Trachyte Lava Flows Research Articles

A multi-methodological approach to record dynamics and timescales of the plumbing system of Zaro (Ischia Island, Italy)

Determining the time spans of processes related to the assembly of eruptible magma at active volcanoes is fundamental to understand magma chamber dynamics and assess volcanic hazard. This information can be recorded in the chemical zoning of crystals. Nevertheless, this kind of study is still poorly employed for the active volcanoes of the Neapolitan area (Southern Italy), in particular, for Ischia island where the risk is extremely high and this information can provide the basis for probabilistic volcanic hazard assessment. For these reasons, we acquired chemical composition on clinopyroxene crystals erupted at Ischia during the Zaro eruption (6.6 ± 2.2 ka) and performed numerical simulations of the input of mafic magma into a trachytic reservoir, in order to investigate various aspects of pre-eruptive dynamics occurring at different timescales. This event emplaced a ~ 0.1 km3 lava complex, in which the main trachytic lava flows host abundant mafic to felsic enclaves. Previous petrological investigation suggested that mafic magma(s) mixed/mingled with a trachytic one, before the eruption. In this work, the clinopyroxene zoning patterns depict the growth of crystals in different magmatic environments, recording sequential changes occurred in the plumbing system before the eruption. The evolution of the plumbing system involved a hierarchy of timescales: a few hours for magma mingling caused by mafic recharge(s) and likely occurred multiple times over a decade during which a dominant magmatic environment was sustained before the eruption. Such timescales must be considered in volcanic hazard assessment at Ischia and similar active volcanoes in densely populated areas.

Single-crystal 40Ar/39Ar dating of the Montagna Grande-Monte Gibele trachytic shield volcano, Pantelleria (Strait of Sicily rift zone), Italy

Montagna Grande and Monte Gibele represent the northwestern and southeastern portions of a shield volcano (hereafter “MGV”) located entirely within the Cinque Denti caldera structure on Pantelleria. The Cinque Denti caldera formed during the 45.7 ± 1.0 ka eruption of the pantelleritic Green Tuff. Subsequent isostatic compensation was achieved by the eruption within the caldera of ∼3 km3 of trachyte lavas that comprise the MGV and are thought to represent renewed tapping of the Green Tuff magma reservoir. The time interval between the eruption of the Green Tuff and the MGV trachytes has not been well constrained, with previous KAr ages from alkali feldspar yielding ages between 44 ± 8 and 28 ± 16 ka. In this study we report the results of new single-crystal anorthoclase incremental heating and total fusion 40Ar/39Ar ages collected from three trachyte lava flows collected at different sites and at different stratigraphic heights that provided ages of 26.2 ± 2.0, 22.5 ± 0.8 ka, and 22.3 ± 2.8 ka (uncertainties at 2σ). The results indicate that there was a repose period of up to ∼20,000 years between the eruption of the caldera-forming ignimbrite and the caldera-filling trachytes. The timing of our oldest dated sample is coincident with renewed basaltic volcanism in the northwest part of the island at ∼28 ka and suggests that the eruption of the trachyte was promoted by mafic recharge. We briefly review the petrogenetic processes operative in the reservoir during the repose period. Modelling of the thermal and compositional evolution of the fractionating system indicates an eruption rate of 7.5 × 10−4 km3/yr over the ∼4 ka eruption period.

Quaternary ring plain- and valley-confined pyroclastic deposits of Aragats stratovolcano (Lesser Caucasus): Lithofacies, geochronology and eruption history

Aragats stratovolcano, located on the South Armenian block, a micro-plate in the Arabian-Eurasian collision zone, has a complex and long-lasting Quaternary eruption history. Cataclysmic eruptions formed widespread pyroclastic deposits, which are interrupted by numerous basaltic to trachytic lava flows as well as lacustrine and lacustrine-alluvial sediments. Contrary to previous concepts, detailed mapping of lithofacies variation combined with stratigraphic documentation and new high-precision 40Ar/39Ar ages revealed four eruption cycles, each started with mafic lavas and generally finalized with pyroclastic deposits. These data contributed to a fundamental reinterpretation of the development of the Aragats Volcanic Province (AVP). Within an estimated activity of one million years, Aragats itself experienced three eruption stages, two of which culminated in pyroclastic deposits of remarkable lithofacies variability with ages of 0.97 to 0.90 Ma and 0.75 to 0.65 Ma, respectively. Vertical and lateral changes in proximal and distal facies distribution of pyroclastic flow deposits indicate temporal variations in eruption intensity and periods of relative volcanic quiescence.Aragats' explosive activities were dominated by Plinian eruptions with columns as high as 25 km, while others are characterized by low eruption columns followed by instantaneous collapse, in one case associated with vent widening, possible caldera collapse and breccia formation. Furthermore, one AVP unit features juvenile lapilli and blocks including banded and armored pumice, indicating magma mingling and their involvement in the erupting mixture before magma fragmentation.

Rifting of the northern margin of the Indian craton in the Early Cretaceous: Insight from the Aulis Trachyte of the Lesser Himalaya (Nepal)

Abstract To reconstruct the early tectonic history of the Himalayan orogen before final India-Asia collision, we carried out geochemical and geochronological studies on the Early Cretaceous Aulis Trachyte of the Lesser Himalaya. The trace-element geochemistry of the trachytic lava flows suggests formation in a rift setting, and zircon U-Pb ages indicate that volcanism occurred in Early Cretaceous time. The felsic volcanics show enrichment of more incompatible elements and rare earth elements, a pattern that is identical to the trachyte from the East African Rift (Kenya rift), with conspicuous negative anomalies of Nb, P, and Ti. Although much of the zircon age data are discordant, they strongly suggest an Early Cretaceous eruption age, which is in agreement with the fossil age of intravolcanic siltstones. The Aulis Trachyte provides the first corroboration of Cretaceous rifting in the Lesser Himalaya as suggested by paleomagnetic data associated with the concept that the northern margin of India separated as a microcontinent and drifted north in the Neo-Tethys before terminal collision of India with Asia.

The Puesto Piris Formation: Evidence of basin-development in the North Patagonian Massif during crustal extension associated with Gondwana breakup

The Marifil Volcanic Complex, exposed in the eastern North Patagonian Massif, Argentina, includes up to 550 m of red conglomerates, sandstones, black siltstones, limestones, and reworked tuff of the Puesto Piris Formation. The basal part of this unit, which was deposited in high-gradient topographic relief, is composed of conglomerates and sandstones with thin layers of reworked tuffs. The lithofacies associations of the basal part indicate that the depositional mechanisms were mantled and gravitational flows. The middle part of the unit consists of fine sandstones, limestones, and black siltstones that were deposited in low-energy fluvial and lacustrine environments. The outcrops are located along the NE–SW direction and the major thickest units represented by limestones and siltstones, occur near the southeastern border of this NE–SW depocenter. Since the rhyolitic and trachytic lava flows and tuffs of the Marifil Volcanic Complex are interbedded with the sedimentary sequences of the Puesto Piris Formation, both units are coeval. Zircon U–Pb age was obtained for a trachytic lava flow (193.4 ± 3.1 Ma) suggesting that sedimentation and volcanism are Sinemurian. This extensional episode was recorded in the eastern, western, and southwestern sectors of the North Patagonian Massif, and is possibly associated with the Gondwana supercontinent breakup.

Constraints of texture and composition of clinopyroxene phenocrysts of Holocene volcanic rocks on a magmatic plumbing system beneath Tengchong, SW China

Understanding processes of magma replenishment in a magma plumbing system is essential to predict eruption potential of a dormant volcano. In this study, we present new petrologic and thermobarometric data for youngest lava flows from the Holocene Heikongshan volcano in the Tengchong area, SW China. Clinopyroxene phenocrysts from the trachytic lava flows display various textural/compositional zoning styles (i.e., normal, reverse and oscillatory). Such zoning patterns are indicative of an open magmatic plumbing system with multiphase magma replenishment and mixing, which were likely a key drive of the volcanic eruptions. Thermobarometric calculations of these zoned clinopyroxene phenocrysts yield crystallization pressures of 3.8–7.1 kbar (peak at 4.5–7.0 kbar), corresponding to a magma chamber at depths of 14–21 km. The calculated depths are consistent with the large low-resistivity body at 12–30 km beneath the Heikongshan volcano, implying that the magmatic plumbing system may still be active. Recent earthquakes in the Tengchong area suggest that the regional strike-slip faulting are still active, and may trigger future volcanic eruptions if the magma chamber(s) beneath the Tengchong volcanic field is disturbed, in spite of the volcanic quiescence since 1609 CE.

Differentiating lava slip planes from tectonic faults: A key issue in structural geology

Abstract The correct geological and structural survey of rock successions containing lava deposits may be biased by the possible confusion between tectonic faults and slip planes produced by lava flow sectors moving at different velocities during emplacement. Microtectonic features on slip planes resulting from lava emplacement, here named “lava slip planes”, can have a very similar appearance to slickensides formed under tectonic deformation. In order to come up with criteria to distinguish between the two types of structures, slip planes of various extent from recent trachytic lava flows on Ischia Island (Italy), and basaltic lavas at Mt. Etna (Italy) and along the Northern Volcanic Zone in Iceland, have been studied in detail in the field, sampled, and analysed also with the Scanning Electrone Microscope. Strike-slip planes dominate along flow margins while reverse slip planes dominate near eruptive centers. Normal slip planes can also be found at the sides and at the front of lava flows. Slip planes form at thermal boundaries that enable plastic/brittle deformation and favour the development of shear zones. The rheological properties of the lava should be such that the material has developed a shear strength. Although the best criterion to recognize a lava slip plane is the evaluation of its confinement to a single depositional unit, this approach can be rarely used, due to the frequently limited (decimetre- to meter-sized) outcrop of slip planes. The best criterion, which can be used directly in the outcrop as well as by means of a microscope, is the individuation of burrs, which are sub-millimeter to centimeter-size portions of the slip surface that get dragged away in the direction of movement of the missing block.

Marco geológico y tectónico de la Isla del Coco y la región marítima circunvecina, Costa Rica

La Isla del Coco es la única isla oceánica y el único afloramiento subaéreo de la Cordillera Volcánica del Coco, el rasgo geográfico y geológico más extenso en aguas territoriales costarricenses. Desde el punto de vista geológico, está conformada por rocas volcánicas, predominantemente coladas de lavas basálticas y traquíticas en menor cantidad, con rocas piroclásticas y epiclásticas subordinadas. Posee suelos, coluvios y depósitos de playas (arenas y cantos) superficiales; su topografía es muy variable pero predomina el relieve quebrado y rugoso. La Isla del Coco es la parte emergida de un volcán submarino de evolución compleja, desarrollado a partir y durante el Pleistoceno Inferior (entre 2.2 y 1.5 millones de años, Ma), producto de una anomalía térmica en el manto a través de varias fisuras, que originaron varios alineamientos de volcanes submarinos. La Isla del Coco se encuentra en un estadio de erosión activo, y sus arcos y plataformas sumergidas (90-110m y 183m) son probablemente el producto de una erosión subaérea durante las dos últimas máximas glaciaciones, combinados quizás con la subsidencia debida al enfriamiento del escudo volcánico y de la corteza oceánica. Al ser la isla geológicamente joven, posee importantes implicaciones para la comprensión de la evolución y el endemismo de su biodiversidad. Las amenazas en la geodinámica principales identificadas son los deslizamientos, los tsunamis y, en menor grado, la sismicidad, poco frecuente y con magnitudes moderadas (≤ 5.8 Mw hasta la fecha) dentro de un radio de alrededor de 300km, predominantemente asociada con fallas dextrales de rumbo N-S. Los pocos datos obtenidos hasta el momento indican que hay un cierto grado de actividad sísmica en los alrededores de la Isla del Coco, relacionados con las fallas locales. Algunos pequeños tsunamis históricos y prehistóricos han afectado a la Isla del Coco y los sismos así como la precipitación pluvial elevada han generado deslizamientos. La licuefacción está restringida a las dos playas arenosas (bahías Chatham y Wafer).

Middle Triassic trachytic lava flows associated with coeval dyke swarm in the North Patagonian Massif: A postorogenic magmatism related to extensional collapse of the Gondwanide orogen

Middle Triassic trachytic lava flows associated with coeval dyke swarm in the North Patagonian Massif: A postorogenic magmatism related to extensional collapse of the Gondwanide orogen

Geochronology and geochemistry of Eocene-aged volcanic rocks around the Bafra (Samsun, N Turkey) area: Constraints for the interaction of lithospheric mantle and crustal melts

40Ar–39Ar age, whole-rock chemical, and Sr–Nd isotope data are presented for the post-collisional, Eocene (51.3–44.1Ma)-aged volcanic rocks from the Bafra (Samsun) area in the western part of the Eastern Pontides (N Turkey) aiming to unravel their sources and evolutionary history. The studied Eocene volcanic rocks can be divided into two groups: analcime-bearing (tephritic lava flows and dykes) and analcime-free (basaltic to trachytic lava flows and basaltic dykes). The analcime-bearing volcanic rocks have a fine-grained porphyritic texture with clinopyroxene phenocrysts, whereas analcime-free volcanic rocks show a variety of textures including hyalo-microlitic microgranular porphyritic, intersertal, trachytic, fluidal, and glomeroporphyritic. The volcanic rocks also show evidence of mineral-melt disequilibrium textures such as sieved, rounded, and corroded plagioclases, partially melted and dissolved clinopyroxenes and poikilitic texture. Petrochemically, the parental magmas of the volcanic rocks evolved from alkaline to calc-alkaline lava suites and include high-K and shoshonitic compositions. They display enrichments in light rare earth and large ion lithophile elements such as Sr, K, and Rb, as well as depletions in high field strength elements such as Nb, Ta, Zr, and Ti, resembling subduction-related magmas. The analcime-bearing and -free volcanic rocks share similar incompatible element ratios and chondrite-normalised rare rearth element patterns, indicating that they originated from similar sources. They also have relatively low to moderate initial 87Sr/86Sr (0.7042–0.7051), high positive εNd(t) values (+0.20 to +3.32), and depleted mantle Nd model ages (TDM1=0.63–0.93Ga, TDM2=0.58–0.84Ga). The bulk-rock chemical and Sr–Nd isotope features as well as the high Rb/Y and Th/Zr, but low Nb/Zr and Nb/Y ratios, indicate that the volcanic rocks were derived from a lithospheric mantle source that had been metasomatised by slab-derived fluids. Trace element modelling suggests that the parental magma(s) of the volcanic rocks represent mixtures of melts derived by low-degree (~5–10%) partial melting of spinel–lherzolite (40–85%) and garnet–lherzolite (15–60%) mantle sources. Sr–Nd isotopic modelling also suggests that a 25–35% lower crustal component was added in the parental magmas; AFC modelling additionally indicates minor upper crustal contamination during the evolution of the volcanic rocks. In conclusion, integration of the geochemical, petrologic, and isotopic data with regional geology suggests that the analcime-bearing and -free volcanic rocks evolved from parental magma(s) derived from melts of a subcontinental lithospheric mantle and lower crustal sources.

Groundwater composition in perched-water bodies in the north flank of Fogo volcano (São Miguel, Azores): main causes and comparison with river water chemistry

A set of 46 springs located in the north flank of Fogo volcano (Sao Miguel, Azores) were sampled to characterize groundwater chemistry. The majority of the discharge from volcanic rocks with acidic character (95 %), at an elevation range of 200–400 m a.s.l., and the average discharge are usually not higher than 2 L/s. Nevertheless, the discharge in a few springs from trachytic lava flows can be higher (21.3 L/s). A group of 12 surface water samples were also collected twice along the Ribeira Grande River. Groundwater average temperature ranges between 19.6 and 20.7 °C, and it is possible to depict two groups of discharges: thermal waters (16.7 %), with a temperature range from 27.8 to 71.2 °C, and cold waters, with temperature under 20 °C. In this latter group, temperature is inversely proportional to elevation. The average pH and conductivity range, respectively, between 5.7 and 7.8 and 291–425 μS/cm. Nevertheless, higher conductivity values were measured in boiling pools. Groundwater mainly belongs to the Na–HCO3 to Na–Cl types. The major-ion relative composition of the cold springs shows a decreasing order for cations as Na > Ca > K > Mg and for anions HCO3 > Cl > SO4. In thermal discharges the relative composition is, respectively, Na > Ca > K > Mg and SO4 > HCO3 > Cl. The average nitrate content ranges from 2.09 to 2.57 mg/L comparing all the sampling periods, and the cumulative probability plot shows that about 75 % of the samples present values lower than 1.98 mg/L. Results show clearly that the higher nitrate measurements are observed in river waters and springs sampled in areas where pasture lands are dominant.

Geomagnetic field intensity determination from Pleistocene trachytic lava flows in Jeju Geopark

A composite of 28 trachytic lava flows were recovered from the Jeju Geopark Drilling Project (JGDP) in Jeju Geopark, one of the new seven wonders of Nature declared by UNESCO in 2011. Each trachytic lava flow has a tendency to increase in magnetic grain size from the rapidly cooled brecciated margin and vesicle streaked zone downward into the massive crystalline flow interiors. The brecciated margin and vesicle streaked zone of individual trachytic lava flow contains exclusively fine‐grained magnetite as inclusions in plagioclase. High‐fidelity paleointensity determinations were obtained from 26 (out of 224 examined) samples from JGDP cores. Temporal variation of virtual axial dipole moments (VADMs) calculated from the absolute paleointensity estimates follows the trend of sint‐800 data for the interval from ∼80 to ∼360 ka. High VADM from flow 21 possibly represents real intensity peak, as previously recognized high VADM in Japan at ∼336 ka, in Trans‐Mexican volcanism ∼339, and in Hawaii ∼340–350 ka. Perhaps such a strong magnetic intensity near ∼325–350 ka might be smoothed out in relative paleointensity records.

Volatiles in pantellerite magmas: A case study of the Green Tuff Plinian eruption (Island of Pantelleria, Italy)

The Green Tuff (GT) Plinian eruption, the largest in magnitude at Pantelleria, erupted 3 to 7km3 DRE of pantellerite magma and a small volume of trachyte. Fifty-nine anorthoclase-hosted melt inclusions from the two basal pumice members were analyzed by FT-IR spectroscopy in order to assess the pre-eruptive H2O content in the pantellerite melt. Microanalytical methods were used to determine major element, Cl, F and S contents. Melt inclusions and glassy groundmasses have a nearly homogeneous pantelleritic composition (peralkaline index=1.9-2.2) and variable water contents ranging from 1.4 to as high as 4.2 wt %, i.e. much higher than the 1.4 wt % of earlier published studies. The chlorine content is constant at about 1 wt %. Combined Cl and H2O data were used to estimate a confining pressure of about 50MPa (depth around 2-3km) for the GT magma chamber. The chamber was characterized by a compositional zoning with a dominant pantellerite overlying a trachyte magma. Soon after the GT eruption, intra-caldera volcanism was dominated by the eruption of voluminous trachyte lava flows, while pantellerite melt production resumed after about 20ka with numerous low-volume, mildly explosive (Strombolian) to effusive eruptions. Comparison with data from the literature reveals that, despite the different explosivity, the post-caldera Strombolian eruptions and the GT Plinian eruption were fed by pantelleritic magmas with similar water contents. Chlorine and CO2 contents suggest that the young magma reservoirs feeding the Strombolian to effusive activity were deeper (h≥4.5km) than the much larger (based on erupted volumes) magma chamber which fed the GT eruption.

Possible Late Pleistocene volcanic activity on Nightingale Island, South Atlantic Ocean, based on geoelectrical resistivity measurements, sediment corings and 14C dating

Tristan da Cunha is a volcanic island group situated in the central South Atlantic. The oldest of these islands, Nightingale Island, has an age of about 18 Ma. In the interior of the island, there are several wetlands situated in topographic depressions. The ages of these basins have been unknown, and their genesis has been debated. Aiming towards the reconstruction of the geomorphological history of these basins, we conducted geoelectrical resistivity measurements to map the subsurface topography, extracted peat and sediment cores and dated the onset of sedimentation applying the radiocarbon method. The irregular shapes of the basins and the lack of clear erosional features indicate that they are not eruption craters and were not formed by erosion. Instead, we regard them as morphological depressions formed between ridges of trachytic lava flows and domes at a late stage of the formation of the volcanic edifice. The onset of sedimentation within these basins appears to have occurred between 24 and 37 ka with the highest situated wetland yielding the highest ages. These ages are very young compared to the timing of the main phase of the formation of the island, implying volcanic activity on the island during the Late Pleistocene.

Caldera-forming eruptions of the Quaternary Kone Volcanic Complex, Ethiopia

Quaternary volcanism in the northern Main Ethiopian Rift is characterized by rift-axis eruptive centres with complicated collapse structures. Despite their association with major explosive eruptions, few modern studies have been carried out on them. We present the results of fieldwork and laboratory analyses of one of the largest of these centres, the Kone Volcanic Complex (KVC). It consists of nested calderas, trachyte lava flows, trachyte and alkali rhyolite pyroclastic deposits, alkali rhyolite lava domes and numerous small transitional to alkali basalt lava vents and flows. The largest subsidence structure, Birenti Caldera, has a diameter of ∼11 km and an area of ∼95 km 2. Post-collapse volcanism in the interior of Birenti Caldera produced a predominantly trachytic cone that was partially destroyed by a further eruption, forming the well-preserved but irregularly-shaped Kone Caldera (diameter of ∼8 km and an area of ∼24 km 2). Rhyolite lava dome eruptions and tectonically-controlled basalt lava effusion represent the most recent volcanism in the KVC. We report here the first detailed geologic map, along with petrologic and stratigraphic analyses, in order to reconstruct the eruptive history of this large but previously little-known volcanic complex. The findings have implications for understanding the relationships between rifting, tectonics and volcanism in the northern Main Ethiopian Rift, and for future efforts to trace widespread tephra horizons for regional stratigraphic correlation.

Bimodal back-arc alkaline magmatism after ridge subduction: Pliocene felsic rocks from Central Patagonia (47°S)

Volumetrically minor microsyenites, alkali microgranite and related trachytic dykes intrude early Pliocene OIB-like alkali basaltic and basanitic flows of the Meseta del Lago Buenos Aires in Central Patagonia (47°S–71°30′W), and occur together with scarce trachytic lava flows. Whole-rock K–Ar ages between 3.98 and 3.08 Ma indicate that the emplacement of these felsic rocks occurred more or less synchronously with that of the post-plateau basaltic sequence that they intrude, during a bimodal mafic–felsic magmatic episode devoid of intermediate compositions. Chemically, these rocks have A 1-type granitoid affinities and are characterized by high silica and alkali contents (60–68 wt.% SiO 2; 8.7–10.8 wt.% Na 2O + K 2O), major and trace elements patterns evidencing evolution by low-pressure fractional crystallization, and Sr and Nd isotopic signatures similar to those of coeval basalts (( 87Sr/ 86Sr) o = 0.70488–0.70571; ( 143Nd/ 144Nd) o = 0.512603–0.512645). Nevertheless, some of them have the most radiogenic Sr values ever reported for a magmatic rock in the Meseta and even in the whole Neogene Patagonian Plateau Lavas province (( 87Sr/ 86Sr) o = 0.70556–0.70571; ( 143Nd/ 144Nd) o = 0.512603–0.512608). In addition, very high contents of strongly incompatible elements in the most evolved rocks, together with Sr isotopic ratios higher than those of coeval basalts, suggest the occurrence of open-system magmatic processes. Continuous fractional crystallization from a primitive basaltic source, similar to post-plateau coeval basalts, towards alkali granites combined with small rates of assimilation of host Jurassic tuffs (AFC) in a shallow magmatic reservoir, best explains the geochemical and petrographic features of the felsic rocks. Therefore, A 1-type magmatic rocks can be generated by open-system crystallization of deep asthenospheric melts in back-arc tectonic settings. In Central Patagonia, these ∼ 3–4 Ma old alkaline intrusions occur aligned along a ∼ N160–170 trending lineament, the Zeballos Fault Zone, stacking the morphotectonic front of one segment of the Patagonian Cordillera. Intrusion along this fault zone occurred during the onset of a new transtensional or extensional event in the area, related to major regional tectonics occurring in possible relation with the collision of one segment of the Chile Spreading Ridge with the trench.

Thick lava flows of Karisimbi Volcano, Rwanda: insights from SIR-C interferometric topography

We use a digital elevation model (DEM) derived from interferometrically processed SIR-C radar data to estimate the thickness of massive trachyte lava flows on the east flank of Karisimbi Volcano, Rwanda. The flows are as long as 12 km and average 40–60 m (up to >140 m) in thickness. By calculating and subtracting a reference surface from the DEM, we derived a map of flow thickness, which we used to calculate the volume (up to 1 km3 for an individual flow, and 1.8 km3 for all the identified flows) and yield strength of several flows (23–124 kPa). Using the DEM we estimated apparent viscosity based on the spacing of large folds (1.2×1012 to 5.5×1012 Pa s for surface viscosity, and 7.5×1010 to 5.2×1011 Pa s for interior viscosity, for a strain interval of 24 h). We use shaded-relief images of the DEM to map basic flow structures such as channels, shear zones, and surface folds, as well as flow boundaries. The flow thickness map also proves invaluable in mapping flows where flow boundaries are indistinct and poorly expressed in the radar backscatter and shaded-relief images.

Mineralogy and Genesis of the Çanakçi (Ulukisla-Nigde) Baryte Vein Deposits, Central Turkey

The studied area is located around (~anakqi (Ulukisla) Village in the north of Bolkardag Unit, Central Turkey. The baryte mineralization occurs within Elmali vulcanite member belonging to UpperPalaeocene aged ~iftehan complex which overlies unconformably the Alihoca ophiolites. The complex on the basis of lithological features can be divided into four members; Kodak shale-sandstone, Elmali volcanite, Aktastepe limestone and Ugtepe monzonite-diorite. The Elmali volcanites consists of basalt, andesite and trachyte lava flows, hypabyssal rocks, pillow lava and agglomerate. The lava flows have a E W strike and are cut by hypabyssal rocks of different phases. The baryte mineralizations occur nearly 2km southeast-east and 3km southwest of ~anakgi village. The baryte mineralization in SW of ~anakgi village has nearly lm thick and 33m length. Many small veins of baryte, nearly parallel to the main ore, are also present in the surrounding areas. Baryte veins contain mainly some galena. Minerals were identified by ore microscopy, and major and trace element analyses were performed by the XRF method. Mineralogy Megascopically, baryte ore is crystall ine, white and p ink i sh white in colour . Microscopically, small amount of quartz is accompanied with baryte mineral. Baryte occurs mainly as euhedral but sometimes anhedral crystals. Baryte crystals are colourless, perpendicular sets of cleavage and greyish interference colours indicating small brefringence. Crystals are characterised by polysynthetic twinning, undulatory extinction due to mechanic terrain in common. Mechanic twinning and like moiler texture are also observed in minerals. Baryte is cut by very thin quartz, sphalerite and galena veins and some baryte inclusion formed in some quartz crystals, all of which indicates that baryte is formed early than quartz, sphalerite and galena. The baryte crystals may also show fibrous or prismatic habit with patchy extinction reflecting the presence of considerable stresses during or after the deposition of baryte. The common opaque minerals are mainly galena, and rare sphalerite, chalcopyrite, pyrite, covellite, ilmenite and fahlore. Secondary minerals are anglesite, malachite, azurite, smitsonite, iron oxide and cerussite. The gangue minerals are represented by quartz, calcite chlorite and muscovite. Chemistry Selected 4 samples from baryte occurrences were analysed for major and trace elements (Table 1). As seen in Table 1, there is a negative correlation between the Ba and Al203. Feldspar, muscovite and possibly some clay minerals are present in order to account for the A1203 content that ranges between 3.07-12.35%. FeO is very low (0 .1-0 .5%), suggesting that baryte mineralization occurred at very low temperatures. Extremely low Na20 and CaO contents show clearly that calcium and sodium enriched fluids did not come to deposition environment during the baryte mineralization. Na20 and K20 occur principally as soluble salts mostly taking the form of chlorides, MgO does not behave as a soluble salt. The main impurity is SiO2 (mainly in the form of quartz mineral) and ranges between 2.1-17.81%. High ratio of Cu (109-161ppm) and TiO2 (0.12-1.56 %) reveals that the mineralising fluids are of hydrothermal origin and mineralising fluids came up from through volcanics and intrusive rocks. S content of the baryte minerals are between 63984 and 74306 ppm suggesting that the baryte mineralization is epithermal in origin. In addition, the high Sr values (9605-13603ppm) reflect that the origin of Sr is related with volcanic activity. The very high BaJSr ratio implies that the origin of Ba is the hydrothermal (especially epithermal) origin (Werner, 1958; Starke, 1969). Zn content ranges from 2 to 793 ppm, suggesting that the baryte occurrence is related with volcanic activity (Marchig et al., 1985). Pb

Magma Chamber Evolution as Revealed by 226Ra-230Th Disequilibrium at Longonot Volcano, Kenya

The rates at which magmatic processes operate are seldom determined, yet they offer powerful tests of competing physical models of magmatic evolution. U-series disequilibria have the potential to place constraints on the rates of geological processes and highlight significant differences in magmatic timescales. Recent advances in thermal ionisation mass spectrometry have made it possible to detect small variations in 226Ra-23~ disequilibria and here we apply these techniques to a suite of peralkaline trachytes from Longonot volcano in Kenya to evaluate the timescales of closed system magma chamber fractionation. Longonot volcano is situated in central Kenya in the vicinity of Lake Naivasha and has erupted both pyroclastic material and lava flows of almost exclusively trachyte composition for about 10,000 years. The latest phase of activity started around 5650 _+ 120 y BP with the cessation of pyroclastic eruptions and the production of a sequence of trachyte lava flows. The earliest eruptions in this lava pile are mixed peralkaline trachytes and hawaiites and were followed by successive lava flows of peralkaline trachyte which terminated at 3280 _+ 120 y with the eruption of a large (2 km 3) ash horizon. The lava pile stage is estimated to have a total volume of 18-20 km 3 (Scott 1980) of which the main trachyte sequence accounts for 16 km 3. The lava pile shows minimal variation in major elements (61.4-62.2% SiO2) and radiogenic isotopes (average 2~176 19.65 + 0.02, n = 17; 143Nd/144Nd: 0.51262 _+ 1.8 x 10 -5, n = 21). By contrast, incompatible elements increase dramatically (e.g. Th: 12.5-23.6 ppm; Zr: 576-1126 ppm) whereas Ba, which is compatible in sanidine, the major fractionating phase, decreases rapidly from 198-4.7 ppm (Fig. 1). It is well established that Zr behaves as an incompatible element in F-rich silicic peralkaline systems (Leat, 1984), and the strong linear trend shown by Th and Zr can be modeled as the result of-50% closed system sanidine fractionation (DTh ~ Dzr Dna and that it has a value of~12. Fig. 2 shows how the initial 226Ra/Ba ratio is reduced by fractionation if DRa = 12 and consequently how the ages calculated from the measured 226Ra/Ba ratios are likely to be greater than that calculated assuming the linear array represents an isochron. Applying this model to the data from Longonot yields a number of results. Firstly, all the data points now give distinct model ages that are greater then 3 000 y and are therefore consistent with the 14C ages. Secondly, the calculated ages of Ra/Th fractionation decrease as the amount of fractionation increases. Thus the two least evolved samples have ages of -9000 y because they lie close to the equiline,

Tchabal Nganha volcano in Adamawa (Cameroon): petrology of a continental alkaline lava series

Abstract The Mio-Pliocene Tchabal Nganha volcano in the Adamawa Plateau (Cameroon) contains a broad range of rock types from ankaramites to phonolites and trachytes. Tchabal Nganha is located on large N70°E strike-slip faults that have been active since the Cretaceous and delimit the Adamawa Plateau horst. Geological mapping revealed a succession of basaltic-hawaiitic lava flow units, mugearite-benmoreite breccias, phonolite and trachyte lava flows and plugs, and finally tiny basaltic and intermediate outpourings. Olivine is Fo-85 in basaltic lavas and has altered fayalitic compositions in trachytes. Clinopyroxene compositions range from Ti-rich salite in basaltic rocks to hedenbergite in phonolites and aegirine in trachytes. Kaersutite is present as megacrysts (almost completely replaced by rhonite+phlogopite+salite+olivine+nepheline) in ankaramites and as phenocrysts in phonolites. The Fe-Ti oxides indicate decreasing equilibration temperatures and oxygen fugacities from basalts to trachytes. Spinels occur as inclusions in ol and/or cpx phenocrysts in ankaramitic basalts where Al-rich Cr-poor spinels are found exclusively in clinopyroxene phenocrysts. Feldspar occurs as phenocrysts of plagioclase in intermediate lavas, and as sanidine and anorthoclase in phonolites and trachytes. Apatite is ubiquitous in intermediate and felsic lavas. Sphene is common in some phonolites and chevkinite in some trachytes. Differentiation indices vary from 17 to 93 with a gap in the range 60–75. Ba may attain 4650 ppm in phonolites. Transition elements decrease strongly with differentiation in the basaltic lavas. Zr/Ta is constant along the series except in sphene-bearing phonolites, which are depleted in Ta. REE patterns are similar for basaltic and intermediate lavas. Some phonolites exhibit overall-enriched REE patterns, whereas peralkaline phonolites are strongly depleted in MREE. Major- and trace-element variations confirm the dominance of crystal fractionation in the differentiation of the basaltic and phonolite series. Due to high Ta contents along the series, and to low Sr-isotope ratios (