Petrogenesis of Cenozoic volcanic rocks in the NW sector of the Gharyan volcanic field, Libya

The north-western sector of the Gharyan volcanic field (northern Libya) consists of trachytic-phonolitic domes emplaced between similar to 41 and 38 Ma, and small-volume mafic alkaline volcanic centres (basanites, tephrites. alkali basalts. hawaiites and rare benmoreites) of Middle Miocene-Pliocene age (similar to 12-2 Ma). Two types of trachytes and phonolites have been recognized on the basis of petrography, mineralogy and geochemistry. Type-1 trachytes and phonolites display a smooth spoon-shaped REE pattern without negative Europium anomalies. Type-2 trachytes and phonolites show a remarkable Eu negative anomaly, higher concentration in HFSE (Nb-Ta-Zr-Hf), REE and Ti than Type-1 rocks. The origin of Type-1 trachytes and phonolites is compatible with removal of clinopyroxene, plagioclase, alkali feldspar, amphibole. magnetite and titanite starting from benmoreitic magmas. found in the same outcrops. Type-2 trachytes and phonolites could be the result of extensive fractional crystallization starting from mafic alkaline magma, without removal of titanite. In primitive mantle-normalized diagrams, the mafic rocks (Mg#= 62-68, Cr up to 514 ppm, Ni up to 425 ppm) show peaks at Nb and Ta and troughs at K. These characteristics, coupled with low Sr-87/Sr-86(i) (0.7033-0.7038) and positive epsilon(Nd) (from +4.2 to + 5.3) features typical of the mafic anorogenic magmas of the northern African plate and of HIMU-OIB-like magma in general. The origin of the mafic rocks is compatible from a derivation from low degree partial melting (3-9%) shallow mantle sources in the spinel/gamet facies. placed just below the rigid plate in the uppermost low-velocity zone. The origin of the igneous activity is considered linked to passive lithospheric thinning related to the development of continental rifts like those of Sicily Channel (e.g.. Pantelleria and Linosa) and Sardinia (e.g., Campidano Graben) in the Central-Western Mediterranean Sea. (C) 2012 Elsevier B.V. All rights reserved.

The Banhado Alkaline Complex, Southeastern Brazil: source and evolution of potassic SiO2-undersaturated high-Ca and low-Ca magmatic series

The Cretaceous Banhado alkaline complex in southeastern Brazil presents two potassic SiO2-undersaturated series. The high-Ca magmatic series consist of initially fractionated olivine (Fo(92-91)) + diopside (Wo(48-43)En(49-35)Ae(0-7)), as evidenced by the presence of xenocrysts and xenoliths. In that sequence, diopside (Wo(47-38)En(46-37)Ae(0-8)) + phlogopite + apatite + perovskite (Prv(> 92)) crystallized to form the phlogopite melteigite and led to the Ca enrichment of the magma. Diopside (Wo(47-41)En(32-24) Ae(3-14)) continued to crystallize as an early mafic mineral, followed by nepheline (Ne(74.8-70.1)Ks(26.3-21.2)Qz(7.6-0.9)) and leucite (Lc(65-56)) and subsequently by melanite and potassic feldspar (Or(85-99)Ab(1-7)) to form melanite ijolites, wollastonite-melanite urtites and melanite-nepheline syenites. Melanite-pseudoleucite-nepheline syenites are interpreted to be a leucite accumulation. Melanite nephelinite dykes are believed to represent some of the magmatic differentiation steps. The low-Ca magmatic series is representative of a typical fractionation of aegirine-augite (Wo(36-29)En(25-4)Ae(39-18)) + alkali feldspar (Or(57-96)Ab(3-43)) + nepheline (Ne(76.5-69.0)Ks(19.9-14.4)Qz(15.1-7.7)) + titanite from phonolite magma. The evolution of this series from potassic nepheline syenites to sodic sodalite syenites and sodalitolites is attributed to an extensive fractionation of potassic feldspar, which led to an increase of the NaCl activity in the melt during the final stages forming sodalite-rich rocks. Phonolite dykes followed a similar evolutionary process and also registered some crustal assimilation. The mesocratic nepheline syenites showed interactions with phlogopite melteigites, such as compatible trace element enrichments and the presence of diopside xenocrysts, which were interpreted to be due to a mixing/mingling process of phonolite and nephelinite magmas. The geochemical data show higher TiO2 and P2O5 contents and lower SiO2 contents for the high-Ca series and different LILE evolution trends and REE chondrite-normalized patterns as compared to the low-Ca series. The Sr-87/Sr-86, Nd-143/Nd-144, Pb-206/Pb-204 and Pb-208/Pb-204 initial ratios for the high-Ca series (0.70407-0.70526, 0.51242-0.51251, 17.782-19.266 and 38.051-39.521, respectively) were slightly different from those of the low-Ca series (0.70542-0.70583, 0.51232-0.51240, 17.758-17.772 and 38.021-38.061, respectively). For both series, a CO2-rich potassic metasomatized lithospheric mantle enriched the source with rutile-bearing phlogopite clinopyroxenite veins. Kamafugite-like parental magma is attributed to the high-Ca series with major contributions from the melting of the veins. Potassic nephelinite-like parental magma is assigned to the low-Ca series, where the metasomatized wall-rock played a more significant role in the melting process.

Petrology and Sr-Nd characteristics of the Nova Lacerda dike swarm, SW Amazonian Craton: new insights regarding its subcontinental mantle source and Mesoproterozoic geodynamics

The NNW-trending Nova Lacerda tholeiitic dike swarm in Mato Grosso State, Central Brazil, intrudes the Nova Lacerda granite (1.46 Ga) and the Jauru granite-greenstone terrain (ca. 1.79-1.77 Ga). The swarm comprises diabases I and II and amphibolites emplaced at ca. 1.38 Ga. Geochemical data indicate that these are evolved tholeiites characterized by high LILE/HSFE and LREE/HSFE ratios. Isotopic modelling yields positive epsilon(Nd)(T) values (+0.86 to +2.65), whereas values for epsilon(Sr)(T) range from positive to negative (+1.96 to -5.56). Crustal contamination did not play a significant petrogenetic role, as indicated by a comparison of isotopic data (Sr-Nd) from both dikes and country rocks, and by the relationship between isotopic and geochemical parameters (SiO2, K2O, Rb/Sr, and La/Yb) of the dikes. We attribute the origin of these tholeiites to fractional crystallization of evolved melts derived from a heterogeneous mantle source. Comparison of the geochemical and isotopic data of the studied swarm and other tholeiitic Mesoproterozoic mafic intrusions of the SWAmazonian Craton the Serra da Providencia, Colorado, and Nova Brasilandia bimodal suites - indicates that parental melts of the Nova Lacerda swarm were derived from the most enriched mantle source. This enrichment was probably caused by the stronger influence of the EMI component on the DMM end-member. These data, coupled with trace element bulk-rock geochemistry of the country rocks, and comparisons with the Colorado Complex of similar age, suggest a continental-margin arc setting for the emplacement of the Nova Lacerda dikes.

New constraints on the genesis and long-term stability of Os-rich alloys in the Earth's mantle

A variety of seemingly unrelated processes, such as core-mantle interaction, desulfurization, and direct precipitation from a silicate melt have been proposed to explain the formation of Ru-Os-Ir alloys (here referred to as osmiridiums) found in terrestrial mantle rocks. However, no consensus has yet been reached on how these important micrometer-sized phases form. In this paper we report the results of an experimental study on the solubilities of Ru, Os and Ir in sulfide melts (or mattes) as a function of alloy composition at 1300 degrees C. Considering the low solubilities of Ru, Os, and Ir in silicate melts, coupled with their high matte/silicate-melt partition coefficients, our results indicate that these elements concentrate initially at the ppm level in a matte phase in the mantle source region. During partial melting, the extraction of sulfur into silicate melt leads to a decrease in fS(2) that triggers the exsolution of osmiridiums from the refractory matte in the residue. The newly formed osmiridiums may persist in the terrestrial mantle for periods exceeding billions of years. (C) 2012 Elsevier Ltd. All rights reserved.

The trophic role of the squid Loligo plei as a keystone species in the South Brazil Bight ecosystem

The issue of whether loliginid squid can influence the average structure of marine ecosystems in a keystone role, i.e. a strong effect with relatively low biomass, has not yet been examined. Here, the diet of Loligo plei in inner shelf waters of the South Brazil Bight was examined, as a first step, based on the stomach contents of 2200 squid hand-jigged in shallow water (, 30 m) and taken as bycatch of shrimp trawlers in deeper water (30-100 m). Diet varied by size, season, and fishing zone. Stomachs were not empty in similar to 12%, with more empty during winter. The range of mantle lengths of squid caught by jigging (101-356 mm) appeared to differ from the squid trawled (30-236 mm), and the diet also differed. Food categories recorded in deeper water did not include amphipods or polychaetes, but in both fishing areas, fish were the most common prey. The fish prey identified included Trachurus lathami, small pelagic species, trichiurids, and Merluccius hubbsi. Demersal species, such as Ctenosciaena gracilicirrhus, and flatfish were also present. An ecosystem network model is updated through which a mixed-trophic impact matrix and ""keystoneness"" indicators were calculated. Loligo plei represents an important link between pelagic and demersal energy pathways, with high indices of keystoneness.

A new species of Chlamydoconcha Dall, 1884, from southeastern Brazil (Bivalvia: Chlamydoconchidae)

The second species in the genus Chlamydoconcha is described. Chlamydoconcha avalvis new species, occurs off the coast of Rio de Janeiro coast, in southeastern Brazil. The new species has very reduced valves acid a mantle surrounding the entire body, two features of the genus. The outer surface of the mantle lacks papillae except for a single cite located close to the excurrent siphon. These are distinctive characters of Chlamydoconcha orcutti Dall, 1884, from the eastern Pacific coast of North America, the single other known species of the genus. Some of the more interesting anatomical characters of the new species are: posterior pair of retractor muscles Of foot free front valves, absence of adductor muscles, gastric style sac totally separated from intestine, and the presence of a single (excurrent) siphon.

A New Species of Fissurella from Sao Pedro e Sao Paulo Archipelago, Brazil (Vetigastropoda, Fissurellidae)

Fissurella mesoatlantica n. sp. is endemic to the Saint Peter and Saint Paul Archipelago, Brazil, located approximately in the middle Atlantic (00 degrees 55N. 29 degrees 20W). The species is very similar to F. clenchi from the mainland Brazilian coast, differing in having a taller. more richly sculptured shell and by anatomical details. such its the papillae of mantle border and epipodial tentacles. A complete anatomical description is included.

PLICATULOSTREA, A NEW GENUS OF PLICATULIDAE (BIVALVIA: PECTINOIDEA) FROM THAILAND

Plicatulostrea onca, a new genus and species of Plicatulidae, is described based on shell and anatomy. The samples were collected under intertidal rocks at Kungkrabaen Bay, Thailand. Plicatulostrea differs from other plicatulids mainly in lacking undulations or shell sculpture, by long projections of the hinge teeth of the right valve and extreme body compression. Plicatulostrea onca has a radially arranged spots at the shell surface and typical plicatulid anatomy, differing mainly by the richness of papillae on the mantle edge and by invasion of the gonad along the mantle lobes. Some comments on the anatomy of the family are also provided.

Anatomical description of Solen cf. exiguus DUNKER from Thailand (Bivalvia: Solenidae)

An anatomical description of the solenid Solen cf. exiguus DUNKER 1862, collected in the Gulf of Thailand, is provided. The anatomical features include high antero-posterior elongation; mantle lobes widely fused with each other; powerful pallial muscles; fused siphons with capacity of autotomy; wide and partially hollow foot; complete separation between gastric style sac and adjacent portion of intestine; all possible common characters of the family. Some so far exclusive attributes include short anterior pallial tentacles; pair of muscular valves at base of siphons; and arrangement of tentacles at siphonal tip. A short discussion on the taxonomy of the Solen exiguus/leanus complex is also included.

COMPARATIVE ANATOMICAL STUDY OF TWO SPECIES OF SEMELE FROM THAILAND (BIVALVIA: TELLINOIDEA)

An anatomical description of two semelids from Thailand is presented, based on samples from Kungkrabaen Bay, Gulf of Thailand. The species are Semele sinensis A. Adams, 1853, and S. carnicolor (Hanley, 1845), both with Indo-Pacific distributions. Morphology in these two species is typically tellinoidean, each with a long internal ligamental element (resilium), a distance between the inner fold of the mantle edge and the other two folds, long and branched gastric ducts to the digestive diverticula, and a stomach diverticulum located in the posterodorsal corner of the gastric chamber, projecting posteriorly. The main anatomical differentiation between the two species is in the character of the labial palps.

Contrasted crustal sources as defined by whole-rock and Sr-Nd-Pb isotope geochemistry of neoproterozoic early post-collisional granitic magmatism within the Southern Brazilian Shear Belt, Camboriu, Brazil

The early phase of post-collisional granitic magmatism in the Camboriu region, south Brazil, is represented by the porphyritic biotite +/- hornblende Rio Pequeno Granite (RPG; 630-620 Ma) and the younger (similar to 610 Ma), equigranular, biotite +/- muscovite Serra dos Macacos Granite (SMG). The two granite types share some geochemical characteristics, but the more felsic SMG constitutes a distinctive group not related to RPG by simple fractionation processes, as indicated by its lower FeOt, TiO2, K2O/Na2O and higher Zr Al2O3, Na2O, Ba and Sr when compared to RPG of similar SiO2 range. Sr-Nd-Pb isotopes require different sources. The SMG derives from old crustal sources, possibly related to the Paleoproterozoic protoliths of the Camboriu Complex, as indicated by strongly negative epsilon Nd-t (-23 to -24) and unradiogenic Pb (e.g., Pb-206/Pb-204 = 16.0-16.3; Pb-207/Pb-204 = 15.3-15.4) and confirmed by previous LA-MC-ICPMS data showing dominant zircon inheritance of Archean to Paleoproterozoic age. In contrast, the RPG shows less negative epsilon Nd-t (-12 to -15) and a distinctive zircon inheritance pattern with no traces of post-1.6 Ga sources. This is indicative of younger sources whose significance in the regional context is still unclear; some contribution of mantle-derived magmas is indicated by coeval mafic dykes and may account for some of the geochemical and isotopic characteristics of the least differentiated varieties of the RPG. The transcurrent tectonics seems to have played an essential role in the generation of mantle-derived magmas despite their emplacement within a low-strain zone. It may have facilitated their interaction with crustal melts which seem to be to a large extent the products of reworking of Paleoproterozoic orthogneisses from the Camboriu Complex. (C) 2012 Elsevier Ltd. All rights reserved.

Magmatism and fenitization in the Cretaceous potassium-alkaline-carbonatitic complex of Ipanema So Paulo State, Brazil

The Ipanema alkaline-carbonatitic complex is part of the Meso-Cenozoic alkaline magmatism located within the southeastern part of the Brazilian Platform. Drill-core and field sampling have indicated the occurrence of glimmerites, with subordinate shonkinites (mela-syenites), clinopyroxene-bearing glimmerites, diorites and syenites. The glimmerites are cross-cut by lamprophyric dykes and calciocarbonatites. Fenitization has deeply affected the country rocks, originating dioritic and syenitic rocks. The Ipanema rocks show a distinct potassic affinity. The initial Sr-Nd- isotopic composition of the Ipanema rocks ((87)Sr/(86)Sr = 0.70661-0.70754 and (143)Nd/(144)Nd = 0.51169-0.51181) is similar to that of tholeiitic and potassium-rich-alkaline rocks of the Eastern Paraguay. Stable isotope data for the Ipanema calciocarbonatite suggest interaction with fluids at temperatures typical of hydrothermal stages, as hypothesized for other carbonatite complexes from southeastern Brazil. The chemical differences between the lamprophyre, glimmerites, carbonatites, apatitites and magnetitites, and the absence of marked REE enrichment in the evolved lithologies, all indicate that fractional crystallization and accumulus of liquidus phases in a magma reservoir, likely coupled with liquid immiscibility processes, may have played an important role in the genesis of the Ipanema rocks.

Ordovician A-type granitoid magmatism on the Ceara Central Domain, Borborema Province, NE-Brazil

We present field relationships, major and trace element geochemistry and U-Pb SHRIMP and ID-TIMS geochronology of the A-type Ordovician Quintas pluton located in the Ceara Central Domain of the Borborema Province, in northeastern Brazil. This pluton presents a concentric geometry and is composed mainly of syenogranite, monzogranite, quartz syenite to quartz monzodiorite, monzogabbro and diorite. Its geochemical characteristics [SiO2 (52-70%), Na2O/K2O (1.55-0.65), Fe2O3/MgO (2.2-7.3), metaluminous to sligthly alkaline affinity, post-collisional type in (Y + Nb) x Rb diagram, and A-type affinity (Ga > 22 ppm, Nb > 20 ppm, Zn > 60 ppm), REE fractioned pattern with negative Eu anomaly] are coherent with post-collisional A(2)-type granitoids. However, the emplacement of this pluton is to some extent temporally associated with the deposition of the first strata of the Parnaiba intracratonic basin, attesting also to a purely anorogenic character (A(1)-type granitoid). The emplacement of this pluton is preceded by one of the largest known orogenesis of the planet (Neoproterozoic Pan-African/Brasiliano) and, if it is classified as an A(2)-type granitoid, it provides interesting constraints about how long can last A(2)-type magmatic activity after a major collisional episode, arguably triggered by disturbance of the underlying mantle, a topic extensively debated in the geoscience community. (C) 2011 Elsevier Ltd. All rights reserved.

Magnetostratigraphy and mid-palaeolatitude VGP dispersion during the Permo-Carboniferous Superchron: results from Parana Basin (Southern Brazil) rhythmites

A detailed magnetostratigraphic and rock-magnetism study of two Late Palaeozoic rhythmite exposures (Itu and Rio do Sul) from the Itarare Group (Parana Basin, Brazil) is presented in this paper. After stepwise alterning-field procedures and thermal cleaning were performed, samples from both collections show reversed characteristic magnetization components, which is expected for Late Palaeozoic rocks. However, the Itu rocks presented an odd, flat inclination pattern that could not be corrected with mathematical methods based on the virtual geomagnetic pole (VGP) distributions. Correlation tests between the maximum anisotropy of the magnetic susceptibility axis (K1) and the magnetic declination indicated a possible mechanical influence on the remanence acquisition. The Rio do Sul sequence displayed medium to high inclinations and provided a high-quality palaeomagnetic pole (after shallowing corrections of f = 0.8) of 347.5 degrees E 63.2 degrees S (N = 119; A95 = 3.3; K = 31), which is in accordance with the Palaeozoic apparent wander pole path of South America. The angular dispersion (Sb) for the distribution of the VGPs calculated on the basis of both the 45 degrees cut-off angle and Vandamme method was compared to the best-fit Model G for mid-latitudes. Both of the Sb results are in reasonable agreement with the predicted (palaeo) latitudinal S-? relationship during the Cretaceous Normal Superchron (CNS), although the Sb value after the Vandamme cut-off has been applied is a little lower than expected. This result, in addition to those for low palaeolatitudes during the Permo-Carboniferous Reversed Superchron (PCRS) previously reported, indicates that the low secular variation regime for the geodynamo that has already been discovered in the CNS might have also been predominant during the PCRS.

Geochemistry and Pb-207/Pb-206 zircon ages of granitoids from the southern portion of the Tamboril-Santa Quiteria granitic-migmatitic complex, Ceara Central Domain, Borborema Province (NE Brazil)

The Tamboril-Santa Quiteria Complex is an important Neoproterozoic granitic-migmatitic unit from the Ceara Central Domain that developed from ca. 650 to 610 Ma. In general the granitoids range in composition from diorite to granite with predominance (up to 85%) of granitic to monzogranitic composition with biotite as the main mafic AFM phase. Geochemical and Pb-207/Pb-206 evaporation zircon geochronology studies were applied in a group of these abundant monzogranitic rocks from the region of Novo Oriente in the southern portion of the Ceara Central Domain. In this area the granitoids are weakly peraluminous biotite granitoids and deformed biotite granitoids of high-K calc-alkaline and ferroan composition, which we interpreted as primary magmas (segregated diatexites) derived from the partial melting of crustal material. The close temporal relation of this magmatism with local eclogitic and regional high temperature metamorphism in Ceara Central Domain point out to an orogenic setting, arguably emplaced during the collisional stage. Subordinate coeval juvenile mantle incursions are also present. This crustally derived magmatism is the primary product of the continental thickening that resulted from the collision between the rocks represented by the Amazonian-West African craton (Sao Luiz cratonic fragment) to the northwest and the Paleoproterozoic-Archean basement of the Borborema Province to the southeast along the Transbrasiliano tectonic corridor. (C) 2011 Elsevier Ltd. All rights reserved.