Intrusion features of some hypabyssal south African kimberlites

Kimberlite in certain dykes and in the deepest parts of some diatremes show textural and other features which contrast with those in the breccia diatremes. Some hypabyssal kimberlite intrusions show relatively high-temperature contact phenomena including baking of country-rock sediments and sedimentary xenoliths, and contrasting with the brecciated texture of most diatreme-facies kimberlites, in the hypabyssal kimberlites are numerous examples of preferred orientation of inequidimensional minerals (? trachytic flow texture), and rapid mineralogical gradients from the contact towards the dyke centres that may be attributable to flowage differentiation. In the Benfontein sill (Kimberley area) there is well-developed horizontal banding due to gravitational settling, and pseudo-sedimentary structures are also present. The accumulated evidence indicates that kimberlite existed as a relatively hot fluid up to depths of 2–3 km below the land-surface at the time of intrusion; above this level, gas release caused diatreme formation, brecciation and adiabatic cooling. These views are contrary to those of geologists who postulate eruption of kimberlite as a cold breccia directly from the mantle or deep within the crust, but accords with the views of many Russian geologsts who accept the existence of kimberlite magma, the extrusive equivalent of which is the ultrabasic lava meimechite.     

Volcanological features of a low-viscosity melt: the carbonatitic Gross Brukkaros Volcanic Field, Namibia

 In the Upper Cretaceous Gross Brukkaros Volcanic Field, southern Namibia, a radial dyke system surrounds a dome structure and its 74 closely related carbonatite diatremes. This paper focuses on volcanological features which seem to be typical for a low-viscosity melt in various settings such as dykes, sills and diatremes. The total or near absence of vesicles in carbonatite ash grains and lapilli inside the diatremes is evidence against explosive exsolution of volatile phases and in favour of a phreatomagmatic fragmentation mechanism and thus for a phreatomagmatic eruption mechanism of the carbonatite diatremes. Received: 15 August 1996 / Accepted: 13 January 1997     

Volcanology of the Aries micaceous kimberlite,central Kimberley Basin,Western Australia

The Neoproterozoic (815.4 ± 4.3 Ma) Aries kimberlite intrudes the King Leopold Sandstone and the Carson Volcanics in the central Kimberley Basin, northern Western Australia. Aries is comprised of a N–NNE-trending series of three diatremes and associated hypabyssal kimberlite dykes and plugs. The diatremes are volumetrically dominated by massive, clast-supported, accidental lithic-rich kimberlite breccias that were intruded by hypabyssal macrocrystic phlogopite kimberlite dykes and plugs with variably uniform- to globular segregationary-textured groundmasses. Lower-diatreme facies, accidental lithic-rich breccias probably formed through fall-back of debris into the vent with a major contribution from the collapse of the vent walls. These massive breccias are overlain by a sequence of bedded volcaniclastic breccias in the upper part of the north lobe diatreme. Abundant, poorly vesicular to nonvesicular, juvenile kimberlite ash and lapilli, with morphologies that are indicative of phreatomagmatic fragmentation processes, occur in a reversely graded volcaniclastic kimberlite breccia unit at the base of this sequence. This unit and overlying bedded accidental lithic-rich breccias are interpreted to be sediment gravity-flow deposits (including possible debris flows) derived from the collapse of the crater walls and/or tephra ring deposits that surrounded the crater. Diatreme-forming eruptions may have been initiated by magma–water interactions along fracture and joint-controlled aquifers within the King Leopold Sandstone. The current level of exposure of the diatremes probably extends from the lower-diatreme facies up into the base of a bedded upper-diatreme sequence.     

On the origin of diamantiferous diatremes

Diamantiferous diatremes usually occur in the old platforms and shields where deep fractures are «blind»,i.e., these fractures do not come out to the earth surface. Alkaline-ultrabasic magma ascending along these fractures and encountering an impervious cap of sedimentary and/or volcanic rocks had formed, between the cap and the basemnet rocks, intermediate chambers in which the crystallization of diamonds took place. Under the influence of the increasing pressures in these chambers, the roofs were destroyed and diamantiferous diatremes, dykes and veins of kimberlite have been formed. These diatremes are filled with a typical eruptive breccia in which the fragmental material, formed by the destructive explosion of the magma chamber roof, is cemented by a porphyritic, alkaline-ultrabasic rock known under the name of kimberlite.     

Lamproites and kimberlites in China and the genesis of diamond deposit

It is known that the lamproites occur in the southeastern Guizhou Province and in the Dahong Mountains area, Hubei Province; and many para-lamproite occurrences spread in the west half part of the South China landmass. The para-larnproite diatremes in Ningxiang County, Hunan Province, contain a few of fine grains of diamond. Parts of the kimberlite pipes and dykes in Mengyin County, Shandong Province, consist locally of basic kimberlite; and the kimberlite dykes in the Maping kimberlite area, Zhenyuan County, Guizhou Province, consist of basic kimberlite principally. Although the diamondiferous kimberlites and lamproites occur always in the cratons within continental plate, both the potassium-rich ultramafic rocks display the geochemical features of the magmatism of post-collision in orogenic belt. Both the kimberlite and lamproite magmas may originate from the local parts of the mantle transition zone, where the mantle contains the matter of ancestor slab of ancient subduction zone. And, both the K-rich ultramafic magmas generated in an active mantle plume, which came from the boundary between the core and the lower mantle. The basic kimberlite magma may be more capable of preserving the crystals of the diamond type 11.     

A rare occurrence of a crater-filling clastogenic extrusive coherent kimberlite,Victor Northwest (Ontario,Canada)

Kimberlite pipes can contain significant proportions of dark and dense kimberlite that have mostly been interpreted as intrusive coherent (hypabyssal) in origin. This study reports a well-documented occurrence of a fresh intra-crater clastogenic extrusive coherent kimberlite that is concluded to have formed as a result of lava fountaining. This paper focuses on a dark, dense, competent, generally crystal-rich, massive kimberlite unit within the Victor Northwest kimberlite pipe (Ontario, Canada). Using a comprehensive volcanological and petrographic analysis of all available drill cores, it is shown that this unit has a fresh well-crystallised coherent groundmass and is extrusive and pyroclastic in origin. The proposed clastogenic coherent extrusive origin is based on deposit morphology, gradational contacts to enveloping pyroclastic units, as well as the presence of remnant pyroclast outlines and angular broken olivines. This paper, and an increasing number of other studies, suggest that fragmental extrusive coherent kimberlite in intra-crater settings may be more common than previously thought. The emplacement history and volcanology of these pipes need to be reconsidered based on the emerging importance of this particular kimberlite facies.     

Reconstruction of a kimberlite eruption,using an integrated volcanological,geochemical and numerical approach: A case study of the Fox Kimberlite,NWT, Canada

An integrated approach involving volcanology, geochemistry and numerical modelling has enabled the reconstruction of the volcanic history of the Fox kimberlite pipe. The observed deposits within the vent include a basal massive, poorly sorted, matrix supported, lithic fragment rich, eruption column collapse lapilli tuff. Extensive vent widening during the climactic magmatic phase of the eruption led to overloading of the eruption column with cold dense country rock lithic fragments, dense juvenile pyroclasts and olivine crystals, triggering column collapse. > 40% dilution of the kimberlite by granodiorite country rock lithic fragments is observed both in the physical componentry of the rocks and in the geochemical signature, where enrichment in Al₂O₃ and Na₂O compared to average values for coherent kimberlite is seen. The wide, deep, open vent provided a trap for a significant proportion of the collapsing column material, preventing large scale run-away in the form of pyroclastic flow onto the ground surface, although minor flows probably also occurred. A massive to diffusely bedded, poorly sorted, matrix supported, accretionary-lapilli bearing, lithic fragment rich, lapilli tuff overlies the column collapse deposit providing evidence for a late phreatomagmatic eruption stage, caused by the explosive interaction of external water with residual magma. Correlation of pipe morphology and internal stratigraphy indicate that widening of the pipe occurred during this latter stage and a thick granodiorite cobble-boulder breccia was deposited. Ash- and accretionary lapilli-rich tephra, deposited on the crater rim during the late phreatomagmatic stage, was subsequently resedimented into the vent. Incompatible elements such as Nb are used as indicators of the proportion of the melt fraction, or kimberlite ash, retained or removed by eruptive processes. When compared to average coherent kimberlite the ash-rich deposits exhibit ~ 30% loss of fines whereas the column collapse deposit exhibits ~ 50% loss. This shows that despite the poorly sorted nature of the column collapse deposit significant elutriation has occurred during the eruption, indicating the existence of a high sustained eruption column. The deposits within Fox record a complex eruption sequence showing a transition from a probable violent sub-plinian style eruption, driven by instantaneous exsolution of magmatic volatiles, to a late phreatomagmatic eruption phase. Mass eruption rate and duration of the sub-plinian phase of the eruption have been determined based on the dimensions of milled country-rock boulders found within the intra-vent deposits. Calculations show a short lived eruption of one to eleven days for the sub-plinian magmatic phase, which is similar in duration to small volume basaltic eruptions. This is in general agreement with durations of kimberlite eruptions calculated using entirely different approaches and parameters, such as predictions of magma ascent rates in kimberlite dykes.     

Criteria for interpreting kimberlite as coherent: insights from the Muskox and Jericho kimberlites (Nunavut,Canada)

The Jurassic Muskox and Jericho kimberlites (Northern Slave Province, Nunavut, Canada) contain a variety of facies exhibiting different geometries, contact relationships, internal organisation, country rock abundance and olivine shapes, although many have similar matrix/groundmass mineralogies and textures. Five facies are examined that either have characteristics consistent with coherent rocks in general (i.e. intrusive and extrusive non-fragmental rocks) or are mineralogically and texturally similar to kimberlite described as coherent (or apparent coherent). Three facies are interpreted as coherent on the basis of: (1) geological setting, (2) apparent-porphyritic texture, (3) sharp contacts with fragmental kimberlite, (4) relative abundance of elongate and unbroken olivine crystals and (5) paucity of country rock xenoliths, while the remaining two facies are interpreted as fragmental on the basis of: (1) the gradational contacts with demonstrably fragmental kimberlite, (2) relative abundance and range of sizes of country rock lithic clasts and (3) numerous broken olivine crystals. Comparisons are made with coherent and apparent-coherent kimberlite from the literature. Our three coherent facies are similar to literature reported coherent kimberlite dykes hosted in country rock (CK_d) in terms of internal organisation, low abundance of country rock xenoliths, and apparent-porphyritic texture. Conversely, our two fragmental facies share attributes with previously described pipe-filling coherent and apparent-coherent kimberlite (CK_pf) in terms of geometry, internal organisation and abundance of country rock xenoliths. We conclude that CK_d and most CK_pf, although similar in matrix/groundmass mineralogy and texture, can be distinguished on the basis of internal organisation, country rock lithic clast abundance, texture (e.g. apparent-porphyritic texture) and possibly olivine crystal shapes and suggest that fragmental kimberlite is more common than reported.     

Fragmentation in kimberlite: products and intensity of explosive eruption

The explosive eruption of kimberlite magma is capable of producing a variety of pyroclast sizes, shapes, and textures. However, all pyroclastic deposits of kimberlite comprise two main types of pyroclasts: (1) pyroclasts of kimberlite with or without enclosed olivine crystals and (2) olivine crystals which lack coatings of kimberlite. Here, we propose two hypotheses for how kimberlite magmas are modified due to explosive eruption: (1) olivine crystals break during kimberlite eruption, and (2) kimberlite melt can be efficiently separated from crystals during eruption. These ideas are tested against data collected from field study and image analysis of coherent kimberlite and fragmental kimberlite from kimberlite pipes at Diavik, NT. Olivines are expected to break because of rapid pressure changes during the explosive eruption. Disruption of kimberlite magma, and pyroclast production, is driven by ductile deformation processes, rather than by brittle fragmentation. The extent to which melt separates from olivine crystals to produce kimberlite-free crystals is a direct consequence of the relative proportions of gas, melt and crystals. Lastly, the properties of juvenile pyroclasts in deposits of pyroclastic kimberlite are used to index the relative intensity of kimberlite eruptions. A fragmentation index is proposed for kimberlite eruption based on: (a) crystal size distributions of olivine and on (b) ratios of selvage-free olivine pyroclasts to pyroclasts of kimberlite with or without olivine crystals.     

The Fekete-hegy (Balaton Highland Hungary) “soft-substrate” and “hard-substrate” maar volcanoes in an aligned volcanic complex – Implications for vent geometry,subsurface stratigraphy and the palaeoenvironmental setting

The Fekete-hegy volcanic complex is located in the centre of the Bakony Balaton Highland Volcanic Field, in the Pannonian Basin, which formed from the late Miocene to Pliocene period. The eruption of at least four very closely clustered maar volcanoes into two clearly distinct types of prevolcanic rocks allows the observation and comparison of hard-substrate and soft-substrate maars in one volcanic complex. The analyses of bedding features, determination of the proportion of accidental lithic clasts, granulometry and age determination helped to identify and distinguish the two types of maar volcanoes. Ascending magma interacted with meteoric water in karst aquifers in Mesozoic carbonates, as well as in porous media aquifers in Neogene unconsolidated, wet, siliciclastic sediments. The divided basement setting is reflected by distinct bedding characteristics and morphological features of the individual volcanic edifices as well as a distinct composition of pyroclastic rocks. Country rocks in hard-substrate maars have a steep angle of repose, leading to the formation of steep sided cone-shaped diatremes. Enlargement and filling of these diatreme is mainly a result of shattering material by FCI related shock waves and wall-rock collapse during downward penetration of the explosion locus. Country rocks in soft-substrate maars have much shallower angles of repose, leading to the formation of broad, bowl shaped structures or irregular depressions. Enlargement and filling of these diatremes is mainly the result of substrate collapse, for example due to liquefaction of unconsolidated material by FCI-related shock waves, and its emplacement by gravity flows. The Fekete-hegy is an important example illustrating that the substrate of a volcanic edifice has to be taken into account as an important interface, which can have major control on phreatomagmatic eruptions and the resulting characteristics of the volcanic complex.     

Emplacement mechanisms of sill complexes: Information from the geochemical architecture of the Golden Valley Sill Complex,South Africa

The well-exposed Golden Valley Sill Complex, Karoo basin, South Africa, consists of four large sills (ca. 100 m thick; long axes: 13–24 km), one small sill (55–80 m thick; long axis: 4 km; forming an appendix to one of the large sills), and two large dykes (15–20 m thick; 25 and 70 km long), plus some minor intrusions. Field mapping shows physical connections between the small sill and one of the large sills, but no other connection between the large sills, or connections between the sills and the large dykes.     

The influence of complex intra- and extra-vent processes on facies characteristics of the Koala Kimberlite,NWT, Canada: volcanology,sedimentology and intrusive processes

The Koala kimberlite, Northwest Territories, Canada, is a small pipe-like body that was emplaced into the Archean Koala granodiorite batholith and the overlying Cretaceous to Tertiary sediments at ~53 Ma. Koala is predominantly in-filled by a series of six distinct clastic deposits, the lowermost of which has been intruded by a late stage coherent kimberlite body. The clastic facies are easily distinguished from each other by variations in texture, and in the abundance and distribution of the dominant components. From facies analysis, we infer that the pipe was initially partially filled by a massive, poorly sorted, matrix-supported, olivine-rich lapilli tuff formed from a collapsing eruption column during the waning stage of the pipe-forming eruption. This unit is overlain by a granodiorite cobble-boulder breccia and a massive, poorly sorted, mud-rich pebbly-sandstone. These deposits represent post-eruptive gravitational collapse of the unstable pipe walls and mass wasting of tephra forming the crater rim. The crater then filled with water within which ~20 m of non-kimberlitic, wood-rich, silty sand accumulated, representing up to 47,000 years of quiescence. The upper two units in the Koala pipe are both olivine rich and show distinct grain-size grading. These units are interpreted to have been deposited sub-aqueously, from pyroclastic flows sourced from one or more other kimberlite volcanoes. The uppermost units in the Koala pipe highlight the likelihood that some kimberlite pipes may be only partially filled by their own eruptive products at the cessation of volcanic activity, enabling them to act as depocentres for pyroclastic and sedimentary deposits from the surrounding volcanic landscape. Recognition of these exotic kimberlite deposits has implications for kimberlite eruption and emplacement processes.     

Geology of the BK9 kimberlite (Damtshaa,Botswana): implications for the formation of dark volcaniclastic kimberlite

The BK9 kimberlite consists of three overlapping pipes. It contains two dark varieties of massive volcaniclastic kimberlite, informally termed dark volcaniclastic kimberlite (DVK). DVK(ns) is present in the north and south pipes and is interbedded with lenses of basalt breccia at the margins of the pipes. DVK(c) is present within the central pipe where it is overlain by a sequence of basalt breccias with interbedded volcanogenic sediments. The features observed within the DVK units of the BK9 kimberlite provide strong evidence for gas fluidisation of the accumulating pyroclastic material. These include the massive interior of the pipes, marginal epiclastic units, well-dispersed country-rock xenoliths and small-scale heterogeneities in lithic clast abundance. The upper portions of the central pipe provide a record of the transition from pyroclastic eruption and infill to passive epiclastic infilling of the crater, after the eruption has ceased. The wall-rock of the BK9 kimberlite dips inwards and is interpreted as post pipe-fill subsidence of the adjacent country rock. The two DVK units contain interstitial, silt-sized pyroclasts. The DVK(ns) has a higher fraction of former melt and displays evidence of incipient welding, as a result of differences in eruption dynamics. These units demonstrate that whilst DVK is comparable in many respects to MVK and forms part of a spectrum of volcaniclastic rocks formed by fluidisation, it differs in frequently containing silt-sized particles and including agglutinated and welded varieties with a high melt fraction. The DVK varieties, studied here, also have a distinctive hydrothermal assemblage, resulting from the abundance of low-silica accidental lithic clasts. Both the hydrothermal alteration and the abundance of silt-sized particles contribute to the DVKs distinctive dark colour.     

The Karoo igneous province — A problem area for inferring tectonic setting from basalt geochemistry

Tholeiitic basalts and associated intrusives are the major component of the Karoo igneous province. They are of Mesozoic age and constitute one of the world's classic continental flood basalt (CFB) provinces. It has been argued that most Karoo basalts have not undergone significant contamination with continental crust and that their lithospheric mantle source areas were enriched in incompatible minor and trace elements during the Proterozoic. The only exceptions to this are late-stage MORB-like dolerites near the present-day continental margins which are considered to be of asthenospheric origin.When data for the “southern” Karoo basalts are plotted on many of the geochemical discriminant diagrams which have been used to infer tectonic setting, essentially all of them would be classified as calc-alkali basalts (CAB's) or low-K tholeiites. Virtually none of them plot in the compositional fields designated as characteristic of “within-plate” basalts. There is little likelihood that the compositions of the Karoo basalts can be controlled by active subduction at the time of their eruption and no convincing evidence that a “subduction component” has been added to the subcontinental lithospheric mantle under the entire area in which the basalts crop out. It must be concluded that the mantle source areas for CAB's and the southern Karoo basalts have marked similarities.In contrast, the data for “northern” Karoo basalts largely plot in the “within-plate” field on geochemical discriminant diagrams. Available data suggest that the source composition and/or the restite mineralogy and degree of partial melting are different for southern and northern Karoo basalts. There is no evidence for any difference in tectonic setting between the southern and northern Karoo basalts at the time they were erupted. This appears to be clear evidence that specific mantle source characteristics and/or magmatic processes can vary within a single CFB province to an extent that renders at least some geochemical discriminant diagrams most unreliable for classifying tectonic environment with respect to continental volcanic rocks.     

Magnetic and Electromagnetic Signatures around Polile Tshisa Hot Spring in the Northern Neotectonic Belt in the Eastern Cape Province,South Africa

Finding productive boreholes in the Karoo fractured aquifers has never been an easy task. Fractured Karoo aquifers in the neotectonic zones in the Eastern Cape Province can be targeted for groundwater exploration. The Polile Tshisa hot spring is located in a seismo-tectonic region beset by neotectonics. Hot springs are indicative of circulation of groundwater at great depths along fault zones, and accordingly of neotectonics. The characterisation of hot springs by means of magnetic and electromagnetic methods can help infer the occurrence of structures which are favourable for groundwater potential. The Polile Tshisa hot spring is characterised by faults, fractures, and dolerite dykes. All these structures make the hot spring a good case study for groundwater exploration.     

QUANTITATIVE INTERPRETATION OF PULSE EM MEASUREMENTS OVER A WEATHERED KIMBERLITE DIATREME*

The application of pulse EM and MAXMIN II systems in search of a kimberlite diatremes under conducting cover has been demonstrated through a field example. The effectiveness of the PEM system in the search for conductive zones of economic importance and in discrimination against geologic noise such as overburden has been investigated. The transient observation has been transformed to equivalent half-space resistivities at various times. The feasibility of mapping thin conducting overburden using the PEM system has been studied by inverting the transient measurement to the thin infinite horizontal sheet conductance. The PEM measurement is shown to be an economic and effective tool for shallow geoelectric mapping.     

On the conditions of sheet injections and eruptions in stratovolcanoes

Nearly all eruptions in stratovolcanoes (composite volcanoes, central volcanoes) are supplied with magma through fractures. Consequently, a primary physical condition for an eruption to occur in a stratovolcano is that a magma-driven fracture is able to propagate to the surface. Magma-filled fractures, frozen or fluid, are referred to as sheet intrusions. More specifically, they are named dykes when subvertical, and inclined (or cone) sheets when inclined. Field observations indicate that most sheet intrusions do not reach the surface to feed eruptions but rather become arrested at various crustal depths. For this reason periods of volcanic unrest with sheet injections are much more common than volcanic eruptions. Whether a sheet intrusion becomes arrested or, alternatively, propagates to the surface depends primarily on the stress field in the stratovolcano. A stratovolcano normally consists of layers of contrasting mechanical properties, such as soft (low Youngs modulus) pyroclastic units and stiff (high Youngs modulus) lava flows. We present numerical models indicating that volcanoes composed of such layers commonly develop stress fields encouraging sheet and dyke arrest. The models indicate that a necessary condition for a sheet intrusion to reach the surface and feed a volcanic eruption is that the stress field along the sheet pathway becomes homogenised. We propose that much of the activity in a stratovolcano during a volcanic cycle encourages stress-field homogenisation. Field studies show that the sheet intrusions in individual stratovolcanoes have various dips: some are vertical dykes, others inclined sheets, and still others horizontal sills. Analytical models indicate that the dip of a sheet reaching the surface can have great effects on the magma transport during an eruption. This effect is normally greater for a flat volcano such as a collapse caldera than for a stratovolcano that forms a topographic high. We conclude that the shallower the dip of a sheet intrusion, the less will be its volumetric magma transport to the surface of a stratovolcano.Editorial responsibility: D Dingwell     

Mode of eruption of the deccan trap basalts

The Deccan Trap basalts have long been considered to be products of fissure eruptions and the dykes intruding them have been supposed to represent the fissures of eruption. However, the question of the mode of eruption of Deccan Trap lavas seems to need more careful consideration in view of the features brought to light by detailed field work in Western Maharashtra. Detailed studies of dykes suggest that majority of the dykes could not have acted as feeders as previously supposed. When examined in detail the basalt flows have more often been found to have only a limited lateral extent and are not always quite horizontal. Thin irregular flows with ropy surfaces, dipping in different directions and piled up into a chaotic mass are frequently met with, indicating eruption from local vents of the central type. Volcanic vents are found at a number of widely separated localities. All this suggests that many lavas are products of central type of volcanicity. However, central type of volcanoes would be inadequate to account for the vast amounts of lavas, and as the known dykes are not likely to have acted as feeders, and dykes still remain to be reported from large portions of the Deccan Trap area, the question of how the lavas came to the surface largely remains unanswered. Extensive beds of volcanic breccia traceable over a few miles and upto 50 feet thick are met with. As such extensive beds are more likely to be associated with fissure cruptions the question arises whether these fissure eruptions were accompanied by considerable explosive activity.     

The dynamics of magma withdrawal from a density stratified dyke

Eruptions from the top of a dyke containing two layers of magma can selectively withdraw the upper layer, leaving the dense lower layer undisturbed. Alternatively, if the upper layer is thinner than some critical depth, d, then both layers will be tapped simultaneously. Laboratory experiments yield an equation giving the draw-up depth, d, as a function of dyke geometry, eruption rate, and magma properties. This equation is valid for low to moderate Reynolds numbers and applies to dykes which are much longer than the draw-up depth. Short dykes will yield larger draw-up depths than are predicted by the equation. A large draw-up depth is favoured when the eruption rate, upper layer magma viscosity, or dyke length/breadth ratio is large or the density difference is small. Calculations show that rhyolite-capped dykes can contain several hundred metres thickness of rhyolite when a lower layer is first tapped. Draw-up depths in a dyke are as much as an order of magnitude greater than those for an identical eruption from a large cylindrical chamber tapped by a central vent. Nonetheless, for low effusion rate eruptions from small dykes, as at Inyo Domes, California, relatively small draw-up heights are calculated (e.g. 70 m). This is compatible with the small amounts of mixed magmas found at the transition between the two rhyolite magmas erupted there [11].     

Eruption of kimberlite magmas: physical volcanology, geomorphology and age of the youngest kimberlitic volcanoes known on earth (the Upper Pleistocene/Holocene Igwisi Hills volcanoes, Tanzania)

The Igwisi Hills volcanoes (IHV), Tanzania, are unique and important in preserving extra-crater lavas and pyroclastic edifices. They provide critical insights into the eruptive behaviour of kimberlite magmas that are not available at other known kimberlite volcanoes. Cosmogenic ³He dating of olivine crystals from IHV lavas and palaeomagnetic analyses indicates that they are Upper Pleistocene to Holocene in age. This makes them the youngest known kimberlite bodies on Earth by >30?Ma and may indicate a new phase of kimberlite volcanism on the Tanzania craton. Geological mapping, Global Positioning System surveying and field investigations reveal that each volcano comprises partially eroded pyroclastic edifices, craters and lavas. The volcanoes stand <40?m above the surrounding ground and are comparable in size to small monogenetic basaltic volcanoes. Pyroclastic cones consist of diffusely layered pyroclastic fall deposits comprising scoriaceous, pelletal and dense juvenile pyroclasts. Pyroclasts are similar to those documented in many ancient kimberlite pipes, indicating overlap in magma fragmentation dynamics between the Igwisi eruptions and other kimberlite eruptions. Characteristics of the pyroclastic cone deposits, including an absence of ballistic clasts and dominantly poorly vesicular scoria lapillistones and lapilli tuffs, indicate relatively weak explosive activity. Lava flow features indicate unexpectedly high viscosities (estimated at >10² to 10⁶?Pa?s) for kimberlite, attributed to degassing and in-vent cooling. Each volcano is inferred to be the result of a small-volume, short-lived (days to weeks) monogenetic eruption. The eruptive processes of each Igwisi volcano were broadly similar and developed through three phases: (1) fallout of lithic-bearing pyroclastic rocks during explosive excavation of craters and conduits; (2) fallout of juvenile lapilli from unsteady eruption columns and the construction of pyroclastic edifices around the vent; and (3) effusion of degassed viscous magma as lava flows. These processes are similar to those observed for other small-volume monogenetic eruptions (e.g. of basaltic magma).