Isotope geochemistry of Paleoproterozoic metacarbonates from Itatuba,Borborema Province,Northeastern Brazil: Evidence of marble melting within a collisional suture

Strongly-deformed marbles may be easily confused with linear and elongated carbonatite intrusions. Both rocks may present similar texture and foliation to the host rock, or even cross cutting field relationships, which could be interpreted either as igneous or high-grade metamorphosed marble. Diagnostic criteria are even more complex when there is evidence of melting of the metasedimentary carbonate rock, such as has been described in the Himalayas and in the Eastern Ghats, India.In the Alto Moxotó Terrane, a high-grade gneissic domain of the Borborema Province, Northeastern Brazil, there are metacarbonates associated with banded gneisses and different metaplutonic rocks. Field evidence indicates the absence of other metasedimentary rocks associated with these marbles, thus suggesting that these carbonates were separated from other siliciclastic metasedimentary rocks. The presence of marble also suggests that it may represent the initial stage of a crustal carbon recycling into the mantle. These marbles present many field similarities to carbonatites (e.g., fluid-flow structure) and, together with metagranites and metamafic intrusions, may represent a major collisional tectonic suture.A detailed study of the carbon, oxygen and strontium isotopic composition of these marbles is presented. This study aims to identify the origin of the different isotopic components. It is argued that these rocks were subjected to temperature and pressure conditions that were sufficiently high to have melted them. The isotopic data presented here support this interpretation and indicate the mixing of two components: (i) one characterized by radiogenic Sr isotopes and mantle-like carbon isotopes, which is associated with the gneissic and mafic rocks, and (ii) another characterized by low ⁸⁷Sr/⁸⁶Sr ratios and highly positive δ¹³C values. Available geochemical data for the upper Paleoproterozoic indicate that the ⁸⁷Sr/⁸⁶Sr ratio of ocean water, varying between 0.7050 (2.25 ± 0.25 Ga) and 0.7047 (1.91 Ga), falls within the lower range of the samples from Itatuba and thus reinforces the interpretation that these marbles are sedimentary-derived and were partially contaminated by interaction with the host gneissic and mafic rocks.     

A review of new interpretations of the tectonostratigraphy,geochemistry and evolution of the Onverwacht Suite,Barberton Greenstone Belt,South Africa

The Paleoarchean (ca. 3.5–3.3 Ga) Onverwacht Suite (OS) of the Barberton Greenstone Belt consists of a 15‐km thick imbricate tectonic stack of seven complexes consisting predominantly of volcanic rocks and intrusions. Tectonostratigraphically from base to top they are the Sandspruit, Theespruit, Komati, Hooggenoeg, Noisy, Kromberg and Mendon Complexes. The Hooggenoeg and Noisy Complexes in the middle of the OS are separated by a significant unconformity resulting from the uplift of the submarine lavas and deep erosion, demonstrating the onset of tectonic accretion prior to 3455 Ma. The basic lavas of the tectonostratigraphic lower (Theespruit, Sandspruit and Komati) and upper (Mendon) complexes are composed of komatiite, komatiitic basalt and high-MgO basalt, whereas those in the middle part (Hooggenoeg and Kromberg) are predominantly high- to low-MgO tholeiitic basalts. Felsic volcanic rocks and intrusions are important in two of the complexes (Theespruit and Noisy). The ultramafic to basaltic lavas show REE patterns that are almost flat and resemble those of modern MORB, whereas those of the felsic rocks are flat from Lu to Gd and moderately to strongly enriched in LREE, similar to modern arcs. Average ε_{Nd (T)} values are close to depleted mantle growth curves. In MORB-normalised multi-element diagrams, the komatiitic to basaltic rocks exhibit flat patterns from Lu through La and consistent relative enrichment in the elements Pb, U, Th, Ba and Cs. Apart from the Komati Complex, the majority of the lavas show significant negative Nb and Ta anomalies. Enrichment in non-conservative incompatible elements (Cs, Ba, Th, LREE) relative to conservative elements (Ta, Nb, Zr, Hf, Ti, Y, HREE) shows that the komatiitic to basaltic magmas were generated from metasomatised mantle above subducting altered oceanic crust. The geochemistry of the felsic rocks indicates an origin by melting of subducted amphibolite and eclogite. The tectonostratigraphy and the geochemical characteristics of the lavas and intrusions are consistent with successive obduction and accretion of segments of oceanic crust formed in back-arc basins and volcanic arcs.     

Earth's oldest preserved unconformity — Prospect of a beginning in the tectono-sedimentary continental cycle?

Earth's oldest preserved conglomerates and basaltic pillow lavas at Isua, Greenland, provide robust field evidence for deep- and shallow-water environments on our planet within the first billion years of its formation. The conglomerates represent the first Paleoarchaean archive of sub-aerial erosion and shallow water sedimentation. These ca. 3700 million year old sedimentary rocks, now metamorphosed at amphibolite grade, comprise units of rounded quartz pebbles set in a sandy to muddy matrix that unconformably overlie an ophiolite sequence. The pillow lavas of the ophiolite are variolitic and essentially vesicle-free, indicative of formation in a deep water environment. Locally, an unconformity separates the conglomerates from the deformed ophiolite-related rocks; elsewhere the contact between these units is tectonic. Such field relationship between deep and shallow water environments resemble those preserved in younger orogenic belts, where obducted oceanic crust has been tectonically emplaced across terrestrial platforms or subaerial parts of forearc and backarc basins.     

Seasonal Diet Shifts and Overlap Between Two Sympatric Catfishes in an Estuarine Nursery

This study describes the seasonal feeding habits of different size classes of Cathorops spixii and Cathorops agassizii along an estuarine ecocline and the food overlap when different size classes occur together. These species were essentially zoobenthivorous, feeding mainly on Polychaeta Nereis sp., Copepoda Pseudodiaptomus acutus, Ostracoda Asterope sp., Gastropoda: Littorinidae, and Bivalvia Mytilus sp. and Anomalocardia brasiliana. However, during their life cycle and between different habitats and seasons, their trophic guild can change to zooplanktivore. The competition for resources was observed among C. spixii and C. agassizii, but was significantly reduced due to the seasonal diference in habitat use by different ontogenetic phases in the main channel of the estuary. The food niche separation was strongly influenced by environmental fluctuations, principally of salinity, resulting from rainfall and river inflow. High abundance of some preys, such as P. acutus (all estuary), Asterope sp. (upper and middle estuary), and Littorinidae (upper and lower estuary), could influence the significant diet overlap, principally during the rainy season, and call for more detailed studies of the benthic community structure. Moreover, dietary overlap was observed mainly between smaller sizes (intraspecific and/or interspecific) or between corresponding ontogenetic phases (interspecific), suggesting some differentiation in the diet in relation to the size class. Differences in prey type and size between the different ontogenetic phases of these ariids, principally among juveniles and adults, could be related to the size of the mouth, since adults are able to successfully capture larger preys or larger quantities of particular items.     

The Contribution of Groundwater Discharge to Nutrient Exports from a Coastal Catchment: Post-Flood Seepage Increases Estuarine N/P Ratios

Four months of daily nutrient and radon (a natural groundwater tracer) observations at the outlet of a heavily drained coastal wetland illustrated how episodic floods and diffuse groundwater seepage influence the biogeochemistry of a sub-tropical estuary (Richmond River, New South Wales, Australia). Our observations downstream of the Tuckean Swamp (an acid sulphate soil floodplain) covered a dry stage, a flood triggered by a 213-mm rain event and a post-flood stage when surface water chemistry was dominated by groundwater discharge. Significant correlations were found between radon and ammonium and N/P ratios and between radon and dissolved organic nitrogen (DON) during the post-flood stage. While the flood lasted for 14?% of the time of the surface water time series, it accounted for 18?% of NH₄, 32?% of NO _x , 66?% of DON, 58?% of PO₄ and 55?% of dissolved organic phosphorus (DOP) catchment exports. Over the 4-month study period, groundwater fluxes of 35.0, 3.6, 36.3, 0.5 and 0.7?mmol?m^?2?day^?1 for NH₄, NO _x , DON, PO₄ and DOP, respectively, were estimated. The groundwater contribution to the total surface water catchment exports was nearly 100?% for ammonium, and <20?% for the other nutrients. Post-flood groundwater seepage shifted the system from a DON to a dissolved inorganic N-dominated system and doubled N/P ratios in surface waters. We hypothesise that the Richmond River Estuary N/P ratios may reflect a widespread trend of tidal rivers and estuaries becoming more groundwater-dominated and phosphorus-limited as coastal wetlands are drained for agriculture, grazing and development.     

Assessing Decomposition Rates of Rhizophora mangle and Laguncularia racemosa Leaves in a Tropical Mangrove

Studies exploring leaf decomposition in mangroves in the Brazilian northeast are scarce. Therefore, in a mangrove of the Cachoeira River estuary at Ilhéus, Bahia, the decomposition of senescent leaves of Rhizophora mangle L. and Laguncularia racemosa Gaertn was studied. The litter-bag method and the Olson exponential model were applied to estimate the decomposition rates. The decay pattern of leaf material of both species was similar to those reported in the literature, with rapid initial weight loss, followed by a deceleration and period of slow weight loss. L. racemosa had the highest decay constants at all the study sites which suggest that their leaves are more easily degraded than those of R. mangle. The constantly submerged site presented the highest decay constants of 0.022 g g^?1 day^?1 for R. mangle and 0.031 g g^?1 day^?1 for L. racemosa. Most (95 %) of the leaf material of the two species was lost in 135 and 98 days, respectively. Benthic macrofauna organisms were recorded in the litter bags from the fifth day after the start of the experiment. Such organisms play a key role in the first phase of decomposition in this mangrove system. The decay constants of the leaf litter were generally higher than those reported in other tropical and subtropical regions. This suggests that differences in the process of decomposition are in response to environmental conditions at each location.     

Performance enhancement of a GIS-based facility location problem using desktop grid infrastructure

This paper presents the integration of desktop grid infrastructure with GIS technologies, by proposing a parallel resolution method in a generic distributed environment. A case study focused on a discrete facility location problem, in the biomass area, exemplifies the high amount of computing resources (CPU, memory, HDD) required to solve the spatial problem. A comprehensive analysis is undertaken in order to analyse the behaviour of the grid-enabled GIS system. This analysis, consisting of a set of the experiments on the case study, concludes that the desktop grid infrastructure is able to use a commercial GIS system to solve the spatial problem achieving high speedup and computational resource utilization. Particularly, the results of the experiments showed an increase in speedup of fourteen times using sixteen computers and a computational efficiency greater than 87 % compared with the sequential procedure.     

A Top-Cut Model for Deposits with Heavy-Tailed Grade Distribution

In some ore deposits, the grade distribution is heavy-tailed and high values make the inference of first- and second-order statistics nonrobust. In the case of gold data, high values are usually cut and the block estimation is performed using truncated grades. With this method, a part of the deposit is omitted, resulting in a potential bias on resources figures. Ad-hoc corrections can be applied on the final figures to take into account the tail of the grade distribution, but no theoretical guidelines are available. A geostatistical model has been developed to handle high values based on the assumption that for high grade zones, the only tangible information is the geometry. The grade variable z can be split into three parts: the truncated grade ( $\operatorname{Min} (z, z_{\mathrm{e}})$ where z _e is the top-cut grade), a weighted indicator above top-cut grade (1{z≥z _e}), and a residual. Within this framework, the residual is poorly structured, and in most cases is a pure nugget-effect. Moreover, it has no spatial correlation with the truncated grade and the indicator above top-cut grade. This decomposition makes the variographic study more robust because variables (indicator and truncated grade) do not keep high grade values. The underlying hypotheses can be tested on experimental indicator variograms and the top-cut grade can be justified. Finally, the estimation is based on a truncated grade and indicator cokriging. The model is applied to blast holes from a gold deposit and on a simulated example. Both cases illustrate the benefits of keeping the high values in the estimation process via an appropriate mathematical model.     

New geological model of the Lagoa Real uraniferous albitites from Bahia (Brazil)

New evidence supported by petrography (including mineral chemistry), lithogeochemistry, U-Pb geochronology by Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS), and physicochemical study of fluid and melt inclusions by LA-ICP-MS and microthermometry, point to an orogenic setting of Lagoa Real (Bahia-Brazil) involving uraniferous mineralization. Unlike the previous models in which uraniferous albitites represent Na-metasomatised 1.75 Ga anorogenic granitic rocks, it is understood here that they correspond to metamorphosed sodium-rich and quartz-free 1.9 Ga late-orogenic syenitic rocks (Na-metasyenites). These syenitic rocks are rich not only in albite, but also in U-rich titanite (source of uranium). The interpretation of geochemical data points to a petrogenetic connection between alkali-diorite (local amphibolite protolith) and sodic syenite by fractional crystallization through a transalkaline series. This magmatic differentiation occurred either before or during shear processes, which in turn led to albitite and amphibolite formation. The metamorphic reactions, which include intense recrystallization of magmatic minerals, led uraninite to precipitate at 1.87 Ga under Oxidation/Reduction control. A second population of uraninites was also generated by the reactivation of shear zones during the 0.6 Ga Brasiliano Orogeny. The geotectonic implications include the importance of the Orosirian event in the Paramirim Block during paleoproterozoic S?o Francisco Craton edification and the influence of the Brasiliano event in the Paramirim Block during the West-Gondwana assembly processes. The regional microcline-gneiss, whose protolith is a 2.0 Ga syn-collisional potassic granite, represents the albitite host rock. The microcilne-gneiss has no petrogenetic association to the syenite (albitite protolith) in magmatic evolutionary terms.