remains closed, fluid–rock interactions may induce hydration of the calc-silicate assemblages and the in situ precipitation of graphite. The interplay between these two contrasting processes (production of CO₂-rich fluids vs. carbon sequestration through graphite precipitation) must be considered when dealing with a global estimate of the role exerted by decarbonation processes on the orogenic CO₂ cycle.     

Late Neogene–Quaternary evolution of the intermontane Clusone Basin (Southern Alps,Italy): integration of seismic and geological data

In the Clusone Basin (a large intermontane basin filled by thick late Neogene–Quaternary sediments in the Middle Val Seriana, Southern Alps), two high‐resolution seismic profiles have been acquired in order to reconstruct the geometries of the sediments that fill the depression, with a maximum thickness of more than 200 m as documented by available well data, and to define their relationships with the bedrock, consisting of Late Triassic carbonates. In addition to standard seismic reflection processing, a seismic refraction inversion technique has been applied. The integration of geological (both surface and well data) and seismic data indicates a complex history of the drainage patterns of the Clusone Basin, documenting a shift of the Serio River from a palaeodrainage toward the southeast (Val Borlezza) to the present situation, toward the south (Val Seriana): between the older and the present‐day drainages an important depositional stage occurred, as documented by the thick sediments that fill the Clusone Basin, controlling the capture of the Serio River along the Val Seriana. Copyright © 2004 John Wiley & Sons, Ltd.     

Metazoan/microbial biostalactites from present‐day submarine caves in the Mediterranean Sea

Biostalactites formed by metazoan–microbialite associations from three submerged marine caves in the Plemmirio Peninsula (south of Syracuse, Ionian Sea) are randomly distributed and show different sizes and morphologies, as well as variations in surface roughness/smoothness. The biostalactites consist of crusts a few centimeters thick of small serpulids and other metazoans, associated with fine‐grained carbonate; the larger ones often include a nucleus of serpulid tubes (Protula). The metazoans include mainly serpuloideans, sponges, bryozoans and foraminifers but microbial carbonates are also significant components. The composition of both the living communities and thanatocoenoses on the outer surfaces, as well as the composition and fabric of the internal framework, were analysed and used to reconstruct the history of the caves. All of the identified sessile faunas mainly consist of cryptic and sciaphilic dwellers that reflect cave conditions and their variations through time. The distribution pattern, composition and abundance of the present‐day dwellers largely depend on the degree of roughness of the biostalactite surfaces and their positions within the caves. It has been suggested that the Protula specimens in the nuclei represent pioneer populations that formed aggregates during the early cave colonization phase, in response to relatively high food supply from seawater inflow and intruding continental waters. By contrast, the outer metazoan–microbialite carbonates reflect more confined conditions in the caves caused by Holocene sea‐level rises. Hypotheses are proposed for biostalactite growth, taking into account information about the growth rates of some constituents, and evidence of dissolution effects. Similarities and differences between these biostalactites and other Holocene deposits previously described from submarine caves in the Mediterranean Sea and in tropical reefs are discussed.     

Environmental monitoring of magnetic iron phases of urban water reservoir lake sediments (Taiaçupeba Lake,metropolitan region of São Paulo,Brazil) by using Mössbauer spectroscopy

In the present work, we investigate the iron oxides and oxyhydroxides behavior and evolution, related to the geochemical behavior of some metals, which could be retained as solid phases in the sediments from an urban water reservoir lake, placed in Taiaçupeba, Great São Paulo, Brazil. These tasks were performed by the establishment of a proceduring setting for environmental monitoring analysis through Mössbauer spectroscopy measurements associated to hysteresis loops measurements and chemical analysis [X-ray fluorescence (XRF)]. We inferred the possibility of goethite occurrence in broad particle size distribution (5–50 nm), and related to ferrihydrite, and small grain-size hematite (about 8 nm). The magnetometry results pointed to the paramagnetic/superparamagnetic behavior of the magnetic phases present in the samples and also suggested the occurrence of small grain-size magnetite. We also verified the presence of clay minerals related to Fe, as well as the occurrence of Fe³⁺ and/or Fe²⁺ in short-range structural order. Through a straight correlation among Mössbauer spectra data gained at T = 77 K and Al-metal, metal-Fe molar ratios, provided through XRF data, we found remarkable indications of interference on meta-stable phases evolution to its final products. Such results can be pointing for evidences about the possible isomorphic replacing and/or adsorption of Al and other metals in goethite and hematite.