Mantle source compositions of the Paraná Magmatic Province (southern Brazil): evidence from trace element and Sr–Nd–Pb isotope geochemistry

An integrated study based on incompatible trace elements and Sr–Nd–Pb isotopes is presented in order to assess the mantle sources involved in the genesis of the Paraná Magmatic Province (PMP) tholeiites. Particular emphasis is given to 33 new Pb isotope and concentration data obtained in representative samples of low-TiO₂ (LTiB) and high-TiO₂ (HTiB) flood basalts that occur in the province. Results show important differences with respect to type and location of these rocks. The LTiB and HTiB from northern PMP exhibit very similar initial Pb isotope ratios (average LTiB vs HTiB: ²⁰⁶Pb/²⁰⁴Pb_i=17.78±0.03 vs 17.65±0.02; ²⁰⁷Pb/²⁰⁴Pb_i=15.53±0.01 vs 15.52±0.01; ²⁰⁸Pb/²⁰⁴Pb_i=38.12±0.03 vs 38.05±0.04). The LTiB from southern PMP, with initial ⁸⁷Sr/⁸⁶Sr_i≤0.7060, show small variation in initial Pb isotope compositions (average ²⁰⁶Pb/²⁰⁴Pb_i=18.20±0.07; ²⁰⁷Pb/²⁰⁴Pb_i=15.61±0.01; ²⁰⁸Pb/²⁰⁴Pb_i=38.32±0.10), which are highly enriched in radiogenic Pb in comparison to the northern PMP analogues. The HTiB from southern PMP have initial Pb isotope ratios (average ²⁰⁶Pb/²⁰⁴Pb_i=17.45±0.09; ²⁰⁷Pb/²⁰⁴Pb_i=15.50±0.01; ²⁰⁸Pb/²⁰⁴Pb_i=37.89±0.03) slightly less radiogenic compared with the HTiB from northern PMP. The data cover a large range of isotope compositions, which are accompanied by systematic changes in incompatible trace element ratios and Sr–Nd isotopes, indicating contributions from different mantle sources. The remarkable chemical and isotope differences between PMP basalts, N-MORB and Tristan da Cunha least evolved volcanics indicate that these asthenospheric sources did not play a significant role in the basalt genesis, suggesting generation from the melting of heterogeneous lithospheric mantle sources. The close similarity between the radiogenic isotopes of the Cretaceous carbonatites that surround the PMP and those of the HTiB rock-types and the LTiB from the northern Paraná suggests the involvement of the same mantle components in their genesis: a dominant EMI end member and a radiogenic isotope enriched component of EMII-type, as some phlogopite-peridotite mantle xenoliths (Japecanga) from the Alto Paranaíba Igneous Province. The latter component seems also to have an important role in the origin of the LTiB from the southern Paraná, where the other end member is highly depleted in radiogenic lead similar to DMM.     

Analytical approximate wave form for asymmetric waves

A simple analytical formulation that reproduces a skewed, nonlinear near-bed wave orbital velocity is presented. It contains four free parameters, where two are solely related to the velocity and acceleration skewnesses. The equation is compared with other models and is validated against field and laboratory experiments. The results reveal that it can simulate a wide range of nonlinear wave shapes, reproducing satisfactorily the measured nonlinear wave particle velocity. Also, the new expression overcomes some limitations of the other models. The new formulation is therefore capable of being used in many engineering applications that require the use of representative wave forms.     

Low-pressure evolution of basalt sills from Bore-Holes in the Paranà basin,Brazil

Summary Basalt sills (up to 200 metres thick) of tholeiitic and transitional chemistry are widespread in the Paleozoic sedimentary rocks of the Paranà basin. They closely correspond in composition to the overlying Lower Cretaceous flood basalts, and are systematically characterized by pigeonite.All sills underwent low-pressure evolurionary processes (ca. 1 kb; max. depth of intrusion about 3000 metres), essentially throughin situ crystallization of plagioclase, pyroxene, magnetite, and olivine, leading to the formation of quartz-alkali feldspar intergrowths. Mass-balance calculations indicate that differentiation proceded essentially under closed system conditions and imply a major role of Ca-rich pyroxene (liquidus phase) consistent with petrographic data.Bulk rock compositions of the sills distinguish the Paranà Lower Cretaceous magmatism into northern and southern regions characterized by high- and low-TiO₂ contents, respectively.

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Die Entwicklung von Basalt-Gängen aus dem Paranà-Becken, Brasilien: Ergebnisse der Untersuchung von Bohrkernen

Zusammenfassung Basaltische Sills von tholeiitischer und Ubergangszusammensetzung sind in paläozoischen Sedimentgesteinen des Paranà-Beckens weit verbreitet. Sie können bis zu 200 m mächtig werden. In geochemischer Hinsicht sind sie mit den darüberliegenden Plateau-Basalten unterkretazischen Alters eng verwandt und durch das Auftreten von Pigeonit charakterisiert.Alle Sills haben in niedrigen Druckbereichen (maximale Intrusionstiefe ca. 3000 m) eine Kristall-Fraktionierung durchgemacht, wahrend der es hauptsächlich zurin situ-Kristallisation von Plagioklas, Pyroxen, Magnetit und Olivin kam. Dies führte zur Bildung von Verwachsungen aus Quarz und Alkalifeldspat. Massenbilanz-Berechnungen zeigen, daß die Differentiation im wesentlichen in einem geschlossenen System stattfand, und bestätigen die petrographischen Beobachtungen, nach denen Ca-reicher Pyroxen als Liquidus-Phase eine bestimmende Rolle spielte. Die chemische Pauschalzusammensetzung der. Sills untermauert die Annahme einer Trennung des Paranà-Magmatismus in eine nördliche und eine südliche Zone, welche jeweils durch hohe bzw, niedrige TiO₂-Gehalte gekennzeichnet sind.

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With 11 Figures     

Controls on the geometry of tails around rigid circular inclusions: insights from analogue modelling in simple shear

We used analogue modelling to investigate the factors controlling tail geometry in porphyroclast systems. Results show that: (1) σ inclusions can develop in both slipping and non-slipping modes, but δ-inclusions only form in the latter. (2) σ inclusions develop when the mantle production rate is constant and the mantle is transected by the separatrix. δ inclusions form when the mantle is initially outside the separatrix and later comes inside this line. (3) In the slipping mode, the wedge-shaped tail of σ-inclusions always has a straight external surface parallel to the shear plane, whereas in the non-slipping mode the external surface is curved inwards (external embayments). (4) Together with earlier theoretical results, σ- and δ inclusions always show stair-stepping of tails when embedded in a viscous matrix under homogeneous simple shear deformation. (5) Maximum stair-stepping occurs in the slipping mode and is at least equal to the inclusion diameter. If our models bear significant similarity to nature, then (i) the straight or curved character of σ-inclusions could mean that they had, respectively, a slipping or non-slipping interface with the surrounding recrystallized matrix, and (ii) δ-inclusions may result from shear deformation under retrogressive metamorphic conditions in thrust systems.     

Post-Jurassic tectono-sedimentary evolution of the Northern Lusitanian Basin (Western Iberian margin)

Seismic reflection and well stratigraphic data are used to investigate the post‐Jurassic evolution of the Northern Lusitanian Basin, offshore west Iberia. Stratigraphic correlations between 11 exploration wells were attained in order to characterize the variations in depositional facies associated with salt tectonics. Latest Triassic–Hettangian salt, which generated multiple salt pillows during the Jurassic rifting, was reactivated after the early Aptian in two main phases. The first phase stretches from the late Turonian to the Maastrichtian. The second relates to Miocene tectonic inversion. The compression of the post‐salt overburden caused the amplification of Jurassic detachment folds, forming barriers to the westward progradation of sediment into distinct salt‐withdrawal sub‐basins. Particularly during the Miocene, thin‐skinned overburden shortening was accommodated by growing salt structures that suffered thrusting and extrusion. This structural style contrasts with that of salt‐scarce areas where a simple westerly tilted, fault‐bounded monocline was generated.     

Traditional fisherfolk and no-take protected areas: The Peixe Lagoon National Park dilemma

This study challenges the ‘National Park’ as the appropriate management model for the Peixe Lagoon area in southern Brazil through an investigation of local fisherfolk livelihoods and traditional ecological knowledge. We argue that top-down management policies implemented through a non-participatory process, which have resulted in conflict between government officials and fisherfolk, disregarded the fisherfolk’s cultural practices and particular knowledge, thereby violating their rights as traditional people. Multiple suggestions are provided to achieve environmental conservation schemes without compromising the local traditional fishing livelihoods.     

Offset continuation (OCO) ray tracing using OCO trajectories

Offset continuation (OCO) is a seismic configuration transform designed to simulate a seismic section as if obtained with a certain source-receiver offset using the data measured with another offset. Since OCO is dependent on the velocity model used in the process, comparison of the simulated section to an acquired section allows for the extraction of velocity information. An algorithm for such a horizon-oriented velocity analysis is based on so-called OCO rays. These OCO rays describe the output point of an OCO as a function of the Root Mean Square (RMS) velocity. The intersection point of an OCO ray with the picked traveltime curve in the acquired data corresponding to the output half-offset defines the RMS velocity at that position. We theoretically relate the OCO rays to the kinematic properties of OCO image waves that describe the continuous transformation of the common-offset reflection event from one offset to another. By applying the method of characteristics to the OCO image-wave equation, we obtain a raytracing-like procedure that allows to construct OCO trajectories describing the position of the OCO output point under varying offset. The endpoints of these OCO trajectories for a single input point and different values of the RMS velocity form then the OCO rays. A numerical example demonstrates that the developed ray-tracing procedure leads to reliable OCO rays, which in turn provide high-quality RMS velocities. The proposed procedure can be carried out fully automatically, while conventional velocity analysis needs human intervention. Moreover, since velocities are extracted using offset sections, more redundancy is available or, alternatively, OCO velocities can be studied as a function of offset.     

Seasonal and interannual variation of the phytoplankton and copepod dynamics in Liverpool Bay

The seasonal and interannual variability in the phytoplankton community in Liverpool Bay between 2003 and 2009 has been examined using results from high frequency, in situ measurements combined with discrete samples collected at one location in the bay. The spring phytoplankton bloom (up to 29.4 mg chlorophyll m⁻³) is an annual feature at the study site and its timing may vary by up to 50 days between years. The variability in the underwater light climate and turbulent mixing are identified as key factors controlling the timing of phytoplankton blooms. Modelled average annual gross and net production are estimated to be 223 and 56 g C m⁻² year⁻¹, respectively. Light microscope counts showed that the phytoplankton community is dominated by diatoms, with dinoflagellates appearing annually for short periods of time between July and October. The zooplankton community at the study site is dominated by copepods and use of a fine mesh (80 μm) resulted in higher abundances of copepods determined (up to 2.5 × 10⁶ ind. m⁻²) than has previously reported for this location. There is a strong seasonal cycle in copepod biomass and copepods greater than 270 μm contribute less than 10% of the total biomass. Seasonal trends in copepod biomass lag those in the phytoplankton community with a delay of 3 to 4 months between the maximum phytoplankton biomass and the maximum copepod biomass. Grazing by copepods exceeds net primary production at the site and indicates that an additional advective supply of carbon is required to support the copepod community.