Rhyolitic Lava Flows Research Articles

Mineral chemistry, crystallization conditions and magma mixing‐mingling at Orduzu volcano (Malatya), Eastern Anatolia, Turkey

AbstractThe Middle Miocene Orduzu volcanic suite, which is a part of the widespread Neogene Yamadağ volcanism of Eastern Anatolia, consists of a rhyolitic lava flow, rhyolitic dykes, a trachyandesitic lava flow and basaltic trachyandesitic dykes. Existence of mafic enclaves and globules in some of the volcanic rocks, and microtextures in phenocrysts indicate that magma mingling and mixing between andesitic and basaltic melts played an important role in the evolution of the volcanic suite. Major and trace element characteristics of the volcanic rocks are similar to those formed in convergent margin settings. In particular, incompatible trace element patterns exhibit large depletions in high field strength elements (Nb and Ta) and strong enrichments in both large ion lithofile elements (Ba, Th and U) and light rare earth elements, indicating a strong subduction signature in the source of the volcanic rocks. Furthermore, petrochemical data obtained suggest that parental magmas of rhyolite lava and dykes, and trachyandesite lava and basaltic trachyandesite dykes were derived from subduction‐related enriched lithospheric mantle and metasomatized mantle (± asthenosphere), respectively. A detailed mineralogical study of the volcanic suite shows that plagioclase is the principal phenocryst phase in all of the rock units from the Orduzu volcano. The plagioclase phenocrysts are accompanied by quartz in the rhyolitic lava flows and by two pyroxenes in the trachyandesitic lava flows and basaltic trachyandesitic dykes. Oxide phases in all rocks are magnetite and ilmenite. Calculated crystallization temperatures range from 650°C to 800°C for plagioclase, 745°C–1054°C for biotite, 888°C–915°C for pyroxene and 736°C–841°C for magnetite–ilmenite pairs. Calculated crystallization pressures of pyroxenes vary between 1.24–5.81 kb, and oxygen fugacity range from −14.47 to −12.39. The estimates of magmatic intensive parameters indicate that the initial magma forming the Orduzu volcanic unit began to crystallize in a high‐level magma chamber and then was stored in a shallow reservoir where it underwent intermediate‐mafic mixing. The rhyolitic lava flow and dykes evolved in relatively shallower crustal magma chambers. Copyright © 2007 John Wiley & Sons, Ltd.

Multi-method approach to dating glass: The case of Basiluzzo Islet (Aeolian archipelago, Italy)

A rhyolitic lava flow from Basiluzzo islet (Aeolian Islands), has been analysed with the Fission tracks (FT) and 40Ar– 39Ar methods on glass, and with the U/Th method on whole rock to constrain its age and to compare the behaviour of different dating methodologies on glass samples late Quaternary in age. Laser 40Ar– 39Ar total fusion analyses were performed on populations of grains. Due to the low yields of radiogenic 40Ar the age data are characterised by very high errors. The weighted average of the ages of the whole population is 55.7 ± 8.7 ka (MSWD = 0.7). The isochron age calculated on all points is 40.6 ± 11.4 ka (MSWD = 0.6), with an initial 40Ar/ 36Ar ratio of 297.8 ± 1.8; the isochron is characterised by very little spread among points. The procedure named ‘point-counting technique’ was adopted in FT dating. Spontaneous track mean size resulted reduced by around 20% compared to induced tracks, which indicates that the determined FT age, 28.6 ± 3.6 ka, is a reduced age, due to a certain amount of track annealing. For this reason the plateau technique for correcting thermally lowered ages was applied. We determined a plateau age (commonly assumed as a reliable estimate of the glass formation age) of 43.4 ± 7.1 ka. Four sub-samples of whole rock from Basiluzzo lava flow have been analysed using U/Th isochron method. The 238U/ 232Th and 230Th/ 232Th activity ratios of sub-samples have been determined by alpha counting and plotted on an isochron diagram. The resulting age is 46 ± 8 ka and the 234U/ 238U activity ratios are always close to one, demonstrating that no significant processes of alteration have occurred. The relatively high error associated with the age is due to a low fractionation of U/Th ratio in the analysed whole rocks. The ages obtained with different methods, 43.4 ± 7.1 ka (FT plateau age), 40.6 ± 11.4 ka ( 40Ar– 39Ar isochron age of all grains), and 46 ± 8 ka (U/Th isochron) agree at the 1 σ level, excluding a Holocene age for this sample. This could be valuable information for the Department of Civil Protection because it seems to mitigate the potential risk for present volcanic activity in the area. All ages are affected by very high analytical errors, which are due to the characteristics of the material analysed. Young ages result in low tracks numbers (FT dating) and barely detectable amounts of radiogenic 40Ar in the presence of high atmospheric contamination ( 40Ar– 39Ar dating). Stratigraphic successions without strict chronologic constraints might however benefit even from age data with low precision.

Insights into magma chamber processes from the analysis of size distribution of enclaves in lava flows: A case study from Vulcano Island (Southern Italy)

The size distribution of latitic enclaves dispersed in a rhyolitic lava flow is studied. Enclave size distribution is self-similar, a feature that can be explained by a fractal fragmentation process, and estimated fractal dimension of fragmentation is 2.50. Fragment size distribution of enclaves generated by disruption of viscous fingering structures in granitoid rocks is analyzed. It is shown that this distribution is fractal with a value of fractal dimension of fragmentation of 2.55, in close agreement with the value estimated for enclaves in the lava flow. It is suggested that the fragmentation process producing the size distribution of enclaves in the lava flow may have occurred in response to the disruption of viscous fingering morphologies generated by the injection of the more mafic magma into the felsic one. Accordingly, the size distribution of enclaves dispersed in the lava flow is considered a feature inherited from deep magma chamber processes and gives insights into magma chamber dynamics.

From flood basalts to the inception of oceanization: Example from the 40Ar/39Ar high‐resolution picture of the Karoo large igneous province

The Jurassic Karoo large igneous province consists of vast quantities of basaltic lava flows, giant dike swarms, and poorly dated silicic magmatism. The Karoo magmatism occurred over more than 3 × 106 km2 and was associated with the opening of the Indian Ocean. We present new 40Ar/39Ar geochronological data concerning the easternmost part (i.e., the Mwenezi and Lebombo areas) of the Karoo province, close to the future rifted margin. These data allow documenting the final history of the province and the time elapsed between the end of the Karoo magmatism and the inception of oceanization. This important type of information is poorly constrained for most of the large igneous provinces. Eighteen plagioclase separates yielded 14 plateau and “mini‐plateau” ages obtained on 3 basaltic dikes (178.1 ± 1.1 to 177.2 ± 2.4 Ma; 2σ), 3 gabbroic (178.2 ± 1.7 to 176.8 ± 0.7 Ma) and 2 silicic (175.8 ± 0.7 to 174.4 ± 0.7 Ma) plutons, and 1 rhyolitic lava flow (177.8 ± 0.7 Ma). We also obtained three concordant plateau and mini‐plateau ages ranging from 173.9 ± 0.7 Ma to 172.1 ± 2.3 Ma on the atypical E‐MORB‐like N–S striking Rooi Rand dikes. One dike from the Save‐Limpopo N70°‐oriented giant dike swarm provides a mini‐plateau age of 177.7 ± 0.8 Ma in agreement with the dates previously determined on this branch and possibly assessing the same age and short duration (≤1–2 Myr) as for the 179 Ma giant Okavango dike swarm. New and previous selected age data obtained on the Karoo province show that the magmatism was active over ∼10 Myr from 184 to 174 Ma. The main basaltic phase occurred mostly over the first ∼5 Myr and was progressively followed by a more differentiated and less voluminous magmatism over the last 4 Myr. The easternmost Lebombo‐Mwenezi long‐lasting magmatism is interpreted as being triggered by the progressive lithospheric extension preceding the continental disruption. The transition from rifting to oceanization is probably illustrated by the E‐MORB‐like Rooi Rand dikes which are likely to be emplaced during or shortly after the final stage of the Karoo magmatism. A geodynamic evolution of the province is proposed. These data indicate that the Karoo magmatism represents an atypical province (admitting that other large igneous provinces are sufficiently known) with a long‐lasting and relatively low‐emission rate magmatic activity. It appears from a reevaluation of major Phanerozoic Continental Flood Basalts (CFBs) that for most of them, including the Karoo, the onset of oceanization shortly follows (or is coeval to) the latest CFB‐related activity more closely than previously thought.

Uplift, thermal unrest and magma intrusion at Yellowstone caldera

The Yellowstone caldera, in the western United States, formed approximately 640,000 years ago when an explosive eruption ejected approximately 1,000 km3 of material. It is the youngest of a series of large calderas that formed during sequential cataclysmic eruptions that began approximately 16 million years ago in eastern Oregon and northern Nevada. The Yellowstone caldera was largely buried by rhyolite lava flows during eruptions that occurred from approximately 150,000 to approximately 70,000 years ago. Since the last eruption, Yellowstone has remained restless, with high seismicity, continuing uplift/subsidence episodes with movements of approximately 70 cm historically to several metres since the Pleistocene epoch, and intense hydrothermal activity. Here we present observations of a new mode of surface deformation in Yellowstone, based on radar interferometry observations from the European Space Agency ERS-2 satellite. We infer that the observed pattern of uplift and subsidence results from variations in the movement of molten basalt into and out of the Yellowstone volcanic system.

南長岡ガス田周辺の火山岩貯留岩の分布と特性

The distribution pattern of the Green Tuff rock type and facies in the Niigata Sedimentary Basin, with a central focus on the ‘Nagaoka Area’, may have been controlled by rifting and other structural development in the Nanatani Stage. The term “Volcanic Sequence” model is now propounded as a volcanostratigraphic framework for the compound Green Tuff succession which encountered in the Minami Nagaoka Gas Field, based on the comprehensive interpretation and crossed understanding of a large variety of volcanic rock types and facies and their changes in physicality and geochemical composition. The model comprises five volcanic sequences such as the Volcanic Sequence I, II, IIIR, IIIB and IV in ascending order. The Volcanic Sequence I and II consist of non-acidic rocks, and the former characterized with an extremely high resistivity is sharply distinguished from the latter. The acidic Sequence IIIR which has a bimodal relationship with the basaltic Sequence IIIB consists of multi-stacked rhyolitic lava-flows and/or lava-domes. Those are able to be identified based on the composite interpretation of the presumed rock facies together with dipmeter data in each well. The Sequence IV, a thin basaltic flow unit, is limited its distribution to the northern part of the field. Reservoir geological interpretation based on the “Volcanic Sequence” model might be able to explain heterogeneous facies distribution, and different productivities and pressure performances among wells..In the ‘Nagaoka Area’, some of the vintage wells encountered acidic rocks corresponding to the Volcanic Sequence IIIR and the lowermost part of the underlying non-acidic rocks are extremely resistive like the Volcanic Sequence I. In order to predict the distribution of acidic volcanic bodies which remain undiscovered, it is worth trying to re-interpret the vintage wells and seismic data from the aspect of the “Volcanic Sequence”

Volcanostratigraphy and petrogenesis of the Nemrut stratovolcano (East Anatolian High Plateau): The most recent post-collisional volcanism in Turkey

The Nemrut stratovolcano, with its well defined summit caldera, is the most recent volcanic eruption center (1441 A.D.) in the East Anatolian High Plateau, one of the best examples of an active colllision zone. Widespread volcanism has been active in this region for the last ∼ 10 Ma, producing large volumes of lavas and pyroclastics covering a broad belt, across the Bitlis Suture Zone, from the Anatolian plate in the north to the Arabian plate in the south. Three major evolutionary stages have been identified in the evolution of the Nemrut stratovolcano: pre-caldera, post-caldera and late stages. The pre-caldera stage is further differentiated into effusive, extrusive and explosive phases, the latter being responsible for the caldera formation. The products of this stage cover a broad compositional range from basalts to rhyolites, and consist of lava flows, domes, large volumes of ignimbrites and associated pyroclastics. The post-caldera stage consists of three phases comprising phreatomagmatic eruptions, vitrophyric rhyolitic lava flows and dykes. The late stage consists of the explosive and the following effusive phases with vitrophyric rhyolitic and basaltic lava flows, respectively. The post-caldera and late stages show a compositional gap between the (younger) mafic and the (older) felsic members. The products of all three stages are silica-saturated and display a gradual transition in nature from sodic in the mafic to potassic in the felsic members. The overlaps in the compositional spectra and the geochemical trends on bivariate plots suggest the derivation of the products of all three stages from a single parental magma (or similar parental magmas), yet the stratigraphic position of the mafic and felsic members points to a periodically replenished magma chamber. The major and trace element systematics, along with petrographic evidences, suggest clinopyroxene + plagioclase ± olivine ± amphibole as the major fractionating phases governing the liquid evolutionary paths, with a progressive increase in the proportions of plagioclase and amphibole (joined by K-feldspar, biotite and apatite) in the fractionating assemblages during the course of magmatic differentiation. The volcanics of all three stages exhibit enrichment in light rare earth elements (LREE) over heavy rare earth elements (HREE), and large ion lithophile elements (LILE) over high field strength elements (HFSE). The multi-element patterns and low Th/Y and Nb/Y ratios imply a within-plate type mantle asthenospheric source for this volcanics. Trace element modelling suggests derivation of parental magma(s) from 10% to 30% partial melting of a spinel–lherzolite source. Assimilation-fractional crystallization (AFC) modelling reveals significant but variable degrees of crustal contamination in the evolution of the volcanics with a range of r values (assimilation rate/crystallization rate) from 0.2 to 0.8. These interpretations are in conformity with the recent findings from seismic studies showing that lithospheric mantle is absent in eastern Anatolia. The crust in the region is about 45 km thick and directly underlain by the asthenosphere, possibly due to slab breakoff following the collision of the Arabian and the Anatolian plates along the Bitlis Suture Zone.

Reactivation of a rhyolitic magma body by new rhyolitic intrusion before the 15.8 ka Rotorua eruptive episode: implications for magma storage in the Okataina Volcanic Centre, New Zealand

The 15.8 ka Rotorua rhyolite eruptive episode from the Okataina Volcanic Centre comprises a plinian pumice fall deposit followed by the extrusion of two rhyolitic lava domes or flows and their associated block-and-ash flows, with a total volume >1 km 3 (dense rock equivalent). Variations in mineralogy, whole-rock, glass and mineral chemistry, and calculated magmatic properties suggest that two distinct rhyolitic magmas were sequentially tapped during the eruption. The first magma erupted (T1) is characterized by low SiO 2 ( c . 76.5 wt% in glass), calcic feldspars (An 44 ), magnesian hornblendes (MgO c . 14.45 wt%), clinopyroxene, and high temperatures ( c . 835 °C) and f O 2 (1.8–2.1 ΔFMQ (where FMQ is the fayalite–magnetite–quartz buffer)). The second magma (T2) was more evolved and is characterized by higher SiO 2 ( c . 77.4 wt% in glass), Na-rich feldspars (An 24 ), less magnesian hornblendes (MgO c . 11.8 wt%), biotite, and low temperatures ( c . 750 °C) and f O 2 (0.65–1.1 ΔFMQ). Both magmas are homogeneous, but evidence for some magma mingling indicates that they were in contact during eruption. However, there was only a minor degree of hybridization, perhaps reflecting the contrasting temperatures and viscosities of the two magmas. The crystal-rich, poorly vesicular nature of the T2 ejecta indicates that it originated from a cooling, high-level magma chamber that was reactivated by intrusion of hotter, volatile-rich T1 magma. The ponding of rhyolite magmas at shallow depth and their subsequent reactivation by later rhyolitic intrusion may be an important process in the compositional evolution and eruption dynamics of many Okataina Volcanic Centre rhyolite magma bodies.

A bimodal volcanic–plutonic system: the Zarembo Island extrusive suite and the Burnett Inlet intrusive complex

The Zarembo Island volcanic rocks and the Burnett Inlet plutonic complex in central southeastern Alaska were investigated to determine if they are genetically related. The Zarembo Island volcanic suite consists of basalt, andesite, and rhyolite lava flows, which exhibit features that suggest simultaneous eruptions of mafic and felsic lavas. Five kilometres to the southeast, the broadly layered Burnett Inlet plutonic complex consists of gabbro–diorite and granite plutons that also show characteristics of contemporaneous mafic and felsic magmatism. These bimodal volcanic and plutonic rocks are similar in age, ranging from 18.5 to 21.5 Ma. Both suites show a gap in silica concentration between 60 and 65 wt.% and have similar major, trace, and rare-earth element composition. Both suites also show igneous layering, either as interlayered basalt and rhyolite flows or as alternating gabbro and granite sheets. Additionally, both groups contain magma mingling and mixing textures, including mafic enclaves in felsic members and quartz xenocrysts rimmed by clinopyroxene in enclaves. These characteristics suggest that the Burnett Inlet intrusive complex and the Zarembo Island volcanic suite represent an eroded, shallow-level plutonic center and its eruptive cover. The style of volcanism and the bimodal nature of magmatism suggest that igneous activity occurred during crustal extension and thinning that accompanied strike-slip tectonic motion in southeastern Alaska during the Tertiary. The volcanic–plutonic rock associations now exposed at the surface indicate that at least 7° of post-20 Ma crustal tilting has affected the region and can help to explain aberrant paleomagnetic poles in mid-Cretaceous intrusions of the Cordillera Coast belt.

Exploration and discovery in Yellowstone Lake: results from high-resolution sonar imaging, seismic reflection profiling, and submersible studies

‘No portion of the American continent is perhaps so rich in wonders as the Yellow Stone’ (F.V. Hayden, September 2, 1874) Discoveries from multi-beam sonar mapping and seismic reflection surveys of the northern, central, and West Thumb basins of Yellowstone Lake provide new insight into the extent of post-collapse volcanism and active hydrothermal processes occurring in a large lake environment above a large magma chamber. Yellowstone Lake has an irregular bottom covered with dozens of features directly related to hydrothermal, tectonic, volcanic, and sedimentary processes. Detailed bathymetric, seismic reflection, and magnetic evidence reveals that rhyolitic lava flows underlie much of Yellowstone Lake and exert fundamental control on lake bathymetry and localization of hydrothermal activity. Many previously unknown features have been identified and include over 250 hydrothermal vents, several very large (>500 m diameter) hydrothermal explosion craters, many small hydrothermal vent craters (∼1–200 m diameter), domed lacustrine sediments related to hydrothermal activity, elongate fissures cutting post-glacial sediments, siliceous hydrothermal spire structures, sublacustrine landslide deposits, submerged former shorelines, and a recently active graben. Sampling and observations with a submersible remotely operated vehicle confirm and extend our understanding of the identified features. Faults, fissures, hydrothermally inflated domal structures, hydrothermal explosion craters, and sublacustrine landslides constitute potentially significant geologic hazards. Toxic elements derived from hydrothermal processes also may significantly affect the Yellowstone ecosystem.

Origin of rhyolitic lavas in the Mesa Central, Mexico, by crustal melting related to extension

The emplacement of a voluminous sequence of rhyolitic lava flows and domes characterizes Oligocene volcanism in the Mesa Central (MC) of Mexico. Its dominant effusive style of emplacement contrasts deeply with the predominantly explosive volcanism of the Sierra Madre Occidental Volcanic Province toward the west. Whole rock geochemical (major- and trace elements) and Sr–Nd isotopic data of the MC Oligocene rhyolitic lavas document a marked change in magma composition at around 30 Ma, allowing us to distinguish between a lower and an upper sequence. Lavas from the lower sequence are geochemically similar to the high-K rhyolitic rocks of the eastern Sierra Madre Occidental. Major- and trace-element variations are characteristic of mantle-derived magmas evolving through fractional crystallization. The initial 87Sr/ 86Sr and ϵ Nd values are nearly constant (0.70644–0.70770 and −1.2 to −2.1 respectively) and indicate some contribution from crustal material. Lavas from the upper sequence are high-silica, peraluminous rhyolites, with strong enrichment in fluorine and in some incompatible lithophile elements (Rb, La, Sm, Yb, Y, Th, U, Nb, Ta), and strong depletion in the feldspar-compatible elements Sr, Ba, Eu. Initial 87Sr/ 86Sr ratios of the upper sequence lavas are high and variable (0.70812–0.72190), and decrease as silica content increases, whereas the ϵ Nd values are relatively constant (−1.4 to −2.8). The trace element behavior indicates an origin by variable degrees of non-modal partial melting of granulitic low-crustal rocks and chemical disequilibrium during melting processes. The high and variable Sr isotopic ratios could also be related to isotope disequilibrium melting processes if the isotopic heterogeneities between individual mineral phases were preserved during heating of the source rocks. The changes in geochemical compositions are related to the onset of crustal extension at high strain rates documented for the MC. Crustal extension promoted crustal melting at high melting rates, high melt segregation rates, rapid ascent of low-viscosity fluorine-rich magmas, and inhibited melt stagnation in magma chambers. Such conditions favored the effusive volcanic style and support the possibility of melting under disequilibrium conditions.

Rhyolitic volcanism in extension zone associated with strike-slip tectonics in the Taxco region, southern Mexico

The Taxco Volcanic Field (TVF) is part of a broad magmatic province in southern Mexico. It constitutes an isolated zone of deeply dissected volcanic rocks encircled by outcrops of Mesozoic sedimentary and volcano-sedimentary units. A thick unit of rhyolitic lava flows associated with domes and at least two ignimbrite units forms the TVF. This volcanic sequence is distributed within a well defined zone, it overlies and is in part contemporaneous with continental sedimentary beds limited by major faults. Geochronologic data indicate that most rhyolitic volcanism in the area is Oligocene in age and synchronous with episodes of strike-slip faulting. We document two successive phases of strike-slip faulting for the late Eocene–early Oligocene interval, the first with NNW extension and the second with NE extension. In both cases pre-existing structures were reactivated and sedimentary basins were developed in response to displacement along major faults. The stratigraphic sequence gives evidence that the TVF is located in an extensional basin associated to strike-slip faults. The evolution of the basin underwent a change from sedimentary deposition with subsidence to piling up by volcanism. The result of this change was the development of a volcanic pile with elevations higher than the surrounding Mesozoic rocks. According to the fault kinematics, stratigraphy and the volume of volcanic rocks, the rhyolitic volcanism was emplaced in the area of maximum extension, showing that magma flowed into low pressure zones. The small number of faults within the Oligocene volcanic sequence suggests that volcanism inhibited normal faulting and that magma partially filled the space generated in the extended zone produced by the strike-slip faulting.

The Amazcala caldera, Queretaro, Mexico. Geology and geochronology

The Amazcala caldera is located 30km NE of Querétaro City, near Amazcala, state of Querétaro. This caldera is the northernmost caldera within the central sector of the Mexican Volcanic Belt (MVB). It has a 11×14km2 elliptical shape, and was formed 7.3–6.6Ma ago. All caldera products are rhyolites. The oldest caldera unit, the Ezequiel Montes pumice (EMP), is a widespread pumice fallout emplaced around 7.3Ma ago. An isopach map of the EMP shows two dispersal axes, oriented to the SW and SE with respect to the caldera. The EMP is 5m thick 40km to the SW and 35km to the SE of the caldera. An isopleth map of the EMP shows that pumice fragments increase in size toward the caldera, from 1cm at 40km to 25cm near the caldera rim. The EMP is a regional stratigraphic marker. The Colón ignimbrite, dated at 7.3±0.5Ma, is stratigraphically above the EMP. It consists of several ash-flow units interbedded with minor pumice fall lapilli and ash, with a minimum thickness of about 80m at Colón. The caldera rim is occupied by several rhyolite lava domes and flows, some of which extend 10km from the rim. These domes contain parts of fresh, aphyric obsidian. The last caldera event is an intracaldera rhyolitic dome near the NE rim at about 6.6Ma. The dome is 4×2km2 and is elongated in the NE direction. The Amazcala caldera is 480km from the Middle America Trench and represents the farthest inland caldera in the central sector of the MVB. Its age of 7.3–6.6Ma indicates that it is the oldest caldera of the MVB so far reported. This confirms the general view that the volcanic activity of the MVB initiated at its northern margin, and then migrated southward in time.

Controls on magmatism in the Munster Basin, SW Ireland

Abstract The following paper is intended as a review of the magmatism in the Munster Basin and an initial attempt to link it with the basin’s history. During the initiation, evolution and inversion of the Devono-Carboniferous Munster Basin magmatic activity was widespread, although small in volume. The episodic occurrence and diversity of the magmatism gives insights into the basement involvement and structural controls relating to the basin’s history. The differences in composition, location and structural relationship of the magmatic bodies are used as indicators for the timing of their emplacement and their relationship to the basin’s evolution. On the western Beara Peninsula in SW Ireland alone, more than 160 sheet-like intrusions, a wide variety of tuff bands and a deep-seated pipe-like structure of lamprophyric affinity occur. Other centres of magmatism in the basin are the Lough Guitane area, where rhyolitic lava flows and acidic pyroclastic rocks are associated with contemporaneous basin faults, and the Valentia Harbour area, where doleritic sills are associated with a volcanic breccia. A rhyodacitic tuff can be traced as far as St Finan’s Bay on the western shore of the Iveragh Peninsula. The chemical composition of the extrusive and intrusive magmatic bodies ranges from tholeiitic dolerites (Valentia Harbour) and subalkaline rhyolites (Lough Guitane) in Mid-Devonian time to subalkaline basalts and tuffs (e.g. Beara Peninsula) in Late Devonian time. In Late Carboniferous and possibly Permian time alkali basalts, trachytes and phonolites (e.g. Beara Peninsula) occur. The igneous activity in the Munster Basin is linked to the basin’s history by the interaction between active faults and fractures opening up during various stages of stress imposed on the basin and exploitation of these faults and fractures by rising magma.

P-164 一枚の流紋岩溶岩中に見られる組成的不均質性の成因 : 隠岐島後, 末期中新世流紋岩の場合

P-164 一枚の流紋岩溶岩中に見られる組成的不均質性の成因 : 隠岐島後, 末期中新世流紋岩の場合

SOIL GENESIS ON FELSIC ROCKS IN THE ST. FRANCOIS MOUNTAINS. I. THE ROLE OF SAPROLITE AND ITS INFLUENCE ON SOIL PROPERTIES.

One soil profile from Taum Sauk Mountain and two soil profiles from Mudlick Mountain in the St. Francois Mountains of Southeast Missouri were investigated to determine if the soils evolved from the underlying rock unit and to characterize the role of saprolite on soil genesis. Taum Sauk Mountain is composed of rhyolite lava and ash flow of the Van East Group. The rock is quartz and orthoclase phenocrysts in an orthoclase groundmass. Mudlick Mountain consists of rock composed of orthoclase, oligoclase, and a minor amount of quartz phenocrysts in a groundmass of quartz and orthoclase. The shallow Aquic Hapludult formed on the summit of Taum Sauk Mountain shows no evidence of saprolite, whereas the two Typic Paleudalfs formed on Mudlick Mountain rest on a thick layer of saprolite. The presence of minerals in the host rock that weather more easily is largely responsible for the development of the saprolite on Mudlick Mountain. The very strongly acid Typic Paleudalfs have textures ranging from cobbly silt loam to cobbly sandy loam for the eluvial horizons, whereas the argillic and C horizons are generally fine or clayey-skeletal. The extremely acid Aquic Hapludults have a fine-silty texture. The clay mineralogy is mixed, with hydroxy-Al interlayered vermiculite and hydrous mica prominent in the eluvial horizons and kaolinite dominating the Bt and C horizons. The aluminum hydroxide potentials of soil solutions extracted from selected horizons are saturated or slightly supersaturated with respect to kaolinite. Elemental analysis demonstrates that the soil and saprolite originate from the same parent material and that elemental depletion is important to the formation of the saprolite. Iron oxides represent nearly all of the total Fe, and Fe is correlated highly with clay content. The rare earth elements are more abundant in the soil than in the host rock. The weathered thin loess capping is suggested as the source of the rare earth elements.

Large volume silicic volcanism along the proto-Pacific margin of Gondwana: lithological and stratigraphical investigations from the Antarctic Peninsula

Jurassic magmatism in western Gondwana produced the most voluminous episode of continental volcanism in the Phanerozoic era. During the Early to Middle Jurassic, some 2.5–3 million km3 of dominantly basalt, and to a lesser extent rhyolite, were erupted onto a supercontinent in the early stages of break-up. The major silicic portion of the Gondwana magmatic province is exposed in Patagonian South America. The volcanic rocks of Patagonia have been collectively termed the Chon-Aike Province and constitute one of the world's most voluminous silicic provinces. The volcanic rocks are predominantly pyroclastic, dominated by ignimbrite units of rhyolite composition. Volcanic rocks crop out sporadically across much of the once contiguous Antarctic Peninsula, and are considered to form an extension of the Chon-Aike Province. A continuation of the province to include the Antarctic Peninsula would extend its strike length along the active Pacific margin by c. 2000 km.Volcanic rocks exposed along the east coast of the Antarctic Peninsula, defined here as the Mapple Formation, are also dominated by rhyolitic ignimbrite flows, with individual units up to 80 m in thickness, and a total thickness of c. 1 km. The ignimbrites vary in degree of welding, from high-grade rheomorphic ignimbrites with parataxitic textures, to unwelded, lithic-rich ignimbrites. Rhyolite lava flows, air-fall horizons, debris flow deposits and epiclastic deposits are volumetrically minor, occurring as interbedded units within the ignimbrite succession.The lithology and stratigraphy of the Jurassic volcanic rocks of the Mapple Formation are presented, and comparisons are made to the Chon-Aike Province. A consistent stratigraphy of Permo-Triassic metasedimentary rocks, unconformably overlain by terrestrial mudstone–siltstone sequences, which are in turn conformably overlain by largely silicic, subaerial volcanic rocks, is present at several localities along the Antarctic Peninsula, and at localities in the Chon-Aike Province. Precise (zircon U–Pb) Middle Jurassic ages exist for two volcanic formations from the Antarctic Peninsula, and a Middle–Lower Jurassic age has been suggested for the underlying sedimentary formations based on fossil flora analysis. The Antarctic Peninsula chronostratigraphy, coupled with lithological similarities, indicate a close relationship to those sequences of the Chon-Aike province.

Facies architecture and origin of a submarine rhyolitic lava flow‐dome complex, Ponza, Italy

The Islands of Ponza and Gavi (western Pontine Archipelago, Italy) preserve parts of a subaqueously emplaced, high‐silica (75–77% SiO2) rhyolitic lava flow that overlies the margins of three older domes. Exposures of the lava alone cover 24 km2, and with a thickness of 150 m exposed in the cliffs, the preserved volume of the lava flow is 3.6 km3, and the original volume is likely to have exceeded 7 km3. The lava flow consists of coherent obsidian and several hyaloclastite facies, including in situ and clast‐rotated breccias and sandstones, which all exhibit gradational relationships. The coherent obsidian occurs as separate domains and is often flow banded. Although flow folds occur, the banding is generally subhorizontal, dipping gently to the NNE. The hyaloclastite, representing 90% of the lava by volume, has a diffuse layering varying from 0.5 to 2 m in thickness. This layering also dips gently to the NNE and is marked in places by alternation of some coherent horizons, coarse and finely fragmented in situ hyaloclastite, pumiceous hyaloclastite, and clast‐rotated hyaloclastite. The layering is laterally continuous and consistent in orientation over most of the 13 km length of Ponza: the subhorizontal orientation of the flow banding and presence of resedimented deposits in the upper parts of the sequence indicate that the unit is a lava flow. The resedimented deposits comprise both proximal and distal deposits of debris flows and turbidity currents initiated by gravitational instability of the upper and marginal parts of the lava flow and include stratified monomictic obsidian breccias and bedded vitric siltstones and sandstones. The lava flow has been intruded and crosscut by rhyolitic bodies of various ages, including discordant, coherent to internally fractured contemporaneous rhyolite bodies, and late, penecontemporaneous dikes with glassy margins and alteration haloes.

Hydrothermal uranium deposits containing molybdenum and fluorite in the Marysvale volcanic field, west-central Utah

Uranium deposits containing molybdenum and fluorite occur in the Central Mining Area, near Marysvale, Utah, and formed in an epithermal vein system that is part of a volcanic/hypabyssal complex. They represent a known, but uncommon, type of deposit; relative to other commonly described volcanic-related uranium deposits, they are young, well-exposed and well-documented. Hydrothermal uranium-bearing quartz and fluorite veins are exposed over a 300 m vertical range in the mines. Molybdenum, as jordisite (amorphous MoS2), together with fluorite and pyrite, increase with depth, and uranium decreases with depth. The veins cut 23-Ma quartz monzonite, 20-Ma granite, and 19-Ma rhyolite ash-flow tuff. The veins formed at 19-18 Ma in a 1 km2 area, above a cupola of a composite, recurrent, magma chamber at least 24 × 5 km across that fed a sequence of 21- to 14-Ma hypabyssal granitic stocks, rhyolite lava flows, ash-flow tuffs, and volcanic domes. Formation of the Central Mining Area began when the intrusion of a rhyolite stock, and related molybdenite-bearing, uranium-rich, glassy rhyolite dikes, lifted the fractured roof above the stock. A breccia pipe formed and relieved magmatic pressures, and as blocks of the fractured roof began to settle back in place, flat-lying, concave-downward, “pull-apart” fractures were formed. Uranium-bearing, quartz and fluorite veins were deposited by a shallow hydrothermal system in the disarticulated carapace. The veins, which filled open spaces along the high-angle fault zones and flat-lying fractures, were deposited within 115 m of the ground surface above the concealed rhyolite stock. Hydrothermal fluids with temperatures near 200 °C, 18OH2O∼−1.5, DH2O∼−130, log f O2 about −47 to −50, and pH about 6 to 7, permeated the fractured rocks; these fluids were rich in fluorine, molybdenum, potassium, and hydrogen sulfide, and contained uranium as fluoride complexes. The hydrothermal fluids reacted with the wallrock resulting in precipitation of uranium minerals. At the deepest exposed levels, wallrocks were altered to sericite; and uraninite, coffinite, jordisite, fluorite, molybdenite, quartz, and pyrite were deposited in the veins. The fluids were progressively oxidized and cooled at higher levels in the system by boiling and degassing; iron-bearing minerals in wall rocks were oxidized to hematite, and quartz, fluorite, minor siderite, and uraninite were deposited in the veins. Near the ground surface, the fluids were acidified by condensation of volatiles and oxidation of hydrogen sulfide in near-surface, steam-heated, ground waters; wall rocks were altered to kaolinite, and quartz, fluorite, and uraninite were deposited in veins. Secondary uranium minerals, hematite, and gypsum formed during supergene alteration later in the Cenozoic when the upper part of the mineralized system was exposed by erosion.

隠岐島後における末期中新世隠岐アルカリ火山岩類の成因

A large volume of lavas and pyroclastic rocks with varying composition erupted in the latest Miocene in the Oki-Dogo island. In this paper, the origin of shoshonite, trachyte, and low and high-SiO2 rhyolites of the Oki Alkaline Volcanic Group is examined. On the basis of model calculation, trachyte magma is inferred to be generated by fractional crystallization of plagioclase, clinopyroxene, magnetite and apatite from shoshonite magma, or by partial melting of crustal material or solidified shoshonite magma with complete melting of K-feldspar and/or biotite. From consideration of variation diagrams, low-SiO2 rhyolites magma was not generated by fractional crystallization from trachyte magma or by magma mixing between trachyte and high-SiO2 rhyolites magmas. High-SiO2 rhyolites magmas were not generated by fractional crystallization from low-SiO2 rhyolites magma. Aphyric low- and high-SiO2 rhyolites plot in restricted, but separate areas on the normative Qz-Ab-Or diagram. This suggests that these magmas were generated by partial melting of crustal materials under different PH2O conditions: that is, low-SiO2 rhyolites were generated under higher PH2O than were the high-SiO2 rhyolites. Fe, Ce, Pb, Ba, Nb, Rb, Zr, Y and Th contents of both low- and high-SiO2 rhyolites vary between factors of 1.1 to 6 times, compared with restricted SiO2 variation of only 71.4 to 72.7 wt% for the low-SiO2 rhyolites, and 75.0 to 75.3 wt% for high-SiO2 rhyolites. The variations in element concentration are interpreted to be due to the following process: during partial melting at the cotectic point, in the some source areas following minerals such as apatite (affecting Y, Ce due to high partition coefficients), zircon (Zr, Th, Ce), sphene (Nb), allanite (Ce, Th), garnet (Y), magnetite (Fe) and biotite (Rb, Ba, Nb) remained as residual phases, in others they did not. This produced separate magmas with low or high concentrations of the elements concerned. This interpretation is supported by the observation that partial melt glasses in crustal xenoliths from Cenozoic basalts in Oki-Dogo Island also show similar compositional trends to the bulk composition of the low- and high-SiO2 rhyolites. Similar compositional variation is recognized in one high viscosity rhyolite lava flow unit, suggesting that incomplete magma mixing took place between separate magmas generated by different degrees of partial melting.