U-Pb zircon geochronology of basaltic pyroclastic rocks from the basement beneath the Xisha Islands in the northwestern South China Sea and its geological significance

As one of the micro-blocks dispersed in the South China Sea(SCS), the basement of the Xisha Islands has rarely been drilled because of the thick overlying Cenozoic sediments, which has led to a confused understanding of the pre-Cenozoic basement of the Xisha Islands. Well CK-1, a kilometer-scale major scientific drill in the Xisha Islands in the northwestern SCS, penetrated thick reefal limestone(0–888.4 m) and the underlying basement rocks(888.4–901.4 m). In this study, we present the zircon U-Pb ages of basement basaltic pyroclastic rocks from Well CK-1 in the Xisha Islands of the northwestern SCS to investigate the basement nature of the Xisha microblock. The basement of Well CK-1 consists of basaltic pyroclastic rocks on the seamount. The zircon grains yielded apparent ages ranging from ca. 2 138.9 Ma to ca. 36 Ma. The old group of zircon grains from Well CK-1 was considered to be inherited zircons. Two Cenozoic zircons gave a weighted mean 206Pb/238U age of(36.3 ± 1.1) Ma,Mean Squared Weighted Deviations(MSWD) = 1.2, which may represent the maximum age of the volcano eruption. The Yanshanian inherited zircons(116.9–105.7 Ma and 146.1–130.2 Ma) from Well CK-1 are consistent with the zircons from Well XK-1, indicating that the basement of Chenhang Island may be similar to that of Well XK-1. We propose that the Xisha micro-block may have developed on a uniform Late Jurassic metamorphic crystalline basement, intruded by Cretaceous granitic magma.

Debris Fan Produced by Failure of Canyon-Blocking Pyroclastic Flows

Ash-rich pyroclastic flows from the cataclysmic eruption of Mount Mazama (~7700 yr. B. P.), Cascade volcanic arc, Oregon, entered and blocked the narrow, bedrock-lined canyon of the Williamson River approximately 35 to 44 km from the source volcano. The blockage impounded a body of water which then released producing four stratigraphic units in the downstream debris fan. The four stratigraphic units are a boulder core comprised of locally sourced bedrock boulders and three sand-rich units including a fine-grained sand unit, a sandy pumice gravel (±basalt/hydrovolcanic tuff) unit, and a pumice pebble-bearing, crystal-rich sand unit. Hand-drilled auger holes up to ~1.6 m deep were used to obtain samples of the sand-rich units. Units were delimited using surface and down-hole observations, composition and texture, estimated density, statistical parameters of grain size, and vertical and lateral distribution of properties. Overtopping followed by rapid incision into the ash-rich pyroclastic flows progressively cleared the canyon, but a bedrock knickpoint near the head of the canyon limited the volume of debris available for transport to about 0.04 km3 to 0.08 km3. Co-deposition of bedrock boulders and lithic-rich sand was followed by rapid deposition with minimal reworking of remobilized pyroclastics. Continued draining of the impounded lake sent hyperconcentrated flows onto the debris fan depositing pumice-rich gravels that graded upward to crystal-rich sands.

Possible Correlation of Oligocene Climate Changes with Afro-Arabian Silicic Pyroclastic Eruptions

The Oligocene Afro-Arabian flood volcanic province contains significant silicic pyroclastic rocks (】60000 km^3 constituting up to 20%of the volcanic stratigraphy).Rhyolitic tephras,synchronous with the Afro-Arabian silicic pyroclastic rocks,are found in Indian Ocean ODP holes 711A.They are geoche-mically akin to the Afro-Arabian silicic pyroclastic rocks.This suggests that the tephras originated from Afro-Arabian silicic eruptions and represents more distal fallout of this volcanism.The temporal coin-

Characterization of Pyroclastic Deposit from Three Different Areas within Foumbot Region (West-Cameroon): Comparative Studies of Their Effects as Pozzolanic Materials in Mortars and Cement Manufacture

Pyroclastics which are known natural pozzolanic materials due to amorphous contents, are present in several areas of the Mbepit Massif in West Cameroon. In this work natural pozzolan from three zones namely Pouoloum, Njimbouot and Nkouonja were characterized. A comparative study was then developed to attest the effect of these pyroclastics as partial substitution in portland cement. The mixtures were made at different pozzolanic proportions (00%, 10%, 15%, 25% and 35%) of substitution of the cement. The compressive and flexibility strength was carried out at 7, 28 and 90 days on mortar specimens (4 × 4 × 16 cm3). The results revealed apozzolanicactivity index of 81.99 %, 83.47% and 74.54% respectively for rock sample from Pouoloum (PCB), sample from Njimbouot (PCC) and sample from Nkouonja (PCN). After 90 days, for a substitution rate of 25% compressive strength are respectively 55.69 MPa, 60.4 MPa, 53.34 MPa for PCB, PCC and PCN. According to the American Society for Testing and Materials (ASTM) C618 classification, the pyroclastics are in accordance with all the criteria to be classified as pozzolan. Independent of the substitution rate, the mechanical performance increases with age in PCB, PCC and PCN. However PCC is most reactive than PCB and PCN. This may be due to the amorphous reactive content in this material and can be linked to the eruptive dynamisms which were more explosive in some areas than in others. The amorphous content is 32.01%, 36.99%, 24.84% for PCB, PCC and PCN respectively. These results also prove that Natural Pozzolan is interesting in the manufacture of composite cement CEM II, CEM IV in accordance with EN197-1 or can be added in mortar for buildings and sustainable environmental management.

Landslide susceptibility: a statistically-based assessment on a depositional pyroclastic ramp

This study aimed to produce a high-quality landslide susceptibility map for Teziutlán municipality, a landslide-prone region in Mexico, which is characterised by a depositional pyroclastic ramp. The heterogeneous quality of available topographic information(i.e. higher resolution digital elevation model only for a sub-region) encouraged to confront modelling results based on two different study area delineations and two raster resolutions. Input data was based on the larger modelling region L15(163 km2) and smaller S(70 km2; located inside L15) with an associated raster cell size of 15 m(region L15 and S15) and 5 m(region S5). The resulting three data sets(L15, S15 and S5) were included into three differently flexible modelling techniques(Generalized Linear Model-GLM, General Additive Model-GAM, Support Vector Machine-SVM) to produce nine landslide susceptibility models. Preceding variable selection was performed heuristically and supported by an exploratory data analysis. The final models were based on the explanatory variables slope angle, slope aspect, lithology, relative slope position, elevation, convergence index, distance to streams, distance to springs and topographic wetness index. The ability of the models to classify independent test data was elaborated using a k-fold cross validation procedure and the AUROC(Area Under the Receiver Operating Characteristic) metric. In general, all produced landslide susceptibility maps depicted the hillslopes of the ravines, which cut the pyroclastic ramp, as prone to landsliding. The modelling results showed that predictive performances(i.e. AUROC values) slightly increased with an increasing flexibility of the applied modelling technique. Thus, SVM performed best, while the GAM outperformed the GLM. This tendency was most distinctive when modelling with the largest landslide sample size(i.e. data set L15; n = 662 landslides). Non-linear classifiers(GAMs, SVMs) performed slightly better when trained on the basis of lower raster resolution(data set S15) compared to the 5 m counterparts(data set S5). Highest predictive performance was obtained for the model based on data set L15 and the SVM classifier(median AUROC: 0.82). However, SVMs also indicated the highest degree of model overfitting. This study indicates that the decision to delineate a study area, the selection of a raster resolution as well as the chosen classification technique can affect varying aspects of subsequent modelling results. The results do not support the assumption that a higher raster resolution(i.e. a more detailed digital representation of the terrain) inevitably leads to better performing or geomorphically more plausible landslide susceptibility maps.

Development characteristics and petroleum geological significance of Permian pyroclastic flow volcanic rocks in Western Sichuan Basin,SW China

By examining field outcrops, drilling cores and seismic data, it is concluded that the Middle and Late Permian “Emeishan basalts” in Western Sichuan Basin were developed in two large eruption cycles, and the two sets of igneous rocks are in unconformable contact. The lower cycle is dominated by overflow volcanic rocks;while the upper cycle made up of pyroclastic flow volcanic breccia and pyroclastic lava is typical explosive facies accumulation. With high-quality micro-dissolution pores and ultra-fine dissolution pores, the upper cycle is a set of high-quality porous reservoir. Based on strong heterogeneity and great differences of pyroclastic flow subfacies from surrounding rocks in lithology and physical properties, the volcanic facies and volcanic edifices in Western Sichuan were effectively predicted and characterized by using seismic attribute analysis method and instantaneous amplitude and instantaneous frequency coherence analysis. The pyroclastic flow volcanic rocks are widely distributed in the Jianyang area. Centering around wells YT1, TF2 and TF8, the volcanic rocks in Jianyang area had 3edifice groups and an area of about 500 km^(2), which is the most favorable area for oil and gas exploration in volcanic rocks.

Subaqueous early eruptive phase of the late Aptian Rajmahal volcanism,India:Evidence from volcaniclastic rocks,bentonite,black shales,and oolite

The late Aptian（118-115 Ma） continental flood basalts of the Rajmahal Volcanic Province（RVP） are part of the Kerguelen Large Igneous Province,and constitute the uppermost part of the Gondwana Supergroup on the eastern Indian shield margin.The lower one-third of the Rajmahal volcanic succession contains thin layers of plant fossil-rich inter-trappean sedimentary rocks with pyroclasts,bentonite,grey and black shale/mudstone and oolite,whereas the upper two-thirds consist of sub-aerial fine-grained aphyric basalts with no inter-trappean material.At the eastern margin and the north-central sector of the RVP,the volcanics in the lower part include rhyolites and dacites overlain by enstatite-bearing basalts and enstatite-andesites.The pyroclastic rocks are largely felsic in composition,and comprise ignimbrite as well as coarse-grained tuff with lithic clasts,and tuff breccia with bombs,lapilli and ash that indicate explosive eruption of viscous rhyolitic magma.The rhyolites/dacites（〉68 wt.%） are separated from the andesites（〈 60 wt.%） by a gap in silica content indicating their formation through upper crustal anatexis with only heat supplied by the basaltic magma.On the other hand,partially melted siltstone xenoliths in enstatite-bearing basalts suggest that the enstatite-andesites originated through mixing of the upper crust with basaltic magma,crystallizing orthopyroxene at a pressure-temperature of ~3 kb/1150℃.In contrast,the northwestern sector of the RVP is devoid of felsic-intermediate rocks,and the volcaniclastic rocks are predominantly mafic（basaltic） in composition.Here,the presence of fine-grained tuffs,tuff breccia containing sideromelane shards and quenched texture,welded tuff breccia,peperite,shale/mudstone and oolite substantiates a subaqueous environment.Based on these observations,we conclude that the early phase of Rajmahal volcanism occurred under predominantly subaqueous conditions.The presence of grey and black shale/mudstone in the lower one-third of the succession across the entire Rajmahal basin provides unequivocal evidence of a shallow-marine continental shelf-type environment.Alignment of the Rajmahal eruptive centers with a major N—S mid-Neoproterozoic lineament and the presence of a gravity high on the RVP suggest a tectonic control for the eruption of melts associated with the Kerguelen plume that was active in a post-Gondwana rift between India and Australia-Antarctica.

Distribution and gas-bearing properties of Permian igneous rocks in Sichuan Basin, SW China

Based on the analysis of outcrop, seismic, logging and drilling data, combined with exploration practice, the characteristics,distribution, reservoir performance and gas-bearing properties of Permian igneous rocks in Sichuan Basin are studied. The study shows that central volcanic eruptive facies are developed in Sichuan Basin, and their lithological assemblages and distribution characteristics show obvious differences. The igneous rocks are mainly distributed in three regions: the southwestern part of the basin has dominantly largescale overflow facies basalts; the central and western part of the basin, Jianyang-Santai area, develop intrusive rocks, volcanic lavas(basalts)and pyroclastic rocks; and the eastern part of Sichuan, Dazhou-Liangping area, only develop diabase and basalts. Five aspects of understandings are achieved:(1) The Upper Permian igneous rocks can be divided into intrusive rocks and extrusive rocks, with the extrusive rocks as the main body. The chemical compositions of the extrusive rocks are characterized by both alkaline basalt and tholeiitic basalt, and belong to the subalkaline type of transitional basalt magma eruption.(2) There are obvious rhythmic structures vertically among overflow facies basalt, and the single rhythmic layer consists of, from bottom up, pyroclastic rocks(undeveloped), gray and dark gray porphyritic basalts(unstable), dark gray and purple microcrystalline-cryptocrystalline basalts, dark greyish green porous and amygdaloid basalts; the central volcanic eruption shows the rhythm and the vertical sequence of volcanic clastic rocks(agglomerates and breccias), volcanic lava, tuffaceous lava from bottom to top.(3) The pore types of basalt and pyroclastic rocks are diverse, mainly dissolution pore and de-vitrification micropore, but their physical properties are different. Basalt is characterized by ultra-low pore permeability, small reservoir thickness, and reservoirs are distributed in the upper and middle parts of the cycle, with poor lateral comparability. Volcanic clastic rocks are medium to high porous reservoirs(Well YT1: porosity: 8.66%?16.48%, average 13.76%) with large thickness and good reservoir quality.(4) Natural gas in basalts in southwestern basin mainly comes from Middle Permian, and natural gas in volcanic clastic rocks in central and western basin comes from Cambrian Qiongzhusi Formation.(5) Analysis of igneous reservoir-forming conditions in different areas shows that there are relatively insufficient gas sources and great differences in preservation conditions in southwestern basin.Reservoirs are poorly developed and gas-bearing is complex. The Jianyang-Santai area in the central and western part of Sichuan Basin has abundant hydrocarbon sources, developed reservoir, favorable preservation conditions and favorable gas geological conditions, and it is a favorable area for gas exploration.

Pyroclasts of the First Phases of the Explosive-Effusive PCCVC Volcanic Eruption: Physicochemical Analysis

The morphology, texture, grain size and other physicochemical characteristics of pyroclastic material from the first phases of the Puyehue-Cordon Caulle volcanic complex (PCCVC) eruption, (Southern Andes, Chile), can be associated to the model recently reported for the magma storage and its ascent conditions. The eruption started June 4th 2011, and the studied volcanic material corresponds to that collected in Argentine territory at different distances from the source, between 4 and 12 June 2011. The explosive-effusive volcanic process of the first days occurred with the simultaneous emplacement of lava flows and the venting of pyroclastic material, ejecting two well differentiated types of particles. The more abundant was constituted by rhyolitic and light color pumice fragments, characterized by a typical vesicular texture, easy fragmentation and absence of occluded crystalline phases. Particles found in minor proportion were dark color, different in shape and texture and rich in Fe and Ti. They seemed to be more effective for the interaction with emitted gases in the upper part of the column, for this reason, they appeared partially covered by condensation products. The ascent conditions of the magma affected its rheological behavior through variations in the degassing, viscosity and fragmentation. On the other hand, distance to the source, depositional time, volcanic evolution and environmental conditions are factors that affect the chemical composition of collected ash. So, the SiO2/FeO ratio not only increases with the distance but also with the deposition time and volcanic activity. The work was done with the aid of several techniques such as a laser-sediment analyzer, X-ray diffraction (XRD), chemical analysis (bulk and surface), SEM microscopy and Raman “microprobe” spectroscopy. On the other hand, the physicochemical behavior of the pyroclastic material allows us to suggest eventual applications.

Structural traits of woody plants and geomorphological conditions to the vegetation recovery at Ksudach caldera(Southern Kamchatka) since the explosive eruption in 1907

This study considers the restoration of the plant cover destroyed by a catastrophic volcano eruption in the south of Kamchatka Peninsula(the Ksudach caldera,Shtyubel cone),with a particular attention to the role of woody plants.It aims to qualitatively estimate(at a qualitative level)the rate of the primary succession development depending on the microrelief and the modern relief-forming processes.The vegetation recovery in the caldera after the Shtyubel cone eruption 109 years ago proceeds slowly and irregularly.It proceeds most slowly on a thick tephra mantle continuously disturbed by water and wind erosion,lahars,debris flows,landslides,etc.Such conditions are most typical of the western part of the caldera dominated by pumice fields with occasional patches of pioneer herbs and dwarf shrubs.Alpine tundra communities with gradually expanding patches of shrub alder develop in the southern,eastern and northeastern parts of the Ksudach caldera.Since 1997 the area of the shrub alder communities has increased by 1.9 times,while the alpine tundra–by 1.3 times only.The woody plants(dwarf shrubs and prostrate shrubs)are of particular importance in the process of the vegetation recovery on pumice fields.We suggest that the bark thickness of woody plants is an integrated indicator,which may serve for estimating the state of ecosystem in the course of the primary succession development on juvenile sediments.The extremely inhospitable environments cause changes in physiological processes in the plant body and in its bark in particular.The results show that woody plants have a"thin"bark and develop some structural-functional arrangements for keeping bark tissues at their minimum.So,the woody plant structure and the rate of changes in plant communities are under control of the geomorphic conditions and primarily–of physicochemical features of the pyroclastic cover.

Constraints on unconsolidated pyroclastic flow sediments related REE enrichments originated from potassic-alkaline Gölcük stratovolcano: Darıdere-Direkli-Yakaören (DDY) table 4deposits, southwestern Anatolia of Turkey

In the last five decades,Rare Earth Elements(REE)are mostly produced from carbonatite complexes and alkaline magmatic environments.In this respect,pyroclastic flows produced by the potassic-alkaline Gölcük volcanism in the Isparta(SW,Turkey)angle stand out as an important target area where REE enrichments can be observed.This study focuses on the REE potential,geochemical and mineralogical characteristics of REE-bearing minerals as well as formation modeling of unconsolidated pyroclastic flows in Darıdere-Direkli-Yakaören(DDY)deposits in Turkey.REE-bearing minerals of DDY deposits which contain abundant trachytic-trachyandesitic rock fragments were determined as fluorapatite,britholite,and chevkinite by using XRD and SEM–EDX analyses.According to the geochemical analysis,the vast majority of RREE(up to 996 ppm)content is LREE such as La,Ce and Nd.Darıdere is the most REE enriched region followed by Yakaören and Direkli regions in terms of LREE(574.9 ppm),relative to MREE(38.5 ppm)and HREE(5.28 ppm)contents.These results are also on par with subduction-related Italian extrusive carbonatites such as Polino,Cupaello and San Venenzo.The DDY deposits can also be comparable with intraplate extrusive REE deposits in terms of LREE enrichments,however,HREE,Nb,Th and Zr values are generally lower.The RREE concentrations tend to rise with the increasing contents of alkaline elements(Na and K),HFSE(such as Nb and U),as well as LILE(Ba,Sr and Rb)and are negatively affected by sedimentary carbonate involvement.The economic value of the DDY deposits can gain importance due to the increasing global demand for LREE’s.

Geochemical Characteristics of Early Permian Pyroclastic Rocks in the Jimunai Basin,West Junggar,Xinjiang(NW China):Implications for Provenance and Tectonic Setting

During the Late Paleozoic,West Junggar(Xinjiang,NW China)experienced a shift in tectonic setting from compression to extension.Ha'erjiao is an important area for investigating collisional structures,post–collisional structures,and magmatic activities.Based on the petrological and geochemical characteristics of pyroclastic and other volcanic rocks in the Permian Kalagang Formation from the borehole ZKH1205 in the Jimunai Basin,the main types of source rock for the pyroclastic rocks deposited in the basin are identified and their implications for the Early Permian tectonic setting examined.The abundance of basalt and andesite lithic fragments in the pyroclastic rocks,together with the REE characteristics and the contents of transition and high field strength elements show that the source rocks were chiefly intermediate–basic volcanic rocks.High ICV values,low CIA values,low Rb/Sr ratios,low Th/U ratios and the mineralogical features suggest weak chemical weathering of the source rocks;the geochemical patterns of the pyroclastic rocks might not only have been impacted by crustal contamination but also might be related to the nature of the magma from the source area.The geochemical properties of the pyroclastic rocks distinguish them from arc-related ones,and such samples plot in the within-plate basalt(WPB)field in some diagrams.This is consistent with the formation background of the Early Permian volcanic rocks in this region.

Chemical Weathering of New Pyroclastic Deposits from Mt. Merapi (Java), Indonesia

The Java Island, Indonesia with abundant amount of pyroclastic deposits is located in the very active and dynamic Pacific Ring of Fires. Studying the geochemical weathering indices of these pyroclastic deposits is important to get a clear picture about weathering profiles on deposits resulting from the eruption of Mt. Merapi. Immediately after the first phase of the eruption （March to June 2006）, moist and leached pyroclastic deposits were collected. These pyroclastic deposits were found to be composed of volcanic glass, plagioclase feldspar in various propoitions, orthopyroxene, clinopyroxene, olivine, amphibole and titanomagnetite. The total elemental composition of the bulk samples （including trace elements and heavy metals） was determined by wet chemical methods and X-ray fluorescence （XRF） analyses. Weathering of the pyroelastic deposits was studied using various weathering indices. The Ruxton ratio, weathering index of Parker, Vought resudual index and chemical index of weathering of moist pyroclastic deposits were lower than those of the leached samples, but the alteration indices （chemical and plagioclase） were slightly higher in the moist compared to the leached pyroelastic deposits.

Transient infiltration tests in pyroclastic soils with double porosity

Fallout volcanic deposits of SommaVesuvius(Campania,southern Italy),characterized by the presence of layers with contrasting textural and hydraulic properties,are frequently affected by shallow landslides during rainwater infiltration.The soils of the stratigraphic sequence present intraparticle pores,originated by the gases escaped during magma decompression in the volcanic conduit,thus are characterized by double porosity(i.e.,intraparticle and interparticle pores),which is expected to affect their hydraulic behaviour,and to play a key role in rainwater infiltration through layered deposits.To understand the effect of double porosity on the hydraulic behaviour of the involved soils,controlled experiments have been carried out in an infiltration column.The experimental apparatus is provided with newly designed non-invasive Time Domain Reflectometry(TDR)probes,not buried in the investigated soil layers so as to minimize disturbance to the flow,allowing water content measurement during vertical flow processes.Specifically,transient flow experiments are carried out through reconstituted specimens of black scoriae and grey pumices,both loose pyroclastic granular soils from fallout deposits of Somma-Vesuvius,featuring double porosity with different pore size distributions,that were estimated by X-ray tomography and Mercury Intrusion Porosimetry.The experimental results highlight the effects of the double porosity and clearly indicate the different behaviour of the two soils during wetting and drying processes,mainly related to the different dimensions of intraparticle pores.

Fluid interaction mechanism and diagenetic reformation of basement reservoirs in Beier Sag, Hailar Basin, China

Based on analysis of core observation, thin sections, cathodoluminescence, scanning electron microscope(SEM), etc., and geochemical testing of stable carbon and oxygen isotopes composition, element content, fluid inclusions, and formation water, the fluid interaction mechanism and diagenetic reformation of fracture-pore basement reservoirs of epimetamorphic pyroclastic rock in the Beier Sag, Hailar Basin were studied. The results show that:(1) Two suites of reservoirs were developed in the basement, the weathering section and interior section, the interior section has a good reservoir zone reaching the standard of type I reservoir.(2) The secondary pores are formed by dissolution of carbonate minerals, feldspar, and tuff etc.(3) The diagenetic fluids include atmospheric freshwater, deep magmatic hydrothermal fluid, organic acids and hydrocarbon-bearing fluids.(4) The reservoir diagenetic reformation can be divided into four stages: initial consolidation, early supergene weathering-leaching, middle structural fracture-cementation-dissolution, and late organic acid-magmatic hydrothermal superimposed dissolution. Among them, the second and fourth stages are the stages for the formation of weathering crust and interior dissolution pore-cave reservoirs, respectively.

Genetic Classification of Pyroclastic Ejecta Based on Physical Volcanology of Possible Large Cauldron in Bombay Volcanic Complex, Western Deccan Trap Province, India

Many Propositions are made about the mechanism of emplacement of volcanoclastic material in the Bombay volcanic complex. The present paper deals exclusively with the physical features of the deposits laid by a complex tectono-magmatic process by making detailed inventory of the different kind of volcanic ejecta exposed in the Bomay Volcanic Complex (BVC), and an attempt has been made to classify the deposits genetically. A subsidenace which was hinted at earlier, may be a possible cauldron in BVC has been proposed, which might be responsible for producing such a varied and complex lithology.

The Nuée Ardentes of Sabalan Volcano in Iran

This article deals with the introduction and explanation of the two nuéeardentes of “Mue’el Valley” and “Shirvandareh Valley” which are one type of the pyroclastic flows of Sabalan volcano. One of the features of nuéeardente in these two valleys is the presence of large blocks in upstream of valley and the abundance of ash in downstream which it has happened in one stage in Mue’el and in several stages in Shirvandareh Valley. Considering the antiquity of the great Sabalan volcano and numerous changes that have happened during these long years in the region, determining the temperature of nuéeardente was impossible. But what could be mentioned as a sign of high temperature because of nuéeardente is the presence of rust ground mass on piece levels which indicate hot water steam (500°C to 600°C). Also puzzle breakings like those in these rocks should be the result of severe explosion blast and very fast movements of nuéeardente that has broken these rocks instantly. These nuéeardentes are peléan type and they are more or less synchronous, and the main reason could be the huge explosion which happened because of the second caldra’s subsidence. By comparing deposits of the two nuéeardentes of Mue’el Valley and Shirvandareh Valley, it is observed that the extent, thickness, and size of blocks are higher in Shirvandareh than those in Mue’el Valley. Also considering higher levels of pieces’ adherence and hardness of deposits, it can be concluded that the temperature of this complex was higher than that of Mue’el Valley, and in general the power of Shirvandareh’s nuéeardente is more than the Mue’el Valley’s.

Constrains of eruption environment and hydrothermal fluid on the Permian pyroclastic reservoirs in the Sichuan Basin,SW China

A breakthrough has been recently made in the hydrocarbon exploration of the Permian pyroclastic rocks in the Jianyang Area,western Sichuan Basin,China.With an aim to decipher the impacts of the eruption environment,the temperature of hydrothermal fluids,and the paleo-salinity on the formation of authigenic mineral assemblage and pores,this study implements comprehensive petrographic and geochemical studies through the integrated core and thin section analyses.The data presented demonstrate that the Permian volcanic intervals are intensively affected by an event of Emei taphrogeny.During basaltic magma upward migration,fractional crystallization of anorthose results in slightlyalkaline magma.The specific pyroclastic rocks are formed by the eruption of slightly-alkaline magma in the sea or a salt lake and subsequent hydrothermal alteration.During deposition and diagenesis,the authigenic mineral association is constrained jointly by the sodium-rich and high salinity water environment,and mid-high temperature,high-salinity hydrothermal fluid.Specifically,the sodium-rich hydrothermal fluid,which may sustain till the late diagenesis stage,caused pervasive albitization of pyroclastic rocks,then leading to mineral transformation and formation of a series of mineral associations.Therefore,zeolitization of volcanic glass and vesicle-infillings of zeolite is an essential condition for later mineral transformation and dissolution.Albitization of analcite,recrystallization induced by deep hydrothermal fluids,and both meteoric and deep burial dissolution expanding the micro-pore space ultimately formed porous pyroclastic reservoirs.

Define the Energy Cone Threshold and Extent of Tianchi Volcano

Energy cone is a unique but characteristic slope that describes the extent of deposits left around a volcano by various fiowage phenomena, usually regarded as the boundary of pyroclastic and the source region of lahar. The energy cone value is determined as 0.07 by the energy-Hne model com- bined with the parameters of plume height and gradient, and the energy cone spread extent is defined by the numerical simulation method LAHARZ to simulate with this value based on the 1 ： 50 000 digi-tal elevation model of Tianchi （天池） Volcano, and the source region profiles in the north and south slope can prove the correctness of this threshold. This energy cone threshold and extent can be used as the reference of pyroelastic flow and lahar simulation.

CHARACTERISTICS, DIAGNOSTIC CRITERIA AND GEOLOGICAL SIGNIFICANCE OF PYROCLASTIC SURGE DEPOSITS

Study has been performed on the characteristics of the pyroclastic surge deposits in the northern Yandangshan area of Zhejiang Province. Based on this study and the relative research of other scholars, five major diagnostic criteria of the deposits have been proposed. Moreover, the geological significance of the study on the deposits has been discussed.