Eutaxitic Texture Research Articles

The subaqueous felsic volcanism from the Upper Member of the Cordón de Lila Complex, Antofagasta region, northern Chile

This contribution presents the study of an Ordovician subaqueous felsic succession, formed by the ~400 m-thick Upper Member of the Cordón de Lila Complex (CISL), northern Chile. From bottom to top, the succession starts with two dacitic sills that intruded into the sediments of the lowermost part of the Upper Member. Then, it is followed by a thick sedimentary deposit, two rhyolitic lavas, a volcanoclastic felsic breccia with pyroclastic clasts, and ends with a red rhyolitic lava. The first rhyolitic lava shows centimetric to metric folds with a north-northwestern vergence, evidence of flow in that direction. In the volcanoclastic breccia, an abundant pumice-and-fiamme-rich green matrix wraps around lithic clasts, giving the rock an eutaxitic texture at the outcrop scale. This matrix and texture evidence is characteristic of hot pyroclastic flows deposited under subaqueous conditions. The described section of the breccia is formed by four coarsening-up sequences, reflecting respectively four pulses of building up energy. The lack of internal erosional features points to a single and continuous explosive eruption. An accidental granite clast in the upper part of the breccia suggests a connection with a granitic body, possibly the Pingo-Pingo monzogranite. The geochemical character of the felsic succession is calc-alkaline. The tectonic setting diagram shows a within-plate setting for the succession. This suggests a shift from the arc position inferred for the Lower Member of the CISL to a forearc continental setting for the Upper Member of the CISL.

Lithofacies and petrochemical characterization of volcano-sedimentary sequence of Chandil Formation around Kharidih-Bareda area, Seraikela-Kharsawan District, Jharkhand: implications for uranium mineralization

Mesoproterozoic Chandil Formation (ca. 1600 Ma) of North Singhbhum Mobile Belt record numerous features of felsic volcaniclastics and felsic to intermediate volcanics preserved in the central sector of the fold belt around Kharidih-Bareda area, Seraikela-Kharsawan district, Jharkhand. The felsic volcanic rocks exhibit flow bands, autoclasts and layering of crystal mushes revealing viscous nature of eruptives. The volcaniclastic sediments comprise of significant proportion of volcanic epiclasts and accidental lithic fragments. These volcaniclastics have been categorized into five prominent lithofacies viz, stratified lapilli tuff, banded tuff, tuff with penecontemporaneous deformation, welded lapilli stones, vitric tuff and volcanic bombs by field and petrographic studies of outcrops and subsurface borehole cores. The welded lapilli tuffs display fiamme and eutaxitic texture. Interlayering of the volcaniclastics, which are most often pyrite-rich, with psamo-pelitic lithology like carbonaceous phyllite, variegated phyllite, quartzite and minor limestone is suggestive of marine euxenic depositional environment. Petrographic study of the volcaniclastics indicated presence of glass shards, garnet phenocrysts, spherules of tremolite, ovoid to lenticular accretionary lapilli along with devitrified glassy material. Compositionally these felsic volcanics and volcaniclastics are rhyodacitic to andesitic in nature with peraluminous to meta aluminous in character. A/CNK values vary from 0.52 to 2.42 in felsic volcanics and from 0.12 to 1.63 in volcaniclastics. Signatures of arc magmatism is indicated by low concentration of HFS elements such as Nb (5-17 ppm), Ga (11-17 ppm) and Y (5-28 ppm). Elevated intrinsic content of uranium (3-8 ppm), Th/U ratio ranging from 1.2 to 13.2, presence of metamict allanite and zircon in volcanics and volcaniclastics reveal their suitability as a prospective source for search of uranium mineralization. The volcanic-volcaniclastic-clastic association of the Chandil Formation provides an ideal situation where provenance and province both are available. Thus, suitable litho-structural locales such as the concealed shear zones sympathetic to the Dalma thrust and South Purulia Shear Zone within the volcano-sedimentary package of Chandil Formation may be targeted as preferable sites for locating concealed uranium mineralization.

Petrological characteristics and K-Ar age of Sakiura welded tuff from the Komegawaki Formation, Fukui prefecture, central Japan

Major and trace element compositions and a K-Ar age are reported for the Sakiura welded tuff of the Miocene Komegawaki Formation in the Mikuni coastal area, Fukui prefecture, central Japan. The rock samples are densely welded, generally show eutaxitic texture, and contain 9-17 vol.% plagioclase, minor (<1 vol.%) hornblende, and 1-14 vol.% lithic fragments. Plagioclase in the Sakiura welded tuff yields a K-Ar age of 13 Ma, which is within the 12-14 Ma age range of Komegawaki volcanic rocks. Sakiura welded tuff samples are rhyolitic (SiO2 = 73.3-74.5 wt.%) and are near the SiO2-rich end of the compositional trend of Komegawaki volcanic rocks on major element versus SiO2 cross-plots. The MORB-normalized trace element patterns of Sakiura welded tuff samples are similar to those of the Komegawaki volcanic rocks. These observations indicate that the origin of the rhyolite magma that produced the Sakiura welded tuff is associated with the Komegawaki volcanic rocks. Evidently, multiple rhyolitic eruptions occurred concurrently with the formation of the Miocene volcanic rocks after the opening of the Japan Sea.

The Marifil Volcanic Complex at Sierra de Pailemán: implications for the Early Jurassic magmatic evolution of the Eastern North Patagonian Region

ABSTRACT The Sinemurian Marifil Volcanic Complex at Sierra de Pailemán, located in the east part of the North Patagonian Region, Argentina, includes five lithofacies and three volcanic events. The volcanic activity begins with subaerial pyroclastic-fall deposits, massive lapilli tuffs with eutaxitic textures, and subaerial silicic domes’ emplacement. The upper part of the sequence is formed by pyroclastic fall deposits that pass to massive lapilli tuffs and rhyolitic lava flows. These lithofacies indicate the collapse of explosive eruption columns, coeval with small silicic domes’ emplacement, and lava-flows’ eruption. The volcanic episode at Sierra de Pailemán was generated during continuous magmatic activity because no erosional or unconformity surfaces were recognized. The Marifil Volcanic Complex’s age at Sierra de Pailemán is 191.2 ± 1.3 Ma (Sinemurian) and belongs to the V0 phase of volcanism. These rocks are high-K rhyolites, possibly produced by partial melting of the lower crust, promoted by fluids derived from a subducted plate in back-arc settings that change to an intraplate setting. The Lu-Hf isotopes suggest the possibility of magma mixing in the source and recycling or re-melting of a more evolved crust related to subduction zones. The Lower Jurassic magmatic activity in the western North Patagonian Region and the Patagonian Cordillera of Argentina and Chile correspond to continental arcs.

Lavas or ignimbrites? Permian felsic volcanic rocks of the Tisza Mega-unit (SE Hungary) revisited: A petrographic study.

Permian felsic volcanic rocks were encountered in petroleum exploration boreholes in SE Hungary (eastern Pannonian Basin, Tisza Mega-unit, Békés-Codru Unit) during the second half of the 20th century. They were considered to be predominantly lavas (the so-called "Battonya quartz-porphyry") and were genetically connected to the underlying "Battonya granite." New petrographic observations, however, showed that the presumed lavas are crystal-poor (8-20 vol%) rhyolitic ignimbrites near Battonya and resedimented pyroclastic or volcanogenic sedimentary rocks in the Tótkomlós and the Biharugra areas, respectively. The studied ignimbrites are usually massive, matrix-supported, fiamme-bearing lapilli tuffs with eutaxitic texture as a result of welding processes. Some samples lack vitroclastic matrix and show low crystal breakage, but consist of oriented, devitrified fiammes as well. Textural features suggest that the latter are high-grade rheomorphic ignimbrites.Felsic volcanic rocks in SE Hungary belong to the Permian volcanic system of the Tisza Mega-unit; however, they show remarkable petrographic differences as compared to the other Permian felsic volcanic rocks of the mega-unit. In contrast to the crystal-poor rhyolitic ignimbrites of SE Hungary with rare biotite, the predominantly rhyodacitic-dacitic pyroclastic rocks of the Tisza Mega-unit are crystal-rich (40-45 vol%) and often contain biotite, pyroxene, and garnet. Additionally, some geochemical and geochronological differences between them were also observed by previous studies. Therefore, the Permian felsic volcanic rocks in SE Hungary might represent the most evolved, crystal-poor rhyolitic melt of a large-volume felsic (rhyodacitic-dacitic) volcanic system.The Permian volcanic rocks of the studied area do not show any evident correlations with either the Permian felsic ignimbrites in the Finiş Nappe (Apuseni Mts, Romania), as was supposed so far, or the similar rocks in any nappe of the Codru Nappe System. Moreover, no relevant plutonic-volcanic connection was found between the studied samples and the underlying "Battonya granite."

Petrology and geochemistry of ignimbrites and associated enclaves from Mount Bambouto, West-Cameroon, Cameroon Volcanic Line

Geochemical and mineralogical analyses data are presented for ignimbritic deposits from the Bambouto volcano. These deposits are characterized by their welded and non-welded massive lapilli tuff and massive lithic breccias facies. The ignimbrites generally display vitroclastic and/or eutaxitic texture characterized by whole or broken minerals, subangular fragments of trachytes, rhyolites, ignimbrites, as well as fiammes, and fragments of glass. The geochemistry of the ignimbrites of Mount Bambouto indicates that they are alkaline and peralkaline with rhyolitic and trachytic composition. The behaviour of the major and trace elements suggest that ignimbrites, as well as felsic and mafic lavas of the Bambouto volcano have an evolution dominated by fractional crystallization (FC). The La/Yb, Ce/Zr and Th/Nb ratios for Mt Bambouto ignimbrites (19.92–52.66; 0–0.48; 0.10–0.12, respectively) and the whole Western Cameroon Highlands (WCH) and Mt Manengouba lavas (15.58–34.29; 0–0.61; 0–0.27, respectively) also reflect the dominant role of FC. This suggests a relatively constant degree of partial melting during the petrogenesis of the lavas and deposits. The high Rb/Sr (0.52–28.98) and the low ratios of LILE/HFSE (Rb/Nb: 0.08–0.95; Rb/Zr: 0.01–0.18; Th/Ta: 1.38–2.1; Ba/Nb: 0.06–8.27) in Mt. Bambouto ignimbrites, compared with the very high value of these ratios in the granitic basement rocks of the area, preclude the petrogenesis of the ignimbrites from the melting of the Pan-African basement. The weak values of Th/La and La/Nb in the studied ignimbrites and other WCH lavas suggest that crustal contamination was insignificant during ascent and storage of the magma in shallow magma chambers prior to their release to the surface. The similarities in Nb/Ta, Zr/Hf and Nb/U ratios in the Mt Bambouto ignimbrites (Nb/Ta: 13.54–20.93; Zr/Hf: 37.39–49.95; Nb/U: 26.04–55.70) and the lavas of the WCH (Nb/Ta: 11.3–19.57; Zr/Hf: 33.16–55.01; Nb/U: 19.83–75.35) corroborated with their similar source origin. The enrichment of light REE relative to heavy REE or their Dy/Yb > 2 (2.03–2.97), (La/Yb)N > 5 (14.29–46.69) and (Tb/Yb)N > 1.9 (1.91–2.9) suggest melting in a garnet-bearing mantle.

Characteristics of Mesoproterozoic Felsic Meta-Volcanics from the Shillong Group of Rocks, Meghalaya, North East India

Shillong Group comprises a thick meta-volcano-sedimentary sequence forming a part of Shillong Plateau and Mikir Hills. The litho variants of felsic meta-volcanics are characterized in detail based on their constituent material, textural variation, depositional feature and mode of eruption. Lava flows are characterized as porphyritic, spherulitic and aphyric type whereas pyroclastic deposits are classified as tuff breccia, lapilli tuff, tuffand ignimbrite. Fractured crystal and brecciated lithic fragments in tuff; fiamme and eutaxitic texture of ignimbrite suggest for subaerial eruption. The felsic meta-volcanics are rhyolitic to dacitic in composition and interbedded with tuffaceous phyllite, phyllite and quartzite.

Sandstone petrography and provenance of the Chubut Group (Cretaceous) in the Cañadón Matasiete (Golfo San Jorge Basin, central Patagonia): Implications for basin evolution and alluvial organization

Sandstone petrographic data have been used to investigate the provenance of the Matasiete, Pozo D-129, and Castillo formations (Chubut Group) in the Cañadón Matasiete, San Bernardo Fold Belt at Golfo San Jorge Basin, Argentina. Available petrographical data provides evidence of several distinctive volcanic source areas, with minor contributions of metamorphic, plutonic, or sedimentary components. Paleo-volcanic contributions from basic-intermediate (lathwork and microlithic textures) and acidic compositions (felsitic, seriate, granular, and vitric textures) were mainly derived from the Middle-Upper Jurassic rocks of the Lonco Trapial Formation. Pyroclastic components (glass shards, pumice fragments, and eutaxitic textures) derived from coeval volcanic activity in the Andes Cordillera, generated either gradual or abrupt changes in the source of the lithic fragments along the study succession. The observed trends in detrital lithic provenance can be integrated with previous sedimentological and architectural studies of the Matasiete and Castillo formations, revealing that narrow-sheet geometries of the channel fill characterize stratigraphic intervals dominated by pyroclastic components, whereas stratigraphic intervals that contain a low content of pyroclastic materials in the channel fill show ribbon-shaped geometries. Sandstone petrography in volcaniclastic basins can be used to complement other indicators of the evolution of the fluvial systems, providing reliable information about the nature of the changes in the sediment supply within the drainage catchment.

Minero-petrographical, physical, and mechanical properties of moderately welded ignimbrite as a traditional building stone from Uşak Region (SW Turkey)

Ignimbrites are commonly used building stone since the ancient times because of its lightweight, soft, and insulating properties. However, they are prone to deteriorate due to their weak structure. The moderately welded Usak ignimbrite has a wide range of exposure in the Usak region of the inner western part of Anatolia. It is mainly composed of plagioclase (oligoclase, andesine), biotite, Fe-Ti oxides (magnetite, hematite), and low amount of amphibole (brown hornblende), quartz mineral composition having hypocrystalline and eutaxitic texture. Geochemical data reveal that the ignimbrite is characterized by trachyandesite composition, calc-alkaline and peraluminous in character. According to the calculated weathering index values, the ignimbrite shows slight to moderate weathering. Apparent dry and partially saturated unit weights are 10.49 and 13.90 kN/m3, respectively. Open porosity is 35.4%. This high value results in high water absorption capacity and low uniaxial compressive strength. Capillary water absorption capacity was determined as 51.84 g/m2 s0.5; with this value, the ignimbrite is in “high” water absorption rock class. Dry and partially saturated uniaxial compressive strength values were determined as 6.89 MPa and 3.45 MPa respectively. Tested ignimbrite specimens are in “weak rock class” according to UCS values both in dry and partially saturated conditions. Therefore, the stone loses about 50% of its strength in partially saturated condition. Longitudinal and shear wave velocities of the specimens were determined as 2.4 and 1.32 km/s respectively. Based on these values, dynamic shear modulus, Poisson’s ratio, and Young’s modulus were calculated as 1.91 GPa, 0.27 GPa, and 4.87 GPa, respectively. Slake durability tests were carried out on ignimbrite specimens after the second cycle 91.33%; Usak ignimbrite is characterized by “medium-high” durability. Specimens were exposed to 25 freeze and thaw cycles and the average of the weight lost values were determined as 0.89%. Thermal conductivity properties were investigated by the thermal conductivity coefficient which was ranged between 0.352 and 0.449 W/mK. Minero-petrographic investigations and geochemical and physico-mechanical test results indicate that the structure of Usak ignimbrite is weak and sensitive to environmental and utilization condition. Therefore, it is necessary to pay attention to this feature in the areas of utilization.

Lithofacies architecture, composition, and age of the Carboniferous Teplice Rhyolite (German–Czech border): Insights into the evolution of the Altenberg-Teplice Caldera

The Teplice Rhyolite (TR) intra-caldera deposits of the Altenberg-Teplice Caldera (ATC, Central Europe) were investigated by lithofacies analysis, stratigraphy, and whole-rock chemistry as well as laser ablation U–Pb zircon dating. The aim was to demonstrate that the joint application of these methods can discern complex stratigraphy of ancient volcanic successions, susceptible to erosion and alteration, taking the TR as case study. The TR was regarded as a formation and divided into nine members that represent three main eruptions, each one separated by periods of volcanic quiescence. The first two eruptions mainly portrayed opening and clearing of vents, which were dominated by lithic-rich ignimbrites with lateral lithofacies changes (fiamme-rich to crystal-rich) covered by lava flows/domes. Eruption style was low pyroclastic fountaining alternated with effusive lava outpour. A sustained buoyant eruption column was exclusively interpreted at the outset of the second eruption based on the presence of a basal fallout deposit. The third eruption recorded three different phases: further vent opening, a waxing phase, and final lava extrusion. The vent opening phase was similar to those occurred in previous eruptions. The waxing phase produced voluminous, massive, poorly-sorted, very crystal-rich (>50vol.%) ignimbrites, exhibiting progressive increase in crystal contents with height and chemical composition variations that indicate deposition by progressive aggradation. Low pyroclastic fountaining formed these high-particle concentration currents that waxed and reached quasi-steady conditions. Eruption ended with extrusion of a lava flow. The TR ignimbrites were emplaced hot and were partially to densely welded, as evidenced by eutaxitic texture, deformed glass shards, spherulites, perlitic cracks, and microcrystalline to granophyric textures. Horizontal and vertical lithofacies variations in pyroclastic deposits, together with progressive infill and association between TR feeding systems and faulting, demonstrate that eruptions developed through several fissure vents or faults. A volcanic explosivity index of 7 was estimated according to a minimum ejecta volume of ∼320km3 (DRE). Volcanic architecture of the TR can be linked to sequential caldera subsidence that occurred synchronously with progressive deposition. Regional faulting (NE–SW-trending rhyolitic dykes) related to ATC feeding conduits suggests that progressive subsidence and final trap-door caldera collapse, possibly with a piece-meal component (faulted caldera floor), originated from post-Variscan tensional stress regime (∼NNW–SSE direction). The TR Formation has a general rhyolitic composition with A-type granite affinities and a normal chemical zoning. U–Pb zircon dating yielded ages from 323 to 312Ma, which, compared to age data from other ATC rocks, indicate that the TR was generated within a time span of ∼325–317Ma.

High‐silica Ediacaran volcanism in the Dom Feliciano Belt, southernmost Brazil

The Cerro Chato region is located in southernmost Brazil and is characterized by associations of acid volcanic and subvolcanic rocks. The region is affected by NW and NE‐ trending faults and is grouped into 2 geomorphologically distinct features: Cerro Chato and Cerro Partido. Cerro Chato is represented by ignimbrites that occur in 2 main facies: lithic‐rich ignimbrites and crystal‐rich ignimbrites. They are poorly sorted and consist of lapilli‐sized pyroclasts in a tuffaceous matrix with eutaxitic texture. Hemi‐crystalline rhyolitic flows represent effusive events, with porphyritic texture, flow structures, and spherulites. Cerro Partido is characterized by a subvolcanic rhyolitic body, with porphyritic texture, elongated in the NE–SW direction. Through geochemical data, the lithologies were characterized as high‐silica type rhyolites, correlated to the alkaline series, close to the subalkaline series limit; display metaluminous to peraluminous character and high contents of alkalis, FeOt/FeOt + MgO and agpaitic index. The Cerro Partido rhyolites were classified as high‐Ti with higher CaO, P2O5, FeOt, MgO, and K2O contents than the Cerro Chato low‐Ti rhyolites. The REE pattern is slightly enriched in LREE in relation to the HREE and has a strong negative Eu anomaly. U–Pb zircon dating indicates an age of 561 ± 2 Ma for Cerro Partido, suggesting contemporaneity with the granitoids of Dom Feliciano Suite. Additionally, Cerro Chato rhyolites indicate an age of 630.4 ± 2.8 Ma. These ages are in agreement with those obtained in the syn‐sedimentary volcanism at the base of the Maricá Formation, Camaquã Basin.

Mineralogical and petrological features of the Cemilköy ignimbrite, Cappadocia, Turkey

The Cemilköy ignimbrite is one of the voluminous ignimbrite deposits in Cappadocia. The Cemilköy ignimbrite contains pumice and lithic clasts of volcanic and ophiolitic origin in an ash matrix. The unwelded Cemilköy ignimbrite is distinguished from other deposits in Cappadocia by flattened pumices, elongate vesicles and a slaty fabric. The mineral assemblage of the Cemilköy ignimbrite is plagioclase, biotite, quartz and oxides (magnetite and Ti-magnetite) and the matrix is glassy. Eutaxitic texture is dominant and all pumice clasts have a vitrophyric-porphyritic texture. The same textural properties were observed throughout the spatial distribution of Cemilköy ignimbrite. Microprobe studies reveal that plagioclase compositions range from albite through oligoclase-andesine. Estimated plagioclase-liquid temperatures (T) and pressures (P) are varying between 806 and 847 °C and 4.2–7.1 (kbar), and the H2O content of the melt is estimated to have been 5 wt.% from the pumice clasts. Based on geochemical data, the Cemilköy ignimbrite is rhyolitic and calc-alkaline in character, and all pumice clasts are enriched in LIL and LRE elements relative to HFS elements. Negative Nb, Ta and Ti anomalies, ratios of Ba/Nb >28 (56–77), Ba/Ta ˃450 (590–700) and Th/Yb vs. Ta/Yb are consistent with a subduction-related origin. According to the geochemical and mineralogical-petrographical data, the Cemilköy ignimbrite originated from partial melting of a mantle source which was enriched during previous subduction processes with variable degrees of assimilation fractional crystallization (AFC) through time and Cemilköy ignimbrite erupted from a crustal magma chamber at shallow to intermediate depth.

Geochemistry of Mesoproterozoic Deonar Pyroclastics from Vindhyan Supergroup of Central India: Evidences of Felsic Magmatism in the Son Valley

ABSTRACT Deonar Pyroclastics of Semri Group in the Vindhyan Supergroup originated as a result of violent and explosive intra-basinal submarine volcanism during the Mesoproterozoic period. These pyroclastics are rhyolitic to rhyodacitic in composition, comprised of banded, massive, pumiceous flow, breccia, vitric tuff, lapilli and volcanic bomb. The pyroclastic deposits represent welded and non-welded ignimbrites, exhibit typical eutaxitic texture. Mantle normalized multi-element patterns show enrichment in LILs and depletion in HFSFs. Ti, Nb and REE contents show close correlation with Zr, indicating their immobile character. HFSEs and Th/Nb, La/Nb and Zr/Nb values indicate contamination and these signatures represent mixing between mantle-derived rocks and the average continental crust. Deonar Pyroclastics reflect continental rift environment. Felsic magma plausibly generated by underplating of the mature Proterozoic crust of the Indian craton (which acted like a ‘heating lens’) resulted in extensive melting of metabasalt in the lower crustal levels. The high heat flow beneath the Indian shield accentuated heat generation which led to extensive partial melting of metabasalts. Thus, generation of rhyolitic magma occurred along the reactivated deep seated fractures and rifting of the craton, resulting in the explosive intra-basinal felsic vulcanicity in the Vindhyan basin.

Magnetic Fabrics and Source Implications of Chisulryoung Ignimbrites, South Korea

The anisotropy of magnetic susceptibility (AMS) of late Cretaceous ash-flow tuffs in Chisulryoung Volcanic Formation, southeastern Korea was studied to define the primary pyroclastic flow azimuth. AMS data revealed a dominant oblate fabric with a tight clustering of k3 (minimum axis of magnetic susceptibility) and shallow dispersal of k1 (maximum axis of magnetic susceptibility) and k2 (intermediate axis of magnetic susceptibility). Dominance of oblate fabrics indicates clast imbrications imposed by compaction and welding. Flow azimuth inferred from AMS data indicates the nearby intrusive welded tuff (IWT) as the source of calderas for ignimbrites. Such an inference is supported by geologic investigations, in which the IWT displays eutaxitic textures nearly parallel to its subvertical contacts. The results are compatible with a unique prolate fabric and an anomalously high inclination observed for the IWT, possibly produced by rheomorphic flows as the welded tuff is squeezed along the rough-surfaced dyke walls due to agglutination.

Mihi Breccia: A stack of lacustrine sediments and subaqueous pyroclastic flows within the Taupo Volcanic Zone, New Zealand

The Taupo Volcanic Zone (TVZ), New Zealand, encompasses a wide variety of arc-related strata, although most of its small-volume (non-caldera-forming) eruptions are poorly-exposed and extensively hydrothermally altered. The Mihi Breccia is a stratigraphic sequence consisting of interbedded rhyolitic pyroclastic flows and lacustrine sediments with eruption ages of 281±18 to at least 239±6ka (uncertainties at 2σ). In contrast to other small-volume rhyolitic eruptions within the TVZ, Mihi Breccia is relatively well-exposed within the Paeroa fault block, and contains minimal hydrothermal alteration. Pyroclastic flow characteristics and textures include: 1) prismatically jointed juvenile clasts, 2) lack of welding, 3) abundant ash-rich matrix, 4) lack of fiamme and eutaxitic textures, 5) lack of thermal oxidation colors, 6) lack of cooling joints, 7) exclusive lacustrine sediment lithic clasts, and 8) interbedding with lacustrine sediments, all indicating that Mihi Breccia strata originated in a paleo-lake system. This ephemeral paleo-lake system is inferred to have lasted for >50kyr (based on Mihi Breccia age constraints), and referred to as Huka Lake. Mihi Breccia pyroclastic flow juvenile clast geochemistry and petrography correspond with similar-aged (264±8, 263±10, and 247±4ka) intra-caldera rhyolite domes filling the Reporoa caldera (source of the ~ 281 ka Kaingaroa Formation ignimbrite). These exposed intra-caldera rhyolite domes (as well as geophysically inferred subsurface domes) are proposed to be source vents for the Mihi Breccia pyroclastic flows. Soft-sediment deformation associated with Mihi Breccia strata indicates either seismic shock, rapid sediment loading during pyroclastic flow emplacement, or both. Thus, the Mihi Breccia reflects a prolonged series of subaqueous rhyolite dome building and associated pyroclastic flows, accompanied by seismic activity, emplaced into a large paleo-lake system within the TVZ.

Commercial bentonite from the Kopernica deposit (Tertiary, Slovakia): a petrographic and mineralogical approach

AbstractCommercial bentonite from the Kopernica deposit, Slovakia, is currently mined by the CERTECH Company, Poland, to produce materials intended for animal-care applications and other industrial purposes. The present study aimed to assess the mineralogical, petrographic and physicochemical characteristics of three bentonite varieties from Kopernica used by the company. The X-ray diffraction, optical microscopy, field emission scanning electron microscopy (FESEM), nuclear magnetic resonance, thermal analysis, infrared spectroscopy and chemical analyses showed that the main component of the rocks is montmorillonite with the average structural formula Ca0.31K0.08Na0.04(Al3.23Mg0.54Fe0.23)[(Si7.80Al0.20)O20](OH)4. In addition, opal-C/CT, biotite, potassium feldspar and plagioclase, quartz, clinoptilolite and kaolinite are present. Key information about the textural relationships between the mineral components identified was obtained from detailed thin-section petrography and FESEM studies. The rocks studied have fragmented, eutaxitic texture. They are composed of pumice fragments collapsed into lenticular masses (fiamme) which were strongly deformed and altered, though the shard structures were retained. The compressed glass shards were moulded around pyroclastic grains such as crystal fragments of quartz, biotite and zoned plagioclases, and clasts of volcanic rocks. Observations by FESEM showed that the axes of shards and the walls of the flattened vacuoles are outlined by the inward-growing microlites of silica (axiolitic texture), whilst the interiors of shards are altered to clay. Grain-size distribution, textural properties and microscope observations of grain-size fractions reveal that the Kopernica bentonite contains montmorillonite-opal aggregates difficult to disperse in water.

Recognition features of felsic pyroclastics of Serra do Tombo formation, Minas Gerais, Brazil

In this contribution we report a study of poorly exposed, rhyodacitic welded-ignimbrite deposit from Minas Gerais. A petrographic study of textures indicate high temperature of emplacement. Key features include eutaxitic texture, flattened and agglutinated lapilli and glass menisci. Most of the feldspar minerals and glass are extensively altered to clay minerals, which pseudomorph the original volcanic textures. Glass menisci and spherules suggest a possible process of liquid immiscibility. Immobile trace element distribution indicates a possible link with other post-Palaeozoic felsic volcanic rocks in Brazil, a magmatism interpreted as due to basaltic underplating and partial melting of a hydrous continental crust. A peculiar feature is a high Light REE/Heavy REE ratio. Depletion in heavy rare earth elements is possibly due to a residual HREE-bearing phase in the source. The geologic context of these rocks suggests a Lower Cretaceous age and a tectonic relationship with a continental rifting event.

O vulcanismo alta-sílica da região do Tupanci, NW do Escudo Sul-Rio-Grandense: faciologia, petrografia e litoquímica

A região do Tupanci, noroeste do Escudo Sul-Rio-Grandense (ESRG), apresenta depósi- tos vulcânicos ácidos, estratigraficamente correlacionados à Formação Acampamento Velho (~550 Ma), da Bacia do Camaquã. Esta região é a exposição mais setentrional deste episódio vulcânico, caracterizado por sequências efusivas e piroclásticas, dominantemente ácidas, afinidade alcalina alta sílica sódica e vinculadas ao magmatismo pós-colisional do ciclo orogênico Brasiliano/Pan- -Africano no ESRG. As vulcânicas ácidas ocorrem na forma de depósitos efusivos e, em menor volu- me, piroclásticos, distribuídas em dois cerros: Cerro Tupanci e Cerro dos Picados. O Cerro Tupanci é uma intrusão subvulcânica alongada N-S, representada por riolitos porfiríticos, com fenocristais de sanidina e quartzo e uma matriz quartzo-feldspática equigranular fina a afanítica, com intensa foliação de fluxo nas regiões de borda. No Cerro dos Picados ocorrem riolitos texturalmente se- melhantes, porém com matriz afanítica de aspecto vítreo e presença de biotita, além de depósitos piroclásticos de ignimbritos riolíticos, divididos em duas fácies: uma rica em líticos, com púmices devitrificados e pouco estirados, além de pequenos e raros cristaloclastos; e outra de ignimbritos reomórficos, com abundantes púmices devitrificados, destacada textura eutaxítica, mais rica em cristaloclastos e raros litoclastos. O comportamento geoquímico de ambas as fácies classifica o magmatismo como supersaturado em sílica, semelhante aos sistemas de alta sílica, afinidade alcali- na sódica, e tendência metaluminosa a levemente peralcalina, com características dos granitos tipo “A”. Os dados faciológicos, petrográficos e litoquímicos obtidos indicam vinculação genética destas rochas com o vulcanismo da Formação Acampamento Velho.

Primary welding and crystallisation textures preserved in the intra-caldera ignimbrites of the Permian Ora Formation, northern Italy: implications for deposit thermal state and cooling history

Exceptional exposure through a Permian intra-caldera ignimbrite fill within the 42 × 40 km Ora caldera (>1,290 km3 erupted volume) provides an opportunity to study welding textures in a thick intra-caldera ignimbrite succession. The ignimbrite succession records primary dense welding, a simple cooling unit structure, common crystallisation zones, and remarkably preserves fresh to slightly hydrated glass in local vitrophyre zones. Evidence for primary syn- and post-emplacement welding consists of (a) viscously deformed and sintered juvenile glass and relict shard textures; (b) complete deposit welding; (c) subtle internal welding intensity variations; (d) vitrophyre preserved locally at the base of the ignimbrite succession; (e) persistent fiamme juvenile clast shapes throughout the succession at the macroscopic and microscopic scales, defining a moderate to well-developed eutaxitic texture; (f) common undulating juvenile clast (pumice) margins and feathery terminations; (g) a general loss of deposit porosity; and (h) perlitic fracturing. A low collapsing or fountaining explosive eruption column model is proposed to have facilitated the ubiquitous welding of the deposit, which in turn helped preserve original textures. The ignimbrite succession preserves no evidence of a time break through the sequence and columnar joints cross-gradational ignimbrite lithofacies boundaries, so the ignimbrite is interpreted to represent a simple cooling unit. Aspect ratio and anisotropy of magnetic susceptibility (AMS) analyses through stratigraphic sections within the thick intra-caldera succession and at the caldera margin reveal variable welding compaction and strain profiles. Significantly, these data show that welding degree/intensity may vary in an apparently simple cooling unit because of variations in eruption process recorded in differing lithofacies. These data imply complex eruption, emplacement, and cooling processes. Three main crystallisation textural zones are identified in the ignimbrite succession: localised vitrophyre zones, widespread microcrystalline to sparsely spherulitic pseudomorphed vitriclastic textural zones, and thick granophyric zones. These zones record a typical spectrum from rapid to prolonged cooling. The non-uniform crystallisation patterns reflect variations in deposit thickness, the relative position of deposits within the larger ignimbrite succession, the type of substrate material, and the degree of confinement in the intra-caldera setting. We support previous work suggesting that traditional welding classifications (e.g. Smith, US Geological Survey Professional Paper 354-F 1960b) are most appropriate for use within altered and/or ancient ignimbrite successions.

The effect of welding degree on geotechnical properties of an ignimbrite flow unit: the Bitlis castle case (eastern Turkey)

Ignimbrites are associated with nearly most of the world’s volcanoes and are defined as a deposit from pyroclastic density currents. They consist predominantly of pumiceous lapilli and blocks, and glass shards, which shows evidence of having been emplaced as a concentrated hot and dry particulate flow. These rocks are widely used as building stone especially in ancient buildings. Bitlis valley is covered by ignimbritic products, derived from Nemrut stratovolcano, one of the significant volcanic centers in Eastern Anatolia. The Bitlis ignimbrite is separated into lower level (LL), middle level (ML) and upper level (UL) according to color, welding degree and structural features. All three levels were used extensively in many parts of the Bitlis castle as masonry materials. Studies were carried out on mineralogical and geochemical composition and on physical and mechanical properties of the ignimbrites. In addition, a freeze–thaw cycle test was executed. There are no considerable differences in mineralogical composition among the levels of ignimbrite. All levels contain plagioclase, sanidine, pyroxene, and opaque mineral. In addition, anorthoclase and quartz are seen. In general, the LL of ignimbrite shows relic perlitic and eutaxitic texture, whereas eutaxitic and vesicular texture are commonly developed in the ML and UL, respectively. Lower, middle, and upper level ignimbrite samples display similar and limited compositional spread in terms of major oxide elements. They have trachyte composition. Building stones can be classified according to mineralogy, mechanical and physical properties and processing types. Mechanical and physical properties are very important with respect to stone quality/durability. The mechanical and physical properties of the ignimbrites are controlled by the welding degree. It was found that increasing welding degree from UL to LL correlates with increasing density, compressive strength and slake durability index and with decreasing porosity. The Bitlis ignimbrites have turned out as susceptible to freeze–thaw cycles.