Tertiary Volcanism Research Articles

Sub-basalt coal seam structure imaging - results from numerical modelling

Tertiary volcanic basalts (high velocity layers) exist in both the Bowen and Sydney Basins of Eastern Australia. The use of conventional seismic for coal mine planning is often compromised and produces sub-surface images of variable quality because the wave propagation through these heterogeneous basalt layers becomes complex. The difficulty of exploring beneath the Tertiary volcanics makes these areas less attractive for coal mining exploration. Workers involved in sub-basalt imaging in the petroleum sector, mainly give the following reasons for the difficulties in imaging beneath high-speed surface layers: 1. The strong impendance contrast between the basalts and the underlying sedimentary rocks prevents the penetration of seismic energy into target zones (Fruehn et al., 1998; Behera, 2006). 2. The roughness of the basalt boundaries causes significant disruption and scattering of the transmitted wavefield (Behera, 2006). 3. The scattering of heterogeneity of the basalt (Ziolkowski et al., 2001; Hobbs, 2002). To improve seismic imaging in these cases, modifications to data acquisition and data processing procedures which improve data quality and increase S/N were proposed (Fliedner and White, 1999; Hu et al., 2003). Commonly used techniques involve long offset, low frequency and converted wave data acquisition (Ryu, 1997; Wombell et al., 1999; Hanssen et al., 2003; Lau et al., 2007; Spitzer et al., 2008). In data processing, procedures such as pre-stack depth migration, have improved the quality of sub-basalt images (Fruehn et al., 1999; Reshef et al., 2003; Gallagher and Dromgoole, 2008). Wave-equation modelling techniques have also been used to investigate the influence of high speed layers on seismic wave propagation (Battig and Hearn, 2001; Hanssen et al., 2003). In this paper, we use a 2D finite-difference-based full elastic wave-equation forward modelling algorithm to further investigate the effects and characteristics of seismic wave propagation under different basalt settings. Our objective is to provide some insights into the seismic wave propagation and guidance for improving the quality of seismic sections for coal mine investigations.

Tertiary volcanism in Iceland

Tertiary volcanism in Iceland

Eruptive and noneruptive calderas, northeastern San Juan Mountains, Colorado: Where did the ignimbrites come from?

The northeastern San Juan Mountains, the least studied portion of this well-known segment of the Southern Rocky Mountains Volcanic Field are the site of several newly identified and reinterpreted ignimbrite calderas. These calderas document some unique eruptive features not described before from large volcanic systems elsewhere, as based on recent mapping, petrologic data, and a large array of newly determined high-precision, laser-fusion 40Ar/39Ar ages (140 samples). Tightly grouped sanidine ages document exceptionally brief durations of 50–100 k.y. or less for individual Oligocene caldera cycles; biotite ages are more variable and commonly as much as several hundred k.y. older than sanidine from the same volcanic unit. A previously unknown ignimbrite caldera at North Pass, along the Continental Divide in the Cochetopa Hills, was the source of the newly distinguished 32.25-Ma Saguache Creek Tuff (~400–500 km3). This regionally distinctive crystal-poor alkalic rhyolite helps fill an apparent gap in the southwestward migration from older explosive activity, from calderas along the N-S Sawatch locus in central Colorado (youngest, Bonanza Tuff at 33.2 Ma), to the culmination of Tertiary volcanism in the San Juan region, where large-volume ignimbrite eruptions started at ca. 29.5 Ma and peaked with the enormous Fish Canyon Tuff (5000 km3) at 28.0 Ma. The entire North Pass cycle, including caldera-forming Saguache Creek Tuff, thick caldera-filling lavas, and a smaller volume late tuff sheet, is tightly bracketed at 32.25–32.17 Ma. No large ignimbrites were erupted in the interval 32–29 Ma, but a previously unmapped cluster of dacite-rhyolite lava flows and small tuffs, areally associated with a newly recognized intermediate-composition intrusion 5 × 10 km across (largest subvolcanic intrusion in San Juan region) centered 15 km north of the North Pass caldera, marks a near- caldera-size silicic system active at 29.8 Ma. In contrast to the completely filled North Pass caldera that has little surviving topographic expression, no voluminous tuffs vented directly from the adjacent Cochetopa Park caldera, which is morphologically beautifully preserved. Instead, Cochetopa Park subsided passively as the >500 km3 Nelson Mountain Tuff vented at 26.9 Ma from an “underfit” caldera (youngest of the San Luis complex) 30 km to the SW. Three separate regional ignimbrites were erupted sequentially from San Luis calderas within an interval of less than 50–100 k.y., a more rapid recurrence rate for large explosive eruptions than previously documented elsewhere. In eruptive processes, volcanic compositions, areal extent, duration of activity, and magmatic production rates and volumes, the Southern Rocky Mountains Volcanic Field represents present-day erosional remnants of a composite volcanic field, comparable to younger ignimbrite terranes of the Central Andes.

Seismicity and Tectonics of the Blue Ridge Area of the Mogollon Plateau, Arizona

Abstract The seismicity of the Blue Ridge area of the Mogollon Plateau exists within a framework of Tertiary volcanics and normal faulting. The seismicity includes earthquake sequences in 1989 and 2005 with principal events as large as Richter local magnitude ( M L ) 4.6 (National Earthquake Information Center [NEIC]). Analysis of this seismicity indicates that it is characterized by normal faulting on northwest-trending planes. The normal faults form part of the Cataract Creek fault system that has historically generated earthquakes as large as surface wave magnitude ( M S ) 6.2 (Brumbaugh, 1991; Bausch and Brumbaugh, 1997). Regionally these northwest-trending fracture systems/faults of the southern Colorado plateau appear to pose the greatest seismic hazard to the Grand Canyon National Park with almost five million visitors per year and the rapidly growing Flagstaff area.

Application of Numerical Modeling for Groundwater Flow System Analysis in the Akaki Catchment, Central Ethiopia

A three dimensional steady-state finite difference groundwater flow model is used to quantify the groundwater fluxes and analyze the subsurface hydrodynamics in the Akaki catchment by giving particular emphasis to the well field that supplies water to the city of Addis Ababa. The area is characterized by Tertiary volcanics covered with thick residual and alluvial soils. The model is calibrated using head observations from 131 wells. The simulation is made in a two layer unconfined aquifer with spatially variable recharge and hydraulic conductivities under well-defined boundary conditions. The calibrated model is used to forecast groundwater flow pattern, the interaction of groundwater and surface water, and the effect of pumping on the well field under different scenarios. The result indicates that the groundwater flows regionally to the south converging to the major well field. Reservoirs and rivers play an important role in recharging the aquifer. Simulations made under different pumping rate indicate that an increase in pumping rate results in substantial regional groundwater level decline, which will lead to the drying of springs and shallow hand dug wells. Also, it has implications of reversal of flow from contaminated rivers into productive aquifers close to main river courses. The scenario analysis shows that the groundwater potential is not enough to sustain the ever-growing water demand of the city of Addis Ababa. The sensitivity and scenario analysis provided important information on the data gaps and the specific sites to be selected for monitoring, and may be of great help for transient model development. This study has laid the foundation for developing detailed predictive groundwater model, which can be readily used for groundwater management practices.

SOURCE ROCK POTENTIAL OF THE BLUE NILE (ABAY) BASIN, ETHIOPIA

The Blue Nile Basin, a Late Palaeozoic ‐ Mesozoic NW‐SE trending rift basin in central Ethiopia, is filled by up to 3000 m of marine deposits (carbonates, evaporites, black shales and mudstones) and continental siliciclastics. Within this fill, perhaps the most significant source rock potential is associated with the Oxfordian‐Kimmeridgian Upper Hamanlei (Antalo) Limestone Formation which has a TOC of up to 7%. Pyrolysis data indicate that black shales and mudstones in this formation have HI and S2 values up to 613 mgHC/gCorg and 37.4 gHC/kg, respectively. In the Dejen‐Gohatsion area in the centre of the basin, these black shales and mudstones are immature for the generation of oil due to insufficient burial. However, in the Were Ilu area in the NE of the basin, the formation is locally buried to depths of more than 1,500 m beneath Cretaceous sedimentary rocks and Tertiary volcanics. Production index, Tmax, hydrogen index and vitrinite reflectance measurements for shale and mudstone samples from this areas indicate that they are mature for oil generation. Burial history reconstruction and Lopatin modelling indicate that hydrocarbons have been generated in this area from 10Ma to the present day.The presence of an oil seepage at Were Ilu points to the presence of an active petroleum system. Seepage oil samples were analysed using gas chromatography and results indicate that source rock OM was dominated by marine material with some land‐derived organic matter. The Pr/Ph ratio of the seepage oil is less than 1, suggesting a marine depositional environment. n‐alkanes are absent but steranes and triterpanes are present; pentacyclic triterpanes are more abundant than steranes. The black shales and mudstones of the Upper Hamanlei Limestone Formation are inferred to be the source of the seepage oil.Of other formations whose source rock potential was investigated, a sample of the Permian Karroo Group shale was found to be overmature for oil generation; whereas algal‐laminated gypsum samples from the Middle Hamanlei Limestone Formation were organic lean and had little source potential

Petrology and geochemistry of Tertiary volcanic rocks from the İkizce (Ordu) area, NE Turkey: Implications for the evolution of the eastern Pontide paleo-magmatic arc

Tertiary volcanism in the İkizce region at the western edge of the eastern Pontides paleo-magmatic arc is represented by basaltic and andesitic rocks associated with sediments deposited in a shallow basin environment. The basaltic rocks contain plagioclase (An 58–80), olivine (Fo 82–84), clinopyroxene (Wo 44–48En 35–42Fs 7–17), hornblende (Mg# = 0.68–0.76) phenocrysts, and magnetite microcrysts, whereas the andesitic rocks include plagioclase (An 25–61), clinopyroxene (Wo 46–49En 38–43Fs 11–13), hornblende (Mg# = 0.48–0.81), biotite (Mg# = 0.48–0.60) phenocrysts, titanomagnetite, apatite, and zircon microcrysts. Geochemical data indicate magmatic evolution from tholeiitic-alkaline transitional to calc-alkaline characteristics with medium-K contents. The geochemical variation in the rocks can be explained by fractionation of common mineral phases such as clinopyroxene, olivine, hornblende, plagioclase, magnetite, and apatite. The trace elements’ distributions of the volcanic rocks show similarities to those of E-Type MORB, have a shape that is typical of rocks from subduction-related tectonic setting with enrichment in LILE and to a lesser extent in LREE, but depletion in HFSE. The rocks evolved from a parental magma derived from an enriched source formed by subduction induced metasomatism of basaltic rocks, the latter formed through clinopyroxene ± olivine controlled fractionation in a high level magma chamber. The andesitic rocks developed through hornblende ± plagioclase controlled fractionation in shallow level magma chamber(s).

Rayleigh‐wave group‐velocity of the Icelandic crust from correlation of ambient seismic noise

By cross‐correlating 2 years of ambient seismic noise from 31 broadband stations distributed across Iceland, we have been able to measure Rayleigh wave group velocity in three discrete frequency bands centered on 4, 7 and 17 s, sensitive to approximately the top 6, 10 and 25 km of the crust, for 289, 420 and 139 station‐to‐station paths, respectively. These are inverted to yield tomographic group velocity maps of the Icelandic crust. The results correlate with the major tectonic features on Iceland, and agree well with earlier seismic studies based on traditional methods (active source and earthquake). Specifically, we obtain low velocities in the central parts of Iceland, coinciding with the rift zones and young crust. In contrast, the parts of Iceland covered with older Tertiary volcanics reveal relatively higher velocities. At the shorter periods (4 and 7 s) this correlation reflects porosity, degree of fracturing and alteration. At the longest period (17 s), i.e. at greater depth, this correlation may reflect the thermal state of the crust, with warm and seismically slow material near the rifts and cooler and seismically faster material beneath the Tertiary formations to the east and west.

Crustal deformation of the Yellowstone–Snake River Plain volcano‐tectonic system: Campaign and continuous GPS observations, 1987–2004

The Yellowstone–Snake River Plain tectonomagmatic province resulted from Late Tertiary volcanism in western North America, producing three large, caldera‐forming eruptions at the Yellowstone Plateau in the last 2 Myr. To understand the kinematics and geodynamics of this volcanic system, the University of Utah conducted seven GPS campaigns at 140 sites between 1987 and 2003 and installed a network of 15 permanent stations. GPS deployments focused on the Yellowstone caldera, the Hebgen Lake and Teton faults, and the eastern Snake River Plain. The GPS data revealed periods of uplift and subsidence of the Yellowstone caldera at rates up to 15 mm/yr. From 1987 to 1995, the caldera subsided and contracted, implying volume loss. From 1995 to 2000, deformation shifted to inflation and extension northwest of the caldera. From 2000 to 2003, uplift continued to the northwest while caldera subsidence was renewed. The GPS observations also revealed extension across the Hebgen Lake fault and fault‐normal contraction across the Teton fault. Deformation rates of the Yellowstone caldera and Hebgen Lake fault were converted to equivalent total moment rates, which exceeded historic seismic moment release and late Quaternary fault slip‐derived moment release by an order of magnitude. The Yellowstone caldera deformation trends were superimposed on regional southwest extension of the Yellowstone Plateau at up to 4.3 ± 0.2 mm/yr, while the eastern Snake River Plain moved southwest as a slower rate at 2.1 ± 0.2 mm/yr. This southwest extension of the Yellowstone–Snake River Plain system merged into east‐west extension of the Basin‐Range province.

IN MEMORY OF LAURO MORTEN

There are people you can not forget. Lauro Morten was one of them. At a first glance he looked like a rough man, forged in the tradition of mountain people. Indeed, he loved mountains and he spent most of his energy to understand their evolution. He was always enthusiastic in planning new field excursions in every part of the world. We remember his company during long trips on minibus and the lively discussions in the field. We appreciated his honesty and directness in scientific discussion. His scientific career lasted over 40 years and contributed to the improvement of the Italian petrologic community. His first work, dated 1971, dealt with the ultramafic xenoliths collected in the Tertiary volcanics from the Lessini Mts. area. The study of mantle rocks, both as xenoliths enclosed in basaltic volcanics and exposed as orogenic ophiolitic peridotite massifs, was his main research interest that he carried out until his last days. In 1986 he accepted a full professor position at the University of Calabria in Cosenza, where he organized and chaired the Fourth International Eclogite Conference (1993). In those years he addressed other research topics, including the granulite rocks of the Catena Costiera and the ophiolitic units occurring in northern Calabria. In Calabria he was appointed as Head of the Department of Earth Sciences and he was also in charge of the courses for the diploma degree in Geological Sciences. He was deeply involved in the establishment and cultural growth of a local petrological school, which benefited from his brilliant and generous teaching. In his last years Lauro Morten served as full professor of Petrology and Petrography at the University of Bologna. He went back to the basement of his beloved Nonsberg range. His most recent research was devoted to mantle metasomatism and crustmantle interactions. In particular, he always encouraged his post-graduated students to be scientifically critical, honest and thoughtful. Even after he was diagnosed his illness, he continued to work in cooperation with scientists from all over the world. His family, colleagues, students and friends miss his cleverness and company. Thanks a lot, Lauro, for communicating your love of the beauty of rocks and the Nonsberg range, where you now rest in peace.

C.H. Emeleus and B.R. Bell, with contributions from Stephenson, D. British Regional Geology: the Palaeogene Volcanic Districts of Scotland (Fourth edition). British Geological Survey, Nottingham. 2005. ISBN: 0-85272-519-1. Price £18.

This, the latest addition to the ‘new look’ BGS regional handbooks, is the long-awaited replacement for the 3rd edition of The Tertiary Volcanic Districts (1961). During the intervening period there have been so many advances in petrology, geochemistry, geophysics and geology in general that this edition bears only the most superficial resemblance to it predecessors. Although there has been no systematic resurvey of the Palaeogene volcanic districts by British Geological Survey staff since the 1930s, the pioneering work of such giants of geology as, Alfred Harker, J.E. Richey and their contemporaries prepared the ground for the major expansion of interest on the area after the Second World War. The resulting plethora of PhD theses and scientific papers is reflected in the significantly increased number of references in the 4th edition, the vast majority of which are from the 1960s onwards. These are now concentrated near the …

Tertiary Mafic Lavas of Turkana, Kenya: Constraints on East African Plume Structure and the Occurrence of High-μ Volcanism in Africa

The East African Rift System is important to understanding plume-initiated rifting as manifest in the geochemistry of mafic lavas erupted along the rift throughout its evolution. We present new data from high-MgO Tertiary lavas from Turkana, northern Kenya, to investigate regional melt source components, to identify the depths and degrees of melting, and to characterize spatially and temporally the chemical structure of the underlying mantle. The Turkana area is a region of high lithospheric extension that sits between two topographic uplifts thought to be surface expressions of one or more upwelling mantle plumes. Thinning of local crust is believed to be accompanied by widespread removal of the mantle lithosphere, causing the asthenosphere to be in close contact with the overlying crust. New geochemical data on basanites, picrites and basalts (MgO >7 wt %) tightly constrain the primary melt source regions of Tertiary volcanism. Initial isotopic signatures (143Nd/144Nd = 0·51267–0·51283, 87Sr/86Sr = 0·7031–0·7036) and trace element abundances (Ce/Pb ∼30, La/Nb = 0·6–0·8 and Ba/Nb = 3–10) in these lavas are consistent with derivation from sub-lithospheric sources. Basalts and picrites erupted between ∼23 and 20 Ma have Sr–Nd–Pb–He isotopic characteristics indicative of high-μ influence, record high depths and degrees of partial melting, and are associated with rift propagation to the north and south. Accordingly, these lavas sample a source region that is geochemically distinct from that reflected both in Oligocene Ethiopian flood basalts and in the modern Afar region. The geochemical data support numerical and theoretical models as well as tomographic results providing for a complex thermal structure in the mantle beneath East Africa and are interpreted to reflect isotopically distinct plume heads beneath Tanzania and Afar that are derived from the chemically heterogeneous South African superplume.

New gravity maps of the Eastern Alps and significance for the crustal structures

The deep seismic profile T ransalp crosses, from north to south, Germany, Austria and Italy. The gravity measurements for each country were made by national agencies with different reference systems and data reduction methods. Within the frame of the T ransalp-project a comprehensive database of the Eastern Alps was compiled covering an area of 3.5° by 4° in longitude and latitude (275 by 445 km), respectively. To increase the data coverage in the south Alpine area two gravity surveys were carried out, resulting in 469 areally distributed new stations, of which 215 have been measured with the intent to improve the geoid in the area of the planned Brenner Basistunnel (BBT). The resulting gravity database is the best in terms of resolution and data quality presently available for the Eastern Alps. Here the free air, Bouguer and isostatic gravity fields are critically discussed. The spatial density of existing gravity stations in the three countries is discussed. On the Italian side of the Alps the spatial density is rather sparse compared to the Austrian side. The Bouguer-gravity field varies between − 190 * 10 − 5 m/s 2 and + 25 * 10 − 5 m/s 2, with the minimum located along the Alpine high topographic chain, but with a small offset (a few tens of km) to the greatest topographic elevation, showing that the Airy-type local isostatic equilibrium does not fully apply here. The maximum of the Bouguer anomaly has an elongated shape of 100 by 50 km located between the towns of Verona and Vicenza and covers the Venetian Tertiary Volcanic Province (VTVP), a feature not directly related to the plate collision in the Eastern Alps. The gravity high is only partly explainable by high-density magmatic rocks and requires also a deeper source, like a shallowing of the Moho. The isostatic residual anomalies (Airy model) are in the range ± 50 * 10 − 5 m/s 2, with the greatest positive anomaly corresponding to the location of the VTVP, indicating here under-compensation of masses. At last a discussion of a 2D density model based on reflection seismic data and receiver functions is made.

Petrology and geochemistry of Early Tertiary volcanism of the Mendejin area, Iran, and implications for magma genesis and tectonomagmatic setting

The Mendejin area, NW Iran, is part of the western Alborz-Azarbaijan zone which is one of the most structurally—and magmatically-active zones of Iran. Volcanic rocks with calc-alkaline and, locally, alkaline features cover an extensive part of this zone. The Mendejin volcanic rocks, Eocene-Oligocene in age, include tuffs and volcanoclastic rocks of dacite, andesite, basaltic andesite, and basalt composition. Felsic (andesite, dacite, and rhyodacite) and basic rocks (basalt, basaltic andesite and andesite) commonly occur in successive layers. This alternation along with multiple occurrences of various types of tuffs suggests prolonged and successive magmatic activity during Eocene-Oligocene in NW Iran. Fractional crystallization has been the most important factor controlling geochemical characteristics of the magma. However, absence of linear correlations on variation diagrams of some immobile elements (such as Al2O3, TiO2, P2O5 and Ga) and poorly-developed trends on variation diagrams of Na2O, MgO, MnO, CaO, Fe2O3, Nb, Nd, Y, La, Ce, Th, Hf, Sc, Zn, V, Ni and Co versus SiO2 indicate that, other than crystal (olivine, pyroxene, plagioclase, biotite, hornblende, zircon, monazite and apatite) fractionation, crustal processes (such as assimilation) have also affected the chemistry of the Mendejin magma. It appears that the basic magma has originated from the mantle whereas the felsic magma resulted from modification in the mantle-derived magma by assimilation in an active continental margin.

Chapter 8 Tertiary volcanicity

The Centenary of the Netherlands Indies Geological Survey was commemorated by the publication of a synthesis of the geology of Indonesia by van Bemmelen (1949). In his account of the geology of Sumatra van Bemmelen (1949) described three distinct, but continuous, cycles of volcanic activity during the Tertiary and Quaternary: Old Neogene (Late Oligocene-Mid-Miocene); Young Neogene (Mid-Miocene-early in the Quaternary); and Young Quaternary. The first cycle began with the 'Old Andesites', and ended with the Mid-Miocene uplift of the Barisan Mountains. The second cycle commenced with the eruption of basic igneous products and concluded with an acidic phase which coincided with a second episode of uplift of the Barisan Mountains. Subsequently, knowledge of the Tertiary volcanic rocks in Sumatra has been refined as the result of programmes of geological mapping in the early 1970s by the Geological Survey of Indonesia and the United States Geological Survey, and between 1975 and the mid-1990s by the Geological Research and Development Centre, the Directorate of Mineral Resources and the British Geological Survey. Exploration by oil and mineral companies has also provided data concerning the distribution of Tertiary plutonic rocks in the Pre-Tertiary basement and in the Tertiary sedimentary basins, of volcanic units interbedded with sediments. Further contributions to the understanding of Tertiary volcanicity in Sumatra and its forearc islands have been made by academic researchers and post-graduate students from the Institute of Technology, Bandung and the University of London, in collaboration with the Geological Research and Development Centre, LIPI and LEMIGAS, and the

Geochemical and Pb-Sr-Nd Isotopic Constraints Indicating an Enriched-Mantle Source for Late Cretaceous to Early Tertiary Volcanism, Central Anatolia, Turkey

Bulk-rock major, trace, and REE geochemistry and Pb-Sr-Nd isotopic compositions identify mantle sources involved in the genesis of volcanic rocks of the Upper Cretaceous-Lower Tertiary Ulukişla Formation, Çamard-Ulukişla Basin, Nigde Province, central Anatolia. Incompatible trace-element patterns exhibit a large Nb-Ta trough and strong enrichment of LILE such as Ba, Th and U, and LREE, which indicate a subduction-zone signature. Trace-element ratios are compatible with a subcontinental lithospheric source. Isotopic data demonstrate the presence of an EMII-like protolith (87Sr/86Sr = 0.707242-0.707582, 143Nd/144Nd = 0.512336-0.512390, 206Pb/204Pb = 18.70-18.917, 207Pb/204Pb = 15.716-15.796, 208Pb/204Pb = 39.157-39.45). These geochemical and isotopic data indicate the derivation of the studied volcanic rocks from an enriched subcontinental mantle source, modified by earlier subduction events. This petrogenetic conclusion is compatible with a geodynamic setting of post-collisional extension for the Çamard-Ulukişla Basin.

Glassy orthopyroxene granodiorites of the Pannonian Basin: tracers of ultra-high-temperature deep-crustal anatexis triggered by Tertiary basaltic volcanism

Glassy orthopyroxene granodiorite-tonalite (named pincinite after type locality) was described from basaltic lapilli tuffs of the Pliocene maar near Pincina village in the Slovakian part of the Pannonian Basin. Two pincinite types exhibit a qualitatively similar mineral composition (quartz, An20–55 plagioclase, intergranular silicic glass with orthopyroxene and ilmenite, ±K-feldspar), but strongly different redox potential and formation PT conditions. Peraluminous pincinite is reduced (6–7% of total iron as Fe3+ in corundum-normative intergranular dacitic glass) and contains ilmenite with 8–10 mol% Fe2O3 and orthopyroxene dominated by ferrosilite. High-density (up to 0.85 g/cm3) primary CO2 inclusions with minor H2, CH4, H2S, CO and N2 ( 0.705–0.706, δ18O>9‰ V-SMOW) around alkali basalt reservoir in depths between 17 and 20 km, and around late stage derivatives of the basalt fractionation, intruding the crust up to depths of 10–11 km. Low water activity in the pincinite parental melt was caused by CO2-flux from the Tertiary basaltic reservoirs and intrusions. The anatexis leads to generation of a melt-depleted granulitic crust beneath the Pannonian Basin, and the pincinites are interpreted as equivalents of igneous charnockites and enderbites quenched at temperatures above solidus and unaffected by sub-solidus re-equilibration and metamorphic overprint.

Palaeomagnetism and K-Ar and40Ar/39Ar ages in the Ali Sabieh area (Republic of Djibouti and Ethiopia): constraints on the mechanism of Aden ridge propagation into southeastern Afar during the last 10 Myr

SUMMARY A new detailed palaeomagnetic study of Tertiary volcanics, including extensive K-Ar and 40 Ar/ 39 Ar dating, helps constrain the deformation mechanisms related to the opening processes of the Afar depression (Ethiopia and Djibouti). Much of the Afar depression is bounded by 30 Myr old flood basalts and floored by the ca 2 Myr old Stratoid basalts, and evidence for pre-2 Ma deformation processes is accessible only on its borders. K-Ar and 40 Ar/ 39 Ar dating of several mineral phases from rhyolitic samples from the Ali Sabieh block shows indistinguishable ages around 20 Myr. These ages can be linked to separation of this block in relation to continental breakup. Different amounts of rotation are found to the north and south of the Holhol fault zone, which cuts across the northern part of the Ali Sabieh block. The southern domain did not record any rotation for the last 8 Myr, whereas the northern domain experienced approximately 12 ± 9 ◦ of clockwise rotation. We propose to link this rotation to the counter-clockwise rotation observed in the Danakil block since 7 Ma. This provides new constraints on the early phases of rifting and opening of the southern Afar depression in connection with the propagation of the Aden ridge. A kinematic model of propagation and transfer of extension within southern Afar is proposed, with particular emphasis on the previously poorly-known period from 10 to 4 Ma.

Geochemistry of volcanic rocks in Barton and Weaver peninsulas, King George Island, Antarctica: Implications for arc maturity and correlation with fossilized volcanic centers

We investigated geochemical characteristics of the Paleocene-Eocene volcanic rocks in Barton and Weaver peninsulas, King George Island, Antarctica. Volcanic rocks are predominantly tholeiitic, and show geochemical properties typical for island arc volcanism. The volcanic rocks can be subdivided into three groups based on the differences of geochemistry and regional distribution. The group 1 rocks are distributed in Weaver Peninsula and in the central part of Barton Peninsula. They show relatively mafic compositions (basalts to basaltic andesites) with the lowest level of total REEs. The group 2 rocks are widely distributed in Barton Peninsula, and show intermediate compositions (basaltic andesites to andesites) with the highest LILE/HFSE and LREE/HREE ratios. The group 3 rocks occur as intermediate dikes or plugs along the southern coast of Barton Peninsula. They generally show similar compositions to those of the group 2, but have smaller LREE/HREE ratios. The parental magma of the group 1 seems to be most depleted among three groups, whereas that of the group 2 rocks enriched in LILEs and LREEs. Predominance of tholeiite series rocks, general absence of the basement complex, and difficulty of identifying dual volcanic chains suggest that the Early Tertiary volcanism in King George Island occurred in an immature island arc without thickened continental-type crust. Geochemical correlations between volcanic rocks and fossilized volcanic vents suggest that volcanic groups can be linked with vents: the group 1 with Weaver Nunatak, and the group 2 with Three Brothers Hill, Florence Nunatak and/or Czajkowski Needle. However, the group 3 seems to be correlative with the Chottaebawi plug (the Narebski Point), or represent distinct dike swarms.

Geodynamic implications of deep mantle upwelling in the source of Tertiary volcanics from the Veneto region (South-Eastern Alps)

Major and trace element and Sr–Nd–Hf–Pb isotopic data for the most primitive Tertiary lavas from the Veneto region (South-Eastern Alps, Italy) show the typical features of HIMU hotspot volcanism, variably diluted by a depleted asthenospheric mantle component (87Sr/86Sri=0.70306–0.70378; εNdi=+3.9 to +6.8; εHfi=+6.4 to +8.1, 206Pb/204Pbi=18.786–19.574). P-wave seismic tomography of the mantle below the Veneto region shows the presence of low-velocity anomalies at depth, which is consistent with possible upwellings of plume material. Between the depths of 100–250 km the velocity anomalies are approximately 2–2.5% slower than average, implying a temperature excess of about 220–280 K, in agreement with estimates for other mantle plumes in the world. In this context, the Veneto volcanics may represent the shallow expression of a mantle upflow. The presence of a HIMU-DM component in a collision environment has significant geodynamic implications. Slab detachment and ensuing rise of deep mantle material into the lithospheric gap is proposed to be a viable mechanism of hotspot magmatism in a subduction zone setting.