On the genesis of ignimbrites how ignimbrites and other pyroclastic products originate from a flowing melt

All the textural and structural characteristics of the alkaline ignimbrites of Northern Latium have been studied in detail. The study clearly indicates that these «ignimbrites» are the products of differential gas exolution from gas-oversaturated melts during and after their moving on the surface. It was also found that the processes of differential gas expansion, depending on the initial melt viscosity, mechanical stresses, etc., result in the whole series of features recognised as typical of ignimbrites. A careful re-examination of the «classic» Italian acid calc-alkaline ignimbrites, and a critical review of analogous materials known from the international literature allowed the Authors to extend the genetic trend of the alkaline ignimbrites of Latium to all deposits having ignimbritic character, and to rearrange their broad range of variability into a comprehensive genetic picture.     

Correlation of welded ignimbrites on Pantelleria (Strait of Sicily) using paleomagnetism

Although the oldest volcanic rocks exposed at Pantelleria (Strait of Sicily) are older than 300 ka, most of the island is covered by the 45–50 ka Green Tuff ignimbrite, thought to be related to the Cinque Denti caldera, and younger lavas and scoria cones. Pre-50 ka rocks (predominantly rheomorphic ignimbrites) are exposed at isolated sea cliffs, and their stratigraphy and chronology are not completely resolved. Based on volcanic stratigraphy and K/Ar dating, it has been proposed that the older La Vecchia caldera is related to ignimbrite Q (114 ka), and that ignimbrites F, D, and Z (106, 94, and 79 ka, respectively) were erupted after caldera formation. We report here the paleomagnetic directions obtained from 23 sites in ignimbrite P (133 ka) and four younger ignimbrites, and from an uncorrelated (and loosely dated) welded lithic breccia thought to record a caldera-forming eruption. The paleosecular variation of the geomagnetic field recorded by ignimbrites is used as correlative tool, with an estimated time resolution in the order of 100 years. We find that ignimbrites D and Z correspond, in good agreement with recent Ar/Ar ages constraining the D/Z eruption to 87 ka. The welded lithic breccia correlates with a thinner breccia lying just below ignimbrite P at another locality, implying that collapse of the La Vecchia caldera took place at ~130–160 ka. This caldera was subsequently buried by ignimbrites P, Q, F, and D/Z. Paleomagnetic data also show that the northern caldera margin underwent a ~10° west–northwest (outwards) tilting after emplacement of ignimbrite P, possibly recording magma resurgence in the crust.     

Lithic types in ignimbrites as a guide to the evolution of a caldera complex, Taupo volcanic centre, New Zealand

Taupo volcanic centre is one of two active rhyolite centres in the Taupo Volcanic Zone (TVZ), and has been sporadically active over the past ca. 300 ka. At least four large-scale ignimbrites have erupted from the centre, including the well documented 26.5 ka Oruanui ignimbrite and 1.8 ka Taupo ignimbrite. Because stratigraphy of earlier ignimbrites and their sources are masked by later volcanism, disrupted by regional tectonics and obscured by poor exposure, indirect methods must be applied in order to determine their source regions. In this paper detailed componentry, density and petrology of lithic fragments from three ignimbrites (Rangatira Point, Oruanui, Taupo) are used to reveal aspects of the sub-Taupo caldera geology, including the evolution of the Taupo volcanic centre, to assist in ignimbrite correlation and to evaluate structures within the Taupo caldera complex. Lithic fragments identify a complex subsurface geology. The Rangatira Point ignimbrite sampled dominantly rhyolite lavas, plus a variety of welded ignimbrites, rare high-silica dacites and a single dolerite. Most lithic fragments in the Oruanui ignimbrite are andesite with minor rhyolite, welded ignimbrite, dacite and rounded greywacke, while in the Taupo ignimbrite, rhyolite is again the dominant lithic component with subordinate welded ignimbrites, andesite, and greywacke. The densities of lithic fragments indicate similar ranges of values for all lava types, and thus density is a poor indicator of lithology. Care must, therefore, be taken before interpreting subcrustal stratigraphy using density as the sole criterion. The petrography and geochemistry of lithic types are more specific, and the variation can be used to identify sources for the ignimbrites. Both pumice chemistry and rhyolite lithic fragments from the Rangatira Point ignimbrite are comparable to domes exposed at the southern end of the Western Dome Complex and, combined with limited outcrop information, suggest the most likely source for this unit is in the northern part of the Taupo caldera complex. The dominance of andesite lithic fragments in the Oruanui ignimbrite suggests a major andesite cone existed beneath the source area, and the different lithic suites between Oruanui and Taupo ignimbrites suggest these ignimbrites came, at least in part, from mutually exclusive collapse structures. We believe that the Oruanui caldera is sited principally in the northwestern part of present-day Lake Taupo and the Taupo caldera in the northeastern part. Identification of abundant ignimbrite lithics in the Taupo ignimbrite, which are considered to represent an intracaldera facies of an earlier ignimbrite, that is not exposed at the surface, suggest there was a further (pre-Oruanui) ignimbrite caldera in the Taupo ignimbrite eruptive vent region.     

Rhyolite magmas of central America

True rhyolites are found in two contrasting occurrences in Central America: in recent obsidian domes in the basalt-rhyolite association of southeastern Guatemala, and in late Tertiary ignimbrites in Honduras and adjacent parts of Nicaragua. Both are on the inner side of the main volcanic axis in a region that is visibly underlain by older metamorphic and plutonic rocks. They have not been found in southern Central America where the basement series consists only of older volcanic rocks, eugeosynclinal sediments, and peridotite. Rhyolite obsidian has erupted alternately with basalts in the Obrajuelo Complex of southeastern Guatemala. Cumulate rocks and moderate compositional variations in the basalts provide evidence of limited crystal fractionation, but abundant venocrysts and partially fused xenoliths indicate that partial melting of the plutonic and metamorphic basement may also have been important. Extremely siliceous rhyolite ignimbrites with a volume of several thousands of cubic kilometers closely resemble the rhyolite obsidians in their major element compositions, but in contrast to the totally glassy obsidians they contain abundant phenocrysts of quartz and alkali feldspar. A few ignimbrites contain inclusions of partially melted basement rocks. The basalts with which they are associated have a limited compositional range. Melting experiments performed on rocks of the basement series, together with chemical, isotopic, and volumetric relations, indicate that the ignimbrites are products of partial fusion of the basement series, while obsidians of the Obrajuelo Complex have differentiated from more basic magmas by a process other than crystal fractionation.     

Petrology of the Yugu peridotites in the Gyeonggi Massif,South Korea: Implications for its origin and hydration process

Peridotites exposed in the Yugu area in the Gyeonggi Massif, South Korea, near the boundary with the Okcheon Belt, exhibit mylonitic to strongly porphyroclastic textures, and are mostly spinel lherzolites. Subordinate dunites, harzburgites, and websterites are associated with the lherzolites. Amphiboles, often zoned from hornblende in the core to tremolite in the rim, are found only as neoblasts. Porphyroclasts have recorded equilibrium temperatures of about 1000°C, whereas neoblasts denote lower temperatures, about 800°C. Olivines are Fo_90–91 in lherzolites and Fo₉₁ in a dunite and a harzburgite. The Cr# (= Cr/(Cr + Al) atomic ratio) of spinels varies together with the Fo of olivines, being from 0.1 to 0.3 in lherzolites and around 0.5 in the dunite and harzburgite. The Na₂O content of clinopyroxene porphyroclasts is relatively low, around 0.3 to 0.5 wt% in the most fertile lherzolite. The Yugu peridotites are similar in porphyroclast mineral chemistry not to continental spinel peridotites but to sub‐arc or abyssal peridotites. Textural and mineralogical characteristics indicate the successive cooling with hydration from the upper mantle to crustal conditions for the Yugu peridotites. Almost all clinopyroxenes and amphiboles show the same U‐shaped rare earth element (REE) patterns although the level is up to ten times higher for the latter. The hydration was associated with enrichment in light REE, resulting from either a slab‐derived fluid or a fluid circulating in the crust. The mantle‐wedge or abyssal peridotites were emplaced into the continental crust as the Yugu peridotite body during collision of continents to form a high‐pressure metamorphic belt in the Gyeonggi Massif. The peridotites from the Gyeonggi Massif exhibit lower‐pressure equilibration than peridotites, with or without garnets, from the Dabie–Sulu Collision Belt, China, which is possibly a westward extension of the Gyeonggi Massif.     

Neogene ignimbrites of the Nevsehir plateau (Central Turkey): stratigraphy, distribution and source constraints

In Anatolia (Turkey), extensive calc-alkaline volcanism has developed along discontinuous provinces from Neogene to Quaternary times as a consequence of plate convergence and continental collision. In the Nevsehir plateau, which is located in the Central Anatolian Volcanic Province, volcanism consists of numerous monogenetic centres, several large stratovolcanoes and an extensive, mainly Neogene, rhyolitic ignimbrite field. Vent and caldera locations for the Neogene ignimbrites were not well known based on previous studies.In the Neogene ignimbrite sequence of the Nevsehir plateau, we have identified an old group of ignimbrites (Kavak ignimbrites) followed by five major ignimbrite units (Zelve, Sarimaden Tepe, Cemilköy, Gördeles, Kizilkaya) and two smaller, less extensive ones (Tahar, Sofular). Other ignimbrite units at the margin of the plateau occur as outliers of larger ignimbrites whose main distributions are beyond the plateau. Excellent exposure and physical continuity of the units over large areas have allowed establishment of the stratigraphic succession of the ignimbrites as, from bottom to top: Kavak, Zelve, Sarimaden Tepe, Cemilköy, Tahar, Gördeles, Sofular, Kizilkaya. Our stratigraphic scheme refines previous ones by the identification of the Zelve ignimbrite and the correlation of the previously defined ‘Akköy’ ignimbrite with the Sarimaden Tepe ignimbrite. Correlations of distant ignimbrite remnants have been achieved by using a combination a field criteria: (1) sedimentological characterisitics; (2) phenocryst assemblage; (3) pumice vesiculation texture; (4) presence and characteristics of associated plinian fallout deposits; and (5) lithic types. The correlations significantly enlarge the estimates of the original extent and volume of most ignimbrites: volumes range between 80 km³ and 300 km³ for the major ignimbrites, corresponding to 2500–10,000 km³ in areal extent.The major ignimbrites of the Nevsehir plateau have an inferred source area in the Derinkuyu tectonic basin which extends mainly between Nevsehir and the Melendiz Dag volcanic complex. The Kavak ignimbrites and the Zelve ignimbrite have inferred sources located between Nevsehir and Derinkuyu, coincident with a negative gravity anomaly. The younger ignimbrites (Sarimaden Tepe, Cemilköy, Gördeles, Kizilkaya) have inferred sources clustered to the south between the Erdas Dag and the Melendiz Dag volcanic complex. We found evidence of collapse structures on the northern and southern flanks of the Erdas Dag volcanic massif, and of a large updoming structure in the Sahinkalesi Tepe massif. The present-day Derinkuyu tectonic basin is mostly covered with Quaternary sediments and volcanics. The fault system which bounds the basin to the east provides evidence that the ignimbrite volcanism and inferred caldera formation took place in a locally extensional environment while the basin was already subsiding. Drilling and geophysical prospecting are necessary to decipher in detail the presently unknown internal structure of the basin and the inferred, probably coalesced or nested, calderas within it.     

黑龙江佳木斯地块南端及邻区深部三维电性结构及其构造意义

佳木斯地块及邻区位于中亚造山带(CAOB)的东部, 由于受到了古太平洋板块俯冲和牡丹江洋闭合的影响, 该地区形成了复杂的构造. 为了揭示该区深部电性结构, 探讨其动力学机制等科学问题, 我们采用基于非线性共轭梯度算法的三维大地电磁反演, 获得了一个长约400 km的大地电磁剖面电性结构模型, 该剖面反映了松嫩地块东北缘、佳木斯地块南端和完达山地体的深部岩石圈电性结构特征. 并结合研究区地质资料和其他地球物理资料, 我们对剖面所经过的主要地质构造单元展开了综合分析. 结果表明: (1)松嫩地块东北缘的小兴安岭岩石圈电性结构以巨厚的高阻体为主, 可能与多期次形成的花岗岩叠置作用有关; (2)三个地块之间的缝合带下方均存在高导异常, 这些高导异常可能与软流圈物质上涌有关; (3)牡丹江断裂、跃进山断裂和敦化—密山断裂可能为地壳尺度的深大断裂, 这些断裂可能作为微板块的边界, 与块体之间的古缝合带重合; (4)佳木斯地块南端存在大规模高阻异常体, 表明古老的佳木斯地块南端具有较冷的刚性块体特征, 我们推测可能是冈瓦纳大陆或塔里木克拉通的一部分.

     

Anisotropic Measurements in the Rhinegraben Area and the French Massif Central: Geodynamic Implications

—As part of an integrated seismic study, polarization of shear waves has been analyzed for teleseismic events recorded at a set of permanent broadband, semi-permanent long- and short-period and temporary short-period seismological stations located in two geodynamically important areas in western Europe, namely the Rhinegraben-Urach area and the French Massif Central volcanic field.¶While for the semi-permanent and the permanent stations there is a good azimuthal coverage of teleseismic earthquakes which allowed us to investigate the azimuthal dependence and the spatial variation over short distances of an anisotropy direction, no even azimuthal distribution of teleseismic recordings with a clear elliptical (or linear) polarization of the S phases could be obtained in the case of the temporary stations.¶While the mean values of the splitting parameters φ and δt are geographically coherent for adjacent stations, our results show a large scatter of the individual splitting parameters for the set of events used. The magnitude of the splitting time suggests that the deformation extends below the lithosphere and that the thickness of the anisotropic structure is at least 100–200 km.¶For some stations located in the Rhinegraben-Urach area (ECH, RG-N, RG-S, RBG), the variations of φ are consistent with a two-layer anisotropic model as suggested by Vinnik et al. (1994) for the South German Triangle. For the stations ECH (Vosges mountains), RG-N and RG-S (Rhinegraben proper), the resulting estimates of fast direction are around N10°E–N30°E and N80°E–N100°E for the upper and lower layers, respectively. For the station RBG (Urach), the results are N60°E–N70°E and N125°E–N135°E, respectively.¶In the Rhinegraben-Urach area, the estimates of the effective fast direction for a one-layer model show a rotation from a graben-related (30°) pattern to an Alpine belt-related pattern in the eastern part (≈ E–W). In the French Massif Central region, the results reveal two distinct fast polarization patterns. While to the west of the Sillon Houiller, φ is parallel to this late-variscan transformlike fault zone and perpendicular to the variscan belt, it is to the east rather perpendicular to the Alpine belt. The results suggest a mixture of both a lithospheric and an asthenospheric component of the seismic anisotropy for the Rhinegraben-Urach as well as for the French Massif Central areas.     

The cenozoic rhyolite-andesite association of the chilean andes

The eruption centres of late volcanism in Chile are situated in two separate areas in the northern and southern High Cordillera. In the north, the ignimbrites of the Rhyolite Formation and the rocks of the « Andesite » Formation occur in about equal proportions, and recent activity is meagre. In the south, the rocks of the « Andesite » Formation predominate, and many volcanoes are in a highly explosive phase of activity. Field relationships, petrological and geochemical data show that the rocks of both Formations are closely related to each other. There is evidence that the magmas of the Rhyolite Formation were formed by fusion of sialic material in the upper parts of the crust. The data for the volcanics of the « Andesite » Formation are inconsistent with their derivation by fractional crystallization of a basaltic parent or by direct mantle derivation involving a single stage process. The authors suggest that the « andesitic » magmas are products of a primary andesitic magma originated by partial fusion of material of the lower crust. Assuming that the « andesitic » magmas of the central parts of the Andes are derived from the upper mantle, this would mean — in the light of the Sr⁸⁷/Sr⁸⁶ data — that the upper mantle in the central region of the Andes is essentially more radiogenic than in other orogenic areas; moroever, it should be very similar in its chemical and Sr⁸⁷/Sr⁸⁶ composition to that of the lower crust.     

Rhyolitic ignimbrites in the Rogerson Graben,southern Snake River Plain volcanic province: volcanic stratigraphy,eruption history and basin evolution

The 80 km long NNE-trending Rogerson Graben on the southern margin of the central Snake River Plain, Idaho, USA, hosts a rhyolitic pyroclastic succession, 200 m thick, that records a period of successive, late-Miocene, large-volume explosive eruptions from the Yellowstone–Snake River Plain volcanic province, and contemporaneous extension. The succession, here termed the Rogerson Formation, comprises seven members (defined herein) and records at least eight large explosive eruptions with numerous repose periods. Five high-grade and extremely high-grade ignimbrites are intercalated with three non-welded ignimbrites and two volcaniclastic deposits, with numerous repose periods (palaeosols) throughout. Two of the ignimbrites are dominantly rheomorphic and lava-like but contain subordinate non-welded pyroclastic layers. The ignimbrites are typical Snake River Plain high-silica rhyolites, with anhydrous crystal assemblages and high inferred magmatic temperatures (≤ 1,025°C). We tentatively infer that the Jackpot and Rabbit Springs Members may have been emplaced from the Bruneau–Jarbidge eruptive centre on the basis of: (1) flow lineation trends, (2) crystal assemblage, and (3) radiometric age. We infer that the overlying Brown’s View, Grey’s Landing, and Sand Springs Members may have been emplaced from the Twin Falls eruptive centre on the basis of: (1) kinematic indicators (from the east), and (2) crystal assemblage. Furthermore, we have established the contemporaneous evolution of the Rogerson Graben from the emplacement of the Jackpot Member onwards, and infer that it is similar to younger half-graben along the southern margin of the Snake River Plain, formed by local reactivation of Basin and Range structures by the northeastwardly migration of the Yellowstone hot-spot. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Sheathfolds in rheomorphic ignimbrites

Structural reappraisal of several classic rheomorphic ignimbrites in Colorado, Idaho, the Canary Islands and Italy has, for the first time, revealed abundant oblique folds, curvilinear folds and sheathfolds which formed during emplacement. Like their equivalents in tectonic shear-zones, the sheathfold axes lie sub-parallel to a pervasive elongation lineation, and appear as eye structures on rock surfaces normal to the transport direction. With the recognition of sheathfolds, ignimbrites previously inferred to have undergone complex rheomorphic deformation histories are re-interpreted as recording a single, progressive deformation event. In some examples, the trends of sheathfolds and related lineations change with height through a single ignimbrite suggesting that rheomorphism did not affect the entire thickness of ignimbrite synchronously. Instead, we infer that in these ignimbrites a thin ductile shear-zone rose gradually through the aggrading agglutinating mass whilst the flow direction varied with time. This suggests that, in some cases, both welding and rheomorphism can be extremely rapid, with ductile strain rates significantly exceeding rates of ignimbrite aggradation.Editorial responsibility: T. Druitt     

Silicic phreatomagmatism in the Snake River Plain: the Deadeye Member

Non-welded rhyolitic pyroclastic units in the central Snake River Plain are interbedded with the much better exposed, large-volume ‘Snake-River type’ rheomorphic welded rhyolitic ignimbrites and rhyolite lavas. We document one such unit to investigate why it is so different from the interbedded welded ignimbrites. The newly recognised Deadeye Member of southern Idaho is a soil-bounded eruption-unit that comprises ashfall layers and a 4-m-thick ignimbrite that extends for >35 km. The ignimbrite is non-welded, lithic-clast poor and varies from massive to diffuse low-angle cross-bedded. It contains abundant angular clasts of non-vesicular black glass, and upper parts contain accretionary lapilli. The ashfall layers above it contain coated ash pellets and ash clumps, which record moist aggregation of fine ash. The magmas of the Deadeye eruption were closely similar in composition and temperature to those that generated the intensely welded rheomorphic ignimbrites of the central Snake River Plain. We infer that the marked contrast in physical appearance of the Deadeye ignimbrite compared to the other, more typical Snake-River-type welded ignimbrites was the result of emplacement at relatively low temperatures during an eruption in a lacustrine environment. Magmatic volatile-driven fragmentation of the rhyolitic magma was influenced by interaction with lake water that also led to cooling. The Deadeye Member is the first-recorded example of explosive silicic phreatomagmatism in the central Snake River Plain.     

Chemical composition of boulder-2 rocks and soils,Apollo 17, station 2

Forty-two elements have been measured via INAA and RNAA in six samples of five rocks from a 2-m tan-gray boulder-2 breccia (South Massif), in four soils from the South Massif and a valley soil. The chemical composition of the four metaclastic rocks corresponds to “high alumina” (52% Pl) and medium-K KREEP-type rocks. Rock 72335,2 is a medium K anorthositic gabbro (74% Pl). Both the North and South Massifs appear to be medium-K KREEP in composition and thus may represent a single stratigraphic unit of the Serenitatis basin event. Four soils of the South Massif are identical in composition to medium-K KREEP; they are more feldspathic and lower in LIL trace elements relative to the boulder-2 rocks. The valley soil 75081 is like the 10084 soil; both soils are high in TiO₂ and both are deficient in KREEP. Th and U give a sharp distinction between the valley and highland soils. The South Massif rocks and soils contain siderophiles at the 2–4% Cl level and show an ancient meteoritic pattern. Five samples of the four rocks have Ir/Au ratios of ± 0.02 which we assign to the Serenitatis basis planetesimal. The valley soil at Camelot Crater has low siderophiles (1% Cl). Our systematic study of four shadowed and exposed soils does not support the labile hypothesis for Cd, In, Tl and Zn. We observe no volatile (atmophile) movement from the South Massif highland soils to the valley soil 75081. The volatiles Cs and Tl appear to have been fractionated in the boulder-2 rocks during cratering, brecciation and metamorphic processes. The uniform ratio of FeO/MnO = 80–85, observed for all previous mare and highland sites, also holds for the Taurus-Littrow site.     

The role of fluidization in the emplacement of pyroclastic claws: An experimental approach

A series of experiments was carried out to review the process of fluidization for a number of particulate materials having various sorting and grain shape characteristics. The up (increasing gas velocity) curve on a gas velocity/bed-pressure drop plot for a poorly sorted mixture of irregularly shaped particles is divided into three sections; non-expanded, expanded, and segregating. These sections are used to define a threefold genetic classification of pyroclastic flows which can be directly linked to conditions within a semifluidized parent flow. Type 1 flows are ungraded and mostly result from hot-avalanche flows and pyroclastic flows formed of relatively dense material. Type 2 and type 3 flows are mainly pumiceous ignimbrites and are distinguished by expansion induced coarse-tail grading, coupled with segregation structures in the latter. The implications of this classification are discussed with reference to the flow regimes, deposition slope angles, crystal concentration and “fossil fumaroles” variously developed in the flows. The relevance of the source of fluidizing gas is discussed in relation to the zoning of flow types within a single flow unit and it is shown how this, and its attendant structures, can be used to estimate the relative importance of each gas source during and after flow emplacement.     

Chronological and palaeomagnetic constraints on widespread welded ignimbrites of the Taupo volcanic zone,New Zealand

 Large volume (100–1000 km³), widespread rhyolitic ignimbrites are the main products of the Taupo volcanic zone (TVZ) of New Zealand, one of the most active silicic volcanic regions on Earth. Several factors have made correlation and the eruptive history of the ignimbrites difficult to resolve, including limited exposure and chronological data, broadly similar lithologies and the lack of stratigraphic successions visible in the field. We have used the isothermal plateau fission track (ITPFT) method on glass shards from the non-welded basal zones to obtain new eruption ages for the widespread units: Ongatiti (1.25±0.12 Ma), Whakamaru group (0.34±0.03 Ma), Matahina (0.34±0.02 Ma), Chimp (0.33±0.02 Ma), Kaingaroa (0.31±0.01 Ma) and Mamaku (0.23±0.01 Ma) ignimbrites. These glasses show little evidence of geochemical alteration and allow the units to be fingerprinted for correlation. The glass ages we have obtained for the late Quaternary units provide an independent check on chronological data obtained from phenocryst phases. The ITPFT method is a useful dating approach for sanidine-poor eruptives which limit the application of ⁴⁰Ar/³⁹Ar. Errors as limited as 10–30 ka can be obtained from the weighted mean of several age determinations. The thermoremanent magnetic (TRM) direction recorded in the units provides a means of correlation over a wide area of the TVZ, because each ignimbrite can be distinguished by its unique record of palaeosecular variation. These data indicate that the four separately mapped members of the Whakamaru group represent the same phase of activity, occurring within a period of 100 years. The TRM data indicate that the widespread Ahuroa ignimbrite erupted during an excursion in Earth's magnetic field, perhaps associated with the Cobb Mountain subchron (ca. 1.2 Ma). The youngest widespread welded unit, Mamaku ignimbrite (ca. 0.23 Ma), also erupted during an excursion and may represent a southern hemisphere record of the Pringle Falls geomagnetic episode found in the western United States. The palaeomagnetic and ITPFT data for the widespread late Quaternary ignimbrites suggest a major period of caldera formation at 0.34–0.30 Ma. This interval represents the eruption of multiple units from the Whakamaru caldera, followed by the formation of the Okataina and Reporoa calderas in rapid succession. Received: 20 November 1995 / Accepted: 8 May 1996     

Tephrochronology of the Mont-Dore volcanic Massif (Massif Central,France): new 40Ar/39Ar constraints on the Late Pliocene and Early Pleistocene activity

The Mont-Dore Massif (500 km²), the youngest stratovolcano of the French Massif Central, consists of two volcanic edifices: the Guéry and the Sancy. To improve our knowledge of the oldest explosive stages of the Mont-Dore Massif, we studied ⁴⁰Ar/³⁹Ar-dated (through single-grain laser and step-heating experiments) 11 pyroclastic units from the Guéry stratovolcano. We demonstrate that the explosive history of the Guéry can be divided into four cycles of explosive eruption activity between 3.09 and 1.46 Ma (G.I to G.IV). We have also ascertained that deposits associated with the 3.1–3.0-Ma rhyolitic activity, which includes the 5-km³ “Grande Nappe” ignimbrite, are not recorded in the central part of the Mont-Dore Massif. All the pyroclastites found in the left bank of the Dordogne River belong to a later explosive phase (2.86–2.58 Ma, G.II) and were channelled down into valleys or topographic lows where they are currently nested. This later activity also gave rise to most of the volcanic products in the Perrier Plateau (30 km east of the Mont-Dore Massif); three quarters of the volcano-sedimentary sequence (up to 100 m thick) was emplaced within less than 20 ky, associated with several flank collapses in the northeastern part of the Guéry. The age of the “Fournet flora” (2.69?±?0.01 Ma) found within an ash bed belonging to G.II suggests that temperate forests already existed in the French Massif Central before the Pliocene/Pleistocene boundary. The Guéry’s third explosive eruption activity cycle (G.III) lasted between 2.36 and 1.91 Ma. It encompassed the Guéry Lake and Morangie pumice and ash deposits, as well as seven other important events recorded as centimetric ash beds some 60 to 100 km southeast of the Massif in the Velay region. We propose a general tephrochronology for the Mont-Dore stratovolcano covering the last 3.1 My. This chronology is based on 44 ⁴⁰Ar/³⁹Ar-dated events belonging to eight explosive eruption cycles each lasting between 100 and 200 ky. The occurrence of only one pumice deposit in the 800-ky period between 1.9 and 1.1 Ma suggests that volcanic explosive activity was strongly reduced or quiescent.     

Some observations on the ignimbrites,lava domes and lava flows of M. Cimino (Central Italy)

Cimino Volcano, in Northern Latium, is one of the most characteristic centers of the early Quaternary acid post-orogenetic volcanism in Central Italy. Three stages in the volcanic activity are distinguished: extrusion of several lava domes; ignimbrite cruptions; effusion of more or less differentiated lava flows. Ignimbrites, that form a shield of over 300 sq. km, show characters of welding variable with the distance from the source area and, in a single section, with the level in the sheet. Most part of the Cimino volcanics (domes and ignimbrites) have a quartzlatite composition. The olivine-trachyte (selagite) inclusions in the domes and ignimbrites, and the dark quartz-latite to olivine-trachyte lava flows, could be the products of a pneumatolitic differentiation in the various stages of the magmatic cycle.     

Gariboldi volcanic complex,Ethiopia

The Garibaldi Complex is one of a chain of predominantly silicic volcanic cones along the centre of the Main Ethiopian Rift, which form part of the Pleistocene-Recent Aden Series (Mohr, 1962a). The present form of the Complex is largely a result of silicic conebuilding episodes, between which ignimbrites were erupted and areas collapsed to form calderas, and to a lesser extent of recent basalt eruptions. Comparisons are made with other areas of caldera collapse and attention drawn to the possible relationship between caldera complexes and plutonic ring structures.