Multiple plume events in the genesis of the peri-Caribbean Cretaceous oceanic plateau province

The oceanic crust fragments exposed in central America, in north-western South America, and in the Caribbean islands have been considered to represent accreted remnants of the Caribbean-Colombian Oceanic Plateau (CCOP). On the basis of trace element and Nd, Sr, and Pb isotopic compositions we infer that cumulate rocks, basalts, and diabases from coastal Ecuador have a different source than the basalts from the Dominican Republic. The latter suite includes the 86 Ma basalts of the Duarte Complex which are light rare earth element (REE) -enriched and display (relative to normal mid-ocean ridge basalts, NMORB) moderate enrichments in large ion lithophile elements, together with high Nb, Ta, Pb, and low Th contents. Moreover, they exhibit a rather restricted range of Nd and Pb isotopic ratios consistent with their derivation from an ocean island-type mantle source, the composition of which includes the HIMU (high U-238/Pb-204) component characteristic of the Galapagos hotspot. In contrast, the 123 Ma Ecuadorian oceanic rocks have flat REE patterns and (relative to NMORB) are depleted in Zr, Hf, Th, and U. Moreover, they show a wide range of Nd and Pb isotopic ratios intermediate between those of ocean island basalts and NMORB. It is unlikely, on geochemical grounds, that the plume source of the Ecuadorian fragments was similar to that of the Galapagos. In addition, because of the NNE motion of the Farallon plate during the Early Cretaceous, the Ecuadorian oceanic plateau fragments could not have been derived from the Galapagos hotspot but were likely formed at a ridge-centered or near-ridge hotspot somewhere in the SE Pacific.

Oxygen isotope thermometry in carbonatites, Fuerteventura, Canary Islands, Spain

Crystallization temperatures of the oceanic carbonatites of Fuerteventura, Canary Islands, have been determined from oxygen isotope fractionations between calcite, silicate minerals (feldspar, pyroxene, biotite, and zircon) and magnetite. The measured fractionations have been interpreted in the light of late stage interactions with meteoric and/or magmatic water. Cathodoluminescence characteristics were investigated for the carbonatite minerals in order to determine the extent of alteration and to select unaltered samples. Oxygen isotope fractionations of minerals of unaltered samples yield crystallization temperatures between 450 and 960degreesC (average 710degreesC). The highest temperature is obtained from pyroxene-calcite pairs. The above range is in agreement with other carbonatite thermometric Studies. This is the first study that provides oxygen isotope data coupled with a CL study on carbonatite-related zircon. The CL pictures revealed that the zircon is broken and altered in the carbonatites and in associated syenites. Regarding geological field evidences of syenite-carbonatite relationship and the close agreement of published zircon U/Pb and whole rock and biotite K/Ar and Ar-Ar age data, the most probable process is early zircon crystallization from the syenite magma and late-stage reworking during magma evolution and carbonatite segregation. The oxygen isotope fractionations between zircon and other carbonatite minerals (calcite and pyroxene) support the assumption that the zircon would correspond to the early crystallization of syenite-carbonatite magmas.

The tectonically controlled emplacement of a vertically sheeted gabbro-pyroxenite intrusion: Feeder-zone of an ocean-island volcano (Fuerteventura, Canary Islands)

The Miocene PX1 gabbro-pyroxenite pluton, Fuerteventura, Canary Islands, is a 3.5 x 5.5 km shallow-level intrusion (0.15-0.2 GPa and 1100-1120 degrees C), interpreted as the feeder-zone to an ocean-island volcano. It displays a vertical magmatic banding expressed in five 50 to 100 metre-wide NNE-SSW trending alkaline gabbro sequences alternating with pyroxenites. This emplacement geometry was controlled by brittle to ductile shear zones, generated by a regional E-W extensional tectonic setting that affected Fuerteventura during the Miocene. At a smaller scale, the PX1 gabbro and pyroxenite bands consist of metre-thick differentiation units, which suggest emplacement by periodic injection of magma pulses as vertical dykes that amalgamated, similarly to a sub-volcanic sheeted dyke complex. Individual dykes underwent internal differentiation following a solidification front parallel to the dyke edges. This solidification front may have been favoured by a significant lateral/horizontal thermal gradient, expressed by the vertical banding in the gabbros, the fractionation asymmetry within individual dykes and the migmatisation of the wall rocks. Pyroxenitic layers result from the fractionation and accumulation of clinopyroxene +/- olivine +/- plagioclase crystals from a mildly alkaline basaltic liquid. They are interpreted as truncated differentiation sequences, from which residual melts were extracted at various stages of their chemical evolution by subsequent dyke intrusions, either next to or within the crystallising unit. Compaction and squeezing of the crystal mush is ascribed to the incoming and inflating magma pulses. The expelled interstitial liquid was likely collected and erupted along with the magma flowing through the newly injected dykes. Clinopyroxene mineral orientation - as evidenced by EBSD and micro X-ray tomography investigations - displays a marked pure-shear component, supporting the interpretation of the role of compaction in the generation of the pyroxenites. Conversely, gabbro sequences underwent minor melt extraction and are believed to represent crystallised coalesced magma batches emplaced at lower rates at the end of eruptive cycles. Clinopyroxene orientations in gabbros record a simple shear component suggesting syn-magmatic deformation parallel to observed NNE-SSW trending shear zones induced by the regional tensional stress field. This emplacement model implies a crystallisation time of 1 to 5 years for individual dykes, consistent with PX1 emplacement over less than 0.5 My. A minimum amount of approximately 150 km(3) of magma is needed to generate the pluton, part of it having been erupted through the Central Volcanic Centre of Fuerteventura. If the regional extensional tectonic regime controls the PX1 feeder-zone initiation and overall geometry, rates and volumes of magma depend on other, source-related factors. High injection rates are likely to induce intrusion growth rates larger than could be accommodated by the regional extension. In this case, dyke intrusion by propagation of a weak tip, combined with the inability of magma to circulate through previously emplaced and crystallised dykes could result in an increase of non-lithostatic pressure on previously emplaced mushy dyke walls; thus generating strong pure-shear compaction within the pluton feeder-zone and interstitial melt expulsion. These compaction-dominated processes are recorded by the cumulitic pyroxenite bands. (C) 2010 Elsevier B.V. All rights reserved.

Epidemiology and prevention of cardiovascular disease in the Seychelles Islands (Indian Ocean)

This article presents selected findings and lessons from a cardiovascular research and prevention program initiated in 1989 in the Republic of Seychelles, a country in demographic and epidemiological transition. Rapid and sustained aging of the population (e.g., two-fold increase of people aged 30-39 from 1979 to 1995) implies, over the next few decades, further dramatic increase of the burden of chronic diseases, particularly cardiovascular disease (CVD). Epidemiological surveillance shows high age-specific rates of CVD (particularly stroke), high prevalence of peripheral atherosclerosis (plaques in carotid and femoral arteries), high prevalence of classical modifiable risk factors in the adult population (particularly hypertension), and substantial proportions of children with overweight. Stagnant life expectancy in men and an increase in women have been observed over the last two decades; this occurred despite largely improved health services and reduced infant mortality rates, and may reflect the large CVD burden found in middle-aged men (less so in middle-aged women). A national program of prevention of CVD has been initiated since 1991, which includes a mix of interventions to reduce risk factors in the general population and in high-risk individuals. Substantial research to back the prevention program indeed shows, at the moment, epidemiological patterns in Seychelles similar to those observed in Western countries (e.g., an association between peripheral atherosclerosis [as a proxy of CVD] and low density lipoprotein-cholesterol, smoking, diabetes, and [inversely] walking). This clearly supports the view that promotion of healthy lifestyles and control of conventional risk factors should be the main targets for CVD prevention and control.

Shallow-level migmatization of gabbros in a metamorphic contact aureole, Fuerteventura Basal Complex, Canary Islands

Migmatization of gabbroic rocks at 2-3 kbar has occurred in the metamorphic contact aureole of a mafic pluton in the Fuerteventura Basal Complex (Canary Island;). Migmatites are characterized by a dense network: of closely spaced millimetre-wide leucocratic veins with perfectly preserved igneous textures. They are all relatively enriched in Al, Na I: Sr Ba, Nb, Y and the rare earth elements compared with the unaffected country rock beyond the aureole. Migmatization under such low-pressure conditions war possible because of the unusual tectonic and magmatic contact in which ii occurred. Multiple basic intrusions associated with extrusive volcanic activity created high heat flow in a small area. Alkaline and metasomatized rocks present in the country rock of the intruding pluton were leached by high-temperature fluids during contact metamorphism. These enriched fluids then favoured partial melting of the host gabbroic rocks, and contaminated both the leucosomes and melanosomes. A transpressive tectonic setting at the time of intrusion created shearing along the contact between the intrusion and its host rock. This shearing enhanced circulation of the fluids and allowed segregation of the nea-formed melts from their restite by opening tension veins into which the melts migrated. Depending on the relative timing of melt segregation and recrystallization leucosomes range in composition from a 40-60% mixture of clinopyroxene (+/- amphibole) and plagioclase to almost pure feldspathic veins. Comparable occurrences of gabbros migmatized at low pressure are expected only at a snail scale in localized areas of high heat flow in the presence of fluids, such as in. mid-ocean ridges or ocean-islands.

Low-pressure, water-assisted anatexis of basic dykes in a contact metamorphic aureole, Fuerteventura (Canary Islands): oxygen isotope evidence for a meteoric fluid origin

Migmatites produced by low-pressure anatexis of basic dykes are found in a contact metamorphic aureole around a pyroxenite-gabbro intrusion (PX2), on Fuerteventura. Dykes outside and inside the aureole record interaction with meteoric water, with low or negative delta O-18 whole-rock values (+0.2 to -3.4 parts per thousand), decreasing towards the contact. Recrystallised plagioclase, diopside, biotite and oxides, from within the aureole, show a similar evolution with lowest delta O-18 values (-2.8, -4.2, - 4.4 and -7.6 parts per thousand, respectively) in the migmatite zone, close to the intrusion. Relict clinopyroxene phenocrysts preserved in all dykes, retain typically magmatic delta O-18 values up to the anatectic zone, where the values are lower and more heterogeneous. Low delta O-18 values, decreasing towards the intrusion, can be ascribed to the advection of meteoric water during magma emplacement, with increasing fluid/rock ratios (higher dyke intensities towards the intrusion acting as fluid-pathways) and higher temperatures promoting increasing exchange during recrystallisation.

An experimental study on the shallow-level migmatization of ferrogabbros from the Fuerteventura Basal Complex, Canary Islands

Spectacular shallow-level migmatization of ferrogabbroic rocks occurs in a metamorphic contact aureole of a gabbroic pluton of the Tierra Mala massif (TM) on Fuerteventura (Canary Islands). In order to improve our knowledge of the low pressure melting behavior of gabbroic rocks and to constrain the conditions of migmatization of the TM gabbros, we performed partial melting experiments on a natural ferrogabbro, which is assumed as protolith of the migmatites. The experiments were performed in an internally heated pressure vessel (IHPV) at 200 MPa, 930-1150 degreesC at relatively oxidizing conditions. Distinct amounts of water were added to the charge. From 930 to 1000 degreesC, the observed experimental phases are plagioclase (An(60-70)), clinopyroxene, amphibole (titanian magnesiohastingsites), two Fe-Ti oxides, and a basaltic, K-poor melt. Above 1000 degreesC, amphibole is no longer stable. The first melts are very rich in non-native plagioclase (>70 wt.%). This indicates that at the beginning of partial melting plagioclase is the major phase which is consumed to produce melt. In the experiments, plagioclase is stable up to high temperatures (1060 degreesC) showing increasing An content with temperature. This is not compatible with the natural migmatites, in which An-rich plagioclase is absent in the melanosomes, while amphibole is stable. Our results show that the partial melting of the natural rocks cannot be regarded as an ``in-situ'' process that occurred in a closed system. Considerable amounts of alkalis probably transported by water-rich fluids, derived from the mafic pluton underplating the TM gabbro, were necessary to drive the melting reaction out of the stability range of plagioclase. A partial melting experiment with a migmatite gabbro showing typical ``in-situ'' textures as starting material supports this assumption. Crystallization experiments performed at 1000 degreesC on a glass of the fitised ferrogabbro with different water contents added to the charge show that generally high water activities could be achieved (crystallization of amphibole), independently of the bulk water content, even in a system with very low initial bulk water content (0.3 wt.%). Increasing water contents produce plagioclase richer in An, reduces the modal proportion of plagioclase in the crystallizing assemblage and extends the melt fraction. High melt fractions of >30 wt.% could only be observed in systems with high bulk water contents (> - 2 wt.%). This indicates that the migmatites were generated under water-rich conditions (probably water-saturated), since those migmatites, which are characterized as ``in-situ'' formations, show generally high amounts of leucosomes (>30 wt.%). (C) 2003 Elsevier B.V. All rights reserved.

Accreted fragments of the Late Cretaceous Caribbean-Colombian Plateau in Ecuador

The eastern part of the Western Cordillera of Ecuador includes fragments of an Early Cretaceous ( approximate to 123 Ma) oceanic plateau accreted around 85-80 Ma (San Juan unit). West of this unit and in fault contact with it, another oceanic plateau sequence (Guaranda unit) is marked by the occurrence of picrites, ankaramites, basalts, dolerites and shallow level gabbros. A comparable unit is also exposed in northwestern coastal Ecuador (Pedernales unit). Picrites have LREE-depleted patterns, high epsilonNd(i) and very low Pb isotopic ratios, suggesting that they were derived from an extremely depleted source. In contrast, the ankaramites and Mg-rich basalts are LREE-enriched and have radiogenic Pb isotopic compositions similar to the Galapagos HIMU component; their epsilonNd(i) are slightly lower than those of the picrites. Basalts, dolerites and gabbros differ from the picrites and ankaramites by flat rare earth element (REE) patterns and lower epsilonNd; their Pb isotopic compositions are intermediate between those of the picrites and ankaramites. The ankaramites, Mg-rich basalts, and picrites differ from the lavas from the San Juan-Multitud Unit by higher Pb ratios and lower epsilonNd(i). The Ecuadorian and Gorgona 88-86 Ma picrites are geochemically similar. The Ecuadorian ankaramites and Mg-rich basalts share with the 92-86 Ma Mg-rich basalts of the Caribbean-Colombian Oceanic Plateau (CCOP) similar trace element and Nd and Pb isotopic chemistry. This suggests that the Pedernales and Guaranda units belong to the Late Cretaceous CCOR The geochemical diversity of the Guaranda and Pedernales rocks illustrates the heterogeneity of the CCOP plume source and suggests a multi-stage model for the emplacement of these rocks. Stratigraphic and geological relations strongly suggest that the Guaranda unit was accreted in the late Maastrichtian (approximate to 68-65 Ma). (C) 2002 Elsevier Science B.V. All rights reserved.

Late Jurassic oceanic crust and Upper Cretaceous Caribbean plateau picritic basalts exposed in the Duarte igneous complex, Hispaniola

Four distinct rock units have been recognized near El Aguacate, in the Janico-Juncalito-La Vega area of the Duarte complex (Dominican Republic): (1) serpentinites crosscut by numerous diabasic dikes, (2) basalts interbedded with Late Jurassic ribbon cherts, (3) picrites and ankaramites relatively enriched in incompatible trace elements, and (4) amphibolites and gneissic amphibolites chemically similar to Oceanic Plateau Basalts. Similar Ar-Ar ages of late magmatic amphibole from a picrite, and hornblende from an amphibolite (86.1 +/- 1.3 Ma and 86.7 +/- 1.6 Ma, respectively), suggest that the Duarte picrites are contemporaneous with the Deep Sea Drilling Program Leg 15 and Ocean Drilling Program Leg 126 basalts drilled from the Caribbean oceanic plateau. These basalts are associated with sediments containing Late Cretaceous faunas. Sr, Nd, and Pb data show that enriched picrites and amphibolites are isotopically similar to mafic lavas from previously described Caribbean plateau and Galapagos hotspot basalts. Major element, trace element, and lead isotopic features of Late Jurassic basalts and diabases are consistent with those of normal oceanic crust basalt. However, these basalts differ from typical N-MORB because they have lower epsilon Nd ratios that plot within the range of Ocean Island Basalts. These rocks appear to represent remnants of the Caribbean Jurassic oceanic crust formed from an oceanic ridge possibly close to a hotspot. Later, they were tectonically juxtaposed with Late Cretaceous slices of the Caribbean-Colombian plateau.

Genetic structure of Gymnures (genus Hylomys; Erinaceidae) on continental islands of Southeast Asia: Historical effects of fragmentation

Eustatic sea level changes during Pleistocene climatic fluctuations produced several cycles of connection-isolation among continental islands of the Sunda shelf. To explore the potential effects of these fluctuations, we reconstructed a model of the vicariant events that separated these islands, based on bathymetric information. Among many possible scenarios, two opposite phylogenetic patterns of evolution were predicted for terrestrial organisms living in this region: one is based on the classical allopatric speciation mode of evolution, while the other is the outcome of a sequential dispersal colonization of the archipelago. We tested the applicability of these predictions with an analysis of sequence variation of the cytochrome b gene from several taxa of Hylomys. They were sampled throughout SE-Asia and the Sunda islands. High levels of haplotype differentiation characterize the different island taxa. Such levels of differentiation support the existence of several allopatric species, as was suggested by previous allozyme and morphological data. Also in accordance with previous results, the occurrence of two sympatric species from Sumatra is suggested by their strongly divergent haplotypes. One species, Hylomys suillus maxi, is found both on Sumatra and in Peninsular Malaysia, while the other, H. parvus, is endemic to Sumatra. Its closest relative is H. suillus dorsalis from Borneo. Phylogenetic reconstructions also demonstrate the existence of a Sundaic clade composed of all island taxa, as opposed to those from the continent. Although there is no statistical support for either proposed biogeographic model of evolution, we argue that the sequential dispersal scenario is more appropriate to describe the genetic variation found among the Hylomys taxa. However, despite strong differentiation among island haplotypes, the cladistic relationships between some island taxa could not be resolved. We argue that this is evidence of a rapid radiation, suggesting that the separation of the islands may have been perceived as a simultaneous event rather than as a succession of vicariant events. Furthermore, the estimates of divergence times between the haplotypes of these taxa suggest that this radiation may actually have predated the climatic fluctuations of the Pleistocene. Further refinement of the initial palaeogeographic models of evolution are therefore needed to account for these results.

Electrocardiographic findings in a middle-aged African population in the Seychelles islands.

This study describes major electrocardiogram (ECG) measurements and diagnoses in a population of African individuals; most reference data have been collected in Caucasian populations and evidence exists for interethnic differences in ECG findings. This study was conducted in the Seychelles islands (Indian Ocean) and included 709 black individuals (343 men and 366 women) aged 25 to 64 years randomly selected from the general population. Resting ECG were recorded by using a validated ECG unit equipped with a measurement and interpretation software (Cardiovit AT-6, Schiller, Switzerland). The epidemiology of 14 basic ECG measurements, 6 composite criteria for left ventricular hypertrophy and 19 specific ECG diagnoses including abnormal rhythms, conduction abnormalities, repolarization abnormalities, and myocardial infarction were examined. Substantial gender and age differences were found for several ECG parameters. Moreover, tracings recorded in African individuals of the Seychelles differed from those collected similarly in Caucasian populations in many respects. For instance, heart rate was approximately 5 beats per minute lower in the African individuals than in selected Caucasian populations, prevalence of first degree atrio-ventricular block was especially high (4.8%), and the average Sokolow-Lyon voltage was markedly higher in African individuals of the Seychelles compared with black and white Americans. The integrated interpretation software detected "old myocardial infarction" in 3.8% of men and 0% of women and "old myocardial infarction possible" in 6.1% and 3%, respectively. Cardiac infarction injury scores are also provided. In conclusion, the study provides reference values for ECG findings in a specific population of people of African descent and stresses the need to systematically consider gender, age, and ethnicity when interpreting ECG tracings in individuals.

Assessing sedimentary evolution by means of Sr-isotope ratios : 3 case studies on the Caribbean plate : (Cretaceous: Nicoya Peninsula, Costa Rica, Tertiary: Hess Rise, and La Désirade, Guadeloupe, France)

The understanding of sedimentary evolution is intimately related to the knowledge of the exact ages of the sediments. When working on carbonate sediments, age dating is commonly based on paleontological observations and established biozonations, which may prove to be relatively imprecise. Dating by means of strontium isotope ratios in marine bioclasts is the probably best method in order to precisely date carbonate successions, provided that the sample reflects original marine geochemical characteristics. This requires a precise study of the samples including its petrography, SEM and cathodoluminescence observations, stable carbon and oxygen isotope geochemistry and finally the strontium isotope measurement itself. On the Nicoya Peninsula (Northwestern Costa Rica) sediments from the Piedras Blancas Formation, Nambi Formation and Quebrada Pavas Formation were dated by the means of strontium isotope ratios measured in Upper Cretaceous Inoceramus shell fragments. Results have shown average 87Sr/86Sr values of 0.707654 (middle late Campanian) for the Piedras Blancas Formation, 0.707322 (Turonian-Coniacian) for the Nambi Formation and 0.707721 (late Campanian-Maastrichtian) for the Quebrada Pavas Formation. Abundant detrital components in the studied formations constitute a difficulty to strontium isotope dating. In fact, the fossil bearing sediments can easily contaminate the target fossil with strontium mobilized form basalts during diagenesis and thus the obtained strontium isotope ratios may be influenced significantly and so will the obtained ages. The new and more precise age assignments allow for more precision in the chronostratigraphic chart of the sedimentary and tectonic evolution of the Nicoya Peninsula, providing a better insight on the evolution of this region. Meteor Cruise M81 dredged shallow water carbonates from the Hess Rise and Hess Escarpment during March 2010. Several of these shallow water carbonates contain abundant Larger Foraminifera that indicates an Eocene-Oligocene age. In this study the strontium isotope values ranging from 0.707847 to 0.708238 can be interpreted as a Rupelian to Chattian age of these sediments. These platform sediments are placed on seamounts, now located at depths reaching 1600 m. Observation of sedimentologic characteristics of these sediments has helped to resolve apparent discrepancies between fossil and strontium isotope ages. Hence, it is possible to show that the subsidence was active during early Miocene times. On La Désirade (Guadeloupe France), the Neogene to Quaternary carbonate cover has been dated by microfossils and some U/Th-ages. Disagreements subsisted in the paleontological ages of the formations. Strontium isotope ratios ranging from 0.709047 to 0.709076 showed the Limestone Table of La Désirade to range from an Early Pliocene to Late Pliocene/early Pleistocene age. A very late Miocene age (87Sr/86Sr =0.709013) can be determined to the Detrital Offshore Limestone. The flat volcanic basement had to be eroded by wave-action during a long-term stable relative sea-level. Sediments of the Table Limestone on La Désirade show both low-stand and high-stand facies that encroach on the igneous basement, implying deposition during a major phase of subsidence creating accommodation space. Subsidence is followed by tectonic uplift documented by fringing reefs and beach rocks that young from the top of the Table Limestone (180 m) towards the present coastline. Strontium isotope ratios from two different fringing reefs (0.707172 and 0.709145) and from a beach rock (0.709163) allow tentative dating, (125ky, ~ 400ky, 945ky) and indicate an uplift rate of about 5cm/ky for this time period of La Désirade Island. The documented subsidence and uplift history calls for a new model of tectonic evolution of the area.