Alkali Volcanism Research Articles

Melt-modified lithosphere beneath Ross Island and its role in the tectono-magmatic evolution of the West Antarctic Rift System

Mantle lithosphere influences rift system tectonic evolution, yet the age and composition of rifted lithosphere is typically difficult to constrain due to limited sampling. In the West Antarctic Rift System (WARS), Cenozoic to recent alkaline volcanic rocks yield a variety of peridotite and pyroxenite xenoliths that allow sampling of lithosphere. We report osmium and helium isotope data, elemental abundances, and petrology, for a suite of xenoliths and lavas from the Hut Point Peninsula of Ross Island. Recently (<1.3 Ma) erupted basanites yield fresh dunite and harzburgite (olivine forsterite [Fo] 90.1–88.2), lherzolite (Fo90.6–87.4), and pyroxenite xenoliths (Fo89.3–87.3). The basanite lavas contain abundant large olivine xenocrysts (Fo89.7–88.0), with more ferroan matrix olivine grains (Fo83.7–81.2) and have HIMU-like incompatible trace-element signatures. The 3He/4He ratios (6.8 ± 0.3RA; 2SD) defined by co-existing He-rich xenoliths indicate a mantle source distinct from high-3He/4He plume mantle. Pyroxenite and lherzolite xenoliths have similar relative abundances of incompatible trace elements to host lavas, whereas dunite xenoliths have refractory compositions. Melt-rock reaction occurring in the xenoliths is demonstrated by replacement by amphibole or clinopyroxene to form pyroxenite and lherzolite lithologies, or as amphibole-impregnated dunites. The 187Re187Os systematics of the lavas, pyroxenites and lherzolites define an apparent isochron, with an initial 187Os/188Os ratio of 0.1286 ± 0.0001. The initial 187Os/188Os ratio is within uncertainty of dunite and harzburgite xenolith Os isotope compositions (0.1279–0.1303). Pervasive evidence for melt-rock interaction indicates that the straight-line relationship in 187Re/188Os-187Os/188Os space is a mixing line between high Re/Os lavas with radiogenic 187Os/188Os, and dunite and harzburgite. Petrological and geochemical evidence indicates that dunite and harzburgite xenoliths represent young lithosphere, with rhenium depletion ages up to ~250 Ma. The timing of formation and composition of the Hut Point Peninsula xenoliths are consistent with both destruction and creation of mantle lithosphere during or after subduction along the Gondwana margin, prior to WARS formation. Modification of mantle lithosphere by subduction is also consistent with generation of HIMU-like metasomatized mantle reservoirs that fed Cenozoic to recent alkali volcanism of Mount Erebus and the WARS. The presence of young lithosphere within the WARS has collateral implications for rift dynamics and melting processes, especially beneath Mount Erebus, contrasting with older lithospheric mantle beneath the Trans-Antarctic Mountains and Marie Byrd Land.

Geometry, kinematics and regional significance of faulting and related lamprophyric intrusion in the mineralised zone at the Pu Sam Cap complex, Northwest Vietnam

Geometry, kinematics and regional significance of faulting and related lamprophyric intrusion in the mineralised zone at the Pu Sam Cap complex, Northwest Vietnam

Mineralogy of the Martian Surface

The past fifteen years of orbital infrared spectroscopy and in situ exploration have led to a new understanding of the composition and history of Mars. Globally, Mars has a basaltic upper crust with regionally variable quantities of plagioclase, pyroxene, and olivine associated with distinctive terrains. Enrichments in olivine (&gt;20%) are found around the largest basins and within late Noachian–early Hesperian lavas. Alkali volcanics are also locally present, pointing to regional differences in igneous processes. Many materials from ancient Mars bear the mineralogic fingerprints of interaction with water. Clay minerals, found in exposures of Noachian crust across the globe, preserve widespread evidence for early weathering, hydrothermal, and diagenetic aqueous environments. Noachian and Hesperian sediments include paleolake deposits with clays, carbonates, sulfates, and chlorides that are more localized in extent. The late Hesperian to Amazonian mineralogic record of water is sparser, though sulfates and silica in some locations indicate local availability of ground and surface waters even in the most recent geologic epoch.

A volcanic district between the Hoggar uplift and the Tenere Rifts: Volcanology, geochemistry and age of the In-Ezzane lavas (Algerian Sahara)

The In-Ezzane volcanic district (EZD), located at the triple junction of Algeria, Niger and Libya belongs to the Eastern Hoggar, covers 350km2 and includes 9 volcanic edifices that are probably aligned along NW–SE faults, parallel to the Tafassasset valley. The low volume (0.7 and 1km3) of emitted lavas, the similar morphology of the monogenic cones and the lack of differentiated rocks indicate that the volcanic activity of the EZD was restricted in time and volume. The new K–Ar age (i.e. 2.86±0.07Ma) indicates that the EZD is contemporaneous with the last alkali volcanism paroxysm in Hoggar and with the nearby Libyan volcanics. The EZD alkali basalts (mainly basanite) show a remarkable homogeneous compositions both in major elements (44.8⩽SiO2⩽45.8wt.%; 5.2⩽(Na2O+K2O)⩽6.2wt.%), trace elements (4.3⩽Th⩽5.5ppm; 34.7⩽La⩽44.7ppm; 16.1<La/Yb(N)<21.6) and radiogenic isotopes (0.70285<87Sr/86Sr<0.70303; 0.51298<143Nd/144Nd<0.51301; (19.212<206Pb/204Pb<19.340, 15.589<207Pb/204Pb<15.602 and 38.834<208Pb/204Pb<38.903). Relative to the Hoggar alkaly basalts the EZD basalts appear systematically impoverished in incompatible elements and show a depleted signature both in Sr and Nd isotopes (almost MORB-like). The Pb isotopes are relatively enriched and intermediate between tholeiites and alkali basalts of the Hoggar. This unusual geochemical signature, is uneasy to reconcile with the known characteristics of the Hoggar swell, and would rather fingerprint a circum cratonic mantle lying beneath the west border of the Murzuq craton (Libya).

English

In recent years, substantial evidence for the early human occupation of the Caucasus during the Quaternary (the last 2.58 million years) has emerged in the literature. The importance of this region for early hominin migration cannot be underestimated. Finds of Homo erectus (thought to be the first hominin to leave Africa), found close to the Georgian village of Dmanisi (see figure 1, over the page) and associated with volcanic deposits with age estimates of around 1.7–1.8 million years, have ignited debates around the topic of ‘the first Eurasians’ (Gabunia et al. 2000). Within Turkey, the oldest known hominin locality is that of Kocabas, in the Buyuk Menderes valley in western Anatolia, where fragments of a cranium, tentatively attributed to Homo erectus, have been found in travertine deposits dated to around 490–510 thousand years (Kappelman et al. 2008). However, earlier hominins are known to have occupied Turkey, as evidenced by finds of Lower Palaeolithic artefacts (Harmankaya, Tanindi 1996), but their chronology is often poorly constrained and the region generally remains poorly understood in terms of hominin dispersal. This three-year pilot project is designed to help initiate renewed efforts to investigate the record of hominin occupation and migration in Turkey. Specifically, we will investigate the environmental changes which provide the context for early hominin occupation in northeastern Turkey by exploiting the palaeoenvironmental archives contained within sedimentary records. Our study area, the upper catchment of the Kura river, spans the Turkish-Georgian border and lies within 100km of the hominin site of Dmanisi (see the maps over the page), and thus there is every reason to believe that hominins would have been present in the area, early in the Quaternary. The Kura river rises in northeastern Turkey and traverses the Lesser Caucasus mountain range, passing the TurkishGeorgian border en route, and ultimately flows through the Transcaucasus depression, through Georgia and on into Azerbaijan where it debouches into the Caspian Sea (see figure 1). The catchment spans a highly active tectonic zone. The mountain ranges of the Caucasus testify to the largescale continental collision (compressional tectonics) and uplift of the Arabian and Eurasian plates which started in the Oligocene (~ 34–23 million years ago) and continued into the Miocene (~ 23–5 million years ago). However, since the Middle Pliocene (~ the last four million years) the Lesser Caucasus and much of the east Anatolian plateau have been subjected to an extensional tectonic regime dominated by strike-slip faulting which has been accompanied by extensive alkali volcanism (Kocyigit et al. 2001). These transform motions have created a series of pull-apart basins which have subsequently filled with sediment (figure 2). More recently, some of these basins have experienced uplift, with active inversion of the basin fill sequences, forcing rivers to incise through their thick sedimentary fills, in response to the uplift. The Kura river, in our Turkish field area, traverses the still deepening (subsiding) Ardahan Basin, but also flows across areas where extensive river terrace sequences have formed in response to localised uplift (for example along the southern edges of the Hanak Basin). The development of this river system is complicated further by extensive localised faulting (normal and oblique, see figure 3), which often offsets river terraces, and the frequent incursion of basaltic lava flows into the developing valley floors from nearby eruption centres. C L I M AT E A N D I T S H I S T O R I C A L & C U R R E N T I M PA C T

Reactivated lithospheric-scale discontinuities localize dynamic uplift of the Moroccan Atlas Mountains

The Atlas Mountains of Morocco, an example of an intracontinental mountain belt, display only modest tectonic shortening, yet have unusually high topography. We present new evidence from receiver functions and shear-wave splitting for localized, nearly vertical offset deformation of both crust-mantle and lithosphere-asthenosphere interfaces at the fl anks of the High Atlas. These offsets coincide with the locations of Jurassic-aged normal fault reactivation that led to tectonic inversion of the region during the Cenozoic. This suggests that a lithospheric-scale discontinuity is involved in orogeny. Another signifi cant step in lithospheric thickness is inferred within the Middle Atlas. Its location corresponds to the source of regional Quaternary alkali volcanism, where the infl ux of melt induced by the shallow asthenosphere appears to be restricted to the lithospheric-scale fault on the northern side of the range. Inferred stretching axes from shear-wave splitting are aligned with the highest topography, suggesting along-strike asthenospheric shearing in mantle fl ow guided by lithospheric topography. Isostatic modeling based on these improved crustal thickness and offset estimates indicates that lithospheric thinning alone does not explain the anomalous Atlas topography. Instead, an upwelling component induced by a hot mantle anomaly is also required to support the Atlas, suggesting that the timing of uplift is contemporaneous with the recent volcanism in the Middle Atlas. These observations provide a refi ned understanding of intracontinental orogeny and localized volcanism.

Ion microprobe (SHRIMP) U–Pb dating of Upper Cretaceous volcanics from the northern Lord Howe Rise, Tasman Sea

Rock samples recovered during the Australian–French AUSFAIR MD153 Survey in 2006 from the northern Lord Howe Rise and the Fairway Ridge provide new constraints on the tectonostratigraphic evolution of the region and represent significant new information from a region in which few rocks have been dated. SHRIMP U–Pb dating of zircon from rocks from the Lord Howe Rise indicates alkali volcanism in the area at 97 Ma (trachyte) and 74 Ma (latite). The older volcanic activity is probably related to the widespread Late Cretaceous magmatism along the eastern Gondwana margin, whereas the younger activity may be related to the opening of the Tasman Sea or rifting in the New Caledonia Basin. The pebbly clasts and shell fossils in some of the associated volcaniclastic rocks provide evidence for the existence of landmasses as a sediment source area in the northern Lord Howe Rise region, and the initial marine incursion into the area around Campanian time.

Alkali volcanism of the Bathymetrists Seamounts chain (Central Atlantic): Description and comparison

were analyzed for major and trace elements at the chemical laboratories of the Geological Institute (Moscow) and the Institute of Precambrian Geology and Geochronology, Russian Academy of Sciences (table). All volcanic rocks dredged from Carter Seamount are strongly altered. The losses of ignition vary from 10 to 14%. In terms of major element composition, the volcanic rocks of group B1 are ascribed to olivine melilitites, while B2 are classed with nephelinites. Olivine melilitites contain 3‐5% clay pseudomorphs (0.5‐ 5 mm in size) after olivine phenocrysts. Microphenocrysts (up to 0.1‐0.3 mm) of clinopyroxene (augite and Ti-augite) and melilite account for 2‐3%. The matrix is made up of lathlike microlites of melilite (20‐30%) and clinopyroxene (augite) (20‐30%), as well as equant grains of Ti-magnetite (about 5%) embedded in a flaky aggregate of secondary minerals (possibly chlorite and zeolite). About 5‐8% is occupied by 0.5‐3 mm pores filled with various zeolites. Nephelinites contain about 2‐3% microphenocrysts of greenish egirine‐augite 0.2‐1 mm in size. The groundmass consists of microlites of egirine‐augite (15‐20%) and nepheline (20‐30%) and grains of Ti-magnetite (about 5%) set in a flaky matrix of secondary minerals. In the oxide‐MgO variation diagrams, the composi

He–Ar and Nd–Sr isotopic compositions of late Pleistocene felsic plutonic back arc basin rocks from Ulleungdo volcanic island, South Korea: Implications for the genesis of young plutonic rocks in a back arc basin

We report analyses of noble gases and Nd–Sr isotopes in mineral separates and whole rocks of late Pleistocene (< 0.2 Ma) monzonites from Ulleungdo, South Korea, a volcanic island within the back arc basin of the Japan island arc. A Rb–Sr mineral isochron age for the monzonites is 0.12 ± 0.01 Ma. K–Ar biotite ages from the same samples gave relatively concordant ages of 0.19 ± 0.01and 0.22 ± 0.01 Ma. 40Ar/ 39Ar yields a similar age of 0.29 ± 0.09 Ma. Geochemical characteristics of the felsic plutonic rocks, which are silica oversaturated alkali felsic rocks (av., 12.5 wt% in K 2O + Na 2O), are similar to those of 30 alkali volcanics from Ulleungdo in terms of concentrations of major, trace and REE elements. The initial Nd–Sr isotopic ratios of the monzonites ( 87Sr/ 86Sr = 0.70454–0.71264, 143Nd/ 144Nd = 0.512528–0.512577) are comparable with those of the alkali volcanics ( 87Sr/ 86Sr = 0.70466–0.70892, 143Nd/ 144Nd = 0.512521–0.512615) erupted in Stage 3 of Ulleungdo volcanism (0.24–0.47 Ma). The high initial 87Sr/ 86Sr values of the monzonites imply that seawater and crustally contaminated pre-existing trachytes may have been melted or assimilated during differentiation of the alkali basaltic magma. A mantle helium component ( 3He/ 4He ratio of up to 6.5 RA) associated with excess argon was found in the monzonites. Feldspar and biotite have preferentially lost helium during slow cooling at depth and/or during their transportation to the surface in a hot host magma. The source magma noble gas isotopic features are well preserved in fluid inclusions in hornblende, and indicate that the magma may be directly derived from subcontinental lithospheric mantle metasomatized by an ancient subduction process, or may have formed as a mixture of MORB-like mantle and crustal components. The radiometric ages, geochemical and Nd–Sr isotopic signatures of the Ulleungdo monzonites as well as the presence of mantle-derived helium and argon, suggests that these felsic plutonic rocks evolved from alkali basaltic magma that formed by partial melting of subcontinental lithospheric mantle beneath the back arc basin located along the active continental margin of the southeastern part of the Eurasian plate.

Alkaline volcanism in the history of the Norwegian-Greenland basin

The opening of the North Atlantic Ocean began in the Late Paleocene and was accompanied by the eruption of submarine and subaerial basalts, which built up submarine plateau and ridges, islands, and volcanoes. The volcanic rocks are dominated by low-K tholeiitic basalts, which associate with almost coeval alkaline rocks (subalkali and alkali basalts and their derivatives, basanites, nephelinites, and others). The oldest alkaline volcanics (58–56 Ma) were formed during the opening of the oceanic rift at its shoulders, in northeastern Greenland and the western Norwegian shelf. It was recently found that 55–53 Ma-old alkali-ultramafic rocks are much more widespread at the eastern coast of Greenland than it was previously thought. The younger occurrences of alkali volcanism with pulses at 30, 10, 5 Ma, and up to the present day were formed on the young oceanic plate and newly formed islands and seamounts. To compare the oceanic and continental volcanism of this region, oceanic volcanics dredged during Cruise 10 of the R/V Akademik Kurchatov were reanalyzed using modern analytical methods (XRF and ICP-MS). This study showed that the oceanic and continental alkaline rocks are significantly different in petrochemical and geochemical characteristics, which is caused by differences in magma generation depths and compositions of the mantle source material. The primary continental alkaline magmas were initially more enriched in incompatible trace elements than oceanic ones. During the shallow-level differentiation of oceanic magmas, trace elements and alkalis could be accumulated in residual melts, but these processes occurred on a minor scale and depended on tectonic conditions.

He-Ar and Nd-Sr isotopic compositions of ultramafic xenoliths and host alkali basalts from the Korean peninsula

Noble gas, Nd and Sr isotopic ratios and major and trace element compositions were determined for ultramafic xenoliths and their host Cenozoic alkali basalts from Baegdusan, Baegryongdo, Jogokri, Jejudo in the Korean peninsula, and Long Quan, close to the Baegdusan in northeastern China, to characterize the lithospheric mantle and the source of alkali basaltic magmatism beneath the active continental margin of the southeastern part of the Eurasian plate. The xenolith samples yield significantly variable 3He/4He ratios ranging from <0.2 to 16.8 RA, with most samples (3.5-7.9 RA) lower than the MORB value (~8 RA). Among them, high 3He/4He ratios obtained by melting the samples are considered to reflect cosmogenic contribution. The 40Ar/36Ar ratios are much lower than the MORB values. Enriched Nd-Sr isotopic compositions in xenoliths and host basalts from the Baegdusan and Baegryongdo areas suggest assimilation of EMII lithosphere and/or continental crust. Widely ranging trace element concentrations in the xenoliths and highly saturated incompatible elements in the host alkali basalts are observed. K-Ar age data show that Cenozoic alkali volcanism in the Korean peninsula occurred intermittently, ranging in age from 21 Ma through 11.5-5.0 Ma to 0.1 Ma, and becoming gradually younger toward the south of the peninsula. Our geochemical and isotopic data suggest the presence of heterogeneously metasomatized enriched lithospheric mantle generated at an ancient subduction zone within the continental margin of the southeastern end of the Eurasian plate. Degree of enrichments by the metasomatism is discussed based on the observed 3He/4He ratios in the xenolithic olivines.

Reinitiation of subduction and magmatic responses in SW Japan during Neogene time

Southwestern Japan during Paleogene time was affected by subduction of the Kula and Pacifi c plates beneath the eastern margin of Eurasia, followed by a brief episode of transform faulting to allow the Shikoku Basin to open at 27‐15 Ma. Subsequent opening of the Japan Sea back-arc basin in middle Miocene time broke Japan away from Eurasia, and SW Japan rotated clockwise ~45o as it drifted south (17‐15 Ma). Southward drift required subduction of the Philippine Sea plate beneath SW Japan, leading to the formation of a magmatic arc. Subduction of the hot Shikoku Basin lithosphere and spreading ridge resulted in distinctive volcanism in the SW Japan forearc between 17 and 12 Ma. This volcanism included midoceanic-ridge basalt (MORB)-like intrusions and oceanic-island basalt (OIB)‐type alkali basalts, felsic plutons in the accretionary prism, and high-magnesium andesites along the Setouchi forearc basin. Mafi c magmas in the outermost forearc originated from the subducted Shikoku Basin spreading ridge, whereas Setouchi high-magnesium andesites (HMA) may have resulted from the interaction of melts of the subducted Philippine Sea plate with the overlying mantle. Felsic magmas in the forearc resulted from melting of Shimanto Belt sediments caused by intrusion of HMA magmas. Persistent rear-arc volcanism was caused by upwelling asthenosphere associated with opening of the Sea of Japan. Rear-arc volcanism began ca. 25 Ma as rift-fi ll low-alkali tholeiite volcanism and was replaced gradually after 12 Ma by alkali basalt volcanism. Upwelling-related alkali volcanism continues up to the present, whereas forearc volcanism ceased at 12 Ma. The volcanic arc narrowed with time as the Philippine Sea slab descended and slowly cooled. Adakitic dacites erupted after 1.7 Ma above the 100-km-depth contour of the subducted Philippine Sea plate, suggesting that melting resulted from interaction of the slab with upwelling asthenosphere. Interactions between upwelled rear-arc asthenosphere and subduction of the hot Philippine Sea slab appear to have been the main controls of magmatism for the Neogene SW Japan arc.

Helium and argon isotopic composition of cenozoic alkali basalts and Mantle-Derived xenoliths from Kuandian, liaoning province, China

The noble gas isotopic composition and content data of 2 alkali basalts, 3 lherzolite xenoliths and one clinopyroxene megacryst from the Kuandian region have confirmed the occurrence of a fractionation of noble gases during magmatism. Light noble gases such as He and Ne are high in mobility and appear to be incompatible as compared with heavy ones (such as Kr and Xe). Therefore, light noble gases are abundant in volcanics, especially in the volcanics with bubbles; lherzolite xenoliths have relatively high heavy noble gases. The clinopyroxene megacryst has the lowest abundance of noble gases, probably due to its highP-T origin. Noble gas isotopic composition of the clinopyroxene megacryst reveals that the mantle source beneath the Kuandian area has an MORB-like reservoir with3He/4He ratio of ≈ 10 Ra (Ra: atmospheric3He/4He ratio) and40Ar/36 Ar ratio of 345.6. The lherzolite xenoliths possess moderate3He/4He ratios of 2.59–4.53 Ra, reflecting the loss of primary helium during rock deformation or metasomatism caused by enriched mantle fluids during the up-lifting. The alkali volcanics have very low3He/4He ratios (0.47–0.61 Ra), indicating a contribution of radiogenic4He, probably having resulted from crust contamination. Most of the samples have excess21Ne and22Ne as compared with atmospheric neon, but Kr and Xe isotopic compositions are indistinguishable from atmospheric values within uncertainties with only individual samples having excess129Xe,134Xe and136Xe.

Mise en évidence d'un volcanisme alcalin intraplaque d'âge Acadien dans la Meseta nord-occidentale (Maroc)

Abstract In northwestern Meseta, the Sidi-Said Maâchou volcanic complex includes dykes, sills and basaltic flows. This volcanism is synchronous with a Paradoxides schist deposit. Most of the rocks have been affected by post-magmatic alteration processes, which have modified their major and trace elements composition. The geochemical characteristics can be discussed on the basis of the high field strength elements, transition elements, Th, Ta and rare earth elements. Hence, the Sidi-Said Maâchou dolerites consist of within-plate alkali volcanism. This volcanism and those recognised in the Anti-Atlas, High-Atlas and eastern Meseta suggest a Cambrian extensive episode during the break-up of the palaeo-Gondwana supercontinent.

400 my of Basic Magmatism in a Single Lithospheric Block during Cratonization: Ion Microprobe Study of Plagioclase Megacrysts in Mafic Rocks from Transbaikalia, Russia

Following accretion of southern Siberian microcontinents to the Siberian craton in the Early Paleozoic, five cycles of K-rich silicic magmatism, progressively decaying in volume, occurred on batholithic scales throughout the Paleozoic and Early Mesozoic, followed by rift-related alkali volcanism of Jurassic to Recent age. Most of the post-Ordovician magmatism occurred within the Ordovician accreted terrane of Transbaikalia, during its 400 my of cratonization. Basic magmas may be critical in the generation of K-rich silicic magma, yet only subordinate volumes of coeval mafic rocks in the silicic plutons and synchronous volcanics are present. Most of the mafic rocks contain plagioclase megacrysts (1–5 mm), and these were used to reconstruct the primary basic magma chemistry and its evolution with time. Optical and scanning electron microscopy studies, and electron microprobe profiling through plagioclase megacrysts of different ages revealed unzoned, Ca-rich cores in a number of crystals in each sample. Several crystals within each rock in a number of rocks within each age group were studied. Several ion microprobe analyses inside each of these cores were made for concentrations of Li, Be, B, F, Mg, P, Cl, K, Ti, Fe, Co, Rb, Sr, Y, Zr, Nb, Cs, Ba, La, Ce, Pr, Nd, Sm, Eu, and Pb. In addition, partition coefficients for the same trace elements and the relevant compositional range of plagioclase were used to convert trace element concentrations in Transbaikalian plagioclase to parental magmatic values. Whole-rock and whole-plagioclase analyses for oxygen isotopes and trace elements were also made to constrain the amount of contamination of basic magma and study its temporal trends. Plagioclase core compositions reveal up to one order of magnitude variation of some trace elements and ratios between suites, and show a progressive change in trace element concentration with decreasing age. Plagioclase megacrysts and the reconstructed basic magmas exhibit depletion in large ion lithophile elements, volatile elements, light rare earth elements and δ18O, and simultaneous increase in high field strength elements and K. We speculate on tectonic implications of the established chemical trends as reflecting progressive incompatible element depletion and devolatilization of a mantle source and increasing prevalence of alkali basalt from the sublithospheric mantle in the course of cratonization.

The Prince of Wales Formation - post-flood basalt alkali volcanism in the Tertiary of East Greenland

In the nunataks of the Prince of Wales Mountains the tholeiite flood basalts of the East Greenland Tertiary Province are unconformably overlain by alkaline lavas. The majority of the alkaline lavas are strongly porphyritic picrites, ankaramites and hawaiites. These rocks have lower 143Nd/144Nd and higher 87Sr/86Sr than the tholeiitic flood basalts and are isotopically akin to ocean island basalts. The alkaline lavas also have high concentrations of incompatible elements which on normalised plots have a pattern which is similar in shape to that of enriched oceanic island basalts. The isotopic and chemical characteristics of these late-stage representatives of the East Greenland volcanic activity are attributed to their derivation from the peripheral regions of the East Greenland plume, the axial region of which was moving progressively eastwards relative to the westwards drift of the Greenland plate. It is proposed that the incompatible element contents of the magmas so produced were dominated by small degree melts formed beneath a cap of continental lithosphere in the marginal regions of the plume.

Active tectonism in the intracontinental Middle Atlas Mountains of Morocco: synchronous crustal shortening and extension

Geological field observations are integrated with digital topography, LANDSAT imagery, and earthquake focal mechanisms to investigate the Middle and Late Quaternary tectonism in the intracontinental Middle Atlas mountain belt in northern Morocco. The NE–SW-trending Middle Atlas Mountains, approximately 80 km in width and about 200 km long, are part of the Atlas system of northwestern Africa and represent an inverted rift that developed into an intracratonic mountain system in the foreland of the Alpine collisional zone. The Middle Atlas is composed of two provinces, the Folded and Tabular Middle Atlas, representing the palaeo-rift and a flank of the palaeo-rift, respectively. Evidence for Late Quaternary tectonism is provided by the analysis of stream morphology in addition to geological relations. Kinematic analysis of fault-slip data and earthquake focal mechanisms demonstrate the coexistence of both extensional and compressional deformation in different areas of the Middle Atlas with a common sinistral component of slip along NE–SW-striking fault zones. Compressional features dominate the Folded Middle Atlas, whereas extension predominates in the Tabular Middle Atlas. Extension is also manifested by widespread Middle to Late Quaternary alkali volcanism. The observed kinematic variations appear to correlate with the Mesozoic palaeogeography; one possible model may involve tectonic escape. This suggests that differences in the structures inherited from the Mesozoic and Palaeozoic may influence the responses of the different regions to the Cenozoic Alpine collision between Europe and northwest Africa.

238U 230Th 226Ra disequilibria in volcanics: A new insight into melting conditions

Using new mass spectrometry techniques developed for the analysis of Ra isotopes, we present 238U 230Th 226Ra disequilibria data from a variety of volcanic settings, and we compare them with previously published data. Two correlations are observed with alkali volcanic data, one between [ 230 Th 238 U ] and [ 230 Th 226 Ra ] and another between the intensity of the disequilibria and the buoyancy flux of the underlying plume. These two correlations prove that partial melting is the major cause of UThRa fractionations in this volcanic context. The 238U 230Th 226Ra disequilibria then place new constraints on some parameters of the classical melting models (batch melting and dynamic melting). We show, nevertheless, that these estimates remain very speculative. The comparison of 238U 230Th 226Ra disequilibria in alkali volcanics, carbonatites and subduction zones shows a clear parallel between the disequilibria value and the type of volcanic context. Such a parallel reflects the diversity of the conditions of magma generation, and shows that the 238U 230Th 226Ra disequilibria systematics are very dependent on the chemical composition of liquids produced during magmatic processes. A systematic difference is observed between disequilibria in MORB and in alkali volcanics, which could indicate that the melting processes in these two volcanic contexts are very different.

Geochronology of alkali volcanism in Oki-Dogo Island, Southwest Japan: Geochemical evolution of basalts related to the opening of the Japan Sea.

Geochronological and geochemical studies were performed on alkali volcanic rocks from Oki-Dogo Island, locating about 60 km off the coast of Southwest Japan inside the Japan Sea. There are big differences both in age and in geochemistry between the oldest alkali rocks and the younger rocks; the former having significant Nb and Ta-deletion and Ba and K2O-enrichment erupted at 19–18 Ma when the opening of the Japan Sea was ongoing, while the latter with no Ta-depletion were active between 5.5–0.6 Ma considerably after the cessation of the rifting activity. Syn-extensional rocks should not have erupted in the active subduction zone but are probably originated from or at least contaminated by the lithosphere beneath the early Proterozoic crust. The lithosphere is considered to have suffered mantle metasomatism to form phlogopite selectively enriched in Ba and K2O by fluids, which had been released from previous subductions at the Asian continental margin. Partial meting of such lithospheric mantle during the active rifting was essential for the generation of Ba and K2O-enriched alkali basalt magmas. The younger basaltic rocks are probably originated either from the asthenosphere or the deeper plume mantle. A suite of volcanic rocks from mugearite through trachyte to alkali rhyolite gave concordant ages of 5.5–5.4 Ma. These ages suggest that a previously reported Rb-Sr whole rock age of 6.8 ± 0.3 Ma does not represent the age of eruption and is probably disturbed by magma mixing between basic and acidic end members.

Geochemistry and petrology of the late cenozoic alkali basalts of the western Carpathians (Czechoslovakia)

Miocene to Quaternary alkali volcanism in the Western Carpathians followed calcalkali volcanism in the same region. The alkali volcanics have restricted composition ranging from alkali basalts and basanites to rare trachyandesites. They contain ultramafic xenoliths, megacrysts and xenocrysts of olivine, pyroxene and amphibole. Pyroxene xenocryst cores differ in color compared to pyroxene phenocrysts. Alkali basaltic rocks are enriched in LILE, LREE (Lan/Ybn = 10–19) and HFS elements. Based on major and trace element distribution and Nd, Sr and Pb isotopic composition, we suggest that metasomatically-enriched manlte peridotite was the source material of the alkali magmas. The range in composition of rocks and their minerals was probably controlled by (1) inhomogenities in the mantle-source and/or different degrees of melting, and (2) metasomatism associated with infiltration of alkali basalt magma through peridotite during ascent through the subcontinental lithosphere. The origin of fluids metasomatically enriching the mande is unclear. Their derivation from the subducted slab cannot be excluded.