Volcanic Succession Research Articles

The Ediacaran to Cambrian Rift System of Southeastern South America: Tectonic Implications

The tectonic evolution of southeastern South America from the Middle Ediacaran to the Early Cambrian is marked by a series of small fault‐bounded siliciclastic and volcaniclastic basins and voluminous coeval granites traditionally associated with the compressional or transpressional tectonics of the late stages of the Pan‐African‐Brasiliano orogeny. Most existing models consider these basins separately, with distinct tectonic evolutionary histories according to local geological settings. However, new and recently published age constraints, lithological similarities, and structural aspects point to the correlation of all Ediacaran to Cambrian basins in southeastern South America within a common basin system more than 1500 km long. The interpretation of a common origin for all Ediacaran to Cambrian basins of southeastern South America implies that all the different terranes of the Brasiliano orogenic collage in the region were already united in a single plate at approximately 600 Ma. An extensional origin for this basin system is interpreted from the recognition of basin‐forming normal faults (later reactivated as strike‐slip or inverse) feeding alluvial fans and from expressive basic to acidic volcanic successions in several basins. The occurrence of basic, intermediate, and acidic volcanic rocks and voluminous coeval granites indicates that mantle and crustal fusion were simultaneous with the extensional event. Raised temperatures may have caused the thermal weakening of the lithosphere, enabling both extensional deformation and recurring strike‐slip deformation that formed major shear zones in the region. This strike‐slip deformation has been mistaken for basin‐forming tectonics, but it occurred in the Early Cambrian, after the formation of the basins, and most probably was the result of the far‐field propagation of compressional stresses originating in younger collisional orogens at the plate margins.

Eocene potassic and ultrapotassic volcanism in south Tibet: New constraints on mantle source characteristics and geodynamic processes

In the Yangbajing area, southern Tibet, several monogenic volcanoes were conformably superimposed on the Linzizong calc-alkaline volcanic successions. According to their petrologic and geochemical characteristics, these monogenic volcanoes are composed of three rock varieties: tephritic phonolitic plugs and shoshonitic and trachytic lavas. Their geochemical systematics reveals that low-pressure evolutionary processes in the large voluminous Linzizong calc-alkaline magmas were not responsible for the generation of these potassic–ultrapotassic rocks, but the significant change in petrologic and geochemical characteristics from the Linzizong calc-alkaline to potassic–ultrapotassic magma is likely accounted for the change of metasomatic agents in the southern Tibetan lithospheric mantle source during the Paleocene to Eocene. The tephritic phonolites containing both leucite and plagioclase show primary ultrapotassic character similar to that of Mediterranean plagioleucititic magmas. Radiogenic Sr increases with SiO 2 in the xenolith-bearing trachytes strongly suggesting significant crustal assimilation in the shoshonitic magmas. The Yangbajing ultrapotassic rocks have high K 2O and Al 2O 3, and show depletion of high field strength elements (HFSEs) with respect to large ion lithophile elements. In primitive mantle-normalized element diagrams, all samples are characterized by positive spikes at Th (U) and Pb with negative anomalies at Ba, Nb–Ta and Ti, reflecting the orogenic nature of the ultrapotassic rocks. They are characterized by highly radiogenic 87Sr/ 86Sr (i) ratios (0.7061–0.7063) and unradiogenic 143Nd/ 144Nd (i) (0.5125), and Pb isotopic compositions ( 206Pb/ 204Pb = 18.688–18.733, 207Pb/ 204Pb = 15.613–15.637, and 208Pb/ 204Pb = 38.861–38.930) similar to the global subducting sediment. Strong enrichment of incompatible trace elements and high Th fractionation from the other HFSEs (such as Nb and U) clearly indicate that the Th-enriched sedimentary component in a network veined mantle source was mainly introduced by sediment-derived melts. In addition, the ultrapotassic rocks have significant Ce (Ce/Ce* = 0.77–0.84) and Eu (Eu/Eu* = 0.72–0.75) anomalies, suggesting a subduction sediment input into the southern Tibetan lithospheric mantle source. In contrast, high U/Th (> 0.20) and Ba/Th (> 32) and low Th/La (< 0.3) in the shoshonites indicate that the Eocene potassic magma originated from partial melting of the surrounding peridotite mantle pervasively affected by slab-related fluid addition from the dehydration of either the subducting oceanic crust or the sediment. Thus, at least two different subduction-related metasomatic agents re-fertilized the upper mantle. According to the radiometric ages and spatial distribution, the Gangdese magmatic association shows a temporal succession from the Linzizong calc-alkaline to ultrapotassic magmas. This indicates a late arrival of recycled sediments within the Tibetan lithospheric mantle wedge. The most diagnostic signatures for the involvement of continent-derived materials are the super-chondritic Zr/Hf (45.5–49.2) and elevated Hf/Sm values (0.81–0.91) in the ultrapotassic rocks. Therefore, the occurrence of orogenic magmatism in the Gangdese belt likely represents the volcanic expression of the onset of the India–Asia collision, preceding the 10 Ma Neo-Tethyan slab break-off process at 42–40 Ma. The absence of residual garnet in the mantle source for the ultrapotassic volcanism seems to imply that the southern Tibetan lithosphere was not been remarkably thickened until the Eocene (∼ 50 Ma).

A volcanic habitat for early life preserved in the Abitibi Greenstone belt, Canada

A breccia facies of mafic composition in the subaqueous Misema shield volcano building phase and Misema caldera of the Blake River caldera complex, Abitibi Greenstone belt, Canada preserves putative microbial ichnofossils in formerly glassy breccia fragments. These Neoarchean ichnofossils are comparable in morphology, distribution, and mineral association to similar textures found in other Archean cratons as well as Phanerozoic ophiolite successions and are interpreted to represent an early Earth subaqueous biosphere. However, a detailed volcanological facies analysis of the rocks that host these purported microbial ichnofossils is lacking from previous studies. The present field locality (Hurd property) is a 62 m-thick volcanic succession that displays two main lithological facies: (i) a massive to lobate facies and (ii) breccia facies. The 5–25 m-thick massive to lobate facies contains massive basalts that grade into lobate flows toward the top. The 1–15 m-thick breccia facies displays delicate volcanic textures that are preserved due to extensive silica precipitation, a low temperature hydrothermal process, and the subgreenschist metamorphic grade. Geochemically, the mafic flows are tholeiitic basalts with TiO 2 contents of 1–2% but high SiO 2, up to 71%, related to hydrothermal silica precipitation. Microbial ichnofossils are found in the glassy fragments of the breccia facies. The breccia facies that displays evidence of associated hydrothermal activity, as in modern seafloor and summit caldera smoker settings, provided a favourable environment for ancient biological interactions between glassy rocks and microorganisms during formation of the Abitibi Greenstone belt.

Middle Jurassic rhyolite volcanism of eastern Graham Land, Antarctic Peninsula: age correlations and stratigraphic relationships

AbstractSilicic volcanism atc.168 Ma has been identified previously on the Antarctic Peninsula, and the Mapple Formation, which includes those volcanic rocks, has been defined and documented from one area of the east coast of Graham Land. Based on age and geochemical criteria, correlations have been made to the extensive Chon Aike Province of South America, which has been demonstrated to be one of the largest silicic volcanic provinces in the world. Rhyolitic and intermediate composition volcanic successions from six separate localities on the east coast of the Antarctic Peninsula are described here and are confirmed as correlatives of the Mapple Formation, based on newly acquired geochronology and field observations. They are dominantly rhyolitic crystal tuffs and/or ignimbrites with ages in the interval 162–168 Ma, overlapping with the age of the Mapple Formation (167–171 Ma) at the type locality. Andesitic agglomerates are also described, which are included in the same event and demonstrate the occurrence of rare intermediate volcanism, which is also seen in the Chon Aike Province. A new group, the Graham Land Volcanic Group, is defined here, and criteria are established which allow the separation of some volcanic successions out of the previously defined Antarctic Peninsula Volcanic Group, which takes no account of tectonic setting, eruption age or geochemistry.

Geological map of Mount Etna West Rift (Italy)

Please click here to download the map associated with this article. Mt Etna (3340 m a.s.l.) is the most active volcano in Europe, with more than 500 ka of geological history. In this work, a detailed structural and geological field survey of its West Rift was performed at 1:10,000 scale, by using lithostratigraphic criteria and unconformity-bounded units, in accordance with guidelines suggested by the International Stratigraphic Guide. In the studied area, lithostratigraphic units have been mapped and the eruptive fissure configuration identified. Fieldwork data were improved by highresolution orthoimages and DEM analysis, producing a map with detailed lava flow boundaries. In order to synthesize the main phases of the West Rift evolution, lithostratigraphic units were grouped into synthems. The volcanic succession starts with the oldest Etna subaerial lava flows (Adrano Synthem), unconformably covered by lavas belonging to Acireale and Concazze Synthems. Mongibello volcanic succession (Il Piano Synthem) widely crops out in the area as several superposed lava flow fields. They are generated by more than 40 eruptive fissures in the past 15,000 years forming the core of the West Rift. The eruptive fissures (strike 245° to 280°) fed several monogenetic cones located in the central sector of the western flank. Some fissures were formed in the last 2,000 years. The result of this work is a geological map at 1:15,000 scale, highlighting the geological and structural setting of the area and significantly improving the knowledge of West Rift evolution, in order to better assess the entire eruptive history and structural framework of Mt Etna.

Geology and geochemistry of the Karadağ volcanic succession, poliocene - quaternary , Central Anatolia ,Turkey

The Karadağ volcanic succession of Pliocene and Oualernaiy represents typical Continen­ tal volcanisms of central Anatolia. It consists of nıainîy lava flows and pyroclastic depositş. Overall, they Avere produced by magmato-phreatic e.Kplosions in four periods throiıgh the plioce­ ne and Çuaternay times. The product of each period can be easiîy distinguished by vvcathering surfaces and localiy erodcd surfaces. Petrographical and geochemical analysis show that they are mostly andcsitic and dacitic roeks, originated from partial rnelting of Continental crust.

Intra-basalt units and base of the volcanic succession east of the Faroe Islands exemplified by interpretation of offshore 3D seismic data

Abstract Recent exploration activities in the basalt covered part of the offshore Faroe Island region has stressed the need for a better understanding of the volcanic succession, especially the lowermost part and the transition to the underlying non-volcanic succession. Using the proposed interpretation methods, which include careful identification of inclined reflector segments and flattening to various reflectors representing pronounced continuous markers embedded horizontally and 3D visualization, six volcanic units have been identified. The upper three units consist of plane-parallel reflector packages and probably represent subaerially extruded lava flows. The three lower units are characterized by irregular hummocky and inclined reflectors and are interpreted as lava deltas composed mainly of hyaloclastites. Within the data coverage area, the direction of extrusion is inferred from the seismic interpretation and four units originate from the NW and two from the SE. It is believed that the base of the volcanic succession is situated at a deeper level than the base of the parallel bedded units. At the base of the basalt succession and in the lower part of the volcanic succession pronounced reflector segments are ascribed to saucer-shaped sills. Analysing the seismic data using principles of seismic stratigraphy and observing them in a 3D environment, it is possible to obtain a broader understanding of the volcanic succession in the area and thereby divide the volcanic succession into characteristic facies units and separate it from the non-volcanic lithology and sills. The study shows that applying the proposed interpretation technique allows an evolution model to be put forward and it is suggested that the approach used in the study can be of use for hydrocarbon exploration in other basalt-covered areas.

Is Efate (Vanuatu, SW Pacific) a result of subaerial or submarine eruption? An alternative model for the 1 Ma Efate Pumice Formation

AbstractThe Efate Pumice Formation (EPF) is a trachydacitic volcaniclastic succession widespread in the central part of Efate Island and also present on Hat and Lelepa islands to the north. The volcanic succession has been inferred to result from a major, entirely subaqueous explosive event north of Efate Island. The accumulated pumice-rich units were previously interpreted to be subaqueous pyroclastic density current deposits on the basis of their bedding, componentry and stratigraphic characteristics. Here we suggest an alternative eruptive scenario for this widespread succession. The major part of the EPF is distributed in central Efate, where pumiceous pyroclastic rock units several hundred meters thick are found within fault scarp cliffs elevated about 800 m above sea level. The basal 200 m of the pumiceous succession is composed of massive to weakly bedded pumiceous lapilli units, each 2-3 m thick. This succession is interbedded with wavy, undulatory and dune bedded pumiceous ash and fine lapilli units with characteristics of co-ignimbrite surges and ground surges. The presence of the surge beds implies that the intervening units comprise a subaerial ignimbrite-dominated succession. There are no sedimentary indicators in the basal units examined that are consistent with water-supported transportation and/or deposition. The subaerial ignimbrite sequence of the EPF is overlain by a shallow marine volcaniclastic Rentanbau Tuffs. The EPF is topped by reef limestone, which presumably preserved the underlying EPF from erosion. We here propose that the EPF was formed by a combination of initial subaerial ignimbrite-forming eruptions, followed by caldera subsidence. The upper volcaniclastic successions in our model represent intra-caldera pumiceous volcaniclastic deposits accumulated in a shallow marine environment in the resultant caldera. The present day elevated position of the succession is a result of a combination of possible caldera resurgence and ongoing arc-related uplift in the region.

Late Miocene submarine volcanism in ANDRILL AND-1B drill core, Ross Embayment, Antarctica

The ANDRILL McMurdo Ice Shelf initiative recovered a 1285-m-long core (AND-1B) composed of cyclic glacimarine sediments with interbedded volcanic deposits. The thickest continuous volcanic sequence by far is ∼175 m long and is found at mid-core depths from 584.19 to 759.32 m below seafloor. The sequence was logged, and initial interpretations of lithostratigraphic subdivisions were made on ice during drilling in late 2006. Subsequent observations, based on image, petrographic, and scanning electron microscopy–energy dispersive spectroscopy analyses, provide a more detailed, revised interpretation of a thick submarine to emergent volcanic succession. The sequence is subdivided into two main subsequences on the basis of sediment composition, texture, and alteration style. The ∼70-m-thick lower subsequence consists mostly of monothematic stacked volcanic-rich mudstone and sandstone deposits, which are attributed to epiclastic gravity flow turbidite processes. This subsequence is consistent with abundant active volcanism that occurred at a distal site with respect to the drill site. The ∼105-m-thick upper subsequence consists mainly of interbedded tuff, lapilli tuff, and volcanic diamictite. A Late Miocene (6.48 Ma) 2.81-m-thick subaqueously emplaced lava flow occurs within the second subsequence. This second subsequence is attributed to recurring cycles of submarine to emergent volcanic activity that occurred proximal to the drill site. This new data set provides (1) the first rock evidence of significant Late Miocene submarine volcanic activity in the Ross Embayment during a period of no to limited glaciation, and (2) a rich stratigraphic record that elucidates submarine volcano-sedimentary processes in an offshore setting.

New exploration opportunities on the southwest Australian margin—deep-water frontier Mentelle Basin

Acreage release by the Australian Government in 2010 offers exploration opportunities in the frontier Mentelle Basin for the first time. The Mentelle Basin is a large deep-water basin on the southwest Australian margin. It consists of a large, very deep water (2,000—4,000 m) depocentre in the west and several depocentres in the east, in water depths of 500–2,000 m. The major depocentres are estimated to contain 7–11 km of sediments. Initial rifting in the Mentelle Basin occurred in the Early Permian, followed by thermal subsidence during the Triassic to Early Jurassic. In the Middle Jurassic renewed extension led to the accumulation of very thick sedimentary successions in half-graben depocentres. Early Cretaceous continental breakup was accompanied by extensive volcanism resulting in a thick syn-breakup volcanic succession in the western Mentelle Basin. Assessment of the petroleum prospectivity of the Mentelle Basin is based on correlations with the adjacent Vlaming Sub-basin. These correlations suggest that the Mentelle Basin depocentres are likely to contain multiple source rock intervals associated with coals and carbonaceous shales, as well as regionally extensive reservoirs and seals within fluvial, lacustrine and marine strata. Petroleum systems modelling suggests that potential source rocks are thermally mature and commenced generation in the Early Cretaceous. The Mentelle Basin offers a wide range of play types, including faulted anticlines and fault blocks, sub-basalt anticlines and fault blocks, drape and forced fold plays, and a large range of stratigraphic and unconformity plays.

Paleofluid evolution of the fractured basalt hydrocarbon reservoir in the Üllés-Ruzsa-Bordány area, SE Hungary

Volcanic successions of the Kecel Basalt Formation (KBF) occur in the southern part of the Pannonian Basin. As a result of periodic submarine eruptions, the basaltic and pyroclastic rock horizons were intercalated with layers of the Late Miocene Endrod Marl Formation, which is regarded as one of the most important hydrocarbon source rocks in the area. The KBF was discovered through almost 30 wells between 2,200 and 2,900 meters of depth. Due to the high fracture porosity, some parts of the formation show good reservoir characteristics and act as important migration pathways of hydrocarbon-bearing fluids. Since the reservoir is presumably fracture-controlled, this study concentrates on the evolution of fractures crosscutting the rock body. Based on textural and mineralogical features, four distinct vein types can be distinguished, of which the first three types are discussed in this paper. Beside calcite, quartz, feldspar, and chlorite, the veins are cemented by various zeolite minerals. The vertical dimension of the dominant zeolite zone indicates the burial-diagenetic type of zeolite zonation and suggests subsidence of the subaqueous basalt after formation.

Petrology of the Paleocene Picrites and Flood Basalts on Disko and Nuussuaq, West Greenland

The c. 62^60 Ma rift-related Paleocene volcanic succession in West Greenland comprises voluminous picrites (the Vaigat Formation) overlain by flood basalts (the Maliga“ t Formation). A detailed stratigraphy for the c. 3 km combined succession on Disko and Nuussuaq has been established that provides evidence of the evolution of the volcanic systems, processes and sources through time. Picrites constitute about one-third of the total erupted volume. The primary magmas for the whole succession were highly magnesian; the weighted average of 16� 6 wt % MgO for the uncontaminated Vaigat Formation is a lower limit for the primary magmas. During the emplacement of the Vaigat Formation, the magmas fractionated olivine in the feeder channels before eruption but generally did not stop in magma chambers. Only minor magma batches stopped in high-level magma chambers, where many became crustally contaminated. At the transition to the Maliga“ t Formation magma chambers developed, presumably in the lower crust. From then on, the main eruption products were basalts with compositions buffered by refilling^fractionation^tapping processes. Large-scale cyclicity can be seen, with more and less fractionated steady-state compositions. The upper part of the Maliga“ t Formation is contaminated throughout, except for the three youngest flows. Repeated pulses of new picritic magmas can be seen in both formations. The asthenospheric mantle source was heterogeneous and the dominant component was depleted, mid-ocean ridge basalt (MORB)-like mantle with Nb/ La51. An additional, less depleted mantle component akin to the Iceland source is evident in the upper Vaigat Formation but disappeared again at the transition to the Maliga“ t Formation. A third mantle component similar to the source for the E-type lavas on Baffin Island may be present in the earliest rocks. Melting took place in the garnet peridotite facies because the melting column was curtailed by the 100 km thick lithosphere. The picrites show coupled isotopic and elemental variations even within single units, indicating that the accumulated primary melts were not homogenized when they left the melting column. An incompatible-element-rich lithospheric mantle component is seen in the ManaŒ tdlat Member of alkaline picrites and basalts. Single scattered tholeiitic lavas somewhat enriched in the same trace elements are considered to be slightly contaminated with this component.The volume of alkaline and enriched rocks is very small. The crustal contaminants are the sediments in the Nuussuaq Basin. The contamination processes included bulk assimilation, inmixing of partial melts of the sediments, assimilation^fractional crystallization processes, and exchange reactions. When carbon-rich shales were assimilated severe reduction led to formation of graphite or metallic iron. All rocks more evolved than basalt arose by contamination processes. The units of contaminated rocks constitute 5^9% of the Vaigat Formation and around 30% of the Maliga“ t Formation.

Causes of plant community divergence in the early stages of volcanic succession

AbstractQuestion: How do temporal changes in plant communities occur after volcanic eruptions? What characteristics determine successional divergence or convergence?Location: The summit area of Mount Usu, northern Japan, completely destroyed by 1‐3 m of thick ash and pumice during the 1977‐1978 eruptions. Habitats were classified into three types: gullies where the pre‐eruption topsoil was exposed due to the erosion of tephra (EG), gullies covered with tephra (CG), and outside of gullies covered with thick tephra (OG).Methods: Plant community structure was monitored for 15 years from 1983 to 1997 in 14 2 m × 5 m permanent plots. The data were summarized by species diversity, life form, and the detrended correspondence analysis.Results: The common species were perennial herbaceous plants, but habitat preferences differed between species. Seed bank species, including a nitrogen fixer Trifolium repens, were dominant in EG, and excluded the establishment of the later colonists. Pioneer trees slowly increased in cover. The detrended correspondence analysis indicated that species composition in the earlier stages did not differ greatly between plots. Thereafter, three patterns of temporal community changes were observed: seed bank species persisted in EG, and in OG and CG forest development proceeded or community structure did not change greatly.Conclusion: Pre‐eruption topsoil contributed to revegetation by the supply of seed bank and nutrients in the earliest stages, but resulted in the delay of forest development due to the persistence of seed bank species. Plant community divergence was driven by the persistence of earlier colonists.

Contrasting geochemistry of the Cretaceous volcanic suites in Shandong province and its implications for the Mesozoic lower crust delamination in the eastern North China craton

The Qingshan volcanic sequences occur in the Mengyin and Jiaozhou basins, west and east of the Tan-Lu fault zone, respectively, were formed at 128±2 and 106–98Ma. Lithologically the Mengyin succession comprises unimodal andesite (53–65wt.% SiO2), whereas the Jiaozhou succession consists of bimodal andesite (56–59wt.% SiO2) and rhyolite (69–77wt.% SiO2). High-Mg (Mg# >60) and low-Mg (Mg# <60) andesitic suites are recognized in the Mengyin volcanic rocks. They show tholeiitic and calc-alkaline trends, respectively. Though the two suites share common trace element features of LILE enrichment and HFSE depletion, higher incompatible element contents in the high-Mg andesite relative to the low-Mg andesite rule out their genetic connection by magmatic differentiation. Similarly, the Jiaozhou andesites also possess higher incompatible element contents compared to their interbedded rhyolites, suggesting their distinct source rocks. The Qingshan mafic volcanic suites are characterized by radiogenic Sr and unradiogenic Nd isotopic compositions, of which the Mengyin high-Mg andesitic rocks display the lowest εNd(t) and highest initial Sr ratio. By contrast, the Jiaozhou rhyolites possess a positive correlation between Sr–Nd isotopes and relatively unradiogenic Sr isotopic ratios.The Qingshan mafic rocks exhibit geochemical characteristics distinct from those of mantle peridotite- or pyroxenite-derived melt as well as from Fangcheng basalts, the only basalts documented in the Cretaceous volcanic suites in Shandong province. They are inferred to be crust-derived melts. However, a clear decreasing trend in ACNK with Mg# increasing and the elevated MgO, Cr and Ni contents relative to basalt-derived melts indicate assimilation with mantle peridotite during their pristine magmatic evolution. Such a two-step process can be best explained by the Archean lower crust delamination of the North China craton with a ~35% partial melting followed by a variable extent of metasomatic interaction with convecting mantle peridotite, which thus resulted in the low-Mg and high-Mg andesitic suites. Nd–Sr isotopic and HFSE features imply that the convecting mantle was mixed with the foundered lithospheric peridotite, which previously experienced subduction-related melt metasomatism during the collision of the Yangtze-North China cratons in the early Triassic. The Qingshan volcanic suites in western Shandong are synchronous with the Tan-Lu strike-slip fault, inferring that the lower crust delamination was initially triggered by sinistral motion of the Tan-Lu fault at ~130Ma. Due to an eastward development of regional lithospheric delamination, the Jiaozhou bimodal volcanic succession was formed owing to the regional geothermal gradient increase and lithosphere thinning. The recognition of contrasting andesitic suites in the Qingshan volcanic succession and their temporal trend provide additional evidence for lower crust delamination during the early Cretaceous in the North China craton and new constraints on its lithosphere thinning model.

Parental magma of the Skaergaard intrusion: constraints from melt inclusions in primitive troctolite blocks and FG-1 dykes

Troctolite blocks with compositions akin to the Hidden Zone are exposed in a tholeiitic dyke cutting across the Skaergaard intrusion, East Greenland. Plagioclase in these blocks contains finely crystallised melt inclusions that we have homogenised to constrain the parental magma to 47.4–49.0 wt.% SiO2, 13.4–14.9 wt.% Al2O3 and 10.7–14.1 wt.% FeOT. These compositions are lower in FeOT and higher in SiO2 than previous estimates and have distinct La/SmN and Dy/YbN ratios that link them to the lowermost volcanic succession (Milne Land Formation) of the regional East Greenland flood basalt province. New major- and trace element compositions for the FG-1 dyke swarm, previously taken to represent Skaergaard magmas, overlap with the entire range of the regional flood basalt succession and do not form a coherent suite of Skaergaard like melts. These dykes are therefore re-interpreted as feeder dykes throughout the main phase of flood basalt volcanism.

Parameters controlling the density of welded ignimbrites—A case study on the Incesu Ignimbrite, Cappadocia, Central Anatolia

Maximum density of the vitrophyre zone of a welded ignimbrite is commonly taken as a direct measure for the degree of welding and thus indirectly for the reconstruction of the paleo-thickness and emplacement temperature of welded ignimbrites [Cas, R.A.F., Wright, J.V., 1987. Volcanic Successions. Chapman and Hall, London; Riehle, J.R., Miller, T.F., Bailey, R.A., 1995. Cooling, degassing and compaction of rhyolitic ash flow tuffs: a computational model. Bull. Volcanol. 57, 319–336]. Variations in primary lithology with respect to phenocrysts and lithic fragments, however, remain unconsidered. Due to the complex formation processes of ignimbrites, a broad variation in primary lithologies is possible, expressed in the established classification of tuffs after Schmid [1981. Descriptive nomenclature and classification of pyroclastic deposits and fragments; recommendations of the IUGS Subcommission on the systematics of igneous rocks. In: Zankl, H., Michot, P., Schwab, K. (Eds) Alfred-Wegener-Symposium I and II. Geol. Rundsch. 70, 794–799]. Nevertheless, the effect of primary lithology on the vitrophyre density may be neglected at low crystal- and lithic-contents: The results of this study on the Incesu Ignimbrite, Central Anatolia, reveal the negligibility of crystals and lithics as their content is only as high as 11 vol% in the vitrophyre zone of the Incesu Ignimbrite. Thus, the uncorrected bulk rock density of the vitrophyre zone of an ignimbrite may serve as a measure of its paleo-thickness at the point sampled. This correlation may then be used to identify the source of an ignimbrite if it is partly eroded.

Products of neptunian eruptions

A common pyroclastic facies in subaqueous volcanic successions comprises massive to graded, very thick (several to tens of meters), laterally extensive (several kilometers) beds of nonwelded pumice lapilli with volumes ranging to tens of cubic kilometers. This facies may be overlain by laminated ash or bimodal ash and giant (>1 m) pumice clasts, and underlain by coarse lithic breccia. The association is inferred to be the typical product of sustained magmatic volatile–driven explosive eruptions from vents at water depths of ~1300–200 m. We propose the term “neptunian” for such eruptions and their products. The eruption column rapidly mixes with the surrounding water, cools, increases in density, and collapses, while remaining under water. Lithic clasts that are too heavy to be entrained in the column are deposited close to the source, forming a neptunian lithic breccia. Pumice lapilli are rapidly waterlogged and form the dominant component in the collapsing column and in eruption-fed, water-supported density currents (neptunian density currents). Hot, buoyant, giant pumice clasts continue to rise and may reach the water surface before being waterlogged and settling, along with temporarily suspended ash, forming neptunian suspension deposits. Eruption magnitude, fragmentation mechanisms, and juvenile pyroclast characteristics, especially vesicularity, are very similar in neptunian and Plinian-style eruptions, but column behavior differs primarily because of the contrasting physical properties of the ambient fluid (water versus air).

Volcanic succession of the Borovnik Member (Mohorje Formation), Bloke Plateau area, Central Slovenia

A 75 m thick volcanic succession of the Borovnik Member, Mohorje Formation in the Bloke Plateau area consists of dacitic and rhyolitic rocks deposited in a shallow-marine environment. Volcanic activity begun with lava flows that underwent extensive disintegration, autobrecciation and mixing with the underlying unconsolidated fine-grained clastic sediments producing dacite/rhyolite-siltstone peperites. Peperites are very rich in fractured plagioclase phenocrysts, and owing to the incorporation of clastic material, they are commonly depleted in silica. The overlying fining-upward pyroclastic sequence is monotonous. Basal parts mainly consist of coarse-grained vitric tuffs that may contain some smaller pumice lapilli. The overlying volcaniclastics are fine-grained vitric tuffs, and in the uppermost parts of the sequence, they are interbedded with cherts. The study confirms the existence of primary volcaniclastic succession in the Bloke Plateau area and excludes its epiclastic or reworked origin.

Precambrian volcanic-associated massive sulfide deposits

Phanerozoic volcanic-associated massive sulfide deposits (VMSD) were subdivided into the Kuroko, Besshi, Cyprus, and Ural types, which differ in their ore geochemistry and mineralogy, in their composition of volcanic ore-hosting rock associations, and in the proportions of felsic and basaltic volcanics and sedimentary rocks in the volcanic successions. The earliest deposits that can be reliably attributed to the above types originated during the Neoproterozoic during the Pangea supercontinental cycle. Archean and Paleoproterozoic VMSD can be accepted as analogues of these types, resembling them in many respects, although differing from the classic deposits of these types in some characteristics. The formation history of Cyprus-type VMSD is traceable to the Paleoproterozoic when deposits in the Outocumpu area dated at approximately 1970 Ma originated during the early stage of Pangea-I amalgamation, whereas the most ancient Besshi-type deposits 1440 million years old originated during the breakup of that supercontinent. Most of the Neoarchean and Paleoproterozoic VMSD are similar in some of their principal features to those of the Ural and Kuroko types. Deposits of both these types and their ancient analogues evolved in the process of the Earth’s evolution and demonstrated unidirectional changes in their ore composition.

Age and Nd–Hf isotopic constraints on the origin of marginal rocks from the Muskox layered intrusion (Nunavut, Canada) and implications for the evolution of the 1.27 Ga Mackenzie large igneous province

The Muskox mafic–ultramafic layered intrusion, Nunavut, is part of the widespread 1.27 Ga Mackenzie large igneous province in northern Canada. The age and Nd–Hf isotope geochemistry of samples from the marginal zone of the Muskox intrusion and adjacent gneisses allow for an assessment of contamination mechanisms along the floor and walls of the intrusion, relationships between the marginal zone and overlying layered series, and genetic links with the Coppermine River flood basalts and Mackenzie dikes. The marginal zone, intersected in two diamond drill holes at stratigraphically low (West Pyrrhotite Lake) and high (Far West Margin) positions along the western margin of the intrusion, comprises a thick upper subzone (∼100–150 m) of olivine and olivine–chromite cumulates and a relatively thin (<10 m) lower subzone containing granophyre-bearing gabbronoritic rocks. U–Pb baddeleyite geochronology yields a crystallization age of the marginal zone of ca. 1269 Ma based on dating of three samples (1268.8 ± 1.7 Ma, chromite-rich peridotite; 1268.7 ± 2.9 Ma, gabbronorite; 1271.5 ± 3.4 Ma, peridotite), which is synchronous with published ages from the layered series and Mackenzie dikes. The peridotites from both sections mostly span a restricted range of isotopic compositions (initial ɛ Nd = −2.8 to +0.5; initial ɛ Hf = −3.9 to +1.5), which is similar to that observed in the overlying layered series and consistent with the lack of significant chamber-wide crustal contamination of the Muskox parent magmas during differentiation and consolidation. In contrast, the gabbronorites within 10 m of the margin of the intrusion (initial ɛ Nd = −13 to −5.5; initial ɛ Hf = −16 to −5.9) were strongly contaminated by melting and exchange with their respective host rocks; the extent of contamination is greater adjacent to paragneiss wallrocks than adjacent to orthogneiss. Crystallization of this thin, contaminated zone along the margin of the Muskox intrusion likely sealed the contacts with the host gneisses and prevented magmas within the interior of the chamber from interacting with the surrounding crustal rocks. Comparison of the Nd isotopic compositions from the marginal zone with published results from the layered series and the Coppermine River flood basalts indicates that only the lowermost portion of the volcanic succession (lower member of the Copper Creek Formation and lowermost part of the middle member) could represent residual magmas that resided and fractionated in the Muskox magma chamber prior to eruption. Construction of the Muskox layered intrusion was thus restricted to the early stages of emplacement of the Mackenzie large igneous province.