Transitional Series Research Articles

Ontogenetic development of diffusional restriction to protein at the pial surface of the rat brain: an electron microscopical study.

Blood-brain, blood-CSF and ventricular CSF-brain barriers to protein are present very early in brain development. In order to determine whether the outer pial surface of the brain also restricts free penetration of macromolecules, the dorso-lateral part of the sensorimotor cortex from rats at embryonic day 12 (E12), 14, 16, and 18, the day of birth (P0), and adult rat, was studied by electron microscopical techniques. Potassium ferrocyanide, Ruthenium Red and immunogold labelling of endogenous albumin were used to investigate junctional structures and the sites of restriction to albumin diffusion. At E12, large fenestrated sinusoids were present in the pia-arachnoid and the brain surface was formed by an incomplete layer of neuroepithelial and presumptive radial glial end feet, but capillaries in the pia-arachnoid showed no fenestrations at E14 or later. From E14, we observed the progressive appearance of distinct junctional structures between the glial end feet which, to our knowledge, have not been described before. Analysis of albumin distribution from E16 to P0 suggests that the junctions may contribute to restriction of diffusion between the subarachnoid space and the brain extracellular fluid. The restriction to the penetration of protein at both the pial and the ependymal surfaces may ensure the isolation of the neural environment during a critical phase in development of the nervous system. The changes in the structure of the junctions between E12 and P0 suggests a transitional series of embryonic junctional types, which eventually give way to the mature junctions of the adult. Parallels between the embryonic glial junctions and junctions described in adult invertebrate brain, suggest some interesting parallels in junctional development in phylogeny and ontogeny.

Magmatic and hydrothermal processes in the Bouvet Triple Junction Region (South Atlantic)

ABSTRACTResults of electron microprobe and microthermometric studies of samples collected from the Bouvet Triple Junction Region (BTJR) during a joint Russian‐Italian geological expedition on the R/V Academician Nikolaj Strakhov (1994) have revealed new data on the composition of basaltic magmas and oceanic hydrothermal fluids connected with magmatic processes. Detailed analysis of basaltic glasses shows that the modem Mid‐Atlantic Ridge (MAR) rift valley is composed of normal mid‐ocean ridge basalts with low concentrations of K2 O and TiOz (N‐MORB), while its flanks are more enriched with these components approaching E‐MORB. A marked influence of the Bouvet hot spot volcanism on magma generation on the South‐West Indian Ridge (SWIR) near Bouvet Island is observed. Basaltic melts in this area belong to alkalic and transitional series and have maximum contents of K2O, TiO2, H2O.Microthermometric analyses of fluid inclusions in the samples from the BTJR have revealed major differences in the oceanic hydrothermal fluid systems on the MAR and near SWIR, which depends on the peculiarities of magma. In the area of the MAR (with dry melts) only H2O solution inclusions in quartz were found; thus, seawater is probably the only primary source of hydrothermal fluids (NaCl + MgCl2+ H2O; T= 170–200°C). In the SWIR area (with the high content of water in melts) syngenetic liquid CO2 and H2O solution inclusions in quartz indicate the influence of the magmatic fluid component on the ore‐forming water/carbon dioxide solutions (NaCl + CaC12+ H2O + CO2; T= 200–310 °C; P= 900–1700 bar).

Marginal sculpture in medieval France: towards the deciphering of an enigmatic pictorial language

Introduction: deciphering a neglected realm of images. Part 1 Romanesque marginal sculpture - subjects and images. Part 2 Reading the language and texts of marginal Romanesque sculpture: reading the codes marginal and official sculpture - a comparative study the multilayered system - patrons versus artists. Part 3 Gothic marginal sculpture: the new Gothic subjects and images the patrons of Gothic marginal sculpture a Gothic mirror of society - the 150 corbels of Guillaume de Beaumont, Bishop of Angers (1202-1240) the corbels of Semur en Auxois and Hugh IV, Duke of Burgundy image and style of the Gothic language. Part 4 The late Gothic marginal pictorial tradition - a medieval subculture: marginal images in stone marginal wood carvings types of figure in the pictorial tradition. Epilogue: the place of marginal sculpture and imagery in European art. Appendices A catalogue of some Romanesque and Gothic series: Romanesque corbel series (mid-twelfth century) - St Pierre in Aulnay, St Troyan in Retaud, St Hilaire in Foussais, St Etienne in Cahors transitional marginal series - St Etienne Cathedral in Sens Gothic marginal series - St Maurice in Angers, Notre Dame, Semur-en-Auxois, Ste Radegonde, Poitiers, St Etienne Cathedral, Metz, St Pierre, Poitiers, Ste Croix, inner church, Lavoute-Chilhac, St Pierre and St Paul, Villenauxe-la-Grande, Notre Dame de l'Epine.

Gradual morphological changes in some dinoflagellate cysts from the eocene (lower tertiary) of the north sea basin

Abstract Two examples of morphological change in fossil dinoflagellate cysts through successive strata are described and discussed. Firstly, the morphological trend among specimens from the lower part of the stratigraphic range of Eatonicysta ursulae is examined. The oldest known specimens are atypical; however, gradual morphological changes over a relatively short period of time led to typical morphologies. Secondly, a transitional morphological series from Rottnestia borussica to Hystrichosphaeropsis sp. is described. The morphological changes appear to coincide with environmental changes, such that the observed morphological trends may represent phe‐notypic adaptations within single species. In addition, the transformation between Rottnestia and Hystrichosphaeropsis seems to have occurred in both directions, and may have occurred twice in geological history. Conversely, changes in cyst shape may represent speciation events. Such may be the case with Eatonicysta ursulae, where the observed morphological...

Metasomatism in the Generation of Granulite Veins: Mass Balance, Mass Transfer, and Reference Frames

In the Seguin subdomain, Grenville Province, dark hornblende–garnet-rich quartz-absent metagabbro is transected by anastomosing light-coloured veins rich in orthopyroxene, clinopyroxene, plagioclase, and, in some cases, quartz. Three types of veins form a smoothly transitional series from fine-grained diffuse veins and patches to a more defined intermediate vein type, and finally to coarsegrained tonalitic veins with abrupt contacts to the metagabbro host. The textures and contact relations of diffuse and intermediate veins are suggestive of channelized subsolidus dehydration of the metagabbro. Mass-balance calculations show that dehydration was accompanied by metasomatism; all reasonable solutions require the addition of SiO2, Na2O, and large ion lithophile elements (LILE) to the host rock to produce the veins. Reference frames for the evaluation of mass transfers can be represented by two cases: (1) constant volume, and (2) constant FeO, MgO, V, and Zr requiring volume increases by factors of 1·3—3·3. The simplest model (minimizing the number of mobile constituents and giving progressively larger volume increases for more leucocratic veins) is based on constant FeO, MgO, V, and Zr. In this case, model mineral reactions and mass transfers for two types of veins, distinguished on the outcrop as (1) diffuse and (2) intermediate, are: (1)1.1Hbd+0.3Grt+1.5Pl1+12.2SiO2+0.9Na2O+1.0Al2O3+1.0CaO→1.8Opx+2.1Cpx+3.2Pl2+2.6Qtz+0.3Ilm+1.1H2O+0.2K2O(2)1.0Hbd+0.7Grt+1.5Pl1+16.7SiO2+1.4Na2O+0.3K2O+0.9A12O3→2.4Opx+1.4Cpx+4.1Pl2+0.3Ilm+2.9Qtz+1.0H2O These mass balances show that SiO2, Na2O, and Al2O3 were added, and H2O lost, in producing the vein assemblages. Minimum fluid/rock volume ratios of 3–20 imply flow-through. The fluid did not induce melting of hornblende+plagioclase, and hence it must have had relatively low aH2O. The open-system transformation of hornblende–garnet-rich metagabbro to pyroxene-rich veins was driven primarily by the introduction of silica and feldspar components in a low-aH2O supercritical fluid at granulite-facies P–T conditions. The application of quantitative mass balance, taking into account propagated errors and a critical appraisal of the possible reference frames, enhances our ability to resolve open-system processes.

Massive sulfide deposits of the Noranda area, Quebec. V. The Corbet mine

The Corbet deposit is located in the upper part of the Flavrian andesite, the lowest volcanic formation within cycle III of the Noranda Central Mine Sequence. The deposit consisted of one main lens of massive sulfides, and several smaller lenses, and contained 2.7 × 106 t of mineable ore at 2.92% Cu, 1.98% Zn, 20.6 g/t Ag, and 1.0 g/t Au. The orebody had a massive pyrrhotite–chalcopyrite core, which passed laterally into massive pyrite–sphalerite. The northwestern part of the ore-body is overlain by 50–100 m of Flavrian andesite, whereas the southeastern portion is overlain by Northwest rhyolite.Lavas within the Flavrian andesite and the lower Northwest rhyolite are low in K2O (<0.5%), and are partly of tholeiitic affinity, and partly of transitional affinity. The tholeiitic volcanic series has Zr/Y ratios of 2.8 to 4.5, whereas the transitional series has Zr/Y ratios of 4.5 to 7.1. The two series are also distinct in plots of Nb–Zr, Yb–Zr, Nb–Y, and the rare earth elements. These data indicate that two slightly different magma types existed in the chamber that fed this portion of the extrusive Central Mine Sequence.Alteration is most intense in the breccias of the upper Flavrian andesite, within ~50 m of the orebody, and is almost entirely chloritic. There is no zone of silicification, although moderate sericitization occurs lateral to and above the orebody. Mass-change calculations indicate that large amounts of SiO2 and CaO + Na2O were leached from the rocks by hydrothermal solutions, whereas large amounts of hydrothermal FeO and seawater MgO were added.Oxygen isotope depletions are among the largest in the Noranda area and extend laterally and vertically up to 300 m from the orebody. Within this volume, δ18O values of altered volcanic rocks have been decreased to values as low as 2 to −2‰. These depletions result from reactions with seawater at ~ 250–300 °C and from strong silica leaching, as indicated by mass-change calculations. The hydrothermal system at Corbet can be assessed using two lithogeochemical tools: calculated mass changes and oxygen isotope ratios, both of which are sensitive to water–rock ratio and temperature.

Stable isotope ratios of soil carbonate and soil organic matter as indicators of forest invasion of prairie near Ames, Iowa.

Stable isotope ratios of pedogenic carbonate and organic matter were measured in a prairie-transition-forest soil biosequence near Ames, Iowa to determine the vegetation succession. The modern vegetation is dominated by non-native C3 plants which have been introduced by agricultural practices. The δ13C values of soil organic matter from the prairie and forest endmembers indicate C4 and C3 dominated ecosystems, respectively, during the accumulation of soil organic matter. Pedogenic carbonate from all soils, including rare pedogenic carbonate from the forested soil, has an average δ13C of-2.0‰, indicating that the carbonate formed under a C4 vegetation. These results indicate that the ecosystem was a C4-dominated prairie and therefore suggest a recent arrival of forests and other C3 plants in the area. This study also implies that the primary features of the transitional Lester soil series, which has soil properties intermediate between Alfisols and Molisolls, formed under prairie conditions and were overprinted by an invading forest.

THE ROLES OF HETEROCHRONY AND HETEROBLASTY IN THE DIVERSIFICATION OF LEAF SHAPES IN BEGONIA DREGEI (BEGONIACEAE)

The relationship between shape variation in the transitional series of leaves and in adult leaves was examined in seedlings of seven morphs of Begonia dregei using several quantitative methods of shape analysis. There is variation in the shape of adult leaves among individuals as well as in juvenile leaves within individuals in B. dregei. As an individual grows, there is a gradual transition in leaf shape from the symmetrical, oval, smooth‐margined leaves through a series of more than ten transitional leaves to a stable adult leaf shape. There appear to be two basic patterns to the acquisition of adult traits. Traits that differ among morphs are acquired gradually throughout the entire transitional series while those that are similar among morphs are acquired by about leaf 5 and remain stable through the later juvenile leaves. There is no identity of leaf shape between the earlier leaves of some morphs and the later leaves of others. Evolutionary diversification in adult leaf morphology in this species is not related to simple changes in ontogeny of the whole plant.

The Roles of Heterochrony and Heteroblasty in the Diversification of Leaf Shapes in Begonia dregei (Begoniaceae)

The relationship between shape variation in the transitional series of leaves and in adult leaves was examined in seedlings of seven morphs of Begonia dregei using several quantitative methods of shape analysis. There is variation in the shape of adult leaves among individuals as well as in juvenile leaves within individuals in B. dregei. As an individual grows, there is a gradual transition in leaf shape from the symmetrical, oval, smooth-margined leaves through a series of more than ten transitional leaves to a stable adult leaf shape. There appear to be two basic patterns to the acquisition of adult traits. Traits that differ among morphs are acquired gradually throughout the entire transitional series while those that are similar among morphs are acquired by about leaf 5 and remain stable through the later juvenile leaves. There is no identity of leaf shape between the earlier leaves of some morphs and the later leaves of others. Evolutionary diversification in adult leaf morphology in this species is not related to simple changes in ontogeny of the whole plant.

Tectonic origin of carbonate depositional sequences in a strike-slip setting (Aptian, northern Iberia)

The Lower-Upper Aptian transitional series of the Aulesti area in the northeastern Basque-Cantabrian region (northern Iberia) shows a depositional sequence arrangement conditioned by local synsedimentary folds and faults. Three sequence boundaries (Sb1, Sb2, Sb3), dated approximately between the deshayesi and nutfieldiensis Zones, are characterized by erosional surfaces with both palaeokarst and palaeovalley reliefs up to 150 m deep in the downthrow blocks of normal faults; one sequence boundary (Sb2) is also a clear angular unconformity with fossilized folds and faults created by strike-slip action. Facies analyses of the two depositional sequences (S1 and S2), reveal general deepening of the sea upward through the sedimentary column. Only the sequence boundary Sb3 could be eustatic, according to the Haq et al. (1987) curve. However, all sequence boundaries are primarily attributed to local tectonic pulses: sudden relaxations of transtensional stresses and associated tectonic uplifts occur after phases of transtensional stress accumulation associated with relative sea-level rises. The intraplate stress tectonic mechanism of Cloetingh et al. (1985) is thought to offer the best explanation for the origin of the sequential arrangement seen.

Petrology and geochemistry of potassic and ultrapotassic volcanism in central Italy: petrogenesis and inferences on the evolution of the mantle sources

Major, trace element, Sr isotopic and mineral chemical data are reported for mafic volcanic rocks (Mg-value ⩾ 65) from the northern-central sector of the potassic volcanic belt of Central Italy. The rocks investigated range from potassic series (KS) and high-K series (HKS) to lamproitic (LMP) and kamafugitic (KAM) through a transitional series (TRANS), thus covering the entire compositional spectrum of potassic and ultrapotassic magmas. KAM rocks are strongly silica undersaturated and, compared with the other rock series, have low SiO 2, Al 2O 3, Na 2O, Sc and V and high CaO, K/Na, (Na + K)/Al. KS and HKS have high Al 2O 3, CaO and variable enrichment in K 2O and incompatible elements. LMP rocks are saturated in silica and have high SiO 2, K 2O, K 2O/Na 2, MgO, Ni and Cr and low Al 2O 3, CaO, Na 2O, Sc and V. TRANS rocks display intermediate compositional characteristics between LMP and KS. All the rocks under study have fractionated hygromagmaphile element patterns with high LIL/HFS element values and negative anomalies of Ti, Ta, Nb and Ba. Negative Sr anomalies are observed in the LMP and TRANS rocks. LIL elements show overall positive correlations with K 2O, whereas different trends of Sr and HFSE vs. K 2O are defined by LMP-TRANS and KS-HKS-KAM. 87Sr/ 86Sr range from about 0.710 to 0.716. KS, HKS and KAM rocks have similar 87Sr/ 86Sr values clustering around 0.710. LMP and TRANS rocks have the highest 87Sr/ 86Sr values. Geochemical and isotopic data reported for the most primitive Italian potassic and ultrapotassic rocks support the hypothesis that the interaction between crustal and mantle reservoirs was a main process in the genesis of Italian potassic magmatism. Simple mass balance calculations exclude, however, an important role of crustal assimilation during ascent of subcrustal magmas to the surface and indicate that the sources of Central Italy volcanics underwent contamination with fluids and/or melts released by upper crustal material previously brought into the mantle by subduction processes. Different trends of incompatible elements vs. K 2O observed in the studied rocks suggest distinct metasomatic processes for the sources of the investigated magmas. Liquids derived by bulk melting of pelitic sediments are believed to be the most likely contaminants of the source of LMP rocks. Fluids or melts rich in Ca, Sr and with high LILE/HFSE value and Sr isotopic composition around 0.710 are the most likely contaminant of the source region of KS, HKS and KAM volcanics. Variations in CaO, Na 2O and ferromagnesian element abundances and ratios suggest that, in some zones, the mantle source of potassic magmas experienced partial melting with extraction of basaltic liquids prior to metasomatism.

Oligocene basaltic volcanism of the Northern Rio Grande Rift: San Luis Hills, Colorado

The inception of the Rio Grande rift in northern New Mexico and southern Colorado was accompanied by voluminous mafic volcanism preserved in part as erosional remnants on an intrarift horst within the current axial rift graben of the San Luis Valley. Oligocene (∼26 Ma) volcanic rocks of the Hinsdale Formation at San Luis Hills range from 49 to 57 wt % SiO2 and include nepheline and hypersthene normative lavas. A mildly alkalic series consisting of trachybasalt, basaltic trachyandesite, and trachyandesite is volumetrically dominant, olivine tholeiites are subordinate, and xenocrystic trachyandesites containing abundant quartz and plagioclase xenocrysts occur only locally. Relative to the San Luis Hills olivine tholeiites which have La/Smn ∼ 2, the more alkaline series are enriched in light rare earth elements (LREE) and have La/Sm ratios that increase in the trachybasalt‐basaltic trachyandesite suite (La/Smn ∼ 3) to xenocrystic trachyandesites that are the most LREE enriched (La/Smn ∼ 4). Chondrite‐normalized, trace element patterns for the lavas in the San Luis Hills are similar in shape within the mildly alkaline to transitional series; they have characteristic Nb and Ta depletions and high K and Th relative to Ta, Nb, and LREE. Major and trace element constraints support a petrogenetic model of fractionation plus lower crustal assimilation for petrologic suites within the San Luis Hills rocks, although the model cannot relate lavas for the entire series to a common parent. Most mafic lavas of the San Luis Hills were evolved (Mg # <60) and contaminated by LREE‐enriched silicic partial melts of granulitic lower crust depleted in Rb, Th, and U. Pb isotopes are the most sensitive indicators of crustal contamination, whereas shifts in Nd and Sr isotope ratios are associated with large amounts of assimilation. However, relatively noncontaminated lavas can be identified and indicate at least two mantle source regions were involved.

Geology and petrology of Jasper Seamount

Fifteen dredges on the summit and upper flanks of Jasper Seamount (122°44'W; 30°27'N) recovered a wide variety of lithologies, including pillow lavas, vesicular lapillistones from shallow submarine explosive volcanism, and a range of xenoliths. On the basis of dredge locations, geochemical characteristics, and 40Ar/39Ar age data, three distinct phases of volcanism can be distinguished, a shield‐building tholeiitic/transitional phase (Flank Transitional Series, FTS), followed by a flank alkalic series (FAS), and a late‐stage Summit Alkalic Series (SAS). All three series consist exclusively of differentiated (Mg# = 54 to 21; Mg# = Mg2+/(Mg2++Fe2+)) compositions. The FTS represents a low‐pressure differentiation trend from tholeiitic/transitional basalts to quartz‐normative residual liquids and probably accounts for more than 90% of the volume of Jasper. 40Ar/39Ar age data, the dominant reversed polarity of Jasper, and a plausible duration (< 1 m.y.) for shield construction suggest FTS volcanism began about 11 Ma and ended about 10 Ma. FTS lavas probably erupted from a NW trending, hotspot track‐parallel rift system. The intermediate alkalinity FAS lavas, which probably comprise 3–8% of the volume of Jasper, erupted from 8.7 to 7.5 Ma, possibly after a brief volcanic hiatus or period of reduced eruptive activity. Normative projections suggest the FAS lavas are the product of fractionation or equilibration at elevated pressures. The hawaiites and mugearites of the SAS erupted between 4.8 and 4.1 Ma, after a probable 2.7 m.y. period of volcanic quiescence, and probably constitute <1% of the seamount volume. A suite of Xenoliths incorporated in SAS lavas includes (1) tholeiitic basalt fragments from either the ocean crust or seamount interior, (2) a range of differentiated gabbros largely derived from the ocean crust, (3) residual mantle spinel lherzolites, and (4) pyroxenite and peridotite cumulates. The abundance of crustal gabbro and spinel lherzolite xenoliths in evolved lavas of the SAS suggests that these lavas probably fractionated in a magma chamber at the crust‐mantle boundary. The occurrence of orthopyroxene‐bearing alkalic cumulate xenoliths in these lavas, however, is enigmatic and may reflect complexities such as magma mixing or the inappropriateness of pressure estimates. The SAS vents of Jasper define a NE‐SW volcanic trend which is orthogonal to the FTS rift. The pattern of volcanic activity, including periods of volcanic quiescence, and the general increase in alkalinity, as well as the structural reorganization of magmatic feeder systems of Jasper Seamount, is strikingly similar to the patterns observed on Hawaiian volcanoes. Thus our data from Jasper (690 km3) extend the concepts of structural and petrological evolution of hotspot volcanoes based on Hawaii to moderate‐sized seamounts.

Geochronometry of the Bridge River Camp, southwestern British Columbia

Mineralization at the Bralorne mesothermal gold vein deposit is closely related to a suite of early Late Cretaceous to early Tertiary dykes. Premineral albitite dykes (91.4 ± 1.4 Ma by U–Pb on zircons) and postmineral lamprophyre dykes (43.5 ± 1.5 Ma by K–Ar on biotite) set definite age limits on the mineralizing event. A late intra- to post-mineral green hornblende dyke set (85.7 ± 3.0 Ma by K–Ar on hornblende) that forms a transitional series to the albitites may further restrict the age. Thus, mineralization occurred long after emplacement of the host Bralorne intrusions, dated as Early Permian (minimum age of approximately 270 ± 5 Ma by U–Pb on zircons, 284 ± 20 Ma by K–Ar on hornblende, and 40Ar/39Ar plateau at 276 ± 31 Ma). Lithologically similar intrusions 20 km to the north near Gold Bridge are also Early Permian (287 ± 20 Ma by K–Ar on hornblende and 320 ± 80 Ma by a Rb–Sr whole-rock isochron). Geochronology, radiogenic and stable isotopes, and fluid-inclusion studies suggest that there were several pulses of mineralizing activity adjacent to and east of the Coast Plutonic Complex (CPC). Decreasing temperatures and younger age of mineralization with increasing distance from the CPC imply that plutons of the CPC were the main heat source responsible for mineralization. The main pulses were about 90 Ma for mesothermal Au–Ag–As ± W,Mo mineralization at Bralorne near the CPC, ranging outwards to 65 Ma for Ag–Au–Sb–As ± Hg mineralization at the Minto and Congress deposits, to 45 Ma for Ag–Au epithermal mineralization at Blackdome, 100 km east of the CPC.The Bralorne intrusions may have been emplaced below the sea floor in a spreading-ridge oceanic environment, as suggested by the petrology of the intrusive suite, which includes serpentinized ultramafite, hornblende diorite, and soda granite (trondhjemite), typical of an ophiolite association. The chemistry of volcanic rocks mapped as Cadwallader Group, which host these intrusive bodies, is transitional from mid-ocean-ridge basalts to island-arc tholeiite, suggesting a back-arc-basin setting. Gradational contact relations between the hornblende diorite and the volcanic rocks suggest that the diorite intruded its own volcanic products. Intrusive contacts of the diorite with adjacent elongate ultramafic bodies imply that the ultramafic rocks are of Permian or older age and had been structurally emplaced into crustal levels by the time of diorite intrusion. In the Bralorne fault block the Bralorne intrusions appear to cut the adjacent Cadwallader and Bridge River groups, implying an Early Permian or older age for at least parts of these groups. Thus, rocks mapped as Cadwallader Group in the Bralorne area could be distinct from and older than lithologic equivalents exposed elsewhere, although they are similar in terms of their petrology and major- and trace-element chemistry.

Jasper Seamount: Seven million years of volcanism

Jasper Seamount is a young, mid-sized (690 km{sup 3}) oceanic intraplate volcano located about 500 km west-southwest of San Diego, California. Reliable {sup 40}Ar/{sup 39}Ar age data were obtained for several milligram-sized samples of 4 to 10 Ma plagioclase by using a defocused laser beam to clean the samples before fusion. Gee and Staudigel suggested that Jasper Seamount consists of a transitional to tholeiitic shield volcano formed by flank transitional series lavas, overlain by flank alkalic series lavas and summit alkalic series lavas. Twenty-nine individual {sup 40}Ar/{sup 39}Ar laser fusion analyses on nine samples confirm the stratigraphy: 10.3-10.0 Ma for the flank transitonal series, 8.7-7.5 Ma for the flank alkalic series, and 4.8-4.1 Ma for the summit alkalic series. The alkalinity of the lavas clearly increases with time, and there appear to be 1 to 3 m.y. hiatuses between each series. The age data are consistent with the complex magnetic anomaly of Jasper; however the dominant reversed polarity inferred from the anomaly suggests that most of the seamount formed at ca. 11 Ma, prior to the onset of Chron C5N. The duration of volcanism of Jasper Seamount is slightly longer than the duration of volcanism at Hawaiian volcanoes, suggesting thatmore » individual age data from seamounts may constrain the age of a seamount only to within about 7 m.y. unless the stage of volcanism can be unambiguously determined. Extrapolating from the results of our study, similar precision in age determinations should be possible on 50 mg of 1 Ma plagioclase from mid-ocean ridge basalt, opening new possibilities in the geochronology of young, low-potassium volcanic rocks.« less

Petrology and geochemistry of the Kerguelen Archipelago basalts (South Indian Ocean): evolution of the mantle sources from ridge to intraplate position

Kerguelen basic lavas belong to three magmatic series: transitional, mildly alkaline and highly alkaline, showing from the first to the latter, a general increase in alkali and incompatible element contents as well as some of the incompatible trace element ratios [(Ce/Yb) n,Nb/Zr,Nb/Y,Zr/Y,Ti/Zr].Ba/Nb,La/Nb and all ratios involving Th are in the range of those observed for Dupal type OIBs although subtle differences exist mainly reflecting a slight Th enrichment in the Kerguelen basalts relative to other OIBs. New Sr, Nd and Pb isotope data strengthen the previously identified Dupal signature of Kerguelen rocks. While the transitional series, older than 26 Ma, shows slightly depleted Sr and Nd isotopic signatures ( 87Sr/ 86Sr< 0.705 andε Nd > 0), the alkaline series, between 26 and about 8 Ma, show a slightly enriched signature which is strengthened in the highly alkaline series ( 87Sr/ 86Sr up to 0.70056), younger than 12 Ma. Pb isotope ratios do not show any significant difference between these three series. These isotopic data coupled with trace element geochemistry indicate a mixing process between a depleted MORB component and an enriched OIB-type component, the Dupal plume. The Pb isotope characteristics indicate the presence of an old recycled component (continentally derived sediment or crust itself) incorporated into the deep plume source-region which may be responsible for this enrichment. The Kerguelen Islands show a geodynamic evolution from an early ridge centered stage, above or close to the South East Indian Ridge (SEIR) 45 Ma ago, to the present day intraplate setting. The geochemical and isotopic variations reflect this geodynamic evolution and correspond to variable degrees of mixing between the two extreme components (SEIR, MORB and the Kerguelen plume) whose relative proportions evolve with the archipelago position, i.e. with the distance from the ridge at the time of eruption.

The Sept-îles layered mafic intrusion: geophysical expression

Eighty-eight percent of the Cambrian Sept-Îles layered mafic intrusion, possibly the second largest on Earth, lies under the estuary of the St. Lawrence River. A marine geophysical cruise in 1986 tied together all earlier gravity surveys and added gravity measurements along 1100 km of new track lines.The following results were based on the 1986 survey and other geophysical data. The Sept-Îles layered mafic intrusion is nearly circular in plan and about 80 km in diameter. It is funnel shaped and some 5.6 km thick at its centre. It has a volume of about 20 000 km3 and is composed of gabbro and allied rocks in a layered series (nearly [Formula: see text] of the total bulk), ultramafic rocks in a lower series (about [Formula: see text]), and gabbro, anorthosite, and potassium-feldspar-bearing rocks in a transitional and upper series (about [Formula: see text]). Roof rocks on the intrusion may have been restricted to a consanguineous volcanic cap.The Sept-Îles intrusion is located at the northwestern end of a reentrant of the North American continent where Appalachian tectonostratigraphic zones are offset dextrally by 400 km. Emplacement of the intrusion may be related to the initial opening of Iapetus. The southeastern border of the intrusion is downwarped and overlain by a rocky cover 8 km thick. The cover may be composed of Taconian nappes thrust northwestward at the reclosing of Iapetus. If this is so, Logan's line crosses the Sept-Îles layered mafic intrusion about 10 km southeast of its centre.

Evolution in Hoplolaims (Nematoda: Tylenchida)

Genera that show a transitional series are difficult to define: the series discussed is Rotylenchus - Scutellonema - Peltamigratus - Aorolaimus - Hoplolaimus. Two hypotheses are presented. First the evolutionary development of this series is associated with the theory of continental drift. Second the morphology of tylenchs is almost unchanged during the last 200-300 million years, explaining the large genetic variation found within some species.

The Derwent-Hunter submarine volcano: The product of a long defunct subduction zone

The Derwent-Hunter Volcano (31°S, 156°E) is one of a 1400-km-long chain of Oligocene-Miocene undersea volcanoes that transect the Tasman abyssal plain. It developed some 15–16 Ma ago and the rocks it contains form an abbreviated differentiation sequence that range in composition from olivine tholeiite to either mugearite or basaltic andesite. These rocks generally contain normative hypersthene, but do not contain significant quantities of either normative quartz or normative nepheline; and their trace-element abundances generally lie in the area of overlap between the subalkali and alkali rock series. The more primitive rocks contain spinels (36% Cr2O3), olivines (Fo81–83), plagioclases (An63–70) and less commonly augite, set in a glassy mesostasis. Some specimens contain xenocrysts of anorthitic plagioclase (An85–94) and calcic salite (En36 Fs12 Wo52).The volcano is part of the Tasmantid volcanic chain, and the latter is one of a number of parallel volcanic chains of Oligocene-Miocene age that arise in eastern Australia and the Tasman Sea. Both the onshore and offshore volcanoes contain rocks that belong to a transitional petrographic series that comprises tholeiitic, transalkalic and alkalic members. With one exception the ages of the volcanoes across the various chains are similar for the same latitudes. The exception is the easternmost chain (i.e. Lord Howe) where the ages of the individual volcanoes lag behind their counterparts at similar latitudes. When considering the origin of these parallel volcanic chains the fundamental problems are: 1.(a) What processes occurred within the asthenosphere to create a volcanic focus that extends for 1000 km in an east-west direction and has been active for circa 40 Ma?2.(b) Why are the ages of the Lord Howe Seamounts not synchronous with the other volcanic chains?3.(c) What processes and materials are required to generate the geochemically diverse and complex magmas needed to construct these intraplate volcanoes?It is proposed that these magmas evolved from a line of diapirs within the asphenosphere. The diapirs began forming at a depth of ≈ 650 km as the result of the heating and partial melting of pressure-transformed materials that had originally been subducted. After traversing the asthenosphere individual diapirs impinged on the base of the lithosphere where they reactivated pre-existing fractures and produced extensional rock failure. This resulted in the extraction of batches of magma from the diapirs; and the composition of each batch was substantially controlled by, the degree of partial melting, and the ambient pressure at the source of magma extraction. Intratelluric fractional crystallization, limited assimilation of crustal materials, high-level magmatic differentiation and the mixing of different batches of magma, all assisted in the evolution of the more differentiated magmas.The diapirs developed along a trend parallel to the long axis of a long defunct subduction zone. It is proposed that this subduction zone was linear along most of its length, but in the east it terminated in a section that was displaced northwards by a transform fault, and the materials subducted by this displaced segment triggered the string of diapirs that eventually produced the Lord Howe Seamount Chain.

The evolution of the magmatism in the external zones of the Betic Cordilleras during the mesozoic

The Mesozoic igneous rocks from the External Zones of the Betic Cordilleras extend for some 300 km along the Subbetic Zone. They are poorly differentiated basic rocks which, altogether, correspond to a transitional series containing tholeiitic and sodium-alkaline terms. They crop out as small ophite stocks and dykes intruded into middle- and upper-Triassic rocks, or as submarine flows and sills interlayered with Jurassic materials.Geological and radiometric evidence points to an upper-Triassic-Liassic age for the ophite-generating magmatism, while the fissure volcanism began locally in the early-Liassic and extended throughout the Dogger. It reached its climax in the Tithonian and ceased abruptly in the lowermost Cretaceous.The magmas that generated the two groups of rocks originated within the mantle. During the ascent through a continental crust they were contaminated by deep-crust granitoid rocks and by the assimilation of pelites from the basement. The chemical composition and fractional-crystallization differentiation trends of the ophites belong to the tholeiitic series, while those of the middle-uppermost Jurassic magmas to the sodic-alkaline series. This magmatic evolution may be attributed to a geodynamic change from distensive in the Triassic to transtensive from the middle-Jurassic onwards.